Merge remote-tracking branch 'origin/master' into lfs

src/external-api-docs/doxygen.cfg.in

@@ -56,3 +56,5 @@ GENERATE_TREEVIEW = YES
 OPTIMIZE_OUTPUT_FOR_C  = YES
 
 USE_MDFILE_AS_MAINPAGE = doc/external-api/README.md
+
+QUIET = YES

src/external-api-docs/package.nix

@@ -1,8 +1,6 @@
 { lib
 , mkMesonDerivation
 
-, meson
-, ninja
 , doxygen
 
 # Configuration Options
@@ -37,8 +35,6 @@ mkMesonDerivation (finalAttrs: {
     ];
 
   nativeBuildInputs = [
-    meson
-    ninja
     doxygen
   ];
 

src/internal-api-docs/doxygen.cfg.in

@@ -41,21 +41,21 @@ INPUT                  = \
   @src@/libcmd \
   @src@/libexpr \
   @src@/libexpr/flake \
-  @src@/nix-expr-tests \
-  @src@/nix-expr-tests/value \
-  @src@/nix-expr-test-support/test \
-  @src@/nix-expr-test-support/test/value \
+  @src@/libexpr-tests \
+  @src@/libexpr-tests/value \
+  @src@/libexpr-test-support/test \
+  @src@/libexpr-test-support/test/value \
   @src@/libexpr/value \
   @src@/libfetchers \
   @src@/libmain \
   @src@/libstore \
   @src@/libstore/build \
   @src@/libstore/builtins \
-  @src@/nix-store-tests \
-  @src@/nix-store-test-support/test \
+  @src@/libstore-tests \
+  @src@/libstore-test-support/test \
   @src@/libutil \
-  @src@/nix-util-tests \
-  @src@/nix-util-test-support/test \
+  @src@/libutil-tests \
+  @src@/libutil-test-support/test \
   @src@/nix \
   @src@/nix-env \
   @src@/nix-store
@@ -97,3 +97,6 @@ EXPAND_AS_DEFINED      = \
   DECLARE_WORKER_SERIALISER \
   DECLARE_SERVE_SERIALISER \
   LENGTH_PREFIXED_PROTO_HELPER
+
+WARN_IF_UNDOCUMENTED = NO
+QUIET = YES

src/internal-api-docs/package.nix

@@ -1,8 +1,6 @@
 { lib
 , mkMesonDerivation
 
-, meson
-, ninja
 , doxygen
 
 # Configuration Options
@@ -32,8 +30,6 @@ mkMesonDerivation (finalAttrs: {
   ];
 
   nativeBuildInputs = [
-    meson
-    ninja
     doxygen
   ];
 

src/libcmd/command.cc

@@ -1,3 +1,4 @@
+#include <algorithm>
 #include <nlohmann/json.hpp>
 
 #include "command.hh"
@@ -9,8 +10,7 @@
 #include "profiles.hh"
 #include "repl.hh"
 #include "strings.hh"
-
-extern char * * environ __attribute__((weak));
+#include "environment-variables.hh"
 
 namespace nix {
 
@@ -23,7 +23,8 @@ nix::Commands RegisterCommand::getCommandsFor(const std::vector<std::string> & p
         if (name.size() == prefix.size() + 1) {
             bool equal = true;
             for (size_t i = 0; i < prefix.size(); ++i)
-                if (name[i] != prefix[i]) equal = false;
+                if (name[i] != prefix[i])
+                    equal = false;
             if (equal)
                 res.insert_or_assign(name[prefix.size()], command);
         }
@@ -42,16 +43,16 @@ void NixMultiCommand::run()
         std::set<std::string> subCommandTextLines;
         for (auto & [name, _] : commands)
             subCommandTextLines.insert(fmt("- `%s`", name));
-        std::string markdownError = fmt("`nix %s` requires a sub-command. Available sub-commands:\n\n%s\n",
-                commandName, concatStringsSep("\n", subCommandTextLines));
+        std::string markdownError =
+            fmt("`nix %s` requires a sub-command. Available sub-commands:\n\n%s\n",
+                commandName,
+                concatStringsSep("\n", subCommandTextLines));
         throw UsageError(renderMarkdownToTerminal(markdownError));
     }
     command->second->run();
 }
 
-StoreCommand::StoreCommand()
-{
-}
+StoreCommand::StoreCommand() {}
 
 ref<Store> StoreCommand::getStore()
 {
@@ -126,15 +127,8 @@ ref<Store> EvalCommand::getEvalStore()
 ref<EvalState> EvalCommand::getEvalState()
 {
     if (!evalState) {
-        evalState =
-            #if HAVE_BOEHMGC
-            std::allocate_shared<EvalState>(
-                traceable_allocator<EvalState>(),
-            #else
-            std::make_shared<EvalState>(
-            #endif
-                lookupPath, getEvalStore(), fetchSettings, evalSettings, getStore())
-            ;
+        evalState = std::allocate_shared<EvalState>(
+            traceable_allocator<EvalState>(), lookupPath, getEvalStore(), fetchSettings, evalSettings, getStore());
 
         evalState->repair = repair;
 
@@ -149,7 +143,8 @@ MixOperateOnOptions::MixOperateOnOptions()
 {
     addFlag({
         .longName = "derivation",
-        .description = "Operate on the [store derivation](@docroot@/glossary.md#gloss-store-derivation) rather than its outputs.",
+        .description =
+            "Operate on the [store derivation](@docroot@/glossary.md#gloss-store-derivation) rather than its outputs.",
         .category = installablesCategory,
         .handler = {&operateOn, OperateOn::Derivation},
     });
@@ -238,46 +233,48 @@ void StorePathCommand::run(ref<Store> store, StorePaths && storePaths)
 
 MixProfile::MixProfile()
 {
-    addFlag({
-        .longName = "profile",
-        .description = "The profile to operate on.",
-        .labels = {"path"},
-        .handler = {&profile},
-        .completer = completePath
-    });
+    addFlag(
+        {.longName = "profile",
+         .description = "The profile to operate on.",
+         .labels = {"path"},
+         .handler = {&profile},
+         .completer = completePath});
 }
 
 void MixProfile::updateProfile(const StorePath & storePath)
 {
-    if (!profile) return;
+    if (!profile)
+        return;
     auto store = getStore().dynamic_pointer_cast<LocalFSStore>();
-    if (!store) throw Error("'--profile' is not supported for this Nix store");
+    if (!store)
+        throw Error("'--profile' is not supported for this Nix store");
     auto profile2 = absPath(*profile);
-    switchLink(profile2,
-        createGeneration(*store, profile2, storePath));
+    switchLink(profile2, createGeneration(*store, profile2, storePath));
 }
 
 void MixProfile::updateProfile(const BuiltPaths & buildables)
 {
-    if (!profile) return;
+    if (!profile)
+        return;
 
     StorePaths result;
 
     for (auto & buildable : buildables) {
-        std::visit(overloaded {
-            [&](const BuiltPath::Opaque & bo) {
-                result.push_back(bo.path);
+        std::visit(
+            overloaded{
+                [&](const BuiltPath::Opaque & bo) { result.push_back(bo.path); },
+                [&](const BuiltPath::Built & bfd) {
+                    for (auto & output : bfd.outputs) {
+                        result.push_back(output.second);
+                    }
+                },
             },
-            [&](const BuiltPath::Built & bfd) {
-                for (auto & output : bfd.outputs) {
-                    result.push_back(output.second);
-                }
-            },
-        }, buildable.raw());
+            buildable.raw());
     }
 
     if (result.size() != 1)
-        throw UsageError("'--profile' requires that the arguments produce a single store path, but there are %d", result.size());
+        throw UsageError(
+            "'--profile' requires that the arguments produce a single store path, but there are %d", result.size());
 
     updateProfile(result[0]);
 }
@@ -287,51 +284,85 @@ MixDefaultProfile::MixDefaultProfile()
     profile = getDefaultProfile();
 }
 
-MixEnvironment::MixEnvironment() : ignoreEnvironment(false)
+MixEnvironment::MixEnvironment()
+    : ignoreEnvironment(false)
 {
     addFlag({
-        .longName = "ignore-environment",
+        .longName = "ignore-env",
+        .aliases = {"ignore-environment"},
         .shortName = 'i',
-        .description = "Clear the entire environment (except those specified with `--keep`).",
+        .description = "Clear the entire environment, except for those specified with `--keep-env-var`.",
+        .category = environmentVariablesCategory,
         .handler = {&ignoreEnvironment, true},
     });
 
     addFlag({
-        .longName = "keep",
+        .longName = "keep-env-var",
+        .aliases = {"keep"},
         .shortName = 'k',
-        .description = "Keep the environment variable *name*.",
+        .description = "Keep the environment variable *name*, when using `--ignore-env`.",
+        .category = environmentVariablesCategory,
         .labels = {"name"},
-        .handler = {[&](std::string s) { keep.insert(s); }},
+        .handler = {[&](std::string s) { keepVars.insert(s); }},
     });
 
     addFlag({
-        .longName = "unset",
+        .longName = "unset-env-var",
+        .aliases = {"unset"},
         .shortName = 'u',
         .description = "Unset the environment variable *name*.",
+        .category = environmentVariablesCategory,
         .labels = {"name"},
-        .handler = {[&](std::string s) { unset.insert(s); }},
+        .handler = {[&](std::string name) {
+            if (setVars.contains(name))
+                throw UsageError("Cannot unset environment variable '%s' that is set with '%s'", name, "--set-env-var");
+
+            unsetVars.insert(name);
+        }},
+    });
+
+    addFlag({
+        .longName = "set-env-var",
+        .shortName = 's',
+        .description = "Sets an environment variable *name* with *value*.",
+        .category = environmentVariablesCategory,
+        .labels = {"name", "value"},
+        .handler = {[&](std::string name, std::string value) {
+            if (unsetVars.contains(name))
+                throw UsageError(
+                    "Cannot set environment variable '%s' that is unset with '%s'", name, "--unset-env-var");
+
+            if (setVars.contains(name))
+                throw UsageError(
+                    "Duplicate definition of environment variable '%s' with '%s' is ambiguous", name, "--set-env-var");
+
+            setVars.insert_or_assign(name, value);
+        }},
     });
 }
 
-void MixEnvironment::setEnviron() {
-    if (ignoreEnvironment) {
-        if (!unset.empty())
-            throw UsageError("--unset does not make sense with --ignore-environment");
+void MixEnvironment::setEnviron()
+{
+    if (ignoreEnvironment && !unsetVars.empty())
+        throw UsageError("--unset-env-var does not make sense with --ignore-env");
 
-        for (const auto & var : keep) {
-            auto val = getenv(var.c_str());
-            if (val) stringsEnv.emplace_back(fmt("%s=%s", var.c_str(), val));
-        }
+    if (!ignoreEnvironment && !keepVars.empty())
+        throw UsageError("--keep-env-var does not make sense without --ignore-env");
 
-        vectorEnv = stringsToCharPtrs(stringsEnv);
-        environ = vectorEnv.data();
-    } else {
-        if (!keep.empty())
-            throw UsageError("--keep does not make sense without --ignore-environment");
+    auto env = getEnv();
 
-        for (const auto & var : unset)
-            unsetenv(var.c_str());
-    }
+    if (ignoreEnvironment)
+        std::erase_if(env, [&](const auto & var) { return !keepVars.contains(var.first); });
+
+    for (const auto & [name, value] : setVars)
+        env[name] = value;
+
+    if (!unsetVars.empty())
+        std::erase_if(env, [&](const auto & var) { return unsetVars.contains(var.first); });
+
+    replaceEnv(env);
+
+    return;
 }
 
 }

src/libcmd/command.hh

@@ -13,7 +13,7 @@ namespace nix {
 
 extern std::string programPath;
 
-extern char * * savedArgv;
+extern char ** savedArgv;
 
 class EvalState;
 struct Pos;
@@ -24,7 +24,8 @@ static constexpr Command::Category catSecondary = 100;
 static constexpr Command::Category catUtility = 101;
 static constexpr Command::Category catNixInstallation = 102;
 
-static constexpr auto installablesCategory = "Options that change the interpretation of [installables](@docroot@/command-ref/new-cli/nix.md#installables)";
+static constexpr auto installablesCategory =
+    "Options that change the interpretation of [installables](@docroot@/command-ref/new-cli/nix.md#installables)";
 
 struct NixMultiCommand : MultiCommand, virtual Command
 {
@@ -112,7 +113,9 @@ struct MixFlakeOptions : virtual Args, EvalCommand
      * arguments) so that the completions for these flags can use them.
      */
     virtual std::vector<FlakeRef> getFlakeRefsForCompletion()
-    { return {}; }
+    {
+        return {};
+    }
 };
 
 struct SourceExprCommand : virtual Args, MixFlakeOptions
@@ -122,11 +125,9 @@ struct SourceExprCommand : virtual Args, MixFlakeOptions
 
     SourceExprCommand();
 
-    Installables parseInstallables(
-        ref<Store> store, std::vector<std::string> ss);
+    Installables parseInstallables(ref<Store> store, std::vector<std::string> ss);
 
-    ref<Installable> parseInstallable(
-        ref<Store> store, const std::string & installable);
+    ref<Installable> parseInstallable(ref<Store> store, const std::string & installable);
 
     virtual Strings getDefaultFlakeAttrPaths();
 
@@ -272,10 +273,10 @@ struct RegisterCommand
     typedef std::map<std::vector<std::string>, std::function<ref<Command>()>> Commands;
     static Commands * commands;
 
-    RegisterCommand(std::vector<std::string> && name,
-        std::function<ref<Command>()> command)
+    RegisterCommand(std::vector<std::string> && name, std::function<ref<Command>()> command)
     {
-        if (!commands) commands = new Commands;
+        if (!commands)
+            commands = new Commands;
         commands->emplace(name, command);
     }
 
@@ -285,13 +286,13 @@ struct RegisterCommand
 template<class T>
 static RegisterCommand registerCommand(const std::string & name)
 {
-    return RegisterCommand({name}, [](){ return make_ref<T>(); });
+    return RegisterCommand({name}, []() { return make_ref<T>(); });
 }
 
 template<class T>
 static RegisterCommand registerCommand2(std::vector<std::string> && name)
 {
-    return RegisterCommand(std::move(name), [](){ return make_ref<T>(); });
+    return RegisterCommand(std::move(name), []() { return make_ref<T>(); });
 }
 
 struct MixProfile : virtual StoreCommand
@@ -313,19 +314,21 @@ struct MixDefaultProfile : MixProfile
     MixDefaultProfile();
 };
 
-struct MixEnvironment : virtual Args {
+struct MixEnvironment : virtual Args
+{
 
-    StringSet keep, unset;
-    Strings stringsEnv;
-    std::vector<char*> vectorEnv;
+    StringSet keepVars;
+    StringSet unsetVars;
+    std::map<std::string, std::string> setVars;
     bool ignoreEnvironment;
 
     MixEnvironment();
 
     /***
-     * Modify global environ based on `ignoreEnvironment`, `keep`, and
-     * `unset`. It's expected that exec will be called before this class
-     * goes out of scope, otherwise `environ` will become invalid.
+     * Modify global environ based on `ignoreEnvironment`, `keep`,
+     * `unset`, and `added`. It's expected that exec will be called
+     * before this class goes out of scope, otherwise `environ` will
+     * become invalid.
      */
     void setEnviron();
 };
@@ -349,9 +352,6 @@ void completeFlakeRefWithFragment(
 std::string showVersions(const std::set<std::string> & versions);
 
 void printClosureDiff(
-    ref<Store> store,
-    const StorePath & beforePath,
-    const StorePath & afterPath,
-    std::string_view indent);
+    ref<Store> store, const StorePath & beforePath, const StorePath & afterPath, std::string_view indent);
 
 }

src/libcmd/common-eval-args.cc

@@ -18,6 +18,8 @@
 
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 fetchers::Settings fetchSettings;
 
 static GlobalConfig::Register rFetchSettings(&fetchSettings);
@@ -119,8 +121,8 @@ MixEvalArgs::MixEvalArgs()
         .category = category,
         .labels = {"original-ref", "resolved-ref"},
         .handler = {[&](std::string _from, std::string _to) {
-            auto from = parseFlakeRef(fetchSettings, _from, absPath("."));
-            auto to = parseFlakeRef(fetchSettings, _to, absPath("."));
+            auto from = parseFlakeRef(fetchSettings, _from, fs::current_path().string());
+            auto to = parseFlakeRef(fetchSettings, _to, fs::current_path().string());
             fetchers::Attrs extraAttrs;
             if (to.subdir != "") extraAttrs["dir"] = to.subdir;
             fetchers::overrideRegistry(from.input, to.input, extraAttrs);
@@ -171,7 +173,9 @@ SourcePath lookupFileArg(EvalState & state, std::string_view s, const Path * bas
 {
     if (EvalSettings::isPseudoUrl(s)) {
         auto accessor = fetchers::downloadTarball(
-            EvalSettings::resolvePseudoUrl(s)).accessor;
+            state.store,
+            state.fetchSettings,
+            EvalSettings::resolvePseudoUrl(s));
         auto storePath = fetchToStore(*state.store, SourcePath(accessor), FetchMode::Copy);
         return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }

src/libcmd/installable-flake.cc

@@ -104,12 +104,12 @@ DerivedPathsWithInfo InstallableFlake::toDerivedPaths()
 
     auto drvPath = attr->forceDerivation();
 
-    std::optional<NixInt> priority;
+    std::optional<NixInt::Inner> priority;
 
     if (attr->maybeGetAttr(state->sOutputSpecified)) {
     } else if (auto aMeta = attr->maybeGetAttr(state->sMeta)) {
         if (auto aPriority = aMeta->maybeGetAttr("priority"))
-            priority = aPriority->getInt();
+            priority = aPriority->getInt().value;
     }
 
     return {{

src/libcmd/installable-value.hh

@@ -40,7 +40,7 @@ struct ExtraPathInfoValue : ExtraPathInfo
         /**
          * An optional priority for use with "build envs". See Package
          */
-        std::optional<NixInt> priority;
+        std::optional<NixInt::Inner> priority;
 
         /**
          * The attribute path associated with this value. The idea is

src/libcmd/installables.cc

@@ -31,6 +31,8 @@
 
 namespace nix {
 
+namespace fs { using namespace std::filesystem; }
+
 void completeFlakeInputPath(
     AddCompletions & completions,
     ref<EvalState> evalState,
@@ -86,7 +88,7 @@ MixFlakeOptions::MixFlakeOptions()
 
     > **DEPRECATED**
     >
-    > Use [`--no-use-registries`](#opt-no-use-registries) instead.
+    > Use [`--no-use-registries`](@docroot@/command-ref/conf-file.md#conf-use-registries) instead.
         )",
         .category = category,
         .handler = {[&]() {
@@ -341,7 +343,7 @@ void completeFlakeRefWithFragment(
             auto flakeRefS = std::string(prefix.substr(0, hash));
 
             // TODO: ideally this would use the command base directory instead of assuming ".".
-            auto flakeRef = parseFlakeRef(fetchSettings, expandTilde(flakeRefS), absPath("."));
+            auto flakeRef = parseFlakeRef(fetchSettings, expandTilde(flakeRefS), fs::current_path().string());
 
             auto evalCache = openEvalCache(*evalState,
                 std::make_shared<flake::LockedFlake>(lockFlake(

src/libcmd/package.nix

@@ -1,11 +1,6 @@
 { lib
 , stdenv
-, mkMesonDerivation
-, releaseTools
-
-, meson
-, ninja
-, pkg-config
+, mkMesonLibrary
 
 , nix-util
 , nix-store
@@ -38,7 +33,7 @@ let
   inherit (lib) fileset;
 in
 
-mkMesonDerivation (finalAttrs: {
+mkMesonLibrary (finalAttrs: {
   pname = "nix-cmd";
   inherit version;
 
@@ -54,14 +49,6 @@ mkMesonDerivation (finalAttrs: {
     (fileset.fileFilter (file: file.hasExt "hh") ./.)
   ];
 
-  outputs = [ "out" "dev" ];
-
-  nativeBuildInputs = [
-    meson
-    ninja
-    pkg-config
-  ];
-
   buildInputs = [
     ({ inherit editline readline; }.${readlineFlavor})
   ] ++ lib.optional enableMarkdown lowdown;
@@ -93,10 +80,6 @@ mkMesonDerivation (finalAttrs: {
     LDFLAGS = "-fuse-ld=gold";
   };
 
-  separateDebugInfo = !stdenv.hostPlatform.isStatic;
-
-  hardeningDisable = lib.optional stdenv.hostPlatform.isStatic "pie";
-
   meta = {
     platforms = lib.platforms.unix ++ lib.platforms.windows;
   };

src/libcmd/repl.cc

@@ -2,12 +2,12 @@
 #include <cstdlib>
 #include <cstring>
 
+#include "error.hh"
 #include "repl-interacter.hh"
 #include "repl.hh"
 
 #include "ansicolor.hh"
 #include "shared.hh"
-#include "config-global.hh"
 #include "eval.hh"
 #include "eval-settings.hh"
 #include "attr-path.hh"
@@ -29,11 +29,6 @@
 #include "ref.hh"
 #include "value.hh"
 
-#if HAVE_BOEHMGC
-#define GC_INCLUDE_NEW
-#include <gc/gc_cpp.h>
-#endif
-
 #include "strings.hh"
 
 namespace nix {
@@ -62,9 +57,7 @@ enum class ProcessLineResult {
 struct NixRepl
     : AbstractNixRepl
     , detail::ReplCompleterMixin
-    #if HAVE_BOEHMGC
     , gc
-    #endif
 {
     size_t debugTraceIndex;
 
@@ -77,10 +70,14 @@ struct NixRepl
     int displ;
     StringSet varNames;
 
+    RunNix * runNixPtr;
+
+    void runNix(Path program, const Strings & args, const std::optional<std::string> & input = {});
+
     std::unique_ptr<ReplInteracter> interacter;
 
     NixRepl(const LookupPath & lookupPath, nix::ref<Store> store,ref<EvalState> state,
-            std::function<AnnotatedValues()> getValues);
+            std::function<AnnotatedValues()> getValues, RunNix * runNix);
     virtual ~NixRepl() = default;
 
     ReplExitStatus mainLoop() override;
@@ -125,32 +122,16 @@ std::string removeWhitespace(std::string s)
 
 
 NixRepl::NixRepl(const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-            std::function<NixRepl::AnnotatedValues()> getValues)
+            std::function<NixRepl::AnnotatedValues()> getValues, RunNix * runNix = nullptr)
     : AbstractNixRepl(state)
     , debugTraceIndex(0)
     , getValues(getValues)
     , staticEnv(new StaticEnv(nullptr, state->staticBaseEnv.get()))
-    , interacter(make_unique<ReadlineLikeInteracter>(getDataDir() + "/nix/repl-history"))
+    , runNixPtr{runNix}
+    , interacter(make_unique<ReadlineLikeInteracter>(getDataDir() + "/repl-history"))
 {
 }
 
-void runNix(Path program, const Strings & args,
-    const std::optional<std::string> & input = {})
-{
-    auto subprocessEnv = getEnv();
-    subprocessEnv["NIX_CONFIG"] = globalConfig.toKeyValue();
-    //isInteractive avoid grabling interactive commands
-    runProgram2(RunOptions {
-        .program = settings.nixBinDir+ "/" + program,
-        .args = args,
-        .environment = subprocessEnv,
-        .input = input,
-        .isInteractive = true,
-    });
-
-    return;
-}
-
 static std::ostream & showDebugTrace(std::ostream & out, const PosTable & positions, const DebugTrace & dt)
 {
     if (dt.isError)
@@ -635,7 +616,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
                 // When missing, trigger the normal exception
                 // e.g. :doc builtins.foo
                 // behaves like
-                // nix-repl> builtins.foo      
+                // nix-repl> builtins.foo<tab>
                 // error: attribute 'foo' missing
                 evalString(arg, v);
                 assert(false);
@@ -664,7 +645,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
 
             logger->cout(trim(renderMarkdownToTerminal(markdown)));
         } else if (fallbackPos) {
-            std::stringstream ss;
+            std::ostringstream ss;
             ss << "Attribute `" << fallbackName << "`\n\n";
             ss << "  … defined at " << state->positions[fallbackPos] << "\n\n";
             if (fallbackDoc) {
@@ -673,7 +654,7 @@ ProcessLineResult NixRepl::processLine(std::string line)
                 ss << "No documentation found.\n\n";
             }
 
-            auto markdown = ss.str();
+            auto markdown = toView(ss);
             logger->cout(trim(renderMarkdownToTerminal(markdown)));
 
         } else
@@ -733,7 +714,14 @@ void NixRepl::loadFlake(const std::string & flakeRefS)
     if (flakeRefS.empty())
         throw Error("cannot use ':load-flake' without a path specified. (Use '.' for the current working directory.)");
 
-    auto flakeRef = parseFlakeRef(fetchSettings, flakeRefS, absPath("."), true);
+    std::filesystem::path cwd;
+    try {
+        cwd = std::filesystem::current_path();
+    } catch (std::filesystem::filesystem_error & e) {
+        throw SysError("cannot determine current working directory");
+    }
+
+    auto flakeRef = parseFlakeRef(fetchSettings, flakeRefS, cwd.string(), true);
     if (evalSettings.pureEval && !flakeRef.input.isLocked())
         throw Error("cannot use ':load-flake' on locked flake reference '%s' (use --impure to override)", flakeRefS);
 
@@ -833,9 +821,18 @@ void NixRepl::evalString(std::string s, Value & v)
 }
 
 
+void NixRepl::runNix(Path program, const Strings & args, const std::optional<std::string> & input)
+{
+    if (runNixPtr)
+        (*runNixPtr)(program, args, input);
+    else
+        throw Error("Cannot run '%s' because no method of calling the Nix CLI was provided. This is a configuration problem pertaining to how this program was built. See Nix 2.25 release notes", program);
+}
+
+
 std::unique_ptr<AbstractNixRepl> AbstractNixRepl::create(
    const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-   std::function<AnnotatedValues()> getValues)
+   std::function<AnnotatedValues()> getValues, RunNix * runNix)
 {
     return std::make_unique<NixRepl>(
         lookupPath,

src/libcmd/repl.hh

@@ -19,9 +19,19 @@ struct AbstractNixRepl
 
     typedef std::vector<std::pair<Value*,std::string>> AnnotatedValues;
 
+    using RunNix = void(Path program, const Strings & args, const std::optional<std::string> & input);
+
+    /**
+     * @param runNix Function to run the nix CLI to support various
+     * `:<something>` commands. Optional; if not provided,
+     * everything else will still work fine, but those commands won't.
+     */
     static std::unique_ptr<AbstractNixRepl> create(
-        const LookupPath & lookupPath, nix::ref<Store> store, ref<EvalState> state,
-        std::function<AnnotatedValues()> getValues);
+        const LookupPath & lookupPath,
+        nix::ref<Store> store,
+        ref<EvalState> state,
+        std::function<AnnotatedValues()> getValues,
+        RunNix * runNix = nullptr);
 
     static ReplExitStatus runSimple(
         ref<EvalState> evalState,

src/libexpr-c/meson.build

@@ -75,6 +75,7 @@ headers = [config_h] + files(
 headers += files('nix_api_expr_internal.h')
 
 subdir('build-utils-meson/export-all-symbols')
+subdir('build-utils-meson/windows-version')
 
 this_library = library(
   'nixexprc',

src/libexpr-c/nix_api_expr.cc

@@ -16,8 +16,6 @@
 
 #if HAVE_BOEHMGC
 #  include <mutex>
-#  define GC_INCLUDE_NEW 1
-#  include "gc_cpp.h"
 #endif
 
 nix_err nix_libexpr_init(nix_c_context * context)

src/libexpr-c/nix_api_external.cc

@@ -14,12 +14,6 @@
 
 #include <nlohmann/json.hpp>
 
-#if HAVE_BOEHMGC
-# include "gc/gc.h"
-# define GC_INCLUDE_NEW 1
-# include "gc_cpp.h"
-#endif
-
 void nix_set_string_return(nix_string_return * str, const char * c)
 {
     str->str = c;

src/libexpr-c/nix_api_value.cc

@@ -14,12 +14,6 @@
 #include "nix_api_value.h"
 #include "value/context.hh"
 
-#if HAVE_BOEHMGC
-#  include "gc/gc.h"
-#  define GC_INCLUDE_NEW 1
-#  include "gc_cpp.h"
-#endif
-
 // Internal helper functions to check [in] and [out] `Value *` parameters
 static const nix::Value & check_value_not_null(const nix_value * value)
 {
@@ -306,7 +300,7 @@ int64_t nix_get_int(nix_c_context * context, const nix_value * value)
     try {
         auto & v = check_value_in(value);
         assert(v.type() == nix::nInt);
-        return v.integer();
+        return v.integer().value;
     }
     NIXC_CATCH_ERRS_RES(0);
 }

src/libexpr-c/package.nix

@@ -1,10 +1,6 @@
 { lib
 , stdenv
-, mkMesonDerivation
-
-, meson
-, ninja
-, pkg-config
+, mkMesonLibrary
 
 , nix-store-c
 , nix-expr
@@ -18,7 +14,7 @@ let
   inherit (lib) fileset;
 in
 
-mkMesonDerivation (finalAttrs: {
+mkMesonLibrary (finalAttrs: {
   pname = "nix-expr-c";
   inherit version;
 
@@ -35,14 +31,6 @@ mkMesonDerivation (finalAttrs: {
     (fileset.fileFilter (file: file.hasExt "h") ./.)
   ];
 
-  outputs = [ "out" "dev" ];
-
-  nativeBuildInputs = [
-    meson
-    ninja
-    pkg-config
-  ];
-
   propagatedBuildInputs = [
     nix-store-c
     nix-expr
@@ -63,10 +51,6 @@ mkMesonDerivation (finalAttrs: {
     LDFLAGS = "-fuse-ld=gold";
   };
 
-  separateDebugInfo = !stdenv.hostPlatform.isStatic;
-
-  hardeningDisable = lib.optional stdenv.hostPlatform.isStatic "pie";
-
   meta = {
     platforms = lib.platforms.unix ++ lib.platforms.windows;
   };

src/libexpr-test-support/.version

@@ -0,0 +1 @@
+../../.version

src/libexpr-test-support/build-utils-meson

@@ -0,0 +1 @@
+../../build-utils-meson

src/libexpr-test-support/local.mk

@@ -0,0 +1,23 @@
+libraries += libexpr-test-support
+
+libexpr-test-support_NAME = libnixexpr-test-support
+
+libexpr-test-support_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libexpr-test-support_INSTALL_DIR := $(checklibdir)
+else
+  libexpr-test-support_INSTALL_DIR :=
+endif
+
+libexpr-test-support_SOURCES := \
+    $(wildcard $(d)/tests/*.cc) \
+    $(wildcard $(d)/tests/value/*.cc)
+
+libexpr-test-support_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)
+
+libexpr-test-support_LIBS = \
+    libstore-test-support libutil-test-support \
+    libexpr libstore libutil
+
+libexpr-test-support_LDFLAGS := $(THREAD_LDFLAGS) -lrapidcheck

src/libexpr-test-support/meson.build

@@ -0,0 +1,77 @@
+project('nix-expr-test-support', 'cpp',
+  version : files('.version'),
+  default_options : [
+    'cpp_std=c++2a',
+    # TODO(Qyriad): increase the warning level
+    'warning_level=1',
+    'debug=true',
+    'optimization=2',
+    'errorlogs=true', # Please print logs for tests that fail
+  ],
+  meson_version : '>= 1.1',
+  license : 'LGPL-2.1-or-later',
+)
+
+cxx = meson.get_compiler('cpp')
+
+subdir('build-utils-meson/deps-lists')
+
+deps_private_maybe_subproject = [
+]
+deps_public_maybe_subproject = [
+  dependency('nix-util'),
+  dependency('nix-util-test-support'),
+  dependency('nix-store'),
+  dependency('nix-store-test-support'),
+  dependency('nix-expr'),
+]
+subdir('build-utils-meson/subprojects')
+
+subdir('build-utils-meson/threads')
+
+rapidcheck = dependency('rapidcheck')
+deps_public += rapidcheck
+
+add_project_arguments(
+  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  # It would be nice for our headers to be idempotent instead.
+  '-include', 'config-util.hh',
+  '-include', 'config-store.hh',
+  '-include', 'config-expr.hh',
+  language : 'cpp',
+)
+
+subdir('build-utils-meson/diagnostics')
+
+sources = files(
+  'tests/value/context.cc',
+)
+
+include_dirs = [include_directories('.')]
+
+headers = files(
+  'tests/libexpr.hh',
+  'tests/nix_api_expr.hh',
+  'tests/value/context.hh',
+)
+
+subdir('build-utils-meson/export-all-symbols')
+subdir('build-utils-meson/windows-version')
+
+this_library = library(
+  'nix-expr-test-support',
+  sources,
+  dependencies : deps_public + deps_private + deps_other,
+  include_directories : include_dirs,
+  # TODO: Remove `-lrapidcheck` when https://github.com/emil-e/rapidcheck/pull/326
+  #       is available. See also ../libutil/build.meson
+  link_args: linker_export_flags + ['-lrapidcheck'],
+  prelink : true, # For C++ static initializers
+  install : true,
+)
+
+install_headers(headers, subdir : 'nix', preserve_path : true)
+
+libraries_private = []
+
+subdir('build-utils-meson/export')

src/libexpr-test-support/package.nix

@@ -0,0 +1,60 @@
+{ lib
+, stdenv
+, mkMesonLibrary
+
+, nix-store-test-support
+, nix-expr
+
+, rapidcheck
+
+# Configuration Options
+
+, version
+}:
+
+let
+  inherit (lib) fileset;
+in
+
+mkMesonLibrary (finalAttrs: {
+  pname = "nix-util-test-support";
+  inherit version;
+
+  workDir = ./.;
+  fileset = fileset.unions [
+    ../../build-utils-meson
+    ./build-utils-meson
+    ../../.version
+    ./.version
+    ./meson.build
+    # ./meson.options
+    (fileset.fileFilter (file: file.hasExt "cc") ./.)
+    (fileset.fileFilter (file: file.hasExt "hh") ./.)
+  ];
+
+  propagatedBuildInputs = [
+    nix-store-test-support
+    nix-expr
+    rapidcheck
+  ];
+
+  preConfigure =
+    # "Inline" .version so it's not a symlink, and includes the suffix.
+    # Do the meson utils, without modification.
+    ''
+      chmod u+w ./.version
+      echo ${version} > ../../.version
+    '';
+
+  mesonFlags = [
+  ];
+
+  env = lib.optionalAttrs (stdenv.isLinux && !(stdenv.hostPlatform.isStatic && stdenv.system == "aarch64-linux")) {
+    LDFLAGS = "-fuse-ld=gold";
+  };
+
+  meta = {
+    platforms = lib.platforms.unix ++ lib.platforms.windows;
+  };
+
+})

src/libexpr-test-support/tests/libexpr.hh

@@ -0,0 +1,150 @@
+#pragma once
+///@file
+
+#include <gtest/gtest.h>
+#include <gmock/gmock.h>
+
+#include "fetch-settings.hh"
+#include "value.hh"
+#include "nixexpr.hh"
+#include "nixexpr.hh"
+#include "eval.hh"
+#include "eval-gc.hh"
+#include "eval-inline.hh"
+#include "eval-settings.hh"
+
+#include "tests/libstore.hh"
+
+namespace nix {
+    class LibExprTest : public LibStoreTest {
+        public:
+            static void SetUpTestSuite() {
+                LibStoreTest::SetUpTestSuite();
+                initGC();
+            }
+
+        protected:
+            LibExprTest()
+                : LibStoreTest()
+                , state({}, store, fetchSettings, evalSettings, nullptr)
+            {
+                evalSettings.nixPath = {};
+            }
+            Value eval(std::string input, bool forceValue = true) {
+                Value v;
+                Expr * e = state.parseExprFromString(input, state.rootPath(CanonPath::root));
+                assert(e);
+                state.eval(e, v);
+                if (forceValue)
+                    state.forceValue(v, noPos);
+                return v;
+            }
+
+            Symbol createSymbol(const char * value) {
+                return state.symbols.create(value);
+            }
+
+            bool readOnlyMode = true;
+            fetchers::Settings fetchSettings{};
+            EvalSettings evalSettings{readOnlyMode};
+            EvalState state;
+    };
+
+    MATCHER(IsListType, "") {
+        return arg != nList;
+    }
+
+    MATCHER(IsList, "") {
+        return arg.type() == nList;
+    }
+
+    MATCHER(IsString, "") {
+        return arg.type() == nString;
+    }
+
+    MATCHER(IsNull, "") {
+        return arg.type() == nNull;
+    }
+
+    MATCHER(IsThunk, "") {
+        return arg.type() == nThunk;
+    }
+
+    MATCHER(IsAttrs, "") {
+        return arg.type() == nAttrs;
+    }
+
+    MATCHER_P(IsStringEq, s, fmt("The string is equal to \"%1%\"", s)) {
+        if (arg.type() != nString) {
+            return false;
+        }
+        return std::string_view(arg.c_str()) == s;
+    }
+
+    MATCHER_P(IsIntEq, v, fmt("The string is equal to \"%1%\"", v)) {
+        if (arg.type() != nInt) {
+            return false;
+        }
+        return arg.integer().value == v;
+    }
+
+    MATCHER_P(IsFloatEq, v, fmt("The float is equal to \"%1%\"", v)) {
+        if (arg.type() != nFloat) {
+            return false;
+        }
+        return arg.fpoint() == v;
+    }
+
+    MATCHER(IsTrue, "") {
+        if (arg.type() != nBool) {
+            return false;
+        }
+        return arg.boolean() == true;
+    }
+
+    MATCHER(IsFalse, "") {
+        if (arg.type() != nBool) {
+            return false;
+        }
+        return arg.boolean() == false;
+    }
+
+    MATCHER_P(IsPathEq, p, fmt("Is a path equal to \"%1%\"", p)) {
+        if (arg.type() != nPath) {
+            *result_listener << "Expected a path got " << arg.type();
+            return false;
+        } else {
+            auto path = arg.path();
+            if (path.path != CanonPath(p)) {
+                *result_listener << "Expected a path that equals \"" << p << "\" but got: " << path.path;
+                return false;
+            }
+        }
+        return true;
+    }
+
+
+    MATCHER_P(IsListOfSize, n, fmt("Is a list of size [%1%]", n)) {
+        if (arg.type() != nList) {
+            *result_listener << "Expected list got " << arg.type();
+            return false;
+        } else if (arg.listSize() != (size_t)n) {
+            *result_listener << "Expected as list of size " << n << " got " << arg.listSize();
+            return false;
+        }
+        return true;
+    }
+
+    MATCHER_P(IsAttrsOfSize, n, fmt("Is a set of size [%1%]", n)) {
+        if (arg.type() != nAttrs) {
+            *result_listener << "Expected set got " << arg.type();
+            return false;
+        } else if (arg.attrs()->size() != (size_t) n) {
+            *result_listener << "Expected a set with " << n << " attributes but got " << arg.attrs()->size();
+            return false;
+        }
+        return true;
+    }
+
+
+} /* namespace nix */

src/libexpr-test-support/tests/nix_api_expr.hh

@@ -0,0 +1,31 @@
+#pragma once
+///@file
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+#include "tests/nix_api_store.hh"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+class nix_api_expr_test : public nix_api_store_test
+{
+protected:
+
+    nix_api_expr_test()
+    {
+        nix_libexpr_init(ctx);
+        state = nix_state_create(nullptr, nullptr, store);
+        value = nix_alloc_value(nullptr, state);
+    }
+    ~nix_api_expr_test()
+    {
+        nix_gc_decref(nullptr, value);
+        nix_state_free(state);
+    }
+
+    EvalState * state;
+    nix_value * value;
+};
+
+}

src/libexpr-test-support/tests/value/context.cc

@@ -0,0 +1,30 @@
+#include <rapidcheck.h>
+
+#include "tests/path.hh"
+#include "tests/value/context.hh"
+
+namespace rc {
+using namespace nix;
+
+Gen<NixStringContextElem::DrvDeep> Arbitrary<NixStringContextElem::DrvDeep>::arbitrary()
+{
+    return gen::just(NixStringContextElem::DrvDeep {
+        .drvPath = *gen::arbitrary<StorePath>(),
+    });
+}
+
+Gen<NixStringContextElem> Arbitrary<NixStringContextElem>::arbitrary()
+{
+    switch (*gen::inRange<uint8_t>(0, std::variant_size_v<NixStringContextElem::Raw>)) {
+    case 0:
+        return gen::just<NixStringContextElem>(*gen::arbitrary<NixStringContextElem::Opaque>());
+    case 1:
+        return gen::just<NixStringContextElem>(*gen::arbitrary<NixStringContextElem::DrvDeep>());
+    case 2:
+        return gen::just<NixStringContextElem>(*gen::arbitrary<NixStringContextElem::Built>());
+    default:
+        assert(false);
+    }
+}
+
+}

src/libexpr-test-support/tests/value/context.hh

@@ -0,0 +1,31 @@
+#pragma once
+///@file
+
+#include <rapidcheck/gen/Arbitrary.h>
+
+#include "value/context.hh"
+
+namespace rc {
+using namespace nix;
+
+template<>
+struct Arbitrary<NixStringContextElem::Opaque> {
+    static Gen<NixStringContextElem::Opaque> arbitrary();
+};
+
+template<>
+struct Arbitrary<NixStringContextElem::Built> {
+    static Gen<NixStringContextElem::Built> arbitrary();
+};
+
+template<>
+struct Arbitrary<NixStringContextElem::DrvDeep> {
+    static Gen<NixStringContextElem::DrvDeep> arbitrary();
+};
+
+template<>
+struct Arbitrary<NixStringContextElem> {
+    static Gen<NixStringContextElem> arbitrary();
+};
+
+}

src/libexpr-tests/.version

@@ -0,0 +1 @@
+../../.version

src/libexpr-tests/build-utils-meson

@@ -0,0 +1 @@
+../../build-utils-meson

src/libexpr-tests/derived-path.cc

@@ -0,0 +1,68 @@
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+#include <rapidcheck/gtest.h>
+
+#include "tests/derived-path.hh"
+#include "tests/libexpr.hh"
+
+namespace nix {
+
+// Testing of trivial expressions
+class DerivedPathExpressionTest : public LibExprTest {};
+
+// FIXME: `RC_GTEST_FIXTURE_PROP` isn't calling `SetUpTestSuite` because it is
+// no a real fixture.
+//
+// See https://github.com/emil-e/rapidcheck/blob/master/doc/gtest.md#rc_gtest_fixture_propfixture-name-args
+TEST_F(DerivedPathExpressionTest, force_init)
+{
+}
+
+#ifndef COVERAGE
+
+RC_GTEST_FIXTURE_PROP(
+    DerivedPathExpressionTest,
+    prop_opaque_path_round_trip,
+    (const SingleDerivedPath::Opaque & o))
+{
+    auto * v = state.allocValue();
+    state.mkStorePathString(o.path, *v);
+    auto d = state.coerceToSingleDerivedPath(noPos, *v, "");
+    RC_ASSERT(SingleDerivedPath { o } == d);
+}
+
+// TODO use DerivedPath::Built for parameter once it supports a single output
+// path only.
+
+RC_GTEST_FIXTURE_PROP(
+    DerivedPathExpressionTest,
+    prop_derived_path_built_placeholder_round_trip,
+    (const SingleDerivedPath::Built & b))
+{
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "ca-derivations");
+
+    auto * v = state.allocValue();
+    state.mkOutputString(*v, b, std::nullopt, mockXpSettings);
+    auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, *v, "");
+    RC_ASSERT(SingleDerivedPath { b } == d);
+}
+
+RC_GTEST_FIXTURE_PROP(
+    DerivedPathExpressionTest,
+    prop_derived_path_built_out_path_round_trip,
+    (const SingleDerivedPath::Built & b, const StorePath & outPath))
+{
+    auto * v = state.allocValue();
+    state.mkOutputString(*v, b, outPath);
+    auto [d, _] = state.coerceToSingleDerivedPathUnchecked(noPos, *v, "");
+    RC_ASSERT(SingleDerivedPath { b } == d);
+}
+
+#endif
+
+} /* namespace nix */

src/libexpr-tests/eval.cc

@@ -0,0 +1,141 @@
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "eval.hh"
+#include "tests/libexpr.hh"
+
+namespace nix {
+
+TEST(nix_isAllowedURI, http_example_com) {
+    Strings allowed;
+    allowed.push_back("http://example.com");
+
+    ASSERT_TRUE(isAllowedURI("http://example.com", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com/foo", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com/foo/", allowed));
+    ASSERT_FALSE(isAllowedURI("/", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.co", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.como", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.org", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.org/foo", allowed));
+}
+
+TEST(nix_isAllowedURI, http_example_com_foo) {
+    Strings allowed;
+    allowed.push_back("http://example.com/foo");
+
+    ASSERT_TRUE(isAllowedURI("http://example.com/foo", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com/foo/", allowed));
+    ASSERT_FALSE(isAllowedURI("/foo", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.como", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.org/foo", allowed));
+    // Broken?
+    // ASSERT_TRUE(isAllowedURI("http://example.com/foo?ok=1", allowed));
+}
+
+TEST(nix_isAllowedURI, http) {
+    Strings allowed;
+    allowed.push_back("http://");
+
+    ASSERT_TRUE(isAllowedURI("http://", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com/foo", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com/foo/", allowed));
+    ASSERT_TRUE(isAllowedURI("http://example.com", allowed));
+    ASSERT_FALSE(isAllowedURI("/", allowed));
+    ASSERT_FALSE(isAllowedURI("https://", allowed));
+    ASSERT_FALSE(isAllowedURI("http:foo", allowed));
+}
+
+TEST(nix_isAllowedURI, https) {
+    Strings allowed;
+    allowed.push_back("https://");
+
+    ASSERT_TRUE(isAllowedURI("https://example.com", allowed));
+    ASSERT_TRUE(isAllowedURI("https://example.com/foo", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com/https:", allowed));
+}
+
+TEST(nix_isAllowedURI, absolute_path) {
+    Strings allowed;
+    allowed.push_back("/var/evil"); // bad idea
+
+    ASSERT_TRUE(isAllowedURI("/var/evil", allowed));
+    ASSERT_TRUE(isAllowedURI("/var/evil/", allowed));
+    ASSERT_TRUE(isAllowedURI("/var/evil/foo", allowed));
+    ASSERT_TRUE(isAllowedURI("/var/evil/foo/", allowed));
+    ASSERT_FALSE(isAllowedURI("/", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evi", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evilo", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evilo/", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evilo/foo", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com/var/evil", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com//var/evil", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com//var/evil/foo", allowed));
+}
+
+TEST(nix_isAllowedURI, file_url) {
+    Strings allowed;
+    allowed.push_back("file:///var/evil"); // bad idea
+
+    ASSERT_TRUE(isAllowedURI("file:///var/evil", allowed));
+    ASSERT_TRUE(isAllowedURI("file:///var/evil/", allowed));
+    ASSERT_TRUE(isAllowedURI("file:///var/evil/foo", allowed));
+    ASSERT_TRUE(isAllowedURI("file:///var/evil/foo/", allowed));
+    ASSERT_FALSE(isAllowedURI("/", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evi", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evilo", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evilo/", allowed));
+    ASSERT_FALSE(isAllowedURI("/var/evilo/foo", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com/var/evil", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com//var/evil", allowed));
+    ASSERT_FALSE(isAllowedURI("http://example.com//var/evil/foo", allowed));
+    ASSERT_FALSE(isAllowedURI("http://var/evil", allowed));
+    ASSERT_FALSE(isAllowedURI("http:///var/evil", allowed));
+    ASSERT_FALSE(isAllowedURI("http://var/evil/", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///var/evi", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///var/evilo", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///var/evilo/", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///var/evilo/foo", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///", allowed));
+    ASSERT_FALSE(isAllowedURI("file://", allowed));
+}
+
+TEST(nix_isAllowedURI, github_all) {
+    Strings allowed;
+    allowed.push_back("github:");
+    ASSERT_TRUE(isAllowedURI("github:", allowed));
+    ASSERT_TRUE(isAllowedURI("github:foo/bar", allowed));
+    ASSERT_TRUE(isAllowedURI("github:foo/bar/feat-multi-bar", allowed));
+    ASSERT_TRUE(isAllowedURI("github:foo/bar?ref=refs/heads/feat-multi-bar", allowed));
+    ASSERT_TRUE(isAllowedURI("github://foo/bar", allowed));
+    ASSERT_FALSE(isAllowedURI("https://github:443/foo/bar/archive/master.tar.gz", allowed));
+    ASSERT_FALSE(isAllowedURI("file://github:foo/bar/archive/master.tar.gz", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///github:foo/bar/archive/master.tar.gz", allowed));
+    ASSERT_FALSE(isAllowedURI("github", allowed));
+}
+
+TEST(nix_isAllowedURI, github_org) {
+    Strings allowed;
+    allowed.push_back("github:foo");
+    ASSERT_FALSE(isAllowedURI("github:", allowed));
+    ASSERT_TRUE(isAllowedURI("github:foo/bar", allowed));
+    ASSERT_TRUE(isAllowedURI("github:foo/bar/feat-multi-bar", allowed));
+    ASSERT_TRUE(isAllowedURI("github:foo/bar?ref=refs/heads/feat-multi-bar", allowed));
+    ASSERT_FALSE(isAllowedURI("github://foo/bar", allowed));
+    ASSERT_FALSE(isAllowedURI("https://github:443/foo/bar/archive/master.tar.gz", allowed));
+    ASSERT_FALSE(isAllowedURI("file://github:foo/bar/archive/master.tar.gz", allowed));
+    ASSERT_FALSE(isAllowedURI("file:///github:foo/bar/archive/master.tar.gz", allowed));
+}
+
+TEST(nix_isAllowedURI, non_scheme_colon) {
+    Strings allowed;
+    allowed.push_back("https://foo/bar:");
+    ASSERT_TRUE(isAllowedURI("https://foo/bar:", allowed));
+    ASSERT_TRUE(isAllowedURI("https://foo/bar:/baz", allowed));
+    ASSERT_FALSE(isAllowedURI("https://foo/bar:baz", allowed));
+}
+
+} // namespace nix

src/libexpr-tests/json.cc

@@ -0,0 +1,68 @@
+#include "tests/libexpr.hh"
+#include "value-to-json.hh"
+
+namespace nix {
+// Testing the conversion to JSON
+
+    class JSONValueTest : public LibExprTest {
+        protected:
+            std::string getJSONValue(Value& value) {
+                std::stringstream ss;
+                NixStringContext ps;
+                printValueAsJSON(state, true, value, noPos, ss, ps);
+                return ss.str();
+            }
+    };
+
+    TEST_F(JSONValueTest, null) {
+        Value v;
+        v.mkNull();
+        ASSERT_EQ(getJSONValue(v), "null");
+    }
+
+    TEST_F(JSONValueTest, BoolFalse) {
+        Value v;
+        v.mkBool(false);
+        ASSERT_EQ(getJSONValue(v),"false");
+    }
+
+    TEST_F(JSONValueTest, BoolTrue) {
+        Value v;
+        v.mkBool(true);
+        ASSERT_EQ(getJSONValue(v), "true");
+    }
+
+    TEST_F(JSONValueTest, IntPositive) {
+        Value v;
+        v.mkInt(100);
+        ASSERT_EQ(getJSONValue(v), "100");
+    }
+
+    TEST_F(JSONValueTest, IntNegative) {
+        Value v;
+        v.mkInt(-100);
+        ASSERT_EQ(getJSONValue(v), "-100");
+    }
+
+    TEST_F(JSONValueTest, String) {
+        Value v;
+        v.mkString("test");
+        ASSERT_EQ(getJSONValue(v), "\"test\"");
+    }
+
+    TEST_F(JSONValueTest, StringQuotes) {
+        Value v;
+
+        v.mkString("test\"");
+        ASSERT_EQ(getJSONValue(v), "\"test\\\"\"");
+    }
+
+    // The dummy store doesn't support writing files. Fails with this exception message:
+    // C++ exception with description "error: operation 'addToStoreFromDump' is
+    // not supported by store 'dummy'" thrown in the test body.
+    TEST_F(JSONValueTest, DISABLED_Path) {
+        Value v;
+        v.mkPath(state.rootPath(CanonPath("/test")));
+        ASSERT_EQ(getJSONValue(v), "\"/nix/store/g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x\"");
+    }
+} /* namespace nix */

src/libexpr-tests/local.mk

@@ -0,0 +1,45 @@
+check: libexpr-tests_RUN
+
+programs += libexpr-tests
+
+libexpr-tests_NAME := libnixexpr-tests
+
+libexpr-tests_ENV := _NIX_TEST_UNIT_DATA=$(d)/data GTEST_OUTPUT=xml:$$testresults/libexpr-tests.xml
+
+libexpr-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libexpr-tests_INSTALL_DIR := $(checkbindir)
+else
+  libexpr-tests_INSTALL_DIR :=
+endif
+
+libexpr-tests_SOURCES := \
+    $(wildcard $(d)/*.cc) \
+    $(wildcard $(d)/value/*.cc) \
+    $(wildcard $(d)/flake/*.cc)
+
+libexpr-tests_EXTRA_INCLUDES = \
+    -I src/libexpr-test-support \
+    -I src/libstore-test-support \
+    -I src/libutil-test-support \
+    $(INCLUDE_libexpr) \
+    $(INCLUDE_libexprc) \
+    $(INCLUDE_libfetchers) \
+    $(INCLUDE_libstore) \
+    $(INCLUDE_libstorec) \
+    $(INCLUDE_libutil) \
+    $(INCLUDE_libutilc)
+
+libexpr-tests_CXXFLAGS += $(libexpr-tests_EXTRA_INCLUDES)
+
+libexpr-tests_LIBS = \
+    libexpr-test-support libstore-test-support libutil-test-support \
+    libexpr libexprc libfetchers libstore libstorec libutil libutilc
+
+libexpr-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS) -lgmock
+
+ifdef HOST_WINDOWS
+  # Increase the default reserved stack size to 65 MB so Nix doesn't run out of space
+  libexpr-tests_LDFLAGS += -Wl,--stack,$(shell echo $$((65 * 1024 * 1024)))
+endif

src/libexpr-tests/main.cc

@@ -0,0 +1,42 @@
+#include <gtest/gtest.h>
+#include <cstdlib>
+#include "globals.hh"
+#include "logging.hh"
+
+using namespace nix;
+
+int main (int argc, char **argv) {
+    if (argc > 1 && std::string_view(argv[1]) == "__build-remote") {
+        printError("test-build-remote: not supported in libexpr unit tests");
+        return 1;
+    }
+
+    // Disable build hook. We won't be testing remote builds in these unit tests. If we do, fix the above build hook.
+    settings.buildHook = {};
+
+    #if __linux__ // should match the conditional around sandboxBuildDir declaration.
+
+    // When building and testing nix within the host's Nix sandbox, our store dir will be located in the host's sandboxBuildDir, e.g.:
+    // Host
+    //   storeDir = /nix/store
+    //   sandboxBuildDir = /build
+    // This process
+    //   storeDir = /build/foo/bar/store
+    //   sandboxBuildDir = /build
+    // However, we have a rule that the store dir must not be inside the storeDir, so we need to pick a different sandboxBuildDir.
+    settings.sandboxBuildDir = "/test-build-dir-instead-of-usual-build-dir";
+    #endif
+
+    #if __APPLE__
+    // Avoid this error, when already running in a sandbox:
+    // sandbox-exec: sandbox_apply: Operation not permitted
+    settings.sandboxMode = smDisabled;
+    setEnv("_NIX_TEST_NO_SANDBOX", "1");
+    #endif
+
+    // For pipe operator tests in trivial.cc
+    experimentalFeatureSettings.set("experimental-features", "pipe-operators");
+
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}

src/libexpr-tests/meson.build

@@ -0,0 +1,93 @@
+project('nix-expr-tests', 'cpp',
+  version : files('.version'),
+  default_options : [
+    'cpp_std=c++2a',
+    # TODO(Qyriad): increase the warning level
+    'warning_level=1',
+    'debug=true',
+    'optimization=2',
+    'errorlogs=true', # Please print logs for tests that fail
+  ],
+  meson_version : '>= 1.1',
+  license : 'LGPL-2.1-or-later',
+)
+
+cxx = meson.get_compiler('cpp')
+
+subdir('build-utils-meson/deps-lists')
+
+deps_private_maybe_subproject = [
+  dependency('nix-expr'),
+  dependency('nix-expr-c'),
+  dependency('nix-expr-test-support'),
+]
+deps_public_maybe_subproject = [
+]
+subdir('build-utils-meson/subprojects')
+
+subdir('build-utils-meson/threads')
+
+subdir('build-utils-meson/export-all-symbols')
+subdir('build-utils-meson/windows-version')
+
+rapidcheck = dependency('rapidcheck')
+deps_private += rapidcheck
+
+gtest = dependency('gtest')
+deps_private += gtest
+
+gtest = dependency('gmock')
+deps_private += gtest
+
+add_project_arguments(
+  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  # It would be nice for our headers to be idempotent instead.
+  '-include', 'config-util.hh',
+  '-include', 'config-store.hh',
+  '-include', 'config-expr.hh',
+  '-include', 'config-util.h',
+  '-include', 'config-store.h',
+  '-include', 'config-expr.h',
+  language : 'cpp',
+)
+
+subdir('build-utils-meson/diagnostics')
+
+sources = files(
+  'derived-path.cc',
+  'error_traces.cc',
+  'eval.cc',
+  'json.cc',
+  'main.cc',
+  'nix_api_expr.cc',
+  'nix_api_external.cc',
+  'nix_api_value.cc',
+  'primops.cc',
+  'search-path.cc',
+  'trivial.cc',
+  'value/context.cc',
+  'value/print.cc',
+  'value/value.cc',
+)
+
+include_dirs = [include_directories('.')]
+
+
+this_exe = executable(
+  meson.project_name(),
+  sources,
+  dependencies : deps_private_subproject + deps_private + deps_other,
+  include_directories : include_dirs,
+  # TODO: -lrapidcheck, see ../libutil-support/build.meson
+  link_args: linker_export_flags + ['-lrapidcheck'],
+  install : true,
+)
+
+test(
+  meson.project_name(),
+  this_exe,
+  env : {
+    '_NIX_TEST_UNIT_DATA': meson.current_source_dir() / 'data',
+  },
+  protocol : 'gtest',
+)

src/libexpr-tests/nix_api_expr.cc

@@ -0,0 +1,404 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+
+#include "tests/nix_api_expr.hh"
+#include "tests/string_callback.hh"
+
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+TEST_F(nix_api_expr_test, nix_expr_eval_from_string)
+{
+    nix_expr_eval_from_string(nullptr, state, "builtins.nixVersion", ".", value);
+    nix_value_force(nullptr, state, value);
+    std::string result;
+    nix_get_string(nullptr, value, OBSERVE_STRING(result));
+
+    ASSERT_STREQ(PACKAGE_VERSION, result.c_str());
+}
+
+TEST_F(nix_api_expr_test, nix_expr_eval_add_numbers)
+{
+    nix_expr_eval_from_string(nullptr, state, "1 + 1", ".", value);
+    nix_value_force(nullptr, state, value);
+    auto result = nix_get_int(nullptr, value);
+
+    ASSERT_EQ(2, result);
+}
+
+TEST_F(nix_api_expr_test, nix_expr_eval_drv)
+{
+    auto expr = R"(derivation { name = "myname"; builder = "mybuilder"; system = "mysystem"; })";
+    nix_expr_eval_from_string(nullptr, state, expr, ".", value);
+    ASSERT_EQ(NIX_TYPE_ATTRS, nix_get_type(nullptr, value));
+
+    EvalState * stateFn = nix_state_create(nullptr, nullptr, store);
+    nix_value * valueFn = nix_alloc_value(nullptr, state);
+    nix_expr_eval_from_string(nullptr, stateFn, "builtins.toString", ".", valueFn);
+    ASSERT_EQ(NIX_TYPE_FUNCTION, nix_get_type(nullptr, valueFn));
+
+    EvalState * stateResult = nix_state_create(nullptr, nullptr, store);
+    nix_value * valueResult = nix_alloc_value(nullptr, stateResult);
+    nix_value_call(ctx, stateResult, valueFn, value, valueResult);
+    ASSERT_EQ(NIX_TYPE_STRING, nix_get_type(nullptr, valueResult));
+
+    std::string p;
+    nix_get_string(nullptr, valueResult, OBSERVE_STRING(p));
+    std::string pEnd = "-myname";
+    ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
+
+    // Clean up
+    nix_gc_decref(nullptr, valueFn);
+    nix_state_free(stateFn);
+
+    nix_gc_decref(nullptr, valueResult);
+    nix_state_free(stateResult);
+}
+
+TEST_F(nix_api_expr_test, nix_build_drv)
+{
+    auto expr = R"(derivation { name = "myname";
+                                system = builtins.currentSystem;
+                                builder = "/bin/sh";
+                                args = [ "-c" "echo foo > $out" ];
+                              })";
+    nix_expr_eval_from_string(nullptr, state, expr, ".", value);
+
+    nix_value * drvPathValue = nix_get_attr_byname(nullptr, value, state, "drvPath");
+    std::string drvPath;
+    nix_get_string(nullptr, drvPathValue, OBSERVE_STRING(drvPath));
+
+    std::string p = drvPath;
+    std::string pEnd = "-myname.drv";
+    ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
+
+    // NOTE: .drvPath should be usually be ignored. Output paths are more versatile.
+    //       See https://github.com/NixOS/nix/issues/6507
+    //       Use e.g. nix_string_realise to realise the output.
+    StorePath * drvStorePath = nix_store_parse_path(ctx, store, drvPath.c_str());
+    ASSERT_EQ(true, nix_store_is_valid_path(ctx, store, drvStorePath));
+
+    nix_value * outPathValue = nix_get_attr_byname(ctx, value, state, "outPath");
+    std::string outPath;
+    nix_get_string(ctx, outPathValue, OBSERVE_STRING(outPath));
+
+    p = outPath;
+    pEnd = "-myname";
+    ASSERT_EQ(pEnd, p.substr(p.size() - pEnd.size()));
+    ASSERT_EQ(true, drvStorePath->path.isDerivation());
+
+    StorePath * outStorePath = nix_store_parse_path(ctx, store, outPath.c_str());
+    ASSERT_EQ(false, nix_store_is_valid_path(ctx, store, outStorePath));
+
+    nix_store_realise(ctx, store, drvStorePath, nullptr, nullptr);
+    auto is_valid_path = nix_store_is_valid_path(ctx, store, outStorePath);
+    ASSERT_EQ(true, is_valid_path);
+
+    // Clean up
+    nix_store_path_free(drvStorePath);
+    nix_store_path_free(outStorePath);
+}
+
+TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_value)
+{
+    auto expr = "true";
+    nix_expr_eval_from_string(ctx, state, expr, ".", value);
+    assert_ctx_ok();
+    auto r = nix_string_realise(ctx, state, value, false);
+    ASSERT_EQ(nullptr, r);
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("cannot coerce")));
+}
+
+TEST_F(nix_api_expr_test, nix_expr_realise_context_bad_build)
+{
+    auto expr = R"(
+        derivation { name = "letsbuild";
+            system = builtins.currentSystem;
+            builder = "/bin/sh";
+            args = [ "-c" "echo failing a build for testing purposes; exit 1;" ];
+            }
+        )";
+    nix_expr_eval_from_string(ctx, state, expr, ".", value);
+    assert_ctx_ok();
+    auto r = nix_string_realise(ctx, state, value, false);
+    ASSERT_EQ(nullptr, r);
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("failed with exit code 1")));
+}
+
+TEST_F(nix_api_expr_test, nix_expr_realise_context)
+{
+    // TODO (ca-derivations): add a content-addressed derivation output, which produces a placeholder
+    auto expr = R"(
+        ''
+            a derivation output: ${
+                derivation { name = "letsbuild";
+                    system = builtins.currentSystem;
+                    builder = "/bin/sh";
+                    args = [ "-c" "echo foo > $out" ];
+                    }}
+            a path: ${builtins.toFile "just-a-file" "ooh file good"}
+            a derivation path by itself: ${
+                builtins.unsafeDiscardOutputDependency
+                    (derivation {
+                        name = "not-actually-built-yet";
+                        system = builtins.currentSystem;
+                        builder = "/bin/sh";
+                        args = [ "-c" "echo foo > $out" ];
+                    }).drvPath}
+        ''
+        )";
+    nix_expr_eval_from_string(ctx, state, expr, ".", value);
+    assert_ctx_ok();
+    auto r = nix_string_realise(ctx, state, value, false);
+    assert_ctx_ok();
+    ASSERT_NE(nullptr, r);
+
+    auto s = std::string(nix_realised_string_get_buffer_start(r), nix_realised_string_get_buffer_size(r));
+
+    EXPECT_THAT(s, testing::StartsWith("a derivation output:"));
+    EXPECT_THAT(s, testing::HasSubstr("-letsbuild\n"));
+    EXPECT_THAT(s, testing::Not(testing::HasSubstr("-letsbuild.drv")));
+    EXPECT_THAT(s, testing::HasSubstr("a path:"));
+    EXPECT_THAT(s, testing::HasSubstr("-just-a-file"));
+    EXPECT_THAT(s, testing::Not(testing::HasSubstr("-just-a-file.drv")));
+    EXPECT_THAT(s, testing::Not(testing::HasSubstr("ooh file good")));
+    EXPECT_THAT(s, testing::HasSubstr("a derivation path by itself:"));
+    EXPECT_THAT(s, testing::EndsWith("-not-actually-built-yet.drv\n"));
+
+    std::vector<std::string> names;
+    size_t n = nix_realised_string_get_store_path_count(r);
+    for (size_t i = 0; i < n; ++i) {
+        const StorePath * p = nix_realised_string_get_store_path(r, i);
+        ASSERT_NE(nullptr, p);
+        std::string name;
+        nix_store_path_name(p, OBSERVE_STRING(name));
+        names.push_back(name);
+    }
+    std::sort(names.begin(), names.end());
+    ASSERT_EQ(3, names.size());
+    EXPECT_THAT(names[0], testing::StrEq("just-a-file"));
+    EXPECT_THAT(names[1], testing::StrEq("letsbuild"));
+    EXPECT_THAT(names[2], testing::StrEq("not-actually-built-yet.drv"));
+
+    nix_realised_string_free(r);
+}
+
+const char * SAMPLE_USER_DATA = "whatever";
+
+static void
+primop_square(void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
+{
+    assert(context);
+    assert(state);
+    assert(user_data == SAMPLE_USER_DATA);
+    auto i = nix_get_int(context, args[0]);
+    nix_init_int(context, ret, i * i);
+}
+
+TEST_F(nix_api_expr_test, nix_expr_primop)
+{
+    PrimOp * primop =
+        nix_alloc_primop(ctx, primop_square, 1, "square", nullptr, "square an integer", (void *) SAMPLE_USER_DATA);
+    assert_ctx_ok();
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * three = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, three, 3);
+    assert_ctx_ok();
+
+    nix_value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_value_call(ctx, state, primopValue, three, result);
+    assert_ctx_ok();
+
+    auto r = nix_get_int(ctx, result);
+    ASSERT_EQ(9, r);
+}
+
+static void
+primop_repeat(void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
+{
+    assert(context);
+    assert(state);
+    assert(user_data == SAMPLE_USER_DATA);
+
+    // Get the string to repeat
+    std::string s;
+    if (nix_get_string(context, args[0], OBSERVE_STRING(s)) != NIX_OK)
+        return;
+
+    // Get the number of times to repeat
+    auto n = nix_get_int(context, args[1]);
+    if (nix_err_code(context) != NIX_OK)
+        return;
+
+    // Repeat the string
+    std::string result;
+    for (int i = 0; i < n; ++i)
+        result += s;
+
+    nix_init_string(context, ret, result.c_str());
+}
+
+TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_multiple_calls)
+{
+    PrimOp * primop =
+        nix_alloc_primop(ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", (void *) SAMPLE_USER_DATA);
+    assert_ctx_ok();
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * hello = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_string(ctx, hello, "hello");
+    assert_ctx_ok();
+
+    nix_value * three = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, three, 3);
+    assert_ctx_ok();
+
+    nix_value * partial = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_value_call(ctx, state, primopValue, hello, partial);
+    assert_ctx_ok();
+
+    nix_value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_value_call(ctx, state, partial, three, result);
+    assert_ctx_ok();
+
+    std::string r;
+    nix_get_string(ctx, result, OBSERVE_STRING(r));
+    ASSERT_STREQ("hellohellohello", r.c_str());
+}
+
+TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_single_call)
+{
+    PrimOp * primop =
+        nix_alloc_primop(ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", (void *) SAMPLE_USER_DATA);
+    assert_ctx_ok();
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * hello = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_string(ctx, hello, "hello");
+    assert_ctx_ok();
+
+    nix_value * three = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, three, 3);
+    assert_ctx_ok();
+
+    nix_value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    NIX_VALUE_CALL(ctx, state, result, primopValue, hello, three);
+    assert_ctx_ok();
+
+    std::string r;
+    nix_get_string(ctx, result, OBSERVE_STRING(r));
+    assert_ctx_ok();
+
+    ASSERT_STREQ("hellohellohello", r.c_str());
+}
+
+static void
+primop_bad_no_return(void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
+{
+}
+
+TEST_F(nix_api_expr_test, nix_expr_primop_bad_no_return)
+{
+    PrimOp * primop =
+        nix_alloc_primop(ctx, primop_bad_no_return, 1, "badNoReturn", nullptr, "a broken primop", nullptr);
+    assert_ctx_ok();
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * three = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, three, 3);
+    assert_ctx_ok();
+
+    nix_value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_value_call(ctx, state, primopValue, three, result);
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(
+        ctx->last_err,
+        testing::Optional(
+            testing::HasSubstr("Implementation error in custom function: return value was not initialized")));
+    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badNoReturn")));
+}
+
+static void primop_bad_return_thunk(
+    void * user_data, nix_c_context * context, EvalState * state, nix_value ** args, nix_value * ret)
+{
+    nix_init_apply(context, ret, args[0], args[1]);
+}
+TEST_F(nix_api_expr_test, nix_expr_primop_bad_return_thunk)
+{
+    PrimOp * primop =
+        nix_alloc_primop(ctx, primop_bad_return_thunk, 2, "badReturnThunk", nullptr, "a broken primop", nullptr);
+    assert_ctx_ok();
+    nix_value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    nix_value * toString = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_expr_eval_from_string(ctx, state, "builtins.toString", ".", toString);
+    assert_ctx_ok();
+
+    nix_value * four = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, four, 4);
+    assert_ctx_ok();
+
+    nix_value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    NIX_VALUE_CALL(ctx, state, result, primopValue, toString, four);
+
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(
+        ctx->last_err,
+        testing::Optional(
+            testing::HasSubstr("Implementation error in custom function: return value must not be a thunk")));
+    ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badReturnThunk")));
+}
+
+TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
+{
+    nix_value * n = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, n, 3);
+    assert_ctx_ok();
+
+    nix_value * r = nix_alloc_value(ctx, state);
+    nix_value_call_multi(ctx, state, n, 0, nullptr, r);
+    assert_ctx_ok();
+
+    auto rInt = nix_get_int(ctx, r);
+    assert_ctx_ok();
+    ASSERT_EQ(3, rInt);
+}
+} // namespace nixC

src/libexpr-tests/nix_api_external.cc

@@ -0,0 +1,68 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_expr.h"
+#include "nix_api_expr_internal.h"
+#include "nix_api_value.h"
+#include "nix_api_external.h"
+
+#include "tests/nix_api_expr.hh"
+#include "tests/string_callback.hh"
+
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+class MyExternalValueDesc : public NixCExternalValueDesc
+{
+public:
+    MyExternalValueDesc(int x)
+        : _x(x)
+    {
+        print = print_function;
+        showType = show_type_function;
+        typeOf = type_of_function;
+    }
+
+private:
+    int _x;
+    static void print_function(void * self, nix_printer * printer) {}
+
+    static void show_type_function(void * self, nix_string_return * res) {}
+
+    static void type_of_function(void * self, nix_string_return * res)
+    {
+        MyExternalValueDesc * obj = static_cast<MyExternalValueDesc *>(self);
+
+        std::string type_string = "nix-external<MyExternalValueDesc( ";
+        type_string += std::to_string(obj->_x);
+        type_string += " )>";
+        res->str = &*type_string.begin();
+    }
+};
+
+TEST_F(nix_api_expr_test, nix_expr_eval_external)
+{
+    MyExternalValueDesc * external = new MyExternalValueDesc(42);
+    ExternalValue * val = nix_create_external_value(ctx, external, external);
+    nix_init_external(ctx, value, val);
+
+    EvalState * stateResult = nix_state_create(nullptr, nullptr, store);
+    nix_value * valueResult = nix_alloc_value(nullptr, stateResult);
+
+    EvalState * stateFn = nix_state_create(nullptr, nullptr, store);
+    nix_value * valueFn = nix_alloc_value(nullptr, stateFn);
+
+    nix_expr_eval_from_string(nullptr, state, "builtins.typeOf", ".", valueFn);
+
+    ASSERT_EQ(NIX_TYPE_EXTERNAL, nix_get_type(nullptr, value));
+
+    nix_value_call(ctx, state, valueFn, value, valueResult);
+
+    std::string string_value;
+    nix_get_string(nullptr, valueResult, OBSERVE_STRING(string_value));
+    ASSERT_STREQ("nix-external<MyExternalValueDesc( 42 )>", string_value.c_str());
+}
+
+}

src/libexpr-tests/nix_api_value.cc

@@ -0,0 +1,402 @@
+#include "nix_api_store.h"
+#include "nix_api_store_internal.h"
+#include "nix_api_util.h"
+#include "nix_api_util_internal.h"
+#include "nix_api_expr.h"
+#include "nix_api_value.h"
+#include "nix_api_expr_internal.h"
+
+#include "tests/nix_api_expr.hh"
+#include "tests/string_callback.hh"
+
+#include "gmock/gmock.h"
+#include <cstddef>
+#include <cstdlib>
+#include <gtest/gtest.h>
+
+namespace nixC {
+
+TEST_F(nix_api_expr_test, as_nix_value_ptr)
+{
+    // nix_alloc_value casts nix::Value to nix_value
+    // It should be obvious from the decl that that works, but if it doesn't,
+    // the whole implementation would be utterly broken.
+    ASSERT_EQ(sizeof(nix::Value), sizeof(nix_value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_get_int_invalid)
+{
+    ASSERT_EQ(0, nix_get_int(ctx, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(0, nix_get_int(ctx, value));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_int)
+{
+    int myInt = 1;
+    nix_init_int(ctx, value, myInt);
+
+    ASSERT_EQ(myInt, nix_get_int(ctx, value));
+    ASSERT_STREQ("an integer", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_INT, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_float_invalid)
+{
+    ASSERT_DOUBLE_EQ(0.0, nix_get_float(ctx, nullptr));
+    assert_ctx_err();
+    ASSERT_DOUBLE_EQ(0.0, nix_get_float(ctx, value));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_float)
+{
+    double myDouble = 1.0;
+    nix_init_float(ctx, value, myDouble);
+
+    ASSERT_DOUBLE_EQ(myDouble, nix_get_float(ctx, value));
+    ASSERT_STREQ("a float", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_FLOAT, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_bool_invalid)
+{
+    ASSERT_EQ(false, nix_get_bool(ctx, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(false, nix_get_bool(ctx, value));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_bool)
+{
+    bool myBool = true;
+    nix_init_bool(ctx, value, myBool);
+
+    ASSERT_EQ(myBool, nix_get_bool(ctx, value));
+    ASSERT_STREQ("a Boolean", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_BOOL, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_string_invalid)
+{
+    std::string string_value;
+    ASSERT_EQ(NIX_ERR_UNKNOWN, nix_get_string(ctx, nullptr, OBSERVE_STRING(string_value)));
+    assert_ctx_err();
+    ASSERT_EQ(NIX_ERR_UNKNOWN, nix_get_string(ctx, value, OBSERVE_STRING(string_value)));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_string)
+{
+    std::string string_value;
+    const char * myString = "some string";
+    nix_init_string(ctx, value, myString);
+
+    nix_get_string(ctx, value, OBSERVE_STRING(string_value));
+    ASSERT_STREQ(myString, string_value.c_str());
+    ASSERT_STREQ("a string", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_STRING, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_null_invalid)
+{
+    ASSERT_EQ(NULL, nix_get_typename(ctx, value));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_null)
+{
+    nix_init_null(ctx, value);
+
+    ASSERT_STREQ("null", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_NULL, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_value_set_get_path_invalid)
+{
+    ASSERT_EQ(nullptr, nix_get_path_string(ctx, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(nullptr, nix_get_path_string(ctx, value));
+    assert_ctx_err();
+}
+TEST_F(nix_api_expr_test, nix_value_set_get_path)
+{
+    const char * p = "/nix/store/40s0qmrfb45vlh6610rk29ym318dswdr-myname";
+    nix_init_path_string(ctx, state, value, p);
+
+    ASSERT_STREQ(p, nix_get_path_string(ctx, value));
+    ASSERT_STREQ("a path", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_PATH, nix_get_type(ctx, value));
+}
+
+TEST_F(nix_api_expr_test, nix_build_and_init_list_invalid)
+{
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, nullptr, state, 0));
+    assert_ctx_err();
+    ASSERT_EQ(0, nix_get_list_size(ctx, nullptr));
+    assert_ctx_err();
+
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, value, state, 0));
+    assert_ctx_err();
+    ASSERT_EQ(0, nix_get_list_size(ctx, value));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_build_and_init_list)
+{
+    int size = 10;
+    ListBuilder * builder = nix_make_list_builder(ctx, state, size);
+
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_value * intValue2 = nix_alloc_value(ctx, state);
+
+    // `init` and `insert` can be called in any order
+    nix_init_int(ctx, intValue, 42);
+    nix_list_builder_insert(ctx, builder, 0, intValue);
+    nix_list_builder_insert(ctx, builder, 1, intValue2);
+    nix_init_int(ctx, intValue2, 43);
+
+    nix_make_list(ctx, builder, value);
+    nix_list_builder_free(builder);
+
+    ASSERT_EQ(42, nix_get_int(ctx, nix_get_list_byidx(ctx, value, state, 0)));
+    ASSERT_EQ(43, nix_get_int(ctx, nix_get_list_byidx(ctx, value, state, 1)));
+    ASSERT_EQ(nullptr, nix_get_list_byidx(ctx, value, state, 2));
+    ASSERT_EQ(10, nix_get_list_size(ctx, value));
+
+    ASSERT_STREQ("a list", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_LIST, nix_get_type(ctx, value));
+
+    // Clean up
+    nix_gc_decref(ctx, intValue);
+}
+
+TEST_F(nix_api_expr_test, nix_build_and_init_attr_invalid)
+{
+    ASSERT_EQ(nullptr, nix_get_attr_byname(ctx, nullptr, state, 0));
+    assert_ctx_err();
+    ASSERT_EQ(nullptr, nix_get_attr_byidx(ctx, nullptr, state, 0, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(nullptr, nix_get_attr_name_byidx(ctx, nullptr, state, 0));
+    assert_ctx_err();
+    ASSERT_EQ(0, nix_get_attrs_size(ctx, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(false, nix_has_attr_byname(ctx, nullptr, state, "no-value"));
+    assert_ctx_err();
+
+    ASSERT_EQ(nullptr, nix_get_attr_byname(ctx, value, state, 0));
+    assert_ctx_err();
+    ASSERT_EQ(nullptr, nix_get_attr_byidx(ctx, value, state, 0, nullptr));
+    assert_ctx_err();
+    ASSERT_EQ(nullptr, nix_get_attr_name_byidx(ctx, value, state, 0));
+    assert_ctx_err();
+    ASSERT_EQ(0, nix_get_attrs_size(ctx, value));
+    assert_ctx_err();
+    ASSERT_EQ(false, nix_has_attr_byname(ctx, value, state, "no-value"));
+    assert_ctx_err();
+}
+
+TEST_F(nix_api_expr_test, nix_build_and_init_attr)
+{
+    int size = 10;
+    const char ** out_name = (const char **) malloc(sizeof(char *));
+
+    BindingsBuilder * builder = nix_make_bindings_builder(ctx, state, size);
+
+    nix_value * intValue = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, intValue, 42);
+
+    nix_value * stringValue = nix_alloc_value(ctx, state);
+    nix_init_string(ctx, stringValue, "foo");
+
+    nix_bindings_builder_insert(ctx, builder, "a", intValue);
+    nix_bindings_builder_insert(ctx, builder, "b", stringValue);
+    nix_make_attrs(ctx, value, builder);
+    nix_bindings_builder_free(builder);
+
+    ASSERT_EQ(2, nix_get_attrs_size(ctx, value));
+
+    nix_value * out_value = nix_get_attr_byname(ctx, value, state, "a");
+    ASSERT_EQ(42, nix_get_int(ctx, out_value));
+    nix_gc_decref(ctx, out_value);
+
+    out_value = nix_get_attr_byidx(ctx, value, state, 0, out_name);
+    ASSERT_EQ(42, nix_get_int(ctx, out_value));
+    ASSERT_STREQ("a", *out_name);
+    nix_gc_decref(ctx, out_value);
+
+    ASSERT_STREQ("a", nix_get_attr_name_byidx(ctx, value, state, 0));
+
+    ASSERT_EQ(true, nix_has_attr_byname(ctx, value, state, "b"));
+    ASSERT_EQ(false, nix_has_attr_byname(ctx, value, state, "no-value"));
+
+    out_value = nix_get_attr_byname(ctx, value, state, "b");
+    std::string string_value;
+    nix_get_string(ctx, out_value, OBSERVE_STRING(string_value));
+    ASSERT_STREQ("foo", string_value.c_str());
+    nix_gc_decref(nullptr, out_value);
+
+    out_value = nix_get_attr_byidx(ctx, value, state, 1, out_name);
+    nix_get_string(ctx, out_value, OBSERVE_STRING(string_value));
+    ASSERT_STREQ("foo", string_value.c_str());
+    ASSERT_STREQ("b", *out_name);
+    nix_gc_decref(nullptr, out_value);
+
+    ASSERT_STREQ("b", nix_get_attr_name_byidx(ctx, value, state, 1));
+
+    ASSERT_STREQ("a set", nix_get_typename(ctx, value));
+    ASSERT_EQ(NIX_TYPE_ATTRS, nix_get_type(ctx, value));
+
+    // Clean up
+    nix_gc_decref(ctx, intValue);
+    nix_gc_decref(ctx, stringValue);
+    free(out_name);
+}
+
+TEST_F(nix_api_expr_test, nix_value_init)
+{
+    // Setup
+
+    // two = 2;
+    // f = a: a * a;
+
+    nix_value * two = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, two, 2);
+
+    nix_value * f = nix_alloc_value(ctx, state);
+    nix_expr_eval_from_string(
+        ctx,
+        state,
+        R"(
+        a: a * a
+    )",
+        "<test>",
+        f);
+
+    // Test
+
+    // r = f two;
+
+    nix_value * r = nix_alloc_value(ctx, state);
+    nix_init_apply(ctx, r, f, two);
+    assert_ctx_ok();
+
+    ValueType t = nix_get_type(ctx, r);
+    assert_ctx_ok();
+
+    ASSERT_EQ(t, NIX_TYPE_THUNK);
+
+    nix_value_force(ctx, state, r);
+
+    t = nix_get_type(ctx, r);
+    assert_ctx_ok();
+
+    ASSERT_EQ(t, NIX_TYPE_INT);
+
+    int n = nix_get_int(ctx, r);
+    assert_ctx_ok();
+
+    ASSERT_EQ(n, 4);
+
+    // Clean up
+    nix_gc_decref(ctx, two);
+    nix_gc_decref(ctx, f);
+    nix_gc_decref(ctx, r);
+}
+
+TEST_F(nix_api_expr_test, nix_value_init_apply_error)
+{
+    nix_value * some_string = nix_alloc_value(ctx, state);
+    nix_init_string(ctx, some_string, "some string");
+    assert_ctx_ok();
+
+    nix_value * v = nix_alloc_value(ctx, state);
+    nix_init_apply(ctx, v, some_string, some_string);
+    assert_ctx_ok();
+
+    // All ok. Call has not been evaluated yet.
+
+    // Evaluate it
+    nix_value_force(ctx, state, v);
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("attempt to call something which is not a function but"));
+
+    // Clean up
+    nix_gc_decref(ctx, some_string);
+    nix_gc_decref(ctx, v);
+}
+
+TEST_F(nix_api_expr_test, nix_value_init_apply_lazy_arg)
+{
+    // f is a lazy function: it does not evaluate its argument before returning its return value
+    // g is a helper to produce e
+    // e is a thunk that throws an exception
+    //
+    // r = f e
+    // r should not throw an exception, because e is not evaluated
+
+    nix_value * f = nix_alloc_value(ctx, state);
+    nix_expr_eval_from_string(
+        ctx,
+        state,
+        R"(
+        a: { foo = a; }
+    )",
+        "<test>",
+        f);
+    assert_ctx_ok();
+
+    nix_value * e = nix_alloc_value(ctx, state);
+    {
+        nix_value * g = nix_alloc_value(ctx, state);
+        nix_expr_eval_from_string(
+            ctx,
+            state,
+            R"(
+            _ignore: throw "error message for test case nix_value_init_apply_lazy_arg"
+        )",
+            "<test>",
+            g);
+        assert_ctx_ok();
+
+        nix_init_apply(ctx, e, g, g);
+        assert_ctx_ok();
+        nix_gc_decref(ctx, g);
+    }
+
+    nix_value * r = nix_alloc_value(ctx, state);
+    nix_init_apply(ctx, r, f, e);
+    assert_ctx_ok();
+
+    nix_value_force(ctx, state, r);
+    assert_ctx_ok();
+
+    auto n = nix_get_attrs_size(ctx, r);
+    assert_ctx_ok();
+    ASSERT_EQ(1, n);
+
+    // nix_get_attr_byname isn't lazy (it could have been) so it will throw the exception
+    nix_value * foo = nix_get_attr_byname(ctx, r, state, "foo");
+    ASSERT_EQ(nullptr, foo);
+    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("error message for test case nix_value_init_apply_lazy_arg"));
+
+    // Clean up
+    nix_gc_decref(ctx, f);
+    nix_gc_decref(ctx, e);
+}
+
+TEST_F(nix_api_expr_test, nix_copy_value)
+{
+    nix_value * source = nix_alloc_value(ctx, state);
+
+    nix_init_int(ctx, source, 42);
+    nix_copy_value(ctx, value, source);
+
+    ASSERT_EQ(42, nix_get_int(ctx, value));
+
+    // Clean up
+    nix_gc_decref(ctx, source);
+}
+
+}

src/libexpr-tests/package.nix

@@ -0,0 +1,83 @@
+{ lib
+, buildPackages
+, stdenv
+, mkMesonExecutable
+
+, nix-expr
+, nix-expr-c
+, nix-expr-test-support
+
+, rapidcheck
+, gtest
+, runCommand
+
+# Configuration Options
+
+, version
+, resolvePath
+}:
+
+let
+  inherit (lib) fileset;
+in
+
+mkMesonExecutable (finalAttrs: {
+  pname = "nix-expr-tests";
+  inherit version;
+
+  workDir = ./.;
+  fileset = fileset.unions [
+    ../../build-utils-meson
+    ./build-utils-meson
+    ../../.version
+    ./.version
+    ./meson.build
+    # ./meson.options
+    (fileset.fileFilter (file: file.hasExt "cc") ./.)
+    (fileset.fileFilter (file: file.hasExt "hh") ./.)
+  ];
+
+  buildInputs = [
+    nix-expr
+    nix-expr-c
+    nix-expr-test-support
+    rapidcheck
+    gtest
+  ];
+
+  preConfigure =
+    # "Inline" .version so it's not a symlink, and includes the suffix.
+    # Do the meson utils, without modification.
+    ''
+      chmod u+w ./.version
+      echo ${version} > ../../.version
+    '';
+
+  mesonFlags = [
+  ];
+
+  env = lib.optionalAttrs (stdenv.isLinux && !(stdenv.hostPlatform.isStatic && stdenv.system == "aarch64-linux")) {
+    LDFLAGS = "-fuse-ld=gold";
+  };
+
+  passthru = {
+    tests = {
+      run = runCommand "${finalAttrs.pname}-run" {
+        meta.broken = !stdenv.hostPlatform.emulatorAvailable buildPackages;
+      } (lib.optionalString stdenv.hostPlatform.isWindows ''
+        export HOME="$PWD/home-dir"
+        mkdir -p "$HOME"
+      '' + ''
+        export _NIX_TEST_UNIT_DATA=${resolvePath ./data}
+        ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
+        touch $out
+      '');
+    };
+  };
+
+  meta = {
+    platforms = lib.platforms.unix ++ lib.platforms.windows;
+    mainProgram = finalAttrs.pname + stdenv.hostPlatform.extensions.executable;
+  };
+
+})

src/libexpr-tests/primops.cc

@@ -0,0 +1,860 @@
+#include <gmock/gmock.h>
+#include <gtest/gtest.h>
+
+#include "eval-settings.hh"
+#include "memory-source-accessor.hh"
+
+#include "tests/libexpr.hh"
+
+namespace nix {
+    class CaptureLogger : public Logger
+    {
+        std::ostringstream oss;
+
+        public:
+            CaptureLogger() {}
+
+            std::string get() const {
+                return oss.str();
+            }
+
+            void log(Verbosity lvl, std::string_view s) override {
+                oss << s << std::endl;
+            }
+
+            void logEI(const ErrorInfo & ei) override {
+                showErrorInfo(oss, ei, loggerSettings.showTrace.get());
+            }
+    };
+
+    class CaptureLogging {
+        Logger * oldLogger;
+        std::unique_ptr<CaptureLogger> tempLogger;
+        public:
+            CaptureLogging() : tempLogger(std::make_unique<CaptureLogger>()) {
+                oldLogger = logger;
+                logger = tempLogger.get();
+            }
+
+            ~CaptureLogging() {
+                logger = oldLogger;
+            }
+
+            std::string get() const {
+                return tempLogger->get();
+            }
+    };
+
+
+    // Testing eval of PrimOp's
+    class PrimOpTest : public LibExprTest {};
+
+
+    TEST_F(PrimOpTest, throw) {
+        ASSERT_THROW(eval("throw \"foo\""), ThrownError);
+    }
+
+    TEST_F(PrimOpTest, abort) {
+        ASSERT_THROW(eval("abort \"abort\""), Abort);
+    }
+
+    TEST_F(PrimOpTest, ceil) {
+        auto v = eval("builtins.ceil 1.9");
+        ASSERT_THAT(v, IsIntEq(2));
+    }
+
+    TEST_F(PrimOpTest, floor) {
+        auto v = eval("builtins.floor 1.9");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(PrimOpTest, tryEvalFailure) {
+        auto v = eval("builtins.tryEval (throw \"\")");
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+        auto s = createSymbol("success");
+        auto p = v.attrs()->get(s);
+        ASSERT_NE(p, nullptr);
+        ASSERT_THAT(*p->value, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, tryEvalSuccess) {
+        auto v = eval("builtins.tryEval 123");
+        ASSERT_THAT(v, IsAttrs());
+        auto s = createSymbol("success");
+        auto p = v.attrs()->get(s);
+        ASSERT_NE(p, nullptr);
+        ASSERT_THAT(*p->value, IsTrue());
+        s = createSymbol("value");
+        p = v.attrs()->get(s);
+        ASSERT_NE(p, nullptr);
+        ASSERT_THAT(*p->value, IsIntEq(123));
+    }
+
+    TEST_F(PrimOpTest, getEnv) {
+        setEnv("_NIX_UNIT_TEST_ENV_VALUE", "test value");
+        auto v = eval("builtins.getEnv \"_NIX_UNIT_TEST_ENV_VALUE\"");
+        ASSERT_THAT(v, IsStringEq("test value"));
+    }
+
+    TEST_F(PrimOpTest, seq) {
+        ASSERT_THROW(eval("let x = throw \"test\"; in builtins.seq x { }"), ThrownError);
+    }
+
+    TEST_F(PrimOpTest, seqNotDeep) {
+        auto v = eval("let x = { z =  throw \"test\"; }; in builtins.seq x { }");
+        ASSERT_THAT(v, IsAttrs());
+    }
+
+    TEST_F(PrimOpTest, deepSeq) {
+        ASSERT_THROW(eval("let x = { z =  throw \"test\"; }; in builtins.deepSeq x { }"), ThrownError);
+    }
+
+    TEST_F(PrimOpTest, trace) {
+        CaptureLogging l;
+        auto v = eval("builtins.trace \"test string 123\" 123");
+        ASSERT_THAT(v, IsIntEq(123));
+        auto text = l.get();
+        ASSERT_NE(text.find("test string 123"), std::string::npos);
+    }
+
+    TEST_F(PrimOpTest, placeholder) {
+        auto v = eval("builtins.placeholder \"out\"");
+        ASSERT_THAT(v, IsStringEq("/1rz4g4znpzjwh1xymhjpm42vipw92pr73vdgl6xs1hycac8kf2n9"));
+    }
+
+    TEST_F(PrimOpTest, baseNameOf) {
+        auto v = eval("builtins.baseNameOf /some/path");
+        ASSERT_THAT(v, IsStringEq("path"));
+    }
+
+    TEST_F(PrimOpTest, dirOf) {
+        auto v = eval("builtins.dirOf /some/path");
+        ASSERT_THAT(v, IsPathEq("/some"));
+    }
+
+    TEST_F(PrimOpTest, attrValues) {
+        auto v = eval("builtins.attrValues { x = \"foo\";  a = 1; }");
+        ASSERT_THAT(v, IsListOfSize(2));
+        ASSERT_THAT(*v.listElems()[0], IsIntEq(1));
+        ASSERT_THAT(*v.listElems()[1], IsStringEq("foo"));
+    }
+
+    TEST_F(PrimOpTest, getAttr) {
+        auto v = eval("builtins.getAttr \"x\" { x = \"foo\"; }");
+        ASSERT_THAT(v, IsStringEq("foo"));
+    }
+
+    TEST_F(PrimOpTest, getAttrNotFound) {
+        // FIXME: TypeError is really bad here, also the error wording is worse
+        // than on Nix <=2.3
+        ASSERT_THROW(eval("builtins.getAttr \"y\" { }"), TypeError);
+    }
+
+    TEST_F(PrimOpTest, unsafeGetAttrPos) {
+        state.corepkgsFS->addFile(CanonPath("foo.nix"), "\n\r\n\r{ y = \"x\"; }");
+
+        auto expr = "builtins.unsafeGetAttrPos \"y\" (import <nix/foo.nix>)";
+        auto v = eval(expr);
+        ASSERT_THAT(v, IsAttrsOfSize(3));
+
+        auto file = v.attrs()->find(createSymbol("file"));
+        ASSERT_NE(file, nullptr);
+        ASSERT_THAT(*file->value, IsString());
+        auto s = baseNameOf(file->value->string_view());
+        ASSERT_EQ(s, "foo.nix");
+
+        auto line = v.attrs()->find(createSymbol("line"));
+        ASSERT_NE(line, nullptr);
+        state.forceValue(*line->value, noPos);
+        ASSERT_THAT(*line->value, IsIntEq(4));
+
+        auto column = v.attrs()->find(createSymbol("column"));
+        ASSERT_NE(column, nullptr);
+        state.forceValue(*column->value, noPos);
+        ASSERT_THAT(*column->value, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, hasAttr) {
+        auto v = eval("builtins.hasAttr \"x\" { x = 1; }");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, hasAttrNotFound) {
+        auto v = eval("builtins.hasAttr \"x\" { }");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, isAttrs) {
+        auto v = eval("builtins.isAttrs {}");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, isAttrsFalse) {
+        auto v = eval("builtins.isAttrs null");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, removeAttrs) {
+        auto v = eval("builtins.removeAttrs { x = 1; } [\"x\"]");
+        ASSERT_THAT(v, IsAttrsOfSize(0));
+    }
+
+    TEST_F(PrimOpTest, removeAttrsRetains) {
+        auto v = eval("builtins.removeAttrs { x = 1; y = 2; } [\"x\"]");
+        ASSERT_THAT(v, IsAttrsOfSize(1));
+        ASSERT_NE(v.attrs()->find(createSymbol("y")), nullptr);
+    }
+
+    TEST_F(PrimOpTest, listToAttrsEmptyList) {
+        auto v = eval("builtins.listToAttrs []");
+        ASSERT_THAT(v, IsAttrsOfSize(0));
+        ASSERT_EQ(v.type(), nAttrs);
+        ASSERT_EQ(v.attrs()->size(), 0);
+    }
+
+    TEST_F(PrimOpTest, listToAttrsNotFieldName) {
+        ASSERT_THROW(eval("builtins.listToAttrs [{}]"), Error);
+    }
+
+    TEST_F(PrimOpTest, listToAttrs) {
+        auto v = eval("builtins.listToAttrs [ { name = \"key\"; value = 123; } ]");
+        ASSERT_THAT(v, IsAttrsOfSize(1));
+        auto key = v.attrs()->find(createSymbol("key"));
+        ASSERT_NE(key, nullptr);
+        ASSERT_THAT(*key->value, IsIntEq(123));
+    }
+
+    TEST_F(PrimOpTest, intersectAttrs) {
+        auto v = eval("builtins.intersectAttrs { a = 1; b = 2; } { b = 3; c = 4; }");
+        ASSERT_THAT(v, IsAttrsOfSize(1));
+        auto b = v.attrs()->find(createSymbol("b"));
+        ASSERT_NE(b, nullptr);
+        ASSERT_THAT(*b->value, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, catAttrs) {
+        auto v = eval("builtins.catAttrs \"a\" [{a = 1;} {b = 0;} {a = 2;}]");
+        ASSERT_THAT(v, IsListOfSize(2));
+        ASSERT_THAT(*v.listElems()[0], IsIntEq(1));
+        ASSERT_THAT(*v.listElems()[1], IsIntEq(2));
+    }
+
+    TEST_F(PrimOpTest, functionArgs) {
+        auto v = eval("builtins.functionArgs ({ x, y ? 123}: 1)");
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+
+        auto x = v.attrs()->find(createSymbol("x"));
+        ASSERT_NE(x, nullptr);
+        ASSERT_THAT(*x->value, IsFalse());
+
+        auto y = v.attrs()->find(createSymbol("y"));
+        ASSERT_NE(y, nullptr);
+        ASSERT_THAT(*y->value, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, mapAttrs) {
+        auto v = eval("builtins.mapAttrs (name: value: value * 10) { a = 1; b = 2; }");
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+
+        auto a = v.attrs()->find(createSymbol("a"));
+        ASSERT_NE(a, nullptr);
+        ASSERT_THAT(*a->value, IsThunk());
+        state.forceValue(*a->value, noPos);
+        ASSERT_THAT(*a->value, IsIntEq(10));
+
+        auto b = v.attrs()->find(createSymbol("b"));
+        ASSERT_NE(b, nullptr);
+        ASSERT_THAT(*b->value, IsThunk());
+        state.forceValue(*b->value, noPos);
+        ASSERT_THAT(*b->value, IsIntEq(20));
+    }
+
+    TEST_F(PrimOpTest, isList) {
+        auto v = eval("builtins.isList []");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, isListFalse) {
+        auto v = eval("builtins.isList null");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, elemtAt) {
+        auto v = eval("builtins.elemAt [0 1 2 3] 3");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, elemtAtOutOfBounds) {
+        ASSERT_THROW(eval("builtins.elemAt [0 1 2 3] 5"), Error);
+    }
+
+    TEST_F(PrimOpTest, head) {
+        auto v = eval("builtins.head [ 3 2 1 0 ]");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, headEmpty) {
+        ASSERT_THROW(eval("builtins.head [ ]"), Error);
+    }
+
+    TEST_F(PrimOpTest, headWrongType) {
+        ASSERT_THROW(eval("builtins.head { }"), Error);
+    }
+
+    TEST_F(PrimOpTest, tail) {
+        auto v = eval("builtins.tail [ 3 2 1 0 ]");
+        ASSERT_THAT(v, IsListOfSize(3));
+        for (const auto [n, elem] : enumerate(v.listItems()))
+            ASSERT_THAT(*elem, IsIntEq(2 - static_cast<int>(n)));
+    }
+
+    TEST_F(PrimOpTest, tailEmpty) {
+        ASSERT_THROW(eval("builtins.tail []"), Error);
+    }
+
+    TEST_F(PrimOpTest, map) {
+        auto v = eval("map (x: \"foo\" + x) [ \"bar\" \"bla\" \"abc\" ]");
+        ASSERT_THAT(v, IsListOfSize(3));
+        auto elem = v.listElems()[0];
+        ASSERT_THAT(*elem, IsThunk());
+        state.forceValue(*elem, noPos);
+        ASSERT_THAT(*elem, IsStringEq("foobar"));
+
+        elem = v.listElems()[1];
+        ASSERT_THAT(*elem, IsThunk());
+        state.forceValue(*elem, noPos);
+        ASSERT_THAT(*elem, IsStringEq("foobla"));
+
+        elem = v.listElems()[2];
+        ASSERT_THAT(*elem, IsThunk());
+        state.forceValue(*elem, noPos);
+        ASSERT_THAT(*elem, IsStringEq("fooabc"));
+    }
+
+    TEST_F(PrimOpTest, filter) {
+        auto v = eval("builtins.filter (x: x == 2) [ 3 2 3 2 3 2 ]");
+        ASSERT_THAT(v, IsListOfSize(3));
+        for (const auto elem : v.listItems())
+            ASSERT_THAT(*elem, IsIntEq(2));
+    }
+
+    TEST_F(PrimOpTest, elemTrue) {
+        auto v = eval("builtins.elem 3 [ 1 2 3 4 5 ]");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, elemFalse) {
+        auto v = eval("builtins.elem 6 [ 1 2 3 4 5 ]");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, concatLists) {
+        auto v = eval("builtins.concatLists [[1 2] [3 4]]");
+        ASSERT_THAT(v, IsListOfSize(4));
+        for (const auto [i, elem] : enumerate(v.listItems()))
+            ASSERT_THAT(*elem, IsIntEq(static_cast<int>(i)+1));
+    }
+
+    TEST_F(PrimOpTest, length) {
+        auto v = eval("builtins.length [ 1 2 3 ]");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, foldStrict) {
+        auto v = eval("builtins.foldl' (a: b: a + b) 0 [1 2 3]");
+        ASSERT_THAT(v, IsIntEq(6));
+    }
+
+    TEST_F(PrimOpTest, anyTrue) {
+        auto v = eval("builtins.any (x: x == 2) [ 1 2 3 ]");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, anyFalse) {
+        auto v = eval("builtins.any (x: x == 5) [ 1 2 3 ]");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, allTrue) {
+        auto v = eval("builtins.all (x: x > 0) [ 1 2 3 ]");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, allFalse) {
+        auto v = eval("builtins.all (x: x <= 0) [ 1 2 3 ]");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, genList) {
+        auto v = eval("builtins.genList (x: x + 1) 3");
+        ASSERT_EQ(v.type(), nList);
+        ASSERT_EQ(v.listSize(), 3);
+        for (const auto [i, elem] : enumerate(v.listItems())) {
+            ASSERT_THAT(*elem, IsThunk());
+            state.forceValue(*elem, noPos);
+            ASSERT_THAT(*elem, IsIntEq(static_cast<int>(i)+1));
+        }
+    }
+
+    TEST_F(PrimOpTest, sortLessThan) {
+        auto v = eval("builtins.sort builtins.lessThan [ 483 249 526 147 42 77 ]");
+        ASSERT_EQ(v.type(), nList);
+        ASSERT_EQ(v.listSize(), 6);
+
+        const std::vector<int> numbers = { 42, 77, 147, 249, 483, 526 };
+        for (const auto [n, elem] : enumerate(v.listItems()))
+            ASSERT_THAT(*elem, IsIntEq(numbers[n]));
+    }
+
+    TEST_F(PrimOpTest, partition) {
+        auto v = eval("builtins.partition (x: x > 10) [1 23 9 3 42]");
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+
+        auto right = v.attrs()->get(createSymbol("right"));
+        ASSERT_NE(right, nullptr);
+        ASSERT_THAT(*right->value, IsListOfSize(2));
+        ASSERT_THAT(*right->value->listElems()[0], IsIntEq(23));
+        ASSERT_THAT(*right->value->listElems()[1], IsIntEq(42));
+
+        auto wrong = v.attrs()->get(createSymbol("wrong"));
+        ASSERT_NE(wrong, nullptr);
+        ASSERT_EQ(wrong->value->type(), nList);
+        ASSERT_EQ(wrong->value->listSize(), 3);
+        ASSERT_THAT(*wrong->value, IsListOfSize(3));
+        ASSERT_THAT(*wrong->value->listElems()[0], IsIntEq(1));
+        ASSERT_THAT(*wrong->value->listElems()[1], IsIntEq(9));
+        ASSERT_THAT(*wrong->value->listElems()[2], IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, concatMap) {
+        auto v = eval("builtins.concatMap (x: x ++ [0]) [ [1 2] [3 4] ]");
+        ASSERT_EQ(v.type(), nList);
+        ASSERT_EQ(v.listSize(), 6);
+
+        const std::vector<int> numbers = { 1, 2, 0, 3, 4, 0 };
+        for (const auto [n, elem] : enumerate(v.listItems()))
+            ASSERT_THAT(*elem, IsIntEq(numbers[n]));
+    }
+
+    TEST_F(PrimOpTest, addInt) {
+        auto v = eval("builtins.add 3 5");
+        ASSERT_THAT(v, IsIntEq(8));
+    }
+
+    TEST_F(PrimOpTest, addFloat) {
+        auto v = eval("builtins.add 3.0 5.0");
+        ASSERT_THAT(v, IsFloatEq(8.0));
+    }
+
+    TEST_F(PrimOpTest, addFloatToInt) {
+        auto v = eval("builtins.add 3.0 5");
+        ASSERT_THAT(v, IsFloatEq(8.0));
+
+        v = eval("builtins.add 3 5.0");
+        ASSERT_THAT(v, IsFloatEq(8.0));
+    }
+
+    TEST_F(PrimOpTest, subInt) {
+        auto v = eval("builtins.sub 5 2");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, subFloat) {
+        auto v = eval("builtins.sub 5.0 2.0");
+        ASSERT_THAT(v, IsFloatEq(3.0));
+    }
+
+    TEST_F(PrimOpTest, subFloatFromInt) {
+        auto v = eval("builtins.sub 5.0 2");
+        ASSERT_THAT(v, IsFloatEq(3.0));
+
+        v = eval("builtins.sub 4 2.0");
+        ASSERT_THAT(v, IsFloatEq(2.0));
+    }
+
+    TEST_F(PrimOpTest, mulInt) {
+        auto v = eval("builtins.mul 3 5");
+        ASSERT_THAT(v, IsIntEq(15));
+    }
+
+    TEST_F(PrimOpTest, mulFloat) {
+        auto v = eval("builtins.mul 3.0 5.0");
+        ASSERT_THAT(v, IsFloatEq(15.0));
+    }
+
+    TEST_F(PrimOpTest, mulFloatMixed) {
+        auto v = eval("builtins.mul 3 5.0");
+        ASSERT_THAT(v, IsFloatEq(15.0));
+
+        v = eval("builtins.mul 2.0 5");
+        ASSERT_THAT(v, IsFloatEq(10.0));
+    }
+
+    TEST_F(PrimOpTest, divInt) {
+        auto v = eval("builtins.div 5 (-1)");
+        ASSERT_THAT(v, IsIntEq(-5));
+    }
+
+    TEST_F(PrimOpTest, divIntZero) {
+        ASSERT_THROW(eval("builtins.div 5 0"), EvalError);
+    }
+
+    TEST_F(PrimOpTest, divFloat) {
+        auto v = eval("builtins.div 5.0 (-1)");
+        ASSERT_THAT(v, IsFloatEq(-5.0));
+    }
+
+    TEST_F(PrimOpTest, divFloatZero) {
+        ASSERT_THROW(eval("builtins.div 5.0 0.0"), EvalError);
+    }
+
+    TEST_F(PrimOpTest, bitOr) {
+        auto v = eval("builtins.bitOr 1 2");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(PrimOpTest, bitXor) {
+        auto v = eval("builtins.bitXor 3 2");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(PrimOpTest, lessThanFalse) {
+        auto v = eval("builtins.lessThan 3 1");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(PrimOpTest, lessThanTrue) {
+        auto v = eval("builtins.lessThan 1 3");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(PrimOpTest, toStringAttrsThrows) {
+        ASSERT_THROW(eval("builtins.toString {}"), EvalError);
+    }
+
+    TEST_F(PrimOpTest, toStringLambdaThrows) {
+        ASSERT_THROW(eval("builtins.toString (x: x)"), EvalError);
+    }
+
+    class ToStringPrimOpTest :
+        public PrimOpTest,
+        public testing::WithParamInterface<std::tuple<std::string, std::string_view>>
+        {};
+
+    TEST_P(ToStringPrimOpTest, toString) {
+        const auto [input, output] = GetParam();
+        auto v = eval(input);
+        ASSERT_THAT(v, IsStringEq(output));
+    }
+
+#define CASE(input, output) (std::make_tuple(std::string_view("builtins.toString " input), std::string_view(output)))
+    INSTANTIATE_TEST_SUITE_P(
+            toString,
+            ToStringPrimOpTest,
+            testing::Values(
+                CASE(R"("foo")", "foo"),
+                CASE(R"(1)", "1"),
+                CASE(R"([1 2 3])", "1 2 3"),
+                CASE(R"(.123)", "0.123000"),
+                CASE(R"(true)", "1"),
+                CASE(R"(false)", ""),
+                CASE(R"(null)", ""),
+                CASE(R"({ v = "bar"; __toString = self: self.v; })", "bar"),
+                CASE(R"({ v = "bar"; __toString = self: self.v; outPath = "foo"; })", "bar"),
+                CASE(R"({ outPath = "foo"; })", "foo"),
+                CASE(R"(./test)", "/test")
+            )
+    );
+#undef CASE
+
+    TEST_F(PrimOpTest, substring){
+        auto v = eval("builtins.substring 0 3 \"nixos\"");
+        ASSERT_THAT(v, IsStringEq("nix"));
+    }
+
+    TEST_F(PrimOpTest, substringSmallerString){
+        auto v = eval("builtins.substring 0 3 \"n\"");
+        ASSERT_THAT(v, IsStringEq("n"));
+    }
+
+    TEST_F(PrimOpTest, substringEmptyString){
+        auto v = eval("builtins.substring 1 3 \"\"");
+        ASSERT_THAT(v, IsStringEq(""));
+    }
+
+    TEST_F(PrimOpTest, stringLength) {
+        auto v = eval("builtins.stringLength \"123\"");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+    TEST_F(PrimOpTest, hashStringMd5) {
+        auto v = eval("builtins.hashString \"md5\" \"asdf\"");
+        ASSERT_THAT(v, IsStringEq("912ec803b2ce49e4a541068d495ab570"));
+    }
+
+    TEST_F(PrimOpTest, hashStringSha1) {
+        auto v = eval("builtins.hashString \"sha1\" \"asdf\"");
+        ASSERT_THAT(v, IsStringEq("3da541559918a808c2402bba5012f6c60b27661c"));
+    }
+
+    TEST_F(PrimOpTest, hashStringSha256) {
+        auto v = eval("builtins.hashString \"sha256\" \"asdf\"");
+        ASSERT_THAT(v, IsStringEq("f0e4c2f76c58916ec258f246851bea091d14d4247a2fc3e18694461b1816e13b"));
+    }
+
+    TEST_F(PrimOpTest, hashStringSha512) {
+        auto v = eval("builtins.hashString \"sha512\" \"asdf\"");
+        ASSERT_THAT(v, IsStringEq("401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b3727429080fb337591abd3e44453b954555b7a0812e1081c39b740293f765eae731f5a65ed1"));
+    }
+
+    TEST_F(PrimOpTest, hashStringInvalidHashAlgorithm) {
+        ASSERT_THROW(eval("builtins.hashString \"foobar\" \"asdf\""), Error);
+    }
+
+    TEST_F(PrimOpTest, nixPath) {
+        auto v = eval("builtins.nixPath");
+        ASSERT_EQ(v.type(), nList);
+        // We can't test much more as currently the EvalSettings are a global
+        // that we can't easily swap / replace
+    }
+
+    TEST_F(PrimOpTest, langVersion) {
+        auto v = eval("builtins.langVersion");
+        ASSERT_EQ(v.type(), nInt);
+    }
+
+    TEST_F(PrimOpTest, storeDir) {
+        auto v = eval("builtins.storeDir");
+        ASSERT_THAT(v, IsStringEq(settings.nixStore));
+    }
+
+    TEST_F(PrimOpTest, nixVersion) {
+        auto v = eval("builtins.nixVersion");
+        ASSERT_THAT(v, IsStringEq(nixVersion));
+    }
+
+    TEST_F(PrimOpTest, currentSystem) {
+        auto v = eval("builtins.currentSystem");
+        ASSERT_THAT(v, IsStringEq(evalSettings.getCurrentSystem()));
+    }
+
+    TEST_F(PrimOpTest, derivation) {
+        auto v = eval("derivation");
+        ASSERT_EQ(v.type(), nFunction);
+        ASSERT_TRUE(v.isLambda());
+        ASSERT_NE(v.payload.lambda.fun, nullptr);
+        ASSERT_TRUE(v.payload.lambda.fun->hasFormals());
+    }
+
+    TEST_F(PrimOpTest, currentTime) {
+        auto v = eval("builtins.currentTime");
+        ASSERT_EQ(v.type(), nInt);
+        ASSERT_TRUE(v.integer() > 0);
+    }
+
+    TEST_F(PrimOpTest, splitVersion) {
+        auto v = eval("builtins.splitVersion \"1.2.3git\"");
+        ASSERT_THAT(v, IsListOfSize(4));
+
+        const std::vector<std::string_view> strings = { "1", "2", "3", "git" };
+        for (const auto [n, p] : enumerate(v.listItems()))
+            ASSERT_THAT(*p, IsStringEq(strings[n]));
+    }
+
+    class CompareVersionsPrimOpTest :
+        public PrimOpTest,
+        public testing::WithParamInterface<std::tuple<std::string, const int>>
+        {};
+
+    TEST_P(CompareVersionsPrimOpTest, compareVersions) {
+        auto [expression, expectation] = GetParam();
+        auto v = eval(expression);
+        ASSERT_THAT(v, IsIntEq(expectation));
+    }
+
+#define CASE(a, b, expected) (std::make_tuple("builtins.compareVersions \"" #a "\" \"" #b "\"", expected))
+    INSTANTIATE_TEST_SUITE_P(
+            compareVersions,
+            CompareVersionsPrimOpTest,
+            testing::Values(
+                // The first two are weird cases. Intuition tells they should
+                // be the same but they aren't.
+                CASE(1.0, 1.0.0, -1),
+                CASE(1.0.0, 1.0, 1),
+                // the following are from the nix-env manual:
+                CASE(1.0, 2.3, -1),
+                CASE(2.1, 2.3, -1),
+                CASE(2.3, 2.3, 0),
+                CASE(2.5, 2.3, 1),
+                CASE(3.1, 2.3, 1),
+                CASE(2.3.1, 2.3, 1),
+                CASE(2.3.1, 2.3a, 1),
+                CASE(2.3pre1, 2.3, -1),
+                CASE(2.3pre3, 2.3pre12, -1),
+                CASE(2.3a, 2.3c, -1),
+                CASE(2.3pre1, 2.3c, -1),
+                CASE(2.3pre1, 2.3q, -1)
+            )
+    );
+#undef CASE
+
+
+  class ParseDrvNamePrimOpTest :
+      public PrimOpTest,
+      public testing::WithParamInterface<std::tuple<std::string, std::string_view, std::string_view>>
+      {};
+
+    TEST_P(ParseDrvNamePrimOpTest, parseDrvName) {
+        auto [input, expectedName, expectedVersion] = GetParam();
+        const auto expr = fmt("builtins.parseDrvName \"%1%\"", input);
+        auto v = eval(expr);
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+
+        auto name = v.attrs()->find(createSymbol("name"));
+        ASSERT_TRUE(name);
+        ASSERT_THAT(*name->value, IsStringEq(expectedName));
+
+        auto version = v.attrs()->find(createSymbol("version"));
+        ASSERT_TRUE(version);
+        ASSERT_THAT(*version->value, IsStringEq(expectedVersion));
+    }
+
+    INSTANTIATE_TEST_SUITE_P(
+            parseDrvName,
+            ParseDrvNamePrimOpTest,
+            testing::Values(
+                std::make_tuple("nix-0.12pre12876", "nix", "0.12pre12876"),
+                std::make_tuple("a-b-c-1234pre5+git", "a-b-c", "1234pre5+git")
+            )
+    );
+
+    TEST_F(PrimOpTest, replaceStrings) {
+        // FIXME: add a test that verifies the string context is as expected
+        auto v = eval("builtins.replaceStrings [\"oo\" \"a\"] [\"a\" \"i\"] \"foobar\"");
+        ASSERT_EQ(v.type(), nString);
+        ASSERT_EQ(v.string_view(), "fabir");
+    }
+
+    TEST_F(PrimOpTest, concatStringsSep) {
+        // FIXME: add a test that verifies the string context is as expected
+        auto v = eval("builtins.concatStringsSep \"%\" [\"foo\" \"bar\" \"baz\"]");
+        ASSERT_EQ(v.type(), nString);
+        ASSERT_EQ(v.string_view(), "foo%bar%baz");
+    }
+
+    TEST_F(PrimOpTest, split1) {
+        // v = [ "" [ "a" ] "c" ]
+        auto v = eval("builtins.split \"(a)b\" \"abc\"");
+        ASSERT_THAT(v, IsListOfSize(3));
+
+        ASSERT_THAT(*v.listElems()[0], IsStringEq(""));
+
+        ASSERT_THAT(*v.listElems()[1], IsListOfSize(1));
+        ASSERT_THAT(*v.listElems()[1]->listElems()[0], IsStringEq("a"));
+
+        ASSERT_THAT(*v.listElems()[2], IsStringEq("c"));
+    }
+
+    TEST_F(PrimOpTest, split2) {
+        // v is expected to be a list [ "" [ "a" ] "b" [ "c"] "" ]
+        auto v = eval("builtins.split \"([ac])\" \"abc\"");
+        ASSERT_THAT(v, IsListOfSize(5));
+
+        ASSERT_THAT(*v.listElems()[0], IsStringEq(""));
+
+        ASSERT_THAT(*v.listElems()[1], IsListOfSize(1));
+        ASSERT_THAT(*v.listElems()[1]->listElems()[0], IsStringEq("a"));
+
+        ASSERT_THAT(*v.listElems()[2], IsStringEq("b"));
+
+        ASSERT_THAT(*v.listElems()[3], IsListOfSize(1));
+        ASSERT_THAT(*v.listElems()[3]->listElems()[0], IsStringEq("c"));
+
+        ASSERT_THAT(*v.listElems()[4], IsStringEq(""));
+    }
+
+    TEST_F(PrimOpTest, split3) {
+        auto v = eval("builtins.split \"(a)|(c)\" \"abc\"");
+        ASSERT_THAT(v, IsListOfSize(5));
+
+        // First list element
+        ASSERT_THAT(*v.listElems()[0], IsStringEq(""));
+
+        // 2nd list element is a list [ "" null ]
+        ASSERT_THAT(*v.listElems()[1], IsListOfSize(2));
+        ASSERT_THAT(*v.listElems()[1]->listElems()[0], IsStringEq("a"));
+        ASSERT_THAT(*v.listElems()[1]->listElems()[1], IsNull());
+
+        // 3rd element
+        ASSERT_THAT(*v.listElems()[2], IsStringEq("b"));
+
+        // 4th element is a list: [ null "c" ]
+        ASSERT_THAT(*v.listElems()[3], IsListOfSize(2));
+        ASSERT_THAT(*v.listElems()[3]->listElems()[0], IsNull());
+        ASSERT_THAT(*v.listElems()[3]->listElems()[1], IsStringEq("c"));
+
+        // 5th element is the empty string
+        ASSERT_THAT(*v.listElems()[4], IsStringEq(""));
+    }
+
+    TEST_F(PrimOpTest, split4) {
+        auto v = eval("builtins.split \"([[:upper:]]+)\" \" FOO \"");
+        ASSERT_THAT(v, IsListOfSize(3));
+        auto first = v.listElems()[0];
+        auto second = v.listElems()[1];
+        auto third = v.listElems()[2];
+
+        ASSERT_THAT(*first, IsStringEq(" "));
+
+        ASSERT_THAT(*second, IsListOfSize(1));
+        ASSERT_THAT(*second->listElems()[0], IsStringEq("FOO"));
+
+        ASSERT_THAT(*third, IsStringEq(" "));
+    }
+
+    TEST_F(PrimOpTest, match1) {
+        auto v = eval("builtins.match \"ab\" \"abc\"");
+        ASSERT_THAT(v, IsNull());
+    }
+
+    TEST_F(PrimOpTest, match2) {
+        auto v = eval("builtins.match \"abc\" \"abc\"");
+        ASSERT_THAT(v, IsListOfSize(0));
+    }
+
+    TEST_F(PrimOpTest, match3) {
+        auto v = eval("builtins.match \"a(b)(c)\" \"abc\"");
+        ASSERT_THAT(v, IsListOfSize(2));
+        ASSERT_THAT(*v.listElems()[0], IsStringEq("b"));
+        ASSERT_THAT(*v.listElems()[1], IsStringEq("c"));
+    }
+
+    TEST_F(PrimOpTest, match4) {
+        auto v = eval("builtins.match \"[[:space:]]+([[:upper:]]+)[[:space:]]+\" \"  FOO   \"");
+        ASSERT_THAT(v, IsListOfSize(1));
+        ASSERT_THAT(*v.listElems()[0], IsStringEq("FOO"));
+    }
+
+    TEST_F(PrimOpTest, match5) {
+        // The regex "\\{}" is valid and matches the string "{}".
+        // Caused a regression before when trying to switch from std::regex to boost::regex.
+        // See https://github.com/NixOS/nix/pull/7762#issuecomment-1834303659
+        auto v = eval("builtins.match \"\\\\{}\" \"{}\"");
+        ASSERT_THAT(v, IsListOfSize(0));
+    }
+
+    TEST_F(PrimOpTest, attrNames) {
+        auto v = eval("builtins.attrNames { x = 1; y = 2; z = 3; a = 2; }");
+        ASSERT_THAT(v, IsListOfSize(4));
+
+        // ensure that the list is sorted
+        const std::vector<std::string_view> expected { "a", "x", "y", "z" };
+        for (const auto [n, elem] : enumerate(v.listItems()))
+            ASSERT_THAT(*elem, IsStringEq(expected[n]));
+    }
+
+    TEST_F(PrimOpTest, genericClosure_not_strict) {
+        // Operator should not be used when startSet is empty
+        auto v = eval("builtins.genericClosure { startSet = []; }");
+        ASSERT_THAT(v, IsListOfSize(0));
+    }
+} /* namespace nix */

src/libexpr-tests/search-path.cc

@@ -0,0 +1,90 @@
+#include <gtest/gtest.h>
+#include <gmock/gmock.h>
+
+#include "search-path.hh"
+
+namespace nix {
+
+TEST(LookupPathElem, parse_justPath) {
+    ASSERT_EQ(
+        LookupPath::Elem::parse("foo"),
+        (LookupPath::Elem {
+            .prefix = LookupPath::Prefix { .s = "" },
+            .path = LookupPath::Path { .s = "foo" },
+        }));
+}
+
+TEST(LookupPathElem, parse_emptyPrefix) {
+    ASSERT_EQ(
+        LookupPath::Elem::parse("=foo"),
+        (LookupPath::Elem {
+            .prefix = LookupPath::Prefix { .s = "" },
+            .path = LookupPath::Path { .s = "foo" },
+        }));
+}
+
+TEST(LookupPathElem, parse_oneEq) {
+    ASSERT_EQ(
+        LookupPath::Elem::parse("foo=bar"),
+        (LookupPath::Elem {
+            .prefix = LookupPath::Prefix { .s = "foo" },
+            .path = LookupPath::Path { .s = "bar" },
+        }));
+}
+
+TEST(LookupPathElem, parse_twoEqs) {
+    ASSERT_EQ(
+        LookupPath::Elem::parse("foo=bar=baz"),
+        (LookupPath::Elem {
+            .prefix = LookupPath::Prefix { .s = "foo" },
+            .path = LookupPath::Path { .s = "bar=baz" },
+        }));
+}
+
+
+TEST(LookupPathElem, suffixIfPotentialMatch_justPath) {
+    LookupPath::Prefix prefix { .s = "" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("any/thing"), std::optional { "any/thing" });
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_misleadingPrefix1) {
+    LookupPath::Prefix prefix { .s = "foo" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("fooX"), std::nullopt);
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_misleadingPrefix2) {
+    LookupPath::Prefix prefix { .s = "foo" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("fooX/bar"), std::nullopt);
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_partialPrefix) {
+    LookupPath::Prefix prefix { .s = "fooX" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("foo"), std::nullopt);
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_exactPrefix) {
+    LookupPath::Prefix prefix { .s = "foo" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("foo"), std::optional { "" });
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_multiKey) {
+    LookupPath::Prefix prefix { .s = "foo/bar" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("foo/bar/baz"), std::optional { "baz" });
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_trailingSlash) {
+    LookupPath::Prefix prefix { .s = "foo" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("foo/"), std::optional { "" });
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_trailingDoubleSlash) {
+    LookupPath::Prefix prefix { .s = "foo" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("foo//"), std::optional { "/" });
+}
+
+TEST(LookupPathElem, suffixIfPotentialMatch_trailingPath) {
+    LookupPath::Prefix prefix { .s = "foo" };
+    ASSERT_EQ(prefix.suffixIfPotentialMatch("foo/bar/baz"), std::optional { "bar/baz" });
+}
+
+}

src/libexpr-tests/trivial.cc

@@ -0,0 +1,250 @@
+#include "tests/libexpr.hh"
+
+namespace nix {
+    // Testing of trivial expressions
+    class TrivialExpressionTest : public LibExprTest {};
+
+    TEST_F(TrivialExpressionTest, true) {
+        auto v = eval("true");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(TrivialExpressionTest, false) {
+        auto v = eval("false");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(TrivialExpressionTest, null) {
+        auto v = eval("null");
+        ASSERT_THAT(v, IsNull());
+    }
+
+    TEST_F(TrivialExpressionTest, 1) {
+        auto v = eval("1");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, 1plus1) {
+        auto v = eval("1+1");
+        ASSERT_THAT(v, IsIntEq(2));
+    }
+
+    TEST_F(TrivialExpressionTest, minus1) {
+        auto v = eval("-1");
+        ASSERT_THAT(v, IsIntEq(-1));
+    }
+
+    TEST_F(TrivialExpressionTest, 1minus1) {
+        auto v = eval("1-1");
+        ASSERT_THAT(v, IsIntEq(0));
+    }
+
+    TEST_F(TrivialExpressionTest, lambdaAdd) {
+        auto v = eval("let add = a: b: a + b; in add 1 2");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(TrivialExpressionTest, list) {
+        auto v = eval("[]");
+        ASSERT_THAT(v, IsListOfSize(0));
+    }
+
+    TEST_F(TrivialExpressionTest, attrs) {
+        auto v = eval("{}");
+        ASSERT_THAT(v, IsAttrsOfSize(0));
+    }
+
+    TEST_F(TrivialExpressionTest, float) {
+        auto v = eval("1.234");
+        ASSERT_THAT(v, IsFloatEq(1.234));
+    }
+
+    TEST_F(TrivialExpressionTest, updateAttrs) {
+        auto v = eval("{ a = 1; } // { b = 2; a = 3; }");
+        ASSERT_THAT(v, IsAttrsOfSize(2));
+        auto a = v.attrs()->find(createSymbol("a"));
+        ASSERT_NE(a, nullptr);
+        ASSERT_THAT(*a->value, IsIntEq(3));
+
+        auto b = v.attrs()->find(createSymbol("b"));
+        ASSERT_NE(b, nullptr);
+        ASSERT_THAT(*b->value, IsIntEq(2));
+    }
+
+    TEST_F(TrivialExpressionTest, hasAttrOpFalse) {
+        auto v = eval("{} ? a");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(TrivialExpressionTest, hasAttrOpTrue) {
+        auto v = eval("{ a = 123; } ? a");
+        ASSERT_THAT(v, IsTrue());
+    }
+
+    TEST_F(TrivialExpressionTest, withFound) {
+        auto v = eval("with { a = 23; }; a");
+        ASSERT_THAT(v, IsIntEq(23));
+    }
+
+    TEST_F(TrivialExpressionTest, withNotFound) {
+        ASSERT_THROW(eval("with {}; a"), Error);
+    }
+
+    TEST_F(TrivialExpressionTest, withOverride) {
+        auto v = eval("with { a = 23; }; with { a = 42; }; a");
+        ASSERT_THAT(v, IsIntEq(42));
+    }
+
+    TEST_F(TrivialExpressionTest, letOverWith) {
+        auto v = eval("let a = 23; in with { a = 1; }; a");
+        ASSERT_THAT(v, IsIntEq(23));
+    }
+
+    TEST_F(TrivialExpressionTest, multipleLet) {
+        auto v = eval("let a = 23; in let a = 42; in a");
+        ASSERT_THAT(v, IsIntEq(42));
+    }
+
+    TEST_F(TrivialExpressionTest, defaultFunctionArgs) {
+        auto v = eval("({ a ? 123 }: a) {}");
+        ASSERT_THAT(v, IsIntEq(123));
+    }
+
+    TEST_F(TrivialExpressionTest, defaultFunctionArgsOverride) {
+        auto v = eval("({ a ? 123 }: a) { a = 5; }");
+        ASSERT_THAT(v, IsIntEq(5));
+    }
+
+    TEST_F(TrivialExpressionTest, defaultFunctionArgsCaptureBack) {
+        auto v = eval("({ a ? 123 }@args: args) {}");
+        ASSERT_THAT(v, IsAttrsOfSize(0));
+    }
+
+    TEST_F(TrivialExpressionTest, defaultFunctionArgsCaptureFront) {
+        auto v = eval("(args@{ a ? 123 }: args) {}");
+        ASSERT_THAT(v, IsAttrsOfSize(0));
+    }
+
+    TEST_F(TrivialExpressionTest, assertThrows) {
+        ASSERT_THROW(eval("let x = arg: assert arg == 1; 123; in x 2"), Error);
+    }
+
+    TEST_F(TrivialExpressionTest, assertPassed) {
+        auto v = eval("let x = arg: assert arg == 1; 123; in x 1");
+        ASSERT_THAT(v, IsIntEq(123));
+    }
+
+    class AttrSetMergeTrvialExpressionTest :
+        public TrivialExpressionTest,
+        public testing::WithParamInterface<const char*>
+        {};
+
+    TEST_P(AttrSetMergeTrvialExpressionTest, attrsetMergeLazy) {
+        // Usually Nix rejects duplicate keys in an attrset but it does allow
+        // so if it is an attribute set that contains disjoint sets of keys.
+        // The below is equivalent to `{a.b = 1; a.c = 2; }`.
+        // The attribute set `a` will be a Thunk at first as the attribuets
+        // have to be merged (or otherwise computed) and that is done in a lazy
+        // manner.
+
+        auto expr = GetParam();
+        auto v = eval(expr);
+        ASSERT_THAT(v, IsAttrsOfSize(1));
+
+        auto a = v.attrs()->find(createSymbol("a"));
+        ASSERT_NE(a, nullptr);
+
+        ASSERT_THAT(*a->value, IsThunk());
+        state.forceValue(*a->value, noPos);
+
+        ASSERT_THAT(*a->value, IsAttrsOfSize(2));
+
+        auto b = a->value->attrs()->find(createSymbol("b"));
+        ASSERT_NE(b, nullptr);
+        ASSERT_THAT(*b->value, IsIntEq(1));
+
+        auto c = a->value->attrs()->find(createSymbol("c"));
+        ASSERT_NE(c, nullptr);
+        ASSERT_THAT(*c->value, IsIntEq(2));
+    }
+
+    INSTANTIATE_TEST_SUITE_P(
+            attrsetMergeLazy,
+            AttrSetMergeTrvialExpressionTest,
+            testing::Values(
+                "{ a.b = 1; a.c = 2; }",
+                "{ a = { b = 1; }; a = { c = 2; }; }"
+            )
+    );
+
+    TEST_F(TrivialExpressionTest, functor) {
+        auto v = eval("{ __functor = self: arg: self.v + arg; v = 10; } 5");
+        ASSERT_THAT(v, IsIntEq(15));
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipe) {
+        auto v = eval("1 |> builtins.add 2 |> builtins.mul 3");
+        ASSERT_THAT(v, IsIntEq(9));
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipe) {
+        auto v = eval("builtins.add 1 <| builtins.mul 2 <| 3");
+        ASSERT_THAT(v, IsIntEq(7));
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipeEvaluationOrder) {
+        auto v = eval("1 |> null |> (x: 2)");
+        ASSERT_THAT(v, IsIntEq(2));
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipeEvaluationOrder) {
+        auto v = eval("(x: 1) <| null <| 2");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, differentPipeOperatorsDoNotAssociate) {
+        ASSERT_THROW(eval("(x: 1) <| 2 |> (x: 3)"), ParseError);
+    }
+
+    TEST_F(TrivialExpressionTest, differentPipeOperatorsParensLeft) {
+        auto v = eval("((x: 1) <| 2) |> (x: 3)");
+        ASSERT_THAT(v, IsIntEq(3));
+    }
+
+    TEST_F(TrivialExpressionTest, differentPipeOperatorsParensRight) {
+        auto v = eval("(x: 1) <| (2 |> (x: 3))");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipeLowestPrecedence) {
+        auto v = eval("false -> true |> (x: !x)");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipeLowestPrecedence) {
+        auto v = eval("(x: !x) <| false -> true");
+        ASSERT_THAT(v, IsFalse());
+    }
+
+    TEST_F(TrivialExpressionTest, forwardPipeStrongerThanElse) {
+        auto v = eval("if true then 1 else 2 |> 3");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, backwardPipeStrongerThanElse) {
+        auto v = eval("if true then 1 else 2 <| 3");
+        ASSERT_THAT(v, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, bindOr) {
+        auto v = eval("{ or = 1; }");
+        ASSERT_THAT(v, IsAttrsOfSize(1));
+        auto b = v.attrs()->find(createSymbol("or"));
+        ASSERT_NE(b, nullptr);
+        ASSERT_THAT(*b->value, IsIntEq(1));
+    }
+
+    TEST_F(TrivialExpressionTest, orCantBeUsed) {
+        ASSERT_THROW(eval("let or = 1; in or"), Error);
+    }
+} /* namespace nix */

src/libexpr-tests/value/context.cc

@@ -0,0 +1,132 @@
+#include <nlohmann/json.hpp>
+#include <gtest/gtest.h>
+#include <rapidcheck/gtest.h>
+
+#include "tests/path.hh"
+#include "tests/libexpr.hh"
+#include "tests/value/context.hh"
+
+namespace nix {
+
+// Test a few cases of invalid string context elements.
+
+TEST(NixStringContextElemTest, empty_invalid) {
+    EXPECT_THROW(
+        NixStringContextElem::parse(""),
+        BadNixStringContextElem);
+}
+
+TEST(NixStringContextElemTest, single_bang_invalid) {
+    EXPECT_THROW(
+        NixStringContextElem::parse("!"),
+        BadNixStringContextElem);
+}
+
+TEST(NixStringContextElemTest, double_bang_invalid) {
+    EXPECT_THROW(
+        NixStringContextElem::parse("!!/"),
+        BadStorePath);
+}
+
+TEST(NixStringContextElemTest, eq_slash_invalid) {
+    EXPECT_THROW(
+        NixStringContextElem::parse("=/"),
+        BadStorePath);
+}
+
+TEST(NixStringContextElemTest, slash_invalid) {
+    EXPECT_THROW(
+        NixStringContextElem::parse("/"),
+        BadStorePath);
+}
+
+/**
+ * Round trip (string <-> data structure) test for
+ * `NixStringContextElem::Opaque`.
+ */
+TEST(NixStringContextElemTest, opaque) {
+    std::string_view opaque = "g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x";
+    auto elem = NixStringContextElem::parse(opaque);
+    auto * p = std::get_if<NixStringContextElem::Opaque>(&elem.raw);
+    ASSERT_TRUE(p);
+    ASSERT_EQ(p->path, StorePath { opaque });
+    ASSERT_EQ(elem.to_string(), opaque);
+}
+
+/**
+ * Round trip (string <-> data structure) test for
+ * `NixStringContextElem::DrvDeep`.
+ */
+TEST(NixStringContextElemTest, drvDeep) {
+    std::string_view drvDeep = "=g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
+    auto elem = NixStringContextElem::parse(drvDeep);
+    auto * p = std::get_if<NixStringContextElem::DrvDeep>(&elem.raw);
+    ASSERT_TRUE(p);
+    ASSERT_EQ(p->drvPath, StorePath { drvDeep.substr(1) });
+    ASSERT_EQ(elem.to_string(), drvDeep);
+}
+
+/**
+ * Round trip (string <-> data structure) test for a simpler
+ * `NixStringContextElem::Built`.
+ */
+TEST(NixStringContextElemTest, built_opaque) {
+    std::string_view built = "!foo!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
+    auto elem = NixStringContextElem::parse(built);
+    auto * p = std::get_if<NixStringContextElem::Built>(&elem.raw);
+    ASSERT_TRUE(p);
+    ASSERT_EQ(p->output, "foo");
+    ASSERT_EQ(*p->drvPath, ((SingleDerivedPath) SingleDerivedPath::Opaque {
+        .path = StorePath { built.substr(5) },
+    }));
+    ASSERT_EQ(elem.to_string(), built);
+}
+
+/**
+ * Round trip (string <-> data structure) test for a more complex,
+ * inductive `NixStringContextElem::Built`.
+ */
+TEST(NixStringContextElemTest, built_built) {
+    /**
+     * We set these in tests rather than the regular globals so we don't have
+     * to worry about race conditions if the tests run concurrently.
+     */
+    ExperimentalFeatureSettings mockXpSettings;
+    mockXpSettings.set("experimental-features", "dynamic-derivations ca-derivations");
+
+    std::string_view built = "!foo!bar!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv";
+    auto elem = NixStringContextElem::parse(built, mockXpSettings);
+    auto * p = std::get_if<NixStringContextElem::Built>(&elem.raw);
+    ASSERT_TRUE(p);
+    ASSERT_EQ(p->output, "foo");
+    auto * drvPath = std::get_if<SingleDerivedPath::Built>(&*p->drvPath);
+    ASSERT_TRUE(drvPath);
+    ASSERT_EQ(drvPath->output, "bar");
+    ASSERT_EQ(*drvPath->drvPath, ((SingleDerivedPath) SingleDerivedPath::Opaque {
+        .path = StorePath { built.substr(9) },
+    }));
+    ASSERT_EQ(elem.to_string(), built);
+}
+
+/**
+ * Without the right experimental features enabled, we cannot parse a
+ * complex inductive string context element.
+ */
+TEST(NixStringContextElemTest, built_built_xp) {
+    ASSERT_THROW(
+        NixStringContextElem::parse("!foo!bar!g1w7hy3qg1w7hy3qg1w7hy3qg1w7hy3q-x.drv"),        MissingExperimentalFeature);
+}
+
+#ifndef COVERAGE
+
+RC_GTEST_PROP(
+    NixStringContextElemTest,
+    prop_round_rip,
+    (const NixStringContextElem & o))
+{
+    RC_ASSERT(o == NixStringContextElem::parse(o.to_string()));
+}
+
+#endif
+
+}

src/libexpr-tests/value/print.cc

@@ -0,0 +1,788 @@
+#include "tests/libexpr.hh"
+
+#include "value.hh"
+#include "print.hh"
+
+namespace nix {
+
+using namespace testing;
+
+struct ValuePrintingTests : LibExprTest
+{
+    template<class... A>
+    void test(Value v, std::string_view expected, A... args)
+    {
+        std::stringstream out;
+        v.print(state, out, args...);
+        ASSERT_EQ(out.str(), expected);
+    }
+};
+
+TEST_F(ValuePrintingTests, tInt)
+{
+    Value vInt;
+    vInt.mkInt(10);
+    test(vInt, "10");
+}
+
+TEST_F(ValuePrintingTests, tBool)
+{
+    Value vBool;
+    vBool.mkBool(true);
+    test(vBool, "true");
+}
+
+TEST_F(ValuePrintingTests, tString)
+{
+    Value vString;
+    vString.mkString("some-string");
+    test(vString, "\"some-string\"");
+}
+
+TEST_F(ValuePrintingTests, tPath)
+{
+    Value vPath;
+    vPath.mkString("/foo");
+    test(vPath, "\"/foo\"");
+}
+
+TEST_F(ValuePrintingTests, tNull)
+{
+    Value vNull;
+    vNull.mkNull();
+    test(vNull, "null");
+}
+
+TEST_F(ValuePrintingTests, tAttrs)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs, "{ one = 1; two = 2; }");
+}
+
+TEST_F(ValuePrintingTests, tList)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList, "[ 1 2 «nullptr» ]");
+}
+
+TEST_F(ValuePrintingTests, vThunk)
+{
+    Value vThunk;
+    vThunk.mkThunk(nullptr, nullptr);
+
+    test(vThunk, "«thunk»");
+}
+
+TEST_F(ValuePrintingTests, vApp)
+{
+    Value vApp;
+    vApp.mkApp(nullptr, nullptr);
+
+    test(vApp, "«thunk»");
+}
+
+TEST_F(ValuePrintingTests, vLambda)
+{
+    Env env {
+        .up = nullptr,
+        .values = { }
+    };
+    PosTable::Origin origin = state.positions.addOrigin(std::monostate(), 1);
+    auto posIdx = state.positions.add(origin, 0);
+    auto body = ExprInt(0);
+    auto formals = Formals {};
+
+    ExprLambda eLambda(posIdx, createSymbol("a"), &formals, &body);
+
+    Value vLambda;
+    vLambda.mkLambda(&env, &eLambda);
+
+    test(vLambda, "«lambda @ «none»:1:1»");
+
+    eLambda.setName(createSymbol("puppy"));
+
+    test(vLambda, "«lambda puppy @ «none»:1:1»");
+}
+
+TEST_F(ValuePrintingTests, vPrimOp)
+{
+    Value vPrimOp;
+    PrimOp primOp{
+        .name = "puppy"
+    };
+    vPrimOp.mkPrimOp(&primOp);
+
+    test(vPrimOp, "«primop puppy»");
+}
+
+TEST_F(ValuePrintingTests, vPrimOpApp)
+{
+    PrimOp primOp{
+        .name = "puppy"
+    };
+    Value vPrimOp;
+    vPrimOp.mkPrimOp(&primOp);
+
+    Value vPrimOpApp;
+    vPrimOpApp.mkPrimOpApp(&vPrimOp, nullptr);
+
+    test(vPrimOpApp, "«partially applied primop puppy»");
+}
+
+TEST_F(ValuePrintingTests, vExternal)
+{
+    struct MyExternal : ExternalValueBase
+    {
+    public:
+        std::string showType() const override
+        {
+            return "";
+        }
+        std::string typeOf() const override
+        {
+            return "";
+        }
+        virtual std::ostream & print(std::ostream & str) const override
+        {
+            str << "testing-external!";
+            return str;
+        }
+    } myExternal;
+    Value vExternal;
+    vExternal.mkExternal(&myExternal);
+
+    test(vExternal, "testing-external!");
+}
+
+TEST_F(ValuePrintingTests, vFloat)
+{
+    Value vFloat;
+    vFloat.mkFloat(2.0);
+
+    test(vFloat, "2");
+}
+
+TEST_F(ValuePrintingTests, vBlackhole)
+{
+    Value vBlackhole;
+    vBlackhole.mkBlackhole();
+    test(vBlackhole, "«potential infinite recursion»");
+}
+
+TEST_F(ValuePrintingTests, depthAttrs)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builderEmpty(state, state.allocBindings(0));
+    Value vAttrsEmpty;
+    vAttrsEmpty.mkAttrs(builderEmpty.finish());
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+    builder.insert(state.symbols.create("nested"), &vAttrsEmpty);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    BindingsBuilder builder2(state, state.allocBindings(10));
+    builder2.insert(state.symbols.create("one"), &vOne);
+    builder2.insert(state.symbols.create("two"), &vTwo);
+    builder2.insert(state.symbols.create("nested"), &vAttrs);
+
+    Value vNested;
+    vNested.mkAttrs(builder2.finish());
+
+    test(vNested, "{ nested = { ... }; one = 1; two = 2; }", PrintOptions { .maxDepth = 1 });
+    test(vNested, "{ nested = { nested = { ... }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 2 });
+    test(vNested, "{ nested = { nested = { }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 3 });
+    test(vNested, "{ nested = { nested = { }; one = 1; two = 2; }; one = 1; two = 2; }", PrintOptions { .maxDepth = 4 });
+}
+
+TEST_F(ValuePrintingTests, depthList)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    BindingsBuilder builder2(state, state.allocBindings(10));
+    builder2.insert(state.symbols.create("one"), &vOne);
+    builder2.insert(state.symbols.create("two"), &vTwo);
+    builder2.insert(state.symbols.create("nested"), &vAttrs);
+
+    Value vNested;
+    vNested.mkAttrs(builder2.finish());
+
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    list.elems[2] = &vNested;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList, "[ 1 2 { ... } ]", PrintOptions { .maxDepth = 1 });
+    test(vList, "[ 1 2 { nested = { ... }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 2 });
+    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 3 });
+    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 4 });
+    test(vList, "[ 1 2 { nested = { one = 1; two = 2; }; one = 1; two = 2; } ]", PrintOptions { .maxDepth = 5 });
+}
+
+struct StringPrintingTests : LibExprTest
+{
+    template<class... A>
+    void test(std::string_view literal, std::string_view expected, unsigned int maxLength, A... args)
+    {
+        Value v;
+        v.mkString(literal);
+
+        std::stringstream out;
+        printValue(state, out, v, PrintOptions {
+            .maxStringLength = maxLength
+        });
+        ASSERT_EQ(out.str(), expected);
+    }
+};
+
+TEST_F(StringPrintingTests, maxLengthTruncation)
+{
+    test("abcdefghi", "\"abcdefghi\"", 10);
+    test("abcdefghij", "\"abcdefghij\"", 10);
+    test("abcdefghijk", "\"abcdefghij\" «1 byte elided»", 10);
+    test("abcdefghijkl", "\"abcdefghij\" «2 bytes elided»", 10);
+    test("abcdefghijklm", "\"abcdefghij\" «3 bytes elided»", 10);
+}
+
+// Check that printing an attrset shows 'important' attributes like `type`
+// first, but only reorder the attrs when we have a maxAttrs budget.
+TEST_F(ValuePrintingTests, attrsTypeFirst)
+{
+    Value vType;
+    vType.mkString("puppy");
+
+    Value vApple;
+    vApple.mkString("apple");
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("type"), &vType);
+    builder.insert(state.symbols.create("apple"), &vApple);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ type = \"puppy\"; apple = \"apple\"; }",
+         PrintOptions {
+            .maxAttrs = 100
+         });
+
+    test(vAttrs,
+         "{ apple = \"apple\"; type = \"puppy\"; }",
+         PrintOptions { });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsInt)
+{
+    Value v;
+    v.mkInt(10);
+
+    test(v,
+         ANSI_CYAN "10" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsFloat)
+{
+    Value v;
+    v.mkFloat(1.6);
+
+    test(v,
+         ANSI_CYAN "1.6" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsBool)
+{
+    Value v;
+    v.mkBool(true);
+
+    test(v,
+         ANSI_CYAN "true" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsString)
+{
+    Value v;
+    v.mkString("puppy");
+
+    test(v,
+         ANSI_MAGENTA "\"puppy\"" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+        });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsStringElided)
+{
+    Value v;
+    v.mkString("puppy");
+
+    test(v,
+         ANSI_MAGENTA "\"pup\" " ANSI_FAINT "«2 bytes elided»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .maxStringLength = 3
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsPath)
+{
+    Value v;
+    v.mkPath(state.rootPath(CanonPath("puppy")));
+
+    test(v,
+         ANSI_GREEN "/puppy" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsNull)
+{
+    Value v;
+    v.mkNull();
+
+    test(v,
+         ANSI_CYAN "null" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAttrs)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ one = " ANSI_CYAN "1" ANSI_NORMAL "; two = " ANSI_CYAN "2" ANSI_NORMAL "; }",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsDerivation)
+{
+    Value vDerivation;
+    vDerivation.mkString("derivation");
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.sType, &vDerivation);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         ANSI_GREEN "«derivation»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true,
+             .derivationPaths = true
+         });
+
+    test(vAttrs,
+         "{ type = " ANSI_MAGENTA "\"derivation\"" ANSI_NORMAL "; }",
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsError)
+{
+    Value throw_ = state.getBuiltin("throw");
+    Value message;
+    message.mkString("uh oh!");
+    Value vError;
+    vError.mkApp(&throw_, &message);
+
+    test(vError,
+         ANSI_RED
+         "«error: uh oh!»"
+         ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true,
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsDerivationError)
+{
+    Value throw_ = state.getBuiltin("throw");
+    Value message;
+    message.mkString("uh oh!");
+    Value vError;
+    vError.mkApp(&throw_, &message);
+
+    Value vDerivation;
+    vDerivation.mkString("derivation");
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.sType, &vDerivation);
+    builder.insert(state.sDrvPath, &vError);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ drvPath = "
+         ANSI_RED
+         "«error: uh oh!»"
+         ANSI_NORMAL
+         "; type = "
+         ANSI_MAGENTA
+         "\"derivation\""
+         ANSI_NORMAL
+         "; }",
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+
+    test(vAttrs,
+         ANSI_RED
+         "«error: uh oh!»"
+         ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true,
+             .derivationPaths = true,
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAssert)
+{
+    ExprVar eFalse(state.symbols.create("false"));
+    eFalse.bindVars(state, state.staticBaseEnv);
+    ExprInt eInt(1);
+
+    ExprAssert expr(noPos, &eFalse, &eInt);
+
+    Value v;
+    state.mkThunk_(v, &expr);
+
+    test(v,
+         ANSI_RED "«error: assertion 'false' failed»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsList)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList,
+         "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_CYAN "2" ANSI_NORMAL " " ANSI_MAGENTA "«nullptr»" ANSI_NORMAL " ]",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsLambda)
+{
+    Env env {
+        .up = nullptr,
+        .values = { }
+    };
+    PosTable::Origin origin = state.positions.addOrigin(std::monostate(), 1);
+    auto posIdx = state.positions.add(origin, 0);
+    auto body = ExprInt(0);
+    auto formals = Formals {};
+
+    ExprLambda eLambda(posIdx, createSymbol("a"), &formals, &body);
+
+    Value vLambda;
+    vLambda.mkLambda(&env, &eLambda);
+
+    test(vLambda,
+         ANSI_BLUE "«lambda @ «none»:1:1»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+
+    eLambda.setName(createSymbol("puppy"));
+
+    test(vLambda,
+         ANSI_BLUE "«lambda puppy @ «none»:1:1»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true,
+             .force = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsPrimOp)
+{
+    PrimOp primOp{
+        .name = "puppy"
+    };
+    Value v;
+    v.mkPrimOp(&primOp);
+
+    test(v,
+         ANSI_BLUE "«primop puppy»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsPrimOpApp)
+{
+    PrimOp primOp{
+        .name = "puppy"
+    };
+    Value vPrimOp;
+    vPrimOp.mkPrimOp(&primOp);
+
+    Value v;
+    v.mkPrimOpApp(&vPrimOp, nullptr);
+
+    test(v,
+         ANSI_BLUE "«partially applied primop puppy»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsThunk)
+{
+    Value v;
+    v.mkThunk(nullptr, nullptr);
+
+    test(v,
+         ANSI_MAGENTA "«thunk»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsBlackhole)
+{
+    Value v;
+    v.mkBlackhole();
+
+    test(v,
+         ANSI_RED "«potential infinite recursion»" ANSI_NORMAL,
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAttrsRepeated)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vEmpty;
+    vEmpty.mkAttrs(emptyBuilder.finish());
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("a"), &vEmpty);
+    builder.insert(state.symbols.create("b"), &vEmpty);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ a = { }; b = " ANSI_MAGENTA "«repeated»" ANSI_NORMAL "; }",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsListRepeated)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vEmpty;
+    vEmpty.mkAttrs(emptyBuilder.finish());
+
+    auto list = state.buildList(2);
+    list.elems[0] = &vEmpty;
+    list.elems[1] = &vEmpty;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList,
+         "[ { } " ANSI_MAGENTA "«repeated»" ANSI_NORMAL " ]",
+         PrintOptions {
+             .ansiColors = true
+         });
+}
+
+TEST_F(ValuePrintingTests, listRepeated)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vEmpty;
+    vEmpty.mkAttrs(emptyBuilder.finish());
+
+    auto list = state.buildList(2);
+    list.elems[0] = &vEmpty;
+    list.elems[1] = &vEmpty;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList, "[ { } «repeated» ]", PrintOptions { });
+    test(vList,
+         "[ { } { } ]",
+         PrintOptions {
+             .trackRepeated = false
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsAttrsElided)
+{
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    BindingsBuilder builder(state, state.allocBindings(10));
+    builder.insert(state.symbols.create("one"), &vOne);
+    builder.insert(state.symbols.create("two"), &vTwo);
+
+    Value vAttrs;
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ one = " ANSI_CYAN "1" ANSI_NORMAL "; " ANSI_FAINT "«1 attribute elided»" ANSI_NORMAL " }",
+         PrintOptions {
+             .ansiColors = true,
+             .maxAttrs = 1
+         });
+
+    Value vThree;
+    vThree.mkInt(3);
+
+    builder.insert(state.symbols.create("three"), &vThree);
+    vAttrs.mkAttrs(builder.finish());
+
+    test(vAttrs,
+         "{ one = " ANSI_CYAN "1" ANSI_NORMAL "; " ANSI_FAINT "«2 attributes elided»" ANSI_NORMAL " }",
+         PrintOptions {
+             .ansiColors = true,
+             .maxAttrs = 1
+         });
+}
+
+TEST_F(ValuePrintingTests, ansiColorsListElided)
+{
+    BindingsBuilder emptyBuilder(state, state.allocBindings(1));
+
+    Value vOne;
+    vOne.mkInt(1);
+
+    Value vTwo;
+    vTwo.mkInt(2);
+
+    {
+    auto list = state.buildList(2);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList,
+         "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_FAINT "«1 item elided»" ANSI_NORMAL " ]",
+         PrintOptions {
+             .ansiColors = true,
+             .maxListItems = 1
+         });
+    }
+
+    Value vThree;
+    vThree.mkInt(3);
+
+    {
+    auto list = state.buildList(3);
+    list.elems[0] = &vOne;
+    list.elems[1] = &vTwo;
+    list.elems[2] = &vThree;
+    Value vList;
+    vList.mkList(list);
+
+    test(vList,
+         "[ " ANSI_CYAN "1" ANSI_NORMAL " " ANSI_FAINT "«2 items elided»" ANSI_NORMAL " ]",
+         PrintOptions {
+             .ansiColors = true,
+             .maxListItems = 1
+         });
+    }
+}
+
+} // namespace nix

src/libexpr-tests/value/value.cc

@@ -0,0 +1,25 @@
+#include "value.hh"
+
+#include "tests/libstore.hh"
+
+namespace nix {
+
+class ValueTest : public LibStoreTest
+{};
+
+TEST_F(ValueTest, unsetValue)
+{
+    Value unsetValue;
+    ASSERT_EQ(false, unsetValue.isValid());
+    ASSERT_EQ(nThunk, unsetValue.type(true));
+    ASSERT_DEATH(unsetValue.type(), "");
+}
+
+TEST_F(ValueTest, vInt)
+{
+    Value vInt;
+    vInt.mkInt(42);
+    ASSERT_EQ(true, vInt.isValid());
+}
+
+} // namespace nix

src/libexpr/eval-cache.cc

@@ -69,7 +69,7 @@ struct AttrDb
     {
         auto state(_state->lock());
 
-        Path cacheDir = getCacheDir() + "/nix/eval-cache-v5";
+        Path cacheDir = getCacheDir() + "/eval-cache-v5";
         createDirs(cacheDir);
 
         Path dbPath = cacheDir + "/" + fingerprint.to_string(HashFormat::Base16, false) + ".sqlite";
@@ -101,7 +101,7 @@ struct AttrDb
                 state->txn->commit();
             state->txn.reset();
         } catch (...) {
-            ignoreException();
+            ignoreExceptionInDestructor();
         }
     }
 
@@ -112,7 +112,7 @@ struct AttrDb
         try {
             return fun();
         } catch (SQLiteError &) {
-            ignoreException();
+            ignoreExceptionExceptInterrupt();
             failed = true;
             return 0;
         }
@@ -328,7 +328,7 @@ struct AttrDb
             case AttrType::Bool:
                 return {{rowId, queryAttribute.getInt(2) != 0}};
             case AttrType::Int:
-                return {{rowId, int_t{queryAttribute.getInt(2)}}};
+                return {{rowId, int_t{NixInt{queryAttribute.getInt(2)}}}};
             case AttrType::ListOfStrings:
                 return {{rowId, tokenizeString<std::vector<std::string>>(queryAttribute.getStr(2), "\t")}};
             case AttrType::Missing:
@@ -351,7 +351,7 @@ static std::shared_ptr<AttrDb> makeAttrDb(
     try {
         return std::make_shared<AttrDb>(cfg, fingerprint, symbols);
     } catch (SQLiteError &) {
-        ignoreException();
+        ignoreExceptionExceptInterrupt();
         return nullptr;
     }
 }
@@ -471,7 +471,7 @@ Value & AttrCursor::forceValue()
         else if (v.type() == nBool)
             cachedValue = {root->db->setBool(getKey(), v.boolean()), v.boolean()};
         else if (v.type() == nInt)
-            cachedValue = {root->db->setInt(getKey(), v.integer()), int_t{v.integer()}};
+            cachedValue = {root->db->setInt(getKey(), v.integer().value), int_t{v.integer()}};
         else if (v.type() == nAttrs)
             ; // FIXME: do something?
         else

src/libexpr/eval-gc.cc

@@ -32,122 +32,6 @@ static void * oomHandler(size_t requested)
     throw std::bad_alloc();
 }
 
-class BoehmGCStackAllocator : public StackAllocator
-{
-    boost::coroutines2::protected_fixedsize_stack stack{
-        // We allocate 8 MB, the default max stack size on NixOS.
-        // A smaller stack might be quicker to allocate but reduces the stack
-        // depth available for source filter expressions etc.
-        std::max(boost::context::stack_traits::default_size(), static_cast<std::size_t>(8 * 1024 * 1024))};
-
-    // This is specific to boost::coroutines2::protected_fixedsize_stack.
-    // The stack protection page is included in sctx.size, so we have to
-    // subtract one page size from the stack size.
-    std::size_t pfss_usable_stack_size(boost::context::stack_context & sctx)
-    {
-        return sctx.size - boost::context::stack_traits::page_size();
-    }
-
-public:
-    boost::context::stack_context allocate() override
-    {
-        auto sctx = stack.allocate();
-
-        // Stacks generally start at a high address and grow to lower addresses.
-        // Architectures that do the opposite are rare; in fact so rare that
-        // boost_routine does not implement it.
-        // So we subtract the stack size.
-        GC_add_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
-        return sctx;
-    }
-
-    void deallocate(boost::context::stack_context sctx) override
-    {
-        GC_remove_roots(static_cast<char *>(sctx.sp) - pfss_usable_stack_size(sctx), sctx.sp);
-        stack.deallocate(sctx);
-    }
-};
-
-static BoehmGCStackAllocator boehmGCStackAllocator;
-
-/**
- * When a thread goes into a coroutine, we lose its original sp until
- * control flow returns to the thread.
- * While in the coroutine, the sp points outside the thread stack,
- * so we can detect this and push the entire thread stack instead,
- * as an approximation.
- * The coroutine's stack is covered by `BoehmGCStackAllocator`.
- * This is not an optimal solution, because the garbage is scanned when a
- * coroutine is active, for both the coroutine and the original thread stack.
- * However, the implementation is quite lean, and usually we don't have active
- * coroutines during evaluation, so this is acceptable.
- */
-void fixupBoehmStackPointer(void ** sp_ptr, void * _pthread_id)
-{
-    void *& sp = *sp_ptr;
-    auto pthread_id = reinterpret_cast<pthread_t>(_pthread_id);
-#  ifndef __APPLE__
-    pthread_attr_t pattr;
-#  endif
-    size_t osStackSize;
-    // The low address of the stack, which grows down.
-    void * osStackLimit;
-    void * osStackBase;
-
-#  ifdef __APPLE__
-    osStackSize = pthread_get_stacksize_np(pthread_id);
-    osStackLimit = pthread_get_stackaddr_np(pthread_id);
-#  else
-    if (pthread_attr_init(&pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
-    }
-#    ifdef HAVE_PTHREAD_GETATTR_NP
-    if (pthread_getattr_np(pthread_id, &pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_getattr_np failed");
-    }
-#    elif HAVE_PTHREAD_ATTR_GET_NP
-    if (!pthread_attr_init(&pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_init failed");
-    }
-    if (!pthread_attr_get_np(pthread_id, &pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_get_np failed");
-    }
-#    else
-#      error "Need one of `pthread_attr_get_np` or `pthread_getattr_np`"
-#    endif
-    if (pthread_attr_getstack(&pattr, &osStackLimit, &osStackSize)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_getstack failed");
-    }
-    if (pthread_attr_destroy(&pattr)) {
-        throw Error("fixupBoehmStackPointer: pthread_attr_destroy failed");
-    }
-#  endif
-    osStackBase = (char *) osStackLimit + osStackSize;
-    // NOTE: We assume the stack grows down, as it does on all architectures we support.
-    //       Architectures that grow the stack up are rare.
-    if (sp >= osStackBase || sp < osStackLimit) { // sp is outside the os stack
-        sp = osStackLimit;
-    }
-}
-
-/* Disable GC while this object lives. Used by CoroutineContext.
- *
- * Boehm keeps a count of GC_disable() and GC_enable() calls,
- * and only enables GC when the count matches.
- */
-class BoehmDisableGC
-{
-public:
-    BoehmDisableGC()
-    {
-        GC_disable();
-    };
-    ~BoehmDisableGC()
-    {
-        GC_enable();
-    };
-};
-
 static inline void initGCReal()
 {
     /* Initialise the Boehm garbage collector. */
@@ -168,24 +52,6 @@ static inline void initGCReal()
 
     GC_set_oom_fn(oomHandler);
 
-    StackAllocator::defaultAllocator = &boehmGCStackAllocator;
-
-// TODO: Remove __APPLE__ condition.
-//       Comment suggests an implementation that works on darwin and windows
-//       https://github.com/ivmai/bdwgc/issues/362#issuecomment-1936672196
-#  if GC_VERSION_MAJOR >= 8 && GC_VERSION_MINOR >= 2 && GC_VERSION_MICRO >= 4 && !defined(__APPLE__)
-    GC_set_sp_corrector(&fixupBoehmStackPointer);
-
-    if (!GC_get_sp_corrector()) {
-        printTalkative("BoehmGC on this platform does not support sp_corrector; will disable GC inside coroutines");
-        /* Used to disable GC when entering coroutines on macOS */
-        create_coro_gc_hook = []() -> std::shared_ptr<void> { return std::make_shared<BoehmDisableGC>(); };
-    }
-#  else
-#    warning \
-        "BoehmGC version does not support GC while coroutine exists. GC will be disabled inside coroutines. Consider updating bdw-gc to 8.2.4 or later."
-#  endif
-
     /* Set the initial heap size to something fairly big (25% of
        physical RAM, up to a maximum of 384 MiB) so that in most cases
        we don't need to garbage collect at all.  (Collection has a

src/libexpr/eval-gc.hh

@@ -3,6 +3,34 @@
 
 #include <cstddef>
 
+#if HAVE_BOEHMGC
+
+#  define GC_INCLUDE_NEW
+
+#  include <gc/gc.h>
+#  include <gc/gc_cpp.h>
+#  include <gc/gc_allocator.h>
+
+#else
+
+#  include <memory>
+
+/* Some dummy aliases for Boehm GC definitions to reduce the number of
+   #ifdefs. */
+
+template<typename T>
+using traceable_allocator = std::allocator<T>;
+
+template<typename T>
+using gc_allocator = std::allocator<T>;
+
+#  define GC_MALLOC_ATOMIC std::malloc
+
+struct gc
+{};
+
+#endif
+
 namespace nix {
 
 /**

src/libexpr/eval-inline.hh

@@ -4,6 +4,7 @@
 #include "print.hh"
 #include "eval.hh"
 #include "eval-error.hh"
+#include "eval-settings.hh"
 
 namespace nix {
 
@@ -138,5 +139,12 @@ inline void EvalState::forceList(Value & v, const PosIdx pos, std::string_view e
     }
 }
 
+[[gnu::always_inline]]
+inline CallDepth EvalState::addCallDepth(const PosIdx pos) {
+    if (callDepth > settings.maxCallDepth)
+        error<EvalError>("stack overflow; max-call-depth exceeded").atPos(pos).debugThrow();
+
+    return CallDepth(callDepth);
+};
 
 }

src/libexpr/eval-settings.cc

@@ -99,7 +99,7 @@ const std::string & EvalSettings::getCurrentSystem() const
 Path getNixDefExpr()
 {
     return settings.useXDGBaseDirectories
-        ? getStateDir() + "/nix/defexpr"
+        ? getStateDir() + "/defexpr"
         : getHome() + "/.nix-defexpr";
 }
 

src/libexpr/eval-settings.hh

@@ -87,10 +87,19 @@ struct EvalSettings : Config
           If the respective paths are accessible, the default values are:
 
           - `$HOME/.nix-defexpr/channels`
+
+            The [user channel link](@docroot@/command-ref/files/default-nix-expression.md#user-channel-link), pointing to the current state of [channels](@docroot@/command-ref/files/channels.md) for the current user.
+
           - `nixpkgs=$NIX_STATE_DIR/profiles/per-user/root/channels/nixpkgs`
+
+            The current state of the `nixpkgs` channel for the `root` user.
+
           - `$NIX_STATE_DIR/profiles/per-user/root/channels`
 
-          See [`NIX_STATE_DIR`](@docroot@/command-ref/env-common.md#env-NIX_STATE_DIR) for details.
+            The current state of all channels for the `root` user.
+
+          These files are set up by the [Nix installer](@docroot@/installation/installing-binary.md).
+          See [`NIX_STATE_DIR`](@docroot@/command-ref/env-common.md#env-NIX_STATE_DIR) for details on the environment variable.
 
           > **Note**
           >
@@ -185,7 +194,11 @@ struct EvalSettings : Config
         )"};
 
     Setting<bool> useEvalCache{this, true, "eval-cache",
-        "Whether to use the flake evaluation cache."};
+        R"(
+            Whether to use the flake evaluation cache.
+            Certain commands won't have to evaluate when invoked for the second time with a particular version of a flake.
+            Intermediate results are not cached.
+        )"};
 
     Setting<bool> ignoreExceptionsDuringTry{this, false, "ignore-try",
         R"(

src/libexpr/eval.cc

@@ -1,5 +1,4 @@
 #include "eval.hh"
-#include "eval-gc.hh"
 #include "eval-settings.hh"
 #include "primops.hh"
 #include "print-options.hh"
@@ -39,16 +38,6 @@
 #  include <sys/resource.h>
 #endif
 
-#if HAVE_BOEHMGC
-
-#  define GC_INCLUDE_NEW
-
-#  include <gc/gc.h>
-#  include <gc/gc_cpp.h>
-#  include <gc/gc_allocator.h>
-
-#endif
-
 #include "strings-inline.hh"
 
 using json = nlohmann::json;
@@ -58,24 +47,7 @@ namespace nix {
 static char * allocString(size_t size)
 {
     char * t;
-#if HAVE_BOEHMGC
     t = (char *) GC_MALLOC_ATOMIC(size);
-#else
-    t = (char *) malloc(size);
-#endif
-    if (!t) throw std::bad_alloc();
-    return t;
-}
-
-
-static char * dupString(const char * s)
-{
-    char * t;
-#if HAVE_BOEHMGC
-    t = GC_STRDUP(s);
-#else
-    t = strdup(s);
-#endif
     if (!t) throw std::bad_alloc();
     return t;
 }
@@ -99,11 +71,7 @@ static const char * makeImmutableString(std::string_view s)
 
 RootValue allocRootValue(Value * v)
 {
-#if HAVE_BOEHMGC
     return std::allocate_shared<Value *>(traceable_allocator<Value *>(), v);
-#else
-    return std::make_shared<Value *>(v);
-#endif
 }
 
 // Pretty print types for assertion errors
@@ -571,7 +539,7 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
     if (v.isLambda()) {
         auto exprLambda = v.payload.lambda.fun;
 
-        std::stringstream s(std::ios_base::out);
+        std::ostringstream s;
         std::string name;
         auto pos = positions[exprLambda->getPos()];
         std::string docStr;
@@ -603,19 +571,33 @@ std::optional<EvalState::Doc> EvalState::getDoc(Value & v)
 
         s << docStr;
 
-        s << '\0'; // for making a c string below
-        std::string ss = s.str();
-
         return Doc {
             .pos = pos,
             .name = name,
             .arity = 0, // FIXME: figure out how deep by syntax only? It's not semantically useful though...
             .args = {},
-            .doc =
-                // FIXME: this leaks; make the field std::string?
-                strdup(ss.data()),
+            .doc = makeImmutableString(toView(s)), // NOTE: memory leak when compiled without GC
         };
     }
+    if (isFunctor(v)) {
+        try {
+            Value & functor = *v.attrs()->find(sFunctor)->value;
+            Value * vp = &v;
+            Value partiallyApplied;
+            // The first paramater is not user-provided, and may be
+            // handled by code that is opaque to the user, like lib.const = x: y: y;
+            // So preferably we show docs that are relevant to the
+            // "partially applied" function returned by e.g. `const`.
+            // We apply the first argument:
+            callFunction(functor, 1, &vp, partiallyApplied, noPos);
+            auto _level = addCallDepth(noPos);
+            return getDoc(partiallyApplied);
+        }
+        catch (Error & e) {
+            e.addTrace(nullptr, "while partially calling '%1%' to retrieve documentation", "__functor");
+            throw;
+        }
+    }
     return {};
 }
 
@@ -836,9 +818,10 @@ static const char * * encodeContext(const NixStringContext & context)
         size_t n = 0;
         auto ctx = (const char * *)
             allocBytes((context.size() + 1) * sizeof(char *));
-        for (auto & i : context)
-            ctx[n++] = dupString(i.to_string().c_str());
-        ctx[n] = 0;
+        for (auto & i : context) {
+            ctx[n++] = makeImmutableString({i.to_string()});
+        }
+        ctx[n] = nullptr;
         return ctx;
     } else
         return nullptr;
@@ -1471,26 +1454,9 @@ void ExprLambda::eval(EvalState & state, Env & env, Value & v)
     v.mkLambda(&env, this);
 }
 
-namespace {
-/** Increments a count on construction and decrements on destruction.
- */
-class CallDepth {
-  size_t & count;
-public:
-  CallDepth(size_t & count) : count(count) {
-    ++count;
-  }
-  ~CallDepth() {
-    --count;
-  }
-};
-};
-
 void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value & vRes, const PosIdx pos)
 {
-    if (callDepth > settings.maxCallDepth)
-        error<EvalError>("stack overflow; max-call-depth exceeded").atPos(pos).debugThrow();
-    CallDepth _level(callDepth);
+    auto _level = addCallDepth(pos);
 
     auto trace = settings.traceFunctionCalls
         ? std::make_unique<FunctionCallTrace>(positions[pos])
@@ -1834,11 +1800,9 @@ void ExprIf::eval(EvalState & state, Env & env, Value & v)
 void ExprAssert::eval(EvalState & state, Env & env, Value & v)
 {
     if (!state.evalBool(env, cond, pos, "in the condition of the assert statement")) {
-        auto exprStr = ({
-            std::ostringstream out;
-            cond->show(state.symbols, out);
-            out.str();
-        });
+        std::ostringstream out;
+        cond->show(state.symbols, out);
+        auto exprStr = toView(out);
 
         if (auto eq = dynamic_cast<ExprOpEq *>(cond)) {
             try {
@@ -1979,7 +1943,7 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
     NixStringContext context;
     std::vector<BackedStringView> s;
     size_t sSize = 0;
-    NixInt n = 0;
+    NixInt n{0};
     NixFloat nf = 0;
 
     bool first = !forceString;
@@ -2023,17 +1987,22 @@ void ExprConcatStrings::eval(EvalState & state, Env & env, Value & v)
 
         if (firstType == nInt) {
             if (vTmp.type() == nInt) {
-                n += vTmp.integer();
+                auto newN = n + vTmp.integer();
+                if (auto checked = newN.valueChecked(); checked.has_value()) {
+                    n = NixInt(*checked);
+                } else {
+                    state.error<EvalError>("integer overflow in adding %1% + %2%", n, vTmp.integer()).atPos(i_pos).debugThrow();
+                }
             } else if (vTmp.type() == nFloat) {
                 // Upgrade the type from int to float;
                 firstType = nFloat;
-                nf = n;
+                nf = n.value;
                 nf += vTmp.fpoint();
             } else
                 state.error<EvalError>("cannot add %1% to an integer", showType(vTmp)).atPos(i_pos).withFrame(env, *this).debugThrow();
         } else if (firstType == nFloat) {
             if (vTmp.type() == nInt) {
-                nf += vTmp.integer();
+                nf += vTmp.integer().value;
             } else if (vTmp.type() == nFloat) {
                 nf += vTmp.fpoint();
             } else
@@ -2158,7 +2127,7 @@ NixFloat EvalState::forceFloat(Value & v, const PosIdx pos, std::string_view err
     try {
         forceValue(v, pos);
         if (v.type() == nInt)
-            return v.integer();
+            return v.integer().value;
         else if (v.type() != nFloat)
             error<TypeError>(
                 "expected a float but found %1%: %2%",
@@ -2345,7 +2314,7 @@ BackedStringView EvalState::coerceToString(
            shell scripting convenience, just like `null'. */
         if (v.type() == nBool && v.boolean()) return "1";
         if (v.type() == nBool && !v.boolean()) return "";
-        if (v.type() == nInt) return std::to_string(v.integer());
+        if (v.type() == nInt) return std::to_string(v.integer().value);
         if (v.type() == nFloat) return std::to_string(v.fpoint());
         if (v.type() == nNull) return "";
 
@@ -2728,9 +2697,9 @@ bool EvalState::eqValues(Value & v1, Value & v2, const PosIdx pos, std::string_v
 
     // Special case type-compatibility between float and int
     if (v1.type() == nInt && v2.type() == nFloat)
-        return v1.integer() == v2.fpoint();
+        return v1.integer().value == v2.fpoint();
     if (v1.type() == nFloat && v2.type() == nInt)
-        return v1.fpoint() == v2.integer();
+        return v1.fpoint() == v2.integer().value;
 
     // All other types are not compatible with each other.
     if (v1.type() != v2.type()) return false;
@@ -2865,7 +2834,9 @@ void EvalState::printStatistics()
 #endif
 #if HAVE_BOEHMGC
         {GC_is_incremental_mode() ? "gcNonIncremental" : "gc", gcFullOnlyTime},
+#ifndef _WIN32 // TODO implement
         {GC_is_incremental_mode() ? "gcNonIncrementalFraction" : "gcFraction", gcFullOnlyTime / cpuTime},
+#endif
 #endif
     };
     topObj["envs"] = {
@@ -3083,7 +3054,9 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
     if (EvalSettings::isPseudoUrl(value)) {
         try {
             auto accessor = fetchers::downloadTarball(
-                EvalSettings::resolvePseudoUrl(value)).accessor;
+                store,
+                fetchSettings,
+                EvalSettings::resolvePseudoUrl(value));
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
             return finish(store->toRealPath(storePath));
         } catch (Error & e) {

src/libexpr/eval.hh

@@ -41,6 +41,21 @@ namespace eval_cache {
     class EvalCache;
 }
 
+/**
+ * Increments a count on construction and decrements on destruction.
+ */
+class CallDepth {
+  size_t & count;
+
+public:
+  CallDepth(size_t & count) : count(count) {
+    ++count;
+  }
+  ~CallDepth() {
+    --count;
+  }
+};
+
 /**
  * Function that implements a primop.
  */
@@ -124,11 +139,7 @@ struct Constant
     bool impureOnly = false;
 };
 
-#if HAVE_BOEHMGC
-    typedef std::map<std::string, Value *, std::less<std::string>, traceable_allocator<std::pair<const std::string, Value *> > > ValMap;
-#else
-    typedef std::map<std::string, Value *> ValMap;
-#endif
+typedef std::map<std::string, Value *, std::less<std::string>, traceable_allocator<std::pair<const std::string, Value *> > > ValMap;
 
 typedef std::unordered_map<PosIdx, DocComment> DocCommentMap;
 
@@ -314,21 +325,13 @@ private:
     /**
      * A cache from path names to parse trees.
      */
-#if HAVE_BOEHMGC
     typedef std::unordered_map<SourcePath, Expr *, std::hash<SourcePath>, std::equal_to<SourcePath>, traceable_allocator<std::pair<const SourcePath, Expr *>>> FileParseCache;
-#else
-    typedef std::unordered_map<SourcePath, Expr *> FileParseCache;
-#endif
     FileParseCache fileParseCache;
 
     /**
      * A cache from path names to values.
      */
-#if HAVE_BOEHMGC
     typedef std::unordered_map<SourcePath, Value, std::hash<SourcePath>, std::equal_to<SourcePath>, traceable_allocator<std::pair<const SourcePath, Value>>> FileEvalCache;
-#else
-    typedef std::unordered_map<SourcePath, Value> FileEvalCache;
-#endif
     FileEvalCache fileEvalCache;
 
     /**
@@ -625,6 +628,12 @@ public:
         const char * doc;
     };
 
+    /**
+     * Retrieve the documentation for a value. This will evaluate the value if
+     * it is a thunk, and it will partially apply __functor if applicable.
+     *
+     * @param v The value to get the documentation for.
+     */
     std::optional<Doc> getDoc(Value & v);
 
 private:
@@ -649,6 +658,11 @@ private:
 
 public:
 
+    /**
+     * Check that the call depth is within limits, and increment it, until the returned object is destroyed.
+     */
+    inline CallDepth addCallDepth(const PosIdx pos);
+
     /**
      * Do a deep equality test between two values.  That is, list
      * elements and attributes are compared recursively.

src/libexpr/gc-small-vector.hh

@@ -2,28 +2,15 @@
 
 #include <boost/container/small_vector.hpp>
 
-#if HAVE_BOEHMGC
-
-#include <gc/gc.h>
-#include <gc/gc_cpp.h>
-#include <gc/gc_allocator.h>
-
-#endif
+#include "value.hh"
 
 namespace nix {
 
-struct Value;
-
 /**
  * A GC compatible vector that may used a reserved portion of `nItems` on the stack instead of allocating on the heap.
  */
-#if HAVE_BOEHMGC
 template <typename T, size_t nItems>
 using SmallVector = boost::container::small_vector<T, nItems, traceable_allocator<T>>;
-#else
-template <typename T, size_t nItems>
-using SmallVector = boost::container::small_vector<T, nItems>;
-#endif
 
 /**
  * A vector of value pointers. See `SmallVector`.
@@ -39,4 +26,4 @@ using SmallValueVector = SmallVector<Value *, nItems>;
 template <size_t nItems>
 using SmallTemporaryValueVector = SmallVector<Value, nItems>;
 
-}
+}

src/libexpr/get-drvs.cc

@@ -246,8 +246,8 @@ NixInt PackageInfo::queryMetaInt(const std::string & name, NixInt def)
     if (v->type() == nString) {
         /* Backwards compatibility with before we had support for
            integer meta fields. */
-        if (auto n = string2Int<NixInt>(v->c_str()))
-            return *n;
+        if (auto n = string2Int<NixInt::Inner>(v->c_str()))
+            return NixInt{*n};
     }
     return def;
 }
@@ -374,11 +374,12 @@ static void getDerivations(EvalState & state, Value & vIn,
            bound to the attribute with the "lower" name should take
            precedence). */
         for (auto & i : v.attrs()->lexicographicOrder(state.symbols)) {
+            std::string_view symbol{state.symbols[i->name]};
             try {
-                debug("evaluating attribute '%1%'", state.symbols[i->name]);
-                if (!std::regex_match(std::string(state.symbols[i->name]), attrRegex))
+                debug("evaluating attribute '%1%'", symbol);
+                if (!std::regex_match(symbol.begin(), symbol.end(), attrRegex))
                     continue;
-                std::string pathPrefix2 = addToPath(pathPrefix, state.symbols[i->name]);
+                std::string pathPrefix2 = addToPath(pathPrefix, symbol);
                 if (combineChannels)
                     getDerivations(state, *i->value, pathPrefix2, autoArgs, drvs, done, ignoreAssertionFailures);
                 else if (getDerivation(state, *i->value, pathPrefix2, drvs, done, ignoreAssertionFailures)) {
@@ -392,7 +393,7 @@ static void getDerivations(EvalState & state, Value & vIn,
                     }
                 }
             } catch (Error & e) {
-                e.addTrace(state.positions[i->pos], "while evaluating the attribute '%s'", state.symbols[i->name]);
+                e.addTrace(state.positions[i->pos], "while evaluating the attribute '%s'", symbol);
                 throw;
             }
         }

src/libexpr/get-drvs.hh

@@ -83,11 +83,7 @@ public:
 };
 
 
-#if HAVE_BOEHMGC
 typedef std::list<PackageInfo, traceable_allocator<PackageInfo>> PackageInfos;
-#else
-typedef std::list<PackageInfo> PackageInfos;
-#endif
 
 
 /**

src/libexpr/json-to-value.cc

@@ -2,6 +2,7 @@
 #include "value.hh"
 #include "eval.hh"
 
+#include <limits>
 #include <variant>
 #include <nlohmann/json.hpp>
 
@@ -101,8 +102,12 @@ public:
         return true;
     }
 
-    bool number_unsigned(number_unsigned_t val) override
+    bool number_unsigned(number_unsigned_t val_) override
     {
+        if (val_ > std::numeric_limits<NixInt::Inner>::max()) {
+            throw Error("unsigned json number %1% outside of Nix integer range", val_);
+        }
+        NixInt::Inner val = val_;
         rs->value(state).mkInt(val);
         rs->add();
         return true;

src/libexpr/lexer.l

@@ -138,7 +138,7 @@ or          { return OR_KW; }
 {INT}       { errno = 0;
               std::optional<int64_t> numMay = string2Int<int64_t>(yytext);
               if (numMay.has_value()) {
-                  yylval->n = *numMay;
+                  yylval->n = NixInt{*numMay};
               } else {
                   throw ParseError(ErrorInfo{
                       .msg = HintFmt("invalid integer '%1%'", yytext),

src/libexpr/nixexpr.cc

@@ -663,4 +663,32 @@ std::string DocComment::getInnerText(const PosTable & positions) const {
     return docStr;
 }
 
+
+
+/* ‘Cursed or’ handling.
+ *
+ * In parser.y, every use of expr_select in a production must call one of the
+ * two below functions.
+ *
+ * To be removed by https://github.com/NixOS/nix/pull/11121
+ */
+
+void ExprCall::resetCursedOr()
+{
+    cursedOrEndPos.reset();
+}
+
+void ExprCall::warnIfCursedOr(const SymbolTable & symbols, const PosTable & positions)
+{
+    if (cursedOrEndPos.has_value()) {
+        std::ostringstream out;
+        out << "at " << positions[pos] << ": "
+               "This expression uses `or` as an identifier in a way that will change in a future Nix release.\n"
+               "Wrap this entire expression in parentheses to preserve its current meaning:\n"
+               "    (" << positions[pos].getSnippetUpTo(positions[*cursedOrEndPos]).value_or("could not read expression") << ")\n"
+               "Give feedback at https://github.com/NixOS/nix/pull/11121";
+        warn(out.str());
+    }
+}
+
 }

src/libexpr/nixexpr.hh

@@ -96,6 +96,10 @@ struct Expr
     virtual void setName(Symbol name);
     virtual void setDocComment(DocComment docComment) { };
     virtual PosIdx getPos() const { return noPos; }
+
+    // These are temporary methods to be used only in parser.y
+    virtual void resetCursedOr() { };
+    virtual void warnIfCursedOr(const SymbolTable & symbols, const PosTable & positions) { };
 };
 
 #define COMMON_METHODS \
@@ -107,6 +111,7 @@ struct ExprInt : Expr
 {
     Value v;
     ExprInt(NixInt n) { v.mkInt(n); };
+    ExprInt(NixInt::Inner n) { v.mkInt(n); };
     Value * maybeThunk(EvalState & state, Env & env) override;
     COMMON_METHODS
 };
@@ -353,10 +358,16 @@ struct ExprCall : Expr
     Expr * fun;
     std::vector<Expr *> args;
     PosIdx pos;
+    std::optional<PosIdx> cursedOrEndPos; // used during parsing to warn about https://github.com/NixOS/nix/issues/11118
     ExprCall(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args)
-        : fun(fun), args(args), pos(pos)
+        : fun(fun), args(args), pos(pos), cursedOrEndPos({})
+    { }
+    ExprCall(const PosIdx & pos, Expr * fun, std::vector<Expr *> && args, PosIdx && cursedOrEndPos)
+        : fun(fun), args(args), pos(pos), cursedOrEndPos(cursedOrEndPos)
     { }
     PosIdx getPos() const override { return pos; }
+    virtual void resetCursedOr() override;
+    virtual void warnIfCursedOr(const SymbolTable & symbols, const PosTable & positions) override;
     COMMON_METHODS
 };
 

src/libexpr/package.nix

@@ -1,11 +1,7 @@
 { lib
 , stdenv
-, mkMesonDerivation
-, releaseTools
+, mkMesonLibrary
 
-, meson
-, ninja
-, pkg-config
 , bison
 , flex
 , cmake # for resolving toml11 dep
@@ -38,7 +34,7 @@ let
   inherit (lib) fileset;
 in
 
-mkMesonDerivation (finalAttrs: {
+mkMesonLibrary (finalAttrs: {
   pname = "nix-expr";
   inherit version;
 
@@ -55,15 +51,13 @@ mkMesonDerivation (finalAttrs: {
     (fileset.fileFilter (file: file.hasExt "hh") ./.)
     ./lexer.l
     ./parser.y
-    (fileset.fileFilter (file: file.hasExt "nix") ./.)
+    (fileset.difference
+      (fileset.fileFilter (file: file.hasExt "nix") ./.)
+      ./package.nix
+    )
   ];
 
-  outputs = [ "out" "dev" ];
-
   nativeBuildInputs = [
-    meson
-    ninja
-    pkg-config
     bison
     flex
     cmake
@@ -102,10 +96,6 @@ mkMesonDerivation (finalAttrs: {
     LDFLAGS = "-fuse-ld=gold";
   };
 
-  separateDebugInfo = !stdenv.hostPlatform.isStatic;
-
-  hardeningDisable = lib.optional stdenv.hostPlatform.isStatic "pie";
-
   meta = {
     platforms = lib.platforms.unix ++ lib.platforms.windows;
   };

src/libexpr/parser.y

@@ -264,19 +264,28 @@ expr_op
   ;
 
 expr_app
-  : expr_app expr_select { $$ = makeCall(CUR_POS, $1, $2); }
-  | expr_select
+  : expr_app expr_select { $$ = makeCall(CUR_POS, $1, $2); $2->warnIfCursedOr(state->symbols, state->positions); }
+  | /* Once a ‘cursed or’ reaches this nonterminal, it is no longer cursed,
+       because the uncursed parse would also produce an expr_app. But we need
+       to remove the cursed status in order to prevent valid things like
+       `f (g or)` from triggering the warning. */
+    expr_select { $$ = $1; $$->resetCursedOr(); }
   ;
 
 expr_select
   : expr_simple '.' attrpath
     { $$ = new ExprSelect(CUR_POS, $1, std::move(*$3), nullptr); delete $3; }
   | expr_simple '.' attrpath OR_KW expr_select
-    { $$ = new ExprSelect(CUR_POS, $1, std::move(*$3), $5); delete $3; }
-  | /* Backwards compatibility: because Nixpkgs has a rarely used
-       function named ‘or’, allow stuff like ‘map or [...]’. */
+    { $$ = new ExprSelect(CUR_POS, $1, std::move(*$3), $5); delete $3; $5->warnIfCursedOr(state->symbols, state->positions); }
+  | /* Backwards compatibility: because Nixpkgs has a function named ‘or’,
+       allow stuff like ‘map or [...]’. This production is problematic (see
+       https://github.com/NixOS/nix/issues/11118) and will be refactored in the
+       future by treating `or` as a regular identifier. The refactor will (in
+       very rare cases, we think) change the meaning of expressions, so we mark
+       the ExprCall with data (establishing that it is a ‘cursed or’) that can
+       be used to emit a warning when an affected expression is parsed. */
     expr_simple OR_KW
-    { $$ = new ExprCall(CUR_POS, $1, {new ExprVar(CUR_POS, state->s.or_)}); }
+    { $$ = new ExprCall(CUR_POS, $1, {new ExprVar(CUR_POS, state->s.or_)}, state->positions.add(state->origin, @$.endOffset)); }
   | expr_simple
   ;
 
@@ -350,7 +359,7 @@ string_parts_interpolated
 
 path_start
   : PATH {
-    Path path(absPath({$1.p, $1.l}, state->basePath.path.abs()));
+    Path path(absPath(std::string_view{$1.p, $1.l}, state->basePath.path.abs()));
     /* add back in the trailing '/' to the first segment */
     if ($1.p[$1.l-1] == '/' && $1.l > 1)
       path += "/";
@@ -472,7 +481,7 @@ string_attr
   ;
 
 expr_list
-  : expr_list expr_select { $$ = $1; $1->elems.push_back($2); /* !!! dangerous */ }
+  : expr_list expr_select { $$ = $1; $1->elems.push_back($2); /* !!! dangerous */; $2->warnIfCursedOr(state->symbols, state->positions); }
   | { $$ = new ExprList; }
   ;
 

src/libexpr/primops.cc

@@ -40,6 +40,13 @@ namespace nix {
  * Miscellaneous
  *************************************************************/
 
+static inline Value * mkString(EvalState & state, const std::csub_match & match)
+{
+    Value * v = state.allocValue();
+    v->mkString({match.first, match.second});
+    return v;
+}
+
 StringMap EvalState::realiseContext(const NixStringContext & context, StorePathSet * maybePathsOut, bool isIFD)
 {
     std::vector<DerivedPath::Built> drvs;
@@ -171,6 +178,78 @@ static void mkOutputString(
         o.second.path(*state.store, Derivation::nameFromPath(drvPath), o.first));
 }
 
+/**
+ * `import` will parse a derivation when it imports a `.drv` file from the store.
+ *
+ * @param state The evaluation state.
+ * @param pos The position of the `import` call.
+ * @param path The path to the `.drv` to import.
+ * @param storePath The path to the `.drv` to import.
+ * @param v Return value
+ */
+void derivationToValue(EvalState & state, const PosIdx pos, const SourcePath & path, const StorePath & storePath, Value & v) {
+    auto path2 = path.path.abs();
+    Derivation drv = state.store->readDerivation(storePath);
+    auto attrs = state.buildBindings(3 + drv.outputs.size());
+    attrs.alloc(state.sDrvPath).mkString(path2, {
+        NixStringContextElem::DrvDeep { .drvPath = storePath },
+    });
+    attrs.alloc(state.sName).mkString(drv.env["name"]);
+
+    auto list = state.buildList(drv.outputs.size());
+    for (const auto & [i, o] : enumerate(drv.outputs)) {
+        mkOutputString(state, attrs, storePath, o);
+        (list[i] = state.allocValue())->mkString(o.first);
+    }
+    attrs.alloc(state.sOutputs).mkList(list);
+
+    auto w = state.allocValue();
+    w->mkAttrs(attrs);
+
+    if (!state.vImportedDrvToDerivation) {
+        state.vImportedDrvToDerivation = allocRootValue(state.allocValue());
+        state.eval(state.parseExprFromString(
+            #include "imported-drv-to-derivation.nix.gen.hh"
+            , state.rootPath(CanonPath::root)), **state.vImportedDrvToDerivation);
+    }
+
+    state.forceFunction(**state.vImportedDrvToDerivation, pos, "while evaluating imported-drv-to-derivation.nix.gen.hh");
+    v.mkApp(*state.vImportedDrvToDerivation, w);
+    state.forceAttrs(v, pos, "while calling imported-drv-to-derivation.nix.gen.hh");
+}
+
+/**
+ * Import a Nix file with an alternate base scope, as `builtins.scopedImport` does.
+ *
+ * @param state The evaluation state.
+ * @param pos The position of the import call.
+ * @param path The path to the file to import.
+ * @param vScope The base scope to use for the import.
+ * @param v Return value
+ */
+static void scopedImport(EvalState & state, const PosIdx pos, SourcePath & path, Value * vScope, Value & v) {
+    state.forceAttrs(*vScope, pos, "while evaluating the first argument passed to builtins.scopedImport");
+
+    Env * env = &state.allocEnv(vScope->attrs()->size());
+    env->up = &state.baseEnv;
+
+    auto staticEnv = std::make_shared<StaticEnv>(nullptr, state.staticBaseEnv.get(), vScope->attrs()->size());
+
+    unsigned int displ = 0;
+    for (auto & attr : *vScope->attrs()) {
+        staticEnv->vars.emplace_back(attr.name, displ);
+        env->values[displ++] = attr.value;
+    }
+
+    // No need to call staticEnv.sort(), because
+    // args[0]->attrs is already sorted.
+
+    printTalkative("evaluating file '%1%'", path);
+    Expr * e = state.parseExprFromFile(resolveExprPath(path), staticEnv);
+
+    e->eval(state, *env, v);
+}
+
 /* Load and evaluate an expression from path specified by the
    argument. */
 static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * vScope, Value & v)
@@ -189,60 +268,13 @@ static void import(EvalState & state, const PosIdx pos, Value & vPath, Value * v
     };
 
     if (auto storePath = isValidDerivationInStore()) {
-        Derivation drv = state.store->readDerivation(*storePath);
-        auto attrs = state.buildBindings(3 + drv.outputs.size());
-        attrs.alloc(state.sDrvPath).mkString(path2, {
-            NixStringContextElem::DrvDeep { .drvPath = *storePath },
-        });
-        attrs.alloc(state.sName).mkString(drv.env["name"]);
-
-        auto list = state.buildList(drv.outputs.size());
-        for (const auto & [i, o] : enumerate(drv.outputs)) {
-            mkOutputString(state, attrs, *storePath, o);
-            (list[i] = state.allocValue())->mkString(o.first);
-        }
-        attrs.alloc(state.sOutputs).mkList(list);
-
-        auto w = state.allocValue();
-        w->mkAttrs(attrs);
-
-        if (!state.vImportedDrvToDerivation) {
-            state.vImportedDrvToDerivation = allocRootValue(state.allocValue());
-            state.eval(state.parseExprFromString(
-                #include "imported-drv-to-derivation.nix.gen.hh"
-                , state.rootPath(CanonPath::root)), **state.vImportedDrvToDerivation);
-        }
-
-        state.forceFunction(**state.vImportedDrvToDerivation, pos, "while evaluating imported-drv-to-derivation.nix.gen.hh");
-        v.mkApp(*state.vImportedDrvToDerivation, w);
-        state.forceAttrs(v, pos, "while calling imported-drv-to-derivation.nix.gen.hh");
+        derivationToValue(state, pos, path, *storePath, v);
+    }
+    else if (vScope) {
+        scopedImport(state, pos, path, vScope, v);
     }
-
     else {
-        if (!vScope)
-            state.evalFile(path, v);
-        else {
-            state.forceAttrs(*vScope, pos, "while evaluating the first argument passed to builtins.scopedImport");
-
-            Env * env = &state.allocEnv(vScope->attrs()->size());
-            env->up = &state.baseEnv;
-
-            auto staticEnv = std::make_shared<StaticEnv>(nullptr, state.staticBaseEnv.get(), vScope->attrs()->size());
-
-            unsigned int displ = 0;
-            for (auto & attr : *vScope->attrs()) {
-                staticEnv->vars.emplace_back(attr.name, displ);
-                env->values[displ++] = attr.value;
-            }
-
-            // No need to call staticEnv.sort(), because
-            // args[0]->attrs is already sorted.
-
-            printTalkative("evaluating file '%1%'", path);
-            Expr * e = state.parseExprFromFile(resolveExprPath(path), staticEnv);
-
-            e->eval(state, *env, v);
-        }
+        state.evalFile(path, v);
     }
 }
 
@@ -587,9 +619,9 @@ struct CompareValues
     {
         try {
             if (v1->type() == nFloat && v2->type() == nInt)
-                return v1->fpoint() < v2->integer();
+                return v1->fpoint() < v2->integer().value;
             if (v1->type() == nInt && v2->type() == nFloat)
-                return v1->integer() < v2->fpoint();
+                return v1->integer().value < v2->fpoint();
             if (v1->type() != v2->type())
                 state.error<EvalError>("cannot compare %s with %s", showType(*v1), showType(*v2)).debugThrow();
             // Allow selecting a subset of enum values
@@ -631,11 +663,7 @@ struct CompareValues
 };
 
 
-#if HAVE_BOEHMGC
 typedef std::list<Value *, gc_allocator<Value *>> ValueList;
-#else
-typedef std::list<Value *> ValueList;
-#endif
 
 
 static Bindings::const_iterator getAttr(
@@ -941,6 +969,9 @@ static RegisterPrimOp primop_tryEval({
       `let e = { x = throw ""; }; in
       (builtins.tryEval (builtins.deepSeq e e)).success` will be
       `false`.
+
+      `tryEval` intentionally does not return the error message, because that risks bringing non-determinism into the evaluation result, and it would become very difficult to improve error reporting without breaking existing expressions.
+      Instead, use [`builtins.addErrorContext`](@docroot@/language/builtins.md#builtins-addErrorContext) to add context to the error message, and use a Nix unit testing tool for testing.
     )",
     .fun = prim_tryEval,
 });
@@ -2133,7 +2164,7 @@ static void prim_toXML(EvalState & state, const PosIdx pos, Value * * args, Valu
     std::ostringstream out;
     NixStringContext context;
     printValueAsXML(state, true, false, *args[0], out, context, pos);
-    v.mkString(out.str(), context);
+    v.mkString(toView(out), context);
 }
 
 static RegisterPrimOp primop_toXML({
@@ -2241,7 +2272,7 @@ static void prim_toJSON(EvalState & state, const PosIdx pos, Value * * args, Val
     std::ostringstream out;
     NixStringContext context;
     printValueAsJSON(state, true, *args[0], pos, out, context);
-    v.mkString(out.str(), context);
+    v.mkString(toView(out), context);
 }
 
 static RegisterPrimOp primop_toJSON({
@@ -2762,13 +2793,13 @@ static struct LazyPosAcessors {
     PrimOp primop_lineOfPos{
         .arity = 1,
         .fun = [] (EvalState & state, PosIdx pos, Value * * args, Value & v) {
-            v.mkInt(state.positions[PosIdx(args[0]->integer())].line);
+            v.mkInt(state.positions[PosIdx(args[0]->integer().value)].line);
         }
     };
     PrimOp primop_columnOfPos{
         .arity = 1,
         .fun = [] (EvalState & state, PosIdx pos, Value * * args, Value & v) {
-            v.mkInt(state.positions[PosIdx(args[0]->integer())].column);
+            v.mkInt(state.positions[PosIdx(args[0]->integer().value)].column);
         }
     };
 
@@ -3136,7 +3167,7 @@ static void prim_zipAttrsWith(EvalState & state, const PosIdx pos, Value * * arg
         std::optional<ListBuilder> list;
     };
 
-    std::map<Symbol, Item> attrsSeen;
+    std::map<Symbol, Item, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, Item>>> attrsSeen;
 
     state.forceFunction(*args[0], pos, "while evaluating the first argument passed to builtins.zipAttrsWith");
     state.forceList(*args[1], pos, "while evaluating the second argument passed to builtins.zipAttrsWith");
@@ -3244,7 +3275,8 @@ static void elemAt(EvalState & state, const PosIdx pos, Value & list, int n, Val
 /* Return the n-1'th element of a list. */
 static void prim_elemAt(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    elemAt(state, pos, *args[0], state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.elemAt"), v);
+    NixInt::Inner elem = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.elemAt").value;
+    elemAt(state, pos, *args[0], elem, v);
 }
 
 static RegisterPrimOp primop_elemAt({
@@ -3538,10 +3570,12 @@ static RegisterPrimOp primop_all({
 
 static void prim_genList(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    auto len = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.genList");
+    auto len_ = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.genList").value;
 
-    if (len < 0)
-        state.error<EvalError>("cannot create list of size %1%", len).atPos(pos).debugThrow();
+    if (len_ < 0)
+        state.error<EvalError>("cannot create list of size %1%", len_).atPos(pos).debugThrow();
+
+    size_t len = size_t(len_);
 
     // More strict than striclty (!) necessary, but acceptable
     // as evaluating map without accessing any values makes little sense.
@@ -3798,9 +3832,17 @@ static void prim_add(EvalState & state, const PosIdx pos, Value * * args, Value
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
         v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first argument of the addition")
                 + state.forceFloat(*args[1], pos, "while evaluating the second argument of the addition"));
-    else
-        v.mkInt(  state.forceInt(*args[0], pos, "while evaluating the first argument of the addition")
-                + state.forceInt(*args[1], pos, "while evaluating the second argument of the addition"));
+    else {
+        auto i1 = state.forceInt(*args[0], pos, "while evaluating the first argument of the addition");
+        auto i2 = state.forceInt(*args[1], pos, "while evaluating the second argument of the addition");
+
+        auto result_ = i1 + i2;
+        if (auto result = result_.valueChecked(); result.has_value()) {
+            v.mkInt(*result);
+        } else {
+            state.error<EvalError>("integer overflow in adding %1% + %2%", i1, i2).atPos(pos).debugThrow();
+        }
+    }
 }
 
 static RegisterPrimOp primop_add({
@@ -3819,9 +3861,18 @@ static void prim_sub(EvalState & state, const PosIdx pos, Value * * args, Value
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
         v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first argument of the subtraction")
                 - state.forceFloat(*args[1], pos, "while evaluating the second argument of the subtraction"));
-    else
-        v.mkInt(  state.forceInt(*args[0], pos, "while evaluating the first argument of the subtraction")
-                - state.forceInt(*args[1], pos, "while evaluating the second argument of the subtraction"));
+    else {
+        auto i1 = state.forceInt(*args[0], pos, "while evaluating the first argument of the subtraction");
+        auto i2 = state.forceInt(*args[1], pos, "while evaluating the second argument of the subtraction");
+
+        auto result_ = i1 - i2;
+
+        if (auto result = result_.valueChecked(); result.has_value()) {
+            v.mkInt(*result);
+        } else {
+            state.error<EvalError>("integer overflow in subtracting %1% - %2%", i1, i2).atPos(pos).debugThrow();
+        }
+    }
 }
 
 static RegisterPrimOp primop_sub({
@@ -3840,9 +3891,18 @@ static void prim_mul(EvalState & state, const PosIdx pos, Value * * args, Value
     if (args[0]->type() == nFloat || args[1]->type() == nFloat)
         v.mkFloat(state.forceFloat(*args[0], pos, "while evaluating the first of the multiplication")
                 * state.forceFloat(*args[1], pos, "while evaluating the second argument of the multiplication"));
-    else
-        v.mkInt(  state.forceInt(*args[0], pos, "while evaluating the first argument of the multiplication")
-                * state.forceInt(*args[1], pos, "while evaluating the second argument of the multiplication"));
+    else {
+        auto i1 = state.forceInt(*args[0], pos, "while evaluating the first argument of the multiplication");
+        auto i2 = state.forceInt(*args[1], pos, "while evaluating the second argument of the multiplication");
+
+        auto result_ = i1 * i2;
+
+        if (auto result = result_.valueChecked(); result.has_value()) {
+            v.mkInt(*result);
+        } else {
+            state.error<EvalError>("integer overflow in multiplying %1% * %2%", i1, i2).atPos(pos).debugThrow();
+        }
+    }
 }
 
 static RegisterPrimOp primop_mul({
@@ -3869,10 +3929,12 @@ static void prim_div(EvalState & state, const PosIdx pos, Value * * args, Value
         NixInt i1 = state.forceInt(*args[0], pos, "while evaluating the first operand of the division");
         NixInt i2 = state.forceInt(*args[1], pos, "while evaluating the second operand of the division");
         /* Avoid division overflow as it might raise SIGFPE. */
-        if (i1 == std::numeric_limits<NixInt>::min() && i2 == -1)
-            state.error<EvalError>("overflow in integer division").atPos(pos).debugThrow();
-
-        v.mkInt(i1 / i2);
+        auto result_ = i1 / i2;
+        if (auto result = result_.valueChecked(); result.has_value()) {
+            v.mkInt(*result);
+        } else {
+            state.error<EvalError>("integer overflow in dividing %1% / %2%", i1, i2).atPos(pos).debugThrow();
+        }
     }
 }
 
@@ -3887,8 +3949,9 @@ static RegisterPrimOp primop_div({
 
 static void prim_bitAnd(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitAnd")
-            & state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitAnd"));
+    auto i1 = state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitAnd");
+    auto i2 = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitAnd");
+    v.mkInt(i1.value & i2.value);
 }
 
 static RegisterPrimOp primop_bitAnd({
@@ -3902,8 +3965,10 @@ static RegisterPrimOp primop_bitAnd({
 
 static void prim_bitOr(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitOr")
-            | state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitOr"));
+    auto i1 = state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitOr");
+    auto i2 = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitOr");
+
+    v.mkInt(i1.value | i2.value);
 }
 
 static RegisterPrimOp primop_bitOr({
@@ -3917,8 +3982,10 @@ static RegisterPrimOp primop_bitOr({
 
 static void prim_bitXor(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    v.mkInt(state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitXor")
-            ^ state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitXor"));
+    auto i1 = state.forceInt(*args[0], pos, "while evaluating the first argument passed to builtins.bitXor");
+    auto i2 = state.forceInt(*args[1], pos, "while evaluating the second argument passed to builtins.bitXor");
+
+    v.mkInt(i1.value ^ i2.value);
 }
 
 static RegisterPrimOp primop_bitXor({
@@ -3998,13 +4065,19 @@ static RegisterPrimOp primop_toString({
    non-negative. */
 static void prim_substring(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
-    int start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring");
+    NixInt::Inner start = state.forceInt(*args[0], pos, "while evaluating the first argument (the start offset) passed to builtins.substring").value;
 
     if (start < 0)
         state.error<EvalError>("negative start position in 'substring'").atPos(pos).debugThrow();
 
 
-    int len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring");
+    NixInt::Inner len = state.forceInt(*args[1], pos, "while evaluating the second argument (the substring length) passed to builtins.substring").value;
+
+    // Negative length may be idiomatically passed to builtins.substring to get
+    // the tail of the string.
+    if (len < 0) {
+        len = std::numeric_limits<NixInt::Inner>::max();
+    }
 
     // Special-case on empty substring to avoid O(n) strlen
     // This allows for the use of empty substrings to efficently capture string context
@@ -4047,7 +4120,7 @@ static void prim_stringLength(EvalState & state, const PosIdx pos, Value * * arg
 {
     NixStringContext context;
     auto s = state.coerceToString(pos, *args[0], context, "while evaluating the argument passed to builtins.stringLength");
-    v.mkInt(s->size());
+    v.mkInt(NixInt::Inner(s->size()));
 }
 
 static RegisterPrimOp primop_stringLength({
@@ -4230,7 +4303,7 @@ void prim_match(EvalState & state, const PosIdx pos, Value * * args, Value & v)
             if (!match[i + 1].matched)
                 v2 = &state.vNull;
             else
-                (v2 = state.allocValue())->mkString(match[i + 1].str());
+                v2 = mkString(state, match[i + 1]);
         v.mkList(list);
 
     } catch (std::regex_error & e) {
@@ -4314,7 +4387,7 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
             auto match = *i;
 
             // Add a string for non-matched characters.
-            (list[idx++] = state.allocValue())->mkString(match.prefix().str());
+            list[idx++] = mkString(state, match.prefix());
 
             // Add a list for matched substrings.
             const size_t slen = match.size() - 1;
@@ -4325,14 +4398,14 @@ void prim_split(EvalState & state, const PosIdx pos, Value * * args, Value & v)
                 if (!match[si + 1].matched)
                     v2 = &state.vNull;
                 else
-                    (v2 = state.allocValue())->mkString(match[si + 1].str());
+                    v2 = mkString(state, match[si + 1]);
             }
 
             (list[idx++] = state.allocValue())->mkList(list2);
 
             // Add a string for non-matched suffix characters.
             if (idx == 2 * len)
-                (list[idx++] = state.allocValue())->mkString(match.suffix().str());
+                list[idx++] = mkString(state, match.suffix());
         }
 
         assert(idx == 2 * len + 1);
@@ -4531,7 +4604,8 @@ static void prim_compareVersions(EvalState & state, const PosIdx pos, Value * *
 {
     auto version1 = state.forceStringNoCtx(*args[0], pos, "while evaluating the first argument passed to builtins.compareVersions");
     auto version2 = state.forceStringNoCtx(*args[1], pos, "while evaluating the second argument passed to builtins.compareVersions");
-    v.mkInt(compareVersions(version1, version2));
+    auto result = compareVersions(version1, version2);
+    v.mkInt(result < 0 ? -1 : result > 0 ? 1 : 0);
 }
 
 static RegisterPrimOp primop_compareVersions({
@@ -4814,7 +4888,10 @@ void EvalState::createBaseEnv()
     addConstant("__nixPath", v, {
         .type = nList,
         .doc = R"(
-          The value of the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path): a list of search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
+          A list of search path entries used to resolve [lookup paths](@docroot@/language/constructs/lookup-path.md).
+          Its value is primarily determined by the [`nix-path` configuration setting](@docroot@/command-ref/conf-file.md#conf-nix-path), which are
+          - Overridden by the [`NIX_PATH`](@docroot@/command-ref/env-common.md#env-NIX_PATH) environment variable or the `--nix-path` option
+          - Extended by the [`-I` option](@docroot@/command-ref/opt-common.md#opt-I) or `--extra-nix-path`
 
           > **Example**
           >

src/libexpr/primops/context.cc

@@ -86,7 +86,7 @@ static RegisterPrimOp primop_unsafeDiscardOutputDependency({
 
       This is the opposite of [`builtins.addDrvOutputDependencies`](#builtins-addDrvOutputDependencies).
 
-      This is unsafe because it allows us to "forget" store objects we would have otherwise refered to with the string context,
+      This is unsafe because it allows us to "forget" store objects we would have otherwise referred to with the string context,
       whereas Nix normally tracks all dependencies consistently.
       Safe operations "grow" but never "shrink" string contexts.
       [`builtins.addDrvOutputDependencies`] in contrast is safe because "derivation deep" string context element always refers to the underlying derivation (among many more things).

src/libexpr/primops/fetchTree.cc

@@ -11,6 +11,8 @@
 #include "value-to-json.hh"
 #include "fetch-to-store.hh"
 
+#include <nlohmann/json.hpp>
+
 #include <ctime>
 #include <iomanip>
 #include <regex>
@@ -122,9 +124,15 @@ static void fetchTree(
             }
             else if (attr.value->type() == nBool)
                 attrs.emplace(state.symbols[attr.name], Explicit<bool>{attr.value->boolean()});
-            else if (attr.value->type() == nInt)
-                attrs.emplace(state.symbols[attr.name], uint64_t(attr.value->integer()));
-            else if (state.symbols[attr.name] == "publicKeys") {
+            else if (attr.value->type() == nInt) {
+                auto intValue = attr.value->integer().value;
+
+                if (intValue < 0) {
+                    state.error<EvalError>("negative value given for fetchTree attr %1%: %2%", state.symbols[attr.name], intValue).atPos(pos).debugThrow();
+                }
+
+                attrs.emplace(state.symbols[attr.name], uint64_t(intValue));
+            } else if (state.symbols[attr.name] == "publicKeys") {
                 experimentalFeatureSettings.require(Xp::VerifiedFetches);
                 attrs.emplace(state.symbols[attr.name], printValueAsJSON(state, true, *attr.value, pos, context).dump());
             }
@@ -240,7 +248,7 @@ static RegisterPrimOp primop_fetchTree({
       The following source types and associated input attributes are supported.
 
       <!-- TODO: It would be soooo much more predictable to work with (and
-      document) if `fetchTree` was a curried call with the first paramter for
+      document) if `fetchTree` was a curried call with the first parameter for
       `type` or an attribute like `builtins.fetchTree.git`! -->
 
       - `"file"`
@@ -501,7 +509,11 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     //       https://github.com/NixOS/nix/issues/4313
     auto storePath =
         unpack
-        ? fetchToStore(*state.store, fetchers::downloadTarball(*url).accessor, FetchMode::Copy, name)
+        ? fetchToStore(
+            *state.store,
+            fetchers::downloadTarball(state.store, state.fetchSettings, *url),
+            FetchMode::Copy,
+            name)
         : fetchers::downloadFile(state.store, *url, name).storePath;
 
     if (expectedHash) {

src/libexpr/primops/fromTOML.cc

@@ -66,7 +66,7 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
                         attrs.alloc("_type").mkString("timestamp");
                         std::ostringstream s;
                         s << t;
-                        attrs.alloc("value").mkString(s.str());
+                        attrs.alloc("value").mkString(toView(s));
                         v.mkAttrs(attrs);
                     } else {
                         throw std::runtime_error("Dates and times are not supported");

src/libexpr/print.cc

@@ -460,7 +460,7 @@ private:
 
                 std::ostringstream s;
                 s << state.positions[v.payload.lambda.fun->pos];
-                output << " @ " << filterANSIEscapes(s.str());
+                output << " @ " << filterANSIEscapes(toView(s));
             }
         } else if (v.isPrimOp()) {
             if (v.primOp())

src/libexpr/value-to-json.cc

@@ -22,7 +22,7 @@ json printValueAsJSON(EvalState & state, bool strict,
     switch (v.type()) {
 
         case nInt:
-            out = v.integer();
+            out = v.integer().value;
             break;
 
         case nBool:

src/libexpr/value.hh

@@ -4,14 +4,13 @@
 #include <cassert>
 #include <span>
 
+#include "eval-gc.hh"
 #include "symbol-table.hh"
 #include "value/context.hh"
 #include "source-path.hh"
 #include "print-options.hh"
+#include "checked-arithmetic.hh"
 
-#if HAVE_BOEHMGC
-#include <gc/gc_allocator.h>
-#endif
 #include <nlohmann/json_fwd.hpp>
 
 namespace nix {
@@ -73,8 +72,8 @@ class EvalState;
 class XMLWriter;
 class Printer;
 
-typedef int64_t NixInt;
-typedef double NixFloat;
+using NixInt = checked::Checked<int64_t>;
+using NixFloat = double;
 
 /**
  * External values must descend from ExternalValueBase, so that
@@ -304,6 +303,11 @@ public:
         return internalType != tUninitialized;
     }
 
+    inline void mkInt(NixInt::Inner n)
+    {
+        mkInt(NixInt{n});
+    }
+
     inline void mkInt(NixInt n)
     {
         finishValue(tInt, { .integer = n });
@@ -325,9 +329,9 @@ public:
 
     void mkStringMove(const char * s, const NixStringContext & context);
 
-    inline void mkString(const Symbol & s)
+    inline void mkString(const SymbolStr & s)
     {
-        mkString(((const std::string &) s).c_str());
+        mkString(s.c_str());
     }
 
     void mkPath(const SourcePath & path);
@@ -492,15 +496,9 @@ void Value::mkBlackhole()
 }
 
 
-#if HAVE_BOEHMGC
 typedef std::vector<Value *, traceable_allocator<Value *>> ValueVector;
 typedef std::unordered_map<Symbol, Value *, std::hash<Symbol>, std::equal_to<Symbol>, traceable_allocator<std::pair<const Symbol, Value *>>> ValueMap;
 typedef std::map<Symbol, ValueVector, std::less<Symbol>, traceable_allocator<std::pair<const Symbol, ValueVector>>> ValueVectorMap;
-#else
-typedef std::vector<Value *> ValueVector;
-typedef std::unordered_map<Symbol, Value *> ValueMap;
-typedef std::map<Symbol, ValueVector> ValueVectorMap;
-#endif
 
 
 /**

src/libfetchers-tests/.version

@@ -0,0 +1 @@
+../../.version

src/libfetchers-tests/build-utils-meson

@@ -0,0 +1 @@
+../../build-utils-meson

src/libfetchers-tests/data/public-key/defaultType.json

@@ -0,0 +1,4 @@
+{
+  "key": "ABCDE",
+  "type": "ssh-ed25519"
+}

src/libfetchers-tests/data/public-key/noRoundTrip.json

@@ -0,0 +1,3 @@
+{
+  "key": "ABCDE"
+}

src/libfetchers-tests/data/public-key/simple.json

@@ -0,0 +1,4 @@
+{
+  "key": "ABCDE",
+  "type": "ssh-rsa"
+}

src/libfetchers-tests/git-utils.cc

@@ -0,0 +1,112 @@
+#include "git-utils.hh"
+#include "file-system.hh"
+#include "gmock/gmock.h"
+#include <git2/global.h>
+#include <git2/repository.h>
+#include <git2/types.h>
+#include <gtest/gtest.h>
+#include "fs-sink.hh"
+#include "serialise.hh"
+
+namespace nix {
+
+class GitUtilsTest : public ::testing::Test
+{
+    // We use a single repository for all tests.
+    Path tmpDir;
+    std::unique_ptr<AutoDelete> delTmpDir;
+
+public:
+    void SetUp() override
+    {
+        tmpDir = createTempDir();
+        delTmpDir = std::make_unique<AutoDelete>(tmpDir, true);
+
+        // Create the repo with libgit2
+        git_libgit2_init();
+        git_repository * repo = nullptr;
+        auto r = git_repository_init(&repo, tmpDir.c_str(), 0);
+        ASSERT_EQ(r, 0);
+        git_repository_free(repo);
+    }
+
+    void TearDown() override
+    {
+        // Destroy the AutoDelete, triggering removal
+        // not AutoDelete::reset(), which would cancel the deletion.
+        delTmpDir.reset();
+    }
+
+    ref<GitRepo> openRepo()
+    {
+        return GitRepo::openRepo(tmpDir, true, false);
+    }
+};
+
+void writeString(CreateRegularFileSink & fileSink, std::string contents, bool executable)
+{
+    if (executable)
+        fileSink.isExecutable();
+    fileSink.preallocateContents(contents.size());
+    fileSink(contents);
+}
+
+TEST_F(GitUtilsTest, sink_basic)
+{
+    auto repo = openRepo();
+    auto sink = repo->getFileSystemObjectSink();
+
+    // TODO/Question: It seems a little odd that we use the tarball-like convention of requiring a top-level directory
+    // here
+    //                The sync method does not document this behavior, should probably renamed because it's not very
+    //                general, and I can't imagine that "non-conventional" archives or any other source to be handled by
+    //                this sink.
+
+    sink->createDirectory(CanonPath("foo-1.1"));
+
+    sink->createRegularFile(CanonPath("foo-1.1/hello"), [](CreateRegularFileSink & fileSink) {
+        writeString(fileSink, "hello world", false);
+    });
+    sink->createRegularFile(CanonPath("foo-1.1/bye"), [](CreateRegularFileSink & fileSink) {
+        writeString(fileSink, "thanks for all the fish", false);
+    });
+    sink->createSymlink(CanonPath("foo-1.1/bye-link"), "bye");
+    sink->createDirectory(CanonPath("foo-1.1/empty"));
+    sink->createDirectory(CanonPath("foo-1.1/links"));
+    sink->createHardlink(CanonPath("foo-1.1/links/foo"), CanonPath("foo-1.1/hello"));
+
+    // sink->createHardlink("foo-1.1/links/foo-2", CanonPath("foo-1.1/hello"));
+
+    auto result = repo->dereferenceSingletonDirectory(sink->sync());
+    auto accessor = repo->getAccessor(result, false, false);
+    auto entries = accessor->readDirectory(CanonPath::root);
+    ASSERT_EQ(entries.size(), 5);
+    ASSERT_EQ(accessor->readFile(CanonPath("hello")), "hello world");
+    ASSERT_EQ(accessor->readFile(CanonPath("bye")), "thanks for all the fish");
+    ASSERT_EQ(accessor->readLink(CanonPath("bye-link")), "bye");
+    ASSERT_EQ(accessor->readDirectory(CanonPath("empty")).size(), 0);
+    ASSERT_EQ(accessor->readFile(CanonPath("links/foo")), "hello world");
+};
+
+TEST_F(GitUtilsTest, sink_hardlink)
+{
+    auto repo = openRepo();
+    auto sink = repo->getFileSystemObjectSink();
+
+    sink->createDirectory(CanonPath("foo-1.1"));
+
+    sink->createRegularFile(CanonPath("foo-1.1/hello"), [](CreateRegularFileSink & fileSink) {
+        writeString(fileSink, "hello world", false);
+    });
+
+    try {
+        sink->createHardlink(CanonPath("foo-1.1/link"), CanonPath("hello"));
+        FAIL() << "Expected an exception";
+    } catch (const nix::Error & e) {
+        ASSERT_THAT(e.msg(), testing::HasSubstr("cannot find hard link target"));
+        ASSERT_THAT(e.msg(), testing::HasSubstr("/hello"));
+        ASSERT_THAT(e.msg(), testing::HasSubstr("foo-1.1/link"));
+    }
+};
+
+} // namespace nix

src/libfetchers-tests/local.mk

@@ -0,0 +1,37 @@
+check: libfetchers-tests_RUN
+
+programs += libfetchers-tests
+
+libfetchers-tests_NAME = libnixfetchers-tests
+
+libfetchers-tests_ENV := _NIX_TEST_UNIT_DATA=$(d)/data GTEST_OUTPUT=xml:$$testresults/libfetchers-tests.xml
+
+libfetchers-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libfetchers-tests_INSTALL_DIR := $(checkbindir)
+else
+  libfetchers-tests_INSTALL_DIR :=
+endif
+
+libfetchers-tests_SOURCES := $(wildcard $(d)/*.cc)
+
+libfetchers-tests_EXTRA_INCLUDES = \
+    -I src/libstore-test-support \
+    -I src/libutil-test-support \
+    $(INCLUDE_libfetchers) \
+    $(INCLUDE_libstore) \
+    $(INCLUDE_libutil)
+
+libfetchers-tests_CXXFLAGS += $(libfetchers-tests_EXTRA_INCLUDES)
+
+libfetchers-tests_LIBS = \
+    libstore-test-support libutil-test-support \
+    libfetchers libstore libutil
+
+libfetchers-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS) $(LIBGIT2_LIBS)
+
+ifdef HOST_WINDOWS
+  # Increase the default reserved stack size to 65 MB so Nix doesn't run out of space
+  libfetchers-tests_LDFLAGS += -Wl,--stack,$(shell echo $$((65 * 1024 * 1024)))
+endif

src/libfetchers-tests/meson.build

@@ -0,0 +1,74 @@
+project('nix-fetchers-tests', 'cpp',
+  version : files('.version'),
+  default_options : [
+    'cpp_std=c++2a',
+    # TODO(Qyriad): increase the warning level
+    'warning_level=1',
+    'debug=true',
+    'optimization=2',
+    'errorlogs=true', # Please print logs for tests that fail
+  ],
+  meson_version : '>= 1.1',
+  license : 'LGPL-2.1-or-later',
+)
+
+cxx = meson.get_compiler('cpp')
+
+subdir('build-utils-meson/deps-lists')
+
+deps_private_maybe_subproject = [
+  dependency('nix-store-test-support'),
+  dependency('nix-fetchers'),
+]
+deps_public_maybe_subproject = [
+]
+subdir('build-utils-meson/subprojects')
+
+subdir('build-utils-meson/threads')
+
+subdir('build-utils-meson/export-all-symbols')
+subdir('build-utils-meson/windows-version')
+
+rapidcheck = dependency('rapidcheck')
+deps_private += rapidcheck
+
+gtest = dependency('gtest', main : true)
+deps_private += gtest
+
+add_project_arguments(
+  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  # It would be nice for our headers to be idempotent instead.
+  '-include', 'config-util.hh',
+  '-include', 'config-store.hh',
+  # '-include', 'config-fetchers.h',
+  language : 'cpp',
+)
+
+subdir('build-utils-meson/diagnostics')
+
+sources = files(
+  'public-key.cc',
+)
+
+include_dirs = [include_directories('.')]
+
+
+this_exe = executable(
+  meson.project_name(),
+  sources,
+  dependencies : deps_private_subproject + deps_private + deps_other,
+  include_directories : include_dirs,
+  # TODO: -lrapidcheck, see ../libutil-support/build.meson
+  link_args: linker_export_flags + ['-lrapidcheck'],
+  # get main from gtest
+  install : true,
+)
+
+test(
+  meson.project_name(),
+  this_exe,
+  env : {
+    '_NIX_TEST_UNIT_DATA': meson.current_source_dir() / 'data',
+  },
+  protocol : 'gtest',
+)

src/libfetchers-tests/package.nix

@@ -0,0 +1,81 @@
+{ lib
+, buildPackages
+, stdenv
+, mkMesonExecutable
+
+, nix-fetchers
+, nix-store-test-support
+
+, rapidcheck
+, gtest
+, runCommand
+
+# Configuration Options
+
+, version
+, resolvePath
+}:
+
+let
+  inherit (lib) fileset;
+in
+
+mkMesonExecutable (finalAttrs: {
+  pname = "nix-fetchers-tests";
+  inherit version;
+
+  workDir = ./.;
+  fileset = fileset.unions [
+    ../../build-utils-meson
+    ./build-utils-meson
+    ../../.version
+    ./.version
+    ./meson.build
+    # ./meson.options
+    (fileset.fileFilter (file: file.hasExt "cc") ./.)
+    (fileset.fileFilter (file: file.hasExt "hh") ./.)
+  ];
+
+  buildInputs = [
+    nix-fetchers
+    nix-store-test-support
+    rapidcheck
+    gtest
+  ];
+
+  preConfigure =
+    # "Inline" .version so it's not a symlink, and includes the suffix.
+    # Do the meson utils, without modification.
+    ''
+      chmod u+w ./.version
+      echo ${version} > ../../.version
+    '';
+
+  mesonFlags = [
+  ];
+
+  env = lib.optionalAttrs (stdenv.isLinux && !(stdenv.hostPlatform.isStatic && stdenv.system == "aarch64-linux")) {
+    LDFLAGS = "-fuse-ld=gold";
+  };
+
+  passthru = {
+    tests = {
+      run = runCommand "${finalAttrs.pname}-run" {
+        meta.broken = !stdenv.hostPlatform.emulatorAvailable buildPackages;
+      } (lib.optionalString stdenv.hostPlatform.isWindows ''
+        export HOME="$PWD/home-dir"
+        mkdir -p "$HOME"
+      '' + ''
+        export _NIX_TEST_UNIT_DATA=${resolvePath ./data}
+        ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
+        touch $out
+      '');
+    };
+  };
+
+  meta = {
+    platforms = lib.platforms.unix ++ lib.platforms.windows;
+    mainProgram = finalAttrs.pname + stdenv.hostPlatform.extensions.executable;
+  };
+
+})

src/libfetchers-tests/public-key.cc

@@ -0,0 +1,54 @@
+#include <gtest/gtest.h>
+#include "fetchers.hh"
+#include "json-utils.hh"
+#include <nlohmann/json.hpp>
+#include "tests/characterization.hh"
+
+namespace nix {
+
+using nlohmann::json;
+
+class PublicKeyTest : public CharacterizationTest
+{
+    std::filesystem::path unitTestData = getUnitTestData() / "public-key";
+
+public:
+    std::filesystem::path goldenMaster(std::string_view testStem) const override {
+        return unitTestData / testStem;
+    }
+};
+
+#define TEST_JSON(FIXTURE, NAME, VAL)                                     \
+    TEST_F(FIXTURE, PublicKey_ ## NAME ## _from_json) {                   \
+        readTest(#NAME ".json", [&](const auto & encoded_) {              \
+            fetchers::PublicKey expected { VAL };                         \
+            fetchers::PublicKey got = nlohmann::json::parse(encoded_);    \
+            ASSERT_EQ(got, expected);                                     \
+        });                                                               \
+    }                                                                     \
+                                                                          \
+    TEST_F(FIXTURE, PublicKey_ ## NAME ## _to_json) {                     \
+        writeTest(#NAME ".json", [&]() -> json {                          \
+            return nlohmann::json(fetchers::PublicKey { VAL });           \
+        }, [](const auto & file) {                                        \
+            return json::parse(readFile(file));                           \
+        }, [](const auto & file, const auto & got) {                      \
+            return writeFile(file, got.dump(2) + "\n");                   \
+        });                                                               \
+    }
+
+TEST_JSON(PublicKeyTest, simple, (fetchers::PublicKey { .type = "ssh-rsa", .key = "ABCDE" }))
+
+TEST_JSON(PublicKeyTest, defaultType, fetchers::PublicKey { .key = "ABCDE" })
+
+#undef TEST_JSON
+
+TEST_F(PublicKeyTest, PublicKey_noRoundTrip_from_json) {
+    readTest("noRoundTrip.json", [&](const auto & encoded_) {
+        fetchers::PublicKey expected = { .type = "ssh-ed25519", .key = "ABCDE" };
+        fetchers::PublicKey got = nlohmann::json::parse(encoded_);
+        ASSERT_EQ(got, expected);
+    });
+}
+
+}

src/libfetchers/cache.cc

@@ -36,7 +36,7 @@ struct CacheImpl : Cache
     {
         auto state(_state.lock());
 
-        auto dbPath = getCacheDir() + "/nix/fetcher-cache-v2.sqlite";
+        auto dbPath = getCacheDir() + "/fetcher-cache-v2.sqlite";
         createDirs(dirOf(dbPath));
 
         state->db = SQLite(dbPath);

src/libfetchers/fetchers.cc

@@ -170,24 +170,6 @@ std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
     if (!scheme)
         throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
-    /* The tree may already be in the Nix store, or it could be
-       substituted (which is often faster than fetching from the
-       original source). So check that. */
-    if (getNarHash()) {
-        try {
-            auto storePath = computeStorePath(*store);
-
-            store->ensurePath(storePath);
-
-            debug("using substituted/cached input '%s' in '%s'",
-                to_string(), store->printStorePath(storePath));
-
-            return {std::move(storePath), *this};
-        } catch (Error & e) {
-            debug("substitution of input '%s' failed: %s", to_string(), e.what());
-        }
-    }
-
     auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
         try {
             auto [accessor, final] = getAccessorUnchecked(store);

src/libfetchers/git-utils.cc

@@ -14,13 +14,17 @@
 #include <git2/describe.h>
 #include <git2/errors.h>
 #include <git2/global.h>
+#include <git2/indexer.h>
 #include <git2/object.h>
+#include <git2/odb.h>
 #include <git2/refs.h>
 #include <git2/remote.h>
 #include <git2/repository.h>
 #include <git2/revparse.h>
 #include <git2/status.h>
 #include <git2/submodule.h>
+#include <git2/sys/odb_backend.h>
+#include <git2/sys/mempack.h>
 #include <git2/tree.h>
 
 #include <iostream>
@@ -77,6 +81,9 @@ typedef std::unique_ptr<git_status_list, Deleter<git_status_list_free>> StatusLi
 typedef std::unique_ptr<git_remote, Deleter<git_remote_free>> Remote;
 typedef std::unique_ptr<git_config, Deleter<git_config_free>> GitConfig;
 typedef std::unique_ptr<git_config_iterator, Deleter<git_config_iterator_free>> ConfigIterator;
+typedef std::unique_ptr<git_odb, Deleter<git_odb_free>> ObjectDb;
+typedef std::unique_ptr<git_packbuilder, Deleter<git_packbuilder_free>> PackBuilder;
+typedef std::unique_ptr<git_indexer, Deleter<git_indexer_free>> Indexer;
 
 // A helper to ensure that we don't leak objects returned by libgit2.
 template<typename T>
@@ -160,24 +167,101 @@ static Object peelToTreeOrBlob(git_object * obj)
         return peelObject<Object>(obj, GIT_OBJECT_TREE);
 }
 
+struct PackBuilderContext {
+    std::exception_ptr exception;
+
+    void handleException(const char * activity, int errCode)
+    {
+        switch (errCode) {
+            case GIT_OK:
+                break;
+            case GIT_EUSER:
+                if (!exception)
+                    panic("PackBuilderContext::handleException: user error, but exception was not set");
+
+                std::rethrow_exception(exception);
+            default:
+                throw Error("%s: %i, %s", Uncolored(activity), errCode, git_error_last()->message);
+        }
+    }
+};
+
+extern "C" {
+
+/**
+ * A `git_packbuilder_progress` implementation that aborts the pack building if needed.
+ */
+static int packBuilderProgressCheckInterrupt(int stage, uint32_t current, uint32_t total, void *payload)
+{
+    PackBuilderContext & args = * (PackBuilderContext *) payload;
+    try {
+        checkInterrupt();
+        return GIT_OK;
+    } catch (const std::exception & e) {
+        args.exception = std::current_exception();
+        return GIT_EUSER;
+    }
+};
+static git_packbuilder_progress PACKBUILDER_PROGRESS_CHECK_INTERRUPT = &packBuilderProgressCheckInterrupt;
+
+} // extern "C"
+
+static void initRepoAtomically(std::filesystem::path &path, bool bare) {
+    if (pathExists(path.string())) return;
+
+    Path tmpDir = createTempDir(os_string_to_string(PathViewNG { std::filesystem::path(path).parent_path() }));
+    AutoDelete delTmpDir(tmpDir, true);
+    Repository tmpRepo;
+
+    if (git_repository_init(Setter(tmpRepo), tmpDir.c_str(), bare))
+        throw Error("creating Git repository %s: %s", path, git_error_last()->message);
+    try {
+        std::filesystem::rename(tmpDir, path);
+    } catch (std::filesystem::filesystem_error & e) {
+        if (e.code() == std::errc::file_exists) // Someone might race us to create the repository.
+            return;
+        else
+            throw SysError("moving temporary git repository from %s to %s", tmpDir, path);
+    }
+    // we successfully moved the repository, so the temporary directory no longer exists.
+    delTmpDir.cancel();
+}
+
 struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 {
     /** Location of the repository on disk. */
     std::filesystem::path path;
+    /**
+     * libgit2 repository. Note that new objects are not written to disk,
+     * because we are using a mempack backend. For writing to disk, see
+     * `flush()`, which is also called by `GitFileSystemObjectSink::sync()`.
+     */
     Repository repo;
+    /**
+     * In-memory object store for efficient batched writing to packfiles.
+     * Owned by `repo`.
+     */
+    git_odb_backend * mempack_backend;
 
     GitRepoImpl(std::filesystem::path _path, bool create, bool bare)
         : path(std::move(_path))
     {
         initLibGit2();
 
-        if (pathExists(path.string())) {
-            if (git_repository_open(Setter(repo), path.string().c_str()))
-                throw Error("opening Git repository '%s': %s", path, git_error_last()->message);
-        } else {
-            if (git_repository_init(Setter(repo), path.string().c_str(), bare))
-                throw Error("creating Git repository '%s': %s", path, git_error_last()->message);
-        }
+        initRepoAtomically(path, bare);
+        if (git_repository_open(Setter(repo), path.string().c_str()))
+            throw Error("opening Git repository %s: %s", path, git_error_last()->message);
+
+        ObjectDb odb;
+        if (git_repository_odb(Setter(odb), repo.get()))
+            throw Error("getting Git object database: %s", git_error_last()->message);
+
+        // mempack_backend will be owned by the repository, so we are not expected to free it ourselves.
+        if (git_mempack_new(&mempack_backend))
+            throw Error("creating mempack backend: %s", git_error_last()->message);
+
+        if (git_odb_add_backend(odb.get(), mempack_backend, 999))
+            throw Error("adding mempack backend to Git object database: %s", git_error_last()->message);
     }
 
     operator git_repository * ()
@@ -185,6 +269,62 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         return repo.get();
     }
 
+    void flush() override {
+        checkInterrupt();
+
+        git_buf buf = GIT_BUF_INIT;
+        Finally _disposeBuf { [&] { git_buf_dispose(&buf); } };
+        PackBuilder packBuilder;
+        PackBuilderContext packBuilderContext;
+        git_packbuilder_new(Setter(packBuilder), *this);
+        git_packbuilder_set_callbacks(packBuilder.get(), PACKBUILDER_PROGRESS_CHECK_INTERRUPT, &packBuilderContext);
+        git_packbuilder_set_threads(packBuilder.get(), 0 /* autodetect */);
+
+        packBuilderContext.handleException(
+            "preparing packfile",
+            git_mempack_write_thin_pack(mempack_backend, packBuilder.get())
+        );
+        checkInterrupt();
+        packBuilderContext.handleException(
+            "writing packfile",
+            git_packbuilder_write_buf(&buf, packBuilder.get())
+        );
+        checkInterrupt();
+
+        std::string repo_path = std::string(git_repository_path(repo.get()));
+        while (!repo_path.empty() && repo_path.back() == '/')
+            repo_path.pop_back();
+        std::string pack_dir_path = repo_path + "/objects/pack";
+
+        // TODO (performance): could the indexing be done in a separate thread?
+        //                     we'd need a more streaming variation of
+        //                     git_packbuilder_write_buf, or incur the cost of
+        //                     copying parts of the buffer to a separate thread.
+        //                     (synchronously on the git_packbuilder_write_buf thread)
+        Indexer indexer;
+        git_indexer_progress stats;
+        if (git_indexer_new(Setter(indexer), pack_dir_path.c_str(), 0, nullptr, nullptr))
+            throw Error("creating git packfile indexer: %s", git_error_last()->message);
+
+        // TODO: provide index callback for checkInterrupt() termination
+        //       though this is about an order of magnitude faster than the packbuilder
+        //       expect up to 1 sec latency due to uninterruptible git_indexer_append.
+        constexpr size_t chunkSize = 128 * 1024;
+        for (size_t offset = 0; offset < buf.size; offset += chunkSize) {
+            if (git_indexer_append(indexer.get(), buf.ptr + offset, std::min(chunkSize, buf.size - offset), &stats))
+                throw Error("appending to git packfile index: %s", git_error_last()->message);
+            checkInterrupt();
+        }
+
+        if (git_indexer_commit(indexer.get(), &stats))
+            throw Error("committing git packfile index: %s", git_error_last()->message);
+
+        if (git_mempack_reset(mempack_backend))
+            throw Error("resetting git mempack backend: %s", git_error_last()->message);
+
+        checkInterrupt();
+    }
+
     uint64_t getRevCount(const Hash & rev) override
     {
         std::unordered_set<git_oid> done;
@@ -461,7 +601,13 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
         std::string re = R"(Good "git" signature for \* with .* key SHA256:[)";
         for (const fetchers::PublicKey & k : publicKeys){
             // Calculate sha256 fingerprint from public key and escape the regex symbol '+' to match the key literally
-            auto fingerprint = trim(hashString(HashAlgorithm::SHA256, base64Decode(k.key)).to_string(nix::HashFormat::Base64, false), "=");
+            std::string keyDecoded;
+            try {
+                keyDecoded = base64Decode(k.key);
+            } catch (Error & e) {
+                e.addTrace({}, "while decoding public key '%s' used for git signature", k.key);
+            }
+            auto fingerprint = trim(hashString(HashAlgorithm::SHA256, keyDecoded).to_string(nix::HashFormat::Base64, false), "=");
             auto escaped_fingerprint = std::regex_replace(fingerprint, std::regex("\\+"), "\\+" );
             re += "(" + escaped_fingerprint + ")";
         }
@@ -641,12 +787,16 @@ struct GitSourceAccessor : SourceAccessor
         return readBlob(path, true);
     }
 
-    Hash getSubmoduleRev(const CanonPath & path)
+    /**
+     * If `path` exists and is a submodule, return its
+     * revision. Otherwise return nothing.
+     */
+    std::optional<Hash> getSubmoduleRev(const CanonPath & path)
     {
-        auto entry = need(path);
+        auto entry = lookup(path);
 
-        if (git_tree_entry_type(entry) != GIT_OBJECT_COMMIT)
-            throw Error("'%s' is not a submodule", showPath(path));
+        if (!entry || git_tree_entry_type(entry) != GIT_OBJECT_COMMIT)
+            return std::nullopt;
 
         return toHash(*git_tree_entry_id(entry));
     }
@@ -870,8 +1020,24 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
 
     void pushBuilder(std::string name)
     {
+        const git_tree_entry * entry;
+        Tree prevTree = nullptr;
+
+        if (!pendingDirs.empty() &&
+            (entry = git_treebuilder_get(pendingDirs.back().builder.get(), name.c_str())))
+        {
+            /* Clone a tree that we've already finished. This happens
+               if a tarball has directory entries that are not
+               contiguous. */
+            if (git_tree_entry_type(entry) != GIT_OBJECT_TREE)
+                throw Error("parent of '%s' is not a directory", name);
+
+            if (git_tree_entry_to_object((git_object * *) (git_tree * *) Setter(prevTree), *repo, entry))
+                throw Error("looking up parent of '%s': %s", name, git_error_last()->message);
+        }
+
         git_treebuilder * b;
-        if (git_treebuilder_new(&b, *repo, nullptr))
+        if (git_treebuilder_new(&b, *repo, prevTree.get()))
             throw Error("creating a tree builder: %s", git_error_last()->message);
         pendingDirs.push_back({ .name = std::move(name), .builder = TreeBuilder(b) });
     };
@@ -1045,12 +1211,14 @@ struct GitFileSystemObjectSinkImpl : GitFileSystemObjectSink
             git_tree_entry_filemode(entry));
     }
 
-    Hash sync() override
+    Hash flush() override
     {
         updateBuilders({});
 
         auto [oid, _name] = popBuilder();
 
+        repo->flush();
+
         return toHash(oid);
     }
 };
@@ -1123,8 +1291,10 @@ std::vector<std::tuple<GitRepoImpl::Submodule, Hash>> GitRepoImpl::getSubmodules
     auto rawAccessor = getRawAccessor(rev, false);
 
     for (auto & submodule : parseSubmodules(pathTemp)) {
-        auto rev = rawAccessor->getSubmoduleRev(submodule.path);
-        result.push_back({std::move(submodule), rev});
+        /* Filter out .gitmodules entries that don't exist or are not
+           submodules. */
+        if (auto rev = rawAccessor->getSubmoduleRev(submodule.path))
+            result.push_back({std::move(submodule), *rev});
     }
 
     return result;
@@ -1132,7 +1302,7 @@ std::vector<std::tuple<GitRepoImpl::Submodule, Hash>> GitRepoImpl::getSubmodules
 
 ref<GitRepo> getTarballCache()
 {
-    static auto repoDir = std::filesystem::path(getCacheDir()) / "nix" / "tarball-cache";
+    static auto repoDir = std::filesystem::path(getCacheDir()) / "tarball-cache";
 
     return GitRepo::openRepo(repoDir, true, true);
 }

src/libfetchers/git-utils.hh

@@ -7,12 +7,16 @@ namespace nix {
 
 namespace fetchers { struct PublicKey; }
 
+/**
+ * A sink that writes into a Git repository. Note that nothing may be written
+ * until `flush()` is called.
+ */
 struct GitFileSystemObjectSink : ExtendedFileSystemObjectSink
 {
     /**
      * Flush builder and return a final Git hash.
      */
-    virtual Hash sync() = 0;
+    virtual Hash flush() = 0;
 };
 
 struct GitRepo
@@ -80,6 +84,8 @@ struct GitRepo
 
     virtual ref<GitFileSystemObjectSink> getFileSystemObjectSink() = 0;
 
+    virtual void flush() = 0;
+
     virtual void fetch(
         const std::string & url,
         const std::string & refspec,

src/libfetchers/git.cc

@@ -13,8 +13,8 @@
 #include "git-utils.hh"
 #include "logging.hh"
 #include "finally.hh"
-
 #include "fetch-settings.hh"
+#include "json-utils.hh"
 
 #include <regex>
 #include <string.h>
@@ -44,7 +44,7 @@ bool isCacheFileWithinTtl(time_t now, const struct stat & st)
 Path getCachePath(std::string_view key, bool shallow)
 {
     return getCacheDir()
-    + "/nix/gitv3/"
+    + "/gitv3/"
     + hashString(HashAlgorithm::SHA256, key).to_string(HashFormat::Nix32, false)
     + (shallow ? "-shallow" : "");
 }
@@ -590,9 +590,10 @@ struct GitInputScheme : InputScheme
                 }
 
                 try {
-                    setWriteTime(localRefFile, now, now);
+                    if (!input.getRev())
+                        setWriteTime(localRefFile, now, now);
                 } catch (Error & e) {
-                    warn("could not update mtime for file '%s': %s", localRefFile, e.msg());
+                    warn("could not update mtime for file '%s': %s", localRefFile, e.info().msg);
                 }
                 if (!originalRef && !storeCachedHead(repoInfo.url, ref))
                     warn("could not update cached head '%s' for '%s'", ref, repoInfo.url);

src/libfetchers/github.cc

@@ -261,11 +261,12 @@ struct GitArchiveInputScheme : InputScheme
         auto tarballCache = getTarballCache();
         auto parseSink = tarballCache->getFileSystemObjectSink();
         auto lastModified = unpackTarfileToSink(archive, *parseSink);
+        auto tree = parseSink->flush();
 
         act.reset();
 
         TarballInfo tarballInfo {
-            .treeHash = tarballCache->dereferenceSingletonDirectory(parseSink->sync()),
+            .treeHash = tarballCache->dereferenceSingletonDirectory(tree),
             .lastModified = lastModified
         };
 

src/libfetchers/mercurial.cc

@@ -263,7 +263,7 @@ struct MercurialInputScheme : InputScheme
                 return makeResult(res->value, res->storePath);
         }
 
-        Path cacheDir = fmt("%s/nix/hg/%s", getCacheDir(), hashString(HashAlgorithm::SHA256, actualUrl).to_string(HashFormat::Nix32, false));
+        Path cacheDir = fmt("%s/hg/%s", getCacheDir(), hashString(HashAlgorithm::SHA256, actualUrl).to_string(HashFormat::Nix32, false));
 
         /* If this is a commit hash that we already have, we don't
            have to pull again. */

src/libfetchers/package.nix

@@ -1,17 +1,11 @@
 { lib
 , stdenv
-, mkMesonDerivation
-, releaseTools
-
-, meson
-, ninja
-, pkg-config
+, mkMesonLibrary
 
 , nix-util
 , nix-store
 , nlohmann_json
 , libgit2
-, man
 
 # Configuration Options
 
@@ -22,7 +16,7 @@ let
   inherit (lib) fileset;
 in
 
-mkMesonDerivation (finalAttrs: {
+mkMesonLibrary (finalAttrs: {
   pname = "nix-fetchers";
   inherit version;
 
@@ -37,14 +31,6 @@ mkMesonDerivation (finalAttrs: {
     (fileset.fileFilter (file: file.hasExt "hh") ./.)
   ];
 
-  outputs = [ "out" "dev" ];
-
-  nativeBuildInputs = [
-    meson
-    ninja
-    pkg-config
-  ];
-
   buildInputs = [
     libgit2
   ];
@@ -67,10 +53,6 @@ mkMesonDerivation (finalAttrs: {
     LDFLAGS = "-fuse-ld=gold";
   };
 
-  separateDebugInfo = !stdenv.hostPlatform.isStatic;
-
-  hardeningDisable = lib.optional stdenv.hostPlatform.isStatic "pie";
-
   meta = {
     platforms = lib.platforms.unix ++ lib.platforms.windows;
   };

src/libfetchers/path.cc

@@ -157,7 +157,11 @@ struct PathInputScheme : InputScheme
             });
             storePath = store->addToStoreFromDump(*src, "source");
         }
-        input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
+
+        /* Trust the lastModified value supplied by the user, if
+           any. It's not a "secure" attribute so we don't care. */
+        if (!input.getLastModified())
+            input.attrs.insert_or_assign("lastModified", uint64_t(mtime));
 
         return {makeStorePathAccessor(store, *storePath), std::move(input)};
     }

src/libfetchers/registry.cc

@@ -116,7 +116,7 @@ static std::shared_ptr<Registry> getSystemRegistry(const Settings & settings)
 
 Path getUserRegistryPath()
 {
-    return getConfigDir() + "/nix/registry.json";
+    return getConfigDir() + "/registry.json";
 }
 
 std::shared_ptr<Registry> getUserRegistry(const Settings & settings)
@@ -159,7 +159,7 @@ static std::shared_ptr<Registry> getGlobalRegistry(const Settings & settings, re
         if (!hasPrefix(path, "/")) {
             auto storePath = downloadFile(store, path, "flake-registry.json").storePath;
             if (auto store2 = store.dynamic_pointer_cast<LocalFSStore>())
-                store2->addPermRoot(storePath, getCacheDir() + "/nix/flake-registry.json");
+                store2->addPermRoot(storePath, getCacheDir() + "/flake-registry.json");
             path = store->toRealPath(storePath);
         }
 

src/libfetchers/tarball.cc

@@ -90,6 +90,7 @@ DownloadFileResult downloadFile(
     /* Cache metadata for all URLs in the redirect chain. */
     for (auto & url : res.urls) {
         key.second.insert_or_assign("url", url);
+        assert(!res.urls.empty());
         infoAttrs.insert_or_assign("url", *res.urls.rbegin());
         getCache()->upsert(key, *store, infoAttrs, *storePath);
     }
@@ -102,7 +103,7 @@ DownloadFileResult downloadFile(
     };
 }
 
-DownloadTarballResult downloadTarball(
+static DownloadTarballResult downloadTarball_(
     const std::string & url,
     const Headers & headers)
 {
@@ -170,6 +171,7 @@ DownloadTarballResult downloadTarball(
     auto tarballCache = getTarballCache();
     auto parseSink = tarballCache->getFileSystemObjectSink();
     auto lastModified = unpackTarfileToSink(archive, *parseSink);
+    auto tree = parseSink->flush();
 
     act.reset();
 
@@ -184,7 +186,7 @@ DownloadTarballResult downloadTarball(
     } else {
         infoAttrs.insert_or_assign("etag", res->etag);
         infoAttrs.insert_or_assign("treeHash",
-            tarballCache->dereferenceSingletonDirectory(parseSink->sync()).gitRev());
+            tarballCache->dereferenceSingletonDirectory(tree).gitRev());
         infoAttrs.insert_or_assign("lastModified", uint64_t(lastModified));
         if (res->immutableUrl)
             infoAttrs.insert_or_assign("immutableUrl", *res->immutableUrl);
@@ -202,6 +204,22 @@ DownloadTarballResult downloadTarball(
     return attrsToResult(infoAttrs);
 }
 
+ref<SourceAccessor> downloadTarball(
+    ref<Store> store,
+    const Settings & settings,
+    const std::string & url)
+{
+    /* Go through Input::getAccessor() to ensure that the resulting
+       accessor has a fingerprint. */
+    fetchers::Attrs attrs;
+    attrs.insert_or_assign("type", "tarball");
+    attrs.insert_or_assign("url", url);
+
+    auto input = Input::fromAttrs(settings, std::move(attrs));
+
+    return input.getAccessor(store).first;
+}
+
 // An input scheme corresponding to a curl-downloadable resource.
 struct CurlInputScheme : InputScheme
 {
@@ -353,7 +371,7 @@ struct TarballInputScheme : CurlInputScheme
     {
         auto input(_input);
 
-        auto result = downloadTarball(getStrAttr(input.attrs, "url"), {});
+        auto result = downloadTarball_(getStrAttr(input.attrs, "url"), {});
 
         result.accessor->setPathDisplay("«" + input.to_string() + "»");
 

src/libfetchers/tarball.hh

@@ -14,6 +14,8 @@ struct SourceAccessor;
 
 namespace nix::fetchers {
 
+struct Settings;
+
 struct DownloadFileResult
 {
     StorePath storePath;
@@ -40,8 +42,9 @@ struct DownloadTarballResult
  * Download and import a tarball into the Git cache. The result is the
  * Git tree hash of the root directory.
  */
-DownloadTarballResult downloadTarball(
-    const std::string & url,
-    const Headers & headers = {});
+ref<SourceAccessor> downloadTarball(
+    ref<Store> store,
+    const Settings & settings,
+    const std::string & url);
 
 }

src/libflake-tests/.version

@@ -0,0 +1 @@
+../../.version

src/libflake-tests/build-utils-meson

@@ -0,0 +1 @@
+../../build-utils-meson

src/libflake-tests/flakeref.cc

@@ -0,0 +1,24 @@
+#include <gtest/gtest.h>
+
+#include "fetch-settings.hh"
+#include "flake/flakeref.hh"
+
+namespace nix {
+
+/* ----------- tests for flake/flakeref.hh --------------------------------------------------*/
+
+    /* ----------------------------------------------------------------------------
+     * to_string
+     * --------------------------------------------------------------------------*/
+
+    TEST(to_string, doesntReencodeUrl) {
+        fetchers::Settings fetchSettings;
+        auto s = "http://localhost:8181/test/+3d.tar.gz";
+        auto flakeref = parseFlakeRef(fetchSettings, s);
+        auto parsed = flakeref.to_string();
+        auto expected = "http://localhost:8181/test/%2B3d.tar.gz";
+
+        ASSERT_EQ(parsed, expected);
+    }
+
+}

src/libflake-tests/local.mk

@@ -0,0 +1,43 @@
+check: libflake-tests_RUN
+
+programs += libflake-tests
+
+libflake-tests_NAME := libnixflake-tests
+
+libflake-tests_ENV := _NIX_TEST_UNIT_DATA=$(d)/data GTEST_OUTPUT=xml:$$testresults/libflake-tests.xml
+
+libflake-tests_DIR := $(d)
+
+ifeq ($(INSTALL_UNIT_TESTS), yes)
+  libflake-tests_INSTALL_DIR := $(checkbindir)
+else
+  libflake-tests_INSTALL_DIR :=
+endif
+
+libflake-tests_SOURCES := \
+    $(wildcard $(d)/*.cc) \
+    $(wildcard $(d)/value/*.cc) \
+    $(wildcard $(d)/flake/*.cc)
+
+libflake-tests_EXTRA_INCLUDES = \
+    -I src/libflake-test-support \
+    -I src/libstore-test-support \
+    -I src/libutil-test-support \
+    $(INCLUDE_libflake) \
+    $(INCLUDE_libexpr) \
+    $(INCLUDE_libfetchers) \
+    $(INCLUDE_libstore) \
+    $(INCLUDE_libutil) \
+
+libflake-tests_CXXFLAGS += $(libflake-tests_EXTRA_INCLUDES)
+
+libflake-tests_LIBS = \
+    libexpr-test-support libstore-test-support libutil-test-support \
+    libflake libexpr libfetchers libstore libutil
+
+libflake-tests_LDFLAGS := -lrapidcheck $(GTEST_LIBS) -lgmock
+
+ifdef HOST_WINDOWS
+  # Increase the default reserved stack size to 65 MB so Nix doesn't run out of space
+  libflake-tests_LDFLAGS += -Wl,--stack,$(shell echo $$((65 * 1024 * 1024)))
+endif

src/libflake-tests/meson.build

@@ -0,0 +1,75 @@
+project('nix-flake-tests', 'cpp',
+  version : files('.version'),
+  default_options : [
+    'cpp_std=c++2a',
+    # TODO(Qyriad): increase the warning level
+    'warning_level=1',
+    'debug=true',
+    'optimization=2',
+    'errorlogs=true', # Please print logs for tests that fail
+  ],
+  meson_version : '>= 1.1',
+  license : 'LGPL-2.1-or-later',
+)
+
+cxx = meson.get_compiler('cpp')
+
+subdir('build-utils-meson/deps-lists')
+
+deps_private_maybe_subproject = [
+  dependency('nix-expr-test-support'),
+  dependency('nix-flake'),
+]
+deps_public_maybe_subproject = [
+]
+subdir('build-utils-meson/subprojects')
+
+subdir('build-utils-meson/threads')
+
+subdir('build-utils-meson/export-all-symbols')
+subdir('build-utils-meson/windows-version')
+
+rapidcheck = dependency('rapidcheck')
+deps_private += rapidcheck
+
+gtest = dependency('gtest', main : true)
+deps_private += gtest
+
+add_project_arguments(
+  # TODO(Qyriad): Yes this is how the autoconf+Make system did it.
+  # It would be nice for our headers to be idempotent instead.
+  '-include', 'config-util.hh',
+  '-include', 'config-store.hh',
+  '-include', 'config-expr.hh',
+  language : 'cpp',
+)
+
+subdir('build-utils-meson/diagnostics')
+
+sources = files(
+  'flakeref.cc',
+  'url-name.cc',
+)
+
+include_dirs = [include_directories('.')]
+
+
+this_exe = executable(
+  meson.project_name(),
+  sources,
+  dependencies : deps_private_subproject + deps_private + deps_other,
+  include_directories : include_dirs,
+  # TODO: -lrapidcheck, see ../libutil-support/build.meson
+  link_args: linker_export_flags + ['-lrapidcheck'],
+  # get main from gtest
+  install : true,
+)
+
+test(
+  meson.project_name(),
+  this_exe,
+  env : {
+    '_NIX_TEST_UNIT_DATA': meson.current_source_dir() / 'data',
+  },
+  protocol : 'gtest',
+)

src/libflake-tests/package.nix

@@ -0,0 +1,81 @@
+{ lib
+, buildPackages
+, stdenv
+, mkMesonExecutable
+
+, nix-flake
+, nix-expr-test-support
+
+, rapidcheck
+, gtest
+, runCommand
+
+# Configuration Options
+
+, version
+, resolvePath
+}:
+
+let
+  inherit (lib) fileset;
+in
+
+mkMesonExecutable (finalAttrs: {
+  pname = "nix-flake-tests";
+  inherit version;
+
+  workDir = ./.;
+  fileset = fileset.unions [
+    ../../build-utils-meson
+    ./build-utils-meson
+    ../../.version
+    ./.version
+    ./meson.build
+    # ./meson.options
+    (fileset.fileFilter (file: file.hasExt "cc") ./.)
+    (fileset.fileFilter (file: file.hasExt "hh") ./.)
+  ];
+
+  buildInputs = [
+    nix-flake
+    nix-expr-test-support
+    rapidcheck
+    gtest
+  ];
+
+  preConfigure =
+    # "Inline" .version so it's not a symlink, and includes the suffix.
+    # Do the meson utils, without modification.
+    ''
+      chmod u+w ./.version
+      echo ${version} > ../../.version
+    '';
+
+  mesonFlags = [
+  ];
+
+  env = lib.optionalAttrs (stdenv.isLinux && !(stdenv.hostPlatform.isStatic && stdenv.system == "aarch64-linux")) {
+    LDFLAGS = "-fuse-ld=gold";
+  };
+
+  passthru = {
+    tests = {
+      run = runCommand "${finalAttrs.pname}-run" {
+        meta.broken = !stdenv.hostPlatform.emulatorAvailable buildPackages;
+      } (lib.optionalString stdenv.hostPlatform.isWindows ''
+        export HOME="$PWD/home-dir"
+        mkdir -p "$HOME"
+      '' + ''
+        export _NIX_TEST_UNIT_DATA=${resolvePath ./data}
+        ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe finalAttrs.finalPackage}
+        touch $out
+      '');
+    };
+  };
+
+  meta = {
+    platforms = lib.platforms.unix ++ lib.platforms.windows;
+    mainProgram = finalAttrs.pname + stdenv.hostPlatform.extensions.executable;
+  };
+
+})