Merge remote-tracking branch 'origin/master' into relative-flakes

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

@@ -41,7 +41,7 @@ INPUT                  = \
   @src@/src/libutil-c \
   @src@/src/libexpr-c \
   @src@/src/libstore-c \
-  @src@/doc/external-api/README.md
+  @src@/src/external-api-docs/README.md
 
 FILE_PATTERNS = nix_api_*.h *.md
 
@@ -55,4 +55,8 @@ EXCLUDE_PATTERNS = *_internal.h
 GENERATE_TREEVIEW = YES
 OPTIMIZE_OUTPUT_FOR_C  = YES
 
-USE_MDFILE_AS_MAINPAGE = doc/external-api/README.md
+USE_MDFILE_AS_MAINPAGE = @src@/src/external-api-docs/README.md
+
+WARN_IF_UNDOCUMENTED = NO
+WARN_IF_INCOMPLETE_DOC = NO
+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 +41,22 @@ 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/tests \
+  @src@/libexpr-test-support/tests/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/tests \
   @src@/libutil \
-  @src@/nix-util-tests \
-  @src@/nix-util-test-support/test \
+  @src@/libutil/args \
+  @src@/libutil-tests \
+  @src@/libutil-test-support/tests \
   @src@/nix \
   @src@/nix-env \
   @src@/nix-store
@@ -83,7 +84,9 @@ EXPAND_ONLY_PREDEF     = YES
 # RECURSIVE has no effect here.
 # This tag requires that the tag SEARCH_INCLUDES is set to YES.
 
-INCLUDE_PATH           =
+INCLUDE_PATH           = \
+  @BUILD_ROOT@/src/libexpr/libnixexpr.so.p \
+  @BUILD_ROOT@/src/nix/nix.p \
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The
@@ -96,4 +99,18 @@ EXPAND_AS_DEFINED      = \
   DECLARE_COMMON_SERIALISER \
   DECLARE_WORKER_SERIALISER \
   DECLARE_SERVE_SERIALISER \
-  LENGTH_PREFIXED_PROTO_HELPER
+  LENGTH_PREFIXED_PROTO_HELPER \
+  LENGTH_PREFIXED_PROTO_HELPER_X \
+  WORKER_USE_LENGTH_PREFIX_SERIALISER \
+  WORKER_USE_LENGTH_PREFIX_SERIALISER_COMMA \
+  SERVE_USE_LENGTH_PREFIX_SERIALISER \
+  SERVE_USE_LENGTH_PREFIX_SERIALISER_COMMA \
+  COMMON_METHODS \
+  JSON_IMPL \
+  MakeBinOp
+
+PREDEFINED = DOXYGEN_SKIP
+
+WARN_IF_UNDOCUMENTED = NO
+WARN_IF_INCOMPLETE_DOC = NO
+QUIET = YES

src/internal-api-docs/meson.build

@@ -12,6 +12,7 @@ doxygen_cfg = configure_file(
   configuration : {
     'PROJECT_NUMBER': meson.project_version(),
     'OUTPUT_DIRECTORY' : meson.current_build_dir(),
+    'BUILD_ROOT' : meson.build_root(),
     'src' : fs.parent(fs.parent(meson.project_source_root())) / 'src',
   },
 )

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()
 {
@@ -126,10 +127,8 @@ ref<Store> EvalCommand::getEvalStore()
 ref<EvalState> EvalCommand::getEvalState()
 {
     if (!evalState) {
-        evalState =
-            std::allocate_shared<EvalState>(
-                traceable_allocator<EvalState>(),
-                lookupPath, getEvalStore(), fetchSettings, evalSettings, getStore());
+        evalState = std::allocate_shared<EvalState>(
+            traceable_allocator<EvalState>(), lookupPath, getEvalStore(), fetchSettings, evalSettings, getStore());
 
         evalState->repair = repair;
 
@@ -144,7 +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},
     });
@@ -179,30 +179,34 @@ BuiltPathsCommand::BuiltPathsCommand(bool recursive)
 
 void BuiltPathsCommand::run(ref<Store> store, Installables && installables)
 {
-    BuiltPaths paths;
+    BuiltPaths rootPaths, allPaths;
+
     if (all) {
         if (installables.size())
             throw UsageError("'--all' does not expect arguments");
         // XXX: Only uses opaque paths, ignores all the realisations
         for (auto & p : store->queryAllValidPaths())
-            paths.emplace_back(BuiltPath::Opaque{p});
+            rootPaths.emplace_back(BuiltPath::Opaque{p});
+        allPaths = rootPaths;
     } else {
-        paths = Installable::toBuiltPaths(getEvalStore(), store, realiseMode, operateOn, installables);
+        rootPaths = Installable::toBuiltPaths(getEvalStore(), store, realiseMode, operateOn, installables);
+        allPaths = rootPaths;
+
         if (recursive) {
             // XXX: This only computes the store path closure, ignoring
             // intermediate realisations
             StorePathSet pathsRoots, pathsClosure;
-            for (auto & root : paths) {
+            for (auto & root : rootPaths) {
                 auto rootFromThis = root.outPaths();
                 pathsRoots.insert(rootFromThis.begin(), rootFromThis.end());
             }
             store->computeFSClosure(pathsRoots, pathsClosure);
             for (auto & path : pathsClosure)
-                paths.emplace_back(BuiltPath::Opaque{path});
+                allPaths.emplace_back(BuiltPath::Opaque{path});
         }
     }
 
-    run(store, std::move(paths));
+    run(store, std::move(allPaths), std::move(rootPaths));
 }
 
 StorePathsCommand::StorePathsCommand(bool recursive)
@@ -210,10 +214,10 @@ StorePathsCommand::StorePathsCommand(bool recursive)
 {
 }
 
-void StorePathsCommand::run(ref<Store> store, BuiltPaths && paths)
+void StorePathsCommand::run(ref<Store> store, BuiltPaths && allPaths, BuiltPaths && rootPaths)
 {
     StorePathSet storePaths;
-    for (auto & builtPath : paths)
+    for (auto & builtPath : allPaths)
         for (auto & p : builtPath.outPaths())
             storePaths.insert(p);
 
@@ -233,46 +237,48 @@ 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;
-    auto store = getStore().dynamic_pointer_cast<LocalFSStore>();
-    if (!store) throw Error("'--profile' is not supported for this Nix store");
+    if (!profile)
+        return;
+    auto store = getDstStore().dynamic_pointer_cast<LocalFSStore>();
+    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]);
 }
@@ -282,50 +288,111 @@ 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;
+}
+
+void createOutLinks(const std::filesystem::path & outLink, const BuiltPaths & buildables, LocalFSStore & store)
+{
+    for (const auto & [_i, buildable] : enumerate(buildables)) {
+        auto i = _i;
+        std::visit(
+            overloaded{
+                [&](const BuiltPath::Opaque & bo) {
+                    auto symlink = outLink;
+                    if (i)
+                        symlink += fmt("-%d", i);
+                    store.addPermRoot(bo.path, absPath(symlink.string()));
+                },
+                [&](const BuiltPath::Built & bfd) {
+                    for (auto & output : bfd.outputs) {
+                        auto symlink = outLink;
+                        if (i)
+                            symlink += fmt("-%d", i);
+                        if (output.first != "out")
+                            symlink += fmt("-%s", output.first);
+                        store.addPermRoot(output.second, absPath(symlink.string()));
+                    }
+                },
+            },
+            buildable.raw());
     }
 }
 

src/libcmd/command.hh

@@ -13,18 +13,20 @@ namespace nix {
 
 extern std::string programPath;
 
-extern char * * savedArgv;
+extern char ** savedArgv;
 
 class EvalState;
 struct Pos;
 class Store;
+class LocalFSStore;
 
 static constexpr Command::Category catHelp = -1;
 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
 {
@@ -45,7 +47,20 @@ struct StoreCommand : virtual Command
 {
     StoreCommand();
     void run() override;
+
+    /**
+     * Return the default Nix store.
+     */
     ref<Store> getStore();
+
+    /**
+     * Return the destination Nix store.
+     */
+    virtual ref<Store> getDstStore()
+    {
+        return getStore();
+    }
+
     virtual ref<Store> createStore();
     /**
      * Main entry point, with a `Store` provided
@@ -68,7 +83,7 @@ struct CopyCommand : virtual StoreCommand
 
     ref<Store> createStore() override;
 
-    ref<Store> getDstStore();
+    ref<Store> getDstStore() override;
 };
 
 /**
@@ -112,7 +127,9 @@ 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 +139,9 @@ 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();
 
@@ -238,7 +253,7 @@ public:
 
     BuiltPathsCommand(bool recursive = false);
 
-    virtual void run(ref<Store> store, BuiltPaths && paths) = 0;
+    virtual void run(ref<Store> store, BuiltPaths && allPaths, BuiltPaths && rootPaths) = 0;
 
     void run(ref<Store> store, Installables && installables) override;
 
@@ -251,7 +266,7 @@ struct StorePathsCommand : public BuiltPathsCommand
 
     virtual void run(ref<Store> store, StorePaths && storePaths) = 0;
 
-    void run(ref<Store> store, BuiltPaths && paths) override;
+    void run(ref<Store> store, BuiltPaths && allPaths, BuiltPaths && rootPaths) override;
 };
 
 /**
@@ -272,10 +287,10 @@ 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 +300,13 @@ 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 +328,21 @@ 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 +366,12 @@ 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);
+
+/**
+ * Create symlinks prefixed by `outLink` to the store paths in
+ * `buildables`.
+ */
+void createOutLinks(const std::filesystem::path & outLink, const BuiltPaths & buildables, LocalFSStore & store);
 
 }

src/libcmd/common-eval-args.cc

@@ -29,13 +29,13 @@ EvalSettings evalSettings {
     {
         {
             "flake",
-            [](ref<Store> store, std::string_view rest) {
+            [](EvalState & state, std::string_view rest) {
                 experimentalFeatureSettings.require(Xp::Flakes);
                 // FIXME `parseFlakeRef` should take a `std::string_view`.
                 auto flakeRef = parseFlakeRef(fetchSettings, std::string { rest }, {}, true, false);
                 debug("fetching flake search path element '%s''", rest);
-                auto storePath = flakeRef.resolve(store).fetchTree(store).first;
-                return store->toRealPath(storePath);
+                auto storePath = flakeRef.resolve(state.store).fetchTree(state.store).first;
+                return state.rootPath(state.store->toRealPath(storePath));
             },
         },
     },

src/libcmd/installable-derived-path.cc

@@ -32,16 +32,6 @@ InstallableDerivedPath InstallableDerivedPath::parse(
         // store path.
         [&](const ExtendedOutputsSpec::Default &) -> DerivedPath {
             auto storePath = store->followLinksToStorePath(prefix);
-            // Remove this prior to stabilizing the new CLI.
-            if (storePath.isDerivation()) {
-                auto oldDerivedPath = DerivedPath::Built {
-                    .drvPath = makeConstantStorePathRef(storePath),
-                    .outputs = OutputsSpec::All { },
-                };
-                warn(
-                    "The interpretation of store paths arguments ending in `.drv` recently changed. If this command is now failing try again with '%s'",
-                    oldDerivedPath.to_string(*store));
-            };
             return DerivedPath::Opaque {
                 .path = std::move(storePath),
             };

src/libcmd/installable-flake.hh

@@ -26,7 +26,7 @@ struct ExtraPathInfoFlake : ExtraPathInfoValue
     Flake flake;
 
     ExtraPathInfoFlake(Value && v, Flake && f)
-        : ExtraPathInfoValue(std::move(v)), flake(f)
+        : ExtraPathInfoValue(std::move(v)), flake(std::move(f))
     { }
 };
 

src/libcmd/installable-value.hh

@@ -59,7 +59,7 @@ struct ExtraPathInfoValue : ExtraPathInfo
     Value value;
 
     ExtraPathInfoValue(Value && v)
-        : value(v)
+        : value(std::move(v))
     { }
 
     virtual ~ExtraPathInfoValue() = default;

src/libcmd/installables.cc

@@ -88,7 +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 = {[&]() {
@@ -857,6 +857,7 @@ std::vector<FlakeRef> RawInstallablesCommand::getFlakeRefsForCompletion()
 {
     applyDefaultInstallables(rawInstallables);
     std::vector<FlakeRef> res;
+    res.reserve(rawInstallables.size());
     for (auto i : rawInstallables)
         res.push_back(parseFlakeRefWithFragment(
             fetchSettings,
@@ -917,4 +918,12 @@ void BuiltPathsCommand::applyDefaultInstallables(std::vector<std::string> & rawI
         rawInstallables.push_back(".");
 }
 
+BuiltPaths toBuiltPaths(const std::vector<BuiltPathWithResult> & builtPathsWithResult)
+{
+    BuiltPaths res;
+    for (auto & i : builtPathsWithResult)
+        res.push_back(i.path);
+    return res;
+}
+
 }

src/libcmd/installables.hh

@@ -86,6 +86,8 @@ struct BuiltPathWithResult
     std::optional<BuildResult> result;
 };
 
+BuiltPaths toBuiltPaths(const std::vector<BuiltPathWithResult> & builtPathsWithResult);
+
 /**
  * Shorthand, for less typing and helping us keep the choice of
  * collection in sync.

src/libcmd/local.mk

@@ -1,15 +0,0 @@
-libraries += libcmd
-
-libcmd_NAME = libnixcmd
-
-libcmd_DIR := $(d)
-
-libcmd_SOURCES := $(wildcard $(d)/*.cc)
-
-libcmd_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) $(INCLUDE_libexpr) $(INCLUDE_libflake) $(INCLUDE_libmain)
-
-libcmd_LDFLAGS = $(EDITLINE_LIBS) $(LOWDOWN_LIBS) $(THREAD_LDFLAGS)
-
-libcmd_LIBS = libutil libstore libfetchers libflake libexpr libmain
-
-$(eval $(call install-file-in, $(buildprefix)$(d)/nix-cmd.pc, $(libdir)/pkgconfig, 0644))

src/libcmd/meson.build

@@ -30,8 +30,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('build-utils-meson/subprojects')
 
-subdir('build-utils-meson/threads')
-
 nlohmann_json = dependency('nlohmann_json', version : '>= 3.9')
 deps_public += nlohmann_json
 
@@ -72,7 +70,7 @@ add_project_arguments(
   language : 'cpp',
 )
 
-subdir('build-utils-meson/diagnostics')
+subdir('build-utils-meson/common')
 
 sources = files(
   'built-path.cc',

src/libcmd/nix-cmd.pc.in

@@ -1,9 +0,0 @@
-prefix=@prefix@
-libdir=@libdir@
-includedir=@includedir@
-
-Name: Nix
-Description: Nix Package Manager
-Version: @PACKAGE_VERSION@
-Libs: -L${libdir} -lnixcmd
-Cflags: -I${includedir}/nix -std=c++2a

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

@@ -645,7 +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) {
@@ -654,7 +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
@@ -826,7 +826,7 @@ void NixRepl::runNix(Path program, const Strings & args, const std::optional<std
     if (runNixPtr)
         (*runNixPtr)(program, args, input);
     else
-        throw Error("Cannot run '%s', no method of calling the Nix CLI provided", program);
+        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);
 }
 
 

src/libexpr-c/local.mk

@@ -1,25 +0,0 @@
-libraries += libexprc
-
-libexprc_NAME = libnixexprc
-
-libexprc_DIR := $(d)
-
-libexprc_SOURCES := \
-  $(wildcard $(d)/*.cc) \
-
-# Not just for this library itself, but also for downstream libraries using this library
-
-INCLUDE_libexprc := -I $(d)
-libexprc_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libutilc) \
-                     $(INCLUDE_libfetchers) \
-                     $(INCLUDE_libstore) $(INCLUDE_libstorec) \
-                     $(INCLUDE_libexpr) $(INCLUDE_libexprc)
-
-libexprc_LIBS = libutil libutilc libstore libstorec libfetchers libexpr
-
-libexprc_LDFLAGS += $(THREAD_LDFLAGS)
-
-$(eval $(call install-file-in, $(d)/nix-expr-c.pc, $(libdir)/pkgconfig, 0644))
-
-libexprc_FORCE_INSTALL := 1
-

src/libexpr-c/meson.build

@@ -29,8 +29,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('build-utils-meson/subprojects')
 
-subdir('build-utils-meson/threads')
-
 # TODO rename, because it will conflict with downstream projects
 configdata.set_quoted('PACKAGE_VERSION', meson.project_version())
 
@@ -55,7 +53,7 @@ add_project_arguments(
   language : 'cpp',
 )
 
-subdir('build-utils-meson/diagnostics')
+subdir('build-utils-meson/common')
 
 sources = files(
   'nix_api_expr.cc',
@@ -75,6 +73,7 @@ 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-expr-c.pc.in

@@ -1,10 +0,0 @@
-prefix=@prefix@
-libdir=@libdir@
-includedir=@includedir@
-
-Name: Nix
-Description: Nix Language Evaluator - C API
-Version: @PACKAGE_VERSION@
-Requires: nix-store-c
-Libs: -L${libdir} -lnixexprc
-Cflags: -I${includedir}/nix

src/libexpr-c/nix_api_expr.cc

@@ -67,7 +67,7 @@ nix_err nix_value_call_multi(nix_c_context * context, EvalState * state, nix_val
     if (context)
         context->last_err_code = NIX_OK;
     try {
-        state->state.callFunction(fn->value, nargs, (nix::Value * *)args, value->value, nix::noPos);
+        state->state.callFunction(fn->value, {(nix::Value * *) args, nargs}, value->value, nix::noPos);
         state->state.forceValue(value->value, nix::noPos);
     }
     NIXC_CATCH_ERRS

src/libexpr-c/nix_api_expr.h

@@ -129,7 +129,7 @@ nix_err nix_value_call_multi(
  * @param[in] state The state of the evaluation.
  * @param[out] value The result of the function call.
  * @param[in] fn The Nix function to call.
- * @param[in] args The arguments to pass to the function.
+ * @param[in] ... The arguments to pass to the function.
  *
  * @see nix_value_call_multi
  */

src/libexpr-c/nix_api_value.h

@@ -77,8 +77,7 @@ typedef struct ExternalValue ExternalValue;
  */
 typedef struct nix_realised_string nix_realised_string;
 
-/** @defgroup primops
- * @brief Create your own primops
+/** @defgroup primops Adding primops
  * @{
  */
 /** @brief Function pointer for primops
@@ -252,7 +251,7 @@ int64_t nix_get_int(nix_c_context * context, const nix_value * value);
  * @param[in] value Nix value to inspect
  * @return reference to external, NULL in case of error
  */
-ExternalValue * nix_get_external(nix_c_context * context, nix_value *);
+ExternalValue * nix_get_external(nix_c_context * context, nix_value * value);
 
 /** @brief Get the ix'th element of a list
  *
@@ -423,7 +422,7 @@ nix_list_builder_insert(nix_c_context * context, ListBuilder * list_builder, uns
 /** @brief Free a list builder
  *
  * Does not fail.
- * @param[in] builder the builder to free
+ * @param[in] list_builder The builder to free.
  */
 void nix_list_builder_free(ListBuilder * list_builder);
 

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/meson.build

@@ -0,0 +1,75 @@
+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')
+
+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/common')
+
+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 +1,156 @@
+#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;
+            }
+
+            Value * maybeThunk(std::string input, bool forceValue = true) {
+                Expr * e = state.parseExprFromString(input, state.rootPath(CanonPath::root));
+                assert(e);
+                return e->maybeThunk(state, state.baseEnv);
+            }
+
+            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 +1,164 @@
+#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));
+}
+
+class EvalStateTest : public LibExprTest {};
+
+TEST_F(EvalStateTest, getBuiltins_ok) {
+    auto evaled = maybeThunk("builtins");
+    auto & builtins = state.getBuiltins();
+    ASSERT_TRUE(builtins.type() == nAttrs);
+    ASSERT_EQ(evaled, &builtins);
+}
+
+TEST_F(EvalStateTest, getBuiltin_ok) {
+    auto & builtin = state.getBuiltin("toString");
+    ASSERT_TRUE(builtin.type() == nFunction);
+    // FIXME
+    // auto evaled = maybeThunk("builtins.toString");
+    // ASSERT_EQ(evaled, &builtin);
+    auto & builtin2 = state.getBuiltin("true");
+    ASSERT_EQ(state.forceBool(builtin2, noPos, "in unit test"), true);
+}
+
+TEST_F(EvalStateTest, getBuiltin_fail) {
+    ASSERT_THROW(state.getBuiltin("nonexistent"), EvalError);
+}
+
+} // 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/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 +1,91 @@
+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/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/common')
+
+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/call-flake.nix

@@ -10,6 +10,9 @@ lockFileStr:
 # unlocked trees.
 overrides:
 
+# This is `prim_fetchFinalTree`.
+fetchTreeFinal:
+
 let
 
   lockFile = builtins.fromJSON lockFileStr;
@@ -51,7 +54,8 @@ let
               }
             else
               # FIXME: remove obsolete node.info.
-              fetchTree (node.info or {} // removeAttrs node.locked ["dir"]);
+              # Note: lock file entries are always final.
+              fetchTreeFinal (node.info or {} // removeAttrs node.locked ["dir"]);
 
           subdir = overrides.${key}.dir or node.locked.dir or "";
 

src/libexpr/eval-cache.cc

@@ -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;
         }
@@ -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;
     }
 }

src/libexpr/eval-gc.hh

@@ -13,6 +13,8 @@
 
 #else
 
+#  include <memory>
+
 /* Some dummy aliases for Boehm GC definitions to reduce the number of
    #ifdefs. */
 
@@ -23,7 +25,6 @@ template<typename T>
 using gc_allocator = std::allocator<T>;
 
 #  define GC_MALLOC_ATOMIC std::malloc
-#  define GC_STRDUP strdup
 
 struct gc
 {};

src/libexpr/eval-inline.hh

@@ -87,11 +87,15 @@ void EvalState::forceValue(Value & v, const PosIdx pos)
 {
     if (v.isThunk()) {
         Env * env = v.payload.thunk.env;
+        assert(env || v.isBlackhole());
         Expr * expr = v.payload.thunk.expr;
         try {
             v.mkBlackhole();
             //checkInterrupt();
-            expr->eval(*this, *env, v);
+            if (env) [[likely]]
+                expr->eval(*this, *env, v);
+            else
+                ExprBlackHole::throwInfiniteRecursionError(*this, v);
         } catch (...) {
             v.mkThunk(env, expr);
             tryFixupBlackHolePos(v, pos);

src/libexpr/eval-settings.hh

@@ -3,10 +3,11 @@
 
 #include "config.hh"
 #include "ref.hh"
+#include "source-path.hh"
 
 namespace nix {
 
-class Store;
+class EvalState;
 
 struct EvalSettings : Config
 {
@@ -18,11 +19,8 @@ struct EvalSettings : Config
      *
      * The return value is (a) whether the entry was valid, and, if so,
      * what does it map to.
-     *
-     * @todo Return (`std::optional` of) `SourceAccssor` or something
-     * more structured instead of mere `std::string`?
      */
-    using LookupPathHook = std::optional<std::string>(ref<Store> store, std::string_view);
+    using LookupPathHook = std::optional<SourcePath>(EvalState & state, std::string_view);
 
     /**
      * Map from "scheme" to a `LookupPathHook`.

src/libexpr/eval.cc

@@ -53,15 +53,6 @@ static char * allocString(size_t size)
 }
 
 
-static char * dupString(const char * s)
-{
-    char * t;
-    t = GC_STRDUP(s);
-    if (!t) throw std::bad_alloc();
-    return t;
-}
-
-
 // When there's no need to write to the string, we can optimize away empty
 // string allocations.
 // This function handles makeImmutableString(std::string_view()) by returning
@@ -457,7 +448,7 @@ void EvalState::addConstant(const std::string & name, Value * v, Constant info)
         /* Install value the base environment. */
         staticBaseEnv->vars.emplace_back(symbols.create(name), baseEnvDispl);
         baseEnv.values[baseEnvDispl++] = v;
-        baseEnv.values[0]->payload.attrs->push_back(Attr(symbols.create(name2), v));
+        getBuiltins().payload.attrs->push_back(Attr(symbols.create(name2), v));
     }
 }
 
@@ -519,16 +510,32 @@ Value * EvalState::addPrimOp(PrimOp && primOp)
 
     Value * v = allocValue();
     v->mkPrimOp(new PrimOp(primOp));
-    staticBaseEnv->vars.emplace_back(envName, baseEnvDispl);
-    baseEnv.values[baseEnvDispl++] = v;
-    baseEnv.values[0]->payload.attrs->push_back(Attr(symbols.create(primOp.name), v));
+
+    if (primOp.internal)
+        internalPrimOps.emplace(primOp.name, v);
+    else {
+        staticBaseEnv->vars.emplace_back(envName, baseEnvDispl);
+        baseEnv.values[baseEnvDispl++] = v;
+        getBuiltins().payload.attrs->push_back(Attr(symbols.create(primOp.name), v));
+    }
+
     return v;
 }
 
 
+Value & EvalState::getBuiltins()
+{
+    return *baseEnv.values[0];
+}
+
+
 Value & EvalState::getBuiltin(const std::string & name)
 {
-    return *baseEnv.values[0]->attrs()->find(symbols.create(name))->value;
+    auto it = getBuiltins().attrs()->get(symbols.create(name));
+    if (it)
+        return *it->value;
+    else
+        error<EvalError>("builtin '%1%' not found", name).debugThrow();
 }
 
 
@@ -548,7 +555,7 @@ 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;
@@ -580,30 +587,25 @@ 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 * 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);
+            callFunction(functor, vp, partiallyApplied, noPos);
             auto _level = addCallDepth(noPos);
             return getDoc(partiallyApplied);
         }
@@ -832,9 +834,10 @@ 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;
@@ -1467,7 +1470,7 @@ void ExprLambda::eval(EvalState & state, Env & env, Value & v)
     v.mkLambda(&env, this);
 }
 
-void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value & vRes, const PosIdx pos)
+void EvalState::callFunction(Value & fun, std::span<Value *> args, Value & vRes, const PosIdx pos)
 {
     auto _level = addCallDepth(pos);
 
@@ -1482,16 +1485,16 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
     auto makeAppChain = [&]()
     {
         vRes = vCur;
-        for (size_t i = 0; i < nrArgs; ++i) {
+        for (auto arg : args) {
             auto fun2 = allocValue();
             *fun2 = vRes;
-            vRes.mkPrimOpApp(fun2, args[i]);
+            vRes.mkPrimOpApp(fun2, arg);
         }
     };
 
     const Attr * functor;
 
-    while (nrArgs > 0) {
+    while (args.size() > 0) {
 
         if (vCur.isLambda()) {
 
@@ -1594,15 +1597,14 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 throw;
             }
 
-            nrArgs--;
-            args += 1;
+            args = args.subspan(1);
         }
 
         else if (vCur.isPrimOp()) {
 
             size_t argsLeft = vCur.primOp()->arity;
 
-            if (nrArgs < argsLeft) {
+            if (args.size() < argsLeft) {
                 /* We don't have enough arguments, so create a tPrimOpApp chain. */
                 makeAppChain();
                 return;
@@ -1614,15 +1616,14 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                 if (countCalls) primOpCalls[fn->name]++;
 
                 try {
-                    fn->fun(*this, vCur.determinePos(noPos), args, vCur);
+                    fn->fun(*this, vCur.determinePos(noPos), args.data(), vCur);
                 } catch (Error & e) {
                     if (fn->addTrace)
                         addErrorTrace(e, pos, "while calling the '%1%' builtin", fn->name);
                     throw;
                 }
 
-                nrArgs -= argsLeft;
-                args += argsLeft;
+                args = args.subspan(argsLeft);
             }
         }
 
@@ -1638,7 +1639,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             auto arity = primOp->primOp()->arity;
             auto argsLeft = arity - argsDone;
 
-            if (nrArgs < argsLeft) {
+            if (args.size() < argsLeft) {
                 /* We still don't have enough arguments, so extend the tPrimOpApp chain. */
                 makeAppChain();
                 return;
@@ -1670,8 +1671,7 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
                     throw;
                 }
 
-                nrArgs -= argsLeft;
-                args += argsLeft;
+                args = args.subspan(argsLeft);
             }
         }
 
@@ -1682,13 +1682,12 @@ void EvalState::callFunction(Value & fun, size_t nrArgs, Value * * args, Value &
             Value * args2[] = {allocValue(), args[0]};
             *args2[0] = vCur;
             try {
-                callFunction(*functor->value, 2, args2, vCur, functor->pos);
+                callFunction(*functor->value, args2, vCur, functor->pos);
             } catch (Error & e) {
                 e.addTrace(positions[pos], "while calling a functor (an attribute set with a '__functor' attribute)");
                 throw;
             }
-            nrArgs--;
-            args++;
+            args = args.subspan(1);
         }
 
         else
@@ -1731,7 +1730,7 @@ void ExprCall::eval(EvalState & state, Env & env, Value & v)
     for (size_t i = 0; i < args.size(); ++i)
         vArgs[i] = args[i]->maybeThunk(state, env);
 
-    state.callFunction(vFun, args.size(), vArgs.data(), v, pos);
+    state.callFunction(vFun, vArgs, v, pos);
 }
 
 
@@ -1743,7 +1742,7 @@ void EvalState::incrFunctionCall(ExprLambda * fun)
 }
 
 
-void EvalState::autoCallFunction(Bindings & args, Value & fun, Value & res)
+void EvalState::autoCallFunction(const Bindings & args, Value & fun, Value & res)
 {
     auto pos = fun.determinePos(noPos);
 
@@ -1813,11 +1812,9 @@ 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 {
@@ -2055,9 +2052,12 @@ void ExprPos::eval(EvalState & state, Env & env, Value & v)
     state.mkPos(v, pos);
 }
 
-
-void ExprBlackHole::eval(EvalState & state, Env & env, Value & v)
+void ExprBlackHole::eval(EvalState & state, [[maybe_unused]] Env & env, Value & v)
 {
+    throwInfiniteRecursionError(state, v);
+}
+
+[[gnu::noinline]] [[noreturn]] void ExprBlackHole::throwInfiniteRecursionError(EvalState & state, Value &v) {
     state.error<InfiniteRecursionError>("infinite recursion encountered")
         .atPos(v.determinePos(noPos))
         .debugThrow();
@@ -2849,7 +2849,9 @@ 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"] = {
@@ -3036,8 +3038,8 @@ SourcePath EvalState::findFile(const LookupPath & lookupPath, const std::string_
         if (!rOpt) continue;
         auto r = *rOpt;
 
-        Path res = suffix == "" ? r : concatStrings(r, "/", suffix);
-        if (pathExists(res)) return rootPath(CanonPath(canonPath(res)));
+        auto res = (r / CanonPath(suffix)).resolveSymlinks();
+        if (res.pathExists()) return res;
     }
 
     if (hasPrefix(path, "nix/"))
@@ -3052,13 +3054,13 @@ SourcePath EvalState::findFile(const LookupPath & lookupPath, const std::string_
 }
 
 
-std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Path & value0, bool initAccessControl)
+std::optional<SourcePath> EvalState::resolveLookupPathPath(const LookupPath::Path & value0, bool initAccessControl)
 {
     auto & value = value0.s;
     auto i = lookupPathResolved.find(value);
     if (i != lookupPathResolved.end()) return i->second;
 
-    auto finish = [&](std::string res) {
+    auto finish = [&](SourcePath res) {
         debug("resolved search path element '%s' to '%s'", value, res);
         lookupPathResolved.emplace(value, res);
         return res;
@@ -3071,7 +3073,7 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
                 fetchSettings,
                 EvalSettings::resolvePseudoUrl(value));
             auto storePath = fetchToStore(*store, SourcePath(accessor), FetchMode::Copy);
-            return finish(store->toRealPath(storePath));
+            return finish(rootPath(store->toRealPath(storePath)));
         } catch (Error & e) {
             logWarning({
                 .msg = HintFmt("Nix search path entry '%1%' cannot be downloaded, ignoring", value)
@@ -3083,29 +3085,29 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
         auto scheme = value.substr(0, colPos);
         auto rest = value.substr(colPos + 1);
         if (auto * hook = get(settings.lookupPathHooks, scheme)) {
-            auto res = (*hook)(store, rest);
+            auto res = (*hook)(*this, rest);
             if (res)
                 return finish(std::move(*res));
         }
     }
 
     {
-        auto path = absPath(value);
+        auto path = rootPath(value);
 
         /* Allow access to paths in the search path. */
         if (initAccessControl) {
-            allowPath(path);
-            if (store->isInStore(path)) {
+            allowPath(path.path.abs());
+            if (store->isInStore(path.path.abs())) {
                 try {
                     StorePathSet closure;
-                    store->computeFSClosure(store->toStorePath(path).first, closure);
+                    store->computeFSClosure(store->toStorePath(path.path.abs()).first, closure);
                     for (auto & p : closure)
                         allowPath(p);
                 } catch (InvalidPath &) { }
             }
         }
 
-        if (pathExists(path))
+        if (path.pathExists())
             return finish(std::move(path));
         else {
             logWarning({
@@ -3116,7 +3118,6 @@ std::optional<std::string> EvalState::resolveLookupPathPath(const LookupPath::Pa
 
     debug("failed to resolve search path element '%s'", value);
     return std::nullopt;
-
 }
 
 

src/libexpr/eval.hh

@@ -91,7 +91,7 @@ struct PrimOp
     const char * doc = nullptr;
 
     /**
-     * Add a trace item, `while calling the '<name>' builtin`
+     * Add a trace item, while calling the `<name>` builtin.
      *
      * This is used to remove the redundant item for `builtins.addErrorContext`.
      */
@@ -107,6 +107,11 @@ struct PrimOp
      */
     std::optional<ExperimentalFeature> experimentalFeature;
 
+    /**
+     * If true, this primop is not exposed to the user.
+     */
+    bool internal = false;
+
     /**
      * Validity check to be performed by functions that introduce primops,
      * such as RegisterPrimOp() and Value::mkPrimOp().
@@ -342,7 +347,7 @@ private:
 
     LookupPath lookupPath;
 
-    std::map<std::string, std::optional<std::string>> lookupPathResolved;
+    std::map<std::string, std::optional<SourcePath>> lookupPathResolved;
 
     /**
      * Cache used by prim_match().
@@ -447,9 +452,9 @@ public:
      *
      * If the specified search path element is a URI, download it.
      *
-     * If it is not found, return `std::nullopt`
+     * If it is not found, return `std::nullopt`.
      */
-    std::optional<std::string> resolveLookupPathPath(
+    std::optional<SourcePath> resolveLookupPathPath(
         const LookupPath::Path & elem,
         bool initAccessControl = false);
 
@@ -591,6 +596,11 @@ public:
      */
     std::shared_ptr<StaticEnv> staticBaseEnv; // !!! should be private
 
+    /**
+     * Internal primops not exposed to the user.
+     */
+    std::unordered_map<std::string, Value *, std::hash<std::string>, std::equal_to<std::string>, traceable_allocator<std::pair<const std::string, Value *>>> internalPrimOps;
+
     /**
      * Name and documentation about every constant.
      *
@@ -613,8 +623,19 @@ private:
 
 public:
 
+    /**
+     * Retrieve a specific builtin, equivalent to evaluating `builtins.${name}`.
+     * @param name The attribute name of the builtin to retrieve.
+     * @throws EvalError if the builtin does not exist.
+     */
     Value & getBuiltin(const std::string & name);
 
+    /**
+     * Retrieve the `builtins` attrset, equivalent to evaluating the reference `builtins`.
+     * Always returns an attribute set value.
+     */
+    Value & getBuiltins();
+
     struct Doc
     {
         Pos pos;
@@ -680,20 +701,19 @@ public:
 
     bool isFunctor(Value & fun);
 
-    // FIXME: use std::span
-    void callFunction(Value & fun, size_t nrArgs, Value * * args, Value & vRes, const PosIdx pos);
+    void callFunction(Value & fun, std::span<Value *> args, Value & vRes, const PosIdx pos);
 
     void callFunction(Value & fun, Value & arg, Value & vRes, const PosIdx pos)
     {
         Value * args[] = {&arg};
-        callFunction(fun, 1, args, vRes, pos);
+        callFunction(fun, args, vRes, pos);
     }
 
     /**
      * Automatically call a function for which each argument has a
      * default value or has a binding in the `args` map.
      */
-    void autoCallFunction(Bindings & args, Value & fun, Value & res);
+    void autoCallFunction(const Bindings & args, Value & fun, Value & res);
 
     /**
      * Allocation primitives.
@@ -799,7 +819,6 @@ public:
     bool callPathFilter(
         Value * filterFun,
         const SourcePath & path,
-        std::string_view pathArg,
         PosIdx pos);
 
     DocComment getDocCommentForPos(PosIdx pos);

src/libexpr/get-drvs.cc

@@ -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/local.mk

@@ -1,50 +0,0 @@
-libraries += libexpr
-
-libexpr_NAME = libnixexpr
-
-libexpr_DIR := $(d)
-
-libexpr_SOURCES := \
-  $(wildcard $(d)/*.cc) \
-  $(wildcard $(d)/value/*.cc) \
-  $(wildcard $(d)/primops/*.cc) \
-  $(d)/lexer-tab.cc \
-  $(d)/parser-tab.cc
-# Not just for this library itself, but also for downstream libraries using this library
-
-INCLUDE_libexpr := -I $(d)
-
-libexpr_CXXFLAGS += \
-	$(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers) $(INCLUDE_libexpr) \
-	-DGC_THREADS
-
-libexpr_LIBS = libutil libstore libfetchers
-
-libexpr_LDFLAGS += -lboost_context $(THREAD_LDFLAGS)
-ifdef HOST_LINUX
-  libexpr_LDFLAGS += -ldl
-endif
-
-# The dependency on libgc must be propagated (i.e. meaning that
-# programs/libraries that use libexpr must explicitly pass -lgc),
-# because inline functions in libexpr's header files call libgc.
-libexpr_LDFLAGS_PROPAGATED = $(BDW_GC_LIBS)
-
-libexpr_ORDER_AFTER := $(d)/parser-tab.cc $(d)/parser-tab.hh $(d)/lexer-tab.cc $(d)/lexer-tab.hh
-
-$(d)/parser-tab.cc $(d)/parser-tab.hh: $(d)/parser.y
-	$(trace-gen) bison -v -o $(libexpr_DIR)/parser-tab.cc $< -d
-
-$(d)/lexer-tab.cc $(d)/lexer-tab.hh: $(d)/lexer.l
-	$(trace-gen) flex --outfile $(libexpr_DIR)/lexer-tab.cc --header-file=$(libexpr_DIR)/lexer-tab.hh $<
-
-clean-files += $(d)/parser-tab.cc $(d)/parser-tab.hh $(d)/lexer-tab.cc $(d)/lexer-tab.hh
-
-$(eval $(call install-file-in, $(buildprefix)$(d)/nix-expr.pc, $(libdir)/pkgconfig, 0644))
-
-$(foreach i, $(wildcard src/libexpr/value/*.hh), \
-  $(eval $(call install-file-in, $(i), $(includedir)/nix/value, 0644)))
-
-$(d)/primops.cc: $(d)/imported-drv-to-derivation.nix.gen.hh
-
-$(d)/eval.cc: $(d)/primops/derivation.nix.gen.hh $(d)/fetchurl.nix.gen.hh $(d)/call-flake.nix.gen.hh

src/libexpr/meson.build

@@ -27,8 +27,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('build-utils-meson/subprojects')
 
-subdir('build-utils-meson/threads')
-
 boost = dependency(
   'boost',
   modules : ['container', 'context'],
@@ -79,7 +77,7 @@ add_project_arguments(
   language : 'cpp',
 )
 
-subdir('build-utils-meson/diagnostics')
+subdir('build-utils-meson/common')
 
 parser_tab = custom_target(
   input : 'parser.y',

src/libexpr/nix-expr.pc.in

@@ -1,10 +0,0 @@
-prefix=@prefix@
-libdir=@libdir@
-includedir=@includedir@
-
-Name: Nix
-Description: Nix Package Manager
-Version: @PACKAGE_VERSION@
-Requires: nix-store bdw-gc
-Libs: -L${libdir} -lnixexpr
-Cflags: -I${includedir}/nix -std=c++2a

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 \
@@ -202,7 +206,7 @@ struct ExprSelect : Expr
     /**
      * Evaluate the `a.b.c` part of `a.b.c.d`. This exists mostly for the purpose of :doc in the repl.
      *
-     * @param[out] v The attribute set that should contain the last attribute name (if it exists).
+     * @param[out] attrs The attribute set that should contain the last attribute name (if it exists).
      * @return The last attribute name in `attrPath`
      *
      * @note This does *not* evaluate the final attribute, and does not fail if that's the only attribute that does not exist.
@@ -354,10 +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
 };
 
@@ -458,6 +468,7 @@ struct ExprBlackHole : Expr
     void show(const SymbolTable & symbols, std::ostream & str) const override {}
     void eval(EvalState & state, Env & env, Value & v) override;
     void bindVars(EvalState & es, const std::shared_ptr<const StaticEnv> & env) override {}
+    [[noreturn]] static void throwInfiniteRecursionError(EvalState & state, Value & v);
 };
 
 extern ExprBlackHole eBlackHole;

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
@@ -77,6 +71,10 @@ mkMesonDerivation (finalAttrs: {
     nix-util
     nix-store
     nix-fetchers
+  ] ++ finalAttrs.passthru.externalPropagatedBuildInputs;
+
+  # Hack for sake of the dev shell
+  passthru.externalPropagatedBuildInputs = [
     boost
     nlohmann_json
   ] ++ lib.optional enableGC boehmgc;
@@ -102,10 +100,6 @@ 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
   ;
 
@@ -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;
@@ -84,6 +91,7 @@ StringMap EvalState::realiseContext(const NixStringContext & context, StorePathS
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;
+    buildReqs.reserve(drvs.size());
     for (auto & d : drvs) buildReqs.emplace_back(DerivedPath { d });
     buildStore->buildPaths(buildReqs, bmNormal, store);
 
@@ -171,6 +179,78 @@ 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 +269,13 @@ 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);
     }
 }
 
@@ -691,7 +724,7 @@ static void prim_genericClosure(EvalState & state, const PosIdx pos, Value * * a
 
         /* Call the `operator' function with `e' as argument. */
         Value newElements;
-        state.callFunction(*op->value, 1, &e, newElements, noPos);
+        state.callFunction(*op->value, {&e, 1}, newElements, noPos);
         state.forceList(newElements, noPos, "while evaluating the return value of the `operator` passed to builtins.genericClosure");
 
         /* Add the values returned by the operator to the work set. */
@@ -937,6 +970,9 @@ 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,
 });
@@ -1567,7 +1603,8 @@ static RegisterPrimOp primop_placeholder({
  *************************************************************/
 
 
-/* Convert the argument to a path.  !!! obsolete? */
+/* Convert the argument to a path and then to a string (confusing,
+   eh?).  !!! obsolete? */
 static void prim_toPath(EvalState & state, const PosIdx pos, Value * * args, Value & v)
 {
     NixStringContext context;
@@ -2129,7 +2166,7 @@ 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({
@@ -2237,7 +2274,7 @@ 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({
@@ -2401,7 +2438,6 @@ static RegisterPrimOp primop_toFile({
 bool EvalState::callPathFilter(
     Value * filterFun,
     const SourcePath & path,
-    std::string_view pathArg,
     PosIdx pos)
 {
     auto st = path.lstat();
@@ -2409,12 +2445,12 @@ bool EvalState::callPathFilter(
     /* Call the filter function.  The first argument is the path, the
        second is a string indicating the type of the file. */
     Value arg1;
-    arg1.mkString(pathArg);
+    arg1.mkString(path.path.abs());
 
     // assert that type is not "unknown"
     Value * args []{&arg1, fileTypeToString(*this, st.type)};
     Value res;
-    callFunction(*filterFun, 2, args, res, pos);
+    callFunction(*filterFun, args, res, pos);
 
     return forceBool(res, pos, "while evaluating the return value of the path filter function");
 }
@@ -2452,7 +2488,7 @@ static void addPath(
         if (filterFun)
             filter = std::make_unique<PathFilter>([&](const Path & p) {
                 auto p2 = CanonPath(p);
-                return state.callPathFilter(filterFun, {path.accessor, p2}, p2.abs(), pos);
+                return state.callPathFilter(filterFun, {path.accessor, p2}, pos);
             });
 
         std::optional<StorePath> expectedStorePath;
@@ -2578,13 +2614,13 @@ static void prim_path(EvalState & state, const PosIdx pos, Value * * args, Value
             expectedHash = newHashAllowEmpty(state.forceStringNoCtx(*attr.value, attr.pos, "while evaluating the `sha256` attribute passed to builtins.path"), HashAlgorithm::SHA256);
         else
             state.error<EvalError>(
-                "unsupported argument '%1%' to 'addPath'",
+                "unsupported argument '%1%' to 'builtins.path'",
                 state.symbols[attr.name]
             ).atPos(attr.pos).debugThrow();
     }
     if (!path)
         state.error<EvalError>(
-            "missing required 'path' attribute in the first argument to builtins.path"
+            "missing required 'path' attribute in the first argument to 'builtins.path'"
         ).atPos(pos).debugThrow();
     if (name.empty())
         name = path->baseName();
@@ -3451,7 +3487,7 @@ static void prim_foldlStrict(EvalState & state, const PosIdx pos, Value * * args
         for (auto [n, elem] : enumerate(args[2]->listItems())) {
             Value * vs []{vCur, elem};
             vCur = n == args[2]->listSize() - 1 ? &v : state.allocValue();
-            state.callFunction(*args[0], 2, vs, *vCur, pos);
+            state.callFunction(*args[0], vs, *vCur, pos);
         }
         state.forceValue(v, pos);
     } else {
@@ -3601,7 +3637,7 @@ static void prim_sort(EvalState & state, const PosIdx pos, Value * * args, Value
 
         Value * vs[] = {a, b};
         Value vBool;
-        state.callFunction(*args[0], 2, vs, vBool, noPos);
+        state.callFunction(*args[0], vs, vBool, noPos);
         return state.forceBool(vBool, pos, "while evaluating the return value of the sorting function passed to builtins.sort");
     };
 
@@ -4268,7 +4304,7 @@ 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) {
@@ -4352,7 +4388,7 @@ 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;
@@ -4363,14 +4399,14 @@ 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);
@@ -4901,7 +4937,7 @@ void EvalState::createBaseEnv()
 
     /* Now that we've added all primops, sort the `builtins' set,
        because attribute lookups expect it to be sorted. */
-    baseEnv.values[0]->payload.attrs->sort();
+    getBuiltins().payload.attrs->sort();
 
     staticBaseEnv->sort();
 

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>
@@ -75,6 +77,7 @@ struct FetchTreeParams {
     bool emptyRevFallback = false;
     bool allowNameArgument = false;
     bool isFetchGit = false;
+    bool isFinal = false;
 };
 
 static void fetchTree(
@@ -192,6 +195,13 @@ static void fetchTree(
 
     state.checkURI(input.toURLString());
 
+    if (params.isFinal) {
+        input.attrs.insert_or_assign("__final", Explicit<bool>(true));
+    } else {
+        if (input.isFinal())
+            throw Error("input '%s' is not allowed to use the '__final' attribute", input.to_string());
+    }
+
     auto [storePath, input2] = input.fetchToStore(state.store);
 
     state.allowPath(storePath);
@@ -245,7 +255,7 @@ 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"`
@@ -428,6 +438,18 @@ static RegisterPrimOp primop_fetchTree({
     .experimentalFeature = Xp::FetchTree,
 });
 
+void prim_fetchFinalTree(EvalState & state, const PosIdx pos, Value * * args, Value & v)
+{
+    fetchTree(state, pos, args, v, {.isFinal = true});
+}
+
+static RegisterPrimOp primop_fetchFinalTree({
+    .name = "fetchFinalTree",
+    .args = {"input"},
+    .fun = prim_fetchFinalTree,
+    .internal = true,
+});
+
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,
     const std::string & who, bool unpack, std::string name)
 {

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.hh

@@ -141,7 +141,9 @@ public:
     Value * * elems;
     ListBuilder(EvalState & state, size_t size);
 
-    ListBuilder(ListBuilder && x)
+    // NOTE: Can be noexcept because we are just copying integral values and
+    // raw pointers.
+    ListBuilder(ListBuilder && x) noexcept
         : size(x.size)
         , inlineElems{x.inlineElems[0], x.inlineElems[1]}
         , elems(size <= 2 ? inlineElems : x.elems)

src/libexpr/value/context.hh

@@ -28,7 +28,7 @@ struct NixStringContextElem {
     /**
      * Plain opaque path to some store object.
      *
-     * Encoded as just the path: ‘<path>’.
+     * Encoded as just the path: `<path>`.
      */
     using Opaque = SingleDerivedPath::Opaque;
 
@@ -39,7 +39,7 @@ struct NixStringContextElem {
      * also all outputs of all derivations in that closure (including the
      * root derivation).
      *
-     * Encoded in the form ‘=<drvPath>’.
+     * Encoded in the form `=<drvPath>`.
      */
     struct DrvDeep {
         StorePath drvPath;
@@ -50,7 +50,7 @@ struct NixStringContextElem {
     /**
      * Derivation output.
      *
-     * Encoded in the form ‘!<output>!<drvPath>’.
+     * Encoded in the form `!<output>!<drvPath>`.
      */
     using Built = SingleDerivedPath::Built;
 
@@ -68,9 +68,9 @@ struct NixStringContextElem {
 
     /**
      * Decode a context string, one of:
-     * - ‘<path>’
-     * - ‘=<path>’
-     * - ‘!<name>!<path>’
+     * - `<path>`
+     * - `=<path>`
+     * - `!<name>!<path>`
      *
      * @param xpSettings Stop-gap to avoid globals during unit tests.
      */

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->flush());
+    auto accessor = repo->getAccessor(result, 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/meson.build

@@ -0,0 +1,72 @@
+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/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/common')
+
+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() + "/fetcher-cache-v2.sqlite";
+        auto dbPath = getCacheDir() + "/fetcher-cache-v3.sqlite";
         createDirs(dirOf(dbPath));
 
         state->db = SQLite(dbPath);

src/libfetchers/fetch-to-store.cc

@@ -44,6 +44,8 @@ StorePath fetchToStore(
         : store.addToStore(
             name, path, method, HashAlgorithm::SHA256, {}, filter2, repair);
 
+    debug(mode == FetchMode::DryRun ? "hashed '%s'" : "copied '%s' to '%s'", path, store.printStorePath(storePath));
+
     if (cacheKey && mode == FetchMode::Copy)
         fetchers::getCache()->upsert(*cacheKey, store, {}, storePath);
 

src/libfetchers/fetchers.cc

@@ -3,6 +3,7 @@
 #include "source-path.hh"
 #include "fetch-to-store.hh"
 #include "json-utils.hh"
+#include "store-path-accessor.hh"
 
 #include <nlohmann/json.hpp>
 
@@ -100,7 +101,7 @@ Input Input::fromAttrs(const Settings & settings, Attrs && attrs)
     auto allowedAttrs = inputScheme->allowedAttrs();
 
     for (auto & [name, _] : attrs)
-        if (name != "type" && allowedAttrs.count(name) == 0)
+        if (name != "type" && name != "__final" && allowedAttrs.count(name) == 0)
             throw Error("input attribute '%s' not supported by scheme '%s'", name, schemeName);
 
     auto res = inputScheme->inputFromAttrs(settings, attrs);
@@ -145,6 +146,11 @@ bool Input::isLocked() const
     return scheme && scheme->isLocked(*this);
 }
 
+bool Input::isFinal() const
+{
+    return maybeGetBoolAttr(attrs, "__final").value_or(false);
+}
+
 std::optional<std::string> Input::isRelative() const
 {
     assert(scheme);
@@ -178,16 +184,24 @@ std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
 
     auto [storePath, input] = [&]() -> std::pair<StorePath, Input> {
         try {
-            auto [accessor, final] = getAccessorUnchecked(store);
+            auto [accessor, result] = getAccessorUnchecked(store);
 
-            auto storePath = nix::fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, final.getName());
+            auto storePath = nix::fetchToStore(*store, SourcePath(accessor), FetchMode::Copy, result.getName());
 
             auto narHash = store->queryPathInfo(storePath)->narHash;
-            final.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
+            result.attrs.insert_or_assign("narHash", narHash.to_string(HashFormat::SRI, true));
 
-            scheme->checkLocks(*this, final);
+            // FIXME: we would like to mark inputs as final in
+            // getAccessorUnchecked(), but then we can't add
+            // narHash. Or maybe narHash should be excluded from the
+            // concept of "final" inputs?
+            result.attrs.insert_or_assign("__final", Explicit<bool>(true));
 
-            return {storePath, final};
+            assert(result.isFinal());
+
+            checkLocks(*this, result);
+
+            return {storePath, result};
         } catch (Error & e) {
             e.addTrace({}, "while fetching the input '%s'", to_string());
             throw;
@@ -197,13 +211,40 @@ std::pair<StorePath, Input> Input::fetchToStore(ref<Store> store) const
     return {std::move(storePath), input};
 }
 
-void InputScheme::checkLocks(const Input & specified, const Input & final) const
+void Input::checkLocks(Input specified, Input & result)
 {
+    /* If the original input is final, then we just return the
+       original attributes, dropping any new fields returned by the
+       fetcher. However, any fields that are in both the specified and
+       result input must be identical. */
+    if (specified.isFinal()) {
+
+        /* Backwards compatibility hack: we had some lock files in the
+           past that 'narHash' fields with incorrect base-64
+           formatting (lacking the trailing '=', e.g. 'sha256-ri...Mw'
+           instead of ''sha256-ri...Mw='). So fix that. */
+        if (auto prevNarHash = specified.getNarHash())
+            specified.attrs.insert_or_assign("narHash", prevNarHash->to_string(HashFormat::SRI, true));
+
+        for (auto & field : specified.attrs) {
+            auto field2 = result.attrs.find(field.first);
+            if (field2 != result.attrs.end() && field.second != field2->second)
+                throw Error("mismatch in field '%s' of input '%s', got '%s'",
+                    field.first,
+                    attrsToJSON(specified.attrs),
+                    attrsToJSON(result.attrs));
+        }
+
+        result.attrs = specified.attrs;
+
+        return;
+    }
+
     if (auto prevNarHash = specified.getNarHash()) {
-        if (final.getNarHash() != prevNarHash) {
-            if (final.getNarHash())
+        if (result.getNarHash() != prevNarHash) {
+            if (result.getNarHash())
                 throw Error((unsigned int) 102, "NAR hash mismatch in input '%s', expected '%s' but got '%s'",
-                    specified.to_string(), prevNarHash->to_string(HashFormat::SRI, true), final.getNarHash()->to_string(HashFormat::SRI, true));
+                    specified.to_string(), prevNarHash->to_string(HashFormat::SRI, true), result.getNarHash()->to_string(HashFormat::SRI, true));
             else
                 throw Error((unsigned int) 102, "NAR hash mismatch in input '%s', expected '%s' but got none",
                     specified.to_string(), prevNarHash->to_string(HashFormat::SRI, true));
@@ -211,32 +252,32 @@ void InputScheme::checkLocks(const Input & specified, const Input & final) const
     }
 
     if (auto prevLastModified = specified.getLastModified()) {
-        if (final.getLastModified() != prevLastModified)
-            throw Error("'lastModified' attribute mismatch in input '%s', expected %d",
-                final.to_string(), *prevLastModified);
+        if (result.getLastModified() != prevLastModified)
+            throw Error("'lastModified' attribute mismatch in input '%s', expected %d, got %d",
+                result.to_string(), *prevLastModified, result.getLastModified().value_or(-1));
     }
 
     if (auto prevRev = specified.getRev()) {
-        if (final.getRev() != prevRev)
+        if (result.getRev() != prevRev)
             throw Error("'rev' attribute mismatch in input '%s', expected %s",
-                final.to_string(), prevRev->gitRev());
+                result.to_string(), prevRev->gitRev());
     }
 
     if (auto prevRevCount = specified.getRevCount()) {
-        if (final.getRevCount() != prevRevCount)
+        if (result.getRevCount() != prevRevCount)
             throw Error("'revCount' attribute mismatch in input '%s', expected %d",
-                final.to_string(), *prevRevCount);
+                result.to_string(), *prevRevCount);
     }
 }
 
 std::pair<ref<SourceAccessor>, Input> Input::getAccessor(ref<Store> store) const
 {
     try {
-        auto [accessor, final] = getAccessorUnchecked(store);
+        auto [accessor, result] = getAccessorUnchecked(store);
 
-        scheme->checkLocks(*this, final);
+        checkLocks(*this, result);
 
-        return {accessor, std::move(final)};
+        return {accessor, std::move(result)};
     } catch (Error & e) {
         e.addTrace({}, "while fetching the input '%s'", to_string());
         throw;
@@ -250,12 +291,42 @@ std::pair<ref<SourceAccessor>, Input> Input::getAccessorUnchecked(ref<Store> sto
     if (!scheme)
         throw Error("cannot fetch unsupported input '%s'", attrsToJSON(toAttrs()));
 
-    auto [accessor, final] = scheme->getAccessor(store, *this);
+    /* 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. We only do this for final
+       inputs, otherwise there is a risk that we don't return the
+       same attributes (like `lastModified`) that the "real" fetcher
+       would return.
+
+       FIXME: add a setting to disable this.
+       FIXME: substituting may be slower than fetching normally,
+       e.g. for fetchers like Git that are incremental!
+    */
+    if (isFinal() && getNarHash()) {
+        try {
+            auto storePath = computeStorePath(*store);
+
+            store->ensurePath(storePath);
+
+            debug("using substituted/cached input '%s' in '%s'",
+                to_string(), store->printStorePath(storePath));
+
+            auto accessor = makeStorePathAccessor(store, storePath);
+
+            accessor->fingerprint = scheme->getFingerprint(store, *this);
+
+            return {accessor, *this};
+        } catch (Error & e) {
+            debug("substitution of input '%s' failed: %s", to_string(), e.what());
+        }
+    }
+
+    auto [accessor, result] = scheme->getAccessor(store, *this);
 
     assert(!accessor->fingerprint);
-    accessor->fingerprint = scheme->getFingerprint(store, final);
+    accessor->fingerprint = scheme->getFingerprint(store, result);
 
-    return {accessor, std::move(final)};
+    return {accessor, std::move(result)};
 }
 
 Input Input::applyOverrides(
@@ -410,7 +481,10 @@ namespace nlohmann {
 
 using namespace nix;
 
-fetchers::PublicKey adl_serializer<fetchers::PublicKey>::from_json(const json & json) {
+#ifndef DOXYGEN_SKIP
+
+fetchers::PublicKey adl_serializer<fetchers::PublicKey>::from_json(const json & json)
+{
     fetchers::PublicKey res = { };
     if (auto type = optionalValueAt(json, "type"))
         res.type = getString(*type);
@@ -420,9 +494,12 @@ fetchers::PublicKey adl_serializer<fetchers::PublicKey>::from_json(const json &
     return res;
 }
 
-void adl_serializer<fetchers::PublicKey>::to_json(json & json, fetchers::PublicKey p) {
+void adl_serializer<fetchers::PublicKey>::to_json(json & json, fetchers::PublicKey p)
+{
     json["type"] = p.type;
     json["key"] = p.key;
 }
 
+#endif
+
 }

src/libfetchers/fetchers.hh

@@ -73,14 +73,15 @@ public:
     Attrs toAttrs() const;
 
     /**
-     * Check whether this is a "direct" input, that is, not
+     * Return whether this is a "direct" input, that is, not
      * one that goes through a registry.
      */
     bool isDirect() const;
 
     /**
-     * Check whether this is a "locked" input, that is,
-     * one that contains a commit hash or content hash.
+     * Return whether this is a "locked" input, that is, it has
+     * attributes like a Git revision or NAR hash that uniquely
+     * identify its contents.
      */
     bool isLocked() const;
 
@@ -90,6 +91,18 @@ public:
      */
     std::optional<std::string> isRelative() const;
 
+    /**
+     * Return whether this is a "final" input, meaning that fetching
+     * it will not add, remove or change any attributes. (See
+     * `checkLocks()` for the semantics.) Only "final" inputs can be
+     * substituted from a binary cache.
+     *
+     * The "final" state is denoted by the presence of an attribute
+     * `__final = true`. This attribute is currently undocumented and
+     * for internal use only.
+     */
+    bool isFinal() const;
+
     bool operator ==(const Input & other) const noexcept;
 
     bool contains(const Input & other) const;
@@ -100,6 +113,19 @@ public:
      */
     std::pair<StorePath, Input> fetchToStore(ref<Store> store) const;
 
+    /**
+     * Check the locking attributes in `result` against
+     * `specified`. E.g. if `specified` has a `rev` attribute, then
+     * `result` must have the same `rev` attribute. Throw an exception
+     * if there is a mismatch.
+     *
+     * If `specified` is marked final (i.e. has the `__final`
+     * attribute), then the intersection of attributes in `specified`
+     * and `result` must be equal, and `final.attrs` is set to
+     * `specified.attrs` (i.e. we discard any new attributes).
+     */
+    static void checkLocks(Input specified, Input & result);
+
     /**
      * Return a `SourceAccessor` that allows access to files in the
      * input without copying it to the store. Also return a possibly
@@ -145,6 +171,10 @@ public:
     /**
      * For locked inputs, return a string that uniquely specifies the
      * content of the input (typically a commit hash or content hash).
+     *
+     * Only known-equivalent inputs should return the same fingerprint.
+     *
+     * This is not a stable identifier between Nix versions, but not guaranteed to change either.
      */
     std::optional<std::string> getFingerprint(ref<Store> store) const;
 };
@@ -216,32 +246,12 @@ struct InputScheme
     virtual bool isDirect(const Input & input) const
     { return true; }
 
-    /**
-     * A sufficiently unique string that can be used as a cache key to identify the `input`.
-     *
-     * Only known-equivalent inputs should return the same fingerprint.
-     *
-     * This is not a stable identifier between Nix versions, but not guaranteed to change either.
-     */
     virtual std::optional<std::string> getFingerprint(ref<Store> store, const Input & input) const
     { return std::nullopt; }
 
-    /**
-     * Return `true` if this input is considered "locked", i.e. it has
-     * attributes like a Git revision or NAR hash that uniquely
-     * identify its contents.
-     */
     virtual bool isLocked(const Input & input) const
     { return false; }
 
-    /**
-     * Check the locking attributes in `final` against
-     * `specified`. E.g. if `specified` has a `rev` attribute, then
-     * `final` must have the same `rev` attribute. Throw an exception
-     * if there is a mismatch.
-     */
-    virtual void checkLocks(const Input & specified, const Input & final) const;
-
     virtual std::optional<std::string> isRelative(const Input & input) const
     { return std::nullopt; }
 };

src/libfetchers/git-utils.cc

@@ -205,6 +205,31 @@ static git_packbuilder_progress PACKBUILDER_PROGRESS_CHECK_INTERRUPT = &packBuil
 
 } // 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) {
+        // Someone may race us to create the repository.
+        if (e.code() == std::errc::file_exists
+            // `path` may be attempted to be deleted by s::f::rename, in which case the code is:
+            || e.code() == std::errc::directory_not_empty) {
+            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. */
@@ -226,13 +251,9 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
     {
         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()))
@@ -583,7 +604,13 @@ 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 + ")";
         }
@@ -954,8 +981,24 @@ 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) });
     };

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>

src/libfetchers/local.mk

@@ -1,17 +0,0 @@
-libraries += libfetchers
-
-libfetchers_NAME = libnixfetchers
-
-libfetchers_DIR := $(d)
-
-libfetchers_SOURCES := $(wildcard $(d)/*.cc)
-
-# Not just for this library itself, but also for downstream libraries using this library
-
-INCLUDE_libfetchers := -I $(d)
-
-libfetchers_CXXFLAGS += $(INCLUDE_libutil) $(INCLUDE_libstore) $(INCLUDE_libfetchers)
-
-libfetchers_LDFLAGS += $(THREAD_LDFLAGS) $(LIBGIT2_LIBS) -larchive
-
-libfetchers_LIBS = libutil libstore

src/libfetchers/meson.build

@@ -26,8 +26,6 @@ deps_public_maybe_subproject = [
 ]
 subdir('build-utils-meson/subprojects')
 
-subdir('build-utils-meson/threads')
-
 nlohmann_json = dependency('nlohmann_json', version : '>= 3.9')
 deps_public += nlohmann_json
 
@@ -43,7 +41,7 @@ add_project_arguments(
   language : 'cpp',
 )
 
-subdir('build-utils-meson/diagnostics')
+subdir('build-utils-meson/common')
 
 sources = files(
   'attrs.cc',
@@ -52,15 +50,15 @@ sources = files(
   'fetch-to-store.cc',
   'fetchers.cc',
   'filtering-source-accessor.cc',
-  'git.cc',
   'git-utils.cc',
+  'git.cc',
   'github.cc',
   'indirect.cc',
   'mercurial.cc',
   'mounted-source-accessor.cc',
   'path.cc',
-  'store-path-accessor.cc',
   'registry.cc',
+  'store-path-accessor.cc',
   'tarball.cc',
 )
 
@@ -71,10 +69,10 @@ headers = files(
   'cache.hh',
   'fetch-settings.hh',
   'fetch-to-store.hh',
+  'fetchers.hh',
   'filtering-source-accessor.hh',
   'git-utils.hh',
   'mounted-source-accessor.hh',
-  'fetchers.hh',
   'registry.hh',
   'store-path-accessor.hh',
   'tarball.hh',

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

@@ -72,6 +72,7 @@ struct PathInputScheme : InputScheme
         auto query = attrsToQuery(input.attrs);
         query.erase("path");
         query.erase("type");
+        query.erase("__final");
         return ParsedURL {
             .scheme = "path",
             .path = getStrAttr(input.attrs, "path"),
@@ -140,7 +141,11 @@ 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/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);
     }

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/meson.build

@@ -0,0 +1,73 @@
+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/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/common')
+
+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;
+  };
+
+})