Support flake references in the old CLI

Fixes #7026.

doc/manual/src/command-ref/env-common.md

@@ -8,41 +8,10 @@ Most Nix commands interpret the following environment variables:
 
   - [`NIX_PATH`]{#env-NIX_PATH}\
     A colon-separated list of directories used to look up Nix
-    expressions enclosed in angle brackets (i.e., `<path>`). For
-    instance, the value
-
-        /home/eelco/Dev:/etc/nixos
-
-    will cause Nix to look for paths relative to `/home/eelco/Dev` and
-    `/etc/nixos`, in this order. It is also possible to match paths
-    against a prefix. For example, the value
-
-        nixpkgs=/home/eelco/Dev/nixpkgs-branch:/etc/nixos
-
-    will cause Nix to search for `<nixpkgs/path>` in
-    `/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.
-
-    If a path in the Nix search path starts with `http://` or
-    `https://`, it is interpreted as the URL of a tarball that will be
-    downloaded and unpacked to a temporary location. The tarball must
-    consist of a single top-level directory. For example, setting
-    `NIX_PATH` to
-
-        nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
-
-    tells Nix to download and use the current contents of the
-    `master` branch in the `nixpkgs` repository.
-
-    The URLs of the tarballs from the official nixos.org channels (see
-    [the manual for `nix-channel`](nix-channel.md)) can be abbreviated
-    as `channel:<channel-name>`.  For instance, the following two
-    values of `NIX_PATH` are equivalent:
-
-        nixpkgs=channel:nixos-21.05
-        nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
-
-    The Nix search path can also be extended using the `-I` option to
-    many Nix commands, which takes precedence over `NIX_PATH`.
+    expressions enclosed in angle brackets (i.e., `<path>`),
+     e.g. `/home/eelco/Dev:/etc/nixos`. It can be extended using the
+    `-I` option. For more information about the semantics of the Nix
+    search path, see the documentation for `-I`.
 
   - [`NIX_IGNORE_SYMLINK_STORE`]{#env-NIX_IGNORE_SYMLINK_STORE}\
     Normally, the Nix store directory (typically `/nix/store`) is not

doc/manual/src/release-notes/rl-next.md

@@ -1,2 +1,10 @@
 # Release X.Y (202?-??-??)
 
+* You can now use flake references in the old CLI, e.g.
+
+  ```
+  # nix-build flake:nixpkgs -A hello
+  # nix-build -I nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05 \
+      '<nixpkgs>' -A hello
+  # NIX_PATH=nixpkgs=flake:nixpkgs nix-build '<nixpkgs>' -A hello
+  ```

src/libcmd/common-eval-args.cc

@@ -36,7 +36,68 @@ MixEvalArgs::MixEvalArgs()
     addFlag({
         .longName = "include",
         .shortName = 'I',
-        .description = "Add *path* to the list of locations used to look up `<...>` file names.",
+        .description = R"(
+  Add *path* to the Nix search path. The Nix search path is
+  initialized from the colon-separated `NIX_PATH` environment
+  variable, and is used to look up Nix expressions enclosed in angle
+  brackets (i.e., `<nixpkgs>`). For instance, if the Nix search path
+  consists of the entries
+
+  ```
+  /home/eelco/Dev
+  /etc/nixos
+  ```
+
+  Nix will look for paths relative to `/home/eelco/Dev` and
+  `/etc/nixos`, in this order. It is also possible to match paths
+  against a prefix. For example, the search path
+
+  ```
+  nixpkgs=/home/eelco/Dev/nixpkgs-branch
+  /etc/nixos
+  ```
+
+  will cause Nix to search for `<nixpkgs/path>` in
+  `/home/eelco/Dev/nixpkgs-branch/path` and `/etc/nixos/nixpkgs/path`.
+
+  If a path in the Nix search path starts with `http://` or `https://`,
+  it is interpreted as the URL of a tarball that will be downloaded and
+  unpacked to a temporary location. The tarball must consist of a single
+  top-level directory. For example, setting `NIX_PATH` to
+
+  ```
+  nixpkgs=https://github.com/NixOS/nixpkgs/archive/master.tar.gz
+  ```
+
+  tells Nix to download and use the current contents of the `master`
+  branch in the `nixpkgs` repository.
+
+  The URLs of the tarballs from the official `nixos.org` channels
+  (see [the manual page for `nix-channel`](nix-channel.md)) can be
+  abbreviated as `channel:<channel-name>`.  For instance, the
+  following two values of `NIX_PATH` are equivalent:
+
+  ```
+  nixpkgs=channel:nixos-21.05
+  nixpkgs=https://nixos.org/channels/nixos-21.05/nixexprs.tar.xz
+  ```
+
+  You can also use refer to source trees looked up in the flake
+  registry. For instance,
+
+  ```
+  nixpkgs=flake:nixpkgs
+  ```
+
+  specifies that the prefix `nixpkgs` shall refer to the source tree
+  downloaded from the `nixpkgs` entry in the flake registry. Similarly,
+
+  ```
+  nixpkgs=flake:github:NixOS/nixpkgs/nixos-22.05
+
+  makes `<nixpkgs>` refer to a particular branch of the
+  `NixOS/nixpkgs` repository on GitHub.
+  ```)",
         .category = category,
         .labels = {"path"},
         .handler = {[&](std::string s) { searchPath.push_back(s); }}
@@ -98,11 +159,21 @@ SourcePath lookupFileArg(EvalState & state, std::string_view s)
             state.store, EvalSettings::resolvePseudoUrl(s), "source", false).first;
         auto accessor = makeStorePathAccessor(state.store, storePath);
         state.registerAccessor(accessor);
-        return {accessor, CanonPath::root};
-    } else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
+        return accessor->root();
+    }
+
+    else if (hasPrefix(s, "flake:")) {
+        auto flakeRef = parseFlakeRef(std::string(s.substr(6)), {}, true, false);
+        auto [accessor, _] = flakeRef.resolve(state.store).lazyFetch(state.store);
+        return accessor->root();
+    }
+
+    else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
         Path p(s.substr(1, s.size() - 2));
         return state.findFile(p);
-    } else
+    }
+
+    else
         return state.rootPath(absPath(std::string(s)));
 }
 

src/libexpr/eval.cc

@@ -405,7 +405,8 @@ static Strings parseNixPath(const std::string & s)
         }
 
         if (*p == ':') {
-            if (EvalSettings::isPseudoUrl(std::string(start2, s.end()))) {
+            auto prefix = std::string(start2, s.end());
+            if (EvalSettings::isPseudoUrl(prefix) || hasPrefix(prefix, "flake:")) {
                 ++p;
                 while (p != s.end() && *p != ':') ++p;
             }

src/libexpr/parser.y

@@ -637,6 +637,7 @@ formal
 #include "fs-input-accessor.hh"
 #include "tarball.hh"
 #include "store-api.hh"
+#include "flake/flake.hh"
 
 
 namespace nix {
@@ -789,13 +790,22 @@ std::optional<SourcePath> EvalState::resolveSearchPathElem(const SearchPathElem
                 store, EvalSettings::resolvePseudoUrl(elem.second), "source", false).first;
             auto accessor = makeStorePathAccessor(store, storePath);
             registerAccessor(accessor);
-            res.emplace(SourcePath {accessor, CanonPath::root});
+            res.emplace(accessor->root());
         } catch (FileTransferError & e) {
             logWarning({
                 .msg = hintfmt("Nix search path entry '%1%' cannot be downloaded, ignoring", elem.second)
             });
         }
-    } else {
+    }
+
+    else if (hasPrefix(elem.second, "flake:")) {
+        auto flakeRef = parseFlakeRef(elem.second.substr(6), {}, true, false);
+        debug("fetching flake search path element '%s''", elem.second);
+        auto [accessor, _] = flakeRef.resolve(store).lazyFetch(store);
+        res.emplace(accessor->root());
+    }
+
+    else {
         auto path = rootPath(absPath(elem.second));
 
         /* Allow access to paths in the search path. */

tests/flakes/flakes.sh

@@ -53,7 +53,11 @@ cat > $flake3Dir/flake.nix <<EOF
 }
 EOF
 
-git -C $flake3Dir add flake.nix
+cat > $flake3Dir/default.nix <<EOF
+{ x = 123; }
+EOF
+
+git -C $flake3Dir add flake.nix default.nix
 git -C $flake3Dir commit -m 'Initial'
 
 cat > $nonFlakeDir/README.md <<EOF
@@ -461,7 +465,7 @@ nix flake lock $flake3Dir --update-input flake2/flake1
 # Test 'nix flake metadata --json'.
 nix flake metadata $flake3Dir --json | jq .
 
-# Test flake in store does not evaluate
+# Test flake in store does not evaluate.
 rm -rf $badFlakeDir
 mkdir $badFlakeDir
 echo INVALID > $badFlakeDir/flake.nix
@@ -469,3 +473,9 @@ nix store delete $(nix store add-path $badFlakeDir)
 
 [[ $(nix path-info      $(nix store add-path $flake1Dir)) =~ flake1 ]]
 [[ $(nix path-info path:$(nix store add-path $flake1Dir)) =~ simple ]]
+
+# Test fetching flakerefs in the legacy CLI.
+[[ $(nix-instantiate --eval flake:flake3 -A x) = 123 ]]
+[[ $(nix-instantiate --eval flake:git+file://$flake3Dir -A x) = 123 ]]
+[[ $(nix-instantiate -I flake3=flake:flake3 --eval '<flake3>' -A x) = 123 ]]
+[[ $(NIX_PATH=flake3=flake:flake3 nix-instantiate --eval '<flake3>' -A x) = 123 ]]