From fd0ed7511818ba871dc3e28796ec1d0ca57b22ec Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 7 Dec 2022 15:23:01 +0100
Subject: [PATCH] Support flake references in the old CLI

Fixes #7026.
---
 doc/manual/src/release-notes/rl-next.md |  9 +++++++++
 src/libcmd/common-eval-args.cc          | 15 +++++++++++++--
 src/libexpr/eval.cc                     |  3 ++-
 src/libexpr/parser.y                    | 13 ++++++++++++-
 tests/flakes/flakes.sh                  |  6 ++++++
 5 files changed, 42 insertions(+), 4 deletions(-)

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index c1f4de76f..160537f41 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -2,3 +2,12 @@
 
 * The `repeat` and `enforce-determinism` options have been removed
   since they had been broken under many circumstances for a long time.
+
+* 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
+  ```
diff --git a/src/libcmd/common-eval-args.cc b/src/libcmd/common-eval-args.cc
index 782a25964..64be2629b 100644
--- a/src/libcmd/common-eval-args.cc
+++ b/src/libcmd/common-eval-args.cc
@@ -146,10 +146,21 @@ Path lookupFileArg(EvalState & state, std::string_view s)
         auto storePath = fetchers::downloadTarball(
             state.store, EvalSettings::resolvePseudoUrl(s), "source", false).first.storePath;
         return state.store->toRealPath(storePath);
-    } else if (s.size() > 2 && s.at(0) == '<' && s.at(s.size() - 1) == '>') {
+    }
+
+    else if (hasPrefix(s, "flake:")) {
+        settings.requireExperimentalFeature(Xp::Flakes);
+        auto flakeRef = parseFlakeRef(std::string(s.substr(6)), {}, true, false);
+        auto storePath = flakeRef.resolve(state.store).fetchTree(state.store).first.storePath;
+        return state.store->toRealPath(storePath);
+    }
+
+    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 absPath(std::string(s));
 }
 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index ca8d634da..6955aacbf 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -402,7 +402,8 @@ 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;
             }
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index 6ef9407cd..fbf865719 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -643,6 +643,7 @@ formal
 #include "filetransfer.hh"
 #include "fetchers.hh"
 #include "store-api.hh"
+#include "flake/flake.hh"
 
 
 namespace nix {
@@ -816,7 +817,17 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
             });
             res = { false, "" };
         }
-    } else {
+    }
+
+    else if (hasPrefix(elem.second, "flake:")) {
+        settings.requireExperimentalFeature(Xp::Flakes);
+        auto flakeRef = parseFlakeRef(elem.second.substr(6), {}, true, false);
+        debug("fetching flake search path element '%s''", elem.second);
+        auto storePath = flakeRef.resolve(store).fetchTree(store).first.storePath;
+        res = { true, store->toRealPath(storePath) };
+    }
+
+    else {
         auto path = absPath(elem.second);
         if (pathExists(path))
             res = { true, path };
diff --git a/tests/flakes/flakes.sh b/tests/flakes/flakes.sh
index 8cdc320fb..5ef4d0a7a 100644
--- a/tests/flakes/flakes.sh
+++ b/tests/flakes/flakes.sh
@@ -473,3 +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 ]]