libexpr: throw a more helpful eval-error if a builtin is not available due to a missing feature-flag

I found it somewhat confusing to have an error like

    error: attribute 'getFlake' missing

if the required experimental-feature (`flakes`) is not enabled. Instead,
I'd expect Nix to throw an error just like it's the case when using e.g. `nix
flake` without `flakes` being enabled.

With this change, the error looks like this:

    $ nix-instantiate -E 'builtins.getFlake "nixpkgs"'
    error: Cannot call 'builtins.getFlake' because experimental Nix feature 'flakes' is disabled. You can enable it via '--extra-experimental-features flakes'.

           at «string»:1:1:

                1| builtins.getFlake "nixpkgs"
                 | ^

I didn't use `settings.requireExperimentalFeature` here on purpose
because this doesn't contain a position. Also, it doesn't seem as if we
need to catch the error and check for the missing feature here since
this already happens at evaluation time.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/libexpr/flake/flake.cc b/src/libexpr/flake/flake.cc
index 492b47115..1a1fa6938 100644
--- a/src/libexpr/flake/flake.cc
+++ b/src/libexpr/flake/flake.cc
@@ -688,6 +688,8 @@ void callFlake(EvalState & state,
 
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
+    state.requireExperimentalFeatureOnEvaluation("flakes", "builtins.getFlake", pos);
+
     auto flakeRefS = state.forceStringNoCtx(*args[0], pos);
     auto flakeRef = parseFlakeRef(flakeRefS, {}, true);
     if (evalSettings.pureEval && !flakeRef.input.isImmutable())
@@ -703,7 +705,7 @@ static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Va
         v);
 }
 
-static RegisterPrimOp r2("__getFlake", 1, prim_getFlake, "flakes");
+static RegisterPrimOp r2("__getFlake", 1, prim_getFlake);
 
 }