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

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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.
This commit is contained in:
Maximilian Bosch 2021-09-28 22:12:41 +02:00
parent a0bb5c4130
commit 2b02ce0e48
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GPG key ID: 091DBF4D1FC46B8E
5 changed files with 35 additions and 15 deletions
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4
src/libexpr/flake/flake.cc
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@ -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);
}