Make fetchTree locked input error message clearer

[jvns]

Motivation

Today I was trying to use Nix and I got the error message

       error: in pure evaluation mode, 'fetchTree' requires a locked input, at «none»:0

This was completely baffling to me -- what's "pure evaluation mode"? What's fetchTree? What's a locked input? It didn't mention any line number in my code, so I couldn't figure out which part of my code it even related to.

Someone on the Discord helped me understand that the problem was in my call to fetchGit (I'd forgotten to set rev = ..., because the fetchGit builtin calls fetchTree.

So here's an idea for how to improve the error message, to mention fetchGit so that users can figure out which function the error message is coming from. It's a little messy but seems potentially worth it to help people out.

Hopefully it compiles, I don't have a toolchain set up.

Priorities

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[roberth]

I don't have a toolchain set up.

That should be easy with nix develop. See https://nixos.org/manual/nix/stable/contributing/hacking#building-nix-with-flakes

[roberth]

Test cases in the form of tests/functional/lang/eval-fail-* seem like the easiest way to add a test for this error message.

[thufschmitt]

Discussed during the Nix maintainers meeting on 2024-01-22. Approved and merged.

- Idea is good

• Oops @Ericson2314 didn't notice that @roberth had asked for tests

• Still want those tests.

• @roberth will write the tests because contributor may have disappeared.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2024-01-22-nix-team-meeting-minutes-117/38838/1