Add warn-short-path-literals setting

[oknyshuk]

Add warn-short-path-literals setting

Closes: #13374

Problem

Path literals that don't start with ./ or ../ (like foo/bar) can be confusing and less explicit than their prefixed counterparts. Even experienced Nix developers are sometimes unaware that bare path literals are valid syntax, leading to inconsistent code style and potential confusion about the role of path expressions.

Solution

This PR adds a new warn-short-path-literals setting to EvalSettings that emits warnings when encountering relative path literals that don't start with . or /. When enabled, expressions like foo/bar will suggest using ./foo/bar instead.

The implementation follows the same pattern as the existing no-url-literals experimental feature but uses a regular setting instead, as requested in the issue.

Examples

Without the setting (default behavior):

$ nix eval --expr 'foo/bar'
/current/path/foo/bar

With the setting enabled:

$ nix eval --expr 'foo/bar' --warn-short-path-literals
warning: Short path literal 'foo/bar' is deprecated. Use './foo/bar' instead.
         at «string»:1:1:
              1| foo/bar
               | ^
/current/path/foo/bar

P.S.: Sorry if I've got something wrong, it's my first contribution to OSS in a while, and first one to the Nix project. Thanks.

[roberth]

We'd much appreciate a test case as well. I see that no-url-literals is not tested unfortunately. The way we'd do it is to add it to tests/functional.
Could you help with that?

[oknyshuk]

Could you help with that?

Never done tests in Meson, but I can try to. Should I commit the test for warn-short-path-literals here and the one for no-url-literals in the issue You opened?

[roberth]

@k1gen Sounds good to me.

Unrelated: looks like the formatter needs to be run on the current diff.

[oknyshuk]

Unrelated: looks like the formatter needs to be run on the current diff.

Yeah, my bad. Thought I did that...

How does the test look?

[oknyshuk]

Oops. Works on my machine:

[olk@think:~/dev/nix/build]$ ../outputs/out/bin/nix eval --expr 'test/subdir'
/home/olk/dev/nix/build/test/subdir
[olk@think:~/dev/nix]$ checkPhase
...
 65/205 nix-functional-tests:main / short-path-literals                              OK              0.84s
...
Ok:                 199
Fail:               0

Any ideas, @roberth?

[oknyshuk]

The test passes even if I don't create and populate test directories, should I remove this part in order not to litter in the tests dir?

# Create test directories
mkdir -p "$TEST_ROOT/test/subdir"
echo "test content" > "$TEST_ROOT/test/file.txt"

[roberth]

should I remove this part

That'd be good, thanks!

[roberth]

Thanks again!

[nixos-discourse]

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

https://discourse.nixos.org/t/nix-2-31-0-released/68465/1