[bug] parse_shebang can return an empty list

[feliblo]

Summary

I encountered a panic when running against the ardupilot repository in this PR #915

Insrc/identity.rs this line caused prek to panic

    tags.extend(tags_from_filename(path));
    if executable {
        if let Ok(shebang) = parse_shebang(path) {
            tags.extend(tags_from_interpreter(&shebang[0]));
        }
    }

output:

prek run check-executables-have-shebangs --all-files --verbose
thread '<unnamed>' panicked at src/identify.rs:767:55:
index out of bounds: the len is 0 but the index is 0
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

It panics because of illegal access of the first element in what could still be an empty list (&shebang[0]).
I adressed the issue in #915, as well, but only adressed the illegal access, not the parse_shebang function itself.

Platform

Darwin 24.6.0 arm64

Version

v0.2.9

.pre-commit-config.yaml

No response

Log file

No response

[j178]

I think this is TODO support for nix-shell as the shebang. We should make sure parse_shebang never returns an empty list, rather than checking if the return list is empty at the calling site.

prek/src/identify.rs

Lines 886 to 890 in 9256926

	// TODO
	if cmd[0] == "nix-shell" {
	return Ok(vec![]);
	}
	Ok(cmd)