`self_named_module_files` implementation is broken

[TimNN]

Summary

The self_named_module_files implementation operates on path components, not paths. This leads to both, false negatives and super-confusing false positives.

The logic, as far as I can tell, is approximately this: The lint triggers if a file named foo.rs exists, a folder named foo exists, and foo/mod.rs doesn't exist. The prefixes of those paths, if any, are completely ignored.

(The lint also cannot be allowed for individual modules).

---

This could probably be fixed by tracking things by their full paths, not path components, though maybe a completely different implementation might be the better solution.

E.g. my first idea for how something like this could be implemented was along the lines of:

• For every module file x.rs that is not a mod.rs file and not the crate root:
  • If x.rs contains a mod that is neither inline nor has a #[path="..."] attribute:
    • Trigger self_named_module_files for x.rs.

I think this should cover the desired semantics, with no manipulation of file paths being required.

Reproducer

False negative:

Consider this directory layout:

src
├── foo
│   └── bar.rs
├── foo.rs
├── lib.rs
└── other
    ├── foo
    │   └── mod.rs
    └── mod.rs

The lint should trigger for foo.rs, but doesn't trigger because other/foo/mod.rs exists.

If you remove other, the lint triggers correctly.

False positive:

(There's probably a simpler repro possible, but I like the absurdity of --release making the lint trigger / not trigger, and this is essentially the situation I ran into, just with build instead release).

build.rs:

use std::{env, fs};

fn main() {
    let mut out_file = env::var("OUT_DIR").unwrap();
    out_file.push_str("/data.txt");

    fs::write(&out_file, "").unwrap();

    println!("cargo::rustc-env=DATA={out_file}");

    println!("cargo::warning=DATA: {out_file}");
}

src/lib.rs:

pub static DATA: &str = include_str!(env!("DATA"));

mod release;

src/release.rs: empty file

cargo clippy -- -D clippy::self-named-module-files will not trigger the lint.

cargo clippy --release -- -D clippy::self-named-module-files will trigger the lint.

This is because for --release, $OUT_DIR contains a directory named /release/ and there's no release/mod.rs file anywhere, so release.rs triggers the lint.

Version

rustc 1.88.0-nightly (10fa3c449 2025-04-26)
binary: rustc
commit-hash: 10fa3c449f6b1613b352a6cbf78d3d91fd9a1d81
commit-date: 2025-04-26
host: aarch64-apple-darwin
release: 1.88.0-nightly
LLVM version: 20.1.2

Additional Labels

No response

[TimNN]

@rustbot label +I-false-positive +I-false-negative

[rustbot]

Error: Label D-confusing can only be set by Rust team members

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