some #[allow(...)] outer attributes do not apply to just_underscores_and_digits lint

Summary

Clippy produces false warnings for the following code. The code is not expected to produce warnings as the #[allow(...)] outer attribute should apply to the following line

fn main() {}

fn _just_underscores_and_digits_0() {
    #[allow(clippy::just_underscores_and_digits)]
    let _1 = 1; // warning
}

fn _just_underscores_and_digits_1() {
    #[allow(warnings)]
    let _1 = 1; // warning
}

These examples work fine

#[allow(clippy::just_underscores_and_digits)]
fn _just_underscores_and_digits_2() {
    let _1 = 1;
}

fn _just_underscores_and_digits_3() {
    #![allow(clippy::just_underscores_and_digits)]
    let _1 = 1;
}

Try it in the rust playground

just_underscores_and_digits can be found in clippy_lints/src/non_expressive_names.rs, which houses 2 other related lints (many_single_char_names and similar_names) .

many_single_char_names shares the same problem as just_underscores_and_digits (run with -W clippy::many_single_char_names since the lint group is pedantic)

fn _many_single_char_names_0() {
    #[allow(clippy::many_single_char_names)]
    let (a, b, c, d, e, f, g) = (1, 2, 3, 4, 5, 6, 7); // warning
}

fn _many_single_char_names_1() {
    #[allow(warnings)]
    let (a, b, c, d, e, f, g) = (1, 2, 3, 4, 5, 6, 7); // warning
}

The remaining lint (similar_names) seems to be a false negative (use -W clippy::similar_names)

// this should warn but does not
fn _similar_names() {
    let checked_exp = 1;
    let checked_expr = 1;
}

I've skimmed through the code but frankly the more I look the more I wonder if it works at all:

• similar_names is not working as levenstein_not_1 doesn't seem to work as expected (try assert!(!levenstein_not_1("checked_exp", "checked_expr")))
• just_underscores_and_digits and many_single_char_names AST walks share the same visitor as similar_names (SimilarNamesLocalVisitor). The custom visit_local implementation in the visitor seems to prevent outer attributes from applying correctly

To be clear, I don't use these lints so they don't bother me, but I did spend good time trying to understand why #[allow(...)] wasn't working. Perhaps if we can determine that these lints are somewhat unused deprecating them can also be an option

Lint Name

just_underscores_and_digits, many_single_char_names, similar_names

Version

rustc 1.77.0-nightly (7ffc697ce 2024-01-24)
binary: rustc
commit-hash: 7ffc697ce10f19447c0ce338428ae4b9bc0c041c
commit-date: 2024-01-24
host: x86_64-unknown-linux-gnu
release: 1.77.0-nightly
LLVM version: 17.0.6

Start in clippy_lints/src/non_expressive_names.rs and inspect SimilarNamesLocalVisitor, especially its custom visit_local implementation. Reproduce the reported examples for just_underscores_and_digits, many_single_char_names, and similar_names, then verify that outer allow attributes behave correctly and that the similar_names result matches the issue's expectations.