Lint regression: `dead_code` ignores `#[allow(dead_code)]` on traits

[tgross35]

Code

#[allow(dead_code)]
trait Foo {
    const FOO: u32;
}

impl Foo for u32 {
    const FOO: u32 = roundtrip(0);
}

const fn roundtrip(x: u32) -> u32 {
    x
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=1991c4121122ea00f92d73867bb5f048

Current output

warning: function `roundtrip` is never used
  --> src/lib.rs:10:10
   |
10 | const fn roundtrip(x: u32) -> u32 {
   |          ^^^^^^^^^
   |
   = note: `#[warn(dead_code)]` on by default

Desired output

No lint

Rationale and extra context

roundtrip is used in an instance of trait Foo so it isn't dead. Foo isn't used, but with it being allowed I don't think the check should propagate back elsewhere.

E.g. bar doesn't get a lint in this case:

#[allow(dead_code)]
const X: u32 = bar(0);

const fn bar(x: u32) -> u32 {
    x
}

This regressed in #143519, cc @mu001999

Rust Version

searched nightlies: from nightly-2025-05-31 to nightly-2025-07-15
regressed nightly: nightly-2025-07-14
searched commit range: bfc046a...e9182f1
regressed commit: 56835d7

bisected with cargo-bisect-rustc v0.6.10

Host triple: aarch64-apple-darwin
Reproduce with:

cargo bisect-rustc --start 2025-05-31 --end 2025-07-15 --without-cargo --script=rustc -- a.rs

[wesleywiser]

@rustbot label +I-lang-nominated

Nominating for Lang team discussion. #144113 implements this proposed change but as @compiler-errors notes:

This behavior seems really non-local and pretty broad. #[allow(dead_code)] on a trait definition suppresses all dead code warnings in all impls with this change?

Given that the trait marked #[allow(dead_code)] could be in a third-party dependency and the dead code in a local, leaf level crate, I think the "non-local" point is especially important as users could be confused why the lint is "broken". (Thanks for the correction @mu001999!)

Should we implement this change?

[mu001999]

@wesleywiser said:

Given that the trait marked #[allow(dead_code)] could be in a third-party dependency and the dead code in a local, leaf level crate

such traits' lint level won't spread to local impls, see #144113 (comment)