extra_unused_lifetimes false positive regression from 1.82 to 1.83

[davidhewitt]

Summary

I believe the following to be a regression in a new false positive for the extra_unused_lifetimes lint for 1.83.

The suggestion is to remove a lifetime 'a, but it is used in the where bound for &'a T: Foo. It's not valid to use where &'_ T: Foo and I also believe where for<'a> &'a T: Foo to have a different meaning, so I do not see a way to remove 'a from the impl.

I am of course happy to be wrong, in which case TIL! 😄

Reproducer

I tried this code:

pub trait Foo {}

impl<'a, T: 'a> Foo for Option<T> where &'a T: Foo {}

I expected to see this happen: no lint warning. (This is the case on 1.82 stable.)

Instead, this happened:

warning: this lifetime isn't used in the impl
 --> src/lib.rs:5:6
  |
5 | impl<'a, T: 'a> Foo for Vec<T> where &'a T: Foo {}
  |      ^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes
  = note: `#[warn(clippy::extra_unused_lifetimes)]` on by default

I observe this on 1.83.0-beta.2

Version

rustc 1.83.0-beta.2 (88c1c3c11 2024-10-18)
binary: rustc
commit-hash: 88c1c3c1102bbf3860891bfa52b7ddd9f26aec2f
commit-date: 2024-10-18
host: aarch64-apple-darwin
release: 1.83.0-beta.2
LLVM version: 19.1.1

Additional Labels

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