Function arguments in trait functions are "unused" if unused in default impl

[birkenfeld]

E.g.

trait Foo {
    fn frob(id: usize) -> Option<usize> { None }
}

triggers "unused variable: id".

I would argue that this shouldn't trigger the lint, since it isn't triggered either when there is no default impl. The argument name has two roles here, one of which the lint doesn't consider.

[sfackler]

Duplicate of #26487.

[estebank]

Current output is the same:

warning: unused variable: `id`
 --> file5.rs:2:13
  |
2 |     fn frob(id: usize) -> Option<usize> { None }
  |             ^^ help: consider prefixing with an underscore: `_id`
  |
  = note: `#[warn(unused_variables)]` on by default

As per #26487 (comment), this is not something we would like to change, but I'll let @rust-lang/lang decide whether that is still the case and close, or prioritize.

[Centril]

I'm a bit conflicted but ultimately I agree with the prior consensus. It seems to me nothing has changed from before and that #[allow(...)] (enhanced by #60406) or let _ = id; works.