Traits allow unused generic parameters

[steveklabnik]

Consider this trait:

trait Foo<'a> {
  fn foo(x: i32, y: i32) -> i32;
}

rustc will happily let this compile, even though 'a is never used for anything. The same goes for type parameters. This feels incorrect to me, though I might be wrong.

[sanxiyn]

Bear with me if I sound bitter, because I am. No one other than @steveklabnik closed #25871 last February. I will not close this issue, because I am better than him.

[steveklabnik]

@sanxiyn that issue was closed as part of a blanket policy on lints, it wasn't anything personal. If this issue were "we should make a lint for this", I wouldn't have made it in the first place. Furthermore, I didn't close your PR at all. it was closed due to a lack of activity by alex.

This issue is "I'm not even sure this code should be valid." If it is, then this should be closed, just as that issue was closed.

[arielb1]

I don't see why this should not be valid - for example, marker traits have an unused Self by this standard.

[steveklabnik]

@arielb1 would it? Marker traits don't take any generic parameters. Is it that Self is treated as one or something?

[sanxiyn]

It was implemented as a lint because unused generic parameters, in general, is not an error. An example is _assert_error_is_sync_send in libstd. I think it is a legitimate use case.

That said, maybe unused lifetime parameters, or unused generic parameters in traits, are useless. I can easily rebase my PR restricted to these cases.

[arielb1]

@steveklabnik

Sure enough. Self is conceptually a generic parameter.

[eddyb]

They're not useless, they can have semantic implications, for example unsafe traits which may be used to determine the safety of an otherwise problematic operation.
One such trait (although not unsafe, as only the compiler can implement it) is Unsize.
T: Unsize<U> means that any (even smart) pointer to T may be coerced to the same pointer to U, turning a thin pointer into a fat pointer most of the time.

[arielb1]

Unsize is a magic trait, but I don't think this is anything more than a Clippy lint.