Warn when a higher-ranked trait bound variable is unused

[SNCPlay42]

This use of HRTB currently compiles without any warnings:

pub fn foo<T>() where for<'a> T: Sized {}

It would be helpful for the compiler to warn that the 'a lifetime is unnecessary.

@rustbot label A-lint A-lifetimes C-feature-request

[Aaron1011]

This produces a warning with #![warn(unused_lifetimes)]. See #41960

[Mark-Simulacrum]

Going to go ahead and close this in favor of #94038 since this is otherwise working as intended (just needs that to be enabled).