[feature request] Auto-fix for `extra_unused_lifetimes`

Description

extra_unused_lifetimes doesn't have an auto-fix for the unused lifetimes it flags, but it'd be nice if it did.

fn unused_lt<'a>(x: u8) {}

could become

fn unused_lt<>(x: u8) {}

which I've seen my editor change to

fn unused_lt(x: u8) {}

on save.

---

I've thought about widening the range of the lint so it changes

fn unused_lt<'a>(x: u8) {}

to

fn unused_lt(x: u8) {}

but that could become messy in the case with

fn unused_lt<'a, 'b, 'c, 'd>(x: u8) {}

so I think it's fine to just have the auto-fix remove the unused lifetime and have a formatter remove any stray <>.

Edge Case

I've looked into this, and there's an edge case with

trait _Foo {
    fn hey();
}
impl<'a, T: ?Sized> _Foo for &mut T {
    fn hey() {}
}

since just removing 'a' results in

trait _Foo {
    fn hey();
}
impl<, T: ?Sized> _Foo for &mut T {
    fn hey() {}
}

and an error.

Version

rustc 1.81.0 (eeb90cda1 2024-09-04)
binary: rustc
commit-hash: eeb90cda1969383f56a2637cbd3037bdf598841c
commit-date: 2024-09-04
host: x86_64-unknown-linux-gnu
release: 1.81.0
LLVM version: 18.1.7

Additional Labels

@rustbot label +

Start by locating the extra_unused_lifetimes lint implementation and its existing autofix tests. Check the single- and multiple-lifetime examples, including the impl<'a, T: ?Sized> edge case, and consider the work complete when the autofix removes unused lifetimes without producing invalid syntax.