Single-use lifetime in return type cannot be removed or replaced with '_

[scottmcm]

This, of course, has worked for ages:

fn foo1<'a>(s: String) -> Cow<'a, str> { s.into() }

With in-band lifetimes, it can be simplified to

fn foo2(s: String) -> Cow<'a, str> { s.into() }

But that's a single-use lifetime, which feels weird, so I went to replace it with '_

fn foo3(s: String) -> Cow<'_, str> { s.into() }

But that doesn't work, giving

error[E0106]: missing lifetime specifier
 --> src/lib.rs:6:27
  |
6 | fn foo3(s: String) -> Cow<'_, str> { s.into() }
  |                           ^^ expected lifetime parameter
  |
  = help: this function's return type contains a borrowed value with an elided lifetime, but the lifetime cannot be derived from the arguments
  = help: consider giving it an explicit bounded or 'static lifetime

Needing a single-use-lifetime with no declaration feels really odd to me, like it should be coming from somewhere else. Not that this is that common of a case...

Context: https://users.rust-lang.org/t/is-static-okay/18636?u=scottmcm

[Spoonbender]

triage: no change

[cjgillot]

In-band lifetimes do not exist any more. This bug has become irrelevant.