Argument-position `impl Trait` requires a named lifetime

[cramertj]

This function produces an "expected lifetime parameter" error:

fn foo(_: impl Iterator<Item = &u8>) {}

This code should instead be accepted and bound the impl Trait parameter by the elided lifetime.

cc #34511

[nikomatsakis]

In my opinion, we should accept '_, in-band lifetimes, and friends in where clauses as well. I'm not 100% sure whether this was controversial though, have to check.

[alexreg]

Wait, was this fixed in #49251 or not?

[cramertj]

Still generates an error for me on nightly.

[alexreg]

@cramertj Hmm yes, I should have tried first, my bad. This is independent of the two return-position impl Trait lifetime-related bugs that you're fixing, right?

[CAD97]

Ran into this today. Providing the '_ lifetime gives a very fun error:

fn f(_: impl Iterator<Item = &'_ ()>) {}

error[E0106]: missing lifetime specifier
 --> src/lib.rs:1:31
  |
1 | fn f(_: impl Iterator<Item = &'_ ()>) {}
  |                               ^^ expected lifetime parameter

[estebank]

It's actually gotten slightly worse:

error[E0106]: missing lifetime specifier
 --> src/lib.rs:1:31
  |
1 | fn f(_: impl Iterator<Item = &'_ ()>) {}
  |                               ^^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 | fn f(_: 'a, impl Iterator<Item = &'a ()>) {}
  |         ^^                       ^^^

#68583 will address that. After it gets merged the output will be

I feel that's going to be as good a diagnostic we're gonna have for this.