Disallow non-explicit elided lifetimes in async fn

[cramertj]

Fix #60203

r? @nikomatsakis

[nikomatsakis]

Generally positive, but the error wording is a bit confusing. If we don't want to do the proper fix, then perhaps something like "help: use '_ to represent the elided lifetime explicitly"?

[nikomatsakis]

This suggestion seems a bit confusing. It would be nicer, of course, to suggest HasLifetime<'_>, but this is also complicated to do. @zackmdavis put a quite a bit of work into these sorts of things as part of the lint, I wonder if we can reuse that code?

[cramertj]

I've put up a change to reuse the logic from the rust_2018_idioms elided lifetime parameters in types lint-- this gives a nice improvement for the impl headers case as well-- thanks for the recommendation! As a drive-by, I did notice that the error prints somewhat surprisingly:

#![deny(rust_2018_idioms)]

struct Foo<'a>(&'a u8);

fn foo(_: Foo) {}

gives

error: hidden lifetime parameters in types are deprecated
 --> src/main.rs:5:11
  |
5 | fn foo(_: Foo) {}
  |           ^^^- help: indicate the anonymous lifetime: `<'_>`

Strangely, adding a type parameter makes it spell out the whole type:

#![deny(rust_2018_idioms)]

struct Foo<'a, T>(&'a T);

fn foo<T>(_: Foo<T>) {}

error: hidden lifetime parameters in types are deprecated
 --> src/main.rs:5:14
  |
5 | fn foo<T>(_: Foo<T>) {}
  |              ^^^^^^ help: indicate the anonymous lifetime: `Foo<'_, T>`

Not sure if that's intended, but I found : <'_> a bit odd.

[nikomatsakis]

Nice!

[nikomatsakis]

@bors r+

[bors]

📌 Commit c6e13bc has been approved by nikomatsakis