unused-lifetimes lint false-positives with async functions

[Seeker14491]

Async functions that take references seem to always trigger the unused-lifetimes lint (which is allow-by-default). Example:

#![feature(async_await)]

#![warn(unused_lifetimes)]

pub async fn print(s: &str) {
    println!("{}", s);
}

warning: lifetime parameter `'_` never used
 --> src/lib.rs:5:23
  |
5 | pub async fn print(s: &str) {
  |                       ^
  |

[Seeker14491]

I found this clippy issue which describes basically the same problem.

[nikomatsakis]

Seeing as this lint is not enabled by default, marking as deferred, though this would be good to fix and probably not too hard.

[gilescope]

@rustbot claim

[gilescope]

Looks the same issue as #64493.

A little inelegant but if

    !"5 | pub async fn print(s: &str) {".contains("'_")

we can suppress the lint I think...