Support trait with lifetime

[taiki-e]

Fixes #28

[dtolnay]

Thanks!

[dtolnay]

This causes the impl to mismatch the trait when the impl generics contains more lifetimes.

#[async_trait]
trait Trait {
    async fn f();
}

#[async_trait]
impl<'a> Trait for &'a () {
    async fn f() {}
}

error[E0195]: lifetime parameters or bounds on method `f` do not match the trait declaration
   --> tests/test.rs:315:5
    |
312 |         async fn f();
    |                   - lifetimes in impl do not match this method in trait
...
315 |     #[async_trait]
    |     ^^^^^^^^^^^^^^ lifetimes do not match method in trait

You will need to chain only those lifetimes that appear on the impl block's trait, i.e. impl Trait<'a> for Type<'b> should chain 'a only.

[dtolnay]

Also, I'd like it if these lifetimes could be filtered down per-function.

#[async_trait]
trait Trait<'a, 'b> {
    async fn f(_: &'a &'b ()); // chain 'a and 'b
    async fn g(_: &'b ()); // chain 'b only
    async fn h(); // do not chain
}

I have a feeling this will reduce the occurrence of lifetime headaches from this change.

[taiki-e]

Done in 8ff8ec2.

[dtolnay]

Thanks, looks good.

[dtolnay]

Published in 0.1.12.