Unsize coercion fails if end of method is unreachable

[dtolnay]

The following async trait fails to compile. Notice that the same method body is fine in an ordinary async fn outside of an async trait.

This worked prior to #143.

use async_trait::async_trait;

struct Thing;
trait Ret {}
impl Ret for Thing {}

async fn ok() -> &'static dyn Ret {
    return &Thing;
}

#[async_trait]
trait Trait {
    async fn fail() -> &'static dyn Ret {
        return &Thing;
    }
}

error[E0308]: mismatched types
  --> src/main.rs:13:41
   |
13 |       async fn fail() -> &'static dyn Ret {
   |  _________________________________________^
14 | |         return &Thing;
15 | |     }
   | |_____^ expected struct `Thing`, found trait object `dyn Ret`
   |
   = note:   expected type `&Thing`
           found reference `&'static (dyn Ret + 'static)`
note: return type inferred to be `&Thing` here
  --> src/main.rs:14:16
   |
14 |         return &Thing;
   |                ^^^^^^

error[E0271]: type mismatch resolving `<impl Future as Future>::Output == &'static (dyn Ret + 'static)`
  --> src/main.rs:13:41
   |
13 |       async fn fail() -> &'static dyn Ret {
   |  _________________________________________^
14 | |         return &Thing;
15 | |     }
   | |_____^ expected trait object `dyn Ret`, found struct `Thing`
   |
   = note: expected reference `&'static (dyn Ret + 'static)`
              found reference `&Thing`
   = note: required for the cast to the object type `dyn Future<Output = &'static (dyn Ret + 'static)> + Send`