Rollup merge of #63796 - estebank:opaque_future, r=Centril

Rollup merge of #63796 - estebank:opaque_future, r=Centril

Tweak E0308 on opaque types

```
error[E0308]: if and else have incompatible types
  --> file.rs:21:9
   |
18 | /     if true {
19 | |         thing_one()
   | |         ----------- expected because of this
20 | |     } else {
21 | |         thing_two()
   | |         ^^^^^^^^^^^ expected opaque type, found a different opaque type
22 | |     }.await
   | |_____- if and else have incompatible types
   |
   = note: expected type `impl std::future::Future` (opaque type)
              found type `impl std::future::Future` (opaque type)
   = note: distinct uses of `impl Trait` result in different opaque types
   = help: if both futures resolve to the same type, consider `await`ing on both of them
```

r? @Centril
CC #63167