Tweak E0308 on opaque types

[estebank]

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

[Centril]

Code looks OK aside from the FIXME -- some tests would be nice tho :)

[Centril]

r=me rollup with a test that exercises the behaviors we're adding (e.g. from the issue) and with or without changing the message per #63796 (comment).

[estebank]

@bors r=Centril rollup

[bors]

📌 Commit 02a80f79c160435a19d7f769c710d183329efdf0 has been approved by Centril

[estebank]

@bors r=Centril rollup

cleaned up an unnecessary "missing main" error

[bors]

📌 Commit 4889e2e1ab72d2038d8413a39bef664cd0387e2d has been approved by Centril

[estebank]

@bors r=Centril

[bors]

📌 Commit a710c61 has been approved by Centril

[uberjay]

= help: if both futures resolve to the same type, consider awaiting on both of them

Could rustc provide that suggestion iff both futures resolve to the same type? Sounds challenging to me, but I know nothing about rustc diagnostics! 😬

[estebank]

@uberjay it could but first we need to set some extra machinery around associated types to be able to do that.