async functions with `unimplemented!()` are marked as unreachable

[tacomorphism]

can reproduce with

async fn example(){
 unimplemented!()
}

results in

warning: unreachable expression
 --> src/main.rs:1:19
  |
1 |   async fn example(){
  |  ___________________^
2 | |   unimplemented!()
3 | | }
  | |_^
  |
  = note: `#[warn(unreachable_code)]` on by default

using nightly-2019-09-18

Seems related to #61798

[tmandry]

This also happens with loops which return (first reported in #61798 (comment)).

Minimal example:

async fn foo() {
    loop {
        return;
    }
}

warning: unreachable expression
 --> src/lib.rs:1:16
  |
1 |   async fn foo() {
  |  ________________^
2 | |     loop {
3 | |         return;
4 | |     }
5 | | }
  | |_^
  |
  = note: `#[warn(unreachable_code)]` on by default

These might be separate issues, but they appeared around the same time so I'm going to assume they're related.