Rollup merge of #58678 - doctorn:refuse-async-fn-2015-edition, r=varkor

Deny `async fn` in 2015 edition

This commit prevents code using `async fn` from being compiled in Rust 2015 edition.

Compiling code of the form:

```rust
async fn foo() {}
```

Will now result in the error:

```
error[E0670]: `async fn` is not permitted in the 2015 edition
 --> async.rs:1:1
  |
1 | async fn foo() {}
  | ^^^^^

error: aborting due to error

For more information about an error, try `rustc --explain E0670`.
```

This resolves #58652 and also resolves #53714.

r? @varkor

0 files changed, 0 insertions, 0 deletions