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

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libsyntax_ext/test.rs b/src/libsyntax_ext/test.rs
index 37186246548..f4b625f8ea2 100644
--- a/src/libsyntax_ext/test.rs
+++ b/src/libsyntax_ext/test.rs
@@ -257,7 +257,7 @@ fn has_test_signature(cx: &ExtCtxt<'_>, i: &ast::Item) -> bool {
             );
             return false
         }
-        if header.asyncness.is_async() {
+        if header.asyncness.node.is_async() {
             sd.span_err(
                 i.span,
                 "async functions cannot be used for tests"