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/build.rs b/src/libsyntax/ext/build.rs
index 27b0cfb1630..2f88749ace8 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -1017,7 +1017,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
                   ast::ItemKind::Fn(self.fn_decl(inputs, ast::FunctionRetTy::Ty(output)),
                               ast::FnHeader {
                                   unsafety: ast::Unsafety::Normal,
-                                  asyncness: ast::IsAsync::NotAsync,
+                                  asyncness: dummy_spanned(ast::IsAsync::NotAsync),
                                   constness: dummy_spanned(ast::Constness::NotConst),
                                   abi: Abi::Rust,
                               },