Auto merge of #118457 - eholk:genfn, r=compiler-errors

Add support for `gen fn`

This builds on #116447 to add support for `gen fn` functions. For the most part we follow the same approach as desugaring `async fn`, but replacing `Future` with `Iterator` and `async {}` with `gen {}` for the body.

The version implemented here uses the return type of a `gen fn` as the yield type. For example:

```rust
gen fn count_to_three() -> i32 {
    yield 1;
    yield 2;
    yield 3;
}
```

In the future, I think we should experiment with a syntax like `gen fn count_to_three() yield i32 { ... }`, but that can go in another PR.

cc `@oli-obk` `@compiler-errors`

1 files changed, 7 insertions, 0 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 03e047b297d..4bd7c99de9a 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -563,6 +563,13 @@ pub(crate) struct GenFn {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_async_gen_fn)]
+pub(crate) struct AsyncGenFn {
+    #[primary_span]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_comma_after_base_struct)]
 #[note]
 pub(crate) struct CommaAfterBaseStruct {