Auto merge of #116447 - oli-obk:gen_fn, r=compiler-errors

Implement `gen` blocks in the 2024 edition

Coroutines tracking issue https://github.com/rust-lang/rust/issues/43122
`gen` block tracking issue https://github.com/rust-lang/rust/issues/117078

This PR implements `gen` blocks that implement `Iterator`. Most of the logic with `async` blocks is shared, and thus I renamed various types that were referring to `async` specifically.

An example usage of `gen` blocks is

```rust
fn foo() -> impl Iterator<Item = i32> {
    gen {
        yield 42;
        for i in 5..18 {
            if i.is_even() { continue }
            yield i * 2;
        }
    }
}
```

The limitations (to be resolved) of the implementation are listed in the tracking issue

compiler/rustc_parse/src/parser/mod.rs

@@ -11,6 +11,7 @@ mod stmt;
 mod ty;
 
 use crate::lexer::UnmatchedDelim;
+use ast::Gen;
 pub use attr_wrapper::AttrWrapper;
 pub use diagnostics::AttemptLocalParseRecovery;
 pub(crate) use expr::ForbiddenLetReason;
@@ -1128,6 +1129,16 @@ impl<'a> Parser<'a> {
         }
     }
 
+    /// Parses genness: `gen` or nothing.
+    fn parse_genness(&mut self, case: Case) -> Gen {
+        if self.token.span.at_least_rust_2024() && self.eat_keyword_case(kw::Gen, case) {
+            let span = self.prev_token.uninterpolated_span();
+            Gen::Yes { span, closure_id: DUMMY_NODE_ID, return_impl_trait_id: DUMMY_NODE_ID }
+        } else {
+            Gen::No
+        }
+    }
+
     /// Parses unsafety: `unsafe` or nothing.
     fn parse_unsafety(&mut self, case: Case) -> Unsafe {
         if self.eat_keyword_case(kw::Unsafe, case) {