Make async gen fn an error

compiler/rustc_parse/src/errors.rs

@@ -562,6 +562,13 @@ pub(crate) struct GenFn {
     pub span: Span,
 }
 
+#[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]

compiler/rustc_parse/src/parser/item.rs

@@ -2414,6 +2414,12 @@ impl<'a> Parser<'a> {
             self.sess.gated_spans.gate(sym::gen_blocks, span);
         }
 
+        if let (Async::Yes { span: async_span, .. }, Gen::Yes { span: gen_span, .. }) =
+            (asyncness, genness)
+        {
+            self.sess.emit_err(errors::AsyncGenFn { span: async_span.to(gen_span) });
+        }
+
         if !self.eat_keyword_case(kw::Fn, case) {
             // It is possible for `expect_one_of` to recover given the contents of
             // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't