Lower return types for gen fn to impl Iterator

compiler/rustc_parse/src/parser/item.rs

@@ -2410,10 +2410,6 @@ impl<'a> Parser<'a> {
             }
         }
 
-        if let Gen::Yes { span, .. } = genness {
-            self.sess.emit_err(errors::GenFn { 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