{"ast_node_type":"function","tokens":[{"end":65,"kind":"keyword","start":0,"text":"r#\"fn"}]}
            r#"fn f(iter: &mut $0impl Iterator<Item = i32>) {}"#,
            introduce_named_type_parameter,
            r#"fn f<$0I: Iterator<Item = i32>>(iter: &mut I) {}"#,