error: expected one of `(`, `+`, `,`, `::`, `<`, or `{`, found `->` --> $DIR/issue-108109-fn-trait-missing-paren.rs:3:34 | LL | pub fn func() where F: FnOnce -> () {} | -------^^ expected one of `(`, `+`, `,`, `::`, `<`, or `{` | | | | | help: try adding parentheses | `Fn` bounds require arguments in parentheses error: aborting due to 1 previous error