error[E0046]: not all trait items implemented, missing: `Output` --> $DIR/issue-23827.rs:26:1 | LL | impl FnOnce<(C,)> for Prototype { | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `Output` in implementation | = help: implement the missing item: `type Output = /* Type */;` error: aborting due to 1 previous error For more information about this error, try `rustc --explain E0046`.