error: implementation of `FnOnce` is not general enough --> $DIR/issue-57843.rs:25:9 | LL | Foo(Box::new(|_| ())); | ^^^^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough | = note: closure with signature `fn(&'2 bool)` must implement `FnOnce<(&'1 bool,)>`, for any lifetime `'1`... = note: ...but it actually implements `FnOnce<(&'2 bool,)>`, for some specific lifetime `'2` error: aborting due to 1 previous error