error: implementation of `FnOnce` is not general enough --> $DIR/rfc1623.rs:24:8 | LL | f: &id, | ^^^ implementation of `FnOnce` is not general enough | = note: `fn(&'2 Foo<'_>) -> &'2 Foo<'_> {id::<&'2 Foo<'_>>}` must implement `FnOnce<(&'1 Foo<'b>,)>`, for any lifetime `'1`... = note: ...but it actually implements `FnOnce<(&'2 Foo<'_>,)>`, for some specific lifetime `'2` error: aborting due to previous error