Fn(Mut,Once)::call(mut,once)
unboxed_closures
They are already gated with feature `fn_traits`
<>
Fn
detect UFCS drop and allow UFCS methods to have explicit type parameters. Work towards #18875. Since code could previously call the methods & implement the traits manually, this is a [breaking-change] Closes #19586. Closes #19375.