fnbox -

# `fnbox`

The tracking issue for this feature is #28796

This had been a temporary alternative to the following impls:

    impl<A, F> FnOnce for Box<F> where F: FnOnce<A> + ?Sized {}
    impl<A, F> FnMut for Box<F> where F: FnMut<A> + ?Sized {}
    impl<A, F> Fn for Box<F> where F: Fn<A> + ?Sized {}

The impls are parallel to these (relatively old) impls:

    impl<A, F> FnOnce for &mut F where F: FnMut<A> + ?Sized {}
    impl<A, F> FnMut for &mut F where F: FnMut<A> + ?Sized {}
    impl<A, F> Fn for &mut F where F: Fn<A> + ?Sized {}
    impl<A, F> FnOnce for &F where F: Fn<A> + ?Sized {}
    impl<A, F> FnMut for &F where F: Fn<A> + ?Sized {}
    impl<A, F> Fn for &F where F: Fn<A> + ?Sized {}

Before the introduction of `unsized_locals`, we had been unable to provide the former impls. That means, unlike `&dyn Fn()` or `&mut dyn FnMut()` we could not use `Box<dyn FnOnce()>` at that time.

`FnBox()` is an alternative approach to `Box<dyn FnBox()>` is delegated to `FnBox::call_box` which doesn't need unsized locals. As we now have `Box<dyn FnOnce()>` working, the `fnbox` feature is going to be removed.