Feature request: From<&[T]> for NonNull<T>

[tspiteri]

While NonNull<T> implements From<&T>, it does not implement From<&[T]>. Current options:

1. Use NonNull::new(slice.as_ptr() as *mut T).unwrap(), and rely on the optimizer to remove the unwrap.
2. Use unsafe { NonNull::new_unchecked(slice.as_ptr() as *mut T) }, which is correct but unfortunately has to use unsafe.
3. It is tempting to use NonNull::from(&slice[0]), which looks more ergonomic than 1 and 2, but has incorrect pointer provenance.

Since both references and slices have as_ptr and as_mut_ptr methods, I think that NonNull should implement From for slices as well as for references.

[jonas-schievink]

It implements <'a, T: ?Sized> From<&'a T>, so this should already work, right?

[tspiteri]

But NonNull::from(slice) returns NonNull<[T]>, it does not return NonNull<T>. By comparison, slice::as_ptr returns *const T, not *const [T].

[jonas-schievink]

Ah, I see

[tspiteri]

As pointed out in #69319 (review), this use case is already solved by NonNull::from(slice).cast::<T>().