Add array transpose From implementations: `[MaybeUninit<T>; N]` <-> `MaybeUninit<[T; N]>`

[SUPERCILEX]

Problem statement

The primary use case for MaybeUninit is to initialize arrays of stuff (typically u8s). This is currently poorly supported, requiring the user to enable unstable features (e.g. uninit_array) or immediately mark an array as assume_init which is very confusing to beginners.

Motivation, use-cases

let data = [MaybeUninit::<u8>::uninit(); 2048]; // This could instead be MaybeUninit::<[u8; 2048]>::uninit(), but now you need to use raw pointers to write the data
for ... {
  data.write(...);
}
let data: [u8; 2048] = UH_OH;

UH_OH can be replaced with a transmute (Requires unsafe. Also is that actually safe? It is, but no one wants to spend hours digging around to figure that out.) or with array_assume_init (Requires nightly).

Having a From implementation between [MaybeUninit<T>; N] and MaybeUninit<[T; N]> makes it easy to move between filling an uninitialized array and then marking it as initialized.

Solution sketches

See rust-lang/rust#102023

Links and related work

rust-lang/rust#88837

Other proposals have suggested implementing Index{,Mut}. While I think that's probably a slightly nicer API in its limited scope, users are very quickly going to be sad when they see that their favorite MaybeUninit method isn't available on [MaybeUninit<T>; N]. Eventually that will lead to API duplication as [MaybeUninit<T>; N] methods are added to mirror the MaybeUninit ones. [MaybeUninit<T>; N] <-> MaybeUninit<[T; N]> conversion avoids this entirely.

[thomcc]

I think this is a good idea. The other reason this is superior to MaybeUninit::uninit().assume_uninit() is that this doesn't require unsafe.

[thomcc]

On further though, I wonder if just a transpose() function is a clearer way to spell this than having it be via From. It seems a decent bit clearer about the operation that's being performed.

[SUPERCILEX]

On further though, I wonder if just a transpose() function is a clearer way to spell this than having it be via From. It seems a decent bit clearer about the operation that's being performed.

Been going back and forth on this and I think you're right. Also after seeing rust-lang/rust#48711, I'm now properly scared: if we later decide the From impls are bad we're screwed, whereas adding transpose methods doesn't prevent us from adding From impls in the future and will give the impls time to sit.