Issues with MaybeUninit, MaybeValid, TryFromBytes, and KnownLayout

[joshlf]

These are old notes from when we were designing TryFromBytes based on a MaybeValid type, and trying to support MaybeValid<T> for T: ?Sized. They're no longer relevant, but I'm putting them here for posterity.

---

It seems like, even with a KnownLayout custom derive, there'll be no way to teach Rust that T: Sized => MaybeUninit<T>: Sized.

One potential alternative: Instead of a KnownLayout w/ custom derive, just wait until ptr_metadata is stabilized. Until then, TryFromBytes supports T: Sized and [T].

One potential problem with this approach: As currently implemented, Sized does not imply Pointee<Metadata = ()>. Thus, the following has an impl conflict without the Pointee<Metadata = ()> bound in the first impl block. This implies that uses of MaybeUninit which require MaybeUninit<T>: Sized will be impossible in a generic context without a separate Pointee<Metadata = ()> bound.

trait Sizedness {
    type SliceOf<T>: ?Sized;
}

impl<T: Pointee<Metadata = ()>> Sizedness for T {
    type SliceOf<U> = ();
}

impl<T: ?Sized + Pointee<Metadata = usize>> Sizedness for T {
    type SliceOf<U> = [U];
}