Add MaybeUninit::(slice_)as_bytes(_mut)

[Amanieu]

This adds methods to convert between MaybeUninit<T> and a slice of MaybeUninit<u8>. This is safe since MaybeUninit<u8> can correctly handle padding bytes in any T.

These methods are added:

impl<T> MaybeUninit<T> {
	pub fn as_bytes(&self) -> &[MaybeUninit<u8>];
	pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>];
	pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>];
	pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>];
}

[rust-highfive]

r? @m-ou-se

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[joshtriplett]

This seems related to some of the questions that have come up in the safe transmute work.

For the mut versions, this allows writing arbitrary data to a value or slice. That data still won't become usable until a call to assume_init, which is unsafe, so this isn't unsound, but as far as I know, doing something like that would previously have required mem::transmute.

In either case, this would make it very easy to rely on the internal layout of a type, which a crate may not have intended to make public. So at the very least, I think we need a comment noting that the ability to access the bytes of private fields or the exact layout of a structure does not necessarily make those details part of a crate's stable ABI.

I don't have any objections to these methods existing; I just want to make sure we note (and where appropriate document) what new capabilities these methods expose, and the potential hazards associated with them.

[Amanieu]

The main use case I have for these methods is reading bytes from a file or memory and turning them into structured types (which I have previously checked to be valid for all bit patterns).

I don't think this exposes anything that wasn't already possible with or without unsafe code.

[Amanieu]

ping @m-ou-se

[m-ou-se]

Looks good to me. Can you open a tracking issue?

r=me with the tracking issue number added.

[Amanieu]

@bors r=m-ou-se

[bors]

📌 Commit 5c96dcf has been approved by m-ou-se