Accessing `&mut MaybeUninit<T>` as `&mut [u8]` by zeroing

[kupiakos]

I have a &mut MaybeUninit<T>. T: FromBytes and is very large. I want to initialize it with data from an io::Read::read method in a closure and output the initialized &mut T.

Today, my options are:

• FromBytes::read_from_io, unpack the Result, and write that to the &mut MaybeUninit<T>. However, I've already proven that return-value optimization doesn't optimize away the temporary objects and memory copies, which is why I'm using a &mut MaybeUninit<T> in the first place. We're trying to do zero copies after all 😄
• Use the FromZeros::zero method to zero out all of the bytes, and then assume_init_mut().as_mut_bytes() to pass into read. This requires using unsafe.

There are two improvements to zerocopy I can consider to solve this problem:

• Provide a zeroing method on FromBytes that outputs a &mut [u8]. This would be valuable for my above situation and for an extra reason: it allows users to get a [u8] out of a non-IntoBytes type by first ensuring all of the padding bytes are zeroed. While the &mut exists, the object cannot be overwritten with a T containing padding.

• As part of Support MaybeUninit<T: ?Sized> via KnownLayout #1797, add a zero method to the zerocopy MaybeUninit that outputs &mut T just like write does. That'd solve this particular problem.

[kupiakos]

Having this method would also make issues like #2319 less likely, being provided a well-lit path to get a zeroed &mut [u8] out of a &mut T