Write generic transmute

[joshlf]

Migrated from https://fxbug.dev/82795

See also: #1316.

The zerocopy transmute! macro can only be called in a context in which both the input and output types are concrete. As suggested in this comment (reproduced below), it should be possible to make a transmute function which can operate on generic types. This would require a substantial increase to zerocopy's API surface, and more code in zerocopy-derive, so it should only be done if a use case arises.

Copy of the comment:

One way you could get rid of this is by adding a

unsafe trait FromByteSized<const SIZE: usize>: FromBytes + Sized {}

that your derive macros generate, and then implement your transmute function as

fn transmute<T, U, const N: usize>(x: T)
where
  T: AsBytesSized<N>,
  U: FromBytesSized<N>,
{
  let eyepatch = ManuallyDrop::new(x);
  unsafe { mem::transmute_copy(&*eyepatch) }
}

If no type parameters are provided, Rust will correctly infer this, although you cannot yet write something like transmute::<_, U, _>(). This behavior isn't any better than the macro, but it makes it a Real Function at least. It is unclear to what degree this can be made to work in a generic context... probably propagating those bounds and the extra const parameter is "enough".

In the future, you could have AsBytesSized<N> provide an into_bytes() function that produces an unaligned [u8; N].

See: https://godbolt.org/z/vnbx9Y843

I took a quick stab at implementing a prototype of this (using a separate Size<const N: usize> trait), and ran into a few problems:

• While it technically supports generics, if one of the types is concrete and the other is generic, the generic type must have a bound of the form Size<N> where N is a compile-time constant. You'd end up with a bound like T: FromBytes + Size<60>. Not the end of the world, but kind of ugly.
• I can't figure out how to implement Size for arrays. There's no way to perform multiplication in a const generic context, so the following is illegal:

  impl<T> Size<0> for [T; 0] {}                                  // OK
  impl<T: Size<N>, const N: usize> Size<N> for [T; 1] {}         // OK
  impl<T: Size<N>, const N: usize> Size<{ N * 2 }> for [T; 2] {} // Illegal

[InternetOfTofu]

We (crosvm) are looking at using zerocopy crate's FromBytes and AsBytes to replace DataInit (our legacy creation similar to pod, requires manually marking types using unsafe impl DataInit for xxx {}).

We have made major progress on it (crrev.com/c/4237765) but for instances like https://chromium.googlesource.com/crosvm/crosvm/+/HEAD/devices/src/usb/xhci/xhci_abi.rs#683 it seems hard to use zerocopy's current safe transmute macro to implement.