Features to support Ti50

[joshlf]

TODO: Link these to the appropriate issues

• MaybeValid<T> at least for T: Sized
  • MaybeValid<T> #1833
• Separate "no UnsafeCell" property into separate Immutable trait; allow FromZeros, FromBytes, and AsBytes on types with UnsafeCells #251
• Convenience method for going from &[u8] to Option<&T> (ie, shorthand for Ref::new(bytes).map(Ref::into_ref))
• For tpm-rs:
  • Slice DSTs
• Field projection inside Unalign
• Over-align Align type
  • Add Align type #249
  • Need a type that doesn't have padding bytes so that you can go from &T -> &Align<T, U>
  • Need a type that does have padding bytes so that you can construct an Align<T, U> by value
• Support prefix transmutes in transmute! et al #1852
• Support desugaring union into struct for safe field access #1853
• Support private impls of zerocopy traits #1855

cc @kupiakos

[joshlf]

Alyssa, let me know which of these is needed for tpm-rs as opposed to just Ti50.

[joshlf]

Triage: @kupiakos Are all of these still needed?

For MaybeValid - how does this differ from MaybeUninit? I presume it has the same bit validity as [u8; size_of::<T>()]? (ie, all bytes must be initialized)

[kupiakos]

#1860 applies to "Field projection inside Unalign"

All of these would still be quite useful to various degrees