Sizeness Trait

[HeroicKatora]

There are some parts of the language with special-cased interaction based on the size information ot types. The plain core::mem::transmute requires the compiler to prove size-equivalence of two types, in a much less generic setting than the type bounds themselves suggest. The core::mem::transmute_copy requires the caller to unsafely uphold that the first parameter is not smaller than the second. And zero sized types (ZST) have additional properties:

• They are not really allocated but instead are expressed as dangling pointers.
• Transparent wrapper types are allowed to have an arbitrary number of zero-sized fields without losing the property of inheriting the layout of a single non-zero-sized field, especially if the alignment requirement of the ZST itself is 1.
• All ZSTs have the exact same validity requirements: none.

These properties allow skipping or entirely eliding some validity checks. This is the size equivalent of Unaligned for alignment. For example, transmuting into a ZST can never violate its validity requirements and is always sound, although it might violate a type's safety requirements. This does not imply that all ZSTs implement ZeroSafe, which is also concerned with safety requirements.

Prior art

• zerocopy and bytemuck do not have this concept.
• typic has SizeOf which is an alias to <T as Layout>::Size where the Size associated type of Layout is an typenum::marker_traits::Unsigned.

[Lokathor]

Small point of order: violating safety invariants isn't actually sound if you don't clean up your mess properly before things go wrong.

That's why the ZeroSafe proposal is about safe, not just valid.