Add initial support for unsized MaybeUninit wrapper type

[jswrenn]

This is achieved by adding a MaybeUninit associated type to KnownLayout, whose layout is identical to Self except that it admits uninitialized bytes in all positions.

For sized types, this is bound to mem::MaybeUninit<Self>. For potentially unsized structs, we synthesize a doppelganger with the same repr, whose leading fields are wrapped in mem::MaybeUninit and whose trailing field is the MaybeUninit associated type of struct's original trailing field type. This type-level recursion bottoms out at [T], whose MaybeUninit associated type is bound to [mem::MaybeUninit<T>].

Next Steps and Future Possibilities

MaybeUninit<T: ?Sized>

In the near term, we may remove doc(hidden) from our MaybeUninit wrapper. In doing so, we'd be quick-to-ship a gadget that extends the present capabilities of Rust. We might, then, be able to use this feature as a demonstration of our approach, potentially suitable for upstreaming into rustc.

UnalignUnsized<T: ?Sized>

Presently, our Unalign wrapper requires T: Sized, because we could not figure out how to drop unsized values. For sized values, we copy them onto the stack, then run their destructor. With MaybeUninit<T: ?Sized>, we can extend unalign support to unsized values, by first copying them into a Box<MaybeUninit<T>>, casting the box to Box<T>, and dropping it. This process would be skipped for types with trivial drops, keeping the common case simple and efficient.

Value<T, I> where I: Invariants<Validity = Any>

Combining the above two approaches, we could create an invariant-parameterized Ptr analogue for values that generalizes over initialization and alignment.

[joshlf]

Haven't looked at most of zerocopy-derive yet, but looked at all of zerocopy.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.54545% with 36 lines in your changes missing coverage. Please review.

Project coverage is 87.42%. Comparing base (664b976) to head (60f0a43).

Files with missing lines	Patch %	Lines
src/wrappers.rs	61.95%	35 Missing ⚠️
src/util/mod.rs	98.48%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           v0.8.x    #2055      +/-   ##
==========================================
- Coverage   88.04%   87.42%   -0.63%     
==========================================
  Files          16       16              
  Lines        5983     6115     +132     
==========================================
+ Hits         5268     5346      +78     
- Misses        715      769      +54     

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[jswrenn]

The remaining CI failures are from cargo-semver-checks; it complains that we've added a new item to KnownLayout:

--- failure trait_associated_type_added: non-sealed public trait added associated type without default value ---

Description:
A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/trait_associated_type_added.ron

Failed in:
  trait associated type zerocopy::KnownLayout::MaybeUninit in file /home/runner/work/zerocopy/zerocopy/src/lib.rs:742

[joshlf]

The remaining CI failures are from cargo-semver-checks; it complains that we've added a new item to KnownLayout:

--- failure trait_associated_type_added: non-sealed public trait added associated type without default value ---

Description:
A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.36.0/src/lints/trait_associated_type_added.ron

Failed in:
  trait associated type zerocopy::KnownLayout::MaybeUninit in file /home/runner/work/zerocopy/zerocopy/src/lib.rs:742

We should report this upstream. In particular, given that KnownLayout already has #[doc(hidden)] items, we know that there can be no downstream implementations of KnownLayout which have not already opted out of semver stability.

[joshlf]

A few more nits, but otherwise LGTM!

[joshlf]

Also need to prove that such a transmute is sound (guaranteed by the caller).

[jswrenn]

Done. Also note this change.

[joshlf]

Is that latter change semantically meaningful? Or do you just prefer that wording?

[jswrenn]

We should report this upstream. In particular, given that KnownLayout already has #[doc(hidden)] items, we know that there can be no downstream implementations of KnownLayout which have not already opted out of semver stability.

Filed: obi1kenobi/cargo-semver-checks#997

In the meantime, I think we should permit cargo-semver-checks to fail in CI. Otherwise, our only recourse is to bypass branch protections altogether, which would allow this PR to skip some of our more intensive CI steps that are run only in the merge queue.