Teach transmute_{ref,mut}! to handle slice DSTs

[joshlf]

This requires us to generalize our prior support for transmuting between
unsized types. In particular, we previously used the SizeEq trait to
denote that two types have equal sizes in the face of a cast operation
(in particular, that *const T as *const U preserves referent size). In
this commit, we add support for metadata fix-up, which means that we
support casts for which *const T as *const U does not preserve
referent size. Instead, we compute an affine function at compile time
and apply it at runtime - computing the destination type's metadata as a
function of the source metadata, dst_meta = A + src_meta * B. A and
B are computed at compile time.

We generalize SizeEq to permit its cast_from_raw method to perform a
runtime metadata fix-up operation.

Makes progress on #1817

Co-authored-by: Jack Wrenn [email protected]

---

This PR is on branch transmute-ref-dst.

• [WIP] transmute_from!(@allow_shrink) #2487
• [WIP] Update TransmuteFrom safety proofs #2469
• [WIP] Generalize SizeEq to SizeCompat #2472
• Teach transmute_{ref,mut}! to handle slice DSTs #2428

[jswrenn]

We don't need to do this now, but it'd be more idiomatic to name this CastFrom, after its method (and source of safety conditions). That name also conveys directionality, unlike SizeCompat.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 72.80453% with 96 lines in your changes missing coverage. Please review.

Project coverage is 89.80%. Comparing base (c2bfb85) to head (454f974).

Files with missing lines	Patch %	Lines
src/layout.rs	40.29%	40 Missing ⚠️
src/macros.rs	75.49%	25 Missing ⚠️
src/impls.rs	0.00%	14 Missing ⚠️
src/doctests.rs	0.00%	5 Missing ⚠️
src/pointer/transmute.rs	44.44%	5 Missing ⚠️
src/lib.rs	55.55%	4 Missing ⚠️
src/util/macros.rs	91.89%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2428      +/-   ##
==========================================
- Coverage   90.36%   89.80%   -0.57%     
==========================================
  Files          19       20       +1     
  Lines        7808     8059     +251     
==========================================
+ Hits         7056     7237     +181     
- Misses        752      822      +70     

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[joshlf]

TODO

Dismissed because we forgot this important TODO: #2428 (comment)