Generalize SizeEq to SizeFrom, support shrinking

[joshlf]

Generalize SizeEq, renaming it to SizeFrom, and supporting casts
which shrink rather than preserve referent size.

This requires fairly deep surgery in our transmute taxonomy. We can no
longer rely on SizeFrom::cast_from_raw preserving referent size, which
means that all pointer transmutes must now reason about two types of
transmutes:

• Size-shrinking transmutes (in which the source value is larger than
  the destination value)
• Overwriting transmutes (in which the source value is larger than the
  destination value and the destination permits mutation). In an
  overwriting transmute, the resulting value is a concatenation of a
  valid T and the suffix of a valid U.

(Note that size-preserving transmutes can be viewed as a special case of
size-shrinking transmutes.)

In order to support these semantics, we split TransmuteFrom into two
traits: TransmuteFrom (which supports shriking transmutes) and
TransmuteOverwrite (which supports overwriting transmutes).

While we're here, we simplify the relationship between
TryTransmuteFromPtr, MutationCompatible, and Read. In particular,
TryTransmuteFromPtr now has a single blanket impl which is bounded by
MutationCompatible, and MutationCompatible now directly supports
three cases (and no longer delegates to Read):

• Exclusive reference transmutes in which the destination type may be
  transmuted into the source type
• Shared reference transmutes in which both the source and the
  destination type are Immutable
• Shared reference transmutes which permit interior mutation, in which
  the source and destination types may both be transmuted into one
  another, and in which the source and destination types support
  compatible interior mutation

Makes progress on #1817

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

---

This PR is on branch transmute-ref-dst.

• [macros] Support shrinking reference transmutes #2487
• Generalize SizeEq to SizeFrom, support shrinking #2472
• [macros] Support shrinking value transmutes #2570

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.61017% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.93%. Comparing base (71bc399) to head (3d8da5b).

Files with missing lines	Patch %	Lines
src/lib.rs	92.30%	1 Missing ⚠️
src/util/macros.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2472      +/-   ##
==========================================
+ Coverage   88.91%   88.93%   +0.02%     
==========================================
  Files          20       20              
  Lines        5313     5316       +3     
==========================================
+ Hits         4724     4728       +4     
+ Misses        589      588       -1     

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[jswrenn]

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