[WIP] Break TransmuteFrom into Shrink vs Overwrite #2567
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This was referenced Jun 5, 2025
Generalize `SizeEq`, renaming it to `SizeCompat`, and supporting casts which do not preserve referent size. This requires fairly deep surgery in our transmute taxonomy. We can no longer rely on `SizeCompat::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) - Tearing transmutes (in which the source value is larger than the destination value *and* the destination permits mutation). In a tearing 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.) Makes progress on #1817 Co-authored-by: Jack Wrenn <[email protected]> gherrit-pr-id: I6c793a9620ad75bdc0d26ab7c7cd1a0c7bef1b8b
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Also, only implement TryTransmuteFromPtr once in terms of MutationCompatible. gherrit-pr-id: I1e15473bf871e5b53a6c093e6a79f48e6498aa04
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Also, only implement TryTransmuteFromPtr once in terms of
MutationCompatible.
This PR is on branch transmute-ref-dst.