[WIP] Update TransmuteFrom safety proofs
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## I6c793a9620ad75bdc0d26ab7c7cd1a0c7bef1b8b #2469 +/- ##
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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]> gherrit-pr-id: I6c793a9620ad75bdc0d26ab7c7cd1a0c7bef1b8b
gherrit-pr-id: I32ffeea758b53073aa461ab41c217e5b8f0bc4e4
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This PR is on branch transmute-ref-dst.
TransmuteFromsafety proofs #2469