Commits on Apr 8, 2024

1. Compute transmutability from rustc_target::abi::Layout …

   In its first step of computing transmutability, `rustc_transmutability`
   constructs a byte-level representation of type layout (`Tree`). Previously, this
   representation was computed for ADTs by inspecting the ADT definition and
   performing our own layout computations. This process was error-prone, verbose,
   and limited our ability to analyze many types (particularly default-repr types).
   
   In this PR, we instead construct `Tree`s from `rustc_target::abi::Layout`s. This
   helps ensure that layout optimizations are reflected our analyses, and increases
   the kinds of types we can now analyze, including:
   - default repr ADTs
   - transparent unions
   - `UnsafeCell`-containing types
   
   Overall, this PR expands the expressvity of `rustc_transmutability` to be much
   closer to the transmutability analysis performed by miri. Future PRs will work
   to close the remaining gaps (e.g., support for `Box`, raw pointers, `NonZero*`,
   coroutines, etc.).

   

   jswrenn committed Apr 8, 2024
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