Fix UB in MaybeOwned by deriving the &mut T from a raw pointer, not a &T #471
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The previous implementation converts a &T to a &mut T by casting through raw pointers, which is UB. Miri can detect this with -Zmiri-tag-raw-pointers, though the wasmparser crate itself doesn't have the test coverage to hit it.
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The previous implementation converts a
&Tto a&mut Tby casting through raw pointers, which is UB. Miri can detect this withMIRIFLAGS=-Zmiri-tag-raw-pointers, though thewasmparsercrate itself doesn't have the test coverage to hit it. I became aware of this code because it is hit in the test suites ofwasm-bindgen-wasm-interpreterandcranelift-wasm.As this is only one of two uses of
unsafein this project, I don't think it's worth adding Miri to CI. The test suite for this whole workspace is pretty complex and uses a number of things which Miri is very slow at running or doesn't support at all.I tried replacing this with an all-safe implementation, and saw a 1-3% performance hit on the benchmarks.