The following code should optimize to a single store of the new discriminant, but doesn't (Godbolt).

enum State { A([u8; 753]), B([u8; 753]), //C, } unsafe fn update(s: *mut State) { let S::A(v) = s.read() else { unreachable_unchecked() }; s.write(S::B(v)); }

Instead, LLVM copies the value to the stack, copies it again and then writes it back. Interestingly, LLVM optimizes this correctly if a third state without value is added.

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rustc --version --verbose:

rustc 1.78.0-nightly (c67326b06 2024-03-15) binary: rustc commit-hash: c67326b063bd27ed04f306ba2e372cd92e0a8751 commit-date: 2024-03-15 host: x86_64-unknown-linux-gnu release: 1.78.0-nightly LLVM version: 18.1.0