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e61403aa4c
Lower to a memset(undef) when Rvalue::Repeat repeats uninit Fixes https://github.com/rust-lang/rust/issues/138625. It is technically correct to just do nothing. But if we actually do nothing, we may miss that this is de-initializing something, so instead we just lower to a single memset that writes undef. This is still superior to the memcpy loop, in both quality of code we hand to the backend and LLVM's final output.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.