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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.
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