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codegen: skip stores for entirely-uninit constant aggregate fields MIR GVN (since rust-lang/rust#147827) propagates MaybeUninit::uninit() as `const <uninit>` in aggregate constructions. Without this fix, codegen would emit a memcpy from an `[N x i8] undef` global for each such field, which LLVM materializes as zero-initialization. This mirrors the existing `all_bytes_uninit` skip already present for `Rvalue::Use` (added in rust-lang/rust#147827) into the `Rvalue::Aggregate` field loop. Fixes: rust-lang/rust#157743
The codegen crate contains the code to convert from MIR into LLVM IR,
and then from LLVM IR into machine code. In general it contains code
that runs towards the end of the compilation process.
For more information about how codegen works, see the rustc dev guide.