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./x.py build
./x.py check

invalidates the build cache for the rustc_llvm crate.
This means that after

./x.py build
./x.py check
./x.py build

the last x.py build will have to rebuild rustc_llvm, rustc_codegen_llvm, rustc_interface, rustc_driver and rustc-main which is less than optional given no code was changed.