reproduce (this is a W=1 build):
        # https://github.com/Rust-for-Linux/linux/commit/96c8b5a5b896bdc21c2792c824e999abcf7113b7
        git remote add rust https://github.com/Rust-for-Linux/linux
        git fetch --no-tags rust staging/rust-device
        git checkout 96c8b5a5b896bdc21c2792c824e999abcf7113b7
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 olddefconfig
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash CLIPPY=1 rustdoc