Macro rustc_target::asm::def_regs

macro_rules! def_regs {
    ($arch:ident $arch_reg:ident $arch_regclass:ident {
        $(
            $reg:ident: $class:ident $(, $extra_class:ident)* = [$reg_name:literal $(, $alias:literal)*] $(% $filter:ident)?,
        )*
        $(
            #error = [$($bad_reg:literal),+] => $error:literal,
        )*
    }) => { ... };
}