Macro simd_type 

macro_rules! simd_type {
    (
        $(
            $(#[$attr: meta])*
            $vis: vis struct $name: ident {
                $($feature_vis: vis $ident: ident: $feature: tt),* $(,)?
            }
        )*
    ) => { ... };
    ({
        $(
            $(#[$attr: meta])*
            $vis: vis struct $name: ident {
                $($feature_vis: vis $ident: ident: $_: ident!($feature: tt)),* $(,)?
            }
        )*
    }) => { ... };
}