Macro num_traits::cast::impl_as_primitive

macro_rules! impl_as_primitive {
    (@ $T: ty => $(#[$cfg:meta])* impl $U: ty ) => { ... };
    (@ $T: ty => { $( $U: ty ),* } ) => { ... };
    ($T: ty => { $( $U: ty ),* } ) => { ... };
}