Macro assert_field 

macro_rules! assert_field {
    (
        $base_ty: ty,
        $impl_ty: ty,
        $scalar: ty,
        $field_name: expr $(,)?
    ) => { ... };
}

Asserts validness of [Field] [Arguments] and returned [Type].

This assertion is a combination of [assert_subtype] and [assert_field_args].

See spec for more info.