Macro impl_altlist_from_data_generic 

macro_rules! impl_altlist_from_data_generic {
    ({$($gen:tt)*} $ty:ty {$($whr:tt)*}) => { ... };
    ({$($gen:tt)*} $ty:ty {$($whr:tt)*}, $guard:ident) => { ... };
}

Generic form of impl_altlist_from_data!: accepts an optional generic parameter list and where-clause so it can target struct Foo<T> { .. } types, not just concrete ones. The non-generic macro above forwards here with empty {} brackets so there is exactly one emission body.