Macro impl_iter_scan_tuple 

macro_rules! impl_iter_scan_tuple {
    (@impl $($T:ident)*) => { ... };
    (@inner $($T:ident)*,) => { ... };
    (@inner $($T:ident)*, $U:ident $($Rest:ident)*) => { ... };
    ($($T:ident)*) => { ... };
}