Macro assert_impl 

macro_rules! assert_impl {
    (impl $(<$($a:ident),*>)? $tr:ident$(<$($tra:ident),*>)? for $ty:ty where $($w:tt)*) => { ... };
    (impl $(<$($a:ident),*>)? $tr:ident$(<$($tra:ident),*>)? for $ty:ty {}) => { ... };
}