Macro identity_subst 

macro_rules! identity_subst {
    (for $l:lifetime { $($t:ty,)* }) => { ... };
}

For types that do not contain any potentially substitutable content, we can use a simple impl. Note that while these types default [ir::subst::Term][] type to SymGenericTerm, that is just for convenience – they implement SubstWith for any type Term.