Type Alias AliasExpander 

pub type AliasExpander<'a> = &'a dyn Fn(&Type) -> Type;

One-level expander for recursion back-edges, mirroring json_strategy::AliasExpander. Given a Type::AliasRef it returns the alias body (args substituted); for any other type it returns ty.peel() cloned. Lets the emitter resolve a named alias without owning the type namespace.