Function type_param_subst 

pub fn type_param_subst(decl: &TypeDecl, args: &[TyId]) -> HashMap<String, TyId>

v0.157 (ADR 0183): the substitution mapping a generic record’s declared type parameters onto a concrete instantiation’s arguments. Empty when the type is non-generic or args is empty (an under-applied reference — the resolver reports that separately).