Function instantiate_field_ty 

pub fn instantiate_field_ty(
    decl: &TypeDecl,
    args: &[TyId],
    field_ref: &TypeRef,
    types: &HashMap<String, Arc<TypeDecl>>,
    tys: &Types,
) -> Option<TyId>

v0.157 (ADR 0183): the type of a generic record’s field at a concrete instantiation. The field’s declared type is resolved with the declaration’s type parameters in scope as rigid vars, then those vars are replaced by the instantiation’s args. For a non-generic record this is a plain resolve.