Callee

const Callee = union(enum) {
    field: struct {
        /// A *pointer* to the object the field is fetched on, so that we can
        /// promote the lvalue to an address if the first parameter requires it.
        obj_ptr: Zir.Inst.Ref,
        /// Offset into `string_bytes`.
        field_name_start: Zir.NullTerminatedString,
    },
    direct: Zir.Inst.Ref,
}