Module#ruby2_keywords

static VALUE
rb_mod_ruby2_keywords(int argc, VALUE *argv, VALUE module)
{
    int i;
    VALUE origin_class = RCLASS_ORIGIN(module);

    rb_check_arity(argc, 1, UNLIMITED_ARGUMENTS);
    rb_check_frozen(module);
    rb_class_owner_check(module);

    for (i = 0; i < argc; i++) {
        VALUE v = argv[i];
        ID name = rb_check_id(&v);
        rb_method_entry_t *me;
        VALUE defined_class;

        if (!name) {
            rb_print_undef_str(module, v);
        }

        me = search_method(origin_class, name, &defined_class);
        if (!me && RB_TYPE_P(module, T_MODULE)) {
            me = search_method(rb_cObject, name, &defined_class);
        }

        if (UNDEFINED_METHOD_ENTRY_P(me) ||
            UNDEFINED_REFINED_METHOD_P(me->def)) {
            rb_print_undef(module, name, METHOD_VISI_UNDEF);
        }

        if (module == defined_class || origin_class == defined_class) {
            switch (me->def->type) {
              case VM_METHOD_TYPE_ISEQ:
                if (ISEQ_BODY(me->def->body.iseq.iseqptr)->param.flags.has_rest &&
                        !ISEQ_BODY(me->def->body.iseq.iseqptr)->param.flags.has_post &&
                        !ISEQ_BODY(me->def->body.iseq.iseqptr)->param.flags.has_kw &&
                        !ISEQ_BODY(me->def->body.iseq.iseqptr)->param.flags.has_kwrest) {
                    ISEQ_BODY(me->def->body.iseq.iseqptr)->param.flags.ruby2_keywords = 1;
                    rb_clear_method_cache(module, name);
                }
                else {
                    rb_warn("Skipping set of ruby2_keywords flag for %"PRIsVALUE" (method accepts keywords or post arguments or method does not accept argument splat)", QUOTE_ID(name));
                }
                break;
              case VM_METHOD_TYPE_BMETHOD: {
                VALUE procval = me->def->body.bmethod.proc;
                if (vm_block_handler_type(procval) == block_handler_type_proc) {
                    procval = vm_proc_to_block_handler(VM_BH_TO_PROC(procval));
                }

                if (vm_block_handler_type(procval) == block_handler_type_iseq) {
                    const struct rb_captured_block *captured = VM_BH_TO_ISEQ_BLOCK(procval);
                    const rb_iseq_t *iseq = rb_iseq_check(captured->code.iseq);
                    if (ISEQ_BODY(iseq)->param.flags.has_rest &&
                            !ISEQ_BODY(iseq)->param.flags.has_post &&
                            !ISEQ_BODY(iseq)->param.flags.has_kw &&
                            !ISEQ_BODY(iseq)->param.flags.has_kwrest) {
                        ISEQ_BODY(iseq)->param.flags.ruby2_keywords = 1;
                        rb_clear_method_cache(module, name);
                    }
                    else {
                        rb_warn("Skipping set of ruby2_keywords flag for %"PRIsVALUE" (method accepts keywords or post arguments or method does not accept argument splat)", QUOTE_ID(name));
                    }
                    break;
                }
              }
              /* fallthrough */
              default:
                rb_warn("Skipping set of ruby2_keywords flag for %"PRIsVALUE" (method not defined in Ruby)", QUOTE_ID(name));
                break;
            }
        }
        else {
            rb_warn("Skipping set of ruby2_keywords flag for %"PRIsVALUE" (can only set in method defining module)", QUOTE_ID(name));
        }
    }
    return Qnil;
}