Introduce ftl::ignore

`ftl::ignore` works like `std::ignore`, but one can also ignore multiple
values using `operator()`. `ftl::ignore(a, b);` is more compact than
`std::ignore = a; std::ignore = b;`, and conveys the same idea.

Bug: 185536303
Flag: EXEMPT New library code
Test: atest ftl_test

Change-Id: Ice549328eeffb90f59f7b902809def48f583d157

Author: adhoc-bobcat

include/ftl/ignore.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+namespace android::ftl {
+
+// An alternative to `std::ignore` that makes it easy to ignore multiple values.
+//
+// Examples:
+//
+//   void ftl_ignore_multiple(int arg1, const char* arg2, std::string arg3) {
+//     // When invoked, all the arguments are ignored.
+//     ftl::ignore(arg1, arg2, arg3);
+//   }
+//
+//   void ftl_ignore_single(int arg) {
+//     // It can be used like std::ignore to ignore a single value
+//     ftl::ignore = arg;
+//   }
+//
+inline constexpr struct {
+  // NOLINTNEXTLINE(misc-unconventional-assign-operator, readability-named-parameter)
+  constexpr auto operator=(auto&&) const -> decltype(*this) { return *this; }
+  // NOLINTNEXTLINE(readability-named-parameter)
+  constexpr void operator()(auto&&...) const {}
+} ignore;
+
+}  // namespace android::ftl

libs/ftl/Android.bp

@@ -26,6 +26,7 @@ cc_test {
         "function_test.cpp",
         "future_test.cpp",
         "hash_test.cpp",
+        "ignore_test.cpp",
         "match_test.cpp",
         "mixins_test.cpp",
         "non_null_test.cpp",

libs/ftl/ignore_test.cpp

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <string>
+
+#include <ftl/ignore.h>
+#include <gtest/gtest.h>
+
+namespace android::test {
+namespace {
+
+// Keep in sync with the example usage in the header file.
+
+void ftl_ignore_multiple(int arg1, const char* arg2, std::string arg3) {
+  // When invoked, all the arguments are ignored.
+  ftl::ignore(arg1, arg2, arg3);
+}
+
+void ftl_ignore_single(int arg) {
+  // It can be used like std::ignore to ignore a single value
+  ftl::ignore = arg;
+}
+
+}  // namespace
+
+TEST(Ignore, Example) {
+  // The real example test is that there are no compiler warnings for unused arguments above.
+
+  // Use the example functions to avoid a compiler warning about unused functions.
+  ftl_ignore_multiple(0, "a", "b");
+  ftl_ignore_single(0);
+}
+
+}  // namespace android::test