Change the macro ABSL_IS_TRIVIALLY_RELOCATABLE into a type trait - absl::is_trivially_relocatable - and move it from optimization.h to type_traits.h.

Example of how to change to the new type trait:
```
#include "absl/base/attributes.h"

if (ABSL_IS_TRIVIALLY_RELOCATABLE(T)) { DoSomething(); }
```
to
```
#include "absl/meta/type_traits.h"

if (absl::is_trivially_relocatable<T>::value) { DoSomething(); }
```

Note that optimization.h is also built in C mode so we can't put a type trait there.

PiperOrigin-RevId: 475633715
Change-Id: I2bc85f3f6711d1280049fd01eb97c497d2d0d929

4 files changed, 49 insertions, 0 deletions

diff --git a/absl/meta/BUILD.bazel b/absl/meta/BUILD.bazel
index fb379251..4478ccf7 100644
--- a/absl/meta/BUILD.bazel
+++ b/absl/meta/BUILD.bazel
@@ -42,6 +42,7 @@ cc_test(
     linkopts = ABSL_DEFAULT_LINKOPTS,
     deps = [
         ":type_traits",
+        "//absl/base:core_headers",
         "@com_google_googletest//:gtest_main",
     ],
 )
diff --git a/absl/meta/CMakeLists.txt b/absl/meta/CMakeLists.txt
index 9de4bd37..f16f17bd 100644
--- a/absl/meta/CMakeLists.txt
+++ b/absl/meta/CMakeLists.txt
@@ -34,6 +34,7 @@ absl_cc_test(
   COPTS
     ${ABSL_TEST_COPTS}
   DEPS
+    absl::core_headers
     absl::type_traits
     GTest::gmock_main
 )
diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index 46b76906..6e6001fe 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -813,6 +813,34 @@ using swap_internal::Swap;
 using swap_internal::StdSwapIsUnconstrained;
 
 }  // namespace type_traits_internal
+
+// absl::is_trivially_relocatable<T>
+// Detects whether a type is "trivially relocatable" -- meaning it can be
+// relocated without invoking the constructor/destructor, using a form of move
+// elision.
+//
+// Example:
+//
+// if constexpr (absl::is_trivially_relocatable<T>::value) {
+//   memcpy(new_location, old_location, sizeof(T));
+// } else {
+//   new(new_location) T(std::move(*old_location));
+//   old_location->~T();
+// }
+//
+// Upstream documentation:
+//
+// https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=__is_trivially_relocatable
+//
+#if ABSL_HAVE_BUILTIN(__is_trivially_relocatable)
+template <class T>
+struct is_trivially_relocatable
+    : std::integral_constant<bool, __is_trivially_relocatable(T)> {};
+#else
+template <class T>
+struct is_trivially_relocatable : std::integral_constant<bool, false> {};
+#endif
+
 ABSL_NAMESPACE_END
 }  // namespace absl
 
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index fe96554d..d08d9ad9 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -21,6 +21,7 @@
 #include <vector>
 
 #include "gtest/gtest.h"
+#include "absl/base/attributes.h"
 
 namespace {
 
@@ -1394,4 +1395,22 @@ TEST(TypeTraitsTest, IsNothrowSwappable) {
   EXPECT_TRUE(IsNothrowSwappable<adl_namespace::SpecialNoexceptSwap>::value);
 }
 
+TEST(TrivallyRelocatable, Sanity) {
+#if !defined(ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI) || \
+    !ABSL_HAVE_BUILTIN(__is_trivially_relocatable)
+  GTEST_SKIP() << "No trivial ABI support.";
+#endif
+
+  struct Trivial {};
+  struct NonTrivial {
+    NonTrivial(const NonTrivial&) {}  // NOLINT
+  };
+  struct ABSL_ATTRIBUTE_TRIVIAL_ABI TrivialAbi {
+    TrivialAbi(const TrivialAbi&) {}  // NOLINT
+  };
+  EXPECT_TRUE(absl::is_trivially_relocatable<Trivial>::value);
+  EXPECT_FALSE(absl::is_trivially_relocatable<NonTrivial>::value);
+  EXPECT_TRUE(absl::is_trivially_relocatable<TrivialAbi>::value);
+}
+
 }  // namespace