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

1 files changed, 28 insertions, 0 deletions

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