Backport std::bit_cast

Summary: Backport `std::bit_cast` from C++20.

Reviewed By: yfeldblum

Differential Revision: D13705428

fbshipit-source-id: a4b284563be67bec3fe4ddb54fed299650458d30

folly/lang/Bits.h

@@ -55,6 +55,7 @@
 
 #include <folly/ConstexprMath.h>
 #include <folly/Portability.h>
+#include <folly/Traits.h>
 #include <folly/Utility.h>
 #include <folly/lang/Assume.h>
 #include <folly/portability/Builtins.h>
@@ -374,4 +375,26 @@ T bitReverse(T n) {
   return static_cast<T>(Endian::swap(m));
 }
 
+#if __cpp_lib_bit_cast
+
+using std::bit_cast;
+
+#else
+
+//  mimic: std::bit_cast, C++20
+template <
+    typename To,
+    typename From,
+    std::enable_if_t<
+        sizeof(From) == sizeof(To) && std::is_trivial<To>::value &&
+            is_trivially_copyable<From>::value,
+        int> = 0>
+To bit_cast(const From& src) noexcept {
+  To to;
+  std::memcpy(&to, &src, sizeof(From));
+  return to;
+}
+
+#endif
+
 } // namespace folly

folly/lang/test/BitsTest.cpp

@@ -25,7 +25,7 @@
 
 #include <folly/portability/GTest.h>
 
-using namespace folly;
+namespace folly {
 
 // Test constexpr-ness.
 #if !defined(__clang__) && !defined(_MSC_VER)
@@ -231,3 +231,21 @@ TEST(Bits, PartialLoadUnaligned) {
     }
   }
 }
+
+TEST(Bits, BitCastBasic) {
+  auto one = std::make_unique<int>();
+  auto two = folly::bit_cast<std::uintptr_t>(one.get());
+  EXPECT_EQ(folly::bit_cast<int*>(two), one.get());
+}
+
+} // namespace folly
+
+TEST(Bits, BitCastCompatibilityTest) {
+  using namespace folly;
+  using namespace std;
+
+  auto one = folly::Random::rand64();
+  auto pointer = bit_cast<std::uintptr_t>(one);
+  auto two = bit_cast<std::uint64_t>(pointer);
+  EXPECT_EQ(one, two);
+}