PR #1618: inlined_vector: Use trivial relocation for `SwapInlinedElements`

Imported from GitHub PR https://github.com/abseil/abseil-cpp/pull/1618

I noticed while working on #1615 that `inlined_vector` could use the trivial relocatability trait here, too.
Here the memcpy codepath already exists; we just have to opt in to using it.
Merge 567a1dd9b6b3352f649e900b24834b59e39cfa14 into a7012a5bfcf26a41b9dd32d4c429004773503dd6

Merging this change closes #1618

COPYBARA_INTEGRATE_REVIEW=https://github.com/abseil/abseil-cpp/pull/1618 from Quuxplusone:trivial-swap 567a1dd9b6b3352f649e900b24834b59e39cfa14
PiperOrigin-RevId: 609019296
Change-Id: I4055ab790245752179e405b490fcd479e7389726

2 files changed, 47 insertions, 17 deletions

diff --git a/absl/container/inlined_vector_test.cc b/absl/container/inlined_vector_test.cc
index 241389ae..5ecf88a9 100644
--- a/absl/container/inlined_vector_test.cc
+++ b/absl/container/inlined_vector_test.cc
@@ -304,6 +304,35 @@ TEST(UniquePtr, MoveAssign) {
   }
 }
 
+// Swapping containers of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+// TODO(absl-team): Using unique_ptr here is technically correct, but
+// a trivially relocatable struct would be less semantically confusing.
+TEST(UniquePtr, Swap) {
+  for (size_t size1 = 0; size1 < 5; ++size1) {
+    for (size_t size2 = 0; size2 < 5; ++size2) {
+      absl::InlinedVector<std::unique_ptr<size_t>, 2> a;
+      absl::InlinedVector<std::unique_ptr<size_t>, 2> b;
+      for (size_t i = 0; i < size1; ++i) {
+        a.push_back(std::make_unique<size_t>(i + 10));
+      }
+      for (size_t i = 0; i < size2; ++i) {
+        b.push_back(std::make_unique<size_t>(i + 20));
+      }
+      a.swap(b);
+      ASSERT_THAT(a, SizeIs(size2));
+      ASSERT_THAT(b, SizeIs(size1));
+      for (size_t i = 0; i < a.size(); ++i) {
+        ASSERT_THAT(a[i], Pointee(i + 20));
+      }
+      for (size_t i = 0; i < b.size(); ++i) {
+        ASSERT_THAT(b[i], Pointee(i + 10));
+      }
+    }
+  }
+}
+
 // At the end of this test loop, the elements between [erase_begin, erase_end)
 // should have reference counts == 0, and all others elements should have
 // reference counts == 1.
@@ -783,7 +812,9 @@ TEST(OverheadTest, Storage) {
   // The union should be absorbing some of the allocation bookkeeping overhead
   // in the larger vectors, leaving only the size_ field as overhead.
 
-  struct T { void* val; };
+  struct T {
+    void* val;
+  };
   size_t expected_overhead = sizeof(T);
 
   EXPECT_EQ((2 * expected_overhead),
diff --git a/absl/container/internal/inlined_vector.h b/absl/container/internal/inlined_vector.h
index 0eb9c34d..90a74dc7 100644
--- a/absl/container/internal/inlined_vector.h
+++ b/absl/container/internal/inlined_vector.h
@@ -322,14 +322,13 @@ class Storage {
 
   // The policy to be used specifically when swapping inlined elements.
   using SwapInlinedElementsPolicy = absl::conditional_t<
-      // Fast path: if the value type can be trivially move constructed/assigned
-      // and destroyed, and we know the allocator doesn't do anything fancy,
-      // then it's safe for us to simply swap the bytes in the inline storage.
-      // It's as if we had move-constructed a temporary vector, move-assigned
-      // one to the other, then move-assigned the first from the temporary.
-      absl::conjunction<absl::is_trivially_move_constructible<ValueType<A>>,
-                        absl::is_trivially_move_assignable<ValueType<A>>,
-                        absl::is_trivially_destructible<ValueType<A>>,
+      // Fast path: if the value type can be trivially relocated, and we
+      // know the allocator doesn't do anything fancy, then it's safe for us
+      // to simply swap the bytes in the inline storage. It's as if we had
+      // relocated the first vector's elements into temporary storage,
+      // relocated the second's elements into the (now-empty) first's,
+      // and then relocated from temporary storage into the second.
+      absl::conjunction<absl::is_trivially_relocatable<ValueType<A>>,
                         std::is_same<A, std::allocator<ValueType<A>>>>::value,
       MemcpyPolicy,
       absl::conditional_t<IsSwapOk<A>::value, ElementwiseSwapPolicy,
@@ -624,8 +623,8 @@ void Storage<T, N, A>::InitFrom(const Storage& other) {
 
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Initialize(ValueAdapter values, SizeType<A> new_size)
-    -> void {
+auto Storage<T, N, A>::Initialize(ValueAdapter values,
+                                  SizeType<A> new_size) -> void {
   // Only callable from constructors!
   ABSL_HARDENING_ASSERT(!GetIsAllocated());
   ABSL_HARDENING_ASSERT(GetSize() == 0);
@@ -656,8 +655,8 @@ auto Storage<T, N, A>::Initialize(ValueAdapter values, SizeType<A> new_size)
 
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Assign(ValueAdapter values, SizeType<A> new_size)
-    -> void {
+auto Storage<T, N, A>::Assign(ValueAdapter values,
+                              SizeType<A> new_size) -> void {
   StorageView<A> storage_view = MakeStorageView();
 
   AllocationTransaction<A> allocation_tx(GetAllocator());
@@ -699,8 +698,8 @@ auto Storage<T, N, A>::Assign(ValueAdapter values, SizeType<A> new_size)
 
 template <typename T, size_t N, typename A>
 template <typename ValueAdapter>
-auto Storage<T, N, A>::Resize(ValueAdapter values, SizeType<A> new_size)
-    -> void {
+auto Storage<T, N, A>::Resize(ValueAdapter values,
+                              SizeType<A> new_size) -> void {
   StorageView<A> storage_view = MakeStorageView();
   Pointer<A> const base = storage_view.data;
   const SizeType<A> size = storage_view.size;
@@ -885,8 +884,8 @@ auto Storage<T, N, A>::EmplaceBackSlow(Args&&... args) -> Reference<A> {
 }
 
 template <typename T, size_t N, typename A>
-auto Storage<T, N, A>::Erase(ConstIterator<A> from, ConstIterator<A> to)
-    -> Iterator<A> {
+auto Storage<T, N, A>::Erase(ConstIterator<A> from,
+                             ConstIterator<A> to) -> Iterator<A> {
   StorageView<A> storage_view = MakeStorageView();
 
   auto erase_size = static_cast<SizeType<A>>(std::distance(from, to));