inlined_vector: Use trivial relocation for erase

Prior art for the `vector::erase` optimization:
https://github.com/AmadeusITGroup/amc/blob/efcb7be/include/amc/vectorcommon.hpp#L176-L180
https://github.com/bloomberg/bde/blob/e15f05be6/groups/bsl/bslalg/bslalg_arrayprimitives.h#L3787-L3799
https://github.com/facebook/folly/blob/d24bf04/folly/FBVector.h#L1254-L1262
https://github.com/qt/qtbase/blob/fbfee2d/src/corelib/tools/qarraydataops.h#L856-L861

Author: Quuxplusone

absl/container/inlined_vector_test.cc

@@ -304,6 +304,53 @@ TEST(UniquePtr, MoveAssign) {
   }
 }
 
+// Erasing from a container of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+TEST(UniquePtr, EraseSingle) {
+  for (size_t size = 4; size < 16; ++size) {
+    absl::InlinedVector<std::unique_ptr<size_t>, 8> a;
+    for (size_t i = 0; i < size; ++i) {
+      a.push_back(std::make_unique<size_t>(i));
+    }
+    a.erase(a.begin());
+    ASSERT_THAT(a, SizeIs(size - 1));
+    for (size_t i = 0; i < size - 1; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 1));
+    }
+    a.erase(a.begin() + 2);
+    ASSERT_THAT(a, SizeIs(size - 2));
+    ASSERT_THAT(a[0], Pointee(1));
+    ASSERT_THAT(a[1], Pointee(2));
+    for (size_t i = 2; i < size - 2; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 2));
+    }
+  }
+}
+
+// Erasing from a container of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+TEST(UniquePtr, EraseMulti) {
+  for (size_t size = 5; size < 16; ++size) {
+    absl::InlinedVector<std::unique_ptr<size_t>, 8> a;
+    for (size_t i = 0; i < size; ++i) {
+      a.push_back(std::make_unique<size_t>(i));
+    }
+    a.erase(a.begin(), a.begin() + 2);
+    ASSERT_THAT(a, SizeIs(size - 2));
+    for (size_t i = 0; i < size - 2; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 2));
+    }
+    a.erase(a.begin() + 1, a.begin() + 3);
+    ASSERT_THAT(a, SizeIs(size - 4));
+    ASSERT_THAT(a[0], Pointee(2));
+    for (size_t i = 1; i < size - 4; ++i) {
+      ASSERT_THAT(a[i], Pointee(i + 4));
+    }
+  }
+}
+
 // 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.

absl/container/internal/inlined_vector.h

@@ -894,16 +894,30 @@ auto Storage<T, N, A>::Erase(ConstIterator<A> from, ConstIterator<A> to)
       std::distance(ConstIterator<A>(storage_view.data), from));
   SizeType<A> erase_end_index = erase_index + erase_size;
 
-  IteratorValueAdapter<A, MoveIterator<A>> move_values(
-      MoveIterator<A>(storage_view.data + erase_end_index));
-
-  AssignElements<A>(storage_view.data + erase_index, move_values,
-                    storage_view.size - erase_end_index);
+  // Fast path: if the value type is trivially relocatable and we know
+  // the allocator doesn't do anything fancy, then we know it is legal for us to
+  // simply destroy the elements in the "erasure window" (which cannot throw)
+  // and then memcpy downward to close the window.
+  if (absl::is_trivially_relocatable<ValueType<A>>::value &&
+      std::is_nothrow_destructible<ValueType<A>>::value &&
+      std::is_same<A, std::allocator<ValueType<A>>>::value) {
+    DestroyAdapter<A>::DestroyElements(
+        GetAllocator(), storage_view.data + erase_index,
+        erase_size);
+    std::memmove(reinterpret_cast<char*>(storage_view.data + erase_index),
+                 reinterpret_cast<const char*>(storage_view.data + erase_end_index),
+                 (storage_view.size - erase_end_index) * sizeof(ValueType<A>));
+  } else {
+    IteratorValueAdapter<A, MoveIterator<A>> move_values(
+        MoveIterator<A>(storage_view.data + erase_end_index));
 
-  DestroyAdapter<A>::DestroyElements(
-      GetAllocator(), storage_view.data + (storage_view.size - erase_size),
-      erase_size);
+    AssignElements<A>(storage_view.data + erase_index, move_values,
+                      storage_view.size - erase_end_index);
 
+    DestroyAdapter<A>::DestroyElements(
+        GetAllocator(), storage_view.data + (storage_view.size - erase_size),
+        erase_size);
+  }
   SubtractSize(erase_size);
   return Iterator<A>(storage_view.data + erase_index);
 }