Emphasize move semantics isn't needed for cross-kind raw_ptr conversion

This CL doesn't change the code, just adds comments and tests.

Cross-kind move will degrade to cross-kind copy, which is what we want,
as different traits may use different ref-count, which a move operation
wouldn't update.

There is a worry that an operation would silently kick off a chain of
implicit conversions: raw_ptr<T, X> -> T* -> raw_ptr<T, Y>, but the
tests confirm that this doesn't happen with a move.

Bug: 1407995
Change-Id: I6adcbe3c699d768283626dcd7ad4dfb26cc54d3e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4263931
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Sergei Glazunov <glazunov@google.com>
Cr-Commit-Position: refs/heads/main@{#1107673}

base/allocator/partition_allocator/pointers/raw_ptr.h

@@ -839,6 +839,9 @@ class PA_TRIVIAL_ABI PA_GSL_POINTER raw_ptr {
 #endif  // BUILDFLAG(ENABLE_BACKUP_REF_PTR_SUPPORT) ||
         // BUILDFLAG(USE_ASAN_UNOWNED_PTR)
 
+  // Cross-kind copy constructor.
+  // Move is not supported as different traits may use different ref-counts, so
+  // let move operations degrade to copy, which handles it well.
   template <RawPtrTraits PassedTraits,
             typename Unused = std::enable_if_t<Traits != PassedTraits>>
   PA_ALWAYS_INLINE explicit raw_ptr(const raw_ptr<T, PassedTraits>& p) noexcept
@@ -853,6 +856,9 @@ class PA_TRIVIAL_ABI PA_GSL_POINTER raw_ptr {
     static_assert(Traits == (PassedTraits | RawPtrTraits::kMayDangle));
   }
 
+  // Cross-kind assignment.
+  // Move is not supported as different traits may use different ref-counts, so
+  // let move operations degrade to copy, which handles it well.
   template <RawPtrTraits PassedTraits,
             typename Unused = std::enable_if_t<Traits != PassedTraits>>
   PA_ALWAYS_INLINE raw_ptr& operator=(

base/allocator/partition_allocator/pointers/raw_ptr_unittest.cc

@@ -1360,14 +1360,15 @@ TEST_F(RawPtrTest, CrossKindConversion) {
   RawPtrCountingMayDangleImpl::ClearCounters();
 
   CountingRawPtrMayDangle<int> ptr2(ptr1);
+  CountingRawPtrMayDangle<int> ptr3(std::move(ptr1));  // Falls back to copy.
 
   EXPECT_THAT((CountingRawPtrExpectations<RawPtrCountingImpl>{
                   .get_for_dereference_cnt = 0,
                   .get_for_extraction_cnt = 0,
-                  .get_for_duplication_cnt = 1}),
+                  .get_for_duplication_cnt = 2}),
               CountersMatch());
   EXPECT_THAT((CountingRawPtrExpectations<RawPtrCountingMayDangleImpl>{
-                  .wrap_raw_ptr_cnt = 0, .wrap_raw_ptr_for_dup_cnt = 1}),
+                  .wrap_raw_ptr_cnt = 0, .wrap_raw_ptr_for_dup_cnt = 2}),
               CountersMatch());
 }
 
@@ -1379,15 +1380,17 @@ TEST_F(RawPtrTest, CrossKindAssignment) {
   RawPtrCountingMayDangleImpl::ClearCounters();
 
   CountingRawPtrMayDangle<int> ptr2;
+  CountingRawPtrMayDangle<int> ptr3;
   ptr2 = ptr1;
+  ptr3 = std::move(ptr1);  // Falls back to copy.
 
   EXPECT_THAT((CountingRawPtrExpectations<RawPtrCountingImpl>{
                   .get_for_dereference_cnt = 0,
                   .get_for_extraction_cnt = 0,
-                  .get_for_duplication_cnt = 1}),
+                  .get_for_duplication_cnt = 2}),
               CountersMatch());
   EXPECT_THAT((CountingRawPtrExpectations<RawPtrCountingMayDangleImpl>{
-                  .wrap_raw_ptr_cnt = 0, .wrap_raw_ptr_for_dup_cnt = 1}),
+                  .wrap_raw_ptr_cnt = 0, .wrap_raw_ptr_for_dup_cnt = 2}),
               CountersMatch());
 }
 

base/allocator/partition_allocator/pointers/raw_ptr_unittest.nc

@@ -241,6 +241,20 @@ void WontCompile() {
   [[maybe_unused]] raw_ptr<int, base::RawPtrTraits::kMayDangle> ptr2(ptr);
 }
 
+#elif defined(NCTEST_CROSS_KIND_MOVE_CONVERSION_FROM_MAY_DANGLE) // [r"static assertion failed due to requirement '\(base::RawPtrTraits\)0U == \(\(base::RawPtrTraits\)1U | RawPtrTraits::kMayDangle\)'"]
+
+void WontCompile() {
+  raw_ptr<int, base::RawPtrTraits::kMayDangle> ptr = new int(3);
+  [[maybe_unused]] raw_ptr<int> ptr2(std::move(ptr));
+}
+
+#elif defined(NCTEST_CROSS_KIND_MOVE_CONVERSION_FROM_DUMMY) // [r"static assertion failed due to requirement '\(base::RawPtrTraits\)0U == \(\(base::RawPtrTraits\)1U | RawPtrTraits::kMayDangle\)'"]
+
+void WontCompile() {
+  raw_ptr<int, base::RawPtrTraits::kDummyForTest> ptr = new int(3);
+  [[maybe_unused]] raw_ptr<int, base::RawPtrTraits::kMayDangle> ptr2(std::move(ptr));
+}
+
 #elif defined(NCTEST_CROSS_KIND_ASSIGNMENT_FROM_MAY_DANGLE) // [r"static assertion failed due to requirement '\(base::RawPtrTraits\)1U == \(\(base::RawPtrTraits\)16U | RawPtrTraits::kMayDangle\)'"]
 
 void WontCompile() {
@@ -257,6 +271,22 @@ void WontCompile() {
   ptr2 = ptr;
 }
 
+#elif defined(NCTEST_CROSS_KIND_MOVE_ASSIGNMENT_FROM_MAY_DANGLE) // [r"static assertion failed due to requirement '\(base::RawPtrTraits\)1U == \(\(base::RawPtrTraits\)16U | RawPtrTraits::kMayDangle\)'"]
+
+void WontCompile() {
+  raw_ptr<int, base::RawPtrTraits::kMayDangle> ptr = new int(3);
+  raw_ptr<int> ptr2;
+  ptr2 = std::move(ptr);
+}
+
+#elif defined(NCTEST_CROSS_KIND_MOVE_ASSIGNMENT_FROM_DUMMY) // [r"static assertion failed due to requirement '\(base::RawPtrTraits\)1U == \(\(base::RawPtrTraits\)16U | RawPtrTraits::kMayDangle\)'"]
+
+void WontCompile() {
+  raw_ptr<int, base::RawPtrTraits::kDummyForTest> ptr = new int(3);
+  raw_ptr<int, base::RawPtrTraits::kMayDangle> ptr2;
+  ptr2 = std::move(ptr);
+}
+
 #endif
 
 }  // namespace