MiraclePtr: Avoid storing ExtractAsDangling() into a variable.

ExtractAsDangling() converts the raw_ptr into a raw_ptr allowed to
dangle. This is expected to be used immediately as a temporary object,
and not stored as a naked pointer.

This relies on C++ calling destructor of temporaries at the end of the
full expression. It means MiraclePtr will continue to consider the
object "referenced" during the expression and continue to protect
against UAF during the callback call.

Bug: None
Change-Id: If9436fb9626253b24c51d2c1db1701d22513fcd3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/206274
Auto-Submit: Arthur Sonzogni <[email protected]>
Commit-Queue: Arthur Sonzogni <[email protected]>
Reviewed-by: Corentin Wallez <[email protected]>
Commit-Queue: Corentin Wallez <[email protected]>

src/dawn/wire/client/Device.cpp

@@ -67,10 +67,9 @@ class PopErrorScopeEvent final : public TrackedEvent {
             mStatus = WGPUPopErrorScopeStatus_InstanceDropped;
             mMessage = std::nullopt;
         }
-        void* userdata1 = mUserdata1.ExtractAsDangling();
-        void* userdata2 = mUserdata2.ExtractAsDangling();
         if (mCallback) {
-            mCallback(mStatus, mType, mMessage ? mMessage->c_str() : nullptr, userdata1, userdata2);
+            mCallback(mStatus, mType, mMessage ? mMessage->c_str() : nullptr,
+                      mUserdata1.ExtractAsDangling(), mUserdata2.ExtractAsDangling());
         }
     }