Bug 1628715 - Part 3: Make fallible nsTArray::Assign nodiscard. r=xpcom-reviewers,erahm

Differential Revision: https://phabricator.services.mozilla.com/D70384

dom/prio/test/gtest/TestPrioEncoder.cpp

@@ -44,7 +44,7 @@ TEST(PrioEncoder, BooleanLimitExceeded)
     *(sequence.AppendElement(mozilla::fallible)) = rand() % 2;
   }
 
-  prioParams.mBooleans.Assign(sequence);
+  ASSERT_TRUE(prioParams.mBooleans.Assign(sequence));
 
   mozilla::dom::RootedDictionary<mozilla::dom::PrioEncodedData> prioEncodedData(
       cx);
@@ -183,7 +183,7 @@ TEST(PrioEncoder, VerifyFull)
   *(sequence.AppendElement(mozilla::fallible)) = dataItems[0];
   *(sequence.AppendElement(mozilla::fallible)) = dataItems[1];
   *(sequence.AppendElement(mozilla::fallible)) = dataItems[2];
-  prioParams.mBooleans.Assign(sequence);
+  ASSERT_TRUE(prioParams.mBooleans.Assign(sequence));
 
   mozilla::dom::RootedDictionary<mozilla::dom::PrioEncodedData> prioEncodedData(
       cx);

xpcom/ds/nsTArray.h

@@ -1467,7 +1467,7 @@ class nsTArray_Impl
   //
 
   template <class Allocator, typename ActualAlloc = Alloc>
-  typename ActualAlloc::ResultType Assign(
+  [[nodiscard]] typename ActualAlloc::ResultType Assign(
       const nsTArray_Impl<E, Allocator>& aOther) {
     return ActualAlloc::ConvertBoolToResultType(
         !!ReplaceElementsAt<E, ActualAlloc>(0, Length(), aOther.Elements(),