diff --git a/dom/media/webaudio/AnalyserNode.cpp b/dom/media/webaudio/AnalyserNode.cpp
index 62258b3c56b..229a14746e8 100644
--- a/dom/media/webaudio/AnalyserNode.cpp
+++ b/dom/media/webaudio/AnalyserNode.cpp
@@ -93,7 +93,7 @@ AnalyserNode::AnalyserNode(AudioContext* aContext)
   // Enough chunks must be recorded to handle the case of fftSize being
   // increased to maximum immediately before getFloatTimeDomainData() is
   // called, for example.
-  (void)mChunks.SetLength(CHUNK_COUNT, fallible);
+  unused << mChunks.SetLength(CHUNK_COUNT, fallible);
 
   AllocateBuffer();
 }
diff --git a/mfbt/Assertions.h b/mfbt/Assertions.h
index c72e020806c..f10af874028 100644
--- a/mfbt/Assertions.h
+++ b/mfbt/Assertions.h
@@ -494,8 +494,18 @@ struct AssertionConditionType
 #  define MOZ_ALWAYS_TRUE(expr)      MOZ_ASSERT((expr))
 #  define MOZ_ALWAYS_FALSE(expr)     MOZ_ASSERT(!(expr))
 #else
-#  define MOZ_ALWAYS_TRUE(expr)      ((void)(expr))
-#  define MOZ_ALWAYS_FALSE(expr)     ((void)(expr))
+#  define MOZ_ALWAYS_TRUE(expr) \
+  do { \
+    if ( ( expr ) ) {                       \
+      /* Silence MOZ_WARN_UNUSED_RESULT. */ \
+    } \
+  } while (0)
+#  define MOZ_ALWAYS_FALSE(expr) \
+  do { \
+    if ( ( expr ) ) {                       \
+      /* Silence MOZ_WARN_UNUSED_RESULT. */ \
+    } \
+  } while (0)
 #endif
 
 #undef MOZ_DUMP_ASSERTION_STACK
diff --git a/storage/mozStorageBindingParams.cpp b/storage/mozStorageBindingParams.cpp
index c036b78a645..ab0317d228a 100644
--- a/storage/mozStorageBindingParams.cpp
+++ b/storage/mozStorageBindingParams.cpp
@@ -110,7 +110,7 @@ BindingParams::BindingParams(mozIStorageBindingParamsArray *aOwningArray,
 , mOwningStatement(aOwningStatement)
 {
   (void)mOwningStatement->GetParameterCount(&mParamCount);
-  (void)mParameters.SetCapacity(mParamCount);
+  mParameters.SetCapacity(mParamCount);
 }
 
 BindingParams::BindingParams(mozIStorageBindingParamsArray *aOwningArray)
diff --git a/toolkit/components/places/History.cpp b/toolkit/components/places/History.cpp
index 655c9e9cdcc..bb16e09d7ff 100644
--- a/toolkit/components/places/History.cpp
+++ b/toolkit/components/places/History.cpp
@@ -987,8 +987,8 @@ private:
   {
     MOZ_ASSERT(NS_IsMainThread(), "This should be called on the main thread");
 
-    (void)mPlaces.SwapElements(aPlaces);
-    (void)mReferrers.SetLength(mPlaces.Length());
+    mPlaces.SwapElements(aPlaces);
+    mReferrers.SetLength(mPlaces.Length());
 
     for (nsTArray<VisitData>::size_type i = 0; i < mPlaces.Length(); i++) {
       mReferrers[i].spec = mPlaces[i].referrerSpec;
diff --git a/xpcom/glue/nsTArray.h b/xpcom/glue/nsTArray.h
index 4c3919a211f..d427b33efa6 100644
--- a/xpcom/glue/nsTArray.h
+++ b/xpcom/glue/nsTArray.h
@@ -1193,7 +1193,7 @@ public:
   }
 
   template<class Allocator>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   bool Assign(const nsTArray_Impl<E, Allocator>& aOther,
               const mozilla::fallible_t&)
   {
@@ -1274,7 +1274,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* ReplaceElementsAt(index_type aStart, size_type aCount,
                                const Item* aArray, size_type aArrayLen,
                                const mozilla::fallible_t&)
@@ -1295,7 +1295,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* ReplaceElementsAt(index_type aStart, size_type aCount,
                                const nsTArray<Item>& aArray,
                                const mozilla::fallible_t&)
@@ -1314,7 +1314,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* ReplaceElementsAt(index_type aStart, size_type aCount,
                                const Item& aItem, const mozilla::fallible_t&)
   {
@@ -1339,7 +1339,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementsAt(index_type aIndex, const Item* aArray,
                               size_type aArrayLen, const mozilla::fallible_t&)
   {
@@ -1358,7 +1358,7 @@ protected:
 public:
 
   template<class Item, class Allocator>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementsAt(index_type aIndex,
                               const nsTArray_Impl<Item, Allocator>& aArray,
                               const mozilla::fallible_t&)
@@ -1385,7 +1385,7 @@ protected:
   }
 public:
 
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementAt(index_type aIndex, const mozilla::fallible_t&)
   {
     return InsertElementAt<FallibleAlloc>(aIndex);
@@ -1409,7 +1409,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementAt(index_type aIndex, Item&& aItem,
                              const mozilla::fallible_t&)
   {
@@ -1466,7 +1466,7 @@ protected:
 public:
 
   template<class Item, class Comparator>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementSorted(Item&& aItem, const Comparator& aComp,
                                  const mozilla::fallible_t&)
   {
@@ -1486,7 +1486,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementSorted(Item&& aItem, const mozilla::fallible_t&)
   {
     return InsertElementSorted<Item, FallibleAlloc>(
@@ -1728,7 +1728,7 @@ protected:
   }
 public:
 
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   bool SetCapacity(size_type aCapacity, const mozilla::fallible_t&)
   {
     return SetCapacity<FallibleAlloc>(aCapacity);
@@ -1757,7 +1757,7 @@ protected:
   }
 public:
 
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   bool SetLength(size_type aNewLen, const mozilla::fallible_t&)
   {
     return SetLength<FallibleAlloc>(aNewLen);
@@ -1796,7 +1796,7 @@ protected:
   }
 public:
 
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   bool EnsureLengthAtLeast(size_type aMinLen, const mozilla::fallible_t&)
   {
     return EnsureLengthAtLeast<FallibleAlloc>(aMinLen);
@@ -1828,7 +1828,7 @@ protected:
   }
 public:
 
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementsAt(index_type aIndex, size_type aCount,
                               const mozilla::fallible_t&)
   {
@@ -1865,7 +1865,7 @@ protected:
 public:
 
   template<class Item>
-  /* MOZ_WARN_UNUSED_RESULT */
+  MOZ_WARN_UNUSED_RESULT
   elem_type* InsertElementsAt(index_type aIndex, size_type aCount,
                               const Item& aItem, const mozilla::fallible_t&)
   {
diff --git a/xpcom/tests/TestTArray.cpp b/xpcom/tests/TestTArray.cpp
index e475ace43c3..a75f0e0e5f2 100644
--- a/xpcom/tests/TestTArray.cpp
+++ b/xpcom/tests/TestTArray.cpp
@@ -5,6 +5,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/ArrayUtils.h"
+#include "mozilla/unused.h"
 
 #include <stdlib.h>
 #include <stdio.h>
@@ -1115,9 +1116,9 @@ static bool test_SetLengthAndRetainStorage_no_ctor() {
     pre t post;                                                \
     pre tauto post;                                            \
   } while (0)
-  
+
   // Setup test arrays.
-  FOR_EACH(;, .SetLength(N, fallible));
+  FOR_EACH(; unused << , .SetLength(N, fallible));
   for (int n = 0; n < N; ++n) {
     FOR_EACH(;, [n] = n);
   }