--- a/dom/ipc/Blob.cpp
+++ b/dom/ipc/Blob.cpp
@@ -3569,20 +3569,17 @@ BlobChild::SetMysteryBlobInfo(const nsSt
 
 bool
 BlobChild::SetMysteryBlobInfo(const nsString& aContentType, uint64_t aLength)
 {
   AssertIsOnOwningThread();
   MOZ_ASSERT(mBlobImpl);
   MOZ_ASSERT(mRemoteBlobImpl);
 
-  nsString voidString;
-  voidString.SetIsVoid(true);
-
-  mBlobImpl->SetLazyData(voidString, aContentType, aLength, INT64_MAX);
+  mBlobImpl->SetLazyData(NullString(), aContentType, aLength, INT64_MAX);
 
   NormalBlobConstructorParams params(aContentType,
                                      aLength,
                                      void_t() /* optionalBlobData */);
   return SendResolveMystery(params);
 }
 
 void
@@ -4395,20 +4392,17 @@ BlobParent::RecvResolveMystery(const Res
       const NormalBlobConstructorParams& params =
         aParams.get_NormalBlobConstructorParams();
 
       if (NS_WARN_IF(params.length() == UINT64_MAX)) {
         ASSERT_UNLESS_FUZZING();
         return false;
       }
 
-      nsString voidString;
-      voidString.SetIsVoid(true);
-
-      mBlobImpl->SetLazyData(voidString,
+      mBlobImpl->SetLazyData(NullString(),
                              params.contentType(),
                              params.length(),
                              INT64_MAX);
       return true;
     }
 
     case ResolveMysteryParams::TFileBlobConstructorParams: {
       const FileBlobConstructorParams& params =

--- a/dom/json/nsJSON.cpp
+++ b/dom/json/nsJSON.cpp
@@ -65,30 +65,28 @@ nsJSON::Encode(JS::Handle<JS::Value> aVa
                nsAString &aJSON)
 {
   // This function should only be called from JS.
   nsresult rv = WarnDeprecatedMethod(EncodeWarning);
   if (NS_FAILED(rv))
     return rv;
 
   if (aArgc == 0) {
-    aJSON.Truncate();
     aJSON.SetIsVoid(true);
     return NS_OK;
   }
 
   nsJSONWriter writer;
   rv = EncodeInternal(cx, aValue, &writer);
 
   // FIXME: bug 408838. Get exception types sorted out
   if (NS_SUCCEEDED(rv) || rv == NS_ERROR_INVALID_ARG) {
     rv = NS_OK;
     // if we didn't consume anything, it's not JSON, so return null
     if (!writer.DidWrite()) {
-      aJSON.Truncate();
       aJSON.SetIsVoid(true);
     } else {
       writer.FlushBuffer();
       aJSON.Append(writer.mOutputString);
     }
   }
 
   return rv;

--- a/dom/plugins/ipc/PluginMessageUtils.h
+++ b/dom/plugins/ipc/PluginMessageUtils.h
@@ -248,19 +248,17 @@ inline bool IsDrawingModelDirect(int16_t
 }
 
 // in NPAPI, char* == nullptr is sometimes meaningful.  the following is
 // helper code for dealing with nullable nsCString's
 inline nsCString
 NullableString(const char* aString)
 {
     if (!aString) {
-        nsCString str;
-        str.SetIsVoid(true);
-        return str;
+        return NullCString();
     }
     return nsCString(aString);
 }
 
 inline const char*
 NullableStringGet(const nsCString& str)
 {
   if (str.IsVoid())

--- a/js/xpconnect/src/XPCConvert.cpp
+++ b/js/xpconnect/src/XPCConvert.cpp
@@ -635,17 +635,16 @@ XPCConvert::JSData2Native(void* d, Handl
 
         return true;
     }
 
     case nsXPTType::T_CSTRING:
     {
         if (s.isNull() || s.isUndefined()) {
             nsACString* rs = *((nsACString**)d);
-            rs->Truncate();
             rs->SetIsVoid(true);
             return true;
         }
 
         // The JS val is neither null nor void...
         JSString* str = ToString(cx, s);
         if (!str) {
             return false;

--- a/parser/html/nsHtml5TreeOperation.cpp
+++ b/parser/html/nsHtml5TreeOperation.cpp
@@ -560,24 +560,22 @@ nsresult
 nsHtml5TreeOperation::AppendDoctypeToDocument(nsIAtom* aName,
                                               const nsAString& aPublicId,
                                               const nsAString& aSystemId,
                                               nsHtml5DocumentBuilder* aBuilder)
 {
   // Adapted from nsXMLContentSink
   // Create a new doctype node
   nsCOMPtr<nsIDOMDocumentType> docType;
-  nsAutoString voidString;
-  voidString.SetIsVoid(true);
   NS_NewDOMDocumentType(getter_AddRefs(docType),
                         aBuilder->GetNodeInfoManager(),
                         aName,
                         aPublicId,
                         aSystemId,
-                        voidString);
+                        NullString());
   NS_ASSERTION(docType, "Doctype creation failed.");
   nsCOMPtr<nsIContent> asContent = do_QueryInterface(docType);
   return AppendToDocument(asContent, aBuilder);
 }
 
 nsIContent*
 nsHtml5TreeOperation::GetDocumentFragmentForTemplate(nsIContent* aNode)
 {

--- a/storage/Variant.h
+++ b/storage/Variant.h
@@ -352,25 +352,23 @@ public:
     NS_ENSURE_ARG_POINTER(_type);
     *_type = nsIDataType::VTYPE_EMPTY;
     return NS_OK;
   }
 
   NS_IMETHOD GetAsAUTF8String(nsACString &_str)
   {
     // Return a void string.
-    _str.Truncate(0);
     _str.SetIsVoid(true);
     return NS_OK;
   }
 
   NS_IMETHOD GetAsAString(nsAString &_str)
   {
     // Return a void string.
-    _str.Truncate(0);
     _str.SetIsVoid(true);
     return NS_OK;
   }
 };
 
 ////////////////////////////////////////////////////////////////////////////////
 //// Template Implementation
 

--- a/storage/mozStorageArgValueArray.cpp
+++ b/storage/mozStorageArgValueArray.cpp
@@ -103,17 +103,16 @@ NS_IMETHODIMP
 ArgValueArray::GetUTF8String(uint32_t aIndex,
                              nsACString &_value)
 {
   ENSURE_INDEX_VALUE(aIndex, mArgc);
 
   if (::sqlite3_value_type(mArgv[aIndex]) == SQLITE_NULL) {
     // NULL columns should have IsVoid set to distinguish them from an empty
     // string.
-    _value.Truncate(0);
     _value.SetIsVoid(true);
   }
   else {
     _value.Assign(reinterpret_cast<const char *>(::sqlite3_value_text(mArgv[aIndex])),
                   ::sqlite3_value_bytes(mArgv[aIndex]));
   }
   return NS_OK;
 }
@@ -122,17 +121,16 @@ NS_IMETHODIMP
 ArgValueArray::GetString(uint32_t aIndex,
                          nsAString &_value)
 {
   ENSURE_INDEX_VALUE(aIndex, mArgc);
 
   if (::sqlite3_value_type(mArgv[aIndex]) == SQLITE_NULL) {
     // NULL columns should have IsVoid set to distinguish them from an empty
     // string.
-    _value.Truncate(0);
     _value.SetIsVoid(true);
   } else {
     _value.Assign(static_cast<const char16_t *>(::sqlite3_value_text16(mArgv[aIndex])),
                   ::sqlite3_value_bytes16(mArgv[aIndex]) / 2);
   }
   return NS_OK;
 }
 

--- a/storage/mozStorageStatement.cpp
+++ b/storage/mozStorageStatement.cpp
@@ -767,17 +767,16 @@ Statement::GetUTF8String(uint32_t aIndex
 {
   // Get type of Index will check aIndex for us, so we don't have to.
   int32_t type;
   nsresult rv = GetTypeOfIndex(aIndex, &type);
   NS_ENSURE_SUCCESS(rv, rv);
   if (type == mozIStorageStatement::VALUE_TYPE_NULL) {
     // NULL columns should have IsVoid set to distinguish them from the empty
     // string.
-    _value.Truncate(0);
     _value.SetIsVoid(true);
   }
   else {
     const char *value =
       reinterpret_cast<const char *>(::sqlite3_column_text(mDBStatement,
                                                            aIndex));
     _value.Assign(value, ::sqlite3_column_bytes(mDBStatement, aIndex));
   }
@@ -790,17 +789,16 @@ Statement::GetString(uint32_t aIndex,
 {
   // Get type of Index will check aIndex for us, so we don't have to.
   int32_t type;
   nsresult rv = GetTypeOfIndex(aIndex, &type);
   NS_ENSURE_SUCCESS(rv, rv);
   if (type == mozIStorageStatement::VALUE_TYPE_NULL) {
     // NULL columns should have IsVoid set to distinguish them from the empty
     // string.
-    _value.Truncate(0);
     _value.SetIsVoid(true);
   } else {
     const char16_t *value =
       static_cast<const char16_t *>(::sqlite3_column_text16(mDBStatement,
                                                              aIndex));
     _value.Assign(value, ::sqlite3_column_bytes16(mDBStatement, aIndex) / 2);
   }
   return NS_OK;

--- a/toolkit/components/places/nsNavBookmarks.cpp
+++ b/toolkit/components/places/nsNavBookmarks.cpp
@@ -794,32 +794,30 @@ nsNavBookmarks::InsertSeparator(int64_t 
     index = aIndex;
     // Create space for the insertion.
     rv = AdjustIndices(aParent, index, INT32_MAX, 1);
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
   *aNewItemId = -1;
   // Set a NULL title rather than an empty string.
-  nsCString voidString;
-  voidString.SetIsVoid(true);
   nsAutoCString guid(aGUID);
   PRTime dateAdded = RoundedPRNow();
-  rv = InsertBookmarkInDB(-1, SEPARATOR, aParent, index, voidString, dateAdded,
+  rv = InsertBookmarkInDB(-1, SEPARATOR, aParent, index, NullCString(), dateAdded,
                           0, folderGuid, grandParentId, nullptr, aSource,
                           aNewItemId, guid);
   NS_ENSURE_SUCCESS(rv, rv);
 
   rv = transaction.Commit();
   NS_ENSURE_SUCCESS(rv, rv);
 
   NOTIFY_OBSERVERS(mCanNotify, mCacheObservers, mObservers,
                    nsINavBookmarkObserver,
                    OnItemAdded(*aNewItemId, aParent, index, TYPE_SEPARATOR,
-                               nullptr, voidString, dateAdded, guid, folderGuid,
+                               nullptr, NullCString(), dateAdded, guid, folderGuid,
                                aSource));
 
   return NS_OK;
 }
 
 
 nsresult
 nsNavBookmarks::GetLastChildId(int64_t aFolderId, int64_t* aItemId)

--- a/xpcom/ds/nsVariant.cpp
+++ b/xpcom/ds/nsVariant.cpp
@@ -761,17 +761,16 @@ nsDiscriminatedUnion::ToString(nsACStrin
     case nsIDataType::VTYPE_WCHAR:
       NS_ERROR("ToString being called for a string type - screwy logic!");
       MOZ_FALLTHROUGH;
 
     // XXX We might want stringified versions of these... ???
 
     case nsIDataType::VTYPE_VOID:
     case nsIDataType::VTYPE_EMPTY:
-      aOutString.Truncate();
       aOutString.SetIsVoid(true);
       return NS_OK;
 
     case nsIDataType::VTYPE_EMPTY_ARRAY:
     case nsIDataType::VTYPE_ARRAY:
     case nsIDataType::VTYPE_INTERFACE:
     case nsIDataType::VTYPE_INTERFACE_IS:
     default: