Bug 1390036 (part 2) - Remove nsXPIDLString use from NullString(). r=dbaron.

This requires adding a new constructor for ns[C]String that can be used to
create an IsVoid string.

--HG--
extra : rebase_source : 8cb078bd0a41e63af0d9d144b9eef369875a05e0

xpcom/string/nsReadableUtils.cpp

@@ -1278,7 +1278,7 @@ EmptyCString()
 const nsString&
 NullString()
 {
-  static const nsXPIDLString sNull;
+  static const nsString sNull(mozilla::detail::StringDataFlags::VOIDED);
 
   return sNull;
 }

xpcom/string/nsTString.h

@@ -462,6 +462,15 @@ protected:
   {
   }
 
+  friend const nsTString_CharT& TNullString_CharT();
+
+  // Used by Null[C]String.
+  explicit nsTString_CharT(DataFlags aDataFlags)
+    : substring_type(char_traits::sEmptyBuffer, 0,
+                     aDataFlags | DataFlags::TERMINATED,
+                     ClassFlags::NULL_TERMINATED)
+  {}
+
   struct Segment {
     uint32_t mBegin, mLength;
     Segment(uint32_t aBegin, uint32_t aLength)

xpcom/string/string-template-def-char.h

@@ -25,3 +25,4 @@
 #define nsTPromiseFlatString_CharT          nsPromiseFlatCString
 #define TPromiseFlatString_CharT            PromiseFlatCString
 #define nsTSubstringSplitter_CharT          nsCSubstringSplitter
+#define TNullString_CharT                   NullCString

xpcom/string/string-template-def-unichar.h

@@ -25,3 +25,4 @@
 #define nsTPromiseFlatString_CharT          nsPromiseFlatString
 #define TPromiseFlatString_CharT            PromiseFlatString
 #define nsTSubstringSplitter_CharT          nsSubstringSplitter
+#define TNullString_CharT                   NullString

xpcom/string/string-template-undef.h

@@ -26,3 +26,4 @@
 #undef nsTPromiseFlatString_CharT
 #undef TPromiseFlatString_CharT
 #undef nsTSubstringSplitter_CharT
+#undef TNullString_CharT