Bug 1485161: Make gfxPlatform::GetDefaultFontName() return nsAutoString, to enable possibility of Return Value Optimization & to address build warning. r=jfkthame

This function uses a nsAutoString internally, and its caller stores its
returned value in a nsAutoString. So it's silly for us to have it return a
different type (nsString). With this change, the compiler should be able to
perform return value optimization and avoid the need for any
copying/reallocation of this function's return value.

Differential Revision: https://phabricator.services.mozilla.com/D3926

--HG--
extra : moz-landing-system : lando

gfx/thebes/gfxPlatform.cpp

@@ -1653,17 +1653,20 @@ gfxPlatform::GetStandardFamilyName(const nsAString& aFontName,
     return NS_OK;
 }
 
-nsString
+nsAutoString
 gfxPlatform::GetDefaultFontName(const nsACString& aLangGroup,
                                 const nsACString& aGenericFamily)
 {
+    // To benefit from Return Value Optimization, all paths here must return
+    // this one variable:
+    nsAutoString result;
+
     gfxFontFamily* fontFamily = gfxPlatformFontList::PlatformFontList()->
         GetDefaultFontFamily(aLangGroup, aGenericFamily);
-    if (!fontFamily) {
-      return EmptyString();
-    }
-    nsAutoString result;
-    fontFamily->LocalizedName(result);
+    if (fontFamily) {
+      fontFamily->LocalizedName(result);
+    } // (else, leave 'result' empty)
+
     return result;
 }
 

gfx/thebes/gfxPlatform.h

@@ -377,8 +377,8 @@ public:
      * available in the system, this may return second or later font in the
      * pref.  If there are no available fonts in the pref, returns empty string.
      */
-    nsString GetDefaultFontName(const nsACString& aLangGroup,
-                                const nsACString& aGenericFamily);
+    nsAutoString GetDefaultFontName(const nsACString& aLangGroup,
+                                    const nsACString& aGenericFamily);
 
     /**
      * Create the appropriate platform font group