Bug 1855074 - Avoid some string copies in WebIDL/XPCOM->JS string conversion. r=edgar

We can convert strings to jsval without mutating the string nowadays.
Make that efficient.

Differential Revision: https://phabricator.services.mozilla.com/D189171

dom/bindings/Codegen.py

@@ -21360,15 +21360,7 @@ class CallbackMember(CGNativeMember):
             if arg.canHaveMissingValue():
                 argval += ".Value()"
 
-        if arg.type.isDOMString():
-            # XPConnect string-to-JS conversion wants to mutate the string.  So
-            # let's give it a string it can mutate
-            # XXXbz if we try to do a sequence of strings, this will kinda fail.
-            result = "mutableStr"
-            prepend = "nsString mutableStr(%s);\n" % argval
-        else:
-            result = argval
-            prepend = ""
+        prepend = ""
 
         wrapScope = self.wrapScope
         if arg.type.isUnion() and wrapScope is None:
@@ -21381,7 +21373,7 @@ class CallbackMember(CGNativeMember):
             arg.type,
             self.descriptorProvider,
             {
-                "result": result,
+                "result": argval,
                 "successCode": "continue;\n" if arg.variadic else "break;\n",
                 "jsvalRef": "argv[%s]" % jsvalIndex,
                 "jsvalHandle": "argv[%s]" % jsvalIndex,

js/xpconnect/src/XPCString.cpp

@@ -112,4 +112,18 @@ bool NonVoidStringToJsval(JSContext* cx, nsAString& str,
   return true;
 }
 
+bool NonVoidStringToJsval(JSContext* cx, const nsAString& str,
+                          MutableHandleValue rval) {
+  nsStringBuffer* sharedBuffer;
+  if (!XPCStringConvert::ReadableToJSVal(cx, str, &sharedBuffer, rval)) {
+    return false;
+  }
+
+  if (sharedBuffer) {
+    // The string was shared but ReadableToJSVal didn't addref it.
+    sharedBuffer->AddRef();
+  }
+  return true;
+}
+
 }  // namespace xpc

js/xpconnect/src/xpcpublic.h

@@ -339,6 +339,8 @@ bool Base64Decode(JSContext* cx, JS::Handle<JS::Value> val,
  */
 bool NonVoidStringToJsval(JSContext* cx, nsAString& str,
                           JS::MutableHandle<JS::Value> rval);
+bool NonVoidStringToJsval(JSContext* cx, const nsAString& str,
+                          JS::MutableHandle<JS::Value> rval);
 inline bool StringToJsval(JSContext* cx, nsAString& str,
                           JS::MutableHandle<JS::Value> rval) {
   // From the T_ASTRING case in XPCConvert::NativeData2JS.
@@ -349,24 +351,14 @@ inline bool StringToJsval(JSContext* cx, nsAString& str,
   return NonVoidStringToJsval(cx, str, rval);
 }
 
-inline bool NonVoidStringToJsval(JSContext* cx, const nsAString& str,
-                                 JS::MutableHandle<JS::Value> rval) {
-  nsString mutableCopy;
-  if (!mutableCopy.Assign(str, mozilla::fallible)) {
-    JS_ReportOutOfMemory(cx);
-    return false;
-  }
-  return NonVoidStringToJsval(cx, mutableCopy, rval);
-}
-
 inline bool StringToJsval(JSContext* cx, const nsAString& str,
                           JS::MutableHandle<JS::Value> rval) {
-  nsString mutableCopy;
-  if (!mutableCopy.Assign(str, mozilla::fallible)) {
-    JS_ReportOutOfMemory(cx);
-    return false;
+  // From the T_ASTRING case in XPCConvert::NativeData2JS.
+  if (str.IsVoid()) {
+    rval.setNull();
+    return true;
   }
-  return StringToJsval(cx, mutableCopy, rval);
+  return NonVoidStringToJsval(cx, str, rval);
 }
 
 /**