Bug 1838703 - [dom/bindings] Mark SetAsFoo() as nodiscard. r=peterv

Differential Revision: https://phabricator.services.mozilla.com/D181140

dom/bindings/Codegen.py

@@ -6340,7 +6340,9 @@ def getJSToNativeConversionInfo(
                     ctorArgs = ""
                 # It's enough to set us to the right type; that will
                 # create an empty array, which is all we need here.
-                default = CGGeneric("%s.RawSetAs%s(%s);\n" % (value, name, ctorArgs))
+                default = CGGeneric(
+                    "Unused << %s.RawSetAs%s(%s);\n" % (value, name, ctorArgs)
+                )
             elif defaultValue.type.isEnum():
                 name = getUnionMemberName(defaultValue.type)
                 # Make sure we actually construct the thing inside the nullable.
@@ -12867,6 +12869,7 @@ class CGUnionStruct(CGThing):
                         vars["ctorArgList"],
                         bodyInHeader=not self.ownsMembers,
                         body=body % "MOZ_ASSERT(mType == eUninitialized);",
+                        noDiscard=True,
                     )
                 )
                 methods.append(
@@ -12876,6 +12879,7 @@ class CGUnionStruct(CGThing):
                         vars["ctorArgList"],
                         bodyInHeader=not self.ownsMembers,
                         body=body % uninit,
+                        noDiscard=True,
                     )
                 )
 
@@ -13295,6 +13299,7 @@ class ClassMethod(ClassItem):
         breakAfterSelf="\n",
         override=False,
         canRunScript=False,
+        noDiscard=False,
     ):
         """
         override indicates whether to flag the method as override
@@ -13314,10 +13319,13 @@ class ClassMethod(ClassItem):
         self.breakAfterSelf = breakAfterSelf
         self.override = override
         self.canRunScript = canRunScript
+        self.noDiscard = noDiscard
         ClassItem.__init__(self, name, visibility)
 
     def getDecorators(self, declaring):
         decorators = []
+        if self.noDiscard:
+            decorators.append("[[nodiscard]]")
         if self.canRunScript:
             decorators.append("MOZ_CAN_RUN_SCRIPT")
         if self.inline:

dom/crypto/WebCryptoTask.h

@@ -91,7 +91,7 @@ class WebCryptoTask : public CancelableRunnable {
                                        CryptoKey& aKey,
                                        const CryptoOperationData& aData) {
     CryptoOperationData dummy;
-    dummy.SetAsArrayBuffer(aCx);
+    Unused << dummy.SetAsArrayBuffer(aCx);
     return CreateSignVerifyTask(aCx, aAlgorithm, aKey, dummy, aData, true);
   }
 

dom/media/MediaManager.cpp

@@ -2528,7 +2528,7 @@ static void ReduceConstraint(
     aConstraint.SetAsMediaTrackConstraints().mMediaSource.Construct(
         *mediaSource);
   } else {
-    aConstraint.SetAsMediaTrackConstraints();
+    Unused << aConstraint.SetAsMediaTrackConstraints();
   }
 }
 

dom/xml/XMLDocument.cpp

@@ -151,7 +151,7 @@ nsresult NS_NewDOMDocument(Document** aInstancePtrResult,
   if (!aQualifiedName.IsEmpty()) {
     ErrorResult result;
     ElementCreationOptionsOrString options;
-    options.SetAsString();
+    Unused << options.SetAsString();
 
     nsCOMPtr<Element> root =
         d->CreateElementNS(aNamespaceURI, aQualifiedName, options, result);

dom/xslt/xslt/txMozillaXSLTProcessor.cpp

@@ -988,7 +988,7 @@ void txMozillaXSLTProcessor::notifyError() {
 
   IgnoredErrorResult rv;
   ElementCreationOptionsOrString options;
-  options.SetAsString();
+  Unused << options.SetAsString();
 
   nsCOMPtr<Element> element =
       document->CreateElementNS(ns, u"parsererror"_ns, options, rv);
@@ -1010,7 +1010,7 @@ void txMozillaXSLTProcessor::notifyError() {
 
   if (!mSourceText.IsEmpty()) {
     ElementCreationOptionsOrString options;
-    options.SetAsString();
+    Unused << options.SetAsString();
 
     nsCOMPtr<Element> sourceElement =
         document->CreateElementNS(ns, u"sourcetext"_ns, options, rv);