Issue #2342: Use [[nodiscard]] in /dom

21 files changed, 113 insertions, 122 deletions

diff --git a/dom/base/FragmentOrElement.h b/dom/base/FragmentOrElement.h
index 2f65fe5174..331a526e9a 100644
--- a/dom/base/FragmentOrElement.h
+++ b/dom/base/FragmentOrElement.h
@@ -147,8 +147,7 @@ public:
   virtual bool TextIsOnlyWhitespace() override;
   virtual bool HasTextForTranslation() override;
   virtual void AppendTextTo(nsAString& aResult) override;
-  MOZ_MUST_USE
-  virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) override;
+  [[nodiscard]] virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) override;
   virtual nsIContent *GetBindingParent() const override;
   virtual nsXBLBinding *GetXBLBinding() const override;
   virtual void SetXBLBinding(nsXBLBinding* aBinding,
diff --git a/dom/base/nsContentAreaDragDrop.cpp b/dom/base/nsContentAreaDragDrop.cpp
index 1734ab378d..49aeb2a774 100644
--- a/dom/base/nsContentAreaDragDrop.cpp
+++ b/dom/base/nsContentAreaDragDrop.cpp
@@ -84,7 +84,7 @@ private:
                                             nsIContent **outImageOrLinkNode,
                                             bool* outDragSelectedText);
   static already_AddRefed<nsIContent> FindParentLinkNode(nsIContent* inNode);
-  static MOZ_MUST_USE nsresult
+  static [[nodiscard]] nsresult
   GetAnchorURL(nsIContent* inNode, nsAString& outURL);
   static void GetNodeString(nsIContent* inNode, nsAString & outNodeString);
   static void CreateLinkText(const nsAString& inURL, const nsAString & inText,
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 6e9f23054a..eac80efc6b 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -1508,9 +1508,10 @@ public:
    * @param aResult the result. Out param.
    * @return false on out of memory errors, true otherwise.
    */
-  MOZ_MUST_USE
-  static bool GetNodeTextContent(nsINode* aNode, bool aDeep,
-                                 nsAString& aResult, const mozilla::fallible_t&);
+  [[nodiscard]] static bool GetNodeTextContent(nsINode* aNode,
+                                               bool aDeep,
+                                               nsAString& aResult,
+                                               const mozilla::fallible_t&);
 
   static void GetNodeTextContent(nsINode* aNode, bool aDeep,
                                  nsAString& aResult);
@@ -1869,28 +1870,29 @@ public:
    */
   static bool CanAccessNativeAnon();
 
-  MOZ_MUST_USE
-  static nsresult WrapNative(JSContext *cx, nsISupports *native,
-                             const nsIID* aIID, JS::MutableHandle<JS::Value> vp,
-                             bool aAllowWrapping = true)
+  [[nodiscard]] static nsresult WrapNative(JSContext* cx,
+                                           nsISupports* native,
+                                           const nsIID* aIID,
+                                           JS::MutableHandle<JS::Value> vp,
+                                           bool aAllowWrapping = true)
   {
     return WrapNative(cx, native, nullptr, aIID, vp, aAllowWrapping);
   }
 
   // Same as the WrapNative above, but use this one if aIID is nsISupports' IID.
-  MOZ_MUST_USE
-  static nsresult WrapNative(JSContext *cx, nsISupports *native,
-                             JS::MutableHandle<JS::Value> vp,
-                             bool aAllowWrapping = true)
+  [[nodiscard]] static nsresult WrapNative(JSContext* cx,
+                                           nsISupports* native,
+                                           JS::MutableHandle<JS::Value> vp,
+                                           bool aAllowWrapping = true)
   {
     return WrapNative(cx, native, nullptr, nullptr, vp, aAllowWrapping);
   }
 
-  MOZ_MUST_USE
-  static nsresult WrapNative(JSContext *cx, nsISupports *native,
-                             nsWrapperCache *cache,
-                             JS::MutableHandle<JS::Value> vp,
-                             bool aAllowWrapping = true)
+  [[nodiscard]] static nsresult WrapNative(JSContext* cx,
+                                           nsISupports* native,
+                                           nsWrapperCache* cache,
+                                           JS::MutableHandle<JS::Value> vp,
+                                           bool aAllowWrapping = true)
   {
     return WrapNative(cx, native, cache, nullptr, vp, aAllowWrapping);
   }
@@ -1914,9 +1916,8 @@ public:
    * @param aString the string to convert the newlines inside [in/out]
    */
   static void PlatformToDOMLineBreaks(nsString &aString);
-  MOZ_MUST_USE
-  static bool PlatformToDOMLineBreaks(nsString &aString,
-                                      const mozilla::fallible_t&);
+  [[nodiscard]] static bool PlatformToDOMLineBreaks(nsString &aString,
+                                                    const mozilla::fallible_t&);
 
   /**
    * Populates aResultString with the contents of the string-buffer aBuf, up
diff --git a/dom/base/nsGenericDOMDataNode.h b/dom/base/nsGenericDOMDataNode.h
index c3e8ae4740..cffed2cb9e 100644
--- a/dom/base/nsGenericDOMDataNode.h
+++ b/dom/base/nsGenericDOMDataNode.h
@@ -144,9 +144,8 @@ public:
   virtual bool TextIsOnlyWhitespace() override;
   virtual bool HasTextForTranslation() override;
   virtual void AppendTextTo(nsAString& aResult) override;
-  MOZ_MUST_USE
-  virtual bool AppendTextTo(nsAString& aResult,
-                            const mozilla::fallible_t&) override;
+  [[nodiscard]] virtual bool AppendTextTo(nsAString& aResult,
+                                          const mozilla::fallible_t&) override;
   virtual void SaveSubtreeState() override;
 
 #ifdef DEBUG
diff --git a/dom/base/nsHTMLContentSerializer.h b/dom/base/nsHTMLContentSerializer.h
index de63f7ee54..288abd8273 100644
--- a/dom/base/nsHTMLContentSerializer.h
+++ b/dom/base/nsHTMLContentSerializer.h
@@ -37,18 +37,16 @@ class nsHTMLContentSerializer final : public nsXHTMLContentSerializer {
                                  nsAString& aStr) override;
  protected:
 
-  MOZ_MUST_USE
-  virtual bool SerializeHTMLAttributes(nsIContent* aContent,
-                                       nsIContent *aOriginalElement,
-                                       nsAString& aTagPrefix,
-                                       const nsAString& aTagNamespaceURI,
-                                       nsIAtom* aTagName,
-                                       int32_t aNamespace,
-                                       nsAString& aStr);
-
-  MOZ_MUST_USE
-  virtual bool AppendAndTranslateEntities(const nsAString& aStr,
-                                          nsAString& aOutputStr) override;
+  [[nodiscard]] virtual bool SerializeHTMLAttributes(nsIContent* aContent,
+                                                     nsIContent* aOriginalElement,
+                                                     nsAString& aTagPrefix,
+                                                     const nsAString& aTagNamespaceURI,
+                                                     nsIAtom* aTagName,
+                                                     int32_t aNamespace,
+                                                     nsAString& aStr);
+
+  [[nodiscard]] virtual bool AppendAndTranslateEntities(const nsAString& aStr,
+                                                        nsAString& aOutputStr) override;
 
 };
 
diff --git a/dom/base/nsIContent.h b/dom/base/nsIContent.h
index 4d7d76e251..a0d81b9ade 100644
--- a/dom/base/nsIContent.h
+++ b/dom/base/nsIContent.h
@@ -576,8 +576,7 @@ public:
    * Append the text content to aResult.
    * NOTE: This asserts and returns for elements
    */
-  MOZ_MUST_USE
-  virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) = 0;
+  [[nodiscard]] virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) = 0;
 
   /**
    * Check if this content is focusable and in the current tab order.
diff --git a/dom/base/nsIDocument.h b/dom/base/nsIDocument.h
index 6ae4eb4f55..174bc4cdd9 100644
--- a/dom/base/nsIDocument.h
+++ b/dom/base/nsIDocument.h
@@ -2513,8 +2513,8 @@ public:
                 mozilla::ErrorResult& rv);
   virtual mozilla::dom::DOMImplementation*
     GetImplementation(mozilla::ErrorResult& rv) = 0;
-  MOZ_MUST_USE nsresult GetURL(nsString& retval) const;
-  MOZ_MUST_USE nsresult GetDocumentURI(nsString& retval) const;
+  [[nodiscard]] nsresult GetURL(nsString& retval) const;
+  [[nodiscard]] nsresult GetDocumentURI(nsString& retval) const;
   // Return the URI for the document.
   // The returned value may differ if the document is loaded via XHR, and
   // when accessed from chrome privileged script and
diff --git a/dom/base/nsINode.h b/dom/base/nsINode.h
index 5268767a3e..857eccc4ef 100644
--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -1873,7 +1873,7 @@ public:
     aNodeName.SetStringBuffer(nsStringBuffer::FromString(nodeName),
                               nodeName.Length());
   }
-  MOZ_MUST_USE nsresult GetBaseURI(nsAString& aBaseURI) const;
+  [[nodiscard]] nsresult GetBaseURI(nsAString& aBaseURI) const;
   // Return the base URI for the document.
   // The returned value may differ if the document is loaded via XHR, and
   // when accessed from chrome privileged script and
diff --git a/dom/base/nsJSUtils.h b/dom/base/nsJSUtils.h
index db70cd924d..53b1330ed7 100644
--- a/dom/base/nsJSUtils.h
+++ b/dom/base/nsJSUtils.h
@@ -143,7 +143,7 @@ public:
     // After getting a notification that an off-thread compilation terminated,
     // this function will take the result of the parser and move it to the main
     // thread.
-    MOZ_MUST_USE nsresult JoinCompile(void** aOffThreadToken);
+    [[nodiscard]] nsresult JoinCompile(void** aOffThreadToken);
 
     // Compile a script contained in a SourceText.
     nsresult Compile(JS::CompileOptions& aCompileOptions,
@@ -167,7 +167,7 @@ public:
     JSScript* GetScript();
 
     // Execute the compiled script and ignore the return value.
-    MOZ_MUST_USE nsresult ExecScript();
+    [[nodiscard]] nsresult ExecScript();
 
     // Execute the compiled script a get the return value.
     //
@@ -180,10 +180,10 @@ public:
     // compartment given as argument to the ExecutionContext constructor. If the
     // caller is in a different compartment, then the out-param value should be
     // wrapped by calling |JS_WrapValue|.
-    MOZ_MUST_USE nsresult
+    [[nodiscard]] nsresult
     ExtractReturnValue(JS::MutableHandle<JS::Value> aRetValue);
 
-    MOZ_MUST_USE nsresult ExecScript(JS::MutableHandle<JS::Value> aRetValue);
+    [[nodiscard]] nsresult ExecScript(JS::MutableHandle<JS::Value> aRetValue);
   };
 
   static nsresult CompileModule(JSContext* aCx,
diff --git a/dom/base/nsObjectLoadingContent.h b/dom/base/nsObjectLoadingContent.h
index 24f80dd083..3ac9914ae7 100644
--- a/dom/base/nsObjectLoadingContent.h
+++ b/dom/base/nsObjectLoadingContent.h
@@ -382,7 +382,7 @@ class nsObjectLoadingContent : public nsImageLoadingContent
     void GetNestedParams(nsTArray<mozilla::dom::MozPluginParameter>& aParameters,
                          bool aIgnoreCodebase);
 
-    MOZ_MUST_USE nsresult BuildParametersArray();
+    [[nodiscard]] nsresult BuildParametersArray();
 
     /**
      * Loads fallback content with the specified FallbackType
diff --git a/dom/base/nsTextFragment.h b/dom/base/nsTextFragment.h
index 725d9c6753..2845a14e1a 100644
--- a/dom/base/nsTextFragment.h
+++ b/dom/base/nsTextFragment.h
@@ -136,7 +136,7 @@ public:
    * Append the contents of this string fragment to aString
    * @return false if an out of memory condition is detected, true otherwise
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendTo(nsAString& aString,
                 const mozilla::fallible_t& aFallible) const {
     if (mState.mIs2b) {
@@ -170,7 +170,7 @@ public:
    * @param aLength the length of the substring
    * @return false if an out of memory condition is detected, true otherwise
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendTo(nsAString& aString, int32_t aOffset, int32_t aLength,
                 const mozilla::fallible_t& aFallible) const
   {
diff --git a/dom/base/nsXHTMLContentSerializer.h b/dom/base/nsXHTMLContentSerializer.h
index d79f588b84..bc2ee87533 100644
--- a/dom/base/nsXHTMLContentSerializer.h
+++ b/dom/base/nsXHTMLContentSerializer.h
@@ -46,7 +46,7 @@ class nsXHTMLContentSerializer : public nsXMLContentSerializer {
                           nsAString& aStr,
                           nsresult& aResult) override;
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AfterElementStart(nsIContent* aContent,
                                  nsIContent* aOriginalElement,
                                  nsAString& aStr) override;
@@ -70,7 +70,7 @@ class nsXHTMLContentSerializer : public nsXMLContentSerializer {
   virtual void MaybeEnterInPreContent(nsIContent* aNode) override;
   virtual void MaybeLeaveFromPreContent(nsIContent* aNode) override;
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool SerializeAttributes(nsIContent* aContent,
                            nsIContent *aOriginalElement,
                            nsAString& aTagPrefix,
@@ -82,13 +82,13 @@ class nsXHTMLContentSerializer : public nsXMLContentSerializer {
 
   bool IsFirstChildOfOL(nsIContent* aElement);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SerializeLIValueAttribute(nsIContent* aElement,
                                  nsAString& aStr);
   bool IsShorthandAttr(const nsIAtom* aAttrName,
                          const nsIAtom* aElementName);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AppendAndTranslateEntities(const nsAString& aStr,
                                           nsAString& aOutputStr) override;
 
diff --git a/dom/base/nsXMLContentSerializer.h b/dom/base/nsXMLContentSerializer.h
index b48f9411d5..c9a4b4002a 100644
--- a/dom/base/nsXMLContentSerializer.h
+++ b/dom/base/nsXMLContentSerializer.h
@@ -72,23 +72,21 @@ class nsXMLContentSerializer : public nsIContentSerializer {
   /**
    * Appends a char16_t character and increments the column position
    */
-  MOZ_MUST_USE
-  bool AppendToString(const char16_t aChar,
-                      nsAString& aOutputStr);
+  [[nodiscard]] bool AppendToString(const char16_t aChar,
+                                    nsAString& aOutputStr);
 
   /**
    * Appends a nsAString string and increments the column position
    */
-  MOZ_MUST_USE
-  bool AppendToString(const nsAString& aStr,
-                      nsAString& aOutputStr);
+  [[nodiscard]] bool AppendToString(const nsAString& aStr,
+                                    nsAString& aOutputStr);
 
   /**
    * Appends a string by replacing all line-endings
    * by mLineBreak, except in the case of raw output.
    * It increments the column position.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendToStringConvertLF(const nsAString& aStr,
                                nsAString& aOutputStr);
 
@@ -96,7 +94,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * Appends a string by wrapping it when necessary.
    * It updates the column position.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendToStringWrapped(const nsASingleFragmentString& aStr,
                              nsAString& aOutputStr);
 
@@ -104,12 +102,12 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * Appends a string by formating and wrapping it when necessary
    * It updates the column position.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendToStringFormatedWrapped(const nsASingleFragmentString& aStr,
                                      nsAString& aOutputStr);
 
   // used by AppendToStringWrapped
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendWrapped_WhitespaceSequence(
           nsASingleFragmentString::const_char_iterator &aPos,
           const nsASingleFragmentString::const_char_iterator aEnd,
@@ -117,7 +115,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
           nsAString &aOutputStr);
 
   // used by AppendToStringFormatedWrapped
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendFormatedWrapped_WhitespaceSequence(
           nsASingleFragmentString::const_char_iterator &aPos,
           const nsASingleFragmentString::const_char_iterator aEnd,
@@ -126,7 +124,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
           nsAString &aOutputStr);
 
   // used by AppendToStringWrapped and AppendToStringFormatedWrapped
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendWrapped_NonWhitespaceSequence(
           nsASingleFragmentString::const_char_iterator &aPos,
           const nsASingleFragmentString::const_char_iterator aEnd,
@@ -139,7 +137,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * add mLineBreak to the string
    * It updates the column position and other flags.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendNewLineToString(nsAString& aOutputStr);
 
 
@@ -147,7 +145,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * Appends a string by translating entities
    * It doesn't increment the column position
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AppendAndTranslateEntities(const nsAString& aStr,
                                           nsAString& aOutputStr);
 
@@ -197,7 +195,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
                                      nsIContent *aOriginalElement,
                                      const nsAString& aTagNamespaceURI);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool SerializeAttributes(nsIContent* aContent,
                                    nsIContent *aOriginalElement,
                                    nsAString& aTagPrefix,
@@ -207,7 +205,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
                                    uint32_t aSkipAttr,
                                    bool aAddNSAttr);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SerializeAttr(const nsAString& aPrefix,
                      const nsAString& aName,
                      const nsAString& aValue,
@@ -239,7 +237,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * aElement and aOriginalElement are the same as the corresponding arguments
    * to AppendElementStart.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendEndOfElementStart(mozilla::dom::Element* aEleemnt,
                                mozilla::dom::Element* aOriginalElement,
                                nsAString& aStr);
@@ -249,7 +247,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * after the serialization ot the start tag.
    * (called at the end of AppendElementStart)
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AfterElementStart(nsIContent* aContent,
                                  nsIContent* aOriginalElement,
                                  nsAString& aStr) { return true; };
@@ -299,17 +297,14 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * add intendation. Call only in the case of formating and if the current
    * position is at 0. It updates the column position.
    */
-  MOZ_MUST_USE
-  bool AppendIndentation(nsAString& aStr);
+  [[nodiscard]] bool AppendIndentation(nsAString& aStr);
 
-  MOZ_MUST_USE
-  bool IncrIndentation(nsIAtom* aName);
+  [[nodiscard]] bool IncrIndentation(nsIAtom* aName);
   void DecrIndentation(nsIAtom* aName);
 
   // Functions to check for newlines that needs to be added between nodes in
   // the root of a document. See mAddNewlineForRootNode
-  MOZ_MUST_USE
-  bool MaybeAddNewlineForRootNode(nsAString& aStr);
+  [[nodiscard]] bool MaybeAddNewlineForRootNode(nsAString& aStr);
   void MaybeFlagNewlineForRootNode(nsINode* aNode);
 
   // Functions to check if we enter in or leave from a preformated content
diff --git a/dom/bindings/ToJSValue.h b/dom/bindings/ToJSValue.h
index 25bd7639fe..0b643315ec 100644
--- a/dom/bindings/ToJSValue.h
+++ b/dom/bindings/ToJSValue.h
@@ -24,7 +24,7 @@ class Promise;
 // JSContext.
 
 // Accept strings.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const nsAString& aArgument,
           JS::MutableHandle<JS::Value> aValue);
@@ -35,7 +35,7 @@ ToJSValue(JSContext* aCx,
 // desirable.  So make this a template that only gets used if the argument type
 // is actually boolean
 template<typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsSame<T, bool>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           T aArgument,
@@ -123,7 +123,7 @@ ToJSValue(JSContext* aCx,
 }
 
 // Accept CallbackObjects
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx,
           CallbackObject& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -139,7 +139,7 @@ ToJSValue(JSContext* aCx,
 // Accept objects that inherit from nsWrapperCache (e.g. most
 // DOM objects).
 template <class T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<nsWrapperCache, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           T& aArgument,
@@ -155,7 +155,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept typed arrays built from appropriate nsTArray values
 template<typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<AllTypedArraysBase, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           const TypedArrayCreator<T>& aArgument,
@@ -175,7 +175,7 @@ ToJSValue(JSContext* aCx,
 // Accept objects that inherit from nsISupports but not nsWrapperCache (e.g.
 // DOM File).
 template <class T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<!IsBaseOf<nsWrapperCache, T>::value &&
                   !IsBaseOf<CallbackObject, T>::value &&
                   IsBaseOf<nsISupports, T>::value, bool>::Type
@@ -193,7 +193,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept nsRefPtr/nsCOMPtr
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const nsCOMPtr<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -202,7 +202,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const RefPtr<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -211,7 +211,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const NonNull<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -221,7 +221,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept WebIDL dictionaries
 template <class T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<DictionaryBase, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           const T& aArgument,
@@ -231,7 +231,7 @@ ToJSValue(JSContext* aCx,
 }
 
 // Accept existing JS values (which may not be same-compartment with us
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, JS::Handle<JS::Value> aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -240,7 +240,7 @@ ToJSValue(JSContext* aCx, JS::Handle<JS::Value> aArgument,
 }
 
 // Accept existing JS values on the Heap (which may not be same-compartment with us
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, const JS::Heap<JS::Value>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -249,7 +249,7 @@ ToJSValue(JSContext* aCx, const JS::Heap<JS::Value>& aArgument,
 }
 
 // Accept existing rooted JS values (which may not be same-compartment with us
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, const JS::Rooted<JS::Value>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -259,7 +259,7 @@ ToJSValue(JSContext* aCx, const JS::Rooted<JS::Value>& aArgument,
 
 // Accept existing rooted JS objects (which may not be same-compartment with
 // us).
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, const JS::Rooted<JSObject*>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -269,7 +269,7 @@ ToJSValue(JSContext* aCx, const JS::Rooted<JSObject*>& aArgument,
 
 // Accept nsresult, for use in rejections, and create an XPCOM
 // exception object representing that nsresult.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           nsresult aArgument,
           JS::MutableHandle<JS::Value> aValue);
@@ -277,14 +277,14 @@ ToJSValue(JSContext* aCx,
 // Accept ErrorResult, for use in rejections, and create an exception
 // representing the failure.  Note, the ErrorResult must indicate a failure
 // with aArgument.Failure() returning true.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           ErrorResult& aArgument,
           JS::MutableHandle<JS::Value> aValue);
 
 // Accept owning WebIDL unions.
 template <typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<AllOwningUnionBase, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           const T& aArgument,
@@ -296,7 +296,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept pointers to other things we accept
 template <typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsPointer<T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           T aArgument,
@@ -306,14 +306,14 @@ ToJSValue(JSContext* aCx,
 }
 
 // Accept Promise objects, which need special handling.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           Promise& aArgument,
           JS::MutableHandle<JS::Value> aValue);
 
 // Accept arrays of other things we accept
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           T* aArguments,
           size_t aLength,
@@ -340,7 +340,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const nsTArray<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -350,7 +350,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const FallibleTArray<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -360,7 +360,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T, int N>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const T(&aArgument)[N],
           JS::MutableHandle<JS::Value> aValue)
diff --git a/dom/html/nsTextEditorState.h b/dom/html/nsTextEditorState.h
index 5abc88d44e..a5d2482387 100644
--- a/dom/html/nsTextEditorState.h
+++ b/dom/html/nsTextEditorState.h
@@ -154,7 +154,7 @@ public:
     // Whether the value change should be notified to the frame/contet nor not.
     eSetValue_Notify                = 1 << 2
   };
-  MOZ_MUST_USE bool SetValue(const nsAString& aValue, uint32_t aFlags);
+  [[nodiscard]] bool SetValue(const nsAString& aValue, uint32_t aFlags);
   void GetValue(nsAString& aValue, bool aIgnoreWrap) const;
   bool HasNonEmptyValue();
   // The following methods are for textarea element to use whether default
diff --git a/dom/ipc/nsIContentParent.h b/dom/ipc/nsIContentParent.h
index 6fae9060cb..fb9c31ff92 100644
--- a/dom/ipc/nsIContentParent.h
+++ b/dom/ipc/nsIContentParent.h
@@ -63,11 +63,11 @@ public:
   virtual bool IsForApp() const = 0;
   virtual bool IsForBrowser() const = 0;
 
-  MOZ_MUST_USE virtual PBlobParent*
+  [[nodiscard]] virtual PBlobParent*
   SendPBlobConstructor(PBlobParent* aActor,
                        const BlobConstructorParams& aParams) = 0;
 
-  MOZ_MUST_USE virtual PBrowserParent*
+  [[nodiscard]] virtual PBrowserParent*
   SendPBrowserConstructor(PBrowserParent* actor,
                           const TabId& aTabId,
                           const IPCTabContext& context,
diff --git a/dom/media/webaudio/AlignedTArray.h b/dom/media/webaudio/AlignedTArray.h
index 6d144d4e5c..5b9c880f65 100644
--- a/dom/media/webaudio/AlignedTArray.h
+++ b/dom/media/webaudio/AlignedTArray.h
@@ -36,7 +36,7 @@ public:
     base_type::SetLength(newLen + sExtra);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SetLength(size_type newLen, const mozilla::fallible_t&)
   {
     return base_type::SetLength(newLen + sExtra, mozilla::fallible);
@@ -89,7 +89,7 @@ public:
     base_type::SetLength(newLen + sExtra);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SetLength(size_type newLen, const mozilla::fallible_t&)
   {
     return base_type::SetLength(newLen + sExtra, mozilla::fallible);
diff --git a/dom/script/ScriptLoader.h b/dom/script/ScriptLoader.h
index 6cf108e331..54a28021b5 100644
--- a/dom/script/ScriptLoader.h
+++ b/dom/script/ScriptLoader.h
@@ -287,14 +287,14 @@ public:
     insertBack(aElem);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   already_AddRefed<ScriptLoadRequest> Steal(ScriptLoadRequest* aElem)
   {
     aElem->removeFrom(*this);
     return dont_AddRef(aElem);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   already_AddRefed<ScriptLoadRequest> StealFirst()
   {
     MOZ_ASSERT(!isEmpty());
diff --git a/dom/script/ScriptSettings.h b/dom/script/ScriptSettings.h
index f2e12f0be9..381f923e6e 100644
--- a/dom/script/ScriptSettings.h
+++ b/dom/script/ScriptSettings.h
@@ -224,11 +224,11 @@ public:
   // If aGlobalObject represents a web-visible global, errors reported by this
   // AutoJSAPI as it comes off the stack will fire the relevant error events and
   // show up in the corresponding web console.
-  MOZ_MUST_USE bool Init(nsIGlobalObject* aGlobalObject);
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject);
 
   // This is a helper that grabs the native global associated with aObject and
   // invokes the above Init() with that.
-  MOZ_MUST_USE bool Init(JSObject* aObject);
+  [[nodiscard]] bool Init(JSObject* aObject);
 
   // Unsurprisingly, this uses aCx and enters the compartment of aGlobalObject.
   // If aGlobalObject or its associated JS global are null then it returns
@@ -238,15 +238,15 @@ public:
   // If aGlobalObject represents a web-visible global, errors reported by this
   // AutoJSAPI as it comes off the stack will fire the relevant error events and
   // show up in the corresponding web console.
-  MOZ_MUST_USE bool Init(nsIGlobalObject* aGlobalObject, JSContext* aCx);
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject, JSContext* aCx);
 
   // Convenience functions to take an nsPIDOMWindow* or nsGlobalWindow*,
   // when it is more easily available than an nsIGlobalObject.
-  MOZ_MUST_USE bool Init(nsPIDOMWindowInner* aWindow);
-  MOZ_MUST_USE bool Init(nsPIDOMWindowInner* aWindow, JSContext* aCx);
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow);
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow, JSContext* aCx);
 
-  MOZ_MUST_USE bool Init(nsGlobalWindow* aWindow);
-  MOZ_MUST_USE bool Init(nsGlobalWindow* aWindow, JSContext* aCx);
+  [[nodiscard]] bool Init(nsGlobalWindow* aWindow);
+  [[nodiscard]] bool Init(nsGlobalWindow* aWindow, JSContext* aCx);
 
   JSContext* cx() const {
     MOZ_ASSERT(mCx, "Must call Init before using an AutoJSAPI");
@@ -272,12 +272,12 @@ public:
   //
   // Note that this fails if and only if we OOM while wrapping the exception
   // into the current compartment.
-  MOZ_MUST_USE bool StealException(JS::MutableHandle<JS::Value> aVal);
+  [[nodiscard]] bool StealException(JS::MutableHandle<JS::Value> aVal);
 
   // As for StealException(), but put the saved frames for any stack trace
   // associated with the point the exception was thrown into aStack.
   // aVal will be in the current compartment, but aStack might not be.
-  MOZ_MUST_USE bool StealExceptionAndStack(JS::MutableHandle<JS::Value> aVal,
+  [[nodiscard]] bool StealExceptionAndStack(JS::MutableHandle<JS::Value> aVal,
                                            JS::MutableHandle<JSObject*> aStack);
 
   // Peek the current exception from the JS engine, without stealing it.
@@ -286,7 +286,7 @@ public:
   //
   // Note that this fails if and only if we OOM while wrapping the exception
   // into the current compartment.
-  MOZ_MUST_USE bool PeekException(JS::MutableHandle<JS::Value> aVal);
+  [[nodiscard]] bool PeekException(JS::MutableHandle<JS::Value> aVal);
 
   void ClearException() {
     MOZ_ASSERT(IsStackTop());
diff --git a/dom/xhr/XMLHttpRequestString.h b/dom/xhr/XMLHttpRequestString.h
index b02040b7ea..df2fc76668 100644
--- a/dom/xhr/XMLHttpRequestString.h
+++ b/dom/xhr/XMLHttpRequestString.h
@@ -44,7 +44,7 @@ public:
   // This method should be called only when the string is really needed because
   // it can cause the duplication of the strings in case the loading of the XHR
   // is not completed yet.
-  MOZ_MUST_USE bool GetAsString(nsAString& aString) const;
+  [[nodiscard]] bool GetAsString(nsAString& aString) const;
 
   size_t SizeOfThis(MallocSizeOf aMallocSizeOf) const;
 
@@ -121,7 +121,7 @@ public:
     return !mLength;
   }
 
-  MOZ_MUST_USE bool GetAsString(DOMString& aString) const;
+  [[nodiscard]] bool GetAsString(DOMString& aString) const;
 
 private:
   XMLHttpRequestStringSnapshot(const XMLHttpRequestStringSnapshot&) = delete;
@@ -193,7 +193,7 @@ public:
     mData.Append(aString);
   }
 
-  MOZ_MUST_USE bool
+  [[nodiscard]] bool
   GetAsString(nsAString& aString)
   {
     MutexAutoLock lock(mMutex);
@@ -206,7 +206,7 @@ public:
     return mData.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
   }
 
-  MOZ_MUST_USE bool
+  [[nodiscard]] bool
   GetAsString(DOMString& aString, uint32_t aLength)
   {
     MutexAutoLock lock(mMutex);
diff --git a/dom/xslt/xslt/txExecutionState.h b/dom/xslt/xslt/txExecutionState.h
index cd54671791..2a7a7912b0 100644
--- a/dom/xslt/xslt/txExecutionState.h
+++ b/dom/xslt/xslt/txExecutionState.h
@@ -60,7 +60,7 @@ public:
     {
     }
     ~txLoadedDocumentsHash();
-    MOZ_MUST_USE nsresult init(const txXPathNode& aSource);
+    [[nodiscard]] nsresult init(const txXPathNode& aSource);
 
 private:
     friend class txExecutionState;