Bug 1664100 - Replace MOZ_MUST_USE with [[nodiscard]] in caps. r=ckerschb

The MOZ_MUST_USE macro is defined as clang's and gcc's nonstandard __attribute__((warn_unused_result)). Now that we compile as C++17 by default (bug 1560664), we can replace MOZ_MUST_USE with C++17's standard [[nodiscard]] attribute.

The [[nodiscard]] attribute must precede a function declaration's declaration specifiers (like static, extern, inline, or virtual). The __attribute__((warn_unused_result)) attribute does not have this order restriction.

Differential Revision: https://phabricator.services.mozilla.com/D89697

caps/OriginAttributes.h

@@ -75,7 +75,7 @@ class OriginAttributes : public dom::OriginAttributesDictionary {
     return !(*this == aOther);
   }
 
-  MOZ_MUST_USE bool EqualsIgnoringFPD(const OriginAttributes& aOther) const {
+  [[nodiscard]] bool EqualsIgnoringFPD(const OriginAttributes& aOther) const {
     return mInIsolatedMozBrowser == aOther.mInIsolatedMozBrowser &&
            mUserContextId == aOther.mUserContextId &&
            mPrivateBrowsingId == aOther.mPrivateBrowsingId &&
@@ -90,12 +90,12 @@ class OriginAttributes : public dom::OriginAttributesDictionary {
   // Don't use this method for anything else than debugging!
   void CreateAnonymizedSuffix(nsACString& aStr) const;
 
-  MOZ_MUST_USE bool PopulateFromSuffix(const nsACString& aStr);
+  [[nodiscard]] bool PopulateFromSuffix(const nsACString& aStr);
 
   // Populates the attributes from a string like
   // |uri!key1=value1&key2=value2| and returns the uri without the suffix.
-  MOZ_MUST_USE bool PopulateFromOrigin(const nsACString& aOrigin,
-                                       nsACString& aOriginNoSuffix);
+  [[nodiscard]] bool PopulateFromOrigin(const nsACString& aOrigin,
+                                        nsACString& aOriginNoSuffix);
 
   // Helper function to match mIsPrivateBrowsing to existing private browsing
   // flags. Once all other flags are removed, this can be removed too.
@@ -125,7 +125,7 @@ class OriginAttributes : public dom::OriginAttributesDictionary {
 
   // Check whether we block the postMessage across different FPDs when the
   // targetOrigin is '*'.
-  static inline MOZ_MUST_USE bool IsBlockPostMessageForFPI() {
+  [[nodiscard]] static inline bool IsBlockPostMessageForFPI() {
     return StaticPrefs::privacy_firstparty_isolate() &&
            StaticPrefs::privacy_firstparty_isolate_block_post_message();
   }