Bug 1434686 part 1. Introduce a mozilla::IgnoreErrors which can be used as a temporary to pass to an ErrorResult& arg when the error should be ignored. r=mystor

Once bug 1434689 is fixed, we can mark this thing as being required to be a temporary.

MozReview-Commit-ID: 7VX0XSYVOc4

dom/bindings/ErrorResult.h

@@ -602,6 +602,21 @@ class IgnoredErrorResult :
 {
 };
 
+// A class for use when an ErrorResult should just automatically be
+// ignored.  This is designed to be passed as a temporary only, like
+// so:
+//
+//    foo->Bar(IgnoreErrors());
+class IgnoreErrors {
+public:
+  operator ErrorResult&() && { return mInner; }
+private:
+  // We don't use an ErrorResult member here so we don't make two separate calls
+  // to SuppressException (one from us, one from the ErrorResult destructor
+  // after asserting).
+  binding_danger::TErrorResult<binding_danger::JustSuppressCleanupPolicy> mInner;
+};
+
 namespace dom {
 namespace binding_detail {
 class FastErrorResult :

js/src/devtools/rootAnalysis/annotations.js

@@ -336,6 +336,7 @@ function extraRootedPointers()
 
         'mozilla::ErrorResult',
         'mozilla::IgnoredErrorResult',
+        'mozilla::IgnoreErrors',
         'mozilla::dom::binding_detail::FastErrorResult',
     ];
 }