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diff --git a/dom/promise/PromiseNativeHandler.h b/dom/promise/PromiseNativeHandler.h
new file mode 100644
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+++ b/dom/promise/PromiseNativeHandler.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_PromiseNativeHandler_h
+#define mozilla_dom_PromiseNativeHandler_h
+
+#include <functional>
+#include "js/TypeDecls.h"
+#include "js/Value.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/Maybe.h"
+#include "nsISupports.h"
+
+namespace mozilla::dom {
+
+/*
+ * PromiseNativeHandler allows C++ to react to a Promise being
+ * rejected/resolved. A PromiseNativeHandler can be appended to a Promise using
+ * Promise::AppendNativeHandler().
+ */
+class PromiseNativeHandler : public nsISupports {
+ protected:
+  virtual ~PromiseNativeHandler() = default;
+
+ public:
+  MOZ_CAN_RUN_SCRIPT
+  virtual void ResolvedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue,
+                                ErrorResult& aRv) = 0;
+
+  MOZ_CAN_RUN_SCRIPT
+  virtual void RejectedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue,
+                                ErrorResult& aRv) = 0;
+};
+
+// This base class exists solely to use NS_IMPL_ISUPPORTS because it doesn't
+// support template classes.
+class MozPromiseRejectOnDestructionBase : public PromiseNativeHandler {
+  NS_DECL_ISUPPORTS
+
+  void ResolvedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue,
+                        ErrorResult& aRv) override {}
+  void RejectedCallback(JSContext* aCx, JS::Handle<JS::Value> aValue,
+                        ErrorResult& aRv) override {}
+
+ protected:
+  ~MozPromiseRejectOnDestructionBase() override = default;
+};
+
+// Use this when you subscribe to a JS promise to settle a MozPromise that is
+// not guaranteed to be settled by anyone else.
+template <typename T>
+class MozPromiseRejectOnDestruction final
+    : public MozPromiseRejectOnDestructionBase {
+ public:
+  // (Accepting RefPtr<T> instead of T* because compiler fails to implicitly
+  // convert it at call sites)
+  MozPromiseRejectOnDestruction(const RefPtr<T>& aMozPromise,
+                                const char* aCallSite)
+      : mMozPromise(aMozPromise), mCallSite(aCallSite) {
+    MOZ_ASSERT(aMozPromise);
+  }
+
+ protected:
+  ~MozPromiseRejectOnDestruction() override {
+    // Rejecting will be no-op if the promise is already settled
+    mMozPromise->Reject(NS_BINDING_ABORTED, mCallSite);
+  }
+
+  RefPtr<T> mMozPromise;
+  const char* mCallSite;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_PromiseNativeHandler_h