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diff --git a/js/src/frontend/ParseNodeVisitor.h b/js/src/frontend/ParseNodeVisitor.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 frontend_ParseNodeVisitor_h
+#define frontend_ParseNodeVisitor_h
+
+#include "mozilla/Assertions.h"
+#include "mozilla/Attributes.h"
+
+#include "jsfriendapi.h"
+
+#include "frontend/ParseNode.h"
+#include "js/friend/StackLimits.h"  // js::CheckRecursionLimit
+
+namespace js {
+namespace frontend {
+
+/**
+ * Utility class for walking a JS AST.
+ *
+ * Simple usage:
+ *
+ *     class HowTrueVisitor : public ParseNodeVisitor<HowTrueVisitor> {
+ *     public:
+ *       bool visitTrueExpr(BooleanLiteral* pn) {
+ *         std::cout << "How true.\n";
+ *         return true;
+ *       }
+ *       bool visitClassDecl(ClassNode* pn) {
+ *         // The base-class implementation of each visit method
+ *         // simply visits the node's children. So the subclass
+ *         // gets to decide whether to descend into a subtree
+ *         // and can do things either before or after:
+ *         std::cout << "How classy.\n";
+ *         return ParseNodeVisitor::visitClassDecl(pn);
+ *       }
+ *     };
+ *
+ *     HowTrueVisitor v;
+ *     v.visit(programRootNode);  // walks the entire tree
+ *
+ * A ParseNodeVisitor can modify nodes, but it can't replace the current node
+ * with a different one; for that, use a RewritingParseNodeVisitor.
+ *
+ * Note that the Curiously Recurring Template Pattern is used for performance,
+ * as it eliminates the need for virtual method calls. Some rough testing shows
+ * about a 12% speedup in the FoldConstants.cpp pass.
+ * https://en.wikipedia.org/wiki/Curiously_recurring_template_pattern
+ */
+template <typename Derived>
+class ParseNodeVisitor {
+ public:
+  JSContext* cx_;
+
+  explicit ParseNodeVisitor(JSContext* cx) : cx_(cx) {}
+
+  MOZ_MUST_USE bool visit(ParseNode* pn) {
+    if (!CheckRecursionLimit(cx_)) {
+      return false;
+    }
+
+    switch (pn->getKind()) {
+#define VISIT_CASE(KIND, TYPE) \
+  case ParseNodeKind::KIND:    \
+    return static_cast<Derived*>(this)->visit##KIND(&pn->as<TYPE>());
+      FOR_EACH_PARSE_NODE_KIND(VISIT_CASE)
+#undef VISIT_CASE
+      default:
+        MOZ_CRASH("invalid node kind");
+    }
+  }
+
+  // using static_cast<Derived*> here allows plain visit() to be overridden.
+#define VISIT_METHOD(KIND, TYPE)                         \
+  MOZ_MUST_USE bool visit##KIND(TYPE* pn) { /* NOLINT */ \
+    return pn->accept(*static_cast<Derived*>(this));     \
+  }
+  FOR_EACH_PARSE_NODE_KIND(VISIT_METHOD)
+#undef VISIT_METHOD
+};
+
+/*
+ * Utility class for walking a JS AST that allows for replacing nodes.
+ *
+ * The API is the same as ParseNodeVisitor, except that visit methods take an
+ * argument of type `ParseNode*&`, a reference to the field in the parent node
+ * that points down to the node being visited. Methods can replace the current
+ * node by assigning to this reference.
+ *
+ * All visit methods take a `ParseNode*&` rather than more specific types like
+ * `BinaryNode*&`, to allow replacing the current node with one of a different
+ * type. Constant folding makes use of this.
+ */
+template <typename Derived>
+class RewritingParseNodeVisitor {
+ public:
+  JSContext* cx_;
+
+  explicit RewritingParseNodeVisitor(JSContext* cx) : cx_(cx) {}
+
+  MOZ_MUST_USE bool visit(ParseNode*& pn) {
+    if (!CheckRecursionLimit(cx_)) {
+      return false;
+    }
+
+    switch (pn->getKind()) {
+#define VISIT_CASE(KIND, _type) \
+  case ParseNodeKind::KIND:     \
+    return static_cast<Derived*>(this)->visit##KIND(pn);
+      FOR_EACH_PARSE_NODE_KIND(VISIT_CASE)
+#undef VISIT_CASE
+      default:
+        MOZ_CRASH("invalid node kind");
+    }
+  }
+
+  // using static_cast<Derived*> here allows plain visit() to be overridden.
+#define VISIT_METHOD(KIND, TYPE)                                 \
+  MOZ_MUST_USE bool visit##KIND(ParseNode*& pn) {                \
+    MOZ_ASSERT(pn->is<TYPE>(),                                   \
+               "Node of kind " #KIND " was not of type " #TYPE); \
+    return pn->as<TYPE>().accept(*static_cast<Derived*>(this));  \
+  }
+  FOR_EACH_PARSE_NODE_KIND(VISIT_METHOD)
+#undef VISIT_METHOD
+};
+
+}  // namespace frontend
+}  // namespace js
+
+#endif  // frontend_ParseNodeVisitor_h