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diff --git a/js/public/StableStringChars.h b/js/public/StableStringChars.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+/*
+ * Safely access the contents of a string even as GC can cause the string's
+ * contents to move around in memory.
+ */
+
+#ifndef js_StableStringChars_h
+#define js_StableStringChars_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+#include "mozilla/Attributes.h"  // MOZ_INIT_OUTSIDE_CTOR, MOZ_STACK_CLASS, MOZ_MUST_USE
+#include "mozilla/Maybe.h"  // mozilla::Maybe
+#include "mozilla/Range.h"  // mozilla::Range
+
+#include <stddef.h>  // size_t
+#include <stdint.h>  // uint8_t
+
+#include "jstypes.h"  // JS_FRIEND_API
+
+#include "js/RootingAPI.h"  // JS::Handle, JS::Rooted
+#include "js/String.h"      // JS::GetStringLength
+#include "js/TypeDecls.h"   // JSContext, JS::Latin1Char, JSString
+#include "js/Vector.h"      // js::Vector
+
+class JSLinearString;
+
+namespace JS {
+
+/**
+ * This class provides safe access to a string's chars across a GC. If we ever
+ * nursery allocate strings' out of line chars, this class will have to make a
+ * copy, so it's best to avoid using this class unless you really need it. It's
+ * usually more efficient to use the latin1Chars/twoByteChars JSString methods
+ * and often the code can be rewritten so that only indexes instead of char
+ * pointers are used in parts of the code that can GC.
+ */
+class MOZ_STACK_CLASS JS_FRIEND_API AutoStableStringChars final {
+  /*
+   * When copying string char, use this many bytes of inline storage.  This is
+   * chosen to allow the inline string types to be copied without allocating.
+   * This is asserted in AutoStableStringChars::allocOwnChars.
+   */
+  static const size_t InlineCapacity = 24;
+
+  /* Ensure the string is kept alive while we're using its chars. */
+  Rooted<JSString*> s_;
+  union MOZ_INIT_OUTSIDE_CTOR {
+    const char16_t* twoByteChars_;
+    const Latin1Char* latin1Chars_;
+  };
+  mozilla::Maybe<js::Vector<uint8_t, InlineCapacity>> ownChars_;
+  enum State { Uninitialized, Latin1, TwoByte };
+  State state_;
+
+ public:
+  explicit AutoStableStringChars(JSContext* cx)
+      : s_(cx), state_(Uninitialized) {}
+
+  MOZ_MUST_USE bool init(JSContext* cx, JSString* s);
+
+  /* Like init(), but Latin1 chars are inflated to TwoByte. */
+  MOZ_MUST_USE bool initTwoByte(JSContext* cx, JSString* s);
+
+  bool isLatin1() const { return state_ == Latin1; }
+  bool isTwoByte() const { return state_ == TwoByte; }
+
+  const Latin1Char* latin1Chars() const {
+    MOZ_ASSERT(state_ == Latin1);
+    return latin1Chars_;
+  }
+  const char16_t* twoByteChars() const {
+    MOZ_ASSERT(state_ == TwoByte);
+    return twoByteChars_;
+  }
+
+  mozilla::Range<const Latin1Char> latin1Range() const {
+    MOZ_ASSERT(state_ == Latin1);
+    return mozilla::Range<const Latin1Char>(latin1Chars_, GetStringLength(s_));
+  }
+
+  mozilla::Range<const char16_t> twoByteRange() const {
+    MOZ_ASSERT(state_ == TwoByte);
+    return mozilla::Range<const char16_t>(twoByteChars_, GetStringLength(s_));
+  }
+
+  /* If we own the chars, transfer ownership to the caller. */
+  bool maybeGiveOwnershipToCaller() {
+    MOZ_ASSERT(state_ != Uninitialized);
+    if (!ownChars_.isSome() || !ownChars_->extractRawBuffer()) {
+      return false;
+    }
+    state_ = Uninitialized;
+    ownChars_.reset();
+    return true;
+  }
+
+ private:
+  AutoStableStringChars(const AutoStableStringChars& other) = delete;
+  void operator=(const AutoStableStringChars& other) = delete;
+
+  bool baseIsInline(Handle<JSLinearString*> linearString);
+  template <typename T>
+  T* allocOwnChars(JSContext* cx, size_t count);
+  bool copyLatin1Chars(JSContext* cx, Handle<JSLinearString*> linearString);
+  bool copyTwoByteChars(JSContext* cx, Handle<JSLinearString*> linearString);
+  bool copyAndInflateLatin1Chars(JSContext*,
+                                 Handle<JSLinearString*> linearString);
+};
+
+}  // namespace JS
+
+#endif /* js_StableStringChars_h */