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diff --git a/dom/base/nsJSUtils.h b/dom/base/nsJSUtils.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 nsJSUtils_h__
+#define nsJSUtils_h__
+
+/**
+ * This is not a generated file. It contains common utility functions
+ * invoked from the JavaScript code generated from IDL interfaces.
+ * The goal of the utility functions is to cut down on the size of
+ * the generated code itself.
+ */
+
+#include "mozilla/Assertions.h"
+
+#include "jsapi.h"
+#include "js/CompileOptions.h"
+#include "js/Conversions.h"
+#include "js/SourceText.h"
+#include "js/String.h"  // JS::{,Lossy}CopyLinearStringChars, JS::CopyStringChars, JS::Get{,Linear}StringLength, JS::MaxStringLength, JS::StringHasLatin1Chars
+#include "nsString.h"
+#include "xpcpublic.h"
+
+class nsIScriptContext;
+class nsIScriptElement;
+class nsIScriptGlobalObject;
+class nsXBLPrototypeBinding;
+
+namespace mozilla {
+union Utf8Unit;
+
+namespace dom {
+class AutoJSAPI;
+class Element;
+}  // namespace dom
+}  // namespace mozilla
+
+class nsJSUtils {
+ public:
+  static bool GetCallingLocation(JSContext* aContext, nsACString& aFilename,
+                                 uint32_t* aLineno = nullptr,
+                                 uint32_t* aColumn = nullptr);
+  static bool GetCallingLocation(JSContext* aContext, nsAString& aFilename,
+                                 uint32_t* aLineno = nullptr,
+                                 uint32_t* aColumn = nullptr);
+
+  /**
+   * Retrieve the inner window ID based on the given JSContext.
+   *
+   * @param JSContext aContext
+   *        The JSContext from which you want to find the inner window ID.
+   *
+   * @returns uint64_t the inner window ID.
+   */
+  static uint64_t GetCurrentlyRunningCodeInnerWindowID(JSContext* aContext);
+
+  static nsresult CompileFunction(mozilla::dom::AutoJSAPI& jsapi,
+                                  JS::HandleVector<JSObject*> aScopeChain,
+                                  JS::CompileOptions& aOptions,
+                                  const nsACString& aName, uint32_t aArgCount,
+                                  const char** aArgArray,
+                                  const nsAString& aBody,
+                                  JSObject** aFunctionObject);
+
+  static nsresult UpdateFunctionDebugMetadata(
+      mozilla::dom::AutoJSAPI& jsapi, JS::Handle<JSObject*> aFun,
+      JS::CompileOptions& aOptions, JS::Handle<JSString*> aElementAttributeName,
+      JS::Handle<JS::Value> aPrivateValue);
+
+  static bool IsScriptable(JS::Handle<JSObject*> aEvaluationGlobal);
+
+  // Returns false if an exception got thrown on aCx.  Passing a null
+  // aElement is allowed; that wil produce an empty aScopeChain.
+  static bool GetScopeChainForElement(
+      JSContext* aCx, mozilla::dom::Element* aElement,
+      JS::MutableHandleVector<JSObject*> aScopeChain);
+
+  static void ResetTimeZone();
+
+  static bool DumpEnabled();
+
+  // A helper function that receives buffer pointer, creates ArrayBuffer, and
+  // convert it to Uint8Array.
+  // Note that the buffer needs to be created by JS_malloc (or at least can be
+  // freed by JS_free), as the resulting Uint8Array takes the ownership of the
+  // buffer.
+  static JSObject* MoveBufferAsUint8Array(JSContext* aCx, size_t aSize,
+                                          mozilla::UniquePtr<uint8_t>& aBuffer);
+};
+
+inline void AssignFromStringBuffer(nsStringBuffer* buffer, size_t len,
+                                   nsAString& dest) {
+  buffer->ToString(len, dest);
+}
+
+template <typename T, typename std::enable_if_t<std::is_same<
+                          typename T::char_type, char16_t>::value>* = nullptr>
+inline bool AssignJSString(JSContext* cx, T& dest, JSString* s) {
+  size_t len = JS::GetStringLength(s);
+  static_assert(JS::MaxStringLength < (1 << 30),
+                "Shouldn't overflow here or in SetCapacity");
+
+  const char16_t* chars;
+  if (XPCStringConvert::MaybeGetDOMStringChars(s, &chars)) {
+    // The characters represent an existing string buffer that we shared with
+    // JS.  We can share that buffer ourselves if the string corresponds to the
+    // whole buffer; otherwise we have to copy.
+    if (chars[len] == '\0') {
+      AssignFromStringBuffer(
+          nsStringBuffer::FromData(const_cast<char16_t*>(chars)), len, dest);
+      return true;
+    }
+  } else if (XPCStringConvert::MaybeGetLiteralStringChars(s, &chars)) {
+    // The characters represent a literal char16_t string constant
+    // compiled into libxul; we can just use it as-is.
+    dest.AssignLiteral(chars, len);
+    return true;
+  }
+
+  // We don't bother checking for a dynamic-atom external string, because we'd
+  // just need to copy out of it anyway.
+
+  if (MOZ_UNLIKELY(!dest.SetLength(len, mozilla::fallible))) {
+    JS_ReportOutOfMemory(cx);
+    return false;
+  }
+  return JS::CopyStringChars(cx, dest.BeginWriting(), s, len);
+}
+
+// Specialization for UTF8String.
+template <typename T, typename std::enable_if_t<std::is_same<
+                          typename T::char_type, char>::value>* = nullptr>
+inline bool AssignJSString(JSContext* cx, T& dest, JSString* s) {
+  using namespace mozilla;
+  CheckedInt<size_t> bufLen(JS::GetStringLength(s));
+  // From the contract for JS_EncodeStringToUTF8BufferPartial, to guarantee that
+  // the whole string is converted.
+  if (JS::StringHasLatin1Chars(s)) {
+    bufLen *= 2;
+  } else {
+    bufLen *= 3;
+  }
+
+  if (MOZ_UNLIKELY(!bufLen.isValid())) {
+    JS_ReportOutOfMemory(cx);
+    return false;
+  }
+
+  // Shouldn't really matter, but worth being safe.
+  const bool kAllowShrinking = true;
+
+  auto handleOrErr = dest.BulkWrite(bufLen.value(), 0, kAllowShrinking);
+  if (MOZ_UNLIKELY(handleOrErr.isErr())) {
+    JS_ReportOutOfMemory(cx);
+    return false;
+  }
+
+  auto handle = handleOrErr.unwrap();
+
+  auto maybe = JS_EncodeStringToUTF8BufferPartial(cx, s, handle.AsSpan());
+  if (MOZ_UNLIKELY(!maybe)) {
+    JS_ReportOutOfMemory(cx);
+    return false;
+  }
+
+  size_t read;
+  size_t written;
+  std::tie(read, written) = *maybe;
+
+  MOZ_ASSERT(read == JS::GetStringLength(s));
+  handle.Finish(written, kAllowShrinking);
+  return true;
+}
+
+inline void AssignJSLinearString(nsAString& dest, JSLinearString* s) {
+  size_t len = JS::GetLinearStringLength(s);
+  static_assert(JS::MaxStringLength < (1 << 30),
+                "Shouldn't overflow here or in SetCapacity");
+  dest.SetLength(len);
+  JS::CopyLinearStringChars(dest.BeginWriting(), s, len);
+}
+
+inline void AssignJSLinearString(nsACString& dest, JSLinearString* s) {
+  size_t len = JS::GetLinearStringLength(s);
+  static_assert(JS::MaxStringLength < (1 << 30),
+                "Shouldn't overflow here or in SetCapacity");
+  dest.SetLength(len);
+  JS::LossyCopyLinearStringChars(dest.BeginWriting(), s, len);
+}
+
+template <typename T>
+class nsTAutoJSLinearString : public nsTAutoString<T> {
+ public:
+  explicit nsTAutoJSLinearString(JSLinearString* str) {
+    AssignJSLinearString(*this, str);
+  }
+};
+
+using nsAutoJSLinearString = nsTAutoJSLinearString<char16_t>;
+using nsAutoJSLinearCString = nsTAutoJSLinearString<char>;
+
+template <typename T>
+class nsTAutoJSString : public nsTAutoString<T> {
+ public:
+  /**
+   * nsTAutoJSString should be default constructed, which leaves it empty
+   * (this->IsEmpty()), and initialized with one of the init() methods below.
+   */
+  nsTAutoJSString() = default;
+
+  bool init(JSContext* aContext, JSString* str) {
+    return AssignJSString(aContext, *this, str);
+  }
+
+  bool init(JSContext* aContext, const JS::Value& v) {
+    if (v.isString()) {
+      return init(aContext, v.toString());
+    }
+
+    // Stringify, making sure not to run script.
+    JS::Rooted<JSString*> str(aContext);
+    if (v.isObject()) {
+      str = JS_NewStringCopyZ(aContext, "[Object]");
+    } else {
+      JS::Rooted<JS::Value> rootedVal(aContext, v);
+      str = JS::ToString(aContext, rootedVal);
+    }
+
+    return str && init(aContext, str);
+  }
+
+  bool init(JSContext* aContext, jsid id) {
+    JS::Rooted<JS::Value> v(aContext);
+    return JS_IdToValue(aContext, id, &v) && init(aContext, v);
+  }
+
+  bool init(const JS::Value& v);
+
+  ~nsTAutoJSString() = default;
+};
+
+using nsAutoJSString = nsTAutoJSString<char16_t>;
+
+// Note that this is guaranteed to be UTF-8.
+using nsAutoJSCString = nsTAutoJSString<char>;
+
+#endif /* nsJSUtils_h__ */