Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 1454 insertions, 0 deletions

diff --git a/xpcom/string/nsTSubstring.h b/xpcom/string/nsTSubstring.h
new file mode 100644
index 0000000000..d54dcafd7b
--- /dev/null
+++ b/xpcom/string/nsTSubstring.h
@@ -0,0 +1,1454 @@
+/* -*- 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/. */
+// IWYU pragma: private, include "nsString.h"
+
+#ifndef nsTSubstring_h
+#define nsTSubstring_h
+
+#include <iterator>
+#include <type_traits>
+
+#include "mozilla/Casting.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/IntegerPrintfMacros.h"
+#include "mozilla/UniquePtr.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/IntegerTypeTraits.h"
+#include "mozilla/Result.h"
+#include "mozilla/ResultExtensions.h"
+#include "mozilla/Span.h"
+#include "mozilla/Unused.h"
+
+#include "nsTStringRepr.h"
+
+#ifndef MOZILLA_INTERNAL_API
+#  error "Using XPCOM strings is limited to code linked into libxul."
+#endif
+
+// The max number of logically uninitialized code units to
+// fill with a marker byte or to mark as unintialized for
+// memory checking. (Limited to avoid quadratic behavior.)
+const size_t kNsStringBufferMaxPoison = 16;
+
+class nsStringBuffer;
+template <typename T>
+class nsTSubstringSplitter;
+template <typename T>
+class nsTString;
+template <typename T>
+class nsTSubstring;
+
+namespace mozilla {
+
+/**
+ * This handle represents permission to perform low-level writes
+ * the storage buffer of a string in a manner that's aware of the
+ * actual capacity of the storage buffer allocation and that's
+ * cache-friendly in the sense that the writing of zero terminator
+ * for C compatibility can happen in linear memory access order
+ * (i.e. the zero terminator write takes place after writing
+ * new content to the string as opposed to the zero terminator
+ * write happening first causing a non-linear memory write for
+ * cache purposes).
+ *
+ * If you requested a prefix to be preserved when starting
+ * or restarting the bulk write, the prefix is present at the
+ * start of the buffer exposed by this handle as Span or
+ * as a raw pointer, and it's your responsibility to start
+ * writing after after the preserved prefix (which you
+ * presumably wanted not to overwrite since you asked for
+ * it to be preserved).
+ *
+ * In a success case, you must call Finish() with the new
+ * length of the string. In failure cases, it's OK to return
+ * early from the function whose local variable this handle is.
+ * The destructor of this class takes care of putting the
+ * string in a valid and mostly harmless state in that case
+ * by setting the value of a non-empty string to a single
+ * REPLACEMENT CHARACTER or in the case of nsACString that's
+ * too short for a REPLACEMENT CHARACTER to fit, an ASCII
+ * SUBSTITUTE.
+ *
+ * You must not allow this handle to outlive the string you
+ * obtained it from.
+ *
+ * You must not access the string you obtained this handle
+ * from in any way other than through this handle until
+ * you call Finish() on the handle or the handle goes out
+ * of scope.
+ *
+ * Once you've called Finish(), you must not call any
+ * methods on this handle and must not use values previously
+ * obtained.
+ *
+ * Once you call RestartBulkWrite(), you must not use
+ * values previously obtained from this handle and must
+ * reobtain the new corresponding values.
+ */
+template <typename T>
+class BulkWriteHandle final {
+  friend class nsTSubstring<T>;
+
+ public:
+  typedef typename mozilla::detail::nsTStringRepr<T> base_string_type;
+  typedef typename base_string_type::size_type size_type;
+
+  /**
+   * Pointer to the start of the writable buffer. Never nullptr.
+   *
+   * This pointer is valid until whichever of these happens first:
+   *  1) Finish() is called
+   *  2) RestartBulkWrite() is called
+   *  3) BulkWriteHandle goes out of scope
+   */
+  T* Elements() const {
+    MOZ_ASSERT(mString);
+    return mString->mData;
+  }
+
+  /**
+   * How many code units can be written to the buffer.
+   * (Note: This is not the same as the string's Length().)
+   *
+   * This value is valid until whichever of these happens first:
+   *  1) Finish() is called
+   *  2) RestartBulkWrite() is called
+   *  3) BulkWriteHandle goes out of scope
+   */
+  size_type Length() const {
+    MOZ_ASSERT(mString);
+    return mCapacity;
+  }
+
+  /**
+   * Pointer past the end of the buffer.
+   *
+   * This pointer is valid until whichever of these happens first:
+   *  1) Finish() is called
+   *  2) RestartBulkWrite() is called
+   *  3) BulkWriteHandle goes out of scope
+   */
+  T* End() const { return Elements() + Length(); }
+
+  /**
+   * The writable buffer as Span.
+   *
+   * This Span is valid until whichever of these happens first:
+   *  1) Finish() is called
+   *  2) RestartBulkWrite() is called
+   *  3) BulkWriteHandle goes out of scope
+   */
+  auto AsSpan() const { return mozilla::Span<T>{Elements(), Length()}; }
+
+  /**
+   * Autoconvert to the buffer as writable Span.
+   *
+   * This Span is valid until whichever of these happens first:
+   *  1) Finish() is called
+   *  2) RestartBulkWrite() is called
+   *  3) BulkWriteHandle goes out of scope
+   */
+  operator mozilla::Span<T>() const { return AsSpan(); }
+
+  /**
+   * Restart the bulk write with a different capacity.
+   *
+   * This method invalidates previous return values
+   * of the other methods above.
+   *
+   * Can fail if out of memory leaving the buffer
+   * in the state before this call.
+   *
+   * @param aCapacity the new requested capacity
+   * @param aPrefixToPreserve the number of code units at
+   *                          the start of the string to
+   *                          copy over to the new buffer
+   * @param aAllowShrinking whether the string is
+   *                        allowed to attempt to
+   *                        allocate a smaller buffer
+   *                        for its content and copy
+   *                        the data over.
+   */
+  mozilla::Result<mozilla::Ok, nsresult> RestartBulkWrite(
+      size_type aCapacity, size_type aPrefixToPreserve, bool aAllowShrinking) {
+    MOZ_ASSERT(mString);
+    MOZ_TRY_VAR(mCapacity, mString->StartBulkWriteImpl(
+                               aCapacity, aPrefixToPreserve, aAllowShrinking));
+    return mozilla::Ok();
+  }
+
+  /**
+   * Indicate that the bulk write finished successfully.
+   *
+   * @param aLength the number of code units written;
+   *                must not exceed Length()
+   * @param aAllowShrinking whether the string is
+   *                        allowed to attempt to
+   *                        allocate a smaller buffer
+   *                        for its content and copy
+   *                        the data over.
+   */
+  void Finish(size_type aLength, bool aAllowShrinking) {
+    MOZ_ASSERT(mString);
+    MOZ_ASSERT(aLength <= mCapacity);
+    if (!aLength) {
+      // Truncate is safe even when the string is in an invalid state
+      mString->Truncate();
+      mString = nullptr;
+      return;
+    }
+    if (aAllowShrinking) {
+      mozilla::Unused << mString->StartBulkWriteImpl(aLength, aLength, true);
+    }
+    mString->FinishBulkWriteImpl(aLength);
+    mString = nullptr;
+  }
+
+  BulkWriteHandle(BulkWriteHandle&& aOther)
+      : mString(aOther.Forget()), mCapacity(aOther.mCapacity) {}
+
+  ~BulkWriteHandle() {
+    if (!mString || !mCapacity) {
+      return;
+    }
+    // The old zero terminator may be gone by now, so we need
+    // to write a new one somewhere and make length match.
+    // We can use a length between 1 and self.capacity.
+    // The contents of the string can be partially uninitialized
+    // or partially initialized in a way that would be dangerous
+    // if parsed by some recipient. It's prudent to write something
+    // same as the contents of the string. U+FFFD is the safest
+    // placeholder, but when it doesn't fit, let's use ASCII
+    // substitute. Merely truncating the string to a zero-length
+    // string might be dangerous in some scenarios. See
+    // https://www.unicode.org/reports/tr36/#Substituting_for_Ill_Formed_Subsequences
+    // for closely related scenario.
+    auto ptr = Elements();
+    // Cast the pointer below to silence warnings
+    if (sizeof(T) == 1) {
+      unsigned char* charPtr = reinterpret_cast<unsigned char*>(ptr);
+      if (mCapacity >= 3) {
+        *charPtr++ = 0xEF;
+        *charPtr++ = 0xBF;
+        *charPtr++ = 0xBD;
+        mString->mLength = 3;
+      } else {
+        *charPtr++ = 0x1A;
+        mString->mLength = 1;
+      }
+      *charPtr = 0;
+    } else if (sizeof(T) == 2) {
+      char16_t* charPtr = reinterpret_cast<char16_t*>(ptr);
+      *charPtr++ = 0xFFFD;
+      *charPtr = 0;
+      mString->mLength = 1;
+    } else {
+      MOZ_ASSERT_UNREACHABLE("Only 8-bit and 16-bit code units supported.");
+    }
+  }
+
+  BulkWriteHandle() = delete;
+  BulkWriteHandle(const BulkWriteHandle&) = delete;
+  BulkWriteHandle& operator=(const BulkWriteHandle&) = delete;
+
+ private:
+  BulkWriteHandle(nsTSubstring<T>* aString, size_type aCapacity)
+      : mString(aString), mCapacity(aCapacity) {}
+
+  nsTSubstring<T>* Forget() {
+    auto string = mString;
+    mString = nullptr;
+    return string;
+  }
+
+  nsTSubstring<T>* mString;  // nullptr upon finish
+  size_type mCapacity;
+};
+
+}  // namespace mozilla
+
+/**
+ * nsTSubstring is an abstract string class. From an API perspective, this
+ * class is the root of the string class hierarchy. It represents a single
+ * contiguous array of characters, which may or may not be null-terminated.
+ * This type is not instantiated directly. A sub-class is instantiated
+ * instead. For example, see nsTString.
+ *
+ * NAMES:
+ *   nsAString for wide characters
+ *   nsACString for narrow characters
+ *
+ */
+template <typename T>
+class nsTSubstring : public mozilla::detail::nsTStringRepr<T> {
+  friend class mozilla::BulkWriteHandle<T>;
+  friend class nsStringBuffer;
+
+ public:
+  typedef nsTSubstring<T> self_type;
+
+  typedef nsTString<T> string_type;
+
+  typedef typename mozilla::detail::nsTStringRepr<T> base_string_type;
+  typedef typename base_string_type::substring_type substring_type;
+
+  typedef typename base_string_type::fallible_t fallible_t;
+
+  typedef typename base_string_type::char_type char_type;
+  typedef typename base_string_type::char_traits char_traits;
+  typedef
+      typename base_string_type::incompatible_char_type incompatible_char_type;
+
+  typedef typename base_string_type::substring_tuple_type substring_tuple_type;
+
+  typedef typename base_string_type::const_iterator const_iterator;
+  typedef typename base_string_type::iterator iterator;
+
+  typedef typename base_string_type::comparator_type comparator_type;
+
+  typedef typename base_string_type::const_char_iterator const_char_iterator;
+
+  typedef typename base_string_type::string_view string_view;
+
+  typedef typename base_string_type::index_type index_type;
+  typedef typename base_string_type::size_type size_type;
+
+  // These are only for internal use within the string classes:
+  typedef typename base_string_type::DataFlags DataFlags;
+  typedef typename base_string_type::ClassFlags ClassFlags;
+  typedef typename base_string_type::LengthStorage LengthStorage;
+
+  // this acts like a virtual destructor
+  ~nsTSubstring() { Finalize(); }
+
+  /**
+   * writing iterators
+   *
+   * BeginWriting() makes the string mutable (if it isn't
+   * already) and returns (or writes into an outparam) a
+   * pointer that provides write access to the string's buffer.
+   *
+   * Note: Consider if BulkWrite() suits your use case better
+   * than BeginWriting() combined with SetLength().
+   *
+   * Note: Strings autoconvert into writable mozilla::Span,
+   * which may suit your use case better than calling
+   * BeginWriting() directly.
+   *
+   * When writing via the pointer obtained from BeginWriting(),
+   * you are allowed to write at most the number of code units
+   * indicated by Length() or, alternatively, write up to, but
+   * not including, the position indicated by EndWriting().
+   *
+   * In particular, calling SetCapacity() does not affect what
+   * the above paragraph says.
+   */
+
+  iterator BeginWriting() {
+    if (!EnsureMutable()) {
+      AllocFailed(base_string_type::mLength);
+    }
+
+    return base_string_type::mData;
+  }
+
+  iterator BeginWriting(const fallible_t&) {
+    return EnsureMutable() ? base_string_type::mData : iterator(0);
+  }
+
+  iterator EndWriting() {
+    if (!EnsureMutable()) {
+      AllocFailed(base_string_type::mLength);
+    }
+
+    return base_string_type::mData + base_string_type::mLength;
+  }
+
+  iterator EndWriting(const fallible_t&) {
+    return EnsureMutable()
+               ? (base_string_type::mData + base_string_type::mLength)
+               : iterator(0);
+  }
+
+  /**
+   * Perform string to int conversion.
+   * @param   aErrorCode will contain error if one occurs
+   * @param   aRadix is the radix to use. Only 10 and 16 are supported.
+   * @return  int rep of string value, and possible (out) error code
+   */
+  int32_t ToInteger(nsresult* aErrorCode, uint32_t aRadix = 10) const;
+
+  /**
+   * Perform string to 64-bit int conversion.
+   * @param   aErrorCode will contain error if one occurs
+   * @param   aRadix is the radix to use. Only 10 and 16 are supported.
+   * @return  64-bit int rep of string value, and possible (out) error code
+   */
+  int64_t ToInteger64(nsresult* aErrorCode, uint32_t aRadix = 10) const;
+
+  /**
+   * assignment
+   */
+
+  void NS_FASTCALL Assign(char_type aChar);
+  [[nodiscard]] bool NS_FASTCALL Assign(char_type aChar, const fallible_t&);
+
+  void NS_FASTCALL Assign(const char_type* aData,
+                          size_type aLength = size_type(-1));
+  [[nodiscard]] bool NS_FASTCALL Assign(const char_type* aData,
+                                        const fallible_t&);
+  [[nodiscard]] bool NS_FASTCALL Assign(const char_type* aData,
+                                        size_type aLength, const fallible_t&);
+
+  void NS_FASTCALL Assign(const self_type&);
+  [[nodiscard]] bool NS_FASTCALL Assign(const self_type&, const fallible_t&);
+
+  void NS_FASTCALL Assign(self_type&&);
+  [[nodiscard]] bool NS_FASTCALL Assign(self_type&&, const fallible_t&);
+
+  void NS_FASTCALL Assign(const substring_tuple_type&);
+  [[nodiscard]] bool NS_FASTCALL Assign(const substring_tuple_type&,
+                                        const fallible_t&);
+
+#if defined(MOZ_USE_CHAR16_WRAPPER)
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  void Assign(char16ptr_t aData) {
+    Assign(static_cast<const char16_t*>(aData));
+  }
+
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  void Assign(char16ptr_t aData, size_type aLength) {
+    Assign(static_cast<const char16_t*>(aData), aLength);
+  }
+
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  [[nodiscard]] bool Assign(char16ptr_t aData, size_type aLength,
+                            const fallible_t& aFallible) {
+    return Assign(static_cast<const char16_t*>(aData), aLength, aFallible);
+  }
+#endif
+
+  void NS_FASTCALL AssignASCII(const char* aData, size_type aLength);
+  [[nodiscard]] bool NS_FASTCALL AssignASCII(const char* aData,
+                                             size_type aLength,
+                                             const fallible_t&);
+
+  void NS_FASTCALL AssignASCII(const char* aData) {
+    AssignASCII(aData, strlen(aData));
+  }
+  [[nodiscard]] bool NS_FASTCALL AssignASCII(const char* aData,
+                                             const fallible_t& aFallible) {
+    return AssignASCII(aData, strlen(aData), aFallible);
+  }
+
+  // AssignLiteral must ONLY be called with an actual literal string, or
+  // a character array *constant* of static storage duration declared
+  // without an explicit size and with an initializer that is a string
+  // literal or is otherwise null-terminated.
+  // Use Assign or AssignASCII for other character array variables.
+  //
+  // This method does not need a fallible version, because it uses the
+  // POD buffer of the literal as the string's buffer without allocating.
+  // The literal does not need to be ASCII. If this a 16-bit string, this
+  // method takes a u"" literal. (The overload on 16-bit strings that takes
+  // a "" literal takes only ASCII.)
+  template <int N>
+  void AssignLiteral(const char_type (&aStr)[N]) {
+    AssignLiteral(aStr, N - 1);
+  }
+
+  // AssignLiteral must ONLY be called with an actual literal string, or
+  // a char array *constant* declared without an explicit size and with an
+  // initializer that is a string literal or is otherwise null-terminated.
+  // Use AssignASCII for other char array variables.
+  //
+  // This method takes an 8-bit (ASCII-only!) string that is expanded
+  // into a 16-bit string at run time causing a run-time allocation.
+  // To avoid the run-time allocation (at the cost of the literal
+  // taking twice the size in the binary), use the above overload that
+  // takes a u"" string instead. Using the overload that takes a u""
+  // literal is generally preferred when working with 16-bit strings.
+  //
+  // There is not a fallible version of this method because it only really
+  // applies to small allocations that we wouldn't want to check anyway.
+  template <int N, typename Q = T,
+            typename EnableIfChar16 = typename mozilla::Char16OnlyT<Q>>
+  void AssignLiteral(const incompatible_char_type (&aStr)[N]) {
+    AssignASCII(aStr, N - 1);
+  }
+
+  self_type& operator=(char_type aChar) {
+    Assign(aChar);
+    return *this;
+  }
+  self_type& operator=(const char_type* aData) {
+    Assign(aData);
+    return *this;
+  }
+#if defined(MOZ_USE_CHAR16_WRAPPER)
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  self_type& operator=(char16ptr_t aData) {
+    Assign(aData);
+    return *this;
+  }
+#endif
+  self_type& operator=(const self_type& aStr) {
+    Assign(aStr);
+    return *this;
+  }
+  self_type& operator=(self_type&& aStr) {
+    Assign(std::move(aStr));
+    return *this;
+  }
+  self_type& operator=(const substring_tuple_type& aTuple) {
+    Assign(aTuple);
+    return *this;
+  }
+
+  // Adopt a heap-allocated char sequence for this string; is Voided if aData
+  // is null. Useful for e.g. converting an strdup'd C string into an
+  // nsCString. See also getter_Copies(), which is a useful wrapper.
+  void NS_FASTCALL Adopt(char_type* aData, size_type aLength = size_type(-1));
+
+  /**
+   * buffer manipulation
+   */
+
+  void NS_FASTCALL Replace(index_type aCutStart, size_type aCutLength,
+                           char_type aChar);
+  [[nodiscard]] bool NS_FASTCALL Replace(index_type aCutStart,
+                                         size_type aCutLength, char_type aChar,
+                                         const fallible_t&);
+  void NS_FASTCALL Replace(index_type aCutStart, size_type aCutLength,
+                           const char_type* aData,
+                           size_type aLength = size_type(-1));
+  [[nodiscard]] bool NS_FASTCALL Replace(index_type aCutStart,
+                                         size_type aCutLength,
+                                         const char_type* aData,
+                                         size_type aLength, const fallible_t&);
+  void Replace(index_type aCutStart, size_type aCutLength,
+               const self_type& aStr) {
+    Replace(aCutStart, aCutLength, aStr.Data(), aStr.Length());
+  }
+  [[nodiscard]] bool Replace(index_type aCutStart, size_type aCutLength,
+                             const self_type& aStr,
+                             const fallible_t& aFallible) {
+    return Replace(aCutStart, aCutLength, aStr.Data(), aStr.Length(),
+                   aFallible);
+  }
+  void NS_FASTCALL Replace(index_type aCutStart, size_type aCutLength,
+                           const substring_tuple_type& aTuple);
+
+  // ReplaceLiteral must ONLY be called with an actual literal string, or
+  // a character array *constant* of static storage duration declared
+  // without an explicit size and with an initializer that is a string
+  // literal or is otherwise null-terminated.
+  // Use Replace for other character array variables.
+  template <int N>
+  void ReplaceLiteral(index_type aCutStart, size_type aCutLength,
+                      const char_type (&aStr)[N]) {
+    ReplaceLiteral(aCutStart, aCutLength, aStr, N - 1);
+  }
+
+  /**
+   * |Left|, |Mid|, and |Right| are annoying signatures that seem better almost
+   * any _other_ way than they are now.  Consider these alternatives
+   *
+   * // ...a member function that returns a |Substring|
+   * aWritable = aReadable.Left(17);
+   * // ...a global function that returns a |Substring|
+   * aWritable = Left(aReadable, 17);
+   * // ...a global function that does the assignment
+   * Left(aReadable, 17, aWritable);
+   *
+   * as opposed to the current signature
+   *
+   * // ...a member function that does the assignment
+   * aReadable.Left(aWritable, 17);
+   *
+   * or maybe just stamping them out in favor of |Substring|, they are just
+   * duplicate functionality
+   *
+   * aWritable = Substring(aReadable, 0, 17);
+   */
+  size_type Mid(self_type& aResult, index_type aStartPos,
+                size_type aCount) const;
+
+  size_type Left(self_type& aResult, size_type aCount) const {
+    return Mid(aResult, 0, aCount);
+  }
+
+  size_type Right(self_type& aResult, size_type aCount) const {
+    aCount = XPCOM_MIN(this->Length(), aCount);
+    return Mid(aResult, this->mLength - aCount, aCount);
+  }
+
+  /**
+   *  This method strips whitespace throughout the string.
+   */
+  void StripWhitespace();
+  bool StripWhitespace(const fallible_t&);
+
+  /**
+   *  This method is used to remove all occurrences of aChar from this
+   * string.
+   *
+   *  @param  aChar -- char to be stripped
+   */
+  void StripChar(char_type aChar);
+
+  /**
+   *  This method is used to remove all occurrences of aChars from this
+   * string.
+   *
+   *  @param  aChars -- chars to be stripped
+   */
+  void StripChars(const char_type* aChars);
+
+  /**
+   * This method is used to remove all occurrences of some characters this
+   * from this string.  The characters removed have the corresponding
+   * entries in the bool array set to true; we retain all characters
+   * with code beyond 127.
+   * THE CALLER IS RESPONSIBLE for making sure the complete boolean
+   * array, 128 entries, is properly initialized.
+   *
+   * See also: ASCIIMask class.
+   *
+   *  @param  aToStrip -- Array where each entry is true if the
+   *          corresponding ASCII character is to be stripped.  All
+   *          characters beyond code 127 are retained.  Note that this
+   *          parameter is of ASCIIMaskArray type, but we expand the typedef
+   *          to avoid having to include nsASCIIMask.h in this include file
+   *          as it brings other includes.
+   */
+  void StripTaggedASCII(const std::array<bool, 128>& aToStrip);
+
+  /**
+   * A shortcut to strip \r and \n.
+   */
+  void StripCRLF();
+
+  /**
+   * swaps occurence of 1 string for another
+   */
+  void ReplaceChar(char_type aOldChar, char_type aNewChar);
+  void ReplaceChar(const string_view& aSet, char_type aNewChar);
+
+  /**
+   * Replace all occurrences of aTarget with aNewValue.
+   * The complexity of this function is O(n+m), n being the length of the string
+   * and m being the length of aNewValue.
+   */
+  void ReplaceSubstring(const self_type& aTarget, const self_type& aNewValue);
+  void ReplaceSubstring(const char_type* aTarget, const char_type* aNewValue);
+  [[nodiscard]] bool ReplaceSubstring(const self_type& aTarget,
+                                      const self_type& aNewValue,
+                                      const fallible_t&);
+  [[nodiscard]] bool ReplaceSubstring(const char_type* aTarget,
+                                      const char_type* aNewValue,
+                                      const fallible_t&);
+
+  /**
+   *  This method trims characters found in aSet from either end of the
+   *  underlying string.
+   *
+   *  @param   aSet -- contains chars to be trimmed from both ends
+   *  @param   aTrimLeading
+   *  @param   aTrimTrailing
+   *  @param   aIgnoreQuotes -- if true, causes surrounding quotes to be ignored
+   *  @return  this
+   */
+  void Trim(const std::string_view& aSet, bool aTrimLeading = true,
+            bool aTrimTrailing = true, bool aIgnoreQuotes = false);
+
+  /**
+   *  This method strips whitespace from string.
+   *  You can control whether whitespace is yanked from start and end of
+   *  string as well.
+   *
+   *  @param   aTrimLeading controls stripping of leading ws
+   *  @param   aTrimTrailing controls stripping of trailing ws
+   */
+  void CompressWhitespace(bool aTrimLeading = true, bool aTrimTrailing = true);
+
+  void Append(char_type aChar);
+
+  [[nodiscard]] bool Append(char_type aChar, const fallible_t& aFallible);
+
+  void Append(const char_type* aData, size_type aLength = size_type(-1));
+
+  [[nodiscard]] bool Append(const char_type* aData, size_type aLength,
+                            const fallible_t& aFallible);
+
+#if defined(MOZ_USE_CHAR16_WRAPPER)
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  void Append(char16ptr_t aData, size_type aLength = size_type(-1)) {
+    Append(static_cast<const char16_t*>(aData), aLength);
+  }
+#endif
+
+  void Append(const self_type& aStr);
+
+  [[nodiscard]] bool Append(const self_type& aStr, const fallible_t& aFallible);
+
+  void Append(const substring_tuple_type& aTuple);
+
+  [[nodiscard]] bool Append(const substring_tuple_type& aTuple,
+                            const fallible_t& aFallible);
+
+  void AppendASCII(const char* aData, size_type aLength = size_type(-1));
+
+  [[nodiscard]] bool AppendASCII(const char* aData,
+                                 const fallible_t& aFallible);
+
+  [[nodiscard]] bool AppendASCII(const char* aData, size_type aLength,
+                                 const fallible_t& aFallible);
+
+  // Appends a literal string ("" literal in the 8-bit case and u"" literal
+  // in the 16-bit case) to the string.
+  //
+  // AppendLiteral must ONLY be called with an actual literal string, or
+  // a character array *constant* of static storage duration declared
+  // without an explicit size and with an initializer that is a string
+  // literal or is otherwise null-terminated.
+  // Use Append or AppendASCII for other character array variables.
+  template <int N>
+  void AppendLiteral(const char_type (&aStr)[N]) {
+    // The case where base_string_type::mLength is zero is intentionally
+    // left unoptimized (could be optimized as call to AssignLiteral),
+    // because it's rare/nonexistent. If you add that optimization,
+    // please be sure to also check that
+    // !(base_string_type::mDataFlags & DataFlags::REFCOUNTED)
+    // to avoid undoing the effects of SetCapacity().
+    Append(aStr, N - 1);
+  }
+
+  template <int N>
+  void AppendLiteral(const char_type (&aStr)[N], const fallible_t& aFallible) {
+    // The case where base_string_type::mLength is zero is intentionally
+    // left unoptimized (could be optimized as call to AssignLiteral),
+    // because it's rare/nonexistent. If you add that optimization,
+    // please be sure to also check that
+    // !(base_string_type::mDataFlags & DataFlags::REFCOUNTED)
+    // to avoid undoing the effects of SetCapacity().
+    return Append(aStr, N - 1, aFallible);
+  }
+
+  // Only enable for T = char16_t
+  //
+  // Appends an 8-bit literal string ("" literal) to a 16-bit string by
+  // expanding it. The literal must only contain ASCII.
+  //
+  // Using u"" literals with 16-bit strings is generally preferred.
+  template <int N, typename Q = T,
+            typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  void AppendLiteral(const incompatible_char_type (&aStr)[N]) {
+    AppendASCII(aStr, N - 1);
+  }
+
+  // Only enable for T = char16_t
+  template <int N, typename Q = T,
+            typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  [[nodiscard]] bool AppendLiteral(const incompatible_char_type (&aStr)[N],
+                                   const fallible_t& aFallible) {
+    return AppendASCII(aStr, N - 1, aFallible);
+  }
+
+  /**
+   * Append a formatted string to the current string. Uses the
+   * standard printf format codes.  This uses NSPR formatting, which will be
+   * locale-aware for floating-point values.  You probably don't want to use
+   * this with floating-point values as a result.
+   */
+  void AppendPrintf(const char* aFormat, ...) MOZ_FORMAT_PRINTF(2, 3);
+  void AppendVprintf(const char* aFormat, va_list aAp) MOZ_FORMAT_PRINTF(2, 0);
+  void AppendInt(int32_t aInteger) { AppendIntDec(aInteger); }
+  void AppendInt(int32_t aInteger, int aRadix) {
+    if (aRadix == 10) {
+      AppendIntDec(aInteger);
+    } else if (aRadix == 8) {
+      AppendIntOct(static_cast<uint32_t>(aInteger));
+    } else {
+      AppendIntHex(static_cast<uint32_t>(aInteger));
+    }
+  }
+  void AppendInt(uint32_t aInteger) { AppendIntDec(aInteger); }
+  void AppendInt(uint32_t aInteger, int aRadix) {
+    if (aRadix == 10) {
+      AppendIntDec(aInteger);
+    } else if (aRadix == 8) {
+      AppendIntOct(aInteger);
+    } else {
+      AppendIntHex(aInteger);
+    }
+  }
+  void AppendInt(int64_t aInteger) { AppendIntDec(aInteger); }
+  void AppendInt(int64_t aInteger, int aRadix) {
+    if (aRadix == 10) {
+      AppendIntDec(aInteger);
+    } else if (aRadix == 8) {
+      AppendIntOct(static_cast<uint64_t>(aInteger));
+    } else {
+      AppendIntHex(static_cast<uint64_t>(aInteger));
+    }
+  }
+  void AppendInt(uint64_t aInteger) { AppendIntDec(aInteger); }
+  void AppendInt(uint64_t aInteger, int aRadix) {
+    if (aRadix == 10) {
+      AppendIntDec(aInteger);
+    } else if (aRadix == 8) {
+      AppendIntOct(aInteger);
+    } else {
+      AppendIntHex(aInteger);
+    }
+  }
+
+ private:
+  void AppendIntDec(int32_t);
+  void AppendIntDec(uint32_t);
+  void AppendIntOct(uint32_t);
+  void AppendIntHex(uint32_t);
+  void AppendIntDec(int64_t);
+  void AppendIntDec(uint64_t);
+  void AppendIntOct(uint64_t);
+  void AppendIntHex(uint64_t);
+
+ public:
+  /**
+   * Append the given float to this string
+   */
+  void NS_FASTCALL AppendFloat(float aFloat);
+  void NS_FASTCALL AppendFloat(double aFloat);
+
+  self_type& operator+=(char_type aChar) {
+    Append(aChar);
+    return *this;
+  }
+  self_type& operator+=(const char_type* aData) {
+    Append(aData);
+    return *this;
+  }
+#if defined(MOZ_USE_CHAR16_WRAPPER)
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  self_type& operator+=(char16ptr_t aData) {
+    Append(aData);
+    return *this;
+  }
+#endif
+  self_type& operator+=(const self_type& aStr) {
+    Append(aStr);
+    return *this;
+  }
+  self_type& operator+=(const substring_tuple_type& aTuple) {
+    Append(aTuple);
+    return *this;
+  }
+
+  void Insert(char_type aChar, index_type aPos) { Replace(aPos, 0, aChar); }
+  void Insert(const char_type* aData, index_type aPos,
+              size_type aLength = size_type(-1)) {
+    Replace(aPos, 0, aData, aLength);
+  }
+#if defined(MOZ_USE_CHAR16_WRAPPER)
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  void Insert(char16ptr_t aData, index_type aPos,
+              size_type aLength = size_type(-1)) {
+    Insert(static_cast<const char16_t*>(aData), aPos, aLength);
+  }
+#endif
+  void Insert(const self_type& aStr, index_type aPos) {
+    Replace(aPos, 0, aStr);
+  }
+  void Insert(const substring_tuple_type& aTuple, index_type aPos) {
+    Replace(aPos, 0, aTuple);
+  }
+
+  // InsertLiteral must ONLY be called with an actual literal string, or
+  // a character array *constant* of static storage duration declared
+  // without an explicit size and with an initializer that is a string
+  // literal or is otherwise null-terminated.
+  // Use Insert for other character array variables.
+  template <int N>
+  void InsertLiteral(const char_type (&aStr)[N], index_type aPos) {
+    ReplaceLiteral(aPos, 0, aStr, N - 1);
+  }
+
+  void Cut(index_type aCutStart, size_type aCutLength) {
+    Replace(aCutStart, aCutLength, char_traits::sEmptyBuffer, 0);
+  }
+
+  nsTSubstringSplitter<T> Split(const char_type aChar) const;
+
+  /**
+   * buffer sizing
+   */
+
+  /**
+   * Attempts to set the capacity to the given size in number of
+   * code units without affecting the length of the string in
+   * order to avoid reallocation during a subsequent sequence of
+   * appends.
+   *
+   * This method is appropriate to use before a sequence of multiple
+   * operations from the following list (without operations that are
+   * not on the list between the SetCapacity() call and operations
+   * from the list):
+   *
+   * Append()
+   * AppendASCII()
+   * AppendLiteral() (except if the string is empty: bug 1487606)
+   * AppendPrintf()
+   * AppendInt()
+   * AppendFloat()
+   * LossyAppendUTF16toASCII()
+   * AppendASCIItoUTF16()
+   *
+   * DO NOT call SetCapacity() if the subsequent operations on the
+   * string do not meet the criteria above. Operations that undo
+   * the benefits of SetCapacity() include but are not limited to:
+   *
+   * SetLength()
+   * Truncate()
+   * Assign()
+   * AssignLiteral()
+   * Adopt()
+   * CopyASCIItoUTF16()
+   * LossyCopyUTF16toASCII()
+   * AppendUTF16toUTF8()
+   * AppendUTF8toUTF16()
+   * CopyUTF16toUTF8()
+   * CopyUTF8toUTF16()
+   *
+   * If your string is an nsAuto[C]String and you are calling
+   * SetCapacity() with a constant N, please instead declare the
+   * string as nsAuto[C]StringN<N+1> without calling SetCapacity().
+   *
+   * There is no need to include room for the null terminator: it is
+   * the job of the string class.
+   *
+   * Note: Calling SetCapacity() does not give you permission to
+   * use the pointer obtained from BeginWriting() to write
+   * past the current length (as returned by Length()) of the
+   * string. Please use either BulkWrite() or SetLength()
+   * instead.
+   *
+   * Note: SetCapacity() won't make the string shorter if
+   * called with an argument smaller than the length of the
+   * string.
+   *
+   * Note: You must not use previously obtained iterators
+   * or spans after calling SetCapacity().
+   */
+  void NS_FASTCALL SetCapacity(size_type aNewCapacity);
+  [[nodiscard]] bool NS_FASTCALL SetCapacity(size_type aNewCapacity,
+                                             const fallible_t&);
+
+  /**
+   * Changes the logical length of the string, potentially
+   * allocating a differently-sized buffer for the string.
+   *
+   * When making the string shorter, this method never
+   * reports allocation failure.
+   *
+   * Exposes uninitialized memory if the string got longer.
+   *
+   * If called with the argument 0, releases the
+   * heap-allocated buffer, if any. (But the no-argument
+   * overload of Truncate() is a more idiomatic and efficient
+   * option than SetLength(0).)
+   *
+   * Note: You must not use previously obtained iterators
+   * or spans after calling SetLength().
+   */
+  void NS_FASTCALL SetLength(size_type aNewLength);
+  [[nodiscard]] bool NS_FASTCALL SetLength(size_type aNewLength,
+                                           const fallible_t&);
+
+  /**
+   * Like SetLength() but asserts in that the string
+   * doesn't become longer. Never fails, so doesn't need a
+   * fallible variant.
+   *
+   * Note: You must not use previously obtained iterators
+   * or spans after calling Truncate().
+   */
+  void Truncate(size_type aNewLength) {
+    MOZ_RELEASE_ASSERT(aNewLength <= base_string_type::mLength,
+                       "Truncate cannot make string longer");
+    mozilla::DebugOnly<bool> success = SetLength(aNewLength, mozilla::fallible);
+    MOZ_ASSERT(success);
+  }
+
+  /**
+   * A more efficient overload for Truncate(0). Releases the
+   * heap-allocated buffer if any.
+   */
+  void Truncate();
+
+  /**
+   * buffer access
+   */
+
+  /**
+   * Get a const pointer to the string's internal buffer.  The caller
+   * MUST NOT modify the characters at the returned address.
+   *
+   * @returns The length of the buffer in characters.
+   */
+  inline size_type GetData(const char_type** aData) const {
+    *aData = base_string_type::mData;
+    return base_string_type::mLength;
+  }
+
+  /**
+   * Get a pointer to the string's internal buffer, optionally resizing
+   * the buffer first.  If size_type(-1) is passed for newLen, then the
+   * current length of the string is used.  The caller MAY modify the
+   * characters at the returned address (up to but not exceeding the
+   * length of the string).
+   *
+   * @returns The length of the buffer in characters or 0 if unable to
+   * satisfy the request due to low-memory conditions.
+   */
+  size_type GetMutableData(char_type** aData,
+                           size_type aNewLen = size_type(-1)) {
+    if (!EnsureMutable(aNewLen)) {
+      AllocFailed(aNewLen == size_type(-1) ? base_string_type::Length()
+                                           : aNewLen);
+    }
+
+    *aData = base_string_type::mData;
+    return base_string_type::Length();
+  }
+
+  size_type GetMutableData(char_type** aData, size_type aNewLen,
+                           const fallible_t&) {
+    if (!EnsureMutable(aNewLen)) {
+      *aData = nullptr;
+      return 0;
+    }
+
+    *aData = base_string_type::mData;
+    return base_string_type::mLength;
+  }
+
+#if defined(MOZ_USE_CHAR16_WRAPPER)
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  size_type GetMutableData(wchar_t** aData, size_type aNewLen = size_type(-1)) {
+    return GetMutableData(reinterpret_cast<char16_t**>(aData), aNewLen);
+  }
+
+  template <typename Q = T, typename EnableIfChar16 = mozilla::Char16OnlyT<Q>>
+  size_type GetMutableData(wchar_t** aData, size_type aNewLen,
+                           const fallible_t& aFallible) {
+    return GetMutableData(reinterpret_cast<char16_t**>(aData), aNewLen,
+                          aFallible);
+  }
+#endif
+
+  mozilla::Span<char_type> GetMutableData(size_type aNewLen = size_type(-1)) {
+    if (!EnsureMutable(aNewLen)) {
+      AllocFailed(aNewLen == size_type(-1) ? base_string_type::Length()
+                                           : aNewLen);
+    }
+
+    return mozilla::Span{base_string_type::mData, base_string_type::Length()};
+  }
+
+  mozilla::Maybe<mozilla::Span<char_type>> GetMutableData(size_type aNewLen,
+                                                          const fallible_t&) {
+    if (!EnsureMutable(aNewLen)) {
+      return mozilla::Nothing();
+    }
+    return Some(
+        mozilla::Span{base_string_type::mData, base_string_type::Length()});
+  }
+
+  /**
+   * Span integration
+   */
+
+  operator mozilla::Span<const char_type>() const {
+    return mozilla::Span{base_string_type::BeginReading(),
+                         base_string_type::Length()};
+  }
+
+  void Append(mozilla::Span<const char_type> aSpan) {
+    Append(aSpan.Elements(), aSpan.Length());
+  }
+
+  [[nodiscard]] bool Append(mozilla::Span<const char_type> aSpan,
+                            const fallible_t& aFallible) {
+    return Append(aSpan.Elements(), aSpan.Length(), aFallible);
+  }
+
+  void NS_FASTCALL AssignASCII(mozilla::Span<const char> aData) {
+    AssignASCII(aData.Elements(), aData.Length());
+  }
+  [[nodiscard]] bool NS_FASTCALL AssignASCII(mozilla::Span<const char> aData,
+                                             const fallible_t& aFallible) {
+    return AssignASCII(aData.Elements(), aData.Length(), aFallible);
+  }
+
+  void AppendASCII(mozilla::Span<const char> aData) {
+    AppendASCII(aData.Elements(), aData.Length());
+  }
+
+  template <typename Q = T, typename EnableIfChar = mozilla::CharOnlyT<Q>>
+  operator mozilla::Span<const uint8_t>() const {
+    return mozilla::Span{
+        reinterpret_cast<const uint8_t*>(base_string_type::BeginReading()),
+        base_string_type::Length()};
+  }
+
+  template <typename Q = T, typename EnableIfChar = mozilla::CharOnlyT<Q>>
+  void Append(mozilla::Span<const uint8_t> aSpan) {
+    Append(reinterpret_cast<const char*>(aSpan.Elements()), aSpan.Length());
+  }
+
+  template <typename Q = T, typename EnableIfChar = mozilla::CharOnlyT<Q>>
+  [[nodiscard]] bool Append(mozilla::Span<const uint8_t> aSpan,
+                            const fallible_t& aFallible) {
+    return Append(reinterpret_cast<const char*>(aSpan.Elements()),
+                  aSpan.Length(), aFallible);
+  }
+
+  void Insert(mozilla::Span<const char_type> aSpan, index_type aPos) {
+    Insert(aSpan.Elements(), aPos, aSpan.Length());
+  }
+
+  /**
+   * string data is never null, but can be marked void.  if true, the
+   * string will be truncated.  @see nsTSubstring::IsVoid
+   */
+
+  void NS_FASTCALL SetIsVoid(bool);
+
+  /**
+   * If the string uses a shared buffer, this method
+   * clears the pointer without releasing the buffer.
+   */
+  void ForgetSharedBuffer() {
+    if (base_string_type::mDataFlags & DataFlags::REFCOUNTED) {
+      SetToEmptyBuffer();
+    }
+  }
+
+ protected:
+  void AssertValid() {
+    MOZ_DIAGNOSTIC_ASSERT(!(this->mClassFlags & ClassFlags::INVALID_MASK));
+    MOZ_DIAGNOSTIC_ASSERT(!(this->mDataFlags & DataFlags::INVALID_MASK));
+    MOZ_ASSERT(!(this->mClassFlags & ClassFlags::NULL_TERMINATED) ||
+                   (this->mDataFlags & DataFlags::TERMINATED),
+               "String classes whose static type guarantees a null-terminated "
+               "buffer must not be assigned a non-null-terminated buffer.");
+  }
+
+ public:
+  /**
+   * this is public to support automatic conversion of tuple to string
+   * base type, which helps avoid converting to nsTAString.
+   */
+  MOZ_IMPLICIT nsTSubstring(const substring_tuple_type& aTuple)
+      : base_string_type(nullptr, 0, DataFlags(0), ClassFlags(0)) {
+    AssertValid();
+    Assign(aTuple);
+  }
+
+  size_t SizeOfExcludingThisIfUnshared(
+      mozilla::MallocSizeOf aMallocSizeOf) const;
+  size_t SizeOfIncludingThisIfUnshared(
+      mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  /**
+   * WARNING: Only use these functions if you really know what you are
+   * doing, because they can easily lead to double-counting strings.  If
+   * you do use them, please explain clearly in a comment why it's safe
+   * and won't lead to double-counting.
+   */
+  size_t SizeOfExcludingThisEvenIfShared(
+      mozilla::MallocSizeOf aMallocSizeOf) const;
+  size_t SizeOfIncludingThisEvenIfShared(
+      mozilla::MallocSizeOf aMallocSizeOf) const;
+
+  template <class N>
+  void NS_ABORT_OOM(T) {
+    struct never {};  // a compiler-friendly way to do static_assert(false)
+    static_assert(
+        std::is_same_v<N, never>,
+        "In string classes, use AllocFailed to account for sizeof(char_type). "
+        "Use the global ::NS_ABORT_OOM if you really have a count of bytes.");
+  }
+
+  MOZ_ALWAYS_INLINE void AllocFailed(size_t aLength) {
+    ::NS_ABORT_OOM(aLength * sizeof(char_type));
+  }
+
+ protected:
+  // default initialization
+  nsTSubstring()
+      : base_string_type(char_traits::sEmptyBuffer, 0, DataFlags::TERMINATED,
+                         ClassFlags(0)) {
+    AssertValid();
+  }
+
+  // copy-constructor, constructs as dependent on given object
+  // (NOTE: this is for internal use only)
+  nsTSubstring(const self_type& aStr)
+      : base_string_type(aStr.base_string_type::mData,
+                         aStr.base_string_type::mLength,
+                         aStr.base_string_type::mDataFlags &
+                             (DataFlags::TERMINATED | DataFlags::VOIDED),
+                         ClassFlags(0)) {
+    AssertValid();
+  }
+
+  // initialization with ClassFlags
+  explicit nsTSubstring(ClassFlags aClassFlags)
+      : base_string_type(char_traits::sEmptyBuffer, 0, DataFlags::TERMINATED,
+                         aClassFlags) {
+    AssertValid();
+  }
+
+  /**
+   * allows for direct initialization of a nsTSubstring object.
+   */
+  nsTSubstring(char_type* aData, size_type aLength, DataFlags aDataFlags,
+               ClassFlags aClassFlags)
+#if defined(NS_BUILD_REFCNT_LOGGING)
+#  define XPCOM_STRING_CONSTRUCTOR_OUT_OF_LINE
+      ;
+#else
+#  undef XPCOM_STRING_CONSTRUCTOR_OUT_OF_LINE
+      : base_string_type(aData, aLength, aDataFlags, aClassFlags) {
+    AssertValid();
+  }
+#endif /* NS_BUILD_REFCNT_LOGGING */
+
+  void SetToEmptyBuffer() {
+    base_string_type::mData = char_traits::sEmptyBuffer;
+    base_string_type::mLength = 0;
+    base_string_type::mDataFlags = DataFlags::TERMINATED;
+    AssertValid();
+  }
+
+  void SetData(char_type* aData, LengthStorage aLength, DataFlags aDataFlags) {
+    base_string_type::mData = aData;
+    base_string_type::mLength = aLength;
+    base_string_type::mDataFlags = aDataFlags;
+    AssertValid();
+  }
+
+  /**
+   * this function releases mData and does not change the value of
+   * any of its member variables.  in other words, this function acts
+   * like a destructor.
+   */
+  void NS_FASTCALL Finalize();
+
+ public:
+  /**
+   * Starts a low-level write transaction to the string.
+   *
+   * Prepares the string for mutation such that the capacity
+   * of the string is at least aCapacity. The returned handle
+   * exposes the actual, potentially larger, capacity.
+   *
+   * If meeting the capacity or mutability requirement requires
+   * reallocation, aPrefixToPreserve code units are copied from the
+   * start of the old buffer to the start of the new buffer.
+   * aPrefixToPreserve must not be greater than the string's current
+   * length or greater than aCapacity.
+   *
+   * aAllowShrinking indicates whether an allocation may be
+   * performed when the string is already mutable and the requested
+   * capacity is smaller than the current capacity.
+   *
+   * If this method returns successfully, you must not access
+   * the string except through the returned BulkWriteHandle
+   * until either the BulkWriteHandle goes out of scope or
+   * you call Finish() on the BulkWriteHandle.
+   *
+   * Compared to SetLength() and BeginWriting(), this more
+   * complex API accomplishes two things:
+   *  1) It exposes the actual capacity which may be larger
+   *     than the requested capacity, which is useful in some
+   *     multi-step write operations that don't allocate for
+   *     the worst case up front.
+   *  2) It writes the zero terminator after the string
+   *     content has been written, which results in a
+   *     cache-friendly linear write pattern.
+   */
+  mozilla::Result<mozilla::BulkWriteHandle<T>, nsresult> NS_FASTCALL BulkWrite(
+      size_type aCapacity, size_type aPrefixToPreserve, bool aAllowShrinking);
+
+  /**
+   * THIS IS NOT REALLY A PUBLIC METHOD! DO NOT CALL FROM OUTSIDE
+   * THE STRING IMPLEMENTATION. (It's public only because friend
+   * declarations don't allow extern or static and this needs to
+   * be called from Rust FFI glue.)
+   *
+   * Prepares mData to be mutated such that the capacity of the string
+   * (not counting the zero-terminator) is at least aCapacity.
+   * Returns the actual capacity, which may be larger than what was
+   * requested or Err(NS_ERROR_OUT_OF_MEMORY) on allocation failure.
+   *
+   * mLength is ignored by this method. If the buffer is reallocated,
+   * aUnitsToPreserve specifies how many code units to copy over to
+   * the new buffer. The old buffer is freed if applicable.
+   *
+   * Unless the return value is Err(NS_ERROR_OUT_OF_MEMORY) to signal
+   * failure or 0 to signal that the string has been set to
+   * the special empty state, this method leaves the string in an
+   * invalid state! The caller is responsible for calling
+   * FinishBulkWrite() (or in Rust calling
+   * nsA[C]StringBulkWriteHandle::finish()), which put the string
+   * into a valid state by setting mLength and zero-terminating.
+   * This method sets the flag to claim that the string is
+   * zero-terminated before it actually is.
+   *
+   * Once this method has been called and before FinishBulkWrite()
+   * has been called, only accessing mData or calling this method
+   * again are valid operations. Do not call any other methods or
+   * access other fields between calling this method and
+   * FinishBulkWrite().
+   *
+   * @param aCapacity The requested capacity. The return value
+   *                  will be greater than or equal to this value.
+   * @param aPrefixToPreserve The number of code units at the start
+   *                          of the old buffer to copy into the
+   *                          new buffer.
+   * @parem aAllowShrinking If true, an allocation may be performed
+   *                        if the requested capacity is smaller
+   *                        than the current capacity.
+   * @param aSuffixLength The length, in code units, of a suffix
+   *                      to move.
+   * @param aOldSuffixStart The old start index of the suffix to
+   *                        move.
+   * @param aNewSuffixStart The new start index of the suffix to
+   *                        move.
+   *
+   */
+  mozilla::Result<size_type, nsresult> NS_FASTCALL StartBulkWriteImpl(
+      size_type aCapacity, size_type aPrefixToPreserve = 0,
+      bool aAllowShrinking = true, size_type aSuffixLength = 0,
+      size_type aOldSuffixStart = 0, size_type aNewSuffixStart = 0);
+
+ private:
+  void AssignOwned(self_type&& aStr);
+  bool AssignNonDependent(const substring_tuple_type& aTuple,
+                          size_type aTupleLength,
+                          const mozilla::fallible_t& aFallible);
+
+  /**
+   * Do not call this except from within FinishBulkWriteImpl() and
+   * SetCapacity().
+   */
+  MOZ_ALWAYS_INLINE void NS_FASTCALL
+  FinishBulkWriteImplImpl(LengthStorage aLength) {
+    base_string_type::mData[aLength] = char_type(0);
+    base_string_type::mLength = aLength;
+#ifdef DEBUG
+    // ifdefed in order to avoid the call to Capacity() in non-debug
+    // builds.
+    //
+    // Our string is mutable, so Capacity() doesn't return zero.
+    // Capacity() doesn't include the space for the zero terminator,
+    // but we want to unitialize that slot, too. Since we start
+    // counting after the zero terminator the we just wrote above,
+    // we end up overwriting the space for terminator not reflected
+    // in the capacity number.
+    char_traits::uninitialize(
+        base_string_type::mData + aLength + 1,
+        XPCOM_MIN(size_t(Capacity() - aLength), kNsStringBufferMaxPoison));
+#endif
+  }
+
+ protected:
+  /**
+   * Restores the string to a valid state after a call to StartBulkWrite()
+   * that returned a non-error result. The argument to this method
+   * must be less than or equal to the value returned by the most recent
+   * StartBulkWrite() call.
+   */
+  void NS_FASTCALL FinishBulkWriteImpl(size_type aLength);
+
+  /**
+   * this function prepares a section of mData to be modified.  if
+   * necessary, this function will reallocate mData and possibly move
+   * existing data to open up the specified section.
+   *
+   * @param aCutStart    specifies the starting offset of the section
+   * @param aCutLength   specifies the length of the section to be replaced
+   * @param aNewLength   specifies the length of the new section
+   *
+   * for example, suppose mData contains the string "abcdef" then
+   *
+   *   ReplacePrep(2, 3, 4);
+   *
+   * would cause mData to look like "ab____f" where the characters
+   * indicated by '_' have an unspecified value and can be freely
+   * modified.  this function will null-terminate mData upon return.
+   *
+   * this function returns false if is unable to allocate sufficient
+   * memory.
+   */
+  [[nodiscard]] bool ReplacePrep(index_type aCutStart, size_type aCutLength,
+                                 size_type aNewLength);
+
+  [[nodiscard]] bool NS_FASTCALL ReplacePrepInternal(index_type aCutStart,
+                                                     size_type aCutLength,
+                                                     size_type aNewFragLength,
+                                                     size_type aNewTotalLength);
+
+  /**
+   * returns the number of writable storage units starting at mData.
+   * the value does not include space for the null-terminator character.
+   *
+   * NOTE: this function returns 0 if mData is immutable (or the buffer
+   *       is 0-sized).
+   */
+  size_type NS_FASTCALL Capacity() const;
+
+  /**
+   * this helper function can be called prior to directly manipulating
+   * the contents of mData.  see, for example, BeginWriting.
+   */
+  [[nodiscard]] bool NS_FASTCALL
+  EnsureMutable(size_type aNewLen = size_type(-1));
+
+  void NS_FASTCALL ReplaceLiteral(index_type aCutStart, size_type aCutLength,
+                                  const char_type* aData, size_type aLength);
+
+ public:
+  // NOTE: this method is declared public _only_ for convenience for
+  // callers who don't have access to the original nsLiteralString_CharT.
+  void NS_FASTCALL AssignLiteral(const char_type* aData, size_type aLength);
+};
+
+extern template class nsTSubstring<char>;
+extern template class nsTSubstring<char16_t>;
+
+static_assert(sizeof(nsTSubstring<char>) ==
+                  sizeof(mozilla::detail::nsTStringRepr<char>),
+              "Don't add new data fields to nsTSubstring_CharT. "
+              "Add to nsTStringRepr<T> instead.");
+
+#include "nsCharSeparatedTokenizer.h"
+#include "nsTDependentSubstring.h"
+
+/**
+ * Span integration
+ */
+namespace mozilla {
+Span(const nsTSubstring<char>&)->Span<const char>;
+Span(const nsTSubstring<char16_t>&)->Span<const char16_t>;
+
+}  // namespace mozilla
+
+#endif