--- a/xpcom/string/nsTSubstring.cpp
+++ b/xpcom/string/nsTSubstring.cpp
@@ -9,19 +9,17 @@
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Printf.h"
 
 using double_conversion::DoubleToStringConverter;
 
 #ifdef XPCOM_STRING_CONSTRUCTOR_OUT_OF_LINE
 nsTSubstring_CharT::nsTSubstring_CharT(char_type* aData, size_type aLength,
                                        uint32_t aFlags)
-  : mData(aData),
-    mLength(aLength),
-    mFlags(aFlags)
+  : nsTStringRepr_CharT(aData, aLength, aFlags)
 {
   if (aFlags & F_OWNED) {
     STRING_STAT_INCREMENT(Adopt);
     MOZ_LOG_CTOR(mData, "StringAdopt", 1);
   }
 }
 #endif /* XPCOM_STRING_CONSTRUCTOR_OUT_OF_LINE */
 

--- a/xpcom/string/nsTSubstring.h
+++ b/xpcom/string/nsTSubstring.h
@@ -45,35 +45,79 @@ public:
   }
 
   virtual int operator()(const char_type*, const char_type*,
                          uint32_t, uint32_t) const override;
 };
 
 class nsTSubstringSplitter_CharT;
 
+namespace mozilla {
+namespace detail {
+
 /**
- * nsTSubstring is the most abstract class in the string 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.
+ * nsTStringRepr defines a string's memory layout and some accessor methods.
+ * This class exists so that nsTLiteralString can avoid inheriting
+ * nsTSubstring's destructor. All methods on this class must be const because
+ * literal strings are not writable.
+ *
+ * This class is an implementation detail and should not be instantiated
+ * directly, nor used in any way outside of the string code itself. It is
+ * buried in a namespace to discourage its use in function parameters.
+ * If you need to take a parameter, use [const] ns[C]Substring&.
+ * If you need to instantiate a string, use ns[C]String or descendents.
+ *
+ * NAMES:
+ *   nsStringRepr for wide characters
+ *   nsCStringRepr for narrow characters
+ *
+ */
+class nsTStringRepr_CharT
+{
+public:
+  typedef CharT                               char_type;
+
+  typedef uint32_t                            size_type;
+
+protected:
+  nsTStringRepr_CharT() = delete; // Never instantiate directly
+
+  constexpr
+  nsTStringRepr_CharT(char_type* aData, size_type aLength, uint32_t aFlags)
+    : mData(aData)
+    , mLength(aLength)
+    , mFlags(aFlags)
+  {
+  }
+
+  char_type*  mData;
+  size_type   mLength;
+  uint32_t    mFlags;
+};
+
+} // namespace detail
+} // 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
  *
- * Many of the accessors on nsTSubstring are inlined as an optimization.
  */
-class nsTSubstring_CharT
+class nsTSubstring_CharT : public mozilla::detail::nsTStringRepr_CharT
 {
 public:
   typedef mozilla::fallible_t                 fallible_t;
 
-  typedef CharT                               char_type;
-
   typedef nsCharTraits<char_type>             char_traits;
   typedef char_traits::incompatible_char_type incompatible_char_type;
 
   typedef nsTSubstring_CharT                  self_type;
   typedef self_type                           base_string_type;
 
   typedef self_type                           substring_type;
   typedef nsTSubstringTuple_CharT             substring_tuple_type;
@@ -82,17 +126,16 @@ public:
   typedef nsReadingIterator<char_type>        const_iterator;
   typedef nsWritingIterator<char_type>        iterator;
 
   typedef nsTStringComparator_CharT           comparator_type;
 
   typedef char_type*                          char_iterator;
   typedef const char_type*                    const_char_iterator;
 
-  typedef uint32_t                            size_type;
   typedef uint32_t                            index_type;
 
 public:
 
   // this acts like a virtual destructor
   ~nsTSubstring_CharT()
   {
     Finalize();
@@ -845,19 +888,17 @@ public:
 
 public:
 
   /**
    * this is public to support automatic conversion of tuple to string
    * base type, which helps avoid converting to nsTAString.
    */
   MOZ_IMPLICIT nsTSubstring_CharT(const substring_tuple_type& aTuple)
-    : mData(nullptr)
-    , mLength(0)
-    , mFlags(F_NONE)
+    : nsTStringRepr_CharT(nullptr, 0, F_NONE)
   {
     Assign(aTuple);
   }
 
   size_t SizeOfExcludingThisIfUnshared(mozilla::MallocSizeOf aMallocSizeOf)
   const;
   size_t SizeOfIncludingThisIfUnshared(mozilla::MallocSizeOf aMallocSizeOf)
   const;
@@ -886,50 +927,41 @@ public:
   {
     ::NS_ABORT_OOM(aLength * sizeof(char_type));
   }
 
 protected:
 
   friend class nsTSubstringTuple_CharT;
 
-  char_type*  mData;
-  size_type   mLength;
-  uint32_t    mFlags;
-
   // default initialization
   nsTSubstring_CharT()
-    : mData(char_traits::sEmptyBuffer)
-    ,  mLength(0)
-    ,  mFlags(F_TERMINATED)
+    : nsTStringRepr_CharT(char_traits::sEmptyBuffer, 0, F_TERMINATED)
   {
   }
 
   // copy-constructor, constructs as dependent on given object
   // (NOTE: this is for internal use only)
   nsTSubstring_CharT(const self_type& aStr)
-    : mData(aStr.mData)
-    ,  mLength(aStr.mLength)
-    ,  mFlags(aStr.mFlags & (F_TERMINATED | F_VOIDED))
+    : nsTStringRepr_CharT(aStr.mData, aStr.mLength,
+                          aStr.mFlags & (F_TERMINATED | F_VOIDED))
   {
   }
 
  /**
    * allows for direct initialization of a nsTSubstring object.
    */
   // XXXbz or can I just include nscore.h and use NS_BUILD_REFCNT_LOGGING?
 #if defined(DEBUG) || defined(FORCE_BUILD_REFCNT_LOGGING)
 #define XPCOM_STRING_CONSTRUCTOR_OUT_OF_LINE
   nsTSubstring_CharT(char_type* aData, size_type aLength, uint32_t aFlags);
 #else
 #undef XPCOM_STRING_CONSTRUCTOR_OUT_OF_LINE
   nsTSubstring_CharT(char_type* aData, size_type aLength, uint32_t aFlags)
-    : mData(aData)
-    , mLength(aLength)
-    , mFlags(aFlags)
+    : nsTStringRepr_CharT(aData, aLength, aFlags)
   {
   }
 #endif /* DEBUG || FORCE_BUILD_REFCNT_LOGGING */
 
   /**
    * 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.
@@ -1089,16 +1121,21 @@ public:
   //   non-terminated substrings are always dependent.
   //
   //   F_VOIDED implies F_TERMINATED, and moreover it implies that mData
   //   points to char_traits::sEmptyBuffer.  Therefore, F_VOIDED is
   //   mutually exclusive with F_SHARED, F_OWNED, and F_FIXED.
   //
 };
 
+static_assert(sizeof(nsTSubstring_CharT) ==
+              sizeof(mozilla::detail::nsTStringRepr_CharT),
+              "Don't add new data fields to nsTSubstring_CharT. "
+              "Add to nsTStringRepr_CharT instead.");
+
 int NS_FASTCALL
 Compare(const nsTSubstring_CharT::base_string_type& aLhs,
         const nsTSubstring_CharT::base_string_type& aRhs,
         const nsTStringComparator_CharT& = nsTDefaultStringComparator_CharT());
 
 
 inline bool
 operator!=(const nsTSubstring_CharT::base_string_type& aLhs,

--- a/xpcom/string/string-template-def-char.h
+++ b/xpcom/string/string-template-def-char.h
@@ -4,16 +4,17 @@
  * 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"
 
 #define CharT                               char
 #define CharT_is_char                       1
 #define nsTAString_IncompatibleCharT        nsAString
 #define nsTString_CharT                     nsCString
+#define nsTStringRepr_CharT                 nsCStringRepr
 #define nsTFixedString_CharT                nsFixedCString
 #define nsTAutoString_CharT                 nsAutoCString
 #define nsTSubstring_CharT                  nsACString
 #define PrintfAppend_CharT                  PrintfAppend_nsACString
 #define nsTSubstringTuple_CharT             nsCSubstringTuple
 #define nsTStringComparator_CharT           nsCStringComparator
 #define nsTDefaultStringComparator_CharT    nsDefaultCStringComparator
 #define nsTDependentString_CharT            nsDependentCString

--- a/xpcom/string/string-template-def-unichar.h
+++ b/xpcom/string/string-template-def-unichar.h
@@ -4,16 +4,17 @@
  * 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"
 
 #define CharT                               char16_t
 #define CharT_is_PRUnichar                  1
 #define nsTAString_IncompatibleCharT        nsACString
 #define nsTString_CharT                     nsString
+#define nsTStringRepr_CharT                 nsStringRepr
 #define nsTFixedString_CharT                nsFixedString
 #define nsTAutoString_CharT                 nsAutoString
 #define nsTSubstring_CharT                  nsAString
 #define PrintfAppend_CharT                  PrintfAppend_nsAString
 #define nsTSubstringTuple_CharT             nsSubstringTuple
 #define nsTStringComparator_CharT           nsStringComparator
 #define nsTDefaultStringComparator_CharT    nsDefaultStringComparator
 #define nsTDependentString_CharT            nsDependentString

--- a/xpcom/string/string-template-undef.h
+++ b/xpcom/string/string-template-undef.h
@@ -5,16 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 // IWYU pragma: private, include "nsString.h"
 
 #undef CharT
 #undef CharT_is_PRUnichar
 #undef CharT_is_char
 #undef nsTAString_IncompatibleCharT
 #undef nsTString_CharT
+#undef nsTStringRepr_CharT
 #undef nsTFixedString_CharT
 #undef nsTAutoString_CharT
 #undef nsTSubstring_CharT
 #undef PrintfAppend_CharT
 #undef nsTSubstringTuple_CharT
 #undef nsTStringComparator_CharT
 #undef nsTDefaultStringComparator_CharT
 #undef nsTDependentString_CharT