Introduce a new non-heap-allocated type for holding nsStringBuffer* in the HTML parser.

An innerHTML setter profile shows about 10% of the time being spent under nsHtml5HtmlAttributes::clear, mostly deleting nsStrings.

parser/html/nsHtml5String.h

@@ -0,0 +1,95 @@
+/* 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 nsHtml5String_h
+#define nsHtml5String_h
+
+#include "nsString.h"
+
+class nsHtml5TreeBuilder;
+
+/**
+ * A pass-by-value type that combines an unsafe `nsStringBuffer*` with its
+ * logical length (`uint32_t`). (`nsStringBuffer` knows its capacity but not
+ * its logical length, i.e. how much of the capacity is in use.)
+ *
+ * Holding or passing this type is as unsafe as holding or passing
+ * `nsStringBuffer*`.
+ *
+ * Empty strings and null strings are distinct. Since an empty nsString does
+ * not have a an `nsStringBuffer`, both empty and null `nsHtml5String` have
+ * `nullptr` as `mBuffer`. If `mBuffer` is `nullptr`, the empty case is marked
+ * with `mLength` being zero and the null case with `mLength` being non-zero.
+ */
+class nsHtml5String final
+{
+public:
+  /**
+   * Default constructor.
+   */
+  inline nsHtml5String()
+    : nsHtml5String(nullptr)
+  {
+  }
+
+  /**
+   * Constructor from nullptr.
+   */
+  inline MOZ_IMPLICIT nsHtml5String(decltype(nullptr))
+    : mBuffer(nullptr)
+    , mLength(UINT32_MAX)
+  {
+  }
+
+  inline uint32_t Length() const { return mBuffer ? mLength : 0; }
+
+  /**
+   * False iff the string is logically null
+   */
+  inline MOZ_IMPLICIT operator bool() const { return !(!mBuffer && mLength); }
+
+  void ToString(nsAString& aString);
+
+  void CopyToBuffer(char16_t* aBuffer);
+
+  bool LowerCaseEqualsASCII(const char* aLowerCaseLiteral);
+
+  bool EqualsASCII(const char* aLiteral);
+
+  bool LowerCaseStartsWithASCII(const char* aLowerCaseLiteral);
+
+  bool Equals(nsHtml5String aOther);
+
+  nsHtml5String Clone();
+
+  void Release();
+
+  static nsHtml5String FromBuffer(char16_t* aBuffer,
+                                  int32_t aLength,
+                                  nsHtml5TreeBuilder* aTreeBuilder);
+
+  static nsHtml5String FromLiteral(const char* aLiteral);
+
+  static nsHtml5String FromString(const nsAString& aString);
+
+  static nsHtml5String EmptyString();
+
+private:
+  /**
+   * Constructor from raw parts.
+   */
+  nsHtml5String(already_AddRefed<nsStringBuffer> aBuffer, uint32_t aLength);
+
+  /**
+   * nullptr if the string is logically null or logically empty
+   */
+  nsStringBuffer* mBuffer;
+
+  /**
+   * The length of the string. non-zero if the string is logically null.
+   */
+  uint32_t mLength;
+};
+
+#endif // nsHtml5String_h