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diff --git a/js/src/frontend/Token.h b/js/src/frontend/Token.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/. */
+
+/*
+ * Token-affiliated data structures except for TokenKind (defined in its own
+ * header).
+ */
+
+#ifndef frontend_Token_h
+#define frontend_Token_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+
+#include <stdint.h>  // uint32_t
+
+#include "frontend/ParserAtom.h"  // TaggedParserAtomIndex, TrivialTaggedParserAtomIndex
+#include "frontend/TokenKind.h"  // js::frontend::TokenKind
+#include "js/RegExpFlags.h"      // JS::RegExpFlags
+
+namespace js {
+
+namespace frontend {
+
+struct TokenPos {
+  uint32_t begin = 0;  // Offset of the token's first code unit.
+  uint32_t end = 0;    // Offset of 1 past the token's last code unit.
+
+  TokenPos() = default;
+  TokenPos(uint32_t begin, uint32_t end) : begin(begin), end(end) {}
+
+  // Return a TokenPos that covers left, right, and anything in between.
+  static TokenPos box(const TokenPos& left, const TokenPos& right) {
+    MOZ_ASSERT(left.begin <= left.end);
+    MOZ_ASSERT(left.end <= right.begin);
+    MOZ_ASSERT(right.begin <= right.end);
+    return TokenPos(left.begin, right.end);
+  }
+
+  bool operator==(const TokenPos& bpos) const {
+    return begin == bpos.begin && end == bpos.end;
+  }
+
+  bool operator!=(const TokenPos& bpos) const {
+    return begin != bpos.begin || end != bpos.end;
+  }
+
+  bool operator<(const TokenPos& bpos) const { return begin < bpos.begin; }
+
+  bool operator<=(const TokenPos& bpos) const { return begin <= bpos.begin; }
+
+  bool operator>(const TokenPos& bpos) const { return !(*this <= bpos); }
+
+  bool operator>=(const TokenPos& bpos) const { return !(*this < bpos); }
+
+  bool encloses(const TokenPos& pos) const {
+    return begin <= pos.begin && pos.end <= end;
+  }
+};
+
+enum DecimalPoint { NoDecimal = false, HasDecimal = true };
+
+// The only escapes found in IdentifierName are of the Unicode flavor.
+enum class IdentifierEscapes { None, SawUnicodeEscape };
+
+enum class NameVisibility { Public, Private };
+
+class TokenStreamShared;
+
+struct Token {
+ private:
+  // The lexical grammar of JavaScript has a quirk around the '/' character.
+  // As the spec puts it:
+  //
+  // > There are several situations where the identification of lexical input
+  // > elements is sensitive to the syntactic grammar context that is consuming
+  // > the input elements. This requires multiple goal symbols for the lexical
+  // > grammar. [...] The InputElementRegExp goal symbol is used in all
+  // > syntactic grammar contexts where a RegularExpressionLiteral is permitted
+  // > [...]  In all other contexts, InputElementDiv is used as the lexical
+  // > goal symbol.
+  //
+  // https://tc39.github.io/ecma262/#sec-lexical-and-regexp-grammars
+  //
+  // What "sensitive to the syntactic grammar context" means is, the parser has
+  // to tell the TokenStream whether to interpret '/' as division or
+  // RegExp. Because only one or the other (or neither) will be legal at that
+  // point in the program, and only the parser knows which one.
+  //
+  // But there's a problem: the parser often gets a token, puts it back, then
+  // consumes it later; or (equivalently) peeks at a token, leaves it, peeks
+  // again later, then finally consumes it. Of course we don't actually re-scan
+  // the token every time; we cache it in the TokenStream. This leads to the
+  // following rule:
+  //
+  // The parser must not pass SlashIsRegExp when getting/peeking at a token
+  // previously scanned with SlashIsDiv; or vice versa.
+  //
+  // That way, code that asks for a SlashIsRegExp mode will never get a cached
+  // Div token. But this rule is easy to screw up, because tokens are so often
+  // peeked at on Parser.cpp line A and consumed on line B, where |A-B| is
+  // thousands of lines. We therefore enforce it with the frontend's most
+  // annoying assertion (in verifyConsistentModifier), and provide
+  // Modifier::SlashIsInvalid to help avoid tripping it.
+  //
+  // This enum belongs in TokenStream, but C++, so we define it here and
+  // typedef it there.
+  enum Modifier {
+    // Parse `/` and `/=` as the division operators. (That is, use
+    // InputElementDiv as the goal symbol.)
+    SlashIsDiv,
+
+    // Parse `/` as the beginning of a RegExp literal. (That is, use
+    // InputElementRegExp.)
+    SlashIsRegExp,
+
+    // Neither a Div token nor a RegExp token is syntactically valid here. When
+    // the parser calls `getToken(SlashIsInvalid)`, it must be prepared to see
+    // either one (and throw a SyntaxError either way).
+    //
+    // It's OK to use SlashIsInvalid to get a token that was originally scanned
+    // with SlashIsDiv or SlashIsRegExp. The reverse--peeking with
+    // SlashIsInvalid, then getting with another mode--is not OK. If either Div
+    // or RegExp is syntactically valid here, use the appropriate modifier.
+    SlashIsInvalid,
+  };
+  friend class TokenStreamShared;
+
+ public:
+  /** The type of this token. */
+  TokenKind type;
+
+  /** The token's position in the overall script. */
+  TokenPos pos;
+
+  union {
+   private:
+    friend struct Token;
+
+    TrivialTaggedParserAtomIndex atom;
+
+    struct {
+      /** Numeric literal's value. */
+      double value;
+
+      /** Does the numeric literal contain a '.'? */
+      DecimalPoint decimalPoint;
+    } number;
+
+    /** Regular expression flags; use charBuffer to access source chars. */
+    JS::RegExpFlags reflags;
+  } u;
+
+#ifdef DEBUG
+  /** The modifier used to get this token. */
+  Modifier modifier;
+#endif
+
+  // Mutators
+
+  void setName(TaggedParserAtomIndex name) {
+    MOZ_ASSERT(type == TokenKind::Name || type == TokenKind::PrivateName);
+    u.atom = TrivialTaggedParserAtomIndex::from(name);
+  }
+
+  void setAtom(TaggedParserAtomIndex atom) {
+    MOZ_ASSERT(type == TokenKind::String || type == TokenKind::TemplateHead ||
+               type == TokenKind::NoSubsTemplate);
+    u.atom = TrivialTaggedParserAtomIndex::from(atom);
+  }
+
+  void setRegExpFlags(JS::RegExpFlags flags) {
+    MOZ_ASSERT(type == TokenKind::RegExp);
+    u.reflags = flags;
+  }
+
+  void setNumber(double n, DecimalPoint decimalPoint) {
+    MOZ_ASSERT(type == TokenKind::Number);
+    u.number.value = n;
+    u.number.decimalPoint = decimalPoint;
+  }
+
+  // Type-safe accessors
+
+  TaggedParserAtomIndex name() const {
+    MOZ_ASSERT(type == TokenKind::Name || type == TokenKind::PrivateName);
+    return u.atom;
+  }
+
+  TaggedParserAtomIndex atom() const {
+    MOZ_ASSERT(type == TokenKind::String || type == TokenKind::TemplateHead ||
+               type == TokenKind::NoSubsTemplate);
+    return u.atom;
+  }
+
+  JS::RegExpFlags regExpFlags() const {
+    MOZ_ASSERT(type == TokenKind::RegExp);
+    return u.reflags;
+  }
+
+  double number() const {
+    MOZ_ASSERT(type == TokenKind::Number);
+    return u.number.value;
+  }
+
+  DecimalPoint decimalPoint() const {
+    MOZ_ASSERT(type == TokenKind::Number);
+    return u.number.decimalPoint;
+  }
+};
+
+}  // namespace frontend
+
+}  // namespace js
+
+#endif  // frontend_Token_h