tools/gn/parser.cc

// Copyright (c) 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "tools/gn/parser.h"

#include <utility>

#include "base/logging.h"
#include "base/memory/ptr_util.h"
#include "tools/gn/functions.h"
#include "tools/gn/operators.h"
#include "tools/gn/token.h"

const char kGrammar_Help[] =
    "GN build language grammar\n"
    "\n"
    "Tokens\n"
    "\n"
    "  GN build files are read as sequences of tokens.  While splitting the\n"
    "  file into tokens, the next token is the longest sequence of characters\n"
    "  that form a valid token.\n"
    "\n"
    "White space and comments\n"
    "\n"
    "  White space is comprised of spaces (U+0020), horizontal tabs (U+0009),\n"
    "  carriage returns (U+000D), and newlines (U+000A).\n"
    "\n"
    "  Comments start at the character \"#\" and stop at the next newline.\n"
    "\n"
    "  White space and comments are ignored except that they may separate\n"
    "  tokens that would otherwise combine into a single token.\n"
    "\n"
    "Identifiers\n"
    "\n"
    "  Identifiers name variables and functions.\n"
    "\n"
    "      identifier = letter { letter | digit } .\n"
    "      letter     = \"A\" ... \"Z\" | \"a\" ... \"z\" | \"_\" .\n"
    "      digit      = \"0\" ... \"9\" .\n"
    "\n"
    "Keywords\n"
    "\n"
    "  The following keywords are reserved and may not be used as\n"
    "  identifiers:\n"
    "\n"
    "          else    false   if      true\n"
    "\n"
    "Integer literals\n"
    "\n"
    "  An integer literal represents a decimal integer value.\n"
    "\n"
    "      integer = [ \"-\" ] digit { digit } .\n"
    "\n"
    "  Leading zeros and negative zero are disallowed.\n"
    "\n"
    "String literals\n"
    "\n"
    "  A string literal represents a string value consisting of the quoted\n"
    "  characters with possible escape sequences and variable expansions.\n"
    "\n"
    "      string           = `\"` { char | escape | expansion } `\"` .\n"
    "      escape           = `\\` ( \"$\" | `\"` | char ) .\n"
    "      BracketExpansion = \"{\" ( identifier | ArrayAccess | ScopeAccess "
                              ") \"}\" .\n"
    "      Hex              = \"0x\" [0-9A-Fa-f][0-9A-Fa-f]\n"
    "      expansion        = \"$\" ( identifier | BracketExpansion | Hex ) .\n"
    "      char             = /* any character except \"$\", `\"`, or newline "
                             "*/ .\n"
    "\n"
    "  After a backslash, certain sequences represent special characters:\n"
    "\n"
    "          \\\"    U+0022    quotation mark\n"
    "          \\$    U+0024    dollar sign\n"
    "          \\\\    U+005C    backslash\n"
    "\n"
    "  All other backslashes represent themselves.\n"
    "\n"
    "  To insert an arbitrary byte value, use $0xFF. For example, to\n"
    "  insert a newline character: \"Line one$0x0ALine two\".\n"
    "\n"
    "Punctuation\n"
    "\n"
    "  The following character sequences represent punctuation:\n"
    "\n"
    "          +       +=      ==      !=      (       )\n"
    "          -       -=      <       <=      [       ]\n"
    "          !       =       >       >=      {       }\n"
    "                          &&      ||      .       ,\n"
    "\n"
    "Grammar\n"
    "\n"
    "  The input tokens form a syntax tree following a context-free grammar:\n"
    "\n"
    "      File = StatementList .\n"
    "\n"
    "      Statement     = Assignment | Call | Condition .\n"
    "      Assignment    = identifier AssignOp Expr .\n"
    "      Call          = identifier \"(\" [ ExprList ] \")\" [ Block ] .\n"
    "      Condition     = \"if\" \"(\" Expr \")\" Block\n"
    "                      [ \"else\" ( Condition | Block ) ] .\n"
    "      Block         = \"{\" StatementList \"}\" .\n"
    "      StatementList = { Statement } .\n"
    "\n"
    "      ArrayAccess = identifier \"[\" { identifier | integer } \"]\" .\n"
    "      ScopeAccess = identifier \".\" identifier .\n"
    "      Expr        = UnaryExpr | Expr BinaryOp Expr .\n"
    "      UnaryExpr   = PrimaryExpr | UnaryOp UnaryExpr .\n"
    "      PrimaryExpr = identifier | integer | string | Call\n"
    "                  | ArrayAccess | ScopeAccess\n"
    "                  | \"(\" Expr \")\"\n"
    "                  | \"[\" [ ExprList [ \",\" ] ] \"]\" .\n"
    "      ExprList    = Expr { \",\" Expr } .\n"
    "\n"
    "      AssignOp = \"=\" | \"+=\" | \"-=\" .\n"
    "      UnaryOp  = \"!\" .\n"
    "      BinaryOp = \"+\" | \"-\"                  // highest priority\n"
    "               | \"<\" | \"<=\" | \">\" | \">=\"\n"
    "               | \"==\" | \"!=\"\n"
    "               | \"&&\"\n"
    "               | \"||\" .                     // lowest priority\n"
    "\n"
    "  All binary operators are left-associative.\n";

enum Precedence {
  PRECEDENCE_ASSIGNMENT = 1,  // Lowest precedence.
  PRECEDENCE_OR = 2,
  PRECEDENCE_AND = 3,
  PRECEDENCE_EQUALITY = 4,
  PRECEDENCE_RELATION = 5,
  PRECEDENCE_SUM = 6,
  PRECEDENCE_PREFIX = 7,
  PRECEDENCE_CALL = 8,
  PRECEDENCE_DOT = 9,         // Highest precedence.
};

// The top-level for blocks/ifs is recursive descent, the expression parser is
// a Pratt parser. The basic idea there is to have the precedences (and
// associativities) encoded relative to each other and only parse up until you
// hit something of that precedence. There's a dispatch table in expressions_
// at the top of parser.cc that describes how each token dispatches if it's
// seen as either a prefix or infix operator, and if it's infix, what its
// precedence is.
//
// Refs:
// - http://javascript.crockford.com/tdop/tdop.html
// - http://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-expression-parsing-made-easy/

// Indexed by Token::Type.
ParserHelper Parser::expressions_[] = {
    {nullptr, nullptr, -1},                                   // INVALID
    {&Parser::Literal, nullptr, -1},                          // INTEGER
    {&Parser::Literal, nullptr, -1},                          // STRING
    {&Parser::Literal, nullptr, -1},                          // TRUE_TOKEN
    {&Parser::Literal, nullptr, -1},                          // FALSE_TOKEN
    {nullptr, &Parser::Assignment, PRECEDENCE_ASSIGNMENT},    // EQUAL
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_SUM},       // PLUS
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_SUM},       // MINUS
    {nullptr, &Parser::Assignment, PRECEDENCE_ASSIGNMENT},    // PLUS_EQUALS
    {nullptr, &Parser::Assignment, PRECEDENCE_ASSIGNMENT},    // MINUS_EQUALS
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_EQUALITY},  // EQUAL_EQUAL
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_EQUALITY},  // NOT_EQUAL
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_RELATION},  // LESS_EQUAL
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_RELATION},  // GREATER_EQUAL
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_RELATION},  // LESS_THAN
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_RELATION},  // GREATER_THAN
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_AND},       // BOOLEAN_AND
    {nullptr, &Parser::BinaryOperator, PRECEDENCE_OR},        // BOOLEAN_OR
    {&Parser::Not, nullptr, -1},                              // BANG
    {nullptr, &Parser::DotOperator, PRECEDENCE_DOT},          // DOT
    {&Parser::Group, nullptr, -1},                            // LEFT_PAREN
    {nullptr, nullptr, -1},                                   // RIGHT_PAREN
    {&Parser::List, &Parser::Subscript, PRECEDENCE_CALL},     // LEFT_BRACKET
    {nullptr, nullptr, -1},                                   // RIGHT_BRACKET
    {nullptr, nullptr, -1},                                   // LEFT_BRACE
    {nullptr, nullptr, -1},                                   // RIGHT_BRACE
    {nullptr, nullptr, -1},                                   // IF
    {nullptr, nullptr, -1},                                   // ELSE
    {&Parser::Name, &Parser::IdentifierOrCall, PRECEDENCE_CALL},  // IDENTIFIER
    {nullptr, nullptr, -1},                                       // COMMA
    {nullptr, nullptr, -1},                // UNCLASSIFIED_COMMENT
    {nullptr, nullptr, -1},                // LINE_COMMENT
    {nullptr, nullptr, -1},                // SUFFIX_COMMENT
    {&Parser::BlockComment, nullptr, -1},  // BLOCK_COMMENT
};

Parser::Parser(const std::vector<Token>& tokens, Err* err)
    : err_(err), cur_(0) {
  for (const auto& token : tokens) {
    switch (token.type()) {
      case Token::LINE_COMMENT:
        line_comment_tokens_.push_back(token);
        break;
      case Token::SUFFIX_COMMENT:
        suffix_comment_tokens_.push_back(token);
        break;
      default:
        // Note that BLOCK_COMMENTs (top-level standalone comments) are passed
        // through the real parser.
        tokens_.push_back(token);
        break;
    }
  }
}

Parser::~Parser() {
}

// static
std::unique_ptr<ParseNode> Parser::Parse(const std::vector<Token>& tokens,
                                         Err* err) {
  Parser p(tokens, err);
  return p.ParseFile();
}

// static
std::unique_ptr<ParseNode> Parser::ParseExpression(
    const std::vector<Token>& tokens,
    Err* err) {
  Parser p(tokens, err);
  std::unique_ptr<ParseNode> expr = p.ParseExpression();
  if (!p.at_end() && !err->has_error()) {
    *err = Err(p.cur_token(), "Trailing garbage");
    return nullptr;
  }
  return expr;
}

// static
std::unique_ptr<ParseNode> Parser::ParseValue(const std::vector<Token>& tokens,
                                              Err* err) {
  for (const Token& token : tokens) {
    switch (token.type()) {
      case Token::INTEGER:
      case Token::STRING:
      case Token::TRUE_TOKEN:
      case Token::FALSE_TOKEN:
      case Token::LEFT_BRACKET:
      case Token::RIGHT_BRACKET:
      case Token::COMMA:
        continue;
      default:
        *err = Err(token, "Invalid token in literal value");
        return nullptr;
    }
  }

  return ParseExpression(tokens, err);
}

bool Parser::IsAssignment(const ParseNode* node) const {
  return node && node->AsBinaryOp() &&
         (node->AsBinaryOp()->op().type() == Token::EQUAL ||
          node->AsBinaryOp()->op().type() == Token::PLUS_EQUALS ||
          node->AsBinaryOp()->op().type() == Token::MINUS_EQUALS);
}

bool Parser::IsStatementBreak(Token::Type token_type) const {
  switch (token_type) {
    case Token::IDENTIFIER:
    case Token::LEFT_BRACE:
    case Token::RIGHT_BRACE:
    case Token::IF:
    case Token::ELSE:
      return true;
    default:
      return false;
  }
}

bool Parser::LookAhead(Token::Type type) {
  if (at_end())
    return false;
  return cur_token().type() == type;
}

bool Parser::Match(Token::Type type) {
  if (!LookAhead(type))
    return false;
  Consume();
  return true;
}

Token Parser::Consume(Token::Type type, const char* error_message) {
  Token::Type types[1] = { type };
  return Consume(types, 1, error_message);
}

Token Parser::Consume(Token::Type* types,
                      size_t num_types,
                      const char* error_message) {
  if (has_error()) {
    // Don't overwrite current error, but make progress through tokens so that
    // a loop that's expecting a particular token will still terminate.
    cur_++;
    return Token(Location(), Token::INVALID, base::StringPiece());
  }
  if (at_end()) {
    const char kEOFMsg[] = "I hit EOF instead.";
    if (tokens_.empty())
      *err_ = Err(Location(), error_message, kEOFMsg);
    else
      *err_ = Err(tokens_[tokens_.size() - 1], error_message, kEOFMsg);
    return Token(Location(), Token::INVALID, base::StringPiece());
  }

  for (size_t i = 0; i < num_types; ++i) {
    if (cur_token().type() == types[i])
      return Consume();
  }
  *err_ = Err(cur_token(), error_message);
  return Token(Location(), Token::INVALID, base::StringPiece());
}

Token Parser::Consume() {
  return tokens_[cur_++];
}

std::unique_ptr<ParseNode> Parser::ParseExpression() {
  return ParseExpression(0);
}

std::unique_ptr<ParseNode> Parser::ParseExpression(int precedence) {
  if (at_end())
    return std::unique_ptr<ParseNode>();

  Token token = Consume();
  PrefixFunc prefix = expressions_[token.type()].prefix;

  if (prefix == nullptr) {
    *err_ = Err(token,
                std::string("Unexpected token '") + token.value().as_string() +
                    std::string("'"));
    return std::unique_ptr<ParseNode>();
  }

  std::unique_ptr<ParseNode> left = (this->*prefix)(token);
  if (has_error())
    return left;

  while (!at_end() && !IsStatementBreak(cur_token().type()) &&
         precedence <= expressions_[cur_token().type()].precedence) {
    token = Consume();
    InfixFunc infix = expressions_[token.type()].infix;
    if (infix == nullptr) {
      *err_ = Err(token,
                  std::string("Unexpected token '") +
                      token.value().as_string() + std::string("'"));
      return std::unique_ptr<ParseNode>();
    }
    left = (this->*infix)(std::move(left), token);
    if (has_error())
      return std::unique_ptr<ParseNode>();
  }

  return left;
}

std::unique_ptr<ParseNode> Parser::Literal(Token token) {
  return base::WrapUnique(new LiteralNode(token));
}

std::unique_ptr<ParseNode> Parser::Name(Token token) {
  return IdentifierOrCall(std::unique_ptr<ParseNode>(), token);
}

std::unique_ptr<ParseNode> Parser::BlockComment(Token token) {
  std::unique_ptr<BlockCommentNode> comment(new BlockCommentNode());
  comment->set_comment(token);
  return std::move(comment);
}

std::unique_ptr<ParseNode> Parser::Group(Token token) {
  std::unique_ptr<ParseNode> expr = ParseExpression();
  if (has_error())
    return std::unique_ptr<ParseNode>();
  Consume(Token::RIGHT_PAREN, "Expected ')'");
  return expr;
}

std::unique_ptr<ParseNode> Parser::Not(Token token) {
  std::unique_ptr<ParseNode> expr = ParseExpression(PRECEDENCE_PREFIX + 1);
  if (has_error())
    return std::unique_ptr<ParseNode>();
  if (!expr) {
    if (!has_error())
      *err_ = Err(token, "Expected right-hand side for '!'.");
    return std::unique_ptr<ParseNode>();
  }
  std::unique_ptr<UnaryOpNode> unary_op(new UnaryOpNode);
  unary_op->set_op(token);
  unary_op->set_operand(std::move(expr));
  return std::move(unary_op);
}

std::unique_ptr<ParseNode> Parser::List(Token node) {
  std::unique_ptr<ParseNode> list(ParseList(node, Token::RIGHT_BRACKET, true));
  if (!has_error() && !at_end())
    Consume(Token::RIGHT_BRACKET, "Expected ']'");
  return list;
}

std::unique_ptr<ParseNode> Parser::BinaryOperator(
    std::unique_ptr<ParseNode> left,
    Token token) {
  std::unique_ptr<ParseNode> right =
      ParseExpression(expressions_[token.type()].precedence + 1);
  if (!right) {
    if (!has_error()) {
      *err_ = Err(token, "Expected right-hand side for '" +
                             token.value().as_string() + "'");
    }
    return std::unique_ptr<ParseNode>();
  }
  std::unique_ptr<BinaryOpNode> binary_op(new BinaryOpNode);
  binary_op->set_op(token);
  binary_op->set_left(std::move(left));
  binary_op->set_right(std::move(right));
  return std::move(binary_op);
}

std::unique_ptr<ParseNode> Parser::IdentifierOrCall(
    std::unique_ptr<ParseNode> left,
    Token token) {
  std::unique_ptr<ListNode> list(new ListNode);
  list->set_begin_token(token);
  list->set_end(base::WrapUnique(new EndNode(token)));
  std::unique_ptr<BlockNode> block;
  bool has_arg = false;
  if (LookAhead(Token::LEFT_PAREN)) {
    Token start_token = Consume();
    // Parsing a function call.
    has_arg = true;
    if (Match(Token::RIGHT_PAREN)) {
      // Nothing, just an empty call.
    } else {
      list = ParseList(start_token, Token::RIGHT_PAREN, false);
      if (has_error())
        return std::unique_ptr<ParseNode>();
      Consume(Token::RIGHT_PAREN, "Expected ')' after call");
    }
    // Optionally with a scope.
    if (LookAhead(Token::LEFT_BRACE)) {
      block = ParseBlock();
      if (has_error())
        return std::unique_ptr<ParseNode>();
    }
  }

  if (!left && !has_arg) {
    // Not a function call, just a standalone identifier.
    return std::unique_ptr<ParseNode>(new IdentifierNode(token));
  }
  std::unique_ptr<FunctionCallNode> func_call(new FunctionCallNode);
  func_call->set_function(token);
  func_call->set_args(std::move(list));
  if (block)
    func_call->set_block(std::move(block));
  return std::move(func_call);
}

std::unique_ptr<ParseNode> Parser::Assignment(std::unique_ptr<ParseNode> left,
                                              Token token) {
  if (left->AsIdentifier() == nullptr) {
    *err_ = Err(left.get(), "Left-hand side of assignment must be identifier.");
    return std::unique_ptr<ParseNode>();
  }
  std::unique_ptr<ParseNode> value = ParseExpression(PRECEDENCE_ASSIGNMENT);
  if (!value) {
    if (!has_error())
      *err_ = Err(token, "Expected right-hand side for assignment.");
    return std::unique_ptr<ParseNode>();
  }
  std::unique_ptr<BinaryOpNode> assign(new BinaryOpNode);
  assign->set_op(token);
  assign->set_left(std::move(left));
  assign->set_right(std::move(value));
  return std::move(assign);
}

std::unique_ptr<ParseNode> Parser::Subscript(std::unique_ptr<ParseNode> left,
                                             Token token) {
  // TODO: Maybe support more complex expressions like a[0][0]. This would
  // require work on the evaluator too.
  if (left->AsIdentifier() == nullptr) {
    *err_ = Err(left.get(), "May only subscript identifiers.",
        "The thing on the left hand side of the [] must be an identifier\n"
        "and not an expression. If you need this, you'll have to assign the\n"
        "value to a temporary before subscripting. Sorry.");
    return std::unique_ptr<ParseNode>();
  }
  std::unique_ptr<ParseNode> value = ParseExpression();
  Consume(Token::RIGHT_BRACKET, "Expecting ']' after subscript.");
  std::unique_ptr<AccessorNode> accessor(new AccessorNode);
  accessor->set_base(left->AsIdentifier()->value());
  accessor->set_index(std::move(value));
  return std::move(accessor);
}

std::unique_ptr<ParseNode> Parser::DotOperator(std::unique_ptr<ParseNode> left,
                                               Token token) {
  if (left->AsIdentifier() == nullptr) {
    *err_ = Err(left.get(), "May only use \".\" for identifiers.",
        "The thing on the left hand side of the dot must be an identifier\n"
        "and not an expression. If you need this, you'll have to assign the\n"
        "value to a temporary first. Sorry.");
    return std::unique_ptr<ParseNode>();
  }

  std::unique_ptr<ParseNode> right = ParseExpression(PRECEDENCE_DOT);
  if (!right || !right->AsIdentifier()) {
    *err_ = Err(token, "Expected identifier for right-hand-side of \".\"",
        "Good: a.cookies\nBad: a.42\nLooks good but still bad: a.cookies()");
    return std::unique_ptr<ParseNode>();
  }

  std::unique_ptr<AccessorNode> accessor(new AccessorNode);
  accessor->set_base(left->AsIdentifier()->value());
  accessor->set_member(std::unique_ptr<IdentifierNode>(
      static_cast<IdentifierNode*>(right.release())));
  return std::move(accessor);
}

// Does not Consume the start or end token.
std::unique_ptr<ListNode> Parser::ParseList(Token start_token,
                                            Token::Type stop_before,
                                            bool allow_trailing_comma) {
  std::unique_ptr<ListNode> list(new ListNode);
  list->set_begin_token(start_token);
  bool just_got_comma = false;
  bool first_time = true;
  while (!LookAhead(stop_before)) {
    if (!first_time) {
      if (!just_got_comma) {
        // Require commas separate things in lists.
        *err_ = Err(cur_token(), "Expected comma between items.");
        return std::unique_ptr<ListNode>();
      }
    }
    first_time = false;

    // Why _OR? We're parsing things that are higher precedence than the ,
    // that separates the items of the list. , should appear lower than
    // boolean expressions (the lowest of which is OR), but above assignments.
    list->append_item(ParseExpression(PRECEDENCE_OR));
    if (has_error())
      return std::unique_ptr<ListNode>();
    if (at_end()) {
      *err_ =
          Err(tokens_[tokens_.size() - 1], "Unexpected end of file in list.");
      return std::unique_ptr<ListNode>();
    }
    if (list->contents().back()->AsBlockComment()) {
      // If there was a comment inside the list, we don't need a comma to the
      // next item, so pretend we got one, if we're expecting one.
      just_got_comma = allow_trailing_comma;
    } else {
      just_got_comma = Match(Token::COMMA);
    }
  }
  if (just_got_comma && !allow_trailing_comma) {
    *err_ = Err(cur_token(), "Trailing comma");
    return std::unique_ptr<ListNode>();
  }
  list->set_end(base::WrapUnique(new EndNode(cur_token())));
  return list;
}

std::unique_ptr<ParseNode> Parser::ParseFile() {
  std::unique_ptr<BlockNode> file(new BlockNode);
  for (;;) {
    if (at_end())
      break;
    std::unique_ptr<ParseNode> statement = ParseStatement();
    if (!statement)
      break;
    file->append_statement(std::move(statement));
  }
  if (!at_end() && !has_error())
    *err_ = Err(cur_token(), "Unexpected here, should be newline.");
  if (has_error())
    return std::unique_ptr<ParseNode>();

  // TODO(scottmg): If this is measurably expensive, it could be done only
  // when necessary (when reformatting, or during tests). Comments are
  // separate from the parse tree at this point, so downstream code can remain
  // ignorant of them.
  AssignComments(file.get());

  return std::move(file);
}

std::unique_ptr<ParseNode> Parser::ParseStatement() {
  if (LookAhead(Token::IF)) {
    return ParseCondition();
  } else if (LookAhead(Token::BLOCK_COMMENT)) {
    return BlockComment(Consume());
  } else {
    // TODO(scottmg): Is this too strict? Just drop all the testing if we want
    // to allow "pointless" expressions and return ParseExpression() directly.
    std::unique_ptr<ParseNode> stmt = ParseExpression();
    if (stmt) {
      if (stmt->AsFunctionCall() || IsAssignment(stmt.get()))
        return stmt;
    }
    if (!has_error()) {
      Token token = at_end() ? tokens_[tokens_.size() - 1] : cur_token();
      *err_ = Err(token, "Expecting assignment or function call.");
    }
    return std::unique_ptr<ParseNode>();
  }
}

std::unique_ptr<BlockNode> Parser::ParseBlock() {
  Token begin_token =
      Consume(Token::LEFT_BRACE, "Expected '{' to start a block.");
  if (has_error())
    return std::unique_ptr<BlockNode>();
  std::unique_ptr<BlockNode> block(new BlockNode);
  block->set_begin_token(begin_token);

  for (;;) {
    if (LookAhead(Token::RIGHT_BRACE)) {
      block->set_end(base::WrapUnique(new EndNode(Consume())));
      break;
    }

    std::unique_ptr<ParseNode> statement = ParseStatement();
    if (!statement)
      return std::unique_ptr<BlockNode>();
    block->append_statement(std::move(statement));
  }
  return block;
}

std::unique_ptr<ParseNode> Parser::ParseCondition() {
  std::unique_ptr<ConditionNode> condition(new ConditionNode);
  condition->set_if_token(Consume(Token::IF, "Expected 'if'"));
  Consume(Token::LEFT_PAREN, "Expected '(' after 'if'.");
  condition->set_condition(ParseExpression());
  if (IsAssignment(condition->condition()))
    *err_ = Err(condition->condition(), "Assignment not allowed in 'if'.");
  Consume(Token::RIGHT_PAREN, "Expected ')' after condition of 'if'.");
  condition->set_if_true(ParseBlock());
  if (Match(Token::ELSE)) {
    if (LookAhead(Token::LEFT_BRACE)) {
      condition->set_if_false(ParseBlock());
    } else if (LookAhead(Token::IF)) {
      condition->set_if_false(ParseStatement());
    } else {
      *err_ = Err(cur_token(), "Expected '{' or 'if' after 'else'.");
      return std::unique_ptr<ParseNode>();
    }
  }
  if (has_error())
    return std::unique_ptr<ParseNode>();
  return std::move(condition);
}

void Parser::TraverseOrder(const ParseNode* root,
                           std::vector<const ParseNode*>* pre,
                           std::vector<const ParseNode*>* post) {
  if (root) {
    pre->push_back(root);

    if (const AccessorNode* accessor = root->AsAccessor()) {
      TraverseOrder(accessor->index(), pre, post);
      TraverseOrder(accessor->member(), pre, post);
    } else if (const BinaryOpNode* binop = root->AsBinaryOp()) {
      TraverseOrder(binop->left(), pre, post);
      TraverseOrder(binop->right(), pre, post);
    } else if (const BlockNode* block = root->AsBlock()) {
      for (const auto& statement : block->statements())
        TraverseOrder(statement.get(), pre, post);
      TraverseOrder(block->End(), pre, post);
    } else if (const ConditionNode* condition = root->AsConditionNode()) {
      TraverseOrder(condition->condition(), pre, post);
      TraverseOrder(condition->if_true(), pre, post);
      TraverseOrder(condition->if_false(), pre, post);
    } else if (const FunctionCallNode* func_call = root->AsFunctionCall()) {
      TraverseOrder(func_call->args(), pre, post);
      TraverseOrder(func_call->block(), pre, post);
    } else if (root->AsIdentifier()) {
      // Nothing.
    } else if (const ListNode* list = root->AsList()) {
      for (const auto& node : list->contents())
        TraverseOrder(node.get(), pre, post);
      TraverseOrder(list->End(), pre, post);
    } else if (root->AsLiteral()) {
      // Nothing.
    } else if (const UnaryOpNode* unaryop = root->AsUnaryOp()) {
      TraverseOrder(unaryop->operand(), pre, post);
    } else if (root->AsBlockComment()) {
      // Nothing.
    } else if (root->AsEnd()) {
      // Nothing.
    } else {
      CHECK(false) << "Unhandled case in TraverseOrder.";
    }

    post->push_back(root);
  }
}

void Parser::AssignComments(ParseNode* file) {
  // Start by generating a pre- and post- order traversal of the tree so we
  // can determine what's before and after comments.
  std::vector<const ParseNode*> pre;
  std::vector<const ParseNode*> post;
  TraverseOrder(file, &pre, &post);

  // Assign line comments to syntax immediately following.
  int cur_comment = 0;
  for (const auto& node : pre) {
    const Location& start = node->GetRange().begin();
    while (cur_comment < static_cast<int>(line_comment_tokens_.size())) {
      if (start.byte() >= line_comment_tokens_[cur_comment].location().byte()) {
        const_cast<ParseNode*>(node)->comments_mutable()->append_before(
            line_comment_tokens_[cur_comment]);
        ++cur_comment;
      } else {
        break;
      }
    }
  }

  // Remaining line comments go at end of file.
  for (; cur_comment < static_cast<int>(line_comment_tokens_.size());
       ++cur_comment)
    file->comments_mutable()->append_after(line_comment_tokens_[cur_comment]);

  // Assign suffix to syntax immediately before.
  cur_comment = static_cast<int>(suffix_comment_tokens_.size() - 1);
  for (std::vector<const ParseNode*>::const_reverse_iterator i = post.rbegin();
       i != post.rend();
       ++i) {
    // Don't assign suffix comments to the function, list, or block, but instead
    // to the last thing inside.
    if ((*i)->AsFunctionCall() || (*i)->AsList() || (*i)->AsBlock())
      continue;

    const Location& start = (*i)->GetRange().begin();
    const Location& end = (*i)->GetRange().end();

    // Don't assign suffix comments to something that starts on an earlier
    // line, so that in:
    //
    // sources = [ "a",
    //     "b" ] # comment
    //
    // it's attached to "b", not sources = [ ... ].
    if (start.line_number() != end.line_number())
      continue;

    while (cur_comment >= 0) {
      if (end.byte() <= suffix_comment_tokens_[cur_comment].location().byte()) {
        const_cast<ParseNode*>(*i)->comments_mutable()->append_suffix(
            suffix_comment_tokens_[cur_comment]);
        --cur_comment;
      } else {
        break;
      }
    }

    // Suffix comments were assigned in reverse, so if there were multiple on
    // the same node, they need to be reversed.
    if ((*i)->comments() && !(*i)->comments()->suffix().empty())
      const_cast<ParseNode*>(*i)->comments_mutable()->ReverseSuffix();
  }
}