carbon-lang/toolchain/parser/parser.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/parser/parser.h"

#include <cstdlib>
#include <memory>

#include "common/check.h"
#include "llvm/ADT/Optional.h"
#include "llvm/Support/PrettyStackTrace.h"
#include "toolchain/lexer/token_kind.h"
#include "toolchain/lexer/tokenized_buffer.h"
#include "toolchain/parser/parse_node_kind.h"
#include "toolchain/parser/parse_tree.h"

namespace Carbon {

// May be emitted a couple different ways as part of operator parsing.
CARBON_DIAGNOSTIC(
    OperatorRequiresParentheses, Error,
    "Parentheses are required to disambiguate operator precedence.");

CARBON_DIAGNOSTIC(ExpectedSemiAfterExpression, Error,
                  "Expected `;` after expression.");

// A relative location for characters in errors.
enum class RelativeLocation : int8_t {
  Around,
  After,
  Before,
};

// Adapts RelativeLocation for use with formatv.
static auto operator<<(llvm::raw_ostream& out, RelativeLocation loc)
    -> llvm::raw_ostream& {
  switch (loc) {
    case RelativeLocation::Around:
      out << "around";
      break;
    case RelativeLocation::After:
      out << "after";
      break;
    case RelativeLocation::Before:
      out << "before";
      break;
  }
  return out;
}

class Parser::PrettyStackTraceParseState : public llvm::PrettyStackTraceEntry {
 public:
  explicit PrettyStackTraceParseState(const Parser* parser) : parser_(parser) {}
  ~PrettyStackTraceParseState() override = default;

  auto print(llvm::raw_ostream& output) const -> void override {
    output << "Parser stack:\n";
    for (int i = 0; i < static_cast<int>(parser_->state_stack_.size()); ++i) {
      const auto& entry = parser_->state_stack_[i];
      output << "\t" << i << ".\t" << entry.state;
      Print(output, entry.token);
    }
    output << "\tcursor\tposition_";
    Print(output, *parser_->position_);
  }

 private:
  auto Print(llvm::raw_ostream& output, TokenizedBuffer::Token token) const
      -> void {
    auto line = parser_->tokens_->GetLine(token);
    output << " @ " << parser_->tokens_->GetLineNumber(line) << ":"
           << parser_->tokens_->GetColumnNumber(token) << ":"
           << " token " << token << " : "
           << parser_->tokens_->GetKind(token).Name() << "\n";
  }

  const Parser* parser_;
};

Parser::Parser(ParseTree& tree, TokenizedBuffer& tokens,
               TokenDiagnosticEmitter& emitter)
    : tree_(&tree),
      tokens_(&tokens),
      emitter_(&emitter),
      position_(tokens_->tokens().begin()),
      end_(tokens_->tokens().end()) {
  CARBON_CHECK(position_ != end_) << "Empty TokenizedBuffer";
  --end_;
  CARBON_CHECK(tokens_->GetKind(*end_) == TokenKind::EndOfFile())
      << "TokenizedBuffer should end with EndOfFile, ended with "
      << tokens_->GetKind(*end_).Name();
}

auto Parser::AddLeafNode(ParseNodeKind kind, TokenizedBuffer::Token token,
                         bool has_error) -> void {
  tree_->node_impls_.push_back(
      ParseTree::NodeImpl(kind, has_error, token, /*subtree_size=*/1));
  if (has_error) {
    tree_->has_errors_ = true;
  }
}

auto Parser::AddNode(ParseNodeKind kind, TokenizedBuffer::Token token,
                     int subtree_start, bool has_error) -> void {
  int subtree_size = tree_->size() - subtree_start + 1;
  tree_->node_impls_.push_back(
      ParseTree::NodeImpl(kind, has_error, token, subtree_size));
  if (has_error) {
    tree_->has_errors_ = true;
  }
}

auto Parser::ConsumeAndAddCloseParen(TokenizedBuffer::Token open_paren,
                                     ParseNodeKind close_kind) -> bool {
  if (ConsumeAndAddLeafNodeIf(TokenKind::CloseParen(), close_kind)) {
    return true;
  }

  // TODO: Include the location of the matching open_paren in the diagnostic.
  CARBON_DIAGNOSTIC(ExpectedCloseParen, Error, "Unexpected tokens before `)`.");
  emitter_->Emit(*position_, ExpectedCloseParen);

  SkipTo(tokens_->GetMatchedClosingToken(open_paren));
  AddLeafNode(close_kind, Consume());
  return false;
}

auto Parser::ConsumeAndAddLeafNodeIf(TokenKind token_kind,
                                     ParseNodeKind node_kind) -> bool {
  auto token = ConsumeIf(token_kind);
  if (!token) {
    return false;
  }

  AddLeafNode(node_kind, *token);
  return true;
}

auto Parser::ConsumeIf(TokenKind kind)
    -> llvm::Optional<TokenizedBuffer::Token> {
  if (!PositionIs(kind)) {
    return llvm::None;
  }
  return Consume();
}

auto Parser::FindNextOf(std::initializer_list<TokenKind> desired_kinds)
    -> llvm::Optional<TokenizedBuffer::Token> {
  auto new_position = position_;
  while (true) {
    TokenizedBuffer::Token token = *new_position;
    TokenKind kind = tokens_->GetKind(token);
    if (kind.IsOneOf(desired_kinds)) {
      return token;
    }

    // Step to the next token at the current bracketing level.
    if (kind.IsClosingSymbol() || kind == TokenKind::EndOfFile()) {
      // There are no more tokens at this level.
      return llvm::None;
    } else if (kind.IsOpeningSymbol()) {
      new_position = TokenizedBuffer::TokenIterator(
          tokens_->GetMatchedClosingToken(token));
      // Advance past the closing token.
      ++new_position;
    } else {
      ++new_position;
    }
  }
}

auto Parser::SkipMatchingGroup() -> bool {
  if (!PositionKind().IsOpeningSymbol()) {
    return false;
  }

  SkipTo(tokens_->GetMatchedClosingToken(*position_));
  ++position_;
  return true;
}

auto Parser::SkipPastLikelyEnd(TokenizedBuffer::Token skip_root)
    -> llvm::Optional<TokenizedBuffer::Token> {
  if (position_ == end_) {
    return llvm::None;
  }

  TokenizedBuffer::Line root_line = tokens_->GetLine(skip_root);
  int root_line_indent = tokens_->GetIndentColumnNumber(root_line);

  // We will keep scanning through tokens on the same line as the root or
  // lines with greater indentation than root's line.
  auto is_same_line_or_indent_greater_than_root =
      [&](TokenizedBuffer::Token t) {
        TokenizedBuffer::Line l = tokens_->GetLine(t);
        if (l == root_line) {
          return true;
        }

        return tokens_->GetIndentColumnNumber(l) > root_line_indent;
      };

  do {
    if (PositionIs(TokenKind::CloseCurlyBrace())) {
      // Immediately bail out if we hit an unmatched close curly, this will
      // pop us up a level of the syntax grouping.
      return llvm::None;
    }

    // We assume that a semicolon is always intended to be the end of the
    // current construct.
    if (auto semi = ConsumeIf(TokenKind::Semi())) {
      return semi;
    }

    // Skip over any matching group of tokens_->
    if (SkipMatchingGroup()) {
      continue;
    }

    // Otherwise just step forward one token.
    ++position_;
  } while (position_ != end_ &&
           is_same_line_or_indent_greater_than_root(*position_));

  return llvm::None;
}

auto Parser::SkipTo(TokenizedBuffer::Token t) -> void {
  CARBON_CHECK(t >= *position_) << "Tried to skip backwards from " << position_
                                << " to " << TokenizedBuffer::TokenIterator(t);
  position_ = TokenizedBuffer::TokenIterator(t);
  CARBON_CHECK(position_ != end_) << "Skipped past EOF.";
}

auto Parser::HandleCodeBlockState() -> void {
  PopAndDiscardState();

  PushState(ParserState::CodeBlockFinish());
  if (ConsumeAndAddLeafNodeIf(TokenKind::OpenCurlyBrace(),
                              ParseNodeKind::CodeBlockStart())) {
    PushState(ParserState::StatementScopeLoop());
  } else {
    AddLeafNode(ParseNodeKind::CodeBlockStart(), *position_,
                /*has_error=*/true);

    // Recover by parsing a single statement.
    CARBON_DIAGNOSTIC(ExpectedCodeBlock, Error, "Expected braced code block.");
    emitter_->Emit(*position_, ExpectedCodeBlock);

    PushState(ParserState::Statement());
  }
}

// Determines whether the given token is considered to be the start of an
// operand according to the rules for infix operator parsing.
static auto IsAssumedStartOfOperand(TokenKind kind) -> bool {
  return kind.IsOneOf({TokenKind::OpenParen(), TokenKind::Identifier(),
                       TokenKind::IntegerLiteral(), TokenKind::RealLiteral(),
                       TokenKind::StringLiteral()});
}

// Determines whether the given token is considered to be the end of an
// operand according to the rules for infix operator parsing.
static auto IsAssumedEndOfOperand(TokenKind kind) -> bool {
  return kind.IsOneOf({TokenKind::CloseParen(), TokenKind::CloseCurlyBrace(),
                       TokenKind::CloseSquareBracket(), TokenKind::Identifier(),
                       TokenKind::IntegerLiteral(), TokenKind::RealLiteral(),
                       TokenKind::StringLiteral()});
}

// Determines whether the given token could possibly be the start of an
// operand. This is conservatively correct, and will never incorrectly return
// `false`, but can incorrectly return `true`.
static auto IsPossibleStartOfOperand(TokenKind kind) -> bool {
  return !kind.IsOneOf({TokenKind::CloseParen(), TokenKind::CloseCurlyBrace(),
                        TokenKind::CloseSquareBracket(), TokenKind::Comma(),
                        TokenKind::Semi(), TokenKind::Colon()});
}

auto Parser::IsLexicallyValidInfixOperator() -> bool {
  CARBON_CHECK(position_ != end_) << "Expected an operator token.";

  bool leading_space = tokens_->HasLeadingWhitespace(*position_);
  bool trailing_space = tokens_->HasTrailingWhitespace(*position_);

  // If there's whitespace on both sides, it's an infix operator.
  if (leading_space && trailing_space) {
    return true;
  }

  // If there's whitespace on exactly one side, it's not an infix operator.
  if (leading_space || trailing_space) {
    return false;
  }

  // Otherwise, for an infix operator, the preceding token must be any close
  // bracket, identifier, or literal and the next token must be an open paren,
  // identifier, or literal.
  if (position_ == tokens_->tokens().begin() ||
      !IsAssumedEndOfOperand(tokens_->GetKind(*(position_ - 1))) ||
      !IsAssumedStartOfOperand(tokens_->GetKind(*(position_ + 1)))) {
    return false;
  }

  return true;
}

auto Parser::IsTrailingOperatorInfix() -> bool {
  if (position_ == end_) {
    return false;
  }

  // An operator that follows the infix operator rules is parsed as
  // infix, unless the next token means that it can't possibly be.
  if (IsLexicallyValidInfixOperator() &&
      IsPossibleStartOfOperand(tokens_->GetKind(*(position_ + 1)))) {
    return true;
  }

  // A trailing operator with leading whitespace that's not valid as infix is
  // not valid at all. If the next token looks like the start of an operand,
  // then parse as infix, otherwise as postfix. Either way we'll produce a
  // diagnostic later on.
  if (tokens_->HasLeadingWhitespace(*position_) &&
      IsAssumedStartOfOperand(tokens_->GetKind(*(position_ + 1)))) {
    return true;
  }

  return false;
}

auto Parser::DiagnoseOperatorFixity(OperatorFixity fixity) -> void {
  if (fixity == OperatorFixity::Infix) {
    // Infix operators must satisfy the infix operator rules.
    if (!IsLexicallyValidInfixOperator()) {
      CARBON_DIAGNOSTIC(BinaryOperatorRequiresWhitespace, Error,
                        "Whitespace missing {0} binary operator.",
                        RelativeLocation);
      emitter_->Emit(*position_, BinaryOperatorRequiresWhitespace,
                     tokens_->HasLeadingWhitespace(*position_)
                         ? RelativeLocation::After
                         : (tokens_->HasTrailingWhitespace(*position_)
                                ? RelativeLocation::Before
                                : RelativeLocation::Around));
    }
  } else {
    bool prefix = fixity == OperatorFixity::Prefix;

    // Whitespace is not permitted between a symbolic pre/postfix operator and
    // its operand.
    if (PositionKind().IsSymbol() &&
        (prefix ? tokens_->HasTrailingWhitespace(*position_)
                : tokens_->HasLeadingWhitespace(*position_))) {
      CARBON_DIAGNOSTIC(UnaryOperatorHasWhitespace, Error,
                        "Whitespace is not allowed {0} this unary operator.",
                        RelativeLocation);
      emitter_->Emit(
          *position_, UnaryOperatorHasWhitespace,
          prefix ? RelativeLocation::After : RelativeLocation::Before);
    }
    // Pre/postfix operators must not satisfy the infix operator rules.
    if (IsLexicallyValidInfixOperator()) {
      CARBON_DIAGNOSTIC(UnaryOperatorRequiresWhitespace, Error,
                        "Whitespace is required {0} this unary operator.",
                        RelativeLocation);
      emitter_->Emit(
          *position_, UnaryOperatorRequiresWhitespace,
          prefix ? RelativeLocation::Before : RelativeLocation::After);
    }
  }
}

auto Parser::ConsumeListToken(ParseNodeKind comma_kind, TokenKind close_kind,
                              bool already_has_error) -> ListTokenKind {
  if (!PositionIs(TokenKind::Comma()) && !PositionIs(close_kind)) {
    // Don't error a second time on the same element.
    if (!already_has_error) {
      CARBON_DIAGNOSTIC(UnexpectedTokenAfterListElement, Error,
                        "Expected `,` or `{0}`.", TokenKind);
      emitter_->Emit(*position_, UnexpectedTokenAfterListElement, close_kind);
      ReturnErrorOnState();
    }

    // Recover from the invalid token.
    auto end_of_element = FindNextOf({TokenKind::Comma(), close_kind});
    // The lexer guarantees that parentheses are balanced.
    CARBON_CHECK(end_of_element)
        << "missing matching `" << close_kind.GetOpeningSymbol() << "` for `"
        << close_kind << "`";

    SkipTo(*end_of_element);
  }

  if (PositionIs(close_kind)) {
    return ListTokenKind::Close;
  } else {
    AddLeafNode(comma_kind, Consume());
    return PositionIs(close_kind) ? ListTokenKind::CommaClose
                                  : ListTokenKind::Comma;
  }
}

auto Parser::Parse() -> void {
  // Traces state_stack_. This runs even in opt because it's low overhead.
  PrettyStackTraceParseState pretty_stack(this);

  PushState(ParserState::DeclarationLoop());
  while (!state_stack_.empty()) {
    switch (state_stack_.back().state) {
#define CARBON_PARSER_STATE(Name) \
  case ParserState::Name():       \
    Handle##Name##State();        \
    break;
#include "toolchain/parser/parser_state.def"
    }
  }

  AddLeafNode(ParseNodeKind::FileEnd(), *position_);
}

auto Parser::HandleBraceExpressionState() -> void {
  auto state = PopState();

  state.state = ParserState::BraceExpressionFinishAsUnknown();
  PushState(state);

  CARBON_CHECK(ConsumeAndAddLeafNodeIf(
      TokenKind::OpenCurlyBrace(),
      ParseNodeKind::StructLiteralOrStructTypeLiteralStart()));
  if (!PositionIs(TokenKind::CloseCurlyBrace())) {
    PushState(ParserState::BraceExpressionParameterAsUnknown());
  }
}

auto Parser::BraceExpressionKindToParserState(BraceExpressionKind kind,
                                              ParserState type,
                                              ParserState value,
                                              ParserState unknown)
    -> ParserState {
  switch (kind) {
    case BraceExpressionKind::Type: {
      return type;
    }
    case BraceExpressionKind::Value: {
      return value;
    }
    case BraceExpressionKind::Unknown: {
      return unknown;
    }
  }
}

auto Parser::HandleBraceExpressionParameterError(StateStackEntry state,
                                                 BraceExpressionKind kind)
    -> void {
  CARBON_DIAGNOSTIC(ExpectedStructLiteralField, Error, "Expected {0}{1}{2}.",
                    llvm::StringRef, llvm::StringRef, llvm::StringRef);
  bool can_be_type = kind != BraceExpressionKind::Value;
  bool can_be_value = kind != BraceExpressionKind::Type;
  emitter_->Emit(*position_, ExpectedStructLiteralField,
                 can_be_type ? "`.field: type`" : "",
                 (can_be_type && can_be_value) ? " or " : "",
                 can_be_value ? "`.field = value`" : "");

  state.state = BraceExpressionKindToParserState(
      kind, ParserState::BraceExpressionParameterFinishAsType(),
      ParserState::BraceExpressionParameterFinishAsValue(),
      ParserState::BraceExpressionParameterFinishAsUnknown());
  state.has_error = true;
  PushState(state);
}

auto Parser::HandleBraceExpressionParameter(BraceExpressionKind kind) -> void {
  auto state = PopState();

  if (!PositionIs(TokenKind::Period())) {
    HandleBraceExpressionParameterError(state, kind);
    return;
  }

  state.state = BraceExpressionKindToParserState(
      kind, ParserState::BraceExpressionParameterAfterDesignatorAsType(),
      ParserState::BraceExpressionParameterAfterDesignatorAsValue(),
      ParserState::BraceExpressionParameterAfterDesignatorAsUnknown());
  PushState(state);
  PushState(ParserState::DesignatorAsStruct());
}

auto Parser::HandleBraceExpressionParameterAsTypeState() -> void {
  HandleBraceExpressionParameter(BraceExpressionKind::Type);
}

auto Parser::HandleBraceExpressionParameterAsValueState() -> void {
  HandleBraceExpressionParameter(BraceExpressionKind::Value);
}

auto Parser::HandleBraceExpressionParameterAsUnknownState() -> void {
  HandleBraceExpressionParameter(BraceExpressionKind::Unknown);
}

auto Parser::HandleBraceExpressionParameterAfterDesignator(
    BraceExpressionKind kind) -> void {
  auto state = PopState();

  if (state.has_error) {
    auto recovery_pos = FindNextOf(
        {TokenKind::Equal(), TokenKind::Colon(), TokenKind::Comma()});
    if (!recovery_pos ||
        tokens_->GetKind(*recovery_pos) == TokenKind::Comma()) {
      state.state = BraceExpressionKindToParserState(
          kind, ParserState::BraceExpressionParameterFinishAsType(),
          ParserState::BraceExpressionParameterFinishAsValue(),
          ParserState::BraceExpressionParameterFinishAsUnknown());
      PushState(state);
      return;
    }
    SkipTo(*recovery_pos);
  }

  // Work out the kind of this element.
  auto elem_kind = BraceExpressionKind::Unknown;
  if (PositionIs(TokenKind::Colon())) {
    elem_kind = BraceExpressionKind::Type;
  } else if (PositionIs(TokenKind::Equal())) {
    elem_kind = BraceExpressionKind::Value;
  }
  // Unknown kinds and changes between type and value are errors.
  if (elem_kind == BraceExpressionKind::Unknown ||
      (kind != BraceExpressionKind::Unknown && elem_kind != kind)) {
    HandleBraceExpressionParameterError(state, kind);
    return;
  }

  // If we're setting the kind, update the BraceExpressionFinish state.
  if (kind == BraceExpressionKind::Unknown) {
    kind = elem_kind;
    auto finish_state = PopState();
    CARBON_CHECK(finish_state.state ==
                 ParserState::BraceExpressionFinishAsUnknown());
    finish_state.state = BraceExpressionKindToParserState(
        kind, ParserState::BraceExpressionFinishAsType(),
        ParserState::BraceExpressionFinishAsValue(),
        ParserState::BraceExpressionFinishAsUnknown());
    PushState(finish_state);
  }

  state.state = BraceExpressionKindToParserState(
      kind, ParserState::BraceExpressionParameterFinishAsType(),
      ParserState::BraceExpressionParameterFinishAsValue(),
      ParserState::BraceExpressionParameterFinishAsUnknown());

  state.token = Consume();

  // Struct type fields and value fields use the same grammar except
  // that one has a `:` separator and the other has an `=` separator.
  PushState(state);
  PushState(ParserState::Expression());
}

auto Parser::HandleBraceExpressionParameterAfterDesignatorAsTypeState()
    -> void {
  HandleBraceExpressionParameterAfterDesignator(BraceExpressionKind::Type);
}

auto Parser::HandleBraceExpressionParameterAfterDesignatorAsValueState()
    -> void {
  HandleBraceExpressionParameterAfterDesignator(BraceExpressionKind::Value);
}

auto Parser::HandleBraceExpressionParameterAfterDesignatorAsUnknownState()
    -> void {
  HandleBraceExpressionParameterAfterDesignator(BraceExpressionKind::Unknown);
}

auto Parser::HandleBraceExpressionParameterFinish(BraceExpressionKind kind)
    -> void {
  auto state = PopState();

  AddNode(kind == BraceExpressionKind::Type ? ParseNodeKind::StructFieldType()
                                            : ParseNodeKind::StructFieldValue(),
          state.token, state.subtree_start, state.has_error);

  if (ConsumeListToken(ParseNodeKind::StructComma(),
                       TokenKind::CloseCurlyBrace(),
                       state.has_error) == ListTokenKind::Comma) {
    PushState(BraceExpressionKindToParserState(
        kind, ParserState::BraceExpressionParameterAsType(),
        ParserState::BraceExpressionParameterAsValue(),
        ParserState::BraceExpressionParameterAsUnknown()));
  }
}

auto Parser::HandleBraceExpressionParameterFinishAsTypeState() -> void {
  HandleBraceExpressionParameterFinish(BraceExpressionKind::Type);
}

auto Parser::HandleBraceExpressionParameterFinishAsValueState() -> void {
  HandleBraceExpressionParameterFinish(BraceExpressionKind::Value);
}

auto Parser::HandleBraceExpressionParameterFinishAsUnknownState() -> void {
  HandleBraceExpressionParameterFinish(BraceExpressionKind::Unknown);
}

auto Parser::HandleBraceExpressionFinish(BraceExpressionKind kind) -> void {
  auto state = PopState();

  AddNode(kind == BraceExpressionKind::Type ? ParseNodeKind::StructTypeLiteral()
                                            : ParseNodeKind::StructLiteral(),
          Consume(), state.subtree_start, state.has_error);
}

auto Parser::HandleBraceExpressionFinishAsTypeState() -> void {
  HandleBraceExpressionFinish(BraceExpressionKind::Type);
}

auto Parser::HandleBraceExpressionFinishAsValueState() -> void {
  HandleBraceExpressionFinish(BraceExpressionKind::Value);
}

auto Parser::HandleBraceExpressionFinishAsUnknownState() -> void {
  HandleBraceExpressionFinish(BraceExpressionKind::Unknown);
}

auto Parser::HandleCallExpressionState() -> void {
  auto state = PopState();

  // TODO: When swapping () start/end, this should AddLeafNode the open before
  // continuing.
  state.state = ParserState::CallExpressionFinish();
  PushState(state);
  // Advance past the open paren.
  ++position_;
  if (!PositionIs(TokenKind::CloseParen())) {
    PushState(ParserState::CallExpressionParameterFinish());
    PushState(ParserState::Expression());
  }
}

auto Parser::HandleCallExpressionParameterFinishState() -> void {
  auto state = PopState();

  if (state.has_error) {
    ReturnErrorOnState();
  }

  if (ConsumeListToken(ParseNodeKind::CallExpressionComma(),
                       TokenKind::CloseParen(),
                       state.has_error) == ListTokenKind::Comma) {
    PushState(ParserState::CallExpressionParameterFinish());
    PushState(ParserState::Expression());
  }
}

auto Parser::HandleCallExpressionFinishState() -> void {
  auto state = PopState();

  AddLeafNode(ParseNodeKind::CallExpressionEnd(), Consume());
  AddNode(ParseNodeKind::CallExpression(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandleCodeBlockFinishState() -> void {
  auto state = PopState();

  // If the block started with an open curly, this is a close curly.
  if (tokens_->GetKind(state.token) == TokenKind::OpenCurlyBrace()) {
    AddNode(ParseNodeKind::CodeBlock(), Consume(), state.subtree_start,
            state.has_error);
  } else {
    AddNode(ParseNodeKind::CodeBlock(), state.token, state.subtree_start,
            /*has_error=*/true);
  }
}

auto Parser::HandleDeclarationLoopState() -> void {
  // This maintains the current state unless we're at the end of the file.

  switch (PositionKind()) {
    case TokenKind::EndOfFile(): {
      PopAndDiscardState();
      break;
    }
    case TokenKind::Fn(): {
      PushState(ParserState::FunctionIntroducer());
      AddLeafNode(ParseNodeKind::FunctionIntroducer(), Consume());
      break;
    }
    case TokenKind::Package(): {
      PushState(ParserState::Package());
      ++position_;
      break;
    }
    case TokenKind::Semi(): {
      AddLeafNode(ParseNodeKind::EmptyDeclaration(), Consume());
      break;
    }
    case TokenKind::Var(): {
      PushState(ParserState::VarAsRequireSemicolon());
      break;
    }
    case TokenKind::Interface(): {
      PushState(ParserState::InterfaceIntroducer());
      ++position_;
      break;
    }
    default: {
      CARBON_DIAGNOSTIC(UnrecognizedDeclaration, Error,
                        "Unrecognized declaration introducer.");
      emitter_->Emit(*position_, UnrecognizedDeclaration);
      auto cursor = *position_;
      auto semi = SkipPastLikelyEnd(cursor);
      // Locate the EmptyDeclaration at the semi when found, but use the
      // original cursor location for an error when not.
      AddLeafNode(ParseNodeKind::EmptyDeclaration(), semi ? *semi : cursor,
                  /*has_error=*/true);
      break;
    }
  }
}

auto Parser::HandleDesignator(bool as_struct) -> void {
  auto state = PopState();

  // `.` identifier
  auto dot = ConsumeIf(TokenKind::Period());
  CARBON_CHECK(dot);
  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier(),
                               ParseNodeKind::DesignatedName())) {
    CARBON_DIAGNOSTIC(ExpectedIdentifierAfterDot, Error,
                      "Expected identifier after `.`.");
    emitter_->Emit(*position_, ExpectedIdentifierAfterDot);
    // If we see a keyword, assume it was intended to be the designated name.
    // TODO: Should keywords be valid in designators?
    if (PositionKind().IsKeyword()) {
      AddLeafNode(ParseNodeKind::DesignatedName(), Consume(),
                  /*has_error=*/true);
    } else {
      state.has_error = true;
      ReturnErrorOnState();
    }
  }

  AddNode(as_struct ? ParseNodeKind::StructFieldDesignator()
                    : ParseNodeKind::DesignatorExpression(),
          *dot, state.subtree_start, state.has_error);
}

auto Parser::HandleDesignatorAsExpressionState() -> void {
  HandleDesignator(/*as_struct=*/false);
}

auto Parser::HandleDesignatorAsStructState() -> void {
  HandleDesignator(/*as_struct=*/true);
}

auto Parser::HandleExpressionState() -> void {
  auto state = PopState();

  // Check for a prefix operator.
  if (auto operator_precedence = PrecedenceGroup::ForLeading(PositionKind())) {
    if (PrecedenceGroup::GetPriority(state.ambient_precedence,
                                     *operator_precedence) !=
        OperatorPriority::RightFirst) {
      // The precedence rules don't permit this prefix operator in this
      // context. Diagnose this, but carry on and parse it anyway.
      emitter_->Emit(*position_, OperatorRequiresParentheses);
    } else {
      // Check that this operator follows the proper whitespace rules.
      DiagnoseOperatorFixity(OperatorFixity::Prefix);
    }

    PushStateForExpressionLoop(ParserState::ExpressionLoopForPrefix(),
                               state.ambient_precedence, *operator_precedence);
    ++position_;
    PushStateForExpression(*operator_precedence);
  } else {
    PushStateForExpressionLoop(ParserState::ExpressionLoop(),
                               state.ambient_precedence,
                               PrecedenceGroup::ForPostfixExpression());
    PushState(ParserState::ExpressionInPostfix());
  }
}

auto Parser::HandleExpressionInPostfixState() -> void {
  auto state = PopState();

  // Continue to the loop state.
  state.state = ParserState::ExpressionInPostfixLoop();

  // Parses a primary expression, which is either a terminal portion of an
  // expression tree, such as an identifier or literal, or a parenthesized
  // expression.
  switch (PositionKind()) {
    case TokenKind::Identifier(): {
      AddLeafNode(ParseNodeKind::NameReference(), Consume());
      PushState(state);
      break;
    }
    case TokenKind::IntegerLiteral():
    case TokenKind::RealLiteral():
    case TokenKind::StringLiteral():
    case TokenKind::IntegerTypeLiteral():
    case TokenKind::UnsignedIntegerTypeLiteral():
    case TokenKind::FloatingPointTypeLiteral(): {
      AddLeafNode(ParseNodeKind::Literal(), Consume());
      PushState(state);
      break;
    }
    case TokenKind::OpenCurlyBrace(): {
      PushState(state);
      PushState(ParserState::BraceExpression());
      break;
    }
    case TokenKind::OpenParen(): {
      PushState(state);
      PushState(ParserState::ParenExpression());
      break;
    }
    default: {
      CARBON_DIAGNOSTIC(ExpectedExpression, Error, "Expected expression.");
      emitter_->Emit(*position_, ExpectedExpression);
      ReturnErrorOnState();
      break;
    }
  }
}

auto Parser::HandleExpressionInPostfixLoopState() -> void {
  // This is a cyclic state that repeats, so this state is typically pushed back
  // on.
  auto state = PopState();

  state.token = *position_;

  switch (PositionKind()) {
    case TokenKind::Period(): {
      PushState(state);
      state.state = ParserState::DesignatorAsExpression();
      PushState(state);
      break;
    }
    case TokenKind::OpenParen(): {
      PushState(state);
      state.state = ParserState::CallExpression();
      PushState(state);
      break;
    }
    default: {
      if (state.has_error) {
        ReturnErrorOnState();
      }
      break;
    }
  }
}

auto Parser::HandleExpressionLoopState() -> void {
  auto state = PopState();

  auto trailing_operator =
      PrecedenceGroup::ForTrailing(PositionKind(), IsTrailingOperatorInfix());
  if (!trailing_operator) {
    if (state.has_error) {
      ReturnErrorOnState();
    }
    return;
  }
  auto [operator_precedence, is_binary] = *trailing_operator;

  // TODO: If this operator is ambiguous with either the ambient precedence
  // or the LHS precedence, and there's a variant with a different fixity
  // that would work, use that one instead for error recovery.
  if (PrecedenceGroup::GetPriority(state.ambient_precedence,
                                   operator_precedence) !=
      OperatorPriority::RightFirst) {
    // The precedence rules don't permit this operator in this context. Try
    // again in the enclosing expression context.
    if (state.has_error) {
      ReturnErrorOnState();
    }
    return;
  }

  if (PrecedenceGroup::GetPriority(state.lhs_precedence, operator_precedence) !=
      OperatorPriority::LeftFirst) {
    // Either the LHS operator and this operator are ambiguous, or the
    // LHS operator is a unary operator that can't be nested within
    // this operator. Either way, parentheses are required.
    emitter_->Emit(*position_, OperatorRequiresParentheses);
    state.has_error = true;
  } else {
    DiagnoseOperatorFixity(is_binary ? OperatorFixity::Infix
                                     : OperatorFixity::Postfix);
  }

  state.token = Consume();
  state.lhs_precedence = operator_precedence;

  if (is_binary) {
    state.state = ParserState::ExpressionLoopForBinary();
    PushState(state);
    PushStateForExpression(operator_precedence);
  } else {
    AddNode(ParseNodeKind::PostfixOperator(), state.token, state.subtree_start,
            state.has_error);
    state.has_error = false;
    PushState(state);
  }
}

auto Parser::HandleExpressionLoopForBinaryState() -> void {
  auto state = PopState();

  AddNode(ParseNodeKind::InfixOperator(), state.token, state.subtree_start,
          state.has_error);
  state.state = ParserState::ExpressionLoop();
  state.has_error = false;
  PushState(state);
}

auto Parser::HandleExpressionLoopForPrefixState() -> void {
  auto state = PopState();

  AddNode(ParseNodeKind::PrefixOperator(), state.token, state.subtree_start,
          state.has_error);
  state.state = ParserState::ExpressionLoop();
  state.has_error = false;
  PushState(state);
}

auto Parser::HandleExpressionStatementFinishState() -> void {
  auto state = PopState();

  if (auto semi = ConsumeIf(TokenKind::Semi())) {
    AddNode(ParseNodeKind::ExpressionStatement(), *semi, state.subtree_start,
            state.has_error);
    return;
  }

  if (!state.has_error) {
    emitter_->Emit(*position_, ExpectedSemiAfterExpression);
  }

  if (auto semi_token = SkipPastLikelyEnd(state.token)) {
    AddNode(ParseNodeKind::ExpressionStatement(), *semi_token,
            state.subtree_start,
            /*has_error=*/true);
    return;
  }

  // Found junk not even followed by a `;`, no node to add.
  ReturnErrorOnState();
}

auto Parser::HandleFunctionError(StateStackEntry state,
                                 bool skip_past_likely_end) -> void {
  auto token = state.token;
  if (skip_past_likely_end) {
    if (auto semi = SkipPastLikelyEnd(token)) {
      token = *semi;
    }
  }
  AddNode(ParseNodeKind::FunctionDeclaration(), token, state.subtree_start,
          /*has_error=*/true);
}

auto Parser::HandleFunctionIntroducerState() -> void {
  auto state = PopState();

  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier(),
                               ParseNodeKind::DeclaredName())) {
    CARBON_DIAGNOSTIC(ExpectedFunctionName, Error,
                      "Expected function name after `fn` keyword.");
    emitter_->Emit(*position_, ExpectedFunctionName);
    // TODO: We could change the lexer to allow us to synthesize certain
    // kinds of tokens and try to "recover" here, but unclear that this is
    // really useful.
    HandleFunctionError(state, true);
    return;
  }

  if (!PositionIs(TokenKind::OpenParen())) {
    CARBON_DIAGNOSTIC(ExpectedFunctionParams, Error,
                      "Expected `(` after function name.");
    emitter_->Emit(*position_, ExpectedFunctionParams);
    HandleFunctionError(state, true);
    return;
  }

  // Parse the parameter list as its own subtree; once that pops, resume
  // function parsing.
  state.state = ParserState::FunctionAfterParameterList();
  PushState(state);
  PushState(ParserState::FunctionParameterListFinish());
  AddLeafNode(ParseNodeKind::ParameterListStart(), Consume());
  if (!PositionIs(TokenKind::CloseParen())) {
    PushState(ParserState::FunctionParameter());
  }
}

auto Parser::HandleFunctionParameterState() -> void {
  PopAndDiscardState();

  PushState(ParserState::FunctionParameterFinish());
  PushState(ParserState::PatternAsFunctionParameter());
}

auto Parser::HandleFunctionParameterFinishState() -> void {
  auto state = PopState();

  if (state.has_error) {
    ReturnErrorOnState();
  }

  if (ConsumeListToken(ParseNodeKind::ParameterListComma(),
                       TokenKind::CloseParen(),
                       state.has_error) == ListTokenKind::Comma) {
    PushState(ParserState::PatternAsFunctionParameter());
  }
}

auto Parser::HandleFunctionParameterListFinishState() -> void {
  auto state = PopState();

  CARBON_CHECK(PositionIs(TokenKind::CloseParen())) << PositionKind().Name();
  AddNode(ParseNodeKind::ParameterList(), Consume(), state.subtree_start,
          state.has_error);
}

auto Parser::HandleFunctionAfterParameterListState() -> void {
  auto state = PopState();

  // Regardless of whether there's a return type, we'll finish the signature.
  state.state = ParserState::FunctionSignatureFinish();
  PushState(state);

  // If there is a return type, parse the expression before adding the return
  // type nod.e
  if (PositionIs(TokenKind::MinusGreater())) {
    PushState(ParserState::FunctionReturnTypeFinish());
    ++position_;
    PushStateForExpression(PrecedenceGroup::ForType());
  }
}

auto Parser::HandleFunctionReturnTypeFinishState() -> void {
  auto state = PopState();

  AddNode(ParseNodeKind::ReturnType(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandleFunctionSignatureFinishState() -> void {
  auto state = PopState();

  switch (PositionKind()) {
    case TokenKind::Semi(): {
      AddNode(ParseNodeKind::FunctionDeclaration(), Consume(),
              state.subtree_start, state.has_error);
      break;
    }
    case TokenKind::OpenCurlyBrace(): {
      AddNode(ParseNodeKind::FunctionDefinitionStart(), Consume(),
              state.subtree_start, state.has_error);
      // Any error is recorded on the FunctionDefinitionStart.
      state.has_error = false;
      state.state = ParserState::FunctionDefinitionFinish();
      PushState(state);
      PushState(ParserState::StatementScopeLoop());
      break;
    }
    default: {
      CARBON_DIAGNOSTIC(
          ExpectedFunctionBodyOrSemi, Error,
          "Expected function definition or `;` after function declaration.");
      emitter_->Emit(*position_, ExpectedFunctionBodyOrSemi);
      // Only need to skip if we've not already found a new line.
      bool skip_past_likely_end =
          tokens_->GetLine(*position_) == tokens_->GetLine(state.token);
      HandleFunctionError(state, skip_past_likely_end);
      break;
    }
  }
}

auto Parser::HandleFunctionDefinitionFinishState() -> void {
  auto state = PopState();
  AddNode(ParseNodeKind::FunctionDefinition(), Consume(), state.subtree_start,
          state.has_error);
}

auto Parser::HandlePackageState() -> void {
  auto state = PopState();

  auto exit_on_parse_error = [&]() {
    if (auto semi_token = SkipPastLikelyEnd(state.token)) {
      AddLeafNode(ParseNodeKind::PackageEnd(), *semi_token);
    }
    return AddNode(ParseNodeKind::PackageDirective(), state.token,
                   state.subtree_start, /*has_error=*/true);
  };

  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier(),
                               ParseNodeKind::DeclaredName())) {
    CARBON_DIAGNOSTIC(ExpectedIdentifierAfterPackage, Error,
                      "Expected identifier after `package`.");
    emitter_->Emit(*position_, ExpectedIdentifierAfterPackage);
    exit_on_parse_error();
    return;
  }

  bool library_parsed = false;
  if (auto library_token = ConsumeIf(TokenKind::Library())) {
    auto library_start = tree_->size();

    if (!ConsumeAndAddLeafNodeIf(TokenKind::StringLiteral(),
                                 ParseNodeKind::Literal())) {
      CARBON_DIAGNOSTIC(
          ExpectedLibraryName, Error,
          "Expected a string literal to specify the library name.");
      emitter_->Emit(*position_, ExpectedLibraryName);
      exit_on_parse_error();
      return;
    }

    AddNode(ParseNodeKind::PackageLibrary(), *library_token, library_start,
            /*has_error=*/false);
    library_parsed = true;
  }

  switch (auto api_or_impl_token = tokens_->GetKind(*(position_))) {
    case TokenKind::Api(): {
      AddLeafNode(ParseNodeKind::PackageApi(), Consume());
      break;
    }
    case TokenKind::Impl(): {
      AddLeafNode(ParseNodeKind::PackageImpl(), Consume());
      break;
    }
    default: {
      if (!library_parsed && api_or_impl_token == TokenKind::StringLiteral()) {
        // If we come acroess a string literal and we didn't parse `library
        // "..."` yet, then most probably the user forgot to add `library`
        // before the library name.
        CARBON_DIAGNOSTIC(MissingLibraryKeyword, Error,
                          "Missing `library` keyword.");
        emitter_->Emit(*position_, MissingLibraryKeyword);
      } else {
        CARBON_DIAGNOSTIC(ExpectedApiOrImpl, Error,
                          "Expected a `api` or `impl`.");
        emitter_->Emit(*position_, ExpectedApiOrImpl);
      }
      exit_on_parse_error();
      return;
    }
  }

  if (!ConsumeAndAddLeafNodeIf(TokenKind::Semi(),
                               ParseNodeKind::PackageEnd())) {
    CARBON_DIAGNOSTIC(ExpectedSemiToEndPackageDirective, Error,
                      "Expected `;` to end package directive.");
    emitter_->Emit(*position_, ExpectedSemiToEndPackageDirective);
    exit_on_parse_error();
    return;
  }

  AddNode(ParseNodeKind::PackageDirective(), state.token, state.subtree_start,
          /*has_error=*/false);
}

auto Parser::HandleParenConditionState() -> void {
  auto state = PopState();

  auto open_paren = ConsumeIf(TokenKind::OpenParen());
  if (open_paren) {
    state.token = *open_paren;
  } else {
    CARBON_DIAGNOSTIC(ExpectedParenAfter, Error, "Expected `(` after `{0}`.",
                      TokenKind);
    emitter_->Emit(*position_, ExpectedParenAfter,
                   tokens_->GetKind(state.token));
  }

  // TODO: This should be adding a ConditionStart here instead of ConditionEnd
  // later, so this does state modification instead of a simpler push.
  state.state = ParserState::ParenConditionFinish();
  PushState(state);
  PushState(ParserState::Expression());
}

auto Parser::HandleParenConditionFinishState() -> void {
  auto state = PopState();

  if (tokens_->GetKind(state.token) != TokenKind::OpenParen()) {
    // Don't expect a matching closing paren if there wasn't an opening paren.
    // TODO: Should probably push nodes on this state in order to have the
    // condition wrapped, but it wasn't before, so not doing it for consistency.
    ReturnErrorOnState();
    return;
  }

  bool close_paren =
      ConsumeAndAddCloseParen(state.token, ParseNodeKind::ConditionEnd());

  return AddNode(ParseNodeKind::Condition(), state.token, state.subtree_start,
                 /*has_error=*/state.has_error || !close_paren);
}

auto Parser::HandleParenExpressionState() -> void {
  auto state = PopState();

  // TODO: When swapping () start/end, this should AddLeafNode the open before
  // continuing.

  // Advance past the open paren.
  CARBON_CHECK(PositionIs(TokenKind::OpenParen()));
  ++position_;
  if (PositionIs(TokenKind::CloseParen())) {
    state.state = ParserState::ParenExpressionFinishAsTuple();
    PushState(state);
  } else {
    state.state = ParserState::ParenExpressionFinish();
    PushState(state);
    PushState(ParserState::ParenExpressionParameterFinishAsUnknown());
    PushState(ParserState::Expression());
  }
}

auto Parser::HandleParenExpressionParameterFinish(bool as_tuple) -> void {
  auto state = PopState();

  auto list_token_kind =
      ConsumeListToken(ParseNodeKind::TupleLiteralComma(),
                       TokenKind::CloseParen(), state.has_error);
  if (list_token_kind == ListTokenKind::Close) {
    return;
  }

  // If this is the first item and a comma was found, switch to tuple handling.
  // Note this could be `(expr,)` so we may not reuse the current state, but
  // it's still necessary to switch the parent.
  if (!as_tuple) {
    state.state = ParserState::ParenExpressionParameterFinishAsTuple();

    auto finish_state = PopState();
    CARBON_CHECK(finish_state.state == ParserState::ParenExpressionFinish())
        << "Unexpected parent state, found: " << finish_state.state;
    finish_state.state = ParserState::ParenExpressionFinishAsTuple();
    PushState(finish_state);
  }

  // On a comma, push another expression handler.
  if (list_token_kind == ListTokenKind::Comma) {
    PushState(state);
    PushState(ParserState::Expression());
  }
}

auto Parser::HandleParenExpressionParameterFinishAsUnknownState() -> void {
  HandleParenExpressionParameterFinish(/*as_tuple=*/false);
}

auto Parser::HandleParenExpressionParameterFinishAsTupleState() -> void {
  HandleParenExpressionParameterFinish(/*as_tuple=*/true);
}

auto Parser::HandleParenExpressionFinishState() -> void {
  auto state = PopState();

  AddLeafNode(ParseNodeKind::ParenExpressionEnd(), Consume());
  AddNode(ParseNodeKind::ParenExpression(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandleParenExpressionFinishAsTupleState() -> void {
  auto state = PopState();

  AddLeafNode(ParseNodeKind::TupleLiteralEnd(), Consume());
  AddNode(ParseNodeKind::TupleLiteral(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandlePattern(PatternKind pattern_kind) -> void {
  auto state = PopState();

  // Ensure the finish state always follows.
  state.state = ParserState::PatternFinish();

  // Handle an invalid pattern introducer.
  if (!PositionIs(TokenKind::Identifier()) ||
      tokens_->GetKind(*(position_ + 1)) != TokenKind::Colon()) {
    switch (pattern_kind) {
      case PatternKind::Parameter: {
        CARBON_DIAGNOSTIC(ExpectedParameterName, Error,
                          "Expected parameter declaration.");
        emitter_->Emit(*position_, ExpectedParameterName);
        break;
      }
      case PatternKind::Variable: {
        CARBON_DIAGNOSTIC(ExpectedVariableName, Error,
                          "Expected pattern in `var` declaration.");
        emitter_->Emit(*position_, ExpectedVariableName);
        break;
      }
    }
    state.has_error = true;
    PushState(state);
    return;
  }

  // Switch the context token to the colon, so that it'll be used for the root
  // node.
  state.token = *(position_ + 1);
  PushState(state);
  PushStateForExpression(PrecedenceGroup::ForType());
  AddLeafNode(ParseNodeKind::DeclaredName(), *position_);
  position_ += 2;
}

auto Parser::HandlePatternAsFunctionParameterState() -> void {
  HandlePattern(PatternKind::Parameter);
}

auto Parser::HandlePatternAsVariableState() -> void {
  HandlePattern(PatternKind::Variable);
}

auto Parser::HandlePatternFinishState() -> void {
  auto state = PopState();

  // If an error was encountered, propagate it without adding a node.
  if (state.has_error) {
    ReturnErrorOnState();
    return;
  }

  // TODO: may need to mark has_error if !type.
  AddNode(ParseNodeKind::PatternBinding(), state.token, state.subtree_start,
          /*has_error=*/false);
}

auto Parser::HandleStatementState() -> void {
  PopAndDiscardState();

  switch (PositionKind()) {
    case TokenKind::Break(): {
      PushState(ParserState::StatementBreakFinish());
      AddLeafNode(ParseNodeKind::BreakStatementStart(), Consume());
      break;
    }
    case TokenKind::Continue(): {
      PushState(ParserState::StatementContinueFinish());
      AddLeafNode(ParseNodeKind::ContinueStatementStart(), Consume());
      break;
    }
    case TokenKind::For(): {
      // Process the header as a child of the for so that we can get consistent
      // starts.
      // TODO: When reorganizing components, we can probably make this flatter.
      PushState(ParserState::StatementForFinish());
      ++position_;
      PushState(ParserState::StatementForHeader());
      break;
    }
    case TokenKind::If(): {
      PushState(ParserState::StatementIf());
      break;
    }
    case TokenKind::Return(): {
      PushState(ParserState::StatementReturn());
      break;
    }
    case TokenKind::Var(): {
      PushState(ParserState::VarAsRequireSemicolon());
      break;
    }
    case TokenKind::While(): {
      PushState(ParserState::StatementWhile());
      break;
    }
    default: {
      PushState(ParserState::ExpressionStatementFinish());
      PushState(ParserState::Expression());
      break;
    }
  }
}

auto Parser::HandleStatementBreakFinishState() -> void {
  HandleStatementKeywordFinish(ParseNodeKind::BreakStatement());
}

auto Parser::HandleStatementContinueFinishState() -> void {
  HandleStatementKeywordFinish(ParseNodeKind::ContinueStatement());
}

auto Parser::HandleStatementForHeaderState() -> void {
  auto state = PopState();

  auto open_paren = ConsumeIf(TokenKind::OpenParen());
  if (!open_paren) {
    CARBON_DIAGNOSTIC(ExpectedParenAfter, Error,
                      "Expected `(` after `{0}`. Recovering from missing `(` "
                      "not implemented yet!",
                      TokenKind);
    emitter_->Emit(*position_, ExpectedParenAfter, TokenKind::For());
    // TODO: A proper recovery strategy is needed here. For now, I assume
    // that all brackets are properly balanced (i.e. each open bracket has a
    // closing one).
    // This is temporary until we come to a conclusion regarding the
    // recovery tokens strategy.
    ReturnErrorOnState();
    PushState(ParserState::CodeBlock());
    return;
  }

  state.state = ParserState::StatementForHeaderIn();

  if (PositionIs(TokenKind::Var())) {
    PushState(state);
    PushState(ParserState::VarAsNoSemicolon());
  } else {
    CARBON_DIAGNOSTIC(ExpectedVariableDeclaration, Error,
                      "Expected `var` declaration.");
    emitter_->Emit(*position_, ExpectedVariableDeclaration);

    if (auto next_in = FindNextOf({TokenKind::In()})) {
      SkipTo(*next_in);
    }
    state.has_error = true;
    PushState(state);
  }
}

auto Parser::HandleStatementForHeaderInState() -> void {
  auto state = PopState();

  state.state = ParserState::StatementForHeaderFinish();

  if (!ConsumeAndAddLeafNodeIf(TokenKind::In(), ParseNodeKind::ForIn())) {
    if (auto colon = ConsumeIf(TokenKind::Colon())) {
      CARBON_DIAGNOSTIC(ExpectedIn, Error, "`:` should be replaced by `in`.");
      emitter_->Emit(*colon, ExpectedIn);
      AddLeafNode(ParseNodeKind::ForIn(), *colon, /*has_error=*/true);
    } else {
      CARBON_DIAGNOSTIC(ExpectedIn, Error,
                        "Expected `in` after loop `var` declaration.");
      emitter_->Emit(*position_, ExpectedIn);
      SkipTo(tokens_->GetMatchedClosingToken(state.token));

      state.has_error = true;
      PushState(state);
      return;
    }
  }

  PushState(state);
  PushState(ParserState::Expression());
}

auto Parser::HandleStatementForHeaderFinishState() -> void {
  auto state = PopState();

  if (!ConsumeAndAddCloseParen(state.token, ParseNodeKind::ForHeaderEnd())) {
    state.has_error = true;
  }

  AddNode(ParseNodeKind::ForHeader(), state.token, state.subtree_start,
          state.has_error);

  PushState(ParserState::CodeBlock());
}

auto Parser::HandleStatementForFinishState() -> void {
  auto state = PopState();

  AddNode(ParseNodeKind::ForStatement(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandleStatementIfState() -> void {
  PopAndDiscardState();

  PushState(ParserState::StatementIfConditionFinish());
  PushState(ParserState::ParenCondition());
  ++position_;
}

auto Parser::HandleStatementIfConditionFinishState() -> void {
  auto state = PopState();

  state.state = ParserState::StatementIfThenBlockFinish();
  PushState(state);
  PushState(ParserState::CodeBlock());
}

auto Parser::HandleStatementIfThenBlockFinishState() -> void {
  auto state = PopState();

  if (ConsumeAndAddLeafNodeIf(TokenKind::Else(),
                              ParseNodeKind::IfStatementElse())) {
    state.state = ParserState::StatementIfElseBlockFinish();
    PushState(state);
    // `else if` is permitted as a special case.
    PushState(PositionIs(TokenKind::If()) ? ParserState::StatementIf()
                                          : ParserState::CodeBlock());
  } else {
    AddNode(ParseNodeKind::IfStatement(), state.token, state.subtree_start,
            state.has_error);
  }
}

auto Parser::HandleStatementIfElseBlockFinishState() -> void {
  auto state = PopState();
  AddNode(ParseNodeKind::IfStatement(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandleStatementKeywordFinish(ParseNodeKind node_kind) -> void {
  auto state = PopState();

  auto semi = ConsumeIf(TokenKind::Semi());
  if (!semi) {
    CARBON_DIAGNOSTIC(ExpectedSemiAfter, Error, "Expected `;` after `{0}`.",
                      TokenKind);
    emitter_->Emit(*position_, ExpectedSemiAfter,
                   tokens_->GetKind(state.token));
    state.has_error = true;
    // Recover to the next semicolon if possible, otherwise indicate the
    // keyword for the error.
    semi = SkipPastLikelyEnd(state.token);
    if (!semi) {
      semi = state.token;
    }
  }
  AddNode(node_kind, *semi, state.subtree_start, state.has_error);
}

auto Parser::HandleStatementReturnState() -> void {
  auto state = PopState();
  state.state = ParserState::StatementReturnFinish();
  PushState(state);

  AddLeafNode(ParseNodeKind::ReturnStatementStart(), Consume());
  if (!PositionIs(TokenKind::Semi())) {
    PushState(ParserState::Expression());
  }
}

auto Parser::HandleStatementReturnFinishState() -> void {
  HandleStatementKeywordFinish(ParseNodeKind::ReturnStatement());
}

auto Parser::HandleStatementScopeLoopState() -> void {
  // This maintains the current state until we're at the end of the scope.

  auto token_kind = PositionKind();
  if (token_kind == TokenKind::CloseCurlyBrace()) {
    auto state = PopState();
    if (state.has_error) {
      ReturnErrorOnState();
    }
  } else {
    PushState(ParserState::Statement());
  }
}

auto Parser::HandleStatementWhileState() -> void {
  PopAndDiscardState();

  PushState(ParserState::StatementWhileConditionFinish());
  PushState(ParserState::ParenCondition());
  ++position_;
}

auto Parser::HandleStatementWhileConditionFinishState() -> void {
  auto state = PopState();

  state.state = ParserState::StatementWhileBlockFinish();
  PushState(state);
  PushState(ParserState::CodeBlock());
}

auto Parser::HandleStatementWhileBlockFinishState() -> void {
  auto state = PopState();

  AddNode(ParseNodeKind::WhileStatement(), state.token, state.subtree_start,
          state.has_error);
}

auto Parser::HandleVar(bool require_semicolon) -> void {
  PopAndDiscardState();

  PushState(require_semicolon ? ParserState::VarFinishAsRequireSemicolon()
                              : ParserState::VarFinishAsNoSemicolon());
  PushState(ParserState::VarAfterPattern());
  ++position_;
  PushState(ParserState::PatternAsVariable());
}

auto Parser::HandleVarAsRequireSemicolonState() -> void {
  HandleVar(/*require_semicolon=*/true);
}

auto Parser::HandleVarAsNoSemicolonState() -> void {
  HandleVar(/*require_semicolon=*/false);
}

auto Parser::HandleVarAfterPatternState() -> void {
  auto state = PopState();

  if (state.has_error) {
    if (auto after_pattern =
            FindNextOf({TokenKind::Equal(), TokenKind::Semi()})) {
      SkipTo(*after_pattern);
    }
  }

  if (PositionIs(TokenKind::Equal())) {
    PushState(ParserState::VarAfterInitializer());
    ++position_;
    PushState(ParserState::Expression());
    return;
  }
}

auto Parser::HandleVarAfterInitializerState() -> void {
  auto state = PopState();

  AddNode(ParseNodeKind::VariableInitializer(), state.token,
          state.subtree_start, state.has_error);
}

auto Parser::HandleVarFinish(bool require_semicolon) -> void {
  auto state = PopState();

  if (require_semicolon) {
    auto semi = ConsumeAndAddLeafNodeIf(TokenKind::Semi(),
                                        ParseNodeKind::DeclarationEnd());
    if (!semi) {
      emitter_->Emit(*position_, ExpectedSemiAfterExpression);
      if (auto semi_token = SkipPastLikelyEnd(state.token)) {
        AddLeafNode(ParseNodeKind::DeclarationEnd(), *semi_token,
                    /*has_error=*/true);
      } else {
        state.has_error = true;
      }
    }
  }

  return AddNode(ParseNodeKind::VariableDeclaration(), state.token,
                 state.subtree_start, state.has_error);
}

auto Parser::HandleVarFinishAsRequireSemicolonState() -> void {
  HandleVarFinish(/*require_semicolon=*/true);
}

auto Parser::HandleVarFinishAsNoSemicolonState() -> void {
  HandleVarFinish(/*require_semicolon=*/false);
}

auto Parser::HandleInterfaceIntroducerState() -> void {
  auto state = PopState();

  if (!ConsumeAndAddLeafNodeIf(TokenKind::Identifier(),
                               ParseNodeKind::DeclaredName())) {
    CARBON_DIAGNOSTIC(ExpectedInterfaceName, Error,
                      "Expected interface name after `interface` keyword.");
    emitter_->Emit(*position_, ExpectedInterfaceName);
    state.has_error = true;
  }

  bool parse_body = true;

  if (!PositionIs(TokenKind::OpenCurlyBrace())) {
    CARBON_DIAGNOSTIC(ExpectedInterfaceOpenCurlyBrace, Error,
                      "Expected `{{` to start interface definition.");
    emitter_->Emit(*position_, ExpectedInterfaceOpenCurlyBrace);
    state.has_error = true;

    SkipPastLikelyEnd(state.token);
    parse_body = false;
  }

  state.state = ParserState::InterfaceDefinitionFinish();
  PushState(state);

  if (parse_body) {
    PushState(ParserState::InterfaceDefinitionLoop());
    AddLeafNode(ParseNodeKind::InterfaceBodyStart(), Consume());
  }
}

auto Parser::HandleInterfaceDefinitionLoopState() -> void {
  // This maintains the current state unless we're at the end of the interface
  // definition.

  switch (PositionKind()) {
    case TokenKind::CloseCurlyBrace(): {
      auto state = PopState();

      AddNode(ParseNodeKind::InterfaceBody(), Consume(), state.subtree_start,
              state.has_error);

      break;
    }
      // TODO: Handle possible declarations inside interface body.
    default: {
      CARBON_DIAGNOSTIC(UnrecognizedDeclaration, Error,
                        "Unrecognized declaration introducer.");
      emitter_->Emit(*position_, UnrecognizedDeclaration);
      if (auto semi = SkipPastLikelyEnd(*position_)) {
        AddLeafNode(ParseNodeKind::EmptyDeclaration(), *semi,
                    /*has_error=*/true);
      } else {
        ReturnErrorOnState();
      }
      break;
    }
  }
}

auto Parser::HandleInterfaceDefinitionFinishState() -> void {
  auto state = PopState();
  AddNode(ParseNodeKind::InterfaceDefinition(), state.token,
          state.subtree_start, state.has_error);
}

}  // namespace Carbon