carbon-lang/toolchain/semantics/semantics_parse_tree_handler.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/semantics/semantics_parse_tree_handler.h"

#include <functional>
#include <utility>

#include "common/vlog.h"
#include "llvm/Support/PrettyStackTrace.h"
#include "toolchain/diagnostics/diagnostic_kind.h"
#include "toolchain/lexer/token_kind.h"
#include "toolchain/lexer/tokenized_buffer.h"
#include "toolchain/parser/parse_node_kind.h"
#include "toolchain/semantics/semantics_ir.h"
#include "toolchain/semantics/semantics_node.h"
#include "toolchain/semantics/semantics_node_block_stack.h"

namespace Carbon {

CARBON_DIAGNOSTIC(SemanticsTodo, Error, "Semantics TODO: {0}", std::string);

class PrettyStackTraceFunction : public llvm::PrettyStackTraceEntry {
 public:
  explicit PrettyStackTraceFunction(std::function<void(llvm::raw_ostream&)> fn)
      : fn_(std::move(fn)) {}
  ~PrettyStackTraceFunction() override = default;

  auto print(llvm::raw_ostream& output) const -> void override { fn_(output); }

 private:
  const std::function<void(llvm::raw_ostream&)> fn_;
};

auto SemanticsParseTreeHandler::Build() -> void {
  PrettyStackTraceFunction pretty_node_stack([&](llvm::raw_ostream& output) {
    node_stack_.PrintForStackDump(output);
  });
  PrettyStackTraceFunction pretty_node_block_stack(
      [&](llvm::raw_ostream& output) {
        node_block_stack_.PrintForStackDump(output);
      });

  // Add a block for the ParseTree.
  node_block_stack_.Push();
  PushScope();

  // Loops over all nodes in the tree. On some errors, this may return early,
  // for example if an unrecoverable state is encountered.
  for (auto parse_node : parse_tree_->postorder()) {
    switch (auto parse_kind = parse_tree_->node_kind(parse_node)) {
#define CARBON_PARSE_NODE_KIND(Name) \
  case ParseNodeKind::Name: {        \
    if (!Handle##Name(parse_node)) { \
      return;                        \
    }                                \
    break;                           \
  }
#include "toolchain/parser/parse_node_kind.def"
    }
  }

  // Pop information for the file-level scope.
  node_block_stack_.Pop();
  PopScope();

  // Information in all the various context objects should be cleaned up as
  // various pieces of context go out of scope. At this point, nothing should
  // remain.
  // node_stack_ will still contain top-level entities.
  CARBON_CHECK(name_lookup_.empty()) << name_lookup_.size();
  CARBON_CHECK(scope_stack_.empty()) << scope_stack_.size();
  CARBON_CHECK(node_block_stack_.empty()) << node_block_stack_.size();
  CARBON_CHECK(params_or_args_stack_.empty()) << params_or_args_stack_.size();
}

auto SemanticsParseTreeHandler::AddNode(SemanticsNode node) -> SemanticsNodeId {
  auto block = node_block_stack_.PeekForAdd();
  CARBON_VLOG() << "AddNode " << block << ": " << node << "\n";
  return semantics_->AddNode(block, node);
}

auto SemanticsParseTreeHandler::AddNodeAndPush(ParseTree::Node parse_node,
                                               SemanticsNode node) -> void {
  auto node_id = AddNode(node);
  node_stack_.Push(parse_node, node_id);
}

auto SemanticsParseTreeHandler::BindName(ParseTree::Node name_node,
                                         SemanticsNodeId type_id,
                                         SemanticsNodeId target_id)
    -> SemanticsStringId {
  CARBON_CHECK(parse_tree_->node_kind(name_node) == ParseNodeKind::DeclaredName)
      << parse_tree_->node_kind(name_node);
  auto name_str = parse_tree_->GetNodeText(name_node);
  auto name_id = semantics_->AddString(name_str);

  AddNode(SemanticsNode::MakeBindName(name_node, type_id, name_id, target_id));
  auto [it, inserted] = current_scope().names.insert(name_id);
  if (inserted) {
    name_lookup_[name_id].push_back(target_id);
  } else {
    CARBON_DIAGNOSTIC(NameRedefined, Error, "Redefining {0} in the same scope.",
                      llvm::StringRef);
    CARBON_DIAGNOSTIC(PreviousDefinition, Note, "Previous definition is here.");
    auto prev_def_id = name_lookup_[name_id].back();
    auto prev_def = semantics_->GetNode(prev_def_id);

    emitter_->Build(name_node, NameRedefined, name_str)
        .Note(prev_def.parse_node(), PreviousDefinition)
        .Emit();
  }
  return name_id;
}

auto SemanticsParseTreeHandler::PushScope() -> void {
  scope_stack_.push_back({});
}

auto SemanticsParseTreeHandler::PopScope() -> void {
  auto scope = scope_stack_.pop_back_val();
  for (const auto& str_id : scope.names) {
    auto it = name_lookup_.find(str_id);
    if (it->second.size() == 1) {
      // Erase names that no longer resolve.
      name_lookup_.erase(it);
    } else {
      it->second.pop_back();
    }
  }
}

auto SemanticsParseTreeHandler::CanTypeConvert(SemanticsNodeId from_type,
                                               SemanticsNodeId to_type)
    -> SemanticsNodeId {
  // TODO: This should attempt implicit conversions, but there's not enough
  // implemented to do that right now.
  if (from_type == SemanticsNodeId::BuiltinInvalidType ||
      to_type == SemanticsNodeId::BuiltinInvalidType) {
    return SemanticsNodeId::BuiltinInvalidType;
  }
  if (from_type == to_type) {
    return from_type;
  }
  return SemanticsNodeId::Invalid;
}

auto SemanticsParseTreeHandler::TryTypeConversion(ParseTree::Node parse_node,
                                                  SemanticsNodeId lhs_id,
                                                  SemanticsNodeId rhs_id,
                                                  bool /*can_convert_lhs*/)
    -> SemanticsNodeId {
  auto lhs_type = semantics_->GetType(lhs_id);
  auto rhs_type = semantics_->GetType(rhs_id);
  // TODO: CanTypeConvert can be assumed to handle rhs conversions, and we'll
  // either want to call it twice or refactor it to be aware of lhs conversions.
  auto type = CanTypeConvert(rhs_type, lhs_type);
  if (type.is_valid()) {
    return type;
  }
  // TODO: This should use type names instead of nodes.
  CARBON_DIAGNOSTIC(TypeMismatch, Error,
                    "Type mismatch: lhs is {0}, rhs is {1}", SemanticsNodeId,
                    SemanticsNodeId);
  emitter_->Emit(parse_node, TypeMismatch, lhs_type, rhs_type);
  return SemanticsNodeId::BuiltinInvalidType;
}

auto SemanticsParseTreeHandler::TryTypeConversionOnArgs(
    ParseTree::Node arg_parse_node, SemanticsNodeBlockId /*arg_ir_id*/,
    SemanticsNodeBlockId arg_refs_id, ParseTree::Node param_parse_node,
    SemanticsNodeBlockId param_refs_id) -> bool {
  CARBON_DIAGNOSTIC(NoMatchingCall, Error, "No matching callable was found.");

  // If both arguments and parameters are empty, return quickly. Otherwise,
  // we'll fetch both so that errors are consistent.
  if (arg_refs_id == SemanticsNodeBlockId::Empty &&
      param_refs_id == SemanticsNodeBlockId::Empty) {
    return true;
  }

  auto arg_refs = semantics_->GetNodeBlock(arg_refs_id);
  auto param_refs = semantics_->GetNodeBlock(param_refs_id);

  // If sizes mismatch, fail early.
  if (arg_refs.size() != param_refs.size()) {
    CARBON_DIAGNOSTIC(CallArgCountMismatch, Note,
                      "Received {0} argument(s), but require {1} argument(s).",
                      int, int);
    emitter_->Build(arg_parse_node, NoMatchingCall)
        .Note(param_parse_node, CallArgCountMismatch, arg_refs.size(),
              param_refs.size())
        .Emit();
    return false;
  }

  // Check type conversions per-element.
  // TODO: arg_ir_id is passed so that implicit conversions can be inserted.
  // It's currently not supported, but will be needed.
  for (size_t i = 0; i < arg_refs.size(); ++i) {
    const auto& arg_ref = arg_refs[i];
    auto arg_ref_type = semantics_->GetType(arg_ref);
    const auto& param_ref = param_refs[i];
    auto param_ref_type = semantics_->GetType(param_ref);

    auto result_type = CanTypeConvert(arg_ref_type, param_ref_type);
    if (!result_type.is_valid()) {
      // TODO: This should use type names instead of nodes.
      CARBON_DIAGNOSTIC(
          CallArgTypeMismatch, Note,
          "Type mismatch: cannot convert argument {0} from {1} to {2}.", size_t,
          SemanticsNodeId, SemanticsNodeId);
      emitter_->Build(arg_parse_node, NoMatchingCall)
          .Note(param_parse_node, CallArgTypeMismatch, i, arg_ref_type,
                param_ref_type)
          .Emit();
      return false;
    }
  }

  return true;
}

auto SemanticsParseTreeHandler::ParamOrArgStart() -> void {
  params_or_args_stack_.Push();
  node_block_stack_.Push();
}

auto SemanticsParseTreeHandler::ParamOrArgComma(ParseTree::Node parse_node)
    -> bool {
  node_stack_.Push(parse_node);

  // Copy the last node added to the IR block into the params block.
  if (!ParamOrArgSave()) {
    emitter_->Emit(
        parse_node, SemanticsTodo,
        "Should have a param before comma, will need error recovery");
    return false;
  }

  return true;
}

auto SemanticsParseTreeHandler::ParamOrArgEnd(
    ParseNodeKind start_kind, ParseNodeKind comma_kind,
    std::function<bool(SemanticsNodeBlockId, SemanticsNodeBlockId)> on_start)
    -> bool {
  // If there's a node in the IR block that has yet to be added to the params
  // block, add it now.
  ParamOrArgSave();

  while (true) {
    auto parse_kind = parse_tree_->node_kind(node_stack_.PeekParseNode());
    if (parse_kind == start_kind) {
      return on_start(node_block_stack_.Pop(), params_or_args_stack_.Pop());
    } else if (parse_kind == comma_kind) {
      node_stack_.PopAndDiscardSoloParseNode(comma_kind);
    } else {
      node_stack_.PopAndIgnore();
    }
  }
}

auto SemanticsParseTreeHandler::ParamOrArgSave() -> bool {
  // Copy the last node added to the IR block into the params block.
  auto ir_id = node_block_stack_.Peek();
  if (!ir_id.is_valid()) {
    return false;
  }

  // We get params before ir because it may add a node block, which can
  // invalidate the ir reference.
  auto& params = semantics_->GetNodeBlock(params_or_args_stack_.PeekForAdd());

  auto& ir = semantics_->GetNodeBlock(ir_id);
  CARBON_CHECK(!ir.empty())
      << "Should only have a valid ID if a node was added";
  auto& param = ir.back();

  if (!params.empty() && param == params.back()) {
    // The param was already added after a comma.
    return false;
  }
  params.push_back(ir.back());
  return true;
}

auto SemanticsParseTreeHandler::HandleAddress(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleAddress");
  return false;
}

auto SemanticsParseTreeHandler::HandleBreakStatement(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleBreakStatement");
  return false;
}

auto SemanticsParseTreeHandler::HandleBreakStatementStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleBreakStatementStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleCallExpression(ParseTree::Node parse_node)
    -> bool {
  auto on_start = [&](SemanticsNodeBlockId ir_id,
                      SemanticsNodeBlockId refs_id) -> bool {
    // TODO: Convert to call expression.
    auto [call_expr_parse_node, name_id] = node_stack_.PopForParseNodeAndNodeId(
        ParseNodeKind::CallExpressionStart);
    auto name_node = semantics_->GetNode(name_id);
    if (name_node.kind() != SemanticsNodeKind::FunctionDeclaration) {
      // TODO: Work on error.
      emitter_->Emit(parse_node, SemanticsTodo, "Not a callable name");
      node_stack_.Push(parse_node, name_id);
      return true;
    }

    auto callable_id = name_node.GetAsFunctionDeclaration();
    auto callable = semantics_->GetCallable(callable_id);

    if (!TryTypeConversionOnArgs(call_expr_parse_node, ir_id, refs_id,
                                 name_node.parse_node(),
                                 callable.param_refs_id)) {
      node_stack_.Push(parse_node, SemanticsNodeId::BuiltinInvalidType);
      return true;
    }

    auto call_id = semantics_->AddCall({ir_id, refs_id});
    // TODO: Propagate return types from callable.
    auto call_node_id = AddNode(SemanticsNode::MakeCall(
        call_expr_parse_node, SemanticsNodeId::BuiltinEmptyTuple, call_id,
        callable_id));

    node_stack_.Push(parse_node, call_node_id);
    return true;
  };
  return ParamOrArgEnd(ParseNodeKind::CallExpressionStart,
                       ParseNodeKind::CallExpressionComma, on_start);
}

auto SemanticsParseTreeHandler::HandleCallExpressionComma(
    ParseTree::Node parse_node) -> bool {
  return ParamOrArgComma(parse_node);
}

auto SemanticsParseTreeHandler::HandleCallExpressionStart(
    ParseTree::Node parse_node) -> bool {
  auto name_id = node_stack_.PopForNodeId(ParseNodeKind::NameReference);
  node_stack_.Push(parse_node, name_id);
  ParamOrArgStart();
  return true;
}

auto SemanticsParseTreeHandler::HandleCodeBlock(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleCodeBlock");
  return false;
}

auto SemanticsParseTreeHandler::HandleCodeBlockStart(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleCodeBlockStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleContinueStatement(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleContinueStatement");
  return false;
}

auto SemanticsParseTreeHandler::HandleContinueStatementStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleContinueStatementStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleDeclaredName(ParseTree::Node parse_node)
    -> bool {
  // The parent is responsible for binding the name.
  node_stack_.Push(parse_node);
  return true;
}

auto SemanticsParseTreeHandler::HandleDeducedParameterList(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleDeducedParameterList");
  return false;
}

auto SemanticsParseTreeHandler::HandleDeducedParameterListStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleDeducedParameterListStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleDesignatedName(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleDesignatedName");
  return false;
}

auto SemanticsParseTreeHandler::HandleDesignatorExpression(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleDesignatorExpression");
  return false;
}

auto SemanticsParseTreeHandler::HandleEmptyDeclaration(
    ParseTree::Node parse_node) -> bool {
  // Empty declarations have no actions associated, but we still balance the
  // tree.
  node_stack_.Push(parse_node);
  return true;
}

auto SemanticsParseTreeHandler::HandleExpressionStatement(
    ParseTree::Node parse_node) -> bool {
  // Pop the expression without investigating its contents.
  // TODO: This will probably eventually need to do some "do not discard"
  // analysis.
  node_stack_.PopAndDiscardId();
  node_stack_.Push(parse_node);
  return true;
}

auto SemanticsParseTreeHandler::HandleFileEnd(ParseTree::Node /*parse_node*/)
    -> bool {
  // Do nothing, no need to balance this node.
  return true;
}

auto SemanticsParseTreeHandler::HandleForHeader(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleForHeader");
  return false;
}

auto SemanticsParseTreeHandler::HandleForHeaderStart(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleForHeaderStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleForIn(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleForIn");
  return false;
}

auto SemanticsParseTreeHandler::HandleForStatement(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleForStatement");
  return false;
}

auto SemanticsParseTreeHandler::HandleFunctionDeclaration(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleFunctionDeclaration");
  return false;
}

auto SemanticsParseTreeHandler::HandleFunctionDefinition(
    ParseTree::Node parse_node) -> bool {
  // Merges code block children up under the FunctionDefinitionStart.
  while (parse_tree_->node_kind(node_stack_.PeekParseNode()) !=
         ParseNodeKind::FunctionDefinitionStart) {
    node_stack_.PopAndIgnore();
  }
  auto decl_id =
      node_stack_.PopForNodeId(ParseNodeKind::FunctionDefinitionStart);

  return_scope_stack_.pop_back();
  PopScope();
  auto block_id = node_block_stack_.Pop();
  AddNode(SemanticsNode::MakeFunctionDefinition(parse_node, decl_id, block_id));
  node_stack_.Push(parse_node);

  return true;
}

auto SemanticsParseTreeHandler::HandleFunctionDefinitionStart(
    ParseTree::Node parse_node) -> bool {
  SemanticsNodeId return_type_id = SemanticsNodeId::Invalid;
  if (parse_tree_->node_kind(node_stack_.PeekParseNode()) ==
      ParseNodeKind::ReturnType) {
    return_type_id = node_stack_.PopForNodeId(ParseNodeKind::ReturnType);
  }
  node_stack_.PopForSoloParseNode(ParseNodeKind::ParameterList);
  auto [param_ir_id, param_refs_id] = finished_params_stack_.pop_back_val();
  auto name_node = node_stack_.PopForSoloParseNode(ParseNodeKind::DeclaredName);
  auto fn_node =
      node_stack_.PopForSoloParseNode(ParseNodeKind::FunctionIntroducer);

  auto callable_id =
      semantics_->AddCallable({.param_ir_id = param_ir_id,
                               .param_refs_id = param_refs_id,
                               .return_type_id = return_type_id});
  auto decl_id =
      AddNode(SemanticsNode::MakeFunctionDeclaration(fn_node, callable_id));
  // TODO: Propagate the type of the function.
  BindName(name_node, SemanticsNodeId::Invalid, decl_id);

  node_block_stack_.Push();
  PushScope();
  return_scope_stack_.push_back(decl_id);
  node_stack_.Push(parse_node, decl_id);

  return true;
}

auto SemanticsParseTreeHandler::HandleFunctionIntroducer(
    ParseTree::Node parse_node) -> bool {
  // No action, just a bracketing node.
  node_stack_.Push(parse_node);
  return true;
}

auto SemanticsParseTreeHandler::HandleIfCondition(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleIfCondition");
  return false;
}

auto SemanticsParseTreeHandler::HandleIfConditionStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleIfConditionStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleIfStatement(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleIfStatement");
  return false;
}

auto SemanticsParseTreeHandler::HandleIfStatementElse(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleIfStatementElse");
  return false;
}

auto SemanticsParseTreeHandler::HandleInfixOperator(ParseTree::Node parse_node)
    -> bool {
  auto rhs_id = node_stack_.PopForNodeId();
  auto lhs_id = node_stack_.PopForNodeId();
  SemanticsNodeId result_type =
      TryTypeConversion(parse_node, lhs_id, rhs_id, /*can_convert_lhs=*/true);

  // Figure out the operator for the token.
  auto token = parse_tree_->node_token(parse_node);
  switch (auto token_kind = tokens_->GetKind(token)) {
    case TokenKind::Plus:
      AddNodeAndPush(parse_node, SemanticsNode::MakeBinaryOperatorAdd(
                                     parse_node, result_type, lhs_id, rhs_id));
      break;
    default:
      emitter_->Emit(parse_node, SemanticsTodo,
                     llvm::formatv("Handle {0}", token_kind));
      return false;
  }

  return true;
}

auto SemanticsParseTreeHandler::HandleInterfaceBody(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleInterfaceBody");
  return false;
}

auto SemanticsParseTreeHandler::HandleInterfaceBodyStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleInterfaceBodyStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleInterfaceDefinition(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleInterfaceDefinition");
  return false;
}

auto SemanticsParseTreeHandler::HandleLiteral(ParseTree::Node parse_node)
    -> bool {
  auto token = parse_tree_->node_token(parse_node);
  switch (auto token_kind = tokens_->GetKind(token)) {
    case TokenKind::IntegerLiteral: {
      auto id =
          semantics_->AddIntegerLiteral(tokens_->GetIntegerLiteral(token));
      AddNodeAndPush(parse_node,
                     SemanticsNode::MakeIntegerLiteral(parse_node, id));
      break;
    }
    case TokenKind::RealLiteral: {
      auto token_value = tokens_->GetRealLiteral(token);
      auto id =
          semantics_->AddRealLiteral({.mantissa = token_value.Mantissa(),
                                      .exponent = token_value.Exponent(),
                                      .is_decimal = token_value.IsDecimal()});
      AddNodeAndPush(parse_node,
                     SemanticsNode::MakeRealLiteral(parse_node, id));
      break;
    }
    case TokenKind::StringLiteral: {
      auto id = semantics_->AddString(tokens_->GetStringLiteral(token));
      AddNodeAndPush(parse_node,
                     SemanticsNode::MakeStringLiteral(parse_node, id));
      break;
    }
    case TokenKind::IntegerTypeLiteral: {
      auto text = tokens_->GetTokenText(token);
      if (text != "i32") {
        emitter_->Emit(parse_node, SemanticsTodo,
                       "Currently only i32 is allowed");
        return false;
      }
      node_stack_.Push(parse_node, SemanticsNodeId::BuiltinIntegerType);
      break;
    }
    case TokenKind::FloatingPointTypeLiteral: {
      auto text = tokens_->GetTokenText(token);
      if (text != "f64") {
        emitter_->Emit(parse_node, SemanticsTodo,
                       "Currently only f64 is allowed");
        return false;
      }
      node_stack_.Push(parse_node, SemanticsNodeId::BuiltinFloatingPointType);
      break;
    }
    case TokenKind::StringTypeLiteral: {
      node_stack_.Push(parse_node, SemanticsNodeId::BuiltinStringType);
      break;
    }
    default: {
      emitter_->Emit(parse_node, SemanticsTodo,
                     llvm::formatv("Handle {0}", token_kind));
      return false;
    }
  }

  return true;
}

auto SemanticsParseTreeHandler::HandleNameReference(ParseTree::Node parse_node)
    -> bool {
  auto name_str = parse_tree_->GetNodeText(parse_node);

  auto name_not_found = [&] {
    CARBON_DIAGNOSTIC(NameNotFound, Error, "Name {0} not found",
                      llvm::StringRef);
    emitter_->Emit(parse_node, NameNotFound, name_str);
    node_stack_.Push(parse_node, SemanticsNodeId::BuiltinInvalidType);
  };

  auto name_id = semantics_->GetString(name_str);
  if (!name_id) {
    name_not_found();
    return true;
  }

  auto it = name_lookup_.find(*name_id);
  if (it == name_lookup_.end()) {
    name_not_found();
    return true;
  }
  CARBON_CHECK(!it->second.empty()) << "Should have been erased: " << name_str;

  // TODO: Check for ambiguous lookups.
  node_stack_.Push(parse_node, it->second.back());

  return true;
}

auto SemanticsParseTreeHandler::HandlePackageApi(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePackageApi");
  return false;
}

auto SemanticsParseTreeHandler::HandlePackageDirective(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePackageDirective");
  return false;
}

auto SemanticsParseTreeHandler::HandlePackageImpl(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePackageImpl");
  return false;
}

auto SemanticsParseTreeHandler::HandlePackageIntroducer(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePackageIntroducer");
  return false;
}

auto SemanticsParseTreeHandler::HandlePackageLibrary(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePackageLibrary");
  return false;
}

auto SemanticsParseTreeHandler::HandleParameterList(ParseTree::Node parse_node)
    -> bool {
  auto on_start = [&](SemanticsNodeBlockId ir_id,
                      SemanticsNodeBlockId refs_id) -> bool {
    PopScope();
    node_stack_.PopAndDiscardSoloParseNode(ParseNodeKind::ParameterListStart);
    finished_params_stack_.push_back({ir_id, refs_id});
    node_stack_.Push(parse_node);
    return true;
  };
  return ParamOrArgEnd(ParseNodeKind::ParameterListStart,
                       ParseNodeKind::ParameterListComma, on_start);
}

auto SemanticsParseTreeHandler::HandleParameterListComma(
    ParseTree::Node parse_node) -> bool {
  return ParamOrArgComma(parse_node);
}

auto SemanticsParseTreeHandler::HandleParameterListStart(
    ParseTree::Node parse_node) -> bool {
  PushScope();
  node_stack_.Push(parse_node);
  ParamOrArgStart();
  return true;
}

auto SemanticsParseTreeHandler::HandleParenExpression(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleParenExpression");
  return false;
}

auto SemanticsParseTreeHandler::HandleParenExpressionOrTupleLiteralStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo,
                 "HandleParenExpressionOrTupleLiteralStart");
  return false;
}

auto SemanticsParseTreeHandler::HandlePatternBinding(ParseTree::Node parse_node)
    -> bool {
  auto type = node_stack_.PopForNodeId();

  // Get the name.
  auto name_node = node_stack_.PopForSoloParseNode();

  // Allocate storage, linked to the name for error locations.
  auto storage_id = AddNode(SemanticsNode::MakeVarStorage(name_node, type));

  // Bind the name to storage.
  auto name_id = BindName(name_node, type, storage_id);

  // If this node's result is used, it'll be for either the name or the storage
  // address. The storage address can be found through the name, so we push the
  // name.
  node_stack_.Push(parse_node, name_id);

  return true;
}

auto SemanticsParseTreeHandler::HandlePostfixOperator(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePostfixOperator");
  return false;
}

auto SemanticsParseTreeHandler::HandlePrefixOperator(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandlePrefixOperator");
  return false;
}

auto SemanticsParseTreeHandler::HandleReturnStatement(
    ParseTree::Node parse_node) -> bool {
  CARBON_CHECK(!return_scope_stack_.empty());
  const auto& fn_node = semantics_->GetNode(return_scope_stack_.back());
  const auto callable =
      semantics_->GetCallable(fn_node.GetAsFunctionDeclaration());

  if (parse_tree_->node_kind(node_stack_.PeekParseNode()) ==
      ParseNodeKind::ReturnStatementStart) {
    node_stack_.PopAndDiscardSoloParseNode(ParseNodeKind::ReturnStatementStart);

    if (callable.return_type_id.is_valid()) {
      // TODO: Stringify types, add a note pointing at the return
      // type's parse node.
      CARBON_DIAGNOSTIC(ReturnStatementMissingExpression, Error,
                        "Must return a {0}.", SemanticsNodeId);
      emitter_
          ->Build(parse_node, ReturnStatementMissingExpression,
                  callable.return_type_id)
          .Emit();
    }

    AddNodeAndPush(parse_node, SemanticsNode::MakeReturn(parse_node));
  } else {
    const auto arg = node_stack_.PopForNodeId();
    auto arg_type = semantics_->GetType(arg);
    node_stack_.PopAndDiscardSoloParseNode(ParseNodeKind::ReturnStatementStart);

    if (!callable.return_type_id.is_valid()) {
      CARBON_DIAGNOSTIC(
          ReturnStatementDisallowExpression, Error,
          "No return expression should be provided in this context.");
      CARBON_DIAGNOSTIC(ReturnStatementImplicitNote, Note,
                        "There was no return type provided.");
      emitter_->Build(parse_node, ReturnStatementDisallowExpression)
          .Note(fn_node.parse_node(), ReturnStatementImplicitNote)
          .Emit();
    } else {
      const auto new_type = CanTypeConvert(arg_type, callable.return_type_id);
      if (!new_type.is_valid()) {
        // TODO: Stringify types, add a note pointing at the return
        // type's parse node.
        CARBON_DIAGNOSTIC(ReturnStatementTypeMismatch, Error,
                          "Cannot convert {0} to {1}.", SemanticsNodeId,
                          SemanticsNodeId);
        emitter_
            ->Build(parse_node, ReturnStatementTypeMismatch, arg_type,
                    callable.return_type_id)
            .Emit();
      }
      arg_type = new_type;
    }

    AddNodeAndPush(parse_node, SemanticsNode::MakeReturnExpression(
                                   parse_node, arg_type, arg));
  }
  return true;
}

auto SemanticsParseTreeHandler::HandleReturnStatementStart(
    ParseTree::Node parse_node) -> bool {
  // No action, just a bracketing node.
  node_stack_.Push(parse_node);
  return true;
}

auto SemanticsParseTreeHandler::HandleReturnType(ParseTree::Node parse_node)
    -> bool {
  // Propagate the type expression.
  node_stack_.Push(parse_node, node_stack_.PopForNodeId());
  return true;
}

auto SemanticsParseTreeHandler::HandleSelfDeducedParameter(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleSelfDeducedParameter");
  return false;
}

auto SemanticsParseTreeHandler::HandleSelfType(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleSelfType");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructComma(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructComma");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructFieldDesignator(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructFieldDesignator");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructFieldType(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructFieldType");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructFieldUnknown(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructFieldUnknown");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructFieldValue(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructFieldValue");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructLiteral(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructLiteral");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructLiteralOrStructTypeLiteralStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo,
                 "HandleStructLiteralOrStructTypeLiteralStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleStructTypeLiteral(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleStructTypeLiteral");
  return false;
}

auto SemanticsParseTreeHandler::HandleTupleLiteral(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleTupleLiteral");
  return false;
}

auto SemanticsParseTreeHandler::HandleTupleLiteralComma(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleTupleLiteralComma");
  return false;
}

auto SemanticsParseTreeHandler::HandleVariableDeclaration(
    ParseTree::Node parse_node) -> bool {
  auto last_child = node_stack_.PopForParseNodeAndNodeId();

  if (parse_tree_->node_kind(last_child.first) !=
      ParseNodeKind::PatternBinding) {
    auto storage_id =
        node_stack_.PopForNodeId(ParseNodeKind::VariableInitializer);

    auto binding = node_stack_.PopForParseNodeAndNameId();

    // Restore the name now that the initializer is complete.
    AddNameToLookup(binding.second, storage_id);

    auto storage_type =
        TryTypeConversion(parse_node, storage_id, last_child.second,
                          /*can_convert_lhs=*/false);
    AddNode(SemanticsNode::MakeAssign(parse_node, storage_type, storage_id,
                                      last_child.second));
  }

  node_stack_.PopAndDiscardSoloParseNode(ParseNodeKind::VariableIntroducer);
  node_stack_.Push(parse_node);

  return true;
}

auto SemanticsParseTreeHandler::HandleVariableIntroducer(
    ParseTree::Node parse_node) -> bool {
  // No action, just a bracketing node.
  node_stack_.Push(parse_node);
  return true;
}

auto SemanticsParseTreeHandler::HandleVariableInitializer(
    ParseTree::Node parse_node) -> bool {
  // Temporarily remove name lookup entries added by the `var`. These will be
  // restored by `VariableDeclaration`.

  // Save the storage ID.
  auto it = name_lookup_.find(node_stack_.PeekForNameId());
  CARBON_CHECK(it != name_lookup_.end());
  CARBON_CHECK(!it->second.empty());
  auto storage_id = it->second.back();

  // Pop the name from lookup.
  if (it->second.size() == 1) {
    // Erase names that no longer resolve.
    name_lookup_.erase(it);
  } else {
    it->second.pop_back();
  }

  node_stack_.Push(parse_node, storage_id);

  return true;
}

auto SemanticsParseTreeHandler::HandleWhileCondition(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleWhileCondition");
  return false;
}

auto SemanticsParseTreeHandler::HandleWhileConditionStart(
    ParseTree::Node parse_node) -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleWhileConditionStart");
  return false;
}

auto SemanticsParseTreeHandler::HandleWhileStatement(ParseTree::Node parse_node)
    -> bool {
  emitter_->Emit(parse_node, SemanticsTodo, "HandleWhileStatement");
  return false;
}

}  // namespace Carbon