carbon-lang/toolchain/check/check.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/check/check.h"

#include <variant>

#include "common/check.h"
#include "common/error.h"
#include "common/map.h"
#include "common/variant_helpers.h"
#include "common/vlog.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/base/pretty_stack_trace_function.h"
#include "toolchain/check/context.h"
#include "toolchain/check/diagnostic_helpers.h"
#include "toolchain/check/function.h"
#include "toolchain/check/handle.h"
#include "toolchain/check/import.h"
#include "toolchain/check/import_ref.h"
#include "toolchain/check/sem_ir_diagnostic_converter.h"
#include "toolchain/diagnostics/diagnostic.h"
#include "toolchain/diagnostics/diagnostic_emitter.h"
#include "toolchain/lex/token_kind.h"
#include "toolchain/parse/node_ids.h"
#include "toolchain/parse/tree.h"
#include "toolchain/parse/tree_node_diagnostic_converter.h"
#include "toolchain/sem_ir/file.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Check {

namespace {
struct UnitInfo {
  // A given import within the file, with its destination.
  struct Import {
    Parse::Tree::PackagingNames names;
    UnitInfo* unit_info;
  };
  // A file's imports corresponding to a single package, for the map.
  struct PackageImports {
    // Use the constructor so that the SmallVector is only constructed
    // as-needed.
    explicit PackageImports(IdentifierId package_id,
                            Parse::ImportDeclId node_id)
        : package_id(package_id), node_id(node_id) {}

    // The identifier of the imported package.
    IdentifierId package_id;
    // The first `import` declaration in the file, which declared the package's
    // identifier (even if the import failed). Used for associating diagnostics
    // not specific to a single import.
    Parse::ImportDeclId node_id;
    // The associated `import` instruction. Only valid once a file is checked.
    SemIR::InstId import_decl_id = SemIR::InstId::Invalid;
    // Whether there's an import that failed to load.
    bool has_load_error = false;
    // The list of valid imports.
    llvm::SmallVector<Import> imports;
  };

  explicit UnitInfo(SemIR::CheckIRId check_ir_id, Unit& unit,
                    Parse::NodeLocConverter& converter)
      : check_ir_id(check_ir_id),
        unit(&unit),
        err_tracker(*unit.consumer),
        emitter(converter, err_tracker) {}

  SemIR::CheckIRId check_ir_id;
  Unit* unit;

  // Emitter information.
  ErrorTrackingDiagnosticConsumer err_tracker;
  DiagnosticEmitter<Parse::NodeLoc> emitter;

  // List of the outgoing imports. If a package includes unavailable library
  // imports, it has an entry with has_load_error set. Invalid imports (for
  // example, `import Main;`) aren't added because they won't add identifiers to
  // name lookup.
  llvm::SmallVector<PackageImports> package_imports;

  // A map of the package names to the outgoing imports above.
  Map<IdentifierId, int32_t> package_imports_map;

  // The remaining number of imports which must be checked before this unit can
  // be processed.
  int32_t imports_remaining = 0;

  // A list of incoming imports. This will be empty for `impl` files, because
  // imports only touch `api` files.
  llvm::SmallVector<UnitInfo*> incoming_imports;

  // The corresponding `api` unit if this is an `impl` file. The entry should
  // also be in the corresponding `PackageImports`.
  UnitInfo* api_for_impl = nullptr;
};
}  // namespace

// Collects direct imports, for CollectTransitiveImports.
static auto CollectDirectImports(llvm::SmallVector<SemIR::ImportIR>& results,
                                 llvm::MutableArrayRef<int> ir_to_result_index,
                                 SemIR::InstId import_decl_id,
                                 const UnitInfo::PackageImports& imports,
                                 bool is_local) -> void {
  for (const auto& import : imports.imports) {
    const auto& direct_ir = **import.unit_info->unit->sem_ir;
    auto& index = ir_to_result_index[direct_ir.check_ir_id().index];
    if (index != -1) {
      // This should only happen when doing API imports for an implementation
      // file. Don't change the entry; is_export doesn't matter.
      continue;
    }
    index = results.size();
    results.push_back({.decl_id = import_decl_id,
                       // Only tag exports in API files, ignoring the value in
                       // implementation files.
                       .is_export = is_local && import.names.is_export,
                       .sem_ir = &direct_ir});
  }
}

// Collects transitive imports, handling deduplication. These will be unified
// between local_imports and api_imports.
static auto CollectTransitiveImports(
    SemIR::InstId import_decl_id, const UnitInfo::PackageImports* local_imports,
    const UnitInfo::PackageImports* api_imports, int total_ir_count)
    -> llvm::SmallVector<SemIR::ImportIR> {
  llvm::SmallVector<SemIR::ImportIR> results;

  // Track whether an IR was imported in full, including `export import`. This
  // distinguishes from IRs that are indirectly added without all names being
  // exported to this IR.
  llvm::SmallVector<int> ir_to_result_index(total_ir_count, -1);

  // First add direct imports. This means that if an entity is imported both
  // directly and indirectly, the import path will reflect the direct import.
  if (local_imports) {
    CollectDirectImports(results, ir_to_result_index, import_decl_id,
                         *local_imports,
                         /*is_local=*/true);
  }
  if (api_imports) {
    CollectDirectImports(results, ir_to_result_index, import_decl_id,
                         *api_imports,
                         /*is_local=*/false);
  }

  // Loop through direct imports for any indirect exports. The underlying vector
  // is appended during iteration, so take the size first.
  const int direct_imports = results.size();
  for (int direct_index : llvm::seq(direct_imports)) {
    bool is_export = results[direct_index].is_export;

    for (const auto& indirect_ir :
         results[direct_index].sem_ir->import_irs().array_ref()) {
      if (!indirect_ir.is_export) {
        continue;
      }

      auto& indirect_index =
          ir_to_result_index[indirect_ir.sem_ir->check_ir_id().index];
      if (indirect_index == -1) {
        indirect_index = results.size();
        // TODO: In the case of a recursive `export import`, this only points at
        // the outermost import. May want something that better reflects the
        // recursion.
        results.push_back({.decl_id = results[direct_index].decl_id,
                           .is_export = is_export,
                           .sem_ir = indirect_ir.sem_ir});
      } else if (is_export) {
        results[indirect_index].is_export = true;
      }
    }
  }

  return results;
}

// Imports the current package.
static auto ImportCurrentPackage(Context& context, UnitInfo& unit_info,
                                 int total_ir_count,
                                 SemIR::InstId package_inst_id,
                                 SemIR::TypeId namespace_type_id) -> void {
  // Add imports from the current package.
  auto import_map_lookup =
      unit_info.package_imports_map.Lookup(IdentifierId::Invalid);
  if (!import_map_lookup) {
    // Push the scope; there are no names to add.
    context.scope_stack().Push(package_inst_id, SemIR::NameScopeId::Package);
    return;
  }
  UnitInfo::PackageImports& self_import =
      unit_info.package_imports[import_map_lookup.value()];

  if (self_import.has_load_error) {
    context.name_scopes().Get(SemIR::NameScopeId::Package).has_error = true;
  }

  ImportLibrariesFromCurrentPackage(
      context, namespace_type_id,
      CollectTransitiveImports(self_import.import_decl_id, &self_import,
                               /*api_imports=*/nullptr, total_ir_count));

  context.scope_stack().Push(
      package_inst_id, SemIR::NameScopeId::Package, SemIR::SpecificId::Invalid,
      context.name_scopes().Get(SemIR::NameScopeId::Package).has_error);
}

// Imports all other packages (excluding the current package).
static auto ImportOtherPackages(Context& context, UnitInfo& unit_info,
                                int total_ir_count,
                                SemIR::TypeId namespace_type_id) -> void {
  // api_imports_list is initially the size of the current file's imports,
  // including for API files, for simplicity in iteration. It's only really used
  // when processing an implementation file, in order to combine the API file
  // imports.
  //
  // For packages imported by the API file, the IdentifierId is the package name
  // and the index is into the API's import list. Otherwise, the initial
  // {Invalid, -1} state remains.
  llvm::SmallVector<std::pair<IdentifierId, int32_t>> api_imports_list;
  api_imports_list.resize(unit_info.package_imports.size(),
                          {IdentifierId::Invalid, -1});

  // When there's an API file, add the mapping to api_imports_list.
  if (unit_info.api_for_impl) {
    const auto& api_identifiers =
        unit_info.api_for_impl->unit->value_stores->identifiers();
    auto& impl_identifiers = unit_info.unit->value_stores->identifiers();

    for (auto [api_imports_index, api_imports] :
         llvm::enumerate(unit_info.api_for_impl->package_imports)) {
      // Skip the current package.
      if (!api_imports.package_id.is_valid()) {
        continue;
      }
      // Translate the package ID from the API file to the implementation file.
      auto impl_package_id =
          impl_identifiers.Add(api_identifiers.Get(api_imports.package_id));
      if (auto lookup = unit_info.package_imports_map.Lookup(impl_package_id)) {
        // On a hit, replace the entry to unify the API and implementation
        // imports.
        api_imports_list[lookup.value()] = {impl_package_id, api_imports_index};
      } else {
        // On a miss, add the package as API-only.
        api_imports_list.push_back({impl_package_id, api_imports_index});
      }
    }
  }

  for (auto [i, api_imports_entry] : llvm::enumerate(api_imports_list)) {
    // These variables are updated after figuring out which imports are present.
    auto import_decl_id = SemIR::InstId::Invalid;
    IdentifierId package_id = IdentifierId::Invalid;
    bool has_load_error = false;

    // Identify the local package imports if present.
    UnitInfo::PackageImports* local_imports = nullptr;
    if (i < unit_info.package_imports.size()) {
      local_imports = &unit_info.package_imports[i];
      if (!local_imports->package_id.is_valid()) {
        // Skip the current package.
        continue;
      }
      import_decl_id = local_imports->import_decl_id;

      package_id = local_imports->package_id;
      has_load_error |= local_imports->has_load_error;
    }

    // Identify the API package imports if present.
    UnitInfo::PackageImports* api_imports = nullptr;
    if (api_imports_entry.second != -1) {
      api_imports =
          &unit_info.api_for_impl->package_imports[api_imports_entry.second];

      if (local_imports) {
        CARBON_CHECK(package_id == api_imports_entry.first);
      } else {
        auto import_ir_inst_id = context.import_ir_insts().Add(
            {.ir_id = SemIR::ImportIRId::ApiForImpl,
             .inst_id = api_imports->import_decl_id});
        import_decl_id =
            context.AddInst(context.MakeImportedLocAndInst<SemIR::ImportDecl>(
                import_ir_inst_id, {.package_id = SemIR::NameId::ForIdentifier(
                                        api_imports_entry.first)}));
        package_id = api_imports_entry.first;
      }
      has_load_error |= api_imports->has_load_error;
    }

    // Do the actual import.
    ImportLibrariesFromOtherPackage(
        context, namespace_type_id, import_decl_id, package_id,
        CollectTransitiveImports(import_decl_id, local_imports, api_imports,
                                 total_ir_count),
        has_load_error);
  }
}

// Add imports to the root block.
static auto InitPackageScopeAndImports(Context& context, UnitInfo& unit_info,
                                       int total_ir_count) -> void {
  // First create the constant values map for all imported IRs. We'll populate
  // these with mappings for namespaces as we go.
  size_t num_irs = 0;
  for (auto& package_imports : unit_info.package_imports) {
    num_irs += package_imports.imports.size();
  }
  if (!unit_info.api_for_impl) {
    // Leave an empty slot for ImportIRId::ApiForImpl.
    ++num_irs;
  }

  context.import_irs().Reserve(num_irs);
  context.import_ir_constant_values().reserve(num_irs);

  context.SetTotalIRCount(total_ir_count);

  // Importing makes many namespaces, so only canonicalize the type once.
  auto namespace_type_id =
      context.GetBuiltinType(SemIR::BuiltinInstKind::NamespaceType);

  // Define the package scope, with an instruction for `package` expressions to
  // reference.
  auto package_scope_id = context.name_scopes().Add(
      SemIR::InstId::PackageNamespace, SemIR::NameId::PackageNamespace,
      SemIR::NameScopeId::Invalid);
  CARBON_CHECK(package_scope_id == SemIR::NameScopeId::Package);

  auto package_inst_id = context.AddInst<SemIR::Namespace>(
      Parse::NodeId::Invalid, {.type_id = namespace_type_id,
                               .name_scope_id = SemIR::NameScopeId::Package,
                               .import_id = SemIR::InstId::Invalid});
  CARBON_CHECK(package_inst_id == SemIR::InstId::PackageNamespace);

  // If there is an implicit `api` import, set it first so that it uses the
  // ImportIRId::ApiForImpl when processed for imports.
  if (unit_info.api_for_impl) {
    const auto& names = context.parse_tree().packaging_decl()->names;
    auto import_decl_id = context.AddInst<SemIR::ImportDecl>(
        names.node_id,
        {.package_id = SemIR::NameId::ForIdentifier(names.package_id)});
    SetApiImportIR(context,
                   {.decl_id = import_decl_id,
                    .is_export = false,
                    .sem_ir = &**unit_info.api_for_impl->unit->sem_ir});
  } else {
    SetApiImportIR(context,
                   {.decl_id = SemIR::InstId::Invalid, .sem_ir = nullptr});
  }

  // Add import instructions for everything directly imported. Implicit imports
  // are handled separately.
  for (auto& package_imports : unit_info.package_imports) {
    CARBON_CHECK(!package_imports.import_decl_id.is_valid());
    package_imports.import_decl_id = context.AddInst<SemIR::ImportDecl>(
        package_imports.node_id, {.package_id = SemIR::NameId::ForIdentifier(
                                      package_imports.package_id)});
  }

  // Process the imports.
  if (unit_info.api_for_impl) {
    ImportApiFile(context, namespace_type_id,
                  **unit_info.api_for_impl->unit->sem_ir);
  }
  ImportCurrentPackage(context, unit_info, total_ir_count, package_inst_id,
                       namespace_type_id);
  CARBON_CHECK(context.scope_stack().PeekIndex() == ScopeIndex::Package);
  ImportOtherPackages(context, unit_info, total_ir_count, namespace_type_id);
}

namespace {
// State used to track the next deferred function definition that we will
// encounter and need to reorder.
class NextDeferredDefinitionCache {
 public:
  explicit NextDeferredDefinitionCache(const Parse::Tree* tree) : tree_(tree) {
    SkipTo(Parse::DeferredDefinitionIndex(0));
  }

  // Set the specified deferred definition index as being the next one that will
  // be encountered.
  auto SkipTo(Parse::DeferredDefinitionIndex next_index) -> void {
    index_ = next_index;
    if (static_cast<std::size_t>(index_.index) ==
        tree_->deferred_definitions().size()) {
      start_id_ = Parse::NodeId::Invalid;
    } else {
      start_id_ = tree_->deferred_definitions().Get(index_).start_id;
    }
  }

  // Returns the index of the next deferred definition to be encountered.
  auto index() const -> Parse::DeferredDefinitionIndex { return index_; }

  // Returns the ID of the start node of the next deferred definition.
  auto start_id() const -> Parse::NodeId { return start_id_; }

 private:
  const Parse::Tree* tree_;
  Parse::DeferredDefinitionIndex index_ =
      Parse::DeferredDefinitionIndex::Invalid;
  Parse::NodeId start_id_ = Parse::NodeId::Invalid;
};
}  // namespace

// Determines whether this node kind is the start of a deferred definition
// scope.
static auto IsStartOfDeferredDefinitionScope(Parse::NodeKind kind) -> bool {
  switch (kind) {
    case Parse::NodeKind::ClassDefinitionStart:
    case Parse::NodeKind::ImplDefinitionStart:
    case Parse::NodeKind::InterfaceDefinitionStart:
    case Parse::NodeKind::NamedConstraintDefinitionStart:
      // TODO: Mixins.
      return true;
    default:
      return false;
  }
}

// Determines whether this node kind is the end of a deferred definition scope.
static auto IsEndOfDeferredDefinitionScope(Parse::NodeKind kind) -> bool {
  switch (kind) {
    case Parse::NodeKind::ClassDefinition:
    case Parse::NodeKind::ImplDefinition:
    case Parse::NodeKind::InterfaceDefinition:
    case Parse::NodeKind::NamedConstraintDefinition:
      // TODO: Mixins.
      return true;
    default:
      return false;
  }
}

namespace {
// A worklist of pending tasks to perform to check deferred function definitions
// in the right order.
class DeferredDefinitionWorklist {
 public:
  // A worklist task that indicates we should check a deferred function
  // definition that we previously skipped.
  struct CheckSkippedDefinition {
    // The definition that we skipped.
    Parse::DeferredDefinitionIndex definition_index;
    // The suspended function.
    SuspendedFunction suspended_fn;
  };

  // A worklist task that indicates we should enter a nested deferred definition
  // scope.
  struct EnterDeferredDefinitionScope {
    // The suspended scope. This is only set once we reach the end of the scope.
    std::optional<DeclNameStack::SuspendedName> suspended_name;
    // Whether this scope is itself within an outer deferred definition scope.
    // If so, we'll delay processing its contents until we reach the end of the
    // parent scope. For example:
    //
    // ```
    // class A {
    //   class B {
    //     fn F() -> A { return {}; }
    //   }
    // } // A.B.F is type-checked here, with A complete.
    //
    // fn F() {
    //   class C {
    //     fn G() {}
    //   } // C.G is type-checked here.
    // }
    // ```
    bool in_deferred_definition_scope;
  };

  // A worklist task that indicates we should leave a deferred definition scope.
  struct LeaveDeferredDefinitionScope {
    // Whether this scope is within another deferred definition scope.
    bool in_deferred_definition_scope;
  };

  // A pending type-checking task.
  using Task =
      std::variant<CheckSkippedDefinition, EnterDeferredDefinitionScope,
                   LeaveDeferredDefinitionScope>;

  explicit DeferredDefinitionWorklist(llvm::raw_ostream* vlog_stream)
      : vlog_stream_(vlog_stream) {
    // See declaration of `worklist_`.
    worklist_.reserve(64);
  }

  static constexpr llvm::StringLiteral VlogPrefix =
      "DeferredDefinitionWorklist ";

  // Suspend the current function definition and push a task onto the worklist
  // to finish it later.
  auto SuspendFunctionAndPush(Context& context,
                              Parse::DeferredDefinitionIndex index,
                              Parse::FunctionDefinitionStartId node_id)
      -> void {
    worklist_.push_back(CheckSkippedDefinition{
        index, HandleFunctionDefinitionSuspend(context, node_id)});
    CARBON_VLOG("{0}Push CheckSkippedDefinition {1}\n", VlogPrefix,
                index.index);
  }

  // Push a task to re-enter a function scope, so that functions defined within
  // it are type-checked in the right context.
  auto PushEnterDeferredDefinitionScope(Context& context) -> void {
    bool nested = !entered_scopes_.empty() &&
                  entered_scopes_.back().scope_index ==
                      context.decl_name_stack().PeekInitialScopeIndex();
    entered_scopes_.push_back(
        {.worklist_start_index = worklist_.size(),
         .scope_index = context.scope_stack().PeekIndex()});
    worklist_.push_back(
        EnterDeferredDefinitionScope{.suspended_name = std::nullopt,
                                     .in_deferred_definition_scope = nested});
    CARBON_VLOG("{0}Push EnterDeferredDefinitionScope {1}\n", VlogPrefix,
                nested ? "(nested)" : "(non-nested)");
  }

  // Suspend the current deferred definition scope, which is finished but still
  // on the decl_name_stack, and push a task to leave the scope when we're
  // type-checking deferred definitions. Returns `true` if the current list of
  // deferred definitions should be type-checked immediately.
  auto SuspendFinishedScopeAndPush(Context& context) -> bool;

  // Pop the next task off the worklist.
  auto Pop() -> Task {
    if (vlog_stream_) {
      VariantMatch(
          worklist_.back(),
          [&](CheckSkippedDefinition& definition) {
            CARBON_VLOG("{0}Handle CheckSkippedDefinition {1}\n", VlogPrefix,
                        definition.definition_index.index);
          },
          [&](EnterDeferredDefinitionScope& enter) {
            CARBON_CHECK(enter.in_deferred_definition_scope);
            CARBON_VLOG("{0}Handle EnterDeferredDefinitionScope (nested)\n",
                        VlogPrefix);
          },
          [&](LeaveDeferredDefinitionScope& leave) {
            bool nested = leave.in_deferred_definition_scope;
            CARBON_VLOG("{0}Handle LeaveDeferredDefinitionScope {1}\n",
                        VlogPrefix, nested ? "(nested)" : "(non-nested)");
          });
    }

    return worklist_.pop_back_val();
  }

  // CHECK that the work list has no further work.
  auto VerifyEmpty() {
    CARBON_CHECK(worklist_.empty() && entered_scopes_.empty())
        << "Tasks left behind on worklist.";
  }

 private:
  llvm::raw_ostream* vlog_stream_;

  // A worklist of type-checking tasks we'll need to do later.
  //
  // Don't allocate any inline storage here. A Task is fairly large, so we never
  // want this to live on the stack. Instead, we reserve space in the
  // constructor for a fairly large number of deferred definitions.
  llvm::SmallVector<Task, 0> worklist_;

  // A deferred definition scope that is currently still open.
  struct EnteredScope {
    // The index in worklist_ of the EnterDeferredDefinitionScope task.
    size_t worklist_start_index;
    // The corresponding lexical scope index.
    ScopeIndex scope_index;
  };

  // The deferred definition scopes for the current checking actions.
  llvm::SmallVector<EnteredScope> entered_scopes_;
};
}  // namespace

auto DeferredDefinitionWorklist::SuspendFinishedScopeAndPush(Context& context)
    -> bool {
  auto start_index = entered_scopes_.pop_back_val().worklist_start_index;

  // If we've not found any deferred definitions in this scope, clean up the
  // stack.
  if (start_index == worklist_.size() - 1) {
    context.decl_name_stack().PopScope();
    worklist_.pop_back();
    CARBON_VLOG("{0}Pop EnterDeferredDefinitionScope (empty)\n", VlogPrefix);
    return false;
  }

  // If we're finishing a nested deferred definition scope, keep track of that
  // but don't type-check deferred definitions now.
  auto& enter_scope = get<EnterDeferredDefinitionScope>(worklist_[start_index]);
  if (enter_scope.in_deferred_definition_scope) {
    // This is a nested deferred definition scope. Suspend the inner scope so we
    // can restore it when we come to type-check the deferred definitions.
    enter_scope.suspended_name = context.decl_name_stack().Suspend();

    // Enqueue a task to leave the nested scope.
    worklist_.push_back(
        LeaveDeferredDefinitionScope{.in_deferred_definition_scope = true});
    CARBON_VLOG("{0}Push LeaveDeferredDefinitionScope (nested)\n", VlogPrefix);
    return false;
  }

  // We're at the end of a non-nested deferred definition scope. Prepare to
  // start checking deferred definitions. Enqueue a task to leave this outer
  // scope and end checking deferred definitions.
  worklist_.push_back(
      LeaveDeferredDefinitionScope{.in_deferred_definition_scope = false});
  CARBON_VLOG("{0}Push LeaveDeferredDefinitionScope (non-nested)\n",
              VlogPrefix);

  // We'll process the worklist in reverse index order, so reverse the part of
  // it we're about to execute so we run our tasks in the order in which they
  // were pushed.
  std::reverse(worklist_.begin() + start_index, worklist_.end());

  // Pop the `EnterDeferredDefinitionScope` that's now on the end of the
  // worklist. We stay in that scope rather than suspending then immediately
  // resuming it.
  CARBON_CHECK(
      holds_alternative<EnterDeferredDefinitionScope>(worklist_.back()))
      << "Unexpected task in worklist.";
  worklist_.pop_back();
  CARBON_VLOG("{0}Handle EnterDeferredDefinitionScope (non-nested)\n",
              VlogPrefix);
  return true;
}

namespace {
// A traversal of the node IDs in the parse tree, in the order in which we need
// to check them.
class NodeIdTraversal {
 public:
  explicit NodeIdTraversal(Context& context, llvm::raw_ostream* vlog_stream)
      : context_(context),
        next_deferred_definition_(&context.parse_tree()),
        worklist_(vlog_stream) {
    auto range = context.parse_tree().postorder();
    chunks_.push_back(
        {.it = range.begin(),
         .end = range.end(),
         .next_definition = Parse::DeferredDefinitionIndex::Invalid});
  }

  // Finds the next `NodeId` to type-check. Returns nullopt if the traversal is
  // complete.
  auto Next() -> std::optional<Parse::NodeId>;

  // Performs any processing necessary after we type-check a node.
  auto Handle(Parse::NodeKind parse_kind) -> void {
    // When we reach the start of a deferred definition scope, add a task to the
    // worklist to check future skipped definitions in the new context.
    if (IsStartOfDeferredDefinitionScope(parse_kind)) {
      worklist_.PushEnterDeferredDefinitionScope(context_);
    }

    // When we reach the end of a deferred definition scope, add a task to the
    // worklist to leave the scope. If this is not a nested scope, start
    // checking the deferred definitions now.
    if (IsEndOfDeferredDefinitionScope(parse_kind)) {
      chunks_.back().checking_deferred_definitions =
          worklist_.SuspendFinishedScopeAndPush(context_);
    }
  }

 private:
  // A chunk of the parse tree that we need to type-check.
  struct Chunk {
    Parse::Tree::PostorderIterator it;
    Parse::Tree::PostorderIterator end;
    // The next definition that will be encountered after this chunk completes.
    Parse::DeferredDefinitionIndex next_definition;
    // Whether we are currently checking deferred definitions, rather than the
    // tokens of this chunk. If so, we'll pull tasks off `worklist` and execute
    // them until we're done with this batch of deferred definitions. Otherwise,
    // we'll pull node IDs from `*it` until it reaches `end`.
    bool checking_deferred_definitions = false;
  };

  // Re-enter a nested deferred definition scope.
  auto PerformTask(
      DeferredDefinitionWorklist::EnterDeferredDefinitionScope&& enter)
      -> void {
    CARBON_CHECK(enter.suspended_name)
        << "Entering a scope with no suspension information.";
    context_.decl_name_stack().Restore(std::move(*enter.suspended_name));
  }

  // Leave a nested or top-level deferred definition scope.
  auto PerformTask(
      DeferredDefinitionWorklist::LeaveDeferredDefinitionScope&& leave)
      -> void {
    if (!leave.in_deferred_definition_scope) {
      // We're done with checking deferred definitions.
      chunks_.back().checking_deferred_definitions = false;
    }
    context_.decl_name_stack().PopScope();
  }

  // Resume checking a deferred definition.
  auto PerformTask(
      DeferredDefinitionWorklist::CheckSkippedDefinition&& parse_definition)
      -> void {
    auto& [definition_index, suspended_fn] = parse_definition;
    const auto& definition_info =
        context_.parse_tree().deferred_definitions().Get(definition_index);
    HandleFunctionDefinitionResume(context_, definition_info.start_id,
                                   std::move(suspended_fn));
    auto range = Parse::Tree::PostorderIterator::MakeRange(
        definition_info.start_id, definition_info.definition_id);
    chunks_.push_back({.it = range.begin() + 1,
                       .end = range.end(),
                       .next_definition = next_deferred_definition_.index()});
    ++definition_index.index;
    next_deferred_definition_.SkipTo(definition_index);
  }

  Context& context_;
  NextDeferredDefinitionCache next_deferred_definition_;
  DeferredDefinitionWorklist worklist_;
  llvm::SmallVector<Chunk> chunks_;
};
}  // namespace

auto NodeIdTraversal::Next() -> std::optional<Parse::NodeId> {
  while (true) {
    // If we're checking deferred definitions, find the next definition we
    // should check, restore its suspended state, and add a corresponding
    // `Chunk` to the top of the chunk list.
    if (chunks_.back().checking_deferred_definitions) {
      std::visit(
          [&](auto&& task) { PerformTask(std::forward<decltype(task)>(task)); },
          worklist_.Pop());
      continue;
    }

    // If we're not checking deferred definitions, produce the next parse node
    // for this chunk. If we've run out of parse nodes, we're done with this
    // chunk of the parse tree.
    if (chunks_.back().it == chunks_.back().end) {
      auto old_chunk = chunks_.pop_back_val();

      // If we're out of chunks, then we're done entirely.
      if (chunks_.empty()) {
        worklist_.VerifyEmpty();
        return std::nullopt;
      }

      next_deferred_definition_.SkipTo(old_chunk.next_definition);
      continue;
    }

    auto node_id = *chunks_.back().it;

    // If we've reached the start of a deferred definition, skip to the end of
    // it, and track that we need to check it later.
    if (node_id == next_deferred_definition_.start_id()) {
      const auto& definition_info =
          context_.parse_tree().deferred_definitions().Get(
              next_deferred_definition_.index());
      worklist_.SuspendFunctionAndPush(context_,
                                       next_deferred_definition_.index(),
                                       definition_info.start_id);

      // Continue type-checking the parse tree after the end of the definition.
      chunks_.back().it =
          Parse::Tree::PostorderIterator(definition_info.definition_id) + 1;
      next_deferred_definition_.SkipTo(definition_info.next_definition_index);
      continue;
    }

    ++chunks_.back().it;
    return node_id;
  }
}

// Emits a diagnostic for each declaration in context.definitions_required()
// that doesn't have a definition.
static auto DiagnoseMissingDefinitions(Context& context,
                                       Context::DiagnosticEmitter& emitter)
    -> void {
  CARBON_DIAGNOSTIC(MissingDefinitionInImpl, Error,
                    "No definition found for declaration in impl file");
  for (SemIR::InstId decl_inst_id : context.definitions_required()) {
    SemIR::Inst decl_inst = context.insts().Get(decl_inst_id);
    CARBON_KIND_SWITCH(context.insts().Get(decl_inst_id)) {
      case CARBON_KIND(SemIR::ClassDecl class_decl): {
        if (!context.classes().Get(class_decl.class_id).is_defined()) {
          emitter.Emit(decl_inst_id, MissingDefinitionInImpl);
        }
        break;
      }
      case CARBON_KIND(SemIR::FunctionDecl function_decl): {
        if (context.functions().Get(function_decl.function_id).definition_id ==
            SemIR::InstId::Invalid) {
          emitter.Emit(decl_inst_id, MissingDefinitionInImpl);
        }
        break;
      }
      case CARBON_KIND(SemIR::ImplDecl impl_decl): {
        if (!context.impls().Get(impl_decl.impl_id).is_defined()) {
          emitter.Emit(decl_inst_id, MissingDefinitionInImpl);
        }
        break;
      }
      case SemIR::InterfaceDecl::Kind: {
        // TODO: handle `interface` as well, once we can test it without
        // triggering https://github.com/carbon-language/carbon-lang/issues/4071
        CARBON_FATAL()
            << "TODO: Support interfaces in DiagnoseMissingDefinitions";
      }
      default: {
        CARBON_FATAL() << "Unexpected inst in definitions_required: "
                       << decl_inst;
      }
    }
  }
}

// Loops over all nodes in the tree. On some errors, this may return early,
// for example if an unrecoverable state is encountered.
// NOLINTNEXTLINE(readability-function-size)
static auto ProcessNodeIds(Context& context, llvm::raw_ostream* vlog_stream,
                           ErrorTrackingDiagnosticConsumer& err_tracker,
                           Parse::NodeLocConverter& converter) -> bool {
  NodeIdTraversal traversal(context, vlog_stream);

  Parse::NodeId node_id = Parse::NodeId::Invalid;

  // On crash, report which token we were handling.
  PrettyStackTraceFunction node_dumper([&](llvm::raw_ostream& output) {
    auto loc = converter.ConvertLoc(
        node_id, [](DiagnosticLoc, const Internal::DiagnosticBase<>&) {});
    loc.FormatLocation(output);
    output << ": checking " << context.parse_tree().node_kind(node_id) << "\n";
    // Crash output has a tab indent; try to indent slightly past that.
    loc.FormatSnippet(output, /*indent=*/10);
  });

  while (auto maybe_node_id = traversal.Next()) {
    node_id = *maybe_node_id;
    auto parse_kind = context.parse_tree().node_kind(node_id);

    switch (parse_kind) {
#define CARBON_PARSE_NODE_KIND(Name)                                         \
  case Parse::NodeKind::Name: {                                              \
    if (!HandleParseNode(context, Parse::Name##Id(node_id))) {               \
      CARBON_CHECK(err_tracker.seen_error())                                 \
          << "Handle" #Name " returned false without printing a diagnostic"; \
      return false;                                                          \
    }                                                                        \
    break;                                                                   \
  }
#include "toolchain/parse/node_kind.def"
    }

    traversal.Handle(parse_kind);
  }
  return true;
}

// Produces and checks the IR for the provided Parse::Tree.
static auto CheckParseTree(
    llvm::MutableArrayRef<Parse::NodeLocConverter> node_converters,
    UnitInfo& unit_info, int total_ir_count, llvm::raw_ostream* vlog_stream)
    -> void {
  auto package_id = IdentifierId::Invalid;
  auto library_id = StringLiteralValueId::Invalid;
  if (const auto& packaging = unit_info.unit->parse_tree->packaging_decl()) {
    package_id = packaging->names.package_id;
    library_id = packaging->names.library_id;
  }
  unit_info.unit->sem_ir->emplace(
      unit_info.check_ir_id, package_id,
      SemIR::LibraryNameId::ForStringLiteralValueId(library_id),
      *unit_info.unit->value_stores,
      unit_info.unit->tokens->source().filename().str());

  SemIR::File& sem_ir = **unit_info.unit->sem_ir;
  SemIRDiagnosticConverter converter(node_converters, &sem_ir);
  Context::DiagnosticEmitter emitter(converter, unit_info.err_tracker);
  Context context(*unit_info.unit->tokens, emitter, *unit_info.unit->parse_tree,
                  unit_info.unit->get_parse_tree_and_subtrees, sem_ir,
                  vlog_stream);
  PrettyStackTraceFunction context_dumper(
      [&](llvm::raw_ostream& output) { context.PrintForStackDump(output); });

  // Add a block for the file.
  context.inst_block_stack().Push();

  InitPackageScopeAndImports(context, unit_info, total_ir_count);

  // Import all impls declared in imports.
  // TODO: Do this selectively when we see an impl query.
  ImportImpls(context);

  if (!ProcessNodeIds(context, vlog_stream, unit_info.err_tracker,
                      node_converters[unit_info.check_ir_id.index])) {
    context.sem_ir().set_has_errors(true);
    return;
  }

  context.Finalize();

  DiagnoseMissingDefinitions(context, emitter);

  context.VerifyOnFinish();

  sem_ir.set_has_errors(unit_info.err_tracker.seen_error());

#ifndef NDEBUG
  if (auto verify = sem_ir.Verify(); !verify.ok()) {
    CARBON_FATAL() << sem_ir << "Built invalid semantics IR: " << verify.error()
                   << "\n";
  }
#endif
}

// The package and library names, used as map keys.
using ImportKey = std::pair<llvm::StringRef, llvm::StringRef>;

// Returns a key form of the package object. file_package_id is only used for
// imports, not the main package declaration; as a consequence, it will be
// invalid for the main package declaration.
static auto GetImportKey(UnitInfo& unit_info, IdentifierId file_package_id,
                         Parse::Tree::PackagingNames names) -> ImportKey {
  auto* stores = unit_info.unit->value_stores;
  llvm::StringRef package_name =
      names.package_id.is_valid()  ? stores->identifiers().Get(names.package_id)
      : file_package_id.is_valid() ? stores->identifiers().Get(file_package_id)
                                   : "";
  llvm::StringRef library_name =
      names.library_id.is_valid()
          ? stores->string_literal_values().Get(names.library_id)
          : "";
  return {package_name, library_name};
}

static constexpr llvm::StringLiteral ExplicitMainName = "Main";

static auto RenderImportKey(ImportKey import_key) -> std::string {
  if (import_key.first.empty()) {
    import_key.first = ExplicitMainName;
  }
  if (import_key.second.empty()) {
    return import_key.first.str();
  }
  return llvm::formatv("{0}//{1}", import_key.first, import_key.second).str();
}

// Marks an import as required on both the source and target file.
//
// The ID comparisons between the import and unit are okay because they both
// come from the same file.
static auto TrackImport(Map<ImportKey, UnitInfo*>& api_map,
                        Map<ImportKey, Parse::NodeId>* explicit_import_map,
                        UnitInfo& unit_info, Parse::Tree::PackagingNames import)
    -> void {
  const auto& packaging = unit_info.unit->parse_tree->packaging_decl();

  IdentifierId file_package_id =
      packaging ? packaging->names.package_id : IdentifierId::Invalid;
  auto import_key = GetImportKey(unit_info, file_package_id, import);

  // True if the import has `Main` as the package name, even if it comes from
  // the file's packaging (diagnostics may differentiate).
  bool is_explicit_main = import_key.first == ExplicitMainName;

  // Explicit imports need more validation than implicit ones. We try to do
  // these in an order of imports that should be removed, followed by imports
  // that might be valid with syntax fixes.
  if (explicit_import_map) {
    // Diagnose redundant imports.
    if (auto insert_result =
            explicit_import_map->Insert(import_key, import.node_id);
        !insert_result.is_inserted()) {
      CARBON_DIAGNOSTIC(RepeatedImport, Error,
                        "Library imported more than once.");
      CARBON_DIAGNOSTIC(FirstImported, Note, "First import here.");
      unit_info.emitter.Build(import.node_id, RepeatedImport)
          .Note(insert_result.value(), FirstImported)
          .Emit();
      return;
    }

    // True if the file's package is implicitly `Main` (by omitting an explicit
    // package name).
    bool is_file_implicit_main =
        !packaging || !packaging->names.package_id.is_valid();
    // True if the import is using implicit "current package" syntax (by
    // omitting an explicit package name).
    bool is_import_implicit_current_package = !import.package_id.is_valid();
    // True if the import is using `default` library syntax.
    bool is_import_default_library = !import.library_id.is_valid();
    // True if the import and file point at the same package, even by
    // incorrectly specifying the current package name to `import`.
    bool is_same_package = is_import_implicit_current_package ||
                           import.package_id == file_package_id;
    // True if the import points at the same library as the file's library.
    bool is_same_library =
        is_same_package &&
        (packaging ? import.library_id == packaging->names.library_id
                   : is_import_default_library);

    // Diagnose explicit imports of the same library, whether from `api` or
    // `impl`.
    if (is_same_library) {
      CARBON_DIAGNOSTIC(ExplicitImportApi, Error,
                        "Explicit import of `api` from `impl` file is "
                        "redundant with implicit import.");
      CARBON_DIAGNOSTIC(ImportSelf, Error, "File cannot import itself.");
      bool is_impl = !packaging || packaging->is_impl;
      unit_info.emitter.Emit(import.node_id,
                             is_impl ? ExplicitImportApi : ImportSelf);
      return;
    }

    // Diagnose explicit imports of `Main//default`. There is no `api` for it.
    // This lets other diagnostics handle explicit `Main` package naming.
    if (is_file_implicit_main && is_import_implicit_current_package &&
        is_import_default_library) {
      CARBON_DIAGNOSTIC(ImportMainDefaultLibrary, Error,
                        "Cannot import `Main//default`.");
      unit_info.emitter.Emit(import.node_id, ImportMainDefaultLibrary);

      return;
    }

    if (!is_import_implicit_current_package) {
      // Diagnose explicit imports of the same package that use the package
      // name.
      if (is_same_package || (is_file_implicit_main && is_explicit_main)) {
        CARBON_DIAGNOSTIC(
            ImportCurrentPackageByName, Error,
            "Imports from the current package must omit the package name.");
        unit_info.emitter.Emit(import.node_id, ImportCurrentPackageByName);
        return;
      }

      // Diagnose explicit imports from `Main`.
      if (is_explicit_main) {
        CARBON_DIAGNOSTIC(ImportMainPackage, Error,
                          "Cannot import `Main` from other packages.");
        unit_info.emitter.Emit(import.node_id, ImportMainPackage);
        return;
      }
    }
  } else if (is_explicit_main) {
    // An implicit import with an explicit `Main` occurs when a `package` rule
    // has bad syntax, which will have been diagnosed when building the API map.
    // As a consequence, we return silently.
    return;
  }

  // Get the package imports, or create them if this is the first.
  auto create_imports = [&]() -> int32_t {
    int32_t index = unit_info.package_imports.size();
    unit_info.package_imports.push_back(
        UnitInfo::PackageImports(import.package_id, import.node_id));
    return index;
  };
  auto insert_result =
      unit_info.package_imports_map.Insert(import.package_id, create_imports);
  UnitInfo::PackageImports& package_imports =
      unit_info.package_imports[insert_result.value()];

  if (auto api_lookup = api_map.Lookup(import_key)) {
    // Add references between the file and imported api.
    UnitInfo* api = api_lookup.value();
    package_imports.imports.push_back({import, api});
    ++unit_info.imports_remaining;
    api->incoming_imports.push_back(&unit_info);

    // If this is the implicit import, note we have it.
    if (!explicit_import_map) {
      CARBON_CHECK(!unit_info.api_for_impl);
      unit_info.api_for_impl = api;
    }
  } else {
    // The imported api is missing.
    package_imports.has_load_error = true;
    CARBON_DIAGNOSTIC(LibraryApiNotFound, Error,
                      "Corresponding API for '{0}' not found.", std::string);
    CARBON_DIAGNOSTIC(ImportNotFound, Error, "Imported API '{0}' not found.",
                      std::string);
    unit_info.emitter.Emit(
        import.node_id,
        explicit_import_map ? ImportNotFound : LibraryApiNotFound,
        RenderImportKey(import_key));
  }
}

// Builds a map of `api` files which might be imported. Also diagnoses issues
// related to the packaging because the strings are loaded as part of getting
// the ImportKey (which we then do for `impl` files too).
static auto BuildApiMapAndDiagnosePackaging(
    llvm::MutableArrayRef<UnitInfo> unit_infos) -> Map<ImportKey, UnitInfo*> {
  Map<ImportKey, UnitInfo*> api_map;
  for (auto& unit_info : unit_infos) {
    const auto& packaging = unit_info.unit->parse_tree->packaging_decl();
    // An import key formed from the `package` or `library` declaration. Or, for
    // Main//default, a placeholder key.
    auto import_key = packaging ? GetImportKey(unit_info, IdentifierId::Invalid,
                                               packaging->names)
                                // Construct a boring key for Main//default.
                                : ImportKey{"", ""};

    // Diagnose explicit `Main` uses before they become marked as possible
    // APIs.
    if (import_key.first == ExplicitMainName) {
      CARBON_DIAGNOSTIC(ExplicitMainPackage, Error,
                        "`Main//default` must omit `package` declaration.");
      CARBON_DIAGNOSTIC(
          ExplicitMainLibrary, Error,
          "Use `library` declaration in `Main` package libraries.");
      unit_info.emitter.Emit(packaging->names.node_id,
                             import_key.second.empty() ? ExplicitMainPackage
                                                       : ExplicitMainLibrary);
      continue;
    }

    bool is_impl = packaging && packaging->is_impl;

    // Add to the `api` map and diagnose duplicates. This occurs before the
    // file extension check because we might emit both diagnostics in situations
    // where the user forgets (or has syntax errors with) a package line
    // multiple times.
    if (!is_impl) {
      auto insert_result = api_map.Insert(import_key, &unit_info);
      if (!insert_result.is_inserted()) {
        llvm::StringRef prev_filename =
            insert_result.value()->unit->tokens->source().filename();
        if (packaging) {
          CARBON_DIAGNOSTIC(DuplicateLibraryApi, Error,
                            "Library's API previously provided by `{0}`.",
                            std::string);
          unit_info.emitter.Emit(packaging->names.node_id, DuplicateLibraryApi,
                                 prev_filename.str());
        } else {
          CARBON_DIAGNOSTIC(DuplicateMainApi, Error,
                            "Main//default previously provided by `{0}`.",
                            std::string);
          // Use the invalid node because there's no node to associate with.
          unit_info.emitter.Emit(Parse::NodeId::Invalid, DuplicateMainApi,
                                 prev_filename.str());
        }
      }
    }

    // Validate file extensions. Note imports rely the packaging declaration,
    // not the extension. If the input is not a regular file, for example
    // because it is stdin, no filename checking is performed.
    if (unit_info.unit->tokens->source().is_regular_file()) {
      auto filename = unit_info.unit->tokens->source().filename();
      static constexpr llvm::StringLiteral ApiExt = ".carbon";
      static constexpr llvm::StringLiteral ImplExt = ".impl.carbon";
      bool is_api_with_impl_ext = !is_impl && filename.ends_with(ImplExt);
      auto want_ext = is_impl ? ImplExt : ApiExt;
      if (is_api_with_impl_ext || !filename.ends_with(want_ext)) {
        CARBON_DIAGNOSTIC(IncorrectExtension, Error,
                          "File extension of `{0}` required for `{1}`.",
                          llvm::StringLiteral, Lex::TokenKind);
        auto diag = unit_info.emitter.Build(
            packaging ? packaging->names.node_id : Parse::NodeId::Invalid,
            IncorrectExtension, want_ext,
            is_impl ? Lex::TokenKind::Impl : Lex::TokenKind::Api);
        if (is_api_with_impl_ext) {
          CARBON_DIAGNOSTIC(IncorrectExtensionImplNote, Note,
                            "File extension of `{0}` only allowed for `{1}`.",
                            llvm::StringLiteral, Lex::TokenKind);
          diag.Note(Parse::NodeId::Invalid, IncorrectExtensionImplNote, ImplExt,
                    Lex::TokenKind::Impl);
        }
        diag.Emit();
      }
    }
  }
  return api_map;
}

auto CheckParseTrees(
    llvm::MutableArrayRef<Unit> units,
    llvm::MutableArrayRef<Parse::NodeLocConverter> node_converters,
    bool prelude_import, llvm::raw_ostream* vlog_stream) -> void {
  // UnitInfo is big due to its SmallVectors, so we default to 0 on the stack.
  llvm::SmallVector<UnitInfo, 0> unit_infos;
  unit_infos.reserve(units.size());
  for (auto [i, unit] : llvm::enumerate(units)) {
    unit_infos.emplace_back(SemIR::CheckIRId(i), unit, node_converters[i]);
  }

  Map<ImportKey, UnitInfo*> api_map =
      BuildApiMapAndDiagnosePackaging(unit_infos);

  // Mark down imports for all files.
  llvm::SmallVector<UnitInfo*> ready_to_check;
  ready_to_check.reserve(units.size());
  for (auto& unit_info : unit_infos) {
    const auto& packaging = unit_info.unit->parse_tree->packaging_decl();
    if (packaging && packaging->is_impl) {
      // An `impl` has an implicit import of its `api`.
      auto implicit_names = packaging->names;
      implicit_names.package_id = IdentifierId::Invalid;
      TrackImport(api_map, nullptr, unit_info, implicit_names);
    }

    Map<ImportKey, Parse::NodeId> explicit_import_map;

    // Add the prelude import. It's added to explicit_import_map so that it can
    // conflict with an explicit import of the prelude.
    IdentifierId core_ident_id =
        unit_info.unit->value_stores->identifiers().Add("Core");
    if (prelude_import &&
        !(packaging && packaging->names.package_id == core_ident_id)) {
      auto prelude_id =
          unit_info.unit->value_stores->string_literal_values().Add("prelude");
      TrackImport(api_map, &explicit_import_map, unit_info,
                  {.node_id = Parse::InvalidNodeId(),
                   .package_id = core_ident_id,
                   .library_id = prelude_id});
    }

    for (const auto& import : unit_info.unit->parse_tree->imports()) {
      TrackImport(api_map, &explicit_import_map, unit_info, import);
    }

    // If there were no imports, mark the file as ready to check for below.
    if (unit_info.imports_remaining == 0) {
      ready_to_check.push_back(&unit_info);
    }
  }

  // Check everything with no dependencies. Earlier entries with dependencies
  // will be checked as soon as all their dependencies have been checked.
  for (int check_index = 0;
       check_index < static_cast<int>(ready_to_check.size()); ++check_index) {
    auto* unit_info = ready_to_check[check_index];
    CheckParseTree(node_converters, *unit_info, units.size(), vlog_stream);
    for (auto* incoming_import : unit_info->incoming_imports) {
      --incoming_import->imports_remaining;
      if (incoming_import->imports_remaining == 0) {
        ready_to_check.push_back(incoming_import);
      }
    }
  }

  // If there are still units with remaining imports, it means there's a
  // dependency loop.
  if (ready_to_check.size() < unit_infos.size()) {
    // Go through units and mask out unevaluated imports. This breaks everything
    // associated with a loop equivalently, whether it's part of it or depending
    // on a part of it.
    // TODO: Better identify cycles, maybe try to untangle them.
    for (auto& unit_info : unit_infos) {
      if (unit_info.imports_remaining > 0) {
        for (auto& package_imports : unit_info.package_imports) {
          for (auto* import_it = package_imports.imports.begin();
               import_it != package_imports.imports.end();) {
            if (*import_it->unit_info->unit->sem_ir) {
              // The import is checked, so continue.
              ++import_it;
            } else {
              // The import hasn't been checked, indicating a cycle.
              CARBON_DIAGNOSTIC(ImportCycleDetected, Error,
                                "Import cannot be used due to a cycle. Cycle "
                                "must be fixed to import.");
              unit_info.emitter.Emit(import_it->names.node_id,
                                     ImportCycleDetected);
              // Make this look the same as an import which wasn't found.
              package_imports.has_load_error = true;
              if (unit_info.api_for_impl == import_it->unit_info) {
                unit_info.api_for_impl = nullptr;
              }
              import_it = package_imports.imports.erase(import_it);
            }
          }
        }
      }
    }

    // Check the remaining file contents, which are probably broken due to
    // incomplete imports.
    for (auto& unit_info : unit_infos) {
      if (unit_info.imports_remaining > 0) {
        CheckParseTree(node_converters, unit_info, units.size(), vlog_stream);
      }
    }
  }
}

}  // namespace Carbon::Check