pkg/analyzer_plugin/test/integration/support/integration_tests.dart

// Copyright (c) 2017, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

import 'dart:async';
import 'dart:collection';
import 'dart:convert';
import 'dart:io';

import 'package:analyzer_plugin/protocol/protocol_common.dart';
import 'package:analyzer_plugin/protocol/protocol_generated.dart';
import 'package:path/path.dart';
import 'package:test/test.dart';

import 'integration_test_methods.dart';
import 'protocol_matchers.dart';

const Matcher isBool = TypeMatcher<bool>();

const Matcher isInt = TypeMatcher<int>();

const Matcher isNotification = MatchesJsonObject(
    'notification', {'event': isString},
    optionalFields: {'params': isMap});

const Matcher isObject = isMap;

const Matcher isString = TypeMatcher<String>();

final Matcher isResponse = MatchesJsonObject('response', {'id': isString},
    optionalFields: {'result': anything, 'error': isRequestError});

Matcher isListOf(Matcher elementMatcher) => _ListOf(elementMatcher);

Matcher isMapOf(Matcher keyMatcher, Matcher valueMatcher) =>
    _MapOf(keyMatcher, valueMatcher);

Matcher isOneOf(List<Matcher> choiceMatchers) => _OneOf(choiceMatchers);

/// Assert that [actual] matches [matcher].
void outOfTestExpect(actual, Matcher matcher,
    {String? reason, skip, bool verbose = false}) {
  var matchState = {};
  try {
    if (matcher.matches(actual, matchState)) return;
  } catch (e, trace) {
    reason ??= '${(e is String) ? e : e.toString()} at $trace';
  }
  fail(_defaultFailFormatter(actual, matcher, reason, matchState, verbose));
}

String _defaultFailFormatter(
    actual, Matcher matcher, String? reason, Map matchState, bool verbose) {
  var description = StringDescription();
  description.add('Expected: ').addDescriptionOf(matcher).add('\n');
  description.add('  Actual: ').addDescriptionOf(actual).add('\n');

  var mismatchDescription = StringDescription();
  matcher.describeMismatch(actual, mismatchDescription, matchState, verbose);

  if (mismatchDescription.length > 0) {
    description.add('   Which: $mismatchDescription\n');
  }
  if (reason != null) description.add(reason).add('\n');
  return description.toString();
}

/// Type of closures used by LazyMatcher.
typedef MatcherCreator = Matcher Function();

/// Type of closures used by MatchesJsonObject to record field mismatches.
typedef MismatchDescriber = Description Function(
    Description mismatchDescription);

/// Type of callbacks used to process notifications.
typedef NotificationProcessor = void Function(String event, Map params);

/// Base class for analysis server integration tests.
abstract class AbstractAnalysisServerIntegrationTest
    extends IntegrationTestMixin {
  /// Amount of time to give the server to respond to a shutdown request before
  /// forcibly terminating it.
  static const Duration SHUTDOWN_TIMEOUT = Duration(seconds: 5);

  /// Connection to the analysis server.
  @override
  final Server server = Server();

  /// Temporary directory in which source files can be stored.
  Directory? sourceDirectory;

  /// Map from file path to the list of analysis errors which have most recently
  /// been received for the file.
  Map<String, List<AnalysisError>> currentAnalysisErrors =
      HashMap<String, List<AnalysisError>>();

  /// True if the teardown process should skip sending a "server.shutdown"
  /// request (e.g. because the server is known to have already shutdown).
  bool skipShutdown = false;

  AbstractAnalysisServerIntegrationTest() {
    initializeInttestMixin();
  }

//  /**
//   * Return a future which will complete when a 'server.status' notification is
//   * received from the server with 'analyzing' set to false.
//   *
//   * The future will only be completed by 'server.status' notifications that are
//   * received after this function call. So it is safe to use this getter
//   * multiple times in one test; each time it is used it will wait afresh for
//   * analysis to finish.
//   */
//  Future get analysisFinished {
//    Completer completer = new Completer();
//    StreamSubscription subscription;
//    // This will only work if the caller has already subscribed to
//    // SERVER_STATUS (e.g. using sendServerSetSubscriptions(['STATUS']))
//    outOfTestExpect(_subscribedToServerStatus, isTrue);
//    subscription = onServerStatus.listen((PluginStatusParams params) {
//      if (params.analysis != null && !params.analysis.isAnalyzing) {
//        completer.complete(params);
//        subscription.cancel();
//      }
//    });
//    return completer.future;
//  }

  /// Print out any messages exchanged with the server. If some messages have
  /// already been exchanged with the server, they are printed out immediately.
  void debugStdio() {
    server.debugStdio();
  }

  /// The server is automatically started before every test, and a temporary
  /// [sourceDirectory] is created.
  Future setUp() {
    sourceDirectory = Directory.systemTemp.createTempSync('analysisServer');

    onAnalysisErrors.listen((AnalysisErrorsParams params) {
      currentAnalysisErrors[params.file] = params.errors;
    });
    var serverConnected = Completer();
    // TODO(brianwilkerson) Implement this.
//    onServerConnected.listen((_) {
//      outOfTestExpect(serverConnected.isCompleted, isFalse);
//      serverConnected.complete();
//    });
    onPluginError.listen((PluginErrorParams params) {
      // A plugin error should never happen during an integration test.
      fail('${params.message}\n${params.stackTrace}');
    });
    return startServer().then((_) {
      server.listenToOutput(dispatchNotification);
      server.exitCode.then((_) {
        skipShutdown = true;
      });
      return serverConnected.future;
    });
  }

  /// If [skipShutdown] is not set, shut down the server.
  Future shutdownIfNeeded() {
    if (skipShutdown) {
      return Future.value();
    }
    // Give the server a short time to comply with the shutdown request; if it
    // doesn't exit, then forcibly terminate it.
    sendPluginShutdown();
    return server.exitCode.timeout(SHUTDOWN_TIMEOUT, onTimeout: () {
      return server.kill('server failed to exit');
    });
  }

  /// Convert the given [relativePath] to an absolute path, by interpreting it
  /// relative to [sourceDirectory]. On Windows any forward slashes in
  /// [relativePath] are converted to backslashes.
  String sourcePath(String relativePath) {
    return join(sourceDirectory!.path, relativePath.replaceAll('/', separator));
  }

  /// Send the server an 'analysis.setAnalysisRoots' command directing it to
  /// analyze [sourceDirectory]. If [subscribeStatus] is true (the default),
  /// then also enable [SERVER_STATUS] notifications so that [analysisFinished]
  /// can be used.
  Future standardAnalysisSetup({bool subscribeStatus = true}) {
    var futures = <Future>[];
    // TODO(brianwilkerson) Implement this.
//    if (subscribeStatus) {
//      futures.add(sendServerSetSubscriptions([ServerService.STATUS]));
//    }
//    futures.add(sendAnalysisSetAnalysisRoots([sourceDirectory.path], []));
    return Future.wait(futures);
  }

  /// Start [server].
  Future startServer(
          {bool checked = true, int? diagnosticPort, int? servicesPort}) =>
      server.start(
          checked: checked,
          diagnosticPort: diagnosticPort,
          servicesPort: servicesPort);

  /// After every test, the server is stopped and [sourceDirectory] is deleted.
  Future tearDown() {
    return shutdownIfNeeded().then((_) {
      sourceDirectory!.deleteSync(recursive: true);
    });
  }

  /// Write a source file with the given absolute [pathname] and [contents].
  ///
  /// If the file didn't previously exist, it is created. If it did, it is
  /// overwritten.
  ///
  /// Parent directories are created as necessary.
  ///
  /// Return a normalized path to the file (with symbolic links resolved).
  String writeFile(String pathname, String contents) {
    Directory(dirname(pathname)).createSync(recursive: true);
    var file = File(pathname);
    file.writeAsStringSync(contents);
    return file.resolveSymbolicLinksSync();
  }
}

/// Wrapper class for Matcher which doesn't create the underlying Matcher object
/// until it is needed. This is necessary in order to create matchers that can
/// refer to themselves (so that recursive data structures can be represented).
class LazyMatcher implements Matcher {
  /// Callback that will be used to create the matcher the first time it is
  /// needed.
  final MatcherCreator _creator;

  /// The matcher returned by [_creator], if it has already been called.
  /// Otherwise null.
  Matcher? _wrappedMatcher;

  LazyMatcher(this._creator);

  @override
  Description describe(Description description) {
    _createMatcher();
    return _wrappedMatcher!.describe(description);
  }

  @override
  Description describeMismatch(
      item, Description mismatchDescription, Map matchState, bool verbose) {
    _createMatcher();
    return _wrappedMatcher!
        .describeMismatch(item, mismatchDescription, matchState, verbose);
  }

  @override
  bool matches(item, Map matchState) {
    _createMatcher();
    return _wrappedMatcher!.matches(item, matchState);
  }

  /// Create the wrapped matcher object, if it hasn't been created already.
  void _createMatcher() {
    _wrappedMatcher ??= _creator();
  }
}

/// Matcher that matches a String drawn from a limited set.
class MatchesEnum extends Matcher {
  /// Short description of the expected type.
  final String description;

  /// The set of enum values that are allowed.
  final List<String> allowedValues;

  const MatchesEnum(this.description, this.allowedValues);

  @override
  Description describe(Description description) =>
      description.add(this.description);

  @override
  bool matches(item, Map matchState) {
    return allowedValues.contains(item);
  }
}

/// Matcher that matches a JSON object, with a given set of required and
/// optional fields, and their associated types (expressed as [Matcher]s).
class MatchesJsonObject extends _RecursiveMatcher {
  /// Short description of the expected type.
  final String description;

  /// Fields that are required to be in the JSON object, and [Matcher]s describing
  /// their expected types.
  final Map<String, Matcher>? requiredFields;

  /// Fields that are optional in the JSON object, and [Matcher]s describing
  /// their expected types.
  final Map<String, Matcher>? optionalFields;

  const MatchesJsonObject(this.description, this.requiredFields,
      {this.optionalFields});

  @override
  Description describe(Description description) =>
      description.add(this.description);

  @override
  void populateMismatches(item, List<MismatchDescriber> mismatches) {
    if (item is! Map) {
      mismatches.add(simpleDescription('is not a map'));
      return;
    }
    final requiredFields = this.requiredFields;
    if (requiredFields != null) {
      requiredFields.forEach((String key, Matcher valueMatcher) {
        if (!item.containsKey(key)) {
          mismatches.add((Description mismatchDescription) =>
              mismatchDescription
                  .add('is missing field ')
                  .addDescriptionOf(key)
                  .add(' (')
                  .addDescriptionOf(valueMatcher)
                  .add(')'));
        } else {
          _checkField(key, item[key], valueMatcher, mismatches);
        }
      });
    }
    final optionalFields = this.optionalFields;
    item.forEach((key, value) {
      if (requiredFields != null && requiredFields.containsKey(key)) {
        // Already checked this field
      } else if (optionalFields != null && optionalFields.containsKey(key)) {
        _checkField(key as String, value, optionalFields[key]!, mismatches);
      } else {
        mismatches.add((Description mismatchDescription) => mismatchDescription
            .add('has unexpected field ')
            .addDescriptionOf(key));
      }
    });
  }

  /// Check the type of a field called [key], having value [value], using
  /// [valueMatcher]. If it doesn't match, record a closure in [mismatches]
  /// which can describe the mismatch.
  void _checkField(String key, value, Matcher valueMatcher,
      List<MismatchDescriber> mismatches) {
    checkSubstructure(
        value,
        valueMatcher,
        mismatches,
        (Description description) =>
            description.add('field ').addDescriptionOf(key));
  }
}

/// Instances of the class [Server] manage a connection to a server process, and
/// facilitate communication to and from the server.
class Server {
  /// Server process object, or null if server hasn't been started yet.
  Process? _process;

  /// Commands that have been sent to the server but not yet acknowledged, and
  /// the [Completer] objects which should be completed when acknowledgement is
  /// received.
  final Map<String, Completer> _pendingCommands = <String, Completer>{};

  /// Number which should be used to compute the 'id' to send in the next command
  /// sent to the server.
  int _nextId = 0;

  /// Messages which have been exchanged with the server; we buffer these
  /// up until the test finishes, so that they can be examined in the debugger
  /// or printed out in response to a call to [debugStdio].
  final List<String> _recordedStdio = <String>[];

  /// True if we are currently printing out messages exchanged with the server.
  bool _debuggingStdio = false;

  /// True if we've received bad data from the server, and we are aborting the
  /// test.
  bool _receivedBadDataFromServer = false;

  /// Stopwatch that we use to generate timing information for debug output.
  final Stopwatch _time = Stopwatch();

  /// The [currentElapseTime] at which the last communication was received from the server
  /// or `null` if no communication has been received.
  double? lastCommunicationTime;

  /// The current elapse time (seconds) since the server was started.
  double get currentElapseTime => _time.elapsedTicks / _time.frequency;

  /// Future that completes when the server process exits.
  Future<int> get exitCode => _process!.exitCode;

  /// Print out any messages exchanged with the server. If some messages have
  /// already been exchanged with the server, they are printed out immediately.
  void debugStdio() {
    if (_debuggingStdio) {
      return;
    }
    _debuggingStdio = true;
    for (var line in _recordedStdio) {
      print(line);
    }
  }

  /// Find the root directory of the analysis_server package by proceeding
  /// upward to the 'test' dir, and then going up one more directory.
  String findRoot(String pathname) {
    while (!['benchmark', 'test'].contains(basename(pathname))) {
      var parent = dirname(pathname);
      if (parent.length >= pathname.length) {
        throw Exception("Can't find root directory");
      }
      pathname = parent;
    }
    return dirname(pathname);
  }

  /// Return a future that will complete when all commands that have been sent
  /// to the server so far have been flushed to the OS buffer.
  Future flushCommands() {
    return _process!.stdin.flush();
  }

  /// Stop the server.
  Future<int> kill(String reason) {
    debugStdio();
    _recordStdio('FORCIBLY TERMINATING PROCESS: $reason');
    _process!.kill();
    return _process!.exitCode;
  }

  /// Start listening to output from the server, and deliver notifications to
  /// [notificationProcessor].
  void listenToOutput(NotificationProcessor notificationProcessor) {
    _process!.stdout
        .transform((Utf8Codec()).decoder)
        .transform(LineSplitter())
        .listen((String line) {
      lastCommunicationTime = currentElapseTime;
      var trimmedLine = line.trim();
      if (trimmedLine.startsWith('The Dart VM service is listening on ')) {
        return;
      }
      _recordStdio('RECV: $trimmedLine');
      var message;
      try {
        message = json.decoder.convert(trimmedLine);
      } catch (exception) {
        _badDataFromServer('JSON decode failure: $exception');
        return;
      }
      outOfTestExpect(message, isMap);
      var messageAsMap = message as Map;
      if (messageAsMap.containsKey('id')) {
        outOfTestExpect(messageAsMap['id'], isString);
        var id = message['id'] as String;
        var completer = _pendingCommands[id];
        if (completer == null) {
          fail('Unexpected response from server: id=$id');
        } else {
          _pendingCommands.remove(id);
        }
        if (messageAsMap.containsKey('error')) {
          completer.completeError(ServerErrorMessage(messageAsMap));
        } else {
          completer.complete(messageAsMap['result']);
        }
        // Check that the message is well-formed. We do this after calling
        // completer.complete() or completer.completeError() so that we don't
        // stall the test in the event of an error.
        outOfTestExpect(message, isResponse);
      } else {
        // Message is a notification. It should have an event and possibly
        // params.
        outOfTestExpect(messageAsMap, contains('event'));
        outOfTestExpect(messageAsMap['event'], isString);
        notificationProcessor(
            messageAsMap['event'] as String, messageAsMap['params'] as Map);
        // Check that the message is well-formed. We do this after calling
        // notificationController.add() so that we don't stall the test in the
        // event of an error.
        outOfTestExpect(message, isNotification);
      }
    });
    _process!.stderr
        .transform((Utf8Codec()).decoder)
        .transform(LineSplitter())
        .listen((String line) {
      var trimmedLine = line.trim();
      _recordStdio('ERR:  $trimmedLine');
      _badDataFromServer('Message received on stderr', silent: true);
    });
  }

  /// Send a command to the server. An 'id' will be automatically assigned.
  /// The returned [Future] will be completed when the server acknowledges the
  /// command with a response. If the server acknowledges the command with a
  /// normal (non-error) response, the future will be completed with the 'result'
  /// field from the response. If the server acknowledges the command with an
  /// error response, the future will be completed with an error.
  Future send(String method, Map<String, dynamic>? params) {
    var id = '${_nextId++}';
    var command = <String, dynamic>{'id': id, 'method': method};
    if (params != null) {
      command['params'] = params;
    }
    var completer = Completer();
    _pendingCommands[id] = completer;
    var line = json.encode(command);
    _recordStdio('SEND: $line');
    _process!.stdin.add(utf8.encoder.convert('$line\n'));
    return completer.future;
  }

  /// Start the server. If [debugServer] is `true`, the server will be started
  /// with "--debug", allowing a debugger to be attached. If [profileServer] is
  /// `true`, the server will be started with "--observe" and
  /// "--pause-isolates-on-exit", allowing the observatory to be used.
  Future start(
      {bool checked = true,
      bool debugServer = false,
      int? diagnosticPort,
      bool profileServer = false,
      String? sdkPath,
      int? servicesPort,
      bool useAnalysisHighlight2 = false}) {
    if (_process != null) {
      throw Exception('Process already started');
    }
    _time.start();
    var dartBinary = Platform.executable;
    var rootDir =
        findRoot(Platform.script.toFilePath(windows: Platform.isWindows));
    var serverPath = normalize(join(rootDir, 'bin', 'server.dart'));
    var arguments = <String>[];
    //
    // Add VM arguments.
    //
    if (debugServer) {
      arguments.add('--debug');
    }
    if (profileServer) {
      if (servicesPort == null) {
        arguments.add('--observe');
      } else {
        arguments.add('--observe=$servicesPort');
      }
      arguments.add('--pause-isolates-on-exit');
    } else if (servicesPort != null) {
      arguments.add('--enable-vm-service=$servicesPort');
    }
    if (Platform.packageConfig != null) {
      arguments.add('--packages=${Platform.packageConfig}');
    }
    if (checked) {
      arguments.add('--checked');
    }
    //
    // Add the server executable.
    //
    arguments.add(serverPath);
    //
    // Add server arguments.
    //
    if (diagnosticPort != null) {
      arguments.add('--port');
      arguments.add(diagnosticPort.toString());
    }
    if (sdkPath != null) {
      arguments.add('--sdk=$sdkPath');
    }
    if (useAnalysisHighlight2) {
      arguments.add('--useAnalysisHighlight2');
    }
//    print('Launching $serverPath');
//    print('$dartBinary ${arguments.join(' ')}');
    return Process.start(dartBinary, arguments).then((Process process) {
      _process = process;
      process.exitCode.then((int code) {
        if (code != 0) {
          _badDataFromServer('server terminated with exit code $code');
        }
      });
    });
  }

  /// Deal with bad data received from the server.
  void _badDataFromServer(String details, {bool silent = false}) {
    if (!silent) {
      _recordStdio('BAD DATA FROM SERVER: $details');
    }
    if (_receivedBadDataFromServer) {
      // We're already dealing with it.
      return;
    }
    _receivedBadDataFromServer = true;
    debugStdio();
    // Give the server 1 second to continue outputting bad data before we kill
    // the test. This is helpful if the server has had an unhandled exception
    // and is outputting a stacktrace, because it ensures that we see the
    // entire stacktrace. Use expectAsync() to prevent the test from
    // ending during this 1 second.
    Future.delayed(Duration(seconds: 1), expectAsync0(() {
      fail('Bad data received from server: $details');
    }));
  }

  /// Record a message that was exchanged with the server, and print it out if
  /// [debugStdio] has been called.
  void _recordStdio(String line) {
    var elapsedTime = currentElapseTime;
    line = '$elapsedTime: $line';
    if (_debuggingStdio) {
      print(line);
    }
    _recordedStdio.add(line);
  }
}

/// An error result from a server request.
class ServerErrorMessage {
  final Map message;

  ServerErrorMessage(this.message);

  dynamic get error => message['error'];
}

/// Matcher that matches a list of objects, each of which satisfies the given
/// matcher.
class _ListOf extends Matcher {
  /// Matcher which every element of the list must satisfy.
  final Matcher elementMatcher;

  /// Iterable matcher which we use to test the contents of the list.
  final Matcher iterableMatcher;

  _ListOf(Matcher elementMatcher)
      : elementMatcher = elementMatcher,
        iterableMatcher = everyElement(elementMatcher);

  @override
  Description describe(Description description) =>
      description.add('List of ').addDescriptionOf(elementMatcher);

  @override
  Description describeMismatch(
      item, Description mismatchDescription, Map matchState, bool verbose) {
    if (item is! List) {
      return super
          .describeMismatch(item, mismatchDescription, matchState, verbose);
    } else {
      return iterableMatcher.describeMismatch(
          item, mismatchDescription, matchState, verbose);
    }
  }

  @override
  bool matches(item, Map matchState) {
    if (item is! List) {
      return false;
    }
    return iterableMatcher.matches(item, matchState);
  }
}

/// Matcher that matches a map of objects, where each key/value pair in the
/// map satisies the given key and value matchers.
class _MapOf extends _RecursiveMatcher {
  /// Matcher which every key in the map must satisfy.
  final Matcher keyMatcher;

  /// Matcher which every value in the map must satisfy.
  final Matcher valueMatcher;

  _MapOf(this.keyMatcher, this.valueMatcher);

  @override
  Description describe(Description description) => description
      .add('Map from ')
      .addDescriptionOf(keyMatcher)
      .add(' to ')
      .addDescriptionOf(valueMatcher);

  @override
  void populateMismatches(item, List<MismatchDescriber> mismatches) {
    if (item is! Map) {
      mismatches.add(simpleDescription('is not a map'));
      return;
    }
    item.forEach((key, value) {
      checkSubstructure(
          key,
          keyMatcher,
          mismatches,
          (Description description) =>
              description.add('key ').addDescriptionOf(key));
      checkSubstructure(
          value,
          valueMatcher,
          mismatches,
          (Description description) =>
              description.add('field ').addDescriptionOf(key));
    });
  }
}

/// Matcher that matches a union of different types, each of which is described
/// by a matcher.
class _OneOf extends Matcher {
  /// Matchers for the individual choices.
  final List<Matcher> choiceMatchers;

  _OneOf(this.choiceMatchers);

  @override
  Description describe(Description description) {
    for (var i = 0; i < choiceMatchers.length; i++) {
      if (i != 0) {
        if (choiceMatchers.length == 2) {
          description = description.add(' or ');
        } else {
          description = description.add(', ');
          if (i == choiceMatchers.length - 1) {
            description = description.add('or ');
          }
        }
      }
      description = description.addDescriptionOf(choiceMatchers[i]);
    }
    return description;
  }

  @override
  bool matches(item, Map matchState) {
    for (var choiceMatcher in choiceMatchers) {
      var subState = {};
      if (choiceMatcher.matches(item, subState)) {
        return true;
      }
    }
    return false;
  }
}

/// Base class for matchers that operate by recursing through the contents of
/// an object.
abstract class _RecursiveMatcher extends Matcher {
  const _RecursiveMatcher();

  /// Check the type of a substructure whose value is [item], using [matcher].
  /// If it doesn't match, record a closure in [mismatches] which can describe
  /// the mismatch. [describeSubstructure] is used to describe which
  /// substructure did not match.
  void checkSubstructure(
      item,
      Matcher matcher,
      List<MismatchDescriber> mismatches,
      Description Function(Description description) describeSubstructure) {
    var subState = {};
    if (!matcher.matches(item, subState)) {
      mismatches.add((Description mismatchDescription) {
        mismatchDescription = mismatchDescription.add('contains malformed ');
        mismatchDescription = describeSubstructure(mismatchDescription);
        mismatchDescription =
            mismatchDescription.add(' (should be ').addDescriptionOf(matcher);
        var subDescription = matcher
            .describeMismatch(item, StringDescription(), subState, false)
            .toString();
        if (subDescription.isNotEmpty) {
          mismatchDescription =
              mismatchDescription.add('; ').add(subDescription);
        }
        return mismatchDescription.add(')');
      });
    }
  }

  @override
  Description describeMismatch(
      item, Description mismatchDescription, Map matchState, bool verbose) {
    var mismatches = matchState['mismatches'] as List<MismatchDescriber>?;
    if (mismatches != null) {
      for (var i = 0; i < mismatches.length; i++) {
        var mismatch = mismatches[i];
        if (i > 0) {
          if (mismatches.length == 2) {
            mismatchDescription = mismatchDescription.add(' and ');
          } else if (i == mismatches.length - 1) {
            mismatchDescription = mismatchDescription.add(', and ');
          } else {
            mismatchDescription = mismatchDescription.add(', ');
          }
        }
        mismatchDescription = mismatch(mismatchDescription);
      }
      return mismatchDescription;
    } else {
      return super
          .describeMismatch(item, mismatchDescription, matchState, verbose);
    }
  }

  @override
  bool matches(item, Map matchState) {
    var mismatches = <MismatchDescriber>[];
    populateMismatches(item, mismatches);
    if (mismatches.isEmpty) {
      return true;
    } else {
      addStateInfo(matchState, {'mismatches': mismatches});
      return false;
    }
  }

  /// Populate [mismatches] with descriptions of all the ways in which [item]
  /// does not match.
  void populateMismatches(item, List<MismatchDescriber> mismatches);

  /// Create a [MismatchDescriber] describing a mismatch with a simple string.
  MismatchDescriber simpleDescription(String description) =>
      (Description mismatchDescription) => mismatchDescription.add(description);
}