pkg/linter/lib/src/rules/unawaited_futures.dart

// Copyright (c) 2016, 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 'package:analyzer/dart/ast/ast.dart';
import 'package:analyzer/dart/ast/visitor.dart';
import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:analyzer/error/error.dart';

import '../analyzer.dart';
import '../extensions.dart';

const _desc =
    r'`Future` results in `async` function bodies must be '
    '`await`ed or marked `unawaited` using `dart:async`.';

class UnawaitedFutures extends LintRule {
  UnawaitedFutures()
    : super(name: LintNames.unawaited_futures, description: _desc);

  @override
  DiagnosticCode get diagnosticCode => LinterLintCode.unawaited_futures;

  @override
  void registerNodeProcessors(
    NodeLintRegistry registry,
    LinterContext context,
  ) {
    var visitor = _Visitor(this);
    registry.addExpressionStatement(this, visitor);
    registry.addCascadeExpression(this, visitor);
    registry.addInterpolationExpression(this, visitor);
  }
}

class _Visitor extends SimpleAstVisitor<void> {
  final LintRule rule;

  _Visitor(this.rule);

  @override
  void visitCascadeExpression(CascadeExpression node) {
    var sections = node.cascadeSections;
    for (var i = 0; i < sections.length; i++) {
      _visit(sections[i]);
    }
  }

  @override
  void visitExpressionStatement(ExpressionStatement node) {
    var expr = node.expression;
    if (expr is AssignmentExpression) return;

    var type = expr.staticType;
    if (type == null || !type.isOrImplementsFuture) {
      return;
    }

    // Ignore a couple of special known cases.
    if (_isFutureDelayedInstanceCreationWithComputation(expr) ||
        _isMapPutIfAbsentInvocation(expr)) {
      return;
    }

    if (expr.isAwaitNotRequired) {
      return;
    }

    if (_isEnclosedInAsyncFunctionBody(node)) {
      // Future expression statement that isn't awaited in an async function:
      // while this is legal, it's a very frequent sign of an error.
      rule.reportAtNode(node);
    }
  }

  @override
  void visitInterpolationExpression(InterpolationExpression node) {
    _visit(node.expression);
  }

  bool _isEnclosedInAsyncFunctionBody(AstNode node) {
    var enclosingFunctionBody = node.thisOrAncestorOfType<FunctionBody>();
    return enclosingFunctionBody?.isAsynchronous ?? false;
  }

  /// Detects `Future.delayed(duration, [computation])` creations with a
  /// computation.
  bool _isFutureDelayedInstanceCreationWithComputation(Expression expr) =>
      expr is InstanceCreationExpression &&
      (expr.staticType?.isDartAsyncFuture ?? false) &&
      expr.constructorName.name?.name == 'delayed' &&
      expr.argumentList.arguments.length == 2;

  bool _isMapClass(Element? e) =>
      e is ClassElement && e.name3 == 'Map' && e.library2.name3 == 'dart.core';

  /// Detects `Map.putIfAbsent` invocations.
  bool _isMapPutIfAbsentInvocation(Expression expr) =>
      expr is MethodInvocation &&
      expr.methodName.name == 'putIfAbsent' &&
      _isMapClass(expr.methodName.element?.enclosingElement);

  void _visit(Expression expr) {
    if (expr.isAwaitNotRequired) {
      return;
    }

    var type = expr.staticType;
    if (type == null || !type.isOrImplementsFuture) {
      return;
    }

    if (_isEnclosedInAsyncFunctionBody(expr) && expr is! AssignmentExpression) {
      rule.reportAtNode(expr);
    }
  }
}

extension on DartType {
  /// Whether this type is `Future` from dart:async, or is a subtype thereof.
  bool get isOrImplementsFuture {
    var typeElement = element3;
    if (typeElement is! InterfaceElement) return false;
    return isDartAsyncFuture ||
        typeElement.allSupertypes.any((t) => t.isDartAsyncFuture);
  }
}