Add lint implicit_call_tearoffs (#3592)

In an effort to simplify the language we will consider removing the
implicit `call` tearoff which can coerce an instance of any class which
defines a `call` method into a `Function` type. Authors should
explicitly add the `.call` so there is less magic.

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 1.29.0-dev
+
+- new lint: `implicit_call_tearoffs`
+
 # 1.28.0
 
 - update `avoid_redundant_argument_values` to work with enum declarations

example/all.yaml

@@ -78,6 +78,7 @@ linter:
     - flutter_style_todos
     - hash_and_equals
     - implementation_imports
+    - implicit_call_tearoffs
     - iterable_contains_unrelated_type
     - join_return_with_assignment
     - leading_newlines_in_multiline_strings

lib/src/rules.dart

@@ -80,6 +80,7 @@ import 'rules/file_names.dart';
 import 'rules/flutter_style_todos.dart';
 import 'rules/hash_and_equals.dart';
 import 'rules/implementation_imports.dart';
+import 'rules/implicit_call_tearoffs.dart';
 import 'rules/invariant_booleans.dart';
 import 'rules/iterable_contains_unrelated_type.dart';
 import 'rules/join_return_with_assignment.dart';
@@ -297,6 +298,7 @@ void registerLintRules({bool inTestMode = false}) {
     ..register(FlutterStyleTodos())
     ..register(HashAndEquals())
     ..register(ImplementationImports())
+    ..register(ImplicitCallTearoffs())
     ..register(InvariantBooleans())
     ..register(IterableContainsUnrelatedType())
     ..register(JoinReturnWithAssignment())

lib/src/rules/implicit_call_tearoffs.dart

@@ -0,0 +1,72 @@
+// Copyright (c) 2022, 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 '../analyzer.dart';
+
+const _desc =
+    r'Explicitly tear-off `call` methods when using an object as a Function.';
+
+const _details = r'''
+**DO**
+Explicitly tear off `.call` methods from objects when assigning to a Function
+type. There is less magic with an explicit tear off. Future language versions
+may remove the implicit call tear off.
+
+**BAD:**
+```dart
+class Callable {
+  void call() {}
+}
+void callIt(void Function() f) {
+  f();
+}
+
+callIt(Callable());
+```
+
+**GOOD:**
+```dart
+class Callable {
+  void call() {}
+}
+void callIt(void Function() f) {
+  f();
+}
+
+callIt(Callable().call);
+```
+
+''';
+
+class ImplicitCallTearoffs extends LintRule {
+  ImplicitCallTearoffs()
+      : super(
+          name: 'implicit_call_tearoffs',
+          description: _desc,
+          details: _details,
+          group: Group.style,
+          maturity: Maturity.experimental,
+        );
+
+  @override
+  void registerNodeProcessors(
+      NodeLintRegistry registry, LinterContext context) {
+    var visitor = _Visitor(this);
+    registry.addImplicitCallReference(this, visitor);
+  }
+}
+
+class _Visitor extends SimpleAstVisitor<void> {
+  final LintRule rule;
+
+  _Visitor(this.rule);
+
+  @override
+  void visitImplicitCallReference(ImplicitCallReference node) {
+    rule.reportLint(node);
+  }
+}

lib/src/version.dart

@@ -3,4 +3,4 @@
 // BSD-style license that can be found in the LICENSE file.
 
 /// Package version.  Synchronized w/ pubspec.yaml.
-const String version = '1.28.0';
+const String version = '1.29.0-dev';

pubspec.yaml

@@ -1,5 +1,5 @@
 name: linter
-version: 1.28.0
+version: 1.29.0-dev
 
 description: >-
   The implementation of the lint rules supported by the analyzer framework.

test_data/rules/implicit_call_tearoffs.dart

@@ -0,0 +1,123 @@
+// Copyright (c) 2022, 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.
+
+// test w/ `dart test -N implicit_call_tearoffs`
+
+class C {
+  void call() {}
+  void other() {}
+}
+
+class C2 {
+  void call<T>(T arg) {}
+}
+
+void callIt(void Function() f) {
+  f();
+}
+
+void callIt2(void Function(int) f) {
+  f(0);
+}
+
+void Function() r1() => C(); // LINT
+void Function() r2() => C().call; // OK
+void Function() r3(C c) => c; // LINT
+void Function() r4(C c) => c.call; // OK
+
+void Function() r5(C? c1, C c2) {
+  return c1 ?? c2; // LINT
+}
+
+void Function() r6(C? c1, C c2) {
+  return c1?.call ?? c2.call; // OK
+}
+
+void Function() r7() {
+  return C()..other(); // LINT
+}
+
+void Function() r8() {
+  return (C()..other()).call; // OK
+}
+
+List<void Function()> r9(C c) {
+  return [c]; // LINT
+}
+
+List<void Function()> r10(C c) {
+  return [c.call]; // OK
+}
+
+void Function(int) r11(C2 c) => c; // LINT
+void Function(int) r12(C2 c) => c.call; // OK
+
+void main() {
+  callIt(C()); // LINT
+  callIt(C().call); // OK
+  Function f1 = C(); // LINT
+  Function f2 = C().call; // OK
+  void Function() f3 = C(); // LINT
+  void Function() f4 = C().call; // OK
+
+  final c = C();
+  callIt(c); // LINT
+  callIt(c.call); // OK
+  Function f5 = c; // LINT
+  Function f6 = c.call; // OK
+  void Function() f7 = c; // LINT
+  void Function() f8 = c.call; // OK
+
+  <void Function()>[
+    C(), // LINT
+    C().call, //OK
+    c, // LINT
+    c.call, // OK
+  ];
+
+  callIt2(C2()); // LINT
+  callIt2(C2().call); // OK
+  callIt2(C2()<int>); // LINT
+  callIt2(C2().call<int>); // OK
+  Function f9 = C2(); // LINT
+  Function f10 = C2().call; // OK
+  Function f11 = C2()<int>; // LINT
+  Function f12 = C2().call<int>; // OK
+  void Function<T>(T) f13 = C2(); // LINT
+  void Function<T>(T) f14 = C2().call; // OK
+  void Function(int) f15 = C2(); // LINT
+  void Function(int) f16 = C2().call; // OK
+  void Function(int) f17 = C2()<int>; // LINT
+  void Function(int) f18 = C2().call<int>; // OK
+
+  final c2 = C2();
+  callIt2(c2); // LINT
+  callIt2(c2.call); // OK
+  callIt2(c2<int>); // LINT
+  callIt2(c2.call<int>); // OK
+  Function f19 = c2; // LINT
+  Function f20 = c2.call; // OK
+  Function f21 = c2<int>; // LINT
+  Function f22 = c2.call<int>; // OK
+  void Function<T>(T) f23 = c2; // LINT
+  void Function<T>(T) f24 = c2.call; // OK
+  void Function(int) f25 = c2; // LINT
+  void Function(int) f26 = c2.call; // OK
+  void Function(int) f27 = c2<int>; // LINT
+  void Function(int) f28 = c2.call<int>; // OK
+
+  <void Function(int)>[
+    C2(), // LINT
+    C2().call, //OK
+    C2()<int>, // LINT
+    C2().call<int>, //OK
+    c2, // LINT
+    c2.call, // OK
+    c2<int>, // LINT
+    c2.call<int>, // OK
+  ];
+
+  C2()<int>; // LINT
+  c2<int>; // LINT
+}