special case assert initializers and const contexts in prefer_is_empty (#2122)

* special case assert initializers and  const contexts

* ws

* + test

* more const context tests

* check in constructor initializer

Author: pq

lib/src/rules/prefer_is_empty.dart

@@ -3,6 +3,8 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/ast/ast.dart';
+// ignore: implementation_imports
+import 'package:analyzer/src/dart/ast/ast.dart' show ExpressionImpl;
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ -129,6 +131,24 @@ class _Visitor extends SimpleAstVisitor<void> {
     }
     final binaryExpression = search as BinaryExpression;
 
+    // Don't lint if we're in a const constructor initializer.
+    final constructorInitializer =
+        search.parent.thisOrAncestorOfType<ConstructorInitializer>();
+    if (constructorInitializer != null) {
+      final constructorDecl =
+          constructorInitializer.parent as ConstructorDeclaration;
+      if (constructorDecl.constKeyword != null) {
+        return;
+      }
+    }
+
+    // Or in a const context.
+    // See: https://github.com/dart-lang/linter/issues/1719
+    final impl = binaryExpression as ExpressionImpl;
+    if (impl.inConstantContext) {
+      return;
+    }
+
     final operator = binaryExpression.operator;
 
     // Comparing constants with length.

test/rules/prefer_is_empty.dart

@@ -4,17 +4,54 @@
 
 // test w/ `pub run test -N prefer_is_empty`
 
+const l = '';
+const bool empty = l.length == 0; //OK
+
+class A {
+  final List<String> a;
+  const A(this.a) : assert(a.length > 0); //OK
+}
+
+class B {
+  final String b;
+  const B(this.b) : assert(b.length > 0); //OK
+}
+
+class C {
+  final bool empty;
+  const C(dynamic l) : empty = l.length == 0; //OK
+}
+
+class D {
+  final bool emptyString;
+  D(String s) : emptyString = s.length == 0; //LINT
+}
+
+class E {
+  final bool empty;
+  const E(dynamic l) : empty = l.length == 0; // OK
+  const E.a(this.empty);
+  const E.b(dynamic l) : this.a(l.length == 0); // OK
+}
+
+class F {
+  // ignore: avoid_positional_boolean_parameters
+  const F(bool b);
+}
+
+class G extends F {
+  const G(dynamic l) : super(l.length == 0); // OK
+}
+
 const int zero = 0;
 Iterable<int> list = [];
 Map map = {};
 
 Iterable get iterable => [];
 
-typedef Iterable F();
+typedef Fun = Iterable Function();
 
-F a() {
-  return () => [];
-}
+Fun a() => () => [];
 
 bool le = list.length > 0; //LINT
 bool le2 = [].length > 0; //LINT
@@ -30,14 +67,14 @@ bool mixed = list.length + map.length > 0; //OK
 Iterable length = [];
 bool ok = length.first > 0; // OK
 
-condition() {
+void condition() {
   final int a = list.length > 0 ? list.first : 0; //LINT
   list..length;
 }
 
 bool le7 = [].length > 1; //OK
 
-testOperators() {
+void testOperators() {
   [].length == 0; // LINT
   [].length != 0; // LINT
   [].length > 0; // LINT