New whitespace_around_ops lint (#249).

Closes https://github.com/dart-lang/linter/issues/249.

BUG=
[email protected]

Review URL: https://codereview.chromium.org//2159123003 .

Author: pq

lib/src/rules.dart

@@ -28,9 +28,9 @@ import 'package:linter/src/rules/empty_statements.dart';
 import 'package:linter/src/rules/hash_and_equals.dart';
 import 'package:linter/src/rules/implementation_imports.dart';
 import 'package:linter/src/rules/iterable_contains_unrelated_type.dart';
-import 'package:linter/src/rules/list_remove_unrelated_type.dart';
 import 'package:linter/src/rules/library_names.dart';
 import 'package:linter/src/rules/library_prefixes.dart';
+import 'package:linter/src/rules/list_remove_unrelated_type.dart';
 import 'package:linter/src/rules/non_constant_identifier_names.dart';
 import 'package:linter/src/rules/one_member_abstracts.dart';
 import 'package:linter/src/rules/only_throw_errors.dart';
@@ -52,6 +52,7 @@ import 'package:linter/src/rules/unawaited_futures.dart';
 import 'package:linter/src/rules/unnecessary_brace_in_string_interp.dart';
 import 'package:linter/src/rules/unnecessary_getters_setters.dart';
 import 'package:linter/src/rules/unrelated_type_equality_checks.dart';
+import 'package:linter/src/rules/whitespace_around_ops.dart';
 
 final Registry ruleRegistry = new Registry()
   ..register(new AlwaysDeclareReturnTypes())
@@ -99,8 +100,8 @@ final Registry ruleRegistry = new Registry()
   // Disabled pending fix: https://github.com/dart-lang/linter/issues/35
   //..register(new UnnecessaryGetters())
   ..register(new UnnecessaryGettersSetters())
-  ..register(new UnrelatedTypeEqualityChecks());
-
+  ..register(new UnrelatedTypeEqualityChecks())
+  ..register(new WhitespaceAroundOps());
 
 /// Registry of contributed lint rules.
 class Registry extends Object with IterableMixin<LintRule> {

lib/src/rules/whitespace_around_ops.dart

@@ -0,0 +1,71 @@
+// 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.
+
+library linter.src.rules.whitespace_around_ops;
+
+import 'package:analyzer/dart/ast/ast.dart';
+import 'package:analyzer/dart/ast/token.dart';
+import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:linter/src/linter.dart';
+
+const desc = r'Use proper whitespace around operators.';
+
+const details = r'''
+**DO** ensure that there are spaces around binary operators and before any
+unary ones.
+
+Improper whitespace can create confusion, especially when applied to operators
+where it's possible to get a binary operator when you mean a unary one.  For
+example, the mistyping of `5 /~ 10` when you mean `5 ~/ 10` is hidden by the
+improper spacing.  (Properly spaced, the mistake is more clear: `5 / ~10`.)
+Whenever possible, use the formatter to cleanup whitespace.  Otherwise, take
+care to ensure that there are spaces around binary operators and before any
+unary ones.
+
+
+**BAD:**
+```
+print(5 /~ 10); //whoops
+```
+
+**GOOD:**
+```
+print(5 / ~10); //aha!
+```
+''';
+
+class Visitor extends SimpleAstVisitor {
+  final LintRule rule;
+  Visitor(this.rule);
+
+  @override
+  visitBinaryExpression(BinaryExpression node) {
+    if (!spaced(node.leftOperand.endToken, node.operator) ||
+        !spaced(node.operator, node.rightOperand.beginToken)) {
+      rule.reportLintForToken(node.operator);
+    }
+  }
+
+  @override
+  visitPrefixExpression(PrefixExpression node) {
+    if (spaced(node.operator, node.operand.beginToken)) {
+      rule.reportLintForToken(node.operator);
+    }
+  }
+
+  static bool spaced(Token first, Token second) =>
+      first != null && second != null && first.end != second.offset;
+}
+
+class WhitespaceAroundOps extends LintRule {
+  WhitespaceAroundOps()
+      : super(
+            name: 'whitespace_around_ops',
+            description: desc,
+            details: details,
+            group: Group.style);
+
+  @override
+  AstVisitor getVisitor() => new Visitor(this);
+}

test/rules/whitespace_around_ops.dart

@@ -0,0 +1,27 @@
+// 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.
+
+// test w/ `dart test/util/solo_test.dart whitespace_around_ops`
+
+
+// Define our own int so we don't need `int` in our mock SDK.
+class MyInt {
+  MyInt operator -() => this;
+  MyInt operator ~() => this;
+  MyInt operator ~/(MyInt other) => this;
+  MyInt operator /(MyInt other) => this;
+}
+
+void main() {
+  MyInt f, g;
+  print(f ~/ g);
+  print(f~/ g); //LINT
+  print(f ~/g); //LINT
+  print(f~/g); //LINT
+  print(f/ ~g); //LINT
+  print(f /~g); //LINT
+  print(f / ~g); //OK
+  f =- g; //LINT
+  f = -g; //OK
+}