Add redundant nil coalesing operator opt-in rule

CHANGELOG.md

@@ -1,3 +1,19 @@
+## Master
+
+##### Breaking
+
+* None.
+
+##### Enhancements
+
+* Add `RedundantNilCoalesingRule` Opt-In rule that warns against `?? nil`.  
+  [Daniel Beard](https://github.com/daniel-beard)
+  [#764](https://github.com/realm/SwiftLint/issues/764)
+
+##### Bug Fixes
+
+* None.
+
 ## 0.12.0: Vertical Laundry
 
 ##### Breaking

Source/SwiftLintFramework/Models/MasterRuleList.swift

@@ -68,6 +68,7 @@ public let masterRuleList = RuleList(rules:
     OperatorFunctionWhitespaceRule.self,
     PrivateOutletRule.self,
     PrivateUnitTestRule.self,
+    RedundantNilCoalesingRule.self,
     ReturnArrowWhitespaceRule.self,
     StatementPositionRule.self,
     TodoRule.self,

Source/SwiftLintFramework/Rules/RedundantNilCoalesingRule.swift

@@ -0,0 +1,48 @@
+//
+//  RedundantNilCoalesingRule.swift
+//  SwiftLint
+//
+//  Created by Daniel Beard on 8/24/16.
+//  Copyright © 2016 Realm. All rights reserved.
+//
+
+import Foundation
+import SourceKittenFramework
+
+extension File {
+    private func violatingRedundantNilCoalesingRanges() -> [NSRange] {
+        return matchPattern(
+            "\\?\\?\\s*nil\\b", // ?? {whitespace} nil {word boundary}
+            excludingSyntaxKinds: SyntaxKind.commentAndStringKinds()
+        )
+    }
+}
+
+public struct RedundantNilCoalesingRule: OptInRule, ConfigurationProviderRule {
+
+    public var configuration = SeverityConfiguration(.Warning)
+
+    public init() {}
+
+    public static let description = RuleDescription(
+        identifier: "redundant_nil_coalesing",
+        name: "Redundant Nil Coalesing",
+        description: "nil coalescing operator is only evaluated if the lhs is nil " +
+            ", coalesing operator with nil as rhs is redundant",
+        nonTriggeringExamples: [
+            "var myVar: Int?; myVar ?? 0"
+        ],
+        triggeringExamples: [
+            "var myVar = nil; myVar ↓?? nil"
+        ]
+    )
+
+    public func validateFile(file: File) -> [StyleViolation] {
+        return file.violatingRedundantNilCoalesingRanges().map {
+            StyleViolation(ruleDescription: self.dynamicType.description,
+                severity: configuration.severity,
+                location: Location(file: file, characterOffset: $0.location))
+        }
+    }
+
+}

SwiftLint.xcodeproj/project.pbxproj

@@ -9,9 +9,10 @@
 /* Begin PBXBuildFile section */
 		006ECFC41C44E99E00EF6364 /* LegacyConstantRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 006ECFC31C44E99E00EF6364 /* LegacyConstantRule.swift */; };
 		02FD8AEF1BFC18D60014BFFB /* ExtendedNSStringTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 02FD8AEE1BFC18D60014BFFB /* ExtendedNSStringTests.swift */; };
-		1EC163521D5992D900DD2928 /* VerticalWhitespaceRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1EC163511D5992D900DD2928 /* VerticalWhitespaceRule.swift */; };
 		094385041D5D4F7C009168CF /* PrivateOutletRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 094385021D5D4F78009168CF /* PrivateOutletRule.swift */; };
+		1EC163521D5992D900DD2928 /* VerticalWhitespaceRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1EC163511D5992D900DD2928 /* VerticalWhitespaceRule.swift */; };
 		1F11B3CF1C252F23002E8FA8 /* ClosingBraceRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 1F11B3CE1C252F23002E8FA8 /* ClosingBraceRule.swift */; };
+		24B4DF0D1D6DFDE90097803B /* RedundantNilCoalesingRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 24B4DF0B1D6DFA370097803B /* RedundantNilCoalesingRule.swift */; };
 		24E17F721B14BB3F008195BE /* File+Cache.swift in Sources */ = {isa = PBXBuildFile; fileRef = 24E17F701B1481FF008195BE /* File+Cache.swift */; };
 		2E02005F1C54BF680024D09D /* CyclomaticComplexityRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2E02005E1C54BF680024D09D /* CyclomaticComplexityRule.swift */; };
 		2E5761AA1C573B83003271AF /* FunctionParameterCountRule.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2E5761A91C573B83003271AF /* FunctionParameterCountRule.swift */; };
@@ -179,9 +180,10 @@
 /* Begin PBXFileReference section */
 		006ECFC31C44E99E00EF6364 /* LegacyConstantRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = LegacyConstantRule.swift; sourceTree = "<group>"; };
 		02FD8AEE1BFC18D60014BFFB /* ExtendedNSStringTests.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ExtendedNSStringTests.swift; sourceTree = "<group>"; };
-		1EC163511D5992D900DD2928 /* VerticalWhitespaceRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = VerticalWhitespaceRule.swift; sourceTree = "<group>"; };
 		094385021D5D4F78009168CF /* PrivateOutletRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = PrivateOutletRule.swift; sourceTree = "<group>"; };
+		1EC163511D5992D900DD2928 /* VerticalWhitespaceRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = VerticalWhitespaceRule.swift; sourceTree = "<group>"; };
 		1F11B3CE1C252F23002E8FA8 /* ClosingBraceRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ClosingBraceRule.swift; sourceTree = "<group>"; };
+		24B4DF0B1D6DFA370097803B /* RedundantNilCoalesingRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = RedundantNilCoalesingRule.swift; sourceTree = "<group>"; };
 		24E17F701B1481FF008195BE /* File+Cache.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = "File+Cache.swift"; sourceTree = "<group>"; };
 		2E02005E1C54BF680024D09D /* CyclomaticComplexityRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = CyclomaticComplexityRule.swift; sourceTree = "<group>"; };
 		2E5761A91C573B83003271AF /* FunctionParameterCountRule.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = FunctionParameterCountRule.swift; sourceTree = "<group>"; };
@@ -618,6 +620,7 @@
 				E5A167C81B25A0B000CF2D03 /* OperatorFunctionWhitespaceRule.swift */,
 				B2902A0B1D66815600BFCCF7 /* PrivateUnitTestRule.swift */,
 				094385021D5D4F78009168CF /* PrivateOutletRule.swift */,
+				24B4DF0B1D6DFA370097803B /* RedundantNilCoalesingRule.swift */,
 				E57B23C01B1D8BF000DEA512 /* ReturnArrowWhitespaceRule.swift */,
 				3BCC04CE1C4F56D3006073C3 /* RuleConfigurations */,
 				692B60AB1BD8F2E700C7AA22 /* StatementPositionRule.swift */,
@@ -940,6 +943,7 @@
 				E88DEA6F1B09843F00A66CB0 /* Location.swift in Sources */,
 				93E0C3CE1D67BD7F007FA25D /* ConditionalReturnsOnNewline.swift in Sources */,
 				E88DEA771B098D0C00A66CB0 /* Rule.swift in Sources */,
+				24B4DF0D1D6DFDE90097803B /* RedundantNilCoalesingRule.swift in Sources */,
 				7250948A1D0859260039B353 /* StatementPositionConfiguration.swift in Sources */,
 				E81619531BFC162C00946723 /* QueuedPrint.swift in Sources */,
 				E87E4A051BFB927C00FCFE46 /* TrailingSemicolonRule.swift in Sources */,

Tests/SwiftLintFramework/RulesTests.swift

@@ -190,6 +190,10 @@ class RulesTests: XCTestCase {
         verifyRule(PrivateUnitTestRule.description)
     }
 
+    func testRedundantNilCoalesing() {
+        verifyRule(RedundantNilCoalesingRule.description)
+    }
+
     func testReturnArrowWhitespace() {
         verifyRule(ReturnArrowWhitespaceRule.description)
     }