Add unavailable_condition rule

Fixes #3897

CHANGELOG.md

@@ -31,6 +31,12 @@
   [Marcelo Fabri](https://github.com/marcelofabri)
   [#3343](https://github.com/realm/SwiftLint/issues/3343)
 
+* Add `unavailable_condition` rule to prefer using `if #unavailable` instead of
+  `if #available` with an empty body and an `else` condition when using
+  Swift 5.6 or later.  
+  [Marcelo Fabri](https://github.com/marcelofabri)
+  [#3897](https://github.com/realm/SwiftLint/issues/3897)
+
 #### Bug Fixes
 
 * Fix false positives in `unused_closure_parameter` when using parameters with

Source/SwiftLintFramework/Models/PrimaryRuleList.swift

@@ -186,6 +186,7 @@ public let primaryRuleList = RuleList(rules: [
     TypeContentsOrderRule.self,
     TypeNameRule.self,
     TypesafeArrayInitRule.self,
+    UnavailableConditionRule.self,
     UnavailableFunctionRule.self,
     UnneededBreakInSwitchRule.self,
     UnneededParenthesesInClosureArgumentRule.self,

Source/SwiftLintFramework/Rules/Idiomatic/UnavailableConditionRule.swift

@@ -0,0 +1,82 @@
+import SourceKittenFramework
+import SwiftSyntax
+
+public struct UnavailableConditionRule: ConfigurationProviderRule, AutomaticTestableRule {
+    public var configuration = SeverityConfiguration(.warning)
+
+    public init() {}
+
+    public static let description = RuleDescription(
+        identifier: "unavailable_condition",
+        name: "Unavailable Condition",
+        description: "Use #unavailable instead of #available with an empty body.",
+        kind: .idiomatic,
+        minSwiftVersion: .fiveDotSix,
+        nonTriggeringExamples: [
+            Example("""
+            if #unavailable(iOS 13) {
+              loadMainWindow()
+            }
+            """),
+            Example("""
+            if #available(iOS 9.0, *) {
+              doSomething()
+            } else {
+              legacyDoSomething()
+            }
+            """)
+        ],
+        triggeringExamples: [
+            Example("""
+            if ↓#available(iOS 14.0) {
+
+            } else {
+              oldIos13TrackingLogic(isEnabled: ASIdentifierManager.shared().isAdvertisingTrackingEnabled)
+            }
+            """),
+            Example("""
+            if ↓#available(iOS 14.0) {
+              // we don't need to do anything here
+            } else {
+              oldIos13TrackingLogic(isEnabled: ASIdentifierManager.shared().isAdvertisingTrackingEnabled)
+            }
+            """),
+            Example("""
+            if ↓#available(iOS 13, *) {} else {
+              loadMainWindow()
+            }
+            """)
+        ]
+    )
+
+    public func validate(file: SwiftLintFile) -> [StyleViolation] {
+        guard let tree = file.syntaxTree else {
+            return []
+        }
+
+        let visitor = UnavailableConditionRuleVisitor()
+        visitor.walk(tree)
+        return visitor.positions.map { position in
+            StyleViolation(ruleDescription: Self.description,
+                           severity: configuration.severity,
+                           location: Location(file: file, byteOffset: ByteCount(position.utf8Offset)))
+        }
+    }
+}
+
+private final class UnavailableConditionRuleVisitor: SyntaxVisitor {
+    private(set) var positions: [AbsolutePosition] = []
+
+    override func visitPost(_ node: IfStmtSyntax) {
+        guard node.body.statements.withoutTrivia().isEmpty else {
+            return
+        }
+
+        guard node.conditions.count == 1, let condition = node.conditions.first,
+              let availability = condition.condition.as(AvailabilityConditionSyntax.self) else {
+            return
+        }
+
+        positions.append(availability.positionAfterSkippingLeadingTrivia)
+    }
+}

Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift

@@ -791,6 +791,12 @@ class TypesafeArrayInitRuleTests: XCTestCase {
     }
 }
 
+class UnavailableConditionRuleTests: XCTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(UnavailableConditionRule.description)
+    }
+}
+
 class UnavailableFunctionRuleTests: XCTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(UnavailableFunctionRule.description)