From 1752587b7b255ab858cb5ffb457df6a656847731 Mon Sep 17 00:00:00 2001
From: Marcelo Fabri <me@marcelofabri.com>
Date: Sun, 17 Apr 2022 03:30:53 -0700
Subject: [PATCH] Add unavailable_condition rule (#3953)

Fixes #3897
---
 CHANGELOG.md                                  |  6 ++
 .../Models/PrimaryRuleList.swift              |  1 +
 .../Idiomatic/UnavailableConditionRule.swift  | 82 +++++++++++++++++++
 .../AutomaticRuleTests.generated.swift        |  6 ++
 4 files changed, 95 insertions(+)
 create mode 100644 Source/SwiftLintFramework/Rules/Idiomatic/UnavailableConditionRule.swift

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 84173b9a67..edc4cf78e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -35,6 +35,12 @@
   [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
diff --git a/Source/SwiftLintFramework/Models/PrimaryRuleList.swift b/Source/SwiftLintFramework/Models/PrimaryRuleList.swift
index 08b29a876c..da0000d260 100644
--- a/Source/SwiftLintFramework/Models/PrimaryRuleList.swift
+++ b/Source/SwiftLintFramework/Models/PrimaryRuleList.swift
@@ -187,6 +187,7 @@ public let primaryRuleList = RuleList(rules: [
     TypeContentsOrderRule.self,
     TypeNameRule.self,
     TypesafeArrayInitRule.self,
+    UnavailableConditionRule.self,
     UnavailableFunctionRule.self,
     UnneededBreakInSwitchRule.self,
     UnneededParenthesesInClosureArgumentRule.self,
diff --git a/Source/SwiftLintFramework/Rules/Idiomatic/UnavailableConditionRule.swift b/Source/SwiftLintFramework/Rules/Idiomatic/UnavailableConditionRule.swift
new file mode 100644
index 0000000000..c296b78a2e
--- /dev/null
+++ b/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)
+    }
+}
diff --git a/Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift b/Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift
index 9dbbf964c2..1a1a5934fe 100644
--- a/Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift
+++ b/Tests/SwiftLintFrameworkTests/AutomaticRuleTests.generated.swift
@@ -797,6 +797,12 @@ class TypesafeArrayInitRuleTests: XCTestCase {
     }
 }
 
+class UnavailableConditionRuleTests: XCTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(UnavailableConditionRule.description)
+    }
+}
+
 class UnavailableFunctionRuleTests: XCTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(UnavailableFunctionRule.description)