Add redundant_final_actor opt-in rule

Source/SwiftLintBuiltInRules/Models/BuiltInRules.swift

@@ -176,6 +176,7 @@ public let builtInRules: [any Rule.Type] = [
     ReduceBooleanRule.self,
     ReduceIntoRule.self,
     RedundantDiscardableLetRule.self,
+    RedundantFinalActorRule.self,
     RedundantNilCoalescingRule.self,
     RedundantObjcAttributeRule.self,
     RedundantSelfRule.self,

Source/SwiftLintBuiltInRules/Rules/Idiomatic/RedundantFinalActorRule.swift

@@ -0,0 +1,125 @@
+import SwiftSyntax
+
+@SwiftSyntaxRule(correctable: true, optIn: true)
+struct RedundantFinalActorRule: Rule {
+    var configuration = SeverityConfiguration<Self>(.warning)
+
+    static let description = RuleDescription(
+        identifier: "redundant_final_actor",
+        name: "Redundant Final on Actor",
+        description: "`final` is redundant on an actor declaration and its members because actors cannot be subclassed",
+        rationale: """
+            Actors in Swift currently do not support inheritance, making `final` redundant \
+            on both actor declarations and their members. Note that this may change in future \
+            Swift versions if actor inheritance is introduced.
+            """,
+        kind: .idiomatic,
+        nonTriggeringExamples: [
+            Example("actor MyActor {}"),
+            Example("final class MyClass {}"),
+            Example("""
+            @globalActor
+            actor MyGlobalActor {}
+            """),
+            Example("""
+            actor MyActor {
+                func doWork() {}
+                var value: Int { 0 }
+            }
+            """),
+            Example("""
+            class MyClass {
+                final func doWork() {}
+            }
+            """),
+        ],
+        triggeringExamples: [
+            Example("↓final actor MyActor {}"),
+            Example("public ↓final actor DataStore {}"),
+            Example("""
+            @globalActor
+            ↓final actor MyGlobalActor {}
+            """),
+            Example("""
+            actor MyActor {
+                ↓final func doWork() {}
+            }
+            """),
+            Example("""
+            actor MyActor {
+                ↓final var value: Int { 0 }
+            }
+            """),
+        ],
+        corrections: [
+            Example("final actor MyActor {}"):
+                Example("actor MyActor {}"),
+            Example("public final actor DataStore {}"):
+                Example("public actor DataStore {}"),
+            Example("actor MyActor {\n    final func doWork() {}\n}"):
+                Example("actor MyActor {\n    func doWork() {}\n}"),
+        ]
+    )
+}
+
+private extension RedundantFinalActorRule {
+    final class Visitor: ViolationsSyntaxVisitor<ConfigurationType> {
+        private var insideActor = false
+
+        override func visit(_ node: ActorDeclSyntax) -> SyntaxVisitorContinueKind {
+            insideActor = true
+            return .visitChildren
+        }
+
+        override func visitPost(_ node: ActorDeclSyntax) {
+            if let finalModifier = node.modifiers.first(where: { $0.name.text == "final" }) {
+                addViolation(for: finalModifier)
+            }
+            insideActor = false
+        }
+
+        override func visitPost(_ node: FunctionDeclSyntax) {
+            guard insideActor,
+                  let finalModifier = node.modifiers.first(where: { $0.name.text == "final" }) else {
+                return
+            }
+            addViolation(for: finalModifier)
+        }
+
+        override func visitPost(_ node: VariableDeclSyntax) {
+            guard insideActor,
+                  let finalModifier = node.modifiers.first(where: { $0.name.text == "final" }) else {
+                return
+            }
+            addViolation(for: finalModifier)
+        }
+
+        override func visitPost(_ node: SubscriptDeclSyntax) {
+            guard insideActor,
+                  let finalModifier = node.modifiers.first(where: { $0.name.text == "final" }) else {
+                return
+            }
+            addViolation(for: finalModifier)
+        }
+
+        // Don't descend into nested classes where `final` is meaningful
+        override func visit(_: ClassDeclSyntax) -> SyntaxVisitorContinueKind {
+            .skipChildren
+        }
+
+        private func addViolation(for finalModifier: DeclModifierSyntax) {
+            let start = finalModifier.positionAfterSkippingLeadingTrivia
+            let end = finalModifier.endPosition
+            violations.append(
+                ReasonedRuleViolation(
+                    position: start,
+                    correction: .init(
+                        start: start,
+                        end: end,
+                        replacement: ""
+                    )
+                )
+            )
+        }
+    }
+}

Tests/GeneratedTests/GeneratedTests_07.swift

@@ -151,8 +151,8 @@ final class RedundantDiscardableLetRuleGeneratedTests: SwiftLintTestCase {
     }
 }
 
-final class RedundantNilCoalescingRuleGeneratedTests: SwiftLintTestCase {
+final class RedundantFinalActorRuleGeneratedTests: SwiftLintTestCase {
     func testWithDefaultConfiguration() {
-        verifyRule(RedundantNilCoalescingRule.description)
+        verifyRule(RedundantFinalActorRule.description)
     }
 }

Tests/GeneratedTests/GeneratedTests_08.swift

@@ -7,6 +7,12 @@
 @testable import SwiftLintCore
 import TestHelpers
 
+final class RedundantNilCoalescingRuleGeneratedTests: SwiftLintTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(RedundantNilCoalescingRule.description)
+    }
+}
+
 final class RedundantObjcAttributeRuleGeneratedTests: SwiftLintTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(RedundantObjcAttributeRule.description)
@@ -150,9 +156,3 @@ final class StrictFilePrivateRuleGeneratedTests: SwiftLintTestCase {
         verifyRule(StrictFilePrivateRule.description)
     }
 }
-
-final class StrongIBOutletRuleGeneratedTests: SwiftLintTestCase {
-    func testWithDefaultConfiguration() {
-        verifyRule(StrongIBOutletRule.description)
-    }
-}

Tests/GeneratedTests/GeneratedTests_09.swift

@@ -7,6 +7,12 @@
 @testable import SwiftLintCore
 import TestHelpers
 
+final class StrongIBOutletRuleGeneratedTests: SwiftLintTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(StrongIBOutletRule.description)
+    }
+}
+
 final class SuperfluousElseRuleGeneratedTests: SwiftLintTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(SuperfluousElseRule.description)
@@ -150,9 +156,3 @@ final class UnneededSynthesizedInitializerRuleGeneratedTests: SwiftLintTestCase
         verifyRule(UnneededSynthesizedInitializerRule.description)
     }
 }
-
-final class UnneededThrowsRuleGeneratedTests: SwiftLintTestCase {
-    func testWithDefaultConfiguration() {
-        verifyRule(UnneededThrowsRule.description)
-    }
-}

Tests/GeneratedTests/GeneratedTests_10.swift

@@ -7,6 +7,12 @@
 @testable import SwiftLintCore
 import TestHelpers
 
+final class UnneededThrowsRuleGeneratedTests: SwiftLintTestCase {
+    func testWithDefaultConfiguration() {
+        verifyRule(UnneededThrowsRule.description)
+    }
+}
+
 final class UnownedVariableCaptureRuleGeneratedTests: SwiftLintTestCase {
     func testWithDefaultConfiguration() {
         verifyRule(UnownedVariableCaptureRule.description)

Tests/IntegrationTests/Resources/default_rule_configurations.yml

@@ -1006,6 +1006,11 @@ redundant_discardable_let:
   meta:
     opt-in: false
     correctable: true
+redundant_final_actor:
+  severity: warning
+  meta:
+    opt-in: true
+    correctable: true
 redundant_nil_coalescing:
   severity: warning
   meta: