Allow to set configuration elements as deprecated

Automatically print an appropriate warning to the console.

Source/SwiftLintCore/Models/Issue.swift

@@ -5,6 +5,9 @@ public enum Issue: LocalizedError, Equatable {
     /// The configuration didn't match internal expectations.
     case invalidConfiguration(ruleID: String)
 
+    /// Issued when an option is deprecated. Suggests an alternative optionally.
+    case deprecatedConfigurationOption(ruleID: String, key: String, alternative: String? = nil)
+
     /// Used in configuration parsing when no changes have been applied. Use only internally!
     case nothingApplied(ruleID: String)
 
@@ -71,6 +74,10 @@ public enum Issue: LocalizedError, Equatable {
     /// Flag to enable warnings for deprecations being printed to the console. Printing is enabled by default.
     public static var printDeprecationWarnings = true
 
+    /// Hook to redirect all messages to a consumer other than stdout. Shall only be used in tests to verify
+    /// console output.
+    public static var messageConsumer: ((String) -> Void)? = nil
+
     /// Wraps any `Error` into a `SwiftLintError.genericWarning` if it is not already a `SwiftLintError`.
     ///
     /// - parameter error: Any `Error`.
@@ -102,13 +109,23 @@ public enum Issue: LocalizedError, Equatable {
         if case .ruleDeprecated = self, !Self.printDeprecationWarnings {
             return
         }
-        queuedPrintError(errorDescription)
+        if let consumer = Self.messageConsumer {
+            consumer(errorDescription)
+        } else {
+            queuedPrintError(errorDescription)
+        }
     }
 
     private var message: String {
         switch self {
         case let .invalidConfiguration(id):
             return "Invalid configuration for '\(id)' rule. Falling back to default."
+        case let .deprecatedConfigurationOption(id, key, alternative):
+            let baseMessage = "Configuration option '\(key)' in '\(id)' rule is deprecated."
+            if let alternative {
+                return baseMessage + " Use the option '\(alternative)' instead."
+            }
+            return baseMessage
         case let .nothingApplied(ruleID: id):
             return Self.invalidConfiguration(ruleID: id).message
         case let .listedMultipleTime(id, times):

Source/SwiftLintCore/Models/RuleConfigurationDescription.swift

@@ -415,9 +415,18 @@ public protocol InlinableOptionType: AcceptableByConfigurationElement {}
 ///
 @propertyWrapper
 public struct ConfigurationElement<T: AcceptableByConfigurationElement & Equatable>: Equatable {
+    /// A deprecation notice.
+    public enum DeprecationNotice {
+        /// Warning suggesting an alternative option.
+        case suggestAlternative(ruleID: String, name: String)
+    }
+
     /// Wrapped option value.
     public var wrappedValue: T {
         didSet {
+            if case let .suggestAlternative(id, name) = deprecationNotice {
+                Issue.deprecatedConfigurationOption(ruleID: id, key: key, alternative: name).print()
+            }
             if wrappedValue != oldValue {
                 postprocessor(&wrappedValue)
             }
@@ -437,18 +446,28 @@ public struct ConfigurationElement<T: AcceptableByConfigurationElement & Equatab
     /// Whether this configuration element will be inlined into its description.
     public let inline: Bool
 
+    private let deprecationNotice: DeprecationNotice?
     private let postprocessor: (inout T) -> Void
 
     /// Default constructor.
     ///
     /// - Parameters:
     ///   - value: Value to be wrapped.
     ///   - key: Optional name of the option. If not specified, it will be inferred from the attributed property.
+    ///   - deprecationNotice: An optional deprecation notice in case an option is outdated and/or has been replaced by
+    ///                        an alternative.
     ///   - postprocessor: Function to be applied to the wrapped value after parsing to validate and modify it.
     public init(wrappedValue value: T,
                 key: String,
+                deprecationNotice: DeprecationNotice? = nil,
                 postprocessor: @escaping (inout T) -> Void = { _ in }) {
-        self.init(wrappedValue: value, key: key, inline: false, postprocessor: postprocessor)
+        self.init(
+            wrappedValue: value,
+            key: key,
+            inline: false,
+            deprecationNotice: deprecationNotice,
+            postprocessor: postprocessor
+        )
 
         // Modify the set value immediately.
         postprocessor(&wrappedValue)
@@ -488,10 +507,12 @@ public struct ConfigurationElement<T: AcceptableByConfigurationElement & Equatab
     private init(wrappedValue: T,
                  key: String,
                  inline: Bool,
+                 deprecationNotice: DeprecationNotice? = nil,
                  postprocessor: @escaping (inout T) -> Void = { _ in }) {
         self.wrappedValue = wrappedValue
         self.key = key
         self.inline = inline
+        self.deprecationNotice = deprecationNotice
         self.postprocessor = postprocessor
     }
 

Tests/SwiftLintFrameworkTests/IndentationWidthRuleTests.swift

@@ -6,11 +6,20 @@ class IndentationWidthRuleTests: SwiftLintTestCase {
     func testInvalidIndentation() throws {
         var testee = IndentationWidthConfiguration()
         let defaultValue = testee.indentationWidth
+
+        var messages = ""
+        Issue.messageConsumer = { messages = $0 }
+        defer { Issue.messageConsumer = nil }
+
         for indentation in [0, -1, -5] {
             try testee.apply(configuration: ["indentation_width": indentation])
 
             // Value remains the default.
             XCTAssertEqual(testee.indentationWidth, defaultValue)
+            XCTAssertEqual(
+                messages,
+                "warning: Invalid configuration for 'indentation_width' rule. Falling back to default."
+            )
         }
     }
 

Tests/SwiftLintFrameworkTests/RuleConfigurationDescriptionTests.swift

@@ -29,7 +29,7 @@ class RuleConfigurationDescriptionTests: XCTestCase {
             postprocessor: { list in list = list.map { $0.uppercased() } }
         )
         var list = ["string1", "string2"]
-        @ConfigurationElement(key: "set")
+        @ConfigurationElement(key: "set", deprecationNotice: .suggestAlternative(ruleID: "my_rule", name: "other_opt"))
         var set: Set<Int> = [1, 2, 3]
         @ConfigurationElement(inline: true)
         var severityConfig = SeverityConfiguration<Parent>(.error)
@@ -490,6 +490,20 @@ class RuleConfigurationDescriptionTests: XCTestCase {
         XCTAssertEqual(configuration.nestedSeverityLevels, SeverityLevelsConfiguration(warning: 6, error: 7))
     }
 
+    func testDeprecationWarning() throws {
+        var messages = ""
+        Issue.messageConsumer = { messages += $0 }
+        defer { Issue.messageConsumer = nil }
+
+        var configuration = TestConfiguration()
+        try configuration.apply(configuration: ["set": [6, 7]])
+
+        XCTAssertEqual(
+            messages,
+            "warning: Configuration option 'set' in 'my_rule' rule is deprecated. Use the option 'other_opt' instead."
+        )
+    }
+
     func testInvalidKeys() throws {
         var configuration = TestConfiguration()