Add a warning for TARGET_OS_* compilation conditions

Sources/SwiftIfConfig/IfConfigDiagnostic.swift

@@ -34,6 +34,7 @@ enum IfConfigDiagnostic: Error, CustomStringConvertible {
   case ignoredTrailingComponents(version: VersionTuple, syntax: ExprSyntax)
   case integerLiteralCondition(syntax: ExprSyntax, replacement: Bool)
   case likelySimulatorPlatform(syntax: ExprSyntax)
+  case likelyTargetOS(syntax: ExprSyntax, replacement: ExprSyntax?)
   case endiannessDoesNotMatch(syntax: ExprSyntax, argument: String)
   case macabiIsMacCatalyst(syntax: ExprSyntax)
   case expectedModuleName(syntax: ExprSyntax)
@@ -88,6 +89,12 @@ enum IfConfigDiagnostic: Error, CustomStringConvertible {
       return
         "platform condition appears to be testing for simulator environment; use 'targetEnvironment(simulator)' instead"
 
+    case .likelyTargetOS(syntax: _, replacement: let replacement?):
+      return "'TARGET_OS_*' preprocessor macros are not available in Swift; use '\(replacement)' instead"
+
+    case .likelyTargetOS(syntax: _, replacement: nil):
+      return "'TARGET_OS_*' preprocessor macros are not available in Swift; use 'os(...)' conditionals instead"
+
     case .macabiIsMacCatalyst:
       return "'macabi' has been renamed to 'macCatalyst'"
 
@@ -122,6 +129,7 @@ enum IfConfigDiagnostic: Error, CustomStringConvertible {
       .ignoredTrailingComponents(version: _, syntax: let syntax),
       .integerLiteralCondition(syntax: let syntax, replacement: _),
       .likelySimulatorPlatform(syntax: let syntax),
+      .likelyTargetOS(syntax: let syntax, replacement: _),
       .endiannessDoesNotMatch(syntax: let syntax, argument: _),
       .macabiIsMacCatalyst(syntax: let syntax),
       .expectedModuleName(syntax: let syntax),
@@ -145,7 +153,7 @@ extension IfConfigDiagnostic: DiagnosticMessage {
   var severity: SwiftDiagnostics.DiagnosticSeverity {
     switch self {
     case .compilerVersionSecondComponentNotWildcard, .ignoredTrailingComponents,
-      .likelySimulatorPlatform, .endiannessDoesNotMatch, .macabiIsMacCatalyst:
+      .likelySimulatorPlatform, .likelyTargetOS, .endiannessDoesNotMatch, .macabiIsMacCatalyst:
       return .warning
     default: return .error
     }
@@ -192,6 +200,21 @@ extension IfConfigDiagnostic: DiagnosticMessage {
       )
     }
 
+    // For the likely TARGET_OS_* condition we may have a Fix-It.
+    if case .likelyTargetOS(let syntax, let replacement?) = self {
+      return Diagnostic(
+        node: syntax,
+        message: self,
+        fixIt: .replace(
+          message: SimpleFixItMessage(
+            message: "replace with '\(replacement)'"
+          ),
+          oldNode: syntax,
+          newNode: replacement
+        )
+      )
+    }
+
     // For the targetEnvironment(macabi) -> macCatalyst rename we have a Fix-It.
     if case .macabiIsMacCatalyst(syntax: let syntax) = self {
       return Diagnostic(

Sources/SwiftIfConfig/IfConfigEvaluation.swift

@@ -99,6 +99,9 @@ func evaluateIfConfig(
   if let identExpr = condition.as(DeclReferenceExprSyntax.self),
     let ident = identExpr.simpleIdentifier?.name
   {
+    if let targetOSDiagnostic = diagnoseLikelyTargetOSTest(at: identExpr, name: ident) {
+      extraDiagnostics.append(targetOSDiagnostic)
+    }
     // Evaluate the custom condition. If the build configuration cannot answer this query, fail.
     return checkConfiguration(at: identExpr) {
       (active: try configuration.isCustomConditionSet(name: ident), syntaxErrorsAllowed: false)
@@ -627,6 +630,51 @@ private func diagnoseLikelySimulatorEnvironmentTest(
   return IfConfigDiagnostic.likelySimulatorPlatform(syntax: ExprSyntax(binOp)).asDiagnostic
 }
 
+/// If this identifier looks like it is a `TARGET_OS_*` compilation condition,
+/// produce a diagnostic that suggests replacing it with the `os(*)` syntax.
+///
+/// For example, this checks for conditions like:
+///
+/// ```
+/// #if TARGET_OS_IOS
+/// ```
+///
+/// which should be replaced with
+///
+/// ```
+/// #if os(iOS)
+/// ```
+private func diagnoseLikelyTargetOSTest(
+  at reference: DeclReferenceExprSyntax,
+  name: String
+) -> Diagnostic? {
+  let prefix = "TARGET_OS_"
+  guard name.hasPrefix(prefix) else { return nil }
+  let osName = String(name.dropFirst(prefix.count))
+
+  if unmappedTargetOSNames.contains(osName) {
+    return IfConfigDiagnostic.likelyTargetOS(syntax: ExprSyntax(reference), replacement: nil).asDiagnostic
+  }
+
+  guard let replacement = targetOSNameMap[osName] else { return nil }
+
+  return IfConfigDiagnostic.likelyTargetOS(syntax: ExprSyntax(reference), replacement: replacement).asDiagnostic
+}
+
+// TARGET_OS_* macros that don’t have a direct Swift equivalent
+private let unmappedTargetOSNames = ["WIN32", "UNIX", "MAC", "IPHONE", "EMBEDDED"]
+private let targetOSNameMap: [String: ExprSyntax] = [
+  "WINDOWS": "os(Windows)",
+  "LINUX": "os(Linux)",
+  "OSX": "os(macOS)",
+  "IOS": "os(iOS)",
+  "MACCATALYST": "targetEnvironment(macCatalyst)",
+  "TV": "os(tvOS)",
+  "WATCH": "os(watchOS)",
+  "VISION": "os(visionOS)",
+  "SIMULATOR": "targetEnvironment(simulator)",
+]
+
 extension IfConfigClauseSyntax {
   /// Fold the operators within an #if condition, turning sequence expressions
   /// involving the various allowed operators (&&, ||, !) into well-structured

Tests/SwiftIfConfigTest/EvaluateTests.swift

@@ -239,6 +239,72 @@ public class EvaluateTests: XCTestCase {
     )
   }
 
+  func testTargetOS() throws {
+    assertIfConfig("TARGET_OS_DISHWASHER", .inactive)
+
+    assertIfConfig(
+      "TARGET_OS_IOS",
+      .inactive,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "'TARGET_OS_*' preprocessor macros are not available in Swift; use 'os(iOS)' instead",
+          line: 1,
+          column: 1,
+          severity: .warning,
+          fixIts: [
+            FixItSpec(message: "replace with 'os(iOS)'")
+          ]
+        )
+      ]
+    )
+
+    assertIfConfig(
+      "TARGET_OS_OSX",
+      .inactive,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "'TARGET_OS_*' preprocessor macros are not available in Swift; use 'os(macOS)' instead",
+          line: 1,
+          column: 1,
+          severity: .warning,
+          fixIts: [
+            FixItSpec(message: "replace with 'os(macOS)'")
+          ]
+        )
+      ]
+    )
+
+    assertIfConfig(
+      "TARGET_OS_MACCATALYST",
+      .inactive,
+      diagnostics: [
+        DiagnosticSpec(
+          message:
+            "'TARGET_OS_*' preprocessor macros are not available in Swift; use 'targetEnvironment(macCatalyst)' instead",
+          line: 1,
+          column: 1,
+          severity: .warning,
+          fixIts: [
+            FixItSpec(message: "replace with 'targetEnvironment(macCatalyst)'")
+          ]
+        )
+      ]
+    )
+
+    assertIfConfig(
+      "TARGET_OS_WIN32",
+      .inactive,
+      diagnostics: [
+        DiagnosticSpec(
+          message: "'TARGET_OS_*' preprocessor macros are not available in Swift; use 'os(...)' conditionals instead",
+          line: 1,
+          column: 1,
+          severity: .warning
+        )
+      ]
+    )
+  }
+
   func testVersions() throws {
     assertIfConfig("swift(>=5.5)", .active)
     assertIfConfig("swift(<6)", .active)