Add support for platform-specific unconditional availability. (#807)

This PR adds support for the following variant of `@available`:

```swift
@available(macOS, unavailable)
@test func f() {}
```

Previously, we were simply ignoring these availability attributes.

Resolves rdar://137776333.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: grynspan

Diff for: Sources/Testing/Traits/ConditionTrait+Macro.swift

@@ -92,9 +92,16 @@ extension Trait where Self == ConditionTrait {
     _ condition: @escaping @Sendable () -> Bool
   ) -> Self {
     // TODO: Semantic capture of platform name/version (rather than just a comment)
-    Self(
+    let message: Comment = if let message {
+      message
+    } else if let version {
+      "Obsolete as of \(_description(ofPlatformName: platformName, version: version))"
+    } else {
+      "Unavailable on \(_description(ofPlatformName: platformName, version: nil))"
+    }
+    return Self(
       kind: .conditional(condition),
-      comments: [message ?? "Obsolete as of \(_description(ofPlatformName: platformName, version: version))"],
+      comments: [message],
       sourceLocation: sourceLocation
     )
   }

Diff for: Sources/TestingMacros/Support/Additions/WithAttributesSyntaxAdditions.swift

@@ -80,10 +80,14 @@ extension WithAttributesSyntax {
             if let lastPlatformName, whenKeyword == .introduced {
               return Availability(attribute: attribute, platformName: lastPlatformName, version: nil, message: message)
             }
-          } else if case let .keyword(keyword) = token.tokenKind, keyword == whenKeyword, asteriskEncountered {
-            // Match the "always this availability" construct, i.e.
-            // `@available(*, deprecated)` and `@available(*, unavailable)`.
-            return Availability(attribute: attribute, platformName: lastPlatformName, version: nil, message: message)
+          } else if case let .keyword(keyword) = token.tokenKind, keyword == whenKeyword {
+            if asteriskEncountered {
+              // Match the "always this availability" construct, i.e.
+              // `@available(*, deprecated)` and `@available(*, unavailable)`.
+              return Availability(attribute: attribute, platformName: lastPlatformName, version: nil, message: message)
+            } else if keyword == .unavailable {
+              return Availability(attribute: attribute, platformName: lastPlatformName, version: nil, message: message)
+            }
           }
         case let .availabilityLabeledArgument(argument):
           if argument.label.tokenKind == .keyword(whenKeyword), case let .version(version) = argument.value {

Diff for: Sources/TestingMacros/Support/AvailabilityGuards.swift

@@ -114,8 +114,25 @@ private func _createAvailabilityTraitExpr(
     }
     """
 
-  case (.unavailable, _):
-    return ".__unavailable(message: \(message), sourceLocation: \(sourceLocationExpr))"
+  case (.unavailable, true):
+    // @available(swift, unavailable) is unsupported. The compiler emits a
+    // warning but doesn't prevent calling the function. Emit a no-op.
+    return ".enabled(if: true)"
+
+  case (.unavailable, false):
+    if let platformName = availability.platformName {
+      return """
+      .__available(\(literal: platformName.textWithoutBackticks), obsoleted: nil, message: \(message), sourceLocation: \(sourceLocationExpr)) {
+        #if os(\(platformName.trimmed))
+        return false
+        #else
+        return true
+        #endif
+      }
+      """
+    } else {
+      return ".__unavailable(message: \(message), sourceLocation: \(sourceLocationExpr))"
+    }
 
   default:
     fatalError("Unsupported keyword \(whenKeyword) passed to \(#function). Please file a bug report at https://github.com/swiftlang/swift-testing/issues/new")
@@ -203,14 +220,14 @@ func createSyntaxNode(
 
   // As above, but for unavailability (`#unavailable(...)`.)
   do {
-    let unavailableExprs: [ExprSyntax] = decl.availability(when: .obsoleted).lazy
+    let obsoletedExprs: [ExprSyntax] = decl.availability(when: .obsoleted).lazy
       .filter { !$0.isSwift }
       .compactMap(\.platformVersion)
       .map { "#unavailable(\($0))" }
-    if !unavailableExprs.isEmpty {
+    if !obsoletedExprs.isEmpty {
       let conditionList = ConditionElementListSyntax {
-        for unavailableExpr in unavailableExprs {
-          unavailableExpr
+        for obsoletedExpr in obsoletedExprs {
+          obsoletedExpr
         }
       }
       result = """
@@ -220,6 +237,23 @@ func createSyntaxNode(
       \(result)
       """
     }
+
+    let unavailableExprs: [ExprSyntax] = decl.availability(when: .unavailable).lazy
+      .filter { !$0.isSwift }
+      .filter { $0.version == nil }
+      .compactMap(\.platformName)
+      .map { "os(\($0.trimmed))" }
+    if !unavailableExprs.isEmpty {
+      for unavailableExpr in unavailableExprs {
+        result = """
+        #if \(unavailableExpr)
+        \(exitStatement)
+        #else
+        \(result)
+        #endif
+        """
+      }
+    }
   }
 
   // If this function has a minimum or maximum Swift version requirement, we

Diff for: Sources/TestingMacros/TestDeclarationMacro.swift

@@ -252,7 +252,9 @@ public struct TestDeclarationMacro: PeerMacro, Sendable {
 
     // Generate a thunk function that invokes the actual function.
     var thunkBody: CodeBlockItemListSyntax
-    if functionDecl.availability(when: .unavailable).first != nil {
+    if functionDecl.availability(when: .unavailable).first(where: { $0.platformVersion == nil }) != nil {
+      // The function is unconditionally disabled, so don't bother emitting a
+      // thunk body that calls it.
       thunkBody = ""
     } else if let typeName {
       if functionDecl.isStaticOrClass {

Diff for: Tests/TestingMacrosTests/TestDeclarationMacroTests.swift

@@ -307,6 +307,11 @@ struct TestDeclarationMacroTests {
           #".__available("Swift", obsoleted: (2, 0, nil), "#,
           #"#if swift(>=1.0) && swift(<2.0)"#,
         ],
+      #"@available(moofOS, unavailable, message: "Moof!") @Test func f() {}"#:
+        [
+          #"#if os(moofOS)"#,
+          #".__available("moofOS", obsoleted: nil, message: "Moof!", "#,
+        ]
     ]
   )
   func availabilityAttributeCapture(input: String, expectedOutputs: [String]) throws {

Diff for: Tests/TestingTests/RunnerTests.swift

@@ -637,6 +637,14 @@ final class RunnerTests: XCTestCase {
     @available(macOS 999.0, iOS 999.0, watchOS 999.0, tvOS 999.0, visionOS 999.0, *)
     func futureAvailable() {}
 
+    @Test(.hidden)
+    @available(macOS, unavailable)
+    @available(iOS, unavailable)
+    @available(watchOS, unavailable)
+    @available(tvOS, unavailable)
+    @available(visionOS, unavailable)
+    func perPlatformUnavailable() {}
+
     @Test(.hidden)
     @available(macOS, introduced: 999.0)
     @available(iOS, introduced: 999.0)
@@ -674,7 +682,7 @@ final class RunnerTests: XCTestCase {
     let testSkipped = expectation(description: "Test skipped")
 #if SWT_TARGET_OS_APPLE
     testStarted.expectedFulfillmentCount = 4
-    testSkipped.expectedFulfillmentCount = 7
+    testSkipped.expectedFulfillmentCount = 8
 #else
     testStarted.expectedFulfillmentCount = 2
     testSkipped.expectedFulfillmentCount = 2
@@ -719,6 +727,14 @@ final class RunnerTests: XCTestCase {
     func unavailable() {}
 
 #if SWT_TARGET_OS_APPLE
+    @Test(.hidden)
+    @available(macOS, unavailable, message: "Expected Message")
+    @available(iOS, unavailable, message: "Expected Message")
+    @available(watchOS, unavailable, message: "Expected Message")
+    @available(tvOS, unavailable, message: "Expected Message")
+    @available(visionOS, unavailable, message: "Expected Message")
+    func perPlatformUnavailable() {}
+
     @Test(.hidden)
     @available(macOS, introduced: 999.0, message: "Expected Message")
     @available(iOS, introduced: 999.0, message: "Expected Message")