Diagnose when a test or suite is embedded in a non-final class. (#332)

Author: grynspan

Sources/Testing/Testing.docc/OrganizingTests.md

@@ -163,10 +163,6 @@ actor CashRegisterTests: NSObject { ... } // ✅ OK: actors are implicitly final
 class MenuItemTests { ... } // ❌ ERROR: this class is not final
 ```
 
-- Bug: Violations of this requirement are not consistently diagnosed at compile
-  time, and the diagnostic produced when an issue is detected may be confusing
-  to developers. ([105470382](rdar://105470382))
-
 ## Topics
 
 - ``Suite(_:_:)``

Sources/TestingMacros/Support/DiagnosticMessage+Diagnosing.swift

@@ -103,8 +103,20 @@ func diagnoseIssuesWithLexicalContext(
   attribute: AttributeSyntax,
   in context: some MacroExpansionContext
 ) -> [DiagnosticMessage] {
-  context.lexicalContext
-    .filter { !$0.isProtocol((any DeclGroupSyntax).self) }
-    .map { .containingNodeUnsupported($0, whenUsing: attribute) }
+  var diagnostics = [DiagnosticMessage]()
+
+  for lexicalContext in context.lexicalContext {
+    if !lexicalContext.isProtocol((any DeclGroupSyntax).self) {
+      diagnostics.append(.containingNodeUnsupported(lexicalContext, whenUsing: attribute, on: decl))
+    }
+
+    if let classDecl = lexicalContext.as(ClassDeclSyntax.self) {
+      if !classDecl.modifiers.lazy.map(\.name.tokenKind).contains(.keyword(.final)) {
+        diagnostics.append(.containingNodeUnsupported(classDecl, whenUsing: attribute, on: decl))
+      }
+    }
+  }
+
+  return diagnostics
 }
 #endif

Sources/TestingMacros/Support/DiagnosticMessage.swift

@@ -237,17 +237,29 @@ struct DiagnosticMessage: SwiftDiagnostics.DiagnosticMessage {
   /// - Parameters:
   ///   - node: The lexical context preventing the use of `attribute`.
   ///   - attribute: The `@Test` or `@Suite` attribute.
+  ///   - decl: The declaration in question (contained in `node`.)
   ///
   /// - Returns: A diagnostic message.
-  static func containingNodeUnsupported(_ node: some SyntaxProtocol, whenUsing attribute: AttributeSyntax) -> Self {
+  static func containingNodeUnsupported(_ node: some SyntaxProtocol, whenUsing attribute: AttributeSyntax, on decl: some SyntaxProtocol) -> Self {
     // It would be great if the diagnostic pointed to the containing lexical
     // context that was unsupported, but that node may be synthesized and does
     // not have reliable location information.
-    Self(
-      syntax: Syntax(attribute),
-      message: "Attribute \(_macroName(attribute)) cannot be applied within \(_kindString(for: node, includeA: true)).",
-      severity: .error
-    )
+    switch node.kind {
+    case .classDecl:
+      // Special-case class declarations as implicitly non-final (since we would
+      // only diagnose a class here if it were non-final.)
+      return Self(
+        syntax: Syntax(attribute),
+        message: "Attribute \(_macroName(attribute)) cannot be applied to \(_kindString(for: decl, includeA: true)) within a non-final class",
+        severity: .error
+      )
+    default:
+      return Self(
+        syntax: Syntax(attribute),
+        message: "Attribute \(_macroName(attribute)) cannot be applied to \(_kindString(for: decl, includeA: true)) within \(_kindString(for: node, includeA: true))",
+        severity: .error
+      )
+    }
   }
 #endif
 

Tests/TestingMacrosTests/TestDeclarationMacroTests.swift

@@ -120,9 +120,13 @@ struct TestDeclarationMacroTests {
   @Test("Error diagnostics emitted for invalid lexical contexts",
     arguments: [
       "struct S { func f() { @Test func f() {} } }":
-        "The @Test attribute cannot be applied within a function.",
+        "Attribute 'Test' cannot be applied to a function within a function",
       "struct S { func f() { @Suite struct S { } } }":
-        "The @Suite attribute cannot be applied within a function.",
+        "Attribute 'Suite' cannot be applied to a structure within a function",
+      "class C { @Test func f() {} }":
+        "Attribute 'Test' cannot be applied to a function within a non-final class",
+      "class C { @Suite struct S {} }":
+        "Attribute 'Suite' cannot be applied to a structure within a non-final class",
     ]
   )
   func invalidLexicalContext(input: String, expectedMessage: String) throws {