Emit a diagnostic if a display name string is empty

Sources/TestingMacros/Support/AttributeDiscovery.swift

@@ -11,6 +11,7 @@
 import SwiftSyntax
 import SwiftSyntaxBuilder
 import SwiftSyntaxMacros
+import SwiftParser
 
 /// A syntax rewriter that removes leading `Self.` tokens from member access
 /// expressions in a syntax tree.
@@ -149,6 +150,16 @@ struct AttributeInfo {
       }
     }
 
+    // If there was a display name but it's completely empty, emit a warning
+    // diagnostic since this can cause confusion isn't generally recommended.
+    // Note that this is only possible for string literal display names; the
+    // compiler enforces that raw identifiers must be non-empty.
+    if let namedDecl = declaration.asProtocol((any NamedDeclSyntax).self),
+       let displayName, let displayNameArgument,
+        displayName.representedLiteralValue?.isEmpty == true {
+      context.diagnose(.declaration(namedDecl, hasEmptyDisplayName: displayName, fromArgument: displayNameArgument, using: attribute))
+    }
+
     // Remove leading "Self." expressions from the arguments of the attribute.
     // See _SelfRemover for more information. Rewriting a syntax tree discards
     // location information from the copy, so only invoke the rewriter if the

Sources/TestingMacros/Support/DiagnosticMessage.swift

@@ -643,6 +643,37 @@ struct DiagnosticMessage: SwiftDiagnostics.DiagnosticMessage {
     )
   }
 
+  /// Create a diagnostic message stating that a string literal expression
+  /// passed as the display name to a `@Test` or `@Suite` attribute is empty
+  /// but should not be.
+  ///
+  /// - Parameters:
+  ///   - decl: The declaration that has an empty display name.
+  ///   - displayNameExpr: The display name string literal expression.
+  ///   - argumentContainingDisplayName: The argument node containing the node
+  ///     `displayNameExpr`.
+  ///   - attribute: The `@Test` or `@Suite` attribute.
+  ///
+  /// - Returns: A diagnostic message.
+  static func declaration(
+    _ decl: some NamedDeclSyntax,
+    hasEmptyDisplayName displayNameExpr: StringLiteralExprSyntax,
+    fromArgument argumentContainingDisplayName: LabeledExprListSyntax.Element,
+    using attribute: AttributeSyntax
+  ) -> Self {
+    Self(
+      syntax: Syntax(displayNameExpr),
+      message: "Attribute \(_macroName(attribute)) specifies an empty display name for \(_kindString(for: decl))",
+      severity: .warning,
+      fixIts: [
+        FixIt(
+          message: MacroExpansionFixItMessage("Remove '\(displayNameExpr.representedLiteralValue!)'"),
+          changes: [.replace(oldNode: Syntax(argumentContainingDisplayName), newNode: Syntax("" as ExprSyntax))]
+        ),
+      ]
+    )
+  }
+
   /// Create a diagnostic message stating that a declaration has two display
   /// names.
   ///

Tests/TestingMacrosTests/TestDeclarationMacroTests.swift

@@ -304,6 +304,14 @@ struct TestDeclarationMacroTests {
       // .serialized on a non-parameterized test function
       "@Test(.serialized) func f() {}":
         "Trait '.serialized' has no effect when used with a non-parameterized test function",
+
+      // empty display name string literal
+      #"@Test("") func f() {}"#:
+        "Attribute 'Test' specifies an empty display name for function",
+      ##"@Test(#""#) func f() {}"##:
+        "Attribute 'Test' specifies an empty display name for function",
+      #"@Suite("") struct S {}"#:
+        "Attribute 'Suite' specifies an empty display name for structure",
     ]
   )
   func apiMisuseWarnings(input: String, expectedMessage: String) throws {