Fix(MacroSystem): Validate MemberMacro application on non-group declarations (Issue #2206)

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -623,6 +623,7 @@ let diagnosticDomain: String = "SwiftSyntaxMacroExpansion"
 private enum MacroApplicationError: DiagnosticMessage, Error {
   case accessorMacroOnVariableWithMultipleBindings
   case peerMacroOnVariableWithMultipleBindings
+  case memberMacroOnInvalidDecl(macroName: String)
   case malformedAccessor
 
   var diagnosticID: MessageID {
@@ -637,6 +638,8 @@ private enum MacroApplicationError: DiagnosticMessage, Error {
       return "accessor macro can only be applied to a single variable"
     case .peerMacroOnVariableWithMultipleBindings:
       return "peer macro can only be applied to a single variable"
+    case .memberMacroOnInvalidDecl(let macroName):
+      return "macro '\(macroName)' can only be applied to a struct, enum, class, extension, or actor"
     case .malformedAccessor:
       return """
         macro returned a malformed accessor. Accessors should start with an introducer like 'get' or 'set'.
@@ -702,6 +705,18 @@ private class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
       let attributedNode = node.asProtocol(WithAttributesSyntax.self),
       !attributedNode.attributes.isEmpty
     {
+      // Check if there are any member macros generated on a non-group decl.
+      if !(node.isProtocol(DeclGroupSyntax.self) || node.is(ExtensionDeclSyntax.self)) {
+        let memberMacros = self.macroAttributes(attachedTo: declSyntax, ofType: MemberMacro.Type.self)
+        for (attribute, _, _) in memberMacros {
+            let macroName = attribute.attributeName.trimmedDescription
+          contextGenerator(Syntax(node)).addDiagnostics(
+            from: MacroApplicationError.memberMacroOnInvalidDecl(macroName: macroName),
+            node: attribute
+          )
+        }
+      }
+
       // Apply body and preamble macros.
       if let nodeWithBody = node.asProtocol(WithOptionalCodeBlockSyntax.self),
         let declNodeWithBody = nodeWithBody as? any DeclSyntaxProtocol & WithOptionalCodeBlockSyntax

Tests/SwiftSyntaxMacroExpansionTest/Issue2206Tests.swift

@@ -0,0 +1,41 @@
+
+import SwiftDiagnostics
+import SwiftSyntax
+import SwiftSyntaxMacroExpansion
+import SwiftSyntaxMacros
+import SwiftSyntaxMacrosTestSupport
+import XCTest
+
+private struct NoOpMemberMacro: MemberMacro {
+  static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf declaration: some DeclGroupSyntax,
+    conformingTo protocols: [TypeSyntax],
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    return []
+  }
+}
+
+final class Issue2206Tests: XCTestCase {
+  private let indentationWidth: Trivia = .spaces(2)
+
+  func testMemberMacroOnVariable() {
+    // Issue #2206: assertMacroExpansion should emit an error if member macro is applied to declaration that can’t have members
+    // Currently this is expected to FAIL because the diagnostic is swallowed.
+    assertMacroExpansion(
+      """
+      @Test
+      var x: Int
+      """,
+      expandedSource: """
+        var x: Int
+        """,
+      diagnostics: [
+        DiagnosticSpec(message: "macro 'Test' can only be applied to a struct, enum, class, extension, or actor", line: 1, column: 1)
+      ],
+      macros: ["Test": NoOpMemberMacro.self],
+      indentationWidth: indentationWidth
+    )
+  }
+}