MacroSystem should qualify nested types when expanding extension macros

Resolves rdar://119850970.

Author: bnbarham

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -312,19 +312,61 @@ private func expandAccessorMacroWithExistingAccessors(
   return "\(raw: indentedSource)"
 }
 
+private func buildQualifiedType(_ inner: DeclSyntax) -> TypeSyntax? {
+  if let ext = inner.as(ExtensionDeclSyntax.self) {
+    return ext.extendedType.trimmed
+  }
+
+  guard let identified = inner.asProtocol(NamedDeclSyntax.self) else {
+    return nil
+  }
+
+  var typeNames: [TokenSyntax] = []
+  var parentExtension: TypeSyntax? = nil
+  var possibleParent = inner.parent
+  while let parent = possibleParent {
+    possibleParent = parent.parent
+    if let parent = parent.asProtocol(NamedDeclSyntax.self) {
+      typeNames.append(parent.name.trimmed)
+    } else if let parent = parent.as(ExtensionDeclSyntax.self) {
+      // Extensions must be top level
+      if parentExtension != nil {
+        return nil
+      }
+      parentExtension = parent.extendedType.trimmed
+    }
+  }
+
+  // Attached type is within an extension
+  if let parentExtension {
+    // Extensions must be top level
+    if !typeNames.isEmpty {
+      return nil
+    }
+    return TypeSyntax(MemberTypeSyntax(baseType: parentExtension, name: identified.name.trimmed))
+  }
+
+  // Attached type is top level
+  if typeNames.isEmpty {
+    return TypeSyntax(IdentifierTypeSyntax(name: identified.name.trimmed))
+  }
+
+  // Attached type is nested in another type(s)
+  var baseType: TypeSyntax = TypeSyntax(IdentifierTypeSyntax(name: typeNames.popLast()!))
+  while let typeName = typeNames.popLast() {
+    baseType = TypeSyntax(MemberTypeSyntax(baseType: baseType, name: typeName))
+  }
+  return TypeSyntax(MemberTypeSyntax(baseType: baseType, name: identified.name.trimmed))
+}
+
 private func expandExtensionMacro(
   definition: ExtensionMacro.Type,
   attributeNode: AttributeSyntax,
   attachedTo: DeclSyntax,
   in context: some MacroExpansionContext,
   indentationWidth: Trivia
 ) throws -> CodeBlockItemListSyntax? {
-  let extendedType: TypeSyntax
-  if let identified = attachedTo.asProtocol(NamedDeclSyntax.self) {
-    extendedType = "\(identified.name.trimmed)"
-  } else if let ext = attachedTo.as(ExtensionDeclSyntax.self) {
-    extendedType = "\(ext.extendedType.trimmed)"
-  } else {
+  guard let extendedType = buildQualifiedType(attachedTo) else {
     return nil
   }
 

Tests/SwiftSyntaxMacroExpansionTest/ExtensionMacroTests.swift

@@ -99,7 +99,34 @@ final class ExtensionMacroTests: XCTestCase {
           }
         }
 
-        extension MyType: Sendable {
+        extension Wrapper.MyType: Sendable {
+        }
+        """,
+      macros: ["AddSendableExtension": SendableExtensionMacro.self],
+      indentationWidth: indentationWidth
+    )
+
+    assertMacroExpansion(
+      """
+      struct Wrapper {
+        struct AnotherWrapper {}
+      }
+      extension Wrapper.AnotherWrapper {
+        @AddSendableExtension
+        struct MyType {
+        }
+      }
+      """,
+      expandedSource: """
+        struct Wrapper {
+          struct AnotherWrapper {}
+        }
+        extension Wrapper.AnotherWrapper {
+          struct MyType {
+          }
+        }
+
+        extension Wrapper.AnotherWrapper.MyType: Sendable {
         }
         """,
       macros: ["AddSendableExtension": SendableExtensionMacro.self],