MacroSystem should qualify nested types when expanding extension macros

Resolves rdar://119850970.

Author: bnbarham

Sources/SwiftSyntaxMacroExpansion/MacroSystem.swift

@@ -319,12 +319,7 @@ private func expandExtensionMacro(
   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 = attachedTo.syntacticQualifiedTypeContext else {
     return nil
   }
 
@@ -1299,3 +1294,25 @@ private extension AccessorDeclSyntax {
     return accessorSpecifier.tokenKind == .keyword(.get) || accessorSpecifier.tokenKind == .keyword(.set)
   }
 }
+
+private extension SyntaxProtocol {
+  /// Retrieve the qualified type for the nearest extension or name decl.
+  ///
+  /// For example, for `struct Foo { struct Bar {} }`, calling this on the
+  /// inner struct (`Bar`) will return `Foo.Bar`.
+  var syntacticQualifiedTypeContext: TypeSyntax? {
+    if let ext = self.as(ExtensionDeclSyntax.self) {
+      // Don't handle nested 'extension' cases - they are invalid anyway.
+      return ext.extendedType.trimmed
+    }
+
+    let base = self.parent?.syntacticQualifiedTypeContext
+    if let named = self.asProtocol(NamedDeclSyntax.self) {
+      if let base = base {
+        return TypeSyntax(MemberTypeSyntax(baseType: base, name: named.name.trimmed))
+      }
+      return TypeSyntax(IdentifierTypeSyntax(name: named.name.trimmed))
+    }
+    return base
+  }
+}

Tests/SwiftSyntaxMacroExpansionTest/ExtensionMacroTests.swift

@@ -70,13 +70,10 @@ final class ExtensionMacroTests: XCTestCase {
     assertMacroExpansion(
       """
       @AddSendableExtension
-      struct MyType {
-      }
+      struct MyType {}
       """,
       expandedSource: """
-
-        struct MyType {
-        }
+        struct MyType {}
 
         extension MyType: Sendable {
         }
@@ -89,17 +86,65 @@ final class ExtensionMacroTests: XCTestCase {
       """
       struct Wrapper {
         @AddSendableExtension
-        struct MyType {
+        struct MyType {}
+      }
+      """,
+      expandedSource: """
+        struct Wrapper {
+          struct MyType {}
+        }
+
+        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 MyType {
+          struct AnotherWrapper {}
+        }
+        extension Wrapper.AnotherWrapper {
+          struct MyType {}
+        }
+
+        extension Wrapper.AnotherWrapper.MyType: Sendable {
+        }
+        """,
+      macros: ["AddSendableExtension": SendableExtensionMacro.self],
+      indentationWidth: indentationWidth
+    )
+
+    assertMacroExpansion(
+      """
+      struct Wrapper {}
+      extension Wrapper {
+        struct AnotherWrapper {
+          @AddSendableExtension
+          struct MyType {}
+        }
+      }
+      """,
+      expandedSource: """
+        struct Wrapper {}
+        extension Wrapper {
+          struct AnotherWrapper {
+            struct MyType {}
           }
         }
 
-        extension MyType: Sendable {
+        extension Wrapper.AnotherWrapper.MyType: Sendable {
         }
         """,
       macros: ["AddSendableExtension": SendableExtensionMacro.self],