[MacroSystem] Expand extension macros.

Author: hborla

Sources/SwiftSyntaxMacros/MacroSystem.swift

@@ -115,7 +115,8 @@ class MacroApplication<Context: MacroExpansionContext>: SyntaxRewriter {
           || macro is MemberMacro.Type
           || macro is AccessorMacro.Type
           || macro is MemberAttributeMacro.Type
-          || macro is ConformanceMacro.Type)
+          || macro is ConformanceMacro.Type
+          || macro is ExtensionMacro.Type)
       }
 
       if newAttributes.isEmpty {
@@ -438,6 +439,20 @@ extension MacroApplication {
       }
     }
 
+    let extensionMacroAttrs = getMacroAttributes(attachedTo: decl.as(DeclSyntax.self)!, ofType: ExtensionMacro.Type.self)
+    let extendedTypeSyntax = TypeSyntax("\(extendedType.trimmed)")
+    for (attribute, extensionMacro) in extensionMacroAttrs {
+      do {
+        let newExtensions = try extensionMacro.expansion(of: attribute,
+                                                         attachedTo: decl,
+                                                         providingExtensionsOf: extendedTypeSyntax,
+                                                         in: context)
+        extensions.append(contentsOf: newExtensions.map(DeclSyntax.init))
+      } catch {
+        context.addDiagnostics(from: error, node: attribute)
+      }
+    }
+
     return extensions
   }
 

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -682,6 +682,26 @@ public struct SendableConformanceMacro: ConformanceMacro {
   }
 }
 
+public struct SendableExtensionMacro: ExtensionMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    attachedTo: some DeclGroupSyntax,
+    providingExtensionsOf type: some TypeSyntaxProtocol,
+    in context: some MacroExpansionContext
+  ) throws -> [ExtensionDeclSyntax] {
+    let sendableExtension: DeclSyntax =
+      """
+      extension \(type.trimmed): Sendable {}
+      """
+
+    guard let extensionDecl = sendableExtension.as(ExtensionDeclSyntax.self) else {
+      return []
+    }
+
+    return [extensionDecl]
+  }
+}
+
 public struct DeclsFromStringsMacroNoAttrs: DeclarationMacro {
   public static var propagateFreestandingMacroAttributes: Bool { false }
   public static var propagateFreestandingMacroModifiers: Bool { false }
@@ -726,6 +746,7 @@ public let testMacros: [String: Macro.Type] = [
   "customTypeWrapper": CustomTypeWrapperMacro.self,
   "unwrap": UnwrapMacro.self,
   "AddSendable": SendableConformanceMacro.self,
+  "AddSendableExtension": SendableExtensionMacro.self,
 ]
 
 final class MacroSystemTests: XCTestCase {
@@ -1196,4 +1217,23 @@ final class MacroSystemTests: XCTestCase {
       indentationWidth: indentationWidth
     )
   }
+
+  func testExtensionExpansion() {
+    assertMacroExpansion(
+      """
+      @AddSendableExtension
+      struct MyType {
+      }
+      """,
+      expandedSource: """
+
+        struct MyType {
+        }
+        extension MyType: Sendable {
+        }
+        """,
+      macros: testMacros,
+      indentationWidth: indentationWidth
+    )
+  }
 }