Generize MemberAttributeMacro

Author: DougGregor

Sources/_SwiftSyntaxMacros/MacroProtocols/MemberAttributeMacro.swift

@@ -21,14 +21,17 @@ public protocol MemberAttributeMacro: AttachedMacro {
   /// - Parameters:
   ///   - node: The custom attribute describing the attached macro.
   ///   - declaration: The declaration the macro attribute is attached to.
-  ///   - member: The member delcaration to attach the resulting attributes to.
+  ///   - member: The member declaration to attach the resulting attributes to.
   ///   - context: The context in which to perform the macro expansion.
   ///
   /// - Returns: the set of attributes to apply to the given member.
-  static func expansion(
+  static func expansion<
+    Declaration: DeclGroupSyntax,
+    Context: MacroExpansionContext
+  >(
     of node: AttributeSyntax,
-    attachedTo declaration: DeclSyntax,
+    attachedTo declaration: Declaration,
     annotating member: DeclSyntax,
-    in context: any MacroExpansionContext
+    in context: Context
   ) throws -> [AttributeSyntax]
 }

Sources/_SwiftSyntaxMacros/MacroSystem.swift

@@ -425,6 +425,27 @@ extension MacroApplication {
     )
   }
 
+  private func expandMemberAttribute(
+    attribute: AttributeSyntax,
+    macro: MemberAttributeMacro.Type,
+    decl: DeclGroupSyntax,
+    member: DeclSyntax,
+    in context: MacroExpansionContext
+  ) throws -> [AttributeSyntax] {
+    #if false
+    _openExistential(decl) { d in
+      return try! macro.expansion(
+        of: attribute,
+        attachedTo: d,
+        annotating: member,
+        in: context
+      )
+    }
+    #else
+    return []
+    #endif
+  }
+
   private func expandAttributes(
     for macroAttributes: [(AttributeSyntax, MemberAttributeMacro.Type)],
     attachedTo decl: DeclSyntax,
@@ -437,13 +458,19 @@ extension MacroApplication {
     var attributes: [AttributeSyntax] = []
     for (attribute, attributeMacro) in macroAttributes {
       do {
-        try attributes.append(
-          contentsOf: attributeMacro.expansion(
+        let typedDecl = decl.asProtocol(DeclGroupSyntax.self)!
+
+        func expand<Decl: DeclGroupSyntax>(_ decl: Decl) throws -> [AttributeSyntax] {
+          return try attributeMacro.expansion(
             of: attribute,
-            attachedTo: DeclSyntax(decl),
+            attachedTo: decl,
             annotating: member.decl,
             in: context
           )
+        }
+
+        attributes.append(
+          contentsOf: try _openExistential(typedDecl, do: expand)
         )
       } catch {
         // Record the error

Tests/SwiftSyntaxMacrosTest/MacroSystemTests.swift

@@ -455,11 +455,14 @@ public struct AddBackingStorage: MemberMacro {
 }
 
 public struct WrapAllProperties: MemberAttributeMacro {
-  public static func expansion(
+  public static func expansion<
+    Declaration: DeclGroupSyntax,
+    Context: MacroExpansionContext
+  >(
     of node: AttributeSyntax,
-    attachedTo decl: DeclSyntax,
+    attachedTo decl: Declaration,
     annotating member: DeclSyntax,
-    in context: any MacroExpansionContext
+    in context: Context
   ) throws -> [AttributeSyntax] {
     guard member.is(VariableDeclSyntax.self) else {
       return []
@@ -477,11 +480,14 @@ public struct WrapAllProperties: MemberAttributeMacro {
 }
 
 public struct WrapStoredProperties: MemberAttributeMacro {
-  public static func expansion(
+  public static func expansion<
+    Declaration: DeclGroupSyntax,
+    Context: MacroExpansionContext
+  >(
     of node: AttributeSyntax,
-    attachedTo decl: DeclSyntax,
+    attachedTo decl: Declaration,
     annotating member: DeclSyntax,
-    in context: any MacroExpansionContext
+    in context: Context
   ) throws -> [AttributeSyntax] {
     guard let property = member.as(VariableDeclSyntax.self),
       property.bindings.count == 1
@@ -536,11 +542,14 @@ extension CustomTypeWrapperMacro: MemberMacro {
 }
 
 extension CustomTypeWrapperMacro: MemberAttributeMacro {
-  static func expansion(
+  static func expansion<
+    Declaration: DeclGroupSyntax,
+    Context: MacroExpansionContext
+  >(
     of node: AttributeSyntax,
-    attachedTo declaration: DeclSyntax,
+    attachedTo declaration: Declaration,
     annotating member: DeclSyntax,
-    in context: any MacroExpansionContext
+    in context: Context
   ) throws -> [AttributeSyntax] {
     return [
       AttributeSyntax(