[Macros] Allow any decl names in macro expansions when the macro covers arbitrary

names.

Author: hborla

include/swift/AST/Decl.h

@@ -8463,6 +8463,11 @@ class MacroDecl : public GenericContext, public ValueDecl {
   void getIntroducedNames(MacroRole role, ValueDecl *attachedTo,
                           SmallVectorImpl<DeclName> &names) const;
 
+  /// Returns a DeclName that represents arbitrary names.
+  static DeclName getArbitraryName() {
+    return DeclName();
+  }
+
   /// Retrieve the definition of this macro.
   MacroDefinition getDefinition() const;
 

lib/AST/Decl.cpp

@@ -10110,7 +10110,7 @@ void MacroDecl::getIntroducedNames(MacroRole role, ValueDecl *attachedTo,
     }
 
     case MacroIntroducedDeclNameKind::Arbitrary:
-      // FIXME: Indicate that the macro covers arbitrary names.
+      names.push_back(MacroDecl::getArbitraryName());
       break;
     }
   }

lib/Sema/TypeCheckMacros.cpp

@@ -538,10 +538,13 @@ static void validateMacroExpansion(SourceFile *expansionBuffer,
         continue;
       }
 
-      if (!coversName.count(baseName)) {
-        value->diagnose(diag::invalid_macro_introduced_name,
-                        baseName, macro->getBaseName());
+      if (coversName.count(baseName) ||
+          coversName.count(MacroDecl::getArbitraryName())) {
+        continue;
       }
+
+      value->diagnose(diag::invalid_macro_introduced_name,
+                      baseName, macro->getBaseName());
     }
   }
 }

test/Macros/Inputs/syntax_macro_definitions.swift

@@ -494,6 +494,25 @@ public struct AddMembers: MemberMacro {
   }
 }
 
+public struct AddArbitraryMembers: MemberMacro {
+  public static func expansion(
+    of node: AttributeSyntax,
+    providingMembersOf decl: some DeclGroupSyntax,
+    in context: some MacroExpansionContext
+  ) throws -> [DeclSyntax] {
+    guard let identified = decl.asProtocol(IdentifiedDeclSyntax.self) else {
+      return []
+    }
+
+    let parentName = identified.identifier.trimmed
+    return [
+      "struct \(parentName)1 {}",
+      "struct \(parentName)2 {}",
+      "struct \(parentName)3 {}",
+    ]
+  }
+}
+
 /// Implementation of the `wrapStoredProperties` macro, which can be
 /// used to apply an attribute to all of the stored properties of a type.
 ///

test/Macros/macro_expand_synthesized_members.swift

@@ -29,6 +29,19 @@ let s = S()
 // CHECK: Storage
 s.useSynthesized()
 
+@attached(member, names: arbitrary)
+macro addArbitraryMembers() = #externalMacro(module: "MacroDefinition", type: "AddArbitraryMembers")
+
+@addArbitraryMembers
+struct MyType {}
+
+// CHECK: MyType1
+// CHECK: MyType2
+// CHECK: MyType3
+print(MyType.MyType1.self)
+print(MyType.MyType2.self)
+print(MyType.MyType3.self)
+
 @attached(
   member,
   names: named(RawValue), named(rawValue), named(`init`)