[Macros] Ban extension macros attached to local types.

(cherry picked from commit 095542a)

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -7148,6 +7148,9 @@ ERROR(global_arbitrary_name,none,
       "'%0' macros are not allowed to introduce arbitrary names "
       "at global scope",
       (StringRef))
+ERROR(local_extension_macro,none,
+      "local type cannot have attached extension macro",
+      ())
 
 ERROR(macro_resolve_circular_reference, none,
       "circular reference resolving %select{freestanding|attached}0 macro %1",

lib/Sema/TypeCheckMacros.cpp

@@ -1488,6 +1488,11 @@ ExpandExtensionMacros::evaluate(Evaluator &evaluator,
 Optional<unsigned>
 swift::expandExtensions(CustomAttr *attr, MacroDecl *macro,
                         MacroRole role, NominalTypeDecl *nominal) {
+  if (nominal->getDeclContext()->isLocalContext()) {
+    nominal->diagnose(diag::local_extension_macro);
+    return llvm::None;
+  }
+
   auto macroSourceFile = ::evaluateAttachedMacro(macro, nominal, attr,
                                                  /*passParentContext=*/false,
                                                  role);

test/Macros/macro_expand_extensions.swift

@@ -54,3 +54,11 @@ struct Outer {
 
 // CHECK: Wrapped.requirement
 requiresP(Outer.Nested<Wrapped>.self)
+
+#if TEST_DIAGNOSTICS
+func testLocal() {
+  @DelegatedConformance
+  struct Local<Element> {}
+  // expected-error@-1{{local type cannot have attached extension macro}}
+}
+#endif