Sema: Accept @_implementationOnly on non-public structs

Gate it behind CheckImplementationOnly.

Author: xymus

include/swift/AST/DeclAttr.def

@@ -474,7 +474,7 @@ SIMPLE_DECL_ATTR(_alwaysEmitIntoClient, AlwaysEmitIntoClient,
   83)
 
 SIMPLE_DECL_ATTR(_implementationOnly, ImplementationOnly,
-  OnImport | OnFunc | OnConstructor | OnVar | OnSubscript,
+  OnImport | OnFunc | OnConstructor | OnVar | OnSubscript | OnStruct,
   UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   84)
 

include/swift/AST/DiagnosticsSema.def

@@ -3946,6 +3946,9 @@ ERROR(implementation_only_override_import_without_attr,none,
       "override of %kindonly0 imported as implementation-only must be declared "
       "'@_implementationOnly'",
       (const ValueDecl *))
+ERROR(implementation_only_on_structs_feature,none,
+      "'@_implementationOnly' on structs requires "
+      "'-enable-experimental-feature CheckImplementationOnly'", ())
 
 ERROR(import_attr_conflict,none,
       "%0 inconsistently imported with %1",
@@ -4128,7 +4131,7 @@ WARNING(attr_has_no_effect_on_decl_with_access_level,none,
        (DeclAttribute, AccessLevel))
 ERROR(attr_not_on_decl_with_invalid_access_level,none,
       "'%0' may not be used on "
-      "%select{private|fileprivate|internal|package|%error|%error}1 declarations",
+      "%select{private|fileprivate|internal|package|public|%error}1 declarations",
       (DeclAttribute, AccessLevel))
 
 ERROR(attr_has_no_effect_decl_not_available_before,none,

lib/Sema/TypeCheckAttr.cpp

@@ -4995,7 +4995,25 @@ AttributeChecker::visitImplementationOnlyAttr(ImplementationOnlyAttr *attr) {
     return;
   }
 
+  // @_implementationOnly on structs only applies to non-public types.
   auto *VD = cast<ValueDecl>(D);
+  if (isa<StructDecl>(VD)) {
+    if (!Ctx.LangOpts.hasFeature(Feature::CheckImplementationOnly)) {
+      diagnoseAndRemoveAttr(attr,
+          diag::implementation_only_on_structs_feature);
+      return;
+    }
+
+    auto access =
+        VD->getFormalAccessScope(/*useDC=*/nullptr,
+                                 /*treatUsableFromInlineAsPublic=*/true);
+    if (access.isPublicOrPackage())
+      diagnoseAndRemoveAttr(
+          attr, diag::attr_not_on_decl_with_invalid_access_level,
+          attr, access.accessLevelForDiagnostics());
+    return;
+  }
+
   auto *overridden = VD->getOverriddenDecl();
   if (!overridden) {
     diagnoseAndRemoveAttr(attr, diag::implementation_only_decl_non_override);