Warn that @objcImpl extension is deprecated

…only when the ObjCImplementation experimental language feature is enabled. (`@implementation extension` is the preferred form.)

Author: beccadax

include/swift/AST/DiagnosticsSema.def

@@ -1739,6 +1739,9 @@ ERROR(swift_native_objc_runtime_base_not_on_root_class,none,
       "@_swift_native_objc_runtime_base_not_on_root_class can only be applied "
       "to root classes", ())
 
+WARNING(objc_implementation_early_spelling_deprecated,none,
+        "warning-only '@_objcImplementation' spelling is deprecated; use "
+        "'@implementation' instead", ())
 ERROR(attr_objc_implementation_must_be_unconditional,none,
       "only unconditional extensions can implement an Objective-C '@interface'",
       ())

lib/Sema/TypeCheckAttr.cpp

@@ -1519,6 +1519,15 @@ void AttributeChecker::visitNonObjCAttr(NonObjCAttr *attr) {
 static bool hasObjCImplementationFeature(Decl *D, ObjCImplementationAttr *attr,
                                          Feature requiredFeature) {
   if (D->getASTContext().LangOpts.hasFeature(requiredFeature)) {
+    // Encourage early ObjCImplementation adopters to adopt @implementation.
+    // (We'll do this for CImplementation when we're ready to stabilize it.)
+    if (requiredFeature == Feature::ObjCImplementation
+            && attr->isEarlyAdopter())
+      D->getASTContext().Diags.diagnose(
+            attr->getLocation(),
+            diag::objc_implementation_early_spelling_deprecated)
+        .fixItReplace(attr->getLocation(), "implementation");
+
     return true;
   }
 

test/decl/ext/objc_implementation_features.swift

@@ -3,6 +3,8 @@
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h 2>&1 | %FileCheck --check-prefixes NO,CHECK %s
 // RUN: not %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource) -typecheck %s -import-objc-header %S/Inputs/objc_implementation.h -enable-experimental-feature ObjCImplementation 2>&1 | %FileCheck --check-prefixes YES,CHECK %s
 
+// NO-NOT: objc_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: warning: warning-only '@_objcImplementation' spelling is deprecated; use '@implementation' instead
+// YES-DAG: objc_implementation_features.swift:[[@LINE+1]]:{{[0-9]+}}: warning: warning-only '@_objcImplementation' spelling is deprecated; use '@implementation' instead
 @_objcImplementation extension ObjCSubclass {}
 
 // NO-DAG: objc_implementation_features.swift:[[@LINE+2]]:{{[0-9]+}}: error: 'implementation' attribute is only valid when experimental feature ObjCImplementation is enabled