[clang] add a new `swift_attr` attribute

The swift_attr attribute is a generic annotation attribute that's not used by clang,
but is used by the Swift compiler. The Swift compiler can use these annotations to provide
various syntactic and semantic sugars for the imported Objective-C API declarations.

Differential Revision: https://reviews.llvm.org/D92354

clang/include/clang/Basic/Attr.td

@@ -2149,6 +2149,12 @@ def Regparm : TypeAttr {
   let ASTNode = 0;
 }
 
+def SwiftAttr : InheritableAttr {
+  let Spellings = [GNU<"swift_attr">];
+  let Args = [StringArgument<"Attribute">];
+  let Documentation = [SwiftAttrDocs];
+}
+
 def SwiftBridge : InheritableAttr {
   let Spellings = [GNU<"swift_bridge">];
   let Args = [StringArgument<"SwiftType">];

clang/include/clang/Basic/AttrDocs.td

@@ -3628,6 +3628,19 @@ Swift.
   }];
 }
 
+def SwiftAttrDocs : Documentation {
+  let Category = SwiftDocs;
+  let Heading = "swift_attr";
+  let Content = [{
+The ``swift_attr`` provides a Swift-specific annotation for the declaration
+to which the attribute appertains to. It can be used on any declaration
+in Clang. This kind of annotation is ignored by Clang as it doesn't have any
+semantic meaning in languages supported by Clang. The Swift compiler can
+interpret these annotations according to its own rules when importing C or
+Objective-C declarations.
+}];
+}
+
 def SwiftBridgeDocs : Documentation {
   let Category = SwiftDocs;
   let Heading = "swift_bridge";

clang/lib/Sema/SemaDeclAttr.cpp

@@ -5607,6 +5607,16 @@ static void handleObjCPreciseLifetimeAttr(Sema &S, Decl *D,
   D->addAttr(::new (S.Context) ObjCPreciseLifetimeAttr(S.Context, AL));
 }
 
+static void handleSwiftAttrAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
+  // Make sure that there is a string literal as the annotation's single
+  // argument.
+  StringRef Str;
+  if (!S.checkStringLiteralArgumentAttr(AL, 0, Str))
+    return;
+
+  D->addAttr(::new (S.Context) SwiftAttrAttr(S.Context, AL, Str));
+}
+
 static void handleSwiftBridge(Sema &S, Decl *D, const ParsedAttr &AL) {
   // Make sure that there is a string literal as the annotation's single
   // argument.
@@ -7941,6 +7951,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
     break;
 
   // Swift attributes.
+  case ParsedAttr::AT_SwiftAttr:
+    handleSwiftAttrAttr(S, D, AL);
+    break;
   case ParsedAttr::AT_SwiftBridge:
     handleSwiftBridge(S, D, AL);
     break;

clang/test/AST/attr-swift_attr.m

@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -fsyntax-only -ast-dump %s | FileCheck %s
+
+__attribute__((swift_attr("@actor")))
+@interface View
+@end
+
+// CHECK: InterfaceDecl {{.*}} View
+// CHECK-NEXT: SwiftAttrAttr {{.*}} "@actor"

clang/test/SemaObjC/validate-attr-swift_attr.m

@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -verify -fsyntax-only %s
+
+// expected-error@+1 {{'swift_attr' attribute takes one argument}}
+__attribute__((swift_attr))
+@interface I
+@end
+
+// expected-error@+1 {{'swift_attr' attribute requires a string}}
+__attribute__((swift_attr(1)))
+@interface J
+@end