Add @_implements decl attribute.

include/swift/AST/Attr.def

@@ -267,6 +267,11 @@ SIMPLE_DECL_ATTR(discardableResult, DiscardableResult,
 
 SIMPLE_DECL_ATTR(GKInspectable, GKInspectable, OnVar, 66)
 
+DECL_ATTR(_implements, Implements,
+          OnFunc | OnVar | OnSubscript | OnTypeAlias
+          | NotSerialized | UserInaccessible,
+          /* Not serialized */ 67)
+
 #undef TYPE_ATTR
 #undef DECL_ATTR_ALIAS
 #undef SIMPLE_DECL_ATTR

include/swift/AST/Attr.h

@@ -25,10 +25,12 @@
 #include "swift/AST/Identifier.h"
 #include "swift/AST/AttrKind.h"
 #include "swift/AST/ConcreteDeclRef.h"
+#include "swift/AST/DeclNameLoc.h"
 #include "swift/AST/KnownProtocols.h"
 #include "swift/AST/Ownership.h"
 #include "swift/AST/PlatformKind.h"
 #include "swift/AST/Requirement.h"
+#include "swift/AST/TypeLoc.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -43,7 +45,6 @@ class Decl;
 class ClassDecl;
 class GenericFunctionType;
 class TrailingWhereClause;
-struct TypeLoc;
 
 /// TypeAttributes - These are attributes that may be applied to types.
 class TypeAttributes {
@@ -1127,6 +1128,36 @@ public:
   }
 };
 
+/// The @_implements attribute, which treats a decl as the implementation for
+/// some named protocol requirement (but otherwise not-visible by that name).
+class ImplementsAttr : public DeclAttribute {
+
+  TypeLoc ProtocolType;
+  DeclName MemberName;
+  DeclNameLoc MemberNameLoc;
+
+public:
+  ImplementsAttr(SourceLoc atLoc, SourceRange Range,
+                 TypeLoc ProtocolType,
+                 DeclName MemberName,
+                 DeclNameLoc MemberNameLoc);
+
+  static ImplementsAttr *create(ASTContext &Ctx, SourceLoc atLoc,
+                                SourceRange Range,
+                                TypeLoc ProtocolType,
+                                DeclName MemberName,
+                                DeclNameLoc MemberNameLoc);
+
+  TypeLoc getProtocolType() const;
+  TypeLoc &getProtocolType();
+  DeclName getMemberName() const { return MemberName; }
+  DeclNameLoc getMemberNameLoc() const { return MemberNameLoc; }
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_Implements;
+  }
+};
+
 /// \brief Attributes that may be applied to declarations.
 class DeclAttributes {
   /// Linked list of declaration attributes.

include/swift/Serialization/ModuleFormat.h

@@ -1310,6 +1310,7 @@ namespace decls_block {
   using ObjCBridgedDeclAttrLayout = BCRecordLayout<ObjCBridged_DECL_ATTR>;
   using SynthesizedProtocolDeclAttrLayout
     = BCRecordLayout<SynthesizedProtocol_DECL_ATTR>;
+  using ImplementsDeclAttrLayout = BCRecordLayout<Implements_DECL_ATTR>;
 
   using InlineDeclAttrLayout = BCRecordLayout<
     Inline_DECL_ATTR,