Serialization: Stop serializing cached rename decls.

Rename decls are typically derived from the rename strings attached to a
`@available` attributes. It shouldn't be necessary to serialize the cached
rename decls since they can be rederived. The only decls that have rename decls
and don't have reanme strings are synthesized by ClangImporter and don't get
serialized.

lib/Serialization/Deserialization.cpp

@@ -5615,7 +5615,6 @@ DeclDeserializer::readAvailable_DECL_ATTR(SmallVectorImpl<uint64_t> &scratch,
   DEF_VER_TUPLE_PIECES(Introduced);
   DEF_VER_TUPLE_PIECES(Deprecated);
   DEF_VER_TUPLE_PIECES(Obsoleted);
-  DeclID renameDeclID;
   unsigned rawPlatform, messageSize, renameSize;
 
   // Decode the record, pulling the version tuple information.
@@ -5623,7 +5622,7 @@ DeclDeserializer::readAvailable_DECL_ATTR(SmallVectorImpl<uint64_t> &scratch,
       scratch, isImplicit, isUnavailable, isDeprecated, isNoAsync,
       isPackageDescriptionVersionSpecific, isSPI, isForEmbedded,
       LIST_VER_TUPLE_PIECES(Introduced), LIST_VER_TUPLE_PIECES(Deprecated),
-      LIST_VER_TUPLE_PIECES(Obsoleted), rawPlatform, renameDeclID, messageSize,
+      LIST_VER_TUPLE_PIECES(Obsoleted), rawPlatform, messageSize,
       renameSize);
 
   auto maybePlatform = platformFromUnsigned(rawPlatform);
@@ -5632,11 +5631,6 @@ DeclDeserializer::readAvailable_DECL_ATTR(SmallVectorImpl<uint64_t> &scratch,
 
   PlatformKind platform = maybePlatform.value();
 
-  ValueDecl *renameDecl = nullptr;
-  if (renameDeclID) {
-    renameDecl = cast<ValueDecl>(MF.getDecl(renameDeclID));
-  }
-
   StringRef message = blobData.substr(0, messageSize);
   blobData = blobData.substr(messageSize);
   StringRef rename = blobData.substr(0, renameSize);

lib/Serialization/ModuleFormat.h

@@ -58,7 +58,7 @@ const uint16_t SWIFTMODULE_VERSION_MAJOR = 0;
 /// describe what change you made. The content of this comment isn't important;
 /// it just ensures a conflict if two people change the module format.
 /// Don't worry about adhering to the 80-column limit for this line.
-const uint16_t SWIFTMODULE_VERSION_MINOR = 903; // @main attribute moved
+const uint16_t SWIFTMODULE_VERSION_MINOR = 904; // @available renamed decl ID removed
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -2352,7 +2352,6 @@ namespace decls_block {
     BC_AVAIL_TUPLE, // Deprecated
     BC_AVAIL_TUPLE, // Obsoleted
     BCVBR<5>,    // platform
-    DeclIDField, // rename declaration (if any)
     BCVBR<5>,    // number of bytes in message string
     BCVBR<5>,    // number of bytes in rename string
     BCBlob       // message, followed by rename

lib/Serialization/Serialization.cpp

@@ -3038,7 +3038,8 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       ENCODE_VER_TUPLE(Deprecated, theAttr->Deprecated)
       ENCODE_VER_TUPLE(Obsoleted, theAttr->Obsoleted)
 
-      auto renameDeclID = S.addDeclRef(theAttr->RenameDecl);
+      assert(theAttr->Rename.empty() || !theAttr->hasCachedRenamedDecl());
+
       llvm::SmallString<32> blob;
       blob.append(theAttr->Message);
       blob.append(theAttr->Rename);
@@ -3056,7 +3057,6 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
           LIST_VER_TUPLE_PIECES(Deprecated),
           LIST_VER_TUPLE_PIECES(Obsoleted),
           static_cast<unsigned>(theAttr->getPlatform()),
-          renameDeclID,
           theAttr->Message.size(),
           theAttr->Rename.size(),
           blob);

test/Serialization/renamed-cycle.swift

@@ -0,0 +1,30 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module %t/Library.swift -o %t/Library.swiftmodule
+// RUN: %target-swift-frontend -emit-module %t/Renamed.swift -o %t/Renamed.swiftmodule -I %t/
+// RUN: %target-swift-frontend -typecheck %t/Client.swift -I %t/
+
+//--- Library.swift
+
+public struct HasRename {
+  public init(new: Int) { }
+}
+
+//--- Renamed.swift
+
+import Library
+
+extension HasRename {
+  @available(*, renamed: "init(new:)")
+  public init(old: Int) {
+    self.init(new: old)
+  }
+}
+
+//--- Client.swift
+
+import Library
+import Renamed
+
+_ = HasRename(old: 0)