Start serializing @_implements attribute

@_implements is in this odd state where it is dropped from serialization,
but persists everywhere else. Keep it in the serialized modules.

Author: DougGregor

include/swift/AST/Attr.def

@@ -235,7 +235,7 @@ SIMPLE_DECL_ATTR(GKInspectable, GKInspectable,
   OnVar | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   66)
 DECL_ATTR(_implements, Implements,
-  OnFunc | OnAccessor | OnVar | OnSubscript | OnTypeAlias | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
+  OnFunc | OnAccessor | OnVar | OnSubscript | OnTypeAlias | UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,
   67)
 DECL_ATTR(_objcRuntimeName, ObjCRuntimeName,
   OnClass | UserInaccessible | NotSerialized | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,

lib/Serialization/Deserialization.cpp

@@ -5440,6 +5440,47 @@ llvm::Error DeclDeserializer::deserializeDeclCommon() {
         break;
       }
 
+      case decls_block::Implements_DECL_ATTR: {
+        bool isImplicit;
+        DeclContextID contextID;
+        DeclID protocolID;
+        unsigned numNameComponentsBiased;
+        ArrayRef<uint64_t> nameIDs;
+        serialization::decls_block::ImplementsDeclAttrLayout::readRecord(
+            scratch, isImplicit, contextID, protocolID, numNameComponentsBiased,
+            nameIDs);
+
+        // Resolve the context.
+        auto dcOrError = MF.getDeclContextChecked(contextID);
+        if (!dcOrError)
+          return dcOrError.takeError();
+        DeclContext *dc = dcOrError.get();
+
+        // Resolve the member name.
+        DeclName memberName;
+        Identifier baseName = MF.getIdentifier(nameIDs.front());
+        if (numNameComponentsBiased != 0) {
+          SmallVector<Identifier, 2> names;
+          for (auto nameID : nameIDs.slice(1, numNameComponentsBiased-1)) {
+            names.push_back(MF.getIdentifier(nameID));
+          }
+          memberName = DeclName(ctx, baseName, names);
+        } else {
+          memberName = baseName;
+        }
+
+        Expected<Decl *> protocolOrError = MF.getDeclChecked(protocolID);
+        ProtocolDecl *protocol;
+        if (protocolOrError) {
+          protocol = cast<ProtocolDecl>(protocolOrError.get());
+        } else {
+          return protocolOrError.takeError();
+        }
+
+        Attr = ImplementsAttr::create(dc, protocol, memberName);
+        break;
+      }
+
       case decls_block::CDecl_DECL_ATTR: {
         bool isImplicit;
         serialization::decls_block::CDeclDeclAttrLayout::readRecord(

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 = 838; // transferring param
+const uint16_t SWIFTMODULE_VERSION_MINOR = 839; // @_implements
 
 /// A standard hash seed used for all string hashes in a serialized module.
 ///
@@ -2087,6 +2087,16 @@ namespace decls_block {
     BCBlob      // _silgen_name
   >;
 
+  using ImplementsDeclAttrLayout = BCRecordLayout<
+    Implements_DECL_ATTR,
+    BCFixed<1>,  // implicit flag
+    DeclContextIDField,// context decl
+    DeclIDField, // protocol
+    BCVBR<5>,     // 0 for a simple name, otherwise the number of parameter name
+                  // components plus one
+    BCArray<IdentifierIDField> // name components
+  >;
+
   using SPIAccessControlDeclAttrLayout = BCRecordLayout<
     SPIAccessControl_DECL_ATTR,
     BCArray<IdentifierIDField>  // SPI names
@@ -2183,7 +2193,6 @@ namespace decls_block {
   using ObjCBridgedDeclAttrLayout = BCRecordLayout<ObjCBridged_DECL_ATTR>;
   using SynthesizedProtocolDeclAttrLayout
     = BCRecordLayout<SynthesizedProtocol_DECL_ATTR>;
-  using ImplementsDeclAttrLayout = BCRecordLayout<Implements_DECL_ATTR>;
   using ObjCRuntimeNameDeclAttrLayout
     = BCRecordLayout<ObjCRuntimeName_DECL_ATTR>;
   using RestatedObjCConformanceDeclAttrLayout

lib/Serialization/Serialization.cpp

@@ -2681,7 +2681,6 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
     case DAK_SetterAccess:
     case DAK_ObjCBridged:
     case DAK_SynthesizedProtocol:
-    case DAK_Implements:
     case DAK_ObjCRuntimeName:
     case DAK_RestatedObjCConformance:
     case DAK_ClangImporterSynthesizedType:
@@ -2709,6 +2708,29 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
       return;
     }
 
+    case DAK_Implements: {
+      auto *theAttr = cast<ImplementsAttr>(DA);
+      auto abbrCode = S.DeclTypeAbbrCodes[ImplementsDeclAttrLayout::Code];
+
+      DeclName memberName = theAttr->getMemberName();
+      SmallVector<IdentifierID, 4> nameComponents;
+      nameComponents.push_back(
+          S.addDeclBaseNameRef(memberName.getBaseName()));
+      for (auto argName : memberName.getArgumentNames())
+        nameComponents.push_back(S.addDeclBaseNameRef(argName));
+
+      auto dc = D->getDeclContext();
+      ImplementsDeclAttrLayout::emitRecord(
+          S.Out, S.ScratchRecord, abbrCode,
+          theAttr->isImplicit(),
+          S.addDeclContextRef(dc).getOpaqueValue(),
+          S.addDeclRef(theAttr->getProtocol(dc)),
+          memberName.getArgumentNames().size() +
+            memberName.isCompoundName(),
+          nameComponents);
+      return;
+    }
+
     case DAK_CDecl: {
       auto *theAttr = cast<CDeclAttr>(DA);
       auto abbrCode = S.DeclTypeAbbrCodes[CDeclDeclAttrLayout::Code];