slight tweak to how implicit conformance of nominals is generated

Also, only generate those conformances when the move-only feature is on,
for now.

Author: kavon

lib/AST/Module.cpp

@@ -1659,6 +1659,15 @@ LookupConformanceInModuleRequest::evaluate(
       } else {
         return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);
       }
+    } else if (protocol->isSpecificProtocol(KnownProtocolKind::Copyable)) {
+      // Only move-only nominals are not Copyable
+      if (nominal->isMoveOnly()) {
+        return ProtocolConformanceRef::forInvalid();
+      } else {
+        // FIXME: this should probably follow the Sendable case in that
+        // we should synthesize and append a ProtocolConformance to the `conformances` list.
+       return ProtocolConformanceRef(protocol);
+      }
     } else {
       // Was unable to infer the missing conformance.
       return ProtocolConformanceRef::forMissingOrInvalid(type, protocol);

lib/AST/ProtocolConformance.cpp

@@ -1097,11 +1097,6 @@ void NominalTypeDecl::prepareConformanceTable() const {
   if (mutableThis->getAttrs().hasAttribute<GlobalActorAttr>()) {
     addSynthesized(ctx.getProtocol(KnownProtocolKind::GlobalActor));
   }
-
-  // All nominal types that are not move-only conform to Copyable.
-  if (!mutableThis->isMoveOnly()) {
-    addSynthesized(ctx.getProtocol(KnownProtocolKind::Copyable));
-  }
 }
 
 bool NominalTypeDecl::lookupConformance(
@@ -1239,7 +1234,11 @@ static SmallVector<ProtocolConformance *, 2> findSynthesizedConformances(
   // Concrete types may synthesize some conformances
   if (!isa<ProtocolDecl>(nominal)) {
     trySynthesize(KnownProtocolKind::Sendable);
-    trySynthesize(KnownProtocolKind::Copyable);
+
+    // FIXME(kavon): make sure this conformance doesn't show up in swiftinterfaces
+    // before do this synthesis unconditionally.
+    if (dc->getASTContext().LangOpts.hasFeature(Feature::MoveOnly))
+      trySynthesize(KnownProtocolKind::Copyable);
   }
 
   /// Distributed actors can synthesize Encodable/Decodable, so look for those