[Deserialization] Use 'swift::lookupConformance' to resolve implicit conformances

On reading out a 'ProtocolConformanceXrefLayout', instead of querying the protocol on the 'NominalDecl' referenced by the Xref, use the general-purpose lookup that resolves implicit conformances to e.g. 'Sendable'.

Otherwise, we can end up in a situation where a library serializes some type as (implicitly) conforming to Sendable, but the client, upon deserialization, has not yet run implicit Sendable inference logic, so its representation of said type will not match.

Author: artemcm

lib/Serialization/Deserialization.cpp

@@ -18,6 +18,7 @@
 #include "swift/AST/Attr.h"
 #include "swift/AST/AttrKind.h"
 #include "swift/AST/AutoDiff.h"
+#include "swift/AST/ConformanceLookup.h"
 #include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/DistributedDecl.h"
 #include "swift/AST/Expr.h"
@@ -957,16 +958,29 @@ ProtocolConformanceDeserializer::readNormalProtocolConformanceXRef(
   if (!module)
     module = MF.getAssociatedModule();
 
-  SmallVector<ProtocolConformance *, 2> conformances;
-  nominal->lookupConformance(proto, conformances);
   PrettyStackTraceModuleFile traceMsg(
       "If you're seeing a crash here, check that your SDK and dependencies "
       "are at least as new as the versions used to build", MF);
-  // This would normally be an assertion but it's more useful to print the
-  // PrettyStackTrace here even in no-asserts builds.
-  if (conformances.empty())
-    abort();
-  return conformances.front();
+
+  // Because Sendable conformances are currently inferred with
+  // 'ImplicitKnownProtocolConformanceRequest' in swift::lookupConformance,
+  // we may end up in a situation where we are deserializing such inferred
+  // conformance but a lookup on the 'NominalDecl' will not succeed nor
+  // will it run inference logic. For now, special-case 'Sendable' lookups
+  // here.
+  // TODO: Sink Sendable derivation into the conformance lookup table
+  if (proto->isSpecificProtocol(KnownProtocolKind::Sendable)) {
+    auto conformanceRef = lookupConformance(nominal->getDeclaredInterfaceType(), proto);
+    if (!conformanceRef.isConcrete())
+      abort();
+    return conformanceRef.getConcrete();
+  } else {
+    SmallVector<ProtocolConformance *, 2> conformances;
+    nominal->lookupConformance(proto, conformances);
+    if (conformances.empty())
+      abort();
+    return conformances.front();
+  }
 }
 
 Expected<ProtocolConformance *>