NFC: [TypeLowering] Handle @unchecked BC fields.

The appearance of a field whose type's conformance to BitwiseCopyable is
@unchecked permits the type not to conform.

Author: nate-chandler

lib/SIL/IR/TypeLowering.cpp

@@ -28,6 +28,7 @@
 #include "swift/AST/Pattern.h"
 #include "swift/AST/PrettyStackTrace.h"
 #include "swift/AST/PropertyWrappers.h"
+#include "swift/AST/ProtocolConformance.h"
 #include "swift/AST/TypeDifferenceVisitor.h"
 #include "swift/AST/Types.h"
 #include "swift/ClangImporter/ClangModule.h"
@@ -3009,6 +3010,21 @@ void TypeConverter::verifyLexicalLowering(const TypeLowering &lowering,
   }
 }
 
+static bool isUnchecked(ProtocolConformanceRef conformance) {
+  if (!conformance)
+    return false;
+  if (!conformance.isConcrete())
+    return false;
+  auto concrete = conformance.getConcrete();
+  assert(concrete);
+  auto *root = concrete->getRootConformance();
+  assert(root);
+  auto *normal = dyn_cast<NormalProtocolConformance>(root);
+  if (!normal)
+    return false;
+  return normal->isUnchecked();
+}
+
 void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
                                           AbstractionPattern origType,
                                           CanType substType,
@@ -3042,10 +3058,17 @@ void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
           if (!nominal)
             return false;
 
-          // Nominals with generic parameters must be explicitly conformed to
-          // BitwiseCopyable.
-          auto *generic = ty.getAnyGeneric();
-          if (generic && generic->isGenericContext()) {
+          // Don't walk into types whose conformance is unchecked--such
+          // conformances obstruct automatic inference of BitwiseCopyable.
+          auto conformance = M.checkConformance(ty, bitwiseCopyableProtocol);
+          if (isUnchecked(conformance)) {
+            return true;
+          }
+
+          // Nominals with fields that conditionally conform to BitwiseCopyable
+          // must be explicitly conditionally conformed.  Such a field must be a
+          // generic context.
+          if (nominal->isGenericContext()) {
             return true;
           }
 
@@ -3059,10 +3082,12 @@ void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
           // Return false to indicate seeing a leaf which justifies the type
           // being trivial but not conforming to BitwiseCopyable.
 
-          // A BitwiseCopyable conformer appearing within its layout doesn't
-          // explain why substType doesn't itself conform.
-          if (M.checkConformance(ty, bitwiseCopyableProtocol))
-            return true;
+          // A BitwiseCopyable conformer appearing within its layout explains a
+          // non-conformance iff the conformance is @unchecked.
+          auto conformance = M.checkConformance(ty, bitwiseCopyableProtocol);
+          if (conformance) {
+            return !isUnchecked(conformance);
+          }
 
           // ModuleTypes are trivial but don't warrant being given a conformance
           // to BitwiseCopyable.
@@ -3099,15 +3124,15 @@ void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
             return true;
           }
 
-          /// A non-conforming generic nominal type justifies substType not
-          /// conforming.
-          auto *generic = ty.getAnyGeneric();
-          if (generic && generic->isGenericContext()) {
+          /// A field of conditionally-BitwiseCopyable type justifies the
+          /// aggregate not conforming because the aggregate must be conformed
+          /// explicitly in that case.
+          if (nominal->isGenericContext()) {
             return false;
           }
 
           // The field is trivial and the whole type is nonconforming.  That's
-          // legal iff the type is public.
+          // legal iff the field's type is public.
           return !nominal
                       ->getFormalAccessScope(
                           /*useDC=*/nullptr,
@@ -3148,6 +3173,7 @@ void TypeConverter::verifyTrivialLowering(const TypeLowering &lowering,
       llvm::errs() << "Non-trivial type with _BitwiseCopyable conformance!?:\n"
                    << substType << "\n";
       conformance.print(llvm::errs());
+      llvm::errs() << "\n";
       assert(false);
     }
   }