[BitwiseCopyable] Don't derive on @unchecked field

Allow for a type to conform to BitwiseCopyable but not in such a way
that aggregates containing an instance will derive conformance
automatically.  Useful for types which don't involve reference counting
but nonetheless can't simply be copied bit-by-bit.

Author: nate-chandler

lib/Sema/TypeCheckBitwise.cpp

@@ -106,6 +106,7 @@ class BitwiseCopyableStorageVisitor : public StorageVisitor {
 
   bool visitMemberDecl(ValueDecl *storage, Type ty);
   bool visitMemberType(Type type, SourceLoc loc);
+  bool isUnchecked(ProtocolConformanceRef conformance);
   bool visitNonconformingMemberType(Type type, SourceLoc loc);
   void emitNonconformingMemberTypeDiagnostic(Type ty, SourceLoc loc);
 };
@@ -136,6 +137,13 @@ bool BitwiseCopyableStorageVisitor::visitMemberType(Type ty, SourceLoc loc) {
     return visitNonconformingMemberType(ty, loc);
   }
 
+  if (isImplicit(check) && isUnchecked(conformance)) {
+    // Do not automatically derive conformance if one of the field's
+    // conformance is @unchecked.
+    invalid = true;
+    return true;
+  }
+
   // Walk the conformance, diagnosing any missing BitwiseCopyable conformances.
   bool anyMissing = false;
   conformance.forEachMissingConformance(
@@ -150,6 +158,20 @@ bool BitwiseCopyableStorageVisitor::visitMemberType(Type ty, SourceLoc loc) {
   return anyMissing;
 }
 
+bool BitwiseCopyableStorageVisitor::isUnchecked(
+    ProtocolConformanceRef conformance) {
+  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();
+}
+
 bool BitwiseCopyableStorageVisitor::visitNonconformingMemberType(
     Type ty, SourceLoc loc) {
   if (ty->hasError())

lib/Sema/TypeCheckProtocol.cpp

@@ -2165,7 +2165,8 @@ static void diagnoseConformanceImpliedByConditionalConformance(
 /// to the given protocol. This should return true when @unchecked can be
 /// used to disable those semantic checks.
 static bool hasAdditionalSemanticChecks(ProtocolDecl *proto) {
-  return proto->isSpecificProtocol(KnownProtocolKind::Sendable);
+  return proto->isSpecificProtocol(KnownProtocolKind::Sendable) ||
+         proto->isSpecificProtocol(KnownProtocolKind::BitwiseCopyable);
 }
 
 /// Determine whether the type \c T conforms to the protocol \c Proto,

test/Sema/bitwise_copyable.swift

@@ -94,6 +94,21 @@ struct S_Explicit_With_Metatype_Optional_AnyObject : _BitwiseCopyable {
   var ty: Optional<AnyObject>.Type
 }
 
+struct S_Unchecked : @unchecked _BitwiseCopyable {}
+
+struct S_Implicit_With_S_Unchecked {
+  var s: S_Unchecked
+}
+
+func passS_Implicit_With_S_Unchecked(_ s: S_Implicit_With_S_Unchecked) {
+  take1(s) // expected-error{{type_does_not_conform_decl_owner}}
+           // expected-note@-92 {{where_requirement_failure_one_subst}}
+}
+
+struct S_Explicit_With_S_Unchecked : _BitwiseCopyable {
+  var s: S_Unchecked
+}
+
 //==============================================================================
 //===========================DEPENDENCY-FREE TESTS=(END)======================}}
 //==============================================================================