[BitwiseCopyable] Allow conformance via swift_attr

If an imported type's conformance cannot be implicitly inferred, allow
it to be explicitly stated via
`__attribute__((__swift_attr__("_BitwiseCopyable")))`.

Author: nate-chandler

include/swift/AST/DiagnosticsSema.def

@@ -7652,6 +7652,8 @@ ERROR(non_bitwise_copyable_type_cxx_nontrivial,none,
       "non-trivial C++ type cannot conform to 'BitwiseCopyable'", ())
 ERROR(non_bitwise_copyable_c_type_nontrivial,none,
       "type with unrepresentable fields cannot derive conformance to 'BitwiseCopyable'", ())
+NOTE(note_non_bitwise_copyable_c_type_add_attr,none,
+     "annotate the type __attribute__((__swift_attr__(\"_BitwiseCopyable\")))",())
 ERROR(non_bitwise_copyable_type_member,none,
       "%select{stored property %2|associated value %2}1 of "
       "'BitwiseCopyable'-conforming %kind3 has non-bitwise-copyable type %0",

lib/ClangImporter/ImportDecl.cpp

@@ -8039,6 +8039,30 @@ ClangImporter::Implementation::importSwiftAttrAttributes(Decl *MappedDecl) {
         continue;
       }
 
+      if (swiftAttr->getAttribute() == "_BitwiseCopyable") {
+        if (!SwiftContext.LangOpts.hasFeature(Feature::BitwiseCopyable))
+          continue;
+        auto *protocol =
+            SwiftContext.getProtocol(KnownProtocolKind::BitwiseCopyable);
+        auto *nominal = dyn_cast<NominalTypeDecl>(MappedDecl);
+        if (!nominal)
+          continue;
+        auto *module = nominal->getModuleContext();
+        // Don't synthesize a conformance if one already exists.
+        auto ty = nominal->getDeclaredInterfaceType();
+        if (module->lookupConformance(ty, protocol))
+          continue;
+        auto conformance = SwiftContext.getNormalConformance(
+            ty, protocol, nominal->getLoc(), nominal->getDeclContextForModule(),
+            ProtocolConformanceState::Complete,
+            /*isUnchecked=*/false,
+            /*isPreconcurrency=*/false);
+        conformance->setSourceKindAndImplyingConformance(
+            ConformanceEntryKind::Synthesized, nullptr);
+
+        nominal->registerProtocolConformance(conformance, /*synthesized=*/true);
+      }
+
       // Dig out a buffer with the attribute text.
       unsigned bufferID = getClangSwiftAttrSourceBuffer(
           swiftAttr->getAttribute());

lib/Sema/TypeCheckBitwise.cpp

@@ -272,6 +272,7 @@ static bool checkBitwiseCopyableInstanceStorage(NominalTypeDecl *nominal,
   if (sd && sd->hasUnreferenceableStorage()) {
     if (!isImplicit(check)) {
       sd->diagnose(diag::non_bitwise_copyable_c_type_nontrivial);
+      sd->diagnose(diag::note_non_bitwise_copyable_c_type_add_attr);
     }
     return true;
   }

test/Sema/bitwse_copyable_import.swift

@@ -54,6 +54,12 @@ struct IntsTrailing2 {
   int is[];
 };
 
+struct IntsTrailing3 {
+  double d;
+  float f;
+  int is[];
+} __attribute__((__swift_attr__("_BitwiseCopyable")));
+
 //--- Downstream.swift
 
 func take<T : _BitwiseCopyable>(_ t: T) {}
@@ -76,3 +82,6 @@ extension IntsTrailing2 : _BitwiseCopyable {} //expected-error{{bitwise_copyable
 func passIntsTrailing2(_ t: IntsTrailing2) {
   take(t)
 }
+func passIntsTrailing3(_ t: IntsTrailing3) {
+  take(t)
+}