MetadataReader: Add support for NSError toll-free bridging

An Error existential value can directly store a
reference to an NSError instance without wrapping
it in an Error container.

When "projecting" such an existential, the dynamic type
is the NSError's isa pointer, and the payload is the
address of the instance itself.

Author: slavapestov

include/swift/Reflection/ReflectionContext.h

@@ -537,8 +537,9 @@ class ReflectionContext
       if (!OptMetaAndValue)
         return false;
 
-      RemoteAddress InstanceMetadataAddress = OptMetaAndValue->first;
-      RemoteAddress InstanceAddress = OptMetaAndValue->second;
+      RemoteAddress InstanceMetadataAddress = std::get<0>(*OptMetaAndValue);
+      RemoteAddress InstanceAddress = std::get<1>(*OptMetaAndValue);
+      // FIXME: Check third value, 'isBridged'
 
       auto InstanceTR =
           readTypeFromMetadata(InstanceMetadataAddress.getAddressData());

include/swift/Remote/MetadataReader.h

@@ -327,8 +327,10 @@ class MetadataReader {
   }
 
   /// Given a pointer to a known-error existential, attempt to discover the
-  /// pointer to its metadata address and its value address.
-  Optional<std::pair<RemoteAddress, RemoteAddress>>
+  /// pointer to its metadata address, its value address, and whether this
+  /// is a toll-free-bridged NSError or an actual Error existential wrapper
+  /// around a native Swift value.
+  Optional<std::tuple<RemoteAddress, RemoteAddress, bool>>
   readMetadataAndValueErrorExistential(RemoteAddress ExistentialAddress) {
     // An pointer to an error existential is always an heap object.
     auto MetadataAddress =
@@ -337,13 +339,16 @@ class MetadataReader {
       return None;
 
     bool isObjC = false;
+    bool isBridged = false;
 
     // If we can determine the Objective-C class name, this is probably an
     // error existential with NSError-compatible layout.
     std::string ObjCClassName;
     if (readObjCClassName(*MetadataAddress, ObjCClassName)) {
       if (ObjCClassName == "__SwiftNativeNSError")
         isObjC = true;
+      else
+        isBridged = true;
     } else {
       // Otherwise, we can check to see if this is a class metadata with the
       // kind value's least significant bit set, which indicates a pure
@@ -356,6 +361,14 @@ class MetadataReader {
       isObjC = ClassMeta->isPureObjC();
     }
 
+    if (isBridged) {
+      // NSError instances don't need to be unwrapped.
+      return Optional<std::tuple<RemoteAddress, RemoteAddress, bool>>(
+          {RemoteAddress(*MetadataAddress),
+          ExistentialAddress,
+          isBridged});
+    }
+
     // In addition to the isa pointer and two 32-bit reference counts, if the
     // error existential is layout-compatible with NSError, we also need to
     // skip over its three word-sized fields: the error code, the domain,
@@ -387,9 +400,10 @@ class MetadataReader {
     auto Offset = (sizeof(HeapObject) + AlignmentMask) & ~AlignmentMask;
     InstanceAddress += Offset;
 
-    return Optional<std::pair<RemoteAddress, RemoteAddress>>(
+    return Optional<std::tuple<RemoteAddress, RemoteAddress, bool>>(
         {RemoteAddress(*InstanceMetadataAddress),
-         RemoteAddress(InstanceAddress)});
+         RemoteAddress(InstanceAddress),
+         isBridged});
   }
 
   /// Given a known-opaque existential, attemp to discover the pointer to its

lib/RemoteAST/RemoteAST.cpp

@@ -500,8 +500,10 @@ class RemoteASTContextConcreteImpl final : public RemoteASTContextImpl {
         Reader.readMetadataAndValueErrorExistential(RemoteAddress(*pointerval));
     if (!result)
       return getFailure<std::pair<Type, RemoteAddress>>();
-    RemoteAddress metadataAddress = result->first;
-    RemoteAddress valueAddress = result->second;
+
+    RemoteAddress metadataAddress = std::get<0>(*result);
+    RemoteAddress valueAddress = std::get<1>(*result);
+    bool isBridged = std::get<2>(*result);
 
     auto typeResult =
         Reader.readTypeFromMetadata(metadataAddress.getAddressData());

test/RemoteAST/existentials_objc.swift

@@ -31,3 +31,14 @@ printDynamicTypeAndAddressForExistential(NSString("hello") as AnyObject)
 
 // CHECK: NSString
 printDynamicTypeAndAddressForExistential(NSString("hello") as AnyObject)
+
+// Bridged NSError.
+class ClassError : NSError {
+  required init(coder: NSCoder) { fatalError() }
+  init() {
+    super.init(domain: "ClassError", code: 10, userInfo: [:])
+  }
+}
+
+// CHECK: ClassError
+printDynamicTypeAndAddressForExistential(ClassError() as Error)