Runtime: Add swift_getTypePackByMangledName() entry point

For now this has SWIFT_RUNTIME_LIBRARY_VISIBILITY, but in the
future we might want to make it public so that IRGen can use it
to build packs out of concrete types.

Author: slavapestov

include/swift/ABI/Metadata.h

@@ -4917,6 +4917,8 @@ class TargetPackPointer {
   using PointerType = typename Runtime::template Pointer<const Pointee<Runtime>>;
 
 public:
+  explicit TargetPackPointer() : Ptr(0) {}
+
   explicit TargetPackPointer(typename Runtime::StoredSize rawPtr) : Ptr(rawPtr) {}
 
   explicit TargetPackPointer(const void *rawPtr)
@@ -4926,6 +4928,10 @@ class TargetPackPointer {
     : Ptr(reinterpret_cast<typename Runtime::StoredSize>(ptr) |
           (lifetime == PackLifetime::OnHeap ? 1 : 0)) {}
 
+  explicit operator bool() const {
+    return Ptr != 0;
+  }
+
   // Strips off the LSB.
   const PointerType *getElements() const {
     return reinterpret_cast<const PointerType *>(Ptr & ~1);

stdlib/public/runtime/MetadataLookup.cpp

@@ -2301,6 +2301,34 @@ swift_stdlib_getTypeByMangledNameUntrusted(const char *typeNameStart,
                                     {}, {}).getType().getMetadata();
 }
 
+TypeLookupErrorOr<MetadataPackPointer>
+swift::getTypePackByMangledName(StringRef typeName,
+                                const void *const *origArgumentVector,
+                                SubstGenericParameterFn substGenericParam,
+                                SubstDependentWitnessTableFn substWitnessTable) {
+  DemanglerForRuntimeTypeResolution<StackAllocatedDemangler<2048>> demangler;
+
+  NodePointer node = demangler.demangleTypeRef(typeName);
+  if (!node)
+    return TypeLookupError("Demangling failed");
+
+  DecodedMetadataBuilder builder(demangler, substGenericParam,
+                                 substWitnessTable);
+  auto type = Demangle::decodeMangledType(builder, node);
+  if (type.isError()) {
+    return *type.getError();
+  }
+  if (!type.getType()) {
+    return TypeLookupError("NULL type but no error provided");
+  }
+
+  if (!type.getType().isMetadataPack()) {
+    return TypeLookupError("This entry point is only for packs");
+  }
+
+  return type.getType().getMetadataPack();
+}
+
 // ==== Function metadata functions ----------------------------------------------
 
 static llvm::Optional<llvm::StringRef>

stdlib/public/runtime/Private.h

@@ -477,6 +477,20 @@ class TypeInfo {
                                const void * const *arguments,
                                SubstGenericParameterFn substGenericParam,
                                SubstDependentWitnessTableFn substWitnessTable);
+
+  /// Retrieve the type metadata pack described by the given type name.
+  ///
+  /// \p substGenericParam Function that provides generic argument metadata
+  /// given a particular generic parameter specified by depth/index.
+  /// \p substWitnessTable Function that provides witness tables given a
+  /// particular dependent conformance index.
+  SWIFT_RUNTIME_LIBRARY_VISIBILITY
+  TypeLookupErrorOr<MetadataPackPointer> getTypePackByMangledName(
+                               StringRef typeName,
+                               const void * const *arguments,
+                               SubstGenericParameterFn substGenericParam,
+                               SubstDependentWitnessTableFn substWitnessTable);
+
 #pragma clang diagnostic pop
 
   /// Gather generic parameter counts from a context descriptor.