Runtime: Implement wrapper function for metadata access functions

Author: Azoy

include/swift/Runtime/Metadata.h

@@ -843,6 +843,13 @@ void swift_enableDynamicReplacementScope(const DynamicReplacementScope *scope);
 SWIFT_RUNTIME_EXPORT
 void swift_disableDynamicReplacementScope(const DynamicReplacementScope *scope);
 
+// Shim to allow Swift to call metadata accessors.
+SWIFT_RUNTIME_EXPORT SWIFT_CC(swift)
+MetadataResponse _swift_metadataAccessorCall(void *accessor,
+                                             MetadataRequest request,
+                                             const void * const *args,
+                                             size_t count);
+
 #pragma clang diagnostic pop
 
 } // end namespace swift

stdlib/public/runtime/Metadata.cpp

@@ -5591,6 +5591,17 @@ SWIFT_RUNTIME_STDLIB_SPI
 const HeapObject *swift_getKeyPathImpl(const void *pattern,
                                        const void *arguments);
 
+// Shims to call a metadata accessor in Swift.
+SWIFT_RUNTIME_EXPORT SWIFT_CC(swift)
+MetadataResponse _swift_metadataAccessorCall(void *accessor,
+                                             MetadataRequest request,
+                                             const void * const *args,
+                                             size_t count) {
+  using Fn = MetadataResponse (...);
+  auto func = reinterpret_cast<Fn *>(accessor);
+  return MetadataAccessFunction(func)(request, llvm::makeArrayRef(args, count));
+}
+
 #define OVERRIDE_KEYPATH COMPATIBILITY_OVERRIDE
 #define OVERRIDE_WITNESSTABLE COMPATIBILITY_OVERRIDE
 #include "CompatibilityOverride.def"

test/Interpreter/metadata_access.swift

@@ -0,0 +1,50 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+
+@_silgen_name("_swift_metadataAccessorCall")
+func _metadataAccessorCall(
+  fn: UnsafeRawPointer,
+  request: UInt,
+  args: UnsafePointer<Any.Type>?,
+  count: Int
+) -> MetadataResponse
+
+struct MetadataResponse {
+  let type: Any.Type
+
+  let state: UInt8
+}
+
+struct MetadataAccessFunction {
+  let ptr: UnsafeRawPointer
+
+  func callAsFunction(request: UInt, args: [Any.Type]) -> MetadataResponse {
+    args.withUnsafeBufferPointer {
+      _metadataAccessorCall(
+        fn: ptr,
+        request: request,
+        args: $0.baseAddress!,
+        count: $0.count
+      )
+    }
+  }
+}
+
+func callStructAccessor(for type: Any.Type, with generics: Any.Type...) {
+  let metadata = unsafeBitCast(type, to: UnsafeRawPointer.self)
+  let descriptor = metadata.advanced(by: 8).load(as: UnsafeRawPointer.self)
+  let accessorLoc = descriptor.advanced(by: 12)
+  let accessor = accessorLoc.advanced(by: Int(accessorLoc.load(as: Int32.self)))
+
+  let accessFn = MetadataAccessFunction(ptr: accessor)
+  print(accessFn(request: 0, args: generics))
+}
+
+// CHECK: MetadataResponse(type: Swift.Int, state: 0)
+callStructAccessor(for: Int.self)
+
+// CHECK: MetadataResponse(type: Swift.Array<Swift.Double>, state: 0)
+callStructAccessor(for: [Int].self, with: Double.self)
+
+// CHECK: MetadataResponse(type: Swift.Dictionary<Swift.Int, Swift.Array<Swift.Double>>, state: 0)
+callStructAccessor(for: [String: [Int]].self, with: Int.self, [Double].self)