[stdlib] Create an offset based keypath at runtime

stdlib/public/core/KeyPath.swift

@@ -3893,6 +3893,50 @@ internal func _instantiateKeyPathBuffer(
   return offset
 }
 
+@available(SwiftStdlib 5.9, *)
+public func _createOffsetBasedKeyPath(
+  root: Any.Type,
+  value: Any.Type,
+  offset: Int
+) -> AnyKeyPath {
+  func openRoot<Root>(_: Root.Type) -> AnyKeyPath.Type {
+    func openValue<Value>(_: Value.Type) -> AnyKeyPath.Type {
+      KeyPath<Root, Value>.self
+    }
+
+    return _openExistential(value, do: openValue(_:))
+  }
+
+  let kpTy = _openExistential(root, do: openRoot(_:))
+
+  // The buffer header is 32 bits, but components must start on a word
+  // boundary.
+  let kpBufferSize = MemoryLayout<Int>.size + MemoryLayout<Int32>.size
+  return kpTy._create(capacityInBytes: kpBufferSize) {
+    var builder = KeyPathBuffer.Builder($0)
+    let header = KeyPathBuffer.Header(
+      size: kpBufferSize - MemoryLayout<Int>.size,
+      trivial: true,
+      hasReferencePrefix: false
+    )
+
+    builder.pushHeader(header)
+
+    let componentHeader = RawKeyPathComponent.Header(
+      stored: .struct,
+      mutable: false,
+      inlineOffset: UInt32(offset)
+    )
+
+    let component = RawKeyPathComponent(
+      header: componentHeader,
+      body: UnsafeRawBufferPointer(start: nil, count: 0)
+    )
+
+    component.clone(into: &builder.buffer, endOfReferencePrefix: false)
+  }
+}
+
 #if SWIFT_ENABLE_REFLECTION
 
 @_silgen_name("swift_keyPath_copySymbolName")

test/stdlib/KeyPath.swift

@@ -1065,5 +1065,26 @@ keyPath.test("ReferenceWritableKeyPath statically typed as WritableKeyPath") {
   expectEqual(outer[keyPath: upcastKeyPath], 46)
 }
 
+struct Dog {
+  var name: String
+  var age: Int
+}
+
+if #available(SwiftStdlib 5.9, *) {
+  keyPath.test("_createOffsetBasedKeyPath") {
+    let dogAgeKp = _createOffsetBasedKeyPath(
+      root: Dog.self,
+      value: Int.self,
+      offset: 16
+    ) as? KeyPath<Dog, Int>
+
+    expectNotNil(dogAgeKp)
+
+    let sparky = Dog(name: "Sparky", age: 7)
+
+    expectEqual(sparky[keyPath: dogAgeKp!], 7)
+  }
+}
+
 runAllTests()