From 3522a338a28ca0ef501a7e0ed1f1f8250d33b599 Mon Sep 17 00:00:00 2001
From: Alejandro Alonso <alejandro_alonso@apple.com>
Date: Tue, 4 Apr 2023 09:44:18 -0700
Subject: [PATCH 1/2] Create function to create an offset based keypath at
 runtime

---
 stdlib/public/core/KeyPath.swift | 45 ++++++++++++++++++++++++++++++++
 test/stdlib/KeyPath.swift        | 21 +++++++++++++++
 2 files changed, 66 insertions(+)

diff --git a/stdlib/public/core/KeyPath.swift b/stdlib/public/core/KeyPath.swift
index 05102ed10389e..d2e27a520c46d 100644
--- a/stdlib/public/core/KeyPath.swift
+++ b/stdlib/public/core/KeyPath.swift
@@ -3893,6 +3893,51 @@ internal func _instantiateKeyPathBuffer(
   return offset
 }
 
+@available(SwiftStdlib 5.9, *)
+public func _createOffsetBasedKeyPath(
+  root: Any.Type,
+  value: Any.Type,
+  offset: Int,
+  isClass: Bool = false
+) -> 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: isClass ? .class : .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")
diff --git a/test/stdlib/KeyPath.swift b/test/stdlib/KeyPath.swift
index 734de6fc9d064..68ea4257c34c6 100644
--- a/test/stdlib/KeyPath.swift
+++ b/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()
 

From e9c0b32497a623d647d2ae0362355c1352e99803 Mon Sep 17 00:00:00 2001
From: Alejandro Alonso <alejandro_alonso@apple.com>
Date: Tue, 4 Apr 2023 09:54:09 -0700
Subject: [PATCH 2/2] Remove class support

---
 stdlib/public/core/KeyPath.swift | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/stdlib/public/core/KeyPath.swift b/stdlib/public/core/KeyPath.swift
index d2e27a520c46d..d7914b979afe5 100644
--- a/stdlib/public/core/KeyPath.swift
+++ b/stdlib/public/core/KeyPath.swift
@@ -3897,8 +3897,7 @@ internal func _instantiateKeyPathBuffer(
 public func _createOffsetBasedKeyPath(
   root: Any.Type,
   value: Any.Type,
-  offset: Int,
-  isClass: Bool = false
+  offset: Int
 ) -> AnyKeyPath {
   func openRoot<Root>(_: Root.Type) -> AnyKeyPath.Type {
     func openValue<Value>(_: Value.Type) -> AnyKeyPath.Type {
@@ -3924,7 +3923,7 @@ public func _createOffsetBasedKeyPath(
     builder.pushHeader(header)
 
     let componentHeader = RawKeyPathComponent.Header(
-      stored: isClass ? .class : .struct,
+      stored: .struct,
       mutable: false,
       inlineOffset: UInt32(offset)
     )