Avoid sendable key paths in dynamic member lookup (#245)

There are a few compiler bugs that prevent us from declaring sendability
for key paths where it's needed.

First, doing so breaks autocomplete, which really hurts the developer
experience: swiftlang/swift#77035

Second, even though recovering autocomplete might be preferable at the
cost of safety, there is no safety to begin with right now because
sendable diagnostics don't propagate through dynamic member lookup:
swiftlang/swift#77105

Because of this, let's only use non-sendable key paths for now, and
force cast them under the hood.

Sources/SwiftNavigation/Internal/KeyPath+Sendable.swift

@@ -6,3 +6,30 @@
   public typealias _SendableKeyPath<Root, Value> = KeyPath<Root, Value>
   public typealias _SendableWritableKeyPath<Root, Value> = WritableKeyPath<Root, Value>
 #endif
+
+// NB: Dynamic member lookup does not currently support sendable key paths and even breaks
+//     autocomplete.
+//
+//     * https://github.com/swiftlang/swift/issues/77035
+//     * https://github.com/swiftlang/swift/issues/77105
+extension _AppendKeyPath {
+  @_transparent
+  func unsafeSendable<Root, Value>() -> _SendableKeyPath<Root, Value>
+  where Self == KeyPath<Root, Value> {
+    #if compiler(>=6)
+      unsafeBitCast(self, to: _SendableKeyPath<Root, Value>.self)
+    #else
+      self
+    #endif
+  }
+
+  @_transparent
+  func unsafeSendable<Root, Value>() -> _SendableWritableKeyPath<Root, Value>
+  where Self == WritableKeyPath<Root, Value> {
+    #if compiler(>=6)
+      unsafeBitCast(self, to: _SendableWritableKeyPath<Root, Value>.self)
+    #else
+      self
+    #endif
+  }
+}

Sources/SwiftNavigation/UIBinding.swift

@@ -374,11 +374,11 @@ public struct UIBinding<Value>: Sendable {
   /// - Parameter keyPath: A key path to a specific resulting value.
   /// - Returns: A new binding.
   public subscript<Member>(
-    dynamicMember keyPath: _SendableWritableKeyPath<Value, Member>
+    dynamicMember keyPath: WritableKeyPath<Value, Member>
   ) -> UIBinding<Member> {
     func open(_ location: some _UIBinding<Value>) -> UIBinding<Member> {
       UIBinding<Member>(
-        location: _UIBindingAppendKeyPath(base: location, keyPath: keyPath),
+        location: _UIBindingAppendKeyPath(base: location, keyPath: keyPath.unsafeSendable()),
         transaction: transaction
       )
     }
@@ -391,12 +391,12 @@ public struct UIBinding<Value>: Sendable {
   /// - Returns: A new binding.
   @_disfavoredOverload
   public subscript<Member>(
-    dynamicMember keyPath: _SendableKeyPath<Value.AllCasePaths, AnyCasePath<Value, Member>>
+    dynamicMember keyPath: KeyPath<Value.AllCasePaths, AnyCasePath<Value, Member>>
   ) -> UIBinding<Member>?
   where Value: CasePathable {
     func open(_ location: some _UIBinding<Value>) -> UIBinding<Member?> {
       UIBinding<Member?>(
-        location: _UIBindingEnumToOptionalCase(base: location, keyPath: keyPath),
+        location: _UIBindingEnumToOptionalCase(base: location, keyPath: keyPath.unsafeSendable()),
         transaction: transaction
       )
     }
@@ -408,12 +408,12 @@ public struct UIBinding<Value>: Sendable {
   /// - Parameter keyPath: A key path to a specific value.
   /// - Returns: A new binding.
   public subscript<Wrapped, Member>(
-    dynamicMember keyPath: _SendableWritableKeyPath<Wrapped, Member>
+    dynamicMember keyPath: WritableKeyPath<Wrapped, Member>
   ) -> UIBinding<Member?>
   where Value == Wrapped? {
     func open(_ location: some _UIBinding<Value>) -> UIBinding<Member?> {
       UIBinding<Member?>(
-        location: _UIBindingOptionalToMember(base: location, keyPath: keyPath),
+        location: _UIBindingOptionalToMember(base: location, keyPath: keyPath.unsafeSendable()),
         transaction: transaction
       )
     }
@@ -425,12 +425,12 @@ public struct UIBinding<Value>: Sendable {
   /// - Parameter keyPath: A case key path to a specific associated value.
   /// - Returns: A new binding.
   public subscript<V: CasePathable, Member>(
-    dynamicMember keyPath: _SendableKeyPath<V.AllCasePaths, AnyCasePath<V, Member>>
+    dynamicMember keyPath: KeyPath<V.AllCasePaths, AnyCasePath<V, Member>>
   ) -> UIBinding<Member?>
   where Value == V? {
     func open(_ location: some _UIBinding<Value>) -> UIBinding<Member?> {
       UIBinding<Member?>(
-        location: _UIBindingOptionalEnumToCase(base: location, keyPath: keyPath),
+        location: _UIBindingOptionalEnumToCase(base: location, keyPath: keyPath.unsafeSendable()),
         transaction: transaction
       )
     }