very elegant extension API and parts of it used in std as POC

orchid-base/src/box_cow.rs

@@ -0,0 +1,29 @@
+use std::borrow::Borrow;
+use std::ops::Deref;
+use std::sync::Arc;
+
+pub enum ArcCow<'a, T: ?Sized + ToOwned> {
+  Borrowed(&'a T),
+  Owned(Arc<T::Owned>),
+}
+impl<'a, T: ?Sized + ToOwned> ArcCow<'a, T> {
+  pub fn owned(value: T::Owned) -> Self { Self::Owned(Arc::new(value)) }
+}
+impl<'a, T: ?Sized + ToOwned> Clone for ArcCow<'a, T> {
+  fn clone(&self) -> Self {
+    match self {
+      Self::Borrowed(r) => Self::Borrowed(r),
+      Self::Owned(b) => Self::Owned(b.clone()),
+    }
+  }
+}
+
+impl<'a, T: ?Sized + ToOwned> Deref for ArcCow<'a, T> {
+  type Target = T;
+  fn deref(&self) -> &Self::Target {
+    match self {
+      Self::Borrowed(t) => t,
+      Self::Owned(b) => b.as_ref().borrow(),
+    }
+  }
+}