chore: checkpoint before Python removal

vendor/http/src/convert.rs

@@ -0,0 +1,17 @@
+macro_rules! if_downcast_into {
+    ($in_ty:ty, $out_ty:ty, $val:ident, $body:expr) => {{
+        if std::any::TypeId::of::<$in_ty>() == std::any::TypeId::of::<$out_ty>() {
+            // Store the value in an `Option` so we can `take`
+            // it after casting to `&mut dyn Any`.
+            let mut slot = Some($val);
+            // Re-write the `$val` ident with the downcasted value.
+            let $val = (&mut slot as &mut dyn std::any::Any)
+                .downcast_mut::<Option<$out_ty>>()
+                .unwrap()
+                .take()
+                .unwrap();
+            // Run the $body in scope of the replaced val.
+            $body
+        }
+    }};
+}