Currently generics in types only limited to list and any other type can't implement a type with similar constructs.
For instance while this is correct starlark type, starlark-rust does not permit expressing such types.

def try_types(g: http) -> future[http_response]:
   return h.get("https://httpbin.io/ip")


 try_types(http)

FWIW I'd be happy to send patches if this something that starlark-rust maintainers are willing to take as contribution