pub fn get_mut<'a, T: Any>(&'a mut self, name: &str) -> Result<&'a mut T, String> {
        let boxed = self.handles.get_mut(name).ok_or(format!("failed to find handle: {:?}", name))?;
        boxed.downcast_mut::<T>().ok_or(format!("failed to downcast: {}", name))
    }
Acquire a mutable borrow of the value for `name`, if it is of type `T`.
server/src/lib.rs