miniserve | 🌟 For when you really just want to serve some files | HTTP library

Let's look at the requirements. You want to call statsd::Client::incr(&mut client, metric) from inside the closure, so you need mutable access to client. This is a variable you close over with ||.

Now AdHoc::on_request(f: F) requires F: Fn(...) + Send + Sync + 'static. Fn means you only have immutable access to your capture via &self. The 'static bound means the capture cannot be a reference itself, so it requires move ||. Finally Sync means you can't use Cell or RefCell to get a mutable reference from &self.client, since Rocket will share it between threads.

Just like you suspected, the canonical solution to have shared mutable access through a Send + Sync value is to use Arc<_>>. This also solves the problem of "losing access by moving". Your code would look like the following (untested):