Integrate logger into chat route

[willcrichton]

Task

Functionality

In {{ 08-abstraction pr }}, your colleague has implemented the infrastructure for a Logger data structure that collects the messages provided by users, and intermittently writes them to disk. Your task is to integrate this logger into the /chat route. The application should have a single persistent Logger. Every time a user queries /chat, then the user's most recent message is added via Logger::append. Then Logger::save is called immediately after append.

You should implement the logger via a message-passing protocol in the same way you designed the chatbot. The challenge in this task is to reuse as much code between your logger and chatbot as possible. That is, you should design an abstraction that is generic over both use cases. This is fully open-ended — be creative! And we encourage you to compare with our reference solution at the end. Remember that you can see the reference solution by running repo-quest and selecting "View reference solution".

Background

Async in traits

Your abstraction will likely involve a trait that contains an asynchronous method. At the time of writing (August 2024), this is supported to varying degrees in the compiler as detailed in this announcement. In brief, our recommendation is that you should avoid using async fn in traits, and instead prefer the impl Trait representation. For example:

trait MyInterface {
  fn call(&mut self) -> impl Future<Output = i32>;
}

struct Example;
impl MyInterface for Example {
  async fn call(&mut self) -> i32 { // you can still use async in the impl
    0
  }
}