refactor(service): change Service::call to take &self

[rob2244]

change Service::call to take &self instead of &mut self. Because of this change, the trait bound in the service::util::service_fn and the trait bound in the impl for the ServiceFn struct were changed from FnMut to Fn. This change was decided on for the following reasons:

• It prepares the way for async fn, since then the future only borrows &self, and thus a Service can concurrently handle multiple outstanding requests at once.
• It's clearer that Services can likely be cloned
• To share state across clones you generally need Arc<Mutex<_>> that means you're not really using the &mut self and could do with a &self

This commit closses issue: #3040
BREAKING CHANGE: The Service::call function no longer takes a mutable reference to self. The FnMut trait bound on the service::util::service_fn function and the trait bound on the impl for the ServiceFn struct were changed from FnMut to Fn

[seanmonstar]

Awesome, thanks!

[seanmonstar]

@programatik29 what's an adapater in hyper-util going to need to look like, do you think?

[rob2244]

@programatik29 what's an adapater in hyper-util going to need to look like, do you think?

When you say adapter, are there structs in hyper-util that implement this trait? Couldn't we just change them there as well? Or am I missing something?

[programatik29]

@seanmonstar With current tower services, I think our best bet is something like this:

https://github.com/davidpdrsn/tower-hyper-http-body-compat/blob/cbaa7df5246587082b770d91283daf646303c34e/src/service.rs#L31-L46

[seanmonstar]

are there structs in hyper-util that implement this trait?

Not yet, but we could add them. Or there could be a separate crate, as was just linked. 🤷