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

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: hyperium#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

src/service/service.rs

@@ -28,5 +28,13 @@ pub trait Service<Request> {
     type Future: Future<Output = Result<Self::Response, Self::Error>>;
 
     /// Process the request and return the response asynchronously.
-    fn call(&mut self, req: Request) -> Self::Future;
+    /// call takes a &self instead of a mut &self because:
+    /// - 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
+    /// To see the discussion on this see: https://github.com/hyperium/hyper/issues/3040
+    fn call(&self, req: Request) -> Self::Future;
 }

src/service/util.rs

@@ -29,7 +29,7 @@ use crate::{Request, Response};
 /// ```
 pub fn service_fn<F, R, S>(f: F) -> ServiceFn<F, R>
 where
-    F: FnMut(Request<R>) -> S,
+    F: Fn(Request<R>) -> S,
     S: Future,
 {
     ServiceFn {
@@ -46,7 +46,7 @@ pub struct ServiceFn<F, R> {
 
 impl<F, ReqBody, Ret, ResBody, E> Service<Request<ReqBody>> for ServiceFn<F, ReqBody>
 where
-    F: FnMut(Request<ReqBody>) -> Ret,
+    F: Fn(Request<ReqBody>) -> Ret,
     ReqBody: Body,
     Ret: Future<Output = Result<Response<ResBody>, E>>,
     E: Into<Box<dyn StdError + Send + Sync>>,
@@ -56,7 +56,7 @@ where
     type Error = E;
     type Future = Ret;
 
-    fn call(&mut self, req: Request<ReqBody>) -> Self::Future {
+    fn call(&self, req: Request<ReqBody>) -> Self::Future {
         (self.f)(req)
     }
 }