feat(unboxed_closures): allow unboxed closures for handlers and error handlers

[Ryman]

This is a bit of a compromise, allowing unboxed closures to be used for handlers but not for Middleware, while also making the routing a bit less flexible for the moment (can only take unboxed closures and not a Middleware, so you can't have a Router dispatch to another Middleware without wrapping a closure).

The second commit shows an alternative that I encountered while refactoring. If we deem macros acceptable in the time being we could follow that approach, as we maintain flexibility with only a minor syntactic wart. I favor this approach, but wanted to show how the example usage could look without any macros, which is the current state of the PR. Whenever rust-lang/rust#24680 is fixed, we should then be able to depreciate the macro and the user code should be very easy to update middleware! { |req, res| "foo" } becomes |_, res| res.send("foo").

Another alternative is to use Fn(..) as the bound everywhere in future when implementing unboxed closures for a type becomes stable, then the Middleware trait will become redundant and we could implement Fn(..) for Router and such. This approach doesn't help us with the current 1.0 feature-set though.

[Ryman]

Some example usage:

Macro-less version where router is bounded by Fn instead of Middleware

let visits = AtomicUsize::new(0);
router.get("/", move |_, response|
    response.send(format!("{}", visits.fetch_add(1, Relaxed)))
);

// but unable to do something such as:
router.get("/other", SomeMiddleware);

Retaining the macros which now can use an unboxed closure under the hood, everything sticks with Middleware bounds.

let visits = AtomicUsize::new(0);
router.get("/", middleware! { |_req, _res| format!("{}", visits.fetch_add(1, Relaxed)) });

// can also still do this
router.get("/other", SomeMiddleware);

[cburgdorf]

I think a router that doesn't accept a Middleware anymore isn't a good compromise for now.

However, your other attempt looks pretty cool.

let visits = AtomicUsize::new(0);
router.get("/", middleware! { |_req, _res| format!("{}", visits.fetch_add(1, Relaxed)) });

// can also still do this
router.get("/other", SomeMiddleware);

This seems to be a good compromise as it allows closure usage with just a little syntactic bloat (the macro). And as rust gets better over time one will just be able to drop the macro and keep the closure as it is. 👍

[bodokaiser]

Hi,

I still get the error from #136 when trying to compile these two files.

Any ideas?

[Ryman]

@bodokaiser Opened https://github.com/bodokaiser/rust-restful/pull/1 with a fix.