There's probably some opportunity to use custom annotations in Rust to achieve the same thing.

    #[route("/foobar")]
    fn user(_: &mut Request) -> PencilResult {
        // ...
    }

However, procedural macros are unstable, but Rust is able to do amazing things with macros.

I know very little Rust, but I was actually looking at how this could be done in Rust some time ago, but I didn't get anything working. All the routes (e.g. /foobar and accompanying function) needs to be collected by the "router" somehow which begs for a global route-container that the route macro (like Flasks route decorator) would need to add routes to, similar to how it is done is Flask, and, well, yeah, I couldn't find a rusty way to do it.

Bearing in mind that macros er unstable, can this be done in Rust, and if so, what would the process be?

There are a few different ways one could achieve this. The first options would be to generate a struct:

    struct RouteUser {
        route: &'static str,
        f: fn(_: &mut Request) -> PencilResult
    }

Then given

    #[route("/user")]
    fn user(_: &mut Request) -> PencilResult {}

You would hijack the `user` function to return an instance of the struct.

    fn user() -> RouteUser {
        // The actual function the user wrote:
        fn user(_: &mut Request) -> PencilResult {
            // ...
        }

        RouteUser {
            // Generated from the annotation argument
            route: "/user",
            f: user
        }
    }

There is a difference between item decorators (`#[foobar]`) and regular procedural macros and I'm not completely sure if you could in-fact significantly change the given function. I haven't touched procedural macros in a while.

To use the above route, you would simply have a `Route` trait perhaps.

    trait Route {}

And implement it for each generated struct:

    impl Route for RouteUser {}

Then you could use the route as

    app.route(user());

Which could be defined as

    fn route<R>(r: R) where R: Route {
        // ...
    }

That seems like a good approach, although one thing is missing: when you use the route-decorator in Flask, the route is registered to the app object "via magic", whereas in your example, you still have to register it manually (`app.route(user())`). Now, I am partial to liking registering manually, like in your example and in Pencil itself, since that's clearer than having a lot of routes spread around in a whole lot of files that are magically registered, but just for the kicks, would it be possible to not having to manually register the route to the app object?

I'm a huge fan of flask but I actually prefer the approach of not using decorators. Without decorators its easy to see all the paths in one area and they get type-checked in rust.

There is a language plugin for pythonic decorators in Rust:

https://github.com/Manishearth/rust-adorn

You actually can use pythonish decorators in Rust, I wrote a proof of concept library for that: https://github.com/manishearth/rust-adorn (it's not really intended to be used, but if folks want to use it I'm willing to clean it up)

>I always look for the "Flask" of that language.

I always look for the "Sinatra" of that language.

> "gives the bare essentials and nothing else"

Rust is that thing exactly.