Custom `Runner`

Now, let's implement a custom Runner which simply executes scenarios in features sequentially, without considering tags, rules, Backgrounds or other extras, and specifically suitable for our AnimalWorld (for implementation simplicity).
Runner represents anything that transforms a Stream of features into a Stream of cucumber events.
extern crate cucumber;
extern crate futures;
extern crate once_cell;
extern crate tokio;

use std::{
    panic::{self, AssertUnwindSafe},
    path::PathBuf,
    sync::Arc,
    time::Duration,
};

use cucumber::{
    cli, event, gherkin, given, parser, step, then, when, Event, World,
};
use futures::{
    future::{self, FutureExt as _},
    stream::{self, LocalBoxStream, Stream, StreamExt as _, TryStreamExt as _},
};
use once_cell::sync::Lazy;
use tokio::time::sleep;

#[derive(Clone, Copy, Debug, Default)]
struct Animal {
    pub hungry: bool,
}

impl Animal {
    fn feed(&mut self) {
        self.hungry = false;
    }
}

#[derive(Clone, Debug, Default, World)]
pub struct AnimalWorld {
    cat: Animal,
}

#[given(regex = r"^a (hungry|satiated) cat$")]
async fn hungry_cat(world: &mut AnimalWorld, state: String) {
    sleep(Duration::from_secs(2)).await;

    match state.as_str() {
        "hungry" => world.cat.hungry = true,
        "satiated" => world.cat.hungry = false,
        _ => unreachable!(),
    }
}

#[when("I feed the cat")]
async fn feed_cat(world: &mut AnimalWorld) {
    sleep(Duration::from_secs(2)).await;

    world.cat.feed();
}

#[then("the cat is not hungry")]
async fn cat_is_fed(world: &mut AnimalWorld) {
    sleep(Duration::from_secs(2)).await;

    assert!(!world.cat.hungry);
}

struct CustomParser;

impl<I> cucumber::Parser<I> for CustomParser {
    type Cli = cli::Empty;
    type Output = stream::Once<future::Ready<parser::Result<gherkin::Feature>>>;

    fn parse(self, _: I, _: Self::Cli) -> Self::Output {
        let keyword = "Feature";
        let name = "Animal feature";
        stream::once(future::ok(gherkin::Feature {
            keyword: keyword.into(),
            name: name.into(),
            description: None,
            background: None,
            scenarios: vec![gherkin::Scenario {
                keyword: "Scenario".into(),
                name: "If we feed a hungry cat it won't be hungry".into(),
                description: None,
                steps: vec![
                    gherkin::Step {
                        keyword: "Given ".into(),
                        ty: gherkin::StepType::Given,
                        value: "a hungry cat".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 18 },
                        position: gherkin::LineCol { line: 3, col: 5 },
                    },
                    gherkin::Step {
                        keyword: "When ".into(),
                        ty: gherkin::StepType::When,
                        value: "I feed the cat".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 19 },
                        position: gherkin::LineCol { line: 4, col: 5 },
                    },
                    gherkin::Step {
                        keyword: "Then ".into(),
                        ty: gherkin::StepType::Then,
                        value: "the cat is not hungry".into(),
                        docstring: None,
                        table: None,
                        span: gherkin::Span { start: 5, end: 26 },
                        position: gherkin::LineCol { line: 5, col: 5 },
                    },
                ],
                examples: vec![],
                tags: vec![],
                span: gherkin::Span { start: 3, end: 52 },
                position: gherkin::LineCol { line: 2, col: 3 },
            }],
            rules: vec![],
            tags: vec![],
            span: gherkin::Span { start: 1, end: 23 },
            position: gherkin::LineCol { line: 1, col: 1 },
            path: Some(PathBuf::from(file!())),
        }))
    }
}

struct CustomRunner;

impl CustomRunner {
    fn steps_fns() -> &'static step::Collection<AnimalWorld> {
        // Wire the static collection of step matching functions.
        static STEPS: Lazy<step::Collection<AnimalWorld>> =
            Lazy::new(AnimalWorld::collection);
        &STEPS
    }

    async fn execute_step(
        mut world: AnimalWorld,
        step: gherkin::Step,
    ) -> (AnimalWorld, event::Step<AnimalWorld>) {
        let ev = if let Some((step_fn, captures, loc, ctx)) =
            Self::steps_fns().find(&step).expect("Ambiguous match")
        {
            // Panic represents a failed assertion in a step matching
            // function.
            match AssertUnwindSafe(step_fn(&mut world, ctx))
                .catch_unwind()
                .await
            {
                Ok(()) => event::Step::Passed(captures, loc),
                Err(e) => event::Step::Failed(
                    Some(captures),
                    loc,
                    Some(Arc::new(world.clone())),
                    event::StepError::Panic(e.into()),
                ),
            }
        } else {
            event::Step::Skipped
        };
        (world, ev)
    }

    async fn execute_scenario(
        scenario: gherkin::Scenario,
    ) -> impl Stream<Item = event::Feature<AnimalWorld>> {
        // Those panic hook shenanigans are done to avoid console messages like
        // "thread 'main' panicked at ..."
        //
        // 1. We obtain the current panic hook and replace it with an empty one.
        // 2. We run tests, which can panic. In that case we pass all panic info
        //    down the line to the `Writer`, which will print it at right time.
        // 3. We restore original panic hook, because suppressing all panics
        //    doesn't sound like a very good idea.
        let hook = panic::take_hook();
        panic::set_hook(Box::new(|_| {}));

        let mut world = AnimalWorld::new().await.unwrap();
        let mut steps = Vec::with_capacity(scenario.steps.len());

        for step in scenario.steps.clone() {
            let (w, ev) = Self::execute_step(world, step.clone()).await;
            world = w;
            let should_stop = matches!(ev, event::Step::Failed(..));
            steps.push((step, ev));
            if should_stop {
                break;
            }
        }

        panic::set_hook(hook);

        let scenario = Arc::new(scenario);
        stream::once(future::ready(event::Scenario::Started))
            .chain(stream::iter(steps.into_iter().flat_map(|(step, ev)| {
                let step = Arc::new(step);
                [
                    event::Scenario::Step(step.clone(), event::Step::Started),
                    event::Scenario::Step(step, ev),
                ]
            })))
            .chain(stream::once(future::ready(event::Scenario::Finished)))
            .map(move |event| event::Feature::Scenario(
                scenario.clone(), 
                event::RetryableScenario { event, retries: None },
            ))
    }

    fn execute_feature(
        feature: gherkin::Feature,
    ) -> impl Stream<Item = event::Cucumber<AnimalWorld>> {
        let feature = Arc::new(feature);
        stream::once(future::ready(event::Feature::Started))
            .chain(
                stream::iter(feature.scenarios.clone())
                    .then(Self::execute_scenario)
                    .flatten(),
            )
            .chain(stream::once(future::ready(event::Feature::Finished)))
            .map(move |ev| event::Cucumber::Feature(feature.clone(), ev))
    }
}

impl cucumber::Runner<AnimalWorld> for CustomRunner {
    type Cli = cli::Empty; // we provide no CLI options
    type EventStream = LocalBoxStream<
        'static,
        parser::Result<Event<event::Cucumber<AnimalWorld>>>,
    >;

    fn run<S>(self, features: S, _: Self::Cli) -> Self::EventStream
    where
        S: Stream<Item = parser::Result<gherkin::Feature>> + 'static,
    {
        stream::once(future::ok(event::Cucumber::Started))
            .chain(
                features
                    .map_ok(|f| Self::execute_feature(f).map(Ok))
                    .try_flatten(),
            )
            .chain(stream::once(future::ok(event::Cucumber::Finished)))
            .map_ok(Event::new)
            .boxed_local()
    }
}

#[tokio::main]
async fn main() {
    AnimalWorld::cucumber::<&str>() // aiding type inference for `CustomParser`
        .with_parser(CustomParser)
        .with_runner(CustomRunner)
        .run_and_exit("tests/features/book")
        .await;
}
record
NOTE: Output is printed only after all the steps were executed, because we have implemented the CustomRunner in the way to emit event::Cucumbers only after executing all the scenario steps (see execute_scenario() function).
Cucumber Rust Book

Custom Runner

Custom `Runner`
