# The Unstable Book

Welcome to the Unstable Book! This book consists of a number of chapters,
each one organized by a "feature flag." That is, when using an unstable
feature of Rust, you must use a flag, like this:

```rust
#![feature(coroutines, coroutine_trait)]

use std::ops::{Coroutine, CoroutineState};
use std::pin::Pin;

fn main() {
    let mut coroutine = || {
        yield 1;
        return "foo"
    };

    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Yielded(1) => {}
        _ => panic!("unexpected value from resume"),
    }
    match Pin::new(&mut coroutine).resume(()) {
        CoroutineState::Complete("foo") => {}
        _ => panic!("unexpected value from resume"),
    }
}
```

The `coroutines` feature [has a chapter][coroutines] describing how to use it.

[coroutines]: language-features/coroutines.md

Because this documentation relates to unstable features, we make no guarantees
that what is contained here is accurate or up to date. It's developed on a
best-effort basis. Each page will have a link to its tracking issue with the
latest developments; you might want to check those as well.
