'static super-powers

In #[init] and #[idle] local resources have 'static lifetime.

Useful when pre-allocating and/or splitting resources between tasks, drivers or some other object. This comes in handy when drivers, such as USB drivers, need to allocate memory and when using splittable data structures such as heapless::spsc::Queue.

In the following example two different tasks share a heapless::spsc::Queue for lock-free access to the shared queue.

//! examples/static.rs

#![no_main]
#![no_std]
#![deny(warnings)]
#![deny(unsafe_code)]
#![deny(missing_docs)]

use panic_semihosting as _;

#[rtic::app(device = lm3s6965, dispatchers = [UART0])]
mod app {
    use cortex_m_semihosting::{debug, hprintln};
    use heapless::spsc::{Consumer, Producer, Queue};

    #[shared]
    struct Shared {}

    #[local]
    struct Local {
        p: Producer<'static, u32, 5>,
        c: Consumer<'static, u32, 5>,
    }

    #[init(local = [q: Queue<u32, 5> = Queue::new()])]
    fn init(cx: init::Context) -> (Shared, Local) {
        // q has 'static life-time so after the split and return of `init`
        // it will continue to exist and be allocated
        let (p, c) = cx.local.q.split();

        foo::spawn().unwrap();

        (Shared {}, Local { p, c })
    }

    #[idle(local = [c])]
    fn idle(c: idle::Context) -> ! {
        loop {
            // Lock-free access to the same underlying queue!
            if let Some(data) = c.local.c.dequeue() {
                hprintln!("received message: {}", data);

                // Run foo until data
                if data == 3 {
                    debug::exit(debug::EXIT_SUCCESS); // Exit QEMU simulator
                } else {
                    foo::spawn().unwrap();
                }
            }
        }
    }

    #[task(local = [p, state: u32 = 0], priority = 1)]
    async fn foo(c: foo::Context) {
        *c.local.state += 1;

        // Lock-free access to the same underlying queue!
        c.local.p.enqueue(*c.local.state).unwrap();
    }
}

Running this program produces the expected output.

$ cargo xtask qemu --verbose --example static

received message: 1
received message: 2
received message: 3