no_std and Your First Blink

#![no_std]
#![no_main]

use panic_halt as _;
use cortex_m_rt::entry;

#[entry]
fn main() -> ! {
    let dp = pac::Peripherals::take().unwrap();
    let mut led = /* configure a GPIO output */;

    loop {
        led.set_high();
        delay(1_000_000);
        led.set_low();
        delay(1_000_000);
    }
}

#![no_std] drops the standard library. No heap, no String, no Vec, no std::io. You keep all of core: iterators, Option, Result, slices, formatting.

fn main() -> ! never returns — there’s no OS to return to.

You must declare a panic handler; panic_halt spins forever. Flash with probe-rs, and use defmt for logging: it sends format indices instead of strings, which matters when your whole flash budget is 256 KB.