Rust code shared from the playground

playground.rs

#![feature(used)]
#![feature(const_fn)]
#![no_std]

extern crate cortex_m;

#[macro_use(exception)]
extern crate cortex_m_rt;
extern crate stm32f042;
extern crate volatile_register;

fn main() {}

exception!(SYS_TICK, hello_world);

struct Buffer {}

impl core::fmt::Write for Buffer {
    fn write_str(&mut self, s: &str) -> core::fmt::Result {
        let _ = s;
        Ok(())
    }
}

fn hello_world() {
    use core::fmt::Write;

    let mut output = Buffer {};
    writeln!(&mut output, "Hello World!").unwrap();
}