My solution

#![deny(unsafe_code)]
#![no_main]
#![no_std]
#[allow(unused_imports)]
use aux11::{entry, iprint, iprintln};
use heapless::{consts, Vec};
#[entry]
fn main() -> ! {
    let (usart1, mono_timer, itm) = aux11::init();
    // A buffer with 32 bytes of capacity
    let mut buffer: Vec<u8, consts::U32> = Vec::new();
    loop {
        buffer.clear();
        loop {
            while usart1.isr.read().rxne().bit_is_clear() {}
            let byte = usart1.rdr.read().rdr().bits() as u8;
            if buffer.push(byte).is_err() {
                // buffer full
                for byte in b"error: buffer full\n\r" {
                    while usart1.isr.read().txe().bit_is_clear() {}
                    usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
                }
                break;
            }
            // Carriage return
            if byte == 13 {
                // Respond
                for byte in buffer.iter().rev().chain(&[b'\n', b'\r']) {
                    while usart1.isr.read().txe().bit_is_clear() {}
                    usart1.tdr.write(|w| w.tdr().bits(u16::from(*byte)));
                }
                break;
            }
        }
    }
}