You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: buffered.rs
    path of file: ./repos/embassy/embassy-stm32/src/usart
    
    the code of the file until where you have to start completion: use core::future::poll_fn;
use core::slice;
use core::sync::atomic::{AtomicBool, Ordering};
use core::task::Poll;

use embassy_hal_internal::atomic_ring_buffer::RingBuffer;
use embassy_sync::waitqueue::AtomicWaker;

use super::*;
use crate::interrupt::typelevel::Interrupt;

/// Interrupt handler.
pub struct InterruptHandler<T: BasicInstance> {
    _phan
    
    async fn fill_buf(&self) -> Result<&[u8], Error> {
        poll_fn(move |cx| {
            let state = T::buffered_state();
            let mut rx_reader = unsafe { state.rx_buf.reader() };
            let (p, n) = rx_reader.pop_buf();
            if n == 0 {
                state.rx_waker.register(cx.waker());
                return Poll::Pending;
            }

            let buf = unsafe { slice::from_raw_parts(p, n) };
            Poll::Ready(Ok(buf))
        })
        .await
    }