stdin.scan() for competitive programming

main.rs

use std::io::{self, BufRead};
use std::str::{self, FromStr};

fn from_buf<T: FromStr>(buf: &[u8]) -> io::Result<T>
where
    T::Err: std::error::Error + Send + Sync + 'static,
{
    let s = str::from_utf8(buf).map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))?;
    T::from_str(s).map_err(|e| io::Error::new(io::ErrorKind::InvalidInput, e))
}

trait BufReadExt: BufRead {
    fn skip(&mut self) -> io::Result<()> {
        loop {
            let buf = self.fill_buf()?;
            if buf.is_empty() {
                return Ok(());
            }
            if let Some(pos) = buf.iter().position(|&c| !c.is_ascii_whitespace()) {
                self.consume(pos);
                break;
            } else {
                let len = buf.len();
                self.consume(len);
            };
        }
        Ok(())
    }

    fn scan<T: FromStr>(&mut self) -> io::Result<T>
    where
        T::Err: std::error::Error + Send + Sync + 'static,
    {
        self.skip()?;
        let buf = self.fill_buf()?;
        if let Some(pos) = buf.iter().position(|&c| c.is_ascii_whitespace()) {
            let x = from_buf(&buf[..pos])?;
            self.consume(pos);
            Ok(x)
        } else {
            self.scan_slow()
        }
    }

    fn scan_slow<T: FromStr>(&mut self) -> io::Result<T>
    where
        T::Err: std::error::Error + Send + Sync + 'static,
    {
        let mut buf = Vec::<u8>::new();
        loop {
            let next = self.fill_buf()?;
            if next.is_empty() {
                break;
            }
            if let Some(pos) = next.iter().position(|&c| c.is_ascii_whitespace()) {
                buf.extend_from_slice(&next[..pos]);
                self.consume(pos);
                break;
            } else {
                buf.extend_from_slice(next);
                let len = next.len();
                self.consume(len);
            };
        }
        from_buf(&buf)
    }
}
impl<T: BufRead + ?Sized> BufReadExt for T {}

fn main() -> io::Result<()> {
    let stdin = std::io::stdin();
    let mut stdin = stdin.lock();
    let n = stdin.scan::<i32>()?;
    eprintln!("n = {}", n);
    Ok(())
}