compute digest as a side effect of reading file

digestreader.rs

// read and digest at the same time
pub struct DigestReader<R> {
    pub digest: Blake2b<consts::U32>,
    reader: R,
}

impl<R: Read> DigestReader<R> {
    /// Construct a `DigestReader` reader from an existing reader
    pub fn new(r: R) -> DigestReader<R> {
        DigestReader {
            digest: Blake2b::<consts::U32>::new(),
            reader: r,
        }
    }
}

impl<R: Read> Read for DigestReader<R> {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        let num = self.reader.read(buf)?;
        self.digest.update(&buf[..num]);
        Ok(num)
    }
}