BufResult being aliased to a tuple makes it less ergonomic than a standard Result could be.
In future, a std::ops::Try implementation could help, but it needs a local type to be implemented on.

I propose to make use of the standard infrastructure provided by Result and provide the buffer with a new error type:

use std::error::Error;
use std::io;

pub type BufResult<T, B> = Result<(T, B), BufError<B>>;

#[derive(Debug)]
pub struct BufError<B>(pub io::Error, pub B);

impl<B: Debug> Error for BufError<B> {
    // ...
}