FromStr::Err should be bounded by str::error::Error

[malbarbo]

Conceptually FromStr::Err is an error, so the Error bound shouldn't be necessary is the following example

use std::io;
use std::str::FromStr;
use std::borrow::Borrow;
use std::error::Error;

fn from_reader<F, B>(reader: B) -> io::Result<F>
    where F: FromStr,
          F::Err: 'static + Error + Send + Sync,
          B: io::BufRead
{
    let line  = try!(reader.lines().next().unwrap());
    match F::from_str(line.borrow()) {
        Ok(r) => Ok(r),
        Err(r) => Err(io::Error::new(io::ErrorKind::InvalidData, r))
    }
}

I think there is other cases like this in std.

[alexcrichton]

As this trait and associated type are stable there may not be a whole lot we can do about this, but if there are unstable traits it seems reasonable to do this.

[Mark-Simulacrum]

I'm going to close this -- we can't really change it now, but labeled with rust 2 breakage wishlist.