Implement iter_chain on the type that derives Error

[prasannavl]

I think it'd be immensely useful if iter_chain can be added on the type that derives Error and delegated to std::error::Error's iter_chain on nightlies with error_iter feature. Sure, one can always convert it into the std::error trait object and do it from there, but I think this will be ergonomic to have it right on the type.

https://doc.rust-lang.org/std/error/trait.Error.html#method.iter_chain

[dtolnay]

Thanks, I would consider doing this after the signature of iter_chain is stable in std. For now you can get the same behavior by an extension trait:

pub trait IterChain: std::error::Error + 'static {
    fn iter_chain(&self) -> ErrorIter;
}

impl<T: std::error::Error + 'static> IterChain for T {
    fn iter_chain(&self) -> ErrorIter {
        /* ... */
    }
}

use thiserror::Error;

#[derive(Error, Debug)]
pub struct MyError;

fn main() {
    let err = MyError;
    let _ = err.iter_chain(); // works
}