Attribute for delegating Error impl to one field

[dtolnay]

Something to easily delegate the Error trait's source and backtrace methods and the Display impl to a field. This could be useful for hiding error variants from a library's public error type.

use thiserror::Error;

#[derive(Error, Debug)]
#[error(transparent)]  // <--
pub struct Error(ErrorKind);

#[derive(Error, Debug)]
enum ErrorKind {
    #[error("...")]
    E0,
    #[error("...")]
    E1(#[source] io::Error),
}

[dtolnay]

Also useful for enums that need an "anything else" variant.

#[derive(Error, Debug)]
pub enum MyError {
    ...

    #[error(transparent)]
    Other(#[from] anyhow::Error),
}

Such variants would tend not to have their own Display message but just forward through to the underlying error's Display (and source).

They might hold a backtrace of their own though.