Allow #[error(transparent)] on struct fields

[djc]

The crate I'm currently working on has code like the following:

#[derive(Debug, Clone, Error)]
pub struct Error {
    pub(crate) kind: ErrorKind,
    backtrack: Option<ExtBacktrace>,
}

#[derive(Debug, Error)]
pub enum ErrorKind {
    // error variants go here
}

If I understand correctly, this doesn't currently allow the Error impl for Error to return types from the ErrorKind variants, which seems like it would be useful. I thought I might try #[error(transparent)] on Error's kind field, but that's not allowed. Is there currently a way to support this scenario? From the description of error(transparent) it seems like it would be a good fit for this, but maybe there's a good reason not to allow this.

[pjenvey]

This style was common with the failure crate: it recommended it as one of its patterns as it simplifies handling of extra data like the Optional backtrace here in to one centralized place (rather than within each Kind's variants).

I also came across this problem hoping #[error(transparent)] might defer the top level Error's source method to the inner kind's. The solution was to simply not use thiserror::Error for the top level Error struct here, instead implementing custom std Error and Display traits for it that defer to kind's.

#[derive(thiserror::Error)] is not doing a lot for the top level Error in this case otherwise: those two traits are just a few lines of code. I just use #[derive(thiserror::Error)] on the ErrorKind which is where it's doing the heavier lifting.