Generic source compatible with anyhow (or similar)

[Nikolaus Thümmel]

Sometimes, it may make sense to compose an error enum of generic sources. This is easily possible by using std::error::Error + 'static trait bounds:

#[derive(Debug, Error)]
pub enum FetchError<
    L: std::error::Error + 'static,
    S: std::error::Error + 'static,
> {
    #[error(transparent)]
    Load(L),

    #[error("Failed to save new value")]
    Save(#[source] S),
}

This supports errors which implement std::error::Error directly, however, it can't be used when the sources are, say, an anyhow::Error:

FetchError<anyhow::Error, anyhow::Error>> =
    |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `StdError` is not implemented for `anyhow::Error`

I have seen that thiserror internally uses thiserror::private::AsDynError<'a>, but that's (a) a private API and (b) I wouldn't know what to supply as a lifetime parameter when using this as a trait bound.

Could you help me find a way to support both implementations of StdError as well as anyhow::Error (or other types that dereference to a dyn StdError) as generic arguments?
Would it be possible for thiserror to simplify these trait bounds in some way?

[David Tolnay]

Hi @nthuemmel, sorry that no one was able to provide guidance here. If this is still an issue, you could try taking this question to any of the resources shown in https://www.rust-lang.org/community.