`downcast_ref` on `anyhow::Error` vs `&dyn std::error::Error` about anyhow

[dtolnay]

If you need the previous approach of downcasting to Context<C> to work, it should be possible to define your own extension trait which you use instead of anyhow::Context:

pub struct Context<C> {
    context: C,
    source: Error,
}

pub trait DowncastableContext {
    type Ok;

    fn context<C>(self, context: C) -> Result<Self::Ok, Error>
    where
        C: Display + Send + Sync + 'static; // same signature as anyhow::Context
}

impl<T, E> DowncastableContext for Result<T, E>
where
    E: Into<Error>,
{
    type Ok = T;

    fn context<C>(self, context: C) -> Result<Self::Ok, Error>
    where
        C: Display + Send + Sync + 'static,
    {
        self.map_err(|err| {
            let source = err.into();
            Error::new(Context { context, source })
        })
    }
}

[idubrov]

Right, we can do that, basically have our own "context" errors.

But why is there a difference in downcasting behavior? At very least, it looks confusing to me. Is it documented anywhere?

[dtolnay]

It's intentionally different from the simplistic downcasting that std::error::Error comes with.

Notice that the signatures are not even the same, https://doc.rust-lang.org/std/error/trait.Error.html#method.downcast_ref is fn downcast_ref<T: Error + 'static> while https://docs.rs/anyhow/1.0/anyhow/struct.Error.html#method.downcast_ref is fn downcast_ref<E: Display + Debug + Send + Sync + 'static>, since the caller's context types generally would not have a std Error impl.

[idubrov]

Thanks, makes total sense. I just find it a bit confusing.