Can't use downcast in libraries

[s-panferov]

Hello @reem. I have a strange problem with rust-error. To demonstrate it I clone the last Iron version and modify source in-place:

#[deriving(Show, PartialEq)]
pub struct ParseError {
    location: uint,
}

impl StdError for ParseError {
    fn description(&self) -> &str { "Parse Error" }
}

/// `AfterMiddleware` are fired after a `Handler` is called inside of a Chain.
///
/// `AfterMiddleware` receive both a `Request` and a `Response` and are responsible for doing
/// any response post-processing.
///
/// `AfterMiddleware` should *not* overwrite the contents of a Response. In
/// the common case, a complete response is generated by the Chain's `Handler` and
/// `AfterMiddleware` simply do post-processing of that Response, such as
/// adding headers or logging.
pub trait AfterMiddleware: Send + Sync {
    /// Do whatever work this middleware needs to do with both a `Request` and `Response` objects.
    ///
    /// An error here is propagated by the containing Chain down to this and any later
    /// `AfterMiddleware`'s `catch` methods, which can attempt to handle the error or modify
    /// the `Response` to indicate to a client that something went wrong.
    fn after(&self, &mut Request, &mut Response) -> IronResult<()>;

    /// Try to catch an error thrown by previous `AfterMiddleware`, the `Handler`, or a previous
    /// `BeforeMiddleware`.
    ///
    /// This indicates that the `Response` is abnormal in some way, either because it was
    /// generated by a `Handler`s `catch` method or because a previous `AfterMiddleware`
    /// errored.
    fn catch(&self, _: &mut Request, _: &mut Response, err: IronError) -> IronResult<()> {
        let error = err.downcast::<ParseError>();

        Err(err)
    }
}

And when I compile it, I get:

src/middleware.rs:152:25: 152:49 error: type `Box<error::Error + 'static>` does not implement any method in scope named `downcast`
src/middleware.rs:152         let error = err.downcast::<ParseError>();
                                              ^~~~~~~~~~~~~~~~~~~~~~~~

I have the same error in rustless and I don't know how to deal with it. Can you help?

[reem]

This is bizarre and I'm 99% sure that this is an error in the compiler's treatment of inherent impls on trait objects. I can reproduce with similarly short examples in rust-error.

[reem]

Ugh, I was right: rust-lang/rust#19056

[s-panferov]

@reem, thanks. I'll wait until it will be fixed. I think we can close this issue.

[reem]

I'll keep it open, since it makes the library basically unusable :/ I will look into an alternative fix if this isn't fixed upstream soon.

[s-panferov]

It has been fixed already.