Fixed a couple inconsistencies in the MessageError<M> vtable

[Michael-F-Bryan]

I'm doing a review of anyhow and noticed a couple inconsistencies when constructing the vtable for MessageError<M>.

The current code is sound, but I thought I'd make things easier for future reviewers because I needed to spend a couple minutes reassuring myself it's okay to use something like object_drop_front::<M> instead of object_drop_front::<MessageError<M>>.

The lines I'm referring to:

pub(crate) fn from_adhoc<M>(message: M, backtrace: Option<Backtrace>) -> Self
where
    M: Display + Debug + Send + Sync + 'static,
{
    use crate::wrapper::MessageError;
    let error: MessageError<M> = MessageError(message);
    let vtable = &ErrorVTable {
        object_drop: object_drop::<MessageError<M>>,
        object_ref: object_ref::<MessageError<M>>,
        #[cfg(feature = "std")]
        object_mut: object_mut::<MessageError<M>>,
        object_boxed: object_boxed::<MessageError<M>>,
        object_downcast: object_downcast::<M>,                             <-----
        object_drop_rest: object_drop_front::<M>,                            <-----
    };

    // Safety: MessageError is repr(transparent) so it is okay for the
    // vtable to allow casting the MessageError<M> to M.
    unsafe { Error::construct(error, vtable, backtrace) }
}

[dtolnay]

I think the original code is correct and I would prefer to keep it. object_drop_rest is a downcast in general, and we should be using the same type as for the downcast. The vtable setup in Error::from_adhoc is consistent with all the other vtables in Error::from_display, Error::from_context, Error::from_boxed, and Error::context, some of which rely on object_downcast and object_drop_rest being matched.

Thanks anyway!

[Michael-F-Bryan]

Thanks @dtolnay, when you put it that way it makes more sense.

I haven't read the code further down in any detail, so didn't know Error::from_display() and friends use the same convention or rely on the two functions using the same type.