KodrAus commented on Aug 5, 2020 •

Opening this up just to see what it breaks. It's unfortunate that &(impl Error) doesn't actually implement Error. If this direct approach doesn't work out then I'll try something different, like an Error::by_ref method.

EDIT: This is a super low-priority experiment so feel free to cancel it for more important crater runs!

Stabilization Report

We've been working for the last few years to try "fix" the Error trait, which is probably one of the most fundamental in the whole standard library. One of its issues is that we commonly expect you to work with abstract errors through dyn Trait, but references and smart pointers over dyn Trait don't actually implement the Error trait. If you have a &dyn Error or a Box<dyn Error> you simply can't pass it to a method that wants a impl Error.

What does this do?

This stabilizes the following trait impl:

impl<'a, T: Error + ?Sized + 'static> Error for &'a T;

This means that &dyn Error will now satisfy a impl Error bound.

It doesn't do anything with Box<dyn Error> directly. We discussed how we could do Box<dyn Error> in the thread here (and elsewhere in the past), but it seems like we need something like lattice-based specialization or a sprinkling of snowflake compiler magic to make that work. Having said that, with this new impl you can now get a impl Error from a Box<dyn Error> by dereferencing it.

What breaks?

A crater run revealed a few crates broke with something like the following:

// where e: &'short &'long dyn Error
err.source()

previously we'd auto-deref that &'short &'long dyn Error to return a Option<&'long dyn Error> from source, but now will call directly on &'short impl Error, so will return a Option<&'short dyn Error>. The fix is to manually deref:

// where e: &'short &'long dyn Error
(*err).source()

In the recent Libs meeting we considered this acceptable breakage.