Broken code after 2017-09-30 nightly

[pedrohjordao]

The following code was working until the 2017-09-30 nightly:

Playground

Maybe it's catching something I'm not seeing. If that's the case maybe someone could explain it to me?

[pedrohjordao]

Notice that if I change the inner try_from call and call the function

fn a_function<'a, R: io::Read>(_: &'a mut EventReader<R>) {}

everything is fine. The way I understand both calls should work the same way.

[pedrohjordao]

Changing a_function to

fn a_function<'a, R: io::Read>(value: &'a mut EventReader<R>) -> Result<Other, ()> {
    Other::try_from(value)
}

makes everything work. So I believe this is actually a bug.

[nikomatsakis]

Most likely caused by #43880

[nikomatsakis]

I think what is happening is that we are no longer inserting the auto-coercion because of the inner details of how method calls are working after #43880 -- i.e., we used to effectively pass &mut *value in the compiler.

@pedrohjordao if you rewrite to <Other as TryFrom<&'a mut EventReader<R>>::try_from(value), that will work too, right?

[nikomatsakis]

triage: P-high

Calling P-high -- we should decide if we plan to fix. cc @arielb1

[pedrohjordao]

@nikomatsakis huum, I think I get the reasoning behind what you're saying about the change, but I do hope you decide to fix it or make a really nice compiler error pointing to the correct call.

Anyway, it might be my sleepy brain making me confuse, but there might still be something wrong. What you said still doesn't work:

https://play.rust-lang.org/?gist=5d5fc92da3270bf04736f943e8f236a5&version=undefined

[arielb1]

@pedrohjordao

That's because you need <Other as TryFrom<&mut EventReader<R>>>::try_from(value), without the 'a.

[arielb1]

I can't get this code as written to compile with any version of rustc.

[pedrohjordao]

Are you sure? Not even the nightly I said? Em qui, 2 de nov de 2017 19:38, Ariel Ben-Yehuda <notifications@github.com> escreveu:

I can't get this code as written to compile with *any* version of rustc. — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#45142 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AFkDZMOaatqwPWvdFA3HDsPD5JgOtBYEks5syjZYgaJpZM4Pyx9Z> .

-- Att. Pedro Jordão

[arielb1]

@pedrohjordao

I tried again and it worked. I'm not sure what was the problem. It was me using nightlies around #43880, from August 30 with TryFrom copied over instead of nightlies from September 30.