beta regression matching unit-like structs with ..

[durka]

This code

struct Empty;

match Empty {
    Empty(..) => {}
}

compiles on stable 1.5 (no RFC 218 warning) but fails on beta 1.6 with E0164.

[petrochenkov]

This is an expected result of #29383

[durka]

@petrochenkov breaking change with no warning cycle? I expected a warning like there is with a unitary enum variant.

[petrochenkov]

The warning was introduced only for the case UnitVariant(..), other cases were not encountered during crater run.

[durka]

I guess my crate wasn't up during the crater run then :( But this is the point of beta, right, to catch stuff like this?

[petrochenkov]

I don't know, it's possible to turn all errors produced by #29383 into warnings temporarily, but IMHO it's simpler to fix the bug on your side.
This is a policy question, so let's someone else decide.
cc @nikomatsakis as a reviewer of #29383

EDIT: Oh wait, @pnkfelix was the reviewer, but I remember nikomatsakis commented there about errors/warnings.

[alexcrichton]

triage: I-nominated

[durka]

In this case it's in a complex macro (my guard crate) so there really is no workaround until #![feature(braced_empty_structs)]. But you may overrule me by saying tt-based macros are never stable :)

[petrochenkov]

@durka

In this case it's in a complex macro (my guard crate)

https://github.com/durka/guard/blob/master/src/lib.rs#L71
Hmm, does something like self::Empty work as a workaround for this path/identifier problem?

[durka]

Yes (though it's ugly), unless the struct/enum is local to the function/block. So it's a rather niche case I suppose.

[durka]

@petrochenkov actually no, that doesn't seem to help for structs imported from other modules: http://is.gd/TlO9Ro

[petrochenkov]

@durka
Then the module name can be used instead of self - foo::Empty.
I know this is all horrible, but Empty(..) itself is a workaround too.

[pnkfelix]

Hmm, I must have misunderstood #29383 ; I had thought there was a warning cycle associated with this change... e.g. wasn't this comment evidence of such a warning? What did I miss here...