Implement Default for DefId

[aochagavia]

Would it make sense to do this? If so, I would be happy to provide a PR. I guess it would look like the code below:

DefId {
    krate: 0,
    index: DefIndex::new(0)
}

[apasel422]

Is this necessary for something in particular?

[petrochenkov]

@apasel422
By accident, I needed this last week.
Here you can see a manual implementation of Default for some map keyed with IDs. Why can't it be derived? Because of #26925 in the first place, but also because DefId is not Default.

[Manishearth]

I don't think it makes sense to return bad defids. There's a reason it has to be manually derived, since it could have been a case where autoderiving returns an invalid struct with a bad defid.

[aochagavia]

I just needed to create dummy defids, so I wrote a default_defid function. Afterwards, I looked DefId up in the documentation to see if it implemented Default. Since it doesn't, I thought I could ask about it (instead of directly opening a PR). If it doesn't make sense, we can close this issue. I am happy with my default_defid function 😄

[Manishearth]

I just needed to create dummy defids,

But why? 😄

That shouldn't be necessary.

[aochagavia]

@Manishearth I am working on an experimental refactoring tool. The existing code was written by somebody else and he uses dummy defids in some places. I don't yet know if it is necessary, though. I guess I will discover it later on, when I have a better understanding of the code.

[Manishearth]

Ah, cool. Makes sense. As far as rustc goes we shouldn't be using dummy defids, and outside of rustc also it may not always be the best path.

[Manishearth]

(Closing, unless anyone feels that there's a need for this in the compiler)