Detect if a rollup has merge conflicts with the currently pending job

[JonathanBrouwer]

My rollup (rust-lang/rust#151062) was unmergable due to conflicts with the rollup=never PR that was in queue before it, which was only detected when the rollup actually started getting tested.

This is low priority as this is the first time I've seen this happen, it's not a big deal (just make a new rollup without the conflicting PR) and I'm also not sure how easy this is to implement.

[Kobzol]

How should this work? It's possible that the currently pending build will fail. When an auto build starts, should bors try to merge the pending PR with the second/third PR in the queue (but this ordering might change over time!) and post some warning on the PR?

[JonathanBrouwer]

That was also a bit of a question for me, mostly started this as a way to start thinking about this (admittedly minor) problem.
At the very least, this could be a check specifically when creating rollups.

It is indeed possible the current pending build will fail, but then I'd still rather exclude the offending PR from my rollup and get it picked up by the next rollup if the offending PR fails

[Kobzol]

So when you are creating a rollup, and there is currently a pending build, try to merge the rollup with the pending auto branch and report if there's a conflict?

[JonathanBrouwer]

Yes, that seems like a relatively simple fix(???) that would catch most cases