CI: Make sure that we can bootstrap with cfg(parallel_queries).

[michaelwoerister]

For now a docker image that just builds with cfg(parallel_queries) (but without running tests) should be enough.

cc @Zoxc

[Zoxc]

I would also like Travis PRs builds to run x.py check with cfg(parallel_queries) before doing the normal build without cfg(parallel_queries).

[Zoxc]

cc @rust-lang/infra

[alexcrichton]

We unfortunately don't have a lot of spare cycles on CI right now, both in terms of overall capacity and extra time to run more tests on builders (we're already timing out all over the place). If this is "things are on fire we really need this" then this can be prioritized, but otherwise this probably won't happen for awhile.

[alexcrichton]

er, didn't mean to close

[michaelwoerister]

I think it would be great if we could at least make sure that stage 0 builds with cfg(parallel_queries). Down the line we'll have to run test too but that's not so urgent.

[alexcrichton]

That may be easier to do so, yeah. We could probably do something like RUSTFLAGS_STAGE0=... on whatever builder executes the fastest today, such as x86_64-gnu-debug

[Zoxc]

I'd be satisfied with running x.py check on some configuration which should take like 6 minutes (assuming it skips LLVM).

[Zoxc]

Running x.py check first would also serve to quickly reject PRs with compile errors. I'm not sure that matters for the bors queue, but it might help with PR builds.