PR CI: check-build the library for more targets

[RalfJung]

@saethlin and @onur-ozkan made it so that ./x.py check library --target=aarch64-pc-windows-msvc now works on all hosts, without special magic Windows tooling being needed. We should use that in PR CI to catch some more standard library build issues. In particular we are currently not covering

• Windows MSVC (we do check GNU)
• macOS

(Or, well, it turns out we run Miri tests for these targets so we cover them in x86_64-gnu-tools. But that seems to be a bit too accidental. And Miri doesn't complain if there are warnings, only if there are errors. And this sets cfg(miri) which changes some codepaths.)

@rust-lang/infra which runner would be the best one to add this to? AFAIK the mingw-check runner exists mostly to give us some test coverage for library builds on a Windows target, so maybe that would be a good one?

[saethlin]

Sysroot builds are nearly serial, and our runners have 8 vCPUs each. While Cargo supports multi-target builds and parallelizes them effectively, other tools like cargo-miri and bootstrap do not.

I've poked around the code in bootstrap for passing multiple targets down to the cargo invocation, but found it too challenging for me to do personally. I suspect the bootstrap team has enough understanding to do this much more easily.

[saethlin]

For anyone who wants a demo of how close-at-hand this is, you can just run this locally today:

cargo +nightly check -Zbuild-std --target=aarch64-unknown-linux-gnu --target=i686-pc-windows-gnu --target=i686-pc-windows-msvc --target=i686-unknown-linux-gnu --target=x86_64-apple-darwin --target=x86_64-pc-windows-gnu --target=x86_64-pc-windows-msvc --target=x86_64-unknown-linux-gnu

[onur-ozkan]

I believe this is more related to the infra team. Bootstrap team don't have any access to CI environments.

[RalfJung]

It's not about access to any environments, all files that need changing are in this repo. It's about adjusting our CI scripts to run these checks. No idea who's usually maintaining them...

[onur-ozkan]

It's not about access to any environments, all files that need changing are in this repo. It's about adjusting our CI scripts to run these checks. No idea who's usually maintaining them...

Right 🤦 my bad.