CI: add a script for dynamically computing CI job matrix

[Kobzol]

It would be great if was easier to run specific CI workflows locally, and also to allow us to spawn a specific CI workflow by bors, to enable running arbitrary try builds. See discussion here.

This PR is a first step in that direction.

• Moves the definition of CI runners and (for now) PR jobs into a separate jobs.yml file.
• Adds a simple Python script that reads the file, decides which jobs should be active for the current CI workflow, and prints them as JSON to their output.
• The PR job then reads this output and generates its job matrix based on it.

By moving the job definitions from ci.yml into a separate file, we can handle it programmatically, which should make it easier to both do local execution of CI jobs and also to do arbitrary try builds.

[rustbot]

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[Kobzol]

r? @pietroalbini

[pietroalbini]

r=me after applying the suggestions

[Kobzol]

Added comments.

[pietroalbini]

@bors r+

[bors]

📌 Commit 72104e2 has been approved by pietroalbini

It is now in the queue for this repository.

[bors]

⌛ Testing commit 72104e2 with merge ccfcd95...

[bors]

☀️ Test successful - checks-actions
Approved by: pietroalbini
Pushing ccfcd95 to master...

[rust-timer]

Finished benchmarking commit (ccfcd95): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

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Bootstrap: 678.486s -> 678.255s (-0.03%)
Artifact size: 315.96 MiB -> 316.00 MiB (0.01%)

[lqd]

BTW is the procedure described in https://rustc-dev-guide.rust-lang.org/tests/ci.html#using-ci-to-test still recommended? Or should we rather change jobs.toml to temporarily enable PR builders?

[Kobzol]

Currently jobs.yml needs to be modified to change what is executed on PRs, yeah. I plan to move the rest of the jobs to jobs.yml soon, then this section should be rewritten.

[RalfJung]

Wow, I had no idea this is possible. Does that mean we can entirely get rid of the ./x.py run step to generate the yml files, and just compute them on-the-fly instead?

Also, this did had some probably unintended side-effect -- the actually relevant jobs for a PR no longer show up at the top of the list, half of them are hidden behind the scrollbar:

If there's a way to order these CI / {auto,try,master} down again, that'd be nice. :D

[Kobzol]

https://rust-lang.zulipchat.com/#narrow/stream/242791-t-infra/topic/job.20matrix.20re-ordered.20PR.20list It's not possible to reorder, but we plan to get around that by removing the skipped jobs from the list completely.

Yeah, I plan to work on removing the YAML preprocessing step next.