treat setting the number of test-threads to 0 as an error

[battisti]

It is currently possible to call cargo test -- --test-threads=0 which will cause cargo to hang until aborted. This change will fix that and will report an appropriate error to the user.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @brson (or someone else) soon.

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[Mark-Simulacrum]

Some programs treat 0 as "number of cores," should we also do that?

[battisti]

@Mark-Simulacrum do you have an example where this is done?

[Mark-Simulacrum]

Hmm, actually, I can't find such an example right now. I think Cargo defaults to number of cores by default, though, so maybe just documenting that in the --help would be sufficient? For example, cargo test's help currently has: -j N, --jobs N Number of parallel jobs, defaults to # of CPUs. It may be sufficient to just add a note to the documentation (--help) and maybe print something along the lines of "the default is the # of CPU cores, 0 is not a valid number of threads."?

It's worth noting the test --help currently includes the following after the main options summary. I'm not sure that's the best place.

By default, all tests are run in parallel. This can be altered with the
--test-threads flag or the RUST_TEST_THREADS environment variable when running
tests (set it to 1).

[battisti]

Thanks! Next weekend, I will add that to the --help documentation and change the error message for --test-threads=0 accordingly.

[alexcrichton]

@battisti sorry for the long turnaround! It looks like tidy checks may be failing though? If you want to fix that up I'll r+ and we can merge

[battisti]

@alexcrichton looks like I fixed the tidy check, but now the OS X checks fail due to an: "unknown archive format".

[alexcrichton]

@bors: r+

No worries, looks good to me!

[bors]

📌 Commit 0a4c268 has been approved by alexcrichton

[bors]

⌛ Testing commit 0a4c268 with merge 912bc14...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 912bc14 to master...