rust/cargo: add support for a custom target directory

[mathstuf]

This can avoid having to wait for ALE or ALE being blocked on other
cargo actions within the same crate.

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Where are the tests?

Where should I add them? I don't see any existing ones testing this kind of feature already.

Seriously, read :help ale-dev and write tests.

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[mathstuf]

Ping @w0rp to un-stale this :) . It's not clear to me how to test this since it is controlling a cargo behavior.

[mathstuf]

Thanks. Trying to run tests locally seems to hang with no CPU usage from the processes in the container...seeing what CI says.

[w0rp]

Thanks. Trying to run tests locally seems to hang with no CPU usage from the processes in the container...seeing what CI says.

Yeah, I get that on my machine too. I haven't been able to figure out why the tests can do that, but if you just let them run for a while they will eventually exit and give you the results. At some point I'll try to figure out what is going on and fix that.

[w0rp]

The upshot of them taking a while with low CPU usage is that you can go off and do whatever else you want for a while, because your CPUs will be free to do other work.

[mathstuf]

Finally all green :) .

[w0rp]

Cheers! 🍻