Add support for threading compiler flag to Rust target

[jhaye]

Based on the given threading flag, the parallel-runtime feature is enabled for the Rust runtime in the generated code.

The way it is implemented right now allows one to override the flag by setting the feature manually in the LF source. There is however no option to disable it the same way.

[cmnrd]

Looks great!

Do you think we could port the Workers Test as part of this PR to Rust? I think for Rust, the test should also check that threading is indeed enabled (which it should be by default). I think we should also add another test that explicitly sets threading: false and checks that the single threaded runtime is used.

[jhaye]

In that case we would need a similar feature for the runtime that reports back the number of workers.

Is the manual override from the LF file okay? If the parallel-runtime feature is enabled from a file, setting threading to false won't change anything. Otherwise I'll have to change that around.

[cmnrd]

In that case we would need a similar feature for the runtime that reports back the number of workers.

Do you think this would be possible to add with reasonable effort?

What do you mean by "If the parallel-runtime feature is enabled from a file"? If threading is set to false, then parallel execution should be disabled. If the parallel runtime feature is requested manually and threading is set to false, then we should probably stick to the manual specification, but I think it would be a good idea to also issue a warning.

[jhaye]

Do you think this would be possible to add with reasonable effort?

Yes, probably. I already opened an issue for it.

What do you mean by "If the parallel-runtime feature is enabled from a file"?

It looks like this:

target Rust {
    build-type: Release,
    cargo-features: [ "cli" ],
    rust-include: "../lib/pseudo_random.rs",
    cargo-dependencies: {
        reactor_rt: {
            features: ["parallel-runtime"]
        },
    }
};

Right now this would enable the parallel runtime independent of the threading flag.

[cmnrd]

Alright, I think this is Ok and we cannot really avoid it. However, if threading: false is set in your example, then we should print a warning.

[oowekyala]

(nitpicks)

[jhaye]

print a warning

How can I do that?

[cmnrd]

Great!