Suppress unstable config options by default.

[peterhuene]

This PR suppresses the output of unstable config options by default.

Users can specify the --unstable-features option to show the config options
that are unstable.

Fixes #2611.

[peterhuene]

I don't like how brittle this makes the test with respect to the options defined in the create_config! macro above. I'd love some guidance on the best way to test this change.

[nrc]

You could make another usage of create_config in a different module with mock data and test using that, rather than the real one - Config is the only name you can use with that macro, but if you use it in a different module, then it is a different name. You could do this within the test module and then only refer to it using the qualified path, e.g., something like:

mod mock_config {
    create_config! { ... }
}

#[test]
fn test_...() {
    mock_config::Config::print_docs(...);
}

[nrc]

Thanks you, this looks great! I left a comment about testing, but if that doesn't work out, then I think the test you've written are fine - many tests are much more fragile wrt the config options than this.

It would be good if when printing the docs, if an option is unstable then it is marked as such, e.g., with (unstable) after the option name or something similar.

[nrc]

You could make another usage of create_config in a different module with mock data and test using that, rather than the real one - Config is the only name you can use with that macro, but if you use it in a different module, then it is a different name. You could do this within the test module and then only refer to it using the qualified path, e.g., something like:

mod mock_config {
    create_config! { ... }
}

#[test]
fn test_...() {
    mock_config::Config::print_docs(...);
}

[peterhuene]

Thanks for the feedback! I'll amend the commit to include the (unstable) modifier in the output and update the tests to use their own macro invocation.

[peterhuene]

@nrc I've pushed up adding (unstable) to the option output as well as using a separate mock create_options! invocation. Unfortunately the macro call needed several of the actual options because the generated functions have a dependency on them.

Let me know what you think; I'm happy to fix anything that needs fixing.

[nrc]

This looks great thanks! I'm glad the mock module worked out.

If you'd like something else to work in Rustfmt, then adding support for unstable command line flags might be interesting - it is fairly well related to this work, but a little bit more involved. See #1976 (comment) (and the rest of that issue) for details, happy to answer any questions if that sounds interesting.

[peterhuene]

I'd love to help out. I may have some time this week in the evenings or this coming weekend to look into that issue and see how I can help. Thanks for the opportunity to contribute!