switch to non-recursive mode by default

[calebcartwright]

Resolves #3587 and refs #3887

This PR updates the default mode of rustfmt to not format submods in external files, with an accompanying new CLI arg (--recursive)/config option to enable recursive submod formatting mode (which was the default behavior of rustfmt 1.x).

cargo fmt behavior remains unchanged, as cargo fmt will automatically pass the --recursive flag to rustfmt.

I went ahead and marked the new recursive config option as stable because it is relatively simple. Also because it would be pretty weird IMO if both recursive and skip_children were only available on nightly, especially with skip_children now deprecated (that would make it impossible to format sub mods on stable).

Lastly, I did a bit of refactoring in cargo fmt to consolidate/simplify the adding of args to rustfmt (for example the transformation of --message-format to --emit .. for rustfmt, --check, etc.) which had been discussed on some earlier threads.

[calebcartwright]

Thanks for the feedback @scampi!

[topecongiro]

Thank you for the PR!

[topecongiro]

We should consider recursive as one of the internal options. We do not want users to add this to the configuration by themselves.

[calebcartwright]

Will do 👍, though out of curiosity, why should this be a CLI only flag for users?

One downside I see with exposing recursive as a config option is that recursive = false in the config file would functionally be ignored when using cargo fmt since cargo fmt will always pass --recursive flag to rustfmt which will always take precedence over the config file value (which could be confusing for users).

[topecongiro]

The options in the configuration file are meant to be persistent, whereas recursive is more of a one-shot configuration, which best suits as a CLI flag.

[calebcartwright]

Makes sense to me! It wouldn't surprise me if there is a future request to support recursive as a config option since skip_children was available, but there's clearly several good reasons why recursive should be internal/cli-only.

Side note, while making recursive internal I noticed that internal options added to the config file seem to just be silently ignored. Do I have that correct?

If they are silently ignored:

Is that the desired behavior or would it beneficial to emit a warning ("Warning: Unknown configuration option ..)? Would that potential change be tackled (perhaps it already is) as part of #3873 or should I open a new issue?

[topecongiro]

Side note, while making recursive internal I noticed that internal options added to the config file seem to just be silently ignored. Do I have that correct?

Yes.

Is that the desired behavior or would it beneficial to emit a warning ("Warning: Unknown configuration option ..)?

We should make it to a hard error rather than silently ignoring it.

Would that potential change be tackled (perhaps it already is) as part of #3873 or should I open a new issue?

This should be fixed by #3873.

[topecongiro]

My apologies for the late review. LGTM, thank you for the update!