fix(config): search for config file recursively on --config-path - #7115

I still some concerns regarding the distinction between the dir mode and the TOML mode, so reconverting to draft.

Addressed the concern regarding mode switching; updated both the PR description accordingly to reflect the new design. This PR is ready for review 🙏

Secondly, however, if the user wants to use a different name for the config file, taking the example mentioned in #4660 (comment):

rustfmt = { extraArgs = { "+nightly", "--config-path=.rustfmt.unstable.toml" } },

The file (.rustfmt.unstable.toml in the above example) is better placed at CWD, but it may totally be at a different place

Could you expand more on this use case, where is .rustfmt.unstable.toml expected to be? Is the use case something like: I have opened my editor at the root of a repo with several packages, each of which might have different rustfmt.toml, so if I edit a file in one of them I want the config from that package to apply?

my_repo
|
|---- first_crate/
    |---- rustfmt.toml
    |---- src/ # files under here use the rustfmt.toml above
|---- second_crate
    |---- rustfmt.toml
    |---- src/ # files under here use a different config

@matthewhughes934 TLDR, I'll interpret this case as "I want to override the config file name, but at the same time I also want to override the base directory, but unfortunately they are under the same flag".

In the case I posted above, I have my_repo/.rustfmt.unstable.toml for sorting import sections (unfortunately the feature is unstable-only so we want to override the file name to make it clear, the idea is borrowed from rustls).

Unfortunately, rustfmt isn't always invoked at the workspace root. For example, to format my_repo/first_crate/src/foo/bar.rs, the CWD for rust-analyzer is my_repo/first_crate/src/foo/. So the same --config-path=.rustfmt.unstable.toml is not resolved to a valid path in this case. And even if rust-analyzer is updated to not cd the rustfmt instance before formatting, it should still be launched at my_repo/ for this to work. Same thing for cargo +nightly fmt -- --config-path=.rustfmt.unstable.toml, because the config path is forwarded to rustfmt verbatim.

Looking back, of course you can argue that it's not rustfmt's responsibility of getting the base directory right so that this config can work. If that is the case for you, I think I can combine our ideas and propose the following semantics, please tell me if it looks better to you:

1. Base dir mode (identical with my proposal): If path exists and metadata says it's a dir, then search for predefined names ([".rustfmt.toml", "rustfmt.toml"]) under path and all its parents. Bails out if none is found.
2. TOML mode (identical with the current mainline): If path exists and the metadata says it's not a dir, then interpret it as a TOML file. Bails out if none is found.

If you agree with this, I can change the feature commit quite quickly to reflect it.

1. Base dir mode (identical with my proposal): If path exists and metadata says it's a dir, then search for predefined names ([".rustfmt.toml", "rustfmt.toml"]) under path and all its parents. Bails out if none is found.
2. TOML mode (identical with the current mainline): If path exists and the metadata says it's not a dir, then interpret it as a TOML file. Bails out if none is found.

👍 this sounds good to me, I think the directory behaviour sounds closer to what's documented in the help output.

It might be worth further discussing the file behaviour (there's also some discussion on it with #5206), but that's best done separately to this change.

CC @ytmimi since you were discussing on the original issue

@matthewhughes934 No problem, I have rewritten the PR as suggested. Please review!

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

Also probably r? @ytmimi

@rustbot ready

Will take a look at this after work today. Thank you @matthewhughes934 for the initial review 🙏🏼