Use --config-path instead of changing working directory when calling rustfmt #23412

Rationale

In the past, rustfmt didn't implement proper directory walking semantics with the --config-path=<dir> parameter. So the following hack of changing the working directory of the rustfmt subprocess is used instead:

However, the above hack is preventing me from doing the following IDE-specific --config-path override in the rustup repo, because the resolution of --config-path is always relative to the working directory:

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

Proposed solution

After fixing rustfmt so that it has the correct directory walking semantics in rust-lang/rustfmt#7115, I'd love to offer a patch here regarding the above rust-analyzer logic, by replacing the hack with a properly-constructed --config-path= parameter if one doesn't exist already.

Concerns

The replacement should be in theory quite simple but it potentially involves rust-analyzer working with a sufficiently old rustfmt where that parameter was flawed.

Please feel free to let me know if there are any compatibility concerns to be considered and/or any prior art I can look into.

Many thanks in advance :)

References

Related to:

• Config rust-analyzer to use the new formatting style rustup#5069