Use unstable `rustfmt` features to ensure import style

[rami3l]

In rustls we enforce format style in CI by using nightly clippy:

https://github.com/rustls/rustls/blob/main/.github/workflows/build.yml#L392

Maybe that could make sense for rustup?

Originally posted by @djc in #5064 (comment)

[rami3l]

group_imports = "StdExternalCrate"
imports_granularity = "Crate"

Yes, these have been very stable IME.

(I don't think you need unstable_features if you run nightly with these?)

Originally posted by @djc in #5064 (comment)

According to the docs:

To enable unstable options, set unstable_features = true in rustfmt.toml or pass --unstable-features to rustfmt.

So this looks like the semantics was about to change in rust-lang/rustfmt#4353 (comment) but the migration never landed.

OTOH to facilitate use in IDEs so that we can never worry about reorganizing imports, I'm discovering the feasibility of adding a recommended rust-analyzer config that looks like so:

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

[rami3l]

OTOH to facilitate use in IDEs so that we can never worry about reorganizing imports, I'm discovering the feasibility of adding a recommended rust-analyzer config that looks like so:

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

Interesting, I cannot use that yet because rust-analyzer is calling rustfmt at the source file's basedir so .rustfmt.unstable-toml will be "not found":

• Recursive --config-path doesn't recurse rustfmt#4660