Codify how to group our imports (#12063)

I'll actually apply it once these PRs are merged:
* #12044
* #12047
* #12048

Author: emilk

.rustfmt.toml

@@ -0,0 +1,6 @@
+# Enable these if you want to run `cargo +nightly fmt`:
+# group_imports = "StdExternalCrate"
+# imports_granularity = "Module"
+
+# To apply these, uncomment the above lines, then run `cargo +nightly fmt`.
+# Then comment them again, and run `pixi run codegen --force && pixi run codegen-protos && cargo fmt`

CODE_STYLE.md

@@ -130,6 +130,10 @@ When importing a `trait` to use its trait methods, do this: `use Trait as _;`. T
 
 When intentionally ignoring a `Result`, prefer `foo().ok();` over `let _ = foo();`. The former shows what is happening, and will fail to compile if `foo`:s return type ever changes.
 
+We group and order imports (`use` statements) by `std`, other crates, and lastly own `crate` and `super`. This corresponds to [`StdExternalCrate`](https://rust-lang.github.io/rustfmt/?version=v1.8.0&search=group#StdExternalCrate%5C%3A).
+
+We group our `use` statements by module, e.g. `crate_name::module::{a, b, c}`. This is a compromise, being rather terse while still avoiding excessive merge conflicts. See [the cargofmt docs](https://rust-lang.github.io/rustfmt/?version=v1.8.0&search=group#Module%5C%3A) for details.
+
 ### `TODO`:s
 When you must remember to do something before merging a PR, write `TODO` or `FIXME` in any file. The CI will not be green until you either remove them or rewrite them as `TODO(yourname)`.