Update rust toolchain to use rules_rust 0.22.0

[contagnas]

rules_rust 0.22.0 removes rust_toolchain.os in bazelbuild/rules_rust#1960

note this change isn't compatible with earlier versions of rules_rust, because the os field was required.

[aherrmann]

Thank you!
Small comment about the version bump.

[benradf]

Thanks for the PR @contagnas!

It looks like CI is failing with this error:

ERROR: ... BUILD:66:15: @nixpkgs_config_rust//:rust_nix_impl: missing value for mandatory attribute 'os' in 'rust_toolchain' rule

Which is odd, if os has been removed in rules_rust 0.22.0. Maybe it's somehow still using the old version? I haven't had a chance to dig deeper yet.

[contagnas]

I'm not familiar with bzlmod yet, so it's not clear to me if this registry should have multiple version of dependencies, or just the current one?

[aherrmann]

This registry is only used for testing (see readme), so a single version is enough.

[contagnas]

I expected this sha to need an update, but I can't find anything that uses it

[aherrmann]

bazel checks against this sha after downloading the dependency. I think it will also cache based on that sha, so if it's not updated you may get an old artifact.

[benradf]

A different CI error now 🤔

   ERROR: /home/runner/work/rules_nixpkgs/rules_nixpkgs/examples/toolchains/rust/WORKSPACE:112:18: fetching crates_repository rule //external:crate_index: Traceback (most recent call last):
  	File "/home/runner/.cache/bazel/_bazel_runner/962535b5b03441543344ad7194339468/external/rules_rust/crate_universe/private/crates_repository.bzl", line 45, column 28, in _crates_repository_impl
  		repin = determine_repin(
  	File "/home/runner/.cache/bazel/_bazel_runner/962535b5b03441543344ad7194339468/external/rules_rust/crate_universe/private/generate_utils.bzl", line 382, column 13, in determine_repin
  		fail(("\n".join([
  Error in fail: Digests do not match: Digest("59ace66e05f26b5ab71bdc99a930d8b1bafdb232f70f60ad13c15599bc4a49a4") != Digest("a287c27b9422cf61b5995ee3c56060decae072616e470f667c74d3bd1650e80f")
  
  The current `lockfile` is out of date for 'crate_index'. Please re-run bazel using `CARGO_BAZEL_REPIN=true` if this is expected and the lockfile should be updated.

[benradf]

The expected hash that gets printed when there's a mismatch is in hexadecimal, but the hash in source.json should be base64. So I had to do a conversion:

$ (xxd -r -p | base64) <<<f47303cd12b8b8ebfdce0e2ac93f2b72d1d484d7ef9bd6f3b4f307f523e331ebbf3496caed2688f4d5345918344be9f2
9HMDzRK4uOv9zg4qyT8rctHUhNfvm9bztPMH9SPjMeu/NJbK7SaI9NU0WRg0S+ny

[benradf]

Fixed the failure in examples/toolchains/rust by updating the lockfile:

CARGO_BAZEL_REPIN=true bazel run --config=nix :hello