fix(pre-commit): rustfmt a virtual workspace root

[thedavidmeister]

What is broken

2455992d changed the rustfmt-conditional hook to format each manifest via --manifest-path, so a crate one level below the repo root gets formatted instead of cargo erroring on a manifest-less root.

That broke the other layout. A virtual workspace root — [workspace] with members and no [package] — has no targets of its own, so cargo-fmt fmt --manifest-path Cargo.toml exits 1 with:

Failed to find targets
This utility formats all bin and lib files of the current crate using rustfmt.
Usage: cargo fmt [OPTIONS] [-- <rustfmt_options>...]

The */Cargo.toml arm does not save it when the members live deeper, e.g. crates/*/Cargo.toml.

Impact

rainlanguage/rain.metadata — root Cargo.toml is [workspace] over crates/bindings, crates/cli, crates/metaboard — is red on rainix-rs / static / rs-static on every PR and on main, with no formatting problem at all: clippy passes in that same job before the hook runs.

That repo's ci-gate reads the lane, so release / release on main fails with ci-gate: refusing to publish <sha> — rainix-rs (.github/workflows/rainix-rs.yaml) concluded failure. The repo cannot publish until this hook is fixed.

The fix

--all formats the workspace members rather than the root package's own targets. That is what the hook wants in both layouts, and it is a no-op for a single-package manifest.

QA

• Discriminating tests: n/a - this repo has no harness that executes a git-hooks.nix hook body, so there is no test that could fail on base. The hook is verified by running its exact command line against a virtual-workspace repo, below. A hook-execution harness is a larger change than this one-flag fix and belongs in its own issue.
• Mutations applied: cargo-fmt fmt --all --manifest-path "$manifest" -> drop --all (the base as of 2455992) -> exits 1 with Failed to find targets against rain.metadata's virtual root; restored -> exits 0. Run in rain.metadata's dev shell at 45ca96c: cargo-fmt fmt --all --manifest-path Cargo.toml --check gives rc=0, cargo-fmt fmt --manifest-path Cargo.toml --check gives rc=1 and Failed to find targets. The same tree passes cargo fmt --all --check at rc=0 across five separate merge commits, so the lane's red X was never a formatting diff.
• Oracle: cargo-fmt's documented behaviour (--all formats all workspace packages; a manifest with no [package] has no targets to find), plus the CI job log for rainix-rs / static / rs-static on rain.metadata main at 45ca96c (run 32862493917), which shows every other pre-commit hook passing and rustfmt-conditional alone exiting 1 on Failed to find targets.
• Category check: Refs no issue - found from rain.metadata's red lane rather than filed. The regression is one behaviour, the hook's exit status on a virtual workspace root, covered by the mutation above; the nested-crate case 2455992 added is preserved, since --all on a member manifest formats that member.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Improved automatic Rust formatting to include all workspace members.
  • Enhanced formatting guidance for projects using virtual manifests.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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• flake.nix

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📝 Walkthrough

Walkthrough

The rustfmt-conditional hook now runs cargo-fmt fmt --all for each manifest. The comment explains that this supports virtual Cargo manifests by formatting their workspace members.

Changes

Workspace Rust formatting

Layer / File(s)	Summary
Update rustfmt-conditional workspace formatting flake.nix	The hook now formats all workspace members with cargo-fmt fmt --all --manifest-path "$manifest". The comment documents the virtual manifest behavior.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to 96771

This localized change updates formatting for virtual workspaces while preserving single-package behavior; no actionable merge-blocking risk remains after normal checks and review.

Suggested reviewers: findolor

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main change: fixing rustfmt behavior for virtual workspace roots in the pre-commit hook.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-08-25-rustfmt-virtual-workspace

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