modernize the scaffold around crane, rust-overlay, and but.nix

[0xgleb]

Motivation

The template had drifted from the projects started from it. It pinned the previous nixpkgs stable, took its toolchain from fenix while every live consumer uses rust-overlay, built nothing at all (a dev shell and no package), and drove CI through the DeterminateSystems magic-nix-cache -- whose hosted cache throttled builds with HTTP 418, which is why the live repos already moved off it.

The most duplicated piece was the crane build: moneymentum/rust.nix and yielduck/rust.nix are near-identical hand-copies, differing only in crate name, the extra source directories they whitelist, and a bit of env. That belongs in the scaffold, not in each project.

Solution

• Move to nixos-26.05 and the rust-overlay toolchain, and add crane. The duplicated build is now one mkRustPackages in nix/lib.nix, parameterized by pname and the non-Cargo source directories a build reads (extraSrcDirs).
• Expose the crane derivations as checks named cargo-test and cargo-clippy. The names are project-agnostic on purpose: the workflow shipped by templates.ci builds them by name, so it works unchanged in any consuming project instead of needing per-project aliases.
• Take the GitButler CLI and its agent skill from but.nix rather than leaving each project to package them, and expose pr-stack-footer.
• Refresh the hook set to what the live projects run: nil, nixfmt, actionlint, taplo, rustfmt --edition 2024, and deno fmt on markdown. The markdown hook is disabled in interactive shells -- deno's V8 has no reliable aarch64-darwin substitute and builds for hours -- but still gates CI through the git-hooks check.
• Add AGENTS.md with the conventions common to both live projects, CLAUDE.md symlinked to it, and a ## Project Direction placeholder each new project fills in.
• Migrate both CI workflows to nix-quick-install + Cachix + cache-nix-action, and bring templates/nix in line with the new inputs and lib API.

Verification

• cargo-test, cargo-clippy, and ci-template-mirror all build.
• The dev shell provides but 0.22.0, cargo 1.97.1, and cargo-nextest.
• nix flake init -t .#nix produces a flake that evaluates against this checkout.
• Note: nix flake check on aarch64-darwin will build deno from source via the markdown hook. CI runs on linux, where it substitutes.

[coderabbitai]

Important

Review skipped

Draft detected.

Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: b670600f-8cb6-4f0f-b703-844f37f74fa6

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands.}