rust-toolchain

Purpose: One shared Rust toolchain (pinned rustc + mold) consumed as a flake input by projects, so versions don't drift and mold is wired the same everywhere.

Location: /home/caspar/nix/rust-toolchain/ — published as the public repo git+https://git.perhaps.works/caspar/rust-toolchain.git.

---

What it provides

• Rust 1.96 (rust-overlay stable, profile default → rustc, cargo, clippy, rustfmt, rust-src, rust-analyzer).
• mold, gnu-only: sets CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_RUSTFLAGS = "-C link-arg=-fuse-ld=mold". This speeds the everyday gnu/host dev-loop link and never touches musl builds (so static crt-static release builds are unaffected).

Consume it

In a project flake:

inputs.rust-toolchain.url = "git+https://git.perhaps.works/caspar/rust-toolchain.git";
# ...
let rt = rust-toolchain.lib.${system}; in {
  devShells.default = rt.mkDevShell {
    # add a musl target if you ship static binaries:
    toolchain = rt.mkToolchain { targets = [ "x86_64-unknown-linux-musl" ]; };
    packages = [ pkgs.just /* ... */ ];
    env = { /* extra env */ };
  };
}

lib API (rust-toolchain.lib.<system>)

• mkToolchain { targets ? [], extensions ? ["rust-src" "rust-analyzer"] } — the pinned toolchain, optionally with extra cross targets.
• mkDevShell { toolchain ? mkToolchain {}, packages ? [], env ? {} } — mkShell with the toolchain + mold + the gnu mold env merged in.
• rustVersion — the pinned version string.

Bump the version

Edit rustVersion in flake.nix, commit + push this repo, then in each consumer: nix flake update rust-toolchain.

Gotchas

• Must be a git input, not path:. Consumers (e.g. the Forgejo nix:host CI runner) run as users that can't traverse /home/caspar (0700), so a path:/home/caspar/... input is unreadable. The public git URL sidesteps filesystem perms entirely.
• mold is gnu-only by design. Don't add it to musl targets — it gives little on the infrequent cold release link and complicates the static linker wiring.

See Also

• big/docs/forgejo-and-agents.md — the CI build/release pipeline that consumes this (perch → static musl → codeberg).