Cargo Nix Plugin — Development skill for Claude Code

cargo-nix-plugin

A Nix plugin that resolves Cargo workspaces natively, replacing the generated `Cargo.nix` file from crate2nix with a single `builtins.resolveCargoWorkspace` primop.

What It Does

• Reads Cargo.lock and the sparse registry index directly, so there is no cargo binary at eval time (you can also feed it pre-generated cargo metadata JSON)
• Evaluates cfg() target expressions for your platform during resolution
• Produces an attrset that plugs into buildRustCrate
• No more crate2nix generate step or checked-in 50K-100K line Cargo.nix

Install

Add the plugin to your Nix configuration:

# nix.conf or via --option — point at the directory so the right
# extension (.so/.dylib) is picked up automatically
plugin-files = /path/to/cargo-nix-plugin/lib/nix/plugins

Or use the flake output:

{
  inputs.cargo-nix-plugin.url = "github:anthropics/cargo-nix-plugin";
}

Usage

Default (lockfile resolve)

Point at your workspace root:

cargoNix = cargo-nix-plugin.lib {
  inherit pkgs;
  src = ./.;  # must contain Cargo.toml + Cargo.lock
};

The plugin reads `Cargo.lock` plus the sparse registry index directly — no `cargo` binary, no crate sources at eval time. On first use it fetches each crate's index entry (a few hundred bytes) into `$CARGO_HOME` and reuses it on later runs.

If you already redirect cargo to a mirror (`CARGO_REGISTRIES_CRATES_IO_INDEX` or `[source.crates-io] replace-with` in `.cargo/config.toml`), the resolver picks that up too:

# .cargo/config.toml — used by both cargo and the plugin
[source.crates-io]
replace-with = "mirror"
[source.mirror]
registry = "sparse+https://artifactory.example/api/cargo/crates/index/"

If every index lookup fails (e.g. egress to `index.crates.io` is blocked and no mirror is configured), evaluation fails rather than silently producing derivations with missing features.

Explicit metadata

You can also generate cargo's resolution up front an