Nix

The Nix plugin builds reproducible shell environments using Nix, resolving packages from a nixpkgs flake URL and exposing selected programs as tool binaries. The driver locks flake URLs to ensure pure evaluation, builds an environment via nix build, and wraps the resulting programs for use in other build targets. This lets you pin an exact toolchain once and depend on it from anywhere in the graph without polluting the host environment.

Driver

A driver is the component that knows how to execute a target's action, turning a resolved target into runnable work. This plugin registers the nix driver.

Enabling it

Not enabled by default. Register the nix driver in the drivers section of .hephconfig, then reference it from a target's driver field. Requires the nix command-line tool with flakes support on the host PATH.

Configuration

.hephconfig

drivers:
  - name: nix

Usage

Build a small toolchain and consume it from another target via tools:

BUILD

nix_tools = target(
    name = "nix_tools",
    driver = "nix",
    nixpkgs = "github:NixOS/nixpkgs/nixos-unstable",
    packages = ["pkgs.ripgrep"],
    programs = ["rg"],
)

target(
    name = "check",
    driver = "bash",
    tools = nix_tools,
    run = ["rg --version"],
)

Notes

Requires nix on PATH with the nix-command and flakes experimental features. The nixpkgs URL should pin a specific commit for reproducibility (moving branches are resolved once then cached until the URL changes). Programs must exist at {store_path}/bin/{program} after the nix build. Remote caching is disabled because wrappers reference host-local /nix/store paths. Use system to override the default host system detection (e.g. for cross-compilation specs).