evix

A library-first Rust crate using nix-bindings for the Nix C API to evaluate Nix expressions and stream derivation info as JSON lines. It ships with a small CLI, but the evaluation logic is exposed as a reusable library so other tools can drive it programmatically.

Why evix?

• Faster evaluation by using multiple worker processes
• Memory used for evaluation is reclaimed when workers restart, so the build can use it.
• Evaluation of jobs can fail individually
• It's really cool!

Usage

Exactly one input is required:

evix --flake .#hydraJobs
evix --expr 'import <nixpkgs> {}'
evix --file ./default.nix

# Evaluate the hydraJobs attribute of the patchelf flake
$ evix --flake 'github:NixOS/patchelf#hydraJobs'
copying path '/nix/store/jfdpyszsgvsnz68y36qi65irx7r6a52q-source' from 'https://cache.nixos.org'...
{"attr":"tarball","attrPath":["tarball"],"drvPath":"/nix/store/dbhsb9ji8ya2js87v1q5621lx87smw3l-patchelf-tarball-0.18.0.drv","name":"patchelf-tarball-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"coverage","attrPath":["coverage"],"drvPath":"/nix/store/h8fzgxxddi1470vad93j2y5s1lyxsii8-patchelf-coverage-0.18.0.drv","name":"patchelf-coverage-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"patchelf-win32","attrPath":["patchelf-win32"],"drvPath":"/nix/store/8kbg5mf09zyykcjvkmwna621ja8vm5pr-patchelf-i686-w64-mingw32-0.18.0.drv","name":"patchelf-i686-w64-mingw32-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"patchelf-win64","attrPath":["patchelf-win64"],"drvPath":"/nix/store/5zvjbw8y4k1fs3vhbb465ixhl032imgg-patchelf-x86_64-w64-mingw32-0.18.0.drv","name":"patchelf-x86_64-w64-mingw32-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}
{"attr":"release","attrPath":["release"],"drvPath":"/nix/store/4l4cl6w4afn4g4bha5h6z0nm16vddnph-patchelf-0.18.0.drv","name":"patchelf-0.18.0","outputs":{"out":null},"system":"x86_64-linux"}

Options

Flag	Description
--flake REF	Evaluate a flake output
--expr EXPR	Evaluate an inline Nix expression
--file PATH	Evaluate a Nix file
--arg NAME EXPR	Pass a Nix expression argument
--argstr NAME VALUE	Pass a string argument
--override-input NAME REF	Override a flake input while locking (flake inputs only)
--option KEY VALUE	Set a Nix setting (e.g. restrict-eval, allow-import-from-derivation)
--meta	Attach each derivation's meta attribute to the output
--show-input-drvs	Attach each derivation's input derivations (inputDrvs)
--workers N	Worker processes (default: 1)
--max-memory-size MB	Memory limit per worker; restarts when exceeded (default: 4096)
--force-recurse	Recurse into all attrsets, ignoring recurseForDerivations
--gc-roots-dir DIR	Register GC root symlinks for evaluated derivations
-v, --verbose	Increase logging verbosity (info -> debug -> trace)

Logging is powered by tracing. The default level is info; use -v for debug and -vv for trace. The RUST_LOG environment variable overrides --verbose if set. Logs are written to stderr so they do not interfere with the JSON output on stdout.

Output

Each line is a JSON object. Derivation attributes emit:

{
  "attr": "packages.x86_64-linux.hello",
  "attrPath": ["packages", "x86_64-linux", "hello"],
  "name": "hello-2.12.1",
  "system": "x86_64-linux",
  "drvPath": "/nix/store/...",
  "outputs": { "out": "/nix/store/..." }
}

With --meta, the derivation's meta attribute is attached verbatim as a meta object. With --show-input-drvs, input derivations are attached as inputDrvs, keyed by absolute .drv store path with their output-name lists:

{
  "attr": "hello",
  "drvPath": "/nix/store/...-hello.drv",
  "outputs": { "out": "/nix/store/...-hello" },
  "meta": { "description": "...", "license": { "spdxId": "GPL-3.0-or-later" } },
  "inputDrvs": { "/nix/store/...-stdenv-linux.drv": ["out"] }
}

Aggregate jobs that declare constituents emit them as a list of attribute names:

{ "attr": "release", "drvPath": "...", "constituents": ["hello", "world"] }

Non-derivation attrsets emit child attribute names for further recursion:

{
  "attr": "packages.x86_64-linux",
  "attrPath": ["packages", "x86_64-linux"],
  "attrs": ["hello", "git", "vim"]
}

Errors are non-fatal unless "fatal": true:

{"attr": "...", "attrPath": [...], "error": "...", "fatal": false}

Architecture

The library splits work between a master and worker processes. The master maintains a queue of attribute paths, dispatches them to workers over stdin/stdout, and collects Event values. Workers are restarted automatically when they exceed the configured memory limit.

The CLI is a thin wrapper around the library. When the library spawns a worker, it re-executes the current binary with the EVIX_WORKER environment variable set; the binary then calls the worker entrypoint.

Library usage

use evix::{Config, Event, Input};

let config = Config {
    input: Input::Expr("import <nixpkgs> {}".into()),
    auto_args: vec![],
    force_recurse: false,
    gc_roots_dir: None,
    workers: 4,
    max_memory_size: 4096,
    meta: false,
    show_input_drvs: false,
    override_inputs: vec![],
    nix_options: vec![],
};

evix::evaluate(&config, |event| {
    println!("{:?}", event);
    Ok(())
})?;

Building

Requires Rust 1.90.0+. Supported on x86_64-linux and aarch64-linux.

License

EUPL-1.2