libfenix

libfenix is fenix for cargo (libnix): the Rust toolchains of the versions cargo (libnix) has releases for, each with the cargo (libnix) release of exactly that version as its cargo. Everything else (rustc, rust-std, clippy, rustfmt, rust-src, rust-analyzer, …) is the rust-lang dist component, packaged as fenix does.

It is a fork of fenix (MPL-2.0, kept); the toolchain packaging (lib/mk-toolchain.nix, lib/combine.nix) and the API are fenix's, restricted to two pinned versions.

attribute	rustc / dist manifest	cargo
rust-1_87	1.87.0 (2025-05-15)	cargo (libnix) libnix-1.87.0+v5
rust-1_99, latest	1.99.0 (2026-10-01)	cargo (libnix) libnix-1.99.0+v2

There are no channels (stable, beta, nightly) and no dated toolchains: a toolchain needs a cargo (libnix) release of its version. Other versions, dates or channels fail with an error naming the supported ones.

A temporary placeholder

libfenix is meant to be temporary. It gives cargo (libnix) toolchains in a fenix shape while cargo (libnix) is young. Once cargo (libnix) gains more traction, the plan is to merge this work into upstream fenix, e.g. as a cargo (libnix) option for its toolchains, and to retire libfenix. The changes against fenix are listed in Differences to fenix.

Usage

As a flake:

{
  inputs.libfenix.url = "github:nixcloud/libfenix";

  outputs = { self, libfenix }: {
    devShells.x86_64-linux.default = libfenix.devShells.x86_64-linux.rust-1_99;
    packages.x86_64-linux.toolchain = libfenix.packages.x86_64-linux.rust-1_99.defaultToolchain;
  };
}

nix develop github:nixcloud/libfenix             # 1.99, CARGO_BACKEND=nix
nix develop github:nixcloud/libfenix#rust-1_87

Nothing is built: rustc and the other components come from static.rust-lang.org, cargo (libnix) from its nixcloud/cargo release, all patched for nix like fenix does.

Each toolchain set has what a fenix channel has:

• the components: rustc (with rust-std), cargo (cargo (libnix)), clippy, rustfmt, rust-src, rust-analyzer, …
• toolchain (all components), minimalToolchain, defaultToolchain, completeToolchain (rustup's profiles)
• withComponents [ "rustc" "cargo" "rust-src" ]
• cargo-vanilla: the dist cargo of that version
• version ("1.99.0"), libnixRelease ("libnix-1.99.0+v2")

And as in fenix:

libfenix.toolchainOf { channel = "1.99"; }                 # or "1.99.0"
libfenix.fromToolchainFile { dir = ./.; }                 # rust-toolchain(.toml) with channel 1.87 or 1.99
libfenix.fromToolchainName { name = "1.87.0"; }
libfenix.combine [ libfenix.rust-1_99.rustc libfenix.rust-1_99.cargo libfenix.targets.wasm32-unknown-unknown.rust-1_99.rust-std ]

The overlay adds pkgs.libfenix.

Differences to fenix

• cargo is cargo (libnix), prebuilt like the dist components: a release asset of nixcloud/cargo (cargo-<tag>-x86_64-unknown-linux-gnu.tar.xz, the dist cargo tarball with the cargo (libnix) binary), fetched, installed and patchelfed by lib/mk-toolchain.nix; nothing is compiled. Its rpath also has openssl and libgcc (it links libssl dynamically, the dist cargo statically), and combine keeps it a symlink (fenix's patchelf --set-rpath $out/lib would drop that rpath)
• cargo.fromSource builds the release from source: its generated nix build system with the dist rustc/cargo of the same version. cargo.asset packs that build into the release asset (no RUNPATH, the FHS interpreter, no store paths), reproducibly, so nix build .#rust-1_99.cargo.asset gives the file the toolchain downloads
• rustc ≥ 1.90 on x86_64-linux links with its own lld, past nix's ld wrapper (binaries without rpath). The combined rustc/rustdoc of these versions pass -C linker-features=-lld, as cargo (libnix) needs
• x86_64-linux only: the release build systems were generated there, and cargo (libnix) is developed and tested there
• removed: the stable/beta/nightly channels, fromManifest/fromManifestFile, rust-analyzer nightly and its vscode extension, the update bots

Adding a version

1. a cargo (libnix) release of that version exists (libnix-<version>+v<n>)
2. add its manifest hash to update and run it: data/<version>.json
3. add the release (tag, tarball hashes, cargo unit name, assetSha256 = lib.fakeSha256) to lib/releases.nix and the version to versions in default.nix, and its attribute to lines in flake.nix and update
4. run update again: it builds the release asset from source, pins its hash in lib/releases.nix and prints where to upload it (the release of the tag on nixcloud/cargo, as cargo-<tag>-x86_64-unknown-linux-gnu.tar.xz)
5. nix flake check