fullrust

Compile unmodified Rust crates into fully-static, libc-free Linux binaries.

fullrust is a patched Rust toolchain whose standard library talks to the Linux kernel through raw syscall instructions instead of libc. An ordinary crate — no code changes, no attributes, no fullrust dependency — compiles to a static x86_64-unknown-linux-fullrust ELF that links no libc and no C runtime: no interpreter (PT_INTERP), no .dynamic section, zero NEEDED libraries. The only thing the binary needs to run is the Linux kernel.

You use it as a GitHub Action or a Docker image — no local toolchain to install, no nightly, no -Z build-std, no target JSON.

// src/main.rs — a completely ordinary program
use std::collections::BTreeMap;
fn main() {
    let mut counts = BTreeMap::new();
    for w in "the quick brown fox the fox".split_whitespace() {
        *counts.entry(w).or_insert(0) += 1;
    }
    println!("{counts:?}");
}

$ file    target/x86_64-unknown-linux-fullrust/release/wordcount
ELF 64-bit LSB executable, x86-64, statically linked
$ ldd     target/x86_64-unknown-linux-fullrust/release/wordcount
        not a dynamic executable
$ readelf -d target/x86_64-unknown-linux-fullrust/release/wordcount
There is no dynamic section in this file.

Anything that reaches for a C library fails at link time — pure-Rust code links, FFI into a .so does not. That's an intended guard-rail, not a bug.

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Use it in a GitHub workflow

One step. It runs the fullrust toolchain image against your checked-out crate and leaves the binary under target/x86_64-unknown-linux-fullrust/release/.

- uses: actions/checkout@v6
- uses: KarpelesLab/fullrust@master
  with:
    args: --release --bin myapp

Attach a static binary to a GitHub Release

name: Release
on:
  push:
    tags: ["v*"]          # or: release: { types: [published] }

permissions:
  contents: write          # required to upload release assets

jobs:
  linux-x86_64-static:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v6

      - name: Build the libc-free static binary
        uses: KarpelesLab/fullrust@master
        with:
          args: --release --bin myapp

      - name: Package + attach
        run: |
          tar -C target/x86_64-unknown-linux-fullrust/release \
              -czf "myapp-${{ github.ref_name }}-linux-x86_64-static.tar.gz" myapp
      - uses: softprops/action-gh-release@v2
        with:
          files: myapp-*-linux-x86_64-static.tar.gz

Action inputs

input	default	meaning
command	build	cargo subcommand: build, test, run, clippy, …
args	--release	extra cargo args, e.g. --release --bin myapp --no-default-features
working-directory	.	the crate to build
image	ghcr.io/karpeleslab/fullrust:1.88	pin a Rust version (see Versions)
no-ecosystem	false	skip the getrandom/socket2 [patch.crates-io] injection

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Use it as a container: job

The image bakes the toolchain into its environment (RUSTC, CARGO_BUILD_TARGET, a CARGO_HOME carrying the ecosystem [patch]), so a plain cargo build is already a fullrust build — the same static, libc-free binary the action produces:

jobs:
  build:
    runs-on: ubuntu-latest
    container: ghcr.io/karpeleslab/fullrust:latest
    steps:
      - uses: actions/checkout@v6
      - run: cargo build --release

Build locally with Docker

No toolchain install — mount your crate and build:

docker run --rm -v "$PWD":/src ghcr.io/karpeleslab/fullrust:latest build --release
# `test`, `run`, `clippy` work the same way; run `bash` for an interactive shell.

The binary lands in your crate's target/x86_64-unknown-linux-fullrust/release/.

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Versions

Images are published per Rust minor, plus latest (the newest):

ghcr.io/karpeleslab/fullrust:1.88   …   :1.95   :latest

They're public — no login to pull. Pin one via the action's image: input, the container: image, or the docker run tag. The action currently defaults to :1.88; for the newest Rust, set image: ghcr.io/karpeleslab/fullrust:1.95 (or :latest).

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What works

An unmodified crate gets the real standard library, with its platform backend rewritten on raw syscalls:

• Threads & sync — std::thread (clone-backed, per-thread stacks), futex Mutex/Condvar/RwLock, native thread_local! (%fs/TCB TLS).
• Files & directories, and std::process::Command (fork+execve, pipes, wait).
• Networking — TCP/UDP with a self-contained DNS resolver (/etc/hosts + /etc/resolv.conf, no NSS).
• Panics unwind (catch_unwind, Drop during unwind), and RUST_BACKTRACE=1 prints symbolized backtraces — with no libunwind and no libc, via an in-tree pure-Rust unwinder.
• std::os::fd (AsFd/OwnedFd/AsRawFd/…) for the fd-interop ecosystem.
• Dependencies — pure-Rust crates work as-is (e.g. serde/serde_json). The image auto-injects a [patch.crates-io] so the common not-quite-pure gateways build libc-free too: getrandom (and thus rand, uuid), socket2 (and thus mio/async stacks). Opt out with no-ecosystem: true.

Limitations

• Linux + x86-64 only today.
• Static only — no dynamic linking; FFI into a .so cannot link (by design).
• Not the unix target family. cfg(unix) is false — that's exactly what keeps the build graph libc-free — so std::os::unix is absent (std::os::fd is provided instead). A crate whose unix-only path is load-bearing may need the ecosystem [patch] or a small fix; in practice most pure-Rust crates need nothing.

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Why a standalone std (the Go "no-cgo" model)

fullrust gives the standard library its own platform backend on Linux syscalls, instead of routing it through the platform libc. This is the same architecture as the Go runtime: Go issues syscalls itself and only touches libc when you opt into cgo. fullrust is, in effect, Rust with cgo off — the binary's only boundary to the outside world is the syscall instruction. It is still the real std, so ordinary crates compile unchanged.

The deciding principle is keeping Rust's guarantees end-to-end, all the way to the kernel — which a pure-Rust libc shim (the Eyra / c-scape model) gives up at a C-shaped wall in the middle of every OS interaction:

• Zero-cost abstraction survives only without the wall. io::Write::write → syscall inlines and monomorphizes end to end. An extern "C" libc seam is opaque to the optimizer — it can't be inlined or specialized across, so you forfeit exactly that benefit at exactly the wrong place.
• Rust types all the way down. I/O returns Result; buffers are slices; errors are values — not raw *mut u8, c_int, NUL-terminated strings and a thread-local errno, with a double-translation tax on every call.
• A tiny unsafe surface instead of a vast one. The unsafe is the handful of syscall sites, not an entire glibc-shaped ABI (symbol versioning, struct layouts, the TLS model, the __libc_start_main handshake) that a shim must reproduce exactly or hit UB.

The one accepted cost, like Go: programs target fullrust's std rather than being able to link arbitrary C libraries. (Pure no_std + alloc crates still work as-is; only code that needs OS services through libc is excluded — on purpose.)

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How it's built (internals)

fullrust is a source overlay on the Rust compiler that adds the built-in x86_64-unknown-linux-fullrust target and a std::sys backend on raw syscalls (the allocator, native TLS, threads, fs/net/process, the pure-Rust unwinder and backtraces), packaged as a thin Docker image around the prebuilt toolchain. It tracks released Rust versions (currently 1.88–1.95).

See toolchain/README.md for the design, the syscall-backed platform layer, and how the overlay is built and ported across Rust versions.

License

MIT OR Apache-2.0.