Setting Up a Basic Cross-Platform Rust Build Environment with Nix Flake

  1. Add a flake.nix file and run git add flake.nix:

    {
  inputs = {
    utils.url = "github:numtide/flake-utils";
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-24.11";
  };

  outputs = {
    nixpkgs,
    utils,
    ...
  }:
    utils.lib.eachDefaultSystem (system: let
      pkgs = import nixpkgs {inherit system;};
      lib = pkgs.lib;
    in {
      devShells =
        {
          default = pkgs.mkShell {
            nativeBuildInputs = with pkgs; [
              pkg-config
              gcc
            ];
          };
        }
        // builtins.listToAttrs (map (target: {
          name = target;
          value = let
            cross = import nixpkgs {
              inherit system;
              crossSystem = {config = target;};
            };
            cpkgs = cross.pkgsMusl;
          in
            cross.mkShell {
              nativeBuildInputs = with pkgs; [
                pkg-config
                gcc # Required for compiling proc macros and other dependencies
              ];
              env = let
                normalized = lib.strings.toUpper (builtins.replaceStrings ["-"] ["_"] target);
              in
                with cpkgs; {
                  # https://doc.rust-lang.org/cargo/reference/environment-variables.html#configuration-environment-variables
                  "CARGO_TARGET_${normalized}_LINKER" = "${stdenv.cc.targetPrefix}cc";
                };
            };
        }) ["x86_64-unknown-linux-musl" "aarch64-unknown-linux-musl"]);
    });
}

  2. Build the project:

    # Local build
nix develop .#default --command \
  cargo build

# Cross-platform build
#
# Ensure the required Rust targets are installed:
# rustup target add aarch64-unknown-linux-musl
# rustup target add x86_64-unknown-linux-musl

TARGET=x86_64-unknown-linux-musl nix develop .#$TARGET --command \
  cargo build --target $TARGET

Handling C/C++ Dependencies (e.g., OpenSSL, PQ)

Use static linking and vendor-based bindings (which package C/C++ source code) to ensure stable and reproducible builds.

  1. Add target-specific dependencies in Cargo.toml:

    [target.x86_64-unknown-linux-musl.dependencies]
openssl-sys = { version = "0.9", features = ["vendored"] }
pq-sys = { version = "0.7", features = ["bundled"] }

[target.aarch64-unknown-linux-musl.dependencies]
openssl-sys = { version = "0.9", features = ["vendored"] }
pq-sys = { version = "0.7", features = ["bundled"] }

  2. Build the project:

    # Static linking may require setting RUSTFLAGS.
# However, this setting may not work for all architectures.
RUSTFLAGS='-C target-feature=+crt-static' cargo build

# x86_64-unknown-linux-musl enables crt-static by default, so no extra flags are needed.
TARGET=x86_64-unknown-linux-musl nix develop .#$TARGET --command \
  cargo build --target $TARGET

CRT stands for the C runtime. The default crt-static setting varies by target. For example, x86_64-unknown-linux-musl enables it by default, whereas arm-unknown-linux-musleabi does not. 1