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Motivation

libnix: Make cargo use nix internally as a backend using the nix c interface.

we could have a .envrc/direnv like concept with a file build-environment.nix in the cargo based project dir. When you enter the directory /home/joachim/myrustproject you will see the rust tools like: cargo-clippy / cargo-fmt / cargo / rust-analyzer / rust-gdb / rust-lldb / rustc / rustdoc / rustfmt. When you leave this directory the tools won't be in the shell anymore.

In your top level project you have a flake.nix:

{ lib, stdenv, pkgs, rust }:
{
    rustPkgs = pkgs.rustBuilder.makePackageSet {
        rustVersion = "1.79.0";
    };
}

So every crate could have a build-environment.nix like:

{ lib, stdenv, pkgs, rust, rustPkgs  }:
{
    buildInputs = [
        rust
        pkg-config
        openssl                  # required for cargo-edit
    ];
}

This myrustproject specific .envrc should only contain the bare minimum to describe the environment needed for the rust project. So a flake could make use to build an output from the rust project but if said flake could contain several rust projects and each would have a specific definition of rust compiler settings.

The same set of tools will be used by cargo when spawning a build, this gives use reproducability in builds since this might require more than just cargo/rustc. For instance a c compiler or some other useful third-party tool.

See also:

• https://lastlog.de/blog/libnix_roadmap.html#cargo-with-nix-support
• https://github.com/NixOS/nix/pull/8699
• https://github.com/Anillc/nixrs/pull/1

Example usages of cargo/crates

Rust only crate:

• https://doc.rust-lang.org/rust-by-example/crates.html

Rust+C: Bundling c code in the create:

• https://github.com/stainless-steel/sqlite3-sys

Rust+C: External dependencies from the system:

• https://doc.rust-lang.org/cargo/reference/manifest.html#the-links-field
• https://github.com/rust-lang/git2-rs/tree/master/libgit2-sys

Build creates using nix abstractions

• https://github.com/cargo2nix/cargo2nix

• https://github.com/nix-community/crate2nix

• https://github.com/anirudhb/rust2nix

• nixpkgs: buildRustCrate = callPackage ../build-support/rust/build-rust-crate { };

• https://github.com/mozilla/nixpkgs-mozilla

• https://github.com/oxalica/rust-overlay

• https://github.com/nix-community/carnix (obsoleted by create2nix)

• https://github.com/nixcloud/nixcrates (obsolete)

Working branch

https://github.com/nixcloud/cargo/tree/libnix-0.79.0

parts of both folders ~/.cargo and target have been replaced by the nix store.

Let's see what is in them!

~/.cargo

[nixos@nixos:~/cargo]$ du -sh ~/.cargo
239M    /home/nixos/.cargo

[nixos@nixos:~/cargo]$ ls ~/.cargo/
registry

[nixos@nixos:~/cargo]$ ls ~/.cargo/registry/
cache  CACHEDIR.TAG  index  src

cargo/target

The example is the cargo project itself, the tool coming with the rust toolchain, since we hack on it.

[nixos@nixos:~/cargo]$ du -sh target/
2.3G    target/

[nixos@nixos:~/cargo]$ ls target/
CACHEDIR.TAG  debug

[nixos@nixos:~/cargo]$ ls target/debug/
build  cargo  cargo.d  deps  examples  incremental  libcargo.d  libcargo.rlib

Plan

libnix backend in cargo needs to get these things working:

• no use of ~/.cargo, everything in /nix/store
• target/debug/* needs to come from /nix/store, eventually symlinked or transparently from the store

figure out what these are:

• target/debug/build

.fingerprint

target/debug/.fingerprint -> https://doc.rust-lang.org/nightly/nightly-rustc/cargo/core/compiler/fingerprint/index.html

[nixos@nixos:~/cargo]$ ls target/debug/.fingerprint/der-d0714c795318982b/
dep-lib-der  invoked.timestamp  lib-der  lib-der.json

[nixos@nixos:~/cargo]$ cat target/debug/.fingerprint/der-d0714c795318982b/lib-der
6b2cf10d51d11828

[nixos@nixos:~/cargo]$ cat target/debug/.fingerprint/der-d0714c795318982b/lib-der.json  | jq
{
  "rustc": 18217185010275080438,
  "features": "[\"alloc\", \"oid\", \"pem\", \"std\", \"zeroize\"]",
  "declared_features": "",
  "target": 4346314333490370059,
  "profile": 12206360443249279867,
  "path": 16879011525611689376,
  "deps": [
    [
      1724196337906130016,
      "pem_rfc7468",
      false,
      5466269655917579261
    ],
    [
      2676387858222833023,
      "const_oid",
      false,
      2852573652909154018
    ],
    [
      15098350142499636151,
      "zeroize",
      false,
      14469829910961900562
    ]
  ],
  "local": [
    {
      "CheckDepInfo": {
        "dep_info": "debug/.fingerprint/der-d0714c795318982b/dep-lib-der"
      }
    }
  ],
  "rustflags": [],
  "metadata": 12456048820742377390,
  "config": 2202906307356721367,
  "compile_kind": 0
}

[nixos@nixos:~/cargo]$ cat target/debug/.fingerprint/der-d0714c795318982b/dep-lib-der

finally:

• get a crate compiled without dependencies
• get one with nested dependencies
• get one with c library usage (like cargo itself)

BuildContext

Some fields that might be interesting:

let bcx : BuildContext = create_bcx(ws, options, &interner)? // Line 153 in cargo/ops/cargo_compile/mod.rs
bcx.ws.gctx.home_path;
bcx.ws.gctx.cwd;
bcx.ws.gctx.ws_roots;
bcx.ws.current_manifest;
bcx.ws.packages.gctx;
bcx.ws.packages.packages; // HashMap<PackageId, LazyCell<Package>>

Compilation outcomes

Where do we need to grab the compilation results that we want to move into the nix store?

https://docs.rs/cargo/latest/src/cargo/core/compiler/build_runner/mod.rs.html#135

Can we just use the Compilation outcome?

With Rust nightly we can see build plan:

cargo build -Z unstable-options --build-plan | jq