core/pkgs/build-support/rust/sysroot/src.nix

{
  lib,
  stdenv,
  rustPlatform,
  buildPackages,
  originalCargoToml ? null,
}:

stdenv.mkDerivation {
  name = "cargo-src";
  preferLocalBuild = true;

  unpackPhase = "true";
  dontConfigure = true;
  dontBuild = true;

  installPhase =
    ''
      export RUSTC_SRC=${rustPlatform.rustLibSrc.override { }}
    ''
    + lib.optionalString (originalCargoToml != null) ''
      export ORIG_CARGO=${originalCargoToml}
    ''
    + ''
      ${buildPackages.python3.withPackages (ps: with ps; [ toml ])}/bin/python3 ${./cargo.py}
      mkdir -p $out/src
      echo '#![no_std]' > $out/src/lib.rs
      cp Cargo.toml $out/Cargo.toml
      cp ${./Cargo.lock} $out/Cargo.lock
    '';
}