Edolstra's import-cargo without the flake.

import-cargo.nix

# based on https://github.com/edolstra/import-cargo @ 25d40be4a73d40a2572e0cc233b83253554f06c5
{ runCommand, makeSetupHook, writeScript }:
{ lockFile }:
let lockFile' = builtins.fromTOML (builtins.readFile lockFile); in
rec {

  # Fetch and unpack the crates specified in the lock file.
  unpackedCrates = map
    (pkg:
      let
        isGit = builtins.match ''git\+(.*)\?rev=([0-9a-f]+)(#.*)?'' pkg.source;
        registry = "registry+https://github.com/rust-lang/crates.io-index";
      in
      if pkg.source == registry then
        let
          sha256 = pkg.checksum or lockFile'.metadata."checksum ${pkg.name} ${pkg.version} (${registry})";
          tarball = import <nix/fetchurl.nix> {
            url = "https://crates.io/api/v1/crates/${pkg.name}/${pkg.version}/download";
            inherit sha256;
          };
        in
        runCommand "${pkg.name}-${pkg.version}" { }
          ''
            mkdir $out
            tar xvf ${tarball} -C $out --strip-components=1
            # Add just enough metadata to keep Cargo happy.
            printf '{"files":{},"package":"${sha256}"}' > "$out/.cargo-checksum.json"
          ''

      else if isGit != null then
        let
          rev = builtins.elemAt isGit 1;
          url = builtins.elemAt isGit 0;
          tree = builtins.fetchGit { inherit url rev; };
        in
        runCommand "${pkg.name}-${pkg.version}" { }
          ''
            tree=${tree}
            if grep --quiet '\[workspace\]' $tree/Cargo.toml; then
              if [[ -e $tree/${pkg.name} ]]; then
                tree=$tree/${pkg.name}
              fi
            fi
            cp -prvd $tree/ $out
            chmod u+w $out
            # Add just enough metadata to keep Cargo happy.
            printf '{"files":{},"package":null}' > "$out/.cargo-checksum.json"
            cat > $out/.cargo-config <<EOF
            [source."${url}"]
            git = "${url}"
            rev = "${rev}"
            replace-with = "vendored-sources"
            EOF
          ''

      else throw "Unsupported crate source '${pkg.source}' in dependency '${pkg.name}-${pkg.version}'.")

    (builtins.filter (pkg: pkg.source or "" != "") lockFile'.package);

  # Create a directory that symlinks all the crate sources and
  # contains a cargo configuration file that redirects to those
  # sources.
  vendorDir = runCommand "cargo-vendor-dir" { }
    ''
      mkdir -p $out/vendor
      cat > $out/vendor/config <<EOF
      [source.crates-io]
      replace-with = "vendored-sources"
      [source.vendored-sources]
      directory = "vendor"
      EOF
      declare -A keysSeen
      for i in ${toString unpackedCrates}; do
        ln -s $i $out/vendor/$(basename "$i" | cut -c 34-)
        if [[ -e "$i/.cargo-config" ]]; then
          # Ensure we emit TOML keys only once.
          key=$(sed 's/\[source\."\(.*\)"\]/\1/; t; d' < "$i/.cargo-config")
          if [[ -z ''${keysSeen[$key]} ]]; then
            keysSeen[$key]=1
            cat "$i/.cargo-config" >> $out/vendor/config
          fi
        fi
      done
    '';

  # Create a setup hook that will initialize CARGO_HOME. Note:
  # we don't point CARGO_HOME at the vendor tree directly
  # because then we end up with a runtime dependency on it.
  cargoHome = makeSetupHook { }
    (writeScript "make-cargo-home" ''
      if [[ -z "''${CARGO_HOME-}" || "''${CARGO_HOME-}" = /build ]]; then
        export CARGO_HOME=$TMPDIR/vendor
        # FIXME: work around Rust 1.36 wanting a $CARGO_HOME/.package-cache file.
        #ln -s ${vendorDir}/vendor $CARGO_HOME
        cp -prd ${vendorDir}/vendor $CARGO_HOME
        chmod -R u+w $CARGO_HOME
      fi
    '');
}