> +++ command cargo check --release --locked --all-targets
       >     Updating crates.io index
       > error: failed to get `async-trait` as a dependency of package `libkypc v0.1.0 (/nix/var/nix/builds/nix-72346-1214498785/source/crates/libkypc)`
       >
       > Caused by:
       >   download of config.json failed
       >
       > Caused by:
       >   failed to download from `https://index.crates.io/config.json`
       >
       > Caused by:
       >   [60] SSL peer certificate or SSH remote key was not OK (SSL certificate OpenSSL verify result: unable to get local issuer certificate (20))

{ pkgs, libiconv, lib, stdenv, crane }:
let
  craneLib = crane.mkLib pkgs;
  src = craneLib.cleanCargoSource ./.;
  commonArgs = {
    inherit src;
    strictDeps = true;
    buildInputs = lib.optionals stdenv.isDarwin [ libiconv ];
  };
  cargoArtifacts = craneLib.buildDepsOnly commonArgs;
in {
  kypc = craneLib.buildPackage {
    inherit cargoArtifacts;
    pname = "libkypc";
    version = "0.1.0";
    cargoExtraArgs = "-p libkypc";
    doCheck = false;
    src = lib.fileset.toSource {
      root = ./.;
      fileset = lib.fileset.unions [
        ./Cargo.toml
        ./Cargo.lock
        (craneLib.fileset.commonCargoSources ./crates/libkypc)
      ];
    };
  };
}

# This file is automatically @generated by Cargo.
# It is not intended for manual editing.
version = 3

[[package]]
name = "quick-start"
version = "0.1.0"

@tomasajt:matrix.org already reviewed it as the author of fetchCargoVendor and stated:

I'll look into whether your approach could be improved.

In the meantime you could patch out one of the revisions in the lockfile over to the other revision. It's not guaranteed to be correct, but it might be fine as a hotfix.

Or more importantly, the concerns directly above that:

I really don't see a pretty way of solving this.
Your fix seems to do its job in this case.
However, if we had both a duplicate package and a duplicate selector, even your fix wouldn't work, because we'd be back to the situation that #387337 aimed to solve.

1. reference for build perf on my laptop: nom-build -A rustc-unwrapped --check on master: 1h52m17s

2. mrustc: 5m2s

3. minicargo: unused. i can't find a way to get it to apply the right flags while building, so it just gets built as part of the next step.

4. rust-1900: 1h54m18s

1. rust-1911: 17m33s

2. rust-1920: 18m53s

3. rust-1931: 18m1s

4. rustc-1940: 1h48m52s