Reposting here:
I am working on readding support for relibc and redoxos to nixpkgs. That requires adding support for the relibc library which is a replacement for libc compatible with redox.
https://github.com/NixOS/nixpkgs/pull/453786

But I run into an infinite recusion problem when I try nix-shell -A pkgsCross.x86_64-unknown-redox.mkShell.

       … while evaluating the attribute 'stdenv.cc.targetPrefix'
         at /home/anderscs/src/nixpkgs/pkgs/stdenv/generic/default.nix:209:15:
          208|
          209|       inherit cc hasCC;
             |               ^
          210|

       … while evaluating 'targetPackages.stdenv.cc' to select 'targetPrefix' on it
         at /home/anderscs/src/nixpkgs/pkgs/development/compilers/rust/rustc.nix:55:14:
           54| stdenv.mkDerivation (finalAttrs: {
           55|   pname = "${targetPackages.stdenv.cc.targetPrefix}rustc";
             |              ^
           56|   inherit version;

       error: infinite recursion encountered
       at /home/anderscs/src/nixpkgs/pkgs/development/compilers/rust/rustc.nix:55:14:
           54| stdenv.mkDerivation (finalAttrs: {
           55|   pname = "${targetPackages.stdenv.cc.targetPrefix}rustc";
             |              ^

It is a circular dependency which should be solvable.

How do I best solve this?