Trying to build the cross-compiler doesn't evaluate either:

$ nix-build -A pkgsCross.x86_64-unknown-redox.gcc13
...
       … from call site

         at /home/zmitchell/nixpkgs/pkgs/top-level/all-packages.nix:14930:16:

        14929|   gcc6Stdenv = overrideCC gccStdenv buildPackages.gcc6;
        14930|   gcc7Stdenv = overrideCC gccStdenv buildPackages.gcc7;
             |                ^
        14931|   gcc8Stdenv = overrideCC gccStdenv buildPackages.gcc8;

       … while calling 'overrideCC'

         at /home/zmitchell/nixpkgs/pkgs/stdenv/adapters.nix:35:24:

           34|   # Override the compiler in stdenv for specific packages.
           35|   overrideCC = stdenv: cc: stdenv.override {
             |                        ^
           36|     allowedRequisites = null;

       error: infinite recursion encountered

       at «none»:0: (source not available)

A cross compiler would be pkgsCross.x86_64-unknown-redox.stdenv.cc or pkgsCross.x86_64-unknown-redox.buildPackages.gcc13

Ah, it looks like relibc was recently removed from nixpkgs due to lack of maintenance. It also looks like Redox has their own patched Rust compiler.

I'm sifting through the nixpkgs source right now to try to figure out where I would swap out the URL pointing at rustc sources, but I'm a little confused by the layout. In pkgs/build-support/rust I see a bootstrap.nix that calls binary.nix, and that seems to download compiler binaries. But also in pkgs/development/compilers/rust it looks like rustc.nix is where we actually build rustc, but it takes a rustc argument?