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?

How could that be fixed?

either patching the makefiles to find the correct CC, or exporting some environment variable shenanigans.
I suspect this may be an issue too: https://github.com/NixOS/nixpkgs/blob/6a08e6bb4e46ff7fcbb53d409b253f6bad8a28ce/pkgs/by-name/li/libparserutils/package.nix#L21
Build system should probably be in native inputs.

How could that be fixed?

either patching the makefiles to find the correct CC, or exporting some environment variable shenanigans.
I suspect this may be an issue too: https://github.com/NixOS/nixpkgs/blob/6a08e6bb4e46ff7fcbb53d409b253f6bad8a28ce/pkgs/by-name/li/libparserutils/package.nix#L21
Build system should probably be in native inputs, not sure.

In reply to @llamato:matrix.org
Ok so good news. After spending way to much time on this I got dillo to compile and x11 to run. The issue is my mouse does not work in x11 and ideally I would like to use the labwc compositor under wayland, that I also got to run. The mouse works there so that is good. However the one thing I could not get running in the chain is xwayland.

huh libffi is acting funny in openbsd

$ l $(nix-build -A pkgsCross.gnu64.libffi)/lib
total 84K
dr-xr-xr-x  1 root root  88 jan  1  1970 .
dr-xr-xr-x  1 root root  16 jan  1  1970 ..
-r-xr-xr-x  2 root root 957 jan  1  1970 libffi.la
lrwxrwxrwx 13 root root  15 jan  1  1970 libffi.so -> libffi.so.8.2.0
lrwxrwxrwx 13 root root  15 jan  1  1970 libffi.so.8 -> libffi.so.8.2.0
-r-xr-xr-x  2 root root 70K jan  1  1970 libffi.so.8.2.0

$ l $(nix-build -A pkgsCross.x86_64-freebsd.libffi)/lib
total 56K
dr-xr-xr-x  1 root root  88 jan  1  1970 .
dr-xr-xr-x  1 root root   6 jan  1  1970 ..
-r-xr-xr-x  2 root root 971 jan  1  1970 libffi.la
lrwxrwxrwx 13 root root  15 jan  1  1970 libffi.so -> libffi.so.8.2.0
lrwxrwxrwx 13 root root  15 jan  1  1970 libffi.so.8 -> libffi.so.8.2.0
-r-xr-xr-x  2 root root 43K jan  1  1970 libffi.so.8.2.0

$ l $(nix-build -A pkgsCross.x86_64-openbsd.libffi)/lib
total 48K
dr-xr-xr-x 1 root root  46 jan  1  1970 .
dr-xr-xr-x 1 root root   6 jan  1  1970 ..
-r-xr-xr-x 2 root root 966 jan  1  1970 libffi.la
-r-xr-xr-x 2 root root 43K jan  1  1970 libffi.so.10.0