is there a guide for how to cross compile to windows? I have some rust code I'm trying to cross compile from x86_64-linux to mingw-ucrt-x86_64-llvm, but hit a snag on needing pthreads. I added it but then I get the supported platform error since it doesn't include any platforms. What's the intended way of using it? Forgive my sloppy code, but here's what I currently have:

          test-cargo-reaper-build-cross-windows =
            let
              inherit (pkgs.lib.systems.examples.mingw-ucrt-x86_64-llvm.rust) rustcTarget;
              crossPkgs = pkgs.pkgsCross.mingw-ucrt-x86_64-llvm;
              mingwCC = crossPkgs.stdenv.cc;
              crossCC = "${mingwCC}/bin/${mingwCC.targetPrefix}cc";
              crossCXX = "${mingwCC}/bin/${mingwCC.targetPrefix}c++";

              rustWithWindowsTarget = fenix.packages.${system}.combine [
                rustToolchain
                (fenix.packages.${system}.targets.${rustcTarget}.toolchainOf {
                  channel = "1.87.0";
                  sha256 = "sha256-KUm16pHj+cRedf8vxs/Hd2YWxpOrWZ7UOrwhILdSJBU=";
                }).rust-std
              ];
              craneLibCross =
                let
                  craneLib = (crane.mkLib pkgs).overrideToolchain rustWithWindowsTarget;
                in
                craneLib // (cargoReaper.crane { inherit craneLib; });

              crossArgs = {
                src = testFileset ./tests/plugin_manifests/package_manifest;
                strictDeps = true;

                nativeBuildInputs = [
                  mingwCC
                  pkgs.llvmPackages.bintools
                  pkgs.windows.pthreads
                ];

                CARGO_BUILD_TARGET = rustcTarget;
                CARGO_TARGET_X86_64_PC_WINDOWS_GNULLVM_LINKER =
                  "${mingwCC}/bin/${mingwCC.targetPrefix}cc";
                "CC_${rustcTarget}" = crossCC;
                "CXX_${rustcTarget}" = crossCXX;
              };
              cargoArtifactsCross = craneLibCross.buildDepsOnly crossArgs;
            in
            craneLibCross.buildReaperExtension (crossArgs // {
              cargoArtifacts = cargoArtifactsCross;
              package = "package_manifest";
              plugin = "reaper_package_ext";
              target = rustcTarget;
              doInstallCheck = true;
              installCheckPhase = ''
                test -f $out/lib/reaper_package_ext.dll
              '';
            });

Ah so the pthreads problem was with libc, it hasn't even gotten to my build yet:

Cannot build '/nix/store/2qixrw546lxpxqkwyglhzhhnf2m7kv2b-compiler-rt-libc-x86_64-w64-mingw32-21.1.8.drv'