Got my NixOS flake to run under WSL!

profiles/wsl/nixos-wsl/modules/installer.nix

@@ -0,0 +1,73 @@
+{ config, lib, pkgs, ... }:
+with builtins; with lib; {
+
+  config = mkIf config.wsl.enable (
+    let
+      mkTarball = pkgs.callPackage "${lib.cleanSource pkgs.path}/nixos/lib/make-system-tarball.nix";
+
+      pkgs2storeContents = map (x: { object = x; symlink = "none"; });
+
+      rootfs = let tarball = config.system.build.tarball; in "${tarball}/tarball/${tarball.fileName}.tar${tarball.extension}";
+
+      installer = pkgs.writeScript "installer.sh" ''
+        #!${pkgs.busybox}/bin/sh
+        BASEPATH=$PATH
+        export PATH=$BASEPATH:${pkgs.busybox}/bin # Add busybox to path
+
+        set -e
+        cd /
+
+        echo "Unpacking root file system..."
+        ${pkgs.pv}/bin/pv ${rootfs} | tar xz
+
+        echo "Activating nix configuration..."
+        /nix/var/nix/profiles/system/activate
+        PATH=$BASEPATH:/run/current-system/sw/bin # Use packages from target system
+
+        echo "Cleaning up installer files..."
+        nix-collect-garbage
+        rm /nix-path-registration
+
+        echo "Optimizing store..."
+        nix-store --optimize
+
+        # Don't package the shell here, it's contained in the rootfs
+        exec ${builtins.unsafeDiscardStringContext config.users.users.root.shell} "$@"
+      '';
+
+      # Set installer.sh as the root shell
+      passwd = pkgs.writeText "passwd" ''
+        root:x:0:0:System administrator:/root:${installer}
+      '';
+    in
+    {
+
+      system.build.installer = mkTarball {
+        fileName = "nixos-wsl-installer";
+        compressCommand = "gzip";
+        compressionExtension = ".gz";
+        extraArgs = "--hard-dereference";
+
+        storeContents = with pkgs; pkgs2storeContents [
+          installer
+        ];
+
+        contents = [
+          { source = config.environment.etc."wsl.conf".source; target = "/etc/wsl.conf"; }
+          { source = config.environment.etc."fstab".source; target = "/etc/fstab"; }
+          { source = passwd; target = "/etc/passwd"; }
+          { source = "${pkgs.busybox}/bin/busybox"; target = "/bin/sh"; }
+          { source = "${pkgs.busybox}/bin/busybox"; target = "/bin/mount"; }
+        ];
+
+        extraCommands = pkgs.writeShellScript "prepare" ''
+          export PATH=$PATH:${pkgs.coreutils}/bin
+          mkdir -p bin
+          ln -s /init bin/wslpath
+        '';
+      };
+
+    }
+  );
+
+}