nixos-rebuild: Add nixos-rebuild repl

Apologies to the non-flake users; your repl isn't quite as fancy,
but at least evaluates your config exactly as you would expect,
unlike flakes which are only evaluated impurely for now.

nixos/doc/manual/installation/changing-config.chapter.md

@@ -55,6 +55,14 @@ which causes the new configuration (and previous ones created using
 This can be useful to separate test configurations from "stable"
 configurations.
 
+A repl, or read-eval-print loop, is also available. You can inspect your configuration and use the Nix language with
+
+```ShellSession
+# nixos-rebuild repl
+```
+
+Your configuration is loaded into the `config` variable. Use tab for autocompletion, use the `:r` command to reload the configuration files. See `:?` or [`nix repl` in the Nix manual](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-repl.html) to learn more.
+
 Finally, you can do
 
 ```ShellSession

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.8

@@ -10,7 +10,7 @@
 .Sh SYNOPSIS
 .Nm
 .Bro
-.Cm switch | boot | test | build | dry-build | dry-activate | edit | build-vm | build-vm-with-bootloader | list-generations Op Fl -json
+.Cm switch | boot | test | build | dry-build | dry-activate | edit | repl | build-vm | build-vm-with-bootloader | list-generations Op Fl -json
 .Brc
 .br
 .Op Fl -upgrade | -upgrade-all
@@ -143,6 +143,10 @@ Opens
 .Pa configuration.nix
 in the default editor.
 .
+.It Cm repl
+Opens the configuration in
+.Ic nix repl Ns .
+.
 .It Cm build-vm
 Build a script that starts a NixOS virtual machine with the desired
 configuration. It leaves a symlink

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -49,7 +49,7 @@ while [ "$#" -gt 0 ]; do
       --help)
         showSyntax
         ;;
-      switch|boot|test|build|edit|dry-build|dry-run|dry-activate|build-vm|build-vm-with-bootloader|list-generations)
+      switch|boot|test|build|edit|repl|dry-build|dry-run|dry-activate|build-vm|build-vm-with-bootloader|list-generations)
         if [ "$i" = dry-run ]; then i=dry-build; fi
         # exactly one action mandatory, bail out if multiple are given
         if [ -n "$action" ]; then showSyntax; fi
@@ -511,6 +511,68 @@ if [ "$action" = dry-build ]; then
     extraBuildFlags+=(--dry-run)
 fi
 
+if [ "$action" = repl ]; then
+    # This is a very end user command, implemented using sub-optimal means.
+    # You should feel free to improve its behavior, as well as resolve tech
+    # debt in "breaking" ways. Humans adapt quite well.
+    if [[ -z $flake ]]; then
+        exec nix repl '<nixpkgs/nixos>' "${extraBuildFlags[@]}"
+    else
+        if [[ -n "${lockFlags[0]}" ]]; then
+            # nix repl itself does not support locking flags
+            log "nixos-rebuild repl does not support locking flags yet"
+            exit 1
+        fi
+        d='$'
+        q='"'
+        bold="$(echo -e '\033[1m')"
+        blue="$(echo -e '\033[34;1m')"
+        attention="$(echo -e '\033[35;1m')"
+        reset="$(echo -e '\033[0m')"
+        # This nix repl invocation is impure, because usually the flakeref is.
+        # For a solution that preserves the motd and custom scope, we need
+        # something like https://github.com/NixOS/nix/issues/8679.
+        exec nix repl --impure --expr "
+          let flake = builtins.getFlake ''$flake'';
+              configuration = flake.$flakeAttr;
+              motd = ''
+                $d{$q\n$q}
+                Hello and welcome to the NixOS configuration
+                    $flakeAttr
+                    in $flake
+
+                The following is loaded into nix repl's scope:
+
+                    - ${blue}config${reset}   All option values
+                    - ${blue}options${reset}  Option data and metadata
+                    - ${blue}pkgs${reset}     Nixpkgs package set
+                    - other module arguments
+
+                    - ${blue}flake${reset}    Flake outputs, inputs and source info of $flake
+
+                Use tab completion to browse around ${blue}config${reset}.
+
+                Use ${bold}:r${reset} to ${bold}reload${reset} everything after making a change in the flake.
+                  (assuming $flake is a mutable flake ref)
+
+                See ${bold}:?${reset} for more repl commands.
+
+                ${attention}warning:${reset} nixos-rebuild repl does not currently enforce pure evaluation.
+              '';
+              scope =
+                assert configuration._type or null == ''configuration'';
+                assert configuration.class or ''nixos'' == ''nixos'';
+                configuration._module.args //
+                configuration._module.specialArgs //
+                {
+                  inherit (configuration) config options;
+                  inherit flake;
+                };
+          in builtins.seq scope builtins.trace motd scope
+        " "${extraBuildFlags[@]}"
+    fi
+fi
+
 if [ "$action" = list-generations ]; then
     if [ ! -L "$profile" ]; then
         log "No profile \`$(basename "$profile")' found"