home-manager: add repl subcommand (#5600)

Similar to the `nixos-rebuild repl` command, `home-manager repl` will
launch the Nix read-evaluate-print-loop environment with the Home
Manager configuration loaded.

To make that more useful, also add the pkgs and options attributes from
the generated Home Manager configuration to the environment.

This doesn't currently work with flakes, because I don't use them and
I'm not confident I could safely test that function.

home-manager/home-manager

@@ -637,6 +637,28 @@ function doBuild() {
     presentNews
 }
 
+function doRepl() {
+    setFlakeAttribute
+    if [[ -v FLAKE_CONFIG_URI ]]; then
+        _i 'home-manager repl does not (yet) support flakes' >&2
+        return 1
+    fi
+
+    setConfigFile
+
+    extraArgs=()
+    for p in "${EXTRA_NIX_PATH[@]}"; do
+        extraArgs+=(-I "$p")
+    done
+
+    exec nix repl \
+        --file '<home-manager/home-manager/home-manager.nix>' \
+        "${extraArgs[@]}" \
+        "${PASSTHROUGH_OPTS[@]}" \
+        --argstr confPath "$HOME_MANAGER_CONFIG" \
+        --argstr confAttr "$HOME_MANAGER_CONFIG_ATTRIBUTE"
+}
+
 function doSwitch() {
     setWorkDir
 
@@ -941,6 +963,9 @@ function doHelp() {
     echo "      Remove indicated generations. Use 'generations' command to"
     echo "      find suitable generation numbers."
     echo
+    echo "  repl"
+    echo "      Opens the configuration in \`nix repl\`"
+    echo
     echo "  expire-generations TIMESTAMP"
     echo "      Remove generations older than TIMESTAMP where TIMESTAMP is"
     echo "      interpreted as in the -d argument of the date tool. For"
@@ -964,7 +989,7 @@ while [[ $# -gt 0 ]]; do
     opt="$1"
     shift
     case $opt in
-        build|init|instantiate|option|edit|expire-generations|generations|help|news|packages|remove-generations|switch|uninstall)
+        build|init|instantiate|option|edit|expire-generations|generations|help|news|packages|remove-generations|repl|switch|uninstall)
             COMMAND="$opt"
             ;;
         -A)
@@ -1109,6 +1134,9 @@ case $COMMAND in
     packages)
         doListPackages
         ;;
+    repl)
+        doRepl
+        ;;
     news)
         doShowNews --all
         ;;

home-manager/home-manager.nix

@@ -16,5 +16,10 @@ let
 
 in
 {
-  inherit (env) activationPackage config;
+  inherit (env)
+    activationPackage
+    config
+    pkgs
+    options
+    ;
 }