home-environment: make sessionVariables a freeform module

Specifically, introduce explicit options for `PATH` and `NIX_PATH`.

modules/home-environment.nix

@@ -229,7 +229,30 @@ in
 
     home.sessionVariables = mkOption {
       default = {};
-      type = types.attrs;
+      type = types.submodule {
+        freeformType = with types;
+          lazyAttrsOf (oneOf [ package str int float ]);
+
+        options = {
+          PATH = mkOption {
+            type = types.envVar;
+            default = "$PATH";
+            defaultText = literalExample ''"$PATH"'';
+            description = ''
+              Content of the <envar>PATH</envar> variable.
+            '';
+          };
+
+          NIX_PATH = mkOption {
+            type = types.envVar;
+            default = "$NIX_PATH";
+            defaultText = literalExample ''"$NIX_PATH"'';
+            description = ''
+              Content of the <envar>NIX_PATH</envar> variable.
+            '';
+          };
+        };
+      };
       example = { EDITOR = "emacs"; GS_OPTIONS = "-sPAPERSIZE=a4"; };
       description = ''
         Environment variables to always set at login.

tests/modules/home-environment/default.nix

@@ -1,4 +1,5 @@
 {
-  home-session-variables = ./session-variables.nix;
   home-session-path = ./session-path.nix;
+  home-session-variables = ./session-variables.nix;
+  home-session-variables-explicit = ./session-variables-explicit.nix;
 }

tests/modules/home-environment/session-variables-explicit.nix

@@ -0,0 +1,57 @@
+# Test of explicitly defined options inside the `home.sessionVariables` freeform
+# module.
+
+{ config, lib, pkgs, ... }:
+
+let
+
+  inherit (pkgs.stdenv.hostPlatform) isDarwin;
+
+  linuxExpected = ''
+    # Only source this once.
+    if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+    export __HM_SESS_VARS_SOURCED=1
+
+    export LOCALE_ARCHIVE_2_27="${pkgs.glibcLocales}/lib/locale/locale-archive"
+    export NIX_PATH="testpath=$HOME/testpath:$NIX_PATH"
+    export PATH="$PATH:$HOME/bin"
+    export XDG_CACHE_HOME="/home/hm-user/.cache"
+    export XDG_CONFIG_HOME="/home/hm-user/.config"
+    export XDG_DATA_HOME="/home/hm-user/.local/share"
+  '';
+
+  darwinExpected = ''
+    # Only source this once.
+    if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
+    export __HM_SESS_VARS_SOURCED=1
+
+    export NIX_PATH="testpath=$HOME/testpath:$NIX_PATH"
+    export PATH="$PATH:$HOME/bin"
+    export XDG_CACHE_HOME="/home/hm-user/.cache"
+    export XDG_CONFIG_HOME="/home/hm-user/.config"
+    export XDG_DATA_HOME="/home/hm-user/.local/share"
+  '';
+
+  expected = pkgs.writeText "expected"
+    (if isDarwin then darwinExpected else linuxExpected);
+
+in {
+  config = {
+    home.sessionVariables = lib.mkMerge [
+      {
+        PATH = "$PATH";
+        NIX_PATH = "$NIX_PATH";
+      }
+      {
+        PATH = lib.mkAfter "$HOME/bin";
+        NIX_PATH = lib.mkBefore "testpath=$HOME/testpath";
+      }
+    ];
+
+    nmt.script = ''
+      assertFileExists home-path/etc/profile.d/hm-session-vars.sh
+      assertFileContent home-path/etc/profile.d/hm-session-vars.sh \
+        ${expected}
+    '';
+  };
+}

tests/modules/home-environment/session-variables.nix

@@ -10,6 +10,8 @@ let
     export __HM_SESS_VARS_SOURCED=1
 
     export LOCALE_ARCHIVE_2_27="${pkgs.glibcLocales}/lib/locale/locale-archive"
+    export NIX_PATH="$NIX_PATH"
+    export PATH="$PATH"
     export V1="v1"
     export V2="v2-v1"
     export XDG_CACHE_HOME="/home/hm-user/.cache"
@@ -22,6 +24,8 @@ let
     if [ -n "$__HM_SESS_VARS_SOURCED" ]; then return; fi
     export __HM_SESS_VARS_SOURCED=1
 
+    export NIX_PATH="$NIX_PATH"
+    export PATH="$PATH"
     export V1="v1"
     export V2="v2-v1"
     export XDG_CACHE_HOME="/home/hm-user/.cache"