nix: add a declarative alternative to Nix channels (#4031)

* nix: add options 'nixPath' and 'keepOldNixPath'

By default, the system value for $NIX_PATH is kept as a fallback.
To completely override the system value for $NIX_PATH:

    nix.keepOldNixPath = false;

* nix: add more tests

* nix: add a declarative alternative to Nix channels

This adds a new option, 'nix.channels'. It's the Nix channels equivalent
of the 'nix.registry' option, and compatible with pre-Flake Nix tooling
including nix-env and nix-shell. Like 'nix.registry', this option is
useful for pinning Nix channels.

Channels defined in the new option can coexist with channels introduced
through the nix-channel command. If the same channel exists in both, the
one from Home Manager will be prioritized.

* nix: add news entry

* nix: make channels respect use-xdg-base-directories

* nix: remove 'with lib;'

---------

Co-authored-by: Michael Hoang <enzime@users.noreply.github.com>

tests/modules/misc/nix/default.nix

@@ -2,4 +2,7 @@
   nix-empty-settings = ./empty-settings.nix;
   nix-example-settings = ./example-settings.nix;
   nix-example-registry = ./example-registry.nix;
+  nix-keep-old-nix-path = ./keep-old-nix-path.nix;
+  nix-example-channels = ./example-channels.nix;
+  nix-example-channels-xdg = ./example-channels-xdg.nix;
 }

tests/modules/misc/nix/empty-settings.nix

@@ -8,6 +8,7 @@ with lib;
 
     nmt.script = ''
       assertPathNotExists home-files/.config/nix
+      assertPathNotExists home-files/.nix-defexpr/50-home-manager
     '';
   };
 }

tests/modules/misc/nix/example-channels-xdg.nix

@@ -0,0 +1,29 @@
+{ lib, config, pkgs, ... }:
+
+let
+  exampleChannel = pkgs.writeTextDir "default.nix" ''
+    { pkgs ? import <nixpkgs> { } }:
+
+    {
+      example = pkgs.emptyDirectory;
+    }
+  '';
+in {
+  config = {
+    nix = {
+      package = config.lib.test.mkStubPackage {
+        version = lib.getVersion pkgs.nixVersions.stable;
+      };
+      channels.example = exampleChannel;
+      settings.use-xdg-base-directories = true;
+    };
+
+    nmt.script = ''
+      assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+        'export NIX_PATH="/home/hm-user/.local/state/nix/defexpr/50-home-manager''${NIX_PATH:+:$NIX_PATH}"'
+      assertFileContent \
+        home-files/.local/state/nix/defexpr/50-home-manager/example/default.nix \
+        ${exampleChannel}/default.nix
+    '';
+  };
+}

tests/modules/misc/nix/example-channels.nix

@@ -0,0 +1,26 @@
+{ config, pkgs, ... }:
+
+let
+  exampleChannel = pkgs.writeTextDir "default.nix" ''
+    { pkgs ? import <nixpkgs> { } }:
+
+    {
+      example = pkgs.emptyDirectory;
+    }
+  '';
+in {
+  config = {
+    nix = {
+      package = config.lib.test.mkStubPackage { };
+      channels.example = exampleChannel;
+    };
+
+    nmt.script = ''
+      assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+        'export NIX_PATH="/home/hm-user/.nix-defexpr/50-home-manager''${NIX_PATH:+:$NIX_PATH}"'
+      assertFileContent \
+        home-files/.nix-defexpr/50-home-manager/example/default.nix \
+        ${exampleChannel}/default.nix
+    '';
+  };
+}

tests/modules/misc/nix/example-settings.nix

@@ -17,6 +17,8 @@ with lib;
         '';
       };
 
+      nixPath = [ "/a" "/b/c" ];
+
       settings = {
         use-sandbox = true;
         show-trace = true;
@@ -28,6 +30,9 @@ with lib;
       assertFileContent \
         home-files/.config/nix/nix.conf \
         ${./example-settings-expected.conf}
+
+      assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+        'export NIX_PATH="/a:/b/c''${NIX_PATH:+:$NIX_PATH}"'
     '';
   };
 }

tests/modules/misc/nix/keep-old-nix-path.nix

@@ -0,0 +1,16 @@
+{ config, ... }:
+
+{
+  config = {
+    nix = {
+      package = config.lib.test.mkStubPackage { };
+      nixPath = [ "/a" "/b/c" ];
+      keepOldNixPath = false;
+    };
+
+    nmt.script = ''
+      assertFileContains home-path/etc/profile.d/hm-session-vars.sh \
+        'export NIX_PATH="/a:/b/c"'
+    '';
+  };
+}