eval-config: implement release branch checks

eval-config.nix

@@ -1,10 +1,72 @@
+let
+  nixDarwinVersion = builtins.fromJSON (builtins.readFile ./version.json);
+
+  checkRelease = lib:
+    # Avoid breaking configurations when the unstable Nixpkgs version
+    # rolls over.
+    #
+    # TODO: Something more refined than this would be ideal, as this
+    # still means you could be using unstable nix-darwin 25.05 with
+    # Nixpkgs 26.05, which would be unfortunate.
+    if nixDarwinVersion.isReleaseBranch then
+      lib.trivial.release == nixDarwinVersion.release
+    else
+      lib.versionAtLeast lib.trivial.release nixDarwinVersion.release;
+in
+
 { lib
 , modules
 , baseModules ? import ./modules/module-list.nix
 , specialArgs ? { }
 , check ? true
+, enableNixpkgsReleaseCheck ? true
 }@args:
 
+assert enableNixpkgsReleaseCheck -> checkRelease lib || throw ''
+
+  nix-darwin now uses release branches that correspond to Nixpkgs releases.
+  The nix-darwin and Nixpkgs branches in use must match, but you are currently
+  using nix-darwin ${nixDarwinVersion.release} with Nixpkgs ${lib.trivial.release}.
+
+  On macOS, you should use either the `nixpkgs-unstable` or
+  `nixpkgs-YY.MM-darwin` branches of Nixpkgs. These correspond to the
+  `master` and `nix-darwin-YY.MM` branches of nix-darwin, respectively. Check
+  <https://status.nixos.org/> for the currently supported Nixpkgs releases.
+
+  If you’re using flakes, make sure your inputs look like this:
+
+      inputs = {
+        nixpkgs.url = "github:NixOS/nixpkgs/NIXPKGS-BRANCH";
+        nix-darwin.url = "github:LnL7/nix-darwin/NIX-DARWIN-BRANCH";
+        nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
+        # …
+      };
+
+  If you’re using channels, you can check your current channels with:
+
+      $ sudo nix-channel --list
+      nixpkgs https://nixos.org/channels/NIXPKGS-BRANCH
+      darwin https://github.com/LnL7/nix-darwin/archive/NIX-DARWIN-BRANCH.tar.gz
+      …
+      $ nix-channel --list
+      …
+
+  If `darwin` or `nixpkgs` are present in `nix-channel --list` (without
+  `sudo`), you should delete them with `nix-channel --remove NAME`. These can
+  contribute to version mismatch problems.
+
+  You can then fix your channels like this:
+
+      $ sudo nix-channel --add https://nixos.org/channels/NIXPKGS-BRANCH nixpkgs
+      $ sudo nix-channel --add https://github.com/LnL7/nix-darwin/archive/NIX-DARWIN-BRANCH.tar.gz darwin
+      $ sudo nix-channel --update
+
+  After that, activating your system again should work correctly. If it
+  doesn’t, please open an issue at
+  <https://github.com/LnL7/nix-darwin/issues/new> and include as much
+  information as possible.
+'';
+
 let
   argsModule = {
     _file = ./eval-config.nix;