nixos/xserver: abort when encountering unknown xorg drivers (#486149)

doc/release-notes/rl-2605.section.md

@@ -126,6 +126,8 @@
 
 - `pocket-id` has been updated to version 2 that contains [breaking changes](https://pocket-id.org/docs/setup/major-releases/migrate-v2).
 
+- `services.xserver` will now throw an error if an X11 driver specified in  `videoDriver(s)` cannot be found. Previously, unknown drivers would be silently ignored.
+
 - `asio` (standalone version of `boost::asio`) has been updated from 1.24.0 to 1.36.0. Some breaking changes were introduced between these
   two versions, and the one affected most was the removal of `asio::io_service` in favor of `asio::io_context` in 1.33.0. `asio_1_32_0` is
   retained for packages that have not completed migration. `asio_1_10` has been removed as no packages depend on it anymore.

nixos/modules/services/x11/xserver.nix

@@ -830,7 +830,8 @@ in
 
     services.xserver.videoDrivers = mkIf (cfg.videoDriver != null) [ cfg.videoDriver ];
 
-    # FIXME: somehow check for unknown driver names.
+    # We ignore unknown drivers here because they may be resolved by other modules (e.g., the Nvidia
+    # module). We assert that all specified drivers were eventually found in the assertions below.
     services.xserver.drivers = flip concatMap cfg.videoDrivers (
       name:
       lib.optional (videoDrivers ? ${name}) (
@@ -862,7 +863,11 @@ in
         assertion = cfg.upscaleDefaultCursor -> cfg.dpi != null;
         message = "Specify `config.services.xserver.dpi` to upscale the default cursor.";
       }
-    ];
+    ]
+    ++ map (driver: {
+      assertion = builtins.elem driver (builtins.catAttrs "name" cfg.drivers);
+      message = "Unknown X11 driver ‘${driver}’ specified in `services.xserver.videoDrivers`.";
+    }) cfg.videoDrivers;
 
     environment.etc =
       (optionalAttrs cfg.exportConfiguration {