lib/modules: improve mismatching class error mesage (#377345)

Author: roberth

lib/modules.nix

@@ -367,7 +367,14 @@ let
             if m._class == null || m._class == class
             then m
             else
-              throw "The module ${m._file or m.key} was imported into ${class} instead of ${m._class}."
+              throw ''
+                The module `${m._file or m.key}` (class: ${lib.strings.escapeNixString m._class}) cannot be imported into a module evaluation that expects class ${lib.strings.escapeNixString class}.
+
+                Help:
+                - Ensure that you are importing the correct module.
+                - Verify that the module's `_class`, ${lib.strings.escapeNixString m._class} matches the expected `class` ${lib.strings.escapeNixString class}.
+                - If you are using a custom class, make sure it is correctly defined and used consistently across your modules.
+              ''
         else
           m: m;
 

lib/tests/modules.sh

@@ -538,12 +538,12 @@ checkConfigOutput '^1$' config.sub.specialisation.value ./extendModules-168767-i
 # Class checks, evalModules
 checkConfigOutput '^{}$' config.ok.config ./class-check.nix
 checkConfigOutput '"nixos"' config.ok.class ./class-check.nix
-checkConfigError 'The module .*/module-class-is-darwin.nix was imported into nixos instead of darwin.' config.fail.config ./class-check.nix
-checkConfigError 'The module foo.nix#darwinModules.default was imported into nixos instead of darwin.' config.fail-anon.config ./class-check.nix
+checkConfigError 'The module `.*/module-class-is-darwin.nix`.*?expects class "nixos".' config.fail.config ./class-check.nix
+checkConfigError 'The module `foo.nix#darwinModules.default`.*?expects class "nixos".' config.fail-anon.config ./class-check.nix
 
 # Class checks, submoduleWith
 checkConfigOutput '^{}$' config.sub.nixosOk ./class-check.nix
-checkConfigError 'The module .*/module-class-is-darwin.nix was imported into nixos instead of darwin.' config.sub.nixosFail.config ./class-check.nix
+checkConfigError 'The module `.*/module-class-is-darwin.nix`.*?expects class "nixos".' config.sub.nixosFail.config ./class-check.nix
 
 # submoduleWith type merge with different class
 checkConfigError 'A submoduleWith option is declared multiple times with conflicting class values "darwin" and "nixos".' config.sub.mergeFail.config ./class-check.nix