From 5c35166b71353cb32c89e344ba78c37c6d82ff0f Mon Sep 17 00:00:00 2001
From: Johannes Kirschbauer <hsjobeki@gmail.com>
Date: Mon, 27 Jan 2025 17:26:08 +0100
Subject: [PATCH 1/2] lib/modules: improve mismatching class error mesage

---
 lib/modules.nix | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/lib/modules.nix b/lib/modules.nix
index 79b8f25c2f43..4d2d8e0b08b6 100644
--- a/lib/modules.nix
+++ b/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;
 

From b3884c8b7d7f6e7bb3efb2199894b6cbdcea43c7 Mon Sep 17 00:00:00 2001
From: Johannes Kirschbauer <hsjobeki@gmail.com>
Date: Wed, 29 Jan 2025 11:50:16 +0100
Subject: [PATCH 2/2] lib/modules: tests change expected error for class
 mismatch

---
 lib/tests/modules.sh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index beb191aed0c1..c86e0aeec6dc 100755
--- a/lib/tests/modules.sh
+++ b/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