lib.modules: Add prefix to imports type check error

lib/modules.nix

@@ -389,6 +389,7 @@ let
                 value = m;
                 _type = m._type;
                 expectedClass = class;
+                prefix = args._prefix;
               }
             )
         else if isList m then
@@ -1977,6 +1978,10 @@ let
           file != null && file != unknownModule
         ) ", while trying to load a module into ${toString file}";
 
+      into_prefix_maybe =
+        prefix:
+        optionalString (prefix != [ ]) ", while trying to load a module into ${code (showOption prefix)}";
+
       /**
         Format text with one line break between each list item.
       */
@@ -2029,7 +2034,7 @@ let
         paragraphs (
           [
             ''
-              Expected a module, but found a value of type ${warn (escapeNixString _type)}${into_fallback_file_maybe fallbackFile}.
+              Expected a module, but found a value of type ${warn (escapeNixString _type)}${into_fallback_file_maybe fallbackFile}${into_prefix_maybe prefix}.
               A module is typically loaded by adding it to the ${code "imports = [ ... ];"} attribute of an existing module, or in the ${code "modules = [ ... ];"} argument of various functions.
               Please make sure that each of the list items is a module, and not a different kind of value.
             ''

lib/tests/modules.sh

@@ -606,6 +606,7 @@ checkConfigError 'Expected a module, but found a value of type .*"flake".*, whil
 checkConfigOutput '^true$' config.enable ./declare-enable.nix ./define-enable-with-top-level-mkIf.nix
 checkConfigError 'Expected a module, but found a value of type .*"configuration".*, while trying to load a module into .*/import-configuration.nix.' config ./import-configuration.nix
 checkConfigError 'please only import the modules that make up the configuration' config ./import-configuration.nix
+checkConfigError 'Expected a module, but found a value of type "configuration", while trying to load a module into .*/import-error-submodule.nix, while trying to load a module into .*foo.*\.' config.foo ./import-error-submodule.nix
 
 # doRename works when `warnings` does not exist.
 checkConfigOutput '^1234$' config.c.d.e ./doRename-basic.nix

lib/tests/modules/import-error-submodule.nix

@@ -0,0 +1,18 @@
+{ lib, ... }:
+let
+  myconf = lib.evalModules { modules = [ { } ]; };
+in
+{
+  options.foo = lib.mkOption {
+    type = lib.types.submodule { };
+    default = { };
+  };
+  config.foo =
+    { ... }:
+    {
+      imports = [
+        # error, like `import-configuration.nix`, but in a submodule this time
+        myconf
+      ];
+    };
+}