lib/modules: Add context to the "option does not exist" error

Add trace items that provide context for a failed definition that
can not be caught within the Nix language.

This also adds a test for the `tryEval` behavior of `showDefs`.

1 files changed, 12 insertions, 1 deletions

diff --git a/lib/modules.nix b/lib/modules.nix
index 204a2cc1ac1..23e531de5c3 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -284,7 +284,18 @@ rec {
         if config._module.check && config._module.freeformType == null && merged.unmatchedDefns != [] then
           let
             firstDef = head merged.unmatchedDefns;
-            baseMsg = "The option `${showOption (prefix ++ firstDef.prefix)}' does not exist. Definition values:${showDefs [ firstDef ]}";
+            baseMsg =
+              let
+                optText = showOption (prefix ++ firstDef.prefix);
+                defText =
+                  builtins.addErrorContext
+                    "while evaluating the error message for definitions for `${optText}', which is an option that does not exist"
+                    (builtins.addErrorContext
+                      "while evaluating a definition from `${firstDef.file}'"
+                      ( showDefs [ firstDef ])
+                    );
+              in
+                "The option `${optText}' does not exist. Definition values:${defText}";
           in
             if attrNames options == [ "_module" ]
               then