From 9c0ab2f26db26c21950e5c61fb06570cb4c5c79a Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Mon, 24 Feb 2020 00:15:36 +0100
Subject: [PATCH] lib/modules.nix: Add file context to unmerged values in
 mergeDefinitions

This helps with troubleshooting exceptions in config values, which were hard
to track down for options with many definitions.
The trace will look like:

    error: while evaluating the attribute 'config.foo' at undefined position:
    [...]
    while evaluating the option `foo':
    [...]
    while evaluating definitions from `/home/user/mymod.nix':
    while evaluating 'dischargeProperties' at /home/user/nixpkgs/lib/modules.nix:464:25, called from /home/user/nixpkgs/lib/modules.nix:392:137:
    while evaluating the attribute 'value' at /home/user/nixpkgs/lib/modules.nix:277:44:
    Value error!

where the `/home/user/mymod.nix` module is

    { lib, ... }: {
      options.foo = lib.mkOption {
        type = lib.types.lines;
      };
      config.foo = builtins.throw "Value error!";
    }
---
 lib/modules.nix | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/modules.nix b/lib/modules.nix
index 2b1faf4f0c28..6cbef5632bd7 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -389,7 +389,7 @@ rec {
       let
         # Process mkMerge and mkIf properties.
         defs' = concatMap (m:
-          map (value: { inherit (m) file; inherit value; }) (dischargeProperties m.value)
+          map (value: { inherit (m) file; inherit value; }) (builtins.addErrorContext "while evaluating definitions from `${m.file}':" (dischargeProperties m.value))
         ) defs;
 
         # Process mkOverride properties.