Allow modules to define warnings to be shown during evaluation

1 files changed, 17 insertions, 3 deletions

diff --git a/nixos/modules/misc/assertions.nix b/nixos/modules/misc/assertions.nix
index 7ded9785bc10..229f8f278608 100644
--- a/nixos/modules/misc/assertions.nix
+++ b/nixos/modules/misc/assertions.nix
@@ -6,6 +6,8 @@ let
 
   failed = map (x: x.message) (filter (x: !x.assertion) config.assertions);
 
+  showWarnings = res: fold (w: x: builtins.trace "warning: ${w}" x) res config.warnings;
+
 in
 
 {
@@ -24,14 +26,26 @@ in
       '';
     };
 
+    warnings = mkOption {
+      internal = true;
+      default = [];
+      type = types.listOf types.string;
+      example = [ "The `foo' service is deprecated and will go away soon!" ];
+      description = ''
+        This option allows modules to show warnings to users during
+        the evaluation of the system configuration.
+      '';
+    };
+
   };
 
   config = {
 
-    # This option is evaluated always. Thus the assertions are checked as well. hacky!
-    environment.systemPackages =
+    # This option is evaluated always. Thus the assertions are checked
+    # as well. Hacky!
+    environment.systemPackages = showWarnings (
       if [] == failed then []
-      else throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failed)}";
+      else throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failed)}");
 
   };