lib: add showWarnings

lib/default.nix

@@ -59,7 +59,7 @@ let
       stringLength sub substring tail;
     inherit (trivial) id const concat or and bitAnd bitOr bitXor bitNot
       boolToString mergeAttrs flip mapNullable inNixShell min max
-      importJSON warn info nixpkgsVersion version mod compare
+      importJSON warn info showWarnings nixpkgsVersion version mod compare
       splitByAndCompare functionArgs setFunctionArgs isFunction;
     inherit (fixedPoints) fix fix' converge extends composeExtensions
       makeExtensible makeExtensibleWithCustomName;

lib/trivial.nix

@@ -259,9 +259,10 @@ rec {
   # TODO: figure out a clever way to integrate location information from
   # something like __unsafeGetAttrPos.
 
-  warn = msg: builtins.trace "WARNING: ${msg}";
+  warn = msg: builtins.trace "warning: ${msg}";
   info = msg: builtins.trace "INFO: ${msg}";
 
+  showWarnings = warnings: res: lib.fold (w: x: warn w x) res warnings;
 
   ## Function annotations
 

nixos/modules/system/activation/top-level.nix

@@ -130,11 +130,9 @@ let
 
   failedAssertions = map (x: x.message) (filter (x: !x.assertion) config.assertions);
 
-  showWarnings = res: fold (w: x: builtins.trace "warning: ${w}" x) res config.warnings;
-
   baseSystemAssertWarn = if failedAssertions != []
     then throw "\nFailed assertions:\n${concatStringsSep "\n" (map (x: "- ${x}") failedAssertions)}"
-    else showWarnings baseSystem;
+    else showWarnings config.warnings baseSystem;
 
   # Replace runtime dependencies
   system = fold ({ oldDependency, newDependency }: drv: