Log warnings on IFD with new option

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

src/libexpr/include/nix/expr/eval-settings.hh

@@ -152,6 +152,16 @@ struct EvalSettings : Config
         )"
         };
 
+    Setting<bool> traceImportFromDerivation{
+        this, false, "trace-import-from-derivation",
+        R"(
+          By default, Nix allows [Import from Derivation](@docroot@/language/import-from-derivation.md).
+
+          When this setting is `true`, Nix will log a warning indicating that it performed such an import.
+          This option has no effect if `allow-import-from-derivation` is disabled.
+        )"
+        };
+
     Setting<bool> enableImportFromDerivation{
         this, true, "allow-import-from-derivation",
         R"(

src/libexpr/primops.cc

@@ -90,11 +90,19 @@ StringMap EvalState::realiseContext(const NixStringContext & context, StorePathS
 
     if (drvs.empty()) return {};
 
-    if (isIFD && !settings.enableImportFromDerivation)
-        error<IFDError>(
-            "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
-            drvs.begin()->to_string(*store)
-        ).debugThrow();
+    if (isIFD) {
+        if (!settings.enableImportFromDerivation)
+            error<IFDError>(
+                "cannot build '%1%' during evaluation because the option 'allow-import-from-derivation' is disabled",
+                drvs.begin()->to_string(*store)
+            ).debugThrow();
+
+        if (settings.traceImportFromDerivation)
+            warn(
+                "built '%1%' during evaluation due to an import from derivation",
+                drvs.begin()->to_string(*store)
+            );
+    }
 
     /* Build/substitute the context. */
     std::vector<DerivedPath> buildReqs;