Add lazy evaluation for experimental feature reasons

Wrap fmt() calls in lambdas to defer string formatting until the
feature check fails. This avoids unnecessary string formatting in
the common case where the feature is enabled.

Addresses performance concern raised by xokdvium in PR review.

src/libstore/derivations.cc

@@ -426,7 +426,9 @@ Derivation parseDerivation(
         if (*versionS == "xp-dyn-drv"sv) {
             // Only version we have so far
             version = DerivationATermVersion::DynamicDerivations;
-            xpSettings.require(Xp::DynamicDerivations, fmt("derivation '%s', ATerm format version 'xp-dyn-drv'", name));
+            xpSettings.require(Xp::DynamicDerivations, [&] {
+                return fmt("derivation '%s', ATerm format version 'xp-dyn-drv'", name);
+            });
         } else {
             throw FormatError("Unknown derivation ATerm format version '%s'", *versionS);
         }
@@ -1454,7 +1456,8 @@ Derivation Derivation::fromJSON(const nlohmann::json & _json, const Experimental
             node.value = getStringSet(valueAt(json, "outputs"));
             auto drvs = getObject(valueAt(json, "dynamicOutputs"));
             for (auto & [outputId, childNode] : drvs) {
-                xpSettings.require(Xp::DynamicDerivations, fmt("dynamic output '%s' in JSON", outputId));
+                xpSettings.require(
+                    Xp::DynamicDerivations, [&] { return fmt("dynamic output '%s' in JSON", outputId); });
                 node.childMap[outputId] = doInput(childNode);
             }
             return node;

src/libstore/derived-path.cc

@@ -86,9 +86,9 @@ void drvRequireExperiment(const SingleDerivedPath & drv, const ExperimentalFeatu
                 // plain drv path; no experimental features required.
             },
             [&](const SingleDerivedPath::Built & b) {
-                xpSettings.require(
+                xpSettings.require(Xp::DynamicDerivations, [&] {
-                    Xp::DynamicDerivations,
+                    return fmt("building output '%s' of '%s'", b.output, b.drvPath->getBaseStorePath().to_string());
-                    fmt("building output '%s' of '%s'", b.output, b.drvPath->getBaseStorePath().to_string()));
+                });
             },
         },
         drv.raw());

src/libstore/downstream-placeholder.cc

@@ -24,7 +24,8 @@ DownstreamPlaceholder DownstreamPlaceholder::unknownDerivation(
     OutputNameView outputName,
     const ExperimentalFeatureSettings & xpSettings)
 {
-    xpSettings.require(Xp::DynamicDerivations, fmt("placeholder for unknown derivation output '%s'", outputName));
+    xpSettings.require(
+        Xp::DynamicDerivations, [&] { return fmt("placeholder for unknown derivation output '%s'", outputName); });
     auto compressed = compressHash(placeholder.hash, 20);
     auto clearText =
         "nix-computed-output:" + compressed.to_string(HashFormat::Nix32, false) + ":" + std::string{outputName};

src/libutil/include/nix/util/configuration.hh

@@ -465,6 +465,19 @@ struct ExperimentalFeatureSettings : Config
      */
     void require(const ExperimentalFeature &, std::string reason = "") const;
 
+    /**
+     * Require an experimental feature be enabled, throwing an error if it is
+     * not. The reason is lazily evaluated only if the feature is disabled.
+     */
+    template<typename GetReason>
+        requires std::invocable<GetReason> && std::convertible_to<std::invoke_result_t<GetReason>, std::string>
+    void require(const ExperimentalFeature & feature, GetReason && getReason) const
+    {
+        if (isEnabled(feature))
+            return;
+        require(feature, getReason());
+    }
+
     /**
      * `std::nullopt` pointer means no feature, which means there is nothing that could be
      * disabled, and so the function returns true in that case.