Add infra for InputSchemes to be experimental

src/libfetchers/fetchers.cc

@@ -36,6 +36,7 @@ Input Input::fromURL(const ParsedURL & url, bool requireTree)
     for (auto & inputScheme : *inputSchemes) {
         auto res = inputScheme->inputFromURL(url, requireTree);
         if (res) {
+            experimentalFeatureSettings.require(inputScheme->experimentalFeature());
             res->scheme = inputScheme;
             fixupInput(*res);
             return std::move(*res);
@@ -50,6 +51,7 @@ Input Input::fromAttrs(Attrs && attrs)
     for (auto & inputScheme : *inputSchemes) {
         auto res = inputScheme->inputFromAttrs(attrs);
         if (res) {
+            experimentalFeatureSettings.require(inputScheme->experimentalFeature());
             res->scheme = inputScheme;
             fixupInput(*res);
             return std::move(*res);
@@ -309,4 +311,9 @@ void InputScheme::clone(const Input & input, const Path & destDir) const
     throw Error("do not know how to clone input '%s'", input.to_string());
 }
 
+std::optional<ExperimentalFeature> InputScheme::experimentalFeature()
+{
+    return {};
+}
+
 }

src/libfetchers/fetchers.hh

@@ -158,6 +158,11 @@ struct InputScheme
     virtual void markChangedFile(const Input & input, std::string_view file, std::optional<std::string> commitMsg);
 
     virtual std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) = 0;
+
+    /**
+     * Is this `InputScheme` part of an experimental feature?
+     */
+    virtual std::optional<ExperimentalFeature> experimentalFeature();
 };
 
 void registerInputScheme(std::shared_ptr<InputScheme> && fetcher);

src/libutil/args.hh

@@ -236,7 +236,7 @@ struct Command : virtual public Args
 
     static constexpr Category catDefault = 0;
 
-    virtual std::optional<ExperimentalFeature> experimentalFeature ();
+    virtual std::optional<ExperimentalFeature> experimentalFeature();
 
     virtual Category category() { return catDefault; }
 };