Only provide builtin.{getFlake,fetchClosure} is the corresponding experimental feature is enabled

This allows writing fallback code like

  if builtins ? fetchClosure then
    builtins.fetchClose { ... }
  else
    builtins.storePath ...

doc/manual/src/release-notes/rl-next.md

@@ -5,9 +5,12 @@
 * `nix store make-content-addressable` has been renamed to `nix store
   make-content-addressed`.
 
-* New builtin function `builtins.fetchClosure` that copies a closure
-  from a binary cache at evaluation time and rewrites it to
-  content-addressed form (if it isn't already). Like
+* New experimental builtin function `builtins.fetchClosure` that
+  copies a closure from a binary cache at evaluation time and rewrites
+  it to content-addressed form (if it isn't already). Like
   `builtins.storePath`, this allows importing pre-built store paths;
   the difference is that it doesn't require the user to configure
   binary caches and trusted public keys.
+
+  This function is only available if you enable the experimental
+  feature `fetch-closure`.

src/libexpr/eval.cc

@@ -507,23 +507,6 @@ EvalState::~EvalState()
 }
 
 
-void EvalState::requireExperimentalFeatureOnEvaluation(
-    const ExperimentalFeature & feature,
-    const std::string_view fName,
-    const Pos & pos)
-{
-    if (!settings.isExperimentalFeatureEnabled(feature)) {
-        throw EvalError({
-            .msg = hintfmt(
-                "Cannot call '%2%' because experimental Nix feature '%1%' is disabled. You can enable it via '--extra-experimental-features %1%'.",
-                feature,
-                fName
-            ),
-            .errPos = pos
-        });
-    }
-}
-
 void EvalState::allowPath(const Path & path)
 {
     if (allowedPaths)

src/libexpr/eval.hh

@@ -149,12 +149,6 @@ public:
         std::shared_ptr<Store> buildStore = nullptr);
     ~EvalState();
 
-    void requireExperimentalFeatureOnEvaluation(
-        const ExperimentalFeature &,
-        const std::string_view fName,
-        const Pos & pos
-    );
-
     void addToSearchPath(const std::string & s);
 
     SearchPath getSearchPath() { return searchPath; }

src/libexpr/flake/flake.cc

@@ -706,8 +706,6 @@ void callFlake(EvalState & state,
 
 static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    state.requireExperimentalFeatureOnEvaluation(Xp::Flakes, "builtins.getFlake", pos);
-
     std::string flakeRefS(state.forceStringNoCtx(*args[0], pos));
     auto flakeRef = parseFlakeRef(flakeRefS, {}, true);
     if (evalSettings.pureEval && !flakeRef.input.isLocked())
@@ -723,7 +721,12 @@ static void prim_getFlake(EvalState & state, const Pos & pos, Value * * args, Va
         v);
 }
 
-static RegisterPrimOp r2("__getFlake", 1, prim_getFlake);
+static RegisterPrimOp r2({
+    .name =  "__getFlake",
+    .args = {"args"},
+    .fun = prim_getFlake,
+    .experimentalFeature = Xp::Flakes,
+});
 
 }
 

src/libexpr/primops.cc

@@ -3814,7 +3814,7 @@ RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun)
         .name = name,
         .args = {},
         .arity = arity,
-        .fun = fun
+        .fun = fun,
     });
 }
 
@@ -3886,6 +3886,9 @@ void EvalState::createBaseEnv()
 
     if (RegisterPrimOp::primOps)
         for (auto & primOp : *RegisterPrimOp::primOps)
+            if (!primOp.experimentalFeature
+                || settings.isExperimentalFeatureEnabled(*primOp.experimentalFeature))
+            {
                 addPrimOp({
                     .fun = primOp.fun,
                     .arity = std::max(primOp.args.size(), primOp.arity),
@@ -3893,6 +3896,7 @@ void EvalState::createBaseEnv()
                     .args = primOp.args,
                     .doc = primOp.doc,
                 });
+            }
 
     /* Add a wrapper around the derivation primop that computes the
        `drvPath' and `outPath' attributes lazily. */

src/libexpr/primops.hh

@@ -16,6 +16,7 @@ struct RegisterPrimOp
         size_t arity = 0;
         const char * doc;
         PrimOpFun fun;
+        std::optional<ExperimentalFeature> experimentalFeature;
     };
 
     typedef std::vector<Info> PrimOps;
@@ -35,6 +36,7 @@ struct RegisterPrimOp
 /* These primops are disabled without enableNativeCode, but plugins
    may wish to use them in limited contexts without globally enabling
    them. */
+
 /* Load a ValueInitializer from a DSO and return whatever it initializes */
 void prim_importNative(EvalState & state, const Pos & pos, Value * * args, Value & v);
 

src/libexpr/primops/fetchClosure.cc

@@ -7,8 +7,6 @@ namespace nix {
 
 static void prim_fetchClosure(EvalState & state, const Pos & pos, Value * * args, Value & v)
 {
-    settings.requireExperimentalFeature(Xp::FetchClosure);
-
     state.forceAttrs(*args[0], pos);
 
     std::optional<std::string> fromStoreUrl;
@@ -145,8 +143,12 @@ static RegisterPrimOp primop_fetchClosure({
       specifying a binary cache from which the path can be fetched.
       Also, requiring a content-addressed final store path avoids the
       need for users to configure binary cache public keys.
+
+      This function is only available if you enable the experimental
+      feature `fetch-closure`.
     )",
     .fun = prim_fetchClosure,
+    .experimentalFeature = Xp::FetchClosure,
 });
 
 }

src/nix/main.cc

@@ -289,6 +289,7 @@ void mainWrapped(int argc, char * * argv)
     }
 
     if (argc == 2 && std::string(argv[1]) == "__dump-builtins") {
+        settings.experimentalFeatures = {Xp::Flakes, Xp::FetchClosure};
         evalSettings.pureEval = false;
         EvalState state({}, openStore("dummy://"));
         auto res = nlohmann::json::object();