Mark fetchTree as unstable again

As discussed in our last meeting, we need a bit more time, but we are
"time boxing" the work left to do to ensure there is not unbounded
delay.

Rather than putting it back underneath `flakes`, though, put it
underneath its own `fetch-tree` experimental feature (which `flakes`
includes/implies). This signals our commitment to the plan to stabilize
it first without waiting to go through the rest of Flakes, and also will
give users a "release candidate" when we get closer to stabilization.

This reverts commit 4112dd1fc9.

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

@@ -17,7 +17,8 @@
 
 - `nix-shell` shebang lines now support single-quoted arguments.
 
-- `builtins.fetchTree` is now marked as stable.
+- `builtins.fetchTree` is now unstable under its own experimental feature, [`fetch-tree`](@docroot@/contributing/experimental-features.md#xp-fetch-tree).
+  As described in the document for that feature, this is because we anticipate polishing it and then stabilizing it before the rest of Flakes.
 
 - The interface for creating and updating lock files has been overhauled:
 

src/libexpr/primops/fetchTree.cc

@@ -228,6 +228,7 @@ static RegisterPrimOp primop_fetchTree({
           ```
     )",
     .fun = prim_fetchTree,
+    .experimentalFeature = Xp::FetchTree,
 });
 
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,

src/libutil/config.cc

@@ -330,9 +330,13 @@ template<> std::set<ExperimentalFeature> BaseSetting<std::set<ExperimentalFeatur
 {
     std::set<ExperimentalFeature> res;
     for (auto & s : tokenizeString<StringSet>(str)) {
-        if (auto thisXpFeature = parseExperimentalFeature(s); thisXpFeature)
+        if (auto thisXpFeature = parseExperimentalFeature(s); thisXpFeature) {
             res.insert(thisXpFeature.value());
-        else
+            // FIXME: Replace this hack with a proper notion of
+            // experimental feature implications/dependencies.
+            if (thisXpFeature.value() == Xp::Flakes)
+                res.insert(Xp::FetchTree);
+        } else
             warn("unknown experimental feature '%s'", s);
     }
     return res;

src/libutil/experimental-features.cc

@@ -74,6 +74,21 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
             flake`](@docroot@/command-ref/new-cli/nix3-flake.md) for details.
         )",
     },
+    {
+        .tag = Xp::FetchTree,
+        .name = "fetch-tree",
+        .description = R"(
+            Enable the use of the [`fetchTree`](@docroot@/language/builtins.md#builtins-fetchTree) built-in function in the Nix language.
+
+            `fetchTree` exposes a larger suite of fetching functionality in a more systematic way.
+            The same fetching functionality is always used for for
+            [`flakes`](#xp-feature-flakes).
+
+            This built-in was previously guarded by the `flakes` experimental feature because of that overlap,
+            but since the plan is to work on stabilizing this first (due 2024 Q1), we are putting it underneath a separate feature.
+            Once we've made the changes we want to make, enabling just this feature will serve as a "release candidate" --- allowing users to try out the functionality we want to stabilize and not any other functionality we don't yet want to, in isolation.
+        )",
+    },
     {
         .tag = Xp::NixCommand,
         .name = "nix-command",

src/libutil/experimental-features.hh

@@ -20,6 +20,7 @@ enum struct ExperimentalFeature
     CaDerivations,
     ImpureDerivations,
     Flakes,
+    FetchTree,
     NixCommand,
     RecursiveNix,
     NoUrlLiterals,

tests/functional/config.sh

@@ -50,7 +50,8 @@ exp_cores=$(nix show-config | grep '^cores' | cut -d '=' -f 2 | xargs)
 exp_features=$(nix show-config | grep '^experimental-features' | cut -d '=' -f 2 | xargs)
 [[ $prev != $exp_cores ]]
 [[ $exp_cores == "4242" ]]
-[[ $exp_features == "flakes nix-command" ]]
+# flakes implies fetch-tree
+[[ $exp_features == "fetch-tree flakes nix-command" ]]
 
 # Test that it's possible to retrieve a single setting's value
 val=$(nix show-config | grep '^warn-dirty' | cut -d '=' -f  2 | xargs)