Break down the fetch-tree experimental-feature

Add two new features:
- `fetch-tree-git` to enable the `git` fetcher for `fetchTree`
- `fetch-tree-urls` to enable the URL-like syntax for `fetchTree`

`fetch-tree` is kept as a backwards-compatibility alias for the two new
features.

src/libexpr/primops/fetchTree.cc

@@ -159,9 +159,9 @@ static void fetchTree(
             }
             input = fetchers::Input::fromAttrs(std::move(attrs));
         } else {
-            if (!experimentalFeatureSettings.isEnabled(Xp::Flakes))
+            if (!experimentalFeatureSettings.isEnabled(Xp::FetchTreeUrls))
                 state.error<EvalError>(
-                    "passing a string argument to 'fetchTree' requires the 'flakes' experimental feature"
+                    "passing a string argument to 'fetchTree' requires the 'fetch-tree-urls' experimental feature"
                 ).atPos(pos).debugThrow();
             input = fetchers::Input::fromURL(url);
         }

src/libfetchers/git.cc

@@ -282,6 +282,11 @@ struct GitInputScheme : InputScheme
         return res;
     }
 
+    std::optional<ExperimentalFeature> experimentalFeature() const override
+    {
+        return Xp::FetchTreeGit;
+    }
+
     void clone(const Input & input, const Path & destDir) const override
     {
         auto repoInfo = getRepoInfo(input);

src/libutil/config.cc

@@ -334,8 +334,14 @@ template<> std::set<ExperimentalFeature> BaseSetting<std::set<ExperimentalFeatur
     for (auto & s : tokenizeString<StringSet>(str)) {
         if (auto thisXpFeature = parseExperimentalFeature(s); thisXpFeature) {
             res.insert(thisXpFeature.value());
-            if (thisXpFeature.value() == Xp::Flakes)
+            if (thisXpFeature.value() == Xp::Flakes) {
                 res.insert(Xp::FetchTree);
+                res.insert(Xp::FetchTreeGit);
+                res.insert(Xp::FetchTreeUrls);
+            } else if (thisXpFeature.value() == Xp::FetchTree) {
+                res.insert(Xp::FetchTreeGit);
+                res.insert(Xp::FetchTreeUrls);
+            }
         } else
             warn("unknown experimental feature '%s'", s);
     }

src/libutil/experimental-features.cc

@@ -78,13 +78,23 @@ constexpr std::array<ExperimentalFeatureDetails, numXpFeatures> xpFeatureDetails
         .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 generic interface for fetching remote file system trees from different types of remote sources.
-            The [`flakes`](#xp-feature-flakes) feature flag always enables `fetch-tree`.
-            This built-in was previously guarded by the `flakes` experimental feature because of that overlap.
-
-            Enabling just this feature serves as a "release candidate", allowing users to try it out in isolation.
+            Backwards-compatibility alias for
+            [fetch-tree-git](#xp-feature-fetch-tree-git) and
+            [fetch-tree-urls](#xp-feature-fetch-tree-urls).
+        )",
+    },
+    {
+        .tag = Xp::FetchTreeGit,
+        .name = "fetch-tree-git",
+        .description = R"(
+            Enable the use of the `git` [`fetchTree`](@docroot@/language/builtins.md#builtins-fetchTree) fetcher.
+        )",
+    },
+    {
+        .tag = Xp::FetchTreeUrls,
+        .name = "fetch-tree-urls",
+        .description = R"(
+            Enable the use of the [URL-like syntax](@docroot@/command-ref/new-cli/nix3-flake.html#url-like-syntax) in [`fetchTree`](@docroot@/language/builtins.md#builtins-fetchTree).
         )",
     },
     {

src/libutil/experimental-features.hh

@@ -21,6 +21,8 @@ enum struct ExperimentalFeature
     ImpureDerivations,
     Flakes,
     FetchTree,
+    FetchTreeGit,
+    FetchTreeUrls,
     NixCommand,
     GitHashing,
     RecursiveNix,