Expose builtins.fetchFinalTree to the user

This may be more efficient than fetchTree since it allows substitution
from binary caches if `narHash` is specified in the input
attributes. This is what call-flake.nix already used internally.

src/libexpr/primops/fetchTree.cc

@@ -362,10 +362,13 @@ void prim_fetchFinalTree(EvalState & state, const PosIdx pos, Value ** args, Val
 }
 
 static RegisterPrimOp primop_fetchFinalTree({
-    .name = "fetchFinalTree",
+    .name = "__fetchFinalTree",
     .args = {"input"},
+    .doc = R"(
+      Like `fetchTree`, but does not return any additional fetcher attributes (like `revCount`).
+      This allows inputs to be substituted if `narHash` is specified.
+    )",
     .fun = prim_fetchFinalTree,
-    .internal = true,
 });
 
 static void fetch(

src/libflake/call-flake.nix

@@ -10,9 +10,6 @@ lockFileStr:
 # unlocked trees.
 overrides:
 
-# This is `prim_fetchFinalTree`.
-fetchTreeFinal:
-
 let
   inherit (builtins) mapAttrs;
 
@@ -52,7 +49,7 @@ let
         else
           # FIXME: remove obsolete node.info.
           # Note: lock file entries are always final.
-          fetchTreeFinal (node.info or { } // removeAttrs node.locked [ "dir" ]);
+          builtins.fetchFinalTree (node.info or { } // removeAttrs node.locked [ "dir" ]);
 
       subdir = overrides.${key}.dir or node.locked.dir or "";
 

src/libflake/flake.cc

@@ -968,10 +968,7 @@ void callFlake(EvalState & state, const LockedFlake & lockedFlake, Value & vRes)
     auto vLocks = state.allocValue();
     vLocks->mkString(lockFileStr, state.mem);
 
-    auto vFetchFinalTree = get(state.internalPrimOps, "fetchFinalTree");
-    assert(vFetchFinalTree);
-
-    Value * args[] = {vLocks, &vOverrides, *vFetchFinalTree};
+    Value * args[] = {vLocks, &vOverrides};
     state.callFunction(*vCallFlake, args, vRes, noPos);
 }