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 ...

1 files changed, 6 insertions, 3 deletions

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index 33eaa8a2c..2ec864ee4 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/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`.