Mark super_let feature as incomplete.

Co-authored-by: Travis Cross <tc@traviscross.com>

Author: m-ou-se

compiler/rustc_feature/src/unstable.rs

@@ -631,7 +631,7 @@ declare_features! (
     /// Allows string patterns to dereference values to match them.
     (unstable, string_deref_patterns, "1.67.0", Some(87121)),
     /// Allows `super let` statements.
-    (unstable, super_let, "CURRENT_RUSTC_VERSION", Some(139076)),
+    (incomplete, super_let, "CURRENT_RUSTC_VERSION", Some(139076)),
     /// Allows subtrait items to shadow supertrait items.
     (unstable, supertrait_item_shadowing, "1.86.0", Some(89151)),
     /// Allows using `#[thread_local]` on `static` items.