Add feature gate for `const_loop`

src/librustc_feature/active.rs

@@ -538,6 +538,9 @@ declare_features! (
     /// Allows using `&mut` in constant functions.
     (active, const_mut_refs, "1.41.0", Some(57349), None),
 
+    /// Allows the use of `loop` and `while` in constants.
+    (active, const_loop, "1.42.0", Some(52000), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

src/libsyntax_pos/symbol.rs

@@ -213,6 +213,7 @@ symbols! {
         const_indexing,
         const_in_array_repeat_expressions,
         const_let,
+        const_loop,
         const_mut_refs,
         const_panic,
         const_raw_ptr_deref,