Syntatically accept become expressions

compiler/rustc_feature/src/active.rs

@@ -395,6 +395,8 @@ declare_features! (
     (active, exclusive_range_pattern, "1.11.0", Some(37854), None),
     /// Allows exhaustive pattern matching on types that contain uninhabited types.
     (active, exhaustive_patterns, "1.13.0", Some(51085), None),
+    /// Allows explicit tail calls via `become` expression.
+    (incomplete, explicit_tail_calls, "CURRENT_RUSTC_VERSION", Some(112788), None),
     /// Allows using `efiapi`, `sysv64` and `win64` as calling convention
     /// for functions with varargs.
     (active, extended_varargs_abi_support, "1.65.0", Some(100189), None),