Add const_trait_bound_opt_out feature gate

src/librustc_feature/active.rs

@@ -547,6 +547,9 @@ declare_features! (
     /// Allows `impl const Trait for T` syntax.
     (active, const_trait_impl, "1.42.0", Some(67792), None),
 
+    /// Allows `T: ?const Trait` syntax in bounds.
+    (active, const_trait_bound_opt_out, "1.42.0", Some(67794), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
@@ -563,4 +566,5 @@ pub const INCOMPLETE_FEATURES: &[Symbol] = &[
     sym::let_chains,
     sym::raw_dylib,
     sym::const_trait_impl,
+    sym::const_trait_bound_opt_out,
 ];

src/librustc_span/symbol.rs

@@ -219,6 +219,7 @@ symbols! {
         const_raw_ptr_deref,
         const_raw_ptr_to_usize_cast,
         const_transmute,
+        const_trait_bound_opt_out,
         const_trait_impl,
         contents,
         context,

src/libsyntax/feature_gate/check.rs

@@ -909,6 +909,7 @@ pub fn check_crate(
     gate_all!(or_patterns, "or-patterns syntax is experimental");
     gate_all!(const_extern_fn, "`const extern fn` definitions are unstable");
     gate_all!(raw_ref_op, "raw address of syntax is experimental");
+    gate_all!(const_trait_bound_opt_out, "`?const` on trait bounds is experimental");
     gate_all!(const_trait_impl, "const trait impls are experimental");
 
     // All uses of `gate_all!` below this point were added in #65742,