add generator_clone feature gate

compiler/rustc_feature/src/active.rs

@@ -396,6 +396,8 @@ declare_features! (
     (active, fn_align, "1.53.0", Some(82232), None),
     /// Allows defining generators.
     (active, generators, "1.21.0", Some(43122), None),
+    /// Allows generators to be cloned.
+    (active, generator_clone, "1.60.0", None, None),
     /// Infer generic args for both consts and types.
     (active, generic_arg_infer, "1.55.0", Some(85077), None),
     /// Allows associated types to be generic, e.g., `type Foo<T>;` (RFC 1598).

compiler/rustc_span/src/symbol.rs

@@ -763,6 +763,7 @@ symbols! {
         gen_future,
         gen_kill,
         generator,
+        generator_clone,
         generator_state,
         generators,
         generic_arg_infer,

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1938,6 +1938,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
 
             ty::Generator(_, substs, hir::Movability::Movable) => {
+                if self.tcx().features().generator_clone {
                     let resolved_upvars = self.infcx.shallow_resolve(substs.as_generator().tupled_upvars_ty());
                     let resolved_witness = self.infcx.shallow_resolve(substs.as_generator().witness());
                     if {
@@ -1951,10 +1952,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         all.push(substs.as_generator().witness());
                         Where(obligation.predicate.rebind(all))
                     }
+                } else {
+                    None
+                }
             }
 
             ty::GeneratorWitness(binder) => {
-                let tys = binder.no_bound_vars().unwrap();
+                match binder.no_bound_vars() {
+                    Some(tys) => {
                         let mut iter = tys.iter();
                         loop {
                             let ty = match iter.next() {
@@ -1968,6 +1973,9 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                                 break Ambiguous;
                             }
                         }
+                    },
+                    Option::None => None,
+                }
             }
 
             ty::Closure(_, substs) => {