Rollup merge of #122829 - ShoyuVanilla:gen-block-impl-fused-iter, r=compiler-errors

Implement `FusedIterator` for `gen` block

cc #117078

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

@@ -267,6 +267,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 self.copy_clone_conditions(obligation)
             } else if Some(trait_def) == lang_items.clone_trait() {
                 self.copy_clone_conditions(obligation)
+            } else if Some(trait_def) == lang_items.fused_iterator_trait() {
+                self.fused_iterator_conditions(obligation)
             } else {
                 bug!("unexpected builtin trait {:?}", trait_def)
             };