Add the same logic to the old solver

Author: tiif

compiler/rustc_trait_selection/src/traits/fulfill.rs

@@ -404,7 +404,9 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                 ty::PredicateKind::AliasRelate(..) => {
                     bug!("AliasRelate is only used by the new solver")
                 }
-                ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(_)) => todo!(), // TODO: not sure what will happen here. 
+                ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(_)) => {
+                   unreachable!("unexpected higher ranked `UnstableFeature` goal")
+                }
             },
             Some(pred) => match pred {
                 ty::PredicateKind::Clause(ty::ClauseKind::Trait(data)) => {
@@ -762,7 +764,26 @@ impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
                         }
                     }
                 }
-                ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(_)) => todo!(),
+                ty::PredicateKind::Clause(ty::ClauseKind::UnstableFeature(symbol)) => {
+                    // Iterate through all goals in param_env to find the one that has the same symbol.
+                    for pred in obligation.param_env.caller_bounds().iter() {
+                        match pred.kind().skip_binder() {
+                            ty::ClauseKind::UnstableFeature(sym) => {
+                                if sym == symbol {
+                                    return ProcessResult::Changed(Default::default());
+                                }
+                            }
+                            _ => {} // don't care
+                        }
+                    }
+                    // Check if feature is enabled at crate level with #[feature(..)] or if we are currently in codegen.
+                    if self.selcx.tcx().features().enabled(symbol) || (self.selcx.infcx.typing_mode() == TypingMode::PostAnalysis) {
+                        return ProcessResult::Changed(Default::default());
+                    } else {
+                        return ProcessResult::Error(FulfillmentErrorCode::Ambiguity { overflow: None });
+                    }
+
+                }
             },
         }
     }