Change:
location ); <del> let tcx = self.cx.infcx.tcx; let mut types = vec![(dropped_ty, 0)]; let mut known = FxHashSet(); while let Some((ty, depth)) = types.pop() { let span = DUMMY_SP; // FIXME let result = match tcx.dtorck_constraint_for_ty(span, dropped_ty, depth, ty) { Ok(result) => result, Err(ErrorReported) => { continue; } }; let ty::DtorckConstraint { outlives, dtorck_types, } = result; // All things in the `outlives` array may be touched by // the destructor and must be live at this point. for outlive in outlives { let cause = Cause::DropVar(dropped_local, location); self.push_type_live_constraint(outlive, location, cause); } </del> <ins> // If we end visiting the same type twice (usually due to a cycle involving // associated types), we need to ensure that its region types match up with the type // we added to the 'known' map the first time around. For this reason, we need // our infcx to hold onto its calculated region constraints after each call // to dtorck_constraint_for_ty. Otherwise, normalizing the corresponding associated // type will end up instantiating the type with a new set of inference variables // Since this new type will never be in 'known', we end up looping forever. // // For this reason, we avoid calling TypeChecker.normalize, instead doing all normalization // ourselves in one large 'fully_perform_op' callback. let (type_constraints, kind_constraints) = self.cx.fully_perform_op(location.at_self(), |cx| { let tcx = cx.infcx.tcx; let mut selcx = traits::SelectionContext::new(cx.infcx); let cause = cx.misc(cx.last_span); let mut types = vec![(dropped_ty, 0)]; let mut final_obligations = Vec::new(); let mut type_constraints = Vec::new(); let mut kind_constraints = Vec::new(); </ins> <del> // However, there may also be some types that // `dtorck_constraint_for_ty` could not resolve (e.g., // associated types and parameters). We need to normalize // associated types here and possibly recursively process. for ty in dtorck_types { let ty = self.cx.normalize(&ty, location); let ty = self.cx.infcx.resolve_type_and_region_vars_if_possible(&ty); match ty.sty { ty::TyParam(..) | ty::TyProjection(..) | ty::TyAnon(..) => { let cause = Cause::DropVar(dropped_local, location); self.push_type_live_constraint(ty, location, cause); </del> <ins> let mut known = FxHashSet(); while let Some((ty, depth)) = types.pop() { let span = DUMMY_SP; // FIXME let result = match tcx.dtorck_constraint_for_ty(span, dropped_ty, depth, ty) { Ok(result) => result, Err(ErrorReported) => { continue; </ins> } <ins> }; let ty::DtorckConstraint { outlives, dtorck_types, } = result; // All things in the `outlives` array may be touched by // the destructor and must be live at this point. for outlive in outlives { let cause = Cause::DropVar(dropped_local, location); kind_constraints.push((outlive, location, cause)); } </ins> <del> _ => if known.insert(ty) { types.push((ty, depth + 1)); }, </del> <ins> // However, there may also be some types that // `dtorck_constraint_for_ty` could not resolve (e.g., // associated types and parameters). We need to normalize // associated types here and possibly recursively process. for ty in dtorck_types { let traits::Normalized { value: ty, obligations } = traits::normalize(&mut selcx, cx.param_env, cause.clone(), &ty); final_obligations.extend(obligations); let ty = cx.infcx.resolve_type_and_region_vars_if_possible(&ty); match ty.sty { ty::TyParam(..) | ty::TyProjection(..) | ty::TyAnon(..) => { let cause = Cause::DropVar(dropped_local, location); type_constraints.push((ty, location, cause)); } _ => if known.insert(ty) { types.push((ty, depth + 1)); }, } </ins> } } <ins> Ok(InferOk { value: (type_constraints, kind_constraints), obligations: final_obligations }) }).unwrap(); for (ty, location, cause) in type_constraints { self.push_type_live_constraint(ty, location, cause); } for (kind, location, cause) in kind_constraints { self.push_type_live_constraint(kind, location, cause); </ins> } } }

Discussion:
I'd like to work on this.
This appears to be a regular error, not an ICE.