src/librustc_trait_selection/traits/fulfill.rs

   1 use crate::infer::{InferCtxt, TyOrConstInferVar};
   2 use rustc_data_structures::obligation_forest::ProcessResult;
   3 use rustc_data_structures::obligation_forest::{DoCompleted, Error, ForestObligation};
   4 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
   5 use rustc_errors::ErrorReported;
   6 use rustc_infer::traits::{TraitEngine, TraitEngineExt as _};
   7 use rustc_middle::mir::interpret::ErrorHandled;
   8 use rustc_middle::ty::error::ExpectedFound;
   9 use rustc_middle::ty::{self, Const, ToPolyTraitRef, Ty, TypeFoldable};
  10 use std::marker::PhantomData;
  11
  12 use super::project;
  13 use super::select::SelectionContext;
  14 use super::wf;
  15 use super::CodeAmbiguity;
  16 use super::CodeProjectionError;
  17 use super::CodeSelectionError;
  18 use super::{ConstEvalFailure, Unimplemented};
  19 use super::{FulfillmentError, FulfillmentErrorCode};
  20 use super::{ObligationCause, PredicateObligation};
  21
  22 use crate::traits::error_reporting::InferCtxtExt as _;
  23 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  24
  25 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  26     /// Note that we include both the `ParamEnv` and the `Predicate`,
  27     /// as the `ParamEnv` can influence whether fulfillment succeeds
  28     /// or fails.
  29     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  30
  31     fn as_cache_key(&self) -> Self::CacheKey {
  32         self.obligation.param_env.and(self.obligation.predicate)
  33     }
  34 }
  35
  36 /// The fulfillment context is used to drive trait resolution. It
  37 /// consists of a list of obligations that must be (eventually)
  38 /// satisfied. The job is to track which are satisfied, which yielded
  39 /// errors, and which are still pending. At any point, users can call
  40 /// `select_where_possible`, and the fulfillment context will try to do
  41 /// selection, retaining only those obligations that remain
  42 /// ambiguous. This may be helpful in pushing type inference
  43 /// along. Once all type inference constraints have been generated, the
  44 /// method `select_all_or_error` can be used to report any remaining
  45 /// ambiguous cases as errors.
  46 pub struct FulfillmentContext<'tcx> {
  47     // A list of all obligations that have been registered with this
  48     // fulfillment context.
  49     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  50     // Should this fulfillment context register type-lives-for-region
  51     // obligations on its parent infcx? In some cases, region
  52     // obligations are either already known to hold (normalization) or
  53     // hopefully verifed elsewhere (type-impls-bound), and therefore
  54     // should not be checked.
  55     //
  56     // Note that if we are normalizing a type that we already
  57     // know is well-formed, there should be no harm setting this
  58     // to true - all the region variables should be determinable
  59     // using the RFC 447 rules, which don't depend on
  60     // type-lives-for-region constraints, and because the type
  61     // is well-formed, the constraints should hold.
  62     register_region_obligations: bool,
  63     // Is it OK to register obligations into this infcx inside
  64     // an infcx snapshot?
  65     //
  66     // The "primary fulfillment" in many cases in typeck lives
  67     // outside of any snapshot, so any use of it inside a snapshot
  68     // will lead to trouble and therefore is checked against, but
  69     // other fulfillment contexts sometimes do live inside of
  70     // a snapshot (they don't *straddle* a snapshot, so there
  71     // is no trouble there).
  72     usable_in_snapshot: bool,
  73 }
  74
  75 #[derive(Clone, Debug)]
  76 pub struct PendingPredicateObligation<'tcx> {
  77     pub obligation: PredicateObligation<'tcx>,
  78     // This is far more often read than modified, meaning that we
  79     // should mostly optimize for reading speed, while modifying is not as relevant.
  80     //
  81     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  82     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  83 }
  84
  85 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  86 #[cfg(target_arch = "x86_64")]
  87 static_assert_size!(PendingPredicateObligation<'_>, 64);
  88
  89 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  90     /// Creates a new fulfillment context.
  91     pub fn new() -> FulfillmentContext<'tcx> {
  92         FulfillmentContext {
  93             predicates: ObligationForest::new(),
  94             register_region_obligations: true,
  95             usable_in_snapshot: false,
  96         }
  97     }
  98
  99     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 100         FulfillmentContext {
 101             predicates: ObligationForest::new(),
 102             register_region_obligations: true,
 103             usable_in_snapshot: true,
 104         }
 105     }
 106
 107     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 108         FulfillmentContext {
 109             predicates: ObligationForest::new(),
 110             register_region_obligations: false,
 111             usable_in_snapshot: false,
 112         }
 113     }
 114
 115     /// Attempts to select obligations using `selcx`.
 116     fn select(
 117         &mut self,
 118         selcx: &mut SelectionContext<'a, 'tcx>,
 119     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 120         debug!("select(obligation-forest-size={})", self.predicates.len());
 121
 122         let mut errors = Vec::new();
 123
 124         loop {
 125             debug!("select: starting another iteration");
 126
 127             // Process pending obligations.
 128             let outcome = self.predicates.process_obligations(
 129                 &mut FulfillProcessor {
 130                     selcx,
 131                     register_region_obligations: self.register_region_obligations,
 132                 },
 133                 DoCompleted::No,
 134             );
 135             debug!("select: outcome={:#?}", outcome);
 136
 137             // FIXME: if we kept the original cache key, we could mark projection
 138             // obligations as complete for the projection cache here.
 139
 140             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 141
 142             // If nothing new was added, no need to keep looping.
 143             if outcome.stalled {
 144                 break;
 145             }
 146         }
 147
 148         debug!(
 149             "select({} predicates remaining, {} errors) done",
 150             self.predicates.len(),
 151             errors.len()
 152         );
 153
 154         if errors.is_empty() { Ok(()) } else { Err(errors) }
 155     }
 156 }
 157
 158 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 159     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 160     /// creating a fresh type variable `$0` as well as a projection
 161     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 162     /// inference engine runs, it will attempt to find an impl of
 163     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 164     /// something concrete. If this fails, we'll unify `$0` with
 165     /// `projection_ty` again.
 166     fn normalize_projection_type(
 167         &mut self,
 168         infcx: &InferCtxt<'_, 'tcx>,
 169         param_env: ty::ParamEnv<'tcx>,
 170         projection_ty: ty::ProjectionTy<'tcx>,
 171         cause: ObligationCause<'tcx>,
 172     ) -> Ty<'tcx> {
 173         debug!("normalize_projection_type(projection_ty={:?})", projection_ty);
 174
 175         debug_assert!(!projection_ty.has_escaping_bound_vars());
 176
 177         // FIXME(#20304) -- cache
 178
 179         let mut selcx = SelectionContext::new(infcx);
 180         let mut obligations = vec![];
 181         let normalized_ty = project::normalize_projection_type(
 182             &mut selcx,
 183             param_env,
 184             projection_ty,
 185             cause,
 186             0,
 187             &mut obligations,
 188         );
 189         self.register_predicate_obligations(infcx, obligations);
 190
 191         debug!("normalize_projection_type: result={:?}", normalized_ty);
 192
 193         normalized_ty
 194     }
 195
 196     fn register_predicate_obligation(
 197         &mut self,
 198         infcx: &InferCtxt<'_, 'tcx>,
 199         obligation: PredicateObligation<'tcx>,
 200     ) {
 201         // this helps to reduce duplicate errors, as well as making
 202         // debug output much nicer to read and so on.
 203         let obligation = infcx.resolve_vars_if_possible(&obligation);
 204
 205         debug!("register_predicate_obligation(obligation={:?})", obligation);
 206
 207         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 208
 209         self.predicates
 210             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 211     }
 212
 213     fn select_all_or_error(
 214         &mut self,
 215         infcx: &InferCtxt<'_, 'tcx>,
 216     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 217         self.select_where_possible(infcx)?;
 218
 219         let errors: Vec<_> = self
 220             .predicates
 221             .to_errors(CodeAmbiguity)
 222             .into_iter()
 223             .map(to_fulfillment_error)
 224             .collect();
 225         if errors.is_empty() { Ok(()) } else { Err(errors) }
 226     }
 227
 228     fn select_where_possible(
 229         &mut self,
 230         infcx: &InferCtxt<'_, 'tcx>,
 231     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 232         let mut selcx = SelectionContext::new(infcx);
 233         self.select(&mut selcx)
 234     }
 235
 236     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 237         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 238     }
 239 }
 240
 241 struct FulfillProcessor<'a, 'b, 'tcx> {
 242     selcx: &'a mut SelectionContext<'b, 'tcx>,
 243     register_region_obligations: bool,
 244 }
 245
 246 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 247     os.into_iter()
 248         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 249         .collect()
 250 }
 251
 252 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 253     type Obligation = PendingPredicateObligation<'tcx>;
 254     type Error = FulfillmentErrorCode<'tcx>;
 255
 256     /// Processes a predicate obligation and returns either:
 257     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 258     /// - `Unchanged` if we don't have enough info to be sure
 259     /// - `Error(e)` if the predicate does not hold
 260     ///
 261     /// This is always inlined, despite its size, because it has a single
 262     /// callsite and it is called *very* frequently.
 263     #[inline(always)]
 264     fn process_obligation(
 265         &mut self,
 266         pending_obligation: &mut Self::Obligation,
 267     ) -> ProcessResult<Self::Obligation, Self::Error> {
 268         // If we were stalled on some unresolved variables, first check whether
 269         // any of them have been resolved; if not, don't bother doing more work
 270         // yet.
 271         let change = match pending_obligation.stalled_on.len() {
 272             // Match arms are in order of frequency, which matters because this
 273             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 274             1 => {
 275                 let infer_var = pending_obligation.stalled_on[0];
 276                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 277             }
 278             0 => {
 279                 // In this case we haven't changed, but wish to make a change.
 280                 true
 281             }
 282             _ => {
 283                 // This `for` loop was once a call to `all()`, but this lower-level
 284                 // form was a perf win. See #64545 for details.
 285                 (|| {
 286                     for &infer_var in &pending_obligation.stalled_on {
 287                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 288                             return true;
 289                         }
 290                     }
 291                     false
 292                 })()
 293             }
 294         };
 295
 296         if !change {
 297             debug!(
 298                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 299                 self.selcx.infcx().resolve_vars_if_possible(&pending_obligation.obligation),
 300                 pending_obligation.stalled_on
 301             );
 302             return ProcessResult::Unchanged;
 303         }
 304
 305         // This part of the code is much colder.
 306
 307         pending_obligation.stalled_on.truncate(0);
 308
 309         let obligation = &mut pending_obligation.obligation;
 310
 311         if obligation.predicate.has_infer_types_or_consts() {
 312             obligation.predicate =
 313                 self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
 314         }
 315
 316         debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
 317
 318         let infcx = self.selcx.infcx();
 319
 320         match obligation.predicate.kind() {
 321             ty::PredicateKind::Trait(ref data, _) => {
 322                 let trait_obligation = obligation.with(*data);
 323
 324                 if obligation.predicate.is_global() {
 325                     // no type variables present, can use evaluation for better caching.
 326                     // FIXME: consider caching errors too.
 327                     if infcx.predicate_must_hold_considering_regions(&obligation) {
 328                         debug!(
 329                             "selecting trait `{:?}` at depth {} evaluated to holds",
 330                             data, obligation.recursion_depth
 331                         );
 332                         return ProcessResult::Changed(vec![]);
 333                     }
 334                 }
 335
 336                 match self.selcx.select(&trait_obligation) {
 337                     Ok(Some(impl_source)) => {
 338                         debug!(
 339                             "selecting trait `{:?}` at depth {} yielded Ok(Some)",
 340                             data, obligation.recursion_depth
 341                         );
 342                         ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 343                     }
 344                     Ok(None) => {
 345                         debug!(
 346                             "selecting trait `{:?}` at depth {} yielded Ok(None)",
 347                             data, obligation.recursion_depth
 348                         );
 349
 350                         // This is a bit subtle: for the most part, the
 351                         // only reason we can fail to make progress on
 352                         // trait selection is because we don't have enough
 353                         // information about the types in the trait.
 354                         pending_obligation.stalled_on =
 355                             trait_ref_infer_vars(self.selcx, data.to_poly_trait_ref());
 356
 357                         debug!(
 358                             "process_predicate: pending obligation {:?} now stalled on {:?}",
 359                             infcx.resolve_vars_if_possible(obligation),
 360                             pending_obligation.stalled_on
 361                         );
 362
 363                         ProcessResult::Unchanged
 364                     }
 365                     Err(selection_err) => {
 366                         info!(
 367                             "selecting trait `{:?}` at depth {} yielded Err",
 368                             data, obligation.recursion_depth
 369                         );
 370
 371                         ProcessResult::Error(CodeSelectionError(selection_err))
 372                     }
 373                 }
 374             }
 375
 376             &ty::PredicateKind::RegionOutlives(binder) => {
 377                 match infcx.region_outlives_predicate(&obligation.cause, binder) {
 378                     Ok(()) => ProcessResult::Changed(vec![]),
 379                     Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 380                 }
 381             }
 382
 383             ty::PredicateKind::TypeOutlives(ref binder) => {
 384                 // Check if there are higher-ranked vars.
 385                 match binder.no_bound_vars() {
 386                     // If there are, inspect the underlying type further.
 387                     None => {
 388                         // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 389                         let binder = binder.map_bound_ref(|pred| pred.0);
 390
 391                         // Check if the type has any bound vars.
 392                         match binder.no_bound_vars() {
 393                             // If so, this obligation is an error (for now). Eventually we should be
 394                             // able to support additional cases here, like `for<'a> &'a str: 'a`.
 395                             // NOTE: this is duplicate-implemented between here and fulfillment.
 396                             None => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 397                             // Otherwise, we have something of the form
 398                             // `for<'a> T: 'a where 'a not in T`, which we can treat as
 399                             // `T: 'static`.
 400                             Some(t_a) => {
 401                                 let r_static = self.selcx.tcx().lifetimes.re_static;
 402                                 if self.register_region_obligations {
 403                                     self.selcx.infcx().register_region_obligation_with_cause(
 404                                         t_a,
 405                                         r_static,
 406                                         &obligation.cause,
 407                                     );
 408                                 }
 409                                 ProcessResult::Changed(vec![])
 410                             }
 411                         }
 412                     }
 413                     // If there aren't, register the obligation.
 414                     Some(ty::OutlivesPredicate(t_a, r_b)) => {
 415                         if self.register_region_obligations {
 416                             self.selcx.infcx().register_region_obligation_with_cause(
 417                                 t_a,
 418                                 r_b,
 419                                 &obligation.cause,
 420                             );
 421                         }
 422                         ProcessResult::Changed(vec![])
 423                     }
 424                 }
 425             }
 426
 427             ty::PredicateKind::Projection(ref data) => {
 428                 let project_obligation = obligation.with(*data);
 429                 match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 430                     Ok(None) => {
 431                         let tcx = self.selcx.tcx();
 432                         pending_obligation.stalled_on =
 433                             trait_ref_infer_vars(self.selcx, data.to_poly_trait_ref(tcx));
 434                         ProcessResult::Unchanged
 435                     }
 436                     Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
 437                     Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 438                 }
 439             }
 440
 441             &ty::PredicateKind::ObjectSafe(trait_def_id) => {
 442                 if !self.selcx.tcx().is_object_safe(trait_def_id) {
 443                     ProcessResult::Error(CodeSelectionError(Unimplemented))
 444                 } else {
 445                     ProcessResult::Changed(vec![])
 446                 }
 447             }
 448
 449             &ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 450                 match self.selcx.infcx().closure_kind(closure_substs) {
 451                     Some(closure_kind) => {
 452                         if closure_kind.extends(kind) {
 453                             ProcessResult::Changed(vec![])
 454                         } else {
 455                             ProcessResult::Error(CodeSelectionError(Unimplemented))
 456                         }
 457                     }
 458                     None => ProcessResult::Unchanged,
 459                 }
 460             }
 461
 462             &ty::PredicateKind::WellFormed(arg) => {
 463                 match wf::obligations(
 464                     self.selcx.infcx(),
 465                     obligation.param_env,
 466                     obligation.cause.body_id,
 467                     arg,
 468                     obligation.cause.span,
 469                 ) {
 470                     None => {
 471                         pending_obligation.stalled_on =
 472                             vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 473                         ProcessResult::Unchanged
 474                     }
 475                     Some(os) => ProcessResult::Changed(mk_pending(os)),
 476                 }
 477             }
 478
 479             &ty::PredicateKind::Subtype(subtype) => {
 480                 match self.selcx.infcx().subtype_predicate(
 481                     &obligation.cause,
 482                     obligation.param_env,
 483                     subtype,
 484                 ) {
 485                     None => {
 486                         // None means that both are unresolved.
 487                         pending_obligation.stalled_on = vec![
 488                             TyOrConstInferVar::maybe_from_ty(subtype.skip_binder().a).unwrap(),
 489                             TyOrConstInferVar::maybe_from_ty(subtype.skip_binder().b).unwrap(),
 490                         ];
 491                         ProcessResult::Unchanged
 492                     }
 493                     Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 494                     Some(Err(err)) => {
 495                         let expected_found = ExpectedFound::new(
 496                             subtype.skip_binder().a_is_expected,
 497                             subtype.skip_binder().a,
 498                             subtype.skip_binder().b,
 499                         );
 500                         ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 501                             expected_found,
 502                             err,
 503                         ))
 504                     }
 505                 }
 506             }
 507
 508             &ty::PredicateKind::ConstEvaluatable(def_id, substs) => {
 509                 match self.selcx.infcx().const_eval_resolve(
 510                     obligation.param_env,
 511                     def_id,
 512                     substs,
 513                     None,
 514                     Some(obligation.cause.span),
 515                 ) {
 516                     Ok(_) => ProcessResult::Changed(vec![]),
 517                     Err(err) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err))),
 518                 }
 519             }
 520
 521             ty::PredicateKind::ConstEquate(c1, c2) => {
 522                 debug!("equating consts: c1={:?} c2={:?}", c1, c2);
 523
 524                 let stalled_on = &mut pending_obligation.stalled_on;
 525
 526                 let mut evaluate = |c: &'tcx Const<'tcx>| {
 527                     if let ty::ConstKind::Unevaluated(def_id, substs, promoted) = c.val {
 528                         match self.selcx.infcx().const_eval_resolve(
 529                             obligation.param_env,
 530                             def_id,
 531                             substs,
 532                             promoted,
 533                             Some(obligation.cause.span),
 534                         ) {
 535                             Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 536                             Err(ErrorHandled::TooGeneric) => {
 537                                 stalled_on.append(
 538                                     &mut substs
 539                                         .types()
 540                                         .filter_map(|ty| TyOrConstInferVar::maybe_from_ty(ty))
 541                                         .collect(),
 542                                 );
 543                                 Err(ErrorHandled::TooGeneric)
 544                             }
 545                             Err(err) => Err(err),
 546                         }
 547                     } else {
 548                         Ok(c)
 549                     }
 550                 };
 551
 552                 match (evaluate(c1), evaluate(c2)) {
 553                     (Ok(c1), Ok(c2)) => {
 554                         match self
 555                             .selcx
 556                             .infcx()
 557                             .at(&obligation.cause, obligation.param_env)
 558                             .eq(c1, c2)
 559                         {
 560                             Ok(_) => ProcessResult::Changed(vec![]),
 561                             Err(err) => {
 562                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 563                                     ExpectedFound::new(true, c1, c2),
 564                                     err,
 565                                 ))
 566                             }
 567                         }
 568                     }
 569                     (Err(ErrorHandled::Reported(ErrorReported)), _)
 570                     | (_, Err(ErrorHandled::Reported(ErrorReported))) => ProcessResult::Error(
 571                         CodeSelectionError(ConstEvalFailure(ErrorHandled::Reported(ErrorReported))),
 572                     ),
 573                     (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => span_bug!(
 574                         obligation.cause.span(self.selcx.tcx()),
 575                         "ConstEquate: const_eval_resolve returned an unexpected error"
 576                     ),
 577                     (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 578                         ProcessResult::Unchanged
 579                     }
 580                 }
 581             }
 582         }
 583     }
 584
 585     fn process_backedge<'c, I>(
 586         &mut self,
 587         cycle: I,
 588         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 589     ) where
 590         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 591     {
 592         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 593             debug!("process_child_obligations: coinductive match");
 594         } else {
 595             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 596             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 597         }
 598     }
 599 }
 600
 601 /// Returns the set of inference variables contained in a trait ref.
 602 fn trait_ref_infer_vars<'a, 'tcx>(
 603     selcx: &mut SelectionContext<'a, 'tcx>,
 604     trait_ref: ty::PolyTraitRef<'tcx>,
 605 ) -> Vec<TyOrConstInferVar<'tcx>> {
 606     selcx
 607         .infcx()
 608         .resolve_vars_if_possible(&trait_ref)
 609         .skip_binder() // ok b/c this check doesn't care about regions
 610         .substs
 611         .iter()
 612         // FIXME(eddyb) try using `skip_current_subtree` to skip everything that
 613         // doesn't contain inference variables, not just the outermost level.
 614         .filter(|arg| arg.has_infer_types_or_consts())
 615         .flat_map(|arg| arg.walk())
 616         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 617         .collect()
 618 }
 619
 620 fn to_fulfillment_error<'tcx>(
 621     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 622 ) -> FulfillmentError<'tcx> {
 623     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 624     FulfillmentError::new(obligation, error.error)
 625 }