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::{PolyTraitObligation, TraitEngine, TraitEngineExt as _};
   7 use rustc_middle::mir::interpret::ErrorHandled;
   8 use rustc_middle::ty::error::ExpectedFound;
   9 use rustc_middle::ty::ToPredicate;
  10 use rustc_middle::ty::{self, Binder, Const, Ty, TypeFoldable};
  11 use std::marker::PhantomData;
  12
  13 use super::project;
  14 use super::select::SelectionContext;
  15 use super::wf;
  16 use super::CodeAmbiguity;
  17 use super::CodeProjectionError;
  18 use super::CodeSelectionError;
  19 use super::{ConstEvalFailure, Unimplemented};
  20 use super::{FulfillmentError, FulfillmentErrorCode};
  21 use super::{ObligationCause, PredicateObligation};
  22
  23 use crate::traits::error_reporting::InferCtxtExt as _;
  24 use crate::traits::project::PolyProjectionObligation;
  25 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  26
  27 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  28     /// Note that we include both the `ParamEnv` and the `Predicate`,
  29     /// as the `ParamEnv` can influence whether fulfillment succeeds
  30     /// or fails.
  31     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  32
  33     fn as_cache_key(&self) -> Self::CacheKey {
  34         self.obligation.param_env.and(self.obligation.predicate)
  35     }
  36 }
  37
  38 /// The fulfillment context is used to drive trait resolution. It
  39 /// consists of a list of obligations that must be (eventually)
  40 /// satisfied. The job is to track which are satisfied, which yielded
  41 /// errors, and which are still pending. At any point, users can call
  42 /// `select_where_possible`, and the fulfillment context will try to do
  43 /// selection, retaining only those obligations that remain
  44 /// ambiguous. This may be helpful in pushing type inference
  45 /// along. Once all type inference constraints have been generated, the
  46 /// method `select_all_or_error` can be used to report any remaining
  47 /// ambiguous cases as errors.
  48 pub struct FulfillmentContext<'tcx> {
  49     // A list of all obligations that have been registered with this
  50     // fulfillment context.
  51     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  52     // Should this fulfillment context register type-lives-for-region
  53     // obligations on its parent infcx? In some cases, region
  54     // obligations are either already known to hold (normalization) or
  55     // hopefully verifed elsewhere (type-impls-bound), and therefore
  56     // should not be checked.
  57     //
  58     // Note that if we are normalizing a type that we already
  59     // know is well-formed, there should be no harm setting this
  60     // to true - all the region variables should be determinable
  61     // using the RFC 447 rules, which don't depend on
  62     // type-lives-for-region constraints, and because the type
  63     // is well-formed, the constraints should hold.
  64     register_region_obligations: bool,
  65     // Is it OK to register obligations into this infcx inside
  66     // an infcx snapshot?
  67     //
  68     // The "primary fulfillment" in many cases in typeck lives
  69     // outside of any snapshot, so any use of it inside a snapshot
  70     // will lead to trouble and therefore is checked against, but
  71     // other fulfillment contexts sometimes do live inside of
  72     // a snapshot (they don't *straddle* a snapshot, so there
  73     // is no trouble there).
  74     usable_in_snapshot: bool,
  75 }
  76
  77 #[derive(Clone, Debug)]
  78 pub struct PendingPredicateObligation<'tcx> {
  79     pub obligation: PredicateObligation<'tcx>,
  80     // This is far more often read than modified, meaning that we
  81     // should mostly optimize for reading speed, while modifying is not as relevant.
  82     //
  83     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  84     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  85 }
  86
  87 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  88 #[cfg(target_arch = "x86_64")]
  89 static_assert_size!(PendingPredicateObligation<'_>, 64);
  90
  91 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  92     /// Creates a new fulfillment context.
  93     pub fn new() -> FulfillmentContext<'tcx> {
  94         FulfillmentContext {
  95             predicates: ObligationForest::new(),
  96             register_region_obligations: true,
  97             usable_in_snapshot: false,
  98         }
  99     }
 100
 101     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 102         FulfillmentContext {
 103             predicates: ObligationForest::new(),
 104             register_region_obligations: true,
 105             usable_in_snapshot: true,
 106         }
 107     }
 108
 109     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 110         FulfillmentContext {
 111             predicates: ObligationForest::new(),
 112             register_region_obligations: false,
 113             usable_in_snapshot: false,
 114         }
 115     }
 116
 117     /// Attempts to select obligations using `selcx`.
 118     fn select(
 119         &mut self,
 120         selcx: &mut SelectionContext<'a, 'tcx>,
 121     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 122         debug!("select(obligation-forest-size={})", self.predicates.len());
 123
 124         let mut errors = Vec::new();
 125
 126         loop {
 127             debug!("select: starting another iteration");
 128
 129             // Process pending obligations.
 130             let outcome = self.predicates.process_obligations(
 131                 &mut FulfillProcessor {
 132                     selcx,
 133                     register_region_obligations: self.register_region_obligations,
 134                 },
 135                 DoCompleted::No,
 136             );
 137             debug!("select: outcome={:#?}", outcome);
 138
 139             // FIXME: if we kept the original cache key, we could mark projection
 140             // obligations as complete for the projection cache here.
 141
 142             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 143
 144             // If nothing new was added, no need to keep looping.
 145             if outcome.stalled {
 146                 break;
 147             }
 148         }
 149
 150         debug!(
 151             "select({} predicates remaining, {} errors) done",
 152             self.predicates.len(),
 153             errors.len()
 154         );
 155
 156         if errors.is_empty() { Ok(()) } else { Err(errors) }
 157     }
 158 }
 159
 160 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 161     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 162     /// creating a fresh type variable `$0` as well as a projection
 163     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 164     /// inference engine runs, it will attempt to find an impl of
 165     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 166     /// something concrete. If this fails, we'll unify `$0` with
 167     /// `projection_ty` again.
 168     fn normalize_projection_type(
 169         &mut self,
 170         infcx: &InferCtxt<'_, 'tcx>,
 171         param_env: ty::ParamEnv<'tcx>,
 172         projection_ty: ty::ProjectionTy<'tcx>,
 173         cause: ObligationCause<'tcx>,
 174     ) -> Ty<'tcx> {
 175         debug!("normalize_projection_type(projection_ty={:?})", projection_ty);
 176
 177         debug_assert!(!projection_ty.has_escaping_bound_vars());
 178
 179         // FIXME(#20304) -- cache
 180
 181         let mut selcx = SelectionContext::new(infcx);
 182         let mut obligations = vec![];
 183         let normalized_ty = project::normalize_projection_type(
 184             &mut selcx,
 185             param_env,
 186             projection_ty,
 187             cause,
 188             0,
 189             &mut obligations,
 190         );
 191         self.register_predicate_obligations(infcx, obligations);
 192
 193         debug!("normalize_projection_type: result={:?}", normalized_ty);
 194
 195         normalized_ty
 196     }
 197
 198     fn register_predicate_obligation(
 199         &mut self,
 200         infcx: &InferCtxt<'_, 'tcx>,
 201         obligation: PredicateObligation<'tcx>,
 202     ) {
 203         // this helps to reduce duplicate errors, as well as making
 204         // debug output much nicer to read and so on.
 205         let obligation = infcx.resolve_vars_if_possible(&obligation);
 206
 207         debug!("register_predicate_obligation(obligation={:?})", obligation);
 208
 209         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 210
 211         self.predicates
 212             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 213     }
 214
 215     fn select_all_or_error(
 216         &mut self,
 217         infcx: &InferCtxt<'_, 'tcx>,
 218     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 219         self.select_where_possible(infcx)?;
 220
 221         let errors: Vec<_> = self
 222             .predicates
 223             .to_errors(CodeAmbiguity)
 224             .into_iter()
 225             .map(to_fulfillment_error)
 226             .collect();
 227         if errors.is_empty() { Ok(()) } else { Err(errors) }
 228     }
 229
 230     fn select_where_possible(
 231         &mut self,
 232         infcx: &InferCtxt<'_, 'tcx>,
 233     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 234         let mut selcx = SelectionContext::new(infcx);
 235         self.select(&mut selcx)
 236     }
 237
 238     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 239         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 240     }
 241 }
 242
 243 struct FulfillProcessor<'a, 'b, 'tcx> {
 244     selcx: &'a mut SelectionContext<'b, 'tcx>,
 245     register_region_obligations: bool,
 246 }
 247
 248 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 249     os.into_iter()
 250         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 251         .collect()
 252 }
 253
 254 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 255     type Obligation = PendingPredicateObligation<'tcx>;
 256     type Error = FulfillmentErrorCode<'tcx>;
 257
 258     /// Processes a predicate obligation and returns either:
 259     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 260     /// - `Unchanged` if we don't have enough info to be sure
 261     /// - `Error(e)` if the predicate does not hold
 262     ///
 263     /// This is always inlined, despite its size, because it has a single
 264     /// callsite and it is called *very* frequently.
 265     #[inline(always)]
 266     fn process_obligation(
 267         &mut self,
 268         pending_obligation: &mut Self::Obligation,
 269     ) -> ProcessResult<Self::Obligation, Self::Error> {
 270         // If we were stalled on some unresolved variables, first check whether
 271         // any of them have been resolved; if not, don't bother doing more work
 272         // yet.
 273         let change = match pending_obligation.stalled_on.len() {
 274             // Match arms are in order of frequency, which matters because this
 275             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 276             1 => {
 277                 let infer_var = pending_obligation.stalled_on[0];
 278                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 279             }
 280             0 => {
 281                 // In this case we haven't changed, but wish to make a change.
 282                 true
 283             }
 284             _ => {
 285                 // This `for` loop was once a call to `all()`, but this lower-level
 286                 // form was a perf win. See #64545 for details.
 287                 (|| {
 288                     for &infer_var in &pending_obligation.stalled_on {
 289                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 290                             return true;
 291                         }
 292                     }
 293                     false
 294                 })()
 295             }
 296         };
 297
 298         if !change {
 299             debug!(
 300                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 301                 self.selcx.infcx().resolve_vars_if_possible(&pending_obligation.obligation),
 302                 pending_obligation.stalled_on
 303             );
 304             return ProcessResult::Unchanged;
 305         }
 306
 307         // This part of the code is much colder.
 308
 309         pending_obligation.stalled_on.truncate(0);
 310
 311         let obligation = &mut pending_obligation.obligation;
 312
 313         if obligation.predicate.has_infer_types_or_consts() {
 314             obligation.predicate =
 315                 self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
 316         }
 317
 318         debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
 319
 320         let infcx = self.selcx.infcx();
 321
 322         match obligation.predicate.kind() {
 323             ty::PredicateKind::ForAll(binder) => match binder.skip_binder().kind() {
 324                 ty::PredicateKind::ForAll(_) => bug!("unexpected forall"),
 325                 // Evaluation will discard candidates using the leak check.
 326                 // This means we need to pass it the bound version of our
 327                 // predicate.
 328                 &ty::PredicateKind::Atom(atom) => match atom {
 329                     ty::PredicateAtom::Trait(trait_ref, _constness) => {
 330                         let trait_obligation = obligation.with(Binder::bind(trait_ref));
 331
 332                         self.process_trait_obligation(
 333                             obligation,
 334                             trait_obligation,
 335                             &mut pending_obligation.stalled_on,
 336                         )
 337                     }
 338                     ty::PredicateAtom::Projection(projection) => {
 339                         let project_obligation = obligation.with(Binder::bind(projection));
 340
 341                         self.process_projection_obligation(
 342                             project_obligation,
 343                             &mut pending_obligation.stalled_on,
 344                         )
 345                     }
 346                     ty::PredicateAtom::RegionOutlives(_)
 347                     | ty::PredicateAtom::TypeOutlives(_)
 348                     | ty::PredicateAtom::WellFormed(_)
 349                     | ty::PredicateAtom::ObjectSafe(_)
 350                     | ty::PredicateAtom::ClosureKind(..)
 351                     | ty::PredicateAtom::Subtype(_)
 352                     | ty::PredicateAtom::ConstEvaluatable(..)
 353                     | ty::PredicateAtom::ConstEquate(..) => {
 354                         let (pred, _) = infcx.replace_bound_vars_with_placeholders(binder);
 355                         ProcessResult::Changed(mk_pending(vec![obligation.with(pred)]))
 356                     }
 357                 },
 358             },
 359             &ty::PredicateKind::Atom(atom) => match atom {
 360                 ty::PredicateAtom::Trait(ref data, _) => {
 361                     let trait_obligation = obligation.with(Binder::dummy(*data));
 362
 363                     self.process_trait_obligation(
 364                         obligation,
 365                         trait_obligation,
 366                         &mut pending_obligation.stalled_on,
 367                     )
 368                 }
 369
 370                 ty::PredicateAtom::RegionOutlives(data) => {
 371                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 372                         Ok(()) => ProcessResult::Changed(vec![]),
 373                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 374                     }
 375                 }
 376
 377                 ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 378                     if self.register_region_obligations {
 379                         self.selcx.infcx().register_region_obligation_with_cause(
 380                             t_a,
 381                             r_b,
 382                             &obligation.cause,
 383                         );
 384                     }
 385                     ProcessResult::Changed(vec![])
 386                 }
 387
 388                 ty::PredicateAtom::Projection(ref data) => {
 389                     let project_obligation = obligation.with(Binder::dummy(*data));
 390
 391                     self.process_projection_obligation(
 392                         project_obligation,
 393                         &mut pending_obligation.stalled_on,
 394                     )
 395                 }
 396
 397                 ty::PredicateAtom::ObjectSafe(trait_def_id) => {
 398                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 399                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 400                     } else {
 401                         ProcessResult::Changed(vec![])
 402                     }
 403                 }
 404
 405                 ty::PredicateAtom::ClosureKind(_, closure_substs, kind) => {
 406                     match self.selcx.infcx().closure_kind(closure_substs) {
 407                         Some(closure_kind) => {
 408                             if closure_kind.extends(kind) {
 409                                 ProcessResult::Changed(vec![])
 410                             } else {
 411                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 412                             }
 413                         }
 414                         None => ProcessResult::Unchanged,
 415                     }
 416                 }
 417
 418                 ty::PredicateAtom::WellFormed(arg) => {
 419                     match wf::obligations(
 420                         self.selcx.infcx(),
 421                         obligation.param_env,
 422                         obligation.cause.body_id,
 423                         arg,
 424                         obligation.cause.span,
 425                     ) {
 426                         None => {
 427                             pending_obligation.stalled_on =
 428                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 429                             ProcessResult::Unchanged
 430                         }
 431                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 432                     }
 433                 }
 434
 435                 ty::PredicateAtom::Subtype(subtype) => {
 436                     match self.selcx.infcx().subtype_predicate(
 437                         &obligation.cause,
 438                         obligation.param_env,
 439                         Binder::dummy(subtype),
 440                     ) {
 441                         None => {
 442                             // None means that both are unresolved.
 443                             pending_obligation.stalled_on = vec![
 444                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 445                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 446                             ];
 447                             ProcessResult::Unchanged
 448                         }
 449                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 450                         Some(Err(err)) => {
 451                             let expected_found =
 452                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 453                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 454                                 expected_found,
 455                                 err,
 456                             ))
 457                         }
 458                     }
 459                 }
 460
 461                 ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
 462                     match self.selcx.infcx().const_eval_resolve(
 463                         obligation.param_env,
 464                         def_id,
 465                         substs,
 466                         None,
 467                         Some(obligation.cause.span),
 468                     ) {
 469                         Ok(_) => ProcessResult::Changed(vec![]),
 470                         Err(err) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err))),
 471                     }
 472                 }
 473
 474             ty::PredicateAtom::ConstEquate(c1, c2) => {
 475                 debug!("equating consts: c1={:?} c2={:?}", c1, c2);
 476
 477                 let stalled_on = &mut pending_obligation.stalled_on;
 478
 479                 let mut evaluate = |c: &'tcx Const<'tcx>| {
 480                     if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
 481                         match self.selcx.infcx().const_eval_resolve(
 482                             obligation.param_env,
 483                             def,
 484                             substs,
 485                             promoted,
 486                             Some(obligation.cause.span),
 487                         ) {
 488                             Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 489                             Err(ErrorHandled::TooGeneric) => {
 490                                 stalled_on.append(
 491                                     &mut substs
 492                                         .types()
 493                                         .filter_map(|ty| TyOrConstInferVar::maybe_from_ty(ty))
 494                                         .collect(),
 495                                 );
 496                                 Err(ErrorHandled::TooGeneric)
 497                             }
 498                         } else {
 499                             Ok(c)
 500                         }
 501                     };
 502
 503                     match (evaluate(c1), evaluate(c2)) {
 504                         (Ok(c1), Ok(c2)) => {
 505                             match self
 506                                 .selcx
 507                                 .infcx()
 508                                 .at(&obligation.cause, obligation.param_env)
 509                                 .eq(c1, c2)
 510                             {
 511                                 Ok(_) => ProcessResult::Changed(vec![]),
 512                                 Err(err) => ProcessResult::Error(
 513                                     FulfillmentErrorCode::CodeConstEquateError(
 514                                         ExpectedFound::new(true, c1, c2),
 515                                         err,
 516                                     ),
 517                                 ),
 518                             }
 519                         }
 520                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 521                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => {
 522                             ProcessResult::Error(CodeSelectionError(ConstEvalFailure(
 523                                 ErrorHandled::Reported(ErrorReported),
 524                             )))
 525                         }
 526                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 527                             span_bug!(
 528                                 obligation.cause.span(self.selcx.tcx()),
 529                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 530                             )
 531                         }
 532                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 533                             ProcessResult::Unchanged
 534                         }
 535                     }
 536                 }
 537             },
 538         }
 539     }
 540
 541     fn process_backedge<'c, I>(
 542         &mut self,
 543         cycle: I,
 544         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 545     ) where
 546         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 547     {
 548         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 549             debug!("process_child_obligations: coinductive match");
 550         } else {
 551             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 552             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 553         }
 554     }
 555 }
 556
 557 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 558     fn process_trait_obligation(
 559         &mut self,
 560         obligation: &PredicateObligation<'tcx>,
 561         trait_obligation: PolyTraitObligation<'tcx>,
 562         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 563     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 564         let infcx = self.selcx.infcx();
 565         if obligation.predicate.is_global() {
 566             // no type variables present, can use evaluation for better caching.
 567             // FIXME: consider caching errors too.
 568             if infcx.predicate_must_hold_considering_regions(obligation) {
 569                 debug!(
 570                     "selecting trait `{:?}` at depth {} evaluated to holds",
 571                     obligation.predicate, obligation.recursion_depth
 572                 );
 573                 return ProcessResult::Changed(vec![]);
 574             }
 575         }
 576
 577         match self.selcx.select(&trait_obligation) {
 578             Ok(Some(impl_source)) => {
 579                 debug!(
 580                     "selecting trait `{:?}` at depth {} yielded Ok(Some)",
 581                     trait_obligation.predicate, obligation.recursion_depth
 582                 );
 583                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 584             }
 585             Ok(None) => {
 586                 debug!(
 587                     "selecting trait `{:?}` at depth {} yielded Ok(None)",
 588                     trait_obligation.predicate, obligation.recursion_depth
 589                 );
 590
 591                 // This is a bit subtle: for the most part, the
 592                 // only reason we can fail to make progress on
 593                 // trait selection is because we don't have enough
 594                 // information about the types in the trait.
 595                 *stalled_on = trait_ref_infer_vars(
 596                     self.selcx,
 597                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref),
 598                 );
 599
 600                 debug!(
 601                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 602                     infcx.resolve_vars_if_possible(obligation),
 603                     stalled_on
 604                 );
 605
 606                 ProcessResult::Unchanged
 607             }
 608             Err(selection_err) => {
 609                 info!(
 610                     "selecting trait `{:?}` at depth {} yielded Err",
 611                     trait_obligation.predicate, obligation.recursion_depth
 612                 );
 613
 614                 ProcessResult::Error(CodeSelectionError(selection_err))
 615             }
 616         }
 617     }
 618
 619     fn process_projection_obligation(
 620         &mut self,
 621         project_obligation: PolyProjectionObligation<'tcx>,
 622         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 623     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 624         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 625             Ok(None) => {
 626                 *stalled_on = trait_ref_infer_vars(
 627                     self.selcx,
 628                     project_obligation.predicate.to_poly_trait_ref(self.selcx.tcx()),
 629                 );
 630                 ProcessResult::Unchanged
 631             }
 632             Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
 633             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 634         }
 635     }
 636 }
 637
 638 /// Returns the set of inference variables contained in a trait ref.
 639 fn trait_ref_infer_vars<'a, 'tcx>(
 640     selcx: &mut SelectionContext<'a, 'tcx>,
 641     trait_ref: ty::PolyTraitRef<'tcx>,
 642 ) -> Vec<TyOrConstInferVar<'tcx>> {
 643     selcx
 644         .infcx()
 645         .resolve_vars_if_possible(&trait_ref)
 646         .skip_binder()
 647         .substs
 648         .iter()
 649         // FIXME(eddyb) try using `skip_current_subtree` to skip everything that
 650         // doesn't contain inference variables, not just the outermost level.
 651         .filter(|arg| arg.has_infer_types_or_consts())
 652         .flat_map(|arg| arg.walk())
 653         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 654         .collect()
 655 }
 656
 657 fn to_fulfillment_error<'tcx>(
 658     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 659 ) -> FulfillmentError<'tcx> {
 660     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 661     FulfillmentError::new(obligation, error.error)
 662 }