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 _, TraitObligation};
   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() {
 324                 // Evaluation will discard candidates using the leak check.
 325                 // This means we need to pass it the bound version of our
 326                 // predicate.
 327                 ty::PredicateAtom::Trait(trait_ref, _constness) => {
 328                     let trait_obligation = obligation.with(Binder::bind(trait_ref));
 329
 330                     self.process_trait_obligation(
 331                         obligation,
 332                         trait_obligation,
 333                         &mut pending_obligation.stalled_on,
 334                     )
 335                 }
 336                 ty::PredicateAtom::Projection(data) => {
 337                     let project_obligation = obligation.with(Binder::bind(data));
 338
 339                     self.process_projection_obligation(
 340                         project_obligation,
 341                         &mut pending_obligation.stalled_on,
 342                     )
 343                 }
 344                 ty::PredicateAtom::RegionOutlives(_)
 345                 | ty::PredicateAtom::TypeOutlives(_)
 346                 | ty::PredicateAtom::WellFormed(_)
 347                 | ty::PredicateAtom::ObjectSafe(_)
 348                 | ty::PredicateAtom::ClosureKind(..)
 349                 | ty::PredicateAtom::Subtype(_)
 350                 | ty::PredicateAtom::ConstEvaluatable(..)
 351                 | ty::PredicateAtom::ConstEquate(..) => {
 352                     let (pred, _) = infcx.replace_bound_vars_with_placeholders(binder);
 353                     ProcessResult::Changed(mk_pending(vec![
 354                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 355                     ]))
 356                 }
 357             },
 358             &ty::PredicateKind::Atom(atom) => match atom {
 359                 ty::PredicateAtom::Trait(ref data, _) => {
 360                     let trait_obligation = obligation.with(Binder::dummy(*data));
 361
 362                     self.process_trait_obligation(
 363                         obligation,
 364                         trait_obligation,
 365                         &mut pending_obligation.stalled_on,
 366                     )
 367                 }
 368
 369                 ty::PredicateAtom::RegionOutlives(data) => {
 370                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 371                         Ok(()) => ProcessResult::Changed(vec![]),
 372                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 373                     }
 374                 }
 375
 376                 ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 377                     if self.register_region_obligations {
 378                         self.selcx.infcx().register_region_obligation_with_cause(
 379                             t_a,
 380                             r_b,
 381                             &obligation.cause,
 382                         );
 383                     }
 384                     ProcessResult::Changed(vec![])
 385                 }
 386
 387                 ty::PredicateAtom::Projection(ref data) => {
 388                     let project_obligation = obligation.with(Binder::dummy(*data));
 389
 390                     self.process_projection_obligation(
 391                         project_obligation,
 392                         &mut pending_obligation.stalled_on,
 393                     )
 394                 }
 395
 396                 ty::PredicateAtom::ObjectSafe(trait_def_id) => {
 397                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 398                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 399                     } else {
 400                         ProcessResult::Changed(vec![])
 401                     }
 402                 }
 403
 404                 ty::PredicateAtom::ClosureKind(_, closure_substs, kind) => {
 405                     match self.selcx.infcx().closure_kind(closure_substs) {
 406                         Some(closure_kind) => {
 407                             if closure_kind.extends(kind) {
 408                                 ProcessResult::Changed(vec![])
 409                             } else {
 410                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 411                             }
 412                         }
 413                         None => ProcessResult::Unchanged,
 414                     }
 415                 }
 416
 417                 ty::PredicateAtom::WellFormed(arg) => {
 418                     match wf::obligations(
 419                         self.selcx.infcx(),
 420                         obligation.param_env,
 421                         obligation.cause.body_id,
 422                         arg,
 423                         obligation.cause.span,
 424                     ) {
 425                         None => {
 426                             pending_obligation.stalled_on =
 427                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 428                             ProcessResult::Unchanged
 429                         }
 430                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 431                     }
 432                 }
 433
 434                 ty::PredicateAtom::Subtype(subtype) => {
 435                     match self.selcx.infcx().subtype_predicate(
 436                         &obligation.cause,
 437                         obligation.param_env,
 438                         Binder::dummy(subtype),
 439                     ) {
 440                         None => {
 441                             // None means that both are unresolved.
 442                             pending_obligation.stalled_on = vec![
 443                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 444                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 445                             ];
 446                             ProcessResult::Unchanged
 447                         }
 448                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 449                         Some(Err(err)) => {
 450                             let expected_found =
 451                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 452                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 453                                 expected_found,
 454                                 err,
 455                             ))
 456                         }
 457                     }
 458                 }
 459
 460                 ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
 461                     match self.selcx.infcx().const_eval_resolve(
 462                         obligation.param_env,
 463                         def_id,
 464                         substs,
 465                         None,
 466                         Some(obligation.cause.span),
 467                     ) {
 468                         Ok(_) => ProcessResult::Changed(vec![]),
 469                         Err(err) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err))),
 470                     }
 471                 }
 472
 473                 ty::PredicateAtom::ConstEquate(c1, c2) => {
 474                     debug!("equating consts: c1={:?} c2={:?}", c1, c2);
 475
 476                     let stalled_on = &mut pending_obligation.stalled_on;
 477
 478                     let mut evaluate = |c: &'tcx Const<'tcx>| {
 479                         if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
 480                             match self.selcx.infcx().const_eval_resolve(
 481                                 obligation.param_env,
 482                                 def,
 483                                 substs,
 484                                 promoted,
 485                                 Some(obligation.cause.span),
 486                             ) {
 487                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 488                                 Err(ErrorHandled::TooGeneric) => {
 489                                     stalled_on.append(
 490                                         &mut substs
 491                                             .types()
 492                                             .filter_map(|ty| TyOrConstInferVar::maybe_from_ty(ty))
 493                                             .collect(),
 494                                     );
 495                                     Err(ErrorHandled::TooGeneric)
 496                                 }
 497                                 Err(err) => Err(err),
 498                             }
 499                         } else {
 500                             Ok(c)
 501                         }
 502                     };
 503
 504                     match (evaluate(c1), evaluate(c2)) {
 505                         (Ok(c1), Ok(c2)) => {
 506                             match self
 507                                 .selcx
 508                                 .infcx()
 509                                 .at(&obligation.cause, obligation.param_env)
 510                                 .eq(c1, c2)
 511                             {
 512                                 Ok(_) => ProcessResult::Changed(vec![]),
 513                                 Err(err) => ProcessResult::Error(
 514                                     FulfillmentErrorCode::CodeConstEquateError(
 515                                         ExpectedFound::new(true, c1, c2),
 516                                         err,
 517                                     ),
 518                                 ),
 519                             }
 520                         }
 521                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 522                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => {
 523                             ProcessResult::Error(CodeSelectionError(ConstEvalFailure(
 524                                 ErrorHandled::Reported(ErrorReported),
 525                             )))
 526                         }
 527                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 528                             span_bug!(
 529                                 obligation.cause.span(self.selcx.tcx()),
 530                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 531                             )
 532                         }
 533                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 534                             ProcessResult::Unchanged
 535                         }
 536                     }
 537                 }
 538             },
 539         }
 540     }
 541
 542     fn process_backedge<'c, I>(
 543         &mut self,
 544         cycle: I,
 545         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 546     ) where
 547         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 548     {
 549         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 550             debug!("process_child_obligations: coinductive match");
 551         } else {
 552             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 553             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 554         }
 555     }
 556 }
 557
 558 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 559     fn process_trait_obligation(
 560         &mut self,
 561         obligation: &PredicateObligation<'tcx>,
 562         trait_obligation: TraitObligation<'tcx>,
 563         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 564     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 565         let infcx = self.selcx.infcx();
 566         if obligation.predicate.is_global() {
 567             // no type variables present, can use evaluation for better caching.
 568             // FIXME: consider caching errors too.
 569             if infcx.predicate_must_hold_considering_regions(obligation) {
 570                 debug!(
 571                     "selecting trait `{:?}` at depth {} evaluated to holds",
 572                     obligation.predicate, obligation.recursion_depth
 573                 );
 574                 return ProcessResult::Changed(vec![]);
 575             }
 576         }
 577
 578         match self.selcx.select(&trait_obligation) {
 579             Ok(Some(impl_source)) => {
 580                 debug!(
 581                     "selecting trait `{:?}` at depth {} yielded Ok(Some)",
 582                     trait_obligation.predicate, obligation.recursion_depth
 583                 );
 584                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 585             }
 586             Ok(None) => {
 587                 debug!(
 588                     "selecting trait `{:?}` at depth {} yielded Ok(None)",
 589                     trait_obligation.predicate, obligation.recursion_depth
 590                 );
 591
 592                 // This is a bit subtle: for the most part, the
 593                 // only reason we can fail to make progress on
 594                 // trait selection is because we don't have enough
 595                 // information about the types in the trait.
 596                 *stalled_on = trait_ref_infer_vars(
 597                     self.selcx,
 598                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref),
 599                 );
 600
 601                 debug!(
 602                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 603                     infcx.resolve_vars_if_possible(obligation),
 604                     stalled_on
 605                 );
 606
 607                 ProcessResult::Unchanged
 608             }
 609             Err(selection_err) => {
 610                 info!(
 611                     "selecting trait `{:?}` at depth {} yielded Err",
 612                     trait_obligation.predicate, obligation.recursion_depth
 613                 );
 614
 615                 ProcessResult::Error(CodeSelectionError(selection_err))
 616             }
 617         }
 618     }
 619
 620     fn process_projection_obligation(
 621         &mut self,
 622         project_obligation: PolyProjectionObligation<'tcx>,
 623         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 624     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 625         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 626             Ok(None) => {
 627                 *stalled_on = trait_ref_infer_vars(
 628                     self.selcx,
 629                     project_obligation.predicate.to_poly_trait_ref(self.selcx.tcx()),
 630                 );
 631                 ProcessResult::Unchanged
 632             }
 633             Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
 634             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 635         }
 636     }
 637 }
 638
 639 /// Returns the set of inference variables contained in a trait ref.
 640 fn trait_ref_infer_vars<'a, 'tcx>(
 641     selcx: &mut SelectionContext<'a, 'tcx>,
 642     trait_ref: ty::PolyTraitRef<'tcx>,
 643 ) -> Vec<TyOrConstInferVar<'tcx>> {
 644     selcx
 645         .infcx()
 646         .resolve_vars_if_possible(&trait_ref)
 647         .skip_binder()
 648         .substs
 649         .iter()
 650         // FIXME(eddyb) try using `skip_current_subtree` to skip everything that
 651         // doesn't contain inference variables, not just the outermost level.
 652         .filter(|arg| arg.has_infer_types_or_consts())
 653         .flat_map(|arg| arg.walk())
 654         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 655         .collect()
 656 }
 657
 658 fn to_fulfillment_error<'tcx>(
 659     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 660 ) -> FulfillmentError<'tcx> {
 661     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 662     FulfillmentError::new(obligation, error.error)
 663 }