compiler/rustc_trait_selection/src/traits/fulfill.rs

   1 use crate::infer::{InferCtxt, TyOrConstInferVar};
   2 use rustc_data_structures::obligation_forest::ProcessResult;
   3 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
   4 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
   5 use rustc_infer::traits::ProjectionCacheKey;
   6 use rustc_infer::traits::{SelectionError, TraitEngine, TraitObligation};
   7 use rustc_middle::mir::interpret::ErrorHandled;
   8 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
   9 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  10 use rustc_middle::ty::subst::SubstsRef;
  11 use rustc_middle::ty::{self, Binder, Const, TypeVisitable};
  12 use std::marker::PhantomData;
  13
  14 use super::const_evaluatable;
  15 use super::project::{self, ProjectAndUnifyResult};
  16 use super::select::SelectionContext;
  17 use super::wf;
  18 use super::CodeAmbiguity;
  19 use super::CodeProjectionError;
  20 use super::CodeSelectionError;
  21 use super::EvaluationResult;
  22 use super::PredicateObligation;
  23 use super::Unimplemented;
  24 use super::{FulfillmentError, FulfillmentErrorCode};
  25
  26 use crate::traits::project::PolyProjectionObligation;
  27 use crate::traits::project::ProjectionCacheKeyExt as _;
  28 use crate::traits::query::evaluate_obligation::InferCtxtExt;
  29
  30 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  31     /// Note that we include both the `ParamEnv` and the `Predicate`,
  32     /// as the `ParamEnv` can influence whether fulfillment succeeds
  33     /// or fails.
  34     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  35
  36     fn as_cache_key(&self) -> Self::CacheKey {
  37         self.obligation.param_env.and(self.obligation.predicate)
  38     }
  39 }
  40
  41 /// The fulfillment context is used to drive trait resolution. It
  42 /// consists of a list of obligations that must be (eventually)
  43 /// satisfied. The job is to track which are satisfied, which yielded
  44 /// errors, and which are still pending. At any point, users can call
  45 /// `select_where_possible`, and the fulfillment context will try to do
  46 /// selection, retaining only those obligations that remain
  47 /// ambiguous. This may be helpful in pushing type inference
  48 /// along. Once all type inference constraints have been generated, the
  49 /// method `select_all_or_error` can be used to report any remaining
  50 /// ambiguous cases as errors.
  51 pub struct FulfillmentContext<'tcx> {
  52     // A list of all obligations that have been registered with this
  53     // fulfillment context.
  54     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  55
  56     // Is it OK to register obligations into this infcx inside
  57     // an infcx snapshot?
  58     //
  59     // The "primary fulfillment" in many cases in typeck lives
  60     // outside of any snapshot, so any use of it inside a snapshot
  61     // will lead to trouble and therefore is checked against, but
  62     // other fulfillment contexts sometimes do live inside of
  63     // a snapshot (they don't *straddle* a snapshot, so there
  64     // is no trouble there).
  65     usable_in_snapshot: bool,
  66 }
  67
  68 #[derive(Clone, Debug)]
  69 pub struct PendingPredicateObligation<'tcx> {
  70     pub obligation: PredicateObligation<'tcx>,
  71     // This is far more often read than modified, meaning that we
  72     // should mostly optimize for reading speed, while modifying is not as relevant.
  73     //
  74     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  75     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  76 }
  77
  78 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  79 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  80 static_assert_size!(PendingPredicateObligation<'_>, 72);
  81
  82 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  83     /// Creates a new fulfillment context.
  84     pub(super) fn new() -> FulfillmentContext<'tcx> {
  85         FulfillmentContext { predicates: ObligationForest::new(), usable_in_snapshot: false }
  86     }
  87
  88     pub(super) fn new_in_snapshot() -> FulfillmentContext<'tcx> {
  89         FulfillmentContext { predicates: ObligationForest::new(), usable_in_snapshot: true }
  90     }
  91
  92     /// Attempts to select obligations using `selcx`.
  93     fn select(&mut self, selcx: SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
  94         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
  95         let _enter = span.enter();
  96
  97         // Process pending obligations.
  98         let outcome: Outcome<_, _> =
  99             self.predicates.process_obligations(&mut FulfillProcessor { selcx });
 100
 101         // FIXME: if we kept the original cache key, we could mark projection
 102         // obligations as complete for the projection cache here.
 103
 104         let errors: Vec<FulfillmentError<'tcx>> =
 105             outcome.errors.into_iter().map(to_fulfillment_error).collect();
 106
 107         debug!(
 108             "select({} predicates remaining, {} errors) done",
 109             self.predicates.len(),
 110             errors.len()
 111         );
 112
 113         errors
 114     }
 115 }
 116
 117 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 118     fn register_predicate_obligation(
 119         &mut self,
 120         infcx: &InferCtxt<'tcx>,
 121         obligation: PredicateObligation<'tcx>,
 122     ) {
 123         // this helps to reduce duplicate errors, as well as making
 124         // debug output much nicer to read and so on.
 125         let obligation = infcx.resolve_vars_if_possible(obligation);
 126
 127         debug!(?obligation, "register_predicate_obligation");
 128
 129         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 130
 131         self.predicates
 132             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 133     }
 134
 135     fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
 136         {
 137             let errors = self.select_where_possible(infcx);
 138             if !errors.is_empty() {
 139                 return errors;
 140             }
 141         }
 142
 143         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 144     }
 145
 146     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
 147         let selcx = SelectionContext::new(infcx);
 148         self.select(selcx)
 149     }
 150
 151     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 152         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 153     }
 154 }
 155
 156 struct FulfillProcessor<'a, 'tcx> {
 157     selcx: SelectionContext<'a, 'tcx>,
 158 }
 159
 160 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 161     os.into_iter()
 162         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 163         .collect()
 164 }
 165
 166 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 167     type Obligation = PendingPredicateObligation<'tcx>;
 168     type Error = FulfillmentErrorCode<'tcx>;
 169     type OUT = Outcome<Self::Obligation, Self::Error>;
 170
 171     /// Identifies whether a predicate obligation needs processing.
 172     ///
 173     /// This is always inlined, despite its size, because it has a single
 174     /// callsite and it is called *very* frequently.
 175     #[inline(always)]
 176     fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 177         // If we were stalled on some unresolved variables, first check whether
 178         // any of them have been resolved; if not, don't bother doing more work
 179         // yet.
 180         match pending_obligation.stalled_on.len() {
 181             // Match arms are in order of frequency, which matters because this
 182             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 183             1 => {
 184                 let infer_var = pending_obligation.stalled_on[0];
 185                 self.selcx.infcx.ty_or_const_infer_var_changed(infer_var)
 186             }
 187             0 => {
 188                 // In this case we haven't changed, but wish to make a change.
 189                 true
 190             }
 191             _ => {
 192                 // This `for` loop was once a call to `all()`, but this lower-level
 193                 // form was a perf win. See #64545 for details.
 194                 (|| {
 195                     for &infer_var in &pending_obligation.stalled_on {
 196                         if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
 197                             return true;
 198                         }
 199                     }
 200                     false
 201                 })()
 202             }
 203         }
 204     }
 205
 206     /// Processes a predicate obligation and returns either:
 207     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 208     /// - `Unchanged` if we don't have enough info to be sure
 209     /// - `Error(e)` if the predicate does not hold
 210     ///
 211     /// This is called much less often than `needs_process_obligation`, so we
 212     /// never inline it.
 213     #[inline(never)]
 214     #[instrument(level = "debug", skip(self, pending_obligation))]
 215     fn process_obligation(
 216         &mut self,
 217         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 218     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 219         pending_obligation.stalled_on.truncate(0);
 220
 221         let obligation = &mut pending_obligation.obligation;
 222
 223         debug!(?obligation, "pre-resolve");
 224
 225         if obligation.predicate.has_non_region_infer() {
 226             obligation.predicate = self.selcx.infcx.resolve_vars_if_possible(obligation.predicate);
 227         }
 228
 229         let obligation = &pending_obligation.obligation;
 230
 231         let infcx = self.selcx.infcx;
 232
 233         if obligation.predicate.has_projections() {
 234             let mut obligations = Vec::new();
 235             let predicate = crate::traits::project::try_normalize_with_depth_to(
 236                 &mut self.selcx,
 237                 obligation.param_env,
 238                 obligation.cause.clone(),
 239                 obligation.recursion_depth + 1,
 240                 obligation.predicate,
 241                 &mut obligations,
 242             );
 243             if predicate != obligation.predicate {
 244                 obligations.push(obligation.with(infcx.tcx, predicate));
 245                 return ProcessResult::Changed(mk_pending(obligations));
 246             }
 247         }
 248         let binder = obligation.predicate.kind();
 249         match binder.no_bound_vars() {
 250             None => match binder.skip_binder() {
 251                 // Evaluation will discard candidates using the leak check.
 252                 // This means we need to pass it the bound version of our
 253                 // predicate.
 254                 ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => {
 255                     let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
 256
 257                     self.process_trait_obligation(
 258                         obligation,
 259                         trait_obligation,
 260                         &mut pending_obligation.stalled_on,
 261                     )
 262                 }
 263                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 264                     let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
 265
 266                     self.process_projection_obligation(
 267                         obligation,
 268                         project_obligation,
 269                         &mut pending_obligation.stalled_on,
 270                     )
 271                 }
 272                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
 273                 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
 274                 | ty::PredicateKind::WellFormed(_)
 275                 | ty::PredicateKind::ObjectSafe(_)
 276                 | ty::PredicateKind::ClosureKind(..)
 277                 | ty::PredicateKind::Subtype(_)
 278                 | ty::PredicateKind::Coerce(_)
 279                 | ty::PredicateKind::ConstEvaluatable(..)
 280                 | ty::PredicateKind::ConstEquate(..) => {
 281                     let pred =
 282                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 283                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
 284                 }
 285                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 286                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 287                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 288                 }
 289             },
 290             Some(pred) => match pred {
 291                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
 292                     let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
 293
 294                     self.process_trait_obligation(
 295                         obligation,
 296                         trait_obligation,
 297                         &mut pending_obligation.stalled_on,
 298                     )
 299                 }
 300
 301                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => {
 302                     if infcx.considering_regions {
 303                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 304                     }
 305
 306                     ProcessResult::Changed(vec![])
 307                 }
 308
 309                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 310                     t_a,
 311                     r_b,
 312                 ))) => {
 313                     if infcx.considering_regions {
 314                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 315                     }
 316                     ProcessResult::Changed(vec![])
 317                 }
 318
 319                 ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
 320                     let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
 321
 322                     self.process_projection_obligation(
 323                         obligation,
 324                         project_obligation,
 325                         &mut pending_obligation.stalled_on,
 326                     )
 327                 }
 328
 329                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 330                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 331                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 332                     } else {
 333                         ProcessResult::Changed(vec![])
 334                     }
 335                 }
 336
 337                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 338                     match self.selcx.infcx.closure_kind(closure_substs) {
 339                         Some(closure_kind) => {
 340                             if closure_kind.extends(kind) {
 341                                 ProcessResult::Changed(vec![])
 342                             } else {
 343                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 344                             }
 345                         }
 346                         None => ProcessResult::Unchanged,
 347                     }
 348                 }
 349
 350                 ty::PredicateKind::WellFormed(arg) => {
 351                     match wf::obligations(
 352                         self.selcx.infcx,
 353                         obligation.param_env,
 354                         obligation.cause.body_id,
 355                         obligation.recursion_depth + 1,
 356                         arg,
 357                         obligation.cause.span,
 358                     ) {
 359                         None => {
 360                             pending_obligation.stalled_on =
 361                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 362                             ProcessResult::Unchanged
 363                         }
 364                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 365                     }
 366                 }
 367
 368                 ty::PredicateKind::Subtype(subtype) => {
 369                     match self.selcx.infcx.subtype_predicate(
 370                         &obligation.cause,
 371                         obligation.param_env,
 372                         Binder::dummy(subtype),
 373                     ) {
 374                         Err((a, b)) => {
 375                             // None means that both are unresolved.
 376                             pending_obligation.stalled_on =
 377                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 378                             ProcessResult::Unchanged
 379                         }
 380                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 381                         Ok(Err(err)) => {
 382                             let expected_found =
 383                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 384                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 385                                 expected_found,
 386                                 err,
 387                             ))
 388                         }
 389                     }
 390                 }
 391
 392                 ty::PredicateKind::Coerce(coerce) => {
 393                     match self.selcx.infcx.coerce_predicate(
 394                         &obligation.cause,
 395                         obligation.param_env,
 396                         Binder::dummy(coerce),
 397                     ) {
 398                         Err((a, b)) => {
 399                             // None means that both are unresolved.
 400                             pending_obligation.stalled_on =
 401                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 402                             ProcessResult::Unchanged
 403                         }
 404                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 405                         Ok(Err(err)) => {
 406                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 407                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 408                                 expected_found,
 409                                 err,
 410                             ))
 411                         }
 412                     }
 413                 }
 414
 415                 ty::PredicateKind::ConstEvaluatable(uv) => {
 416                     match const_evaluatable::is_const_evaluatable(
 417                         self.selcx.infcx,
 418                         uv,
 419                         obligation.param_env,
 420                         obligation.cause.span,
 421                     ) {
 422                         Ok(()) => ProcessResult::Changed(vec![]),
 423                         Err(NotConstEvaluatable::MentionsInfer) => {
 424                             pending_obligation.stalled_on.clear();
 425                             pending_obligation.stalled_on.extend(
 426                                 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 427                             );
 428                             ProcessResult::Unchanged
 429                         }
 430                         Err(
 431                             e @ NotConstEvaluatable::MentionsParam
 432                             | e @ NotConstEvaluatable::Error(_),
 433                         ) => ProcessResult::Error(CodeSelectionError(
 434                             SelectionError::NotConstEvaluatable(e),
 435                         )),
 436                     }
 437                 }
 438
 439                 ty::PredicateKind::ConstEquate(c1, c2) => {
 440                     let tcx = self.selcx.tcx();
 441                     assert!(
 442                         tcx.features().generic_const_exprs,
 443                         "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
 444                     );
 445                     // FIXME: we probably should only try to unify abstract constants
 446                     // if the constants depend on generic parameters.
 447                     //
 448                     // Let's just see where this breaks :shrug:
 449                     {
 450                         let c1 = tcx.expand_abstract_consts(c1);
 451                         let c2 = tcx.expand_abstract_consts(c2);
 452                         debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
 453
 454                         use rustc_hir::def::DefKind;
 455                         use ty::ConstKind::Unevaluated;
 456                         match (c1.kind(), c2.kind()) {
 457                             (Unevaluated(a), Unevaluated(b))
 458                                 if a.def.did == b.def.did
 459                                     && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
 460                             {
 461                                 if let Ok(new_obligations) = infcx
 462                                     .at(&obligation.cause, obligation.param_env)
 463                                     .trace(c1, c2)
 464                                     .eq(a.substs, b.substs)
 465                                 {
 466                                     return ProcessResult::Changed(mk_pending(
 467                                         new_obligations.into_obligations(),
 468                                     ));
 469                                 }
 470                             }
 471                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
 472                             (_, _) => {
 473                                 if let Ok(new_obligations) =
 474                                     infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
 475                                 {
 476                                     return ProcessResult::Changed(mk_pending(
 477                                         new_obligations.into_obligations(),
 478                                     ));
 479                                 }
 480                             }
 481                         }
 482                     }
 483
 484                     let stalled_on = &mut pending_obligation.stalled_on;
 485
 486                     let mut evaluate = |c: Const<'tcx>| {
 487                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 488                             match self.selcx.infcx.try_const_eval_resolve(
 489                                 obligation.param_env,
 490                                 unevaluated,
 491                                 c.ty(),
 492                                 Some(obligation.cause.span),
 493                             ) {
 494                                 Ok(val) => Ok(val),
 495                                 Err(e) => match e {
 496                                     ErrorHandled::TooGeneric => {
 497                                         stalled_on.extend(
 498                                             unevaluated.substs.iter().filter_map(
 499                                                 TyOrConstInferVar::maybe_from_generic_arg,
 500                                             ),
 501                                         );
 502                                         Err(ErrorHandled::TooGeneric)
 503                                     }
 504                                     _ => Err(e),
 505                                 },
 506                             }
 507                         } else {
 508                             Ok(c)
 509                         }
 510                     };
 511
 512                     match (evaluate(c1), evaluate(c2)) {
 513                         (Ok(c1), Ok(c2)) => {
 514                             match self
 515                                 .selcx
 516                                 .infcx
 517                                 .at(&obligation.cause, obligation.param_env)
 518                                 .eq(c1, c2)
 519                             {
 520                                 Ok(inf_ok) => {
 521                                     ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
 522                                 }
 523                                 Err(err) => ProcessResult::Error(
 524                                     FulfillmentErrorCode::CodeConstEquateError(
 525                                         ExpectedFound::new(true, c1, c2),
 526                                         err,
 527                                     ),
 528                                 ),
 529                             }
 530                         }
 531                         (Err(ErrorHandled::Reported(reported)), _)
 532                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 533                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 534                                 NotConstEvaluatable::Error(reported),
 535                             )),
 536                         ),
 537                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 538                             if c1.has_non_region_infer() || c2.has_non_region_infer() {
 539                                 ProcessResult::Unchanged
 540                             } else {
 541                                 // Two different constants using generic parameters ~> error.
 542                                 let expected_found = ExpectedFound::new(true, c1, c2);
 543                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 544                                     expected_found,
 545                                     TypeError::ConstMismatch(expected_found),
 546                                 ))
 547                             }
 548                         }
 549                     }
 550                 }
 551                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 552                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 553                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 554                 }
 555             },
 556         }
 557     }
 558
 559     #[inline(never)]
 560     fn process_backedge<'c, I>(
 561         &mut self,
 562         cycle: I,
 563         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 564     ) -> Result<(), FulfillmentErrorCode<'tcx>>
 565     where
 566         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 567     {
 568         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 569             debug!("process_child_obligations: coinductive match");
 570             Ok(())
 571         } else {
 572             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 573             Err(FulfillmentErrorCode::CodeCycle(cycle))
 574         }
 575     }
 576 }
 577
 578 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
 579     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 580     fn process_trait_obligation(
 581         &mut self,
 582         obligation: &PredicateObligation<'tcx>,
 583         trait_obligation: TraitObligation<'tcx>,
 584         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 585     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 586         let infcx = self.selcx.infcx;
 587         if obligation.predicate.is_global() {
 588             // no type variables present, can use evaluation for better caching.
 589             // FIXME: consider caching errors too.
 590             if infcx.predicate_must_hold_considering_regions(obligation) {
 591                 debug!(
 592                     "selecting trait at depth {} evaluated to holds",
 593                     obligation.recursion_depth
 594                 );
 595                 return ProcessResult::Changed(vec![]);
 596             }
 597         }
 598
 599         match self.selcx.select(&trait_obligation) {
 600             Ok(Some(impl_source)) => {
 601                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 602                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 603             }
 604             Ok(None) => {
 605                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 606
 607                 // This is a bit subtle: for the most part, the
 608                 // only reason we can fail to make progress on
 609                 // trait selection is because we don't have enough
 610                 // information about the types in the trait.
 611                 stalled_on.clear();
 612                 stalled_on.extend(substs_infer_vars(
 613                     &self.selcx,
 614                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 615                 ));
 616
 617                 debug!(
 618                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 619                     infcx.resolve_vars_if_possible(obligation.clone()),
 620                     stalled_on
 621                 );
 622
 623                 ProcessResult::Unchanged
 624             }
 625             Err(selection_err) => {
 626                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 627
 628                 ProcessResult::Error(CodeSelectionError(selection_err))
 629             }
 630         }
 631     }
 632
 633     fn process_projection_obligation(
 634         &mut self,
 635         obligation: &PredicateObligation<'tcx>,
 636         project_obligation: PolyProjectionObligation<'tcx>,
 637         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 638     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 639         let tcx = self.selcx.tcx();
 640
 641         if obligation.predicate.is_global() {
 642             // no type variables present, can use evaluation for better caching.
 643             // FIXME: consider caching errors too.
 644             if self.selcx.infcx.predicate_must_hold_considering_regions(obligation) {
 645                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 646                     &mut self.selcx,
 647                     project_obligation.predicate,
 648                 ) {
 649                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 650                     // evaluated all sub-obligations. We can therefore mark the 'root'
 651                     // obligation as complete, and skip evaluating sub-obligations.
 652                     self.selcx
 653                         .infcx
 654                         .inner
 655                         .borrow_mut()
 656                         .projection_cache()
 657                         .complete(key, EvaluationResult::EvaluatedToOk);
 658                 }
 659                 return ProcessResult::Changed(vec![]);
 660             } else {
 661                 debug!("Does NOT hold: {:?}", obligation);
 662             }
 663         }
 664
 665         match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
 666             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 667             ProjectAndUnifyResult::FailedNormalization => {
 668                 stalled_on.clear();
 669                 stalled_on.extend(substs_infer_vars(
 670                     &self.selcx,
 671                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 672                 ));
 673                 ProcessResult::Unchanged
 674             }
 675             // Let the caller handle the recursion
 676             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 677                 project_obligation.with(tcx, project_obligation.predicate),
 678             ])),
 679             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 680                 ProcessResult::Error(CodeProjectionError(e))
 681             }
 682         }
 683     }
 684 }
 685
 686 /// Returns the set of inference variables contained in `substs`.
 687 fn substs_infer_vars<'a, 'tcx>(
 688     selcx: &SelectionContext<'a, 'tcx>,
 689     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 690 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 691     selcx
 692         .infcx
 693         .resolve_vars_if_possible(substs)
 694         .skip_binder() // ok because this check doesn't care about regions
 695         .iter()
 696         .filter(|arg| arg.has_non_region_infer())
 697         .flat_map(|arg| {
 698             let mut walker = arg.walk();
 699             while let Some(c) = walker.next() {
 700                 if !c.has_non_region_infer() {
 701                     walker.visited.remove(&c);
 702                     walker.skip_current_subtree();
 703                 }
 704             }
 705             walker.visited.into_iter()
 706         })
 707         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 708 }
 709
 710 fn to_fulfillment_error<'tcx>(
 711     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 712 ) -> FulfillmentError<'tcx> {
 713     let mut iter = error.backtrace.into_iter();
 714     let obligation = iter.next().unwrap().obligation;
 715     // The root obligation is the last item in the backtrace - if there's only
 716     // one item, then it's the same as the main obligation
 717     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 718     FulfillmentError::new(obligation, error.error, root_obligation)
 719 }