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 collect_remaining_errors(&mut self) -> Vec<FulfillmentError<'tcx>> {
 136         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 137     }
 138
 139     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
 140         let selcx = SelectionContext::new(infcx);
 141         self.select(selcx)
 142     }
 143
 144     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 145         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 146     }
 147 }
 148
 149 struct FulfillProcessor<'a, 'tcx> {
 150     selcx: SelectionContext<'a, 'tcx>,
 151 }
 152
 153 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 154     os.into_iter()
 155         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 156         .collect()
 157 }
 158
 159 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 160     type Obligation = PendingPredicateObligation<'tcx>;
 161     type Error = FulfillmentErrorCode<'tcx>;
 162     type OUT = Outcome<Self::Obligation, Self::Error>;
 163
 164     /// Identifies whether a predicate obligation needs processing.
 165     ///
 166     /// This is always inlined, despite its size, because it has a single
 167     /// callsite and it is called *very* frequently.
 168     #[inline(always)]
 169     fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 170         // If we were stalled on some unresolved variables, first check whether
 171         // any of them have been resolved; if not, don't bother doing more work
 172         // yet.
 173         match pending_obligation.stalled_on.len() {
 174             // Match arms are in order of frequency, which matters because this
 175             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 176             1 => {
 177                 let infer_var = pending_obligation.stalled_on[0];
 178                 self.selcx.infcx.ty_or_const_infer_var_changed(infer_var)
 179             }
 180             0 => {
 181                 // In this case we haven't changed, but wish to make a change.
 182                 true
 183             }
 184             _ => {
 185                 // This `for` loop was once a call to `all()`, but this lower-level
 186                 // form was a perf win. See #64545 for details.
 187                 (|| {
 188                     for &infer_var in &pending_obligation.stalled_on {
 189                         if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
 190                             return true;
 191                         }
 192                     }
 193                     false
 194                 })()
 195             }
 196         }
 197     }
 198
 199     /// Processes a predicate obligation and returns either:
 200     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 201     /// - `Unchanged` if we don't have enough info to be sure
 202     /// - `Error(e)` if the predicate does not hold
 203     ///
 204     /// This is called much less often than `needs_process_obligation`, so we
 205     /// never inline it.
 206     #[inline(never)]
 207     #[instrument(level = "debug", skip(self, pending_obligation))]
 208     fn process_obligation(
 209         &mut self,
 210         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 211     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 212         pending_obligation.stalled_on.truncate(0);
 213
 214         let obligation = &mut pending_obligation.obligation;
 215
 216         debug!(?obligation, "pre-resolve");
 217
 218         if obligation.predicate.has_non_region_infer() {
 219             obligation.predicate = self.selcx.infcx.resolve_vars_if_possible(obligation.predicate);
 220         }
 221
 222         let obligation = &pending_obligation.obligation;
 223
 224         let infcx = self.selcx.infcx;
 225
 226         if obligation.predicate.has_projections() {
 227             let mut obligations = Vec::new();
 228             let predicate = crate::traits::project::try_normalize_with_depth_to(
 229                 &mut self.selcx,
 230                 obligation.param_env,
 231                 obligation.cause.clone(),
 232                 obligation.recursion_depth + 1,
 233                 obligation.predicate,
 234                 &mut obligations,
 235             );
 236             if predicate != obligation.predicate {
 237                 obligations.push(obligation.with(infcx.tcx, predicate));
 238                 return ProcessResult::Changed(mk_pending(obligations));
 239             }
 240         }
 241         let binder = obligation.predicate.kind();
 242         match binder.no_bound_vars() {
 243             None => match binder.skip_binder() {
 244                 // Evaluation will discard candidates using the leak check.
 245                 // This means we need to pass it the bound version of our
 246                 // predicate.
 247                 ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => {
 248                     let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
 249
 250                     self.process_trait_obligation(
 251                         obligation,
 252                         trait_obligation,
 253                         &mut pending_obligation.stalled_on,
 254                     )
 255                 }
 256                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 257                     let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
 258
 259                     self.process_projection_obligation(
 260                         obligation,
 261                         project_obligation,
 262                         &mut pending_obligation.stalled_on,
 263                     )
 264                 }
 265                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
 266                 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
 267                 | ty::PredicateKind::WellFormed(_)
 268                 | ty::PredicateKind::ObjectSafe(_)
 269                 | ty::PredicateKind::ClosureKind(..)
 270                 | ty::PredicateKind::Subtype(_)
 271                 | ty::PredicateKind::Coerce(_)
 272                 | ty::PredicateKind::ConstEvaluatable(..)
 273                 | ty::PredicateKind::ConstEquate(..) => {
 274                     let pred =
 275                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 276                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
 277                 }
 278                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 279                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 280                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 281                 }
 282             },
 283             Some(pred) => match pred {
 284                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
 285                     let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
 286
 287                     self.process_trait_obligation(
 288                         obligation,
 289                         trait_obligation,
 290                         &mut pending_obligation.stalled_on,
 291                     )
 292                 }
 293
 294                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => {
 295                     if infcx.considering_regions {
 296                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 297                     }
 298
 299                     ProcessResult::Changed(vec![])
 300                 }
 301
 302                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 303                     t_a,
 304                     r_b,
 305                 ))) => {
 306                     if infcx.considering_regions {
 307                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 308                     }
 309                     ProcessResult::Changed(vec![])
 310                 }
 311
 312                 ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
 313                     let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
 314
 315                     self.process_projection_obligation(
 316                         obligation,
 317                         project_obligation,
 318                         &mut pending_obligation.stalled_on,
 319                     )
 320                 }
 321
 322                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 323                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 324                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 325                     } else {
 326                         ProcessResult::Changed(vec![])
 327                     }
 328                 }
 329
 330                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 331                     match self.selcx.infcx.closure_kind(closure_substs) {
 332                         Some(closure_kind) => {
 333                             if closure_kind.extends(kind) {
 334                                 ProcessResult::Changed(vec![])
 335                             } else {
 336                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 337                             }
 338                         }
 339                         None => ProcessResult::Unchanged,
 340                     }
 341                 }
 342
 343                 ty::PredicateKind::WellFormed(arg) => {
 344                     match wf::obligations(
 345                         self.selcx.infcx,
 346                         obligation.param_env,
 347                         obligation.cause.body_id,
 348                         obligation.recursion_depth + 1,
 349                         arg,
 350                         obligation.cause.span,
 351                     ) {
 352                         None => {
 353                             pending_obligation.stalled_on =
 354                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 355                             ProcessResult::Unchanged
 356                         }
 357                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 358                     }
 359                 }
 360
 361                 ty::PredicateKind::Subtype(subtype) => {
 362                     match self.selcx.infcx.subtype_predicate(
 363                         &obligation.cause,
 364                         obligation.param_env,
 365                         Binder::dummy(subtype),
 366                     ) {
 367                         Err((a, b)) => {
 368                             // None means that both are unresolved.
 369                             pending_obligation.stalled_on =
 370                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 371                             ProcessResult::Unchanged
 372                         }
 373                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 374                         Ok(Err(err)) => {
 375                             let expected_found =
 376                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 377                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 378                                 expected_found,
 379                                 err,
 380                             ))
 381                         }
 382                     }
 383                 }
 384
 385                 ty::PredicateKind::Coerce(coerce) => {
 386                     match self.selcx.infcx.coerce_predicate(
 387                         &obligation.cause,
 388                         obligation.param_env,
 389                         Binder::dummy(coerce),
 390                     ) {
 391                         Err((a, b)) => {
 392                             // None means that both are unresolved.
 393                             pending_obligation.stalled_on =
 394                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 395                             ProcessResult::Unchanged
 396                         }
 397                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 398                         Ok(Err(err)) => {
 399                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 400                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 401                                 expected_found,
 402                                 err,
 403                             ))
 404                         }
 405                     }
 406                 }
 407
 408                 ty::PredicateKind::ConstEvaluatable(uv) => {
 409                     match const_evaluatable::is_const_evaluatable(
 410                         self.selcx.infcx,
 411                         uv,
 412                         obligation.param_env,
 413                         obligation.cause.span,
 414                     ) {
 415                         Ok(()) => ProcessResult::Changed(vec![]),
 416                         Err(NotConstEvaluatable::MentionsInfer) => {
 417                             pending_obligation.stalled_on.clear();
 418                             pending_obligation.stalled_on.extend(
 419                                 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 420                             );
 421                             ProcessResult::Unchanged
 422                         }
 423                         Err(
 424                             e @ NotConstEvaluatable::MentionsParam
 425                             | e @ NotConstEvaluatable::Error(_),
 426                         ) => ProcessResult::Error(CodeSelectionError(
 427                             SelectionError::NotConstEvaluatable(e),
 428                         )),
 429                     }
 430                 }
 431
 432                 ty::PredicateKind::ConstEquate(c1, c2) => {
 433                     let tcx = self.selcx.tcx();
 434                     assert!(
 435                         tcx.features().generic_const_exprs,
 436                         "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
 437                     );
 438                     // FIXME: we probably should only try to unify abstract constants
 439                     // if the constants depend on generic parameters.
 440                     //
 441                     // Let's just see where this breaks :shrug:
 442                     {
 443                         let c1 = tcx.expand_abstract_consts(c1);
 444                         let c2 = tcx.expand_abstract_consts(c2);
 445                         debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
 446
 447                         use rustc_hir::def::DefKind;
 448                         use ty::ConstKind::Unevaluated;
 449                         match (c1.kind(), c2.kind()) {
 450                             (Unevaluated(a), Unevaluated(b))
 451                                 if a.def.did == b.def.did
 452                                     && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
 453                             {
 454                                 if let Ok(new_obligations) = infcx
 455                                     .at(&obligation.cause, obligation.param_env)
 456                                     .trace(c1, c2)
 457                                     .eq(a.substs, b.substs)
 458                                 {
 459                                     return ProcessResult::Changed(mk_pending(
 460                                         new_obligations.into_obligations(),
 461                                     ));
 462                                 }
 463                             }
 464                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
 465                             (_, _) => {
 466                                 if let Ok(new_obligations) =
 467                                     infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
 468                                 {
 469                                     return ProcessResult::Changed(mk_pending(
 470                                         new_obligations.into_obligations(),
 471                                     ));
 472                                 }
 473                             }
 474                         }
 475                     }
 476
 477                     let stalled_on = &mut pending_obligation.stalled_on;
 478
 479                     let mut evaluate = |c: Const<'tcx>| {
 480                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 481                             match self.selcx.infcx.try_const_eval_resolve(
 482                                 obligation.param_env,
 483                                 unevaluated,
 484                                 c.ty(),
 485                                 Some(obligation.cause.span),
 486                             ) {
 487                                 Ok(val) => Ok(val),
 488                                 Err(e) => match e {
 489                                     ErrorHandled::TooGeneric => {
 490                                         stalled_on.extend(
 491                                             unevaluated.substs.iter().filter_map(
 492                                                 TyOrConstInferVar::maybe_from_generic_arg,
 493                                             ),
 494                                         );
 495                                         Err(ErrorHandled::TooGeneric)
 496                                     }
 497                                     _ => Err(e),
 498                                 },
 499                             }
 500                         } else {
 501                             Ok(c)
 502                         }
 503                     };
 504
 505                     match (evaluate(c1), evaluate(c2)) {
 506                         (Ok(c1), Ok(c2)) => {
 507                             match self
 508                                 .selcx
 509                                 .infcx
 510                                 .at(&obligation.cause, obligation.param_env)
 511                                 .eq(c1, c2)
 512                             {
 513                                 Ok(inf_ok) => {
 514                                     ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
 515                                 }
 516                                 Err(err) => ProcessResult::Error(
 517                                     FulfillmentErrorCode::CodeConstEquateError(
 518                                         ExpectedFound::new(true, c1, c2),
 519                                         err,
 520                                     ),
 521                                 ),
 522                             }
 523                         }
 524                         (Err(ErrorHandled::Reported(reported)), _)
 525                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 526                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 527                                 NotConstEvaluatable::Error(reported),
 528                             )),
 529                         ),
 530                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 531                             if c1.has_non_region_infer() || c2.has_non_region_infer() {
 532                                 ProcessResult::Unchanged
 533                             } else {
 534                                 // Two different constants using generic parameters ~> error.
 535                                 let expected_found = ExpectedFound::new(true, c1, c2);
 536                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 537                                     expected_found,
 538                                     TypeError::ConstMismatch(expected_found),
 539                                 ))
 540                             }
 541                         }
 542                     }
 543                 }
 544                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 545                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 546                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 547                 }
 548             },
 549         }
 550     }
 551
 552     #[inline(never)]
 553     fn process_backedge<'c, I>(
 554         &mut self,
 555         cycle: I,
 556         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 557     ) -> Result<(), FulfillmentErrorCode<'tcx>>
 558     where
 559         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 560     {
 561         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 562             debug!("process_child_obligations: coinductive match");
 563             Ok(())
 564         } else {
 565             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 566             Err(FulfillmentErrorCode::CodeCycle(cycle))
 567         }
 568     }
 569 }
 570
 571 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
 572     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 573     fn process_trait_obligation(
 574         &mut self,
 575         obligation: &PredicateObligation<'tcx>,
 576         trait_obligation: TraitObligation<'tcx>,
 577         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 578     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 579         let infcx = self.selcx.infcx;
 580         if obligation.predicate.is_global() {
 581             // no type variables present, can use evaluation for better caching.
 582             // FIXME: consider caching errors too.
 583             if infcx.predicate_must_hold_considering_regions(obligation) {
 584                 debug!(
 585                     "selecting trait at depth {} evaluated to holds",
 586                     obligation.recursion_depth
 587                 );
 588                 return ProcessResult::Changed(vec![]);
 589             }
 590         }
 591
 592         match self.selcx.select(&trait_obligation) {
 593             Ok(Some(impl_source)) => {
 594                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 595                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 596             }
 597             Ok(None) => {
 598                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 599
 600                 // This is a bit subtle: for the most part, the
 601                 // only reason we can fail to make progress on
 602                 // trait selection is because we don't have enough
 603                 // information about the types in the trait.
 604                 stalled_on.clear();
 605                 stalled_on.extend(substs_infer_vars(
 606                     &self.selcx,
 607                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 608                 ));
 609
 610                 debug!(
 611                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 612                     infcx.resolve_vars_if_possible(obligation.clone()),
 613                     stalled_on
 614                 );
 615
 616                 ProcessResult::Unchanged
 617             }
 618             Err(selection_err) => {
 619                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 620
 621                 ProcessResult::Error(CodeSelectionError(selection_err))
 622             }
 623         }
 624     }
 625
 626     fn process_projection_obligation(
 627         &mut self,
 628         obligation: &PredicateObligation<'tcx>,
 629         project_obligation: PolyProjectionObligation<'tcx>,
 630         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 631     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 632         let tcx = self.selcx.tcx();
 633
 634         if obligation.predicate.is_global() {
 635             // no type variables present, can use evaluation for better caching.
 636             // FIXME: consider caching errors too.
 637             if self.selcx.infcx.predicate_must_hold_considering_regions(obligation) {
 638                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 639                     &mut self.selcx,
 640                     project_obligation.predicate,
 641                 ) {
 642                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 643                     // evaluated all sub-obligations. We can therefore mark the 'root'
 644                     // obligation as complete, and skip evaluating sub-obligations.
 645                     self.selcx
 646                         .infcx
 647                         .inner
 648                         .borrow_mut()
 649                         .projection_cache()
 650                         .complete(key, EvaluationResult::EvaluatedToOk);
 651                 }
 652                 return ProcessResult::Changed(vec![]);
 653             } else {
 654                 debug!("Does NOT hold: {:?}", obligation);
 655             }
 656         }
 657
 658         match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
 659             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 660             ProjectAndUnifyResult::FailedNormalization => {
 661                 stalled_on.clear();
 662                 stalled_on.extend(substs_infer_vars(
 663                     &self.selcx,
 664                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 665                 ));
 666                 ProcessResult::Unchanged
 667             }
 668             // Let the caller handle the recursion
 669             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 670                 project_obligation.with(tcx, project_obligation.predicate),
 671             ])),
 672             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 673                 ProcessResult::Error(CodeProjectionError(e))
 674             }
 675         }
 676     }
 677 }
 678
 679 /// Returns the set of inference variables contained in `substs`.
 680 fn substs_infer_vars<'a, 'tcx>(
 681     selcx: &SelectionContext<'a, 'tcx>,
 682     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 683 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 684     selcx
 685         .infcx
 686         .resolve_vars_if_possible(substs)
 687         .skip_binder() // ok because this check doesn't care about regions
 688         .iter()
 689         .filter(|arg| arg.has_non_region_infer())
 690         .flat_map(|arg| {
 691             let mut walker = arg.walk();
 692             while let Some(c) = walker.next() {
 693                 if !c.has_non_region_infer() {
 694                     walker.visited.remove(&c);
 695                     walker.skip_current_subtree();
 696                 }
 697             }
 698             walker.visited.into_iter()
 699         })
 700         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 701 }
 702
 703 fn to_fulfillment_error<'tcx>(
 704     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 705 ) -> FulfillmentError<'tcx> {
 706     let mut iter = error.backtrace.into_iter();
 707     let obligation = iter.next().unwrap().obligation;
 708     // The root obligation is the last item in the backtrace - if there's only
 709     // one item, then it's the same as the main obligation
 710     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 711     FulfillmentError::new(obligation, error.error, root_obligation)
 712 }