compiler/rustc_trait_selection/src/traits/fulfill.rs

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