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 =
 244                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 245         }
 246
 247         let obligation = &pending_obligation.obligation;
 248
 249         let infcx = self.selcx.infcx();
 250
 251         if obligation.predicate.has_projections() {
 252             let mut obligations = Vec::new();
 253             let predicate = crate::traits::project::try_normalize_with_depth_to(
 254                 &mut self.selcx,
 255                 obligation.param_env,
 256                 obligation.cause.clone(),
 257                 obligation.recursion_depth + 1,
 258                 obligation.predicate,
 259                 &mut obligations,
 260             );
 261             if predicate != obligation.predicate {
 262                 obligations.push(obligation.with(infcx.tcx, predicate));
 263                 return ProcessResult::Changed(mk_pending(obligations));
 264             }
 265         }
 266         let binder = obligation.predicate.kind();
 267         match binder.no_bound_vars() {
 268             None => match binder.skip_binder() {
 269                 // Evaluation will discard candidates using the leak check.
 270                 // This means we need to pass it the bound version of our
 271                 // predicate.
 272                 ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => {
 273                     let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
 274
 275                     self.process_trait_obligation(
 276                         obligation,
 277                         trait_obligation,
 278                         &mut pending_obligation.stalled_on,
 279                     )
 280                 }
 281                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 282                     let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
 283
 284                     self.process_projection_obligation(
 285                         obligation,
 286                         project_obligation,
 287                         &mut pending_obligation.stalled_on,
 288                     )
 289                 }
 290                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
 291                 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
 292                 | ty::PredicateKind::WellFormed(_)
 293                 | ty::PredicateKind::ObjectSafe(_)
 294                 | ty::PredicateKind::ClosureKind(..)
 295                 | ty::PredicateKind::Subtype(_)
 296                 | ty::PredicateKind::Coerce(_)
 297                 | ty::PredicateKind::ConstEvaluatable(..)
 298                 | ty::PredicateKind::ConstEquate(..) => {
 299                     let pred =
 300                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 301                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
 302                 }
 303                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 304                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 305                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 306                 }
 307             },
 308             Some(pred) => match pred {
 309                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
 310                     let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
 311
 312                     self.process_trait_obligation(
 313                         obligation,
 314                         trait_obligation,
 315                         &mut pending_obligation.stalled_on,
 316                     )
 317                 }
 318
 319                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => {
 320                     if infcx.considering_regions {
 321                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 322                     }
 323
 324                     ProcessResult::Changed(vec![])
 325                 }
 326
 327                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 328                     t_a,
 329                     r_b,
 330                 ))) => {
 331                     if infcx.considering_regions {
 332                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 333                     }
 334                     ProcessResult::Changed(vec![])
 335                 }
 336
 337                 ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
 338                     let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
 339
 340                     self.process_projection_obligation(
 341                         obligation,
 342                         project_obligation,
 343                         &mut pending_obligation.stalled_on,
 344                     )
 345                 }
 346
 347                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 348                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 349                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 350                     } else {
 351                         ProcessResult::Changed(vec![])
 352                     }
 353                 }
 354
 355                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 356                     match self.selcx.infcx().closure_kind(closure_substs) {
 357                         Some(closure_kind) => {
 358                             if closure_kind.extends(kind) {
 359                                 ProcessResult::Changed(vec![])
 360                             } else {
 361                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 362                             }
 363                         }
 364                         None => ProcessResult::Unchanged,
 365                     }
 366                 }
 367
 368                 ty::PredicateKind::WellFormed(arg) => {
 369                     match wf::obligations(
 370                         self.selcx.infcx(),
 371                         obligation.param_env,
 372                         obligation.cause.body_id,
 373                         obligation.recursion_depth + 1,
 374                         arg,
 375                         obligation.cause.span,
 376                     ) {
 377                         None => {
 378                             pending_obligation.stalled_on =
 379                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 380                             ProcessResult::Unchanged
 381                         }
 382                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 383                     }
 384                 }
 385
 386                 ty::PredicateKind::Subtype(subtype) => {
 387                     match self.selcx.infcx().subtype_predicate(
 388                         &obligation.cause,
 389                         obligation.param_env,
 390                         Binder::dummy(subtype),
 391                     ) {
 392                         Err((a, b)) => {
 393                             // None means that both are unresolved.
 394                             pending_obligation.stalled_on =
 395                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 396                             ProcessResult::Unchanged
 397                         }
 398                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 399                         Ok(Err(err)) => {
 400                             let expected_found =
 401                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 402                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 403                                 expected_found,
 404                                 err,
 405                             ))
 406                         }
 407                     }
 408                 }
 409
 410                 ty::PredicateKind::Coerce(coerce) => {
 411                     match self.selcx.infcx().coerce_predicate(
 412                         &obligation.cause,
 413                         obligation.param_env,
 414                         Binder::dummy(coerce),
 415                     ) {
 416                         Err((a, b)) => {
 417                             // None means that both are unresolved.
 418                             pending_obligation.stalled_on =
 419                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 420                             ProcessResult::Unchanged
 421                         }
 422                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 423                         Ok(Err(err)) => {
 424                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 425                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 426                                 expected_found,
 427                                 err,
 428                             ))
 429                         }
 430                     }
 431                 }
 432
 433                 ty::PredicateKind::ConstEvaluatable(uv) => {
 434                     match const_evaluatable::is_const_evaluatable(
 435                         self.selcx.infcx(),
 436                         uv,
 437                         obligation.param_env,
 438                         obligation.cause.span,
 439                     ) {
 440                         Ok(()) => ProcessResult::Changed(vec![]),
 441                         Err(NotConstEvaluatable::MentionsInfer) => {
 442                             pending_obligation.stalled_on.clear();
 443                             pending_obligation.stalled_on.extend(
 444                                 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 445                             );
 446                             ProcessResult::Unchanged
 447                         }
 448                         Err(
 449                             e @ NotConstEvaluatable::MentionsParam
 450                             | e @ NotConstEvaluatable::Error(_),
 451                         ) => ProcessResult::Error(CodeSelectionError(
 452                             SelectionError::NotConstEvaluatable(e),
 453                         )),
 454                     }
 455                 }
 456
 457                 ty::PredicateKind::ConstEquate(c1, c2) => {
 458                     let tcx = self.selcx.tcx();
 459                     assert!(
 460                         tcx.features().generic_const_exprs,
 461                         "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
 462                     );
 463                     // FIXME: we probably should only try to unify abstract constants
 464                     // if the constants depend on generic parameters.
 465                     //
 466                     // Let's just see where this breaks :shrug:
 467                     {
 468                         let c1 = tcx.expand_abstract_consts(c1);
 469                         let c2 = tcx.expand_abstract_consts(c2);
 470                         debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
 471
 472                         use rustc_hir::def::DefKind;
 473                         use ty::ConstKind::Unevaluated;
 474                         match (c1.kind(), c2.kind()) {
 475                             (Unevaluated(a), Unevaluated(b))
 476                                 if a.def.did == b.def.did
 477                                     && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
 478                             {
 479                                 if let Ok(new_obligations) = infcx
 480                                     .at(&obligation.cause, obligation.param_env)
 481                                     .trace(c1, c2)
 482                                     .eq(a.substs, b.substs)
 483                                 {
 484                                     return ProcessResult::Changed(mk_pending(
 485                                         new_obligations.into_obligations(),
 486                                     ));
 487                                 }
 488                             }
 489                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
 490                             (_, _) => {
 491                                 if let Ok(new_obligations) =
 492                                     infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
 493                                 {
 494                                     return ProcessResult::Changed(mk_pending(
 495                                         new_obligations.into_obligations(),
 496                                     ));
 497                                 }
 498                             }
 499                         }
 500                     }
 501
 502                     let stalled_on = &mut pending_obligation.stalled_on;
 503
 504                     let mut evaluate = |c: Const<'tcx>| {
 505                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 506                             match self.selcx.infcx().try_const_eval_resolve(
 507                                 obligation.param_env,
 508                                 unevaluated,
 509                                 c.ty(),
 510                                 Some(obligation.cause.span),
 511                             ) {
 512                                 Ok(val) => Ok(val),
 513                                 Err(e) => match e {
 514                                     ErrorHandled::TooGeneric => {
 515                                         stalled_on.extend(
 516                                             unevaluated.substs.iter().filter_map(
 517                                                 TyOrConstInferVar::maybe_from_generic_arg,
 518                                             ),
 519                                         );
 520                                         Err(ErrorHandled::TooGeneric)
 521                                     }
 522                                     _ => Err(e),
 523                                 },
 524                             }
 525                         } else {
 526                             Ok(c)
 527                         }
 528                     };
 529
 530                     match (evaluate(c1), evaluate(c2)) {
 531                         (Ok(c1), Ok(c2)) => {
 532                             match self
 533                                 .selcx
 534                                 .infcx()
 535                                 .at(&obligation.cause, obligation.param_env)
 536                                 .eq(c1, c2)
 537                             {
 538                                 Ok(inf_ok) => {
 539                                     ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
 540                                 }
 541                                 Err(err) => ProcessResult::Error(
 542                                     FulfillmentErrorCode::CodeConstEquateError(
 543                                         ExpectedFound::new(true, c1, c2),
 544                                         err,
 545                                     ),
 546                                 ),
 547                             }
 548                         }
 549                         (Err(ErrorHandled::Reported(reported)), _)
 550                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 551                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 552                                 NotConstEvaluatable::Error(reported),
 553                             )),
 554                         ),
 555                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 556                             if c1.has_non_region_infer() || c2.has_non_region_infer() {
 557                                 ProcessResult::Unchanged
 558                             } else {
 559                                 // Two different constants using generic parameters ~> error.
 560                                 let expected_found = ExpectedFound::new(true, c1, c2);
 561                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 562                                     expected_found,
 563                                     TypeError::ConstMismatch(expected_found),
 564                                 ))
 565                             }
 566                         }
 567                     }
 568                 }
 569                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 570                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 571                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 572                 }
 573             },
 574         }
 575     }
 576
 577     #[inline(never)]
 578     fn process_backedge<'c, I>(
 579         &mut self,
 580         cycle: I,
 581         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 582     ) -> Result<(), FulfillmentErrorCode<'tcx>>
 583     where
 584         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 585     {
 586         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 587             debug!("process_child_obligations: coinductive match");
 588             Ok(())
 589         } else {
 590             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 591             Err(FulfillmentErrorCode::CodeCycle(cycle))
 592         }
 593     }
 594 }
 595
 596 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
 597     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 598     fn process_trait_obligation(
 599         &mut self,
 600         obligation: &PredicateObligation<'tcx>,
 601         trait_obligation: TraitObligation<'tcx>,
 602         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 603     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 604         let infcx = self.selcx.infcx();
 605         if obligation.predicate.is_global() {
 606             // no type variables present, can use evaluation for better caching.
 607             // FIXME: consider caching errors too.
 608             if infcx.predicate_must_hold_considering_regions(obligation) {
 609                 debug!(
 610                     "selecting trait at depth {} evaluated to holds",
 611                     obligation.recursion_depth
 612                 );
 613                 return ProcessResult::Changed(vec![]);
 614             }
 615         }
 616
 617         match self.selcx.select(&trait_obligation) {
 618             Ok(Some(impl_source)) => {
 619                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 620                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 621             }
 622             Ok(None) => {
 623                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 624
 625                 // This is a bit subtle: for the most part, the
 626                 // only reason we can fail to make progress on
 627                 // trait selection is because we don't have enough
 628                 // information about the types in the trait.
 629                 stalled_on.clear();
 630                 stalled_on.extend(substs_infer_vars(
 631                     &self.selcx,
 632                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 633                 ));
 634
 635                 debug!(
 636                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 637                     infcx.resolve_vars_if_possible(obligation.clone()),
 638                     stalled_on
 639                 );
 640
 641                 ProcessResult::Unchanged
 642             }
 643             Err(selection_err) => {
 644                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 645
 646                 ProcessResult::Error(CodeSelectionError(selection_err))
 647             }
 648         }
 649     }
 650
 651     fn process_projection_obligation(
 652         &mut self,
 653         obligation: &PredicateObligation<'tcx>,
 654         project_obligation: PolyProjectionObligation<'tcx>,
 655         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 656     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 657         let tcx = self.selcx.tcx();
 658
 659         if obligation.predicate.is_global() {
 660             // no type variables present, can use evaluation for better caching.
 661             // FIXME: consider caching errors too.
 662             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 663                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 664                     &mut self.selcx,
 665                     project_obligation.predicate,
 666                 ) {
 667                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 668                     // evaluated all sub-obligations. We can therefore mark the 'root'
 669                     // obligation as complete, and skip evaluating sub-obligations.
 670                     self.selcx
 671                         .infcx()
 672                         .inner
 673                         .borrow_mut()
 674                         .projection_cache()
 675                         .complete(key, EvaluationResult::EvaluatedToOk);
 676                 }
 677                 return ProcessResult::Changed(vec![]);
 678             } else {
 679                 debug!("Does NOT hold: {:?}", obligation);
 680             }
 681         }
 682
 683         match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
 684             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 685             ProjectAndUnifyResult::FailedNormalization => {
 686                 stalled_on.clear();
 687                 stalled_on.extend(substs_infer_vars(
 688                     &self.selcx,
 689                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 690                 ));
 691                 ProcessResult::Unchanged
 692             }
 693             // Let the caller handle the recursion
 694             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 695                 project_obligation.with(tcx, project_obligation.predicate),
 696             ])),
 697             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 698                 ProcessResult::Error(CodeProjectionError(e))
 699             }
 700         }
 701     }
 702 }
 703
 704 /// Returns the set of inference variables contained in `substs`.
 705 fn substs_infer_vars<'a, 'tcx>(
 706     selcx: &SelectionContext<'a, 'tcx>,
 707     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 708 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 709     selcx
 710         .infcx()
 711         .resolve_vars_if_possible(substs)
 712         .skip_binder() // ok because this check doesn't care about regions
 713         .iter()
 714         .filter(|arg| arg.has_non_region_infer())
 715         .flat_map(|arg| {
 716             let mut walker = arg.walk();
 717             while let Some(c) = walker.next() {
 718                 if !c.has_non_region_infer() {
 719                     walker.visited.remove(&c);
 720                     walker.skip_current_subtree();
 721                 }
 722             }
 723             walker.visited.into_iter()
 724         })
 725         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 726 }
 727
 728 fn to_fulfillment_error<'tcx>(
 729     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 730 ) -> FulfillmentError<'tcx> {
 731     let mut iter = error.backtrace.into_iter();
 732     let obligation = iter.next().unwrap().obligation;
 733     // The root obligation is the last item in the backtrace - if there's only
 734     // one item, then it's the same as the main obligation
 735     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 736     FulfillmentError::new(obligation, error.error, root_obligation)
 737 }