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 drain_unstalled_obligations(
 145         &mut self,
 146         infcx: &InferCtxt<'tcx>,
 147     ) -> Vec<PredicateObligation<'tcx>> {
 148         let mut processor = DrainProcessor { removed_predicates: Vec::new(), infcx };
 149         let outcome: Outcome<_, _> = self.predicates.process_obligations(&mut processor);
 150         assert!(outcome.errors.is_empty());
 151         return processor.removed_predicates;
 152
 153         struct DrainProcessor<'a, 'tcx> {
 154             infcx: &'a InferCtxt<'tcx>,
 155             removed_predicates: Vec<PredicateObligation<'tcx>>,
 156         }
 157
 158         impl<'tcx> ObligationProcessor for DrainProcessor<'_, 'tcx> {
 159             type Obligation = PendingPredicateObligation<'tcx>;
 160             type Error = !;
 161             type OUT = Outcome<Self::Obligation, Self::Error>;
 162
 163             fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 164                 pending_obligation
 165                     .stalled_on
 166                     .iter()
 167                     .any(|&var| self.infcx.ty_or_const_infer_var_changed(var))
 168             }
 169
 170             fn process_obligation(
 171                 &mut self,
 172                 pending_obligation: &mut PendingPredicateObligation<'tcx>,
 173             ) -> ProcessResult<PendingPredicateObligation<'tcx>, !> {
 174                 assert!(self.needs_process_obligation(pending_obligation));
 175                 self.removed_predicates.push(pending_obligation.obligation.clone());
 176                 ProcessResult::Changed(vec![])
 177             }
 178
 179             fn process_backedge<'c, I>(
 180                 &mut self,
 181                 cycle: I,
 182                 _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 183             ) -> Result<(), !>
 184             where
 185                 I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 186             {
 187                 self.removed_predicates.extend(cycle.map(|c| c.obligation.clone()));
 188                 Ok(())
 189             }
 190         }
 191     }
 192
 193     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 194         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 195     }
 196 }
 197
 198 struct FulfillProcessor<'a, 'tcx> {
 199     selcx: SelectionContext<'a, 'tcx>,
 200 }
 201
 202 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 203     os.into_iter()
 204         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 205         .collect()
 206 }
 207
 208 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 209     type Obligation = PendingPredicateObligation<'tcx>;
 210     type Error = FulfillmentErrorCode<'tcx>;
 211     type OUT = Outcome<Self::Obligation, Self::Error>;
 212
 213     /// Identifies whether a predicate obligation needs processing.
 214     ///
 215     /// This is always inlined, despite its size, because it has a single
 216     /// callsite and it is called *very* frequently.
 217     #[inline(always)]
 218     fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 219         // If we were stalled on some unresolved variables, first check whether
 220         // any of them have been resolved; if not, don't bother doing more work
 221         // yet.
 222         match pending_obligation.stalled_on.len() {
 223             // Match arms are in order of frequency, which matters because this
 224             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 225             1 => {
 226                 let infer_var = pending_obligation.stalled_on[0];
 227                 self.selcx.infcx.ty_or_const_infer_var_changed(infer_var)
 228             }
 229             0 => {
 230                 // In this case we haven't changed, but wish to make a change.
 231                 true
 232             }
 233             _ => {
 234                 // This `for` loop was once a call to `all()`, but this lower-level
 235                 // form was a perf win. See #64545 for details.
 236                 (|| {
 237                     for &infer_var in &pending_obligation.stalled_on {
 238                         if self.selcx.infcx.ty_or_const_infer_var_changed(infer_var) {
 239                             return true;
 240                         }
 241                     }
 242                     false
 243                 })()
 244             }
 245         }
 246     }
 247
 248     /// Processes a predicate obligation and returns either:
 249     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 250     /// - `Unchanged` if we don't have enough info to be sure
 251     /// - `Error(e)` if the predicate does not hold
 252     ///
 253     /// This is called much less often than `needs_process_obligation`, so we
 254     /// never inline it.
 255     #[inline(never)]
 256     #[instrument(level = "debug", skip(self, pending_obligation))]
 257     fn process_obligation(
 258         &mut self,
 259         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 260     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 261         pending_obligation.stalled_on.truncate(0);
 262
 263         let obligation = &mut pending_obligation.obligation;
 264
 265         debug!(?obligation, "pre-resolve");
 266
 267         if obligation.predicate.has_non_region_infer() {
 268             obligation.predicate = self.selcx.infcx.resolve_vars_if_possible(obligation.predicate);
 269         }
 270
 271         let obligation = &pending_obligation.obligation;
 272
 273         let infcx = self.selcx.infcx;
 274
 275         if obligation.predicate.has_projections() {
 276             let mut obligations = Vec::new();
 277             let predicate = crate::traits::project::try_normalize_with_depth_to(
 278                 &mut self.selcx,
 279                 obligation.param_env,
 280                 obligation.cause.clone(),
 281                 obligation.recursion_depth + 1,
 282                 obligation.predicate,
 283                 &mut obligations,
 284             );
 285             if predicate != obligation.predicate {
 286                 obligations.push(obligation.with(infcx.tcx, predicate));
 287                 return ProcessResult::Changed(mk_pending(obligations));
 288             }
 289         }
 290         let binder = obligation.predicate.kind();
 291         match binder.no_bound_vars() {
 292             None => match binder.skip_binder() {
 293                 // Evaluation will discard candidates using the leak check.
 294                 // This means we need to pass it the bound version of our
 295                 // predicate.
 296                 ty::PredicateKind::Clause(ty::Clause::Trait(trait_ref)) => {
 297                     let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
 298
 299                     self.process_trait_obligation(
 300                         obligation,
 301                         trait_obligation,
 302                         &mut pending_obligation.stalled_on,
 303                     )
 304                 }
 305                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 306                     let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
 307
 308                     self.process_projection_obligation(
 309                         obligation,
 310                         project_obligation,
 311                         &mut pending_obligation.stalled_on,
 312                     )
 313                 }
 314                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(_))
 315                 | ty::PredicateKind::Clause(ty::Clause::TypeOutlives(_))
 316                 | ty::PredicateKind::WellFormed(_)
 317                 | ty::PredicateKind::ObjectSafe(_)
 318                 | ty::PredicateKind::ClosureKind(..)
 319                 | ty::PredicateKind::Subtype(_)
 320                 | ty::PredicateKind::Coerce(_)
 321                 | ty::PredicateKind::ConstEvaluatable(..)
 322                 | ty::PredicateKind::ConstEquate(..) => {
 323                     let pred =
 324                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 325                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
 326                 }
 327                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 328                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 329                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 330                 }
 331             },
 332             Some(pred) => match pred {
 333                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => {
 334                     let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
 335
 336                     self.process_trait_obligation(
 337                         obligation,
 338                         trait_obligation,
 339                         &mut pending_obligation.stalled_on,
 340                     )
 341                 }
 342
 343                 ty::PredicateKind::Clause(ty::Clause::RegionOutlives(data)) => {
 344                     if infcx.considering_regions {
 345                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 346                     }
 347
 348                     ProcessResult::Changed(vec![])
 349                 }
 350
 351                 ty::PredicateKind::Clause(ty::Clause::TypeOutlives(ty::OutlivesPredicate(
 352                     t_a,
 353                     r_b,
 354                 ))) => {
 355                     if infcx.considering_regions {
 356                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 357                     }
 358                     ProcessResult::Changed(vec![])
 359                 }
 360
 361                 ty::PredicateKind::Clause(ty::Clause::Projection(ref data)) => {
 362                     let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
 363
 364                     self.process_projection_obligation(
 365                         obligation,
 366                         project_obligation,
 367                         &mut pending_obligation.stalled_on,
 368                     )
 369                 }
 370
 371                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 372                     if !self.selcx.tcx().check_is_object_safe(trait_def_id) {
 373                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 374                     } else {
 375                         ProcessResult::Changed(vec![])
 376                     }
 377                 }
 378
 379                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 380                     match self.selcx.infcx.closure_kind(closure_substs) {
 381                         Some(closure_kind) => {
 382                             if closure_kind.extends(kind) {
 383                                 ProcessResult::Changed(vec![])
 384                             } else {
 385                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 386                             }
 387                         }
 388                         None => ProcessResult::Unchanged,
 389                     }
 390                 }
 391
 392                 ty::PredicateKind::WellFormed(arg) => {
 393                     match wf::obligations(
 394                         self.selcx.infcx,
 395                         obligation.param_env,
 396                         obligation.cause.body_id,
 397                         obligation.recursion_depth + 1,
 398                         arg,
 399                         obligation.cause.span,
 400                     ) {
 401                         None => {
 402                             pending_obligation.stalled_on =
 403                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 404                             ProcessResult::Unchanged
 405                         }
 406                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 407                     }
 408                 }
 409
 410                 ty::PredicateKind::Subtype(subtype) => {
 411                     match self.selcx.infcx.subtype_predicate(
 412                         &obligation.cause,
 413                         obligation.param_env,
 414                         Binder::dummy(subtype),
 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 =
 425                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 426                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 427                                 expected_found,
 428                                 err,
 429                             ))
 430                         }
 431                     }
 432                 }
 433
 434                 ty::PredicateKind::Coerce(coerce) => {
 435                     match self.selcx.infcx.coerce_predicate(
 436                         &obligation.cause,
 437                         obligation.param_env,
 438                         Binder::dummy(coerce),
 439                     ) {
 440                         Err((a, b)) => {
 441                             // None means that both are unresolved.
 442                             pending_obligation.stalled_on =
 443                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 444                             ProcessResult::Unchanged
 445                         }
 446                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 447                         Ok(Err(err)) => {
 448                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 449                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 450                                 expected_found,
 451                                 err,
 452                             ))
 453                         }
 454                     }
 455                 }
 456
 457                 ty::PredicateKind::ConstEvaluatable(uv) => {
 458                     match const_evaluatable::is_const_evaluatable(
 459                         self.selcx.infcx,
 460                         uv,
 461                         obligation.param_env,
 462                         obligation.cause.span,
 463                     ) {
 464                         Ok(()) => ProcessResult::Changed(vec![]),
 465                         Err(NotConstEvaluatable::MentionsInfer) => {
 466                             pending_obligation.stalled_on.clear();
 467                             pending_obligation.stalled_on.extend(
 468                                 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 469                             );
 470                             ProcessResult::Unchanged
 471                         }
 472                         Err(
 473                             e @ NotConstEvaluatable::MentionsParam
 474                             | e @ NotConstEvaluatable::Error(_),
 475                         ) => ProcessResult::Error(CodeSelectionError(
 476                             SelectionError::NotConstEvaluatable(e),
 477                         )),
 478                     }
 479                 }
 480
 481                 ty::PredicateKind::ConstEquate(c1, c2) => {
 482                     let tcx = self.selcx.tcx();
 483                     assert!(
 484                         tcx.features().generic_const_exprs,
 485                         "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
 486                     );
 487                     // FIXME: we probably should only try to unify abstract constants
 488                     // if the constants depend on generic parameters.
 489                     //
 490                     // Let's just see where this breaks :shrug:
 491                     {
 492                         let c1 = tcx.expand_abstract_consts(c1);
 493                         let c2 = tcx.expand_abstract_consts(c2);
 494                         debug!("equating consts:\nc1= {:?}\nc2= {:?}", c1, c2);
 495
 496                         use rustc_hir::def::DefKind;
 497                         use ty::ConstKind::Unevaluated;
 498                         match (c1.kind(), c2.kind()) {
 499                             (Unevaluated(a), Unevaluated(b))
 500                                 if a.def.did == b.def.did
 501                                     && tcx.def_kind(a.def.did) == DefKind::AssocConst =>
 502                             {
 503                                 if let Ok(new_obligations) = infcx
 504                                     .at(&obligation.cause, obligation.param_env)
 505                                     .trace(c1, c2)
 506                                     .eq(a.substs, b.substs)
 507                                 {
 508                                     return ProcessResult::Changed(mk_pending(
 509                                         new_obligations.into_obligations(),
 510                                     ));
 511                                 }
 512                             }
 513                             (_, Unevaluated(_)) | (Unevaluated(_), _) => (),
 514                             (_, _) => {
 515                                 if let Ok(new_obligations) =
 516                                     infcx.at(&obligation.cause, obligation.param_env).eq(c1, c2)
 517                                 {
 518                                     return ProcessResult::Changed(mk_pending(
 519                                         new_obligations.into_obligations(),
 520                                     ));
 521                                 }
 522                             }
 523                         }
 524                     }
 525
 526                     let stalled_on = &mut pending_obligation.stalled_on;
 527
 528                     let mut evaluate = |c: Const<'tcx>| {
 529                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 530                             match self.selcx.infcx.try_const_eval_resolve(
 531                                 obligation.param_env,
 532                                 unevaluated,
 533                                 c.ty(),
 534                                 Some(obligation.cause.span),
 535                             ) {
 536                                 Ok(val) => Ok(val),
 537                                 Err(e) => match e {
 538                                     ErrorHandled::TooGeneric => {
 539                                         stalled_on.extend(
 540                                             unevaluated.substs.iter().filter_map(
 541                                                 TyOrConstInferVar::maybe_from_generic_arg,
 542                                             ),
 543                                         );
 544                                         Err(ErrorHandled::TooGeneric)
 545                                     }
 546                                     _ => Err(e),
 547                                 },
 548                             }
 549                         } else {
 550                             Ok(c)
 551                         }
 552                     };
 553
 554                     match (evaluate(c1), evaluate(c2)) {
 555                         (Ok(c1), Ok(c2)) => {
 556                             match self
 557                                 .selcx
 558                                 .infcx
 559                                 .at(&obligation.cause, obligation.param_env)
 560                                 .eq(c1, c2)
 561                             {
 562                                 Ok(inf_ok) => {
 563                                     ProcessResult::Changed(mk_pending(inf_ok.into_obligations()))
 564                                 }
 565                                 Err(err) => ProcessResult::Error(
 566                                     FulfillmentErrorCode::CodeConstEquateError(
 567                                         ExpectedFound::new(true, c1, c2),
 568                                         err,
 569                                     ),
 570                                 ),
 571                             }
 572                         }
 573                         (Err(ErrorHandled::Reported(reported)), _)
 574                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 575                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 576                                 NotConstEvaluatable::Error(reported),
 577                             )),
 578                         ),
 579                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 580                             if c1.has_non_region_infer() || c2.has_non_region_infer() {
 581                                 ProcessResult::Unchanged
 582                             } else {
 583                                 // Two different constants using generic parameters ~> error.
 584                                 let expected_found = ExpectedFound::new(true, c1, c2);
 585                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 586                                     expected_found,
 587                                     TypeError::ConstMismatch(expected_found),
 588                                 ))
 589                             }
 590                         }
 591                     }
 592                 }
 593                 ty::PredicateKind::Ambiguous => ProcessResult::Unchanged,
 594                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 595                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 596                 }
 597             },
 598         }
 599     }
 600
 601     #[inline(never)]
 602     fn process_backedge<'c, I>(
 603         &mut self,
 604         cycle: I,
 605         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 606     ) -> Result<(), FulfillmentErrorCode<'tcx>>
 607     where
 608         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 609     {
 610         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 611             debug!("process_child_obligations: coinductive match");
 612             Ok(())
 613         } else {
 614             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 615             Err(FulfillmentErrorCode::CodeCycle(cycle))
 616         }
 617     }
 618 }
 619
 620 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
 621     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 622     fn process_trait_obligation(
 623         &mut self,
 624         obligation: &PredicateObligation<'tcx>,
 625         trait_obligation: TraitObligation<'tcx>,
 626         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 627     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 628         let infcx = self.selcx.infcx;
 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 infcx.predicate_must_hold_considering_regions(obligation) {
 633                 debug!(
 634                     "selecting trait at depth {} evaluated to holds",
 635                     obligation.recursion_depth
 636                 );
 637                 return ProcessResult::Changed(vec![]);
 638             }
 639         }
 640
 641         match self.selcx.select(&trait_obligation) {
 642             Ok(Some(impl_source)) => {
 643                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 644                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 645             }
 646             Ok(None) => {
 647                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 648
 649                 // This is a bit subtle: for the most part, the
 650                 // only reason we can fail to make progress on
 651                 // trait selection is because we don't have enough
 652                 // information about the types in the trait.
 653                 stalled_on.clear();
 654                 stalled_on.extend(substs_infer_vars(
 655                     &self.selcx,
 656                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 657                 ));
 658
 659                 debug!(
 660                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 661                     infcx.resolve_vars_if_possible(obligation.clone()),
 662                     stalled_on
 663                 );
 664
 665                 ProcessResult::Unchanged
 666             }
 667             Err(selection_err) => {
 668                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 669
 670                 ProcessResult::Error(CodeSelectionError(selection_err))
 671             }
 672         }
 673     }
 674
 675     fn process_projection_obligation(
 676         &mut self,
 677         obligation: &PredicateObligation<'tcx>,
 678         project_obligation: PolyProjectionObligation<'tcx>,
 679         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 680     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 681         let tcx = self.selcx.tcx();
 682
 683         if obligation.predicate.is_global() {
 684             // no type variables present, can use evaluation for better caching.
 685             // FIXME: consider caching errors too.
 686             if self.selcx.infcx.predicate_must_hold_considering_regions(obligation) {
 687                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 688                     &mut self.selcx,
 689                     project_obligation.predicate,
 690                 ) {
 691                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 692                     // evaluated all sub-obligations. We can therefore mark the 'root'
 693                     // obligation as complete, and skip evaluating sub-obligations.
 694                     self.selcx
 695                         .infcx
 696                         .inner
 697                         .borrow_mut()
 698                         .projection_cache()
 699                         .complete(key, EvaluationResult::EvaluatedToOk);
 700                 }
 701                 return ProcessResult::Changed(vec![]);
 702             } else {
 703                 debug!("Does NOT hold: {:?}", obligation);
 704             }
 705         }
 706
 707         match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
 708             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 709             ProjectAndUnifyResult::FailedNormalization => {
 710                 stalled_on.clear();
 711                 stalled_on.extend(substs_infer_vars(
 712                     &self.selcx,
 713                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 714                 ));
 715                 ProcessResult::Unchanged
 716             }
 717             // Let the caller handle the recursion
 718             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 719                 project_obligation.with(tcx, project_obligation.predicate),
 720             ])),
 721             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 722                 ProcessResult::Error(CodeProjectionError(e))
 723             }
 724         }
 725     }
 726 }
 727
 728 /// Returns the set of inference variables contained in `substs`.
 729 fn substs_infer_vars<'a, 'tcx>(
 730     selcx: &SelectionContext<'a, 'tcx>,
 731     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 732 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 733     selcx
 734         .infcx
 735         .resolve_vars_if_possible(substs)
 736         .skip_binder() // ok because this check doesn't care about regions
 737         .iter()
 738         .filter(|arg| arg.has_non_region_infer())
 739         .flat_map(|arg| {
 740             let mut walker = arg.walk();
 741             while let Some(c) = walker.next() {
 742                 if !c.has_non_region_infer() {
 743                     walker.visited.remove(&c);
 744                     walker.skip_current_subtree();
 745                 }
 746             }
 747             walker.visited.into_iter()
 748         })
 749         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 750 }
 751
 752 fn to_fulfillment_error<'tcx>(
 753     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 754 ) -> FulfillmentError<'tcx> {
 755     let mut iter = error.backtrace.into_iter();
 756     let obligation = iter.next().unwrap().obligation;
 757     // The root obligation is the last item in the backtrace - if there's only
 758     // one item, then it's the same as the main obligation
 759     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 760     FulfillmentError::new(obligation, error.error, root_obligation)
 761 }