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, TraitEngineExt as _, 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, Ty, 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::Unimplemented;
  24 use super::{FulfillmentError, FulfillmentErrorCode};
  25 use super::{ObligationCause, PredicateObligation};
  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     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 130     /// creating a fresh type variable `$0` as well as a projection
 131     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 132     /// inference engine runs, it will attempt to find an impl of
 133     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 134     /// something concrete. If this fails, we'll unify `$0` with
 135     /// `projection_ty` again.
 136     #[instrument(level = "debug", skip(self, infcx, param_env, cause))]
 137     fn normalize_projection_type(
 138         &mut self,
 139         infcx: &InferCtxt<'tcx>,
 140         param_env: ty::ParamEnv<'tcx>,
 141         projection_ty: ty::ProjectionTy<'tcx>,
 142         cause: ObligationCause<'tcx>,
 143     ) -> Ty<'tcx> {
 144         debug_assert!(!projection_ty.has_escaping_bound_vars());
 145
 146         // FIXME(#20304) -- cache
 147
 148         let mut selcx = SelectionContext::new(infcx);
 149         let mut obligations = vec![];
 150         let normalized_ty = project::normalize_projection_type(
 151             &mut selcx,
 152             param_env,
 153             projection_ty,
 154             cause,
 155             0,
 156             &mut obligations,
 157         );
 158         self.register_predicate_obligations(infcx, obligations);
 159
 160         debug!(?normalized_ty);
 161
 162         normalized_ty.ty().unwrap()
 163     }
 164
 165     fn register_predicate_obligation(
 166         &mut self,
 167         infcx: &InferCtxt<'tcx>,
 168         obligation: PredicateObligation<'tcx>,
 169     ) {
 170         // this helps to reduce duplicate errors, as well as making
 171         // debug output much nicer to read and so on.
 172         let obligation = infcx.resolve_vars_if_possible(obligation);
 173
 174         debug!(?obligation, "register_predicate_obligation");
 175
 176         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 177
 178         super::relationships::update(self, infcx, &obligation);
 179
 180         self.predicates
 181             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 182     }
 183
 184     fn select_all_or_error(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
 185         {
 186             let errors = self.select_where_possible(infcx);
 187             if !errors.is_empty() {
 188                 return errors;
 189             }
 190         }
 191
 192         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 193     }
 194
 195     fn select_where_possible(&mut self, infcx: &InferCtxt<'tcx>) -> Vec<FulfillmentError<'tcx>> {
 196         let selcx = SelectionContext::new(infcx);
 197         self.select(selcx)
 198     }
 199
 200     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 201         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 202     }
 203
 204     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 205         &mut self.relationships
 206     }
 207 }
 208
 209 struct FulfillProcessor<'a, 'tcx> {
 210     selcx: SelectionContext<'a, 'tcx>,
 211 }
 212
 213 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 214     os.into_iter()
 215         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 216         .collect()
 217 }
 218
 219 impl<'a, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'tcx> {
 220     type Obligation = PendingPredicateObligation<'tcx>;
 221     type Error = FulfillmentErrorCode<'tcx>;
 222     type OUT = Outcome<Self::Obligation, Self::Error>;
 223
 224     /// Identifies whether a predicate obligation needs processing.
 225     ///
 226     /// This is always inlined, despite its size, because it has a single
 227     /// callsite and it is called *very* frequently.
 228     #[inline(always)]
 229     fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 230         // If we were stalled on some unresolved variables, first check whether
 231         // any of them have been resolved; if not, don't bother doing more work
 232         // yet.
 233         match pending_obligation.stalled_on.len() {
 234             // Match arms are in order of frequency, which matters because this
 235             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 236             1 => {
 237                 let infer_var = pending_obligation.stalled_on[0];
 238                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 239             }
 240             0 => {
 241                 // In this case we haven't changed, but wish to make a change.
 242                 true
 243             }
 244             _ => {
 245                 // This `for` loop was once a call to `all()`, but this lower-level
 246                 // form was a perf win. See #64545 for details.
 247                 (|| {
 248                     for &infer_var in &pending_obligation.stalled_on {
 249                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 250                             return true;
 251                         }
 252                     }
 253                     false
 254                 })()
 255             }
 256         }
 257     }
 258
 259     /// Processes a predicate obligation and returns either:
 260     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 261     /// - `Unchanged` if we don't have enough info to be sure
 262     /// - `Error(e)` if the predicate does not hold
 263     ///
 264     /// This is called much less often than `needs_process_obligation`, so we
 265     /// never inline it.
 266     #[inline(never)]
 267     #[instrument(level = "debug", skip(self, pending_obligation))]
 268     fn process_obligation(
 269         &mut self,
 270         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 271     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 272         pending_obligation.stalled_on.truncate(0);
 273
 274         let obligation = &mut pending_obligation.obligation;
 275
 276         debug!(?obligation, "pre-resolve");
 277
 278         if obligation.predicate.has_non_region_infer() {
 279             obligation.predicate =
 280                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 281         }
 282
 283         let obligation = &pending_obligation.obligation;
 284
 285         let infcx = self.selcx.infcx();
 286
 287         if obligation.predicate.has_projections() {
 288             let mut obligations = Vec::new();
 289             let predicate = crate::traits::project::try_normalize_with_depth_to(
 290                 &mut self.selcx,
 291                 obligation.param_env,
 292                 obligation.cause.clone(),
 293                 obligation.recursion_depth + 1,
 294                 obligation.predicate,
 295                 &mut obligations,
 296             );
 297             if predicate != obligation.predicate {
 298                 obligations.push(obligation.with(infcx.tcx, predicate));
 299                 return ProcessResult::Changed(mk_pending(obligations));
 300             }
 301         }
 302         let binder = obligation.predicate.kind();
 303         match binder.no_bound_vars() {
 304             None => match binder.skip_binder() {
 305                 // Evaluation will discard candidates using the leak check.
 306                 // This means we need to pass it the bound version of our
 307                 // predicate.
 308                 ty::PredicateKind::Trait(trait_ref) => {
 309                     let trait_obligation = obligation.with(infcx.tcx, binder.rebind(trait_ref));
 310
 311                     self.process_trait_obligation(
 312                         obligation,
 313                         trait_obligation,
 314                         &mut pending_obligation.stalled_on,
 315                     )
 316                 }
 317                 ty::PredicateKind::Projection(data) => {
 318                     let project_obligation = obligation.with(infcx.tcx, binder.rebind(data));
 319
 320                     self.process_projection_obligation(
 321                         obligation,
 322                         project_obligation,
 323                         &mut pending_obligation.stalled_on,
 324                     )
 325                 }
 326                 ty::PredicateKind::RegionOutlives(_)
 327                 | ty::PredicateKind::TypeOutlives(_)
 328                 | ty::PredicateKind::WellFormed(_)
 329                 | ty::PredicateKind::ObjectSafe(_)
 330                 | ty::PredicateKind::ClosureKind(..)
 331                 | ty::PredicateKind::Subtype(_)
 332                 | ty::PredicateKind::Coerce(_)
 333                 | ty::PredicateKind::ConstEvaluatable(..)
 334                 | ty::PredicateKind::ConstEquate(..) => {
 335                     let pred =
 336                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 337                     ProcessResult::Changed(mk_pending(vec![obligation.with(infcx.tcx, pred)]))
 338                 }
 339                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 340                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 341                 }
 342             },
 343             Some(pred) => match pred {
 344                 ty::PredicateKind::Trait(data) => {
 345                     let trait_obligation = obligation.with(infcx.tcx, Binder::dummy(data));
 346
 347                     self.process_trait_obligation(
 348                         obligation,
 349                         trait_obligation,
 350                         &mut pending_obligation.stalled_on,
 351                     )
 352                 }
 353
 354                 ty::PredicateKind::RegionOutlives(data) => {
 355                     if infcx.considering_regions {
 356                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 357                     }
 358
 359                     ProcessResult::Changed(vec![])
 360                 }
 361
 362                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 363                     if infcx.considering_regions {
 364                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 365                     }
 366                     ProcessResult::Changed(vec![])
 367                 }
 368
 369                 ty::PredicateKind::Projection(ref data) => {
 370                     let project_obligation = obligation.with(infcx.tcx, Binder::dummy(*data));
 371
 372                     self.process_projection_obligation(
 373                         obligation,
 374                         project_obligation,
 375                         &mut pending_obligation.stalled_on,
 376                     )
 377                 }
 378
 379                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 380                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 381                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 382                     } else {
 383                         ProcessResult::Changed(vec![])
 384                     }
 385                 }
 386
 387                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 388                     match self.selcx.infcx().closure_kind(closure_substs) {
 389                         Some(closure_kind) => {
 390                             if closure_kind.extends(kind) {
 391                                 ProcessResult::Changed(vec![])
 392                             } else {
 393                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 394                             }
 395                         }
 396                         None => ProcessResult::Unchanged,
 397                     }
 398                 }
 399
 400                 ty::PredicateKind::WellFormed(arg) => {
 401                     match wf::obligations(
 402                         self.selcx.infcx(),
 403                         obligation.param_env,
 404                         obligation.cause.body_id,
 405                         obligation.recursion_depth + 1,
 406                         arg,
 407                         obligation.cause.span,
 408                     ) {
 409                         None => {
 410                             pending_obligation.stalled_on =
 411                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 412                             ProcessResult::Unchanged
 413                         }
 414                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 415                     }
 416                 }
 417
 418                 ty::PredicateKind::Subtype(subtype) => {
 419                     match self.selcx.infcx().subtype_predicate(
 420                         &obligation.cause,
 421                         obligation.param_env,
 422                         Binder::dummy(subtype),
 423                     ) {
 424                         Err((a, b)) => {
 425                             // None means that both are unresolved.
 426                             pending_obligation.stalled_on =
 427                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 428                             ProcessResult::Unchanged
 429                         }
 430                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 431                         Ok(Err(err)) => {
 432                             let expected_found =
 433                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 434                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 435                                 expected_found,
 436                                 err,
 437                             ))
 438                         }
 439                     }
 440                 }
 441
 442                 ty::PredicateKind::Coerce(coerce) => {
 443                     match self.selcx.infcx().coerce_predicate(
 444                         &obligation.cause,
 445                         obligation.param_env,
 446                         Binder::dummy(coerce),
 447                     ) {
 448                         Err((a, b)) => {
 449                             // None means that both are unresolved.
 450                             pending_obligation.stalled_on =
 451                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 452                             ProcessResult::Unchanged
 453                         }
 454                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 455                         Ok(Err(err)) => {
 456                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 457                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 458                                 expected_found,
 459                                 err,
 460                             ))
 461                         }
 462                     }
 463                 }
 464
 465                 ty::PredicateKind::ConstEvaluatable(uv) => {
 466                     match const_evaluatable::is_const_evaluatable(
 467                         self.selcx.infcx(),
 468                         uv,
 469                         obligation.param_env,
 470                         obligation.cause.span,
 471                     ) {
 472                         Ok(()) => ProcessResult::Changed(vec![]),
 473                         Err(NotConstEvaluatable::MentionsInfer) => {
 474                             pending_obligation.stalled_on.clear();
 475                             pending_obligation.stalled_on.extend(
 476                                 uv.walk().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 477                             );
 478                             ProcessResult::Unchanged
 479                         }
 480                         Err(
 481                             e @ NotConstEvaluatable::MentionsParam
 482                             | e @ NotConstEvaluatable::Error(_),
 483                         ) => ProcessResult::Error(CodeSelectionError(
 484                             SelectionError::NotConstEvaluatable(e),
 485                         )),
 486                     }
 487                 }
 488
 489                 ty::PredicateKind::ConstEquate(c1, c2) => {
 490                     assert!(
 491                         self.selcx.tcx().features().generic_const_exprs,
 492                         "`ConstEquate` without a feature gate: {c1:?} {c2:?}",
 493                     );
 494                     debug!(?c1, ?c2, "equating consts");
 495                     // FIXME: we probably should only try to unify abstract constants
 496                     // if the constants depend on generic parameters.
 497                     //
 498                     // Let's just see where this breaks :shrug:
 499                     if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 500                         (c1.kind(), c2.kind())
 501                     {
 502                         if infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
 503                             return ProcessResult::Changed(vec![]);
 504                         }
 505                     }
 506
 507                     let stalled_on = &mut pending_obligation.stalled_on;
 508
 509                     let mut evaluate = |c: Const<'tcx>| {
 510                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 511                             match self.selcx.infcx().try_const_eval_resolve(
 512                                 obligation.param_env,
 513                                 unevaluated,
 514                                 c.ty(),
 515                                 Some(obligation.cause.span),
 516                             ) {
 517                                 Ok(val) => Ok(val),
 518                                 Err(e) => match e {
 519                                     ErrorHandled::TooGeneric => {
 520                                         stalled_on.extend(
 521                                             unevaluated.substs.iter().filter_map(
 522                                                 TyOrConstInferVar::maybe_from_generic_arg,
 523                                             ),
 524                                         );
 525                                         Err(ErrorHandled::TooGeneric)
 526                                     }
 527                                     _ => Err(e),
 528                                 },
 529                             }
 530                         } else {
 531                             Ok(c)
 532                         }
 533                     };
 534
 535                     match (evaluate(c1), evaluate(c2)) {
 536                         (Ok(c1), Ok(c2)) => {
 537                             match self
 538                                 .selcx
 539                                 .infcx()
 540                                 .at(&obligation.cause, obligation.param_env)
 541                                 .eq(c1, c2)
 542                             {
 543                                 Ok(_) => ProcessResult::Changed(vec![]),
 544                                 Err(err) => ProcessResult::Error(
 545                                     FulfillmentErrorCode::CodeConstEquateError(
 546                                         ExpectedFound::new(true, c1, c2),
 547                                         err,
 548                                     ),
 549                                 ),
 550                             }
 551                         }
 552                         (Err(ErrorHandled::Reported(reported)), _)
 553                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 554                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 555                                 NotConstEvaluatable::Error(reported),
 556                             )),
 557                         ),
 558                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 559                             if c1.has_non_region_infer() || c2.has_non_region_infer() {
 560                                 ProcessResult::Unchanged
 561                             } else {
 562                                 // Two different constants using generic parameters ~> error.
 563                                 let expected_found = ExpectedFound::new(true, c1, c2);
 564                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 565                                     expected_found,
 566                                     TypeError::ConstMismatch(expected_found),
 567                                 ))
 568                             }
 569                         }
 570                     }
 571                 }
 572                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 573                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 574                 }
 575             },
 576         }
 577     }
 578
 579     #[inline(never)]
 580     fn process_backedge<'c, I>(
 581         &mut self,
 582         cycle: I,
 583         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 584     ) -> Result<(), FulfillmentErrorCode<'tcx>>
 585     where
 586         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 587     {
 588         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 589             debug!("process_child_obligations: coinductive match");
 590             Ok(())
 591         } else {
 592             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 593             Err(FulfillmentErrorCode::CodeCycle(cycle))
 594         }
 595     }
 596 }
 597
 598 impl<'a, 'tcx> FulfillProcessor<'a, 'tcx> {
 599     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 600     fn process_trait_obligation(
 601         &mut self,
 602         obligation: &PredicateObligation<'tcx>,
 603         trait_obligation: TraitObligation<'tcx>,
 604         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 605     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 606         let infcx = self.selcx.infcx();
 607         if obligation.predicate.is_global() {
 608             // no type variables present, can use evaluation for better caching.
 609             // FIXME: consider caching errors too.
 610             if infcx.predicate_must_hold_considering_regions(obligation) {
 611                 debug!(
 612                     "selecting trait at depth {} evaluated to holds",
 613                     obligation.recursion_depth
 614                 );
 615                 return ProcessResult::Changed(vec![]);
 616             }
 617         }
 618
 619         match self.selcx.select(&trait_obligation) {
 620             Ok(Some(impl_source)) => {
 621                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 622                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 623             }
 624             Ok(None) => {
 625                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 626
 627                 // This is a bit subtle: for the most part, the
 628                 // only reason we can fail to make progress on
 629                 // trait selection is because we don't have enough
 630                 // information about the types in the trait.
 631                 stalled_on.clear();
 632                 stalled_on.extend(substs_infer_vars(
 633                     &self.selcx,
 634                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 635                 ));
 636
 637                 debug!(
 638                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 639                     infcx.resolve_vars_if_possible(obligation.clone()),
 640                     stalled_on
 641                 );
 642
 643                 ProcessResult::Unchanged
 644             }
 645             Err(selection_err) => {
 646                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 647
 648                 ProcessResult::Error(CodeSelectionError(selection_err))
 649             }
 650         }
 651     }
 652
 653     fn process_projection_obligation(
 654         &mut self,
 655         obligation: &PredicateObligation<'tcx>,
 656         project_obligation: PolyProjectionObligation<'tcx>,
 657         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 658     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 659         let tcx = self.selcx.tcx();
 660
 661         if obligation.predicate.is_global() {
 662             // no type variables present, can use evaluation for better caching.
 663             // FIXME: consider caching errors too.
 664             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 665                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 666                     &mut self.selcx,
 667                     project_obligation.predicate,
 668                 ) {
 669                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 670                     // evaluated all sub-obligations. We can therefore mark the 'root'
 671                     // obligation as complete, and skip evaluating sub-obligations.
 672                     self.selcx
 673                         .infcx()
 674                         .inner
 675                         .borrow_mut()
 676                         .projection_cache()
 677                         .complete(key, EvaluationResult::EvaluatedToOk);
 678                 }
 679                 return ProcessResult::Changed(vec![]);
 680             } else {
 681                 debug!("Does NOT hold: {:?}", obligation);
 682             }
 683         }
 684
 685         match project::poly_project_and_unify_type(&mut self.selcx, &project_obligation) {
 686             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 687             ProjectAndUnifyResult::FailedNormalization => {
 688                 stalled_on.clear();
 689                 stalled_on.extend(substs_infer_vars(
 690                     &self.selcx,
 691                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 692                 ));
 693                 ProcessResult::Unchanged
 694             }
 695             // Let the caller handle the recursion
 696             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 697                 project_obligation.with(tcx, project_obligation.predicate),
 698             ])),
 699             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 700                 ProcessResult::Error(CodeProjectionError(e))
 701             }
 702         }
 703     }
 704 }
 705
 706 /// Returns the set of inference variables contained in `substs`.
 707 fn substs_infer_vars<'a, 'tcx>(
 708     selcx: &SelectionContext<'a, 'tcx>,
 709     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 710 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 711     selcx
 712         .infcx()
 713         .resolve_vars_if_possible(substs)
 714         .skip_binder() // ok because this check doesn't care about regions
 715         .iter()
 716         .filter(|arg| arg.has_non_region_infer())
 717         .flat_map(|arg| {
 718             let mut walker = arg.walk();
 719             while let Some(c) = walker.next() {
 720                 if !c.has_non_region_infer() {
 721                     walker.visited.remove(&c);
 722                     walker.skip_current_subtree();
 723                 }
 724             }
 725             walker.visited.into_iter()
 726         })
 727         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 728 }
 729
 730 fn to_fulfillment_error<'tcx>(
 731     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 732 ) -> FulfillmentError<'tcx> {
 733     let mut iter = error.backtrace.into_iter();
 734     let obligation = iter.next().unwrap().obligation;
 735     // The root obligation is the last item in the backtrace - if there's only
 736     // one item, then it's the same as the main obligation
 737     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 738     FulfillmentError::new(obligation, error.error, root_obligation)
 739 }