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::ToPredicate;
  13 use rustc_middle::ty::{self, Binder, Const, Ty, TypeVisitable};
  14 use std::marker::PhantomData;
  15
  16 use super::const_evaluatable;
  17 use super::project::{self, ProjectAndUnifyResult};
  18 use super::select::SelectionContext;
  19 use super::wf;
  20 use super::CodeAmbiguity;
  21 use super::CodeProjectionError;
  22 use super::CodeSelectionError;
  23 use super::EvaluationResult;
  24 use super::Unimplemented;
  25 use super::{FulfillmentError, FulfillmentErrorCode};
  26 use super::{ObligationCause, PredicateObligation};
  27
  28 use crate::traits::error_reporting::InferCtxtExt as _;
  29 use crate::traits::project::PolyProjectionObligation;
  30 use crate::traits::project::ProjectionCacheKeyExt as _;
  31 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  32
  33 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  34     /// Note that we include both the `ParamEnv` and the `Predicate`,
  35     /// as the `ParamEnv` can influence whether fulfillment succeeds
  36     /// or fails.
  37     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  38
  39     fn as_cache_key(&self) -> Self::CacheKey {
  40         self.obligation.param_env.and(self.obligation.predicate)
  41     }
  42 }
  43
  44 /// The fulfillment context is used to drive trait resolution. It
  45 /// consists of a list of obligations that must be (eventually)
  46 /// satisfied. The job is to track which are satisfied, which yielded
  47 /// errors, and which are still pending. At any point, users can call
  48 /// `select_where_possible`, and the fulfillment context will try to do
  49 /// selection, retaining only those obligations that remain
  50 /// ambiguous. This may be helpful in pushing type inference
  51 /// along. Once all type inference constraints have been generated, the
  52 /// method `select_all_or_error` can be used to report any remaining
  53 /// ambiguous cases as errors.
  54 pub struct FulfillmentContext<'tcx> {
  55     // A list of all obligations that have been registered with this
  56     // fulfillment context.
  57     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  58
  59     relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
  60
  61     // Is it OK to register obligations into this infcx inside
  62     // an infcx snapshot?
  63     //
  64     // The "primary fulfillment" in many cases in typeck lives
  65     // outside of any snapshot, so any use of it inside a snapshot
  66     // will lead to trouble and therefore is checked against, but
  67     // other fulfillment contexts sometimes do live inside of
  68     // a snapshot (they don't *straddle* a snapshot, so there
  69     // is no trouble there).
  70     usable_in_snapshot: bool,
  71 }
  72
  73 #[derive(Clone, Debug)]
  74 pub struct PendingPredicateObligation<'tcx> {
  75     pub obligation: PredicateObligation<'tcx>,
  76     // This is far more often read than modified, meaning that we
  77     // should mostly optimize for reading speed, while modifying is not as relevant.
  78     //
  79     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  80     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  81 }
  82
  83 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  84 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  85 static_assert_size!(PendingPredicateObligation<'_>, 72);
  86
  87 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  88     /// Creates a new fulfillment context.
  89     pub fn new() -> FulfillmentContext<'tcx> {
  90         FulfillmentContext {
  91             predicates: ObligationForest::new(),
  92             relationships: FxHashMap::default(),
  93             usable_in_snapshot: false,
  94         }
  95     }
  96
  97     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
  98         FulfillmentContext {
  99             predicates: ObligationForest::new(),
 100             relationships: FxHashMap::default(),
 101             usable_in_snapshot: true,
 102         }
 103     }
 104
 105     /// Attempts to select obligations using `selcx`.
 106     fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 107         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 108         let _enter = span.enter();
 109
 110         // Process pending obligations.
 111         let outcome: Outcome<_, _> =
 112             self.predicates.process_obligations(&mut FulfillProcessor { selcx });
 113
 114         // FIXME: if we kept the original cache key, we could mark projection
 115         // obligations as complete for the projection cache here.
 116
 117         let errors: Vec<FulfillmentError<'tcx>> =
 118             outcome.errors.into_iter().map(to_fulfillment_error).collect();
 119
 120         debug!(
 121             "select({} predicates remaining, {} errors) done",
 122             self.predicates.len(),
 123             errors.len()
 124         );
 125
 126         errors
 127     }
 128 }
 129
 130 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 131     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 132     /// creating a fresh type variable `$0` as well as a projection
 133     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 134     /// inference engine runs, it will attempt to find an impl of
 135     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 136     /// something concrete. If this fails, we'll unify `$0` with
 137     /// `projection_ty` again.
 138     #[instrument(level = "debug", skip(self, infcx, param_env, cause))]
 139     fn normalize_projection_type(
 140         &mut self,
 141         infcx: &InferCtxt<'_, 'tcx>,
 142         param_env: ty::ParamEnv<'tcx>,
 143         projection_ty: ty::ProjectionTy<'tcx>,
 144         cause: ObligationCause<'tcx>,
 145     ) -> Ty<'tcx> {
 146         debug_assert!(!projection_ty.has_escaping_bound_vars());
 147
 148         // FIXME(#20304) -- cache
 149
 150         let mut selcx = SelectionContext::new(infcx);
 151         let mut obligations = vec![];
 152         let normalized_ty = project::normalize_projection_type(
 153             &mut selcx,
 154             param_env,
 155             projection_ty,
 156             cause,
 157             0,
 158             &mut obligations,
 159         );
 160         self.register_predicate_obligations(infcx, obligations);
 161
 162         debug!(?normalized_ty);
 163
 164         normalized_ty.ty().unwrap()
 165     }
 166
 167     fn register_predicate_obligation(
 168         &mut self,
 169         infcx: &InferCtxt<'_, 'tcx>,
 170         obligation: PredicateObligation<'tcx>,
 171     ) {
 172         // this helps to reduce duplicate errors, as well as making
 173         // debug output much nicer to read and so on.
 174         let obligation = infcx.resolve_vars_if_possible(obligation);
 175
 176         debug!(?obligation, "register_predicate_obligation");
 177
 178         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 179
 180         super::relationships::update(self, infcx, &obligation);
 181
 182         self.predicates
 183             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 184     }
 185
 186     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 187         {
 188             let errors = self.select_where_possible(infcx);
 189             if !errors.is_empty() {
 190                 return errors;
 191             }
 192         }
 193
 194         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 195     }
 196
 197     fn select_where_possible(
 198         &mut self,
 199         infcx: &InferCtxt<'_, 'tcx>,
 200     ) -> Vec<FulfillmentError<'tcx>> {
 201         let mut selcx = SelectionContext::new(infcx);
 202         self.select(&mut selcx)
 203     }
 204
 205     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 206         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 207     }
 208
 209     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 210         &mut self.relationships
 211     }
 212 }
 213
 214 struct FulfillProcessor<'a, 'b, 'tcx> {
 215     selcx: &'a mut SelectionContext<'b, 'tcx>,
 216 }
 217
 218 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 219     os.into_iter()
 220         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 221         .collect()
 222 }
 223
 224 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 225     type Obligation = PendingPredicateObligation<'tcx>;
 226     type Error = FulfillmentErrorCode<'tcx>;
 227
 228     /// Identifies whether a predicate obligation needs processing.
 229     ///
 230     /// This is always inlined, despite its size, because it has a single
 231     /// callsite and it is called *very* frequently.
 232     #[inline(always)]
 233     fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 234         // If we were stalled on some unresolved variables, first check whether
 235         // any of them have been resolved; if not, don't bother doing more work
 236         // yet.
 237         match pending_obligation.stalled_on.len() {
 238             // Match arms are in order of frequency, which matters because this
 239             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 240             1 => {
 241                 let infer_var = pending_obligation.stalled_on[0];
 242                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 243             }
 244             0 => {
 245                 // In this case we haven't changed, but wish to make a change.
 246                 true
 247             }
 248             _ => {
 249                 // This `for` loop was once a call to `all()`, but this lower-level
 250                 // form was a perf win. See #64545 for details.
 251                 (|| {
 252                     for &infer_var in &pending_obligation.stalled_on {
 253                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 254                             return true;
 255                         }
 256                     }
 257                     false
 258                 })()
 259             }
 260         }
 261     }
 262
 263     /// Processes a predicate obligation and returns either:
 264     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 265     /// - `Unchanged` if we don't have enough info to be sure
 266     /// - `Error(e)` if the predicate does not hold
 267     ///
 268     /// This is called much less often than `needs_process_obligation`, so we
 269     /// never inline it.
 270     #[inline(never)]
 271     #[instrument(level = "debug", skip(self, pending_obligation))]
 272     fn process_obligation(
 273         &mut self,
 274         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 275     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 276         pending_obligation.stalled_on.truncate(0);
 277
 278         let obligation = &mut pending_obligation.obligation;
 279
 280         debug!(?obligation, "pre-resolve");
 281
 282         if obligation.predicate.has_infer_types_or_consts() {
 283             obligation.predicate =
 284                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 285         }
 286
 287         let obligation = &pending_obligation.obligation;
 288
 289         let infcx = self.selcx.infcx();
 290
 291         if obligation.predicate.has_projections() {
 292             let mut obligations = Vec::new();
 293             let predicate = crate::traits::project::try_normalize_with_depth_to(
 294                 self.selcx,
 295                 obligation.param_env,
 296                 obligation.cause.clone(),
 297                 obligation.recursion_depth + 1,
 298                 obligation.predicate,
 299                 &mut obligations,
 300             );
 301             if predicate != obligation.predicate {
 302                 obligations.push(obligation.with(predicate));
 303                 return ProcessResult::Changed(mk_pending(obligations));
 304             }
 305         }
 306         let binder = obligation.predicate.kind();
 307         match binder.no_bound_vars() {
 308             None => match binder.skip_binder() {
 309                 // Evaluation will discard candidates using the leak check.
 310                 // This means we need to pass it the bound version of our
 311                 // predicate.
 312                 ty::PredicateKind::Trait(trait_ref) => {
 313                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 314
 315                     self.process_trait_obligation(
 316                         obligation,
 317                         trait_obligation,
 318                         &mut pending_obligation.stalled_on,
 319                     )
 320                 }
 321                 ty::PredicateKind::Projection(data) => {
 322                     let project_obligation = obligation.with(binder.rebind(data));
 323
 324                     self.process_projection_obligation(
 325                         obligation,
 326                         project_obligation,
 327                         &mut pending_obligation.stalled_on,
 328                     )
 329                 }
 330                 ty::PredicateKind::RegionOutlives(_)
 331                 | ty::PredicateKind::TypeOutlives(_)
 332                 | ty::PredicateKind::WellFormed(_)
 333                 | ty::PredicateKind::ObjectSafe(_)
 334                 | ty::PredicateKind::ClosureKind(..)
 335                 | ty::PredicateKind::Subtype(_)
 336                 | ty::PredicateKind::Coerce(_)
 337                 | ty::PredicateKind::ConstEvaluatable(..)
 338                 | ty::PredicateKind::ConstEquate(..) => {
 339                     let pred =
 340                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 341                     ProcessResult::Changed(mk_pending(vec![
 342                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 343                     ]))
 344                 }
 345                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 346                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 347                 }
 348             },
 349             Some(pred) => match pred {
 350                 ty::PredicateKind::Trait(data) => {
 351                     let trait_obligation = obligation.with(Binder::dummy(data));
 352
 353                     self.process_trait_obligation(
 354                         obligation,
 355                         trait_obligation,
 356                         &mut pending_obligation.stalled_on,
 357                     )
 358                 }
 359
 360                 ty::PredicateKind::RegionOutlives(data) => {
 361                     if infcx.considering_regions || data.has_placeholders() {
 362                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 363                     }
 364
 365                     ProcessResult::Changed(vec![])
 366                 }
 367
 368                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 369                     if infcx.considering_regions {
 370                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 371                     }
 372                     ProcessResult::Changed(vec![])
 373                 }
 374
 375                 ty::PredicateKind::Projection(ref data) => {
 376                     let project_obligation = obligation.with(Binder::dummy(*data));
 377
 378                     self.process_projection_obligation(
 379                         obligation,
 380                         project_obligation,
 381                         &mut pending_obligation.stalled_on,
 382                     )
 383                 }
 384
 385                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 386                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 387                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 388                     } else {
 389                         ProcessResult::Changed(vec![])
 390                     }
 391                 }
 392
 393                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 394                     match self.selcx.infcx().closure_kind(closure_substs) {
 395                         Some(closure_kind) => {
 396                             if closure_kind.extends(kind) {
 397                                 ProcessResult::Changed(vec![])
 398                             } else {
 399                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 400                             }
 401                         }
 402                         None => ProcessResult::Unchanged,
 403                     }
 404                 }
 405
 406                 ty::PredicateKind::WellFormed(arg) => {
 407                     match wf::obligations(
 408                         self.selcx.infcx(),
 409                         obligation.param_env,
 410                         obligation.cause.body_id,
 411                         obligation.recursion_depth + 1,
 412                         arg,
 413                         obligation.cause.span,
 414                     ) {
 415                         None => {
 416                             pending_obligation.stalled_on =
 417                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 418                             ProcessResult::Unchanged
 419                         }
 420                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 421                     }
 422                 }
 423
 424                 ty::PredicateKind::Subtype(subtype) => {
 425                     match self.selcx.infcx().subtype_predicate(
 426                         &obligation.cause,
 427                         obligation.param_env,
 428                         Binder::dummy(subtype),
 429                     ) {
 430                         None => {
 431                             // None means that both are unresolved.
 432                             pending_obligation.stalled_on = vec![
 433                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 434                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 435                             ];
 436                             ProcessResult::Unchanged
 437                         }
 438                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 439                         Some(Err(err)) => {
 440                             let expected_found =
 441                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 442                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 443                                 expected_found,
 444                                 err,
 445                             ))
 446                         }
 447                     }
 448                 }
 449
 450                 ty::PredicateKind::Coerce(coerce) => {
 451                     match self.selcx.infcx().coerce_predicate(
 452                         &obligation.cause,
 453                         obligation.param_env,
 454                         Binder::dummy(coerce),
 455                     ) {
 456                         None => {
 457                             // None means that both are unresolved.
 458                             pending_obligation.stalled_on = vec![
 459                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 460                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 461                             ];
 462                             ProcessResult::Unchanged
 463                         }
 464                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 465                         Some(Err(err)) => {
 466                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 467                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 468                                 expected_found,
 469                                 err,
 470                             ))
 471                         }
 472                     }
 473                 }
 474
 475                 ty::PredicateKind::ConstEvaluatable(uv) => {
 476                     match const_evaluatable::is_const_evaluatable(
 477                         self.selcx.infcx(),
 478                         uv,
 479                         obligation.param_env,
 480                         obligation.cause.span,
 481                     ) {
 482                         Ok(()) => ProcessResult::Changed(vec![]),
 483                         Err(NotConstEvaluatable::MentionsInfer) => {
 484                             pending_obligation.stalled_on.clear();
 485                             pending_obligation.stalled_on.extend(
 486                                 uv.substs
 487                                     .iter()
 488                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 489                             );
 490                             ProcessResult::Unchanged
 491                         }
 492                         Err(
 493                             e @ NotConstEvaluatable::MentionsParam
 494                             | e @ NotConstEvaluatable::Error(_),
 495                         ) => ProcessResult::Error(CodeSelectionError(
 496                             SelectionError::NotConstEvaluatable(e),
 497                         )),
 498                     }
 499                 }
 500
 501                 ty::PredicateKind::ConstEquate(c1, c2) => {
 502                     debug!(?c1, ?c2, "equating consts");
 503                     let tcx = self.selcx.tcx();
 504                     if tcx.features().generic_const_exprs {
 505                         // FIXME: we probably should only try to unify abstract constants
 506                         // if the constants depend on generic parameters.
 507                         //
 508                         // Let's just see where this breaks :shrug:
 509                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 510                             (c1.kind(), c2.kind())
 511                         {
 512                             if infcx.try_unify_abstract_consts(
 513                                 a.shrink(),
 514                                 b.shrink(),
 515                                 obligation.param_env,
 516                             ) {
 517                                 return ProcessResult::Changed(vec![]);
 518                             }
 519                         }
 520                     }
 521
 522                     let stalled_on = &mut pending_obligation.stalled_on;
 523
 524                     let mut evaluate = |c: Const<'tcx>| {
 525                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 526                             match self.selcx.infcx().try_const_eval_resolve(
 527                                 obligation.param_env,
 528                                 unevaluated,
 529                                 c.ty(),
 530                                 Some(obligation.cause.span),
 531                             ) {
 532                                 Ok(val) => Ok(val),
 533                                 Err(e) => match e {
 534                                     ErrorHandled::TooGeneric => {
 535                                         stalled_on.extend(
 536                                             unevaluated.substs.iter().filter_map(
 537                                                 TyOrConstInferVar::maybe_from_generic_arg,
 538                                             ),
 539                                         );
 540                                         Err(ErrorHandled::TooGeneric)
 541                                     }
 542                                     _ => Err(e),
 543                                 },
 544                             }
 545                         } else {
 546                             Ok(c)
 547                         }
 548                     };
 549
 550                     match (evaluate(c1), evaluate(c2)) {
 551                         (Ok(c1), Ok(c2)) => {
 552                             match self
 553                                 .selcx
 554                                 .infcx()
 555                                 .at(&obligation.cause, obligation.param_env)
 556                                 .eq(c1, c2)
 557                             {
 558                                 Ok(_) => ProcessResult::Changed(vec![]),
 559                                 Err(err) => ProcessResult::Error(
 560                                     FulfillmentErrorCode::CodeConstEquateError(
 561                                         ExpectedFound::new(true, c1, c2),
 562                                         err,
 563                                     ),
 564                                 ),
 565                             }
 566                         }
 567                         (Err(ErrorHandled::Reported(reported)), _)
 568                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 569                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 570                                 NotConstEvaluatable::Error(reported),
 571                             )),
 572                         ),
 573                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 574                             span_bug!(
 575                                 obligation.cause.span(),
 576                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 577                             )
 578                         }
 579                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 580                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 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::TypeWellFormedFromEnv(..) => {
 594                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 595                 }
 596             },
 597         }
 598     }
 599
 600     #[inline(never)]
 601     fn process_backedge<'c, I>(
 602         &mut self,
 603         cycle: I,
 604         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 605     ) where
 606         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 607     {
 608         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 609             debug!("process_child_obligations: coinductive match");
 610         } else {
 611             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 612             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 613         }
 614     }
 615 }
 616
 617 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 618     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 619     fn process_trait_obligation(
 620         &mut self,
 621         obligation: &PredicateObligation<'tcx>,
 622         trait_obligation: TraitObligation<'tcx>,
 623         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 624     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 625         let infcx = self.selcx.infcx();
 626         if obligation.predicate.is_global() {
 627             // no type variables present, can use evaluation for better caching.
 628             // FIXME: consider caching errors too.
 629             if infcx.predicate_must_hold_considering_regions(obligation) {
 630                 debug!(
 631                     "selecting trait at depth {} evaluated to holds",
 632                     obligation.recursion_depth
 633                 );
 634                 return ProcessResult::Changed(vec![]);
 635             }
 636         }
 637
 638         match self.selcx.select(&trait_obligation) {
 639             Ok(Some(impl_source)) => {
 640                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 641                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 642             }
 643             Ok(None) => {
 644                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 645
 646                 // This is a bit subtle: for the most part, the
 647                 // only reason we can fail to make progress on
 648                 // trait selection is because we don't have enough
 649                 // information about the types in the trait.
 650                 stalled_on.clear();
 651                 stalled_on.extend(substs_infer_vars(
 652                     self.selcx,
 653                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 654                 ));
 655
 656                 debug!(
 657                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 658                     infcx.resolve_vars_if_possible(obligation.clone()),
 659                     stalled_on
 660                 );
 661
 662                 ProcessResult::Unchanged
 663             }
 664             Err(selection_err) => {
 665                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 666
 667                 ProcessResult::Error(CodeSelectionError(selection_err))
 668             }
 669         }
 670     }
 671
 672     fn process_projection_obligation(
 673         &mut self,
 674         obligation: &PredicateObligation<'tcx>,
 675         project_obligation: PolyProjectionObligation<'tcx>,
 676         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 677     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 678         let tcx = self.selcx.tcx();
 679
 680         if obligation.predicate.is_global() {
 681             // no type variables present, can use evaluation for better caching.
 682             // FIXME: consider caching errors too.
 683             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 684                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 685                     &mut self.selcx,
 686                     project_obligation.predicate,
 687                 ) {
 688                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 689                     // evaluated all sub-obligations. We can therefore mark the 'root'
 690                     // obligation as complete, and skip evaluating sub-obligations.
 691                     self.selcx
 692                         .infcx()
 693                         .inner
 694                         .borrow_mut()
 695                         .projection_cache()
 696                         .complete(key, EvaluationResult::EvaluatedToOk);
 697                 }
 698                 return ProcessResult::Changed(vec![]);
 699             } else {
 700                 debug!("Does NOT hold: {:?}", obligation);
 701             }
 702         }
 703
 704         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 705             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 706             ProjectAndUnifyResult::FailedNormalization => {
 707                 stalled_on.clear();
 708                 stalled_on.extend(substs_infer_vars(
 709                     self.selcx,
 710                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 711                 ));
 712                 ProcessResult::Unchanged
 713             }
 714             // Let the caller handle the recursion
 715             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 716                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 717             ])),
 718             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 719                 ProcessResult::Error(CodeProjectionError(e))
 720             }
 721         }
 722     }
 723 }
 724
 725 /// Returns the set of inference variables contained in `substs`.
 726 fn substs_infer_vars<'a, 'tcx>(
 727     selcx: &mut SelectionContext<'a, 'tcx>,
 728     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 729 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 730     selcx
 731         .infcx()
 732         .resolve_vars_if_possible(substs)
 733         .skip_binder() // ok because this check doesn't care about regions
 734         .iter()
 735         .filter(|arg| arg.has_infer_types_or_consts())
 736         .flat_map(|arg| {
 737             let mut walker = arg.walk();
 738             while let Some(c) = walker.next() {
 739                 if !c.has_infer_types_or_consts() {
 740                     walker.visited.remove(&c);
 741                     walker.skip_current_subtree();
 742                 }
 743             }
 744             walker.visited.into_iter()
 745         })
 746         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 747 }
 748
 749 fn to_fulfillment_error<'tcx>(
 750     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 751 ) -> FulfillmentError<'tcx> {
 752     let mut iter = error.backtrace.into_iter();
 753     let obligation = iter.next().unwrap().obligation;
 754     // The root obligation is the last item in the backtrace - if there's only
 755     // one item, then it's the same as the main obligation
 756     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 757     FulfillmentError::new(obligation, error.error, root_obligation)
 758 }