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     #[tracing::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                         match infcx
 363                             .region_outlives_predicate(&obligation.cause, Binder::dummy(data))
 364                         {
 365                             Ok(()) => ProcessResult::Changed(vec![]),
 366                             Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 367                         }
 368                     } else {
 369                         ProcessResult::Changed(vec![])
 370                     }
 371                 }
 372
 373                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 374                     if infcx.considering_regions {
 375                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 376                     }
 377                     ProcessResult::Changed(vec![])
 378                 }
 379
 380                 ty::PredicateKind::Projection(ref data) => {
 381                     let project_obligation = obligation.with(Binder::dummy(*data));
 382
 383                     self.process_projection_obligation(
 384                         obligation,
 385                         project_obligation,
 386                         &mut pending_obligation.stalled_on,
 387                     )
 388                 }
 389
 390                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 391                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 392                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 393                     } else {
 394                         ProcessResult::Changed(vec![])
 395                     }
 396                 }
 397
 398                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 399                     match self.selcx.infcx().closure_kind(closure_substs) {
 400                         Some(closure_kind) => {
 401                             if closure_kind.extends(kind) {
 402                                 ProcessResult::Changed(vec![])
 403                             } else {
 404                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 405                             }
 406                         }
 407                         None => ProcessResult::Unchanged,
 408                     }
 409                 }
 410
 411                 ty::PredicateKind::WellFormed(arg) => {
 412                     match wf::obligations(
 413                         self.selcx.infcx(),
 414                         obligation.param_env,
 415                         obligation.cause.body_id,
 416                         obligation.recursion_depth + 1,
 417                         arg,
 418                         obligation.cause.span,
 419                     ) {
 420                         None => {
 421                             pending_obligation.stalled_on =
 422                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 423                             ProcessResult::Unchanged
 424                         }
 425                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 426                     }
 427                 }
 428
 429                 ty::PredicateKind::Subtype(subtype) => {
 430                     match self.selcx.infcx().subtype_predicate(
 431                         &obligation.cause,
 432                         obligation.param_env,
 433                         Binder::dummy(subtype),
 434                     ) {
 435                         None => {
 436                             // None means that both are unresolved.
 437                             pending_obligation.stalled_on = vec![
 438                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 439                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 440                             ];
 441                             ProcessResult::Unchanged
 442                         }
 443                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 444                         Some(Err(err)) => {
 445                             let expected_found =
 446                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 447                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 448                                 expected_found,
 449                                 err,
 450                             ))
 451                         }
 452                     }
 453                 }
 454
 455                 ty::PredicateKind::Coerce(coerce) => {
 456                     match self.selcx.infcx().coerce_predicate(
 457                         &obligation.cause,
 458                         obligation.param_env,
 459                         Binder::dummy(coerce),
 460                     ) {
 461                         None => {
 462                             // None means that both are unresolved.
 463                             pending_obligation.stalled_on = vec![
 464                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 465                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 466                             ];
 467                             ProcessResult::Unchanged
 468                         }
 469                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 470                         Some(Err(err)) => {
 471                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 472                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 473                                 expected_found,
 474                                 err,
 475                             ))
 476                         }
 477                     }
 478                 }
 479
 480                 ty::PredicateKind::ConstEvaluatable(uv) => {
 481                     match const_evaluatable::is_const_evaluatable(
 482                         self.selcx.infcx(),
 483                         uv,
 484                         obligation.param_env,
 485                         obligation.cause.span,
 486                     ) {
 487                         Ok(()) => ProcessResult::Changed(vec![]),
 488                         Err(NotConstEvaluatable::MentionsInfer) => {
 489                             pending_obligation.stalled_on.clear();
 490                             pending_obligation.stalled_on.extend(
 491                                 uv.substs
 492                                     .iter()
 493                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 494                             );
 495                             ProcessResult::Unchanged
 496                         }
 497                         Err(
 498                             e @ NotConstEvaluatable::MentionsParam
 499                             | e @ NotConstEvaluatable::Error(_),
 500                         ) => ProcessResult::Error(CodeSelectionError(
 501                             SelectionError::NotConstEvaluatable(e),
 502                         )),
 503                     }
 504                 }
 505
 506                 ty::PredicateKind::ConstEquate(c1, c2) => {
 507                     debug!(?c1, ?c2, "equating consts");
 508                     let tcx = self.selcx.tcx();
 509                     if tcx.features().generic_const_exprs {
 510                         // FIXME: we probably should only try to unify abstract constants
 511                         // if the constants depend on generic parameters.
 512                         //
 513                         // Let's just see where this breaks :shrug:
 514                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 515                             (c1.kind(), c2.kind())
 516                         {
 517                             if infcx.try_unify_abstract_consts(
 518                                 a.shrink(),
 519                                 b.shrink(),
 520                                 obligation.param_env,
 521                             ) {
 522                                 return ProcessResult::Changed(vec![]);
 523                             }
 524                         }
 525                     }
 526
 527                     let stalled_on = &mut pending_obligation.stalled_on;
 528
 529                     let mut evaluate = |c: Const<'tcx>| {
 530                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 531                             match self.selcx.infcx().try_const_eval_resolve(
 532                                 obligation.param_env,
 533                                 unevaluated,
 534                                 c.ty(),
 535                                 Some(obligation.cause.span),
 536                             ) {
 537                                 Ok(val) => Ok(val),
 538                                 Err(e) => match e {
 539                                     ErrorHandled::TooGeneric => {
 540                                         stalled_on.extend(
 541                                             unevaluated.substs.iter().filter_map(
 542                                                 TyOrConstInferVar::maybe_from_generic_arg,
 543                                             ),
 544                                         );
 545                                         Err(ErrorHandled::TooGeneric)
 546                                     }
 547                                     _ => Err(e),
 548                                 },
 549                             }
 550                         } else {
 551                             Ok(c)
 552                         }
 553                     };
 554
 555                     match (evaluate(c1), evaluate(c2)) {
 556                         (Ok(c1), Ok(c2)) => {
 557                             match self
 558                                 .selcx
 559                                 .infcx()
 560                                 .at(&obligation.cause, obligation.param_env)
 561                                 .eq(c1, c2)
 562                             {
 563                                 Ok(_) => ProcessResult::Changed(vec![]),
 564                                 Err(err) => ProcessResult::Error(
 565                                     FulfillmentErrorCode::CodeConstEquateError(
 566                                         ExpectedFound::new(true, c1, c2),
 567                                         err,
 568                                     ),
 569                                 ),
 570                             }
 571                         }
 572                         (Err(ErrorHandled::Reported(reported)), _)
 573                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 574                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 575                                 NotConstEvaluatable::Error(reported),
 576                             )),
 577                         ),
 578                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 579                             span_bug!(
 580                                 obligation.cause.span(),
 581                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 582                             )
 583                         }
 584                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 585                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 586                                 ProcessResult::Unchanged
 587                             } else {
 588                                 // Two different constants using generic parameters ~> error.
 589                                 let expected_found = ExpectedFound::new(true, c1, c2);
 590                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 591                                     expected_found,
 592                                     TypeError::ConstMismatch(expected_found),
 593                                 ))
 594                             }
 595                         }
 596                     }
 597                 }
 598                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 599                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 600                 }
 601             },
 602         }
 603     }
 604
 605     fn process_backedge<'c, I>(
 606         &mut self,
 607         cycle: I,
 608         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 609     ) where
 610         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 611     {
 612         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 613             debug!("process_child_obligations: coinductive match");
 614         } else {
 615             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 616             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 617         }
 618     }
 619 }
 620
 621 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 622     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 623     fn process_trait_obligation(
 624         &mut self,
 625         obligation: &PredicateObligation<'tcx>,
 626         trait_obligation: TraitObligation<'tcx>,
 627         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 628     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 629         let infcx = self.selcx.infcx();
 630         if obligation.predicate.is_global() {
 631             // no type variables present, can use evaluation for better caching.
 632             // FIXME: consider caching errors too.
 633             if infcx.predicate_must_hold_considering_regions(obligation) {
 634                 debug!(
 635                     "selecting trait at depth {} evaluated to holds",
 636                     obligation.recursion_depth
 637                 );
 638                 return ProcessResult::Changed(vec![]);
 639             }
 640         }
 641
 642         match self.selcx.select(&trait_obligation) {
 643             Ok(Some(impl_source)) => {
 644                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 645                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 646             }
 647             Ok(None) => {
 648                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 649
 650                 // This is a bit subtle: for the most part, the
 651                 // only reason we can fail to make progress on
 652                 // trait selection is because we don't have enough
 653                 // information about the types in the trait.
 654                 stalled_on.clear();
 655                 stalled_on.extend(substs_infer_vars(
 656                     self.selcx,
 657                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 658                 ));
 659
 660                 debug!(
 661                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 662                     infcx.resolve_vars_if_possible(obligation.clone()),
 663                     stalled_on
 664                 );
 665
 666                 ProcessResult::Unchanged
 667             }
 668             Err(selection_err) => {
 669                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 670
 671                 ProcessResult::Error(CodeSelectionError(selection_err))
 672             }
 673         }
 674     }
 675
 676     fn process_projection_obligation(
 677         &mut self,
 678         obligation: &PredicateObligation<'tcx>,
 679         project_obligation: PolyProjectionObligation<'tcx>,
 680         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 681     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 682         let tcx = self.selcx.tcx();
 683
 684         if obligation.predicate.is_global() {
 685             // no type variables present, can use evaluation for better caching.
 686             // FIXME: consider caching errors too.
 687             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 688                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 689                     &mut self.selcx,
 690                     project_obligation.predicate,
 691                 ) {
 692                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 693                     // evaluated all sub-obligations. We can therefore mark the 'root'
 694                     // obligation as complete, and skip evaluating sub-obligations.
 695                     self.selcx
 696                         .infcx()
 697                         .inner
 698                         .borrow_mut()
 699                         .projection_cache()
 700                         .complete(key, EvaluationResult::EvaluatedToOk);
 701                 }
 702                 return ProcessResult::Changed(vec![]);
 703             } else {
 704                 debug!("Does NOT hold: {:?}", obligation);
 705             }
 706         }
 707
 708         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 709             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 710             ProjectAndUnifyResult::FailedNormalization => {
 711                 stalled_on.clear();
 712                 stalled_on.extend(substs_infer_vars(
 713                     self.selcx,
 714                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 715                 ));
 716                 ProcessResult::Unchanged
 717             }
 718             // Let the caller handle the recursion
 719             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 720                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 721             ])),
 722             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 723                 ProcessResult::Error(CodeProjectionError(e))
 724             }
 725         }
 726     }
 727 }
 728
 729 /// Returns the set of inference variables contained in `substs`.
 730 fn substs_infer_vars<'a, 'tcx>(
 731     selcx: &mut SelectionContext<'a, 'tcx>,
 732     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 733 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 734     selcx
 735         .infcx()
 736         .resolve_vars_if_possible(substs)
 737         .skip_binder() // ok because this check doesn't care about regions
 738         .iter()
 739         .filter(|arg| arg.has_infer_types_or_consts())
 740         .flat_map(|arg| {
 741             let mut walker = arg.walk();
 742             while let Some(c) = walker.next() {
 743                 if !c.has_infer_types_or_consts() {
 744                     walker.visited.remove(&c);
 745                     walker.skip_current_subtree();
 746                 }
 747             }
 748             walker.visited.into_iter()
 749         })
 750         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 751 }
 752
 753 fn to_fulfillment_error<'tcx>(
 754     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 755 ) -> FulfillmentError<'tcx> {
 756     let mut iter = error.backtrace.into_iter();
 757     let obligation = iter.next().unwrap().obligation;
 758     // The root obligation is the last item in the backtrace - if there's only
 759     // one item, then it's the same as the main obligation
 760     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 761     FulfillmentError::new(obligation, error.error, root_obligation)
 762 }