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::thir::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, TypeFoldable};
  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     // Should this fulfillment context register type-lives-for-region
  62     // obligations on its parent infcx? In some cases, region
  63     // obligations are either already known to hold (normalization) or
  64     // hopefully verified elsewhere (type-impls-bound), and therefore
  65     // should not be checked.
  66     //
  67     // Note that if we are normalizing a type that we already
  68     // know is well-formed, there should be no harm setting this
  69     // to true - all the region variables should be determinable
  70     // using the RFC 447 rules, which don't depend on
  71     // type-lives-for-region constraints, and because the type
  72     // is well-formed, the constraints should hold.
  73     register_region_obligations: bool,
  74     // Is it OK to register obligations into this infcx inside
  75     // an infcx snapshot?
  76     //
  77     // The "primary fulfillment" in many cases in typeck lives
  78     // outside of any snapshot, so any use of it inside a snapshot
  79     // will lead to trouble and therefore is checked against, but
  80     // other fulfillment contexts sometimes do live inside of
  81     // a snapshot (they don't *straddle* a snapshot, so there
  82     // is no trouble there).
  83     usable_in_snapshot: bool,
  84 }
  85
  86 #[derive(Clone, Debug)]
  87 pub struct PendingPredicateObligation<'tcx> {
  88     pub obligation: PredicateObligation<'tcx>,
  89     // This is far more often read than modified, meaning that we
  90     // should mostly optimize for reading speed, while modifying is not as relevant.
  91     //
  92     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  93     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  94 }
  95
  96 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  97 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  98 static_assert_size!(PendingPredicateObligation<'_>, 72);
  99
 100 impl<'a, 'tcx> FulfillmentContext<'tcx> {
 101     /// Creates a new fulfillment context.
 102     pub fn new() -> FulfillmentContext<'tcx> {
 103         FulfillmentContext {
 104             predicates: ObligationForest::new(),
 105             relationships: FxHashMap::default(),
 106             register_region_obligations: true,
 107             usable_in_snapshot: false,
 108         }
 109     }
 110
 111     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 112         FulfillmentContext {
 113             predicates: ObligationForest::new(),
 114             relationships: FxHashMap::default(),
 115             register_region_obligations: true,
 116             usable_in_snapshot: true,
 117         }
 118     }
 119
 120     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 121         FulfillmentContext {
 122             predicates: ObligationForest::new(),
 123             relationships: FxHashMap::default(),
 124             register_region_obligations: false,
 125             usable_in_snapshot: false,
 126         }
 127     }
 128
 129     /// Attempts to select obligations using `selcx`.
 130     fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 131         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 132         let _enter = span.enter();
 133
 134         let mut errors = Vec::new();
 135
 136         loop {
 137             debug!("select: starting another iteration");
 138
 139             // Process pending obligations.
 140             let outcome: Outcome<_, _> =
 141                 self.predicates.process_obligations(&mut FulfillProcessor {
 142                     selcx,
 143                     register_region_obligations: self.register_region_obligations,
 144                 });
 145             debug!("select: outcome={:#?}", outcome);
 146
 147             // FIXME: if we kept the original cache key, we could mark projection
 148             // obligations as complete for the projection cache here.
 149
 150             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 151
 152             // If nothing new was added, no need to keep looping.
 153             if outcome.stalled {
 154                 break;
 155             }
 156         }
 157
 158         debug!(
 159             "select({} predicates remaining, {} errors) done",
 160             self.predicates.len(),
 161             errors.len()
 162         );
 163
 164         errors
 165     }
 166 }
 167
 168 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 169     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 170     /// creating a fresh type variable `$0` as well as a projection
 171     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 172     /// inference engine runs, it will attempt to find an impl of
 173     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 174     /// something concrete. If this fails, we'll unify `$0` with
 175     /// `projection_ty` again.
 176     #[tracing::instrument(level = "debug", skip(self, infcx, param_env, cause))]
 177     fn normalize_projection_type(
 178         &mut self,
 179         infcx: &InferCtxt<'_, 'tcx>,
 180         param_env: ty::ParamEnv<'tcx>,
 181         projection_ty: ty::ProjectionTy<'tcx>,
 182         cause: ObligationCause<'tcx>,
 183     ) -> Ty<'tcx> {
 184         debug_assert!(!projection_ty.has_escaping_bound_vars());
 185
 186         // FIXME(#20304) -- cache
 187
 188         let mut selcx = SelectionContext::new(infcx);
 189         let mut obligations = vec![];
 190         let normalized_ty = project::normalize_projection_type(
 191             &mut selcx,
 192             param_env,
 193             projection_ty,
 194             cause,
 195             0,
 196             &mut obligations,
 197         );
 198         self.register_predicate_obligations(infcx, obligations);
 199
 200         debug!(?normalized_ty);
 201
 202         normalized_ty.ty().unwrap()
 203     }
 204
 205     fn register_predicate_obligation(
 206         &mut self,
 207         infcx: &InferCtxt<'_, 'tcx>,
 208         obligation: PredicateObligation<'tcx>,
 209     ) {
 210         // this helps to reduce duplicate errors, as well as making
 211         // debug output much nicer to read and so on.
 212         let obligation = infcx.resolve_vars_if_possible(obligation);
 213
 214         debug!(?obligation, "register_predicate_obligation");
 215
 216         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 217
 218         super::relationships::update(self, infcx, &obligation);
 219
 220         self.predicates
 221             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 222     }
 223
 224     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 225         {
 226             let errors = self.select_where_possible(infcx);
 227             if !errors.is_empty() {
 228                 return errors;
 229             }
 230         }
 231
 232         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 233     }
 234
 235     fn select_where_possible(
 236         &mut self,
 237         infcx: &InferCtxt<'_, 'tcx>,
 238     ) -> Vec<FulfillmentError<'tcx>> {
 239         let mut selcx = SelectionContext::new(infcx);
 240         self.select(&mut selcx)
 241     }
 242
 243     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 244         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 245     }
 246
 247     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 248         &mut self.relationships
 249     }
 250 }
 251
 252 struct FulfillProcessor<'a, 'b, 'tcx> {
 253     selcx: &'a mut SelectionContext<'b, 'tcx>,
 254     register_region_obligations: bool,
 255 }
 256
 257 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 258     os.into_iter()
 259         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 260         .collect()
 261 }
 262
 263 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 264     type Obligation = PendingPredicateObligation<'tcx>;
 265     type Error = FulfillmentErrorCode<'tcx>;
 266
 267     /// Processes a predicate obligation and returns either:
 268     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 269     /// - `Unchanged` if we don't have enough info to be sure
 270     /// - `Error(e)` if the predicate does not hold
 271     ///
 272     /// This is always inlined, despite its size, because it has a single
 273     /// callsite and it is called *very* frequently.
 274     #[inline(always)]
 275     fn process_obligation(
 276         &mut self,
 277         pending_obligation: &mut Self::Obligation,
 278     ) -> ProcessResult<Self::Obligation, Self::Error> {
 279         // If we were stalled on some unresolved variables, first check whether
 280         // any of them have been resolved; if not, don't bother doing more work
 281         // yet.
 282         let change = match pending_obligation.stalled_on.len() {
 283             // Match arms are in order of frequency, which matters because this
 284             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 285             1 => {
 286                 let infer_var = pending_obligation.stalled_on[0];
 287                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 288             }
 289             0 => {
 290                 // In this case we haven't changed, but wish to make a change.
 291                 true
 292             }
 293             _ => {
 294                 // This `for` loop was once a call to `all()`, but this lower-level
 295                 // form was a perf win. See #64545 for details.
 296                 (|| {
 297                     for &infer_var in &pending_obligation.stalled_on {
 298                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 299                             return true;
 300                         }
 301                     }
 302                     false
 303                 })()
 304             }
 305         };
 306
 307         if !change {
 308             debug!(
 309                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 310                 self.selcx.infcx().resolve_vars_if_possible(pending_obligation.obligation.clone()),
 311                 pending_obligation.stalled_on
 312             );
 313             return ProcessResult::Unchanged;
 314         }
 315
 316         self.process_changed_obligations(pending_obligation)
 317     }
 318
 319     fn process_backedge<'c, I>(
 320         &mut self,
 321         cycle: I,
 322         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 323     ) where
 324         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 325     {
 326         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 327             debug!("process_child_obligations: coinductive match");
 328         } else {
 329             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 330             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 331         }
 332     }
 333 }
 334
 335 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 336     // The code calling this method is extremely hot and only rarely
 337     // actually uses this, so move this part of the code
 338     // out of that loop.
 339     #[inline(never)]
 340     fn process_changed_obligations(
 341         &mut self,
 342         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 343     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 344         pending_obligation.stalled_on.truncate(0);
 345
 346         let obligation = &mut pending_obligation.obligation;
 347
 348         debug!(?obligation, "process_obligation pre-resolve");
 349
 350         if obligation.predicate.has_infer_types_or_consts() {
 351             obligation.predicate =
 352                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 353         }
 354
 355         debug!(?obligation, ?obligation.cause, "process_obligation");
 356
 357         let infcx = self.selcx.infcx();
 358
 359         if obligation.predicate.has_projections() {
 360             let mut obligations = Vec::new();
 361             let predicate = crate::traits::project::try_normalize_with_depth_to(
 362                 self.selcx,
 363                 obligation.param_env,
 364                 obligation.cause.clone(),
 365                 obligation.recursion_depth + 1,
 366                 obligation.predicate,
 367                 &mut obligations,
 368             );
 369             if predicate != obligation.predicate {
 370                 obligations.push(obligation.with(predicate));
 371                 return ProcessResult::Changed(mk_pending(obligations));
 372             }
 373         }
 374         let binder = obligation.predicate.kind();
 375         match binder.no_bound_vars() {
 376             None => match binder.skip_binder() {
 377                 // Evaluation will discard candidates using the leak check.
 378                 // This means we need to pass it the bound version of our
 379                 // predicate.
 380                 ty::PredicateKind::Trait(trait_ref) => {
 381                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 382
 383                     self.process_trait_obligation(
 384                         obligation,
 385                         trait_obligation,
 386                         &mut pending_obligation.stalled_on,
 387                     )
 388                 }
 389                 ty::PredicateKind::Projection(data) => {
 390                     let project_obligation = obligation.with(binder.rebind(data));
 391
 392                     self.process_projection_obligation(
 393                         obligation,
 394                         project_obligation,
 395                         &mut pending_obligation.stalled_on,
 396                     )
 397                 }
 398                 ty::PredicateKind::RegionOutlives(_)
 399                 | ty::PredicateKind::TypeOutlives(_)
 400                 | ty::PredicateKind::WellFormed(_)
 401                 | ty::PredicateKind::ObjectSafe(_)
 402                 | ty::PredicateKind::ClosureKind(..)
 403                 | ty::PredicateKind::Subtype(_)
 404                 | ty::PredicateKind::Coerce(_)
 405                 | ty::PredicateKind::ConstEvaluatable(..)
 406                 | ty::PredicateKind::ConstEquate(..) => {
 407                     let pred =
 408                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 409                     ProcessResult::Changed(mk_pending(vec![
 410                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 411                     ]))
 412                 }
 413                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 414                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 415                 }
 416             },
 417             Some(pred) => match pred {
 418                 ty::PredicateKind::Trait(data) => {
 419                     let trait_obligation = obligation.with(Binder::dummy(data));
 420
 421                     self.process_trait_obligation(
 422                         obligation,
 423                         trait_obligation,
 424                         &mut pending_obligation.stalled_on,
 425                     )
 426                 }
 427
 428                 ty::PredicateKind::RegionOutlives(data) => {
 429                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 430                         Ok(()) => ProcessResult::Changed(vec![]),
 431                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 432                     }
 433                 }
 434
 435                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 436                     if self.register_region_obligations {
 437                         self.selcx.infcx().register_region_obligation_with_cause(
 438                             t_a,
 439                             r_b,
 440                             &obligation.cause,
 441                         );
 442                     }
 443                     ProcessResult::Changed(vec![])
 444                 }
 445
 446                 ty::PredicateKind::Projection(ref data) => {
 447                     let project_obligation = obligation.with(Binder::dummy(*data));
 448
 449                     self.process_projection_obligation(
 450                         obligation,
 451                         project_obligation,
 452                         &mut pending_obligation.stalled_on,
 453                     )
 454                 }
 455
 456                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 457                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 458                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 459                     } else {
 460                         ProcessResult::Changed(vec![])
 461                     }
 462                 }
 463
 464                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 465                     match self.selcx.infcx().closure_kind(closure_substs) {
 466                         Some(closure_kind) => {
 467                             if closure_kind.extends(kind) {
 468                                 ProcessResult::Changed(vec![])
 469                             } else {
 470                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 471                             }
 472                         }
 473                         None => ProcessResult::Unchanged,
 474                     }
 475                 }
 476
 477                 ty::PredicateKind::WellFormed(arg) => {
 478                     match wf::obligations(
 479                         self.selcx.infcx(),
 480                         obligation.param_env,
 481                         obligation.cause.body_id,
 482                         obligation.recursion_depth + 1,
 483                         arg,
 484                         obligation.cause.span,
 485                     ) {
 486                         None => {
 487                             pending_obligation.stalled_on =
 488                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 489                             ProcessResult::Unchanged
 490                         }
 491                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 492                     }
 493                 }
 494
 495                 ty::PredicateKind::Subtype(subtype) => {
 496                     match self.selcx.infcx().subtype_predicate(
 497                         &obligation.cause,
 498                         obligation.param_env,
 499                         Binder::dummy(subtype),
 500                     ) {
 501                         None => {
 502                             // None means that both are unresolved.
 503                             pending_obligation.stalled_on = vec![
 504                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 505                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 506                             ];
 507                             ProcessResult::Unchanged
 508                         }
 509                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 510                         Some(Err(err)) => {
 511                             let expected_found =
 512                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 513                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 514                                 expected_found,
 515                                 err,
 516                             ))
 517                         }
 518                     }
 519                 }
 520
 521                 ty::PredicateKind::Coerce(coerce) => {
 522                     match self.selcx.infcx().coerce_predicate(
 523                         &obligation.cause,
 524                         obligation.param_env,
 525                         Binder::dummy(coerce),
 526                     ) {
 527                         None => {
 528                             // None means that both are unresolved.
 529                             pending_obligation.stalled_on = vec![
 530                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 531                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 532                             ];
 533                             ProcessResult::Unchanged
 534                         }
 535                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 536                         Some(Err(err)) => {
 537                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 538                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 539                                 expected_found,
 540                                 err,
 541                             ))
 542                         }
 543                     }
 544                 }
 545
 546                 ty::PredicateKind::ConstEvaluatable(uv) => {
 547                     match const_evaluatable::is_const_evaluatable(
 548                         self.selcx.infcx(),
 549                         uv,
 550                         obligation.param_env,
 551                         obligation.cause.span,
 552                     ) {
 553                         Ok(()) => ProcessResult::Changed(vec![]),
 554                         Err(NotConstEvaluatable::MentionsInfer) => {
 555                             pending_obligation.stalled_on.clear();
 556                             pending_obligation.stalled_on.extend(
 557                                 uv.substs
 558                                     .iter()
 559                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 560                             );
 561                             ProcessResult::Unchanged
 562                         }
 563                         Err(
 564                             e @ NotConstEvaluatable::MentionsParam
 565                             | e @ NotConstEvaluatable::Error(_),
 566                         ) => ProcessResult::Error(CodeSelectionError(
 567                             SelectionError::NotConstEvaluatable(e),
 568                         )),
 569                     }
 570                 }
 571
 572                 ty::PredicateKind::ConstEquate(c1, c2) => {
 573                     debug!(?c1, ?c2, "equating consts");
 574                     let tcx = self.selcx.tcx();
 575                     if tcx.features().generic_const_exprs {
 576                         // FIXME: we probably should only try to unify abstract constants
 577                         // if the constants depend on generic parameters.
 578                         //
 579                         // Let's just see where this breaks :shrug:
 580                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 581                             (c1.val(), c2.val())
 582                         {
 583                             if infcx.try_unify_abstract_consts(
 584                                 a.shrink(),
 585                                 b.shrink(),
 586                                 obligation.param_env,
 587                             ) {
 588                                 return ProcessResult::Changed(vec![]);
 589                             }
 590                         }
 591                     }
 592
 593                     let stalled_on = &mut pending_obligation.stalled_on;
 594
 595                     let mut evaluate = |c: Const<'tcx>| {
 596                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val() {
 597                             match self.selcx.infcx().const_eval_resolve(
 598                                 obligation.param_env,
 599                                 unevaluated,
 600                                 Some(obligation.cause.span),
 601                             ) {
 602                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty())),
 603                                 Err(ErrorHandled::TooGeneric) => {
 604                                     stalled_on.extend(
 605                                         unevaluated
 606                                             .substs
 607                                             .iter()
 608                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 609                                     );
 610                                     Err(ErrorHandled::TooGeneric)
 611                                 }
 612                                 Err(err) => Err(err),
 613                             }
 614                         } else {
 615                             Ok(c)
 616                         }
 617                     };
 618
 619                     match (evaluate(c1), evaluate(c2)) {
 620                         (Ok(c1), Ok(c2)) => {
 621                             match self
 622                                 .selcx
 623                                 .infcx()
 624                                 .at(&obligation.cause, obligation.param_env)
 625                                 .eq(c1, c2)
 626                             {
 627                                 Ok(_) => ProcessResult::Changed(vec![]),
 628                                 Err(err) => ProcessResult::Error(
 629                                     FulfillmentErrorCode::CodeConstEquateError(
 630                                         ExpectedFound::new(true, c1, c2),
 631                                         err,
 632                                     ),
 633                                 ),
 634                             }
 635                         }
 636                         (Err(ErrorHandled::Reported(reported)), _)
 637                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 638                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 639                                 NotConstEvaluatable::Error(reported),
 640                             )),
 641                         ),
 642                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 643                             span_bug!(
 644                                 obligation.cause.span(self.selcx.tcx()),
 645                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 646                             )
 647                         }
 648                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 649                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 650                                 ProcessResult::Unchanged
 651                             } else {
 652                                 // Two different constants using generic parameters ~> error.
 653                                 let expected_found = ExpectedFound::new(true, c1, c2);
 654                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 655                                     expected_found,
 656                                     TypeError::ConstMismatch(expected_found),
 657                                 ))
 658                             }
 659                         }
 660                     }
 661                 }
 662                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 663                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 664                 }
 665             },
 666         }
 667     }
 668
 669     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 670     fn process_trait_obligation(
 671         &mut self,
 672         obligation: &PredicateObligation<'tcx>,
 673         trait_obligation: TraitObligation<'tcx>,
 674         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 675     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 676         let infcx = self.selcx.infcx();
 677         if obligation.predicate.is_global() {
 678             // no type variables present, can use evaluation for better caching.
 679             // FIXME: consider caching errors too.
 680             if infcx.predicate_must_hold_considering_regions(obligation) {
 681                 debug!(
 682                     "selecting trait at depth {} evaluated to holds",
 683                     obligation.recursion_depth
 684                 );
 685                 return ProcessResult::Changed(vec![]);
 686             }
 687         }
 688
 689         match self.selcx.select(&trait_obligation) {
 690             Ok(Some(impl_source)) => {
 691                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 692                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 693             }
 694             Ok(None) => {
 695                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 696
 697                 // This is a bit subtle: for the most part, the
 698                 // only reason we can fail to make progress on
 699                 // trait selection is because we don't have enough
 700                 // information about the types in the trait.
 701                 stalled_on.clear();
 702                 stalled_on.extend(substs_infer_vars(
 703                     self.selcx,
 704                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 705                 ));
 706
 707                 debug!(
 708                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 709                     infcx.resolve_vars_if_possible(obligation.clone()),
 710                     stalled_on
 711                 );
 712
 713                 ProcessResult::Unchanged
 714             }
 715             Err(selection_err) => {
 716                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 717
 718                 ProcessResult::Error(CodeSelectionError(selection_err))
 719             }
 720         }
 721     }
 722
 723     fn process_projection_obligation(
 724         &mut self,
 725         obligation: &PredicateObligation<'tcx>,
 726         project_obligation: PolyProjectionObligation<'tcx>,
 727         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 728     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 729         let tcx = self.selcx.tcx();
 730
 731         if obligation.predicate.is_global() {
 732             // no type variables present, can use evaluation for better caching.
 733             // FIXME: consider caching errors too.
 734             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 735                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 736                     &mut self.selcx,
 737                     project_obligation.predicate,
 738                 ) {
 739                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 740                     // evaluated all sub-obligations. We can therefore mark the 'root'
 741                     // obligation as complete, and skip evaluating sub-obligations.
 742                     self.selcx
 743                         .infcx()
 744                         .inner
 745                         .borrow_mut()
 746                         .projection_cache()
 747                         .complete(key, EvaluationResult::EvaluatedToOk);
 748                 }
 749                 return ProcessResult::Changed(vec![]);
 750             } else {
 751                 tracing::debug!("Does NOT hold: {:?}", obligation);
 752             }
 753         }
 754
 755         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 756             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 757             ProjectAndUnifyResult::FailedNormalization => {
 758                 stalled_on.clear();
 759                 stalled_on.extend(substs_infer_vars(
 760                     self.selcx,
 761                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 762                 ));
 763                 ProcessResult::Unchanged
 764             }
 765             // Let the caller handle the recursion
 766             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 767                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 768             ])),
 769             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 770                 ProcessResult::Error(CodeProjectionError(e))
 771             }
 772         }
 773     }
 774 }
 775
 776 /// Returns the set of inference variables contained in `substs`.
 777 fn substs_infer_vars<'a, 'tcx>(
 778     selcx: &mut SelectionContext<'a, 'tcx>,
 779     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 780 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 781     selcx
 782         .infcx()
 783         .resolve_vars_if_possible(substs)
 784         .skip_binder() // ok because this check doesn't care about regions
 785         .iter()
 786         .filter(|arg| arg.has_infer_types_or_consts())
 787         .flat_map(|arg| {
 788             let mut walker = arg.walk();
 789             while let Some(c) = walker.next() {
 790                 if !c.has_infer_types_or_consts() {
 791                     walker.visited.remove(&c);
 792                     walker.skip_current_subtree();
 793                 }
 794             }
 795             walker.visited.into_iter()
 796         })
 797         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 798 }
 799
 800 fn to_fulfillment_error<'tcx>(
 801     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 802 ) -> FulfillmentError<'tcx> {
 803     let mut iter = error.backtrace.into_iter();
 804     let obligation = iter.next().unwrap().obligation;
 805     // The root obligation is the last item in the backtrace - if there's only
 806     // one item, then it's the same as the main obligation
 807     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 808     FulfillmentError::new(obligation, error.error, root_obligation)
 809 }