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_errors::ErrorReported;
   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;
  18 use super::select::SelectionContext;
  19 use super::wf;
  20 use super::CodeAmbiguity;
  21 use super::CodeProjectionError;
  22 use super::CodeSelectionError;
  23 use super::Unimplemented;
  24 use super::{FulfillmentError, FulfillmentErrorCode};
  25 use super::{ObligationCause, PredicateObligation};
  26
  27 use crate::traits::error_reporting::InferCtxtExt as _;
  28 use crate::traits::project::PolyProjectionObligation;
  29 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  30
  31 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  32     /// Note that we include both the `ParamEnv` and the `Predicate`,
  33     /// as the `ParamEnv` can influence whether fulfillment succeeds
  34     /// or fails.
  35     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  36
  37     fn as_cache_key(&self) -> Self::CacheKey {
  38         self.obligation.param_env.and(self.obligation.predicate)
  39     }
  40 }
  41
  42 /// The fulfillment context is used to drive trait resolution. It
  43 /// consists of a list of obligations that must be (eventually)
  44 /// satisfied. The job is to track which are satisfied, which yielded
  45 /// errors, and which are still pending. At any point, users can call
  46 /// `select_where_possible`, and the fulfillment context will try to do
  47 /// selection, retaining only those obligations that remain
  48 /// ambiguous. This may be helpful in pushing type inference
  49 /// along. Once all type inference constraints have been generated, the
  50 /// method `select_all_or_error` can be used to report any remaining
  51 /// ambiguous cases as errors.
  52 pub struct FulfillmentContext<'tcx> {
  53     // A list of all obligations that have been registered with this
  54     // fulfillment context.
  55     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  56
  57     relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
  58
  59     // Should this fulfillment context register type-lives-for-region
  60     // obligations on its parent infcx? In some cases, region
  61     // obligations are either already known to hold (normalization) or
  62     // hopefully verifed elsewhere (type-impls-bound), and therefore
  63     // should not be checked.
  64     //
  65     // Note that if we are normalizing a type that we already
  66     // know is well-formed, there should be no harm setting this
  67     // to true - all the region variables should be determinable
  68     // using the RFC 447 rules, which don't depend on
  69     // type-lives-for-region constraints, and because the type
  70     // is well-formed, the constraints should hold.
  71     register_region_obligations: bool,
  72     // Is it OK to register obligations into this infcx inside
  73     // an infcx snapshot?
  74     //
  75     // The "primary fulfillment" in many cases in typeck lives
  76     // outside of any snapshot, so any use of it inside a snapshot
  77     // will lead to trouble and therefore is checked against, but
  78     // other fulfillment contexts sometimes do live inside of
  79     // a snapshot (they don't *straddle* a snapshot, so there
  80     // is no trouble there).
  81     usable_in_snapshot: bool,
  82 }
  83
  84 #[derive(Clone, Debug)]
  85 pub struct PendingPredicateObligation<'tcx> {
  86     pub obligation: PredicateObligation<'tcx>,
  87     // This is far more often read than modified, meaning that we
  88     // should mostly optimize for reading speed, while modifying is not as relevant.
  89     //
  90     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  91     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  92 }
  93
  94 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  95 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  96 static_assert_size!(PendingPredicateObligation<'_>, 56);
  97
  98 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  99     /// Creates a new fulfillment context.
 100     pub fn new() -> FulfillmentContext<'tcx> {
 101         FulfillmentContext {
 102             predicates: ObligationForest::new(),
 103             relationships: FxHashMap::default(),
 104             register_region_obligations: true,
 105             usable_in_snapshot: false,
 106         }
 107     }
 108
 109     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 110         FulfillmentContext {
 111             predicates: ObligationForest::new(),
 112             relationships: FxHashMap::default(),
 113             register_region_obligations: true,
 114             usable_in_snapshot: true,
 115         }
 116     }
 117
 118     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 119         FulfillmentContext {
 120             predicates: ObligationForest::new(),
 121             relationships: FxHashMap::default(),
 122             register_region_obligations: false,
 123             usable_in_snapshot: false,
 124         }
 125     }
 126
 127     /// Attempts to select obligations using `selcx`.
 128     fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 129         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 130         let _enter = span.enter();
 131
 132         let mut errors = Vec::new();
 133
 134         loop {
 135             debug!("select: starting another iteration");
 136
 137             // Process pending obligations.
 138             let outcome: Outcome<_, _> =
 139                 self.predicates.process_obligations(&mut FulfillProcessor {
 140                     selcx,
 141                     register_region_obligations: self.register_region_obligations,
 142                 });
 143             debug!("select: outcome={:#?}", outcome);
 144
 145             // FIXME: if we kept the original cache key, we could mark projection
 146             // obligations as complete for the projection cache here.
 147
 148             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 149
 150             // If nothing new was added, no need to keep looping.
 151             if outcome.stalled {
 152                 break;
 153             }
 154         }
 155
 156         debug!(
 157             "select({} predicates remaining, {} errors) done",
 158             self.predicates.len(),
 159             errors.len()
 160         );
 161
 162         errors
 163     }
 164 }
 165
 166 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 167     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 168     /// creating a fresh type variable `$0` as well as a projection
 169     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 170     /// inference engine runs, it will attempt to find an impl of
 171     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 172     /// something concrete. If this fails, we'll unify `$0` with
 173     /// `projection_ty` again.
 174     #[tracing::instrument(level = "debug", skip(self, infcx, param_env, cause))]
 175     fn normalize_projection_type(
 176         &mut self,
 177         infcx: &InferCtxt<'_, 'tcx>,
 178         param_env: ty::ParamEnv<'tcx>,
 179         projection_ty: ty::ProjectionTy<'tcx>,
 180         cause: ObligationCause<'tcx>,
 181     ) -> Ty<'tcx> {
 182         debug_assert!(!projection_ty.has_escaping_bound_vars());
 183
 184         // FIXME(#20304) -- cache
 185
 186         let mut selcx = SelectionContext::new(infcx);
 187         let mut obligations = vec![];
 188         let normalized_ty = project::normalize_projection_type(
 189             &mut selcx,
 190             param_env,
 191             projection_ty,
 192             cause,
 193             0,
 194             &mut obligations,
 195         );
 196         self.register_predicate_obligations(infcx, obligations);
 197
 198         debug!(?normalized_ty);
 199
 200         normalized_ty
 201     }
 202
 203     fn register_predicate_obligation(
 204         &mut self,
 205         infcx: &InferCtxt<'_, 'tcx>,
 206         obligation: PredicateObligation<'tcx>,
 207     ) {
 208         // this helps to reduce duplicate errors, as well as making
 209         // debug output much nicer to read and so on.
 210         let obligation = infcx.resolve_vars_if_possible(obligation);
 211
 212         debug!(?obligation, "register_predicate_obligation");
 213
 214         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 215
 216         super::relationships::update(self, infcx, &obligation);
 217
 218         self.predicates
 219             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 220     }
 221
 222     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 223         {
 224             let errors = self.select_where_possible(infcx);
 225             if !errors.is_empty() {
 226                 return errors;
 227             }
 228         }
 229
 230         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 231     }
 232
 233     fn select_where_possible(
 234         &mut self,
 235         infcx: &InferCtxt<'_, 'tcx>,
 236     ) -> Vec<FulfillmentError<'tcx>> {
 237         let mut selcx = SelectionContext::new(infcx);
 238         self.select(&mut selcx)
 239     }
 240
 241     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 242         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 243     }
 244
 245     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 246         &mut self.relationships
 247     }
 248 }
 249
 250 struct FulfillProcessor<'a, 'b, 'tcx> {
 251     selcx: &'a mut SelectionContext<'b, 'tcx>,
 252     register_region_obligations: bool,
 253 }
 254
 255 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 256     os.into_iter()
 257         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 258         .collect()
 259 }
 260
 261 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 262     type Obligation = PendingPredicateObligation<'tcx>;
 263     type Error = FulfillmentErrorCode<'tcx>;
 264
 265     /// Processes a predicate obligation and returns either:
 266     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 267     /// - `Unchanged` if we don't have enough info to be sure
 268     /// - `Error(e)` if the predicate does not hold
 269     ///
 270     /// This is always inlined, despite its size, because it has a single
 271     /// callsite and it is called *very* frequently.
 272     #[inline(always)]
 273     fn process_obligation(
 274         &mut self,
 275         pending_obligation: &mut Self::Obligation,
 276     ) -> ProcessResult<Self::Obligation, Self::Error> {
 277         // If we were stalled on some unresolved variables, first check whether
 278         // any of them have been resolved; if not, don't bother doing more work
 279         // yet.
 280         let change = match pending_obligation.stalled_on.len() {
 281             // Match arms are in order of frequency, which matters because this
 282             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 283             1 => {
 284                 let infer_var = pending_obligation.stalled_on[0];
 285                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 286             }
 287             0 => {
 288                 // In this case we haven't changed, but wish to make a change.
 289                 true
 290             }
 291             _ => {
 292                 // This `for` loop was once a call to `all()`, but this lower-level
 293                 // form was a perf win. See #64545 for details.
 294                 (|| {
 295                     for &infer_var in &pending_obligation.stalled_on {
 296                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 297                             return true;
 298                         }
 299                     }
 300                     false
 301                 })()
 302             }
 303         };
 304
 305         if !change {
 306             debug!(
 307                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 308                 self.selcx.infcx().resolve_vars_if_possible(pending_obligation.obligation.clone()),
 309                 pending_obligation.stalled_on
 310             );
 311             return ProcessResult::Unchanged;
 312         }
 313
 314         self.progress_changed_obligations(pending_obligation)
 315     }
 316
 317     fn process_backedge<'c, I>(
 318         &mut self,
 319         cycle: I,
 320         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 321     ) where
 322         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 323     {
 324         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 325             debug!("process_child_obligations: coinductive match");
 326         } else {
 327             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 328             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 329         }
 330     }
 331 }
 332
 333 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 334     // The code calling this method is extremely hot and only rarely
 335     // actually uses this, so move this part of the code
 336     // out of that loop.
 337     #[inline(never)]
 338     fn progress_changed_obligations(
 339         &mut self,
 340         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 341     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 342         pending_obligation.stalled_on.truncate(0);
 343
 344         let obligation = &mut pending_obligation.obligation;
 345
 346         if obligation.predicate.has_infer_types_or_consts() {
 347             obligation.predicate =
 348                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 349         }
 350
 351         debug!(?obligation, ?obligation.cause, "process_obligation");
 352
 353         let infcx = self.selcx.infcx();
 354
 355         let binder = obligation.predicate.kind();
 356         match binder.no_bound_vars() {
 357             None => match binder.skip_binder() {
 358                 // Evaluation will discard candidates using the leak check.
 359                 // This means we need to pass it the bound version of our
 360                 // predicate.
 361                 ty::PredicateKind::Trait(trait_ref) => {
 362                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 363
 364                     self.process_trait_obligation(
 365                         obligation,
 366                         trait_obligation,
 367                         &mut pending_obligation.stalled_on,
 368                     )
 369                 }
 370                 ty::PredicateKind::Projection(data) => {
 371                     let project_obligation = obligation.with(binder.rebind(data));
 372
 373                     self.process_projection_obligation(
 374                         obligation,
 375                         project_obligation,
 376                         &mut pending_obligation.stalled_on,
 377                     )
 378                 }
 379                 ty::PredicateKind::RegionOutlives(_)
 380                 | ty::PredicateKind::TypeOutlives(_)
 381                 | ty::PredicateKind::WellFormed(_)
 382                 | ty::PredicateKind::ObjectSafe(_)
 383                 | ty::PredicateKind::ClosureKind(..)
 384                 | ty::PredicateKind::Subtype(_)
 385                 | ty::PredicateKind::Coerce(_)
 386                 | ty::PredicateKind::ConstEvaluatable(..)
 387                 | ty::PredicateKind::ConstEquate(..) => {
 388                     let pred =
 389                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 390                     ProcessResult::Changed(mk_pending(vec![
 391                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 392                     ]))
 393                 }
 394                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 395                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 396                 }
 397             },
 398             Some(pred) => match pred {
 399                 ty::PredicateKind::Trait(data) => {
 400                     let trait_obligation = obligation.with(Binder::dummy(data));
 401
 402                     self.process_trait_obligation(
 403                         obligation,
 404                         trait_obligation,
 405                         &mut pending_obligation.stalled_on,
 406                     )
 407                 }
 408
 409                 ty::PredicateKind::RegionOutlives(data) => {
 410                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 411                         Ok(()) => ProcessResult::Changed(vec![]),
 412                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 413                     }
 414                 }
 415
 416                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 417                     if self.register_region_obligations {
 418                         self.selcx.infcx().register_region_obligation_with_cause(
 419                             t_a,
 420                             r_b,
 421                             &obligation.cause,
 422                         );
 423                     }
 424                     ProcessResult::Changed(vec![])
 425                 }
 426
 427                 ty::PredicateKind::Projection(ref data) => {
 428                     let project_obligation = obligation.with(Binder::dummy(*data));
 429
 430                     self.process_projection_obligation(
 431                         obligation,
 432                         project_obligation,
 433                         &mut pending_obligation.stalled_on,
 434                     )
 435                 }
 436
 437                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 438                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 439                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 440                     } else {
 441                         ProcessResult::Changed(vec![])
 442                     }
 443                 }
 444
 445                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 446                     match self.selcx.infcx().closure_kind(closure_substs) {
 447                         Some(closure_kind) => {
 448                             if closure_kind.extends(kind) {
 449                                 ProcessResult::Changed(vec![])
 450                             } else {
 451                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 452                             }
 453                         }
 454                         None => ProcessResult::Unchanged,
 455                     }
 456                 }
 457
 458                 ty::PredicateKind::WellFormed(arg) => {
 459                     match wf::obligations(
 460                         self.selcx.infcx(),
 461                         obligation.param_env,
 462                         obligation.cause.body_id,
 463                         obligation.recursion_depth + 1,
 464                         arg,
 465                         obligation.cause.span,
 466                     ) {
 467                         None => {
 468                             pending_obligation.stalled_on =
 469                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 470                             ProcessResult::Unchanged
 471                         }
 472                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 473                     }
 474                 }
 475
 476                 ty::PredicateKind::Subtype(subtype) => {
 477                     match self.selcx.infcx().subtype_predicate(
 478                         &obligation.cause,
 479                         obligation.param_env,
 480                         Binder::dummy(subtype),
 481                     ) {
 482                         None => {
 483                             // None means that both are unresolved.
 484                             pending_obligation.stalled_on = vec![
 485                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 486                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 487                             ];
 488                             ProcessResult::Unchanged
 489                         }
 490                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 491                         Some(Err(err)) => {
 492                             let expected_found =
 493                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 494                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 495                                 expected_found,
 496                                 err,
 497                             ))
 498                         }
 499                     }
 500                 }
 501
 502                 ty::PredicateKind::Coerce(coerce) => {
 503                     match self.selcx.infcx().coerce_predicate(
 504                         &obligation.cause,
 505                         obligation.param_env,
 506                         Binder::dummy(coerce),
 507                     ) {
 508                         None => {
 509                             // None means that both are unresolved.
 510                             pending_obligation.stalled_on = vec![
 511                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 512                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 513                             ];
 514                             ProcessResult::Unchanged
 515                         }
 516                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 517                         Some(Err(err)) => {
 518                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 519                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 520                                 expected_found,
 521                                 err,
 522                             ))
 523                         }
 524                     }
 525                 }
 526
 527                 ty::PredicateKind::ConstEvaluatable(uv) => {
 528                     match const_evaluatable::is_const_evaluatable(
 529                         self.selcx.infcx(),
 530                         uv,
 531                         obligation.param_env,
 532                         obligation.cause.span,
 533                     ) {
 534                         Ok(()) => ProcessResult::Changed(vec![]),
 535                         Err(NotConstEvaluatable::MentionsInfer) => {
 536                             pending_obligation.stalled_on.clear();
 537                             pending_obligation.stalled_on.extend(
 538                                 uv.substs(infcx.tcx)
 539                                     .iter()
 540                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 541                             );
 542                             ProcessResult::Unchanged
 543                         }
 544                         Err(
 545                             e @ NotConstEvaluatable::MentionsParam
 546                             | e @ NotConstEvaluatable::Error(_),
 547                         ) => ProcessResult::Error(CodeSelectionError(
 548                             SelectionError::NotConstEvaluatable(e),
 549                         )),
 550                     }
 551                 }
 552
 553                 ty::PredicateKind::ConstEquate(c1, c2) => {
 554                     debug!(?c1, ?c2, "equating consts");
 555                     let tcx = self.selcx.tcx();
 556                     if tcx.features().generic_const_exprs {
 557                         // FIXME: we probably should only try to unify abstract constants
 558                         // if the constants depend on generic parameters.
 559                         //
 560                         // Let's just see where this breaks :shrug:
 561                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 562                             (c1.val, c2.val)
 563                         {
 564                             if infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
 565                                 return ProcessResult::Changed(vec![]);
 566                             }
 567                         }
 568                     }
 569
 570                     let stalled_on = &mut pending_obligation.stalled_on;
 571
 572                     let mut evaluate = |c: &'tcx Const<'tcx>| {
 573                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
 574                             match self.selcx.infcx().const_eval_resolve(
 575                                 obligation.param_env,
 576                                 unevaluated,
 577                                 Some(obligation.cause.span),
 578                             ) {
 579                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 580                                 Err(ErrorHandled::TooGeneric) => {
 581                                     stalled_on.extend(
 582                                         unevaluated
 583                                             .substs(tcx)
 584                                             .iter()
 585                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 586                                     );
 587                                     Err(ErrorHandled::TooGeneric)
 588                                 }
 589                                 Err(err) => Err(err),
 590                             }
 591                         } else {
 592                             Ok(c)
 593                         }
 594                     };
 595
 596                     match (evaluate(c1), evaluate(c2)) {
 597                         (Ok(c1), Ok(c2)) => {
 598                             match self
 599                                 .selcx
 600                                 .infcx()
 601                                 .at(&obligation.cause, obligation.param_env)
 602                                 .eq(c1, c2)
 603                             {
 604                                 Ok(_) => ProcessResult::Changed(vec![]),
 605                                 Err(err) => ProcessResult::Error(
 606                                     FulfillmentErrorCode::CodeConstEquateError(
 607                                         ExpectedFound::new(true, c1, c2),
 608                                         err,
 609                                     ),
 610                                 ),
 611                             }
 612                         }
 613                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 614                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => ProcessResult::Error(
 615                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 616                                 NotConstEvaluatable::Error(ErrorReported),
 617                             )),
 618                         ),
 619                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 620                             span_bug!(
 621                                 obligation.cause.span(self.selcx.tcx()),
 622                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 623                             )
 624                         }
 625                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 626                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 627                                 ProcessResult::Unchanged
 628                             } else {
 629                                 // Two different constants using generic parameters ~> error.
 630                                 let expected_found = ExpectedFound::new(true, c1, c2);
 631                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 632                                     expected_found,
 633                                     TypeError::ConstMismatch(expected_found),
 634                                 ))
 635                             }
 636                         }
 637                     }
 638                 }
 639                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 640                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 641                 }
 642             },
 643         }
 644     }
 645
 646     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 647     fn process_trait_obligation(
 648         &mut self,
 649         obligation: &PredicateObligation<'tcx>,
 650         trait_obligation: TraitObligation<'tcx>,
 651         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 652     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 653         let infcx = self.selcx.infcx();
 654         if obligation.predicate.is_known_global() {
 655             // no type variables present, can use evaluation for better caching.
 656             // FIXME: consider caching errors too.
 657             if infcx.predicate_must_hold_considering_regions(obligation) {
 658                 debug!(
 659                     "selecting trait at depth {} evaluated to holds",
 660                     obligation.recursion_depth
 661                 );
 662                 return ProcessResult::Changed(vec![]);
 663             }
 664         }
 665
 666         match self.selcx.select(&trait_obligation) {
 667             Ok(Some(impl_source)) => {
 668                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 669                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 670             }
 671             Ok(None) => {
 672                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 673
 674                 // This is a bit subtle: for the most part, the
 675                 // only reason we can fail to make progress on
 676                 // trait selection is because we don't have enough
 677                 // information about the types in the trait.
 678                 stalled_on.clear();
 679                 stalled_on.extend(substs_infer_vars(
 680                     self.selcx,
 681                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 682                 ));
 683
 684                 debug!(
 685                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 686                     infcx.resolve_vars_if_possible(obligation.clone()),
 687                     stalled_on
 688                 );
 689
 690                 ProcessResult::Unchanged
 691             }
 692             Err(selection_err) => {
 693                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 694
 695                 ProcessResult::Error(CodeSelectionError(selection_err))
 696             }
 697         }
 698     }
 699
 700     fn process_projection_obligation(
 701         &mut self,
 702         obligation: &PredicateObligation<'tcx>,
 703         project_obligation: PolyProjectionObligation<'tcx>,
 704         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 705     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 706         let tcx = self.selcx.tcx();
 707
 708         if obligation.predicate.is_global(tcx) {
 709             // no type variables present, can use evaluation for better caching.
 710             // FIXME: consider caching errors too.
 711             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 712                 return ProcessResult::Changed(vec![]);
 713             } else {
 714                 tracing::debug!("Does NOT hold: {:?}", obligation);
 715             }
 716         }
 717
 718         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 719             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 720             Ok(Ok(None)) => {
 721                 stalled_on.clear();
 722                 stalled_on.extend(substs_infer_vars(
 723                     self.selcx,
 724                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 725                 ));
 726                 ProcessResult::Unchanged
 727             }
 728             // Let the caller handle the recursion
 729             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 730                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 731             ])),
 732             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 733         }
 734     }
 735 }
 736
 737 /// Returns the set of inference variables contained in `substs`.
 738 fn substs_infer_vars<'a, 'tcx>(
 739     selcx: &mut SelectionContext<'a, 'tcx>,
 740     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 741 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 742     let tcx = selcx.tcx();
 743     selcx
 744         .infcx()
 745         .resolve_vars_if_possible(substs)
 746         .skip_binder() // ok because this check doesn't care about regions
 747         .iter()
 748         .filter(|arg| arg.has_infer_types_or_consts())
 749         .flat_map(move |arg| {
 750             let mut walker = arg.walk(tcx);
 751             while let Some(c) = walker.next() {
 752                 if !c.has_infer_types_or_consts() {
 753                     walker.visited.remove(&c);
 754                     walker.skip_current_subtree();
 755                 }
 756             }
 757             walker.visited.into_iter()
 758         })
 759         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 760 }
 761
 762 fn to_fulfillment_error<'tcx>(
 763     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 764 ) -> FulfillmentError<'tcx> {
 765     let mut iter = error.backtrace.into_iter();
 766     let obligation = iter.next().unwrap().obligation;
 767     // The root obligation is the last item in the backtrace - if there's only
 768     // one item, then it's the same as the main obligation
 769     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 770     FulfillmentError::new(obligation, error.error, root_obligation)
 771 }