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::ProjectionCacheKey;
   8 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
   9 use rustc_middle::mir::interpret::ErrorHandled;
  10 use rustc_middle::thir::abstract_const::NotConstEvaluatable;
  11 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  12 use rustc_middle::ty::subst::SubstsRef;
  13 use rustc_middle::ty::ToPredicate;
  14 use rustc_middle::ty::{self, Binder, Const, Ty, TypeFoldable};
  15 use std::marker::PhantomData;
  16
  17 use super::const_evaluatable;
  18 use super::project;
  19 use super::select::SelectionContext;
  20 use super::wf;
  21 use super::CodeAmbiguity;
  22 use super::CodeProjectionError;
  23 use super::CodeSelectionError;
  24 use super::EvaluationResult;
  25 use super::Unimplemented;
  26 use super::{FulfillmentError, FulfillmentErrorCode};
  27 use super::{ObligationCause, PredicateObligation};
  28
  29 use crate::traits::error_reporting::InferCtxtExt as _;
  30 use crate::traits::project::PolyProjectionObligation;
  31 use crate::traits::project::ProjectionCacheKeyExt as _;
  32 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  33
  34 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  35     /// Note that we include both the `ParamEnv` and the `Predicate`,
  36     /// as the `ParamEnv` can influence whether fulfillment succeeds
  37     /// or fails.
  38     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  39
  40     fn as_cache_key(&self) -> Self::CacheKey {
  41         self.obligation.param_env.and(self.obligation.predicate)
  42     }
  43 }
  44
  45 /// The fulfillment context is used to drive trait resolution. It
  46 /// consists of a list of obligations that must be (eventually)
  47 /// satisfied. The job is to track which are satisfied, which yielded
  48 /// errors, and which are still pending. At any point, users can call
  49 /// `select_where_possible`, and the fulfillment context will try to do
  50 /// selection, retaining only those obligations that remain
  51 /// ambiguous. This may be helpful in pushing type inference
  52 /// along. Once all type inference constraints have been generated, the
  53 /// method `select_all_or_error` can be used to report any remaining
  54 /// ambiguous cases as errors.
  55 pub struct FulfillmentContext<'tcx> {
  56     // A list of all obligations that have been registered with this
  57     // fulfillment context.
  58     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  59
  60     relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
  61
  62     // Should this fulfillment context register type-lives-for-region
  63     // obligations on its parent infcx? In some cases, region
  64     // obligations are either already known to hold (normalization) or
  65     // hopefully verifed elsewhere (type-impls-bound), and therefore
  66     // should not be checked.
  67     //
  68     // Note that if we are normalizing a type that we already
  69     // know is well-formed, there should be no harm setting this
  70     // to true - all the region variables should be determinable
  71     // using the RFC 447 rules, which don't depend on
  72     // type-lives-for-region constraints, and because the type
  73     // is well-formed, the constraints should hold.
  74     register_region_obligations: bool,
  75     // Is it OK to register obligations into this infcx inside
  76     // an infcx snapshot?
  77     //
  78     // The "primary fulfillment" in many cases in typeck lives
  79     // outside of any snapshot, so any use of it inside a snapshot
  80     // will lead to trouble and therefore is checked against, but
  81     // other fulfillment contexts sometimes do live inside of
  82     // a snapshot (they don't *straddle* a snapshot, so there
  83     // is no trouble there).
  84     usable_in_snapshot: bool,
  85 }
  86
  87 #[derive(Clone, Debug)]
  88 pub struct PendingPredicateObligation<'tcx> {
  89     pub obligation: PredicateObligation<'tcx>,
  90     // This is far more often read than modified, meaning that we
  91     // should mostly optimize for reading speed, while modifying is not as relevant.
  92     //
  93     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  94     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  95 }
  96
  97 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  98 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  99 static_assert_size!(PendingPredicateObligation<'_>, 72);
 100
 101 impl<'a, 'tcx> FulfillmentContext<'tcx> {
 102     /// Creates a new fulfillment context.
 103     pub fn new() -> FulfillmentContext<'tcx> {
 104         FulfillmentContext {
 105             predicates: ObligationForest::new(),
 106             relationships: FxHashMap::default(),
 107             register_region_obligations: true,
 108             usable_in_snapshot: false,
 109         }
 110     }
 111
 112     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 113         FulfillmentContext {
 114             predicates: ObligationForest::new(),
 115             relationships: FxHashMap::default(),
 116             register_region_obligations: true,
 117             usable_in_snapshot: true,
 118         }
 119     }
 120
 121     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 122         FulfillmentContext {
 123             predicates: ObligationForest::new(),
 124             relationships: FxHashMap::default(),
 125             register_region_obligations: false,
 126             usable_in_snapshot: false,
 127         }
 128     }
 129
 130     /// Attempts to select obligations using `selcx`.
 131     fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 132         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 133         let _enter = span.enter();
 134
 135         let mut errors = Vec::new();
 136
 137         loop {
 138             debug!("select: starting another iteration");
 139
 140             // Process pending obligations.
 141             let outcome: Outcome<_, _> =
 142                 self.predicates.process_obligations(&mut FulfillProcessor {
 143                     selcx,
 144                     register_region_obligations: self.register_region_obligations,
 145                 });
 146             debug!("select: outcome={:#?}", outcome);
 147
 148             // FIXME: if we kept the original cache key, we could mark projection
 149             // obligations as complete for the projection cache here.
 150
 151             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 152
 153             // If nothing new was added, no need to keep looping.
 154             if outcome.stalled {
 155                 break;
 156             }
 157         }
 158
 159         debug!(
 160             "select({} predicates remaining, {} errors) done",
 161             self.predicates.len(),
 162             errors.len()
 163         );
 164
 165         errors
 166     }
 167 }
 168
 169 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 170     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 171     /// creating a fresh type variable `$0` as well as a projection
 172     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 173     /// inference engine runs, it will attempt to find an impl of
 174     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 175     /// something concrete. If this fails, we'll unify `$0` with
 176     /// `projection_ty` again.
 177     #[tracing::instrument(level = "debug", skip(self, infcx, param_env, cause))]
 178     fn normalize_projection_type(
 179         &mut self,
 180         infcx: &InferCtxt<'_, 'tcx>,
 181         param_env: ty::ParamEnv<'tcx>,
 182         projection_ty: ty::ProjectionTy<'tcx>,
 183         cause: ObligationCause<'tcx>,
 184     ) -> Ty<'tcx> {
 185         debug_assert!(!projection_ty.has_escaping_bound_vars());
 186
 187         // FIXME(#20304) -- cache
 188
 189         let mut selcx = SelectionContext::new(infcx);
 190         let mut obligations = vec![];
 191         let normalized_ty = project::normalize_projection_type(
 192             &mut selcx,
 193             param_env,
 194             projection_ty,
 195             cause,
 196             0,
 197             &mut obligations,
 198         );
 199         self.register_predicate_obligations(infcx, obligations);
 200
 201         debug!(?normalized_ty);
 202
 203         normalized_ty.ty().unwrap()
 204     }
 205
 206     fn register_predicate_obligation(
 207         &mut self,
 208         infcx: &InferCtxt<'_, 'tcx>,
 209         obligation: PredicateObligation<'tcx>,
 210     ) {
 211         // this helps to reduce duplicate errors, as well as making
 212         // debug output much nicer to read and so on.
 213         let obligation = infcx.resolve_vars_if_possible(obligation);
 214
 215         debug!(?obligation, "register_predicate_obligation");
 216
 217         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 218
 219         super::relationships::update(self, infcx, &obligation);
 220
 221         self.predicates
 222             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 223     }
 224
 225     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 226         {
 227             let errors = self.select_where_possible(infcx);
 228             if !errors.is_empty() {
 229                 return errors;
 230             }
 231         }
 232
 233         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 234     }
 235
 236     fn select_where_possible(
 237         &mut self,
 238         infcx: &InferCtxt<'_, 'tcx>,
 239     ) -> Vec<FulfillmentError<'tcx>> {
 240         let mut selcx = SelectionContext::new(infcx);
 241         self.select(&mut selcx)
 242     }
 243
 244     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 245         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 246     }
 247
 248     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 249         &mut self.relationships
 250     }
 251 }
 252
 253 struct FulfillProcessor<'a, 'b, 'tcx> {
 254     selcx: &'a mut SelectionContext<'b, 'tcx>,
 255     register_region_obligations: bool,
 256 }
 257
 258 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 259     os.into_iter()
 260         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 261         .collect()
 262 }
 263
 264 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 265     type Obligation = PendingPredicateObligation<'tcx>;
 266     type Error = FulfillmentErrorCode<'tcx>;
 267
 268     /// Processes a predicate obligation and returns either:
 269     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 270     /// - `Unchanged` if we don't have enough info to be sure
 271     /// - `Error(e)` if the predicate does not hold
 272     ///
 273     /// This is always inlined, despite its size, because it has a single
 274     /// callsite and it is called *very* frequently.
 275     #[inline(always)]
 276     fn process_obligation(
 277         &mut self,
 278         pending_obligation: &mut Self::Obligation,
 279     ) -> ProcessResult<Self::Obligation, Self::Error> {
 280         // If we were stalled on some unresolved variables, first check whether
 281         // any of them have been resolved; if not, don't bother doing more work
 282         // yet.
 283         let change = match pending_obligation.stalled_on.len() {
 284             // Match arms are in order of frequency, which matters because this
 285             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 286             1 => {
 287                 let infer_var = pending_obligation.stalled_on[0];
 288                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 289             }
 290             0 => {
 291                 // In this case we haven't changed, but wish to make a change.
 292                 true
 293             }
 294             _ => {
 295                 // This `for` loop was once a call to `all()`, but this lower-level
 296                 // form was a perf win. See #64545 for details.
 297                 (|| {
 298                     for &infer_var in &pending_obligation.stalled_on {
 299                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 300                             return true;
 301                         }
 302                     }
 303                     false
 304                 })()
 305             }
 306         };
 307
 308         if !change {
 309             debug!(
 310                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 311                 self.selcx.infcx().resolve_vars_if_possible(pending_obligation.obligation.clone()),
 312                 pending_obligation.stalled_on
 313             );
 314             return ProcessResult::Unchanged;
 315         }
 316
 317         self.progress_changed_obligations(pending_obligation)
 318     }
 319
 320     fn process_backedge<'c, I>(
 321         &mut self,
 322         cycle: I,
 323         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 324     ) where
 325         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 326     {
 327         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 328             debug!("process_child_obligations: coinductive match");
 329         } else {
 330             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 331             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 332         }
 333     }
 334 }
 335
 336 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 337     // The code calling this method is extremely hot and only rarely
 338     // actually uses this, so move this part of the code
 339     // out of that loop.
 340     #[inline(never)]
 341     fn progress_changed_obligations(
 342         &mut self,
 343         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 344     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 345         pending_obligation.stalled_on.truncate(0);
 346
 347         let obligation = &mut pending_obligation.obligation;
 348
 349         if obligation.predicate.has_infer_types_or_consts() {
 350             obligation.predicate =
 351                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 352         }
 353
 354         debug!(?obligation, ?obligation.cause, "process_obligation");
 355
 356         let infcx = self.selcx.infcx();
 357
 358         let binder = obligation.predicate.kind();
 359         match binder.no_bound_vars() {
 360             None => match binder.skip_binder() {
 361                 // Evaluation will discard candidates using the leak check.
 362                 // This means we need to pass it the bound version of our
 363                 // predicate.
 364                 ty::PredicateKind::Trait(trait_ref) => {
 365                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 366
 367                     self.process_trait_obligation(
 368                         obligation,
 369                         trait_obligation,
 370                         &mut pending_obligation.stalled_on,
 371                     )
 372                 }
 373                 ty::PredicateKind::Projection(data) => {
 374                     let project_obligation = obligation.with(binder.rebind(data));
 375
 376                     self.process_projection_obligation(
 377                         obligation,
 378                         project_obligation,
 379                         &mut pending_obligation.stalled_on,
 380                     )
 381                 }
 382                 ty::PredicateKind::RegionOutlives(_)
 383                 | ty::PredicateKind::TypeOutlives(_)
 384                 | ty::PredicateKind::WellFormed(_)
 385                 | ty::PredicateKind::ObjectSafe(_)
 386                 | ty::PredicateKind::ClosureKind(..)
 387                 | ty::PredicateKind::Subtype(_)
 388                 | ty::PredicateKind::Coerce(_)
 389                 | ty::PredicateKind::ConstEvaluatable(..)
 390                 | ty::PredicateKind::ConstEquate(..) => {
 391                     let pred =
 392                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 393                     ProcessResult::Changed(mk_pending(vec![
 394                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 395                     ]))
 396                 }
 397                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 398                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 399                 }
 400                 ty::PredicateKind::OpaqueType(..) => {
 401                     todo!("{:#?}", obligation);
 402                 }
 403             },
 404             Some(pred) => match pred {
 405                 ty::PredicateKind::Trait(data) => {
 406                     let trait_obligation = obligation.with(Binder::dummy(data));
 407
 408                     self.process_trait_obligation(
 409                         obligation,
 410                         trait_obligation,
 411                         &mut pending_obligation.stalled_on,
 412                     )
 413                 }
 414
 415                 ty::PredicateKind::RegionOutlives(data) => {
 416                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 417                         Ok(()) => ProcessResult::Changed(vec![]),
 418                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 419                     }
 420                 }
 421
 422                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 423                     if self.register_region_obligations {
 424                         self.selcx.infcx().register_region_obligation_with_cause(
 425                             t_a,
 426                             r_b,
 427                             &obligation.cause,
 428                         );
 429                     }
 430                     ProcessResult::Changed(vec![])
 431                 }
 432
 433                 ty::PredicateKind::Projection(ref data) => {
 434                     let project_obligation = obligation.with(Binder::dummy(*data));
 435
 436                     self.process_projection_obligation(
 437                         obligation,
 438                         project_obligation,
 439                         &mut pending_obligation.stalled_on,
 440                     )
 441                 }
 442
 443                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 444                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 445                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 446                     } else {
 447                         ProcessResult::Changed(vec![])
 448                     }
 449                 }
 450
 451                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 452                     match self.selcx.infcx().closure_kind(closure_substs) {
 453                         Some(closure_kind) => {
 454                             if closure_kind.extends(kind) {
 455                                 ProcessResult::Changed(vec![])
 456                             } else {
 457                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 458                             }
 459                         }
 460                         None => ProcessResult::Unchanged,
 461                     }
 462                 }
 463
 464                 ty::PredicateKind::WellFormed(arg) => {
 465                     match wf::obligations(
 466                         self.selcx.infcx(),
 467                         obligation.param_env,
 468                         obligation.cause.body_id,
 469                         obligation.recursion_depth + 1,
 470                         arg,
 471                         obligation.cause.span,
 472                     ) {
 473                         None => {
 474                             pending_obligation.stalled_on =
 475                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 476                             ProcessResult::Unchanged
 477                         }
 478                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 479                     }
 480                 }
 481
 482                 ty::PredicateKind::Subtype(subtype) => {
 483                     match self.selcx.infcx().subtype_predicate(
 484                         &obligation.cause,
 485                         obligation.param_env,
 486                         Binder::dummy(subtype),
 487                     ) {
 488                         None => {
 489                             // None means that both are unresolved.
 490                             pending_obligation.stalled_on = vec![
 491                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 492                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 493                             ];
 494                             ProcessResult::Unchanged
 495                         }
 496                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 497                         Some(Err(err)) => {
 498                             let expected_found =
 499                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 500                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 501                                 expected_found,
 502                                 err,
 503                             ))
 504                         }
 505                     }
 506                 }
 507
 508                 ty::PredicateKind::Coerce(coerce) => {
 509                     match self.selcx.infcx().coerce_predicate(
 510                         &obligation.cause,
 511                         obligation.param_env,
 512                         Binder::dummy(coerce),
 513                     ) {
 514                         None => {
 515                             // None means that both are unresolved.
 516                             pending_obligation.stalled_on = vec![
 517                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 518                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 519                             ];
 520                             ProcessResult::Unchanged
 521                         }
 522                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 523                         Some(Err(err)) => {
 524                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 525                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 526                                 expected_found,
 527                                 err,
 528                             ))
 529                         }
 530                     }
 531                 }
 532
 533                 ty::PredicateKind::ConstEvaluatable(uv) => {
 534                     match const_evaluatable::is_const_evaluatable(
 535                         self.selcx.infcx(),
 536                         uv,
 537                         obligation.param_env,
 538                         obligation.cause.span,
 539                     ) {
 540                         Ok(()) => ProcessResult::Changed(vec![]),
 541                         Err(NotConstEvaluatable::MentionsInfer) => {
 542                             pending_obligation.stalled_on.clear();
 543                             pending_obligation.stalled_on.extend(
 544                                 uv.substs
 545                                     .iter()
 546                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 547                             );
 548                             ProcessResult::Unchanged
 549                         }
 550                         Err(
 551                             e @ NotConstEvaluatable::MentionsParam
 552                             | e @ NotConstEvaluatable::Error(_),
 553                         ) => ProcessResult::Error(CodeSelectionError(
 554                             SelectionError::NotConstEvaluatable(e),
 555                         )),
 556                     }
 557                 }
 558
 559                 ty::PredicateKind::ConstEquate(c1, c2) => {
 560                     debug!(?c1, ?c2, "equating consts");
 561                     let tcx = self.selcx.tcx();
 562                     if tcx.features().generic_const_exprs {
 563                         // FIXME: we probably should only try to unify abstract constants
 564                         // if the constants depend on generic parameters.
 565                         //
 566                         // Let's just see where this breaks :shrug:
 567                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 568                             (c1.val, c2.val)
 569                         {
 570                             if infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
 571                                 return ProcessResult::Changed(vec![]);
 572                             }
 573                         }
 574                     }
 575
 576                     let stalled_on = &mut pending_obligation.stalled_on;
 577
 578                     let mut evaluate = |c: &'tcx Const<'tcx>| {
 579                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
 580                             match self.selcx.infcx().const_eval_resolve(
 581                                 obligation.param_env,
 582                                 unevaluated,
 583                                 Some(obligation.cause.span),
 584                             ) {
 585                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 586                                 Err(ErrorHandled::TooGeneric) => {
 587                                     stalled_on.extend(
 588                                         unevaluated
 589                                             .substs
 590                                             .iter()
 591                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 592                                     );
 593                                     Err(ErrorHandled::TooGeneric)
 594                                 }
 595                                 Err(err) => Err(err),
 596                             }
 597                         } else {
 598                             Ok(c)
 599                         }
 600                     };
 601
 602                     match (evaluate(c1), evaluate(c2)) {
 603                         (Ok(c1), Ok(c2)) => {
 604                             match self
 605                                 .selcx
 606                                 .infcx()
 607                                 .at(&obligation.cause, obligation.param_env)
 608                                 .eq(c1, c2)
 609                             {
 610                                 Ok(_) => ProcessResult::Changed(vec![]),
 611                                 Err(err) => ProcessResult::Error(
 612                                     FulfillmentErrorCode::CodeConstEquateError(
 613                                         ExpectedFound::new(true, c1, c2),
 614                                         err,
 615                                     ),
 616                                 ),
 617                             }
 618                         }
 619                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 620                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => ProcessResult::Error(
 621                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 622                                 NotConstEvaluatable::Error(ErrorReported),
 623                             )),
 624                         ),
 625                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 626                             span_bug!(
 627                                 obligation.cause.span(self.selcx.tcx()),
 628                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 629                             )
 630                         }
 631                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 632                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 633                                 ProcessResult::Unchanged
 634                             } else {
 635                                 // Two different constants using generic parameters ~> error.
 636                                 let expected_found = ExpectedFound::new(true, c1, c2);
 637                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 638                                     expected_found,
 639                                     TypeError::ConstMismatch(expected_found),
 640                                 ))
 641                             }
 642                         }
 643                     }
 644                 }
 645                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 646                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 647                 }
 648                 ty::PredicateKind::OpaqueType(a, b) => {
 649                     match self.selcx.infcx().handle_opaque_type(
 650                         a,
 651                         b,
 652                         &obligation.cause,
 653                         obligation.param_env,
 654                     ) {
 655                         Ok(value) => ProcessResult::Changed(mk_pending(value.obligations)),
 656                         Err(err) => ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 657                             ExpectedFound::new(true, a, b),
 658                             err,
 659                         )),
 660                     }
 661                 }
 662             },
 663         }
 664     }
 665
 666     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 667     fn process_trait_obligation(
 668         &mut self,
 669         obligation: &PredicateObligation<'tcx>,
 670         trait_obligation: TraitObligation<'tcx>,
 671         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 672     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 673         let infcx = self.selcx.infcx();
 674         if obligation.predicate.is_global() {
 675             // no type variables present, can use evaluation for better caching.
 676             // FIXME: consider caching errors too.
 677             if infcx.predicate_must_hold_considering_regions(obligation) {
 678                 debug!(
 679                     "selecting trait at depth {} evaluated to holds",
 680                     obligation.recursion_depth
 681                 );
 682                 return ProcessResult::Changed(vec![]);
 683             }
 684         }
 685
 686         match self.selcx.select(&trait_obligation) {
 687             Ok(Some(impl_source)) => {
 688                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 689                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 690             }
 691             Ok(None) => {
 692                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 693
 694                 // This is a bit subtle: for the most part, the
 695                 // only reason we can fail to make progress on
 696                 // trait selection is because we don't have enough
 697                 // information about the types in the trait.
 698                 stalled_on.clear();
 699                 stalled_on.extend(substs_infer_vars(
 700                     self.selcx,
 701                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 702                 ));
 703
 704                 debug!(
 705                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 706                     infcx.resolve_vars_if_possible(obligation.clone()),
 707                     stalled_on
 708                 );
 709
 710                 ProcessResult::Unchanged
 711             }
 712             Err(selection_err) => {
 713                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 714
 715                 ProcessResult::Error(CodeSelectionError(selection_err))
 716             }
 717         }
 718     }
 719
 720     fn process_projection_obligation(
 721         &mut self,
 722         obligation: &PredicateObligation<'tcx>,
 723         project_obligation: PolyProjectionObligation<'tcx>,
 724         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 725     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 726         let tcx = self.selcx.tcx();
 727
 728         if obligation.predicate.is_global() {
 729             // no type variables present, can use evaluation for better caching.
 730             // FIXME: consider caching errors too.
 731             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 732                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 733                     &mut self.selcx,
 734                     project_obligation.predicate,
 735                 ) {
 736                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 737                     // evaluated all sub-obligations. We can therefore mark the 'root'
 738                     // obligation as complete, and skip evaluating sub-obligations.
 739                     self.selcx
 740                         .infcx()
 741                         .inner
 742                         .borrow_mut()
 743                         .projection_cache()
 744                         .complete(key, EvaluationResult::EvaluatedToOk);
 745                 }
 746                 return ProcessResult::Changed(vec![]);
 747             } else {
 748                 tracing::debug!("Does NOT hold: {:?}", obligation);
 749             }
 750         }
 751
 752         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 753             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 754             Ok(Ok(None)) => {
 755                 stalled_on.clear();
 756                 stalled_on.extend(substs_infer_vars(
 757                     self.selcx,
 758                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 759                 ));
 760                 ProcessResult::Unchanged
 761             }
 762             // Let the caller handle the recursion
 763             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 764                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 765             ])),
 766             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 767         }
 768     }
 769 }
 770
 771 /// Returns the set of inference variables contained in `substs`.
 772 fn substs_infer_vars<'a, 'tcx>(
 773     selcx: &mut SelectionContext<'a, 'tcx>,
 774     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 775 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 776     selcx
 777         .infcx()
 778         .resolve_vars_if_possible(substs)
 779         .skip_binder() // ok because this check doesn't care about regions
 780         .iter()
 781         .filter(|arg| arg.has_infer_types_or_consts())
 782         .flat_map(|arg| {
 783             let mut walker = arg.walk();
 784             while let Some(c) = walker.next() {
 785                 if !c.has_infer_types_or_consts() {
 786                     walker.visited.remove(&c);
 787                     walker.skip_current_subtree();
 788                 }
 789             }
 790             walker.visited.into_iter()
 791         })
 792         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 793 }
 794
 795 fn to_fulfillment_error<'tcx>(
 796     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 797 ) -> FulfillmentError<'tcx> {
 798     let mut iter = error.backtrace.into_iter();
 799     let obligation = iter.next().unwrap().obligation;
 800     // The root obligation is the last item in the backtrace - if there's only
 801     // one item, then it's the same as the main obligation
 802     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 803     FulfillmentError::new(obligation, error.error, root_obligation)
 804 }