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             },
 401             Some(pred) => match pred {
 402                 ty::PredicateKind::Trait(data) => {
 403                     let trait_obligation = obligation.with(Binder::dummy(data));
 404
 405                     self.process_trait_obligation(
 406                         obligation,
 407                         trait_obligation,
 408                         &mut pending_obligation.stalled_on,
 409                     )
 410                 }
 411
 412                 ty::PredicateKind::RegionOutlives(data) => {
 413                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 414                         Ok(()) => ProcessResult::Changed(vec![]),
 415                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 416                     }
 417                 }
 418
 419                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 420                     if self.register_region_obligations {
 421                         self.selcx.infcx().register_region_obligation_with_cause(
 422                             t_a,
 423                             r_b,
 424                             &obligation.cause,
 425                         );
 426                     }
 427                     ProcessResult::Changed(vec![])
 428                 }
 429
 430                 ty::PredicateKind::Projection(ref data) => {
 431                     let project_obligation = obligation.with(Binder::dummy(*data));
 432
 433                     self.process_projection_obligation(
 434                         obligation,
 435                         project_obligation,
 436                         &mut pending_obligation.stalled_on,
 437                     )
 438                 }
 439
 440                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 441                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 442                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 443                     } else {
 444                         ProcessResult::Changed(vec![])
 445                     }
 446                 }
 447
 448                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 449                     match self.selcx.infcx().closure_kind(closure_substs) {
 450                         Some(closure_kind) => {
 451                             if closure_kind.extends(kind) {
 452                                 ProcessResult::Changed(vec![])
 453                             } else {
 454                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 455                             }
 456                         }
 457                         None => ProcessResult::Unchanged,
 458                     }
 459                 }
 460
 461                 ty::PredicateKind::WellFormed(arg) => {
 462                     match wf::obligations(
 463                         self.selcx.infcx(),
 464                         obligation.param_env,
 465                         obligation.cause.body_id,
 466                         obligation.recursion_depth + 1,
 467                         arg,
 468                         obligation.cause.span,
 469                     ) {
 470                         None => {
 471                             pending_obligation.stalled_on =
 472                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 473                             ProcessResult::Unchanged
 474                         }
 475                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 476                     }
 477                 }
 478
 479                 ty::PredicateKind::Subtype(subtype) => {
 480                     match self.selcx.infcx().subtype_predicate(
 481                         &obligation.cause,
 482                         obligation.param_env,
 483                         Binder::dummy(subtype),
 484                     ) {
 485                         None => {
 486                             // None means that both are unresolved.
 487                             pending_obligation.stalled_on = vec![
 488                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 489                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 490                             ];
 491                             ProcessResult::Unchanged
 492                         }
 493                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 494                         Some(Err(err)) => {
 495                             let expected_found =
 496                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 497                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 498                                 expected_found,
 499                                 err,
 500                             ))
 501                         }
 502                     }
 503                 }
 504
 505                 ty::PredicateKind::Coerce(coerce) => {
 506                     match self.selcx.infcx().coerce_predicate(
 507                         &obligation.cause,
 508                         obligation.param_env,
 509                         Binder::dummy(coerce),
 510                     ) {
 511                         None => {
 512                             // None means that both are unresolved.
 513                             pending_obligation.stalled_on = vec![
 514                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 515                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 516                             ];
 517                             ProcessResult::Unchanged
 518                         }
 519                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 520                         Some(Err(err)) => {
 521                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 522                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 523                                 expected_found,
 524                                 err,
 525                             ))
 526                         }
 527                     }
 528                 }
 529
 530                 ty::PredicateKind::ConstEvaluatable(uv) => {
 531                     match const_evaluatable::is_const_evaluatable(
 532                         self.selcx.infcx(),
 533                         uv,
 534                         obligation.param_env,
 535                         obligation.cause.span,
 536                     ) {
 537                         Ok(()) => ProcessResult::Changed(vec![]),
 538                         Err(NotConstEvaluatable::MentionsInfer) => {
 539                             pending_obligation.stalled_on.clear();
 540                             pending_obligation.stalled_on.extend(
 541                                 uv.substs
 542                                     .iter()
 543                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 544                             );
 545                             ProcessResult::Unchanged
 546                         }
 547                         Err(
 548                             e @ NotConstEvaluatable::MentionsParam
 549                             | e @ NotConstEvaluatable::Error(_),
 550                         ) => ProcessResult::Error(CodeSelectionError(
 551                             SelectionError::NotConstEvaluatable(e),
 552                         )),
 553                     }
 554                 }
 555
 556                 ty::PredicateKind::ConstEquate(c1, c2) => {
 557                     debug!(?c1, ?c2, "equating consts");
 558                     let tcx = self.selcx.tcx();
 559                     if tcx.features().generic_const_exprs {
 560                         // FIXME: we probably should only try to unify abstract constants
 561                         // if the constants depend on generic parameters.
 562                         //
 563                         // Let's just see where this breaks :shrug:
 564                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 565                             (c1.val, c2.val)
 566                         {
 567                             if infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
 568                                 return ProcessResult::Changed(vec![]);
 569                             }
 570                         }
 571                     }
 572
 573                     let stalled_on = &mut pending_obligation.stalled_on;
 574
 575                     let mut evaluate = |c: &'tcx Const<'tcx>| {
 576                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
 577                             match self.selcx.infcx().const_eval_resolve(
 578                                 obligation.param_env,
 579                                 unevaluated,
 580                                 Some(obligation.cause.span),
 581                             ) {
 582                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 583                                 Err(ErrorHandled::TooGeneric) => {
 584                                     stalled_on.extend(
 585                                         unevaluated
 586                                             .substs
 587                                             .iter()
 588                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 589                                     );
 590                                     Err(ErrorHandled::TooGeneric)
 591                                 }
 592                                 Err(err) => Err(err),
 593                             }
 594                         } else {
 595                             Ok(c)
 596                         }
 597                     };
 598
 599                     match (evaluate(c1), evaluate(c2)) {
 600                         (Ok(c1), Ok(c2)) => {
 601                             match self
 602                                 .selcx
 603                                 .infcx()
 604                                 .at(&obligation.cause, obligation.param_env)
 605                                 .eq(c1, c2)
 606                             {
 607                                 Ok(_) => ProcessResult::Changed(vec![]),
 608                                 Err(err) => ProcessResult::Error(
 609                                     FulfillmentErrorCode::CodeConstEquateError(
 610                                         ExpectedFound::new(true, c1, c2),
 611                                         err,
 612                                     ),
 613                                 ),
 614                             }
 615                         }
 616                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 617                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => ProcessResult::Error(
 618                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 619                                 NotConstEvaluatable::Error(ErrorReported),
 620                             )),
 621                         ),
 622                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 623                             span_bug!(
 624                                 obligation.cause.span(self.selcx.tcx()),
 625                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 626                             )
 627                         }
 628                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 629                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 630                                 ProcessResult::Unchanged
 631                             } else {
 632                                 // Two different constants using generic parameters ~> error.
 633                                 let expected_found = ExpectedFound::new(true, c1, c2);
 634                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 635                                     expected_found,
 636                                     TypeError::ConstMismatch(expected_found),
 637                                 ))
 638                             }
 639                         }
 640                     }
 641                 }
 642                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 643                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 644                 }
 645             },
 646         }
 647     }
 648
 649     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 650     fn process_trait_obligation(
 651         &mut self,
 652         obligation: &PredicateObligation<'tcx>,
 653         trait_obligation: TraitObligation<'tcx>,
 654         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 655     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 656         let infcx = self.selcx.infcx();
 657         if obligation.predicate.is_global() {
 658             // no type variables present, can use evaluation for better caching.
 659             // FIXME: consider caching errors too.
 660             if infcx.predicate_must_hold_considering_regions(obligation) {
 661                 debug!(
 662                     "selecting trait at depth {} evaluated to holds",
 663                     obligation.recursion_depth
 664                 );
 665                 return ProcessResult::Changed(vec![]);
 666             }
 667         }
 668
 669         match self.selcx.select(&trait_obligation) {
 670             Ok(Some(impl_source)) => {
 671                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 672                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 673             }
 674             Ok(None) => {
 675                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 676
 677                 // This is a bit subtle: for the most part, the
 678                 // only reason we can fail to make progress on
 679                 // trait selection is because we don't have enough
 680                 // information about the types in the trait.
 681                 stalled_on.clear();
 682                 stalled_on.extend(substs_infer_vars(
 683                     self.selcx,
 684                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 685                 ));
 686
 687                 debug!(
 688                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 689                     infcx.resolve_vars_if_possible(obligation.clone()),
 690                     stalled_on
 691                 );
 692
 693                 ProcessResult::Unchanged
 694             }
 695             Err(selection_err) => {
 696                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 697
 698                 ProcessResult::Error(CodeSelectionError(selection_err))
 699             }
 700         }
 701     }
 702
 703     fn process_projection_obligation(
 704         &mut self,
 705         obligation: &PredicateObligation<'tcx>,
 706         project_obligation: PolyProjectionObligation<'tcx>,
 707         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 708     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 709         let tcx = self.selcx.tcx();
 710
 711         if obligation.predicate.is_global() {
 712             // no type variables present, can use evaluation for better caching.
 713             // FIXME: consider caching errors too.
 714             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 715                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 716                     &mut self.selcx,
 717                     project_obligation.predicate,
 718                 ) {
 719                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 720                     // evaluated all sub-obligations. We can therefore mark the 'root'
 721                     // obligation as complete, and skip evaluating sub-obligations.
 722                     self.selcx
 723                         .infcx()
 724                         .inner
 725                         .borrow_mut()
 726                         .projection_cache()
 727                         .complete(key, EvaluationResult::EvaluatedToOk);
 728                 }
 729                 return ProcessResult::Changed(vec![]);
 730             } else {
 731                 tracing::debug!("Does NOT hold: {:?}", obligation);
 732             }
 733         }
 734
 735         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 736             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 737             Ok(Ok(None)) => {
 738                 stalled_on.clear();
 739                 stalled_on.extend(substs_infer_vars(
 740                     self.selcx,
 741                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 742                 ));
 743                 ProcessResult::Unchanged
 744             }
 745             // Let the caller handle the recursion
 746             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 747                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 748             ])),
 749             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 750         }
 751     }
 752 }
 753
 754 /// Returns the set of inference variables contained in `substs`.
 755 fn substs_infer_vars<'a, 'tcx>(
 756     selcx: &mut SelectionContext<'a, 'tcx>,
 757     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 758 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 759     selcx
 760         .infcx()
 761         .resolve_vars_if_possible(substs)
 762         .skip_binder() // ok because this check doesn't care about regions
 763         .iter()
 764         .filter(|arg| arg.has_infer_types_or_consts())
 765         .flat_map(|arg| {
 766             let mut walker = arg.walk();
 767             while let Some(c) = walker.next() {
 768                 if !c.has_infer_types_or_consts() {
 769                     walker.visited.remove(&c);
 770                     walker.skip_current_subtree();
 771                 }
 772             }
 773             walker.visited.into_iter()
 774         })
 775         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 776 }
 777
 778 fn to_fulfillment_error<'tcx>(
 779     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 780 ) -> FulfillmentError<'tcx> {
 781     let mut iter = error.backtrace.into_iter();
 782     let obligation = iter.next().unwrap().obligation;
 783     // The root obligation is the last item in the backtrace - if there's only
 784     // one item, then it's the same as the main obligation
 785     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 786     FulfillmentError::new(obligation, error.error, root_obligation)
 787 }