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::ErrorGuaranteed;
   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.process_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 process_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         debug!(?obligation, "process_obligation pre-resolve");
 350
 351         if obligation.predicate.has_infer_types_or_consts() {
 352             obligation.predicate =
 353                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 354         }
 355
 356         debug!(?obligation, ?obligation.cause, "process_obligation");
 357
 358         let infcx = self.selcx.infcx();
 359
 360         if obligation.predicate.has_projections() {
 361             let mut obligations = Vec::new();
 362             let predicate = crate::traits::project::try_normalize_with_depth_to(
 363                 self.selcx,
 364                 obligation.param_env,
 365                 obligation.cause.clone(),
 366                 obligation.recursion_depth + 1,
 367                 obligation.predicate,
 368                 &mut obligations,
 369             );
 370             if predicate != obligation.predicate {
 371                 obligations.push(obligation.with(predicate));
 372                 return ProcessResult::Changed(mk_pending(obligations));
 373             }
 374         }
 375         let binder = obligation.predicate.kind();
 376         match binder.no_bound_vars() {
 377             None => match binder.skip_binder() {
 378                 // Evaluation will discard candidates using the leak check.
 379                 // This means we need to pass it the bound version of our
 380                 // predicate.
 381                 ty::PredicateKind::Trait(trait_ref) => {
 382                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 383
 384                     self.process_trait_obligation(
 385                         obligation,
 386                         trait_obligation,
 387                         &mut pending_obligation.stalled_on,
 388                     )
 389                 }
 390                 ty::PredicateKind::Projection(data) => {
 391                     let project_obligation = obligation.with(binder.rebind(data));
 392
 393                     self.process_projection_obligation(
 394                         obligation,
 395                         project_obligation,
 396                         &mut pending_obligation.stalled_on,
 397                     )
 398                 }
 399                 ty::PredicateKind::RegionOutlives(_)
 400                 | ty::PredicateKind::TypeOutlives(_)
 401                 | ty::PredicateKind::WellFormed(_)
 402                 | ty::PredicateKind::ObjectSafe(_)
 403                 | ty::PredicateKind::ClosureKind(..)
 404                 | ty::PredicateKind::Subtype(_)
 405                 | ty::PredicateKind::Coerce(_)
 406                 | ty::PredicateKind::ConstEvaluatable(..)
 407                 | ty::PredicateKind::ConstEquate(..) => {
 408                     let pred =
 409                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 410                     ProcessResult::Changed(mk_pending(vec![
 411                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 412                     ]))
 413                 }
 414                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 415                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 416                 }
 417             },
 418             Some(pred) => match pred {
 419                 ty::PredicateKind::Trait(data) => {
 420                     let trait_obligation = obligation.with(Binder::dummy(data));
 421
 422                     self.process_trait_obligation(
 423                         obligation,
 424                         trait_obligation,
 425                         &mut pending_obligation.stalled_on,
 426                     )
 427                 }
 428
 429                 ty::PredicateKind::RegionOutlives(data) => {
 430                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 431                         Ok(()) => ProcessResult::Changed(vec![]),
 432                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 433                     }
 434                 }
 435
 436                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 437                     if self.register_region_obligations {
 438                         self.selcx.infcx().register_region_obligation_with_cause(
 439                             t_a,
 440                             r_b,
 441                             &obligation.cause,
 442                         );
 443                     }
 444                     ProcessResult::Changed(vec![])
 445                 }
 446
 447                 ty::PredicateKind::Projection(ref data) => {
 448                     let project_obligation = obligation.with(Binder::dummy(*data));
 449
 450                     self.process_projection_obligation(
 451                         obligation,
 452                         project_obligation,
 453                         &mut pending_obligation.stalled_on,
 454                     )
 455                 }
 456
 457                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 458                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 459                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 460                     } else {
 461                         ProcessResult::Changed(vec![])
 462                     }
 463                 }
 464
 465                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 466                     match self.selcx.infcx().closure_kind(closure_substs) {
 467                         Some(closure_kind) => {
 468                             if closure_kind.extends(kind) {
 469                                 ProcessResult::Changed(vec![])
 470                             } else {
 471                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 472                             }
 473                         }
 474                         None => ProcessResult::Unchanged,
 475                     }
 476                 }
 477
 478                 ty::PredicateKind::WellFormed(arg) => {
 479                     match wf::obligations(
 480                         self.selcx.infcx(),
 481                         obligation.param_env,
 482                         obligation.cause.body_id,
 483                         obligation.recursion_depth + 1,
 484                         arg,
 485                         obligation.cause.span,
 486                     ) {
 487                         None => {
 488                             pending_obligation.stalled_on =
 489                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 490                             ProcessResult::Unchanged
 491                         }
 492                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 493                     }
 494                 }
 495
 496                 ty::PredicateKind::Subtype(subtype) => {
 497                     match self.selcx.infcx().subtype_predicate(
 498                         &obligation.cause,
 499                         obligation.param_env,
 500                         Binder::dummy(subtype),
 501                     ) {
 502                         None => {
 503                             // None means that both are unresolved.
 504                             pending_obligation.stalled_on = vec![
 505                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 506                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 507                             ];
 508                             ProcessResult::Unchanged
 509                         }
 510                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 511                         Some(Err(err)) => {
 512                             let expected_found =
 513                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 514                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 515                                 expected_found,
 516                                 err,
 517                             ))
 518                         }
 519                     }
 520                 }
 521
 522                 ty::PredicateKind::Coerce(coerce) => {
 523                     match self.selcx.infcx().coerce_predicate(
 524                         &obligation.cause,
 525                         obligation.param_env,
 526                         Binder::dummy(coerce),
 527                     ) {
 528                         None => {
 529                             // None means that both are unresolved.
 530                             pending_obligation.stalled_on = vec![
 531                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 532                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 533                             ];
 534                             ProcessResult::Unchanged
 535                         }
 536                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 537                         Some(Err(err)) => {
 538                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 539                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 540                                 expected_found,
 541                                 err,
 542                             ))
 543                         }
 544                     }
 545                 }
 546
 547                 ty::PredicateKind::ConstEvaluatable(uv) => {
 548                     match const_evaluatable::is_const_evaluatable(
 549                         self.selcx.infcx(),
 550                         uv,
 551                         obligation.param_env,
 552                         obligation.cause.span,
 553                     ) {
 554                         Ok(()) => ProcessResult::Changed(vec![]),
 555                         Err(NotConstEvaluatable::MentionsInfer) => {
 556                             pending_obligation.stalled_on.clear();
 557                             pending_obligation.stalled_on.extend(
 558                                 uv.substs
 559                                     .iter()
 560                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 561                             );
 562                             ProcessResult::Unchanged
 563                         }
 564                         Err(
 565                             e @ NotConstEvaluatable::MentionsParam
 566                             | e @ NotConstEvaluatable::Error(_),
 567                         ) => ProcessResult::Error(CodeSelectionError(
 568                             SelectionError::NotConstEvaluatable(e),
 569                         )),
 570                     }
 571                 }
 572
 573                 ty::PredicateKind::ConstEquate(c1, c2) => {
 574                     debug!(?c1, ?c2, "equating consts");
 575                     let tcx = self.selcx.tcx();
 576                     if tcx.features().generic_const_exprs {
 577                         // FIXME: we probably should only try to unify abstract constants
 578                         // if the constants depend on generic parameters.
 579                         //
 580                         // Let's just see where this breaks :shrug:
 581                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 582                             (c1.val(), c2.val())
 583                         {
 584                             if infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
 585                                 return ProcessResult::Changed(vec![]);
 586                             }
 587                         }
 588                     }
 589
 590                     let stalled_on = &mut pending_obligation.stalled_on;
 591
 592                     let mut evaluate = |c: Const<'tcx>| {
 593                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val() {
 594                             match self.selcx.infcx().const_eval_resolve(
 595                                 obligation.param_env,
 596                                 unevaluated,
 597                                 Some(obligation.cause.span),
 598                             ) {
 599                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty())),
 600                                 Err(ErrorHandled::TooGeneric) => {
 601                                     stalled_on.extend(
 602                                         unevaluated
 603                                             .substs
 604                                             .iter()
 605                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 606                                     );
 607                                     Err(ErrorHandled::TooGeneric)
 608                                 }
 609                                 Err(err) => Err(err),
 610                             }
 611                         } else {
 612                             Ok(c)
 613                         }
 614                     };
 615
 616                     match (evaluate(c1), evaluate(c2)) {
 617                         (Ok(c1), Ok(c2)) => {
 618                             match self
 619                                 .selcx
 620                                 .infcx()
 621                                 .at(&obligation.cause, obligation.param_env)
 622                                 .eq(c1, c2)
 623                             {
 624                                 Ok(_) => ProcessResult::Changed(vec![]),
 625                                 Err(err) => ProcessResult::Error(
 626                                     FulfillmentErrorCode::CodeConstEquateError(
 627                                         ExpectedFound::new(true, c1, c2),
 628                                         err,
 629                                     ),
 630                                 ),
 631                             }
 632                         }
 633                         (Err(ErrorHandled::Reported(ErrorGuaranteed)), _)
 634                         | (_, Err(ErrorHandled::Reported(ErrorGuaranteed))) => {
 635                             ProcessResult::Error(CodeSelectionError(
 636                                 SelectionError::NotConstEvaluatable(NotConstEvaluatable::Error(
 637                                     ErrorGuaranteed,
 638                                 )),
 639                             ))
 640                         }
 641                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 642                             span_bug!(
 643                                 obligation.cause.span(self.selcx.tcx()),
 644                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 645                             )
 646                         }
 647                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 648                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 649                                 ProcessResult::Unchanged
 650                             } else {
 651                                 // Two different constants using generic parameters ~> error.
 652                                 let expected_found = ExpectedFound::new(true, c1, c2);
 653                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 654                                     expected_found,
 655                                     TypeError::ConstMismatch(expected_found),
 656                                 ))
 657                             }
 658                         }
 659                     }
 660                 }
 661                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 662                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 663                 }
 664             },
 665         }
 666     }
 667
 668     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 669     fn process_trait_obligation(
 670         &mut self,
 671         obligation: &PredicateObligation<'tcx>,
 672         trait_obligation: TraitObligation<'tcx>,
 673         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 674     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 675         let infcx = self.selcx.infcx();
 676         if obligation.predicate.is_global() {
 677             // no type variables present, can use evaluation for better caching.
 678             // FIXME: consider caching errors too.
 679             if infcx.predicate_must_hold_considering_regions(obligation) {
 680                 debug!(
 681                     "selecting trait at depth {} evaluated to holds",
 682                     obligation.recursion_depth
 683                 );
 684                 return ProcessResult::Changed(vec![]);
 685             }
 686         }
 687
 688         match self.selcx.select(&trait_obligation) {
 689             Ok(Some(impl_source)) => {
 690                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 691                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 692             }
 693             Ok(None) => {
 694                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 695
 696                 // This is a bit subtle: for the most part, the
 697                 // only reason we can fail to make progress on
 698                 // trait selection is because we don't have enough
 699                 // information about the types in the trait.
 700                 stalled_on.clear();
 701                 stalled_on.extend(substs_infer_vars(
 702                     self.selcx,
 703                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 704                 ));
 705
 706                 debug!(
 707                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 708                     infcx.resolve_vars_if_possible(obligation.clone()),
 709                     stalled_on
 710                 );
 711
 712                 ProcessResult::Unchanged
 713             }
 714             Err(selection_err) => {
 715                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 716
 717                 ProcessResult::Error(CodeSelectionError(selection_err))
 718             }
 719         }
 720     }
 721
 722     fn process_projection_obligation(
 723         &mut self,
 724         obligation: &PredicateObligation<'tcx>,
 725         project_obligation: PolyProjectionObligation<'tcx>,
 726         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 727     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 728         let tcx = self.selcx.tcx();
 729
 730         if obligation.predicate.is_global() {
 731             // no type variables present, can use evaluation for better caching.
 732             // FIXME: consider caching errors too.
 733             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 734                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 735                     &mut self.selcx,
 736                     project_obligation.predicate,
 737                 ) {
 738                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 739                     // evaluated all sub-obligations. We can therefore mark the 'root'
 740                     // obligation as complete, and skip evaluating sub-obligations.
 741                     self.selcx
 742                         .infcx()
 743                         .inner
 744                         .borrow_mut()
 745                         .projection_cache()
 746                         .complete(key, EvaluationResult::EvaluatedToOk);
 747                 }
 748                 return ProcessResult::Changed(vec![]);
 749             } else {
 750                 tracing::debug!("Does NOT hold: {:?}", obligation);
 751             }
 752         }
 753
 754         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 755             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 756             Ok(Ok(None)) => {
 757                 stalled_on.clear();
 758                 stalled_on.extend(substs_infer_vars(
 759                     self.selcx,
 760                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 761                 ));
 762                 ProcessResult::Unchanged
 763             }
 764             // Let the caller handle the recursion
 765             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 766                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 767             ])),
 768             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 769         }
 770     }
 771 }
 772
 773 /// Returns the set of inference variables contained in `substs`.
 774 fn substs_infer_vars<'a, 'tcx>(
 775     selcx: &mut SelectionContext<'a, 'tcx>,
 776     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 777 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 778     selcx
 779         .infcx()
 780         .resolve_vars_if_possible(substs)
 781         .skip_binder() // ok because this check doesn't care about regions
 782         .iter()
 783         .filter(|arg| arg.has_infer_types_or_consts())
 784         .flat_map(|arg| {
 785             let mut walker = arg.walk();
 786             while let Some(c) = walker.next() {
 787                 if !c.has_infer_types_or_consts() {
 788                     walker.visited.remove(&c);
 789                     walker.skip_current_subtree();
 790                 }
 791             }
 792             walker.visited.into_iter()
 793         })
 794         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 795 }
 796
 797 fn to_fulfillment_error<'tcx>(
 798     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 799 ) -> FulfillmentError<'tcx> {
 800     let mut iter = error.backtrace.into_iter();
 801     let obligation = iter.next().unwrap().obligation;
 802     // The root obligation is the last item in the backtrace - if there's only
 803     // one item, then it's the same as the main obligation
 804     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 805     FulfillmentError::new(obligation, error.error, root_obligation)
 806 }