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_hir as hir;
   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::Unimplemented;
  25 use super::{FulfillmentError, FulfillmentErrorCode};
  26 use super::{ObligationCause, PredicateObligation};
  27
  28 use crate::traits::error_reporting::InferCtxtExt as _;
  29 use crate::traits::project::PolyProjectionObligation;
  30 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  31
  32 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  33     /// Note that we include both the `ParamEnv` and the `Predicate`,
  34     /// as the `ParamEnv` can influence whether fulfillment succeeds
  35     /// or fails.
  36     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  37
  38     fn as_cache_key(&self) -> Self::CacheKey {
  39         self.obligation.param_env.and(self.obligation.predicate)
  40     }
  41 }
  42
  43 /// The fulfillment context is used to drive trait resolution. It
  44 /// consists of a list of obligations that must be (eventually)
  45 /// satisfied. The job is to track which are satisfied, which yielded
  46 /// errors, and which are still pending. At any point, users can call
  47 /// `select_where_possible`, and the fulfillment context will try to do
  48 /// selection, retaining only those obligations that remain
  49 /// ambiguous. This may be helpful in pushing type inference
  50 /// along. Once all type inference constraints have been generated, the
  51 /// method `select_all_or_error` can be used to report any remaining
  52 /// ambiguous cases as errors.
  53 pub struct FulfillmentContext<'tcx> {
  54     // A list of all obligations that have been registered with this
  55     // fulfillment context.
  56     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  57
  58     relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
  59
  60     // Should this fulfillment context register type-lives-for-region
  61     // obligations on its parent infcx? In some cases, region
  62     // obligations are either already known to hold (normalization) or
  63     // hopefully verifed elsewhere (type-impls-bound), and therefore
  64     // should not be checked.
  65     //
  66     // Note that if we are normalizing a type that we already
  67     // know is well-formed, there should be no harm setting this
  68     // to true - all the region variables should be determinable
  69     // using the RFC 447 rules, which don't depend on
  70     // type-lives-for-region constraints, and because the type
  71     // is well-formed, the constraints should hold.
  72     register_region_obligations: bool,
  73     // Is it OK to register obligations into this infcx inside
  74     // an infcx snapshot?
  75     //
  76     // The "primary fulfillment" in many cases in typeck lives
  77     // outside of any snapshot, so any use of it inside a snapshot
  78     // will lead to trouble and therefore is checked against, but
  79     // other fulfillment contexts sometimes do live inside of
  80     // a snapshot (they don't *straddle* a snapshot, so there
  81     // is no trouble there).
  82     usable_in_snapshot: bool,
  83 }
  84
  85 #[derive(Clone, Debug)]
  86 pub struct PendingPredicateObligation<'tcx> {
  87     pub obligation: PredicateObligation<'tcx>,
  88     // This is far more often read than modified, meaning that we
  89     // should mostly optimize for reading speed, while modifying is not as relevant.
  90     //
  91     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  92     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  93 }
  94
  95 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  96 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  97 static_assert_size!(PendingPredicateObligation<'_>, 56);
  98
  99 impl<'a, 'tcx> FulfillmentContext<'tcx> {
 100     /// Creates a new fulfillment context.
 101     pub fn new() -> FulfillmentContext<'tcx> {
 102         FulfillmentContext {
 103             predicates: ObligationForest::new(),
 104             relationships: FxHashMap::default(),
 105             register_region_obligations: true,
 106             usable_in_snapshot: false,
 107         }
 108     }
 109
 110     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 111         FulfillmentContext {
 112             predicates: ObligationForest::new(),
 113             relationships: FxHashMap::default(),
 114             register_region_obligations: true,
 115             usable_in_snapshot: true,
 116         }
 117     }
 118
 119     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 120         FulfillmentContext {
 121             predicates: ObligationForest::new(),
 122             relationships: FxHashMap::default(),
 123             register_region_obligations: false,
 124             usable_in_snapshot: false,
 125         }
 126     }
 127
 128     /// Attempts to select obligations using `selcx`.
 129     fn select(
 130         &mut self,
 131         selcx: &mut SelectionContext<'a, 'tcx>,
 132     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 133         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 134         let _enter = span.enter();
 135
 136         let mut errors = Vec::new();
 137
 138         loop {
 139             debug!("select: starting another iteration");
 140
 141             // Process pending obligations.
 142             let outcome: Outcome<_, _> =
 143                 self.predicates.process_obligations(&mut FulfillProcessor {
 144                     selcx,
 145                     register_region_obligations: self.register_region_obligations,
 146                 });
 147             debug!("select: outcome={:#?}", outcome);
 148
 149             // FIXME: if we kept the original cache key, we could mark projection
 150             // obligations as complete for the projection cache here.
 151
 152             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 153
 154             // If nothing new was added, no need to keep looping.
 155             if outcome.stalled {
 156                 break;
 157             }
 158         }
 159
 160         debug!(
 161             "select({} predicates remaining, {} errors) done",
 162             self.predicates.len(),
 163             errors.len()
 164         );
 165
 166         if errors.is_empty() { Ok(()) } else { Err(errors) }
 167     }
 168 }
 169
 170 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 171     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 172     /// creating a fresh type variable `$0` as well as a projection
 173     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 174     /// inference engine runs, it will attempt to find an impl of
 175     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 176     /// something concrete. If this fails, we'll unify `$0` with
 177     /// `projection_ty` again.
 178     #[tracing::instrument(level = "debug", skip(self, infcx, param_env, cause))]
 179     fn normalize_projection_type(
 180         &mut self,
 181         infcx: &InferCtxt<'_, 'tcx>,
 182         param_env: ty::ParamEnv<'tcx>,
 183         projection_ty: ty::ProjectionTy<'tcx>,
 184         cause: ObligationCause<'tcx>,
 185     ) -> Ty<'tcx> {
 186         debug_assert!(!projection_ty.has_escaping_bound_vars());
 187
 188         // FIXME(#20304) -- cache
 189
 190         let mut selcx = SelectionContext::new(infcx);
 191         let mut obligations = vec![];
 192         let normalized_ty = project::normalize_projection_type(
 193             &mut selcx,
 194             param_env,
 195             projection_ty,
 196             cause,
 197             0,
 198             &mut obligations,
 199         );
 200         self.register_predicate_obligations(infcx, obligations);
 201
 202         debug!(?normalized_ty);
 203
 204         normalized_ty
 205     }
 206
 207     fn register_predicate_obligation(
 208         &mut self,
 209         infcx: &InferCtxt<'_, 'tcx>,
 210         obligation: PredicateObligation<'tcx>,
 211     ) {
 212         // this helps to reduce duplicate errors, as well as making
 213         // debug output much nicer to read and so on.
 214         let obligation = infcx.resolve_vars_if_possible(obligation);
 215
 216         debug!(?obligation, "register_predicate_obligation");
 217
 218         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 219
 220         super::relationships::update(self, infcx, &obligation);
 221
 222         self.predicates
 223             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 224     }
 225
 226     fn select_all_or_error(
 227         &mut self,
 228         infcx: &InferCtxt<'_, 'tcx>,
 229     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 230         self.select_where_possible(infcx)?;
 231
 232         let errors: Vec<_> = self
 233             .predicates
 234             .to_errors(CodeAmbiguity)
 235             .into_iter()
 236             .map(to_fulfillment_error)
 237             .collect();
 238         if errors.is_empty() { Ok(()) } else { Err(errors) }
 239     }
 240
 241     fn select_all_with_constness_or_error(
 242         &mut self,
 243         infcx: &InferCtxt<'_, 'tcx>,
 244         constness: rustc_hir::Constness,
 245     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 246         self.select_with_constness_where_possible(infcx, constness)?;
 247
 248         let errors: Vec<_> = self
 249             .predicates
 250             .to_errors(CodeAmbiguity)
 251             .into_iter()
 252             .map(to_fulfillment_error)
 253             .collect();
 254         if errors.is_empty() { Ok(()) } else { Err(errors) }
 255     }
 256
 257     fn select_where_possible(
 258         &mut self,
 259         infcx: &InferCtxt<'_, 'tcx>,
 260     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 261         let mut selcx = SelectionContext::new(infcx);
 262         self.select(&mut selcx)
 263     }
 264
 265     fn select_with_constness_where_possible(
 266         &mut self,
 267         infcx: &InferCtxt<'_, 'tcx>,
 268         constness: hir::Constness,
 269     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 270         let mut selcx = SelectionContext::with_constness(infcx, constness);
 271         self.select(&mut selcx)
 272     }
 273
 274     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 275         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 276     }
 277
 278     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 279         &mut self.relationships
 280     }
 281 }
 282
 283 struct FulfillProcessor<'a, 'b, 'tcx> {
 284     selcx: &'a mut SelectionContext<'b, 'tcx>,
 285     register_region_obligations: bool,
 286 }
 287
 288 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 289     os.into_iter()
 290         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 291         .collect()
 292 }
 293
 294 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 295     type Obligation = PendingPredicateObligation<'tcx>;
 296     type Error = FulfillmentErrorCode<'tcx>;
 297
 298     /// Processes a predicate obligation and returns either:
 299     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 300     /// - `Unchanged` if we don't have enough info to be sure
 301     /// - `Error(e)` if the predicate does not hold
 302     ///
 303     /// This is always inlined, despite its size, because it has a single
 304     /// callsite and it is called *very* frequently.
 305     #[inline(always)]
 306     fn process_obligation(
 307         &mut self,
 308         pending_obligation: &mut Self::Obligation,
 309     ) -> ProcessResult<Self::Obligation, Self::Error> {
 310         // If we were stalled on some unresolved variables, first check whether
 311         // any of them have been resolved; if not, don't bother doing more work
 312         // yet.
 313         let change = match pending_obligation.stalled_on.len() {
 314             // Match arms are in order of frequency, which matters because this
 315             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 316             1 => {
 317                 let infer_var = pending_obligation.stalled_on[0];
 318                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 319             }
 320             0 => {
 321                 // In this case we haven't changed, but wish to make a change.
 322                 true
 323             }
 324             _ => {
 325                 // This `for` loop was once a call to `all()`, but this lower-level
 326                 // form was a perf win. See #64545 for details.
 327                 (|| {
 328                     for &infer_var in &pending_obligation.stalled_on {
 329                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 330                             return true;
 331                         }
 332                     }
 333                     false
 334                 })()
 335             }
 336         };
 337
 338         if !change {
 339             debug!(
 340                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 341                 self.selcx.infcx().resolve_vars_if_possible(pending_obligation.obligation.clone()),
 342                 pending_obligation.stalled_on
 343             );
 344             return ProcessResult::Unchanged;
 345         }
 346
 347         self.progress_changed_obligations(pending_obligation)
 348     }
 349
 350     fn process_backedge<'c, I>(
 351         &mut self,
 352         cycle: I,
 353         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 354     ) where
 355         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 356     {
 357         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 358             debug!("process_child_obligations: coinductive match");
 359         } else {
 360             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 361             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 362         }
 363     }
 364 }
 365
 366 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 367     // The code calling this method is extremely hot and only rarely
 368     // actually uses this, so move this part of the code
 369     // out of that loop.
 370     #[inline(never)]
 371     fn progress_changed_obligations(
 372         &mut self,
 373         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 374     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 375         pending_obligation.stalled_on.truncate(0);
 376
 377         let obligation = &mut pending_obligation.obligation;
 378
 379         if obligation.predicate.has_infer_types_or_consts() {
 380             obligation.predicate =
 381                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 382         }
 383
 384         debug!(?obligation, ?obligation.cause, "process_obligation");
 385
 386         let infcx = self.selcx.infcx();
 387
 388         let binder = obligation.predicate.kind();
 389         match binder.no_bound_vars() {
 390             None => match binder.skip_binder() {
 391                 // Evaluation will discard candidates using the leak check.
 392                 // This means we need to pass it the bound version of our
 393                 // predicate.
 394                 ty::PredicateKind::Trait(trait_ref) => {
 395                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 396
 397                     self.process_trait_obligation(
 398                         obligation,
 399                         trait_obligation,
 400                         &mut pending_obligation.stalled_on,
 401                     )
 402                 }
 403                 ty::PredicateKind::Projection(data) => {
 404                     let project_obligation = obligation.with(binder.rebind(data));
 405
 406                     self.process_projection_obligation(
 407                         obligation,
 408                         project_obligation,
 409                         &mut pending_obligation.stalled_on,
 410                     )
 411                 }
 412                 ty::PredicateKind::RegionOutlives(_)
 413                 | ty::PredicateKind::TypeOutlives(_)
 414                 | ty::PredicateKind::WellFormed(_)
 415                 | ty::PredicateKind::ObjectSafe(_)
 416                 | ty::PredicateKind::ClosureKind(..)
 417                 | ty::PredicateKind::Subtype(_)
 418                 | ty::PredicateKind::Coerce(_)
 419                 | ty::PredicateKind::ConstEvaluatable(..)
 420                 | ty::PredicateKind::ConstEquate(..) => {
 421                     let pred =
 422                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 423                     ProcessResult::Changed(mk_pending(vec![
 424                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 425                     ]))
 426                 }
 427                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 428                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 429                 }
 430             },
 431             Some(pred) => match pred {
 432                 ty::PredicateKind::Trait(data) => {
 433                     let trait_obligation = obligation.with(Binder::dummy(data));
 434
 435                     self.process_trait_obligation(
 436                         obligation,
 437                         trait_obligation,
 438                         &mut pending_obligation.stalled_on,
 439                     )
 440                 }
 441
 442                 ty::PredicateKind::RegionOutlives(data) => {
 443                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 444                         Ok(()) => ProcessResult::Changed(vec![]),
 445                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 446                     }
 447                 }
 448
 449                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 450                     if self.register_region_obligations {
 451                         self.selcx.infcx().register_region_obligation_with_cause(
 452                             t_a,
 453                             r_b,
 454                             &obligation.cause,
 455                         );
 456                     }
 457                     ProcessResult::Changed(vec![])
 458                 }
 459
 460                 ty::PredicateKind::Projection(ref data) => {
 461                     let project_obligation = obligation.with(Binder::dummy(*data));
 462
 463                     self.process_projection_obligation(
 464                         obligation,
 465                         project_obligation,
 466                         &mut pending_obligation.stalled_on,
 467                     )
 468                 }
 469
 470                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 471                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 472                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 473                     } else {
 474                         ProcessResult::Changed(vec![])
 475                     }
 476                 }
 477
 478                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 479                     match self.selcx.infcx().closure_kind(closure_substs) {
 480                         Some(closure_kind) => {
 481                             if closure_kind.extends(kind) {
 482                                 ProcessResult::Changed(vec![])
 483                             } else {
 484                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 485                             }
 486                         }
 487                         None => ProcessResult::Unchanged,
 488                     }
 489                 }
 490
 491                 ty::PredicateKind::WellFormed(arg) => {
 492                     match wf::obligations(
 493                         self.selcx.infcx(),
 494                         obligation.param_env,
 495                         obligation.cause.body_id,
 496                         obligation.recursion_depth + 1,
 497                         arg,
 498                         obligation.cause.span,
 499                     ) {
 500                         None => {
 501                             pending_obligation.stalled_on =
 502                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 503                             ProcessResult::Unchanged
 504                         }
 505                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 506                     }
 507                 }
 508
 509                 ty::PredicateKind::Subtype(subtype) => {
 510                     match self.selcx.infcx().subtype_predicate(
 511                         &obligation.cause,
 512                         obligation.param_env,
 513                         Binder::dummy(subtype),
 514                     ) {
 515                         None => {
 516                             // None means that both are unresolved.
 517                             pending_obligation.stalled_on = vec![
 518                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 519                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 520                             ];
 521                             ProcessResult::Unchanged
 522                         }
 523                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 524                         Some(Err(err)) => {
 525                             let expected_found =
 526                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 527                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 528                                 expected_found,
 529                                 err,
 530                             ))
 531                         }
 532                     }
 533                 }
 534
 535                 ty::PredicateKind::Coerce(coerce) => {
 536                     match self.selcx.infcx().coerce_predicate(
 537                         &obligation.cause,
 538                         obligation.param_env,
 539                         Binder::dummy(coerce),
 540                     ) {
 541                         None => {
 542                             // None means that both are unresolved.
 543                             pending_obligation.stalled_on = vec![
 544                                 TyOrConstInferVar::maybe_from_ty(coerce.a).unwrap(),
 545                                 TyOrConstInferVar::maybe_from_ty(coerce.b).unwrap(),
 546                             ];
 547                             ProcessResult::Unchanged
 548                         }
 549                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 550                         Some(Err(err)) => {
 551                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 552                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 553                                 expected_found,
 554                                 err,
 555                             ))
 556                         }
 557                     }
 558                 }
 559
 560                 ty::PredicateKind::ConstEvaluatable(uv) => {
 561                     match const_evaluatable::is_const_evaluatable(
 562                         self.selcx.infcx(),
 563                         uv,
 564                         obligation.param_env,
 565                         obligation.cause.span,
 566                     ) {
 567                         Ok(()) => ProcessResult::Changed(vec![]),
 568                         Err(NotConstEvaluatable::MentionsInfer) => {
 569                             pending_obligation.stalled_on.clear();
 570                             pending_obligation.stalled_on.extend(
 571                                 uv.substs(infcx.tcx)
 572                                     .iter()
 573                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 574                             );
 575                             ProcessResult::Unchanged
 576                         }
 577                         Err(
 578                             e @ NotConstEvaluatable::MentionsParam
 579                             | e @ NotConstEvaluatable::Error(_),
 580                         ) => ProcessResult::Error(CodeSelectionError(
 581                             SelectionError::NotConstEvaluatable(e),
 582                         )),
 583                     }
 584                 }
 585
 586                 ty::PredicateKind::ConstEquate(c1, c2) => {
 587                     debug!(?c1, ?c2, "equating consts");
 588                     let tcx = self.selcx.tcx();
 589                     if tcx.features().generic_const_exprs {
 590                         // FIXME: we probably should only try to unify abstract constants
 591                         // if the constants depend on generic parameters.
 592                         //
 593                         // Let's just see where this breaks :shrug:
 594                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 595                             (c1.val, c2.val)
 596                         {
 597                             if infcx.try_unify_abstract_consts(a.shrink(), b.shrink()) {
 598                                 return ProcessResult::Changed(vec![]);
 599                             }
 600                         }
 601                     }
 602
 603                     let stalled_on = &mut pending_obligation.stalled_on;
 604
 605                     let mut evaluate = |c: &'tcx Const<'tcx>| {
 606                         if let ty::ConstKind::Unevaluated(unevaluated) = c.val {
 607                             match self.selcx.infcx().const_eval_resolve(
 608                                 obligation.param_env,
 609                                 unevaluated,
 610                                 Some(obligation.cause.span),
 611                             ) {
 612                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 613                                 Err(ErrorHandled::TooGeneric) => {
 614                                     stalled_on.extend(
 615                                         unevaluated
 616                                             .substs(tcx)
 617                                             .iter()
 618                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 619                                     );
 620                                     Err(ErrorHandled::TooGeneric)
 621                                 }
 622                                 Err(err) => Err(err),
 623                             }
 624                         } else {
 625                             Ok(c)
 626                         }
 627                     };
 628
 629                     match (evaluate(c1), evaluate(c2)) {
 630                         (Ok(c1), Ok(c2)) => {
 631                             match self
 632                                 .selcx
 633                                 .infcx()
 634                                 .at(&obligation.cause, obligation.param_env)
 635                                 .eq(c1, c2)
 636                             {
 637                                 Ok(_) => ProcessResult::Changed(vec![]),
 638                                 Err(err) => ProcessResult::Error(
 639                                     FulfillmentErrorCode::CodeConstEquateError(
 640                                         ExpectedFound::new(true, c1, c2),
 641                                         err,
 642                                     ),
 643                                 ),
 644                             }
 645                         }
 646                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 647                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => ProcessResult::Error(
 648                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 649                                 NotConstEvaluatable::Error(ErrorReported),
 650                             )),
 651                         ),
 652                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 653                             span_bug!(
 654                                 obligation.cause.span(self.selcx.tcx()),
 655                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 656                             )
 657                         }
 658                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 659                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 660                                 ProcessResult::Unchanged
 661                             } else {
 662                                 // Two different constants using generic parameters ~> error.
 663                                 let expected_found = ExpectedFound::new(true, c1, c2);
 664                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 665                                     expected_found,
 666                                     TypeError::ConstMismatch(expected_found),
 667                                 ))
 668                             }
 669                         }
 670                     }
 671                 }
 672                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 673                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 674                 }
 675             },
 676         }
 677     }
 678
 679     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 680     fn process_trait_obligation(
 681         &mut self,
 682         obligation: &PredicateObligation<'tcx>,
 683         trait_obligation: TraitObligation<'tcx>,
 684         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 685     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 686         let infcx = self.selcx.infcx();
 687         if obligation.predicate.is_known_global() {
 688             // no type variables present, can use evaluation for better caching.
 689             // FIXME: consider caching errors too.
 690             //
 691             // If the predicate is considered const, then we cannot use this because
 692             // it will cause false negatives in the ui tests.
 693             if !self.selcx.is_predicate_const(obligation.predicate)
 694                 && infcx.predicate_must_hold_considering_regions(obligation)
 695             {
 696                 debug!(
 697                     "selecting trait at depth {} evaluated to holds",
 698                     obligation.recursion_depth
 699                 );
 700                 return ProcessResult::Changed(vec![]);
 701             }
 702         }
 703
 704         match self.selcx.select(&trait_obligation) {
 705             Ok(Some(impl_source)) => {
 706                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 707                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 708             }
 709             Ok(None) => {
 710                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 711
 712                 // This is a bit subtle: for the most part, the
 713                 // only reason we can fail to make progress on
 714                 // trait selection is because we don't have enough
 715                 // information about the types in the trait.
 716                 stalled_on.clear();
 717                 stalled_on.extend(substs_infer_vars(
 718                     self.selcx,
 719                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 720                 ));
 721
 722                 debug!(
 723                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 724                     infcx.resolve_vars_if_possible(obligation.clone()),
 725                     stalled_on
 726                 );
 727
 728                 ProcessResult::Unchanged
 729             }
 730             Err(selection_err) => {
 731                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 732
 733                 ProcessResult::Error(CodeSelectionError(selection_err))
 734             }
 735         }
 736     }
 737
 738     fn process_projection_obligation(
 739         &mut self,
 740         obligation: &PredicateObligation<'tcx>,
 741         project_obligation: PolyProjectionObligation<'tcx>,
 742         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 743     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 744         let tcx = self.selcx.tcx();
 745
 746         if obligation.predicate.is_global(tcx) {
 747             // no type variables present, can use evaluation for better caching.
 748             // FIXME: consider caching errors too.
 749             //
 750             // If the predicate is considered const, then we cannot use this because
 751             // it will cause false negatives in the ui tests.
 752             if !self.selcx.is_predicate_const(obligation.predicate)
 753                 && self.selcx.infcx().predicate_must_hold_considering_regions(obligation)
 754             {
 755                 return ProcessResult::Changed(vec![]);
 756             } else {
 757                 tracing::debug!("Does NOT hold: {:?}", obligation);
 758             }
 759         }
 760
 761         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 762             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 763             Ok(Ok(None)) => {
 764                 stalled_on.clear();
 765                 stalled_on.extend(substs_infer_vars(
 766                     self.selcx,
 767                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 768                 ));
 769                 ProcessResult::Unchanged
 770             }
 771             // Let the caller handle the recursion
 772             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 773                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 774             ])),
 775             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 776         }
 777     }
 778 }
 779
 780 /// Returns the set of inference variables contained in `substs`.
 781 fn substs_infer_vars<'a, 'tcx>(
 782     selcx: &mut SelectionContext<'a, 'tcx>,
 783     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 784 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 785     let tcx = selcx.tcx();
 786     selcx
 787         .infcx()
 788         .resolve_vars_if_possible(substs)
 789         .skip_binder() // ok because this check doesn't care about regions
 790         .iter()
 791         .filter(|arg| arg.has_infer_types_or_consts())
 792         .flat_map(move |arg| {
 793             let mut walker = arg.walk(tcx);
 794             while let Some(c) = walker.next() {
 795                 if !c.has_infer_types_or_consts() {
 796                     walker.visited.remove(&c);
 797                     walker.skip_current_subtree();
 798                 }
 799             }
 800             walker.visited.into_iter()
 801         })
 802         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 803 }
 804
 805 fn to_fulfillment_error<'tcx>(
 806     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 807 ) -> FulfillmentError<'tcx> {
 808     let mut iter = error.backtrace.into_iter();
 809     let obligation = iter.next().unwrap().obligation;
 810     // The root obligation is the last item in the backtrace - if there's only
 811     // one item, then it's the same as the main obligation
 812     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 813     FulfillmentError::new(obligation, error.error, root_obligation)
 814 }