compiler/rustc_trait_selection/src/traits/fulfill.rs

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