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::{DoCompleted, Error, ForestObligation};
   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<'_>, 64);
  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         debug!("select(obligation-forest-size={})", self.predicates.len());
 124
 125         let mut errors = Vec::new();
 126
 127         loop {
 128             debug!("select: starting another iteration");
 129
 130             // Process pending obligations.
 131             let outcome = self.predicates.process_obligations(
 132                 &mut FulfillProcessor {
 133                     selcx,
 134                     register_region_obligations: self.register_region_obligations,
 135                 },
 136                 DoCompleted::No,
 137             );
 138             debug!("select: outcome={:#?}", outcome);
 139
 140             // FIXME: if we kept the original cache key, we could mark projection
 141             // obligations as complete for the projection cache here.
 142
 143             errors.extend(outcome.errors.into_iter().map(to_fulfillment_error));
 144
 145             // If nothing new was added, no need to keep looping.
 146             if outcome.stalled {
 147                 break;
 148             }
 149         }
 150
 151         debug!(
 152             "select({} predicates remaining, {} errors) done",
 153             self.predicates.len(),
 154             errors.len()
 155         );
 156
 157         if errors.is_empty() { Ok(()) } else { Err(errors) }
 158     }
 159 }
 160
 161 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 162     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 163     /// creating a fresh type variable `$0` as well as a projection
 164     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 165     /// inference engine runs, it will attempt to find an impl of
 166     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 167     /// something concrete. If this fails, we'll unify `$0` with
 168     /// `projection_ty` again.
 169     fn normalize_projection_type(
 170         &mut self,
 171         infcx: &InferCtxt<'_, 'tcx>,
 172         param_env: ty::ParamEnv<'tcx>,
 173         projection_ty: ty::ProjectionTy<'tcx>,
 174         cause: ObligationCause<'tcx>,
 175     ) -> Ty<'tcx> {
 176         debug!("normalize_projection_type(projection_ty={:?})", projection_ty);
 177
 178         debug_assert!(!projection_ty.has_escaping_bound_vars());
 179
 180         // FIXME(#20304) -- cache
 181
 182         let mut selcx = SelectionContext::new(infcx);
 183         let mut obligations = vec![];
 184         let normalized_ty = project::normalize_projection_type(
 185             &mut selcx,
 186             param_env,
 187             projection_ty,
 188             cause,
 189             0,
 190             &mut obligations,
 191         );
 192         self.register_predicate_obligations(infcx, obligations);
 193
 194         debug!("normalize_projection_type: result={:?}", normalized_ty);
 195
 196         normalized_ty
 197     }
 198
 199     fn register_predicate_obligation(
 200         &mut self,
 201         infcx: &InferCtxt<'_, 'tcx>,
 202         obligation: PredicateObligation<'tcx>,
 203     ) {
 204         // this helps to reduce duplicate errors, as well as making
 205         // debug output much nicer to read and so on.
 206         let obligation = infcx.resolve_vars_if_possible(&obligation);
 207
 208         debug!("register_predicate_obligation(obligation={:?})", obligation);
 209
 210         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 211
 212         self.predicates
 213             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 214     }
 215
 216     fn select_all_or_error(
 217         &mut self,
 218         infcx: &InferCtxt<'_, 'tcx>,
 219     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 220         self.select_where_possible(infcx)?;
 221
 222         let errors: Vec<_> = self
 223             .predicates
 224             .to_errors(CodeAmbiguity)
 225             .into_iter()
 226             .map(to_fulfillment_error)
 227             .collect();
 228         if errors.is_empty() { Ok(()) } else { Err(errors) }
 229     }
 230
 231     fn select_where_possible(
 232         &mut self,
 233         infcx: &InferCtxt<'_, 'tcx>,
 234     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 235         let mut selcx = SelectionContext::new(infcx);
 236         self.select(&mut selcx)
 237     }
 238
 239     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 240         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 241     }
 242 }
 243
 244 struct FulfillProcessor<'a, 'b, 'tcx> {
 245     selcx: &'a mut SelectionContext<'b, 'tcx>,
 246     register_region_obligations: bool,
 247 }
 248
 249 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 250     os.into_iter()
 251         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 252         .collect()
 253 }
 254
 255 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 256     type Obligation = PendingPredicateObligation<'tcx>;
 257     type Error = FulfillmentErrorCode<'tcx>;
 258
 259     /// Processes a predicate obligation and returns either:
 260     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 261     /// - `Unchanged` if we don't have enough info to be sure
 262     /// - `Error(e)` if the predicate does not hold
 263     ///
 264     /// This is always inlined, despite its size, because it has a single
 265     /// callsite and it is called *very* frequently.
 266     #[inline(always)]
 267     fn process_obligation(
 268         &mut self,
 269         pending_obligation: &mut Self::Obligation,
 270     ) -> ProcessResult<Self::Obligation, Self::Error> {
 271         // If we were stalled on some unresolved variables, first check whether
 272         // any of them have been resolved; if not, don't bother doing more work
 273         // yet.
 274         let change = match pending_obligation.stalled_on.len() {
 275             // Match arms are in order of frequency, which matters because this
 276             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 277             1 => {
 278                 let infer_var = pending_obligation.stalled_on[0];
 279                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 280             }
 281             0 => {
 282                 // In this case we haven't changed, but wish to make a change.
 283                 true
 284             }
 285             _ => {
 286                 // This `for` loop was once a call to `all()`, but this lower-level
 287                 // form was a perf win. See #64545 for details.
 288                 (|| {
 289                     for &infer_var in &pending_obligation.stalled_on {
 290                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 291                             return true;
 292                         }
 293                     }
 294                     false
 295                 })()
 296             }
 297         };
 298
 299         if !change {
 300             debug!(
 301                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 302                 self.selcx.infcx().resolve_vars_if_possible(&pending_obligation.obligation),
 303                 pending_obligation.stalled_on
 304             );
 305             return ProcessResult::Unchanged;
 306         }
 307
 308         // This part of the code is much colder.
 309
 310         pending_obligation.stalled_on.truncate(0);
 311
 312         let obligation = &mut pending_obligation.obligation;
 313
 314         if obligation.predicate.has_infer_types_or_consts() {
 315             obligation.predicate =
 316                 self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
 317         }
 318
 319         debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
 320
 321         let infcx = self.selcx.infcx();
 322
 323         match obligation.predicate.kind() {
 324             ty::PredicateKind::ForAll(binder) => match binder.skip_binder() {
 325                 // Evaluation will discard candidates using the leak check.
 326                 // This means we need to pass it the bound version of our
 327                 // predicate.
 328                 ty::PredicateAtom::Trait(trait_ref, _constness) => {
 329                     let trait_obligation = obligation.with(Binder::bind(trait_ref));
 330
 331                     self.process_trait_obligation(
 332                         obligation,
 333                         trait_obligation,
 334                         &mut pending_obligation.stalled_on,
 335                     )
 336                 }
 337                 ty::PredicateAtom::Projection(data) => {
 338                     let project_obligation = obligation.with(Binder::bind(data));
 339
 340                     self.process_projection_obligation(
 341                         project_obligation,
 342                         &mut pending_obligation.stalled_on,
 343                     )
 344                 }
 345                 ty::PredicateAtom::RegionOutlives(_)
 346                 | ty::PredicateAtom::TypeOutlives(_)
 347                 | ty::PredicateAtom::WellFormed(_)
 348                 | ty::PredicateAtom::ObjectSafe(_)
 349                 | ty::PredicateAtom::ClosureKind(..)
 350                 | ty::PredicateAtom::Subtype(_)
 351                 | ty::PredicateAtom::ConstEvaluatable(..)
 352                 | ty::PredicateAtom::ConstEquate(..) => {
 353                     let (pred, _) = infcx.replace_bound_vars_with_placeholders(binder);
 354                     ProcessResult::Changed(mk_pending(vec![
 355                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 356                     ]))
 357                 }
 358                 ty::PredicateAtom::TypeWellFormedFromEnv(..) => {
 359                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 360                 }
 361             },
 362             &ty::PredicateKind::Atom(atom) => match atom {
 363                 ty::PredicateAtom::Trait(ref data, _) => {
 364                     let trait_obligation = obligation.with(Binder::dummy(*data));
 365
 366                     self.process_trait_obligation(
 367                         obligation,
 368                         trait_obligation,
 369                         &mut pending_obligation.stalled_on,
 370                     )
 371                 }
 372
 373                 ty::PredicateAtom::RegionOutlives(data) => {
 374                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 375                         Ok(()) => ProcessResult::Changed(vec![]),
 376                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 377                     }
 378                 }
 379
 380                 ty::PredicateAtom::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 381                     if self.register_region_obligations {
 382                         self.selcx.infcx().register_region_obligation_with_cause(
 383                             t_a,
 384                             r_b,
 385                             &obligation.cause,
 386                         );
 387                     }
 388                     ProcessResult::Changed(vec![])
 389                 }
 390
 391                 ty::PredicateAtom::Projection(ref data) => {
 392                     let project_obligation = obligation.with(Binder::dummy(*data));
 393
 394                     self.process_projection_obligation(
 395                         project_obligation,
 396                         &mut pending_obligation.stalled_on,
 397                     )
 398                 }
 399
 400                 ty::PredicateAtom::ObjectSafe(trait_def_id) => {
 401                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 402                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 403                     } else {
 404                         ProcessResult::Changed(vec![])
 405                     }
 406                 }
 407
 408                 ty::PredicateAtom::ClosureKind(_, closure_substs, kind) => {
 409                     match self.selcx.infcx().closure_kind(closure_substs) {
 410                         Some(closure_kind) => {
 411                             if closure_kind.extends(kind) {
 412                                 ProcessResult::Changed(vec![])
 413                             } else {
 414                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 415                             }
 416                         }
 417                         None => ProcessResult::Unchanged,
 418                     }
 419                 }
 420
 421                 ty::PredicateAtom::WellFormed(arg) => {
 422                     match wf::obligations(
 423                         self.selcx.infcx(),
 424                         obligation.param_env,
 425                         obligation.cause.body_id,
 426                         arg,
 427                         obligation.cause.span,
 428                     ) {
 429                         None => {
 430                             pending_obligation.stalled_on =
 431                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 432                             ProcessResult::Unchanged
 433                         }
 434                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 435                     }
 436                 }
 437
 438                 ty::PredicateAtom::Subtype(subtype) => {
 439                     match self.selcx.infcx().subtype_predicate(
 440                         &obligation.cause,
 441                         obligation.param_env,
 442                         Binder::dummy(subtype),
 443                     ) {
 444                         None => {
 445                             // None means that both are unresolved.
 446                             pending_obligation.stalled_on = vec![
 447                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 448                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 449                             ];
 450                             ProcessResult::Unchanged
 451                         }
 452                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 453                         Some(Err(err)) => {
 454                             let expected_found =
 455                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 456                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 457                                 expected_found,
 458                                 err,
 459                             ))
 460                         }
 461                     }
 462                 }
 463
 464                 ty::PredicateAtom::ConstEvaluatable(def_id, substs) => {
 465                     match const_evaluatable::is_const_evaluatable(
 466                         self.selcx.infcx(),
 467                         def_id,
 468                         substs,
 469                         obligation.param_env,
 470                         obligation.cause.span,
 471                     ) {
 472                         Ok(()) => ProcessResult::Changed(vec![]),
 473                         Err(e) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(e))),
 474                     }
 475                 }
 476
 477                 ty::PredicateAtom::ConstEquate(c1, c2) => {
 478                     debug!("equating consts: c1={:?} c2={:?}", c1, c2);
 479                     if self.selcx.tcx().features().const_evaluatable_checked {
 480                         // FIXME: we probably should only try to unify abstract constants
 481                         // if the constants depend on generic parameters.
 482                         //
 483                         // Let's just see where this breaks :shrug:
 484                         if let (
 485                             ty::ConstKind::Unevaluated(a_def, a_substs, None),
 486                             ty::ConstKind::Unevaluated(b_def, b_substs, None),
 487                         ) = (c1.val, c2.val)
 488                         {
 489                             if self
 490                                 .selcx
 491                                 .tcx()
 492                                 .try_unify_abstract_consts(((a_def, a_substs), (b_def, b_substs)))
 493                             {
 494                                 return ProcessResult::Changed(vec![]);
 495                             }
 496                         }
 497                     }
 498
 499                     let stalled_on = &mut pending_obligation.stalled_on;
 500
 501                     let mut evaluate = |c: &'tcx Const<'tcx>| {
 502                         if let ty::ConstKind::Unevaluated(def, substs, promoted) = c.val {
 503                             match self.selcx.infcx().const_eval_resolve(
 504                                 obligation.param_env,
 505                                 def,
 506                                 substs,
 507                                 promoted,
 508                                 Some(obligation.cause.span),
 509                             ) {
 510                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 511                                 Err(ErrorHandled::TooGeneric) => {
 512                                     stalled_on.append(
 513                                         &mut substs
 514                                             .types()
 515                                             .filter_map(|ty| TyOrConstInferVar::maybe_from_ty(ty))
 516                                             .collect(),
 517                                     );
 518                                     Err(ErrorHandled::TooGeneric)
 519                                 }
 520                                 Err(err) => Err(err),
 521                             }
 522                         } else {
 523                             Ok(c)
 524                         }
 525                     };
 526
 527                     match (evaluate(c1), evaluate(c2)) {
 528                         (Ok(c1), Ok(c2)) => {
 529                             match self
 530                                 .selcx
 531                                 .infcx()
 532                                 .at(&obligation.cause, obligation.param_env)
 533                                 .eq(c1, c2)
 534                             {
 535                                 Ok(_) => ProcessResult::Changed(vec![]),
 536                                 Err(err) => ProcessResult::Error(
 537                                     FulfillmentErrorCode::CodeConstEquateError(
 538                                         ExpectedFound::new(true, c1, c2),
 539                                         err,
 540                                     ),
 541                                 ),
 542                             }
 543                         }
 544                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 545                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => {
 546                             ProcessResult::Error(CodeSelectionError(ConstEvalFailure(
 547                                 ErrorHandled::Reported(ErrorReported),
 548                             )))
 549                         }
 550                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 551                             span_bug!(
 552                                 obligation.cause.span(self.selcx.tcx()),
 553                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 554                             )
 555                         }
 556                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 557                             ProcessResult::Unchanged
 558                         }
 559                     }
 560                 }
 561                 ty::PredicateAtom::TypeWellFormedFromEnv(..) => {
 562                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 563                 }
 564             },
 565         }
 566     }
 567
 568     fn process_backedge<'c, I>(
 569         &mut self,
 570         cycle: I,
 571         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 572     ) where
 573         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 574     {
 575         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 576             debug!("process_child_obligations: coinductive match");
 577         } else {
 578             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 579             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 580         }
 581     }
 582 }
 583
 584 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 585     fn process_trait_obligation(
 586         &mut self,
 587         obligation: &PredicateObligation<'tcx>,
 588         trait_obligation: TraitObligation<'tcx>,
 589         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 590     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 591         let infcx = self.selcx.infcx();
 592         if obligation.predicate.is_global() {
 593             // no type variables present, can use evaluation for better caching.
 594             // FIXME: consider caching errors too.
 595             if infcx.predicate_must_hold_considering_regions(obligation) {
 596                 debug!(
 597                     "selecting trait `{:?}` at depth {} evaluated to holds",
 598                     obligation.predicate, obligation.recursion_depth
 599                 );
 600                 return ProcessResult::Changed(vec![]);
 601             }
 602         }
 603
 604         match self.selcx.select(&trait_obligation) {
 605             Ok(Some(impl_source)) => {
 606                 debug!(
 607                     "selecting trait `{:?}` at depth {} yielded Ok(Some)",
 608                     trait_obligation.predicate, obligation.recursion_depth
 609                 );
 610                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 611             }
 612             Ok(None) => {
 613                 debug!(
 614                     "selecting trait `{:?}` at depth {} yielded Ok(None)",
 615                     trait_obligation.predicate, obligation.recursion_depth
 616                 );
 617
 618                 // This is a bit subtle: for the most part, the
 619                 // only reason we can fail to make progress on
 620                 // trait selection is because we don't have enough
 621                 // information about the types in the trait.
 622                 *stalled_on = trait_ref_infer_vars(
 623                     self.selcx,
 624                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref),
 625                 );
 626
 627                 debug!(
 628                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 629                     infcx.resolve_vars_if_possible(obligation),
 630                     stalled_on
 631                 );
 632
 633                 ProcessResult::Unchanged
 634             }
 635             Err(selection_err) => {
 636                 info!(
 637                     "selecting trait `{:?}` at depth {} yielded Err",
 638                     trait_obligation.predicate, obligation.recursion_depth
 639                 );
 640
 641                 ProcessResult::Error(CodeSelectionError(selection_err))
 642             }
 643         }
 644     }
 645
 646     fn process_projection_obligation(
 647         &mut self,
 648         project_obligation: PolyProjectionObligation<'tcx>,
 649         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 650     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 651         let tcx = self.selcx.tcx();
 652         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 653             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 654             Ok(Ok(None)) => {
 655                 *stalled_on = trait_ref_infer_vars(
 656                     self.selcx,
 657                     project_obligation.predicate.to_poly_trait_ref(self.selcx.tcx()),
 658                 );
 659                 ProcessResult::Unchanged
 660             }
 661             // Let the caller handle the recursion
 662             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 663                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 664             ])),
 665             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 666         }
 667     }
 668 }
 669
 670 /// Returns the set of inference variables contained in a trait ref.
 671 fn trait_ref_infer_vars<'a, 'tcx>(
 672     selcx: &mut SelectionContext<'a, 'tcx>,
 673     trait_ref: ty::PolyTraitRef<'tcx>,
 674 ) -> Vec<TyOrConstInferVar<'tcx>> {
 675     selcx
 676         .infcx()
 677         .resolve_vars_if_possible(&trait_ref)
 678         .skip_binder()
 679         .substs
 680         .iter()
 681         // FIXME(eddyb) try using `skip_current_subtree` to skip everything that
 682         // doesn't contain inference variables, not just the outermost level.
 683         .filter(|arg| arg.has_infer_types_or_consts())
 684         .flat_map(|arg| arg.walk())
 685         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 686         .collect()
 687 }
 688
 689 fn to_fulfillment_error<'tcx>(
 690     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 691 ) -> FulfillmentError<'tcx> {
 692     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 693     FulfillmentError::new(obligation, error.error)
 694 }