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::subst::SubstsRef;
  10 use rustc_middle::ty::ToPredicate;
  11 use rustc_middle::ty::{self, Binder, Const, Ty, TypeFoldable};
  12 use std::marker::PhantomData;
  13
  14 use super::const_evaluatable;
  15 use super::project;
  16 use super::select::SelectionContext;
  17 use super::wf;
  18 use super::CodeAmbiguity;
  19 use super::CodeProjectionError;
  20 use super::CodeSelectionError;
  21 use super::{ConstEvalFailure, Unimplemented};
  22 use super::{FulfillmentError, FulfillmentErrorCode};
  23 use super::{ObligationCause, PredicateObligation};
  24
  25 use crate::traits::error_reporting::InferCtxtExt as _;
  26 use crate::traits::project::PolyProjectionObligation;
  27 use crate::traits::query::evaluate_obligation::InferCtxtExt as _;
  28
  29 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  30     /// Note that we include both the `ParamEnv` and the `Predicate`,
  31     /// as the `ParamEnv` can influence whether fulfillment succeeds
  32     /// or fails.
  33     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  34
  35     fn as_cache_key(&self) -> Self::CacheKey {
  36         self.obligation.param_env.and(self.obligation.predicate)
  37     }
  38 }
  39
  40 /// The fulfillment context is used to drive trait resolution. It
  41 /// consists of a list of obligations that must be (eventually)
  42 /// satisfied. The job is to track which are satisfied, which yielded
  43 /// errors, and which are still pending. At any point, users can call
  44 /// `select_where_possible`, and the fulfillment context will try to do
  45 /// selection, retaining only those obligations that remain
  46 /// ambiguous. This may be helpful in pushing type inference
  47 /// along. Once all type inference constraints have been generated, the
  48 /// method `select_all_or_error` can be used to report any remaining
  49 /// ambiguous cases as errors.
  50 pub struct FulfillmentContext<'tcx> {
  51     // A list of all obligations that have been registered with this
  52     // fulfillment context.
  53     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  54     // Should this fulfillment context register type-lives-for-region
  55     // obligations on its parent infcx? In some cases, region
  56     // obligations are either already known to hold (normalization) or
  57     // hopefully verifed elsewhere (type-impls-bound), and therefore
  58     // should not be checked.
  59     //
  60     // Note that if we are normalizing a type that we already
  61     // know is well-formed, there should be no harm setting this
  62     // to true - all the region variables should be determinable
  63     // using the RFC 447 rules, which don't depend on
  64     // type-lives-for-region constraints, and because the type
  65     // is well-formed, the constraints should hold.
  66     register_region_obligations: bool,
  67     // Is it OK to register obligations into this infcx inside
  68     // an infcx snapshot?
  69     //
  70     // The "primary fulfillment" in many cases in typeck lives
  71     // outside of any snapshot, so any use of it inside a snapshot
  72     // will lead to trouble and therefore is checked against, but
  73     // other fulfillment contexts sometimes do live inside of
  74     // a snapshot (they don't *straddle* a snapshot, so there
  75     // is no trouble there).
  76     usable_in_snapshot: bool,
  77 }
  78
  79 #[derive(Clone, Debug)]
  80 pub struct PendingPredicateObligation<'tcx> {
  81     pub obligation: PredicateObligation<'tcx>,
  82     // This is far more often read than modified, meaning that we
  83     // should mostly optimize for reading speed, while modifying is not as relevant.
  84     //
  85     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  86     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  87 }
  88
  89 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  90 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  91 static_assert_size!(PendingPredicateObligation<'_>, 56);
  92
  93 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  94     /// Creates a new fulfillment context.
  95     pub fn new() -> FulfillmentContext<'tcx> {
  96         FulfillmentContext {
  97             predicates: ObligationForest::new(),
  98             register_region_obligations: true,
  99             usable_in_snapshot: false,
 100         }
 101     }
 102
 103     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
 104         FulfillmentContext {
 105             predicates: ObligationForest::new(),
 106             register_region_obligations: true,
 107             usable_in_snapshot: true,
 108         }
 109     }
 110
 111     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
 112         FulfillmentContext {
 113             predicates: ObligationForest::new(),
 114             register_region_obligations: false,
 115             usable_in_snapshot: false,
 116         }
 117     }
 118
 119     /// Attempts to select obligations using `selcx`.
 120     fn select(
 121         &mut self,
 122         selcx: &mut SelectionContext<'a, 'tcx>,
 123     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 124         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 125         let _enter = span.enter();
 126
 127         let mut errors = Vec::new();
 128
 129         loop {
 130             debug!("select: starting another iteration");
 131
 132             // Process pending obligations.
 133             let outcome: Outcome<_, _> =
 134                 self.predicates.process_obligations(&mut FulfillProcessor {
 135                     selcx,
 136                     register_region_obligations: self.register_region_obligations,
 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!(?projection_ty, "normalize_projection_type");
 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!(?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!(?obligation, "register_predicate_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.clone()),
 303                 pending_obligation.stalled_on
 304             );
 305             return ProcessResult::Unchanged;
 306         }
 307
 308         self.progress_changed_obligations(pending_obligation)
 309     }
 310
 311     fn process_backedge<'c, I>(
 312         &mut self,
 313         cycle: I,
 314         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 315     ) where
 316         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 317     {
 318         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 319             debug!("process_child_obligations: coinductive match");
 320         } else {
 321             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 322             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 323         }
 324     }
 325 }
 326
 327 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 328     // The code calling this method is extremely hot and only rarely
 329     // actually uses this, so move this part of the code
 330     // out of that loop.
 331     #[inline(never)]
 332     fn progress_changed_obligations(
 333         &mut self,
 334         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 335     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 336         pending_obligation.stalled_on.truncate(0);
 337
 338         let obligation = &mut pending_obligation.obligation;
 339
 340         if obligation.predicate.has_infer_types_or_consts() {
 341             obligation.predicate =
 342                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 343         }
 344
 345         debug!(?obligation, ?obligation.cause, "process_obligation");
 346
 347         let infcx = self.selcx.infcx();
 348
 349         let binder = obligation.predicate.kind();
 350         match binder.no_bound_vars() {
 351             None => match binder.skip_binder() {
 352                 // Evaluation will discard candidates using the leak check.
 353                 // This means we need to pass it the bound version of our
 354                 // predicate.
 355                 ty::PredicateKind::Trait(trait_ref, _constness) => {
 356                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 357
 358                     self.process_trait_obligation(
 359                         obligation,
 360                         trait_obligation,
 361                         &mut pending_obligation.stalled_on,
 362                     )
 363                 }
 364                 ty::PredicateKind::Projection(data) => {
 365                     let project_obligation = obligation.with(binder.rebind(data));
 366
 367                     self.process_projection_obligation(
 368                         project_obligation,
 369                         &mut pending_obligation.stalled_on,
 370                     )
 371                 }
 372                 ty::PredicateKind::RegionOutlives(_)
 373                 | ty::PredicateKind::TypeOutlives(_)
 374                 | ty::PredicateKind::WellFormed(_)
 375                 | ty::PredicateKind::ObjectSafe(_)
 376                 | ty::PredicateKind::ClosureKind(..)
 377                 | ty::PredicateKind::Subtype(_)
 378                 | ty::PredicateKind::ConstEvaluatable(..)
 379                 | ty::PredicateKind::ConstEquate(..) => {
 380                     let pred = infcx.replace_bound_vars_with_placeholders(binder);
 381                     ProcessResult::Changed(mk_pending(vec![
 382                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 383                     ]))
 384                 }
 385                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 386                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 387                 }
 388             },
 389             Some(pred) => match pred {
 390                 ty::PredicateKind::Trait(data, _) => {
 391                     let trait_obligation = obligation.with(Binder::dummy(data));
 392
 393                     self.process_trait_obligation(
 394                         obligation,
 395                         trait_obligation,
 396                         &mut pending_obligation.stalled_on,
 397                     )
 398                 }
 399
 400                 ty::PredicateKind::RegionOutlives(data) => {
 401                     match infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data)) {
 402                         Ok(()) => ProcessResult::Changed(vec![]),
 403                         Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 404                     }
 405                 }
 406
 407                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 408                     if self.register_region_obligations {
 409                         self.selcx.infcx().register_region_obligation_with_cause(
 410                             t_a,
 411                             r_b,
 412                             &obligation.cause,
 413                         );
 414                     }
 415                     ProcessResult::Changed(vec![])
 416                 }
 417
 418                 ty::PredicateKind::Projection(ref data) => {
 419                     let project_obligation = obligation.with(Binder::dummy(*data));
 420
 421                     self.process_projection_obligation(
 422                         project_obligation,
 423                         &mut pending_obligation.stalled_on,
 424                     )
 425                 }
 426
 427                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 428                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 429                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 430                     } else {
 431                         ProcessResult::Changed(vec![])
 432                     }
 433                 }
 434
 435                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 436                     match self.selcx.infcx().closure_kind(closure_substs) {
 437                         Some(closure_kind) => {
 438                             if closure_kind.extends(kind) {
 439                                 ProcessResult::Changed(vec![])
 440                             } else {
 441                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 442                             }
 443                         }
 444                         None => ProcessResult::Unchanged,
 445                     }
 446                 }
 447
 448                 ty::PredicateKind::WellFormed(arg) => {
 449                     match wf::obligations(
 450                         self.selcx.infcx(),
 451                         obligation.param_env,
 452                         obligation.cause.body_id,
 453                         obligation.recursion_depth + 1,
 454                         arg,
 455                         obligation.cause.span,
 456                     ) {
 457                         None => {
 458                             pending_obligation.stalled_on =
 459                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 460                             ProcessResult::Unchanged
 461                         }
 462                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 463                     }
 464                 }
 465
 466                 ty::PredicateKind::Subtype(subtype) => {
 467                     match self.selcx.infcx().subtype_predicate(
 468                         &obligation.cause,
 469                         obligation.param_env,
 470                         Binder::dummy(subtype),
 471                     ) {
 472                         None => {
 473                             // None means that both are unresolved.
 474                             pending_obligation.stalled_on = vec![
 475                                 TyOrConstInferVar::maybe_from_ty(subtype.a).unwrap(),
 476                                 TyOrConstInferVar::maybe_from_ty(subtype.b).unwrap(),
 477                             ];
 478                             ProcessResult::Unchanged
 479                         }
 480                         Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 481                         Some(Err(err)) => {
 482                             let expected_found =
 483                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 484                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 485                                 expected_found,
 486                                 err,
 487                             ))
 488                         }
 489                     }
 490                 }
 491
 492                 ty::PredicateKind::ConstEvaluatable(def_id, substs) => {
 493                     match const_evaluatable::is_const_evaluatable(
 494                         self.selcx.infcx(),
 495                         def_id,
 496                         substs,
 497                         obligation.param_env,
 498                         obligation.cause.span,
 499                     ) {
 500                         Ok(()) => ProcessResult::Changed(vec![]),
 501                         Err(ErrorHandled::TooGeneric) => {
 502                             pending_obligation.stalled_on.clear();
 503                             pending_obligation.stalled_on.extend(
 504                                 substs.iter().filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 505                             );
 506                             ProcessResult::Unchanged
 507                         }
 508                         Err(e) => ProcessResult::Error(CodeSelectionError(ConstEvalFailure(e))),
 509                     }
 510                 }
 511
 512                 ty::PredicateKind::ConstEquate(c1, c2) => {
 513                     debug!(?c1, ?c2, "equating consts");
 514                     if self.selcx.tcx().features().const_evaluatable_checked {
 515                         // FIXME: we probably should only try to unify abstract constants
 516                         // if the constants depend on generic parameters.
 517                         //
 518                         // Let's just see where this breaks :shrug:
 519                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 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(unevaluated) = c.val {
 536                             match self.selcx.infcx().const_eval_resolve(
 537                                 obligation.param_env,
 538                                 unevaluated,
 539                                 Some(obligation.cause.span),
 540                             ) {
 541                                 Ok(val) => Ok(Const::from_value(self.selcx.tcx(), val, c.ty)),
 542                                 Err(ErrorHandled::TooGeneric) => {
 543                                     stalled_on.extend(
 544                                         unevaluated
 545                                             .substs
 546                                             .iter()
 547                                             .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 548                                     );
 549                                     Err(ErrorHandled::TooGeneric)
 550                                 }
 551                                 Err(err) => Err(err),
 552                             }
 553                         } else {
 554                             Ok(c)
 555                         }
 556                     };
 557
 558                     match (evaluate(c1), evaluate(c2)) {
 559                         (Ok(c1), Ok(c2)) => {
 560                             match self
 561                                 .selcx
 562                                 .infcx()
 563                                 .at(&obligation.cause, obligation.param_env)
 564                                 .eq(c1, c2)
 565                             {
 566                                 Ok(_) => ProcessResult::Changed(vec![]),
 567                                 Err(err) => ProcessResult::Error(
 568                                     FulfillmentErrorCode::CodeConstEquateError(
 569                                         ExpectedFound::new(true, c1, c2),
 570                                         err,
 571                                     ),
 572                                 ),
 573                             }
 574                         }
 575                         (Err(ErrorHandled::Reported(ErrorReported)), _)
 576                         | (_, Err(ErrorHandled::Reported(ErrorReported))) => {
 577                             ProcessResult::Error(CodeSelectionError(ConstEvalFailure(
 578                                 ErrorHandled::Reported(ErrorReported),
 579                             )))
 580                         }
 581                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 582                             span_bug!(
 583                                 obligation.cause.span(self.selcx.tcx()),
 584                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 585                             )
 586                         }
 587                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 588                             ProcessResult::Unchanged
 589                         }
 590                     }
 591                 }
 592                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 593                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 594                 }
 595             },
 596         }
 597     }
 598
 599     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 600     fn process_trait_obligation(
 601         &mut self,
 602         obligation: &PredicateObligation<'tcx>,
 603         trait_obligation: TraitObligation<'tcx>,
 604         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 605     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 606         let infcx = self.selcx.infcx();
 607         if obligation.predicate.is_global() {
 608             // no type variables present, can use evaluation for better caching.
 609             // FIXME: consider caching errors too.
 610             if infcx.predicate_must_hold_considering_regions(obligation) {
 611                 debug!(
 612                     "selecting trait at depth {} evaluated to holds",
 613                     obligation.recursion_depth
 614                 );
 615                 return ProcessResult::Changed(vec![]);
 616             }
 617         }
 618
 619         match self.selcx.select(&trait_obligation) {
 620             Ok(Some(impl_source)) => {
 621                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 622                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 623             }
 624             Ok(None) => {
 625                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 626
 627                 // This is a bit subtle: for the most part, the
 628                 // only reason we can fail to make progress on
 629                 // trait selection is because we don't have enough
 630                 // information about the types in the trait.
 631                 stalled_on.clear();
 632                 stalled_on.extend(substs_infer_vars(
 633                     self.selcx,
 634                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 635                 ));
 636
 637                 debug!(
 638                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 639                     infcx.resolve_vars_if_possible(obligation.clone()),
 640                     stalled_on
 641                 );
 642
 643                 ProcessResult::Unchanged
 644             }
 645             Err(selection_err) => {
 646                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 647
 648                 ProcessResult::Error(CodeSelectionError(selection_err))
 649             }
 650         }
 651     }
 652
 653     fn process_projection_obligation(
 654         &mut self,
 655         project_obligation: PolyProjectionObligation<'tcx>,
 656         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 657     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 658         let tcx = self.selcx.tcx();
 659         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 660             Ok(Ok(Some(os))) => ProcessResult::Changed(mk_pending(os)),
 661             Ok(Ok(None)) => {
 662                 stalled_on.clear();
 663                 stalled_on.extend(substs_infer_vars(
 664                     self.selcx,
 665                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 666                 ));
 667                 ProcessResult::Unchanged
 668             }
 669             // Let the caller handle the recursion
 670             Ok(Err(project::InProgress)) => ProcessResult::Changed(mk_pending(vec![
 671                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 672             ])),
 673             Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 674         }
 675     }
 676 }
 677
 678 /// Returns the set of inference variables contained in `substs`.
 679 fn substs_infer_vars<'a, 'tcx>(
 680     selcx: &mut SelectionContext<'a, 'tcx>,
 681     substs: ty::Binder<SubstsRef<'tcx>>,
 682 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 683     selcx
 684         .infcx()
 685         .resolve_vars_if_possible(substs)
 686         .skip_binder() // ok because this check doesn't care about regions
 687         .iter()
 688         .filter(|arg| arg.has_infer_types_or_consts())
 689         .flat_map(|arg| {
 690             let mut walker = arg.walk();
 691             while let Some(c) = walker.next() {
 692                 if !c.has_infer_types_or_consts() {
 693                     walker.visited.remove(&c);
 694                     walker.skip_current_subtree();
 695                 }
 696             }
 697             walker.visited.into_iter()
 698         })
 699         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 700 }
 701
 702 fn to_fulfillment_error<'tcx>(
 703     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 704 ) -> FulfillmentError<'tcx> {
 705     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 706     FulfillmentError::new(obligation, error.error)
 707 }