src/librustc/traits/fulfill.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use dep_graph::DepGraph;
  12 use infer::{InferCtxt, InferOk};
  13 use ty::{self, Ty, TypeFoldable, ToPolyTraitRef, TyCtxt, ToPredicate};
  14 use ty::error::ExpectedFound;
  15 use rustc_data_structures::obligation_forest::{ObligationForest, Error};
  16 use rustc_data_structures::obligation_forest::{ForestObligation, ObligationProcessor};
  17 use std::marker::PhantomData;
  18 use syntax::ast;
  19 use util::nodemap::{FxHashSet, NodeMap};
  20 use hir::def_id::DefId;
  21
  22 use super::CodeAmbiguity;
  23 use super::CodeProjectionError;
  24 use super::CodeSelectionError;
  25 use super::{FulfillmentError, FulfillmentErrorCode};
  26 use super::{ObligationCause, PredicateObligation, Obligation};
  27 use super::project;
  28 use super::select::SelectionContext;
  29 use super::Unimplemented;
  30
  31 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  32     type Predicate = ty::Predicate<'tcx>;
  33
  34     fn as_predicate(&self) -> &Self::Predicate { &self.obligation.predicate }
  35 }
  36
  37 pub struct GlobalFulfilledPredicates<'tcx> {
  38     set: FxHashSet<ty::PolyTraitPredicate<'tcx>>,
  39     dep_graph: DepGraph,
  40 }
  41
  42 /// The fulfillment context is used to drive trait resolution.  It
  43 /// consists of a list of obligations that must be (eventually)
  44 /// satisfied. The job is to track which are satisfied, which yielded
  45 /// errors, and which are still pending. At any point, users can call
  46 /// `select_where_possible`, and the fulfilment context will try to do
  47 /// selection, retaining only those obligations that remain
  48 /// ambiguous. This may be helpful in pushing type inference
  49 /// along. Once all type inference constraints have been generated, the
  50 /// method `select_all_or_error` can be used to report any remaining
  51 /// ambiguous cases as errors.
  52
  53 pub struct FulfillmentContext<'tcx> {
  54     // A list of all obligations that have been registered with this
  55     // fulfillment context.
  56     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  57
  58     // A set of constraints that regionck must validate. Each
  59     // constraint has the form `T:'a`, meaning "some type `T` must
  60     // outlive the lifetime 'a". These constraints derive from
  61     // instantiated type parameters. So if you had a struct defined
  62     // like
  63     //
  64     //     struct Foo<T:'static> { ... }
  65     //
  66     // then in some expression `let x = Foo { ... }` it will
  67     // instantiate the type parameter `T` with a fresh type `$0`. At
  68     // the same time, it will record a region obligation of
  69     // `$0:'static`. This will get checked later by regionck. (We
  70     // can't generally check these things right away because we have
  71     // to wait until types are resolved.)
  72     //
  73     // These are stored in a map keyed to the id of the innermost
  74     // enclosing fn body / static initializer expression. This is
  75     // because the location where the obligation was incurred can be
  76     // relevant with respect to which sublifetime assumptions are in
  77     // place. The reason that we store under the fn-id, and not
  78     // something more fine-grained, is so that it is easier for
  79     // regionck to be sure that it has found *all* the region
  80     // obligations (otherwise, it's easy to fail to walk to a
  81     // particular node-id).
  82     region_obligations: NodeMap<Vec<RegionObligation<'tcx>>>,
  83 }
  84
  85 #[derive(Clone)]
  86 pub struct RegionObligation<'tcx> {
  87     pub sub_region: ty::Region<'tcx>,
  88     pub sup_type: Ty<'tcx>,
  89     pub cause: ObligationCause<'tcx>,
  90 }
  91
  92 #[derive(Clone, Debug)]
  93 pub struct PendingPredicateObligation<'tcx> {
  94     pub obligation: PredicateObligation<'tcx>,
  95     pub stalled_on: Vec<Ty<'tcx>>,
  96 }
  97
  98 impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> {
  99     /// Creates a new fulfillment context.
 100     pub fn new() -> FulfillmentContext<'tcx> {
 101         FulfillmentContext {
 102             predicates: ObligationForest::new(),
 103             region_obligations: NodeMap(),
 104         }
 105     }
 106
 107     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 108     /// creating a fresh type variable `$0` as well as a projection
 109     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 110     /// inference engine runs, it will attempt to find an impl of
 111     /// `SomeTrait` or a where clause that lets us unify `$0` with
 112     /// something concrete. If this fails, we'll unify `$0` with
 113     /// `projection_ty` again.
 114     pub fn normalize_projection_type(&mut self,
 115                                      infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 116                                      param_env: ty::ParamEnv<'tcx>,
 117                                      projection_ty: ty::ProjectionTy<'tcx>,
 118                                      cause: ObligationCause<'tcx>)
 119                                      -> Ty<'tcx>
 120     {
 121         debug!("normalize_projection_type(projection_ty={:?})",
 122                projection_ty);
 123
 124         assert!(!projection_ty.has_escaping_regions());
 125
 126         // FIXME(#20304) -- cache
 127
 128         let mut selcx = SelectionContext::new(infcx);
 129         let normalized = project::normalize_projection_type(&mut selcx,
 130                                                             param_env,
 131                                                             projection_ty,
 132                                                             cause,
 133                                                             0);
 134
 135         for obligation in normalized.obligations {
 136             self.register_predicate_obligation(infcx, obligation);
 137         }
 138
 139         debug!("normalize_projection_type: result={:?}", normalized.value);
 140
 141         normalized.value
 142     }
 143
 144     /// Requires that `ty` must implement the trait with `def_id` in
 145     /// the given environment. This trait must not have any type
 146     /// parameters (except for `Self`).
 147     pub fn register_bound(&mut self,
 148                           infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 149                           param_env: ty::ParamEnv<'tcx>,
 150                           ty: Ty<'tcx>,
 151                           def_id: DefId,
 152                           cause: ObligationCause<'tcx>)
 153     {
 154         let trait_ref = ty::TraitRef {
 155             def_id: def_id,
 156             substs: infcx.tcx.mk_substs_trait(ty, &[]),
 157         };
 158         self.register_predicate_obligation(infcx, Obligation {
 159             cause: cause,
 160             recursion_depth: 0,
 161             param_env,
 162             predicate: trait_ref.to_predicate()
 163         });
 164     }
 165
 166     pub fn register_region_obligation(&mut self,
 167                                       t_a: Ty<'tcx>,
 168                                       r_b: ty::Region<'tcx>,
 169                                       cause: ObligationCause<'tcx>)
 170     {
 171         register_region_obligation(t_a, r_b, cause, &mut self.region_obligations);
 172     }
 173
 174     pub fn register_predicate_obligation(&mut self,
 175                                          infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 176                                          obligation: PredicateObligation<'tcx>)
 177     {
 178         // this helps to reduce duplicate errors, as well as making
 179         // debug output much nicer to read and so on.
 180         let obligation = infcx.resolve_type_vars_if_possible(&obligation);
 181
 182         debug!("register_predicate_obligation(obligation={:?})", obligation);
 183
 184         assert!(!infcx.is_in_snapshot());
 185
 186         if infcx.tcx.fulfilled_predicates.borrow().check_duplicate(&obligation.predicate) {
 187             debug!("register_predicate_obligation: duplicate");
 188             return
 189         }
 190
 191         self.predicates.register_obligation(PendingPredicateObligation {
 192             obligation: obligation,
 193             stalled_on: vec![]
 194         });
 195     }
 196
 197     pub fn register_predicate_obligations(&mut self,
 198                                           infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 199                                           obligations: Vec<PredicateObligation<'tcx>>)
 200     {
 201         for obligation in obligations {
 202             self.register_predicate_obligation(infcx, obligation);
 203         }
 204     }
 205
 206
 207     pub fn region_obligations(&self,
 208                               body_id: ast::NodeId)
 209                               -> &[RegionObligation<'tcx>]
 210     {
 211         match self.region_obligations.get(&body_id) {
 212             None => Default::default(),
 213             Some(vec) => vec,
 214         }
 215     }
 216
 217     pub fn select_all_or_error(&mut self,
 218                                infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 219                                -> Result<(),Vec<FulfillmentError<'tcx>>>
 220     {
 221         self.select_where_possible(infcx)?;
 222
 223         let errors: Vec<_> =
 224             self.predicates.to_errors(CodeAmbiguity)
 225                            .into_iter()
 226                            .map(|e| to_fulfillment_error(e))
 227                            .collect();
 228         if errors.is_empty() {
 229             Ok(())
 230         } else {
 231             Err(errors)
 232         }
 233     }
 234
 235     pub fn select_where_possible(&mut self,
 236                                  infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 237                                  -> Result<(),Vec<FulfillmentError<'tcx>>>
 238     {
 239         let mut selcx = SelectionContext::new(infcx);
 240         self.select(&mut selcx)
 241     }
 242
 243     pub fn pending_obligations(&self) -> Vec<PendingPredicateObligation<'tcx>> {
 244         self.predicates.pending_obligations()
 245     }
 246
 247     /// Attempts to select obligations using `selcx`. If `only_new_obligations` is true, then it
 248     /// only attempts to select obligations that haven't been seen before.
 249     fn select(&mut self, selcx: &mut SelectionContext<'a, 'gcx, 'tcx>)
 250               -> Result<(),Vec<FulfillmentError<'tcx>>> {
 251         debug!("select(obligation-forest-size={})", self.predicates.len());
 252
 253         let mut errors = Vec::new();
 254
 255         loop {
 256             debug!("select: starting another iteration");
 257
 258             // Process pending obligations.
 259             let outcome = self.predicates.process_obligations(&mut FulfillProcessor {
 260                 selcx: selcx,
 261                 region_obligations: &mut self.region_obligations,
 262             });
 263             debug!("select: outcome={:?}", outcome);
 264
 265             // these are obligations that were proven to be true.
 266             for pending_obligation in outcome.completed {
 267                 let predicate = &pending_obligation.obligation.predicate;
 268                 selcx.tcx().fulfilled_predicates.borrow_mut()
 269                            .add_if_global(selcx.tcx(), predicate);
 270             }
 271
 272             errors.extend(
 273                 outcome.errors.into_iter()
 274                               .map(|e| to_fulfillment_error(e)));
 275
 276             // If nothing new was added, no need to keep looping.
 277             if outcome.stalled {
 278                 break;
 279             }
 280         }
 281
 282         debug!("select({} predicates remaining, {} errors) done",
 283                self.predicates.len(), errors.len());
 284
 285         if errors.is_empty() {
 286             Ok(())
 287         } else {
 288             Err(errors)
 289         }
 290     }
 291 }
 292
 293 struct FulfillProcessor<'a, 'b: 'a, 'gcx: 'tcx, 'tcx: 'b> {
 294     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 295     region_obligations: &'a mut NodeMap<Vec<RegionObligation<'tcx>>>,
 296 }
 297
 298 impl<'a, 'b, 'gcx, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'gcx, 'tcx> {
 299     type Obligation = PendingPredicateObligation<'tcx>;
 300     type Error = FulfillmentErrorCode<'tcx>;
 301
 302     fn process_obligation(&mut self,
 303                           obligation: &mut Self::Obligation)
 304                           -> Result<Option<Vec<Self::Obligation>>, Self::Error>
 305     {
 306         process_predicate(self.selcx,
 307                           obligation,
 308                           self.region_obligations)
 309             .map(|os| os.map(|os| os.into_iter().map(|o| PendingPredicateObligation {
 310                 obligation: o,
 311                 stalled_on: vec![]
 312             }).collect()))
 313     }
 314
 315     fn process_backedge<'c, I>(&mut self, cycle: I,
 316                                _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>)
 317         where I: Clone + Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 318     {
 319         if coinductive_match(self.selcx, cycle.clone()) {
 320             debug!("process_child_obligations: coinductive match");
 321         } else {
 322             let cycle : Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 323             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 324         }
 325     }
 326 }
 327
 328 /// Return the set of type variables contained in a trait ref
 329 fn trait_ref_type_vars<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 330                                        t: ty::PolyTraitRef<'tcx>) -> Vec<Ty<'tcx>>
 331 {
 332     t.skip_binder() // ok b/c this check doesn't care about regions
 333      .input_types()
 334      .map(|t| selcx.infcx().resolve_type_vars_if_possible(&t))
 335      .filter(|t| t.has_infer_types())
 336      .flat_map(|t| t.walk())
 337      .filter(|t| match t.sty { ty::TyInfer(_) => true, _ => false })
 338      .collect()
 339 }
 340
 341 /// Processes a predicate obligation and returns either:
 342 /// - `Ok(Some(v))` if the predicate is true, presuming that `v` are also true
 343 /// - `Ok(None)` if we don't have enough info to be sure
 344 /// - `Err` if the predicate does not hold
 345 fn process_predicate<'a, 'gcx, 'tcx>(
 346     selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 347     pending_obligation: &mut PendingPredicateObligation<'tcx>,
 348     region_obligations: &mut NodeMap<Vec<RegionObligation<'tcx>>>)
 349     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 350               FulfillmentErrorCode<'tcx>>
 351 {
 352     // if we were stalled on some unresolved variables, first check
 353     // whether any of them have been resolved; if not, don't bother
 354     // doing more work yet
 355     if !pending_obligation.stalled_on.is_empty() {
 356         if pending_obligation.stalled_on.iter().all(|&ty| {
 357             let resolved_ty = selcx.infcx().shallow_resolve(&ty);
 358             resolved_ty == ty // nothing changed here
 359         }) {
 360             debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
 361                    selcx.infcx().resolve_type_vars_if_possible(&pending_obligation.obligation),
 362                    pending_obligation.stalled_on);
 363             return Ok(None);
 364         }
 365         pending_obligation.stalled_on = vec![];
 366     }
 367
 368     let obligation = &mut pending_obligation.obligation;
 369
 370     if obligation.predicate.has_infer_types() {
 371         obligation.predicate = selcx.infcx().resolve_type_vars_if_possible(&obligation.predicate);
 372     }
 373
 374     match obligation.predicate {
 375         ty::Predicate::Trait(ref data) => {
 376             if selcx.tcx().fulfilled_predicates.borrow().check_duplicate_trait(data) {
 377                 return Ok(Some(vec![]));
 378             }
 379
 380             let trait_obligation = obligation.with(data.clone());
 381             match selcx.select(&trait_obligation) {
 382                 Ok(Some(vtable)) => {
 383                     debug!("selecting trait `{:?}` at depth {} yielded Ok(Some)",
 384                           data, obligation.recursion_depth);
 385                     Ok(Some(vtable.nested_obligations()))
 386                 }
 387                 Ok(None) => {
 388                     debug!("selecting trait `{:?}` at depth {} yielded Ok(None)",
 389                           data, obligation.recursion_depth);
 390
 391                     // This is a bit subtle: for the most part, the
 392                     // only reason we can fail to make progress on
 393                     // trait selection is because we don't have enough
 394                     // information about the types in the trait. One
 395                     // exception is that we sometimes haven't decided
 396                     // what kind of closure a closure is. *But*, in
 397                     // that case, it turns out, the type of the
 398                     // closure will also change, because the closure
 399                     // also includes references to its upvars as part
 400                     // of its type, and those types are resolved at
 401                     // the same time.
 402                     //
 403                     // FIXME(#32286) logic seems false if no upvars
 404                     pending_obligation.stalled_on =
 405                         trait_ref_type_vars(selcx, data.to_poly_trait_ref());
 406
 407                     debug!("process_predicate: pending obligation {:?} now stalled on {:?}",
 408                            selcx.infcx().resolve_type_vars_if_possible(obligation),
 409                            pending_obligation.stalled_on);
 410
 411                     Ok(None)
 412                 }
 413                 Err(selection_err) => {
 414                     info!("selecting trait `{:?}` at depth {} yielded Err",
 415                           data, obligation.recursion_depth);
 416
 417                     Err(CodeSelectionError(selection_err))
 418                 }
 419             }
 420         }
 421
 422         ty::Predicate::Equate(ref binder) => {
 423             match selcx.infcx().equality_predicate(&obligation.cause,
 424                                                    obligation.param_env,
 425                                                    binder) {
 426                 Ok(InferOk { obligations, value: () }) => {
 427                     Ok(Some(obligations))
 428                 },
 429                 Err(_) => Err(CodeSelectionError(Unimplemented)),
 430             }
 431         }
 432
 433         ty::Predicate::RegionOutlives(ref binder) => {
 434             match selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
 435                 Ok(()) => Ok(Some(Vec::new())),
 436                 Err(_) => Err(CodeSelectionError(Unimplemented)),
 437             }
 438         }
 439
 440         ty::Predicate::TypeOutlives(ref binder) => {
 441             // Check if there are higher-ranked regions.
 442             match selcx.tcx().no_late_bound_regions(binder) {
 443                 // If there are, inspect the underlying type further.
 444                 None => {
 445                     // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 446                     let binder = binder.map_bound_ref(|pred| pred.0);
 447
 448                     // Check if the type has any bound regions.
 449                     match selcx.tcx().no_late_bound_regions(&binder) {
 450                         // If so, this obligation is an error (for now). Eventually we should be
 451                         // able to support additional cases here, like `for<'a> &'a str: 'a`.
 452                         None => {
 453                             Err(CodeSelectionError(Unimplemented))
 454                         }
 455                         // Otherwise, we have something of the form
 456                         // `for<'a> T: 'a where 'a not in T`, which we can treat as `T: 'static`.
 457                         Some(t_a) => {
 458                             let r_static = selcx.tcx().types.re_static;
 459                             register_region_obligation(t_a, r_static,
 460                                                        obligation.cause.clone(),
 461                                                        region_obligations);
 462                             Ok(Some(vec![]))
 463                         }
 464                     }
 465                 }
 466                 // If there aren't, register the obligation.
 467                 Some(ty::OutlivesPredicate(t_a, r_b)) => {
 468                     register_region_obligation(t_a, r_b,
 469                                                obligation.cause.clone(),
 470                                                region_obligations);
 471                     Ok(Some(vec![]))
 472                 }
 473             }
 474         }
 475
 476         ty::Predicate::Projection(ref data) => {
 477             let project_obligation = obligation.with(data.clone());
 478             match project::poly_project_and_unify_type(selcx, &project_obligation) {
 479                 Ok(None) => {
 480                     pending_obligation.stalled_on =
 481                         trait_ref_type_vars(selcx, data.to_poly_trait_ref());
 482                     Ok(None)
 483                 }
 484                 Ok(v) => Ok(v),
 485                 Err(e) => Err(CodeProjectionError(e))
 486             }
 487         }
 488
 489         ty::Predicate::ObjectSafe(trait_def_id) => {
 490             if !selcx.tcx().is_object_safe(trait_def_id) {
 491                 Err(CodeSelectionError(Unimplemented))
 492             } else {
 493                 Ok(Some(Vec::new()))
 494             }
 495         }
 496
 497         ty::Predicate::ClosureKind(closure_def_id, kind) => {
 498             match selcx.infcx().closure_kind(closure_def_id) {
 499                 Some(closure_kind) => {
 500                     if closure_kind.extends(kind) {
 501                         Ok(Some(vec![]))
 502                     } else {
 503                         Err(CodeSelectionError(Unimplemented))
 504                     }
 505                 }
 506                 None => {
 507                     Ok(None)
 508                 }
 509             }
 510         }
 511
 512         ty::Predicate::WellFormed(ty) => {
 513             match ty::wf::obligations(selcx.infcx(),
 514                                       obligation.param_env,
 515                                       obligation.cause.body_id,
 516                                       ty, obligation.cause.span) {
 517                 None => {
 518                     pending_obligation.stalled_on = vec![ty];
 519                     Ok(None)
 520                 }
 521                 s => Ok(s)
 522             }
 523         }
 524
 525         ty::Predicate::Subtype(ref subtype) => {
 526             match selcx.infcx().subtype_predicate(&obligation.cause,
 527                                                   obligation.param_env,
 528                                                   subtype) {
 529                 None => {
 530                     // none means that both are unresolved
 531                     pending_obligation.stalled_on = vec![subtype.skip_binder().a,
 532                                                          subtype.skip_binder().b];
 533                     Ok(None)
 534                 }
 535                 Some(Ok(ok)) => {
 536                     Ok(Some(ok.obligations))
 537                 }
 538                 Some(Err(err)) => {
 539                     let expected_found = ExpectedFound::new(subtype.skip_binder().a_is_expected,
 540                                                             subtype.skip_binder().a,
 541                                                             subtype.skip_binder().b);
 542                     Err(FulfillmentErrorCode::CodeSubtypeError(expected_found, err))
 543                 }
 544             }
 545         }
 546     }
 547 }
 548
 549 /// For defaulted traits, we use a co-inductive strategy to solve, so
 550 /// that recursion is ok. This routine returns true if the top of the
 551 /// stack (`cycle[0]`):
 552 /// - is a defaulted trait, and
 553 /// - it also appears in the backtrace at some position `X`; and,
 554 /// - all the predicates at positions `X..` between `X` an the top are
 555 ///   also defaulted traits.
 556 fn coinductive_match<'a,'c,'gcx,'tcx,I>(selcx: &mut SelectionContext<'a,'gcx,'tcx>,
 557                                         cycle: I) -> bool
 558     where I: Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 559           'tcx: 'c
 560 {
 561     let mut cycle = cycle;
 562     cycle
 563         .all(|bt_obligation| {
 564             let result = coinductive_obligation(selcx, &bt_obligation.obligation);
 565             debug!("coinductive_match: bt_obligation={:?} coinductive={}",
 566                    bt_obligation, result);
 567             result
 568         })
 569 }
 570
 571 fn coinductive_obligation<'a,'gcx,'tcx>(selcx: &SelectionContext<'a,'gcx,'tcx>,
 572                                           obligation: &PredicateObligation<'tcx>)
 573                                           -> bool {
 574     match obligation.predicate {
 575         ty::Predicate::Trait(ref data) => {
 576             selcx.tcx().trait_has_default_impl(data.def_id())
 577         }
 578         _ => {
 579             false
 580         }
 581     }
 582 }
 583
 584 fn register_region_obligation<'tcx>(t_a: Ty<'tcx>,
 585                                     r_b: ty::Region<'tcx>,
 586                                     cause: ObligationCause<'tcx>,
 587                                     region_obligations: &mut NodeMap<Vec<RegionObligation<'tcx>>>)
 588 {
 589     let region_obligation = RegionObligation { sup_type: t_a,
 590                                                sub_region: r_b,
 591                                                cause: cause };
 592
 593     debug!("register_region_obligation({:?}, cause={:?})",
 594            region_obligation, region_obligation.cause);
 595
 596     region_obligations.entry(region_obligation.cause.body_id)
 597                       .or_insert(vec![])
 598                       .push(region_obligation);
 599
 600 }
 601
 602 impl<'a, 'gcx, 'tcx> GlobalFulfilledPredicates<'gcx> {
 603     pub fn new(dep_graph: DepGraph) -> GlobalFulfilledPredicates<'gcx> {
 604         GlobalFulfilledPredicates {
 605             set: FxHashSet(),
 606             dep_graph: dep_graph,
 607         }
 608     }
 609
 610     pub fn check_duplicate(&self, key: &ty::Predicate<'tcx>) -> bool {
 611         if let ty::Predicate::Trait(ref data) = *key {
 612             self.check_duplicate_trait(data)
 613         } else {
 614             false
 615         }
 616     }
 617
 618     pub fn check_duplicate_trait(&self, data: &ty::PolyTraitPredicate<'tcx>) -> bool {
 619         // For the global predicate registry, when we find a match, it
 620         // may have been computed by some other task, so we want to
 621         // add a read from the node corresponding to the predicate
 622         // processing to make sure we get the transitive dependencies.
 623         if self.set.contains(data) {
 624             debug_assert!(data.is_global());
 625             self.dep_graph.read(data.dep_node());
 626             debug!("check_duplicate: global predicate `{:?}` already proved elsewhere", data);
 627
 628             true
 629         } else {
 630             false
 631         }
 632     }
 633
 634     fn add_if_global(&mut self, tcx: TyCtxt<'a, 'gcx, 'tcx>, key: &ty::Predicate<'tcx>) {
 635         if let ty::Predicate::Trait(ref data) = *key {
 636             // We only add things to the global predicate registry
 637             // after the current task has proved them, and hence
 638             // already has the required read edges, so we don't need
 639             // to add any more edges here.
 640             if data.is_global() {
 641                 if let Some(data) = tcx.lift_to_global(data) {
 642                     if self.set.insert(data.clone()) {
 643                         debug!("add_if_global: global predicate `{:?}` added", data);
 644                     }
 645                 }
 646             }
 647         }
 648     }
 649 }
 650
 651 fn to_fulfillment_error<'tcx>(
 652     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>)
 653     -> FulfillmentError<'tcx>
 654 {
 655     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 656     FulfillmentError::new(obligation, error.error)
 657 }