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: &'tcx ty::Region,
  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                                      projection_ty: ty::ProjectionTy<'tcx>,
 117                                      cause: ObligationCause<'tcx>)
 118                                      -> Ty<'tcx>
 119     {
 120         debug!("normalize_projection_type(projection_ty={:?})",
 121                projection_ty);
 122
 123         assert!(!projection_ty.has_escaping_regions());
 124
 125         // FIXME(#20304) -- cache
 126
 127         let mut selcx = SelectionContext::new(infcx);
 128         let normalized = project::normalize_projection_type(&mut selcx, projection_ty, cause, 0);
 129
 130         for obligation in normalized.obligations {
 131             self.register_predicate_obligation(infcx, obligation);
 132         }
 133
 134         debug!("normalize_projection_type: result={:?}", normalized.value);
 135
 136         normalized.value
 137     }
 138
 139     pub fn register_bound(&mut self,
 140                           infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 141                           ty: Ty<'tcx>,
 142                           def_id: DefId,
 143                           cause: ObligationCause<'tcx>)
 144     {
 145         let trait_ref = ty::TraitRef {
 146             def_id: def_id,
 147             substs: infcx.tcx.mk_substs_trait(ty, &[]),
 148         };
 149         self.register_predicate_obligation(infcx, Obligation {
 150             cause: cause,
 151             recursion_depth: 0,
 152             predicate: trait_ref.to_predicate()
 153         });
 154     }
 155
 156     pub fn register_region_obligation(&mut self,
 157                                       t_a: Ty<'tcx>,
 158                                       r_b: &'tcx ty::Region,
 159                                       cause: ObligationCause<'tcx>)
 160     {
 161         register_region_obligation(t_a, r_b, cause, &mut self.region_obligations);
 162     }
 163
 164     pub fn register_predicate_obligation(&mut self,
 165                                          infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 166                                          obligation: PredicateObligation<'tcx>)
 167     {
 168         // this helps to reduce duplicate errors, as well as making
 169         // debug output much nicer to read and so on.
 170         let obligation = infcx.resolve_type_vars_if_possible(&obligation);
 171
 172         debug!("register_predicate_obligation(obligation={:?})", obligation);
 173
 174         infcx.obligations_in_snapshot.set(true);
 175
 176         if infcx.tcx.fulfilled_predicates.borrow().check_duplicate(&obligation.predicate) {
 177             debug!("register_predicate_obligation: duplicate");
 178             return
 179         }
 180
 181         self.predicates.register_obligation(PendingPredicateObligation {
 182             obligation: obligation,
 183             stalled_on: vec![]
 184         });
 185     }
 186
 187     pub fn region_obligations(&self,
 188                               body_id: ast::NodeId)
 189                               -> &[RegionObligation<'tcx>]
 190     {
 191         match self.region_obligations.get(&body_id) {
 192             None => Default::default(),
 193             Some(vec) => vec,
 194         }
 195     }
 196
 197     pub fn select_all_or_error(&mut self,
 198                                infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 199                                -> Result<(),Vec<FulfillmentError<'tcx>>>
 200     {
 201         self.select_where_possible(infcx)?;
 202
 203         let errors: Vec<_> =
 204             self.predicates.to_errors(CodeAmbiguity)
 205                            .into_iter()
 206                            .map(|e| to_fulfillment_error(e))
 207                            .collect();
 208         if errors.is_empty() {
 209             Ok(())
 210         } else {
 211             Err(errors)
 212         }
 213     }
 214
 215     pub fn select_where_possible(&mut self,
 216                                  infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 217                                  -> Result<(),Vec<FulfillmentError<'tcx>>>
 218     {
 219         let mut selcx = SelectionContext::new(infcx);
 220         self.select(&mut selcx)
 221     }
 222
 223     pub fn pending_obligations(&self) -> Vec<PendingPredicateObligation<'tcx>> {
 224         self.predicates.pending_obligations()
 225     }
 226
 227     /// Attempts to select obligations using `selcx`. If `only_new_obligations` is true, then it
 228     /// only attempts to select obligations that haven't been seen before.
 229     fn select(&mut self, selcx: &mut SelectionContext<'a, 'gcx, 'tcx>)
 230               -> Result<(),Vec<FulfillmentError<'tcx>>> {
 231         debug!("select(obligation-forest-size={})", self.predicates.len());
 232
 233         let mut errors = Vec::new();
 234
 235         loop {
 236             debug!("select: starting another iteration");
 237
 238             // Process pending obligations.
 239             let outcome = self.predicates.process_obligations(&mut FulfillProcessor {
 240                 selcx: selcx,
 241                 region_obligations: &mut self.region_obligations,
 242             });
 243             debug!("select: outcome={:?}", outcome);
 244
 245             // these are obligations that were proven to be true.
 246             for pending_obligation in outcome.completed {
 247                 let predicate = &pending_obligation.obligation.predicate;
 248                 selcx.tcx().fulfilled_predicates.borrow_mut()
 249                            .add_if_global(selcx.tcx(), predicate);
 250             }
 251
 252             errors.extend(
 253                 outcome.errors.into_iter()
 254                               .map(|e| to_fulfillment_error(e)));
 255
 256             // If nothing new was added, no need to keep looping.
 257             if outcome.stalled {
 258                 break;
 259             }
 260         }
 261
 262         debug!("select({} predicates remaining, {} errors) done",
 263                self.predicates.len(), errors.len());
 264
 265         if errors.is_empty() {
 266             Ok(())
 267         } else {
 268             Err(errors)
 269         }
 270     }
 271 }
 272
 273 struct FulfillProcessor<'a, 'b: 'a, 'gcx: 'tcx, 'tcx: 'b> {
 274     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 275     region_obligations: &'a mut NodeMap<Vec<RegionObligation<'tcx>>>,
 276 }
 277
 278 impl<'a, 'b, 'gcx, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'gcx, 'tcx> {
 279     type Obligation = PendingPredicateObligation<'tcx>;
 280     type Error = FulfillmentErrorCode<'tcx>;
 281
 282     fn process_obligation(&mut self,
 283                           obligation: &mut Self::Obligation)
 284                           -> Result<Option<Vec<Self::Obligation>>, Self::Error>
 285     {
 286         process_predicate(self.selcx,
 287                           obligation,
 288                           self.region_obligations)
 289             .map(|os| os.map(|os| os.into_iter().map(|o| PendingPredicateObligation {
 290                 obligation: o,
 291                 stalled_on: vec![]
 292             }).collect()))
 293     }
 294
 295     fn process_backedge<'c, I>(&mut self, cycle: I,
 296                                _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>)
 297         where I: Clone + Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 298     {
 299         if coinductive_match(self.selcx, cycle.clone()) {
 300             debug!("process_child_obligations: coinductive match");
 301         } else {
 302             let cycle : Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 303             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 304         }
 305     }
 306 }
 307
 308 /// Return the set of type variables contained in a trait ref
 309 fn trait_ref_type_vars<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 310                                        t: ty::PolyTraitRef<'tcx>) -> Vec<Ty<'tcx>>
 311 {
 312     t.skip_binder() // ok b/c this check doesn't care about regions
 313      .input_types()
 314      .map(|t| selcx.infcx().resolve_type_vars_if_possible(&t))
 315      .filter(|t| t.has_infer_types())
 316      .flat_map(|t| t.walk())
 317      .filter(|t| match t.sty { ty::TyInfer(_) => true, _ => false })
 318      .collect()
 319 }
 320
 321 /// Processes a predicate obligation and returns either:
 322 /// - `Ok(Some(v))` if the predicate is true, presuming that `v` are also true
 323 /// - `Ok(None)` if we don't have enough info to be sure
 324 /// - `Err` if the predicate does not hold
 325 fn process_predicate<'a, 'gcx, 'tcx>(
 326     selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 327     pending_obligation: &mut PendingPredicateObligation<'tcx>,
 328     region_obligations: &mut NodeMap<Vec<RegionObligation<'tcx>>>)
 329     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 330               FulfillmentErrorCode<'tcx>>
 331 {
 332     // if we were stalled on some unresolved variables, first check
 333     // whether any of them have been resolved; if not, don't bother
 334     // doing more work yet
 335     if !pending_obligation.stalled_on.is_empty() {
 336         if pending_obligation.stalled_on.iter().all(|&ty| {
 337             let resolved_ty = selcx.infcx().shallow_resolve(&ty);
 338             resolved_ty == ty // nothing changed here
 339         }) {
 340             debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
 341                    selcx.infcx().resolve_type_vars_if_possible(&pending_obligation.obligation),
 342                    pending_obligation.stalled_on);
 343             return Ok(None);
 344         }
 345         pending_obligation.stalled_on = vec![];
 346     }
 347
 348     let obligation = &mut pending_obligation.obligation;
 349
 350     if obligation.predicate.has_infer_types() {
 351         obligation.predicate = selcx.infcx().resolve_type_vars_if_possible(&obligation.predicate);
 352     }
 353
 354     match obligation.predicate {
 355         ty::Predicate::Trait(ref data) => {
 356             if selcx.tcx().fulfilled_predicates.borrow().check_duplicate_trait(data) {
 357                 return Ok(Some(vec![]));
 358             }
 359
 360             let trait_obligation = obligation.with(data.clone());
 361             match selcx.select(&trait_obligation) {
 362                 Ok(Some(vtable)) => {
 363                     debug!("selecting trait `{:?}` at depth {} yielded Ok(Some)",
 364                           data, obligation.recursion_depth);
 365                     Ok(Some(vtable.nested_obligations()))
 366                 }
 367                 Ok(None) => {
 368                     debug!("selecting trait `{:?}` at depth {} yielded Ok(None)",
 369                           data, obligation.recursion_depth);
 370
 371                     // This is a bit subtle: for the most part, the
 372                     // only reason we can fail to make progress on
 373                     // trait selection is because we don't have enough
 374                     // information about the types in the trait. One
 375                     // exception is that we sometimes haven't decided
 376                     // what kind of closure a closure is. *But*, in
 377                     // that case, it turns out, the type of the
 378                     // closure will also change, because the closure
 379                     // also includes references to its upvars as part
 380                     // of its type, and those types are resolved at
 381                     // the same time.
 382                     //
 383                     // FIXME(#32286) logic seems false if no upvars
 384                     pending_obligation.stalled_on =
 385                         trait_ref_type_vars(selcx, data.to_poly_trait_ref());
 386
 387                     debug!("process_predicate: pending obligation {:?} now stalled on {:?}",
 388                            selcx.infcx().resolve_type_vars_if_possible(obligation),
 389                            pending_obligation.stalled_on);
 390
 391                     Ok(None)
 392                 }
 393                 Err(selection_err) => {
 394                     info!("selecting trait `{:?}` at depth {} yielded Err",
 395                           data, obligation.recursion_depth);
 396
 397                     Err(CodeSelectionError(selection_err))
 398                 }
 399             }
 400         }
 401
 402         ty::Predicate::Equate(ref binder) => {
 403             match selcx.infcx().equality_predicate(&obligation.cause, binder) {
 404                 Ok(InferOk { obligations, value: () }) => {
 405                     Ok(Some(obligations))
 406                 },
 407                 Err(_) => Err(CodeSelectionError(Unimplemented)),
 408             }
 409         }
 410
 411         ty::Predicate::RegionOutlives(ref binder) => {
 412             match selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
 413                 Ok(()) => Ok(Some(Vec::new())),
 414                 Err(_) => Err(CodeSelectionError(Unimplemented)),
 415             }
 416         }
 417
 418         ty::Predicate::TypeOutlives(ref binder) => {
 419             // Check if there are higher-ranked regions.
 420             match selcx.tcx().no_late_bound_regions(binder) {
 421                 // If there are, inspect the underlying type further.
 422                 None => {
 423                     // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 424                     let binder = binder.map_bound_ref(|pred| pred.0);
 425
 426                     // Check if the type has any bound regions.
 427                     match selcx.tcx().no_late_bound_regions(&binder) {
 428                         // If so, this obligation is an error (for now). Eventually we should be
 429                         // able to support additional cases here, like `for<'a> &'a str: 'a`.
 430                         None => {
 431                             Err(CodeSelectionError(Unimplemented))
 432                         }
 433                         // Otherwise, we have something of the form
 434                         // `for<'a> T: 'a where 'a not in T`, which we can treat as `T: 'static`.
 435                         Some(t_a) => {
 436                             let r_static = selcx.tcx().mk_region(ty::ReStatic);
 437                             register_region_obligation(t_a, r_static,
 438                                                        obligation.cause.clone(),
 439                                                        region_obligations);
 440                             Ok(Some(vec![]))
 441                         }
 442                     }
 443                 }
 444                 // If there aren't, register the obligation.
 445                 Some(ty::OutlivesPredicate(t_a, r_b)) => {
 446                     register_region_obligation(t_a, r_b,
 447                                                obligation.cause.clone(),
 448                                                region_obligations);
 449                     Ok(Some(vec![]))
 450                 }
 451             }
 452         }
 453
 454         ty::Predicate::Projection(ref data) => {
 455             let project_obligation = obligation.with(data.clone());
 456             match project::poly_project_and_unify_type(selcx, &project_obligation) {
 457                 Ok(None) => {
 458                     pending_obligation.stalled_on =
 459                         trait_ref_type_vars(selcx, data.to_poly_trait_ref());
 460                     Ok(None)
 461                 }
 462                 Ok(v) => Ok(v),
 463                 Err(e) => Err(CodeProjectionError(e))
 464             }
 465         }
 466
 467         ty::Predicate::ObjectSafe(trait_def_id) => {
 468             if !selcx.tcx().is_object_safe(trait_def_id) {
 469                 Err(CodeSelectionError(Unimplemented))
 470             } else {
 471                 Ok(Some(Vec::new()))
 472             }
 473         }
 474
 475         ty::Predicate::ClosureKind(closure_def_id, kind) => {
 476             match selcx.infcx().closure_kind(closure_def_id) {
 477                 Some(closure_kind) => {
 478                     if closure_kind.extends(kind) {
 479                         Ok(Some(vec![]))
 480                     } else {
 481                         Err(CodeSelectionError(Unimplemented))
 482                     }
 483                 }
 484                 None => {
 485                     Ok(None)
 486                 }
 487             }
 488         }
 489
 490         ty::Predicate::WellFormed(ty) => {
 491             match ty::wf::obligations(selcx.infcx(), obligation.cause.body_id,
 492                                       ty, obligation.cause.span) {
 493                 None => {
 494                     pending_obligation.stalled_on = vec![ty];
 495                     Ok(None)
 496                 }
 497                 s => Ok(s)
 498             }
 499         }
 500
 501         ty::Predicate::Subtype(ref subtype) => {
 502             match selcx.infcx().subtype_predicate(&obligation.cause, subtype) {
 503                 None => {
 504                     // none means that both are unresolved
 505                     pending_obligation.stalled_on = vec![subtype.skip_binder().a,
 506                                                          subtype.skip_binder().b];
 507                     Ok(None)
 508                 }
 509                 Some(Ok(ok)) => {
 510                     Ok(Some(ok.obligations))
 511                 }
 512                 Some(Err(err)) => {
 513                     let expected_found = ExpectedFound::new(subtype.skip_binder().a_is_expected,
 514                                                             subtype.skip_binder().a,
 515                                                             subtype.skip_binder().b);
 516                     Err(FulfillmentErrorCode::CodeSubtypeError(expected_found, err))
 517                 }
 518             }
 519         }
 520     }
 521 }
 522
 523 /// For defaulted traits, we use a co-inductive strategy to solve, so
 524 /// that recursion is ok. This routine returns true if the top of the
 525 /// stack (`cycle[0]`):
 526 /// - is a defaulted trait, and
 527 /// - it also appears in the backtrace at some position `X`; and,
 528 /// - all the predicates at positions `X..` between `X` an the top are
 529 ///   also defaulted traits.
 530 fn coinductive_match<'a,'c,'gcx,'tcx,I>(selcx: &mut SelectionContext<'a,'gcx,'tcx>,
 531                                         cycle: I) -> bool
 532     where I: Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 533           'tcx: 'c
 534 {
 535     let mut cycle = cycle;
 536     cycle
 537         .all(|bt_obligation| {
 538             let result = coinductive_obligation(selcx, &bt_obligation.obligation);
 539             debug!("coinductive_match: bt_obligation={:?} coinductive={}",
 540                    bt_obligation, result);
 541             result
 542         })
 543 }
 544
 545 fn coinductive_obligation<'a,'gcx,'tcx>(selcx: &SelectionContext<'a,'gcx,'tcx>,
 546                                           obligation: &PredicateObligation<'tcx>)
 547                                           -> bool {
 548     match obligation.predicate {
 549         ty::Predicate::Trait(ref data) => {
 550             selcx.tcx().trait_has_default_impl(data.def_id())
 551         }
 552         _ => {
 553             false
 554         }
 555     }
 556 }
 557
 558 fn register_region_obligation<'tcx>(t_a: Ty<'tcx>,
 559                                     r_b: &'tcx ty::Region,
 560                                     cause: ObligationCause<'tcx>,
 561                                     region_obligations: &mut NodeMap<Vec<RegionObligation<'tcx>>>)
 562 {
 563     let region_obligation = RegionObligation { sup_type: t_a,
 564                                                sub_region: r_b,
 565                                                cause: cause };
 566
 567     debug!("register_region_obligation({:?}, cause={:?})",
 568            region_obligation, region_obligation.cause);
 569
 570     region_obligations.entry(region_obligation.cause.body_id)
 571                       .or_insert(vec![])
 572                       .push(region_obligation);
 573
 574 }
 575
 576 impl<'a, 'gcx, 'tcx> GlobalFulfilledPredicates<'gcx> {
 577     pub fn new(dep_graph: DepGraph) -> GlobalFulfilledPredicates<'gcx> {
 578         GlobalFulfilledPredicates {
 579             set: FxHashSet(),
 580             dep_graph: dep_graph,
 581         }
 582     }
 583
 584     pub fn check_duplicate(&self, key: &ty::Predicate<'tcx>) -> bool {
 585         if let ty::Predicate::Trait(ref data) = *key {
 586             self.check_duplicate_trait(data)
 587         } else {
 588             false
 589         }
 590     }
 591
 592     pub fn check_duplicate_trait(&self, data: &ty::PolyTraitPredicate<'tcx>) -> bool {
 593         // For the global predicate registry, when we find a match, it
 594         // may have been computed by some other task, so we want to
 595         // add a read from the node corresponding to the predicate
 596         // processing to make sure we get the transitive dependencies.
 597         if self.set.contains(data) {
 598             debug_assert!(data.is_global());
 599             self.dep_graph.read(data.dep_node());
 600             debug!("check_duplicate: global predicate `{:?}` already proved elsewhere", data);
 601
 602             true
 603         } else {
 604             false
 605         }
 606     }
 607
 608     fn add_if_global(&mut self, tcx: TyCtxt<'a, 'gcx, 'tcx>, key: &ty::Predicate<'tcx>) {
 609         if let ty::Predicate::Trait(ref data) = *key {
 610             // We only add things to the global predicate registry
 611             // after the current task has proved them, and hence
 612             // already has the required read edges, so we don't need
 613             // to add any more edges here.
 614             if data.is_global() {
 615                 if let Some(data) = tcx.lift_to_global(data) {
 616                     if self.set.insert(data.clone()) {
 617                         debug!("add_if_global: global predicate `{:?}` added", data);
 618                     }
 619                 }
 620             }
 621         }
 622     }
 623 }
 624
 625 fn to_fulfillment_error<'tcx>(
 626     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>)
 627     -> FulfillmentError<'tcx>
 628 {
 629     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 630     FulfillmentError::new(obligation, error.error)
 631 }