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