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 infer::{RegionObligation, InferCtxt};
  12 use mir::interpret::GlobalId;
  13 use ty::{self, Ty, TypeFoldable, ToPolyTraitRef, 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 hir::def_id::DefId;
  19 use middle::const_val::{ConstEvalErr, ErrKind};
  20
  21 use super::CodeAmbiguity;
  22 use super::CodeProjectionError;
  23 use super::CodeSelectionError;
  24 use super::engine::TraitEngine;
  25 use super::{FulfillmentError, FulfillmentErrorCode};
  26 use super::{ObligationCause, PredicateObligation, Obligation};
  27 use super::project;
  28 use super::select::SelectionContext;
  29 use super::{Unimplemented, ConstEvalFailure};
  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 /// The fulfillment context is used to drive trait resolution.  It
  38 /// consists of a list of obligations that must be (eventually)
  39 /// satisfied. The job is to track which are satisfied, which yielded
  40 /// errors, and which are still pending. At any point, users can call
  41 /// `select_where_possible`, and the fulfillment context will try to do
  42 /// selection, retaining only those obligations that remain
  43 /// ambiguous. This may be helpful in pushing type inference
  44 /// along. Once all type inference constraints have been generated, the
  45 /// method `select_all_or_error` can be used to report any remaining
  46 /// ambiguous cases as errors.
  47
  48 pub struct FulfillmentContext<'tcx> {
  49     // A list of all obligations that have been registered with this
  50     // fulfillment context.
  51     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  52     // Should this fulfillment context register type-lives-for-region
  53     // obligations on its parent infcx? In some cases, region
  54     // obligations are either already known to hold (normalization) or
  55     // hopefully verifed elsewhere (type-impls-bound), and therefore
  56     // should not be checked.
  57     //
  58     // Note that if we are normalizing a type that we already
  59     // know is well-formed, there should be no harm setting this
  60     // to true - all the region variables should be determinable
  61     // using the RFC 447 rules, which don't depend on
  62     // type-lives-for-region constraints, and because the type
  63     // is well-formed, the constraints should hold.
  64     register_region_obligations: bool,
  65 }
  66
  67 #[derive(Clone, Debug)]
  68 pub struct PendingPredicateObligation<'tcx> {
  69     pub obligation: PredicateObligation<'tcx>,
  70     pub stalled_on: Vec<Ty<'tcx>>,
  71 }
  72
  73 impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> {
  74     /// Creates a new fulfillment context.
  75     pub fn new() -> FulfillmentContext<'tcx> {
  76         FulfillmentContext {
  77             predicates: ObligationForest::new(),
  78             register_region_obligations: true
  79         }
  80     }
  81
  82     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
  83         FulfillmentContext {
  84             predicates: ObligationForest::new(),
  85             register_region_obligations: false
  86         }
  87     }
  88
  89     pub fn register_predicate_obligations<I>(&mut self,
  90                                              infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  91                                              obligations: I)
  92         where I: IntoIterator<Item = PredicateObligation<'tcx>>
  93     {
  94         for obligation in obligations {
  95             self.register_predicate_obligation(infcx, obligation);
  96         }
  97     }
  98
  99     /// Attempts to select obligations using `selcx`. If `only_new_obligations` is true, then it
 100     /// only attempts to select obligations that haven't been seen before.
 101     fn select(&mut self, selcx: &mut SelectionContext<'a, 'gcx, 'tcx>)
 102               -> Result<(),Vec<FulfillmentError<'tcx>>> {
 103         debug!("select(obligation-forest-size={})", self.predicates.len());
 104
 105         let mut errors = Vec::new();
 106
 107         loop {
 108             debug!("select: starting another iteration");
 109
 110             // Process pending obligations.
 111             let outcome = self.predicates.process_obligations(&mut FulfillProcessor {
 112                 selcx,
 113                 register_region_obligations: self.register_region_obligations
 114             });
 115             debug!("select: outcome={:#?}", outcome);
 116
 117             // FIXME: if we kept the original cache key, we could mark projection
 118             // obligations as complete for the projection cache here.
 119
 120             errors.extend(
 121                 outcome.errors.into_iter()
 122                               .map(|e| to_fulfillment_error(e)));
 123
 124             // If nothing new was added, no need to keep looping.
 125             if outcome.stalled {
 126                 break;
 127             }
 128         }
 129
 130         debug!("select({} predicates remaining, {} errors) done",
 131                self.predicates.len(), errors.len());
 132
 133         if errors.is_empty() {
 134             Ok(())
 135         } else {
 136             Err(errors)
 137         }
 138     }
 139 }
 140
 141 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 142     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 143     /// creating a fresh type variable `$0` as well as a projection
 144     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 145     /// inference engine runs, it will attempt to find an impl of
 146     /// `SomeTrait` or a where clause that lets us unify `$0` with
 147     /// something concrete. If this fails, we'll unify `$0` with
 148     /// `projection_ty` again.
 149     fn normalize_projection_type<'a, 'gcx>(&mut self,
 150                                  infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 151                                  param_env: ty::ParamEnv<'tcx>,
 152                                  projection_ty: ty::ProjectionTy<'tcx>,
 153                                  cause: ObligationCause<'tcx>)
 154                                  -> Ty<'tcx>
 155     {
 156         debug!("normalize_projection_type(projection_ty={:?})",
 157                projection_ty);
 158
 159         assert!(!projection_ty.has_escaping_regions());
 160
 161         // FIXME(#20304) -- cache
 162
 163         let mut selcx = SelectionContext::new(infcx);
 164         let normalized = project::normalize_projection_type(&mut selcx,
 165                                                             param_env,
 166                                                             projection_ty,
 167                                                             cause,
 168                                                             0);
 169
 170         for obligation in normalized.obligations {
 171             self.register_predicate_obligation(infcx, obligation);
 172         }
 173
 174         debug!("normalize_projection_type: result={:?}", normalized.value);
 175
 176         normalized.value
 177     }
 178
 179     /// Requires that `ty` must implement the trait with `def_id` in
 180     /// the given environment. This trait must not have any type
 181     /// parameters (except for `Self`).
 182     fn register_bound<'a, 'gcx>(&mut self,
 183                       infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 184                       param_env: ty::ParamEnv<'tcx>,
 185                       ty: Ty<'tcx>,
 186                       def_id: DefId,
 187                       cause: ObligationCause<'tcx>)
 188     {
 189         let trait_ref = ty::TraitRef {
 190             def_id,
 191             substs: infcx.tcx.mk_substs_trait(ty, &[]),
 192         };
 193         self.register_predicate_obligation(infcx, Obligation {
 194             cause,
 195             recursion_depth: 0,
 196             param_env,
 197             predicate: trait_ref.to_predicate()
 198         });
 199     }
 200
 201     fn register_predicate_obligation<'a, 'gcx>(&mut self,
 202                                      infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 203                                      obligation: PredicateObligation<'tcx>)
 204     {
 205         // this helps to reduce duplicate errors, as well as making
 206         // debug output much nicer to read and so on.
 207         let obligation = infcx.resolve_type_vars_if_possible(&obligation);
 208
 209         debug!("register_predicate_obligation(obligation={:?})", obligation);
 210
 211         assert!(!infcx.is_in_snapshot());
 212
 213         self.predicates.register_obligation(PendingPredicateObligation {
 214             obligation,
 215             stalled_on: vec![]
 216         });
 217     }
 218
 219     fn select_all_or_error<'a, 'gcx>(&mut self,
 220                                      infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 221                                      -> Result<(),Vec<FulfillmentError<'tcx>>>
 222     {
 223         self.select_where_possible(infcx)?;
 224
 225         let errors: Vec<_> =
 226             self.predicates.to_errors(CodeAmbiguity)
 227                            .into_iter()
 228                            .map(|e| to_fulfillment_error(e))
 229                            .collect();
 230         if errors.is_empty() {
 231             Ok(())
 232         } else {
 233             Err(errors)
 234         }
 235     }
 236
 237     fn select_where_possible<'a, 'gcx>(&mut self,
 238                              infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 239                              -> Result<(),Vec<FulfillmentError<'tcx>>>
 240     {
 241         let mut selcx = SelectionContext::new(infcx);
 242         self.select(&mut selcx)
 243     }
 244
 245     fn pending_obligations(&self) -> Vec<PendingPredicateObligation<'tcx>> {
 246         self.predicates.pending_obligations()
 247     }
 248 }
 249
 250 struct FulfillProcessor<'a, 'b: 'a, 'gcx: 'tcx, 'tcx: 'b> {
 251     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 252     register_region_obligations: bool
 253 }
 254
 255 impl<'a, 'b, 'gcx, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'gcx, 'tcx> {
 256     type Obligation = PendingPredicateObligation<'tcx>;
 257     type Error = FulfillmentErrorCode<'tcx>;
 258
 259     fn process_obligation(&mut self,
 260                           obligation: &mut Self::Obligation)
 261                           -> Result<Option<Vec<Self::Obligation>>, Self::Error>
 262     {
 263         process_predicate(self.selcx, obligation, self.register_region_obligations)
 264             .map(|os| os.map(|os| os.into_iter().map(|o| PendingPredicateObligation {
 265                 obligation: o,
 266                 stalled_on: vec![]
 267             }).collect()))
 268     }
 269
 270     fn process_backedge<'c, I>(&mut self, cycle: I,
 271                                _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>)
 272         where I: Clone + Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 273     {
 274         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 275             debug!("process_child_obligations: coinductive match");
 276         } else {
 277             let cycle : Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 278             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 279         }
 280     }
 281 }
 282
 283 /// Return the set of type variables contained in a trait ref
 284 fn trait_ref_type_vars<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 285                                        t: ty::PolyTraitRef<'tcx>) -> Vec<Ty<'tcx>>
 286 {
 287     t.skip_binder() // ok b/c this check doesn't care about regions
 288      .input_types()
 289      .map(|t| selcx.infcx().resolve_type_vars_if_possible(&t))
 290      .filter(|t| t.has_infer_types())
 291      .flat_map(|t| t.walk())
 292      .filter(|t| match t.sty { ty::TyInfer(_) => true, _ => false })
 293      .collect()
 294 }
 295
 296 /// Processes a predicate obligation and returns either:
 297 /// - `Ok(Some(v))` if the predicate is true, presuming that `v` are also true
 298 /// - `Ok(None)` if we don't have enough info to be sure
 299 /// - `Err` if the predicate does not hold
 300 fn process_predicate<'a, 'gcx, 'tcx>(
 301     selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 302     pending_obligation: &mut PendingPredicateObligation<'tcx>,
 303     register_region_obligations: bool)
 304     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 305               FulfillmentErrorCode<'tcx>>
 306 {
 307     // if we were stalled on some unresolved variables, first check
 308     // whether any of them have been resolved; if not, don't bother
 309     // doing more work yet
 310     if !pending_obligation.stalled_on.is_empty() {
 311         if pending_obligation.stalled_on.iter().all(|&ty| {
 312             let resolved_ty = selcx.infcx().shallow_resolve(&ty);
 313             resolved_ty == ty // nothing changed here
 314         }) {
 315             debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
 316                    selcx.infcx().resolve_type_vars_if_possible(&pending_obligation.obligation),
 317                    pending_obligation.stalled_on);
 318             return Ok(None);
 319         }
 320         pending_obligation.stalled_on = vec![];
 321     }
 322
 323     let obligation = &mut pending_obligation.obligation;
 324
 325     if obligation.predicate.has_infer_types() {
 326         obligation.predicate = selcx.infcx().resolve_type_vars_if_possible(&obligation.predicate);
 327     }
 328
 329     match obligation.predicate {
 330         ty::Predicate::Trait(ref data) => {
 331             let trait_obligation = obligation.with(data.clone());
 332
 333             if data.is_global() && !data.has_late_bound_regions() {
 334                 // no type variables present, can use evaluation for better caching.
 335                 // FIXME: consider caching errors too.
 336                 if selcx.infcx().predicate_must_hold(&obligation) {
 337                     debug!("selecting trait `{:?}` at depth {} evaluated to holds",
 338                            data, obligation.recursion_depth);
 339                     return Ok(Some(vec![]))
 340                 }
 341             }
 342
 343             match selcx.select(&trait_obligation) {
 344                 Ok(Some(vtable)) => {
 345                     debug!("selecting trait `{:?}` at depth {} yielded Ok(Some)",
 346                            data, obligation.recursion_depth);
 347                     Ok(Some(vtable.nested_obligations()))
 348                 }
 349                 Ok(None) => {
 350                     debug!("selecting trait `{:?}` at depth {} yielded Ok(None)",
 351                            data, obligation.recursion_depth);
 352
 353                     // This is a bit subtle: for the most part, the
 354                     // only reason we can fail to make progress on
 355                     // trait selection is because we don't have enough
 356                     // information about the types in the trait. One
 357                     // exception is that we sometimes haven't decided
 358                     // what kind of closure a closure is. *But*, in
 359                     // that case, it turns out, the type of the
 360                     // closure will also change, because the closure
 361                     // also includes references to its upvars as part
 362                     // of its type, and those types are resolved at
 363                     // the same time.
 364                     //
 365                     // FIXME(#32286) logic seems false if no upvars
 366                     pending_obligation.stalled_on =
 367                         trait_ref_type_vars(selcx, data.to_poly_trait_ref());
 368
 369                     debug!("process_predicate: pending obligation {:?} now stalled on {:?}",
 370                            selcx.infcx().resolve_type_vars_if_possible(obligation),
 371                            pending_obligation.stalled_on);
 372
 373                     Ok(None)
 374                 }
 375                 Err(selection_err) => {
 376                     info!("selecting trait `{:?}` at depth {} yielded Err",
 377                           data, obligation.recursion_depth);
 378
 379                     Err(CodeSelectionError(selection_err))
 380                 }
 381             }
 382         }
 383
 384         ty::Predicate::RegionOutlives(ref binder) => {
 385             match selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
 386                 Ok(()) => Ok(Some(Vec::new())),
 387                 Err(_) => Err(CodeSelectionError(Unimplemented)),
 388             }
 389         }
 390
 391         ty::Predicate::TypeOutlives(ref binder) => {
 392             // Check if there are higher-ranked regions.
 393             match binder.no_late_bound_regions() {
 394                 // If there are, inspect the underlying type further.
 395                 None => {
 396                     // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 397                     let binder = binder.map_bound_ref(|pred| pred.0);
 398
 399                     // Check if the type has any bound regions.
 400                     match binder.no_late_bound_regions() {
 401                         // If so, this obligation is an error (for now). Eventually we should be
 402                         // able to support additional cases here, like `for<'a> &'a str: 'a`.
 403                         None => {
 404                             Err(CodeSelectionError(Unimplemented))
 405                         }
 406                         // Otherwise, we have something of the form
 407                         // `for<'a> T: 'a where 'a not in T`, which we can treat as `T: 'static`.
 408                         Some(t_a) => {
 409                             let r_static = selcx.tcx().types.re_static;
 410                             if register_region_obligations {
 411                                 selcx.infcx().register_region_obligation(
 412                                     obligation.cause.body_id,
 413                                     RegionObligation {
 414                                         sup_type: t_a,
 415                                         sub_region: r_static,
 416                                         cause: obligation.cause.clone(),
 417                                     });
 418                             }
 419                             Ok(Some(vec![]))
 420                         }
 421                     }
 422                 }
 423                 // If there aren't, register the obligation.
 424                 Some(ty::OutlivesPredicate(t_a, r_b)) => {
 425                     if register_region_obligations {
 426                         selcx.infcx().register_region_obligation(
 427                             obligation.cause.body_id,
 428                             RegionObligation {
 429                                 sup_type: t_a,
 430                                 sub_region: r_b,
 431                                 cause: obligation.cause.clone()
 432                             });
 433                     }
 434                     Ok(Some(vec![]))
 435                 }
 436             }
 437         }
 438
 439         ty::Predicate::Projection(ref data) => {
 440             let project_obligation = obligation.with(data.clone());
 441             match project::poly_project_and_unify_type(selcx, &project_obligation) {
 442                 Ok(None) => {
 443                     let tcx = selcx.tcx();
 444                     pending_obligation.stalled_on =
 445                         trait_ref_type_vars(selcx, data.to_poly_trait_ref(tcx));
 446                     Ok(None)
 447                 }
 448                 Ok(v) => Ok(v),
 449                 Err(e) => Err(CodeProjectionError(e))
 450             }
 451         }
 452
 453         ty::Predicate::ObjectSafe(trait_def_id) => {
 454             if !selcx.tcx().is_object_safe(trait_def_id) {
 455                 Err(CodeSelectionError(Unimplemented))
 456             } else {
 457                 Ok(Some(Vec::new()))
 458             }
 459         }
 460
 461         ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 462             match selcx.infcx().closure_kind(closure_def_id, closure_substs) {
 463                 Some(closure_kind) => {
 464                     if closure_kind.extends(kind) {
 465                         Ok(Some(vec![]))
 466                     } else {
 467                         Err(CodeSelectionError(Unimplemented))
 468                     }
 469                 }
 470                 None => {
 471                     Ok(None)
 472                 }
 473             }
 474         }
 475
 476         ty::Predicate::WellFormed(ty) => {
 477             match ty::wf::obligations(selcx.infcx(),
 478                                       obligation.param_env,
 479                                       obligation.cause.body_id,
 480                                       ty, obligation.cause.span) {
 481                 None => {
 482                     pending_obligation.stalled_on = vec![ty];
 483                     Ok(None)
 484                 }
 485                 s => Ok(s)
 486             }
 487         }
 488
 489         ty::Predicate::Subtype(ref subtype) => {
 490             match selcx.infcx().subtype_predicate(&obligation.cause,
 491                                                   obligation.param_env,
 492                                                   subtype) {
 493                 None => {
 494                     // none means that both are unresolved
 495                     pending_obligation.stalled_on = vec![subtype.skip_binder().a,
 496                                                          subtype.skip_binder().b];
 497                     Ok(None)
 498                 }
 499                 Some(Ok(ok)) => {
 500                     Ok(Some(ok.obligations))
 501                 }
 502                 Some(Err(err)) => {
 503                     let expected_found = ExpectedFound::new(subtype.skip_binder().a_is_expected,
 504                                                             subtype.skip_binder().a,
 505                                                             subtype.skip_binder().b);
 506                     Err(FulfillmentErrorCode::CodeSubtypeError(expected_found, err))
 507                 }
 508             }
 509         }
 510
 511         ty::Predicate::ConstEvaluatable(def_id, substs) => {
 512             match selcx.tcx().lift_to_global(&obligation.param_env) {
 513                 None => {
 514                     Ok(None)
 515                 }
 516                 Some(param_env) => {
 517                     match selcx.tcx().lift_to_global(&substs) {
 518                         Some(substs) => {
 519                             let instance = ty::Instance::resolve(
 520                                 selcx.tcx().global_tcx(),
 521                                 param_env,
 522                                 def_id,
 523                                 substs,
 524                             );
 525                             if let Some(instance) = instance {
 526                                 let cid = GlobalId {
 527                                     instance,
 528                                     promoted: None,
 529                                 };
 530                                 match selcx.tcx().at(obligation.cause.span)
 531                                                  .const_eval(param_env.and(cid)) {
 532                                     Ok(_) => Ok(Some(vec![])),
 533                                     Err(err) => Err(CodeSelectionError(ConstEvalFailure(err)))
 534                                 }
 535                             } else {
 536                                 Err(CodeSelectionError(ConstEvalFailure(ConstEvalErr {
 537                                     span: obligation.cause.span,
 538                                     kind: ErrKind::UnimplementedConstVal("could not resolve")
 539                                         .into(),
 540                                 })))
 541                             }
 542                         },
 543                         None => {
 544                             pending_obligation.stalled_on = substs.types().collect();
 545                             Ok(None)
 546                         }
 547                     }
 548                 }
 549             }
 550         }
 551     }
 552 }
 553
 554 fn to_fulfillment_error<'tcx>(
 555     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>)
 556     -> FulfillmentError<'tcx>
 557 {
 558     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 559     FulfillmentError::new(obligation, error.error)
 560 }