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 ty::{self, Ty, TypeFoldable, ToPolyTraitRef, ToPredicate};
  13 use ty::error::ExpectedFound;
  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 hir::def_id::DefId;
  18
  19 use super::CodeAmbiguity;
  20 use super::CodeProjectionError;
  21 use super::CodeSelectionError;
  22 use super::{FulfillmentError, FulfillmentErrorCode};
  23 use super::{ObligationCause, PredicateObligation, Obligation};
  24 use super::project;
  25 use super::select::SelectionContext;
  26 use super::{Unimplemented, ConstEvalFailure};
  27
  28 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  29     type Predicate = ty::Predicate<'tcx>;
  30
  31     fn as_predicate(&self) -> &Self::Predicate { &self.obligation.predicate }
  32 }
  33
  34 /// The fulfillment context is used to drive trait resolution.  It
  35 /// consists of a list of obligations that must be (eventually)
  36 /// satisfied. The job is to track which are satisfied, which yielded
  37 /// errors, and which are still pending. At any point, users can call
  38 /// `select_where_possible`, and the fulfillment context will try to do
  39 /// selection, retaining only those obligations that remain
  40 /// ambiguous. This may be helpful in pushing type inference
  41 /// along. Once all type inference constraints have been generated, the
  42 /// method `select_all_or_error` can be used to report any remaining
  43 /// ambiguous cases as errors.
  44
  45 pub struct FulfillmentContext<'tcx> {
  46     // A list of all obligations that have been registered with this
  47     // fulfillment context.
  48     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  49     // Should this fulfillment context register type-lives-for-region
  50     // obligations on its parent infcx? In some cases, region
  51     // obligations are either already known to hold (normalization) or
  52     // hopefully verifed elsewhere (type-impls-bound), and therefore
  53     // should not be checked.
  54     //
  55     // Note that if we are normalizing a type that we already
  56     // know is well-formed, there should be no harm setting this
  57     // to true - all the region variables should be determinable
  58     // using the RFC 447 rules, which don't depend on
  59     // type-lives-for-region constraints, and because the type
  60     // is well-formed, the constraints should hold.
  61     register_region_obligations: bool,
  62 }
  63
  64 #[derive(Clone, Debug)]
  65 pub struct PendingPredicateObligation<'tcx> {
  66     pub obligation: PredicateObligation<'tcx>,
  67     pub stalled_on: Vec<Ty<'tcx>>,
  68 }
  69
  70 impl<'a, 'gcx, 'tcx> FulfillmentContext<'tcx> {
  71     /// Creates a new fulfillment context.
  72     pub fn new() -> FulfillmentContext<'tcx> {
  73         FulfillmentContext {
  74             predicates: ObligationForest::new(),
  75             register_region_obligations: true
  76         }
  77     }
  78
  79     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
  80         FulfillmentContext {
  81             predicates: ObligationForest::new(),
  82             register_region_obligations: false
  83         }
  84     }
  85
  86     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
  87     /// creating a fresh type variable `$0` as well as a projection
  88     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
  89     /// inference engine runs, it will attempt to find an impl of
  90     /// `SomeTrait` or a where clause that lets us unify `$0` with
  91     /// something concrete. If this fails, we'll unify `$0` with
  92     /// `projection_ty` again.
  93     pub fn normalize_projection_type(&mut self,
  94                                      infcx: &InferCtxt<'a, 'gcx, 'tcx>,
  95                                      param_env: ty::ParamEnv<'tcx>,
  96                                      projection_ty: ty::ProjectionTy<'tcx>,
  97                                      cause: ObligationCause<'tcx>)
  98                                      -> Ty<'tcx>
  99     {
 100         debug!("normalize_projection_type(projection_ty={:?})",
 101                projection_ty);
 102
 103         assert!(!projection_ty.has_escaping_regions());
 104
 105         // FIXME(#20304) -- cache
 106
 107         let mut selcx = SelectionContext::new(infcx);
 108         let normalized = project::normalize_projection_type(&mut selcx,
 109                                                             param_env,
 110                                                             projection_ty,
 111                                                             cause,
 112                                                             0);
 113
 114         for obligation in normalized.obligations {
 115             self.register_predicate_obligation(infcx, obligation);
 116         }
 117
 118         debug!("normalize_projection_type: result={:?}", normalized.value);
 119
 120         normalized.value
 121     }
 122
 123     /// Requires that `ty` must implement the trait with `def_id` in
 124     /// the given environment. This trait must not have any type
 125     /// parameters (except for `Self`).
 126     pub fn register_bound(&mut self,
 127                           infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 128                           param_env: ty::ParamEnv<'tcx>,
 129                           ty: Ty<'tcx>,
 130                           def_id: DefId,
 131                           cause: ObligationCause<'tcx>)
 132     {
 133         let trait_ref = ty::TraitRef {
 134             def_id,
 135             substs: infcx.tcx.mk_substs_trait(ty, &[]),
 136         };
 137         self.register_predicate_obligation(infcx, Obligation {
 138             cause,
 139             recursion_depth: 0,
 140             param_env,
 141             predicate: trait_ref.to_predicate()
 142         });
 143     }
 144
 145     pub fn register_predicate_obligation(&mut self,
 146                                          infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 147                                          obligation: PredicateObligation<'tcx>)
 148     {
 149         // this helps to reduce duplicate errors, as well as making
 150         // debug output much nicer to read and so on.
 151         let obligation = infcx.resolve_type_vars_if_possible(&obligation);
 152
 153         debug!("register_predicate_obligation(obligation={:?})", obligation);
 154
 155         assert!(!infcx.is_in_snapshot());
 156
 157         self.predicates.register_obligation(PendingPredicateObligation {
 158             obligation,
 159             stalled_on: vec![]
 160         });
 161     }
 162
 163     pub fn register_predicate_obligations<I>(&mut self,
 164                                              infcx: &InferCtxt<'a, 'gcx, 'tcx>,
 165                                              obligations: I)
 166         where I: IntoIterator<Item = PredicateObligation<'tcx>>
 167     {
 168         for obligation in obligations {
 169             self.register_predicate_obligation(infcx, obligation);
 170         }
 171     }
 172
 173     pub fn select_all_or_error(&mut self,
 174                                infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 175                                -> Result<(),Vec<FulfillmentError<'tcx>>>
 176     {
 177         self.select_where_possible(infcx)?;
 178
 179         let errors: Vec<_> =
 180             self.predicates.to_errors(CodeAmbiguity)
 181                            .into_iter()
 182                            .map(|e| to_fulfillment_error(e))
 183                            .collect();
 184         if errors.is_empty() {
 185             Ok(())
 186         } else {
 187             Err(errors)
 188         }
 189     }
 190
 191     pub fn select_where_possible(&mut self,
 192                                  infcx: &InferCtxt<'a, 'gcx, 'tcx>)
 193                                  -> Result<(),Vec<FulfillmentError<'tcx>>>
 194     {
 195         let mut selcx = SelectionContext::new(infcx);
 196         self.select(&mut selcx)
 197     }
 198
 199     pub fn pending_obligations(&self) -> Vec<PendingPredicateObligation<'tcx>> {
 200         self.predicates.pending_obligations()
 201     }
 202
 203     /// Attempts to select obligations using `selcx`. If `only_new_obligations` is true, then it
 204     /// only attempts to select obligations that haven't been seen before.
 205     fn select(&mut self, selcx: &mut SelectionContext<'a, 'gcx, 'tcx>)
 206               -> Result<(),Vec<FulfillmentError<'tcx>>> {
 207         debug!("select(obligation-forest-size={})", self.predicates.len());
 208
 209         let mut errors = Vec::new();
 210
 211         loop {
 212             debug!("select: starting another iteration");
 213
 214             // Process pending obligations.
 215             let outcome = self.predicates.process_obligations(&mut FulfillProcessor {
 216                 selcx,
 217                 register_region_obligations: self.register_region_obligations
 218             });
 219             debug!("select: outcome={:?}", outcome);
 220
 221             // FIXME: if we kept the original cache key, we could mark projection
 222             // obligations as complete for the projection cache here.
 223
 224             errors.extend(
 225                 outcome.errors.into_iter()
 226                               .map(|e| to_fulfillment_error(e)));
 227
 228             // If nothing new was added, no need to keep looping.
 229             if outcome.stalled {
 230                 break;
 231             }
 232         }
 233
 234         debug!("select({} predicates remaining, {} errors) done",
 235                self.predicates.len(), errors.len());
 236
 237         if errors.is_empty() {
 238             Ok(())
 239         } else {
 240             Err(errors)
 241         }
 242     }
 243 }
 244
 245 struct FulfillProcessor<'a, 'b: 'a, 'gcx: 'tcx, 'tcx: 'b> {
 246     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 247     register_region_obligations: bool
 248 }
 249
 250 impl<'a, 'b, 'gcx, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'gcx, 'tcx> {
 251     type Obligation = PendingPredicateObligation<'tcx>;
 252     type Error = FulfillmentErrorCode<'tcx>;
 253
 254     fn process_obligation(&mut self,
 255                           obligation: &mut Self::Obligation)
 256                           -> Result<Option<Vec<Self::Obligation>>, Self::Error>
 257     {
 258         process_predicate(self.selcx, obligation, self.register_region_obligations)
 259             .map(|os| os.map(|os| os.into_iter().map(|o| PendingPredicateObligation {
 260                 obligation: o,
 261                 stalled_on: vec![]
 262             }).collect()))
 263     }
 264
 265     fn process_backedge<'c, I>(&mut self, cycle: I,
 266                                _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>)
 267         where I: Clone + Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 268     {
 269         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 270             debug!("process_child_obligations: coinductive match");
 271         } else {
 272             let cycle : Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 273             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 274         }
 275     }
 276 }
 277
 278 /// Return the set of type variables contained in a trait ref
 279 fn trait_ref_type_vars<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 280                                        t: ty::PolyTraitRef<'tcx>) -> Vec<Ty<'tcx>>
 281 {
 282     t.skip_binder() // ok b/c this check doesn't care about regions
 283      .input_types()
 284      .map(|t| selcx.infcx().resolve_type_vars_if_possible(&t))
 285      .filter(|t| t.has_infer_types())
 286      .flat_map(|t| t.walk())
 287      .filter(|t| match t.sty { ty::TyInfer(_) => true, _ => false })
 288      .collect()
 289 }
 290
 291 /// Processes a predicate obligation and returns either:
 292 /// - `Ok(Some(v))` if the predicate is true, presuming that `v` are also true
 293 /// - `Ok(None)` if we don't have enough info to be sure
 294 /// - `Err` if the predicate does not hold
 295 fn process_predicate<'a, 'gcx, 'tcx>(
 296     selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 297     pending_obligation: &mut PendingPredicateObligation<'tcx>,
 298     register_region_obligations: bool)
 299     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 300               FulfillmentErrorCode<'tcx>>
 301 {
 302     // if we were stalled on some unresolved variables, first check
 303     // whether any of them have been resolved; if not, don't bother
 304     // doing more work yet
 305     if !pending_obligation.stalled_on.is_empty() {
 306         if pending_obligation.stalled_on.iter().all(|&ty| {
 307             let resolved_ty = selcx.infcx().shallow_resolve(&ty);
 308             resolved_ty == ty // nothing changed here
 309         }) {
 310             debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
 311                    selcx.infcx().resolve_type_vars_if_possible(&pending_obligation.obligation),
 312                    pending_obligation.stalled_on);
 313             return Ok(None);
 314         }
 315         pending_obligation.stalled_on = vec![];
 316     }
 317
 318     let obligation = &mut pending_obligation.obligation;
 319
 320     if obligation.predicate.has_infer_types() {
 321         obligation.predicate = selcx.infcx().resolve_type_vars_if_possible(&obligation.predicate);
 322     }
 323
 324     match obligation.predicate {
 325         ty::Predicate::Trait(ref data) => {
 326             let trait_obligation = obligation.with(data.clone());
 327
 328             if data.is_global() {
 329                 // no type variables present, can use evaluation for better caching.
 330                 // FIXME: consider caching errors too.
 331                 if
 332                     // make defaulted unit go through the slow path for better warnings,
 333                     // please remove this when the warnings are removed.
 334                     !trait_obligation.predicate.skip_binder().self_ty().is_defaulted_unit() &&
 335                     selcx.evaluate_obligation_conservatively(&obligation) {
 336                     debug!("selecting trait `{:?}` at depth {} evaluated to holds",
 337                            data, obligation.recursion_depth);
 338                     return Ok(Some(vec![]))
 339                 }
 340             }
 341
 342             match selcx.select(&trait_obligation) {
 343                 Ok(Some(vtable)) => {
 344                     debug!("selecting trait `{:?}` at depth {} yielded Ok(Some)",
 345                            data, obligation.recursion_depth);
 346                     Ok(Some(vtable.nested_obligations()))
 347                 }
 348                 Ok(None) => {
 349                     debug!("selecting trait `{:?}` at depth {} yielded Ok(None)",
 350                            data, obligation.recursion_depth);
 351
 352                     // This is a bit subtle: for the most part, the
 353                     // only reason we can fail to make progress on
 354                     // trait selection is because we don't have enough
 355                     // information about the types in the trait. One
 356                     // exception is that we sometimes haven't decided
 357                     // what kind of closure a closure is. *But*, in
 358                     // that case, it turns out, the type of the
 359                     // closure will also change, because the closure
 360                     // also includes references to its upvars as part
 361                     // of its type, and those types are resolved at
 362                     // the same time.
 363                     //
 364                     // FIXME(#32286) logic seems false if no upvars
 365                     pending_obligation.stalled_on =
 366                         trait_ref_type_vars(selcx, data.to_poly_trait_ref());
 367
 368                     debug!("process_predicate: pending obligation {:?} now stalled on {:?}",
 369                            selcx.infcx().resolve_type_vars_if_possible(obligation),
 370                            pending_obligation.stalled_on);
 371
 372                     Ok(None)
 373                 }
 374                 Err(selection_err) => {
 375                     info!("selecting trait `{:?}` at depth {} yielded Err",
 376                           data, obligation.recursion_depth);
 377
 378                     Err(CodeSelectionError(selection_err))
 379                 }
 380             }
 381         }
 382
 383         ty::Predicate::RegionOutlives(ref binder) => {
 384             match selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
 385                 Ok(()) => Ok(Some(Vec::new())),
 386                 Err(_) => Err(CodeSelectionError(Unimplemented)),
 387             }
 388         }
 389
 390         ty::Predicate::TypeOutlives(ref binder) => {
 391             // Check if there are higher-ranked regions.
 392             match binder.no_late_bound_regions() {
 393                 // If there are, inspect the underlying type further.
 394                 None => {
 395                     // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 396                     let binder = binder.map_bound_ref(|pred| pred.0);
 397
 398                     // Check if the type has any bound regions.
 399                     match binder.no_late_bound_regions() {
 400                         // If so, this obligation is an error (for now). Eventually we should be
 401                         // able to support additional cases here, like `for<'a> &'a str: 'a`.
 402                         None => {
 403                             Err(CodeSelectionError(Unimplemented))
 404                         }
 405                         // Otherwise, we have something of the form
 406                         // `for<'a> T: 'a where 'a not in T`, which we can treat as `T: 'static`.
 407                         Some(t_a) => {
 408                             let r_static = selcx.tcx().types.re_static;
 409                             if register_region_obligations {
 410                                 selcx.infcx().register_region_obligation(
 411                                     obligation.cause.body_id,
 412                                     RegionObligation {
 413                                         sup_type: t_a,
 414                                         sub_region: r_static,
 415                                         cause: obligation.cause.clone(),
 416                                     });
 417                             }
 418                             Ok(Some(vec![]))
 419                         }
 420                     }
 421                 }
 422                 // If there aren't, register the obligation.
 423                 Some(ty::OutlivesPredicate(t_a, r_b)) => {
 424                     if register_region_obligations {
 425                         selcx.infcx().register_region_obligation(
 426                             obligation.cause.body_id,
 427                             RegionObligation {
 428                                 sup_type: t_a,
 429                                 sub_region: r_b,
 430                                 cause: obligation.cause.clone()
 431                             });
 432                     }
 433                     Ok(Some(vec![]))
 434                 }
 435             }
 436         }
 437
 438         ty::Predicate::Projection(ref data) => {
 439             let project_obligation = obligation.with(data.clone());
 440             match project::poly_project_and_unify_type(selcx, &project_obligation) {
 441                 Ok(None) => {
 442                     let tcx = selcx.tcx();
 443                     pending_obligation.stalled_on =
 444                         trait_ref_type_vars(selcx, data.to_poly_trait_ref(tcx));
 445                     Ok(None)
 446                 }
 447                 Ok(v) => Ok(v),
 448                 Err(e) => Err(CodeProjectionError(e))
 449             }
 450         }
 451
 452         ty::Predicate::ObjectSafe(trait_def_id) => {
 453             if !selcx.tcx().is_object_safe(trait_def_id) {
 454                 Err(CodeSelectionError(Unimplemented))
 455             } else {
 456                 Ok(Some(Vec::new()))
 457             }
 458         }
 459
 460         ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 461             match selcx.infcx().closure_kind(closure_def_id, closure_substs) {
 462                 Some(closure_kind) => {
 463                     if closure_kind.extends(kind) {
 464                         Ok(Some(vec![]))
 465                     } else {
 466                         Err(CodeSelectionError(Unimplemented))
 467                     }
 468                 }
 469                 None => {
 470                     Ok(None)
 471                 }
 472             }
 473         }
 474
 475         ty::Predicate::WellFormed(ty) => {
 476             match ty::wf::obligations(selcx.infcx(),
 477                                       obligation.param_env,
 478                                       obligation.cause.body_id,
 479                                       ty, obligation.cause.span) {
 480                 None => {
 481                     pending_obligation.stalled_on = vec![ty];
 482                     Ok(None)
 483                 }
 484                 s => Ok(s)
 485             }
 486         }
 487
 488         ty::Predicate::Subtype(ref subtype) => {
 489             match selcx.infcx().subtype_predicate(&obligation.cause,
 490                                                   obligation.param_env,
 491                                                   subtype) {
 492                 None => {
 493                     // none means that both are unresolved
 494                     pending_obligation.stalled_on = vec![subtype.skip_binder().a,
 495                                                          subtype.skip_binder().b];
 496                     Ok(None)
 497                 }
 498                 Some(Ok(ok)) => {
 499                     Ok(Some(ok.obligations))
 500                 }
 501                 Some(Err(err)) => {
 502                     let expected_found = ExpectedFound::new(subtype.skip_binder().a_is_expected,
 503                                                             subtype.skip_binder().a,
 504                                                             subtype.skip_binder().b);
 505                     Err(FulfillmentErrorCode::CodeSubtypeError(expected_found, err))
 506                 }
 507             }
 508         }
 509
 510         ty::Predicate::ConstEvaluatable(def_id, substs) => {
 511             match selcx.tcx().lift_to_global(&obligation.param_env) {
 512                 None => {
 513                     Ok(None)
 514                 }
 515                 Some(param_env) => {
 516                     match selcx.tcx().lift_to_global(&substs) {
 517                         None => {
 518                             pending_obligation.stalled_on = substs.types().collect();
 519                             Ok(None)
 520                         }
 521                         Some(substs) => {
 522                             match selcx.tcx().at(obligation.cause.span)
 523                                              .const_eval(param_env.and((def_id, substs))) {
 524                                 Ok(_) => Ok(Some(vec![])),
 525                                 Err(e) => Err(CodeSelectionError(ConstEvalFailure(e)))
 526                             }
 527                         }
 528                     }
 529                 }
 530             }
 531         }
 532     }
 533 }
 534
 535 fn to_fulfillment_error<'tcx>(
 536     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>)
 537     -> FulfillmentError<'tcx>
 538 {
 539     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 540     FulfillmentError::new(obligation, error.error)
 541 }