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