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