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