src/librustc/traits/fulfill.rs

   1 use crate::infer::{InferCtxt, ShallowResolver};
   2 use crate::mir::interpret::{GlobalId, ErrorHandled};
   3 use crate::ty::{self, Ty, TypeFoldable, ToPolyTraitRef};
   4 use crate::ty::error::ExpectedFound;
   5 use rustc_data_structures::obligation_forest::{DoCompleted, Error, ForestObligation};
   6 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
   7 use rustc_data_structures::obligation_forest::{ProcessResult};
   8 use std::marker::PhantomData;
   9
  10 use super::CodeAmbiguity;
  11 use super::CodeProjectionError;
  12 use super::CodeSelectionError;
  13 use super::engine::{TraitEngine, TraitEngineExt};
  14 use super::{FulfillmentError, FulfillmentErrorCode};
  15 use super::{ObligationCause, PredicateObligation};
  16 use super::project;
  17 use super::select::SelectionContext;
  18 use super::{Unimplemented, ConstEvalFailure};
  19
  20 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  21     type Predicate = ty::Predicate<'tcx>;
  22
  23     fn as_predicate(&self) -> &Self::Predicate { &self.obligation.predicate }
  24 }
  25
  26 /// The fulfillment context is used to drive trait resolution. It
  27 /// consists of a list of obligations that must be (eventually)
  28 /// satisfied. The job is to track which are satisfied, which yielded
  29 /// errors, and which are still pending. At any point, users can call
  30 /// `select_where_possible`, and the fulfillment context will try to do
  31 /// selection, retaining only those obligations that remain
  32 /// ambiguous. This may be helpful in pushing type inference
  33 /// along. Once all type inference constraints have been generated, the
  34 /// method `select_all_or_error` can be used to report any remaining
  35 /// ambiguous cases as errors.
  36 pub struct FulfillmentContext<'tcx> {
  37     // A list of all obligations that have been registered with this
  38     // fulfillment context.
  39     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  40     // Should this fulfillment context register type-lives-for-region
  41     // obligations on its parent infcx? In some cases, region
  42     // obligations are either already known to hold (normalization) or
  43     // hopefully verifed elsewhere (type-impls-bound), and therefore
  44     // should not be checked.
  45     //
  46     // Note that if we are normalizing a type that we already
  47     // know is well-formed, there should be no harm setting this
  48     // to true - all the region variables should be determinable
  49     // using the RFC 447 rules, which don't depend on
  50     // type-lives-for-region constraints, and because the type
  51     // is well-formed, the constraints should hold.
  52     register_region_obligations: bool,
  53     // Is it OK to register obligations into this infcx inside
  54     // an infcx snapshot?
  55     //
  56     // The "primary fulfillment" in many cases in typeck lives
  57     // outside of any snapshot, so any use of it inside a snapshot
  58     // will lead to trouble and therefore is checked against, but
  59     // other fulfillment contexts sometimes do live inside of
  60     // a snapshot (they don't *straddle* a snapshot, so there
  61     // is no trouble there).
  62     usable_in_snapshot: bool
  63 }
  64
  65 #[derive(Clone, Debug)]
  66 pub struct PendingPredicateObligation<'tcx> {
  67     pub obligation: PredicateObligation<'tcx>,
  68     pub stalled_on: Vec<ty::InferTy>,
  69 }
  70
  71 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  72 #[cfg(target_arch = "x86_64")]
  73 static_assert_size!(PendingPredicateObligation<'_>, 136);
  74
  75 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  76     /// Creates a new fulfillment context.
  77     pub fn new() -> FulfillmentContext<'tcx> {
  78         FulfillmentContext {
  79             predicates: ObligationForest::new(),
  80             register_region_obligations: true,
  81             usable_in_snapshot: false,
  82         }
  83     }
  84
  85     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
  86         FulfillmentContext {
  87             predicates: ObligationForest::new(),
  88             register_region_obligations: true,
  89             usable_in_snapshot: true,
  90         }
  91     }
  92
  93     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
  94         FulfillmentContext {
  95             predicates: ObligationForest::new(),
  96             register_region_obligations: false,
  97             usable_in_snapshot: false
  98         }
  99     }
 100
 101     /// Attempts to select obligations using `selcx`.
 102     fn select(
 103         &mut self,
 104         selcx: &mut SelectionContext<'a, 'tcx>,
 105     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 106         debug!("select(obligation-forest-size={})", self.predicates.len());
 107
 108         let mut errors = Vec::new();
 109
 110         loop {
 111             debug!("select: starting another iteration");
 112
 113             // Process pending obligations.
 114             let outcome = self.predicates.process_obligations(&mut FulfillProcessor {
 115                 selcx,
 116                 register_region_obligations: self.register_region_obligations
 117             }, DoCompleted::No);
 118             debug!("select: outcome={:#?}", outcome);
 119
 120             // FIXME: if we kept the original cache key, we could mark projection
 121             // obligations as complete for the projection cache here.
 122
 123             errors.extend(
 124                 outcome.errors.into_iter()
 125                               .map(|e| to_fulfillment_error(e)));
 126
 127             // If nothing new was added, no need to keep looping.
 128             if outcome.stalled {
 129                 break;
 130             }
 131         }
 132
 133         debug!("select({} predicates remaining, {} errors) done",
 134                self.predicates.len(), errors.len());
 135
 136         if errors.is_empty() {
 137             Ok(())
 138         } else {
 139             Err(errors)
 140         }
 141     }
 142 }
 143
 144 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 145     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 146     /// creating a fresh type variable `$0` as well as a projection
 147     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 148     /// inference engine runs, it will attempt to find an impl of
 149     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 150     /// something concrete. If this fails, we'll unify `$0` with
 151     /// `projection_ty` again.
 152     fn normalize_projection_type(
 153         &mut self,
 154         infcx: &InferCtxt<'_, 'tcx>,
 155         param_env: ty::ParamEnv<'tcx>,
 156         projection_ty: ty::ProjectionTy<'tcx>,
 157         cause: ObligationCause<'tcx>,
 158     ) -> Ty<'tcx> {
 159         debug!("normalize_projection_type(projection_ty={:?})",
 160                projection_ty);
 161
 162         debug_assert!(!projection_ty.has_escaping_bound_vars());
 163
 164         // FIXME(#20304) -- cache
 165
 166         let mut selcx = SelectionContext::new(infcx);
 167         let mut obligations = vec![];
 168         let normalized_ty = project::normalize_projection_type(&mut selcx,
 169                                                                param_env,
 170                                                                projection_ty,
 171                                                                cause,
 172                                                                0,
 173                                                                &mut obligations);
 174         self.register_predicate_obligations(infcx, obligations);
 175
 176         debug!("normalize_projection_type: result={:?}", normalized_ty);
 177
 178         normalized_ty
 179     }
 180
 181     fn register_predicate_obligation(
 182         &mut self,
 183         infcx: &InferCtxt<'_, 'tcx>,
 184         obligation: PredicateObligation<'tcx>,
 185     ) {
 186         // this helps to reduce duplicate errors, as well as making
 187         // debug output much nicer to read and so on.
 188         let obligation = infcx.resolve_vars_if_possible(&obligation);
 189
 190         debug!("register_predicate_obligation(obligation={:?})", obligation);
 191
 192         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 193
 194         self.predicates.register_obligation(PendingPredicateObligation {
 195             obligation,
 196             stalled_on: vec![]
 197         });
 198     }
 199
 200     fn select_all_or_error(
 201         &mut self,
 202         infcx: &InferCtxt<'_, 'tcx>,
 203     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 204         self.select_where_possible(infcx)?;
 205
 206         let errors: Vec<_> =
 207             self.predicates.to_errors(CodeAmbiguity)
 208                            .into_iter()
 209                            .map(|e| to_fulfillment_error(e))
 210                            .collect();
 211         if errors.is_empty() {
 212             Ok(())
 213         } else {
 214             Err(errors)
 215         }
 216     }
 217
 218     fn select_where_possible(
 219         &mut self,
 220         infcx: &InferCtxt<'_, 'tcx>,
 221     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 222         let mut selcx = SelectionContext::new(infcx);
 223         self.select(&mut selcx)
 224     }
 225
 226     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 227         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 228     }
 229 }
 230
 231 struct FulfillProcessor<'a, 'b, 'tcx> {
 232     selcx: &'a mut SelectionContext<'b, 'tcx>,
 233     register_region_obligations: bool,
 234 }
 235
 236 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 237     os.into_iter().map(|o| PendingPredicateObligation {
 238         obligation: o,
 239         stalled_on: vec![]
 240     }).collect()
 241 }
 242
 243 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 244     type Obligation = PendingPredicateObligation<'tcx>;
 245     type Error = FulfillmentErrorCode<'tcx>;
 246
 247     /// Processes a predicate obligation and returns either:
 248     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 249     /// - `Unchanged` if we don't have enough info to be sure
 250     /// - `Error(e)` if the predicate does not hold
 251     ///
 252     /// This is always inlined, despite its size, because it has a single
 253     /// callsite and it is called *very* frequently.
 254     #[inline(always)]
 255     fn process_obligation(
 256         &mut self,
 257         pending_obligation: &mut Self::Obligation,
 258     ) -> ProcessResult<Self::Obligation, Self::Error> {
 259         // If we were stalled on some unresolved variables, first check whether
 260         // any of them have been resolved; if not, don't bother doing more work
 261         // yet.
 262         let change = match pending_obligation.stalled_on.len() {
 263             // Match arms are in order of frequency, which matters because this
 264             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 265             1 => {
 266                 let infer = pending_obligation.stalled_on[0];
 267                 ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer)
 268             }
 269             0 => {
 270                 // In this case we haven't changed, but wish to make a change.
 271                 true
 272             }
 273             _ => {
 274                 // This `for` loop was once a call to `all()`, but this lower-level
 275                 // form was a perf win. See #64545 for details.
 276                 (|| {
 277                     for &infer in &pending_obligation.stalled_on {
 278                         if ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer) {
 279                             return true;
 280                         }
 281                     }
 282                     false
 283                 })()
 284             }
 285         };
 286
 287         if !change {
 288             debug!("process_predicate: pending obligation {:?} still stalled on {:?}",
 289                    self.selcx.infcx()
 290                        .resolve_vars_if_possible(&pending_obligation.obligation),
 291                    pending_obligation.stalled_on);
 292             return ProcessResult::Unchanged;
 293         }
 294
 295         // This part of the code is much colder.
 296
 297         pending_obligation.stalled_on.truncate(0);
 298
 299         let obligation = &mut pending_obligation.obligation;
 300
 301         if obligation.predicate.has_infer_types() {
 302             obligation.predicate =
 303                 self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
 304         }
 305
 306         debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
 307
 308         fn infer_ty(ty: Ty<'tcx>) -> ty::InferTy {
 309             match ty.kind {
 310                 ty::Infer(infer) => infer,
 311                 _ => panic!(),
 312             }
 313         }
 314
 315         match obligation.predicate {
 316             ty::Predicate::Trait(ref data) => {
 317                 let trait_obligation = obligation.with(data.clone());
 318
 319                 if data.is_global() {
 320                     // no type variables present, can use evaluation for better caching.
 321                     // FIXME: consider caching errors too.
 322                     if self.selcx.infcx().predicate_must_hold_considering_regions(&obligation) {
 323                         debug!("selecting trait `{:?}` at depth {} evaluated to holds",
 324                                data, obligation.recursion_depth);
 325                         return ProcessResult::Changed(vec![])
 326                     }
 327                 }
 328
 329                 match self.selcx.select(&trait_obligation) {
 330                     Ok(Some(vtable)) => {
 331                         debug!("selecting trait `{:?}` at depth {} yielded Ok(Some)",
 332                                data, obligation.recursion_depth);
 333                         ProcessResult::Changed(mk_pending(vtable.nested_obligations()))
 334                     }
 335                     Ok(None) => {
 336                         debug!("selecting trait `{:?}` at depth {} yielded Ok(None)",
 337                                data, obligation.recursion_depth);
 338
 339                         // This is a bit subtle: for the most part, the
 340                         // only reason we can fail to make progress on
 341                         // trait selection is because we don't have enough
 342                         // information about the types in the trait. One
 343                         // exception is that we sometimes haven't decided
 344                         // what kind of closure a closure is. *But*, in
 345                         // that case, it turns out, the type of the
 346                         // closure will also change, because the closure
 347                         // also includes references to its upvars as part
 348                         // of its type, and those types are resolved at
 349                         // the same time.
 350                         //
 351                         // FIXME(#32286) logic seems false if no upvars
 352                         pending_obligation.stalled_on =
 353                             trait_ref_type_vars(self.selcx, data.to_poly_trait_ref());
 354
 355                         debug!("process_predicate: pending obligation {:?} now stalled on {:?}",
 356                                self.selcx.infcx().resolve_vars_if_possible(obligation),
 357                                pending_obligation.stalled_on);
 358
 359                         ProcessResult::Unchanged
 360                     }
 361                     Err(selection_err) => {
 362                         info!("selecting trait `{:?}` at depth {} yielded Err",
 363                               data, obligation.recursion_depth);
 364
 365                         ProcessResult::Error(CodeSelectionError(selection_err))
 366                     }
 367                 }
 368             }
 369
 370             ty::Predicate::RegionOutlives(ref binder) => {
 371                 match self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
 372                     Ok(()) => ProcessResult::Changed(vec![]),
 373                     Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 374                 }
 375             }
 376
 377             ty::Predicate::TypeOutlives(ref binder) => {
 378                 // Check if there are higher-ranked vars.
 379                 match binder.no_bound_vars() {
 380                     // If there are, inspect the underlying type further.
 381                     None => {
 382                         // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 383                         let binder = binder.map_bound_ref(|pred| pred.0);
 384
 385                         // Check if the type has any bound vars.
 386                         match binder.no_bound_vars() {
 387                             // If so, this obligation is an error (for now). Eventually we should be
 388                             // able to support additional cases here, like `for<'a> &'a str: 'a`.
 389                             // NOTE: this is duplicate-implemented between here and fulfillment.
 390                             None => {
 391                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 392                             }
 393                             // Otherwise, we have something of the form
 394                             // `for<'a> T: 'a where 'a not in T`, which we can treat as
 395                             // `T: 'static`.
 396                             Some(t_a) => {
 397                                 let r_static = self.selcx.tcx().lifetimes.re_static;
 398                                 if self.register_region_obligations {
 399                                     self.selcx.infcx().register_region_obligation_with_cause(
 400                                         t_a,
 401                                         r_static,
 402                                         &obligation.cause,
 403                                     );
 404                                 }
 405                                 ProcessResult::Changed(vec![])
 406                             }
 407                         }
 408                     }
 409                     // If there aren't, register the obligation.
 410                     Some(ty::OutlivesPredicate(t_a, r_b)) => {
 411                         if self.register_region_obligations {
 412                             self.selcx.infcx().register_region_obligation_with_cause(
 413                                 t_a,
 414                                 r_b,
 415                                 &obligation.cause,
 416                             );
 417                         }
 418                         ProcessResult::Changed(vec![])
 419                     }
 420                 }
 421             }
 422
 423             ty::Predicate::Projection(ref data) => {
 424                 let project_obligation = obligation.with(data.clone());
 425                 match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 426                     Ok(None) => {
 427                         let tcx = self.selcx.tcx();
 428                         pending_obligation.stalled_on =
 429                             trait_ref_type_vars(self.selcx, data.to_poly_trait_ref(tcx));
 430                         ProcessResult::Unchanged
 431                     }
 432                     Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
 433                     Err(e) => ProcessResult::Error(CodeProjectionError(e))
 434                 }
 435             }
 436
 437             ty::Predicate::ObjectSafe(trait_def_id) => {
 438                 if !self.selcx.tcx().is_object_safe(trait_def_id) {
 439                     ProcessResult::Error(CodeSelectionError(Unimplemented))
 440                 } else {
 441                     ProcessResult::Changed(vec![])
 442                 }
 443             }
 444
 445             ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 446                 match self.selcx.infcx().closure_kind(closure_def_id, closure_substs) {
 447                     Some(closure_kind) => {
 448                         if closure_kind.extends(kind) {
 449                             ProcessResult::Changed(vec![])
 450                         } else {
 451                             ProcessResult::Error(CodeSelectionError(Unimplemented))
 452                         }
 453                     }
 454                     None => {
 455                         ProcessResult::Unchanged
 456                     }
 457                 }
 458             }
 459
 460             ty::Predicate::WellFormed(ty) => {
 461                 match ty::wf::obligations(
 462                     self.selcx.infcx(),
 463                     obligation.param_env,
 464                     obligation.cause.body_id,
 465                     ty,
 466                     obligation.cause.span,
 467                 ) {
 468                     None => {
 469                         pending_obligation.stalled_on = vec![infer_ty(ty)];
 470                         ProcessResult::Unchanged
 471                     }
 472                     Some(os) => ProcessResult::Changed(mk_pending(os))
 473                 }
 474             }
 475
 476             ty::Predicate::Subtype(ref subtype) => {
 477                 match self.selcx.infcx().subtype_predicate(&obligation.cause,
 478                                                            obligation.param_env,
 479                                                            subtype) {
 480                     None => {
 481                         // None means that both are unresolved.
 482                         pending_obligation.stalled_on = vec![infer_ty(subtype.skip_binder().a),
 483                                                              infer_ty(subtype.skip_binder().b)];
 484                         ProcessResult::Unchanged
 485                     }
 486                     Some(Ok(ok)) => {
 487                         ProcessResult::Changed(mk_pending(ok.obligations))
 488                     }
 489                     Some(Err(err)) => {
 490                         let expected_found = ExpectedFound::new(subtype.skip_binder().a_is_expected,
 491                                                                 subtype.skip_binder().a,
 492                                                                 subtype.skip_binder().b);
 493                         ProcessResult::Error(
 494                             FulfillmentErrorCode::CodeSubtypeError(expected_found, err))
 495                     }
 496                 }
 497             }
 498
 499             ty::Predicate::ConstEvaluatable(def_id, substs) => {
 500                 if obligation.param_env.has_local_value() {
 501                         ProcessResult::Unchanged
 502                 } else {
 503                     if !substs.has_local_value() {
 504                         let instance = ty::Instance::resolve(
 505                             self.selcx.tcx(),
 506                             obligation.param_env,
 507                             def_id,
 508                             substs,
 509                         );
 510                         if let Some(instance) = instance {
 511                             let cid = GlobalId {
 512                                 instance,
 513                                 promoted: None,
 514                             };
 515                             match self.selcx.tcx().at(obligation.cause.span)
 516                                                     .const_eval(obligation.param_env.and(cid)) {
 517                                 Ok(_) => ProcessResult::Changed(vec![]),
 518                                 Err(err) => ProcessResult::Error(
 519                                     CodeSelectionError(ConstEvalFailure(err)))
 520                             }
 521                         } else {
 522                             ProcessResult::Error(CodeSelectionError(
 523                                 ConstEvalFailure(ErrorHandled::TooGeneric)
 524                             ))
 525                         }
 526                     } else {
 527                         pending_obligation.stalled_on =
 528                             substs.types().map(|ty| infer_ty(ty)).collect();
 529                         ProcessResult::Unchanged
 530                     }
 531                 }
 532             }
 533         }
 534     }
 535
 536     fn process_backedge<'c, I>(&mut self, cycle: I,
 537                                _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>)
 538         where I: Clone + Iterator<Item=&'c PendingPredicateObligation<'tcx>>,
 539     {
 540         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 541             debug!("process_child_obligations: coinductive match");
 542         } else {
 543             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 544             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 545         }
 546     }
 547 }
 548
 549 /// Returns the set of type variables contained in a trait ref
 550 fn trait_ref_type_vars<'a, 'tcx>(
 551     selcx: &mut SelectionContext<'a, 'tcx>,
 552     t: ty::PolyTraitRef<'tcx>,
 553 ) -> Vec<ty::InferTy> {
 554     t.skip_binder() // ok b/c this check doesn't care about regions
 555      .input_types()
 556      .map(|t| selcx.infcx().resolve_vars_if_possible(&t))
 557      .filter(|t| t.has_infer_types())
 558      .flat_map(|t| t.walk())
 559      .filter_map(|t| match t.kind { ty::Infer(infer) => Some(infer), _ => None })
 560      .collect()
 561 }
 562
 563 fn to_fulfillment_error<'tcx>(
 564     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>)
 565     -> FulfillmentError<'tcx>
 566 {
 567     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 568     FulfillmentError::new(obligation, error.error)
 569 }