src/librustc/traits/fulfill.rs

   1 use crate::infer::{InferCtxt, ShallowResolver};
   2 use crate::ty::error::ExpectedFound;
   3 use crate::ty::{self, ToPolyTraitRef, Ty, TypeFoldable};
   4 use rustc_data_structures::obligation_forest::ProcessResult;
   5 use rustc_data_structures::obligation_forest::{DoCompleted, Error, ForestObligation};
   6 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
   7 use std::marker::PhantomData;
   8
   9 use super::engine::{TraitEngine, TraitEngineExt};
  10 use super::project;
  11 use super::select::SelectionContext;
  12 use super::wf;
  13 use super::CodeAmbiguity;
  14 use super::CodeProjectionError;
  15 use super::CodeSelectionError;
  16 use super::{ConstEvalFailure, Unimplemented};
  17 use super::{FulfillmentError, FulfillmentErrorCode};
  18 use super::{ObligationCause, PredicateObligation};
  19
  20 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  21     type Predicate = ty::Predicate<'tcx>;
  22
  23     fn as_predicate(&self) -> &Self::Predicate {
  24         &self.obligation.predicate
  25     }
  26 }
  27
  28 /// The fulfillment context is used to drive trait resolution. It
  29 /// consists of a list of obligations that must be (eventually)
  30 /// satisfied. The job is to track which are satisfied, which yielded
  31 /// errors, and which are still pending. At any point, users can call
  32 /// `select_where_possible`, and the fulfillment context will try to do
  33 /// selection, retaining only those obligations that remain
  34 /// ambiguous. This may be helpful in pushing type inference
  35 /// along. Once all type inference constraints have been generated, the
  36 /// method `select_all_or_error` can be used to report any remaining
  37 /// ambiguous cases as errors.
  38 pub struct FulfillmentContext<'tcx> {
  39     // A list of all obligations that have been registered with this
  40     // fulfillment context.
  41     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  42     // Should this fulfillment context register type-lives-for-region
  43     // obligations on its parent infcx? In some cases, region
  44     // obligations are either already known to hold (normalization) or
  45     // hopefully verifed elsewhere (type-impls-bound), and therefore
  46     // should not be checked.
  47     //
  48     // Note that if we are normalizing a type that we already
  49     // know is well-formed, there should be no harm setting this
  50     // to true - all the region variables should be determinable
  51     // using the RFC 447 rules, which don't depend on
  52     // type-lives-for-region constraints, and because the type
  53     // is well-formed, the constraints should hold.
  54     register_region_obligations: bool,
  55     // Is it OK to register obligations into this infcx inside
  56     // an infcx snapshot?
  57     //
  58     // The "primary fulfillment" in many cases in typeck lives
  59     // outside of any snapshot, so any use of it inside a snapshot
  60     // will lead to trouble and therefore is checked against, but
  61     // other fulfillment contexts sometimes do live inside of
  62     // a snapshot (they don't *straddle* a snapshot, so there
  63     // is no trouble there).
  64     usable_in_snapshot: bool,
  65 }
  66
  67 #[derive(Clone, Debug)]
  68 pub struct PendingPredicateObligation<'tcx> {
  69     pub obligation: PredicateObligation<'tcx>,
  70     pub stalled_on: Vec<ty::InferTy>,
  71 }
  72
  73 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  74 #[cfg(target_arch = "x86_64")]
  75 static_assert_size!(PendingPredicateObligation<'_>, 136);
  76
  77 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  78     /// Creates a new fulfillment context.
  79     pub fn new() -> FulfillmentContext<'tcx> {
  80         FulfillmentContext {
  81             predicates: ObligationForest::new(),
  82             register_region_obligations: true,
  83             usable_in_snapshot: false,
  84         }
  85     }
  86
  87     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
  88         FulfillmentContext {
  89             predicates: ObligationForest::new(),
  90             register_region_obligations: true,
  91             usable_in_snapshot: true,
  92         }
  93     }
  94
  95     pub fn new_ignoring_regions() -> FulfillmentContext<'tcx> {
  96         FulfillmentContext {
  97             predicates: ObligationForest::new(),
  98             register_region_obligations: false,
  99             usable_in_snapshot: false,
 100         }
 101     }
 102
 103     /// Attempts to select obligations using `selcx`.
 104     fn select(
 105         &mut self,
 106         selcx: &mut SelectionContext<'a, 'tcx>,
 107     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 108         debug!("select(obligation-forest-size={})", self.predicates.len());
 109
 110         let mut errors = Vec::new();
 111
 112         loop {
 113             debug!("select: starting another iteration");
 114
 115             // Process pending obligations.
 116             let outcome = self.predicates.process_obligations(
 117                 &mut FulfillProcessor {
 118                     selcx,
 119                     register_region_obligations: self.register_region_obligations,
 120                 },
 121                 DoCompleted::No,
 122             );
 123             debug!("select: outcome={:#?}", outcome);
 124
 125             // FIXME: if we kept the original cache key, we could mark projection
 126             // obligations as complete for the projection cache here.
 127
 128             errors.extend(outcome.errors.into_iter().map(|e| to_fulfillment_error(e)));
 129
 130             // If nothing new was added, no need to keep looping.
 131             if outcome.stalled {
 132                 break;
 133             }
 134         }
 135
 136         debug!(
 137             "select({} predicates remaining, {} errors) done",
 138             self.predicates.len(),
 139             errors.len()
 140         );
 141
 142         if errors.is_empty() { Ok(()) } else { Err(errors) }
 143     }
 144 }
 145
 146 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 147     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 148     /// creating a fresh type variable `$0` as well as a projection
 149     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 150     /// inference engine runs, it will attempt to find an impl of
 151     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 152     /// something concrete. If this fails, we'll unify `$0` with
 153     /// `projection_ty` again.
 154     fn normalize_projection_type(
 155         &mut self,
 156         infcx: &InferCtxt<'_, 'tcx>,
 157         param_env: ty::ParamEnv<'tcx>,
 158         projection_ty: ty::ProjectionTy<'tcx>,
 159         cause: ObligationCause<'tcx>,
 160     ) -> Ty<'tcx> {
 161         debug!("normalize_projection_type(projection_ty={:?})", projection_ty);
 162
 163         debug_assert!(!projection_ty.has_escaping_bound_vars());
 164
 165         // FIXME(#20304) -- cache
 166
 167         let mut selcx = SelectionContext::new(infcx);
 168         let mut obligations = vec![];
 169         let normalized_ty = project::normalize_projection_type(
 170             &mut selcx,
 171             param_env,
 172             projection_ty,
 173             cause,
 174             0,
 175             &mut obligations,
 176         );
 177         self.register_predicate_obligations(infcx, obligations);
 178
 179         debug!("normalize_projection_type: result={:?}", normalized_ty);
 180
 181         normalized_ty
 182     }
 183
 184     fn register_predicate_obligation(
 185         &mut self,
 186         infcx: &InferCtxt<'_, 'tcx>,
 187         obligation: PredicateObligation<'tcx>,
 188     ) {
 189         // this helps to reduce duplicate errors, as well as making
 190         // debug output much nicer to read and so on.
 191         let obligation = infcx.resolve_vars_if_possible(&obligation);
 192
 193         debug!("register_predicate_obligation(obligation={:?})", obligation);
 194
 195         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 196
 197         self.predicates
 198             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 199     }
 200
 201     fn select_all_or_error(
 202         &mut self,
 203         infcx: &InferCtxt<'_, 'tcx>,
 204     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 205         self.select_where_possible(infcx)?;
 206
 207         let errors: Vec<_> = self
 208             .predicates
 209             .to_errors(CodeAmbiguity)
 210             .into_iter()
 211             .map(|e| to_fulfillment_error(e))
 212             .collect();
 213         if errors.is_empty() { Ok(()) } else { Err(errors) }
 214     }
 215
 216     fn select_where_possible(
 217         &mut self,
 218         infcx: &InferCtxt<'_, 'tcx>,
 219     ) -> Result<(), Vec<FulfillmentError<'tcx>>> {
 220         let mut selcx = SelectionContext::new(infcx);
 221         self.select(&mut selcx)
 222     }
 223
 224     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 225         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 226     }
 227 }
 228
 229 struct FulfillProcessor<'a, 'b, 'tcx> {
 230     selcx: &'a mut SelectionContext<'b, 'tcx>,
 231     register_region_obligations: bool,
 232 }
 233
 234 fn mk_pending(os: Vec<PredicateObligation<'tcx>>) -> Vec<PendingPredicateObligation<'tcx>> {
 235     os.into_iter()
 236         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 237         .collect()
 238 }
 239
 240 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 241     type Obligation = PendingPredicateObligation<'tcx>;
 242     type Error = FulfillmentErrorCode<'tcx>;
 243
 244     /// Processes a predicate obligation and returns either:
 245     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 246     /// - `Unchanged` if we don't have enough info to be sure
 247     /// - `Error(e)` if the predicate does not hold
 248     ///
 249     /// This is always inlined, despite its size, because it has a single
 250     /// callsite and it is called *very* frequently.
 251     #[inline(always)]
 252     fn process_obligation(
 253         &mut self,
 254         pending_obligation: &mut Self::Obligation,
 255     ) -> ProcessResult<Self::Obligation, Self::Error> {
 256         // If we were stalled on some unresolved variables, first check whether
 257         // any of them have been resolved; if not, don't bother doing more work
 258         // yet.
 259         let change = match pending_obligation.stalled_on.len() {
 260             // Match arms are in order of frequency, which matters because this
 261             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 262             1 => {
 263                 let infer = pending_obligation.stalled_on[0];
 264                 ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer)
 265             }
 266             0 => {
 267                 // In this case we haven't changed, but wish to make a change.
 268                 true
 269             }
 270             _ => {
 271                 // This `for` loop was once a call to `all()`, but this lower-level
 272                 // form was a perf win. See #64545 for details.
 273                 (|| {
 274                     for &infer in &pending_obligation.stalled_on {
 275                         if ShallowResolver::new(self.selcx.infcx()).shallow_resolve_changed(infer) {
 276                             return true;
 277                         }
 278                     }
 279                     false
 280                 })()
 281             }
 282         };
 283
 284         if !change {
 285             debug!(
 286                 "process_predicate: pending obligation {:?} still stalled on {:?}",
 287                 self.selcx.infcx().resolve_vars_if_possible(&pending_obligation.obligation),
 288                 pending_obligation.stalled_on
 289             );
 290             return ProcessResult::Unchanged;
 291         }
 292
 293         // This part of the code is much colder.
 294
 295         pending_obligation.stalled_on.truncate(0);
 296
 297         let obligation = &mut pending_obligation.obligation;
 298
 299         if obligation.predicate.has_infer_types() {
 300             obligation.predicate =
 301                 self.selcx.infcx().resolve_vars_if_possible(&obligation.predicate);
 302         }
 303
 304         debug!("process_obligation: obligation = {:?} cause = {:?}", obligation, obligation.cause);
 305
 306         fn infer_ty(ty: Ty<'tcx>) -> ty::InferTy {
 307             match ty.kind {
 308                 ty::Infer(infer) => infer,
 309                 _ => panic!(),
 310             }
 311         }
 312
 313         match obligation.predicate {
 314             ty::Predicate::Trait(ref data) => {
 315                 let trait_obligation = obligation.with(data.clone());
 316
 317                 if data.is_global() {
 318                     // no type variables present, can use evaluation for better caching.
 319                     // FIXME: consider caching errors too.
 320                     if self.selcx.infcx().predicate_must_hold_considering_regions(&obligation) {
 321                         debug!(
 322                             "selecting trait `{:?}` at depth {} evaluated to holds",
 323                             data, obligation.recursion_depth
 324                         );
 325                         return ProcessResult::Changed(vec![]);
 326                     }
 327                 }
 328
 329                 match self.selcx.select(&trait_obligation) {
 330                     Ok(Some(vtable)) => {
 331                         debug!(
 332                             "selecting trait `{:?}` at depth {} yielded Ok(Some)",
 333                             data, obligation.recursion_depth
 334                         );
 335                         ProcessResult::Changed(mk_pending(vtable.nested_obligations()))
 336                     }
 337                     Ok(None) => {
 338                         debug!(
 339                             "selecting trait `{:?}` at depth {} yielded Ok(None)",
 340                             data, obligation.recursion_depth
 341                         );
 342
 343                         // This is a bit subtle: for the most part, the
 344                         // only reason we can fail to make progress on
 345                         // trait selection is because we don't have enough
 346                         // information about the types in the trait. One
 347                         // exception is that we sometimes haven't decided
 348                         // what kind of closure a closure is. *But*, in
 349                         // that case, it turns out, the type of the
 350                         // closure will also change, because the closure
 351                         // also includes references to its upvars as part
 352                         // of its type, and those types are resolved at
 353                         // the same time.
 354                         //
 355                         // FIXME(#32286) logic seems false if no upvars
 356                         pending_obligation.stalled_on =
 357                             trait_ref_type_vars(self.selcx, data.to_poly_trait_ref());
 358
 359                         debug!(
 360                             "process_predicate: pending obligation {:?} now stalled on {:?}",
 361                             self.selcx.infcx().resolve_vars_if_possible(obligation),
 362                             pending_obligation.stalled_on
 363                         );
 364
 365                         ProcessResult::Unchanged
 366                     }
 367                     Err(selection_err) => {
 368                         info!(
 369                             "selecting trait `{:?}` at depth {} yielded Err",
 370                             data, obligation.recursion_depth
 371                         );
 372
 373                         ProcessResult::Error(CodeSelectionError(selection_err))
 374                     }
 375                 }
 376             }
 377
 378             ty::Predicate::RegionOutlives(ref binder) => {
 379                 match self.selcx.infcx().region_outlives_predicate(&obligation.cause, binder) {
 380                     Ok(()) => ProcessResult::Changed(vec![]),
 381                     Err(_) => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 382                 }
 383             }
 384
 385             ty::Predicate::TypeOutlives(ref binder) => {
 386                 // Check if there are higher-ranked vars.
 387                 match binder.no_bound_vars() {
 388                     // If there are, inspect the underlying type further.
 389                     None => {
 390                         // Convert from `Binder<OutlivesPredicate<Ty, Region>>` to `Binder<Ty>`.
 391                         let binder = binder.map_bound_ref(|pred| pred.0);
 392
 393                         // Check if the type has any bound vars.
 394                         match binder.no_bound_vars() {
 395                             // If so, this obligation is an error (for now). Eventually we should be
 396                             // able to support additional cases here, like `for<'a> &'a str: 'a`.
 397                             // NOTE: this is duplicate-implemented between here and fulfillment.
 398                             None => ProcessResult::Error(CodeSelectionError(Unimplemented)),
 399                             // Otherwise, we have something of the form
 400                             // `for<'a> T: 'a where 'a not in T`, which we can treat as
 401                             // `T: 'static`.
 402                             Some(t_a) => {
 403                                 let r_static = self.selcx.tcx().lifetimes.re_static;
 404                                 if self.register_region_obligations {
 405                                     self.selcx.infcx().register_region_obligation_with_cause(
 406                                         t_a,
 407                                         r_static,
 408                                         &obligation.cause,
 409                                     );
 410                                 }
 411                                 ProcessResult::Changed(vec![])
 412                             }
 413                         }
 414                     }
 415                     // If there aren't, register the obligation.
 416                     Some(ty::OutlivesPredicate(t_a, r_b)) => {
 417                         if self.register_region_obligations {
 418                             self.selcx.infcx().register_region_obligation_with_cause(
 419                                 t_a,
 420                                 r_b,
 421                                 &obligation.cause,
 422                             );
 423                         }
 424                         ProcessResult::Changed(vec![])
 425                     }
 426                 }
 427             }
 428
 429             ty::Predicate::Projection(ref data) => {
 430                 let project_obligation = obligation.with(data.clone());
 431                 match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 432                     Ok(None) => {
 433                         let tcx = self.selcx.tcx();
 434                         pending_obligation.stalled_on =
 435                             trait_ref_type_vars(self.selcx, data.to_poly_trait_ref(tcx));
 436                         ProcessResult::Unchanged
 437                     }
 438                     Ok(Some(os)) => ProcessResult::Changed(mk_pending(os)),
 439                     Err(e) => ProcessResult::Error(CodeProjectionError(e)),
 440                 }
 441             }
 442
 443             ty::Predicate::ObjectSafe(trait_def_id) => {
 444                 if !self.selcx.tcx().is_object_safe(trait_def_id) {
 445                     ProcessResult::Error(CodeSelectionError(Unimplemented))
 446                 } else {
 447                     ProcessResult::Changed(vec![])
 448                 }
 449             }
 450
 451             ty::Predicate::ClosureKind(closure_def_id, closure_substs, kind) => {
 452                 match self.selcx.infcx().closure_kind(closure_def_id, closure_substs) {
 453                     Some(closure_kind) => {
 454                         if closure_kind.extends(kind) {
 455                             ProcessResult::Changed(vec![])
 456                         } else {
 457                             ProcessResult::Error(CodeSelectionError(Unimplemented))
 458                         }
 459                     }
 460                     None => ProcessResult::Unchanged,
 461                 }
 462             }
 463
 464             ty::Predicate::WellFormed(ty) => {
 465                 match wf::obligations(
 466                     self.selcx.infcx(),
 467                     obligation.param_env,
 468                     obligation.cause.body_id,
 469                     ty,
 470                     obligation.cause.span,
 471                 ) {
 472                     None => {
 473                         pending_obligation.stalled_on = vec![infer_ty(ty)];
 474                         ProcessResult::Unchanged
 475                     }
 476                     Some(os) => ProcessResult::Changed(mk_pending(os)),
 477                 }
 478             }
 479
 480             ty::Predicate::Subtype(ref subtype) => {
 481                 match self.selcx.infcx().subtype_predicate(
 482                     &obligation.cause,
 483                     obligation.param_env,
 484                     subtype,
 485                 ) {
 486                     None => {
 487                         // None means that both are unresolved.
 488                         pending_obligation.stalled_on = vec![
 489                             infer_ty(subtype.skip_binder().a),
 490                             infer_ty(subtype.skip_binder().b),
 491                         ];
 492                         ProcessResult::Unchanged
 493                     }
 494                     Some(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 495                     Some(Err(err)) => {
 496                         let expected_found = ExpectedFound::new(
 497                             subtype.skip_binder().a_is_expected,
 498                             subtype.skip_binder().a,
 499                             subtype.skip_binder().b,
 500                         );
 501                         ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 502                             expected_found,
 503                             err,
 504                         ))
 505                     }
 506                 }
 507             }
 508
 509             ty::Predicate::ConstEvaluatable(def_id, substs) => {
 510                 if obligation.param_env.has_local_value() {
 511                     ProcessResult::Unchanged
 512                 } else {
 513                     if !substs.has_local_value() {
 514                         match self.selcx.tcx().const_eval_resolve(
 515                             obligation.param_env,
 516                             def_id,
 517                             substs,
 518                             Some(obligation.cause.span),
 519                         ) {
 520                             Ok(_) => ProcessResult::Changed(vec![]),
 521                             Err(err) => {
 522                                 ProcessResult::Error(CodeSelectionError(ConstEvalFailure(err)))
 523                             }
 524                         }
 525                     } else {
 526                         pending_obligation.stalled_on =
 527                             substs.types().map(|ty| infer_ty(ty)).collect();
 528                         ProcessResult::Unchanged
 529                     }
 530                 }
 531             }
 532         }
 533     }
 534
 535     fn process_backedge<'c, I>(
 536         &mut self,
 537         cycle: I,
 538         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 539     ) where
 540         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 541     {
 542         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 543             debug!("process_child_obligations: coinductive match");
 544         } else {
 545             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 546             self.selcx.infcx().report_overflow_error_cycle(&cycle);
 547         }
 548     }
 549 }
 550
 551 /// Returns the set of type variables contained in a trait ref
 552 fn trait_ref_type_vars<'a, 'tcx>(
 553     selcx: &mut SelectionContext<'a, 'tcx>,
 554     t: ty::PolyTraitRef<'tcx>,
 555 ) -> Vec<ty::InferTy> {
 556     t.skip_binder() // ok b/c this check doesn't care about regions
 557         .input_types()
 558         .map(|t| selcx.infcx().resolve_vars_if_possible(&t))
 559         .filter(|t| t.has_infer_types())
 560         .flat_map(|t| t.walk())
 561         .filter_map(|t| match t.kind {
 562             ty::Infer(infer) => Some(infer),
 563             _ => None,
 564         })
 565         .collect()
 566 }
 567
 568 fn to_fulfillment_error<'tcx>(
 569     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 570 ) -> FulfillmentError<'tcx> {
 571     let obligation = error.backtrace.into_iter().next().unwrap().obligation;
 572     FulfillmentError::new(obligation, error.error)
 573 }