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