src/librustc/traits/fulfill.rs

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