compiler/rustc_trait_selection/src/traits/fulfill.rs

   1 use crate::infer::{InferCtxt, TyOrConstInferVar};
   2 use rustc_data_structures::fx::FxHashMap;
   3 use rustc_data_structures::obligation_forest::ProcessResult;
   4 use rustc_data_structures::obligation_forest::{Error, ForestObligation, Outcome};
   5 use rustc_data_structures::obligation_forest::{ObligationForest, ObligationProcessor};
   6 use rustc_infer::traits::ProjectionCacheKey;
   7 use rustc_infer::traits::{SelectionError, TraitEngine, TraitEngineExt as _, TraitObligation};
   8 use rustc_middle::mir::interpret::ErrorHandled;
   9 use rustc_middle::ty::abstract_const::NotConstEvaluatable;
  10 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  11 use rustc_middle::ty::subst::SubstsRef;
  12 use rustc_middle::ty::ToPredicate;
  13 use rustc_middle::ty::{self, Binder, Const, Ty, TypeVisitable};
  14 use std::marker::PhantomData;
  15
  16 use super::const_evaluatable;
  17 use super::project::{self, ProjectAndUnifyResult};
  18 use super::select::SelectionContext;
  19 use super::wf;
  20 use super::CodeAmbiguity;
  21 use super::CodeProjectionError;
  22 use super::CodeSelectionError;
  23 use super::EvaluationResult;
  24 use super::Unimplemented;
  25 use super::{FulfillmentError, FulfillmentErrorCode};
  26 use super::{ObligationCause, PredicateObligation};
  27
  28 use crate::traits::project::PolyProjectionObligation;
  29 use crate::traits::project::ProjectionCacheKeyExt as _;
  30 use crate::traits::query::evaluate_obligation::InferCtxtExt;
  31
  32 impl<'tcx> ForestObligation for PendingPredicateObligation<'tcx> {
  33     /// Note that we include both the `ParamEnv` and the `Predicate`,
  34     /// as the `ParamEnv` can influence whether fulfillment succeeds
  35     /// or fails.
  36     type CacheKey = ty::ParamEnvAnd<'tcx, ty::Predicate<'tcx>>;
  37
  38     fn as_cache_key(&self) -> Self::CacheKey {
  39         self.obligation.param_env.and(self.obligation.predicate)
  40     }
  41 }
  42
  43 /// The fulfillment context is used to drive trait resolution. It
  44 /// consists of a list of obligations that must be (eventually)
  45 /// satisfied. The job is to track which are satisfied, which yielded
  46 /// errors, and which are still pending. At any point, users can call
  47 /// `select_where_possible`, and the fulfillment context will try to do
  48 /// selection, retaining only those obligations that remain
  49 /// ambiguous. This may be helpful in pushing type inference
  50 /// along. Once all type inference constraints have been generated, the
  51 /// method `select_all_or_error` can be used to report any remaining
  52 /// ambiguous cases as errors.
  53 pub struct FulfillmentContext<'tcx> {
  54     // A list of all obligations that have been registered with this
  55     // fulfillment context.
  56     predicates: ObligationForest<PendingPredicateObligation<'tcx>>,
  57
  58     relationships: FxHashMap<ty::TyVid, ty::FoundRelationships>,
  59
  60     // Is it OK to register obligations into this infcx inside
  61     // an infcx snapshot?
  62     //
  63     // The "primary fulfillment" in many cases in typeck lives
  64     // outside of any snapshot, so any use of it inside a snapshot
  65     // will lead to trouble and therefore is checked against, but
  66     // other fulfillment contexts sometimes do live inside of
  67     // a snapshot (they don't *straddle* a snapshot, so there
  68     // is no trouble there).
  69     usable_in_snapshot: bool,
  70 }
  71
  72 #[derive(Clone, Debug)]
  73 pub struct PendingPredicateObligation<'tcx> {
  74     pub obligation: PredicateObligation<'tcx>,
  75     // This is far more often read than modified, meaning that we
  76     // should mostly optimize for reading speed, while modifying is not as relevant.
  77     //
  78     // For whatever reason using a boxed slice is slower than using a `Vec` here.
  79     pub stalled_on: Vec<TyOrConstInferVar<'tcx>>,
  80 }
  81
  82 // `PendingPredicateObligation` is used a lot. Make sure it doesn't unintentionally get bigger.
  83 #[cfg(all(target_arch = "x86_64", target_pointer_width = "64"))]
  84 static_assert_size!(PendingPredicateObligation<'_>, 72);
  85
  86 impl<'a, 'tcx> FulfillmentContext<'tcx> {
  87     /// Creates a new fulfillment context.
  88     pub fn new() -> FulfillmentContext<'tcx> {
  89         FulfillmentContext {
  90             predicates: ObligationForest::new(),
  91             relationships: FxHashMap::default(),
  92             usable_in_snapshot: false,
  93         }
  94     }
  95
  96     pub fn new_in_snapshot() -> FulfillmentContext<'tcx> {
  97         FulfillmentContext {
  98             predicates: ObligationForest::new(),
  99             relationships: FxHashMap::default(),
 100             usable_in_snapshot: true,
 101         }
 102     }
 103
 104     /// Attempts to select obligations using `selcx`.
 105     fn select(&mut self, selcx: &mut SelectionContext<'a, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 106         let span = debug_span!("select", obligation_forest_size = ?self.predicates.len());
 107         let _enter = span.enter();
 108
 109         // Process pending obligations.
 110         let outcome: Outcome<_, _> =
 111             self.predicates.process_obligations(&mut FulfillProcessor { selcx });
 112
 113         // FIXME: if we kept the original cache key, we could mark projection
 114         // obligations as complete for the projection cache here.
 115
 116         let errors: Vec<FulfillmentError<'tcx>> =
 117             outcome.errors.into_iter().map(to_fulfillment_error).collect();
 118
 119         debug!(
 120             "select({} predicates remaining, {} errors) done",
 121             self.predicates.len(),
 122             errors.len()
 123         );
 124
 125         errors
 126     }
 127 }
 128
 129 impl<'tcx> TraitEngine<'tcx> for FulfillmentContext<'tcx> {
 130     /// "Normalize" a projection type `<SomeType as SomeTrait>::X` by
 131     /// creating a fresh type variable `$0` as well as a projection
 132     /// predicate `<SomeType as SomeTrait>::X == $0`. When the
 133     /// inference engine runs, it will attempt to find an impl of
 134     /// `SomeTrait` or a where-clause that lets us unify `$0` with
 135     /// something concrete. If this fails, we'll unify `$0` with
 136     /// `projection_ty` again.
 137     #[instrument(level = "debug", skip(self, infcx, param_env, cause))]
 138     fn normalize_projection_type(
 139         &mut self,
 140         infcx: &InferCtxt<'_, 'tcx>,
 141         param_env: ty::ParamEnv<'tcx>,
 142         projection_ty: ty::ProjectionTy<'tcx>,
 143         cause: ObligationCause<'tcx>,
 144     ) -> Ty<'tcx> {
 145         debug_assert!(!projection_ty.has_escaping_bound_vars());
 146
 147         // FIXME(#20304) -- cache
 148
 149         let mut selcx = SelectionContext::new(infcx);
 150         let mut obligations = vec![];
 151         let normalized_ty = project::normalize_projection_type(
 152             &mut selcx,
 153             param_env,
 154             projection_ty,
 155             cause,
 156             0,
 157             &mut obligations,
 158         );
 159         self.register_predicate_obligations(infcx, obligations);
 160
 161         debug!(?normalized_ty);
 162
 163         normalized_ty.ty().unwrap()
 164     }
 165
 166     fn register_predicate_obligation(
 167         &mut self,
 168         infcx: &InferCtxt<'_, 'tcx>,
 169         obligation: PredicateObligation<'tcx>,
 170     ) {
 171         // this helps to reduce duplicate errors, as well as making
 172         // debug output much nicer to read and so on.
 173         let obligation = infcx.resolve_vars_if_possible(obligation);
 174
 175         debug!(?obligation, "register_predicate_obligation");
 176
 177         assert!(!infcx.is_in_snapshot() || self.usable_in_snapshot);
 178
 179         super::relationships::update(self, infcx, &obligation);
 180
 181         self.predicates
 182             .register_obligation(PendingPredicateObligation { obligation, stalled_on: vec![] });
 183     }
 184
 185     fn select_all_or_error(&mut self, infcx: &InferCtxt<'_, 'tcx>) -> Vec<FulfillmentError<'tcx>> {
 186         {
 187             let errors = self.select_where_possible(infcx);
 188             if !errors.is_empty() {
 189                 return errors;
 190             }
 191         }
 192
 193         self.predicates.to_errors(CodeAmbiguity).into_iter().map(to_fulfillment_error).collect()
 194     }
 195
 196     fn select_where_possible(
 197         &mut self,
 198         infcx: &InferCtxt<'_, 'tcx>,
 199     ) -> Vec<FulfillmentError<'tcx>> {
 200         let mut selcx = SelectionContext::new(infcx);
 201         self.select(&mut selcx)
 202     }
 203
 204     fn pending_obligations(&self) -> Vec<PredicateObligation<'tcx>> {
 205         self.predicates.map_pending_obligations(|o| o.obligation.clone())
 206     }
 207
 208     fn relationships(&mut self) -> &mut FxHashMap<ty::TyVid, ty::FoundRelationships> {
 209         &mut self.relationships
 210     }
 211 }
 212
 213 struct FulfillProcessor<'a, 'b, 'tcx> {
 214     selcx: &'a mut SelectionContext<'b, 'tcx>,
 215 }
 216
 217 fn mk_pending(os: Vec<PredicateObligation<'_>>) -> Vec<PendingPredicateObligation<'_>> {
 218     os.into_iter()
 219         .map(|o| PendingPredicateObligation { obligation: o, stalled_on: vec![] })
 220         .collect()
 221 }
 222
 223 impl<'a, 'b, 'tcx> ObligationProcessor for FulfillProcessor<'a, 'b, 'tcx> {
 224     type Obligation = PendingPredicateObligation<'tcx>;
 225     type Error = FulfillmentErrorCode<'tcx>;
 226     type OUT = Outcome<Self::Obligation, Self::Error>;
 227
 228     /// Identifies whether a predicate obligation needs processing.
 229     ///
 230     /// This is always inlined, despite its size, because it has a single
 231     /// callsite and it is called *very* frequently.
 232     #[inline(always)]
 233     fn needs_process_obligation(&self, pending_obligation: &Self::Obligation) -> bool {
 234         // If we were stalled on some unresolved variables, first check whether
 235         // any of them have been resolved; if not, don't bother doing more work
 236         // yet.
 237         match pending_obligation.stalled_on.len() {
 238             // Match arms are in order of frequency, which matters because this
 239             // code is so hot. 1 and 0 dominate; 2+ is fairly rare.
 240             1 => {
 241                 let infer_var = pending_obligation.stalled_on[0];
 242                 self.selcx.infcx().ty_or_const_infer_var_changed(infer_var)
 243             }
 244             0 => {
 245                 // In this case we haven't changed, but wish to make a change.
 246                 true
 247             }
 248             _ => {
 249                 // This `for` loop was once a call to `all()`, but this lower-level
 250                 // form was a perf win. See #64545 for details.
 251                 (|| {
 252                     for &infer_var in &pending_obligation.stalled_on {
 253                         if self.selcx.infcx().ty_or_const_infer_var_changed(infer_var) {
 254                             return true;
 255                         }
 256                     }
 257                     false
 258                 })()
 259             }
 260         }
 261     }
 262
 263     /// Processes a predicate obligation and returns either:
 264     /// - `Changed(v)` if the predicate is true, presuming that `v` are also true
 265     /// - `Unchanged` if we don't have enough info to be sure
 266     /// - `Error(e)` if the predicate does not hold
 267     ///
 268     /// This is called much less often than `needs_process_obligation`, so we
 269     /// never inline it.
 270     #[inline(never)]
 271     #[instrument(level = "debug", skip(self, pending_obligation))]
 272     fn process_obligation(
 273         &mut self,
 274         pending_obligation: &mut PendingPredicateObligation<'tcx>,
 275     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 276         pending_obligation.stalled_on.truncate(0);
 277
 278         let obligation = &mut pending_obligation.obligation;
 279
 280         debug!(?obligation, "pre-resolve");
 281
 282         if obligation.predicate.has_infer_types_or_consts() {
 283             obligation.predicate =
 284                 self.selcx.infcx().resolve_vars_if_possible(obligation.predicate);
 285         }
 286
 287         let obligation = &pending_obligation.obligation;
 288
 289         let infcx = self.selcx.infcx();
 290
 291         if obligation.predicate.has_projections() {
 292             let mut obligations = Vec::new();
 293             let predicate = crate::traits::project::try_normalize_with_depth_to(
 294                 self.selcx,
 295                 obligation.param_env,
 296                 obligation.cause.clone(),
 297                 obligation.recursion_depth + 1,
 298                 obligation.predicate,
 299                 &mut obligations,
 300             );
 301             if predicate != obligation.predicate {
 302                 obligations.push(obligation.with(predicate));
 303                 return ProcessResult::Changed(mk_pending(obligations));
 304             }
 305         }
 306         let binder = obligation.predicate.kind();
 307         match binder.no_bound_vars() {
 308             None => match binder.skip_binder() {
 309                 // Evaluation will discard candidates using the leak check.
 310                 // This means we need to pass it the bound version of our
 311                 // predicate.
 312                 ty::PredicateKind::Trait(trait_ref) => {
 313                     let trait_obligation = obligation.with(binder.rebind(trait_ref));
 314
 315                     self.process_trait_obligation(
 316                         obligation,
 317                         trait_obligation,
 318                         &mut pending_obligation.stalled_on,
 319                     )
 320                 }
 321                 ty::PredicateKind::Projection(data) => {
 322                     let project_obligation = obligation.with(binder.rebind(data));
 323
 324                     self.process_projection_obligation(
 325                         obligation,
 326                         project_obligation,
 327                         &mut pending_obligation.stalled_on,
 328                     )
 329                 }
 330                 ty::PredicateKind::RegionOutlives(_)
 331                 | ty::PredicateKind::TypeOutlives(_)
 332                 | ty::PredicateKind::WellFormed(_)
 333                 | ty::PredicateKind::ObjectSafe(_)
 334                 | ty::PredicateKind::ClosureKind(..)
 335                 | ty::PredicateKind::Subtype(_)
 336                 | ty::PredicateKind::Coerce(_)
 337                 | ty::PredicateKind::ConstEvaluatable(..)
 338                 | ty::PredicateKind::ConstEquate(..) => {
 339                     let pred =
 340                         ty::Binder::dummy(infcx.replace_bound_vars_with_placeholders(binder));
 341                     ProcessResult::Changed(mk_pending(vec![
 342                         obligation.with(pred.to_predicate(self.selcx.tcx())),
 343                     ]))
 344                 }
 345                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 346                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 347                 }
 348             },
 349             Some(pred) => match pred {
 350                 ty::PredicateKind::Trait(data) => {
 351                     let trait_obligation = obligation.with(Binder::dummy(data));
 352
 353                     self.process_trait_obligation(
 354                         obligation,
 355                         trait_obligation,
 356                         &mut pending_obligation.stalled_on,
 357                     )
 358                 }
 359
 360                 ty::PredicateKind::RegionOutlives(data) => {
 361                     if infcx.considering_regions || data.has_placeholders() {
 362                         infcx.region_outlives_predicate(&obligation.cause, Binder::dummy(data));
 363                     }
 364
 365                     ProcessResult::Changed(vec![])
 366                 }
 367
 368                 ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(t_a, r_b)) => {
 369                     if infcx.considering_regions {
 370                         infcx.register_region_obligation_with_cause(t_a, r_b, &obligation.cause);
 371                     }
 372                     ProcessResult::Changed(vec![])
 373                 }
 374
 375                 ty::PredicateKind::Projection(ref data) => {
 376                     let project_obligation = obligation.with(Binder::dummy(*data));
 377
 378                     self.process_projection_obligation(
 379                         obligation,
 380                         project_obligation,
 381                         &mut pending_obligation.stalled_on,
 382                     )
 383                 }
 384
 385                 ty::PredicateKind::ObjectSafe(trait_def_id) => {
 386                     if !self.selcx.tcx().is_object_safe(trait_def_id) {
 387                         ProcessResult::Error(CodeSelectionError(Unimplemented))
 388                     } else {
 389                         ProcessResult::Changed(vec![])
 390                     }
 391                 }
 392
 393                 ty::PredicateKind::ClosureKind(_, closure_substs, kind) => {
 394                     match self.selcx.infcx().closure_kind(closure_substs) {
 395                         Some(closure_kind) => {
 396                             if closure_kind.extends(kind) {
 397                                 ProcessResult::Changed(vec![])
 398                             } else {
 399                                 ProcessResult::Error(CodeSelectionError(Unimplemented))
 400                             }
 401                         }
 402                         None => ProcessResult::Unchanged,
 403                     }
 404                 }
 405
 406                 ty::PredicateKind::WellFormed(arg) => {
 407                     match wf::obligations(
 408                         self.selcx.infcx(),
 409                         obligation.param_env,
 410                         obligation.cause.body_id,
 411                         obligation.recursion_depth + 1,
 412                         arg,
 413                         obligation.cause.span,
 414                     ) {
 415                         None => {
 416                             pending_obligation.stalled_on =
 417                                 vec![TyOrConstInferVar::maybe_from_generic_arg(arg).unwrap()];
 418                             ProcessResult::Unchanged
 419                         }
 420                         Some(os) => ProcessResult::Changed(mk_pending(os)),
 421                     }
 422                 }
 423
 424                 ty::PredicateKind::Subtype(subtype) => {
 425                     match self.selcx.infcx().subtype_predicate(
 426                         &obligation.cause,
 427                         obligation.param_env,
 428                         Binder::dummy(subtype),
 429                     ) {
 430                         Err((a, b)) => {
 431                             // None means that both are unresolved.
 432                             pending_obligation.stalled_on =
 433                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 434                             ProcessResult::Unchanged
 435                         }
 436                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 437                         Ok(Err(err)) => {
 438                             let expected_found =
 439                                 ExpectedFound::new(subtype.a_is_expected, subtype.a, subtype.b);
 440                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 441                                 expected_found,
 442                                 err,
 443                             ))
 444                         }
 445                     }
 446                 }
 447
 448                 ty::PredicateKind::Coerce(coerce) => {
 449                     match self.selcx.infcx().coerce_predicate(
 450                         &obligation.cause,
 451                         obligation.param_env,
 452                         Binder::dummy(coerce),
 453                     ) {
 454                         Err((a, b)) => {
 455                             // None means that both are unresolved.
 456                             pending_obligation.stalled_on =
 457                                 vec![TyOrConstInferVar::Ty(a), TyOrConstInferVar::Ty(b)];
 458                             ProcessResult::Unchanged
 459                         }
 460                         Ok(Ok(ok)) => ProcessResult::Changed(mk_pending(ok.obligations)),
 461                         Ok(Err(err)) => {
 462                             let expected_found = ExpectedFound::new(false, coerce.a, coerce.b);
 463                             ProcessResult::Error(FulfillmentErrorCode::CodeSubtypeError(
 464                                 expected_found,
 465                                 err,
 466                             ))
 467                         }
 468                     }
 469                 }
 470
 471                 ty::PredicateKind::ConstEvaluatable(uv) => {
 472                     match const_evaluatable::is_const_evaluatable(
 473                         self.selcx.infcx(),
 474                         uv,
 475                         obligation.param_env,
 476                         obligation.cause.span,
 477                     ) {
 478                         Ok(()) => ProcessResult::Changed(vec![]),
 479                         Err(NotConstEvaluatable::MentionsInfer) => {
 480                             pending_obligation.stalled_on.clear();
 481                             pending_obligation.stalled_on.extend(
 482                                 uv.substs
 483                                     .iter()
 484                                     .filter_map(TyOrConstInferVar::maybe_from_generic_arg),
 485                             );
 486                             ProcessResult::Unchanged
 487                         }
 488                         Err(
 489                             e @ NotConstEvaluatable::MentionsParam
 490                             | e @ NotConstEvaluatable::Error(_),
 491                         ) => ProcessResult::Error(CodeSelectionError(
 492                             SelectionError::NotConstEvaluatable(e),
 493                         )),
 494                     }
 495                 }
 496
 497                 ty::PredicateKind::ConstEquate(c1, c2) => {
 498                     debug!(?c1, ?c2, "equating consts");
 499                     let tcx = self.selcx.tcx();
 500                     if tcx.features().generic_const_exprs {
 501                         // FIXME: we probably should only try to unify abstract constants
 502                         // if the constants depend on generic parameters.
 503                         //
 504                         // Let's just see where this breaks :shrug:
 505                         if let (ty::ConstKind::Unevaluated(a), ty::ConstKind::Unevaluated(b)) =
 506                             (c1.kind(), c2.kind())
 507                         {
 508                             if infcx.try_unify_abstract_consts(a, b, obligation.param_env) {
 509                                 return ProcessResult::Changed(vec![]);
 510                             }
 511                         }
 512                     }
 513
 514                     let stalled_on = &mut pending_obligation.stalled_on;
 515
 516                     let mut evaluate = |c: Const<'tcx>| {
 517                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 518                             match self.selcx.infcx().try_const_eval_resolve(
 519                                 obligation.param_env,
 520                                 unevaluated,
 521                                 c.ty(),
 522                                 Some(obligation.cause.span),
 523                             ) {
 524                                 Ok(val) => Ok(val),
 525                                 Err(e) => match e {
 526                                     ErrorHandled::TooGeneric => {
 527                                         stalled_on.extend(
 528                                             unevaluated.substs.iter().filter_map(
 529                                                 TyOrConstInferVar::maybe_from_generic_arg,
 530                                             ),
 531                                         );
 532                                         Err(ErrorHandled::TooGeneric)
 533                                     }
 534                                     _ => Err(e),
 535                                 },
 536                             }
 537                         } else {
 538                             Ok(c)
 539                         }
 540                     };
 541
 542                     match (evaluate(c1), evaluate(c2)) {
 543                         (Ok(c1), Ok(c2)) => {
 544                             match self
 545                                 .selcx
 546                                 .infcx()
 547                                 .at(&obligation.cause, obligation.param_env)
 548                                 .eq(c1, c2)
 549                             {
 550                                 Ok(_) => ProcessResult::Changed(vec![]),
 551                                 Err(err) => ProcessResult::Error(
 552                                     FulfillmentErrorCode::CodeConstEquateError(
 553                                         ExpectedFound::new(true, c1, c2),
 554                                         err,
 555                                     ),
 556                                 ),
 557                             }
 558                         }
 559                         (Err(ErrorHandled::Reported(reported)), _)
 560                         | (_, Err(ErrorHandled::Reported(reported))) => ProcessResult::Error(
 561                             CodeSelectionError(SelectionError::NotConstEvaluatable(
 562                                 NotConstEvaluatable::Error(reported),
 563                             )),
 564                         ),
 565                         (Err(ErrorHandled::Linted), _) | (_, Err(ErrorHandled::Linted)) => {
 566                             span_bug!(
 567                                 obligation.cause.span(),
 568                                 "ConstEquate: const_eval_resolve returned an unexpected error"
 569                             )
 570                         }
 571                         (Err(ErrorHandled::TooGeneric), _) | (_, Err(ErrorHandled::TooGeneric)) => {
 572                             if c1.has_infer_types_or_consts() || c2.has_infer_types_or_consts() {
 573                                 ProcessResult::Unchanged
 574                             } else {
 575                                 // Two different constants using generic parameters ~> error.
 576                                 let expected_found = ExpectedFound::new(true, c1, c2);
 577                                 ProcessResult::Error(FulfillmentErrorCode::CodeConstEquateError(
 578                                     expected_found,
 579                                     TypeError::ConstMismatch(expected_found),
 580                                 ))
 581                             }
 582                         }
 583                     }
 584                 }
 585                 ty::PredicateKind::TypeWellFormedFromEnv(..) => {
 586                     bug!("TypeWellFormedFromEnv is only used for Chalk")
 587                 }
 588             },
 589         }
 590     }
 591
 592     #[inline(never)]
 593     fn process_backedge<'c, I>(
 594         &mut self,
 595         cycle: I,
 596         _marker: PhantomData<&'c PendingPredicateObligation<'tcx>>,
 597     ) -> Result<(), FulfillmentErrorCode<'tcx>>
 598     where
 599         I: Clone + Iterator<Item = &'c PendingPredicateObligation<'tcx>>,
 600     {
 601         if self.selcx.coinductive_match(cycle.clone().map(|s| s.obligation.predicate)) {
 602             debug!("process_child_obligations: coinductive match");
 603             Ok(())
 604         } else {
 605             let cycle: Vec<_> = cycle.map(|c| c.obligation.clone()).collect();
 606             Err(FulfillmentErrorCode::CodeCycle(cycle))
 607         }
 608     }
 609 }
 610
 611 impl<'a, 'b, 'tcx> FulfillProcessor<'a, 'b, 'tcx> {
 612     #[instrument(level = "debug", skip(self, obligation, stalled_on))]
 613     fn process_trait_obligation(
 614         &mut self,
 615         obligation: &PredicateObligation<'tcx>,
 616         trait_obligation: TraitObligation<'tcx>,
 617         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 618     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 619         let infcx = self.selcx.infcx();
 620         if obligation.predicate.is_global() {
 621             // no type variables present, can use evaluation for better caching.
 622             // FIXME: consider caching errors too.
 623             if infcx.predicate_must_hold_considering_regions(obligation) {
 624                 debug!(
 625                     "selecting trait at depth {} evaluated to holds",
 626                     obligation.recursion_depth
 627                 );
 628                 return ProcessResult::Changed(vec![]);
 629             }
 630         }
 631
 632         match self.selcx.select(&trait_obligation) {
 633             Ok(Some(impl_source)) => {
 634                 debug!("selecting trait at depth {} yielded Ok(Some)", obligation.recursion_depth);
 635                 ProcessResult::Changed(mk_pending(impl_source.nested_obligations()))
 636             }
 637             Ok(None) => {
 638                 debug!("selecting trait at depth {} yielded Ok(None)", obligation.recursion_depth);
 639
 640                 // This is a bit subtle: for the most part, the
 641                 // only reason we can fail to make progress on
 642                 // trait selection is because we don't have enough
 643                 // information about the types in the trait.
 644                 stalled_on.clear();
 645                 stalled_on.extend(substs_infer_vars(
 646                     self.selcx,
 647                     trait_obligation.predicate.map_bound(|pred| pred.trait_ref.substs),
 648                 ));
 649
 650                 debug!(
 651                     "process_predicate: pending obligation {:?} now stalled on {:?}",
 652                     infcx.resolve_vars_if_possible(obligation.clone()),
 653                     stalled_on
 654                 );
 655
 656                 ProcessResult::Unchanged
 657             }
 658             Err(selection_err) => {
 659                 debug!("selecting trait at depth {} yielded Err", obligation.recursion_depth);
 660
 661                 ProcessResult::Error(CodeSelectionError(selection_err))
 662             }
 663         }
 664     }
 665
 666     fn process_projection_obligation(
 667         &mut self,
 668         obligation: &PredicateObligation<'tcx>,
 669         project_obligation: PolyProjectionObligation<'tcx>,
 670         stalled_on: &mut Vec<TyOrConstInferVar<'tcx>>,
 671     ) -> ProcessResult<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>> {
 672         let tcx = self.selcx.tcx();
 673
 674         if obligation.predicate.is_global() {
 675             // no type variables present, can use evaluation for better caching.
 676             // FIXME: consider caching errors too.
 677             if self.selcx.infcx().predicate_must_hold_considering_regions(obligation) {
 678                 if let Some(key) = ProjectionCacheKey::from_poly_projection_predicate(
 679                     &mut self.selcx,
 680                     project_obligation.predicate,
 681                 ) {
 682                     // If `predicate_must_hold_considering_regions` succeeds, then we've
 683                     // evaluated all sub-obligations. We can therefore mark the 'root'
 684                     // obligation as complete, and skip evaluating sub-obligations.
 685                     self.selcx
 686                         .infcx()
 687                         .inner
 688                         .borrow_mut()
 689                         .projection_cache()
 690                         .complete(key, EvaluationResult::EvaluatedToOk);
 691                 }
 692                 return ProcessResult::Changed(vec![]);
 693             } else {
 694                 debug!("Does NOT hold: {:?}", obligation);
 695             }
 696         }
 697
 698         match project::poly_project_and_unify_type(self.selcx, &project_obligation) {
 699             ProjectAndUnifyResult::Holds(os) => ProcessResult::Changed(mk_pending(os)),
 700             ProjectAndUnifyResult::FailedNormalization => {
 701                 stalled_on.clear();
 702                 stalled_on.extend(substs_infer_vars(
 703                     self.selcx,
 704                     project_obligation.predicate.map_bound(|pred| pred.projection_ty.substs),
 705                 ));
 706                 ProcessResult::Unchanged
 707             }
 708             // Let the caller handle the recursion
 709             ProjectAndUnifyResult::Recursive => ProcessResult::Changed(mk_pending(vec![
 710                 project_obligation.with(project_obligation.predicate.to_predicate(tcx)),
 711             ])),
 712             ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 713                 ProcessResult::Error(CodeProjectionError(e))
 714             }
 715         }
 716     }
 717 }
 718
 719 /// Returns the set of inference variables contained in `substs`.
 720 fn substs_infer_vars<'a, 'tcx>(
 721     selcx: &mut SelectionContext<'a, 'tcx>,
 722     substs: ty::Binder<'tcx, SubstsRef<'tcx>>,
 723 ) -> impl Iterator<Item = TyOrConstInferVar<'tcx>> {
 724     selcx
 725         .infcx()
 726         .resolve_vars_if_possible(substs)
 727         .skip_binder() // ok because this check doesn't care about regions
 728         .iter()
 729         .filter(|arg| arg.has_infer_types_or_consts())
 730         .flat_map(|arg| {
 731             let mut walker = arg.walk();
 732             while let Some(c) = walker.next() {
 733                 if !c.has_infer_types_or_consts() {
 734                     walker.visited.remove(&c);
 735                     walker.skip_current_subtree();
 736                 }
 737             }
 738             walker.visited.into_iter()
 739         })
 740         .filter_map(TyOrConstInferVar::maybe_from_generic_arg)
 741 }
 742
 743 fn to_fulfillment_error<'tcx>(
 744     error: Error<PendingPredicateObligation<'tcx>, FulfillmentErrorCode<'tcx>>,
 745 ) -> FulfillmentError<'tcx> {
 746     let mut iter = error.backtrace.into_iter();
 747     let obligation = iter.next().unwrap().obligation;
 748     // The root obligation is the last item in the backtrace - if there's only
 749     // one item, then it's the same as the main obligation
 750     let root_obligation = iter.next_back().map_or_else(|| obligation.clone(), |e| e.obligation);
 751     FulfillmentError::new(obligation, error.error, root_obligation)
 752 }