compiler/rustc_trait_selection/src/traits/project.rs

   1 //! Code for projecting associated types out of trait references.
   2
   3 use super::specialization_graph;
   4 use super::translate_substs;
   5 use super::util;
   6 use super::MismatchedProjectionTypes;
   7 use super::Obligation;
   8 use super::ObligationCause;
   9 use super::PredicateObligation;
  10 use super::Selection;
  11 use super::SelectionContext;
  12 use super::SelectionError;
  13 use super::TraitQueryMode;
  14 use super::{
  15     ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
  16     ImplSourceGeneratorData, ImplSourcePointeeData, ImplSourceUserDefinedData,
  17 };
  18 use super::{Normalized, NormalizedTy, ProjectionCacheEntry, ProjectionCacheKey};
  19
  20 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  21 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
  22 use crate::traits::error_reporting::InferCtxtExt as _;
  23 use rustc_data_structures::sso::SsoHashSet;
  24 use rustc_data_structures::stack::ensure_sufficient_stack;
  25 use rustc_errors::ErrorReported;
  26 use rustc_hir::def_id::DefId;
  27 use rustc_hir::lang_items::LangItem;
  28 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
  29 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
  30 use rustc_middle::ty::subst::Subst;
  31 use rustc_middle::ty::{self, ToPredicate, Ty, TyCtxt};
  32 use rustc_span::symbol::sym;
  33
  34 use std::collections::BTreeMap;
  35
  36 pub use rustc_middle::traits::Reveal;
  37
  38 pub type PolyProjectionObligation<'tcx> = Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
  39
  40 pub type ProjectionObligation<'tcx> = Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
  41
  42 pub type ProjectionTyObligation<'tcx> = Obligation<'tcx, ty::ProjectionTy<'tcx>>;
  43
  44 pub(super) struct InProgress;
  45
  46 /// When attempting to resolve `<T as TraitRef>::Name` ...
  47 #[derive(Debug)]
  48 pub enum ProjectionTyError<'tcx> {
  49     /// ...we found multiple sources of information and couldn't resolve the ambiguity.
  50     TooManyCandidates,
  51
  52     /// ...an error occurred matching `T : TraitRef`
  53     TraitSelectionError(SelectionError<'tcx>),
  54 }
  55
  56 #[derive(PartialEq, Eq, Debug)]
  57 enum ProjectionTyCandidate<'tcx> {
  58     /// From a where-clause in the env or object type
  59     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
  60
  61     /// From the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
  62     TraitDef(ty::PolyProjectionPredicate<'tcx>),
  63
  64     /// Bounds specified on an object type
  65     Object(ty::PolyProjectionPredicate<'tcx>),
  66
  67     /// From an "impl" (or a "pseudo-impl" returned by select)
  68     Select(Selection<'tcx>),
  69 }
  70
  71 enum ProjectionTyCandidateSet<'tcx> {
  72     None,
  73     Single(ProjectionTyCandidate<'tcx>),
  74     Ambiguous,
  75     Error(SelectionError<'tcx>),
  76 }
  77
  78 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
  79     fn mark_ambiguous(&mut self) {
  80         *self = ProjectionTyCandidateSet::Ambiguous;
  81     }
  82
  83     fn mark_error(&mut self, err: SelectionError<'tcx>) {
  84         *self = ProjectionTyCandidateSet::Error(err);
  85     }
  86
  87     // Returns true if the push was successful, or false if the candidate
  88     // was discarded -- this could be because of ambiguity, or because
  89     // a higher-priority candidate is already there.
  90     fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
  91         use self::ProjectionTyCandidate::*;
  92         use self::ProjectionTyCandidateSet::*;
  93
  94         // This wacky variable is just used to try and
  95         // make code readable and avoid confusing paths.
  96         // It is assigned a "value" of `()` only on those
  97         // paths in which we wish to convert `*self` to
  98         // ambiguous (and return false, because the candidate
  99         // was not used). On other paths, it is not assigned,
 100         // and hence if those paths *could* reach the code that
 101         // comes after the match, this fn would not compile.
 102         let convert_to_ambiguous;
 103
 104         match self {
 105             None => {
 106                 *self = Single(candidate);
 107                 return true;
 108             }
 109
 110             Single(current) => {
 111                 // Duplicates can happen inside ParamEnv. In the case, we
 112                 // perform a lazy deduplication.
 113                 if current == &candidate {
 114                     return false;
 115                 }
 116
 117                 // Prefer where-clauses. As in select, if there are multiple
 118                 // candidates, we prefer where-clause candidates over impls.  This
 119                 // may seem a bit surprising, since impls are the source of
 120                 // "truth" in some sense, but in fact some of the impls that SEEM
 121                 // applicable are not, because of nested obligations. Where
 122                 // clauses are the safer choice. See the comment on
 123                 // `select::SelectionCandidate` and #21974 for more details.
 124                 match (current, candidate) {
 125                     (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
 126                     (ParamEnv(..), _) => return false,
 127                     (_, ParamEnv(..)) => unreachable!(),
 128                     (_, _) => convert_to_ambiguous = (),
 129                 }
 130             }
 131
 132             Ambiguous | Error(..) => {
 133                 return false;
 134             }
 135         }
 136
 137         // We only ever get here when we moved from a single candidate
 138         // to ambiguous.
 139         let () = convert_to_ambiguous;
 140         *self = Ambiguous;
 141         false
 142     }
 143 }
 144
 145 /// Evaluates constraints of the form:
 146 ///
 147 ///     for<...> <T as Trait>::U == V
 148 ///
 149 /// If successful, this may result in additional obligations. Also returns
 150 /// the projection cache key used to track these additional obligations.
 151 ///
 152 /// ## Returns
 153 ///
 154 /// - `Err(_)`: the projection can be normalized, but is not equal to the
 155 ///   expected type.
 156 /// - `Ok(Err(InProgress))`: this is called recursively while normalizing
 157 ///   the same projection.
 158 /// - `Ok(Ok(None))`: The projection cannot be normalized due to ambiguity
 159 ///   (resolving some inference variables in the projection may fix this).
 160 /// - `Ok(Ok(Some(obligations)))`: The projection bound holds subject to
 161 ///    the given obligations. If the projection cannot be normalized because
 162 ///    the required trait bound doesn't hold this returned with `obligations`
 163 ///    being a predicate that cannot be proven.
 164 #[instrument(level = "debug", skip(selcx))]
 165 pub(super) fn poly_project_and_unify_type<'cx, 'tcx>(
 166     selcx: &mut SelectionContext<'cx, 'tcx>,
 167     obligation: &PolyProjectionObligation<'tcx>,
 168 ) -> Result<
 169     Result<Option<Vec<PredicateObligation<'tcx>>>, InProgress>,
 170     MismatchedProjectionTypes<'tcx>,
 171 > {
 172     let infcx = selcx.infcx();
 173     infcx.commit_if_ok(|_snapshot| {
 174         let placeholder_predicate =
 175             infcx.replace_bound_vars_with_placeholders(obligation.predicate);
 176
 177         let placeholder_obligation = obligation.with(placeholder_predicate);
 178         let result = project_and_unify_type(selcx, &placeholder_obligation)?;
 179         Ok(result)
 180     })
 181 }
 182
 183 /// Evaluates constraints of the form:
 184 ///
 185 ///     <T as Trait>::U == V
 186 ///
 187 /// If successful, this may result in additional obligations.
 188 ///
 189 /// See [poly_project_and_unify_type] for an explanation of the return value.
 190 fn project_and_unify_type<'cx, 'tcx>(
 191     selcx: &mut SelectionContext<'cx, 'tcx>,
 192     obligation: &ProjectionObligation<'tcx>,
 193 ) -> Result<
 194     Result<Option<Vec<PredicateObligation<'tcx>>>, InProgress>,
 195     MismatchedProjectionTypes<'tcx>,
 196 > {
 197     debug!(?obligation, "project_and_unify_type");
 198
 199     let mut obligations = vec![];
 200     let normalized_ty = match opt_normalize_projection_type(
 201         selcx,
 202         obligation.param_env,
 203         obligation.predicate.projection_ty,
 204         obligation.cause.clone(),
 205         obligation.recursion_depth,
 206         &mut obligations,
 207     ) {
 208         Ok(Some(n)) => n,
 209         Ok(None) => return Ok(Ok(None)),
 210         Err(InProgress) => return Ok(Err(InProgress)),
 211     };
 212
 213     debug!(?normalized_ty, ?obligations, "project_and_unify_type result");
 214
 215     let infcx = selcx.infcx();
 216     match infcx
 217         .at(&obligation.cause, obligation.param_env)
 218         .eq(normalized_ty, obligation.predicate.ty)
 219     {
 220         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
 221             obligations.extend(inferred_obligations);
 222             Ok(Ok(Some(obligations)))
 223         }
 224         Err(err) => {
 225             debug!("project_and_unify_type: equating types encountered error {:?}", err);
 226             Err(MismatchedProjectionTypes { err })
 227         }
 228     }
 229 }
 230
 231 /// Normalizes any associated type projections in `value`, replacing
 232 /// them with a fully resolved type where possible. The return value
 233 /// combines the normalized result and any additional obligations that
 234 /// were incurred as result.
 235 pub fn normalize<'a, 'b, 'tcx, T>(
 236     selcx: &'a mut SelectionContext<'b, 'tcx>,
 237     param_env: ty::ParamEnv<'tcx>,
 238     cause: ObligationCause<'tcx>,
 239     value: T,
 240 ) -> Normalized<'tcx, T>
 241 where
 242     T: TypeFoldable<'tcx>,
 243 {
 244     let mut obligations = Vec::new();
 245     let value = normalize_to(selcx, param_env, cause, value, &mut obligations);
 246     Normalized { value, obligations }
 247 }
 248
 249 pub fn normalize_to<'a, 'b, 'tcx, T>(
 250     selcx: &'a mut SelectionContext<'b, 'tcx>,
 251     param_env: ty::ParamEnv<'tcx>,
 252     cause: ObligationCause<'tcx>,
 253     value: T,
 254     obligations: &mut Vec<PredicateObligation<'tcx>>,
 255 ) -> T
 256 where
 257     T: TypeFoldable<'tcx>,
 258 {
 259     normalize_with_depth_to(selcx, param_env, cause, 0, value, obligations)
 260 }
 261
 262 /// As `normalize`, but with a custom depth.
 263 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
 264     selcx: &'a mut SelectionContext<'b, 'tcx>,
 265     param_env: ty::ParamEnv<'tcx>,
 266     cause: ObligationCause<'tcx>,
 267     depth: usize,
 268     value: T,
 269 ) -> Normalized<'tcx, T>
 270 where
 271     T: TypeFoldable<'tcx>,
 272 {
 273     let mut obligations = Vec::new();
 274     let value = normalize_with_depth_to(selcx, param_env, cause, depth, value, &mut obligations);
 275     Normalized { value, obligations }
 276 }
 277
 278 #[instrument(level = "info", skip(selcx, param_env, cause, obligations))]
 279 pub fn normalize_with_depth_to<'a, 'b, 'tcx, T>(
 280     selcx: &'a mut SelectionContext<'b, 'tcx>,
 281     param_env: ty::ParamEnv<'tcx>,
 282     cause: ObligationCause<'tcx>,
 283     depth: usize,
 284     value: T,
 285     obligations: &mut Vec<PredicateObligation<'tcx>>,
 286 ) -> T
 287 where
 288     T: TypeFoldable<'tcx>,
 289 {
 290     debug!(obligations.len = obligations.len());
 291     let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth, obligations);
 292     let result = ensure_sufficient_stack(|| normalizer.fold(value));
 293     debug!(?result, obligations.len = normalizer.obligations.len());
 294     debug!(?normalizer.obligations,);
 295     result
 296 }
 297
 298 pub(crate) fn needs_normalization<'tcx, T: TypeFoldable<'tcx>>(value: &T, reveal: Reveal) -> bool {
 299     match reveal {
 300         Reveal::UserFacing => value
 301             .has_type_flags(ty::TypeFlags::HAS_TY_PROJECTION | ty::TypeFlags::HAS_CT_PROJECTION),
 302         Reveal::All => value.has_type_flags(
 303             ty::TypeFlags::HAS_TY_PROJECTION
 304                 | ty::TypeFlags::HAS_TY_OPAQUE
 305                 | ty::TypeFlags::HAS_CT_PROJECTION,
 306         ),
 307     }
 308 }
 309
 310 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
 311     selcx: &'a mut SelectionContext<'b, 'tcx>,
 312     param_env: ty::ParamEnv<'tcx>,
 313     cause: ObligationCause<'tcx>,
 314     obligations: &'a mut Vec<PredicateObligation<'tcx>>,
 315     depth: usize,
 316     universes: Vec<Option<ty::UniverseIndex>>,
 317 }
 318
 319 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
 320     fn new(
 321         selcx: &'a mut SelectionContext<'b, 'tcx>,
 322         param_env: ty::ParamEnv<'tcx>,
 323         cause: ObligationCause<'tcx>,
 324         depth: usize,
 325         obligations: &'a mut Vec<PredicateObligation<'tcx>>,
 326     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
 327         AssocTypeNormalizer { selcx, param_env, cause, obligations, depth, universes: vec![] }
 328     }
 329
 330     fn fold<T: TypeFoldable<'tcx>>(&mut self, value: T) -> T {
 331         let value = self.selcx.infcx().resolve_vars_if_possible(value);
 332         debug!(?value);
 333
 334         assert!(
 335             !value.has_escaping_bound_vars(),
 336             "Normalizing {:?} without wrapping in a `Binder`",
 337             value
 338         );
 339
 340         if !needs_normalization(&value, self.param_env.reveal()) {
 341             value
 342         } else {
 343             value.fold_with(self)
 344         }
 345     }
 346 }
 347
 348 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
 349     fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
 350         self.selcx.tcx()
 351     }
 352
 353     fn fold_binder<T: TypeFoldable<'tcx>>(
 354         &mut self,
 355         t: ty::Binder<'tcx, T>,
 356     ) -> ty::Binder<'tcx, T> {
 357         self.universes.push(None);
 358         let t = t.super_fold_with(self);
 359         self.universes.pop();
 360         t
 361     }
 362
 363     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 364         if !needs_normalization(&ty, self.param_env.reveal()) {
 365             return ty;
 366         }
 367
 368         // We try to be a little clever here as a performance optimization in
 369         // cases where there are nested projections under binders.
 370         // For example:
 371         // ```
 372         // for<'a> fn(<T as Foo>::One<'a, Box<dyn Bar<'a, Item=<T as Foo>::Two<'a>>>>)
 373         // ```
 374         // We normalize the substs on the projection before the projecting, but
 375         // if we're naive, we'll
 376         //   replace bound vars on inner, project inner, replace placeholders on inner,
 377         //   replace bound vars on outer, project outer, replace placeholders on outer
 378         //
 379         // However, if we're a bit more clever, we can replace the bound vars
 380         // on the entire type before normalizing nested projections, meaning we
 381         //   replace bound vars on outer, project inner,
 382         //   project outer, replace placeholders on outer
 383         //
 384         // This is possible because the inner `'a` will already be a placeholder
 385         // when we need to normalize the inner projection
 386         //
 387         // On the other hand, this does add a bit of complexity, since we only
 388         // replace bound vars if the current type is a `Projection` and we need
 389         // to make sure we don't forget to fold the substs regardless.
 390
 391         match *ty.kind() {
 392             // This is really important. While we *can* handle this, this has
 393             // severe performance implications for large opaque types with
 394             // late-bound regions. See `issue-88862` benchmark.
 395             ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
 396                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 397                 match self.param_env.reveal() {
 398                     Reveal::UserFacing => ty.super_fold_with(self),
 399
 400                     Reveal::All => {
 401                         let recursion_limit = self.tcx().recursion_limit();
 402                         if !recursion_limit.value_within_limit(self.depth) {
 403                             let obligation = Obligation::with_depth(
 404                                 self.cause.clone(),
 405                                 recursion_limit.0,
 406                                 self.param_env,
 407                                 ty,
 408                             );
 409                             self.selcx.infcx().report_overflow_error(&obligation, true);
 410                         }
 411
 412                         let substs = substs.super_fold_with(self);
 413                         let generic_ty = self.tcx().type_of(def_id);
 414                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 415                         self.depth += 1;
 416                         let folded_ty = self.fold_ty(concrete_ty);
 417                         self.depth -= 1;
 418                         folded_ty
 419                     }
 420                 }
 421             }
 422
 423             ty::Projection(data) if !data.has_escaping_bound_vars() => {
 424                 // This branch is *mostly* just an optimization: when we don't
 425                 // have escaping bound vars, we don't need to replace them with
 426                 // placeholders (see branch below). *Also*, we know that we can
 427                 // register an obligation to *later* project, since we know
 428                 // there won't be bound vars there.
 429
 430                 let data = data.super_fold_with(self);
 431                 let normalized_ty = normalize_projection_type(
 432                     self.selcx,
 433                     self.param_env,
 434                     data,
 435                     self.cause.clone(),
 436                     self.depth,
 437                     &mut self.obligations,
 438                 );
 439                 debug!(
 440                     ?self.depth,
 441                     ?ty,
 442                     ?normalized_ty,
 443                     obligations.len = ?self.obligations.len(),
 444                     "AssocTypeNormalizer: normalized type"
 445                 );
 446                 normalized_ty
 447             }
 448
 449             ty::Projection(data) => {
 450                 // If there are escaping bound vars, we temporarily replace the
 451                 // bound vars with placeholders. Note though, that in the case
 452                 // that we still can't project for whatever reason (e.g. self
 453                 // type isn't known enough), we *can't* register an obligation
 454                 // and return an inference variable (since then that obligation
 455                 // would have bound vars and that's a can of worms). Instead,
 456                 // we just give up and fall back to pretending like we never tried!
 457                 //
 458                 // Note: this isn't necessarily the final approach here; we may
 459                 // want to figure out how to register obligations with escaping vars
 460                 // or handle this some other way.
 461
 462                 let infcx = self.selcx.infcx();
 463                 let (data, mapped_regions, mapped_types, mapped_consts) =
 464                     BoundVarReplacer::replace_bound_vars(infcx, &mut self.universes, data);
 465                 let data = data.super_fold_with(self);
 466                 let normalized_ty = opt_normalize_projection_type(
 467                     self.selcx,
 468                     self.param_env,
 469                     data,
 470                     self.cause.clone(),
 471                     self.depth,
 472                     &mut self.obligations,
 473                 )
 474                 .ok()
 475                 .flatten()
 476                 .map(|normalized_ty| {
 477                     PlaceholderReplacer::replace_placeholders(
 478                         infcx,
 479                         mapped_regions,
 480                         mapped_types,
 481                         mapped_consts,
 482                         &self.universes,
 483                         normalized_ty,
 484                     )
 485                 })
 486                 .unwrap_or_else(|| ty.super_fold_with(self));
 487
 488                 debug!(
 489                     ?self.depth,
 490                     ?ty,
 491                     ?normalized_ty,
 492                     obligations.len = ?self.obligations.len(),
 493                     "AssocTypeNormalizer: normalized type"
 494                 );
 495                 normalized_ty
 496             }
 497
 498             _ => ty.super_fold_with(self),
 499         }
 500     }
 501
 502     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 503         if self.selcx.tcx().lazy_normalization() {
 504             constant
 505         } else {
 506             let constant = constant.super_fold_with(self);
 507             constant.eval(self.selcx.tcx(), self.param_env)
 508         }
 509     }
 510 }
 511
 512 pub struct BoundVarReplacer<'me, 'tcx> {
 513     infcx: &'me InferCtxt<'me, 'tcx>,
 514     // These three maps track the bound variable that were replaced by placeholders. It might be
 515     // nice to remove these since we already have the `kind` in the placeholder; we really just need
 516     // the `var` (but we *could* bring that into scope if we were to track them as we pass them).
 517     mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
 518     mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
 519     mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
 520     // The current depth relative to *this* folding, *not* the entire normalization. In other words,
 521     // the depth of binders we've passed here.
 522     current_index: ty::DebruijnIndex,
 523     // The `UniverseIndex` of the binding levels above us. These are optional, since we are lazy:
 524     // we don't actually create a universe until we see a bound var we have to replace.
 525     universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
 526 }
 527
 528 impl<'me, 'tcx> BoundVarReplacer<'me, 'tcx> {
 529     /// Returns `Some` if we *were* able to replace bound vars. If there are any bound vars that
 530     /// use a binding level above `universe_indices.len()`, we fail.
 531     pub fn replace_bound_vars<T: TypeFoldable<'tcx>>(
 532         infcx: &'me InferCtxt<'me, 'tcx>,
 533         universe_indices: &'me mut Vec<Option<ty::UniverseIndex>>,
 534         value: T,
 535     ) -> (
 536         T,
 537         BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
 538         BTreeMap<ty::PlaceholderType, ty::BoundTy>,
 539         BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
 540     ) {
 541         let mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion> = BTreeMap::new();
 542         let mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy> = BTreeMap::new();
 543         let mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar> = BTreeMap::new();
 544
 545         let mut replacer = BoundVarReplacer {
 546             infcx,
 547             mapped_regions,
 548             mapped_types,
 549             mapped_consts,
 550             current_index: ty::INNERMOST,
 551             universe_indices,
 552         };
 553
 554         let value = value.super_fold_with(&mut replacer);
 555
 556         (value, replacer.mapped_regions, replacer.mapped_types, replacer.mapped_consts)
 557     }
 558
 559     fn universe_for(&mut self, debruijn: ty::DebruijnIndex) -> ty::UniverseIndex {
 560         let infcx = self.infcx;
 561         let index =
 562             self.universe_indices.len() + self.current_index.as_usize() - debruijn.as_usize() - 1;
 563         let universe = self.universe_indices[index].unwrap_or_else(|| {
 564             for i in self.universe_indices.iter_mut().take(index + 1) {
 565                 *i = i.or_else(|| Some(infcx.create_next_universe()))
 566             }
 567             self.universe_indices[index].unwrap()
 568         });
 569         universe
 570     }
 571 }
 572
 573 impl TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
 574     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 575         self.infcx.tcx
 576     }
 577
 578     fn fold_binder<T: TypeFoldable<'tcx>>(
 579         &mut self,
 580         t: ty::Binder<'tcx, T>,
 581     ) -> ty::Binder<'tcx, T> {
 582         self.current_index.shift_in(1);
 583         let t = t.super_fold_with(self);
 584         self.current_index.shift_out(1);
 585         t
 586     }
 587
 588     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 589         match *r {
 590             ty::ReLateBound(debruijn, _)
 591                 if debruijn.as_usize() + 1
 592                     > self.current_index.as_usize() + self.universe_indices.len() =>
 593             {
 594                 bug!("Bound vars outside of `self.universe_indices`");
 595             }
 596             ty::ReLateBound(debruijn, br) if debruijn >= self.current_index => {
 597                 let universe = self.universe_for(debruijn);
 598                 let p = ty::PlaceholderRegion { universe, name: br.kind };
 599                 self.mapped_regions.insert(p, br);
 600                 self.infcx.tcx.mk_region(ty::RePlaceholder(p))
 601             }
 602             _ => r,
 603         }
 604     }
 605
 606     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 607         match *t.kind() {
 608             ty::Bound(debruijn, _)
 609                 if debruijn.as_usize() + 1
 610                     > self.current_index.as_usize() + self.universe_indices.len() =>
 611             {
 612                 bug!("Bound vars outside of `self.universe_indices`");
 613             }
 614             ty::Bound(debruijn, bound_ty) if debruijn >= self.current_index => {
 615                 let universe = self.universe_for(debruijn);
 616                 let p = ty::PlaceholderType { universe, name: bound_ty.var };
 617                 self.mapped_types.insert(p, bound_ty);
 618                 self.infcx.tcx.mk_ty(ty::Placeholder(p))
 619             }
 620             _ if t.has_vars_bound_at_or_above(self.current_index) => t.super_fold_with(self),
 621             _ => t,
 622         }
 623     }
 624
 625     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 626         match *ct {
 627             ty::Const { val: ty::ConstKind::Bound(debruijn, _), ty: _ }
 628                 if debruijn.as_usize() + 1
 629                     > self.current_index.as_usize() + self.universe_indices.len() =>
 630             {
 631                 bug!("Bound vars outside of `self.universe_indices`");
 632             }
 633             ty::Const { val: ty::ConstKind::Bound(debruijn, bound_const), ty }
 634                 if debruijn >= self.current_index =>
 635             {
 636                 let universe = self.universe_for(debruijn);
 637                 let p = ty::PlaceholderConst {
 638                     universe,
 639                     name: ty::BoundConst { var: bound_const, ty },
 640                 };
 641                 self.mapped_consts.insert(p, bound_const);
 642                 self.infcx.tcx.mk_const(ty::Const { val: ty::ConstKind::Placeholder(p), ty })
 643             }
 644             _ if ct.has_vars_bound_at_or_above(self.current_index) => ct.super_fold_with(self),
 645             _ => ct,
 646         }
 647     }
 648 }
 649
 650 // The inverse of `BoundVarReplacer`: replaces placeholders with the bound vars from which they came.
 651 pub struct PlaceholderReplacer<'me, 'tcx> {
 652     infcx: &'me InferCtxt<'me, 'tcx>,
 653     mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
 654     mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
 655     mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
 656     universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
 657     current_index: ty::DebruijnIndex,
 658 }
 659
 660 impl<'me, 'tcx> PlaceholderReplacer<'me, 'tcx> {
 661     pub fn replace_placeholders<T: TypeFoldable<'tcx>>(
 662         infcx: &'me InferCtxt<'me, 'tcx>,
 663         mapped_regions: BTreeMap<ty::PlaceholderRegion, ty::BoundRegion>,
 664         mapped_types: BTreeMap<ty::PlaceholderType, ty::BoundTy>,
 665         mapped_consts: BTreeMap<ty::PlaceholderConst<'tcx>, ty::BoundVar>,
 666         universe_indices: &'me Vec<Option<ty::UniverseIndex>>,
 667         value: T,
 668     ) -> T {
 669         let mut replacer = PlaceholderReplacer {
 670             infcx,
 671             mapped_regions,
 672             mapped_types,
 673             mapped_consts,
 674             universe_indices,
 675             current_index: ty::INNERMOST,
 676         };
 677         value.super_fold_with(&mut replacer)
 678     }
 679 }
 680
 681 impl TypeFolder<'tcx> for PlaceholderReplacer<'_, 'tcx> {
 682     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 683         self.infcx.tcx
 684     }
 685
 686     fn fold_binder<T: TypeFoldable<'tcx>>(
 687         &mut self,
 688         t: ty::Binder<'tcx, T>,
 689     ) -> ty::Binder<'tcx, T> {
 690         if !t.has_placeholders() && !t.has_infer_regions() {
 691             return t;
 692         }
 693         self.current_index.shift_in(1);
 694         let t = t.super_fold_with(self);
 695         self.current_index.shift_out(1);
 696         t
 697     }
 698
 699     fn fold_region(&mut self, r0: ty::Region<'tcx>) -> ty::Region<'tcx> {
 700         let r1 = match r0 {
 701             ty::ReVar(_) => self
 702                 .infcx
 703                 .inner
 704                 .borrow_mut()
 705                 .unwrap_region_constraints()
 706                 .opportunistic_resolve_region(self.infcx.tcx, r0),
 707             _ => r0,
 708         };
 709
 710         let r2 = match *r1 {
 711             ty::RePlaceholder(p) => {
 712                 let replace_var = self.mapped_regions.get(&p);
 713                 match replace_var {
 714                     Some(replace_var) => {
 715                         let index = self
 716                             .universe_indices
 717                             .iter()
 718                             .position(|u| matches!(u, Some(pu) if *pu == p.universe))
 719                             .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
 720                         let db = ty::DebruijnIndex::from_usize(
 721                             self.universe_indices.len() - index + self.current_index.as_usize() - 1,
 722                         );
 723                         self.tcx().mk_region(ty::ReLateBound(db, *replace_var))
 724                     }
 725                     None => r1,
 726                 }
 727             }
 728             _ => r1,
 729         };
 730
 731         debug!(?r0, ?r1, ?r2, "fold_region");
 732
 733         r2
 734     }
 735
 736     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 737         match *ty.kind() {
 738             ty::Placeholder(p) => {
 739                 let replace_var = self.mapped_types.get(&p);
 740                 match replace_var {
 741                     Some(replace_var) => {
 742                         let index = self
 743                             .universe_indices
 744                             .iter()
 745                             .position(|u| matches!(u, Some(pu) if *pu == p.universe))
 746                             .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
 747                         let db = ty::DebruijnIndex::from_usize(
 748                             self.universe_indices.len() - index + self.current_index.as_usize() - 1,
 749                         );
 750                         self.tcx().mk_ty(ty::Bound(db, *replace_var))
 751                     }
 752                     None => ty,
 753                 }
 754             }
 755
 756             _ if ty.has_placeholders() || ty.has_infer_regions() => ty.super_fold_with(self),
 757             _ => ty,
 758         }
 759     }
 760
 761     fn fold_const(&mut self, ct: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 762         if let ty::Const { val: ty::ConstKind::Placeholder(p), ty } = *ct {
 763             let replace_var = self.mapped_consts.get(&p);
 764             match replace_var {
 765                 Some(replace_var) => {
 766                     let index = self
 767                         .universe_indices
 768                         .iter()
 769                         .position(|u| matches!(u, Some(pu) if *pu == p.universe))
 770                         .unwrap_or_else(|| bug!("Unexpected placeholder universe."));
 771                     let db = ty::DebruijnIndex::from_usize(
 772                         self.universe_indices.len() - index + self.current_index.as_usize() - 1,
 773                     );
 774                     self.tcx()
 775                         .mk_const(ty::Const { val: ty::ConstKind::Bound(db, *replace_var), ty })
 776                 }
 777                 None => ct,
 778             }
 779         } else {
 780             ct.super_fold_with(self)
 781         }
 782     }
 783 }
 784
 785 /// The guts of `normalize`: normalize a specific projection like `<T
 786 /// as Trait>::Item`. The result is always a type (and possibly
 787 /// additional obligations). If ambiguity arises, which implies that
 788 /// there are unresolved type variables in the projection, we will
 789 /// substitute a fresh type variable `$X` and generate a new
 790 /// obligation `<T as Trait>::Item == $X` for later.
 791 pub fn normalize_projection_type<'a, 'b, 'tcx>(
 792     selcx: &'a mut SelectionContext<'b, 'tcx>,
 793     param_env: ty::ParamEnv<'tcx>,
 794     projection_ty: ty::ProjectionTy<'tcx>,
 795     cause: ObligationCause<'tcx>,
 796     depth: usize,
 797     obligations: &mut Vec<PredicateObligation<'tcx>>,
 798 ) -> Ty<'tcx> {
 799     opt_normalize_projection_type(
 800         selcx,
 801         param_env,
 802         projection_ty,
 803         cause.clone(),
 804         depth,
 805         obligations,
 806     )
 807     .ok()
 808     .flatten()
 809     .unwrap_or_else(move || {
 810         // if we bottom out in ambiguity, create a type variable
 811         // and a deferred predicate to resolve this when more type
 812         // information is available.
 813
 814         selcx.infcx().infer_projection(param_env, projection_ty, cause, depth + 1, obligations)
 815     })
 816 }
 817
 818 /// The guts of `normalize`: normalize a specific projection like `<T
 819 /// as Trait>::Item`. The result is always a type (and possibly
 820 /// additional obligations). Returns `None` in the case of ambiguity,
 821 /// which indicates that there are unbound type variables.
 822 ///
 823 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
 824 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
 825 /// often immediately appended to another obligations vector. So now this
 826 /// function takes an obligations vector and appends to it directly, which is
 827 /// slightly uglier but avoids the need for an extra short-lived allocation.
 828 #[instrument(level = "debug", skip(selcx, param_env, cause, obligations))]
 829 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 830     selcx: &'a mut SelectionContext<'b, 'tcx>,
 831     param_env: ty::ParamEnv<'tcx>,
 832     projection_ty: ty::ProjectionTy<'tcx>,
 833     cause: ObligationCause<'tcx>,
 834     depth: usize,
 835     obligations: &mut Vec<PredicateObligation<'tcx>>,
 836 ) -> Result<Option<Ty<'tcx>>, InProgress> {
 837     let infcx = selcx.infcx();
 838     // Don't use the projection cache in intercrate mode -
 839     // the `infcx` may be re-used between intercrate in non-intercrate
 840     // mode, which could lead to using incorrect cache results.
 841     let use_cache = !selcx.is_intercrate();
 842
 843     let projection_ty = infcx.resolve_vars_if_possible(projection_ty);
 844     let cache_key = ProjectionCacheKey::new(projection_ty);
 845
 846     // FIXME(#20304) For now, I am caching here, which is good, but it
 847     // means we don't capture the type variables that are created in
 848     // the case of ambiguity. Which means we may create a large stream
 849     // of such variables. OTOH, if we move the caching up a level, we
 850     // would not benefit from caching when proving `T: Trait<U=Foo>`
 851     // bounds. It might be the case that we want two distinct caches,
 852     // or else another kind of cache entry.
 853
 854     let cache_result = if use_cache {
 855         infcx.inner.borrow_mut().projection_cache().try_start(cache_key)
 856     } else {
 857         Ok(())
 858     };
 859     match cache_result {
 860         Ok(()) => debug!("no cache"),
 861         Err(ProjectionCacheEntry::Ambiguous) => {
 862             // If we found ambiguity the last time, that means we will continue
 863             // to do so until some type in the key changes (and we know it
 864             // hasn't, because we just fully resolved it).
 865             debug!("found cache entry: ambiguous");
 866             return Ok(None);
 867         }
 868         Err(ProjectionCacheEntry::InProgress) => {
 869             // Under lazy normalization, this can arise when
 870             // bootstrapping.  That is, imagine an environment with a
 871             // where-clause like `A::B == u32`. Now, if we are asked
 872             // to normalize `A::B`, we will want to check the
 873             // where-clauses in scope. So we will try to unify `A::B`
 874             // with `A::B`, which can trigger a recursive
 875             // normalization.
 876
 877             debug!("found cache entry: in-progress");
 878
 879             // Cache that normalizing this projection resulted in a cycle. This
 880             // should ensure that, unless this happens within a snapshot that's
 881             // rolled back, fulfillment or evaluation will notice the cycle.
 882
 883             if use_cache {
 884                 infcx.inner.borrow_mut().projection_cache().recur(cache_key);
 885             }
 886             return Err(InProgress);
 887         }
 888         Err(ProjectionCacheEntry::Recur) => {
 889             debug!("recur cache");
 890             return Err(InProgress);
 891         }
 892         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
 893             // This is the hottest path in this function.
 894             //
 895             // If we find the value in the cache, then return it along
 896             // with the obligations that went along with it. Note
 897             // that, when using a fulfillment context, these
 898             // obligations could in principle be ignored: they have
 899             // already been registered when the cache entry was
 900             // created (and hence the new ones will quickly be
 901             // discarded as duplicated). But when doing trait
 902             // evaluation this is not the case, and dropping the trait
 903             // evaluations can causes ICEs (e.g., #43132).
 904             debug!(?ty, "found normalized ty");
 905             obligations.extend(ty.obligations);
 906             return Ok(Some(ty.value));
 907         }
 908         Err(ProjectionCacheEntry::Error) => {
 909             debug!("opt_normalize_projection_type: found error");
 910             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 911             obligations.extend(result.obligations);
 912             return Ok(Some(result.value));
 913         }
 914     }
 915
 916     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
 917
 918     match project_type(selcx, &obligation) {
 919         Ok(ProjectedTy::Progress(Progress {
 920             ty: projected_ty,
 921             obligations: mut projected_obligations,
 922         })) => {
 923             // if projection succeeded, then what we get out of this
 924             // is also non-normalized (consider: it was derived from
 925             // an impl, where-clause etc) and hence we must
 926             // re-normalize it
 927
 928             let projected_ty = selcx.infcx().resolve_vars_if_possible(projected_ty);
 929             debug!(?projected_ty, ?depth, ?projected_obligations);
 930
 931             let mut result = if projected_ty.has_projections() {
 932                 let mut normalizer = AssocTypeNormalizer::new(
 933                     selcx,
 934                     param_env,
 935                     cause,
 936                     depth + 1,
 937                     &mut projected_obligations,
 938                 );
 939                 let normalized_ty = normalizer.fold(projected_ty);
 940
 941                 debug!(?normalized_ty, ?depth);
 942
 943                 Normalized { value: normalized_ty, obligations: projected_obligations }
 944             } else {
 945                 Normalized { value: projected_ty, obligations: projected_obligations }
 946             };
 947
 948             let mut deduped: SsoHashSet<_> = Default::default();
 949             let mut canonical =
 950                 SelectionContext::with_query_mode(selcx.infcx(), TraitQueryMode::Canonical);
 951
 952             result.obligations.drain_filter(|projected_obligation| {
 953                 if !deduped.insert(projected_obligation.clone()) {
 954                     return true;
 955                 }
 956                 // If any global obligations always apply, considering regions, then we don't
 957                 // need to include them. The `is_global` check rules out inference variables,
 958                 // so there's no need for the caller of `opt_normalize_projection_type`
 959                 // to evaluate them.
 960                 // Note that we do *not* discard obligations that evaluate to
 961                 // `EvaluatedtoOkModuloRegions`. Evaluating these obligations
 962                 // inside of a query (e.g. `evaluate_obligation`) can change
 963                 // the result to `EvaluatedToOkModuloRegions`, while an
 964                 // `EvaluatedToOk` obligation will never change the result.
 965                 // See #85360 for more details
 966                 projected_obligation.is_global(canonical.tcx())
 967                     && canonical
 968                         .evaluate_root_obligation(projected_obligation)
 969                         .map_or(false, |res| res.must_apply_considering_regions())
 970             });
 971
 972             if use_cache {
 973                 infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
 974             }
 975             obligations.extend(result.obligations);
 976             Ok(Some(result.value))
 977         }
 978         Ok(ProjectedTy::NoProgress(projected_ty)) => {
 979             debug!(?projected_ty, "opt_normalize_projection_type: no progress");
 980             let result = Normalized { value: projected_ty, obligations: vec![] };
 981             if use_cache {
 982                 infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
 983             }
 984             // No need to extend `obligations`.
 985             Ok(Some(result.value))
 986         }
 987         Err(ProjectionTyError::TooManyCandidates) => {
 988             debug!("opt_normalize_projection_type: too many candidates");
 989             if use_cache {
 990                 infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
 991             }
 992             Ok(None)
 993         }
 994         Err(ProjectionTyError::TraitSelectionError(_)) => {
 995             debug!("opt_normalize_projection_type: ERROR");
 996             // if we got an error processing the `T as Trait` part,
 997             // just return `ty::err` but add the obligation `T :
 998             // Trait`, which when processed will cause the error to be
 999             // reported later
1000
1001             if use_cache {
1002                 infcx.inner.borrow_mut().projection_cache().error(cache_key);
1003             }
1004             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
1005             obligations.extend(result.obligations);
1006             Ok(Some(result.value))
1007         }
1008     }
1009 }
1010
1011 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
1012 /// hold. In various error cases, we cannot generate a valid
1013 /// normalized projection. Therefore, we create an inference variable
1014 /// return an associated obligation that, when fulfilled, will lead to
1015 /// an error.
1016 ///
1017 /// Note that we used to return `Error` here, but that was quite
1018 /// dubious -- the premise was that an error would *eventually* be
1019 /// reported, when the obligation was processed. But in general once
1020 /// you see an `Error` you are supposed to be able to assume that an
1021 /// error *has been* reported, so that you can take whatever heuristic
1022 /// paths you want to take. To make things worse, it was possible for
1023 /// cycles to arise, where you basically had a setup like `<MyType<$0>
1024 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
1025 /// Trait>::Foo> to `[type error]` would lead to an obligation of
1026 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
1027 /// an error for this obligation, but we legitimately should not,
1028 /// because it contains `[type error]`. Yuck! (See issue #29857 for
1029 /// one case where this arose.)
1030 fn normalize_to_error<'a, 'tcx>(
1031     selcx: &mut SelectionContext<'a, 'tcx>,
1032     param_env: ty::ParamEnv<'tcx>,
1033     projection_ty: ty::ProjectionTy<'tcx>,
1034     cause: ObligationCause<'tcx>,
1035     depth: usize,
1036 ) -> NormalizedTy<'tcx> {
1037     let trait_ref = ty::Binder::dummy(projection_ty.trait_ref(selcx.tcx()));
1038     let trait_obligation = Obligation {
1039         cause,
1040         recursion_depth: depth,
1041         param_env,
1042         predicate: trait_ref.without_const().to_predicate(selcx.tcx()),
1043     };
1044     let tcx = selcx.infcx().tcx;
1045     let def_id = projection_ty.item_def_id;
1046     let new_value = selcx.infcx().next_ty_var(TypeVariableOrigin {
1047         kind: TypeVariableOriginKind::NormalizeProjectionType,
1048         span: tcx.def_span(def_id),
1049     });
1050     Normalized { value: new_value, obligations: vec![trait_obligation] }
1051 }
1052
1053 enum ProjectedTy<'tcx> {
1054     Progress(Progress<'tcx>),
1055     NoProgress(Ty<'tcx>),
1056 }
1057
1058 struct Progress<'tcx> {
1059     ty: Ty<'tcx>,
1060     obligations: Vec<PredicateObligation<'tcx>>,
1061 }
1062
1063 impl<'tcx> Progress<'tcx> {
1064     fn error(tcx: TyCtxt<'tcx>) -> Self {
1065         Progress { ty: tcx.ty_error(), obligations: vec![] }
1066     }
1067
1068     fn with_addl_obligations(mut self, mut obligations: Vec<PredicateObligation<'tcx>>) -> Self {
1069         debug!(
1070             self.obligations.len = ?self.obligations.len(),
1071             obligations.len = obligations.len(),
1072             "with_addl_obligations"
1073         );
1074
1075         debug!(?self.obligations, ?obligations, "with_addl_obligations");
1076
1077         self.obligations.append(&mut obligations);
1078         self
1079     }
1080 }
1081
1082 /// Computes the result of a projection type (if we can).
1083 ///
1084 /// IMPORTANT:
1085 /// - `obligation` must be fully normalized
1086 #[tracing::instrument(level = "info", skip(selcx))]
1087 fn project_type<'cx, 'tcx>(
1088     selcx: &mut SelectionContext<'cx, 'tcx>,
1089     obligation: &ProjectionTyObligation<'tcx>,
1090 ) -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>> {
1091     if !selcx.tcx().recursion_limit().value_within_limit(obligation.recursion_depth) {
1092         debug!("project: overflow!");
1093         // This should really be an immediate error, but some existing code
1094         // relies on being able to recover from this.
1095         return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
1096     }
1097
1098     if obligation.predicate.references_error() {
1099         return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
1100     }
1101
1102     let mut candidates = ProjectionTyCandidateSet::None;
1103
1104     // Make sure that the following procedures are kept in order. ParamEnv
1105     // needs to be first because it has highest priority, and Select checks
1106     // the return value of push_candidate which assumes it's ran at last.
1107     assemble_candidates_from_param_env(selcx, obligation, &mut candidates);
1108
1109     assemble_candidates_from_trait_def(selcx, obligation, &mut candidates);
1110
1111     assemble_candidates_from_object_ty(selcx, obligation, &mut candidates);
1112
1113     if let ProjectionTyCandidateSet::Single(ProjectionTyCandidate::Object(_)) = candidates {
1114         // Avoid normalization cycle from selection (see
1115         // `assemble_candidates_from_object_ty`).
1116         // FIXME(lazy_normalization): Lazy normalization should save us from
1117         // having to special case this.
1118     } else {
1119         assemble_candidates_from_impls(selcx, obligation, &mut candidates);
1120     };
1121
1122     match candidates {
1123         ProjectionTyCandidateSet::Single(candidate) => {
1124             Ok(ProjectedTy::Progress(confirm_candidate(selcx, obligation, candidate)))
1125         }
1126         ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
1127             selcx
1128                 .tcx()
1129                 .mk_projection(obligation.predicate.item_def_id, obligation.predicate.substs),
1130         )),
1131         // Error occurred while trying to processing impls.
1132         ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
1133         // Inherent ambiguity that prevents us from even enumerating the
1134         // candidates.
1135         ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
1136     }
1137 }
1138
1139 /// The first thing we have to do is scan through the parameter
1140 /// environment to see whether there are any projection predicates
1141 /// there that can answer this question.
1142 fn assemble_candidates_from_param_env<'cx, 'tcx>(
1143     selcx: &mut SelectionContext<'cx, 'tcx>,
1144     obligation: &ProjectionTyObligation<'tcx>,
1145     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1146 ) {
1147     debug!("assemble_candidates_from_param_env(..)");
1148     assemble_candidates_from_predicates(
1149         selcx,
1150         obligation,
1151         candidate_set,
1152         ProjectionTyCandidate::ParamEnv,
1153         obligation.param_env.caller_bounds().iter(),
1154         false,
1155     );
1156 }
1157
1158 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
1159 /// that the definition of `Foo` has some clues:
1160 ///
1161 /// ```
1162 /// trait Foo {
1163 ///     type FooT : Bar<BarT=i32>
1164 /// }
1165 /// ```
1166 ///
1167 /// Here, for example, we could conclude that the result is `i32`.
1168 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
1169     selcx: &mut SelectionContext<'cx, 'tcx>,
1170     obligation: &ProjectionTyObligation<'tcx>,
1171     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1172 ) {
1173     debug!("assemble_candidates_from_trait_def(..)");
1174
1175     let tcx = selcx.tcx();
1176     // Check whether the self-type is itself a projection.
1177     // If so, extract what we know from the trait and try to come up with a good answer.
1178     let bounds = match *obligation.predicate.self_ty().kind() {
1179         ty::Projection(ref data) => tcx.item_bounds(data.item_def_id).subst(tcx, data.substs),
1180         ty::Opaque(def_id, substs) => tcx.item_bounds(def_id).subst(tcx, substs),
1181         ty::Infer(ty::TyVar(_)) => {
1182             // If the self-type is an inference variable, then it MAY wind up
1183             // being a projected type, so induce an ambiguity.
1184             candidate_set.mark_ambiguous();
1185             return;
1186         }
1187         _ => return,
1188     };
1189
1190     assemble_candidates_from_predicates(
1191         selcx,
1192         obligation,
1193         candidate_set,
1194         ProjectionTyCandidate::TraitDef,
1195         bounds.iter(),
1196         true,
1197     )
1198 }
1199
1200 /// In the case of a trait object like
1201 /// `<dyn Iterator<Item = ()> as Iterator>::Item` we can use the existential
1202 /// predicate in the trait object.
1203 ///
1204 /// We don't go through the select candidate for these bounds to avoid cycles:
1205 /// In the above case, `dyn Iterator<Item = ()>: Iterator` would create a
1206 /// nested obligation of `<dyn Iterator<Item = ()> as Iterator>::Item: Sized`,
1207 /// this then has to be normalized without having to prove
1208 /// `dyn Iterator<Item = ()>: Iterator` again.
1209 fn assemble_candidates_from_object_ty<'cx, 'tcx>(
1210     selcx: &mut SelectionContext<'cx, 'tcx>,
1211     obligation: &ProjectionTyObligation<'tcx>,
1212     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1213 ) {
1214     debug!("assemble_candidates_from_object_ty(..)");
1215
1216     let tcx = selcx.tcx();
1217
1218     let self_ty = obligation.predicate.self_ty();
1219     let object_ty = selcx.infcx().shallow_resolve(self_ty);
1220     let data = match object_ty.kind() {
1221         ty::Dynamic(data, ..) => data,
1222         ty::Infer(ty::TyVar(_)) => {
1223             // If the self-type is an inference variable, then it MAY wind up
1224             // being an object type, so induce an ambiguity.
1225             candidate_set.mark_ambiguous();
1226             return;
1227         }
1228         _ => return,
1229     };
1230     let env_predicates = data
1231         .projection_bounds()
1232         .filter(|bound| bound.item_def_id() == obligation.predicate.item_def_id)
1233         .map(|p| p.with_self_ty(tcx, object_ty).to_predicate(tcx));
1234
1235     assemble_candidates_from_predicates(
1236         selcx,
1237         obligation,
1238         candidate_set,
1239         ProjectionTyCandidate::Object,
1240         env_predicates,
1241         false,
1242     );
1243 }
1244
1245 fn assemble_candidates_from_predicates<'cx, 'tcx>(
1246     selcx: &mut SelectionContext<'cx, 'tcx>,
1247     obligation: &ProjectionTyObligation<'tcx>,
1248     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1249     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
1250     env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
1251     potentially_unnormalized_candidates: bool,
1252 ) {
1253     debug!(?obligation, "assemble_candidates_from_predicates");
1254
1255     let infcx = selcx.infcx();
1256     for predicate in env_predicates {
1257         debug!(?predicate);
1258         let bound_predicate = predicate.kind();
1259         if let ty::PredicateKind::Projection(data) = predicate.kind().skip_binder() {
1260             let data = bound_predicate.rebind(data);
1261             let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1262
1263             let is_match = same_def_id
1264                 && infcx.probe(|_| {
1265                     selcx.match_projection_projections(
1266                         obligation,
1267                         data,
1268                         potentially_unnormalized_candidates,
1269                     )
1270                 });
1271
1272             debug!(?data, ?is_match, ?same_def_id);
1273
1274             if is_match {
1275                 candidate_set.push_candidate(ctor(data));
1276
1277                 if potentially_unnormalized_candidates
1278                     && !obligation.predicate.has_infer_types_or_consts()
1279                 {
1280                     // HACK: Pick the first trait def candidate for a fully
1281                     // inferred predicate. This is to allow duplicates that
1282                     // differ only in normalization.
1283                     return;
1284                 }
1285             }
1286         }
1287     }
1288 }
1289
1290 fn assemble_candidates_from_impls<'cx, 'tcx>(
1291     selcx: &mut SelectionContext<'cx, 'tcx>,
1292     obligation: &ProjectionTyObligation<'tcx>,
1293     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1294 ) {
1295     debug!("assemble_candidates_from_impls");
1296
1297     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1298     // start out by selecting the predicate `T as TraitRef<...>`:
1299     let poly_trait_ref = ty::Binder::dummy(obligation.predicate.trait_ref(selcx.tcx()));
1300     let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1301     let _ = selcx.infcx().commit_if_ok(|_| {
1302         let impl_source = match selcx.select(&trait_obligation) {
1303             Ok(Some(impl_source)) => impl_source,
1304             Ok(None) => {
1305                 candidate_set.mark_ambiguous();
1306                 return Err(());
1307             }
1308             Err(e) => {
1309                 debug!(error = ?e, "selection error");
1310                 candidate_set.mark_error(e);
1311                 return Err(());
1312             }
1313         };
1314
1315         let eligible = match &impl_source {
1316             super::ImplSource::Closure(_)
1317             | super::ImplSource::Generator(_)
1318             | super::ImplSource::FnPointer(_)
1319             | super::ImplSource::TraitAlias(_) => {
1320                 debug!(?impl_source);
1321                 true
1322             }
1323             super::ImplSource::UserDefined(impl_data) => {
1324                 // We have to be careful when projecting out of an
1325                 // impl because of specialization. If we are not in
1326                 // codegen (i.e., projection mode is not "any"), and the
1327                 // impl's type is declared as default, then we disable
1328                 // projection (even if the trait ref is fully
1329                 // monomorphic). In the case where trait ref is not
1330                 // fully monomorphic (i.e., includes type parameters),
1331                 // this is because those type parameters may
1332                 // ultimately be bound to types from other crates that
1333                 // may have specialized impls we can't see. In the
1334                 // case where the trait ref IS fully monomorphic, this
1335                 // is a policy decision that we made in the RFC in
1336                 // order to preserve flexibility for the crate that
1337                 // defined the specializable impl to specialize later
1338                 // for existing types.
1339                 //
1340                 // In either case, we handle this by not adding a
1341                 // candidate for an impl if it contains a `default`
1342                 // type.
1343                 //
1344                 // NOTE: This should be kept in sync with the similar code in
1345                 // `rustc_ty_utils::instance::resolve_associated_item()`.
1346                 let node_item =
1347                     assoc_ty_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
1348                         .map_err(|ErrorReported| ())?;
1349
1350                 if node_item.is_final() {
1351                     // Non-specializable items are always projectable.
1352                     true
1353                 } else {
1354                     // Only reveal a specializable default if we're past type-checking
1355                     // and the obligation is monomorphic, otherwise passes such as
1356                     // transmute checking and polymorphic MIR optimizations could
1357                     // get a result which isn't correct for all monomorphizations.
1358                     if obligation.param_env.reveal() == Reveal::All {
1359                         // NOTE(eddyb) inference variables can resolve to parameters, so
1360                         // assume `poly_trait_ref` isn't monomorphic, if it contains any.
1361                         let poly_trait_ref = selcx.infcx().resolve_vars_if_possible(poly_trait_ref);
1362                         !poly_trait_ref.still_further_specializable()
1363                     } else {
1364                         debug!(
1365                             assoc_ty = ?selcx.tcx().def_path_str(node_item.item.def_id),
1366                             ?obligation.predicate,
1367                             "assemble_candidates_from_impls: not eligible due to default",
1368                         );
1369                         false
1370                     }
1371                 }
1372             }
1373             super::ImplSource::DiscriminantKind(..) => {
1374                 // While `DiscriminantKind` is automatically implemented for every type,
1375                 // the concrete discriminant may not be known yet.
1376                 //
1377                 // Any type with multiple potential discriminant types is therefore not eligible.
1378                 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1379
1380                 match self_ty.kind() {
1381                     ty::Bool
1382                     | ty::Char
1383                     | ty::Int(_)
1384                     | ty::Uint(_)
1385                     | ty::Float(_)
1386                     | ty::Adt(..)
1387                     | ty::Foreign(_)
1388                     | ty::Str
1389                     | ty::Array(..)
1390                     | ty::Slice(_)
1391                     | ty::RawPtr(..)
1392                     | ty::Ref(..)
1393                     | ty::FnDef(..)
1394                     | ty::FnPtr(..)
1395                     | ty::Dynamic(..)
1396                     | ty::Closure(..)
1397                     | ty::Generator(..)
1398                     | ty::GeneratorWitness(..)
1399                     | ty::Never
1400                     | ty::Tuple(..)
1401                     // Integers and floats always have `u8` as their discriminant.
1402                     | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1403
1404                     ty::Projection(..)
1405                     | ty::Opaque(..)
1406                     | ty::Param(..)
1407                     | ty::Bound(..)
1408                     | ty::Placeholder(..)
1409                     | ty::Infer(..)
1410                     | ty::Error(_) => false,
1411                 }
1412             }
1413             super::ImplSource::Pointee(..) => {
1414                 // While `Pointee` is automatically implemented for every type,
1415                 // the concrete metadata type may not be known yet.
1416                 //
1417                 // Any type with multiple potential metadata types is therefore not eligible.
1418                 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1419
1420                 // FIXME: should this normalize?
1421                 let tail = selcx.tcx().struct_tail_without_normalization(self_ty);
1422                 match tail.kind() {
1423                     ty::Bool
1424                     | ty::Char
1425                     | ty::Int(_)
1426                     | ty::Uint(_)
1427                     | ty::Float(_)
1428                     | ty::Foreign(_)
1429                     | ty::Str
1430                     | ty::Array(..)
1431                     | ty::Slice(_)
1432                     | ty::RawPtr(..)
1433                     | ty::Ref(..)
1434                     | ty::FnDef(..)
1435                     | ty::FnPtr(..)
1436                     | ty::Dynamic(..)
1437                     | ty::Closure(..)
1438                     | ty::Generator(..)
1439                     | ty::GeneratorWitness(..)
1440                     | ty::Never
1441                     // If returned by `struct_tail_without_normalization` this is a unit struct
1442                     // without any fields, or not a struct, and therefore is Sized.
1443                     | ty::Adt(..)
1444                     // If returned by `struct_tail_without_normalization` this is the empty tuple.
1445                     | ty::Tuple(..)
1446                     // Integers and floats are always Sized, and so have unit type metadata.
1447                     | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1448
1449                     ty::Projection(..)
1450                     | ty::Opaque(..)
1451                     | ty::Param(..)
1452                     | ty::Bound(..)
1453                     | ty::Placeholder(..)
1454                     | ty::Infer(..)
1455                     | ty::Error(_) => false,
1456                 }
1457             }
1458             super::ImplSource::Param(..) => {
1459                 // This case tell us nothing about the value of an
1460                 // associated type. Consider:
1461                 //
1462                 // ```
1463                 // trait SomeTrait { type Foo; }
1464                 // fn foo<T:SomeTrait>(...) { }
1465                 // ```
1466                 //
1467                 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1468                 // : SomeTrait` binding does not help us decide what the
1469                 // type `Foo` is (at least, not more specifically than
1470                 // what we already knew).
1471                 //
1472                 // But wait, you say! What about an example like this:
1473                 //
1474                 // ```
1475                 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1476                 // ```
1477                 //
1478                 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1479                 // resolve `T::Foo`? And of course it does, but in fact
1480                 // that single predicate is desugared into two predicates
1481                 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1482                 // projection. And the projection where clause is handled
1483                 // in `assemble_candidates_from_param_env`.
1484                 false
1485             }
1486             super::ImplSource::Object(_) => {
1487                 // Handled by the `Object` projection candidate. See
1488                 // `assemble_candidates_from_object_ty` for an explanation of
1489                 // why we special case object types.
1490                 false
1491             }
1492             super::ImplSource::AutoImpl(..)
1493             | super::ImplSource::Builtin(..)
1494             | super::ImplSource::TraitUpcasting(_)
1495             | super::ImplSource::ConstDrop(_) => {
1496                 // These traits have no associated types.
1497                 selcx.tcx().sess.delay_span_bug(
1498                     obligation.cause.span,
1499                     &format!("Cannot project an associated type from `{:?}`", impl_source),
1500                 );
1501                 return Err(());
1502             }
1503         };
1504
1505         if eligible {
1506             if candidate_set.push_candidate(ProjectionTyCandidate::Select(impl_source)) {
1507                 Ok(())
1508             } else {
1509                 Err(())
1510             }
1511         } else {
1512             Err(())
1513         }
1514     });
1515 }
1516
1517 fn confirm_candidate<'cx, 'tcx>(
1518     selcx: &mut SelectionContext<'cx, 'tcx>,
1519     obligation: &ProjectionTyObligation<'tcx>,
1520     candidate: ProjectionTyCandidate<'tcx>,
1521 ) -> Progress<'tcx> {
1522     debug!(?obligation, ?candidate, "confirm_candidate");
1523     let mut progress = match candidate {
1524         ProjectionTyCandidate::ParamEnv(poly_projection)
1525         | ProjectionTyCandidate::Object(poly_projection) => {
1526             confirm_param_env_candidate(selcx, obligation, poly_projection, false)
1527         }
1528
1529         ProjectionTyCandidate::TraitDef(poly_projection) => {
1530             confirm_param_env_candidate(selcx, obligation, poly_projection, true)
1531         }
1532
1533         ProjectionTyCandidate::Select(impl_source) => {
1534             confirm_select_candidate(selcx, obligation, impl_source)
1535         }
1536     };
1537     // When checking for cycle during evaluation, we compare predicates with
1538     // "syntactic" equality. Since normalization generally introduces a type
1539     // with new region variables, we need to resolve them to existing variables
1540     // when possible for this to work. See `auto-trait-projection-recursion.rs`
1541     // for a case where this matters.
1542     if progress.ty.has_infer_regions() {
1543         progress.ty = OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty);
1544     }
1545     progress
1546 }
1547
1548 fn confirm_select_candidate<'cx, 'tcx>(
1549     selcx: &mut SelectionContext<'cx, 'tcx>,
1550     obligation: &ProjectionTyObligation<'tcx>,
1551     impl_source: Selection<'tcx>,
1552 ) -> Progress<'tcx> {
1553     match impl_source {
1554         super::ImplSource::UserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
1555         super::ImplSource::Generator(data) => confirm_generator_candidate(selcx, obligation, data),
1556         super::ImplSource::Closure(data) => confirm_closure_candidate(selcx, obligation, data),
1557         super::ImplSource::FnPointer(data) => confirm_fn_pointer_candidate(selcx, obligation, data),
1558         super::ImplSource::DiscriminantKind(data) => {
1559             confirm_discriminant_kind_candidate(selcx, obligation, data)
1560         }
1561         super::ImplSource::Pointee(data) => confirm_pointee_candidate(selcx, obligation, data),
1562         super::ImplSource::Object(_)
1563         | super::ImplSource::AutoImpl(..)
1564         | super::ImplSource::Param(..)
1565         | super::ImplSource::Builtin(..)
1566         | super::ImplSource::TraitUpcasting(_)
1567         | super::ImplSource::TraitAlias(..)
1568         | super::ImplSource::ConstDrop(_) => {
1569             // we don't create Select candidates with this kind of resolution
1570             span_bug!(
1571                 obligation.cause.span,
1572                 "Cannot project an associated type from `{:?}`",
1573                 impl_source
1574             )
1575         }
1576     }
1577 }
1578
1579 fn confirm_generator_candidate<'cx, 'tcx>(
1580     selcx: &mut SelectionContext<'cx, 'tcx>,
1581     obligation: &ProjectionTyObligation<'tcx>,
1582     impl_source: ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>,
1583 ) -> Progress<'tcx> {
1584     let gen_sig = impl_source.substs.as_generator().poly_sig();
1585     let Normalized { value: gen_sig, obligations } = normalize_with_depth(
1586         selcx,
1587         obligation.param_env,
1588         obligation.cause.clone(),
1589         obligation.recursion_depth + 1,
1590         gen_sig,
1591     );
1592
1593     debug!(?obligation, ?gen_sig, ?obligations, "confirm_generator_candidate");
1594
1595     let tcx = selcx.tcx();
1596
1597     let gen_def_id = tcx.require_lang_item(LangItem::Generator, None);
1598
1599     let predicate = super::util::generator_trait_ref_and_outputs(
1600         tcx,
1601         gen_def_id,
1602         obligation.predicate.self_ty(),
1603         gen_sig,
1604     )
1605     .map_bound(|(trait_ref, yield_ty, return_ty)| {
1606         let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1607         let ty = if name == sym::Return {
1608             return_ty
1609         } else if name == sym::Yield {
1610             yield_ty
1611         } else {
1612             bug!()
1613         };
1614
1615         ty::ProjectionPredicate {
1616             projection_ty: ty::ProjectionTy {
1617                 substs: trait_ref.substs,
1618                 item_def_id: obligation.predicate.item_def_id,
1619             },
1620             ty,
1621         }
1622     });
1623
1624     confirm_param_env_candidate(selcx, obligation, predicate, false)
1625         .with_addl_obligations(impl_source.nested)
1626         .with_addl_obligations(obligations)
1627 }
1628
1629 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
1630     selcx: &mut SelectionContext<'cx, 'tcx>,
1631     obligation: &ProjectionTyObligation<'tcx>,
1632     _: ImplSourceDiscriminantKindData,
1633 ) -> Progress<'tcx> {
1634     let tcx = selcx.tcx();
1635
1636     let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1637     // We get here from `poly_project_and_unify_type` which replaces bound vars
1638     // with placeholders
1639     debug_assert!(!self_ty.has_escaping_bound_vars());
1640     let substs = tcx.mk_substs([self_ty.into()].iter());
1641
1642     let discriminant_def_id = tcx.require_lang_item(LangItem::Discriminant, None);
1643
1644     let predicate = ty::ProjectionPredicate {
1645         projection_ty: ty::ProjectionTy { substs, item_def_id: discriminant_def_id },
1646         ty: self_ty.discriminant_ty(tcx),
1647     };
1648
1649     // We get here from `poly_project_and_unify_type` which replaces bound vars
1650     // with placeholders, so dummy is okay here.
1651     confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
1652 }
1653
1654 fn confirm_pointee_candidate<'cx, 'tcx>(
1655     selcx: &mut SelectionContext<'cx, 'tcx>,
1656     obligation: &ProjectionTyObligation<'tcx>,
1657     _: ImplSourcePointeeData,
1658 ) -> Progress<'tcx> {
1659     let tcx = selcx.tcx();
1660
1661     let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1662     let substs = tcx.mk_substs([self_ty.into()].iter());
1663
1664     let metadata_def_id = tcx.require_lang_item(LangItem::Metadata, None);
1665
1666     let predicate = ty::ProjectionPredicate {
1667         projection_ty: ty::ProjectionTy { substs, item_def_id: metadata_def_id },
1668         ty: self_ty.ptr_metadata_ty(tcx),
1669     };
1670
1671     confirm_param_env_candidate(selcx, obligation, ty::Binder::dummy(predicate), false)
1672 }
1673
1674 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1675     selcx: &mut SelectionContext<'cx, 'tcx>,
1676     obligation: &ProjectionTyObligation<'tcx>,
1677     fn_pointer_impl_source: ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>,
1678 ) -> Progress<'tcx> {
1679     let fn_type = selcx.infcx().shallow_resolve(fn_pointer_impl_source.fn_ty);
1680     let sig = fn_type.fn_sig(selcx.tcx());
1681     let Normalized { value: sig, obligations } = normalize_with_depth(
1682         selcx,
1683         obligation.param_env,
1684         obligation.cause.clone(),
1685         obligation.recursion_depth + 1,
1686         sig,
1687     );
1688
1689     confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1690         .with_addl_obligations(fn_pointer_impl_source.nested)
1691         .with_addl_obligations(obligations)
1692 }
1693
1694 fn confirm_closure_candidate<'cx, 'tcx>(
1695     selcx: &mut SelectionContext<'cx, 'tcx>,
1696     obligation: &ProjectionTyObligation<'tcx>,
1697     impl_source: ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>,
1698 ) -> Progress<'tcx> {
1699     let closure_sig = impl_source.substs.as_closure().sig();
1700     let Normalized { value: closure_sig, obligations } = normalize_with_depth(
1701         selcx,
1702         obligation.param_env,
1703         obligation.cause.clone(),
1704         obligation.recursion_depth + 1,
1705         closure_sig,
1706     );
1707
1708     debug!(?obligation, ?closure_sig, ?obligations, "confirm_closure_candidate");
1709
1710     confirm_callable_candidate(selcx, obligation, closure_sig, util::TupleArgumentsFlag::No)
1711         .with_addl_obligations(impl_source.nested)
1712         .with_addl_obligations(obligations)
1713 }
1714
1715 fn confirm_callable_candidate<'cx, 'tcx>(
1716     selcx: &mut SelectionContext<'cx, 'tcx>,
1717     obligation: &ProjectionTyObligation<'tcx>,
1718     fn_sig: ty::PolyFnSig<'tcx>,
1719     flag: util::TupleArgumentsFlag,
1720 ) -> Progress<'tcx> {
1721     let tcx = selcx.tcx();
1722
1723     debug!(?obligation, ?fn_sig, "confirm_callable_candidate");
1724
1725     let fn_once_def_id = tcx.require_lang_item(LangItem::FnOnce, None);
1726     let fn_once_output_def_id = tcx.require_lang_item(LangItem::FnOnceOutput, None);
1727
1728     let predicate = super::util::closure_trait_ref_and_return_type(
1729         tcx,
1730         fn_once_def_id,
1731         obligation.predicate.self_ty(),
1732         fn_sig,
1733         flag,
1734     )
1735     .map_bound(|(trait_ref, ret_type)| ty::ProjectionPredicate {
1736         projection_ty: ty::ProjectionTy {
1737             substs: trait_ref.substs,
1738             item_def_id: fn_once_output_def_id,
1739         },
1740         ty: ret_type,
1741     });
1742
1743     confirm_param_env_candidate(selcx, obligation, predicate, true)
1744 }
1745
1746 fn confirm_param_env_candidate<'cx, 'tcx>(
1747     selcx: &mut SelectionContext<'cx, 'tcx>,
1748     obligation: &ProjectionTyObligation<'tcx>,
1749     poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1750     potentially_unnormalized_candidate: bool,
1751 ) -> Progress<'tcx> {
1752     let infcx = selcx.infcx();
1753     let cause = &obligation.cause;
1754     let param_env = obligation.param_env;
1755
1756     let (cache_entry, _) = infcx.replace_bound_vars_with_fresh_vars(
1757         cause.span,
1758         LateBoundRegionConversionTime::HigherRankedType,
1759         poly_cache_entry,
1760     );
1761
1762     let cache_projection = cache_entry.projection_ty;
1763     let mut nested_obligations = Vec::new();
1764     let obligation_projection = obligation.predicate;
1765     let obligation_projection = ensure_sufficient_stack(|| {
1766         normalize_with_depth_to(
1767             selcx,
1768             obligation.param_env,
1769             obligation.cause.clone(),
1770             obligation.recursion_depth + 1,
1771             obligation_projection,
1772             &mut nested_obligations,
1773         )
1774     });
1775     let cache_projection = if potentially_unnormalized_candidate {
1776         ensure_sufficient_stack(|| {
1777             normalize_with_depth_to(
1778                 selcx,
1779                 obligation.param_env,
1780                 obligation.cause.clone(),
1781                 obligation.recursion_depth + 1,
1782                 cache_projection,
1783                 &mut nested_obligations,
1784             )
1785         })
1786     } else {
1787         cache_projection
1788     };
1789
1790     debug!(?cache_projection, ?obligation_projection);
1791
1792     match infcx.at(cause, param_env).eq(cache_projection, obligation_projection) {
1793         Ok(InferOk { value: _, obligations }) => {
1794             nested_obligations.extend(obligations);
1795             assoc_ty_own_obligations(selcx, obligation, &mut nested_obligations);
1796             Progress { ty: cache_entry.ty, obligations: nested_obligations }
1797         }
1798         Err(e) => {
1799             let msg = format!(
1800                 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1801                 obligation, poly_cache_entry, e,
1802             );
1803             debug!("confirm_param_env_candidate: {}", msg);
1804             let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
1805             Progress { ty: err, obligations: vec![] }
1806         }
1807     }
1808 }
1809
1810 fn confirm_impl_candidate<'cx, 'tcx>(
1811     selcx: &mut SelectionContext<'cx, 'tcx>,
1812     obligation: &ProjectionTyObligation<'tcx>,
1813     impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
1814 ) -> Progress<'tcx> {
1815     let tcx = selcx.tcx();
1816
1817     let ImplSourceUserDefinedData { impl_def_id, substs, mut nested } = impl_impl_source;
1818     let assoc_item_id = obligation.predicate.item_def_id;
1819     let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
1820
1821     let param_env = obligation.param_env;
1822     let assoc_ty = match assoc_ty_def(selcx, impl_def_id, assoc_item_id) {
1823         Ok(assoc_ty) => assoc_ty,
1824         Err(ErrorReported) => return Progress { ty: tcx.ty_error(), obligations: nested },
1825     };
1826
1827     if !assoc_ty.item.defaultness.has_value() {
1828         // This means that the impl is missing a definition for the
1829         // associated type. This error will be reported by the type
1830         // checker method `check_impl_items_against_trait`, so here we
1831         // just return Error.
1832         debug!(
1833             "confirm_impl_candidate: no associated type {:?} for {:?}",
1834             assoc_ty.item.ident, obligation.predicate
1835         );
1836         return Progress { ty: tcx.ty_error(), obligations: nested };
1837     }
1838     // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
1839     //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
1840     //
1841     // * `obligation.predicate.substs` is `[Vec<u32>, S]`
1842     // * `substs` is `[u32]`
1843     // * `substs` ends up as `[u32, S]`
1844     let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
1845     let substs =
1846         translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);
1847     let ty = tcx.type_of(assoc_ty.item.def_id);
1848     if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
1849         let err = tcx.ty_error_with_message(
1850             obligation.cause.span,
1851             "impl item and trait item have different parameter counts",
1852         );
1853         Progress { ty: err, obligations: nested }
1854     } else {
1855         assoc_ty_own_obligations(selcx, obligation, &mut nested);
1856         Progress { ty: ty.subst(tcx, substs), obligations: nested }
1857     }
1858 }
1859
1860 // Get obligations corresponding to the predicates from the where-clause of the
1861 // associated type itself.
1862 // Note: `feature(generic_associated_types)` is required to write such
1863 // predicates, even for non-generic associcated types.
1864 fn assoc_ty_own_obligations<'cx, 'tcx>(
1865     selcx: &mut SelectionContext<'cx, 'tcx>,
1866     obligation: &ProjectionTyObligation<'tcx>,
1867     nested: &mut Vec<PredicateObligation<'tcx>>,
1868 ) {
1869     let tcx = selcx.tcx();
1870     for predicate in tcx
1871         .predicates_of(obligation.predicate.item_def_id)
1872         .instantiate_own(tcx, obligation.predicate.substs)
1873         .predicates
1874     {
1875         let normalized = normalize_with_depth_to(
1876             selcx,
1877             obligation.param_env,
1878             obligation.cause.clone(),
1879             obligation.recursion_depth + 1,
1880             predicate,
1881             nested,
1882         );
1883         nested.push(Obligation::with_depth(
1884             obligation.cause.clone(),
1885             obligation.recursion_depth + 1,
1886             obligation.param_env,
1887             normalized,
1888         ));
1889     }
1890 }
1891
1892 /// Locate the definition of an associated type in the specialization hierarchy,
1893 /// starting from the given impl.
1894 ///
1895 /// Based on the "projection mode", this lookup may in fact only examine the
1896 /// topmost impl. See the comments for `Reveal` for more details.
1897 fn assoc_ty_def(
1898     selcx: &SelectionContext<'_, '_>,
1899     impl_def_id: DefId,
1900     assoc_ty_def_id: DefId,
1901 ) -> Result<specialization_graph::LeafDef, ErrorReported> {
1902     let tcx = selcx.tcx();
1903     let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1904     let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1905     let trait_def = tcx.trait_def(trait_def_id);
1906
1907     // This function may be called while we are still building the
1908     // specialization graph that is queried below (via TraitDef::ancestors()),
1909     // so, in order to avoid unnecessary infinite recursion, we manually look
1910     // for the associated item at the given impl.
1911     // If there is no such item in that impl, this function will fail with a
1912     // cycle error if the specialization graph is currently being built.
1913     let impl_node = specialization_graph::Node::Impl(impl_def_id);
1914     for item in impl_node.items(tcx) {
1915         if matches!(item.kind, ty::AssocKind::Type)
1916             && tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id)
1917         {
1918             return Ok(specialization_graph::LeafDef {
1919                 item: *item,
1920                 defining_node: impl_node,
1921                 finalizing_node: if item.defaultness.is_default() { None } else { Some(impl_node) },
1922             });
1923         }
1924     }
1925
1926     let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
1927     if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
1928         Ok(assoc_item)
1929     } else {
1930         // This is saying that neither the trait nor
1931         // the impl contain a definition for this
1932         // associated type.  Normally this situation
1933         // could only arise through a compiler bug --
1934         // if the user wrote a bad item name, it
1935         // should have failed in astconv.
1936         bug!("No associated type `{}` for {}", assoc_ty_name, tcx.def_path_str(impl_def_id))
1937     }
1938 }
1939
1940 crate trait ProjectionCacheKeyExt<'tcx>: Sized {
1941     fn from_poly_projection_predicate(
1942         selcx: &mut SelectionContext<'cx, 'tcx>,
1943         predicate: ty::PolyProjectionPredicate<'tcx>,
1944     ) -> Option<Self>;
1945 }
1946
1947 impl<'tcx> ProjectionCacheKeyExt<'tcx> for ProjectionCacheKey<'tcx> {
1948     fn from_poly_projection_predicate(
1949         selcx: &mut SelectionContext<'cx, 'tcx>,
1950         predicate: ty::PolyProjectionPredicate<'tcx>,
1951     ) -> Option<Self> {
1952         let infcx = selcx.infcx();
1953         // We don't do cross-snapshot caching of obligations with escaping regions,
1954         // so there's no cache key to use
1955         predicate.no_bound_vars().map(|predicate| {
1956             ProjectionCacheKey::new(
1957                 // We don't attempt to match up with a specific type-variable state
1958                 // from a specific call to `opt_normalize_projection_type` - if
1959                 // there's no precise match, the original cache entry is "stranded"
1960                 // anyway.
1961                 infcx.resolve_vars_if_possible(predicate.projection_ty),
1962             )
1963         })
1964     }
1965 }