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