src/librustc_trait_selection/traits/project.rs

   1 //! Code for projecting associated types out of trait references.
   2
   3 use super::elaborate_predicates;
   4 use super::specialization_graph;
   5 use super::translate_substs;
   6 use super::util;
   7 use super::MismatchedProjectionTypes;
   8 use super::Obligation;
   9 use super::ObligationCause;
  10 use super::PredicateObligation;
  11 use super::Selection;
  12 use super::SelectionContext;
  13 use super::SelectionError;
  14 use super::{
  15     ImplSourceClosureData, ImplSourceDiscriminantKindData, ImplSourceFnPointerData,
  16     ImplSourceGeneratorData, 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;
  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::{
  27     DiscriminantTypeLangItem, FnOnceOutputLangItem, FnOnceTraitLangItem, GeneratorTraitLangItem,
  28 };
  29 use rustc_infer::infer::resolve::OpportunisticRegionResolver;
  30 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
  31 use rustc_middle::ty::subst::Subst;
  32 use rustc_middle::ty::{self, ToPolyTraitRef, ToPredicate, Ty, TyCtxt, WithConstness};
  33 use rustc_span::symbol::sym;
  34 use rustc_span::DUMMY_SP;
  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 /// When attempting to resolve `<T as TraitRef>::Name` ...
  45 #[derive(Debug)]
  46 pub enum ProjectionTyError<'tcx> {
  47     /// ...we found multiple sources of information and couldn't resolve the ambiguity.
  48     TooManyCandidates,
  49
  50     /// ...an error occurred matching `T : TraitRef`
  51     TraitSelectionError(SelectionError<'tcx>),
  52 }
  53
  54 #[derive(PartialEq, Eq, Debug)]
  55 enum ProjectionTyCandidate<'tcx> {
  56     // from a where-clause in the env or object type
  57     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
  58
  59     // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
  60     TraitDef(ty::PolyProjectionPredicate<'tcx>),
  61
  62     // from a "impl" (or a "pseudo-impl" returned by select)
  63     Select(Selection<'tcx>),
  64 }
  65
  66 enum ProjectionTyCandidateSet<'tcx> {
  67     None,
  68     Single(ProjectionTyCandidate<'tcx>),
  69     Ambiguous,
  70     Error(SelectionError<'tcx>),
  71 }
  72
  73 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
  74     fn mark_ambiguous(&mut self) {
  75         *self = ProjectionTyCandidateSet::Ambiguous;
  76     }
  77
  78     fn mark_error(&mut self, err: SelectionError<'tcx>) {
  79         *self = ProjectionTyCandidateSet::Error(err);
  80     }
  81
  82     // Returns true if the push was successful, or false if the candidate
  83     // was discarded -- this could be because of ambiguity, or because
  84     // a higher-priority candidate is already there.
  85     fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
  86         use self::ProjectionTyCandidate::*;
  87         use self::ProjectionTyCandidateSet::*;
  88
  89         // This wacky variable is just used to try and
  90         // make code readable and avoid confusing paths.
  91         // It is assigned a "value" of `()` only on those
  92         // paths in which we wish to convert `*self` to
  93         // ambiguous (and return false, because the candidate
  94         // was not used). On other paths, it is not assigned,
  95         // and hence if those paths *could* reach the code that
  96         // comes after the match, this fn would not compile.
  97         let convert_to_ambiguous;
  98
  99         match self {
 100             None => {
 101                 *self = Single(candidate);
 102                 return true;
 103             }
 104
 105             Single(current) => {
 106                 // Duplicates can happen inside ParamEnv. In the case, we
 107                 // perform a lazy deduplication.
 108                 if current == &candidate {
 109                     return false;
 110                 }
 111
 112                 // Prefer where-clauses. As in select, if there are multiple
 113                 // candidates, we prefer where-clause candidates over impls.  This
 114                 // may seem a bit surprising, since impls are the source of
 115                 // "truth" in some sense, but in fact some of the impls that SEEM
 116                 // applicable are not, because of nested obligations. Where
 117                 // clauses are the safer choice. See the comment on
 118                 // `select::SelectionCandidate` and #21974 for more details.
 119                 match (current, candidate) {
 120                     (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
 121                     (ParamEnv(..), _) => return false,
 122                     (_, ParamEnv(..)) => unreachable!(),
 123                     (_, _) => convert_to_ambiguous = (),
 124                 }
 125             }
 126
 127             Ambiguous | Error(..) => {
 128                 return false;
 129             }
 130         }
 131
 132         // We only ever get here when we moved from a single candidate
 133         // to ambiguous.
 134         let () = convert_to_ambiguous;
 135         *self = Ambiguous;
 136         false
 137     }
 138 }
 139
 140 /// Evaluates constraints of the form:
 141 ///
 142 ///     for<...> <T as Trait>::U == V
 143 ///
 144 /// If successful, this may result in additional obligations. Also returns
 145 /// the projection cache key used to track these additional obligations.
 146 pub fn poly_project_and_unify_type<'cx, 'tcx>(
 147     selcx: &mut SelectionContext<'cx, 'tcx>,
 148     obligation: &PolyProjectionObligation<'tcx>,
 149 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
 150     debug!("poly_project_and_unify_type(obligation={:?})", obligation);
 151
 152     let infcx = selcx.infcx();
 153     infcx.commit_if_ok(|_snapshot| {
 154         let (placeholder_predicate, _) =
 155             infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
 156
 157         let placeholder_obligation = obligation.with(placeholder_predicate);
 158         let result = project_and_unify_type(selcx, &placeholder_obligation)?;
 159         Ok(result)
 160     })
 161 }
 162
 163 /// Evaluates constraints of the form:
 164 ///
 165 ///     <T as Trait>::U == V
 166 ///
 167 /// If successful, this may result in additional obligations.
 168 fn project_and_unify_type<'cx, 'tcx>(
 169     selcx: &mut SelectionContext<'cx, 'tcx>,
 170     obligation: &ProjectionObligation<'tcx>,
 171 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
 172     debug!("project_and_unify_type(obligation={:?})", obligation);
 173
 174     let mut obligations = vec![];
 175     let normalized_ty = match opt_normalize_projection_type(
 176         selcx,
 177         obligation.param_env,
 178         obligation.predicate.projection_ty,
 179         obligation.cause.clone(),
 180         obligation.recursion_depth,
 181         &mut obligations,
 182     ) {
 183         Some(n) => n,
 184         None => return Ok(None),
 185     };
 186
 187     debug!(
 188         "project_and_unify_type: normalized_ty={:?} obligations={:?}",
 189         normalized_ty, obligations
 190     );
 191
 192     let infcx = selcx.infcx();
 193     match infcx
 194         .at(&obligation.cause, obligation.param_env)
 195         .eq(normalized_ty, obligation.predicate.ty)
 196     {
 197         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
 198             obligations.extend(inferred_obligations);
 199             Ok(Some(obligations))
 200         }
 201         Err(err) => {
 202             debug!("project_and_unify_type: equating types encountered error {:?}", err);
 203             Err(MismatchedProjectionTypes { err })
 204         }
 205     }
 206 }
 207
 208 /// Normalizes any associated type projections in `value`, replacing
 209 /// them with a fully resolved type where possible. The return value
 210 /// combines the normalized result and any additional obligations that
 211 /// were incurred as result.
 212 pub fn normalize<'a, 'b, 'tcx, T>(
 213     selcx: &'a mut SelectionContext<'b, 'tcx>,
 214     param_env: ty::ParamEnv<'tcx>,
 215     cause: ObligationCause<'tcx>,
 216     value: &T,
 217 ) -> Normalized<'tcx, T>
 218 where
 219     T: TypeFoldable<'tcx>,
 220 {
 221     let mut obligations = Vec::new();
 222     let value = normalize_to(selcx, param_env, cause, value, &mut obligations);
 223     Normalized { value, obligations }
 224 }
 225
 226 pub fn normalize_to<'a, 'b, 'tcx, T>(
 227     selcx: &'a mut SelectionContext<'b, 'tcx>,
 228     param_env: ty::ParamEnv<'tcx>,
 229     cause: ObligationCause<'tcx>,
 230     value: &T,
 231     obligations: &mut Vec<PredicateObligation<'tcx>>,
 232 ) -> T
 233 where
 234     T: TypeFoldable<'tcx>,
 235 {
 236     normalize_with_depth_to(selcx, param_env, cause, 0, value, obligations)
 237 }
 238
 239 /// As `normalize`, but with a custom depth.
 240 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
 241     selcx: &'a mut SelectionContext<'b, 'tcx>,
 242     param_env: ty::ParamEnv<'tcx>,
 243     cause: ObligationCause<'tcx>,
 244     depth: usize,
 245     value: &T,
 246 ) -> Normalized<'tcx, T>
 247 where
 248     T: TypeFoldable<'tcx>,
 249 {
 250     let mut obligations = Vec::new();
 251     let value = normalize_with_depth_to(selcx, param_env, cause, depth, value, &mut obligations);
 252     Normalized { value, obligations }
 253 }
 254
 255 pub fn normalize_with_depth_to<'a, 'b, 'tcx, T>(
 256     selcx: &'a mut SelectionContext<'b, 'tcx>,
 257     param_env: ty::ParamEnv<'tcx>,
 258     cause: ObligationCause<'tcx>,
 259     depth: usize,
 260     value: &T,
 261     obligations: &mut Vec<PredicateObligation<'tcx>>,
 262 ) -> T
 263 where
 264     T: TypeFoldable<'tcx>,
 265 {
 266     debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
 267     let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth, obligations);
 268     let result = ensure_sufficient_stack(|| normalizer.fold(value));
 269     debug!(
 270         "normalize_with_depth: depth={} result={:?} with {} obligations",
 271         depth,
 272         result,
 273         normalizer.obligations.len()
 274     );
 275     debug!("normalize_with_depth: depth={} obligations={:?}", depth, normalizer.obligations);
 276     result
 277 }
 278
 279 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
 280     selcx: &'a mut SelectionContext<'b, 'tcx>,
 281     param_env: ty::ParamEnv<'tcx>,
 282     cause: ObligationCause<'tcx>,
 283     obligations: &'a mut Vec<PredicateObligation<'tcx>>,
 284     depth: usize,
 285 }
 286
 287 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
 288     fn new(
 289         selcx: &'a mut SelectionContext<'b, 'tcx>,
 290         param_env: ty::ParamEnv<'tcx>,
 291         cause: ObligationCause<'tcx>,
 292         depth: usize,
 293         obligations: &'a mut Vec<PredicateObligation<'tcx>>,
 294     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
 295         AssocTypeNormalizer { selcx, param_env, cause, obligations, depth }
 296     }
 297
 298     fn fold<T: TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
 299         let value = self.selcx.infcx().resolve_vars_if_possible(value);
 300
 301         if !value.has_projections() { value } else { value.fold_with(self) }
 302     }
 303 }
 304
 305 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
 306     fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
 307         self.selcx.tcx()
 308     }
 309
 310     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 311         if !ty.has_projections() {
 312             return ty;
 313         }
 314         // We don't want to normalize associated types that occur inside of region
 315         // binders, because they may contain bound regions, and we can't cope with that.
 316         //
 317         // Example:
 318         //
 319         //     for<'a> fn(<T as Foo<&'a>>::A)
 320         //
 321         // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
 322         // normalize it when we instantiate those bound regions (which
 323         // should occur eventually).
 324
 325         let ty = ty.super_fold_with(self);
 326         match ty.kind {
 327             ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => {
 328                 // (*)
 329                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 330                 match self.param_env.reveal() {
 331                     Reveal::UserFacing => ty,
 332
 333                     Reveal::All => {
 334                         let recursion_limit = self.tcx().sess.recursion_limit();
 335                         if !recursion_limit.value_within_limit(self.depth) {
 336                             let obligation = Obligation::with_depth(
 337                                 self.cause.clone(),
 338                                 recursion_limit.0,
 339                                 self.param_env,
 340                                 ty,
 341                             );
 342                             self.selcx.infcx().report_overflow_error(&obligation, true);
 343                         }
 344
 345                         let generic_ty = self.tcx().type_of(def_id);
 346                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 347                         self.depth += 1;
 348                         let folded_ty = self.fold_ty(concrete_ty);
 349                         self.depth -= 1;
 350                         folded_ty
 351                     }
 352                 }
 353             }
 354
 355             ty::Projection(ref data) if !data.has_escaping_bound_vars() => {
 356                 // (*)
 357
 358                 // (*) This is kind of hacky -- we need to be able to
 359                 // handle normalization within binders because
 360                 // otherwise we wind up a need to normalize when doing
 361                 // trait matching (since you can have a trait
 362                 // obligation like `for<'a> T::B: Fn(&'a i32)`), but
 363                 // we can't normalize with bound regions in scope. So
 364                 // far now we just ignore binders but only normalize
 365                 // if all bound regions are gone (and then we still
 366                 // have to renormalize whenever we instantiate a
 367                 // binder). It would be better to normalize in a
 368                 // binding-aware fashion.
 369
 370                 let normalized_ty = normalize_projection_type(
 371                     self.selcx,
 372                     self.param_env,
 373                     *data,
 374                     self.cause.clone(),
 375                     self.depth,
 376                     &mut self.obligations,
 377                 );
 378                 debug!(
 379                     "AssocTypeNormalizer: depth={} normalized {:?} to {:?}, \
 380                      now with {} obligations",
 381                     self.depth,
 382                     ty,
 383                     normalized_ty,
 384                     self.obligations.len()
 385                 );
 386                 normalized_ty
 387             }
 388
 389             _ => ty,
 390         }
 391     }
 392
 393     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 394         if self.selcx.tcx().lazy_normalization() {
 395             constant
 396         } else {
 397             let constant = constant.super_fold_with(self);
 398             constant.eval(self.selcx.tcx(), self.param_env)
 399         }
 400     }
 401 }
 402
 403 /// The guts of `normalize`: normalize a specific projection like `<T
 404 /// as Trait>::Item`. The result is always a type (and possibly
 405 /// additional obligations). If ambiguity arises, which implies that
 406 /// there are unresolved type variables in the projection, we will
 407 /// substitute a fresh type variable `$X` and generate a new
 408 /// obligation `<T as Trait>::Item == $X` for later.
 409 pub fn normalize_projection_type<'a, 'b, 'tcx>(
 410     selcx: &'a mut SelectionContext<'b, 'tcx>,
 411     param_env: ty::ParamEnv<'tcx>,
 412     projection_ty: ty::ProjectionTy<'tcx>,
 413     cause: ObligationCause<'tcx>,
 414     depth: usize,
 415     obligations: &mut Vec<PredicateObligation<'tcx>>,
 416 ) -> Ty<'tcx> {
 417     opt_normalize_projection_type(
 418         selcx,
 419         param_env,
 420         projection_ty,
 421         cause.clone(),
 422         depth,
 423         obligations,
 424     )
 425     .unwrap_or_else(move || {
 426         // if we bottom out in ambiguity, create a type variable
 427         // and a deferred predicate to resolve this when more type
 428         // information is available.
 429
 430         let tcx = selcx.infcx().tcx;
 431         let def_id = projection_ty.item_def_id;
 432         let ty_var = selcx.infcx().next_ty_var(TypeVariableOrigin {
 433             kind: TypeVariableOriginKind::NormalizeProjectionType,
 434             span: tcx.def_span(def_id),
 435         });
 436         let projection = ty::Binder::dummy(ty::ProjectionPredicate { projection_ty, ty: ty_var });
 437         let obligation =
 438             Obligation::with_depth(cause, depth + 1, param_env, projection.to_predicate(tcx));
 439         obligations.push(obligation);
 440         ty_var
 441     })
 442 }
 443
 444 /// The guts of `normalize`: normalize a specific projection like `<T
 445 /// as Trait>::Item`. The result is always a type (and possibly
 446 /// additional obligations). Returns `None` in the case of ambiguity,
 447 /// which indicates that there are unbound type variables.
 448 ///
 449 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
 450 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
 451 /// often immediately appended to another obligations vector. So now this
 452 /// function takes an obligations vector and appends to it directly, which is
 453 /// slightly uglier but avoids the need for an extra short-lived allocation.
 454 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 455     selcx: &'a mut SelectionContext<'b, 'tcx>,
 456     param_env: ty::ParamEnv<'tcx>,
 457     projection_ty: ty::ProjectionTy<'tcx>,
 458     cause: ObligationCause<'tcx>,
 459     depth: usize,
 460     obligations: &mut Vec<PredicateObligation<'tcx>>,
 461 ) -> Option<Ty<'tcx>> {
 462     let infcx = selcx.infcx();
 463
 464     let projection_ty = infcx.resolve_vars_if_possible(&projection_ty);
 465     let cache_key = ProjectionCacheKey::new(projection_ty);
 466
 467     debug!(
 468         "opt_normalize_projection_type(\
 469          projection_ty={:?}, \
 470          depth={})",
 471         projection_ty, depth
 472     );
 473
 474     // FIXME(#20304) For now, I am caching here, which is good, but it
 475     // means we don't capture the type variables that are created in
 476     // the case of ambiguity. Which means we may create a large stream
 477     // of such variables. OTOH, if we move the caching up a level, we
 478     // would not benefit from caching when proving `T: Trait<U=Foo>`
 479     // bounds. It might be the case that we want two distinct caches,
 480     // or else another kind of cache entry.
 481
 482     let cache_result = infcx.inner.borrow_mut().projection_cache().try_start(cache_key);
 483     match cache_result {
 484         Ok(()) => {}
 485         Err(ProjectionCacheEntry::Ambiguous) => {
 486             // If we found ambiguity the last time, that means we will continue
 487             // to do so until some type in the key changes (and we know it
 488             // hasn't, because we just fully resolved it).
 489             debug!(
 490                 "opt_normalize_projection_type: \
 491                  found cache entry: ambiguous"
 492             );
 493             return None;
 494         }
 495         Err(ProjectionCacheEntry::InProgress) => {
 496             // If while normalized A::B, we are asked to normalize
 497             // A::B, just return A::B itself. This is a conservative
 498             // answer, in the sense that A::B *is* clearly equivalent
 499             // to A::B, though there may be a better value we can
 500             // find.
 501
 502             // Under lazy normalization, this can arise when
 503             // bootstrapping.  That is, imagine an environment with a
 504             // where-clause like `A::B == u32`. Now, if we are asked
 505             // to normalize `A::B`, we will want to check the
 506             // where-clauses in scope. So we will try to unify `A::B`
 507             // with `A::B`, which can trigger a recursive
 508             // normalization. In that case, I think we will want this code:
 509             //
 510             // ```
 511             // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
 512             //                                    projection_ty.substs;
 513             // return Some(NormalizedTy { value: v, obligations: vec![] });
 514             // ```
 515
 516             debug!(
 517                 "opt_normalize_projection_type: \
 518                  found cache entry: in-progress"
 519             );
 520
 521             // But for now, let's classify this as an overflow:
 522             let recursion_limit = selcx.tcx().sess.recursion_limit();
 523             let obligation =
 524                 Obligation::with_depth(cause, recursion_limit.0, param_env, projection_ty);
 525             selcx.infcx().report_overflow_error(&obligation, false);
 526         }
 527         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
 528             // This is the hottest path in this function.
 529             //
 530             // If we find the value in the cache, then return it along
 531             // with the obligations that went along with it. Note
 532             // that, when using a fulfillment context, these
 533             // obligations could in principle be ignored: they have
 534             // already been registered when the cache entry was
 535             // created (and hence the new ones will quickly be
 536             // discarded as duplicated). But when doing trait
 537             // evaluation this is not the case, and dropping the trait
 538             // evaluations can causes ICEs (e.g., #43132).
 539             debug!(
 540                 "opt_normalize_projection_type: \
 541                  found normalized ty `{:?}`",
 542                 ty
 543             );
 544
 545             // Once we have inferred everything we need to know, we
 546             // can ignore the `obligations` from that point on.
 547             if infcx.unresolved_type_vars(&ty.value).is_none() {
 548                 infcx.inner.borrow_mut().projection_cache().complete_normalized(cache_key, &ty);
 549             // No need to extend `obligations`.
 550             } else {
 551                 obligations.extend(ty.obligations);
 552             }
 553
 554             obligations.push(get_paranoid_cache_value_obligation(
 555                 infcx,
 556                 param_env,
 557                 projection_ty,
 558                 cause,
 559                 depth,
 560             ));
 561             return Some(ty.value);
 562         }
 563         Err(ProjectionCacheEntry::Error) => {
 564             debug!(
 565                 "opt_normalize_projection_type: \
 566                  found error"
 567             );
 568             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 569             obligations.extend(result.obligations);
 570             return Some(result.value);
 571         }
 572     }
 573
 574     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
 575     match project_type(selcx, &obligation) {
 576         Ok(ProjectedTy::Progress(Progress {
 577             ty: projected_ty,
 578             obligations: mut projected_obligations,
 579         })) => {
 580             // if projection succeeded, then what we get out of this
 581             // is also non-normalized (consider: it was derived from
 582             // an impl, where-clause etc) and hence we must
 583             // re-normalize it
 584
 585             debug!(
 586                 "opt_normalize_projection_type: \
 587                  projected_ty={:?} \
 588                  depth={} \
 589                  projected_obligations={:?}",
 590                 projected_ty, depth, projected_obligations
 591             );
 592
 593             let result = if projected_ty.has_projections() {
 594                 let mut normalizer = AssocTypeNormalizer::new(
 595                     selcx,
 596                     param_env,
 597                     cause,
 598                     depth + 1,
 599                     &mut projected_obligations,
 600                 );
 601                 let normalized_ty = normalizer.fold(&projected_ty);
 602
 603                 debug!(
 604                     "opt_normalize_projection_type: \
 605                      normalized_ty={:?} depth={}",
 606                     normalized_ty, depth
 607                 );
 608
 609                 Normalized { value: normalized_ty, obligations: projected_obligations }
 610             } else {
 611                 Normalized { value: projected_ty, obligations: projected_obligations }
 612             };
 613
 614             let cache_value = prune_cache_value_obligations(infcx, &result);
 615             infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, cache_value);
 616             obligations.extend(result.obligations);
 617             Some(result.value)
 618         }
 619         Ok(ProjectedTy::NoProgress(projected_ty)) => {
 620             debug!(
 621                 "opt_normalize_projection_type: \
 622                  projected_ty={:?} no progress",
 623                 projected_ty
 624             );
 625             let result = Normalized { value: projected_ty, obligations: vec![] };
 626             infcx.inner.borrow_mut().projection_cache().insert_ty(cache_key, result.clone());
 627             // No need to extend `obligations`.
 628             Some(result.value)
 629         }
 630         Err(ProjectionTyError::TooManyCandidates) => {
 631             debug!(
 632                 "opt_normalize_projection_type: \
 633                  too many candidates"
 634             );
 635             infcx.inner.borrow_mut().projection_cache().ambiguous(cache_key);
 636             None
 637         }
 638         Err(ProjectionTyError::TraitSelectionError(_)) => {
 639             debug!("opt_normalize_projection_type: ERROR");
 640             // if we got an error processing the `T as Trait` part,
 641             // just return `ty::err` but add the obligation `T :
 642             // Trait`, which when processed will cause the error to be
 643             // reported later
 644
 645             infcx.inner.borrow_mut().projection_cache().error(cache_key);
 646             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 647             obligations.extend(result.obligations);
 648             Some(result.value)
 649         }
 650     }
 651 }
 652
 653 /// If there are unresolved type variables, then we need to include
 654 /// any subobligations that bind them, at least until those type
 655 /// variables are fully resolved.
 656 fn prune_cache_value_obligations<'a, 'tcx>(
 657     infcx: &'a InferCtxt<'a, 'tcx>,
 658     result: &NormalizedTy<'tcx>,
 659 ) -> NormalizedTy<'tcx> {
 660     if infcx.unresolved_type_vars(&result.value).is_none() {
 661         return NormalizedTy { value: result.value, obligations: vec![] };
 662     }
 663
 664     let mut obligations: Vec<_> = result
 665         .obligations
 666         .iter()
 667         .filter(|obligation| match obligation.predicate.kind() {
 668             // We found a `T: Foo<X = U>` predicate, let's check
 669             // if `U` references any unresolved type
 670             // variables. In principle, we only care if this
 671             // projection can help resolve any of the type
 672             // variables found in `result.value` -- but we just
 673             // check for any type variables here, for fear of
 674             // indirect obligations (e.g., we project to `?0`,
 675             // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
 676             // ?0>`).
 677             ty::PredicateKind::Projection(ref data) => {
 678                 infcx.unresolved_type_vars(&data.ty()).is_some()
 679             }
 680
 681             // We are only interested in `T: Foo<X = U>` predicates, whre
 682             // `U` references one of `unresolved_type_vars`. =)
 683             _ => false,
 684         })
 685         .cloned()
 686         .collect();
 687
 688     obligations.shrink_to_fit();
 689
 690     NormalizedTy { value: result.value, obligations }
 691 }
 692
 693 /// Whenever we give back a cache result for a projection like `<T as
 694 /// Trait>::Item ==> X`, we *always* include the obligation to prove
 695 /// that `T: Trait` (we may also include some other obligations). This
 696 /// may or may not be necessary -- in principle, all the obligations
 697 /// that must be proven to show that `T: Trait` were also returned
 698 /// when the cache was first populated. But there are some vague concerns,
 699 /// and so we take the precautionary measure of including `T: Trait` in
 700 /// the result:
 701 ///
 702 /// Concern #1. The current setup is fragile. Perhaps someone could
 703 /// have failed to prove the concerns from when the cache was
 704 /// populated, but also not have used a snapshot, in which case the
 705 /// cache could remain populated even though `T: Trait` has not been
 706 /// shown. In this case, the "other code" is at fault -- when you
 707 /// project something, you are supposed to either have a snapshot or
 708 /// else prove all the resulting obligations -- but it's still easy to
 709 /// get wrong.
 710 ///
 711 /// Concern #2. Even within the snapshot, if those original
 712 /// obligations are not yet proven, then we are able to do projections
 713 /// that may yet turn out to be wrong. This *may* lead to some sort
 714 /// of trouble, though we don't have a concrete example of how that
 715 /// can occur yet. But it seems risky at best.
 716 fn get_paranoid_cache_value_obligation<'a, 'tcx>(
 717     infcx: &'a InferCtxt<'a, 'tcx>,
 718     param_env: ty::ParamEnv<'tcx>,
 719     projection_ty: ty::ProjectionTy<'tcx>,
 720     cause: ObligationCause<'tcx>,
 721     depth: usize,
 722 ) -> PredicateObligation<'tcx> {
 723     let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
 724     Obligation {
 725         cause,
 726         recursion_depth: depth,
 727         param_env,
 728         predicate: trait_ref.without_const().to_predicate(infcx.tcx),
 729     }
 730 }
 731
 732 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
 733 /// hold. In various error cases, we cannot generate a valid
 734 /// normalized projection. Therefore, we create an inference variable
 735 /// return an associated obligation that, when fulfilled, will lead to
 736 /// an error.
 737 ///
 738 /// Note that we used to return `Error` here, but that was quite
 739 /// dubious -- the premise was that an error would *eventually* be
 740 /// reported, when the obligation was processed. But in general once
 741 /// you see a `Error` you are supposed to be able to assume that an
 742 /// error *has been* reported, so that you can take whatever heuristic
 743 /// paths you want to take. To make things worse, it was possible for
 744 /// cycles to arise, where you basically had a setup like `<MyType<$0>
 745 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
 746 /// Trait>::Foo> to `[type error]` would lead to an obligation of
 747 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
 748 /// an error for this obligation, but we legitimately should not,
 749 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 750 /// one case where this arose.)
 751 fn normalize_to_error<'a, 'tcx>(
 752     selcx: &mut SelectionContext<'a, 'tcx>,
 753     param_env: ty::ParamEnv<'tcx>,
 754     projection_ty: ty::ProjectionTy<'tcx>,
 755     cause: ObligationCause<'tcx>,
 756     depth: usize,
 757 ) -> NormalizedTy<'tcx> {
 758     let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
 759     let trait_obligation = Obligation {
 760         cause,
 761         recursion_depth: depth,
 762         param_env,
 763         predicate: trait_ref.without_const().to_predicate(selcx.tcx()),
 764     };
 765     let tcx = selcx.infcx().tcx;
 766     let def_id = projection_ty.item_def_id;
 767     let new_value = selcx.infcx().next_ty_var(TypeVariableOrigin {
 768         kind: TypeVariableOriginKind::NormalizeProjectionType,
 769         span: tcx.def_span(def_id),
 770     });
 771     Normalized { value: new_value, obligations: vec![trait_obligation] }
 772 }
 773
 774 enum ProjectedTy<'tcx> {
 775     Progress(Progress<'tcx>),
 776     NoProgress(Ty<'tcx>),
 777 }
 778
 779 struct Progress<'tcx> {
 780     ty: Ty<'tcx>,
 781     obligations: Vec<PredicateObligation<'tcx>>,
 782 }
 783
 784 impl<'tcx> Progress<'tcx> {
 785     fn error(tcx: TyCtxt<'tcx>) -> Self {
 786         Progress { ty: tcx.ty_error(), obligations: vec![] }
 787     }
 788
 789     fn with_addl_obligations(mut self, mut obligations: Vec<PredicateObligation<'tcx>>) -> Self {
 790         debug!(
 791             "with_addl_obligations: self.obligations.len={} obligations.len={}",
 792             self.obligations.len(),
 793             obligations.len()
 794         );
 795
 796         debug!(
 797             "with_addl_obligations: self.obligations={:?} obligations={:?}",
 798             self.obligations, obligations
 799         );
 800
 801         self.obligations.append(&mut obligations);
 802         self
 803     }
 804 }
 805
 806 /// Computes the result of a projection type (if we can).
 807 ///
 808 /// IMPORTANT:
 809 /// - `obligation` must be fully normalized
 810 fn project_type<'cx, 'tcx>(
 811     selcx: &mut SelectionContext<'cx, 'tcx>,
 812     obligation: &ProjectionTyObligation<'tcx>,
 813 ) -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>> {
 814     debug!("project(obligation={:?})", obligation);
 815
 816     if !selcx.tcx().sess.recursion_limit().value_within_limit(obligation.recursion_depth) {
 817         debug!("project: overflow!");
 818         return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
 819     }
 820
 821     let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
 822
 823     debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
 824
 825     if obligation_trait_ref.references_error() {
 826         return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
 827     }
 828
 829     let mut candidates = ProjectionTyCandidateSet::None;
 830
 831     // Make sure that the following procedures are kept in order. ParamEnv
 832     // needs to be first because it has highest priority, and Select checks
 833     // the return value of push_candidate which assumes it's ran at last.
 834     assemble_candidates_from_param_env(selcx, obligation, &obligation_trait_ref, &mut candidates);
 835
 836     assemble_candidates_from_trait_def(selcx, obligation, &obligation_trait_ref, &mut candidates);
 837
 838     assemble_candidates_from_impls(selcx, obligation, &obligation_trait_ref, &mut candidates);
 839
 840     match candidates {
 841         ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
 842             confirm_candidate(selcx, obligation, &obligation_trait_ref, candidate),
 843         )),
 844         ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
 845             selcx
 846                 .tcx()
 847                 .mk_projection(obligation.predicate.item_def_id, obligation.predicate.substs),
 848         )),
 849         // Error occurred while trying to processing impls.
 850         ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
 851         // Inherent ambiguity that prevents us from even enumerating the
 852         // candidates.
 853         ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
 854     }
 855 }
 856
 857 /// The first thing we have to do is scan through the parameter
 858 /// environment to see whether there are any projection predicates
 859 /// there that can answer this question.
 860 fn assemble_candidates_from_param_env<'cx, 'tcx>(
 861     selcx: &mut SelectionContext<'cx, 'tcx>,
 862     obligation: &ProjectionTyObligation<'tcx>,
 863     obligation_trait_ref: &ty::TraitRef<'tcx>,
 864     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 865 ) {
 866     debug!("assemble_candidates_from_param_env(..)");
 867     assemble_candidates_from_predicates(
 868         selcx,
 869         obligation,
 870         obligation_trait_ref,
 871         candidate_set,
 872         ProjectionTyCandidate::ParamEnv,
 873         obligation.param_env.caller_bounds().iter(),
 874     );
 875 }
 876
 877 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
 878 /// that the definition of `Foo` has some clues:
 879 ///
 880 /// ```
 881 /// trait Foo {
 882 ///     type FooT : Bar<BarT=i32>
 883 /// }
 884 /// ```
 885 ///
 886 /// Here, for example, we could conclude that the result is `i32`.
 887 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
 888     selcx: &mut SelectionContext<'cx, 'tcx>,
 889     obligation: &ProjectionTyObligation<'tcx>,
 890     obligation_trait_ref: &ty::TraitRef<'tcx>,
 891     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 892 ) {
 893     debug!("assemble_candidates_from_trait_def(..)");
 894
 895     let tcx = selcx.tcx();
 896     // Check whether the self-type is itself a projection.
 897     // If so, extract what we know from the trait and try to come up with a good answer.
 898     let bounds = match obligation_trait_ref.self_ty().kind {
 899         ty::Projection(ref data) => {
 900             tcx.projection_predicates(data.item_def_id).subst(tcx, data.substs)
 901         }
 902         ty::Opaque(def_id, substs) => tcx.projection_predicates(def_id).subst(tcx, substs),
 903         ty::Infer(ty::TyVar(_)) => {
 904             // If the self-type is an inference variable, then it MAY wind up
 905             // being a projected type, so induce an ambiguity.
 906             candidate_set.mark_ambiguous();
 907             return;
 908         }
 909         _ => return,
 910     };
 911
 912     assemble_candidates_from_predicates(
 913         selcx,
 914         obligation,
 915         obligation_trait_ref,
 916         candidate_set,
 917         ProjectionTyCandidate::TraitDef,
 918         bounds.iter(),
 919     )
 920 }
 921
 922 fn assemble_candidates_from_predicates<'cx, 'tcx>(
 923     selcx: &mut SelectionContext<'cx, 'tcx>,
 924     obligation: &ProjectionTyObligation<'tcx>,
 925     obligation_trait_ref: &ty::TraitRef<'tcx>,
 926     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 927     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
 928     env_predicates: impl Iterator<Item = ty::Predicate<'tcx>>,
 929 ) {
 930     debug!("assemble_candidates_from_predicates(obligation={:?})", obligation);
 931     let infcx = selcx.infcx();
 932     for predicate in env_predicates {
 933         debug!("assemble_candidates_from_predicates: predicate={:?}", predicate);
 934         if let &ty::PredicateKind::Projection(data) = predicate.kind() {
 935             let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
 936
 937             let is_match = same_def_id
 938                 && infcx.probe(|_| {
 939                     let data_poly_trait_ref = data.to_poly_trait_ref(infcx.tcx);
 940                     let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
 941                     infcx
 942                         .at(&obligation.cause, obligation.param_env)
 943                         .sup(obligation_poly_trait_ref, data_poly_trait_ref)
 944                         .map(|InferOk { obligations: _, value: () }| {
 945                             // FIXME(#32730) -- do we need to take obligations
 946                             // into account in any way? At the moment, no.
 947                         })
 948                         .is_ok()
 949                 });
 950
 951             debug!(
 952                 "assemble_candidates_from_predicates: candidate={:?} \
 953                  is_match={} same_def_id={}",
 954                 data, is_match, same_def_id
 955             );
 956
 957             if is_match {
 958                 candidate_set.push_candidate(ctor(data));
 959             }
 960         }
 961     }
 962 }
 963
 964 fn assemble_candidates_from_impls<'cx, 'tcx>(
 965     selcx: &mut SelectionContext<'cx, 'tcx>,
 966     obligation: &ProjectionTyObligation<'tcx>,
 967     obligation_trait_ref: &ty::TraitRef<'tcx>,
 968     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 969 ) {
 970     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
 971     // start out by selecting the predicate `T as TraitRef<...>`:
 972     let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
 973     let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
 974     let _ = selcx.infcx().commit_if_ok(|_| {
 975         let impl_source = match selcx.select(&trait_obligation) {
 976             Ok(Some(impl_source)) => impl_source,
 977             Ok(None) => {
 978                 candidate_set.mark_ambiguous();
 979                 return Err(());
 980             }
 981             Err(e) => {
 982                 debug!("assemble_candidates_from_impls: selection error {:?}", e);
 983                 candidate_set.mark_error(e);
 984                 return Err(());
 985             }
 986         };
 987
 988         let eligible = match &impl_source {
 989             super::ImplSourceClosure(_)
 990             | super::ImplSourceGenerator(_)
 991             | super::ImplSourceFnPointer(_)
 992             | super::ImplSourceObject(_)
 993             | super::ImplSourceTraitAlias(_) => {
 994                 debug!("assemble_candidates_from_impls: impl_source={:?}", impl_source);
 995                 true
 996             }
 997             super::ImplSourceUserDefined(impl_data) => {
 998                 // We have to be careful when projecting out of an
 999                 // impl because of specialization. If we are not in
1000                 // codegen (i.e., projection mode is not "any"), and the
1001                 // impl's type is declared as default, then we disable
1002                 // projection (even if the trait ref is fully
1003                 // monomorphic). In the case where trait ref is not
1004                 // fully monomorphic (i.e., includes type parameters),
1005                 // this is because those type parameters may
1006                 // ultimately be bound to types from other crates that
1007                 // may have specialized impls we can't see. In the
1008                 // case where the trait ref IS fully monomorphic, this
1009                 // is a policy decision that we made in the RFC in
1010                 // order to preserve flexibility for the crate that
1011                 // defined the specializable impl to specialize later
1012                 // for existing types.
1013                 //
1014                 // In either case, we handle this by not adding a
1015                 // candidate for an impl if it contains a `default`
1016                 // type.
1017                 //
1018                 // NOTE: This should be kept in sync with the similar code in
1019                 // `rustc_ty::instance::resolve_associated_item()`.
1020                 let node_item =
1021                     assoc_ty_def(selcx, impl_data.impl_def_id, obligation.predicate.item_def_id)
1022                         .map_err(|ErrorReported| ())?;
1023
1024                 if node_item.is_final() {
1025                     // Non-specializable items are always projectable.
1026                     true
1027                 } else {
1028                     // Only reveal a specializable default if we're past type-checking
1029                     // and the obligation is monomorphic, otherwise passes such as
1030                     // transmute checking and polymorphic MIR optimizations could
1031                     // get a result which isn't correct for all monomorphizations.
1032                     if obligation.param_env.reveal() == Reveal::All {
1033                         // NOTE(eddyb) inference variables can resolve to parameters, so
1034                         // assume `poly_trait_ref` isn't monomorphic, if it contains any.
1035                         let poly_trait_ref =
1036                             selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
1037                         !poly_trait_ref.still_further_specializable()
1038                     } else {
1039                         debug!(
1040                             "assemble_candidates_from_impls: not eligible due to default: \
1041                              assoc_ty={} predicate={}",
1042                             selcx.tcx().def_path_str(node_item.item.def_id),
1043                             obligation.predicate,
1044                         );
1045                         false
1046                     }
1047                 }
1048             }
1049             super::ImplSourceDiscriminantKind(..) => {
1050                 // While `DiscriminantKind` is automatically implemented for every type,
1051                 // the concrete discriminant may not be known yet.
1052                 //
1053                 // Any type with multiple potential discriminant types is therefore not eligible.
1054                 let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1055
1056                 match self_ty.kind {
1057                     ty::Bool
1058                     | ty::Char
1059                     | ty::Int(_)
1060                     | ty::Uint(_)
1061                     | ty::Float(_)
1062                     | ty::Adt(..)
1063                     | ty::Foreign(_)
1064                     | ty::Str
1065                     | ty::Array(..)
1066                     | ty::Slice(_)
1067                     | ty::RawPtr(..)
1068                     | ty::Ref(..)
1069                     | ty::FnDef(..)
1070                     | ty::FnPtr(..)
1071                     | ty::Dynamic(..)
1072                     | ty::Closure(..)
1073                     | ty::Generator(..)
1074                     | ty::GeneratorWitness(..)
1075                     | ty::Never
1076                     | ty::Tuple(..)
1077                     // Integers and floats always have `u8` as their discriminant.
1078                     | ty::Infer(ty::InferTy::IntVar(_) | ty::InferTy::FloatVar(..)) => true,
1079
1080                     ty::Projection(..)
1081                     | ty::Opaque(..)
1082                     | ty::Param(..)
1083                     | ty::Bound(..)
1084                     | ty::Placeholder(..)
1085                     | ty::Infer(..)
1086                     | ty::Error(_) => false,
1087                 }
1088             }
1089             super::ImplSourceParam(..) => {
1090                 // This case tell us nothing about the value of an
1091                 // associated type. Consider:
1092                 //
1093                 // ```
1094                 // trait SomeTrait { type Foo; }
1095                 // fn foo<T:SomeTrait>(...) { }
1096                 // ```
1097                 //
1098                 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1099                 // : SomeTrait` binding does not help us decide what the
1100                 // type `Foo` is (at least, not more specifically than
1101                 // what we already knew).
1102                 //
1103                 // But wait, you say! What about an example like this:
1104                 //
1105                 // ```
1106                 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1107                 // ```
1108                 //
1109                 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1110                 // resolve `T::Foo`? And of course it does, but in fact
1111                 // that single predicate is desugared into two predicates
1112                 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1113                 // projection. And the projection where clause is handled
1114                 // in `assemble_candidates_from_param_env`.
1115                 false
1116             }
1117             super::ImplSourceAutoImpl(..) | super::ImplSourceBuiltin(..) => {
1118                 // These traits have no associated types.
1119                 span_bug!(
1120                     obligation.cause.span,
1121                     "Cannot project an associated type from `{:?}`",
1122                     impl_source
1123                 );
1124             }
1125         };
1126
1127         if eligible {
1128             if candidate_set.push_candidate(ProjectionTyCandidate::Select(impl_source)) {
1129                 Ok(())
1130             } else {
1131                 Err(())
1132             }
1133         } else {
1134             Err(())
1135         }
1136     });
1137 }
1138
1139 fn confirm_candidate<'cx, 'tcx>(
1140     selcx: &mut SelectionContext<'cx, 'tcx>,
1141     obligation: &ProjectionTyObligation<'tcx>,
1142     obligation_trait_ref: &ty::TraitRef<'tcx>,
1143     candidate: ProjectionTyCandidate<'tcx>,
1144 ) -> Progress<'tcx> {
1145     debug!("confirm_candidate(candidate={:?}, obligation={:?})", candidate, obligation);
1146
1147     let mut progress = match candidate {
1148         ProjectionTyCandidate::ParamEnv(poly_projection)
1149         | ProjectionTyCandidate::TraitDef(poly_projection) => {
1150             confirm_param_env_candidate(selcx, obligation, poly_projection)
1151         }
1152
1153         ProjectionTyCandidate::Select(impl_source) => {
1154             confirm_select_candidate(selcx, obligation, obligation_trait_ref, impl_source)
1155         }
1156     };
1157     // When checking for cycle during evaluation, we compare predicates with
1158     // "syntactic" equality. Since normalization generally introduces a type
1159     // with new region variables, we need to resolve them to existing variables
1160     // when possible for this to work. See `auto-trait-projection-recursion.rs`
1161     // for a case where this matters.
1162     if progress.ty.has_infer_regions() {
1163         progress.ty = OpportunisticRegionResolver::new(selcx.infcx()).fold_ty(progress.ty);
1164     }
1165     progress
1166 }
1167
1168 fn confirm_select_candidate<'cx, 'tcx>(
1169     selcx: &mut SelectionContext<'cx, 'tcx>,
1170     obligation: &ProjectionTyObligation<'tcx>,
1171     obligation_trait_ref: &ty::TraitRef<'tcx>,
1172     impl_source: Selection<'tcx>,
1173 ) -> Progress<'tcx> {
1174     match impl_source {
1175         super::ImplSourceUserDefined(data) => confirm_impl_candidate(selcx, obligation, data),
1176         super::ImplSourceGenerator(data) => confirm_generator_candidate(selcx, obligation, data),
1177         super::ImplSourceClosure(data) => confirm_closure_candidate(selcx, obligation, data),
1178         super::ImplSourceFnPointer(data) => confirm_fn_pointer_candidate(selcx, obligation, data),
1179         super::ImplSourceDiscriminantKind(data) => {
1180             confirm_discriminant_kind_candidate(selcx, obligation, data)
1181         }
1182         super::ImplSourceObject(_) => {
1183             confirm_object_candidate(selcx, obligation, obligation_trait_ref)
1184         }
1185         super::ImplSourceAutoImpl(..)
1186         | super::ImplSourceParam(..)
1187         | super::ImplSourceBuiltin(..)
1188         | super::ImplSourceTraitAlias(..) =>
1189         // we don't create Select candidates with this kind of resolution
1190         {
1191             span_bug!(
1192                 obligation.cause.span,
1193                 "Cannot project an associated type from `{:?}`",
1194                 impl_source
1195             )
1196         }
1197     }
1198 }
1199
1200 fn confirm_object_candidate<'cx, 'tcx>(
1201     selcx: &mut SelectionContext<'cx, 'tcx>,
1202     obligation: &ProjectionTyObligation<'tcx>,
1203     obligation_trait_ref: &ty::TraitRef<'tcx>,
1204 ) -> Progress<'tcx> {
1205     let self_ty = obligation_trait_ref.self_ty();
1206     let object_ty = selcx.infcx().shallow_resolve(self_ty);
1207     debug!("confirm_object_candidate(object_ty={:?})", object_ty);
1208     let data = match object_ty.kind {
1209         ty::Dynamic(ref data, ..) => data,
1210         _ => span_bug!(
1211             obligation.cause.span,
1212             "confirm_object_candidate called with non-object: {:?}",
1213             object_ty
1214         ),
1215     };
1216     let env_predicates = data
1217         .projection_bounds()
1218         .map(|p| p.with_self_ty(selcx.tcx(), object_ty).to_predicate(selcx.tcx()));
1219     let env_predicate = {
1220         let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1221
1222         // select only those projections that are actually projecting an
1223         // item with the correct name
1224         let env_predicates = env_predicates.filter_map(|o| match o.predicate.kind() {
1225             &ty::PredicateKind::Projection(data)
1226                 if data.projection_def_id() == obligation.predicate.item_def_id =>
1227             {
1228                 Some(data)
1229             }
1230             _ => None,
1231         });
1232
1233         // select those with a relevant trait-ref
1234         let mut env_predicates = env_predicates.filter(|data| {
1235             let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1236             let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1237             selcx.infcx().probe(|_| {
1238                 selcx
1239                     .infcx()
1240                     .at(&obligation.cause, obligation.param_env)
1241                     .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1242                     .is_ok()
1243             })
1244         });
1245
1246         // select the first matching one; there really ought to be one or
1247         // else the object type is not WF, since an object type should
1248         // include all of its projections explicitly
1249         match env_predicates.next() {
1250             Some(env_predicate) => env_predicate,
1251             None => {
1252                 debug!(
1253                     "confirm_object_candidate: no env-predicate \
1254                      found in object type `{:?}`; ill-formed",
1255                     object_ty
1256                 );
1257                 return Progress::error(selcx.tcx());
1258             }
1259         }
1260     };
1261
1262     confirm_param_env_candidate(selcx, obligation, env_predicate)
1263 }
1264
1265 fn confirm_generator_candidate<'cx, 'tcx>(
1266     selcx: &mut SelectionContext<'cx, 'tcx>,
1267     obligation: &ProjectionTyObligation<'tcx>,
1268     impl_source: ImplSourceGeneratorData<'tcx, PredicateObligation<'tcx>>,
1269 ) -> Progress<'tcx> {
1270     let gen_sig = impl_source.substs.as_generator().poly_sig();
1271     let Normalized { value: gen_sig, obligations } = normalize_with_depth(
1272         selcx,
1273         obligation.param_env,
1274         obligation.cause.clone(),
1275         obligation.recursion_depth + 1,
1276         &gen_sig,
1277     );
1278
1279     debug!(
1280         "confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1281         obligation, gen_sig, obligations
1282     );
1283
1284     let tcx = selcx.tcx();
1285
1286     let gen_def_id = tcx.require_lang_item(GeneratorTraitLangItem, None);
1287
1288     let predicate = super::util::generator_trait_ref_and_outputs(
1289         tcx,
1290         gen_def_id,
1291         obligation.predicate.self_ty(),
1292         gen_sig,
1293     )
1294     .map_bound(|(trait_ref, yield_ty, return_ty)| {
1295         let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1296         let ty = if name == sym::Return {
1297             return_ty
1298         } else if name == sym::Yield {
1299             yield_ty
1300         } else {
1301             bug!()
1302         };
1303
1304         ty::ProjectionPredicate {
1305             projection_ty: ty::ProjectionTy {
1306                 substs: trait_ref.substs,
1307                 item_def_id: obligation.predicate.item_def_id,
1308             },
1309             ty,
1310         }
1311     });
1312
1313     confirm_param_env_candidate(selcx, obligation, predicate)
1314         .with_addl_obligations(impl_source.nested)
1315         .with_addl_obligations(obligations)
1316 }
1317
1318 fn confirm_discriminant_kind_candidate<'cx, 'tcx>(
1319     selcx: &mut SelectionContext<'cx, 'tcx>,
1320     obligation: &ProjectionTyObligation<'tcx>,
1321     _: ImplSourceDiscriminantKindData,
1322 ) -> Progress<'tcx> {
1323     let tcx = selcx.tcx();
1324
1325     let self_ty = selcx.infcx().shallow_resolve(obligation.predicate.self_ty());
1326     let substs = tcx.mk_substs([self_ty.into()].iter());
1327
1328     let discriminant_def_id = tcx.require_lang_item(DiscriminantTypeLangItem, None);
1329
1330     let predicate = ty::ProjectionPredicate {
1331         projection_ty: ty::ProjectionTy { substs, item_def_id: discriminant_def_id },
1332         ty: self_ty.discriminant_ty(tcx),
1333     };
1334
1335     confirm_param_env_candidate(selcx, obligation, ty::Binder::bind(predicate))
1336 }
1337
1338 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1339     selcx: &mut SelectionContext<'cx, 'tcx>,
1340     obligation: &ProjectionTyObligation<'tcx>,
1341     fn_pointer_impl_source: ImplSourceFnPointerData<'tcx, PredicateObligation<'tcx>>,
1342 ) -> Progress<'tcx> {
1343     let fn_type = selcx.infcx().shallow_resolve(fn_pointer_impl_source.fn_ty);
1344     let sig = fn_type.fn_sig(selcx.tcx());
1345     let Normalized { value: sig, obligations } = normalize_with_depth(
1346         selcx,
1347         obligation.param_env,
1348         obligation.cause.clone(),
1349         obligation.recursion_depth + 1,
1350         &sig,
1351     );
1352
1353     confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1354         .with_addl_obligations(fn_pointer_impl_source.nested)
1355         .with_addl_obligations(obligations)
1356 }
1357
1358 fn confirm_closure_candidate<'cx, 'tcx>(
1359     selcx: &mut SelectionContext<'cx, 'tcx>,
1360     obligation: &ProjectionTyObligation<'tcx>,
1361     impl_source: ImplSourceClosureData<'tcx, PredicateObligation<'tcx>>,
1362 ) -> Progress<'tcx> {
1363     let closure_sig = impl_source.substs.as_closure().sig();
1364     let Normalized { value: closure_sig, obligations } = normalize_with_depth(
1365         selcx,
1366         obligation.param_env,
1367         obligation.cause.clone(),
1368         obligation.recursion_depth + 1,
1369         &closure_sig,
1370     );
1371
1372     debug!(
1373         "confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1374         obligation, closure_sig, obligations
1375     );
1376
1377     confirm_callable_candidate(selcx, obligation, closure_sig, util::TupleArgumentsFlag::No)
1378         .with_addl_obligations(impl_source.nested)
1379         .with_addl_obligations(obligations)
1380 }
1381
1382 fn confirm_callable_candidate<'cx, 'tcx>(
1383     selcx: &mut SelectionContext<'cx, 'tcx>,
1384     obligation: &ProjectionTyObligation<'tcx>,
1385     fn_sig: ty::PolyFnSig<'tcx>,
1386     flag: util::TupleArgumentsFlag,
1387 ) -> Progress<'tcx> {
1388     let tcx = selcx.tcx();
1389
1390     debug!("confirm_callable_candidate({:?},{:?})", obligation, fn_sig);
1391
1392     let fn_once_def_id = tcx.require_lang_item(FnOnceTraitLangItem, None);
1393     let fn_once_output_def_id = tcx.require_lang_item(FnOnceOutputLangItem, None);
1394
1395     let predicate = super::util::closure_trait_ref_and_return_type(
1396         tcx,
1397         fn_once_def_id,
1398         obligation.predicate.self_ty(),
1399         fn_sig,
1400         flag,
1401     )
1402     .map_bound(|(trait_ref, ret_type)| ty::ProjectionPredicate {
1403         projection_ty: ty::ProjectionTy {
1404             substs: trait_ref.substs,
1405             item_def_id: fn_once_output_def_id,
1406         },
1407         ty: ret_type,
1408     });
1409
1410     confirm_param_env_candidate(selcx, obligation, predicate)
1411 }
1412
1413 fn confirm_param_env_candidate<'cx, 'tcx>(
1414     selcx: &mut SelectionContext<'cx, 'tcx>,
1415     obligation: &ProjectionTyObligation<'tcx>,
1416     poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1417 ) -> Progress<'tcx> {
1418     let infcx = selcx.infcx();
1419     let cause = &obligation.cause;
1420     let param_env = obligation.param_env;
1421
1422     let (cache_entry, _) = infcx.replace_bound_vars_with_fresh_vars(
1423         cause.span,
1424         LateBoundRegionConversionTime::HigherRankedType,
1425         &poly_cache_entry,
1426     );
1427
1428     let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1429     let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1430     match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1431         Ok(InferOk { value: _, obligations }) => Progress { ty: cache_entry.ty, obligations },
1432         Err(e) => {
1433             let msg = format!(
1434                 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1435                 obligation, poly_cache_entry, e,
1436             );
1437             debug!("confirm_param_env_candidate: {}", msg);
1438             let err = infcx.tcx.ty_error_with_message(obligation.cause.span, &msg);
1439             Progress { ty: err, obligations: vec![] }
1440         }
1441     }
1442 }
1443
1444 fn confirm_impl_candidate<'cx, 'tcx>(
1445     selcx: &mut SelectionContext<'cx, 'tcx>,
1446     obligation: &ProjectionTyObligation<'tcx>,
1447     impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
1448 ) -> Progress<'tcx> {
1449     let tcx = selcx.tcx();
1450
1451     let ImplSourceUserDefinedData { impl_def_id, substs, nested } = impl_impl_source;
1452     let assoc_item_id = obligation.predicate.item_def_id;
1453     let trait_def_id = tcx.trait_id_of_impl(impl_def_id).unwrap();
1454
1455     let param_env = obligation.param_env;
1456     let assoc_ty = match assoc_ty_def(selcx, impl_def_id, assoc_item_id) {
1457         Ok(assoc_ty) => assoc_ty,
1458         Err(ErrorReported) => return Progress { ty: tcx.ty_error(), obligations: nested },
1459     };
1460
1461     if !assoc_ty.item.defaultness.has_value() {
1462         // This means that the impl is missing a definition for the
1463         // associated type. This error will be reported by the type
1464         // checker method `check_impl_items_against_trait`, so here we
1465         // just return Error.
1466         debug!(
1467             "confirm_impl_candidate: no associated type {:?} for {:?}",
1468             assoc_ty.item.ident, obligation.predicate
1469         );
1470         return Progress { ty: tcx.ty_error(), obligations: nested };
1471     }
1472     // If we're trying to normalize `<Vec<u32> as X>::A<S>` using
1473     //`impl<T> X for Vec<T> { type A<Y> = Box<Y>; }`, then:
1474     //
1475     // * `obligation.predicate.substs` is `[Vec<u32>, S]`
1476     // * `substs` is `[u32]`
1477     // * `substs` ends up as `[u32, S]`
1478     let substs = obligation.predicate.substs.rebase_onto(tcx, trait_def_id, substs);
1479     let substs =
1480         translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.defining_node);
1481     let ty = tcx.type_of(assoc_ty.item.def_id);
1482     if substs.len() != tcx.generics_of(assoc_ty.item.def_id).count() {
1483         let err = tcx.ty_error_with_message(
1484             DUMMY_SP,
1485             "impl item and trait item have different parameter counts",
1486         );
1487         Progress { ty: err, obligations: nested }
1488     } else {
1489         Progress { ty: ty.subst(tcx, substs), obligations: nested }
1490     }
1491 }
1492
1493 /// Locate the definition of an associated type in the specialization hierarchy,
1494 /// starting from the given impl.
1495 ///
1496 /// Based on the "projection mode", this lookup may in fact only examine the
1497 /// topmost impl. See the comments for `Reveal` for more details.
1498 fn assoc_ty_def(
1499     selcx: &SelectionContext<'_, '_>,
1500     impl_def_id: DefId,
1501     assoc_ty_def_id: DefId,
1502 ) -> Result<specialization_graph::LeafDef, ErrorReported> {
1503     let tcx = selcx.tcx();
1504     let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1505     let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1506     let trait_def = tcx.trait_def(trait_def_id);
1507
1508     // This function may be called while we are still building the
1509     // specialization graph that is queried below (via TraitDef::ancestors()),
1510     // so, in order to avoid unnecessary infinite recursion, we manually look
1511     // for the associated item at the given impl.
1512     // If there is no such item in that impl, this function will fail with a
1513     // cycle error if the specialization graph is currently being built.
1514     let impl_node = specialization_graph::Node::Impl(impl_def_id);
1515     for item in impl_node.items(tcx) {
1516         if matches!(item.kind, ty::AssocKind::Type)
1517             && tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id)
1518         {
1519             return Ok(specialization_graph::LeafDef {
1520                 item: *item,
1521                 defining_node: impl_node,
1522                 finalizing_node: if item.defaultness.is_default() { None } else { Some(impl_node) },
1523             });
1524         }
1525     }
1526
1527     let ancestors = trait_def.ancestors(tcx, impl_def_id)?;
1528     if let Some(assoc_item) = ancestors.leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
1529         Ok(assoc_item)
1530     } else {
1531         // This is saying that neither the trait nor
1532         // the impl contain a definition for this
1533         // associated type.  Normally this situation
1534         // could only arise through a compiler bug --
1535         // if the user wrote a bad item name, it
1536         // should have failed in astconv.
1537         bug!("No associated type `{}` for {}", assoc_ty_name, tcx.def_path_str(impl_def_id))
1538     }
1539 }
1540
1541 crate trait ProjectionCacheKeyExt<'tcx>: Sized {
1542     fn from_poly_projection_predicate(
1543         selcx: &mut SelectionContext<'cx, 'tcx>,
1544         predicate: ty::PolyProjectionPredicate<'tcx>,
1545     ) -> Option<Self>;
1546 }
1547
1548 impl<'tcx> ProjectionCacheKeyExt<'tcx> for ProjectionCacheKey<'tcx> {
1549     fn from_poly_projection_predicate(
1550         selcx: &mut SelectionContext<'cx, 'tcx>,
1551         predicate: ty::PolyProjectionPredicate<'tcx>,
1552     ) -> Option<Self> {
1553         let infcx = selcx.infcx();
1554         // We don't do cross-snapshot caching of obligations with escaping regions,
1555         // so there's no cache key to use
1556         predicate.no_bound_vars().map(|predicate| {
1557             ProjectionCacheKey::new(
1558                 // We don't attempt to match up with a specific type-variable state
1559                 // from a specific call to `opt_normalize_projection_type` - if
1560                 // there's no precise match, the original cache entry is "stranded"
1561                 // anyway.
1562                 infcx.resolve_vars_if_possible(&predicate.projection_ty),
1563             )
1564         })
1565     }
1566 }