src/librustc/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::Obligation;
   7 use super::ObligationCause;
   8 use super::PredicateObligation;
   9 use super::Selection;
  10 use super::SelectionContext;
  11 use super::SelectionError;
  12 use super::{VtableImplData, VtableClosureData, VtableGeneratorData, VtableFnPointerData};
  13 use super::util;
  14
  15 use crate::hir::def_id::DefId;
  16 use crate::infer::{InferCtxt, InferOk, LateBoundRegionConversionTime};
  17 use crate::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  18 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
  19 use rustc_macros::HashStable;
  20 use syntax::ast::Ident;
  21 use syntax::symbol::sym;
  22 use crate::ty::subst::{Subst, InternalSubsts};
  23 use crate::ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
  24 use crate::ty::fold::{TypeFoldable, TypeFolder};
  25 use crate::util::common::FN_OUTPUT_NAME;
  26
  27 /// Depending on the stage of compilation, we want projection to be
  28 /// more or less conservative.
  29 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
  30 pub enum Reveal {
  31     /// At type-checking time, we refuse to project any associated
  32     /// type that is marked `default`. Non-`default` ("final") types
  33     /// are always projected. This is necessary in general for
  34     /// soundness of specialization. However, we *could* allow
  35     /// projections in fully-monomorphic cases. We choose not to,
  36     /// because we prefer for `default type` to force the type
  37     /// definition to be treated abstractly by any consumers of the
  38     /// impl. Concretely, that means that the following example will
  39     /// fail to compile:
  40     ///
  41     /// ```
  42     /// trait Assoc {
  43     ///     type Output;
  44     /// }
  45     ///
  46     /// impl<T> Assoc for T {
  47     ///     default type Output = bool;
  48     /// }
  49     ///
  50     /// fn main() {
  51     ///     let <() as Assoc>::Output = true;
  52     /// }
  53     UserFacing,
  54
  55     /// At codegen time, all monomorphic projections will succeed.
  56     /// Also, `impl Trait` is normalized to the concrete type,
  57     /// which has to be already collected by type-checking.
  58     ///
  59     /// NOTE: as `impl Trait`'s concrete type should *never*
  60     /// be observable directly by the user, `Reveal::All`
  61     /// should not be used by checks which may expose
  62     /// type equality or type contents to the user.
  63     /// There are some exceptions, e.g., around OIBITS and
  64     /// transmute-checking, which expose some details, but
  65     /// not the whole concrete type of the `impl Trait`.
  66     All,
  67 }
  68
  69 pub type PolyProjectionObligation<'tcx> =
  70     Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
  71
  72 pub type ProjectionObligation<'tcx> =
  73     Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
  74
  75 pub type ProjectionTyObligation<'tcx> =
  76     Obligation<'tcx, ty::ProjectionTy<'tcx>>;
  77
  78 /// When attempting to resolve `<T as TraitRef>::Name` ...
  79 #[derive(Debug)]
  80 pub enum ProjectionTyError<'tcx> {
  81     /// ...we found multiple sources of information and couldn't resolve the ambiguity.
  82     TooManyCandidates,
  83
  84     /// ...an error occurred matching `T : TraitRef`
  85     TraitSelectionError(SelectionError<'tcx>),
  86 }
  87
  88 #[derive(Clone)]
  89 pub struct MismatchedProjectionTypes<'tcx> {
  90     pub err: ty::error::TypeError<'tcx>
  91 }
  92
  93 #[derive(PartialEq, Eq, Debug)]
  94 enum ProjectionTyCandidate<'tcx> {
  95     // from a where-clause in the env or object type
  96     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
  97
  98     // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
  99     TraitDef(ty::PolyProjectionPredicate<'tcx>),
 100
 101     // from a "impl" (or a "pseudo-impl" returned by select)
 102     Select(Selection<'tcx>),
 103 }
 104
 105 enum ProjectionTyCandidateSet<'tcx> {
 106     None,
 107     Single(ProjectionTyCandidate<'tcx>),
 108     Ambiguous,
 109     Error(SelectionError<'tcx>),
 110 }
 111
 112 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
 113     fn mark_ambiguous(&mut self) {
 114         *self = ProjectionTyCandidateSet::Ambiguous;
 115     }
 116
 117     fn mark_error(&mut self, err: SelectionError<'tcx>) {
 118         *self = ProjectionTyCandidateSet::Error(err);
 119     }
 120
 121     // Returns true if the push was successful, or false if the candidate
 122     // was discarded -- this could be because of ambiguity, or because
 123     // a higher-priority candidate is already there.
 124     fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
 125         use self::ProjectionTyCandidateSet::*;
 126         use self::ProjectionTyCandidate::*;
 127
 128         // This wacky variable is just used to try and
 129         // make code readable and avoid confusing paths.
 130         // It is assigned a "value" of `()` only on those
 131         // paths in which we wish to convert `*self` to
 132         // ambiguous (and return false, because the candidate
 133         // was not used). On other paths, it is not assigned,
 134         // and hence if those paths *could* reach the code that
 135         // comes after the match, this fn would not compile.
 136         let convert_to_ambiguous;
 137
 138         match self {
 139             None => {
 140                 *self = Single(candidate);
 141                 return true;
 142             }
 143
 144             Single(current) => {
 145                 // Duplicates can happen inside ParamEnv. In the case, we
 146                 // perform a lazy deduplication.
 147                 if current == &candidate {
 148                     return false;
 149                 }
 150
 151                 // Prefer where-clauses. As in select, if there are multiple
 152                 // candidates, we prefer where-clause candidates over impls.  This
 153                 // may seem a bit surprising, since impls are the source of
 154                 // "truth" in some sense, but in fact some of the impls that SEEM
 155                 // applicable are not, because of nested obligations. Where
 156                 // clauses are the safer choice. See the comment on
 157                 // `select::SelectionCandidate` and #21974 for more details.
 158                 match (current, candidate) {
 159                     (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
 160                     (ParamEnv(..), _) => return false,
 161                     (_, ParamEnv(..)) => unreachable!(),
 162                     (_, _) => convert_to_ambiguous = (),
 163                 }
 164             }
 165
 166             Ambiguous | Error(..) => {
 167                 return false;
 168             }
 169         }
 170
 171         // We only ever get here when we moved from a single candidate
 172         // to ambiguous.
 173         let () = convert_to_ambiguous;
 174         *self = Ambiguous;
 175         false
 176     }
 177 }
 178
 179 /// Evaluates constraints of the form:
 180 ///
 181 ///     for<...> <T as Trait>::U == V
 182 ///
 183 /// If successful, this may result in additional obligations. Also returns
 184 /// the projection cache key used to track these additional obligations.
 185 pub fn poly_project_and_unify_type<'cx, 'tcx>(
 186     selcx: &mut SelectionContext<'cx, 'tcx>,
 187     obligation: &PolyProjectionObligation<'tcx>,
 188 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
 189     debug!("poly_project_and_unify_type(obligation={:?})",
 190            obligation);
 191
 192     let infcx = selcx.infcx();
 193     infcx.commit_if_ok(|snapshot| {
 194         let (placeholder_predicate, placeholder_map) =
 195             infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
 196
 197         let placeholder_obligation = obligation.with(placeholder_predicate);
 198         let result = project_and_unify_type(selcx, &placeholder_obligation)?;
 199         infcx.leak_check(false, &placeholder_map, snapshot)
 200             .map_err(|err| MismatchedProjectionTypes { err })?;
 201         Ok(result)
 202     })
 203 }
 204
 205 /// Evaluates constraints of the form:
 206 ///
 207 ///     <T as Trait>::U == V
 208 ///
 209 /// If successful, this may result in additional obligations.
 210 fn project_and_unify_type<'cx, 'tcx>(
 211     selcx: &mut SelectionContext<'cx, 'tcx>,
 212     obligation: &ProjectionObligation<'tcx>,
 213 ) -> Result<Option<Vec<PredicateObligation<'tcx>>>, MismatchedProjectionTypes<'tcx>> {
 214     debug!("project_and_unify_type(obligation={:?})",
 215            obligation);
 216
 217     let mut obligations = vec![];
 218     let normalized_ty =
 219         match opt_normalize_projection_type(selcx,
 220                                             obligation.param_env,
 221                                             obligation.predicate.projection_ty,
 222                                             obligation.cause.clone(),
 223                                             obligation.recursion_depth,
 224                                             &mut obligations) {
 225             Some(n) => n,
 226             None => return Ok(None),
 227         };
 228
 229     debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
 230            normalized_ty,
 231            obligations);
 232
 233     let infcx = selcx.infcx();
 234     match infcx.at(&obligation.cause, obligation.param_env)
 235                .eq(normalized_ty, obligation.predicate.ty) {
 236         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
 237             obligations.extend(inferred_obligations);
 238             Ok(Some(obligations))
 239         },
 240         Err(err) => {
 241             debug!("project_and_unify_type: equating types encountered error {:?}", err);
 242             Err(MismatchedProjectionTypes { err })
 243         }
 244     }
 245 }
 246
 247 /// Normalizes any associated type projections in `value`, replacing
 248 /// them with a fully resolved type where possible. The return value
 249 /// combines the normalized result and any additional obligations that
 250 /// were incurred as result.
 251 pub fn normalize<'a, 'b, 'tcx, T>(
 252     selcx: &'a mut SelectionContext<'b, 'tcx>,
 253     param_env: ty::ParamEnv<'tcx>,
 254     cause: ObligationCause<'tcx>,
 255     value: &T,
 256 ) -> Normalized<'tcx, T>
 257 where
 258     T: TypeFoldable<'tcx>,
 259 {
 260     normalize_with_depth(selcx, param_env, cause, 0, value)
 261 }
 262
 263 /// As `normalize`, but with a custom depth.
 264 pub fn normalize_with_depth<'a, 'b, 'tcx, T>(
 265     selcx: &'a mut SelectionContext<'b, 'tcx>,
 266     param_env: ty::ParamEnv<'tcx>,
 267     cause: ObligationCause<'tcx>,
 268     depth: usize,
 269     value: &T,
 270 ) -> Normalized<'tcx, T>
 271 where
 272     T: TypeFoldable<'tcx>,
 273 {
 274     debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
 275     let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth);
 276     let result = normalizer.fold(value);
 277     debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
 278            depth, result, normalizer.obligations.len());
 279     debug!("normalize_with_depth: depth={} obligations={:?}",
 280            depth, normalizer.obligations);
 281     Normalized {
 282         value: result,
 283         obligations: normalizer.obligations,
 284     }
 285 }
 286
 287 struct AssocTypeNormalizer<'a, 'b, 'tcx> {
 288     selcx: &'a mut SelectionContext<'b, 'tcx>,
 289     param_env: ty::ParamEnv<'tcx>,
 290     cause: ObligationCause<'tcx>,
 291     obligations: Vec<PredicateObligation<'tcx>>,
 292     depth: usize,
 293 }
 294
 295 impl<'a, 'b, 'tcx> AssocTypeNormalizer<'a, 'b, 'tcx> {
 296     fn new(
 297         selcx: &'a mut SelectionContext<'b, 'tcx>,
 298         param_env: ty::ParamEnv<'tcx>,
 299         cause: ObligationCause<'tcx>,
 300         depth: usize,
 301     ) -> AssocTypeNormalizer<'a, 'b, 'tcx> {
 302         AssocTypeNormalizer {
 303             selcx,
 304             param_env,
 305             cause,
 306             obligations: vec![],
 307             depth,
 308         }
 309     }
 310
 311     fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
 312         let value = self.selcx.infcx().resolve_vars_if_possible(value);
 313
 314         if !value.has_projections() {
 315             value
 316         } else {
 317             value.fold_with(self)
 318         }
 319     }
 320 }
 321
 322 impl<'a, 'b, 'tcx> TypeFolder<'tcx> for AssocTypeNormalizer<'a, 'b, 'tcx> {
 323     fn tcx<'c>(&'c self) -> TyCtxt<'tcx> {
 324         self.selcx.tcx()
 325     }
 326
 327     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 328         // We don't want to normalize associated types that occur inside of region
 329         // binders, because they may contain bound regions, and we can't cope with that.
 330         //
 331         // Example:
 332         //
 333         //     for<'a> fn(<T as Foo<&'a>>::A)
 334         //
 335         // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
 336         // normalize it when we instantiate those bound regions (which
 337         // should occur eventually).
 338
 339         let ty = ty.super_fold_with(self);
 340         match ty.kind {
 341             ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => { // (*)
 342                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 343                 match self.param_env.reveal {
 344                     Reveal::UserFacing => ty,
 345
 346                     Reveal::All => {
 347                         let recursion_limit = *self.tcx().sess.recursion_limit.get();
 348                         if self.depth >= recursion_limit {
 349                             let obligation = Obligation::with_depth(
 350                                 self.cause.clone(),
 351                                 recursion_limit,
 352                                 self.param_env,
 353                                 ty,
 354                             );
 355                             self.selcx.infcx().report_overflow_error(&obligation, true);
 356                         }
 357
 358                         let generic_ty = self.tcx().type_of(def_id);
 359                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 360                         self.depth += 1;
 361                         let folded_ty = self.fold_ty(concrete_ty);
 362                         self.depth -= 1;
 363                         folded_ty
 364                     }
 365                 }
 366             }
 367
 368             ty::Projection(ref data) if !data.has_escaping_bound_vars() => { // (*)
 369
 370                 // (*) This is kind of hacky -- we need to be able to
 371                 // handle normalization within binders because
 372                 // otherwise we wind up a need to normalize when doing
 373                 // trait matching (since you can have a trait
 374                 // obligation like `for<'a> T::B : Fn(&'a int)`), but
 375                 // we can't normalize with bound regions in scope. So
 376                 // far now we just ignore binders but only normalize
 377                 // if all bound regions are gone (and then we still
 378                 // have to renormalize whenever we instantiate a
 379                 // binder). It would be better to normalize in a
 380                 // binding-aware fashion.
 381
 382                 let normalized_ty = normalize_projection_type(self.selcx,
 383                                                               self.param_env,
 384                                                               data.clone(),
 385                                                               self.cause.clone(),
 386                                                               self.depth,
 387                                                               &mut self.obligations);
 388                 debug!("AssocTypeNormalizer: depth={} normalized {:?} to {:?}, \
 389                         now with {} obligations",
 390                        self.depth, ty, normalized_ty, self.obligations.len());
 391                 normalized_ty
 392             }
 393
 394             _ => ty
 395         }
 396     }
 397
 398     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 399         constant.eval(self.selcx.tcx(), self.param_env)
 400     }
 401 }
 402
 403 #[derive(Clone)]
 404 pub struct Normalized<'tcx,T> {
 405     pub value: T,
 406     pub obligations: Vec<PredicateObligation<'tcx>>,
 407 }
 408
 409 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
 410
 411 impl<'tcx,T> Normalized<'tcx,T> {
 412     pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
 413         Normalized { value: value, obligations: self.obligations }
 414     }
 415 }
 416
 417 /// The guts of `normalize`: normalize a specific projection like `<T
 418 /// as Trait>::Item`. The result is always a type (and possibly
 419 /// additional obligations). If ambiguity arises, which implies that
 420 /// there are unresolved type variables in the projection, we will
 421 /// substitute a fresh type variable `$X` and generate a new
 422 /// obligation `<T as Trait>::Item == $X` for later.
 423 pub fn normalize_projection_type<'a, 'b, 'tcx>(
 424     selcx: &'a mut SelectionContext<'b, 'tcx>,
 425     param_env: ty::ParamEnv<'tcx>,
 426     projection_ty: ty::ProjectionTy<'tcx>,
 427     cause: ObligationCause<'tcx>,
 428     depth: usize,
 429     obligations: &mut Vec<PredicateObligation<'tcx>>,
 430 ) -> Ty<'tcx> {
 431     opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
 432                                   obligations)
 433         .unwrap_or_else(move || {
 434             // if we bottom out in ambiguity, create a type variable
 435             // and a deferred predicate to resolve this when more type
 436             // information is available.
 437
 438             let tcx = selcx.infcx().tcx;
 439             let def_id = projection_ty.item_def_id;
 440             let ty_var = selcx.infcx().next_ty_var(
 441                 TypeVariableOrigin {
 442                     kind: TypeVariableOriginKind::NormalizeProjectionType,
 443                     span: tcx.def_span(def_id),
 444                 },
 445             );
 446             let projection = ty::Binder::dummy(ty::ProjectionPredicate {
 447                 projection_ty,
 448                 ty: ty_var
 449             });
 450             let obligation = Obligation::with_depth(
 451                 cause, depth + 1, param_env, projection.to_predicate());
 452             obligations.push(obligation);
 453             ty_var
 454         })
 455 }
 456
 457 /// The guts of `normalize`: normalize a specific projection like `<T
 458 /// as Trait>::Item`. The result is always a type (and possibly
 459 /// additional obligations). Returns `None` in the case of ambiguity,
 460 /// which indicates that there are unbound type variables.
 461 ///
 462 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
 463 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
 464 /// often immediately appended to another obligations vector. So now this
 465 /// function takes an obligations vector and appends to it directly, which is
 466 /// slightly uglier but avoids the need for an extra short-lived allocation.
 467 fn opt_normalize_projection_type<'a, 'b, 'tcx>(
 468     selcx: &'a mut SelectionContext<'b, 'tcx>,
 469     param_env: ty::ParamEnv<'tcx>,
 470     projection_ty: ty::ProjectionTy<'tcx>,
 471     cause: ObligationCause<'tcx>,
 472     depth: usize,
 473     obligations: &mut Vec<PredicateObligation<'tcx>>,
 474 ) -> Option<Ty<'tcx>> {
 475     let infcx = selcx.infcx();
 476
 477     let projection_ty = infcx.resolve_vars_if_possible(&projection_ty);
 478     let cache_key = ProjectionCacheKey { ty: projection_ty };
 479
 480     debug!("opt_normalize_projection_type(\
 481            projection_ty={:?}, \
 482            depth={})",
 483            projection_ty,
 484            depth);
 485
 486     // FIXME(#20304) For now, I am caching here, which is good, but it
 487     // means we don't capture the type variables that are created in
 488     // the case of ambiguity. Which means we may create a large stream
 489     // of such variables. OTOH, if we move the caching up a level, we
 490     // would not benefit from caching when proving `T: Trait<U=Foo>`
 491     // bounds. It might be the case that we want two distinct caches,
 492     // or else another kind of cache entry.
 493
 494     let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
 495     match cache_result {
 496         Ok(()) => { }
 497         Err(ProjectionCacheEntry::Ambiguous) => {
 498             // If we found ambiguity the last time, that generally
 499             // means we will continue to do so until some type in the
 500             // key changes (and we know it hasn't, because we just
 501             // fully resolved it). One exception though is closure
 502             // types, which can transition from having a fixed kind to
 503             // no kind with no visible change in the key.
 504             //
 505             // FIXME(#32286) refactor this so that closure type
 506             // changes
 507             debug!("opt_normalize_projection_type: \
 508                     found cache entry: ambiguous");
 509             if !projection_ty.has_closure_types() {
 510                 return None;
 511             }
 512         }
 513         Err(ProjectionCacheEntry::InProgress) => {
 514             // If while normalized A::B, we are asked to normalize
 515             // A::B, just return A::B itself. This is a conservative
 516             // answer, in the sense that A::B *is* clearly equivalent
 517             // to A::B, though there may be a better value we can
 518             // find.
 519
 520             // Under lazy normalization, this can arise when
 521             // bootstrapping.  That is, imagine an environment with a
 522             // where-clause like `A::B == u32`. Now, if we are asked
 523             // to normalize `A::B`, we will want to check the
 524             // where-clauses in scope. So we will try to unify `A::B`
 525             // with `A::B`, which can trigger a recursive
 526             // normalization. In that case, I think we will want this code:
 527             //
 528             // ```
 529             // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
 530             //                                    projection_ty.substs;
 531             // return Some(NormalizedTy { value: v, obligations: vec![] });
 532             // ```
 533
 534             debug!("opt_normalize_projection_type: \
 535                     found cache entry: in-progress");
 536
 537             // But for now, let's classify this as an overflow:
 538             let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
 539             let obligation = Obligation::with_depth(cause,
 540                                                     recursion_limit,
 541                                                     param_env,
 542                                                     projection_ty);
 543             selcx.infcx().report_overflow_error(&obligation, false);
 544         }
 545         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
 546             // This is the hottest path in this function.
 547             //
 548             // If we find the value in the cache, then return it along
 549             // with the obligations that went along with it. Note
 550             // that, when using a fulfillment context, these
 551             // obligations could in principle be ignored: they have
 552             // already been registered when the cache entry was
 553             // created (and hence the new ones will quickly be
 554             // discarded as duplicated). But when doing trait
 555             // evaluation this is not the case, and dropping the trait
 556             // evaluations can causes ICEs (e.g., #43132).
 557             debug!("opt_normalize_projection_type: \
 558                     found normalized ty `{:?}`",
 559                    ty);
 560
 561             // Once we have inferred everything we need to know, we
 562             // can ignore the `obligations` from that point on.
 563             if infcx.unresolved_type_vars(&ty.value).is_none() {
 564                 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
 565                 // No need to extend `obligations`.
 566             } else {
 567                 obligations.extend(ty.obligations);
 568             }
 569
 570             obligations.push(get_paranoid_cache_value_obligation(infcx,
 571                                                                  param_env,
 572                                                                  projection_ty,
 573                                                                  cause,
 574                                                                  depth));
 575             return Some(ty.value);
 576         }
 577         Err(ProjectionCacheEntry::Error) => {
 578             debug!("opt_normalize_projection_type: \
 579                     found error");
 580             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 581             obligations.extend(result.obligations);
 582             return Some(result.value)
 583         }
 584     }
 585
 586     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
 587     match project_type(selcx, &obligation) {
 588         Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
 589                                             obligations: mut projected_obligations })) => {
 590             // if projection succeeded, then what we get out of this
 591             // is also non-normalized (consider: it was derived from
 592             // an impl, where-clause etc) and hence we must
 593             // re-normalize it
 594
 595             debug!("opt_normalize_projection_type: \
 596                     projected_ty={:?} \
 597                     depth={} \
 598                     projected_obligations={:?}",
 599                    projected_ty,
 600                    depth,
 601                    projected_obligations);
 602
 603             let result = if projected_ty.has_projections() {
 604                 let mut normalizer = AssocTypeNormalizer::new(selcx,
 605                                                                    param_env,
 606                                                                    cause,
 607                                                                    depth+1);
 608                 let normalized_ty = normalizer.fold(&projected_ty);
 609
 610                 debug!("opt_normalize_projection_type: \
 611                         normalized_ty={:?} depth={}",
 612                        normalized_ty,
 613                        depth);
 614
 615                 projected_obligations.extend(normalizer.obligations);
 616                 Normalized {
 617                     value: normalized_ty,
 618                     obligations: projected_obligations,
 619                 }
 620             } else {
 621                 Normalized {
 622                     value: projected_ty,
 623                     obligations: projected_obligations,
 624                 }
 625             };
 626
 627             let cache_value = prune_cache_value_obligations(infcx, &result);
 628             infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
 629             obligations.extend(result.obligations);
 630             Some(result.value)
 631         }
 632         Ok(ProjectedTy::NoProgress(projected_ty)) => {
 633             debug!("opt_normalize_projection_type: \
 634                     projected_ty={:?} no progress",
 635                    projected_ty);
 636             let result = Normalized {
 637                 value: projected_ty,
 638                 obligations: vec![]
 639             };
 640             infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
 641             // No need to extend `obligations`.
 642             Some(result.value)
 643         }
 644         Err(ProjectionTyError::TooManyCandidates) => {
 645             debug!("opt_normalize_projection_type: \
 646                     too many candidates");
 647             infcx.projection_cache.borrow_mut()
 648                                   .ambiguous(cache_key);
 649             None
 650         }
 651         Err(ProjectionTyError::TraitSelectionError(_)) => {
 652             debug!("opt_normalize_projection_type: ERROR");
 653             // if we got an error processing the `T as Trait` part,
 654             // just return `ty::err` but add the obligation `T :
 655             // Trait`, which when processed will cause the error to be
 656             // reported later
 657
 658             infcx.projection_cache.borrow_mut()
 659                                   .error(cache_key);
 660             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 661             obligations.extend(result.obligations);
 662             Some(result.value)
 663         }
 664     }
 665 }
 666
 667 /// If there are unresolved type variables, then we need to include
 668 /// any subobligations that bind them, at least until those type
 669 /// variables are fully resolved.
 670 fn prune_cache_value_obligations<'a, 'tcx>(
 671     infcx: &'a InferCtxt<'a, 'tcx>,
 672     result: &NormalizedTy<'tcx>,
 673 ) -> NormalizedTy<'tcx> {
 674     if infcx.unresolved_type_vars(&result.value).is_none() {
 675         return NormalizedTy { value: result.value, obligations: vec![] };
 676     }
 677
 678     let mut obligations: Vec<_> =
 679         result.obligations
 680               .iter()
 681               .filter(|obligation| match obligation.predicate {
 682                   // We found a `T: Foo<X = U>` predicate, let's check
 683                   // if `U` references any unresolved type
 684                   // variables. In principle, we only care if this
 685                   // projection can help resolve any of the type
 686                   // variables found in `result.value` -- but we just
 687                   // check for any type variables here, for fear of
 688                   // indirect obligations (e.g., we project to `?0`,
 689                   // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
 690                   // ?0>`).
 691                   ty::Predicate::Projection(ref data) =>
 692                       infcx.unresolved_type_vars(&data.ty()).is_some(),
 693
 694                   // We are only interested in `T: Foo<X = U>` predicates, whre
 695                   // `U` references one of `unresolved_type_vars`. =)
 696                   _ => false,
 697               })
 698               .cloned()
 699               .collect();
 700
 701     obligations.shrink_to_fit();
 702
 703     NormalizedTy { value: result.value, obligations }
 704 }
 705
 706 /// Whenever we give back a cache result for a projection like `<T as
 707 /// Trait>::Item ==> X`, we *always* include the obligation to prove
 708 /// that `T: Trait` (we may also include some other obligations). This
 709 /// may or may not be necessary -- in principle, all the obligations
 710 /// that must be proven to show that `T: Trait` were also returned
 711 /// when the cache was first populated. But there are some vague concerns,
 712 /// and so we take the precautionary measure of including `T: Trait` in
 713 /// the result:
 714 ///
 715 /// Concern #1. The current setup is fragile. Perhaps someone could
 716 /// have failed to prove the concerns from when the cache was
 717 /// populated, but also not have used a snapshot, in which case the
 718 /// cache could remain populated even though `T: Trait` has not been
 719 /// shown. In this case, the "other code" is at fault -- when you
 720 /// project something, you are supposed to either have a snapshot or
 721 /// else prove all the resulting obligations -- but it's still easy to
 722 /// get wrong.
 723 ///
 724 /// Concern #2. Even within the snapshot, if those original
 725 /// obligations are not yet proven, then we are able to do projections
 726 /// that may yet turn out to be wrong. This *may* lead to some sort
 727 /// of trouble, though we don't have a concrete example of how that
 728 /// can occur yet. But it seems risky at best.
 729 fn get_paranoid_cache_value_obligation<'a, 'tcx>(
 730     infcx: &'a InferCtxt<'a, 'tcx>,
 731     param_env: ty::ParamEnv<'tcx>,
 732     projection_ty: ty::ProjectionTy<'tcx>,
 733     cause: ObligationCause<'tcx>,
 734     depth: usize,
 735 ) -> PredicateObligation<'tcx> {
 736     let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
 737     Obligation {
 738         cause,
 739         recursion_depth: depth,
 740         param_env,
 741         predicate: trait_ref.to_predicate(),
 742     }
 743 }
 744
 745 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
 746 /// hold. In various error cases, we cannot generate a valid
 747 /// normalized projection. Therefore, we create an inference variable
 748 /// return an associated obligation that, when fulfilled, will lead to
 749 /// an error.
 750 ///
 751 /// Note that we used to return `Error` here, but that was quite
 752 /// dubious -- the premise was that an error would *eventually* be
 753 /// reported, when the obligation was processed. But in general once
 754 /// you see a `Error` you are supposed to be able to assume that an
 755 /// error *has been* reported, so that you can take whatever heuristic
 756 /// paths you want to take. To make things worse, it was possible for
 757 /// cycles to arise, where you basically had a setup like `<MyType<$0>
 758 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
 759 /// Trait>::Foo> to `[type error]` would lead to an obligation of
 760 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
 761 /// an error for this obligation, but we legitimately should not,
 762 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 763 /// one case where this arose.)
 764 fn normalize_to_error<'a, 'tcx>(
 765     selcx: &mut SelectionContext<'a, 'tcx>,
 766     param_env: ty::ParamEnv<'tcx>,
 767     projection_ty: ty::ProjectionTy<'tcx>,
 768     cause: ObligationCause<'tcx>,
 769     depth: usize,
 770 ) -> NormalizedTy<'tcx> {
 771     let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
 772     let trait_obligation = Obligation { cause,
 773                                         recursion_depth: depth,
 774                                         param_env,
 775                                         predicate: trait_ref.to_predicate() };
 776     let tcx = selcx.infcx().tcx;
 777     let def_id = projection_ty.item_def_id;
 778     let new_value = selcx.infcx().next_ty_var(
 779         TypeVariableOrigin {
 780             kind: TypeVariableOriginKind::NormalizeProjectionType,
 781             span: tcx.def_span(def_id),
 782         },
 783     );
 784     Normalized {
 785         value: new_value,
 786         obligations: vec![trait_obligation]
 787     }
 788 }
 789
 790 enum ProjectedTy<'tcx> {
 791     Progress(Progress<'tcx>),
 792     NoProgress(Ty<'tcx>),
 793 }
 794
 795 struct Progress<'tcx> {
 796     ty: Ty<'tcx>,
 797     obligations: Vec<PredicateObligation<'tcx>>,
 798 }
 799
 800 impl<'tcx> Progress<'tcx> {
 801     fn error(tcx: TyCtxt<'tcx>) -> Self {
 802         Progress {
 803             ty: tcx.types.err,
 804             obligations: vec![],
 805         }
 806     }
 807
 808     fn with_addl_obligations(mut self,
 809                              mut obligations: Vec<PredicateObligation<'tcx>>)
 810                              -> Self {
 811         debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
 812                self.obligations.len(), obligations.len());
 813
 814         debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
 815                self.obligations, obligations);
 816
 817         self.obligations.append(&mut obligations);
 818         self
 819     }
 820 }
 821
 822 /// Computes the result of a projection type (if we can).
 823 ///
 824 /// IMPORTANT:
 825 /// - `obligation` must be fully normalized
 826 fn project_type<'cx, 'tcx>(
 827     selcx: &mut SelectionContext<'cx, 'tcx>,
 828     obligation: &ProjectionTyObligation<'tcx>,
 829 ) -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>> {
 830     debug!("project(obligation={:?})",
 831            obligation);
 832
 833     let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
 834     if obligation.recursion_depth >= recursion_limit {
 835         debug!("project: overflow!");
 836         return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
 837     }
 838
 839     let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
 840
 841     debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
 842
 843     if obligation_trait_ref.references_error() {
 844         return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
 845     }
 846
 847     let mut candidates = ProjectionTyCandidateSet::None;
 848
 849     // Make sure that the following procedures are kept in order. ParamEnv
 850     // needs to be first because it has highest priority, and Select checks
 851     // the return value of push_candidate which assumes it's ran at last.
 852     assemble_candidates_from_param_env(selcx,
 853                                        obligation,
 854                                        &obligation_trait_ref,
 855                                        &mut candidates);
 856
 857     assemble_candidates_from_trait_def(selcx,
 858                                        obligation,
 859                                        &obligation_trait_ref,
 860                                        &mut candidates);
 861
 862     assemble_candidates_from_impls(selcx,
 863                                    obligation,
 864                                    &obligation_trait_ref,
 865                                    &mut candidates);
 866
 867     match candidates {
 868         ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
 869             confirm_candidate(selcx,
 870                               obligation,
 871                               &obligation_trait_ref,
 872                               candidate))),
 873         ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
 874             selcx.tcx().mk_projection(
 875                 obligation.predicate.item_def_id,
 876                 obligation.predicate.substs))),
 877         // Error occurred while trying to processing impls.
 878         ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
 879         // Inherent ambiguity that prevents us from even enumerating the
 880         // candidates.
 881         ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
 882
 883     }
 884 }
 885
 886 /// The first thing we have to do is scan through the parameter
 887 /// environment to see whether there are any projection predicates
 888 /// there that can answer this question.
 889 fn assemble_candidates_from_param_env<'cx, 'tcx>(
 890     selcx: &mut SelectionContext<'cx, 'tcx>,
 891     obligation: &ProjectionTyObligation<'tcx>,
 892     obligation_trait_ref: &ty::TraitRef<'tcx>,
 893     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 894 ) {
 895     debug!("assemble_candidates_from_param_env(..)");
 896     assemble_candidates_from_predicates(selcx,
 897                                         obligation,
 898                                         obligation_trait_ref,
 899                                         candidate_set,
 900                                         ProjectionTyCandidate::ParamEnv,
 901                                         obligation.param_env.caller_bounds.iter().cloned());
 902 }
 903
 904 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
 905 /// that the definition of `Foo` has some clues:
 906 ///
 907 /// ```
 908 /// trait Foo {
 909 ///     type FooT : Bar<BarT=i32>
 910 /// }
 911 /// ```
 912 ///
 913 /// Here, for example, we could conclude that the result is `i32`.
 914 fn assemble_candidates_from_trait_def<'cx, 'tcx>(
 915     selcx: &mut SelectionContext<'cx, 'tcx>,
 916     obligation: &ProjectionTyObligation<'tcx>,
 917     obligation_trait_ref: &ty::TraitRef<'tcx>,
 918     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 919 ) {
 920     debug!("assemble_candidates_from_trait_def(..)");
 921
 922     let tcx = selcx.tcx();
 923     // Check whether the self-type is itself a projection.
 924     let (def_id, substs) = match obligation_trait_ref.self_ty().kind {
 925         ty::Projection(ref data) => {
 926             (data.trait_ref(tcx).def_id, data.substs)
 927         }
 928         ty::Opaque(def_id, substs) => (def_id, substs),
 929         ty::Infer(ty::TyVar(_)) => {
 930             // If the self-type is an inference variable, then it MAY wind up
 931             // being a projected type, so induce an ambiguity.
 932             candidate_set.mark_ambiguous();
 933             return;
 934         }
 935         _ => return
 936     };
 937
 938     // If so, extract what we know from the trait and try to come up with a good answer.
 939     let trait_predicates = tcx.predicates_of(def_id);
 940     let bounds = trait_predicates.instantiate(tcx, substs);
 941     let bounds = elaborate_predicates(tcx, bounds.predicates);
 942     assemble_candidates_from_predicates(selcx,
 943                                         obligation,
 944                                         obligation_trait_ref,
 945                                         candidate_set,
 946                                         ProjectionTyCandidate::TraitDef,
 947                                         bounds)
 948 }
 949
 950 fn assemble_candidates_from_predicates<'cx, 'tcx, I>(
 951     selcx: &mut SelectionContext<'cx, 'tcx>,
 952     obligation: &ProjectionTyObligation<'tcx>,
 953     obligation_trait_ref: &ty::TraitRef<'tcx>,
 954     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 955     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
 956     env_predicates: I,
 957 ) where
 958     I: IntoIterator<Item = ty::Predicate<'tcx>>,
 959 {
 960     debug!("assemble_candidates_from_predicates(obligation={:?})",
 961            obligation);
 962     let infcx = selcx.infcx();
 963     for predicate in env_predicates {
 964         debug!("assemble_candidates_from_predicates: predicate={:?}",
 965                predicate);
 966         if let ty::Predicate::Projection(data) = predicate {
 967             let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
 968
 969             let is_match = same_def_id && infcx.probe(|_| {
 970                 let data_poly_trait_ref =
 971                     data.to_poly_trait_ref(infcx.tcx);
 972                 let obligation_poly_trait_ref =
 973                     obligation_trait_ref.to_poly_trait_ref();
 974                 infcx.at(&obligation.cause, obligation.param_env)
 975                      .sup(obligation_poly_trait_ref, data_poly_trait_ref)
 976                      .map(|InferOk { obligations: _, value: () }| {
 977                          // FIXME(#32730) -- do we need to take obligations
 978                          // into account in any way? At the moment, no.
 979                      })
 980                      .is_ok()
 981             });
 982
 983             debug!("assemble_candidates_from_predicates: candidate={:?} \
 984                     is_match={} same_def_id={}",
 985                    data, is_match, same_def_id);
 986
 987             if is_match {
 988                 candidate_set.push_candidate(ctor(data));
 989             }
 990         }
 991     }
 992 }
 993
 994 fn assemble_candidates_from_impls<'cx, 'tcx>(
 995     selcx: &mut SelectionContext<'cx, 'tcx>,
 996     obligation: &ProjectionTyObligation<'tcx>,
 997     obligation_trait_ref: &ty::TraitRef<'tcx>,
 998     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 999 ) {
1000     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1001     // start out by selecting the predicate `T as TraitRef<...>`:
1002     let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1003     let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1004     let _ = selcx.infcx().commit_if_ok(|_| {
1005         let vtable = match selcx.select(&trait_obligation) {
1006             Ok(Some(vtable)) => vtable,
1007             Ok(None) => {
1008                 candidate_set.mark_ambiguous();
1009                 return Err(());
1010             }
1011             Err(e) => {
1012                 debug!("assemble_candidates_from_impls: selection error {:?}", e);
1013                 candidate_set.mark_error(e);
1014                 return Err(());
1015             }
1016         };
1017
1018         let eligible = match &vtable {
1019             super::VtableClosure(_) |
1020             super::VtableGenerator(_) |
1021             super::VtableFnPointer(_) |
1022             super::VtableObject(_) |
1023             super::VtableTraitAlias(_) => {
1024                 debug!("assemble_candidates_from_impls: vtable={:?}",
1025                        vtable);
1026                 true
1027             }
1028             super::VtableImpl(impl_data) => {
1029                 // We have to be careful when projecting out of an
1030                 // impl because of specialization. If we are not in
1031                 // codegen (i.e., projection mode is not "any"), and the
1032                 // impl's type is declared as default, then we disable
1033                 // projection (even if the trait ref is fully
1034                 // monomorphic). In the case where trait ref is not
1035                 // fully monomorphic (i.e., includes type parameters),
1036                 // this is because those type parameters may
1037                 // ultimately be bound to types from other crates that
1038                 // may have specialized impls we can't see. In the
1039                 // case where the trait ref IS fully monomorphic, this
1040                 // is a policy decision that we made in the RFC in
1041                 // order to preserve flexibility for the crate that
1042                 // defined the specializable impl to specialize later
1043                 // for existing types.
1044                 //
1045                 // In either case, we handle this by not adding a
1046                 // candidate for an impl if it contains a `default`
1047                 // type.
1048                 let node_item = assoc_ty_def(selcx,
1049                                              impl_data.impl_def_id,
1050                                              obligation.predicate.item_def_id);
1051
1052                 let is_default = if node_item.node.is_from_trait() {
1053                     // If true, the impl inherited a `type Foo = Bar`
1054                     // given in the trait, which is implicitly default.
1055                     // Otherwise, the impl did not specify `type` and
1056                     // neither did the trait:
1057                     //
1058                     // ```rust
1059                     // trait Foo { type T; }
1060                     // impl Foo for Bar { }
1061                     // ```
1062                     //
1063                     // This is an error, but it will be
1064                     // reported in `check_impl_items_against_trait`.
1065                     // We accept it here but will flag it as
1066                     // an error when we confirm the candidate
1067                     // (which will ultimately lead to `normalize_to_error`
1068                     // being invoked).
1069                     node_item.item.defaultness.has_value()
1070                 } else {
1071                     node_item.item.defaultness.is_default() ||
1072                         selcx.tcx().impl_is_default(node_item.node.def_id())
1073                 };
1074
1075                 // Only reveal a specializable default if we're past type-checking
1076                 // and the obligations is monomorphic, otherwise passes such as
1077                 // transmute checking and polymorphic MIR optimizations could
1078                 // get a result which isn't correct for all monomorphizations.
1079                 if !is_default {
1080                     true
1081                 } else if obligation.param_env.reveal == Reveal::All {
1082                     debug_assert!(!poly_trait_ref.needs_infer());
1083                     if !poly_trait_ref.needs_subst() {
1084                         true
1085                     } else {
1086                         false
1087                     }
1088                 } else {
1089                     false
1090                 }
1091             }
1092             super::VtableParam(..) => {
1093                 // This case tell us nothing about the value of an
1094                 // associated type. Consider:
1095                 //
1096                 // ```
1097                 // trait SomeTrait { type Foo; }
1098                 // fn foo<T:SomeTrait>(...) { }
1099                 // ```
1100                 //
1101                 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1102                 // : SomeTrait` binding does not help us decide what the
1103                 // type `Foo` is (at least, not more specifically than
1104                 // what we already knew).
1105                 //
1106                 // But wait, you say! What about an example like this:
1107                 //
1108                 // ```
1109                 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1110                 // ```
1111                 //
1112                 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1113                 // resolve `T::Foo`? And of course it does, but in fact
1114                 // that single predicate is desugared into two predicates
1115                 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1116                 // projection. And the projection where clause is handled
1117                 // in `assemble_candidates_from_param_env`.
1118                 false
1119             }
1120             super::VtableAutoImpl(..) |
1121             super::VtableBuiltin(..) => {
1122                 // These traits have no associated types.
1123                 span_bug!(
1124                     obligation.cause.span,
1125                     "Cannot project an associated type from `{:?}`",
1126                     vtable);
1127             }
1128         };
1129
1130         if eligible {
1131             if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1132                 Ok(())
1133             } else {
1134                 Err(())
1135             }
1136         } else {
1137             Err(())
1138         }
1139     });
1140 }
1141
1142 fn confirm_candidate<'cx, 'tcx>(
1143     selcx: &mut SelectionContext<'cx, 'tcx>,
1144     obligation: &ProjectionTyObligation<'tcx>,
1145     obligation_trait_ref: &ty::TraitRef<'tcx>,
1146     candidate: ProjectionTyCandidate<'tcx>,
1147 ) -> Progress<'tcx> {
1148     debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1149            candidate,
1150            obligation);
1151
1152     match candidate {
1153         ProjectionTyCandidate::ParamEnv(poly_projection) |
1154         ProjectionTyCandidate::TraitDef(poly_projection) => {
1155             confirm_param_env_candidate(selcx, obligation, poly_projection)
1156         }
1157
1158         ProjectionTyCandidate::Select(vtable) => {
1159             confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1160         }
1161     }
1162 }
1163
1164 fn confirm_select_candidate<'cx, 'tcx>(
1165     selcx: &mut SelectionContext<'cx, 'tcx>,
1166     obligation: &ProjectionTyObligation<'tcx>,
1167     obligation_trait_ref: &ty::TraitRef<'tcx>,
1168     vtable: Selection<'tcx>,
1169 ) -> Progress<'tcx> {
1170     match vtable {
1171         super::VtableImpl(data) =>
1172             confirm_impl_candidate(selcx, obligation, data),
1173         super::VtableGenerator(data) =>
1174             confirm_generator_candidate(selcx, obligation, data),
1175         super::VtableClosure(data) =>
1176             confirm_closure_candidate(selcx, obligation, data),
1177         super::VtableFnPointer(data) =>
1178             confirm_fn_pointer_candidate(selcx, obligation, data),
1179         super::VtableObject(_) =>
1180             confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1181         super::VtableAutoImpl(..) |
1182         super::VtableParam(..) |
1183         super::VtableBuiltin(..) |
1184         super::VtableTraitAlias(..) =>
1185             // we don't create Select candidates with this kind of resolution
1186             span_bug!(
1187                 obligation.cause.span,
1188                 "Cannot project an associated type from `{:?}`",
1189                 vtable),
1190     }
1191 }
1192
1193 fn confirm_object_candidate<'cx, 'tcx>(
1194     selcx: &mut SelectionContext<'cx, 'tcx>,
1195     obligation: &ProjectionTyObligation<'tcx>,
1196     obligation_trait_ref: &ty::TraitRef<'tcx>,
1197 ) -> Progress<'tcx> {
1198     let self_ty = obligation_trait_ref.self_ty();
1199     let object_ty = selcx.infcx().shallow_resolve(self_ty);
1200     debug!("confirm_object_candidate(object_ty={:?})",
1201            object_ty);
1202     let data = match object_ty.kind {
1203         ty::Dynamic(ref data, ..) => data,
1204         _ => {
1205             span_bug!(
1206                 obligation.cause.span,
1207                 "confirm_object_candidate called with non-object: {:?}",
1208                 object_ty)
1209         }
1210     };
1211     let env_predicates = data.projection_bounds().map(|p| {
1212         p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1213     }).collect();
1214     let env_predicate = {
1215         let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1216
1217         // select only those projections that are actually projecting an
1218         // item with the correct name
1219         let env_predicates = env_predicates.filter_map(|p| match p {
1220             ty::Predicate::Projection(data) =>
1221                 if data.projection_def_id() == obligation.predicate.item_def_id {
1222                     Some(data)
1223                 } else {
1224                     None
1225                 },
1226             _ => None
1227         });
1228
1229         // select those with a relevant trait-ref
1230         let mut env_predicates = env_predicates.filter(|data| {
1231             let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1232             let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1233             selcx.infcx().probe(|_|
1234                 selcx.infcx().at(&obligation.cause, obligation.param_env)
1235                              .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1236                              .is_ok()
1237             )
1238         });
1239
1240         // select the first matching one; there really ought to be one or
1241         // else the object type is not WF, since an object type should
1242         // include all of its projections explicitly
1243         match env_predicates.next() {
1244             Some(env_predicate) => env_predicate,
1245             None => {
1246                 debug!("confirm_object_candidate: no env-predicate \
1247                         found in object type `{:?}`; ill-formed",
1248                        object_ty);
1249                 return Progress::error(selcx.tcx());
1250             }
1251         }
1252     };
1253
1254     confirm_param_env_candidate(selcx, obligation, env_predicate)
1255 }
1256
1257 fn confirm_generator_candidate<'cx, 'tcx>(
1258     selcx: &mut SelectionContext<'cx, 'tcx>,
1259     obligation: &ProjectionTyObligation<'tcx>,
1260     vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>,
1261 ) -> Progress<'tcx> {
1262     let gen_sig = vtable.substs.as_generator().poly_sig(vtable.generator_def_id, selcx.tcx());
1263     let Normalized {
1264         value: gen_sig,
1265         obligations
1266     } = normalize_with_depth(selcx,
1267                              obligation.param_env,
1268                              obligation.cause.clone(),
1269                              obligation.recursion_depth+1,
1270                              &gen_sig);
1271
1272     debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1273            obligation,
1274            gen_sig,
1275            obligations);
1276
1277     let tcx = selcx.tcx();
1278
1279     let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1280
1281     let predicate =
1282         tcx.generator_trait_ref_and_outputs(gen_def_id,
1283                                             obligation.predicate.self_ty(),
1284                                             gen_sig)
1285         .map_bound(|(trait_ref, yield_ty, return_ty)| {
1286             let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1287             let ty = if name == sym::Return {
1288                 return_ty
1289             } else if name == sym::Yield {
1290                 yield_ty
1291             } else {
1292                 bug!()
1293             };
1294
1295             ty::ProjectionPredicate {
1296                 projection_ty: ty::ProjectionTy {
1297                     substs: trait_ref.substs,
1298                     item_def_id: obligation.predicate.item_def_id,
1299                 },
1300                 ty: ty
1301             }
1302         });
1303
1304     confirm_param_env_candidate(selcx, obligation, predicate)
1305         .with_addl_obligations(vtable.nested)
1306         .with_addl_obligations(obligations)
1307 }
1308
1309 fn confirm_fn_pointer_candidate<'cx, 'tcx>(
1310     selcx: &mut SelectionContext<'cx, 'tcx>,
1311     obligation: &ProjectionTyObligation<'tcx>,
1312     fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>,
1313 ) -> Progress<'tcx> {
1314     let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1315     let sig = fn_type.fn_sig(selcx.tcx());
1316     let Normalized {
1317         value: sig,
1318         obligations
1319     } = normalize_with_depth(selcx,
1320                              obligation.param_env,
1321                              obligation.cause.clone(),
1322                              obligation.recursion_depth+1,
1323                              &sig);
1324
1325     confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1326         .with_addl_obligations(fn_pointer_vtable.nested)
1327         .with_addl_obligations(obligations)
1328 }
1329
1330 fn confirm_closure_candidate<'cx, 'tcx>(
1331     selcx: &mut SelectionContext<'cx, 'tcx>,
1332     obligation: &ProjectionTyObligation<'tcx>,
1333     vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>,
1334 ) -> Progress<'tcx> {
1335     let tcx = selcx.tcx();
1336     let infcx = selcx.infcx();
1337     let closure_sig_ty = vtable.substs
1338         .as_closure().sig_ty(vtable.closure_def_id, tcx);
1339     let closure_sig = infcx.shallow_resolve(closure_sig_ty).fn_sig(tcx);
1340     let Normalized {
1341         value: closure_sig,
1342         obligations
1343     } = normalize_with_depth(selcx,
1344                              obligation.param_env,
1345                              obligation.cause.clone(),
1346                              obligation.recursion_depth+1,
1347                              &closure_sig);
1348
1349     debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1350            obligation,
1351            closure_sig,
1352            obligations);
1353
1354     confirm_callable_candidate(selcx,
1355                                obligation,
1356                                closure_sig,
1357                                util::TupleArgumentsFlag::No)
1358         .with_addl_obligations(vtable.nested)
1359         .with_addl_obligations(obligations)
1360 }
1361
1362 fn confirm_callable_candidate<'cx, 'tcx>(
1363     selcx: &mut SelectionContext<'cx, 'tcx>,
1364     obligation: &ProjectionTyObligation<'tcx>,
1365     fn_sig: ty::PolyFnSig<'tcx>,
1366     flag: util::TupleArgumentsFlag,
1367 ) -> Progress<'tcx> {
1368     let tcx = selcx.tcx();
1369
1370     debug!("confirm_callable_candidate({:?},{:?})",
1371            obligation,
1372            fn_sig);
1373
1374     // the `Output` associated type is declared on `FnOnce`
1375     let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1376
1377     let predicate =
1378         tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1379                                               obligation.predicate.self_ty(),
1380                                               fn_sig,
1381                                               flag)
1382         .map_bound(|(trait_ref, ret_type)|
1383             ty::ProjectionPredicate {
1384                 projection_ty: ty::ProjectionTy::from_ref_and_name(
1385                     tcx,
1386                     trait_ref,
1387                     Ident::with_dummy_span(FN_OUTPUT_NAME),
1388                 ),
1389                 ty: ret_type
1390             }
1391         );
1392
1393     confirm_param_env_candidate(selcx, obligation, predicate)
1394 }
1395
1396 fn confirm_param_env_candidate<'cx, 'tcx>(
1397     selcx: &mut SelectionContext<'cx, 'tcx>,
1398     obligation: &ProjectionTyObligation<'tcx>,
1399     poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1400 ) -> Progress<'tcx> {
1401     let infcx = selcx.infcx();
1402     let cause = &obligation.cause;
1403     let param_env = obligation.param_env;
1404
1405     let (cache_entry, _) =
1406         infcx.replace_bound_vars_with_fresh_vars(
1407             cause.span,
1408             LateBoundRegionConversionTime::HigherRankedType,
1409             &poly_cache_entry);
1410
1411     let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1412     let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1413     match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1414         Ok(InferOk { value: _, obligations }) => {
1415             Progress {
1416                 ty: cache_entry.ty,
1417                 obligations,
1418             }
1419         }
1420         Err(e) => {
1421             let msg = format!(
1422                 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1423                 obligation,
1424                 poly_cache_entry,
1425                 e,
1426             );
1427             debug!("confirm_param_env_candidate: {}", msg);
1428             infcx.tcx.sess.delay_span_bug(obligation.cause.span, &msg);
1429             Progress {
1430                 ty: infcx.tcx.types.err,
1431                 obligations: vec![],
1432             }
1433         }
1434     }
1435 }
1436
1437 fn confirm_impl_candidate<'cx, 'tcx>(
1438     selcx: &mut SelectionContext<'cx, 'tcx>,
1439     obligation: &ProjectionTyObligation<'tcx>,
1440     impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>,
1441 ) -> Progress<'tcx> {
1442     let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
1443
1444     let tcx = selcx.tcx();
1445     let param_env = obligation.param_env;
1446     let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1447
1448     if !assoc_ty.item.defaultness.has_value() {
1449         // This means that the impl is missing a definition for the
1450         // associated type. This error will be reported by the type
1451         // checker method `check_impl_items_against_trait`, so here we
1452         // just return Error.
1453         debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1454                assoc_ty.item.ident,
1455                obligation.predicate);
1456         return Progress {
1457             ty: tcx.types.err,
1458             obligations: nested,
1459         };
1460     }
1461     let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1462     let ty = if let ty::AssocKind::OpaqueTy = assoc_ty.item.kind {
1463         let item_substs = InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
1464         tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
1465     } else {
1466         tcx.type_of(assoc_ty.item.def_id)
1467     };
1468     Progress {
1469         ty: ty.subst(tcx, substs),
1470         obligations: nested,
1471     }
1472 }
1473
1474 /// Locate the definition of an associated type in the specialization hierarchy,
1475 /// starting from the given impl.
1476 ///
1477 /// Based on the "projection mode", this lookup may in fact only examine the
1478 /// topmost impl. See the comments for `Reveal` for more details.
1479 fn assoc_ty_def(
1480     selcx: &SelectionContext<'_, '_>,
1481     impl_def_id: DefId,
1482     assoc_ty_def_id: DefId,
1483 ) -> specialization_graph::NodeItem<ty::AssocItem> {
1484     let tcx = selcx.tcx();
1485     let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1486     let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1487     let trait_def = tcx.trait_def(trait_def_id);
1488
1489     // This function may be called while we are still building the
1490     // specialization graph that is queried below (via TraidDef::ancestors()),
1491     // so, in order to avoid unnecessary infinite recursion, we manually look
1492     // for the associated item at the given impl.
1493     // If there is no such item in that impl, this function will fail with a
1494     // cycle error if the specialization graph is currently being built.
1495     let impl_node = specialization_graph::Node::Impl(impl_def_id);
1496     for item in impl_node.items(tcx) {
1497         if item.kind == ty::AssocKind::Type &&
1498                 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1499             return specialization_graph::NodeItem {
1500                 node: specialization_graph::Node::Impl(impl_def_id),
1501                 item,
1502             };
1503         }
1504     }
1505
1506     if let Some(assoc_item) = trait_def
1507         .ancestors(tcx, impl_def_id)
1508         .leaf_def(tcx, assoc_ty_name, ty::AssocKind::Type) {
1509
1510         assoc_item
1511     } else {
1512         // This is saying that neither the trait nor
1513         // the impl contain a definition for this
1514         // associated type.  Normally this situation
1515         // could only arise through a compiler bug --
1516         // if the user wrote a bad item name, it
1517         // should have failed in astconv.
1518         bug!("No associated type `{}` for {}",
1519              assoc_ty_name,
1520              tcx.def_path_str(impl_def_id))
1521     }
1522 }
1523
1524 // # Cache
1525
1526 /// The projection cache. Unlike the standard caches, this can include
1527 /// infcx-dependent type variables, therefore we have to roll the
1528 /// cache back each time we roll a snapshot back, to avoid assumptions
1529 /// on yet-unresolved inference variables. Types with placeholder
1530 /// regions also have to be removed when the respective snapshot ends.
1531 ///
1532 /// Because of that, projection cache entries can be "stranded" and left
1533 /// inaccessible when type variables inside the key are resolved. We make no
1534 /// attempt to recover or remove "stranded" entries, but rather let them be
1535 /// (for the lifetime of the infcx).
1536 ///
1537 /// Entries in the projection cache might contain inference variables
1538 /// that will be resolved by obligations on the projection cache entry (e.g.,
1539 /// when a type parameter in the associated type is constrained through
1540 /// an "RFC 447" projection on the impl).
1541 ///
1542 /// When working with a fulfillment context, the derived obligations of each
1543 /// projection cache entry will be registered on the fulfillcx, so any users
1544 /// that can wait for a fulfillcx fixed point need not care about this. However,
1545 /// users that don't wait for a fixed point (e.g., trait evaluation) have to
1546 /// resolve the obligations themselves to make sure the projected result is
1547 /// ok and avoid issues like #43132.
1548 ///
1549 /// If that is done, after evaluation the obligations, it is a good idea to
1550 /// call `ProjectionCache::complete` to make sure the obligations won't be
1551 /// re-evaluated and avoid an exponential worst-case.
1552 //
1553 // FIXME: we probably also want some sort of cross-infcx cache here to
1554 // reduce the amount of duplication. Let's see what we get with the Chalk reforms.
1555 #[derive(Default)]
1556 pub struct ProjectionCache<'tcx> {
1557     map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1558 }
1559
1560 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1561 pub struct ProjectionCacheKey<'tcx> {
1562     ty: ty::ProjectionTy<'tcx>
1563 }
1564
1565 impl<'cx, 'tcx> ProjectionCacheKey<'tcx> {
1566     pub fn from_poly_projection_predicate(
1567         selcx: &mut SelectionContext<'cx, 'tcx>,
1568         predicate: &ty::PolyProjectionPredicate<'tcx>,
1569     ) -> Option<Self> {
1570         let infcx = selcx.infcx();
1571         // We don't do cross-snapshot caching of obligations with escaping regions,
1572         // so there's no cache key to use
1573         predicate.no_bound_vars()
1574             .map(|predicate| ProjectionCacheKey {
1575                 // We don't attempt to match up with a specific type-variable state
1576                 // from a specific call to `opt_normalize_projection_type` - if
1577                 // there's no precise match, the original cache entry is "stranded"
1578                 // anyway.
1579                 ty: infcx.resolve_vars_if_possible(&predicate.projection_ty)
1580             })
1581     }
1582 }
1583
1584 #[derive(Clone, Debug)]
1585 enum ProjectionCacheEntry<'tcx> {
1586     InProgress,
1587     Ambiguous,
1588     Error,
1589     NormalizedTy(NormalizedTy<'tcx>),
1590 }
1591
1592 // N.B., intentionally not Clone
1593 pub struct ProjectionCacheSnapshot {
1594     snapshot: Snapshot,
1595 }
1596
1597 impl<'tcx> ProjectionCache<'tcx> {
1598     pub fn clear(&mut self) {
1599         self.map.clear();
1600     }
1601
1602     pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1603         ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1604     }
1605
1606     pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1607         self.map.rollback_to(snapshot.snapshot);
1608     }
1609
1610     pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
1611         self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_placeholders());
1612     }
1613
1614     pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1615         self.map.commit(snapshot.snapshot);
1616     }
1617
1618     /// Try to start normalize `key`; returns an error if
1619     /// normalization already occurred (this error corresponds to a
1620     /// cache hit, so it's actually a good thing).
1621     fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1622                  -> Result<(), ProjectionCacheEntry<'tcx>> {
1623         if let Some(entry) = self.map.get(&key) {
1624             return Err(entry.clone());
1625         }
1626
1627         self.map.insert(key, ProjectionCacheEntry::InProgress);
1628         Ok(())
1629     }
1630
1631     /// Indicates that `key` was normalized to `value`.
1632     fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1633         debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1634                key, value);
1635         let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1636         assert!(!fresh_key, "never started projecting `{:?}`", key);
1637     }
1638
1639     /// Mark the relevant projection cache key as having its derived obligations
1640     /// complete, so they won't have to be re-computed (this is OK to do in a
1641     /// snapshot - if the snapshot is rolled back, the obligations will be
1642     /// marked as incomplete again).
1643     pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1644         let ty = match self.map.get(&key) {
1645             Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1646                 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1647                        key, ty);
1648                 ty.value
1649             }
1650             ref value => {
1651                 // Type inference could "strand behind" old cache entries. Leave
1652                 // them alone for now.
1653                 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1654                        key, value);
1655                 return
1656             }
1657         };
1658
1659         self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1660             value: ty,
1661             obligations: vec![]
1662         }));
1663     }
1664
1665     /// A specialized version of `complete` for when the key's value is known
1666     /// to be a NormalizedTy.
1667     pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1668         // We want to insert `ty` with no obligations. If the existing value
1669         // already has no obligations (as is common) we don't insert anything.
1670         if !ty.obligations.is_empty() {
1671             self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1672                 value: ty.value,
1673                 obligations: vec![]
1674             }));
1675         }
1676     }
1677
1678     /// Indicates that trying to normalize `key` resulted in
1679     /// ambiguity. No point in trying it again then until we gain more
1680     /// type information (in which case, the "fully resolved" key will
1681     /// be different).
1682     fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1683         let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1684         assert!(!fresh, "never started projecting `{:?}`", key);
1685     }
1686
1687     /// Indicates that trying to normalize `key` resulted in
1688     /// error.
1689     fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1690         let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1691         assert!(!fresh, "never started projecting `{:?}`", key);
1692     }
1693 }