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;
  18 use crate::mir::interpret::{GlobalId, ConstValue};
  19 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
  20 use rustc_macros::HashStable;
  21 use syntax::ast::Ident;
  22 use syntax::symbol::sym;
  23 use crate::ty::subst::{Subst, InternalSubsts};
  24 use crate::ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
  25 use crate::ty::fold::{TypeFoldable, TypeFolder};
  26 use crate::util::common::FN_OUTPUT_NAME;
  27
  28 /// Depending on the stage of compilation, we want projection to be
  29 /// more or less conservative.
  30 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, HashStable)]
  31 pub enum Reveal {
  32     /// At type-checking time, we refuse to project any associated
  33     /// type that is marked `default`. Non-`default` ("final") types
  34     /// are always projected. This is necessary in general for
  35     /// soundness of specialization. However, we *could* allow
  36     /// projections in fully-monomorphic cases. We choose not to,
  37     /// because we prefer for `default type` to force the type
  38     /// definition to be treated abstractly by any consumers of the
  39     /// impl. Concretely, that means that the following example will
  40     /// fail to compile:
  41     ///
  42     /// ```
  43     /// trait Assoc {
  44     ///     type Output;
  45     /// }
  46     ///
  47     /// impl<T> Assoc for T {
  48     ///     default type Output = bool;
  49     /// }
  50     ///
  51     /// fn main() {
  52     ///     let <() as Assoc>::Output = true;
  53     /// }
  54     UserFacing,
  55
  56     /// At codegen time, all monomorphic projections will succeed.
  57     /// Also, `impl Trait` is normalized to the concrete type,
  58     /// which has to be already collected by type-checking.
  59     ///
  60     /// NOTE: as `impl Trait`'s concrete type should *never*
  61     /// be observable directly by the user, `Reveal::All`
  62     /// should not be used by checks which may expose
  63     /// type equality or type contents to the user.
  64     /// There are some exceptions, e.g., around OIBITS and
  65     /// transmute-checking, which expose some details, but
  66     /// not the whole concrete type of the `impl Trait`.
  67     All,
  68 }
  69
  70 pub type PolyProjectionObligation<'tcx> =
  71     Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
  72
  73 pub type ProjectionObligation<'tcx> =
  74     Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
  75
  76 pub type ProjectionTyObligation<'tcx> =
  77     Obligation<'tcx, ty::ProjectionTy<'tcx>>;
  78
  79 /// When attempting to resolve `<T as TraitRef>::Name` ...
  80 #[derive(Debug)]
  81 pub enum ProjectionTyError<'tcx> {
  82     /// ...we found multiple sources of information and couldn't resolve the ambiguity.
  83     TooManyCandidates,
  84
  85     /// ...an error occurred matching `T : TraitRef`
  86     TraitSelectionError(SelectionError<'tcx>),
  87 }
  88
  89 #[derive(Clone)]
  90 pub struct MismatchedProjectionTypes<'tcx> {
  91     pub err: ty::error::TypeError<'tcx>
  92 }
  93
  94 #[derive(PartialEq, Eq, Debug)]
  95 enum ProjectionTyCandidate<'tcx> {
  96     // from a where-clause in the env or object type
  97     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
  98
  99     // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
 100     TraitDef(ty::PolyProjectionPredicate<'tcx>),
 101
 102     // from a "impl" (or a "pseudo-impl" returned by select)
 103     Select(Selection<'tcx>),
 104 }
 105
 106 enum ProjectionTyCandidateSet<'tcx> {
 107     None,
 108     Single(ProjectionTyCandidate<'tcx>),
 109     Ambiguous,
 110     Error(SelectionError<'tcx>),
 111 }
 112
 113 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
 114     fn mark_ambiguous(&mut self) {
 115         *self = ProjectionTyCandidateSet::Ambiguous;
 116     }
 117
 118     fn mark_error(&mut self, err: SelectionError<'tcx>) {
 119         *self = ProjectionTyCandidateSet::Error(err);
 120     }
 121
 122     // Returns true if the push was successful, or false if the candidate
 123     // was discarded -- this could be because of ambiguity, or because
 124     // a higher-priority candidate is already there.
 125     fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
 126         use self::ProjectionTyCandidateSet::*;
 127         use self::ProjectionTyCandidate::*;
 128
 129         // This wacky variable is just used to try and
 130         // make code readable and avoid confusing paths.
 131         // It is assigned a "value" of `()` only on those
 132         // paths in which we wish to convert `*self` to
 133         // ambiguous (and return false, because the candidate
 134         // was not used). On other paths, it is not assigned,
 135         // and hence if those paths *could* reach the code that
 136         // comes after the match, this fn would not compile.
 137         let convert_to_ambiguous;
 138
 139         match self {
 140             None => {
 141                 *self = Single(candidate);
 142                 return true;
 143             }
 144
 145             Single(current) => {
 146                 // Duplicates can happen inside ParamEnv. In the case, we
 147                 // perform a lazy deduplication.
 148                 if current == &candidate {
 149                     return false;
 150                 }
 151
 152                 // Prefer where-clauses. As in select, if there are multiple
 153                 // candidates, we prefer where-clause candidates over impls.  This
 154                 // may seem a bit surprising, since impls are the source of
 155                 // "truth" in some sense, but in fact some of the impls that SEEM
 156                 // applicable are not, because of nested obligations. Where
 157                 // clauses are the safer choice. See the comment on
 158                 // `select::SelectionCandidate` and #21974 for more details.
 159                 match (current, candidate) {
 160                     (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
 161                     (ParamEnv(..), _) => return false,
 162                     (_, ParamEnv(..)) => unreachable!(),
 163                     (_, _) => convert_to_ambiguous = (),
 164                 }
 165             }
 166
 167             Ambiguous | Error(..) => {
 168                 return false;
 169             }
 170         }
 171
 172         // We only ever get here when we moved from a single candidate
 173         // to ambiguous.
 174         let () = convert_to_ambiguous;
 175         *self = Ambiguous;
 176         false
 177     }
 178 }
 179
 180 /// Evaluates constraints of the form:
 181 ///
 182 ///     for<...> <T as Trait>::U == V
 183 ///
 184 /// If successful, this may result in additional obligations. Also returns
 185 /// the projection cache key used to track these additional obligations.
 186 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
 187     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 188     obligation: &PolyProjectionObligation<'tcx>)
 189     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 190               MismatchedProjectionTypes<'tcx>>
 191 {
 192     debug!("poly_project_and_unify_type(obligation={:?})",
 193            obligation);
 194
 195     let infcx = selcx.infcx();
 196     infcx.commit_if_ok(|snapshot| {
 197         let (placeholder_predicate, placeholder_map) =
 198             infcx.replace_bound_vars_with_placeholders(&obligation.predicate);
 199
 200         let placeholder_obligation = obligation.with(placeholder_predicate);
 201         let result = project_and_unify_type(selcx, &placeholder_obligation)?;
 202         infcx.leak_check(false, &placeholder_map, snapshot)
 203             .map_err(|err| MismatchedProjectionTypes { err })?;
 204         Ok(result)
 205     })
 206 }
 207
 208 /// Evaluates constraints of the form:
 209 ///
 210 ///     <T as Trait>::U == V
 211 ///
 212 /// If successful, this may result in additional obligations.
 213 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
 214     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 215     obligation: &ProjectionObligation<'tcx>)
 216     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 217               MismatchedProjectionTypes<'tcx>>
 218 {
 219     debug!("project_and_unify_type(obligation={:?})",
 220            obligation);
 221
 222     let mut obligations = vec![];
 223     let normalized_ty =
 224         match opt_normalize_projection_type(selcx,
 225                                             obligation.param_env,
 226                                             obligation.predicate.projection_ty,
 227                                             obligation.cause.clone(),
 228                                             obligation.recursion_depth,
 229                                             &mut obligations) {
 230             Some(n) => n,
 231             None => return Ok(None),
 232         };
 233
 234     debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
 235            normalized_ty,
 236            obligations);
 237
 238     let infcx = selcx.infcx();
 239     match infcx.at(&obligation.cause, obligation.param_env)
 240                .eq(normalized_ty, obligation.predicate.ty) {
 241         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
 242             obligations.extend(inferred_obligations);
 243             Ok(Some(obligations))
 244         },
 245         Err(err) => {
 246             debug!("project_and_unify_type: equating types encountered error {:?}", err);
 247             Err(MismatchedProjectionTypes { err })
 248         }
 249     }
 250 }
 251
 252 /// Normalizes any associated type projections in `value`, replacing
 253 /// them with a fully resolved type where possible. The return value
 254 /// combines the normalized result and any additional obligations that
 255 /// were incurred as result.
 256 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 257                                         param_env: ty::ParamEnv<'tcx>,
 258                                         cause: ObligationCause<'tcx>,
 259                                         value: &T)
 260                                         -> Normalized<'tcx, T>
 261     where T : TypeFoldable<'tcx>
 262 {
 263     normalize_with_depth(selcx, param_env, cause, 0, value)
 264 }
 265
 266 /// As `normalize`, but with a custom depth.
 267 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
 268     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 269     param_env: ty::ParamEnv<'tcx>,
 270     cause: ObligationCause<'tcx>,
 271     depth: usize,
 272     value: &T)
 273     -> Normalized<'tcx, T>
 274
 275     where T : TypeFoldable<'tcx>
 276 {
 277     debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
 278     let mut normalizer = AssocTypeNormalizer::new(selcx, param_env, cause, depth);
 279     let result = normalizer.fold(value);
 280     debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
 281            depth, result, normalizer.obligations.len());
 282     debug!("normalize_with_depth: depth={} obligations={:?}",
 283            depth, normalizer.obligations);
 284     Normalized {
 285         value: result,
 286         obligations: normalizer.obligations,
 287     }
 288 }
 289
 290 struct AssocTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
 291     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 292     param_env: ty::ParamEnv<'tcx>,
 293     cause: ObligationCause<'tcx>,
 294     obligations: Vec<PredicateObligation<'tcx>>,
 295     depth: usize,
 296 }
 297
 298 impl<'a, 'b, 'gcx, 'tcx> AssocTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
 299     fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 300            param_env: ty::ParamEnv<'tcx>,
 301            cause: ObligationCause<'tcx>,
 302            depth: usize)
 303            -> AssocTypeNormalizer<'a, 'b, 'gcx, 'tcx>
 304     {
 305         AssocTypeNormalizer {
 306             selcx,
 307             param_env,
 308             cause,
 309             obligations: vec![],
 310             depth,
 311         }
 312     }
 313
 314     fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
 315         let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
 316
 317         if !value.has_projections() {
 318             value
 319         } else {
 320             value.fold_with(self)
 321         }
 322     }
 323 }
 324
 325 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssocTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
 326     fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
 327         self.selcx.tcx()
 328     }
 329
 330     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 331         // We don't want to normalize associated types that occur inside of region
 332         // binders, because they may contain bound regions, and we can't cope with that.
 333         //
 334         // Example:
 335         //
 336         //     for<'a> fn(<T as Foo<&'a>>::A)
 337         //
 338         // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
 339         // normalize it when we instantiate those bound regions (which
 340         // should occur eventually).
 341
 342         let ty = ty.super_fold_with(self);
 343         match ty.sty {
 344             ty::Opaque(def_id, substs) if !substs.has_escaping_bound_vars() => { // (*)
 345                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 346                 match self.param_env.reveal {
 347                     Reveal::UserFacing => ty,
 348
 349                     Reveal::All => {
 350                         let recursion_limit = *self.tcx().sess.recursion_limit.get();
 351                         if self.depth >= recursion_limit {
 352                             let obligation = Obligation::with_depth(
 353                                 self.cause.clone(),
 354                                 recursion_limit,
 355                                 self.param_env,
 356                                 ty,
 357                             );
 358                             self.selcx.infcx().report_overflow_error(&obligation, true);
 359                         }
 360
 361                         let generic_ty = self.tcx().type_of(def_id);
 362                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 363                         self.depth += 1;
 364                         let folded_ty = self.fold_ty(concrete_ty);
 365                         self.depth -= 1;
 366                         folded_ty
 367                     }
 368                 }
 369             }
 370
 371             ty::Projection(ref data) if !data.has_escaping_bound_vars() => { // (*)
 372
 373                 // (*) This is kind of hacky -- we need to be able to
 374                 // handle normalization within binders because
 375                 // otherwise we wind up a need to normalize when doing
 376                 // trait matching (since you can have a trait
 377                 // obligation like `for<'a> T::B : Fn(&'a int)`), but
 378                 // we can't normalize with bound regions in scope. So
 379                 // far now we just ignore binders but only normalize
 380                 // if all bound regions are gone (and then we still
 381                 // have to renormalize whenever we instantiate a
 382                 // binder). It would be better to normalize in a
 383                 // binding-aware fashion.
 384
 385                 let normalized_ty = normalize_projection_type(self.selcx,
 386                                                               self.param_env,
 387                                                               data.clone(),
 388                                                               self.cause.clone(),
 389                                                               self.depth,
 390                                                               &mut self.obligations);
 391                 debug!("AssocTypeNormalizer: depth={} normalized {:?} to {:?}, \
 392                         now with {} obligations",
 393                        self.depth, ty, normalized_ty, self.obligations.len());
 394                 normalized_ty
 395             }
 396
 397             _ => ty
 398         }
 399     }
 400
 401     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 402         if let ConstValue::Unevaluated(def_id, substs) = constant.val {
 403             let tcx = self.selcx.tcx().global_tcx();
 404             if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
 405                 if substs.needs_infer() || substs.has_placeholders() {
 406                     let identity_substs = InternalSubsts::identity_for_item(tcx, def_id);
 407                     let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
 408                     if let Some(instance) = instance {
 409                         let cid = GlobalId {
 410                             instance,
 411                             promoted: None
 412                         };
 413                         if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
 414                             let substs = tcx.lift_to_global(&substs).unwrap();
 415                             let evaluated = evaluated.subst(tcx, substs);
 416                             return evaluated;
 417                         }
 418                     }
 419                 } else {
 420                     if let Some(substs) = self.tcx().lift_to_global(&substs) {
 421                         let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
 422                         if let Some(instance) = instance {
 423                             let cid = GlobalId {
 424                                 instance,
 425                                 promoted: None
 426                             };
 427                             if let Ok(evaluated) = tcx.const_eval(param_env.and(cid)) {
 428                                 return evaluated;
 429                             }
 430                         }
 431                     }
 432                 }
 433             }
 434         }
 435         constant
 436     }
 437 }
 438
 439 #[derive(Clone)]
 440 pub struct Normalized<'tcx,T> {
 441     pub value: T,
 442     pub obligations: Vec<PredicateObligation<'tcx>>,
 443 }
 444
 445 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
 446
 447 impl<'tcx,T> Normalized<'tcx,T> {
 448     pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
 449         Normalized { value: value, obligations: self.obligations }
 450     }
 451 }
 452
 453 /// The guts of `normalize`: normalize a specific projection like `<T
 454 /// as Trait>::Item`. The result is always a type (and possibly
 455 /// additional obligations). If ambiguity arises, which implies that
 456 /// there are unresolved type variables in the projection, we will
 457 /// substitute a fresh type variable `$X` and generate a new
 458 /// obligation `<T as Trait>::Item == $X` for later.
 459 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
 460     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 461     param_env: ty::ParamEnv<'tcx>,
 462     projection_ty: ty::ProjectionTy<'tcx>,
 463     cause: ObligationCause<'tcx>,
 464     depth: usize,
 465     obligations: &mut Vec<PredicateObligation<'tcx>>)
 466     -> Ty<'tcx>
 467 {
 468     opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
 469                                   obligations)
 470         .unwrap_or_else(move || {
 471             // if we bottom out in ambiguity, create a type variable
 472             // and a deferred predicate to resolve this when more type
 473             // information is available.
 474
 475             let tcx = selcx.infcx().tcx;
 476             let def_id = projection_ty.item_def_id;
 477             let ty_var = selcx.infcx().next_ty_var(
 478                 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
 479             let projection = ty::Binder::dummy(ty::ProjectionPredicate {
 480                 projection_ty,
 481                 ty: ty_var
 482             });
 483             let obligation = Obligation::with_depth(
 484                 cause, depth + 1, param_env, projection.to_predicate());
 485             obligations.push(obligation);
 486             ty_var
 487         })
 488 }
 489
 490 /// The guts of `normalize`: normalize a specific projection like `<T
 491 /// as Trait>::Item`. The result is always a type (and possibly
 492 /// additional obligations). Returns `None` in the case of ambiguity,
 493 /// which indicates that there are unbound type variables.
 494 ///
 495 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
 496 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
 497 /// often immediately appended to another obligations vector. So now this
 498 /// function takes an obligations vector and appends to it directly, which is
 499 /// slightly uglier but avoids the need for an extra short-lived allocation.
 500 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
 501     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 502     param_env: ty::ParamEnv<'tcx>,
 503     projection_ty: ty::ProjectionTy<'tcx>,
 504     cause: ObligationCause<'tcx>,
 505     depth: usize,
 506     obligations: &mut Vec<PredicateObligation<'tcx>>)
 507     -> Option<Ty<'tcx>>
 508 {
 509     let infcx = selcx.infcx();
 510
 511     let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
 512     let cache_key = ProjectionCacheKey { ty: projection_ty };
 513
 514     debug!("opt_normalize_projection_type(\
 515            projection_ty={:?}, \
 516            depth={})",
 517            projection_ty,
 518            depth);
 519
 520     // FIXME(#20304) For now, I am caching here, which is good, but it
 521     // means we don't capture the type variables that are created in
 522     // the case of ambiguity. Which means we may create a large stream
 523     // of such variables. OTOH, if we move the caching up a level, we
 524     // would not benefit from caching when proving `T: Trait<U=Foo>`
 525     // bounds. It might be the case that we want two distinct caches,
 526     // or else another kind of cache entry.
 527
 528     let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
 529     match cache_result {
 530         Ok(()) => { }
 531         Err(ProjectionCacheEntry::Ambiguous) => {
 532             // If we found ambiguity the last time, that generally
 533             // means we will continue to do so until some type in the
 534             // key changes (and we know it hasn't, because we just
 535             // fully resolved it). One exception though is closure
 536             // types, which can transition from having a fixed kind to
 537             // no kind with no visible change in the key.
 538             //
 539             // FIXME(#32286) refactor this so that closure type
 540             // changes
 541             debug!("opt_normalize_projection_type: \
 542                     found cache entry: ambiguous");
 543             if !projection_ty.has_closure_types() {
 544                 return None;
 545             }
 546         }
 547         Err(ProjectionCacheEntry::InProgress) => {
 548             // If while normalized A::B, we are asked to normalize
 549             // A::B, just return A::B itself. This is a conservative
 550             // answer, in the sense that A::B *is* clearly equivalent
 551             // to A::B, though there may be a better value we can
 552             // find.
 553
 554             // Under lazy normalization, this can arise when
 555             // bootstrapping.  That is, imagine an environment with a
 556             // where-clause like `A::B == u32`. Now, if we are asked
 557             // to normalize `A::B`, we will want to check the
 558             // where-clauses in scope. So we will try to unify `A::B`
 559             // with `A::B`, which can trigger a recursive
 560             // normalization. In that case, I think we will want this code:
 561             //
 562             // ```
 563             // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
 564             //                                    projection_ty.substs;
 565             // return Some(NormalizedTy { value: v, obligations: vec![] });
 566             // ```
 567
 568             debug!("opt_normalize_projection_type: \
 569                     found cache entry: in-progress");
 570
 571             // But for now, let's classify this as an overflow:
 572             let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
 573             let obligation = Obligation::with_depth(cause,
 574                                                     recursion_limit,
 575                                                     param_env,
 576                                                     projection_ty);
 577             selcx.infcx().report_overflow_error(&obligation, false);
 578         }
 579         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
 580             // This is the hottest path in this function.
 581             //
 582             // If we find the value in the cache, then return it along
 583             // with the obligations that went along with it. Note
 584             // that, when using a fulfillment context, these
 585             // obligations could in principle be ignored: they have
 586             // already been registered when the cache entry was
 587             // created (and hence the new ones will quickly be
 588             // discarded as duplicated). But when doing trait
 589             // evaluation this is not the case, and dropping the trait
 590             // evaluations can causes ICEs (e.g., #43132).
 591             debug!("opt_normalize_projection_type: \
 592                     found normalized ty `{:?}`",
 593                    ty);
 594
 595             // Once we have inferred everything we need to know, we
 596             // can ignore the `obligations` from that point on.
 597             if infcx.unresolved_type_vars(&ty.value).is_none() {
 598                 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
 599                 // No need to extend `obligations`.
 600             } else {
 601                 obligations.extend(ty.obligations);
 602             }
 603
 604             obligations.push(get_paranoid_cache_value_obligation(infcx,
 605                                                                  param_env,
 606                                                                  projection_ty,
 607                                                                  cause,
 608                                                                  depth));
 609             return Some(ty.value);
 610         }
 611         Err(ProjectionCacheEntry::Error) => {
 612             debug!("opt_normalize_projection_type: \
 613                     found error");
 614             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 615             obligations.extend(result.obligations);
 616             return Some(result.value)
 617         }
 618     }
 619
 620     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
 621     match project_type(selcx, &obligation) {
 622         Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
 623                                             obligations: mut projected_obligations })) => {
 624             // if projection succeeded, then what we get out of this
 625             // is also non-normalized (consider: it was derived from
 626             // an impl, where-clause etc) and hence we must
 627             // re-normalize it
 628
 629             debug!("opt_normalize_projection_type: \
 630                     projected_ty={:?} \
 631                     depth={} \
 632                     projected_obligations={:?}",
 633                    projected_ty,
 634                    depth,
 635                    projected_obligations);
 636
 637             let result = if projected_ty.has_projections() {
 638                 let mut normalizer = AssocTypeNormalizer::new(selcx,
 639                                                                    param_env,
 640                                                                    cause,
 641                                                                    depth+1);
 642                 let normalized_ty = normalizer.fold(&projected_ty);
 643
 644                 debug!("opt_normalize_projection_type: \
 645                         normalized_ty={:?} depth={}",
 646                        normalized_ty,
 647                        depth);
 648
 649                 projected_obligations.extend(normalizer.obligations);
 650                 Normalized {
 651                     value: normalized_ty,
 652                     obligations: projected_obligations,
 653                 }
 654             } else {
 655                 Normalized {
 656                     value: projected_ty,
 657                     obligations: projected_obligations,
 658                 }
 659             };
 660
 661             let cache_value = prune_cache_value_obligations(infcx, &result);
 662             infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
 663             obligations.extend(result.obligations);
 664             Some(result.value)
 665         }
 666         Ok(ProjectedTy::NoProgress(projected_ty)) => {
 667             debug!("opt_normalize_projection_type: \
 668                     projected_ty={:?} no progress",
 669                    projected_ty);
 670             let result = Normalized {
 671                 value: projected_ty,
 672                 obligations: vec![]
 673             };
 674             infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
 675             // No need to extend `obligations`.
 676             Some(result.value)
 677         }
 678         Err(ProjectionTyError::TooManyCandidates) => {
 679             debug!("opt_normalize_projection_type: \
 680                     too many candidates");
 681             infcx.projection_cache.borrow_mut()
 682                                   .ambiguous(cache_key);
 683             None
 684         }
 685         Err(ProjectionTyError::TraitSelectionError(_)) => {
 686             debug!("opt_normalize_projection_type: ERROR");
 687             // if we got an error processing the `T as Trait` part,
 688             // just return `ty::err` but add the obligation `T :
 689             // Trait`, which when processed will cause the error to be
 690             // reported later
 691
 692             infcx.projection_cache.borrow_mut()
 693                                   .error(cache_key);
 694             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 695             obligations.extend(result.obligations);
 696             Some(result.value)
 697         }
 698     }
 699 }
 700
 701 /// If there are unresolved type variables, then we need to include
 702 /// any subobligations that bind them, at least until those type
 703 /// variables are fully resolved.
 704 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 705                                                  result: &NormalizedTy<'tcx>)
 706                                                  -> NormalizedTy<'tcx> {
 707     if infcx.unresolved_type_vars(&result.value).is_none() {
 708         return NormalizedTy { value: result.value, obligations: vec![] };
 709     }
 710
 711     let mut obligations: Vec<_> =
 712         result.obligations
 713               .iter()
 714               .filter(|obligation| match obligation.predicate {
 715                   // We found a `T: Foo<X = U>` predicate, let's check
 716                   // if `U` references any unresolved type
 717                   // variables. In principle, we only care if this
 718                   // projection can help resolve any of the type
 719                   // variables found in `result.value` -- but we just
 720                   // check for any type variables here, for fear of
 721                   // indirect obligations (e.g., we project to `?0`,
 722                   // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
 723                   // ?0>`).
 724                   ty::Predicate::Projection(ref data) =>
 725                       infcx.unresolved_type_vars(&data.ty()).is_some(),
 726
 727                   // We are only interested in `T: Foo<X = U>` predicates, whre
 728                   // `U` references one of `unresolved_type_vars`. =)
 729                   _ => false,
 730               })
 731               .cloned()
 732               .collect();
 733
 734     obligations.shrink_to_fit();
 735
 736     NormalizedTy { value: result.value, obligations }
 737 }
 738
 739 /// Whenever we give back a cache result for a projection like `<T as
 740 /// Trait>::Item ==> X`, we *always* include the obligation to prove
 741 /// that `T: Trait` (we may also include some other obligations). This
 742 /// may or may not be necessary -- in principle, all the obligations
 743 /// that must be proven to show that `T: Trait` were also returned
 744 /// when the cache was first populated. But there are some vague concerns,
 745 /// and so we take the precautionary measure of including `T: Trait` in
 746 /// the result:
 747 ///
 748 /// Concern #1. The current setup is fragile. Perhaps someone could
 749 /// have failed to prove the concerns from when the cache was
 750 /// populated, but also not have used a snapshot, in which case the
 751 /// cache could remain populated even though `T: Trait` has not been
 752 /// shown. In this case, the "other code" is at fault -- when you
 753 /// project something, you are supposed to either have a snapshot or
 754 /// else prove all the resulting obligations -- but it's still easy to
 755 /// get wrong.
 756 ///
 757 /// Concern #2. Even within the snapshot, if those original
 758 /// obligations are not yet proven, then we are able to do projections
 759 /// that may yet turn out to be wrong. This *may* lead to some sort
 760 /// of trouble, though we don't have a concrete example of how that
 761 /// can occur yet. But it seems risky at best.
 762 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
 763     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 764     param_env: ty::ParamEnv<'tcx>,
 765     projection_ty: ty::ProjectionTy<'tcx>,
 766     cause: ObligationCause<'tcx>,
 767     depth: usize)
 768     -> PredicateObligation<'tcx>
 769 {
 770     let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
 771     Obligation {
 772         cause,
 773         recursion_depth: depth,
 774         param_env,
 775         predicate: trait_ref.to_predicate(),
 776     }
 777 }
 778
 779 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
 780 /// hold. In various error cases, we cannot generate a valid
 781 /// normalized projection. Therefore, we create an inference variable
 782 /// return an associated obligation that, when fulfilled, will lead to
 783 /// an error.
 784 ///
 785 /// Note that we used to return `Error` here, but that was quite
 786 /// dubious -- the premise was that an error would *eventually* be
 787 /// reported, when the obligation was processed. But in general once
 788 /// you see a `Error` you are supposed to be able to assume that an
 789 /// error *has been* reported, so that you can take whatever heuristic
 790 /// paths you want to take. To make things worse, it was possible for
 791 /// cycles to arise, where you basically had a setup like `<MyType<$0>
 792 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
 793 /// Trait>::Foo> to `[type error]` would lead to an obligation of
 794 /// `<MyType<[type error]> as Trait>::Foo`. We are supposed to report
 795 /// an error for this obligation, but we legitimately should not,
 796 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 797 /// one case where this arose.)
 798 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 799                                       param_env: ty::ParamEnv<'tcx>,
 800                                       projection_ty: ty::ProjectionTy<'tcx>,
 801                                       cause: ObligationCause<'tcx>,
 802                                       depth: usize)
 803                                       -> NormalizedTy<'tcx>
 804 {
 805     let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
 806     let trait_obligation = Obligation { cause,
 807                                         recursion_depth: depth,
 808                                         param_env,
 809                                         predicate: trait_ref.to_predicate() };
 810     let tcx = selcx.infcx().tcx;
 811     let def_id = projection_ty.item_def_id;
 812     let new_value = selcx.infcx().next_ty_var(
 813         TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
 814     Normalized {
 815         value: new_value,
 816         obligations: vec![trait_obligation]
 817     }
 818 }
 819
 820 enum ProjectedTy<'tcx> {
 821     Progress(Progress<'tcx>),
 822     NoProgress(Ty<'tcx>),
 823 }
 824
 825 struct Progress<'tcx> {
 826     ty: Ty<'tcx>,
 827     obligations: Vec<PredicateObligation<'tcx>>,
 828 }
 829
 830 impl<'tcx> Progress<'tcx> {
 831     fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
 832         Progress {
 833             ty: tcx.types.err,
 834             obligations: vec![],
 835         }
 836     }
 837
 838     fn with_addl_obligations(mut self,
 839                              mut obligations: Vec<PredicateObligation<'tcx>>)
 840                              -> Self {
 841         debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
 842                self.obligations.len(), obligations.len());
 843
 844         debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
 845                self.obligations, obligations);
 846
 847         self.obligations.append(&mut obligations);
 848         self
 849     }
 850 }
 851
 852 /// Computes the result of a projection type (if we can).
 853 ///
 854 /// IMPORTANT:
 855 /// - `obligation` must be fully normalized
 856 fn project_type<'cx, 'gcx, 'tcx>(
 857     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 858     obligation: &ProjectionTyObligation<'tcx>)
 859     -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
 860 {
 861     debug!("project(obligation={:?})",
 862            obligation);
 863
 864     let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
 865     if obligation.recursion_depth >= recursion_limit {
 866         debug!("project: overflow!");
 867         return Err(ProjectionTyError::TraitSelectionError(SelectionError::Overflow));
 868     }
 869
 870     let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
 871
 872     debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
 873
 874     if obligation_trait_ref.references_error() {
 875         return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
 876     }
 877
 878     let mut candidates = ProjectionTyCandidateSet::None;
 879
 880     // Make sure that the following procedures are kept in order. ParamEnv
 881     // needs to be first because it has highest priority, and Select checks
 882     // the return value of push_candidate which assumes it's ran at last.
 883     assemble_candidates_from_param_env(selcx,
 884                                        obligation,
 885                                        &obligation_trait_ref,
 886                                        &mut candidates);
 887
 888     assemble_candidates_from_trait_def(selcx,
 889                                        obligation,
 890                                        &obligation_trait_ref,
 891                                        &mut candidates);
 892
 893     assemble_candidates_from_impls(selcx,
 894                                    obligation,
 895                                    &obligation_trait_ref,
 896                                    &mut candidates);
 897
 898     match candidates {
 899         ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
 900             confirm_candidate(selcx,
 901                               obligation,
 902                               &obligation_trait_ref,
 903                               candidate))),
 904         ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
 905             selcx.tcx().mk_projection(
 906                 obligation.predicate.item_def_id,
 907                 obligation.predicate.substs))),
 908         // Error occurred while trying to processing impls.
 909         ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
 910         // Inherent ambiguity that prevents us from even enumerating the
 911         // candidates.
 912         ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
 913
 914     }
 915 }
 916
 917 /// The first thing we have to do is scan through the parameter
 918 /// environment to see whether there are any projection predicates
 919 /// there that can answer this question.
 920 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
 921     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 922     obligation: &ProjectionTyObligation<'tcx>,
 923     obligation_trait_ref: &ty::TraitRef<'tcx>,
 924     candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
 925 {
 926     debug!("assemble_candidates_from_param_env(..)");
 927     assemble_candidates_from_predicates(selcx,
 928                                         obligation,
 929                                         obligation_trait_ref,
 930                                         candidate_set,
 931                                         ProjectionTyCandidate::ParamEnv,
 932                                         obligation.param_env.caller_bounds.iter().cloned());
 933 }
 934
 935 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
 936 /// that the definition of `Foo` has some clues:
 937 ///
 938 /// ```
 939 /// trait Foo {
 940 ///     type FooT : Bar<BarT=i32>
 941 /// }
 942 /// ```
 943 ///
 944 /// Here, for example, we could conclude that the result is `i32`.
 945 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
 946     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 947     obligation: &ProjectionTyObligation<'tcx>,
 948     obligation_trait_ref: &ty::TraitRef<'tcx>,
 949     candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
 950 {
 951     debug!("assemble_candidates_from_trait_def(..)");
 952
 953     let tcx = selcx.tcx();
 954     // Check whether the self-type is itself a projection.
 955     let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
 956         ty::Projection(ref data) => {
 957             (data.trait_ref(tcx).def_id, data.substs)
 958         }
 959         ty::Opaque(def_id, substs) => (def_id, substs),
 960         ty::Infer(ty::TyVar(_)) => {
 961             // If the self-type is an inference variable, then it MAY wind up
 962             // being a projected type, so induce an ambiguity.
 963             candidate_set.mark_ambiguous();
 964             return;
 965         }
 966         _ => return
 967     };
 968
 969     // If so, extract what we know from the trait and try to come up with a good answer.
 970     let trait_predicates = tcx.predicates_of(def_id);
 971     let bounds = trait_predicates.instantiate(tcx, substs);
 972     let bounds = elaborate_predicates(tcx, bounds.predicates);
 973     assemble_candidates_from_predicates(selcx,
 974                                         obligation,
 975                                         obligation_trait_ref,
 976                                         candidate_set,
 977                                         ProjectionTyCandidate::TraitDef,
 978                                         bounds)
 979 }
 980
 981 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
 982     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 983     obligation: &ProjectionTyObligation<'tcx>,
 984     obligation_trait_ref: &ty::TraitRef<'tcx>,
 985     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
 986     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
 987     env_predicates: I)
 988     where I: IntoIterator<Item=ty::Predicate<'tcx>>
 989 {
 990     debug!("assemble_candidates_from_predicates(obligation={:?})",
 991            obligation);
 992     let infcx = selcx.infcx();
 993     for predicate in env_predicates {
 994         debug!("assemble_candidates_from_predicates: predicate={:?}",
 995                predicate);
 996         if let ty::Predicate::Projection(data) = predicate {
 997             let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
 998
 999             let is_match = same_def_id && infcx.probe(|_| {
1000                 let data_poly_trait_ref =
1001                     data.to_poly_trait_ref(infcx.tcx);
1002                 let obligation_poly_trait_ref =
1003                     obligation_trait_ref.to_poly_trait_ref();
1004                 infcx.at(&obligation.cause, obligation.param_env)
1005                      .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1006                      .map(|InferOk { obligations: _, value: () }| {
1007                          // FIXME(#32730) -- do we need to take obligations
1008                          // into account in any way? At the moment, no.
1009                      })
1010                      .is_ok()
1011             });
1012
1013             debug!("assemble_candidates_from_predicates: candidate={:?} \
1014                     is_match={} same_def_id={}",
1015                    data, is_match, same_def_id);
1016
1017             if is_match {
1018                 candidate_set.push_candidate(ctor(data));
1019             }
1020         }
1021     }
1022 }
1023
1024 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1025     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1026     obligation: &ProjectionTyObligation<'tcx>,
1027     obligation_trait_ref: &ty::TraitRef<'tcx>,
1028     candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1029 {
1030     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1031     // start out by selecting the predicate `T as TraitRef<...>`:
1032     let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1033     let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1034     let _ = selcx.infcx().commit_if_ok(|_| {
1035         let vtable = match selcx.select(&trait_obligation) {
1036             Ok(Some(vtable)) => vtable,
1037             Ok(None) => {
1038                 candidate_set.mark_ambiguous();
1039                 return Err(());
1040             }
1041             Err(e) => {
1042                 debug!("assemble_candidates_from_impls: selection error {:?}", e);
1043                 candidate_set.mark_error(e);
1044                 return Err(());
1045             }
1046         };
1047
1048         let eligible = match &vtable {
1049             super::VtableClosure(_) |
1050             super::VtableGenerator(_) |
1051             super::VtableFnPointer(_) |
1052             super::VtableObject(_) |
1053             super::VtableTraitAlias(_) => {
1054                 debug!("assemble_candidates_from_impls: vtable={:?}",
1055                        vtable);
1056                 true
1057             }
1058             super::VtableImpl(impl_data) => {
1059                 // We have to be careful when projecting out of an
1060                 // impl because of specialization. If we are not in
1061                 // codegen (i.e., projection mode is not "any"), and the
1062                 // impl's type is declared as default, then we disable
1063                 // projection (even if the trait ref is fully
1064                 // monomorphic). In the case where trait ref is not
1065                 // fully monomorphic (i.e., includes type parameters),
1066                 // this is because those type parameters may
1067                 // ultimately be bound to types from other crates that
1068                 // may have specialized impls we can't see. In the
1069                 // case where the trait ref IS fully monomorphic, this
1070                 // is a policy decision that we made in the RFC in
1071                 // order to preserve flexibility for the crate that
1072                 // defined the specializable impl to specialize later
1073                 // for existing types.
1074                 //
1075                 // In either case, we handle this by not adding a
1076                 // candidate for an impl if it contains a `default`
1077                 // type.
1078                 let node_item = assoc_ty_def(selcx,
1079                                              impl_data.impl_def_id,
1080                                              obligation.predicate.item_def_id);
1081
1082                 let is_default = if node_item.node.is_from_trait() {
1083                     // If true, the impl inherited a `type Foo = Bar`
1084                     // given in the trait, which is implicitly default.
1085                     // Otherwise, the impl did not specify `type` and
1086                     // neither did the trait:
1087                     //
1088                     // ```rust
1089                     // trait Foo { type T; }
1090                     // impl Foo for Bar { }
1091                     // ```
1092                     //
1093                     // This is an error, but it will be
1094                     // reported in `check_impl_items_against_trait`.
1095                     // We accept it here but will flag it as
1096                     // an error when we confirm the candidate
1097                     // (which will ultimately lead to `normalize_to_error`
1098                     // being invoked).
1099                     node_item.item.defaultness.has_value()
1100                 } else {
1101                     node_item.item.defaultness.is_default() ||
1102                         selcx.tcx().impl_is_default(node_item.node.def_id())
1103                 };
1104
1105                 // Only reveal a specializable default if we're past type-checking
1106                 // and the obligations is monomorphic, otherwise passes such as
1107                 // transmute checking and polymorphic MIR optimizations could
1108                 // get a result which isn't correct for all monomorphizations.
1109                 if !is_default {
1110                     true
1111                 } else if obligation.param_env.reveal == Reveal::All {
1112                     debug_assert!(!poly_trait_ref.needs_infer());
1113                     if !poly_trait_ref.needs_subst() {
1114                         true
1115                     } else {
1116                         false
1117                     }
1118                 } else {
1119                     false
1120                 }
1121             }
1122             super::VtableParam(..) => {
1123                 // This case tell us nothing about the value of an
1124                 // associated type. Consider:
1125                 //
1126                 // ```
1127                 // trait SomeTrait { type Foo; }
1128                 // fn foo<T:SomeTrait>(...) { }
1129                 // ```
1130                 //
1131                 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1132                 // : SomeTrait` binding does not help us decide what the
1133                 // type `Foo` is (at least, not more specifically than
1134                 // what we already knew).
1135                 //
1136                 // But wait, you say! What about an example like this:
1137                 //
1138                 // ```
1139                 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1140                 // ```
1141                 //
1142                 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1143                 // resolve `T::Foo`? And of course it does, but in fact
1144                 // that single predicate is desugared into two predicates
1145                 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1146                 // projection. And the projection where clause is handled
1147                 // in `assemble_candidates_from_param_env`.
1148                 false
1149             }
1150             super::VtableAutoImpl(..) |
1151             super::VtableBuiltin(..) => {
1152                 // These traits have no associated types.
1153                 span_bug!(
1154                     obligation.cause.span,
1155                     "Cannot project an associated type from `{:?}`",
1156                     vtable);
1157             }
1158         };
1159
1160         if eligible {
1161             if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1162                 Ok(())
1163             } else {
1164                 Err(())
1165             }
1166         } else {
1167             Err(())
1168         }
1169     });
1170 }
1171
1172 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1173     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1174     obligation: &ProjectionTyObligation<'tcx>,
1175     obligation_trait_ref: &ty::TraitRef<'tcx>,
1176     candidate: ProjectionTyCandidate<'tcx>)
1177     -> Progress<'tcx>
1178 {
1179     debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1180            candidate,
1181            obligation);
1182
1183     match candidate {
1184         ProjectionTyCandidate::ParamEnv(poly_projection) |
1185         ProjectionTyCandidate::TraitDef(poly_projection) => {
1186             confirm_param_env_candidate(selcx, obligation, poly_projection)
1187         }
1188
1189         ProjectionTyCandidate::Select(vtable) => {
1190             confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1191         }
1192     }
1193 }
1194
1195 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1196     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1197     obligation: &ProjectionTyObligation<'tcx>,
1198     obligation_trait_ref: &ty::TraitRef<'tcx>,
1199     vtable: Selection<'tcx>)
1200     -> Progress<'tcx>
1201 {
1202     match vtable {
1203         super::VtableImpl(data) =>
1204             confirm_impl_candidate(selcx, obligation, data),
1205         super::VtableGenerator(data) =>
1206             confirm_generator_candidate(selcx, obligation, data),
1207         super::VtableClosure(data) =>
1208             confirm_closure_candidate(selcx, obligation, data),
1209         super::VtableFnPointer(data) =>
1210             confirm_fn_pointer_candidate(selcx, obligation, data),
1211         super::VtableObject(_) =>
1212             confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1213         super::VtableAutoImpl(..) |
1214         super::VtableParam(..) |
1215         super::VtableBuiltin(..) |
1216         super::VtableTraitAlias(..) =>
1217             // we don't create Select candidates with this kind of resolution
1218             span_bug!(
1219                 obligation.cause.span,
1220                 "Cannot project an associated type from `{:?}`",
1221                 vtable),
1222     }
1223 }
1224
1225 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1226     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1227     obligation:  &ProjectionTyObligation<'tcx>,
1228     obligation_trait_ref: &ty::TraitRef<'tcx>)
1229     -> Progress<'tcx>
1230 {
1231     let self_ty = obligation_trait_ref.self_ty();
1232     let object_ty = selcx.infcx().shallow_resolve(self_ty);
1233     debug!("confirm_object_candidate(object_ty={:?})",
1234            object_ty);
1235     let data = match object_ty.sty {
1236         ty::Dynamic(ref data, ..) => data,
1237         _ => {
1238             span_bug!(
1239                 obligation.cause.span,
1240                 "confirm_object_candidate called with non-object: {:?}",
1241                 object_ty)
1242         }
1243     };
1244     let env_predicates = data.projection_bounds().map(|p| {
1245         p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1246     }).collect();
1247     let env_predicate = {
1248         let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1249
1250         // select only those projections that are actually projecting an
1251         // item with the correct name
1252         let env_predicates = env_predicates.filter_map(|p| match p {
1253             ty::Predicate::Projection(data) =>
1254                 if data.projection_def_id() == obligation.predicate.item_def_id {
1255                     Some(data)
1256                 } else {
1257                     None
1258                 },
1259             _ => None
1260         });
1261
1262         // select those with a relevant trait-ref
1263         let mut env_predicates = env_predicates.filter(|data| {
1264             let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1265             let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1266             selcx.infcx().probe(|_|
1267                 selcx.infcx().at(&obligation.cause, obligation.param_env)
1268                              .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1269                              .is_ok()
1270             )
1271         });
1272
1273         // select the first matching one; there really ought to be one or
1274         // else the object type is not WF, since an object type should
1275         // include all of its projections explicitly
1276         match env_predicates.next() {
1277             Some(env_predicate) => env_predicate,
1278             None => {
1279                 debug!("confirm_object_candidate: no env-predicate \
1280                         found in object type `{:?}`; ill-formed",
1281                        object_ty);
1282                 return Progress::error(selcx.tcx());
1283             }
1284         }
1285     };
1286
1287     confirm_param_env_candidate(selcx, obligation, env_predicate)
1288 }
1289
1290 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1291     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1292     obligation: &ProjectionTyObligation<'tcx>,
1293     vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1294     -> Progress<'tcx>
1295 {
1296     let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1297     let Normalized {
1298         value: gen_sig,
1299         obligations
1300     } = normalize_with_depth(selcx,
1301                              obligation.param_env,
1302                              obligation.cause.clone(),
1303                              obligation.recursion_depth+1,
1304                              &gen_sig);
1305
1306     debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1307            obligation,
1308            gen_sig,
1309            obligations);
1310
1311     let tcx = selcx.tcx();
1312
1313     let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1314
1315     let predicate =
1316         tcx.generator_trait_ref_and_outputs(gen_def_id,
1317                                             obligation.predicate.self_ty(),
1318                                             gen_sig)
1319         .map_bound(|(trait_ref, yield_ty, return_ty)| {
1320             let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1321             let ty = if name == sym::Return {
1322                 return_ty
1323             } else if name == sym::Yield {
1324                 yield_ty
1325             } else {
1326                 bug!()
1327             };
1328
1329             ty::ProjectionPredicate {
1330                 projection_ty: ty::ProjectionTy {
1331                     substs: trait_ref.substs,
1332                     item_def_id: obligation.predicate.item_def_id,
1333                 },
1334                 ty: ty
1335             }
1336         });
1337
1338     confirm_param_env_candidate(selcx, obligation, predicate)
1339         .with_addl_obligations(vtable.nested)
1340         .with_addl_obligations(obligations)
1341 }
1342
1343 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1344     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1345     obligation: &ProjectionTyObligation<'tcx>,
1346     fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1347     -> Progress<'tcx>
1348 {
1349     let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1350     let sig = fn_type.fn_sig(selcx.tcx());
1351     let Normalized {
1352         value: sig,
1353         obligations
1354     } = normalize_with_depth(selcx,
1355                              obligation.param_env,
1356                              obligation.cause.clone(),
1357                              obligation.recursion_depth+1,
1358                              &sig);
1359
1360     confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1361         .with_addl_obligations(fn_pointer_vtable.nested)
1362         .with_addl_obligations(obligations)
1363 }
1364
1365 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1366     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1367     obligation: &ProjectionTyObligation<'tcx>,
1368     vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1369     -> Progress<'tcx>
1370 {
1371     let tcx = selcx.tcx();
1372     let infcx = selcx.infcx();
1373     let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1374     let closure_sig = infcx.shallow_resolve(closure_sig_ty).fn_sig(tcx);
1375     let Normalized {
1376         value: closure_sig,
1377         obligations
1378     } = normalize_with_depth(selcx,
1379                              obligation.param_env,
1380                              obligation.cause.clone(),
1381                              obligation.recursion_depth+1,
1382                              &closure_sig);
1383
1384     debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1385            obligation,
1386            closure_sig,
1387            obligations);
1388
1389     confirm_callable_candidate(selcx,
1390                                obligation,
1391                                closure_sig,
1392                                util::TupleArgumentsFlag::No)
1393         .with_addl_obligations(vtable.nested)
1394         .with_addl_obligations(obligations)
1395 }
1396
1397 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1398     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1399     obligation: &ProjectionTyObligation<'tcx>,
1400     fn_sig: ty::PolyFnSig<'tcx>,
1401     flag: util::TupleArgumentsFlag)
1402     -> Progress<'tcx>
1403 {
1404     let tcx = selcx.tcx();
1405
1406     debug!("confirm_callable_candidate({:?},{:?})",
1407            obligation,
1408            fn_sig);
1409
1410     // the `Output` associated type is declared on `FnOnce`
1411     let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1412
1413     let predicate =
1414         tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1415                                               obligation.predicate.self_ty(),
1416                                               fn_sig,
1417                                               flag)
1418         .map_bound(|(trait_ref, ret_type)|
1419             ty::ProjectionPredicate {
1420                 projection_ty: ty::ProjectionTy::from_ref_and_name(
1421                     tcx,
1422                     trait_ref,
1423                     Ident::with_empty_ctxt(FN_OUTPUT_NAME),
1424                 ),
1425                 ty: ret_type
1426             }
1427         );
1428
1429     confirm_param_env_candidate(selcx, obligation, predicate)
1430 }
1431
1432 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1433     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1434     obligation: &ProjectionTyObligation<'tcx>,
1435     poly_cache_entry: ty::PolyProjectionPredicate<'tcx>,
1436 ) -> Progress<'tcx> {
1437     let infcx = selcx.infcx();
1438     let cause = &obligation.cause;
1439     let param_env = obligation.param_env;
1440
1441     let (cache_entry, _) =
1442         infcx.replace_bound_vars_with_fresh_vars(
1443             cause.span,
1444             LateBoundRegionConversionTime::HigherRankedType,
1445             &poly_cache_entry);
1446
1447     let cache_trait_ref = cache_entry.projection_ty.trait_ref(infcx.tcx);
1448     let obligation_trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1449     match infcx.at(cause, param_env).eq(cache_trait_ref, obligation_trait_ref) {
1450         Ok(InferOk { value: _, obligations }) => {
1451             Progress {
1452                 ty: cache_entry.ty,
1453                 obligations,
1454             }
1455         }
1456         Err(e) => {
1457             let msg = format!(
1458                 "Failed to unify obligation `{:?}` with poly_projection `{:?}`: {:?}",
1459                 obligation,
1460                 poly_cache_entry,
1461                 e,
1462             );
1463             debug!("confirm_param_env_candidate: {}", msg);
1464             infcx.tcx.sess.delay_span_bug(obligation.cause.span, &msg);
1465             Progress {
1466                 ty: infcx.tcx.types.err,
1467                 obligations: vec![],
1468             }
1469         }
1470     }
1471 }
1472
1473 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1474     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1475     obligation: &ProjectionTyObligation<'tcx>,
1476     impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1477     -> Progress<'tcx>
1478 {
1479     let VtableImplData { impl_def_id, substs, nested } = impl_vtable;
1480
1481     let tcx = selcx.tcx();
1482     let param_env = obligation.param_env;
1483     let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1484
1485     if !assoc_ty.item.defaultness.has_value() {
1486         // This means that the impl is missing a definition for the
1487         // associated type. This error will be reported by the type
1488         // checker method `check_impl_items_against_trait`, so here we
1489         // just return Error.
1490         debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1491                assoc_ty.item.ident,
1492                obligation.predicate);
1493         return Progress {
1494             ty: tcx.types.err,
1495             obligations: nested,
1496         };
1497     }
1498     let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1499     let ty = if let ty::AssocKind::Existential = assoc_ty.item.kind {
1500         let item_substs = InternalSubsts::identity_for_item(tcx, assoc_ty.item.def_id);
1501         tcx.mk_opaque(assoc_ty.item.def_id, item_substs)
1502     } else {
1503         tcx.type_of(assoc_ty.item.def_id)
1504     };
1505     Progress {
1506         ty: ty.subst(tcx, substs),
1507         obligations: nested,
1508     }
1509 }
1510
1511 /// Locate the definition of an associated type in the specialization hierarchy,
1512 /// starting from the given impl.
1513 ///
1514 /// Based on the "projection mode", this lookup may in fact only examine the
1515 /// topmost impl. See the comments for `Reveal` for more details.
1516 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1517     selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1518     impl_def_id: DefId,
1519     assoc_ty_def_id: DefId)
1520     -> specialization_graph::NodeItem<ty::AssocItem>
1521 {
1522     let tcx = selcx.tcx();
1523     let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1524     let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1525     let trait_def = tcx.trait_def(trait_def_id);
1526
1527     // This function may be called while we are still building the
1528     // specialization graph that is queried below (via TraidDef::ancestors()),
1529     // so, in order to avoid unnecessary infinite recursion, we manually look
1530     // for the associated item at the given impl.
1531     // If there is no such item in that impl, this function will fail with a
1532     // cycle error if the specialization graph is currently being built.
1533     let impl_node = specialization_graph::Node::Impl(impl_def_id);
1534     for item in impl_node.items(tcx) {
1535         if item.kind == ty::AssocKind::Type &&
1536                 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1537             return specialization_graph::NodeItem {
1538                 node: specialization_graph::Node::Impl(impl_def_id),
1539                 item,
1540             };
1541         }
1542     }
1543
1544     if let Some(assoc_item) = trait_def
1545         .ancestors(tcx, impl_def_id)
1546         .defs(tcx, assoc_ty_name, ty::AssocKind::Type, trait_def_id)
1547         .next() {
1548         assoc_item
1549     } else {
1550         // This is saying that neither the trait nor
1551         // the impl contain a definition for this
1552         // associated type.  Normally this situation
1553         // could only arise through a compiler bug --
1554         // if the user wrote a bad item name, it
1555         // should have failed in astconv.
1556         bug!("No associated type `{}` for {}",
1557              assoc_ty_name,
1558              tcx.def_path_str(impl_def_id))
1559     }
1560 }
1561
1562 // # Cache
1563
1564 /// The projection cache. Unlike the standard caches, this can include
1565 /// infcx-dependent type variables, therefore we have to roll the
1566 /// cache back each time we roll a snapshot back, to avoid assumptions
1567 /// on yet-unresolved inference variables. Types with placeholder
1568 /// regions also have to be removed when the respective snapshot ends.
1569 ///
1570 /// Because of that, projection cache entries can be "stranded" and left
1571 /// inaccessible when type variables inside the key are resolved. We make no
1572 /// attempt to recover or remove "stranded" entries, but rather let them be
1573 /// (for the lifetime of the infcx).
1574 ///
1575 /// Entries in the projection cache might contain inference variables
1576 /// that will be resolved by obligations on the projection cache entry (e.g.,
1577 /// when a type parameter in the associated type is constrained through
1578 /// an "RFC 447" projection on the impl).
1579 ///
1580 /// When working with a fulfillment context, the derived obligations of each
1581 /// projection cache entry will be registered on the fulfillcx, so any users
1582 /// that can wait for a fulfillcx fixed point need not care about this. However,
1583 /// users that don't wait for a fixed point (e.g., trait evaluation) have to
1584 /// resolve the obligations themselves to make sure the projected result is
1585 /// ok and avoid issues like #43132.
1586 ///
1587 /// If that is done, after evaluation the obligations, it is a good idea to
1588 /// call `ProjectionCache::complete` to make sure the obligations won't be
1589 /// re-evaluated and avoid an exponential worst-case.
1590 //
1591 // FIXME: we probably also want some sort of cross-infcx cache here to
1592 // reduce the amount of duplication. Let's see what we get with the Chalk reforms.
1593 #[derive(Default)]
1594 pub struct ProjectionCache<'tcx> {
1595     map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1596 }
1597
1598 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1599 pub struct ProjectionCacheKey<'tcx> {
1600     ty: ty::ProjectionTy<'tcx>
1601 }
1602
1603 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1604     pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1605                                           predicate: &ty::PolyProjectionPredicate<'tcx>)
1606                                           -> Option<Self>
1607     {
1608         let infcx = selcx.infcx();
1609         // We don't do cross-snapshot caching of obligations with escaping regions,
1610         // so there's no cache key to use
1611         predicate.no_bound_vars()
1612             .map(|predicate| ProjectionCacheKey {
1613                 // We don't attempt to match up with a specific type-variable state
1614                 // from a specific call to `opt_normalize_projection_type` - if
1615                 // there's no precise match, the original cache entry is "stranded"
1616                 // anyway.
1617                 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1618             })
1619     }
1620 }
1621
1622 #[derive(Clone, Debug)]
1623 enum ProjectionCacheEntry<'tcx> {
1624     InProgress,
1625     Ambiguous,
1626     Error,
1627     NormalizedTy(NormalizedTy<'tcx>),
1628 }
1629
1630 // N.B., intentionally not Clone
1631 pub struct ProjectionCacheSnapshot {
1632     snapshot: Snapshot,
1633 }
1634
1635 impl<'tcx> ProjectionCache<'tcx> {
1636     pub fn clear(&mut self) {
1637         self.map.clear();
1638     }
1639
1640     pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1641         ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1642     }
1643
1644     pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1645         self.map.rollback_to(snapshot.snapshot);
1646     }
1647
1648     pub fn rollback_placeholder(&mut self, snapshot: &ProjectionCacheSnapshot) {
1649         self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_placeholders());
1650     }
1651
1652     pub fn commit(&mut self, snapshot: ProjectionCacheSnapshot) {
1653         self.map.commit(snapshot.snapshot);
1654     }
1655
1656     /// Try to start normalize `key`; returns an error if
1657     /// normalization already occurred (this error corresponds to a
1658     /// cache hit, so it's actually a good thing).
1659     fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1660                  -> Result<(), ProjectionCacheEntry<'tcx>> {
1661         if let Some(entry) = self.map.get(&key) {
1662             return Err(entry.clone());
1663         }
1664
1665         self.map.insert(key, ProjectionCacheEntry::InProgress);
1666         Ok(())
1667     }
1668
1669     /// Indicates that `key` was normalized to `value`.
1670     fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1671         debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1672                key, value);
1673         let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1674         assert!(!fresh_key, "never started projecting `{:?}`", key);
1675     }
1676
1677     /// Mark the relevant projection cache key as having its derived obligations
1678     /// complete, so they won't have to be re-computed (this is OK to do in a
1679     /// snapshot - if the snapshot is rolled back, the obligations will be
1680     /// marked as incomplete again).
1681     pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1682         let ty = match self.map.get(&key) {
1683             Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1684                 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1685                        key, ty);
1686                 ty.value
1687             }
1688             ref value => {
1689                 // Type inference could "strand behind" old cache entries. Leave
1690                 // them alone for now.
1691                 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1692                        key, value);
1693                 return
1694             }
1695         };
1696
1697         self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1698             value: ty,
1699             obligations: vec![]
1700         }));
1701     }
1702
1703     /// A specialized version of `complete` for when the key's value is known
1704     /// to be a NormalizedTy.
1705     pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1706         // We want to insert `ty` with no obligations. If the existing value
1707         // already has no obligations (as is common) we don't insert anything.
1708         if !ty.obligations.is_empty() {
1709             self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1710                 value: ty.value,
1711                 obligations: vec![]
1712             }));
1713         }
1714     }
1715
1716     /// Indicates that trying to normalize `key` resulted in
1717     /// ambiguity. No point in trying it again then until we gain more
1718     /// type information (in which case, the "fully resolved" key will
1719     /// be different).
1720     fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1721         let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1722         assert!(!fresh, "never started projecting `{:?}`", key);
1723     }
1724
1725     /// Indicates that trying to normalize `key` resulted in
1726     /// error.
1727     fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1728         let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1729         assert!(!fresh, "never started projecting `{:?}`", key);
1730     }
1731 }