src/librustc/traits/project.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Code for projecting associated types out of trait references.
  12
  13 use super::elaborate_predicates;
  14 use super::specialization_graph;
  15 use super::translate_substs;
  16 use super::Obligation;
  17 use super::ObligationCause;
  18 use super::PredicateObligation;
  19 use super::Selection;
  20 use super::SelectionContext;
  21 use super::SelectionError;
  22 use super::VtableClosureData;
  23 use super::VtableGeneratorData;
  24 use super::VtableFnPointerData;
  25 use super::VtableImplData;
  26 use super::util;
  27
  28 use hir::def_id::DefId;
  29 use infer::{InferCtxt, InferOk};
  30 use infer::type_variable::TypeVariableOrigin;
  31 use mir::interpret::ConstValue;
  32 use mir::interpret::{GlobalId};
  33 use rustc_data_structures::snapshot_map::{Snapshot, SnapshotMap};
  34 use syntax::ast::Ident;
  35 use ty::subst::{Subst, Substs};
  36 use ty::{self, ToPredicate, ToPolyTraitRef, Ty, TyCtxt};
  37 use ty::fold::{TypeFoldable, TypeFolder};
  38 use util::common::FN_OUTPUT_NAME;
  39
  40 /// Depending on the stage of compilation, we want projection to be
  41 /// more or less conservative.
  42 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
  43 pub enum Reveal {
  44     /// At type-checking time, we refuse to project any associated
  45     /// type that is marked `default`. Non-`default` ("final") types
  46     /// are always projected. This is necessary in general for
  47     /// soundness of specialization. However, we *could* allow
  48     /// projections in fully-monomorphic cases. We choose not to,
  49     /// because we prefer for `default type` to force the type
  50     /// definition to be treated abstractly by any consumers of the
  51     /// impl. Concretely, that means that the following example will
  52     /// fail to compile:
  53     ///
  54     /// ```
  55     /// trait Assoc {
  56     ///     type Output;
  57     /// }
  58     ///
  59     /// impl<T> Assoc for T {
  60     ///     default type Output = bool;
  61     /// }
  62     ///
  63     /// fn main() {
  64     ///     let <() as Assoc>::Output = true;
  65     /// }
  66     UserFacing,
  67
  68     /// At codegen time, all monomorphic projections will succeed.
  69     /// Also, `impl Trait` is normalized to the concrete type,
  70     /// which has to be already collected by type-checking.
  71     ///
  72     /// NOTE: As `impl Trait`'s concrete type should *never*
  73     /// be observable directly by the user, `Reveal::All`
  74     /// should not be used by checks which may expose
  75     /// type equality or type contents to the user.
  76     /// There are some exceptions, e.g. around OIBITS and
  77     /// transmute-checking, which expose some details, but
  78     /// not the whole concrete type of the `impl Trait`.
  79     All,
  80 }
  81
  82 pub type PolyProjectionObligation<'tcx> =
  83     Obligation<'tcx, ty::PolyProjectionPredicate<'tcx>>;
  84
  85 pub type ProjectionObligation<'tcx> =
  86     Obligation<'tcx, ty::ProjectionPredicate<'tcx>>;
  87
  88 pub type ProjectionTyObligation<'tcx> =
  89     Obligation<'tcx, ty::ProjectionTy<'tcx>>;
  90
  91 /// When attempting to resolve `<T as TraitRef>::Name` ...
  92 #[derive(Debug)]
  93 pub enum ProjectionTyError<'tcx> {
  94     /// ...we found multiple sources of information and couldn't resolve the ambiguity.
  95     TooManyCandidates,
  96
  97     /// ...an error occurred matching `T : TraitRef`
  98     TraitSelectionError(SelectionError<'tcx>),
  99 }
 100
 101 #[derive(Clone)]
 102 pub struct MismatchedProjectionTypes<'tcx> {
 103     pub err: ty::error::TypeError<'tcx>
 104 }
 105
 106 #[derive(PartialEq, Eq, Debug)]
 107 enum ProjectionTyCandidate<'tcx> {
 108     // from a where-clause in the env or object type
 109     ParamEnv(ty::PolyProjectionPredicate<'tcx>),
 110
 111     // from the definition of `Trait` when you have something like <<A as Trait>::B as Trait2>::C
 112     TraitDef(ty::PolyProjectionPredicate<'tcx>),
 113
 114     // from a "impl" (or a "pseudo-impl" returned by select)
 115     Select(Selection<'tcx>),
 116 }
 117
 118 enum ProjectionTyCandidateSet<'tcx> {
 119     None,
 120     Single(ProjectionTyCandidate<'tcx>),
 121     Ambiguous,
 122     Error(SelectionError<'tcx>),
 123 }
 124
 125 impl<'tcx> ProjectionTyCandidateSet<'tcx> {
 126     fn mark_ambiguous(&mut self) {
 127         *self = ProjectionTyCandidateSet::Ambiguous;
 128     }
 129
 130     fn mark_error(&mut self, err: SelectionError<'tcx>) {
 131         *self = ProjectionTyCandidateSet::Error(err);
 132     }
 133
 134     // Returns true if the push was successful, or false if the candidate
 135     // was discarded -- this could be because of ambiguity, or because
 136     // a higher-priority candidate is already there.
 137     fn push_candidate(&mut self, candidate: ProjectionTyCandidate<'tcx>) -> bool {
 138         use self::ProjectionTyCandidateSet::*;
 139         use self::ProjectionTyCandidate::*;
 140
 141         // This wacky variable is just used to try and
 142         // make code readable and avoid confusing paths.
 143         // It is assigned a "value" of `()` only on those
 144         // paths in which we wish to convert `*self` to
 145         // ambiguous (and return false, because the candidate
 146         // was not used). On other paths, it is not assigned,
 147         // and hence if those paths *could* reach the code that
 148         // comes after the match, this fn would not compile.
 149         let convert_to_ambiguous;
 150
 151         match self {
 152             None => {
 153                 *self = Single(candidate);
 154                 return true;
 155             }
 156
 157             Single(current) => {
 158                 // Duplicates can happen inside ParamEnv. In the case, we
 159                 // perform a lazy deduplication.
 160                 if current == &candidate {
 161                     return false;
 162                 }
 163
 164                 // Prefer where-clauses. As in select, if there are multiple
 165                 // candidates, we prefer where-clause candidates over impls.  This
 166                 // may seem a bit surprising, since impls are the source of
 167                 // "truth" in some sense, but in fact some of the impls that SEEM
 168                 // applicable are not, because of nested obligations. Where
 169                 // clauses are the safer choice. See the comment on
 170                 // `select::SelectionCandidate` and #21974 for more details.
 171                 match (current, candidate) {
 172                     (ParamEnv(..), ParamEnv(..)) => convert_to_ambiguous = (),
 173                     (ParamEnv(..), _) => return false,
 174                     (_, ParamEnv(..)) => { unreachable!(); }
 175                     (_, _) => convert_to_ambiguous = (),
 176                 }
 177             }
 178
 179             Ambiguous | Error(..) => {
 180                 return false;
 181             }
 182         }
 183
 184         // We only ever get here when we moved from a single candidate
 185         // to ambiguous.
 186         let () = convert_to_ambiguous;
 187         *self = Ambiguous;
 188         false
 189     }
 190 }
 191
 192 /// Evaluates constraints of the form:
 193 ///
 194 ///     for<...> <T as Trait>::U == V
 195 ///
 196 /// If successful, this may result in additional obligations. Also returns
 197 /// the projection cache key used to track these additional obligations.
 198 pub fn poly_project_and_unify_type<'cx, 'gcx, 'tcx>(
 199     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 200     obligation: &PolyProjectionObligation<'tcx>)
 201     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 202               MismatchedProjectionTypes<'tcx>>
 203 {
 204     debug!("poly_project_and_unify_type(obligation={:?})",
 205            obligation);
 206
 207     let infcx = selcx.infcx();
 208     infcx.commit_if_ok(|snapshot| {
 209         let (skol_predicate, skol_map) =
 210             infcx.skolemize_late_bound_regions(&obligation.predicate);
 211
 212         let skol_obligation = obligation.with(skol_predicate);
 213         let r = match project_and_unify_type(selcx, &skol_obligation) {
 214             Ok(result) => {
 215                 let span = obligation.cause.span;
 216                 match infcx.leak_check(false, span, &skol_map, snapshot) {
 217                     Ok(()) => Ok(infcx.plug_leaks(skol_map, snapshot, result)),
 218                     Err(e) => {
 219                         debug!("poly_project_and_unify_type: leak check encountered error {:?}", e);
 220                         Err(MismatchedProjectionTypes { err: e })
 221                     }
 222                 }
 223             }
 224             Err(e) => {
 225                 Err(e)
 226             }
 227         };
 228
 229         r
 230     })
 231 }
 232
 233 /// Evaluates constraints of the form:
 234 ///
 235 ///     <T as Trait>::U == V
 236 ///
 237 /// If successful, this may result in additional obligations.
 238 fn project_and_unify_type<'cx, 'gcx, 'tcx>(
 239     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 240     obligation: &ProjectionObligation<'tcx>)
 241     -> Result<Option<Vec<PredicateObligation<'tcx>>>,
 242               MismatchedProjectionTypes<'tcx>>
 243 {
 244     debug!("project_and_unify_type(obligation={:?})",
 245            obligation);
 246
 247     let mut obligations = vec![];
 248     let normalized_ty =
 249         match opt_normalize_projection_type(selcx,
 250                                             obligation.param_env,
 251                                             obligation.predicate.projection_ty,
 252                                             obligation.cause.clone(),
 253                                             obligation.recursion_depth,
 254                                             &mut obligations) {
 255             Some(n) => n,
 256             None => return Ok(None),
 257         };
 258
 259     debug!("project_and_unify_type: normalized_ty={:?} obligations={:?}",
 260            normalized_ty,
 261            obligations);
 262
 263     let infcx = selcx.infcx();
 264     match infcx.at(&obligation.cause, obligation.param_env)
 265                .eq(normalized_ty, obligation.predicate.ty) {
 266         Ok(InferOk { obligations: inferred_obligations, value: () }) => {
 267             obligations.extend(inferred_obligations);
 268             Ok(Some(obligations))
 269         },
 270         Err(err) => {
 271             debug!("project_and_unify_type: equating types encountered error {:?}", err);
 272             Err(MismatchedProjectionTypes { err: err })
 273         }
 274     }
 275 }
 276
 277 /// Normalizes any associated type projections in `value`, replacing
 278 /// them with a fully resolved type where possible. The return value
 279 /// combines the normalized result and any additional obligations that
 280 /// were incurred as result.
 281 pub fn normalize<'a, 'b, 'gcx, 'tcx, T>(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 282                                         param_env: ty::ParamEnv<'tcx>,
 283                                         cause: ObligationCause<'tcx>,
 284                                         value: &T)
 285                                         -> Normalized<'tcx, T>
 286     where T : TypeFoldable<'tcx>
 287 {
 288     normalize_with_depth(selcx, param_env, cause, 0, value)
 289 }
 290
 291 /// As `normalize`, but with a custom depth.
 292 pub fn normalize_with_depth<'a, 'b, 'gcx, 'tcx, T>(
 293     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 294     param_env: ty::ParamEnv<'tcx>,
 295     cause: ObligationCause<'tcx>,
 296     depth: usize,
 297     value: &T)
 298     -> Normalized<'tcx, T>
 299
 300     where T : TypeFoldable<'tcx>
 301 {
 302     debug!("normalize_with_depth(depth={}, value={:?})", depth, value);
 303     let mut normalizer = AssociatedTypeNormalizer::new(selcx, param_env, cause, depth);
 304     let result = normalizer.fold(value);
 305     debug!("normalize_with_depth: depth={} result={:?} with {} obligations",
 306            depth, result, normalizer.obligations.len());
 307     debug!("normalize_with_depth: depth={} obligations={:?}",
 308            depth, normalizer.obligations);
 309     Normalized {
 310         value: result,
 311         obligations: normalizer.obligations,
 312     }
 313 }
 314
 315 struct AssociatedTypeNormalizer<'a, 'b: 'a, 'gcx: 'b+'tcx, 'tcx: 'b> {
 316     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 317     param_env: ty::ParamEnv<'tcx>,
 318     cause: ObligationCause<'tcx>,
 319     obligations: Vec<PredicateObligation<'tcx>>,
 320     depth: usize,
 321 }
 322
 323 impl<'a, 'b, 'gcx, 'tcx> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
 324     fn new(selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 325            param_env: ty::ParamEnv<'tcx>,
 326            cause: ObligationCause<'tcx>,
 327            depth: usize)
 328            -> AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx>
 329     {
 330         AssociatedTypeNormalizer {
 331             selcx,
 332             param_env,
 333             cause,
 334             obligations: vec![],
 335             depth,
 336         }
 337     }
 338
 339     fn fold<T:TypeFoldable<'tcx>>(&mut self, value: &T) -> T {
 340         let value = self.selcx.infcx().resolve_type_vars_if_possible(value);
 341
 342         if !value.has_projections() {
 343             value.clone()
 344         } else {
 345             value.fold_with(self)
 346         }
 347     }
 348 }
 349
 350 impl<'a, 'b, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for AssociatedTypeNormalizer<'a, 'b, 'gcx, 'tcx> {
 351     fn tcx<'c>(&'c self) -> TyCtxt<'c, 'gcx, 'tcx> {
 352         self.selcx.tcx()
 353     }
 354
 355     fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
 356         // We don't want to normalize associated types that occur inside of region
 357         // binders, because they may contain bound regions, and we can't cope with that.
 358         //
 359         // Example:
 360         //
 361         //     for<'a> fn(<T as Foo<&'a>>::A)
 362         //
 363         // Instead of normalizing `<T as Foo<&'a>>::A` here, we'll
 364         // normalize it when we instantiate those bound regions (which
 365         // should occur eventually).
 366
 367         let ty = ty.super_fold_with(self);
 368         match ty.sty {
 369             ty::TyAnon(def_id, substs) if !substs.has_escaping_regions() => { // (*)
 370                 // Only normalize `impl Trait` after type-checking, usually in codegen.
 371                 match self.param_env.reveal {
 372                     Reveal::UserFacing => ty,
 373
 374                     Reveal::All => {
 375                         let recursion_limit = *self.tcx().sess.recursion_limit.get();
 376                         if self.depth >= recursion_limit {
 377                             let obligation = Obligation::with_depth(
 378                                 self.cause.clone(),
 379                                 recursion_limit,
 380                                 self.param_env,
 381                                 ty,
 382                             );
 383                             self.selcx.infcx().report_overflow_error(&obligation, true);
 384                         }
 385
 386                         let generic_ty = self.tcx().type_of(def_id);
 387                         let concrete_ty = generic_ty.subst(self.tcx(), substs);
 388                         self.depth += 1;
 389                         let folded_ty = self.fold_ty(concrete_ty);
 390                         self.depth -= 1;
 391                         folded_ty
 392                     }
 393                 }
 394             }
 395
 396             ty::TyProjection(ref data) if !data.has_escaping_regions() => { // (*)
 397
 398                 // (*) This is kind of hacky -- we need to be able to
 399                 // handle normalization within binders because
 400                 // otherwise we wind up a need to normalize when doing
 401                 // trait matching (since you can have a trait
 402                 // obligation like `for<'a> T::B : Fn(&'a int)`), but
 403                 // we can't normalize with bound regions in scope. So
 404                 // far now we just ignore binders but only normalize
 405                 // if all bound regions are gone (and then we still
 406                 // have to renormalize whenever we instantiate a
 407                 // binder). It would be better to normalize in a
 408                 // binding-aware fashion.
 409
 410                 let normalized_ty = normalize_projection_type(self.selcx,
 411                                                               self.param_env,
 412                                                               data.clone(),
 413                                                               self.cause.clone(),
 414                                                               self.depth,
 415                                                               &mut self.obligations);
 416                 debug!("AssociatedTypeNormalizer: depth={} normalized {:?} to {:?}, \
 417                         now with {} obligations",
 418                        self.depth, ty, normalized_ty, self.obligations.len());
 419                 normalized_ty
 420             }
 421
 422             _ => {
 423                 ty
 424             }
 425         }
 426     }
 427
 428     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
 429         if let ConstValue::Unevaluated(def_id, substs) = constant.val {
 430             let tcx = self.selcx.tcx().global_tcx();
 431             if let Some(param_env) = self.tcx().lift_to_global(&self.param_env) {
 432                 if substs.needs_infer() || substs.has_skol() {
 433                     let identity_substs = Substs::identity_for_item(tcx, def_id);
 434                     let instance = ty::Instance::resolve(tcx, param_env, def_id, identity_substs);
 435                     if let Some(instance) = instance {
 436                         let cid = GlobalId {
 437                             instance,
 438                             promoted: None
 439                         };
 440                         match tcx.const_eval(param_env.and(cid)) {
 441                             Ok(evaluated) => {
 442                                 let evaluated = evaluated.subst(self.tcx(), substs);
 443                                 return self.fold_const(evaluated);
 444                             }
 445                             Err(_) => {}
 446                         }
 447                     }
 448                 } else {
 449                     if let Some(substs) = self.tcx().lift_to_global(&substs) {
 450                         let instance = ty::Instance::resolve(tcx, param_env, def_id, substs);
 451                         if let Some(instance) = instance {
 452                             let cid = GlobalId {
 453                                 instance,
 454                                 promoted: None
 455                             };
 456                             match tcx.const_eval(param_env.and(cid)) {
 457                                 Ok(evaluated) => return self.fold_const(evaluated),
 458                                 Err(_) => {}
 459                             }
 460                         }
 461                     }
 462                 }
 463             }
 464         }
 465         constant
 466     }
 467 }
 468
 469 #[derive(Clone)]
 470 pub struct Normalized<'tcx,T> {
 471     pub value: T,
 472     pub obligations: Vec<PredicateObligation<'tcx>>,
 473 }
 474
 475 pub type NormalizedTy<'tcx> = Normalized<'tcx, Ty<'tcx>>;
 476
 477 impl<'tcx,T> Normalized<'tcx,T> {
 478     pub fn with<U>(self, value: U) -> Normalized<'tcx,U> {
 479         Normalized { value: value, obligations: self.obligations }
 480     }
 481 }
 482
 483 /// The guts of `normalize`: normalize a specific projection like `<T
 484 /// as Trait>::Item`. The result is always a type (and possibly
 485 /// additional obligations). If ambiguity arises, which implies that
 486 /// there are unresolved type variables in the projection, we will
 487 /// substitute a fresh type variable `$X` and generate a new
 488 /// obligation `<T as Trait>::Item == $X` for later.
 489 pub fn normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
 490     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 491     param_env: ty::ParamEnv<'tcx>,
 492     projection_ty: ty::ProjectionTy<'tcx>,
 493     cause: ObligationCause<'tcx>,
 494     depth: usize,
 495     obligations: &mut Vec<PredicateObligation<'tcx>>)
 496     -> Ty<'tcx>
 497 {
 498     opt_normalize_projection_type(selcx, param_env, projection_ty.clone(), cause.clone(), depth,
 499                                   obligations)
 500         .unwrap_or_else(move || {
 501             // if we bottom out in ambiguity, create a type variable
 502             // and a deferred predicate to resolve this when more type
 503             // information is available.
 504
 505             let tcx = selcx.infcx().tcx;
 506             let def_id = projection_ty.item_def_id;
 507             let ty_var = selcx.infcx().next_ty_var(
 508                 TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
 509             let projection = ty::Binder::dummy(ty::ProjectionPredicate {
 510                 projection_ty,
 511                 ty: ty_var
 512             });
 513             let obligation = Obligation::with_depth(
 514                 cause, depth + 1, param_env, projection.to_predicate());
 515             obligations.push(obligation);
 516             ty_var
 517         })
 518 }
 519
 520 /// The guts of `normalize`: normalize a specific projection like `<T
 521 /// as Trait>::Item`. The result is always a type (and possibly
 522 /// additional obligations). Returns `None` in the case of ambiguity,
 523 /// which indicates that there are unbound type variables.
 524 ///
 525 /// This function used to return `Option<NormalizedTy<'tcx>>`, which contains a
 526 /// `Ty<'tcx>` and an obligations vector. But that obligation vector was very
 527 /// often immediately appended to another obligations vector. So now this
 528 /// function takes an obligations vector and appends to it directly, which is
 529 /// slightly uglier but avoids the need for an extra short-lived allocation.
 530 fn opt_normalize_projection_type<'a, 'b, 'gcx, 'tcx>(
 531     selcx: &'a mut SelectionContext<'b, 'gcx, 'tcx>,
 532     param_env: ty::ParamEnv<'tcx>,
 533     projection_ty: ty::ProjectionTy<'tcx>,
 534     cause: ObligationCause<'tcx>,
 535     depth: usize,
 536     obligations: &mut Vec<PredicateObligation<'tcx>>)
 537     -> Option<Ty<'tcx>>
 538 {
 539     let infcx = selcx.infcx();
 540
 541     let projection_ty = infcx.resolve_type_vars_if_possible(&projection_ty);
 542     let cache_key = ProjectionCacheKey { ty: projection_ty };
 543
 544     debug!("opt_normalize_projection_type(\
 545            projection_ty={:?}, \
 546            depth={})",
 547            projection_ty,
 548            depth);
 549
 550     // FIXME(#20304) For now, I am caching here, which is good, but it
 551     // means we don't capture the type variables that are created in
 552     // the case of ambiguity. Which means we may create a large stream
 553     // of such variables. OTOH, if we move the caching up a level, we
 554     // would not benefit from caching when proving `T: Trait<U=Foo>`
 555     // bounds. It might be the case that we want two distinct caches,
 556     // or else another kind of cache entry.
 557
 558     let cache_result = infcx.projection_cache.borrow_mut().try_start(cache_key);
 559     match cache_result {
 560         Ok(()) => { }
 561         Err(ProjectionCacheEntry::Ambiguous) => {
 562             // If we found ambiguity the last time, that generally
 563             // means we will continue to do so until some type in the
 564             // key changes (and we know it hasn't, because we just
 565             // fully resolved it). One exception though is closure
 566             // types, which can transition from having a fixed kind to
 567             // no kind with no visible change in the key.
 568             //
 569             // FIXME(#32286) refactor this so that closure type
 570             // changes
 571             debug!("opt_normalize_projection_type: \
 572                     found cache entry: ambiguous");
 573             if !projection_ty.has_closure_types() {
 574                 return None;
 575             }
 576         }
 577         Err(ProjectionCacheEntry::InProgress) => {
 578             // If while normalized A::B, we are asked to normalize
 579             // A::B, just return A::B itself. This is a conservative
 580             // answer, in the sense that A::B *is* clearly equivalent
 581             // to A::B, though there may be a better value we can
 582             // find.
 583
 584             // Under lazy normalization, this can arise when
 585             // bootstrapping.  That is, imagine an environment with a
 586             // where-clause like `A::B == u32`. Now, if we are asked
 587             // to normalize `A::B`, we will want to check the
 588             // where-clauses in scope. So we will try to unify `A::B`
 589             // with `A::B`, which can trigger a recursive
 590             // normalization. In that case, I think we will want this code:
 591             //
 592             // ```
 593             // let ty = selcx.tcx().mk_projection(projection_ty.item_def_id,
 594             //                                    projection_ty.substs;
 595             // return Some(NormalizedTy { value: v, obligations: vec![] });
 596             // ```
 597
 598             debug!("opt_normalize_projection_type: \
 599                     found cache entry: in-progress");
 600
 601             // But for now, let's classify this as an overflow:
 602             let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
 603             let obligation = Obligation::with_depth(cause.clone(),
 604                                                     recursion_limit,
 605                                                     param_env,
 606                                                     projection_ty);
 607             selcx.infcx().report_overflow_error(&obligation, false);
 608         }
 609         Err(ProjectionCacheEntry::NormalizedTy(ty)) => {
 610             // This is the hottest path in this function.
 611             //
 612             // If we find the value in the cache, then return it along
 613             // with the obligations that went along with it. Note
 614             // that, when using a fulfillment context, these
 615             // obligations could in principle be ignored: they have
 616             // already been registered when the cache entry was
 617             // created (and hence the new ones will quickly be
 618             // discarded as duplicated). But when doing trait
 619             // evaluation this is not the case, and dropping the trait
 620             // evaluations can causes ICEs (e.g. #43132).
 621             debug!("opt_normalize_projection_type: \
 622                     found normalized ty `{:?}`",
 623                    ty);
 624
 625             // Once we have inferred everything we need to know, we
 626             // can ignore the `obligations` from that point on.
 627             if !infcx.any_unresolved_type_vars(&ty.value) {
 628                 infcx.projection_cache.borrow_mut().complete_normalized(cache_key, &ty);
 629                 // No need to extend `obligations`.
 630             } else {
 631                 obligations.extend(ty.obligations);
 632             }
 633
 634             obligations.push(get_paranoid_cache_value_obligation(infcx,
 635                                                                  param_env,
 636                                                                  projection_ty,
 637                                                                  cause,
 638                                                                  depth));
 639             return Some(ty.value);
 640         }
 641         Err(ProjectionCacheEntry::Error) => {
 642             debug!("opt_normalize_projection_type: \
 643                     found error");
 644             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 645             obligations.extend(result.obligations);
 646             return Some(result.value)
 647         }
 648     }
 649
 650     let obligation = Obligation::with_depth(cause.clone(), depth, param_env, projection_ty);
 651     match project_type(selcx, &obligation) {
 652         Ok(ProjectedTy::Progress(Progress { ty: projected_ty,
 653                                             obligations: mut projected_obligations })) => {
 654             // if projection succeeded, then what we get out of this
 655             // is also non-normalized (consider: it was derived from
 656             // an impl, where-clause etc) and hence we must
 657             // re-normalize it
 658
 659             debug!("opt_normalize_projection_type: \
 660                     projected_ty={:?} \
 661                     depth={} \
 662                     projected_obligations={:?}",
 663                    projected_ty,
 664                    depth,
 665                    projected_obligations);
 666
 667             let result = if projected_ty.has_projections() {
 668                 let mut normalizer = AssociatedTypeNormalizer::new(selcx,
 669                                                                    param_env,
 670                                                                    cause,
 671                                                                    depth+1);
 672                 let normalized_ty = normalizer.fold(&projected_ty);
 673
 674                 debug!("opt_normalize_projection_type: \
 675                         normalized_ty={:?} depth={}",
 676                        normalized_ty,
 677                        depth);
 678
 679                 projected_obligations.extend(normalizer.obligations);
 680                 Normalized {
 681                     value: normalized_ty,
 682                     obligations: projected_obligations,
 683                 }
 684             } else {
 685                 Normalized {
 686                     value: projected_ty,
 687                     obligations: projected_obligations,
 688                 }
 689             };
 690
 691             let cache_value = prune_cache_value_obligations(infcx, &result);
 692             infcx.projection_cache.borrow_mut().insert_ty(cache_key, cache_value);
 693             obligations.extend(result.obligations);
 694             Some(result.value)
 695         }
 696         Ok(ProjectedTy::NoProgress(projected_ty)) => {
 697             debug!("opt_normalize_projection_type: \
 698                     projected_ty={:?} no progress",
 699                    projected_ty);
 700             let result = Normalized {
 701                 value: projected_ty,
 702                 obligations: vec![]
 703             };
 704             infcx.projection_cache.borrow_mut().insert_ty(cache_key, result.clone());
 705             // No need to extend `obligations`.
 706             Some(result.value)
 707         }
 708         Err(ProjectionTyError::TooManyCandidates) => {
 709             debug!("opt_normalize_projection_type: \
 710                     too many candidates");
 711             infcx.projection_cache.borrow_mut()
 712                                   .ambiguous(cache_key);
 713             None
 714         }
 715         Err(ProjectionTyError::TraitSelectionError(_)) => {
 716             debug!("opt_normalize_projection_type: ERROR");
 717             // if we got an error processing the `T as Trait` part,
 718             // just return `ty::err` but add the obligation `T :
 719             // Trait`, which when processed will cause the error to be
 720             // reported later
 721
 722             infcx.projection_cache.borrow_mut()
 723                                   .error(cache_key);
 724             let result = normalize_to_error(selcx, param_env, projection_ty, cause, depth);
 725             obligations.extend(result.obligations);
 726             Some(result.value)
 727         }
 728     }
 729 }
 730
 731 /// If there are unresolved type variables, then we need to include
 732 /// any subobligations that bind them, at least until those type
 733 /// variables are fully resolved.
 734 fn prune_cache_value_obligations<'a, 'gcx, 'tcx>(infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 735                                                  result: &NormalizedTy<'tcx>)
 736                                                  -> NormalizedTy<'tcx> {
 737     if !infcx.any_unresolved_type_vars(&result.value) {
 738         return NormalizedTy { value: result.value, obligations: vec![] };
 739     }
 740
 741     let mut obligations: Vec<_> =
 742         result.obligations
 743               .iter()
 744               .filter(|obligation| match obligation.predicate {
 745                   // We found a `T: Foo<X = U>` predicate, let's check
 746                   // if `U` references any unresolved type
 747                   // variables. In principle, we only care if this
 748                   // projection can help resolve any of the type
 749                   // variables found in `result.value` -- but we just
 750                   // check for any type variables here, for fear of
 751                   // indirect obligations (e.g., we project to `?0`,
 752                   // but we have `T: Foo<X = ?1>` and `?1: Bar<X =
 753                   // ?0>`).
 754                   ty::Predicate::Projection(ref data) =>
 755                       infcx.any_unresolved_type_vars(&data.ty()),
 756
 757                   // We are only interested in `T: Foo<X = U>` predicates, whre
 758                   // `U` references one of `unresolved_type_vars`. =)
 759                   _ => false,
 760               })
 761               .cloned()
 762               .collect();
 763
 764     obligations.shrink_to_fit();
 765
 766     NormalizedTy { value: result.value, obligations }
 767 }
 768
 769 /// Whenever we give back a cache result for a projection like `<T as
 770 /// Trait>::Item ==> X`, we *always* include the obligation to prove
 771 /// that `T: Trait` (we may also include some other obligations). This
 772 /// may or may not be necessary -- in principle, all the obligations
 773 /// that must be proven to show that `T: Trait` were also returned
 774 /// when the cache was first populated. But there are some vague concerns,
 775 /// and so we take the precautionary measure of including `T: Trait` in
 776 /// the result:
 777 ///
 778 /// Concern #1. The current setup is fragile. Perhaps someone could
 779 /// have failed to prove the concerns from when the cache was
 780 /// populated, but also not have used a snapshot, in which case the
 781 /// cache could remain populated even though `T: Trait` has not been
 782 /// shown. In this case, the "other code" is at fault -- when you
 783 /// project something, you are supposed to either have a snapshot or
 784 /// else prove all the resulting obligations -- but it's still easy to
 785 /// get wrong.
 786 ///
 787 /// Concern #2. Even within the snapshot, if those original
 788 /// obligations are not yet proven, then we are able to do projections
 789 /// that may yet turn out to be wrong.  This *may* lead to some sort
 790 /// of trouble, though we don't have a concrete example of how that
 791 /// can occur yet.  But it seems risky at best.
 792 fn get_paranoid_cache_value_obligation<'a, 'gcx, 'tcx>(
 793     infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
 794     param_env: ty::ParamEnv<'tcx>,
 795     projection_ty: ty::ProjectionTy<'tcx>,
 796     cause: ObligationCause<'tcx>,
 797     depth: usize)
 798     -> PredicateObligation<'tcx>
 799 {
 800     let trait_ref = projection_ty.trait_ref(infcx.tcx).to_poly_trait_ref();
 801     Obligation {
 802         cause,
 803         recursion_depth: depth,
 804         param_env,
 805         predicate: trait_ref.to_predicate(),
 806     }
 807 }
 808
 809 /// If we are projecting `<T as Trait>::Item`, but `T: Trait` does not
 810 /// hold. In various error cases, we cannot generate a valid
 811 /// normalized projection. Therefore, we create an inference variable
 812 /// return an associated obligation that, when fulfilled, will lead to
 813 /// an error.
 814 ///
 815 /// Note that we used to return `TyError` here, but that was quite
 816 /// dubious -- the premise was that an error would *eventually* be
 817 /// reported, when the obligation was processed. But in general once
 818 /// you see a `TyError` you are supposed to be able to assume that an
 819 /// error *has been* reported, so that you can take whatever heuristic
 820 /// paths you want to take. To make things worse, it was possible for
 821 /// cycles to arise, where you basically had a setup like `<MyType<$0>
 822 /// as Trait>::Foo == $0`. Here, normalizing `<MyType<$0> as
 823 /// Trait>::Foo> to `[type error]` would lead to an obligation of
 824 /// `<MyType<[type error]> as Trait>::Foo`.  We are supposed to report
 825 /// an error for this obligation, but we legitimately should not,
 826 /// because it contains `[type error]`. Yuck! (See issue #29857 for
 827 /// one case where this arose.)
 828 fn normalize_to_error<'a, 'gcx, 'tcx>(selcx: &mut SelectionContext<'a, 'gcx, 'tcx>,
 829                                       param_env: ty::ParamEnv<'tcx>,
 830                                       projection_ty: ty::ProjectionTy<'tcx>,
 831                                       cause: ObligationCause<'tcx>,
 832                                       depth: usize)
 833                                       -> NormalizedTy<'tcx>
 834 {
 835     let trait_ref = projection_ty.trait_ref(selcx.tcx()).to_poly_trait_ref();
 836     let trait_obligation = Obligation { cause,
 837                                         recursion_depth: depth,
 838                                         param_env,
 839                                         predicate: trait_ref.to_predicate() };
 840     let tcx = selcx.infcx().tcx;
 841     let def_id = projection_ty.item_def_id;
 842     let new_value = selcx.infcx().next_ty_var(
 843         TypeVariableOrigin::NormalizeProjectionType(tcx.def_span(def_id)));
 844     Normalized {
 845         value: new_value,
 846         obligations: vec![trait_obligation]
 847     }
 848 }
 849
 850 enum ProjectedTy<'tcx> {
 851     Progress(Progress<'tcx>),
 852     NoProgress(Ty<'tcx>),
 853 }
 854
 855 struct Progress<'tcx> {
 856     ty: Ty<'tcx>,
 857     obligations: Vec<PredicateObligation<'tcx>>,
 858 }
 859
 860 impl<'tcx> Progress<'tcx> {
 861     fn error<'a,'gcx>(tcx: TyCtxt<'a,'gcx,'tcx>) -> Self {
 862         Progress {
 863             ty: tcx.types.err,
 864             obligations: vec![],
 865         }
 866     }
 867
 868     fn with_addl_obligations(mut self,
 869                              mut obligations: Vec<PredicateObligation<'tcx>>)
 870                              -> Self {
 871         debug!("with_addl_obligations: self.obligations.len={} obligations.len={}",
 872                self.obligations.len(), obligations.len());
 873
 874         debug!("with_addl_obligations: self.obligations={:?} obligations={:?}",
 875                self.obligations, obligations);
 876
 877         self.obligations.append(&mut obligations);
 878         self
 879     }
 880 }
 881
 882 /// Compute the result of a projection type (if we can).
 883 ///
 884 /// IMPORTANT:
 885 /// - `obligation` must be fully normalized
 886 fn project_type<'cx, 'gcx, 'tcx>(
 887     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 888     obligation: &ProjectionTyObligation<'tcx>)
 889     -> Result<ProjectedTy<'tcx>, ProjectionTyError<'tcx>>
 890 {
 891     debug!("project(obligation={:?})",
 892            obligation);
 893
 894     let recursion_limit = *selcx.tcx().sess.recursion_limit.get();
 895     if obligation.recursion_depth >= recursion_limit {
 896         debug!("project: overflow!");
 897         selcx.infcx().report_overflow_error(&obligation, true);
 898     }
 899
 900     let obligation_trait_ref = &obligation.predicate.trait_ref(selcx.tcx());
 901
 902     debug!("project: obligation_trait_ref={:?}", obligation_trait_ref);
 903
 904     if obligation_trait_ref.references_error() {
 905         return Ok(ProjectedTy::Progress(Progress::error(selcx.tcx())));
 906     }
 907
 908     let mut candidates = ProjectionTyCandidateSet::None;
 909
 910     // Make sure that the following procedures are kept in order. ParamEnv
 911     // needs to be first because it has highest priority, and Select checks
 912     // the return value of push_candidate which assumes it's ran at last.
 913     assemble_candidates_from_param_env(selcx,
 914                                        obligation,
 915                                        &obligation_trait_ref,
 916                                        &mut candidates);
 917
 918     assemble_candidates_from_trait_def(selcx,
 919                                        obligation,
 920                                        &obligation_trait_ref,
 921                                        &mut candidates);
 922
 923     assemble_candidates_from_impls(selcx,
 924                                    obligation,
 925                                    &obligation_trait_ref,
 926                                    &mut candidates);
 927
 928     match candidates {
 929         ProjectionTyCandidateSet::Single(candidate) => Ok(ProjectedTy::Progress(
 930             confirm_candidate(selcx,
 931                               obligation,
 932                               &obligation_trait_ref,
 933                               candidate))),
 934         ProjectionTyCandidateSet::None => Ok(ProjectedTy::NoProgress(
 935             selcx.tcx().mk_projection(
 936                 obligation.predicate.item_def_id,
 937                 obligation.predicate.substs))),
 938         // Error occurred while trying to processing impls.
 939         ProjectionTyCandidateSet::Error(e) => Err(ProjectionTyError::TraitSelectionError(e)),
 940         // Inherent ambiguity that prevents us from even enumerating the
 941         // candidates.
 942         ProjectionTyCandidateSet::Ambiguous => Err(ProjectionTyError::TooManyCandidates),
 943
 944     }
 945 }
 946
 947 /// The first thing we have to do is scan through the parameter
 948 /// environment to see whether there are any projection predicates
 949 /// there that can answer this question.
 950 fn assemble_candidates_from_param_env<'cx, 'gcx, 'tcx>(
 951     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 952     obligation: &ProjectionTyObligation<'tcx>,
 953     obligation_trait_ref: &ty::TraitRef<'tcx>,
 954     candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
 955 {
 956     debug!("assemble_candidates_from_param_env(..)");
 957     assemble_candidates_from_predicates(selcx,
 958                                         obligation,
 959                                         obligation_trait_ref,
 960                                         candidate_set,
 961                                         ProjectionTyCandidate::ParamEnv,
 962                                         obligation.param_env.caller_bounds.iter().cloned());
 963 }
 964
 965 /// In the case of a nested projection like <<A as Foo>::FooT as Bar>::BarT, we may find
 966 /// that the definition of `Foo` has some clues:
 967 ///
 968 /// ```
 969 /// trait Foo {
 970 ///     type FooT : Bar<BarT=i32>
 971 /// }
 972 /// ```
 973 ///
 974 /// Here, for example, we could conclude that the result is `i32`.
 975 fn assemble_candidates_from_trait_def<'cx, 'gcx, 'tcx>(
 976     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
 977     obligation: &ProjectionTyObligation<'tcx>,
 978     obligation_trait_ref: &ty::TraitRef<'tcx>,
 979     candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
 980 {
 981     debug!("assemble_candidates_from_trait_def(..)");
 982
 983     let tcx = selcx.tcx();
 984     // Check whether the self-type is itself a projection.
 985     let (def_id, substs) = match obligation_trait_ref.self_ty().sty {
 986         ty::TyProjection(ref data) => {
 987             (data.trait_ref(tcx).def_id, data.substs)
 988         }
 989         ty::TyAnon(def_id, substs) => (def_id, substs),
 990         ty::TyInfer(ty::TyVar(_)) => {
 991             // If the self-type is an inference variable, then it MAY wind up
 992             // being a projected type, so induce an ambiguity.
 993             candidate_set.mark_ambiguous();
 994             return;
 995         }
 996         _ => { return; }
 997     };
 998
 999     // If so, extract what we know from the trait and try to come up with a good answer.
1000     let trait_predicates = tcx.predicates_of(def_id);
1001     let bounds = trait_predicates.instantiate(tcx, substs);
1002     let bounds = elaborate_predicates(tcx, bounds.predicates);
1003     assemble_candidates_from_predicates(selcx,
1004                                         obligation,
1005                                         obligation_trait_ref,
1006                                         candidate_set,
1007                                         ProjectionTyCandidate::TraitDef,
1008                                         bounds)
1009 }
1010
1011 fn assemble_candidates_from_predicates<'cx, 'gcx, 'tcx, I>(
1012     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1013     obligation: &ProjectionTyObligation<'tcx>,
1014     obligation_trait_ref: &ty::TraitRef<'tcx>,
1015     candidate_set: &mut ProjectionTyCandidateSet<'tcx>,
1016     ctor: fn(ty::PolyProjectionPredicate<'tcx>) -> ProjectionTyCandidate<'tcx>,
1017     env_predicates: I)
1018     where I: IntoIterator<Item=ty::Predicate<'tcx>>
1019 {
1020     debug!("assemble_candidates_from_predicates(obligation={:?})",
1021            obligation);
1022     let infcx = selcx.infcx();
1023     for predicate in env_predicates {
1024         debug!("assemble_candidates_from_predicates: predicate={:?}",
1025                predicate);
1026         match predicate {
1027             ty::Predicate::Projection(data) => {
1028                 let same_def_id = data.projection_def_id() == obligation.predicate.item_def_id;
1029
1030                 let is_match = same_def_id && infcx.probe(|_| {
1031                     let data_poly_trait_ref =
1032                         data.to_poly_trait_ref(infcx.tcx);
1033                     let obligation_poly_trait_ref =
1034                         obligation_trait_ref.to_poly_trait_ref();
1035                     infcx.at(&obligation.cause, obligation.param_env)
1036                          .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1037                          .map(|InferOk { obligations: _, value: () }| {
1038                              // FIXME(#32730) -- do we need to take obligations
1039                              // into account in any way? At the moment, no.
1040                          })
1041                          .is_ok()
1042                 });
1043
1044                 debug!("assemble_candidates_from_predicates: candidate={:?} \
1045                                                              is_match={} same_def_id={}",
1046                        data, is_match, same_def_id);
1047
1048                 if is_match {
1049                     candidate_set.push_candidate(ctor(data));
1050                 }
1051             }
1052             _ => {}
1053         }
1054     }
1055 }
1056
1057 fn assemble_candidates_from_impls<'cx, 'gcx, 'tcx>(
1058     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1059     obligation: &ProjectionTyObligation<'tcx>,
1060     obligation_trait_ref: &ty::TraitRef<'tcx>,
1061     candidate_set: &mut ProjectionTyCandidateSet<'tcx>)
1062 {
1063     // If we are resolving `<T as TraitRef<...>>::Item == Type`,
1064     // start out by selecting the predicate `T as TraitRef<...>`:
1065     let poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1066     let trait_obligation = obligation.with(poly_trait_ref.to_poly_trait_predicate());
1067     let _ = selcx.infcx().commit_if_ok(|_| {
1068         let vtable = match selcx.select(&trait_obligation) {
1069             Ok(Some(vtable)) => vtable,
1070             Ok(None) => {
1071                 candidate_set.mark_ambiguous();
1072                 return Err(());
1073             }
1074             Err(e) => {
1075                 debug!("assemble_candidates_from_impls: selection error {:?}",
1076                        e);
1077                 candidate_set.mark_error(e);
1078                 return Err(());
1079             }
1080         };
1081
1082         let eligible = match &vtable {
1083             super::VtableClosure(_) |
1084             super::VtableGenerator(_) |
1085             super::VtableFnPointer(_) |
1086             super::VtableObject(_) => {
1087                 debug!("assemble_candidates_from_impls: vtable={:?}",
1088                        vtable);
1089                 true
1090             }
1091             super::VtableImpl(impl_data) => {
1092                 // We have to be careful when projecting out of an
1093                 // impl because of specialization. If we are not in
1094                 // codegen (i.e., projection mode is not "any"), and the
1095                 // impl's type is declared as default, then we disable
1096                 // projection (even if the trait ref is fully
1097                 // monomorphic). In the case where trait ref is not
1098                 // fully monomorphic (i.e., includes type parameters),
1099                 // this is because those type parameters may
1100                 // ultimately be bound to types from other crates that
1101                 // may have specialized impls we can't see. In the
1102                 // case where the trait ref IS fully monomorphic, this
1103                 // is a policy decision that we made in the RFC in
1104                 // order to preserve flexibility for the crate that
1105                 // defined the specializable impl to specialize later
1106                 // for existing types.
1107                 //
1108                 // In either case, we handle this by not adding a
1109                 // candidate for an impl if it contains a `default`
1110                 // type.
1111                 let node_item = assoc_ty_def(selcx,
1112                                              impl_data.impl_def_id,
1113                                              obligation.predicate.item_def_id);
1114
1115                 let is_default = if node_item.node.is_from_trait() {
1116                     // If true, the impl inherited a `type Foo = Bar`
1117                     // given in the trait, which is implicitly default.
1118                     // Otherwise, the impl did not specify `type` and
1119                     // neither did the trait:
1120                     //
1121                     // ```rust
1122                     // trait Foo { type T; }
1123                     // impl Foo for Bar { }
1124                     // ```
1125                     //
1126                     // This is an error, but it will be
1127                     // reported in `check_impl_items_against_trait`.
1128                     // We accept it here but will flag it as
1129                     // an error when we confirm the candidate
1130                     // (which will ultimately lead to `normalize_to_error`
1131                     // being invoked).
1132                     node_item.item.defaultness.has_value()
1133                 } else {
1134                     node_item.item.defaultness.is_default() ||
1135                         selcx.tcx().impl_is_default(node_item.node.def_id())
1136                 };
1137
1138                 // Only reveal a specializable default if we're past type-checking
1139                 // and the obligations is monomorphic, otherwise passes such as
1140                 // transmute checking and polymorphic MIR optimizations could
1141                 // get a result which isn't correct for all monomorphizations.
1142                 if !is_default {
1143                     true
1144                 } else if obligation.param_env.reveal == Reveal::All {
1145                     debug_assert!(!poly_trait_ref.needs_infer());
1146                     if !poly_trait_ref.needs_subst() {
1147                         true
1148                     } else {
1149                         false
1150                     }
1151                 } else {
1152                     false
1153                 }
1154             }
1155             super::VtableParam(..) => {
1156                 // This case tell us nothing about the value of an
1157                 // associated type. Consider:
1158                 //
1159                 // ```
1160                 // trait SomeTrait { type Foo; }
1161                 // fn foo<T:SomeTrait>(...) { }
1162                 // ```
1163                 //
1164                 // If the user writes `<T as SomeTrait>::Foo`, then the `T
1165                 // : SomeTrait` binding does not help us decide what the
1166                 // type `Foo` is (at least, not more specifically than
1167                 // what we already knew).
1168                 //
1169                 // But wait, you say! What about an example like this:
1170                 //
1171                 // ```
1172                 // fn bar<T:SomeTrait<Foo=usize>>(...) { ... }
1173                 // ```
1174                 //
1175                 // Doesn't the `T : Sometrait<Foo=usize>` predicate help
1176                 // resolve `T::Foo`? And of course it does, but in fact
1177                 // that single predicate is desugared into two predicates
1178                 // in the compiler: a trait predicate (`T : SomeTrait`) and a
1179                 // projection. And the projection where clause is handled
1180                 // in `assemble_candidates_from_param_env`.
1181                 false
1182             }
1183             super::VtableAutoImpl(..) |
1184             super::VtableBuiltin(..) => {
1185                 // These traits have no associated types.
1186                 span_bug!(
1187                     obligation.cause.span,
1188                     "Cannot project an associated type from `{:?}`",
1189                     vtable);
1190             }
1191         };
1192
1193         if eligible {
1194             if candidate_set.push_candidate(ProjectionTyCandidate::Select(vtable)) {
1195                 Ok(())
1196             } else {
1197                 Err(())
1198             }
1199         } else {
1200             Err(())
1201         }
1202     });
1203 }
1204
1205 fn confirm_candidate<'cx, 'gcx, 'tcx>(
1206     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1207     obligation: &ProjectionTyObligation<'tcx>,
1208     obligation_trait_ref: &ty::TraitRef<'tcx>,
1209     candidate: ProjectionTyCandidate<'tcx>)
1210     -> Progress<'tcx>
1211 {
1212     debug!("confirm_candidate(candidate={:?}, obligation={:?})",
1213            candidate,
1214            obligation);
1215
1216     match candidate {
1217         ProjectionTyCandidate::ParamEnv(poly_projection) |
1218         ProjectionTyCandidate::TraitDef(poly_projection) => {
1219             confirm_param_env_candidate(selcx, obligation, poly_projection)
1220         }
1221
1222         ProjectionTyCandidate::Select(vtable) => {
1223             confirm_select_candidate(selcx, obligation, obligation_trait_ref, vtable)
1224         }
1225     }
1226 }
1227
1228 fn confirm_select_candidate<'cx, 'gcx, 'tcx>(
1229     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1230     obligation: &ProjectionTyObligation<'tcx>,
1231     obligation_trait_ref: &ty::TraitRef<'tcx>,
1232     vtable: Selection<'tcx>)
1233     -> Progress<'tcx>
1234 {
1235     match vtable {
1236         super::VtableImpl(data) =>
1237             confirm_impl_candidate(selcx, obligation, data),
1238         super::VtableGenerator(data) =>
1239             confirm_generator_candidate(selcx, obligation, data),
1240         super::VtableClosure(data) =>
1241             confirm_closure_candidate(selcx, obligation, data),
1242         super::VtableFnPointer(data) =>
1243             confirm_fn_pointer_candidate(selcx, obligation, data),
1244         super::VtableObject(_) =>
1245             confirm_object_candidate(selcx, obligation, obligation_trait_ref),
1246         super::VtableAutoImpl(..) |
1247         super::VtableParam(..) |
1248         super::VtableBuiltin(..) =>
1249             // we don't create Select candidates with this kind of resolution
1250             span_bug!(
1251                 obligation.cause.span,
1252                 "Cannot project an associated type from `{:?}`",
1253                 vtable),
1254     }
1255 }
1256
1257 fn confirm_object_candidate<'cx, 'gcx, 'tcx>(
1258     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1259     obligation:  &ProjectionTyObligation<'tcx>,
1260     obligation_trait_ref: &ty::TraitRef<'tcx>)
1261     -> Progress<'tcx>
1262 {
1263     let self_ty = obligation_trait_ref.self_ty();
1264     let object_ty = selcx.infcx().shallow_resolve(self_ty);
1265     debug!("confirm_object_candidate(object_ty={:?})",
1266            object_ty);
1267     let data = match object_ty.sty {
1268         ty::TyDynamic(ref data, ..) => data,
1269         _ => {
1270             span_bug!(
1271                 obligation.cause.span,
1272                 "confirm_object_candidate called with non-object: {:?}",
1273                 object_ty)
1274         }
1275     };
1276     let env_predicates = data.projection_bounds().map(|p| {
1277         p.with_self_ty(selcx.tcx(), object_ty).to_predicate()
1278     }).collect();
1279     let env_predicate = {
1280         let env_predicates = elaborate_predicates(selcx.tcx(), env_predicates);
1281
1282         // select only those projections that are actually projecting an
1283         // item with the correct name
1284         let env_predicates = env_predicates.filter_map(|p| match p {
1285             ty::Predicate::Projection(data) =>
1286                 if data.projection_def_id() == obligation.predicate.item_def_id {
1287                     Some(data)
1288                 } else {
1289                     None
1290                 },
1291             _ => None
1292         });
1293
1294         // select those with a relevant trait-ref
1295         let mut env_predicates = env_predicates.filter(|data| {
1296             let data_poly_trait_ref = data.to_poly_trait_ref(selcx.tcx());
1297             let obligation_poly_trait_ref = obligation_trait_ref.to_poly_trait_ref();
1298             selcx.infcx().probe(|_| {
1299                 selcx.infcx().at(&obligation.cause, obligation.param_env)
1300                              .sup(obligation_poly_trait_ref, data_poly_trait_ref)
1301                              .is_ok()
1302             })
1303         });
1304
1305         // select the first matching one; there really ought to be one or
1306         // else the object type is not WF, since an object type should
1307         // include all of its projections explicitly
1308         match env_predicates.next() {
1309             Some(env_predicate) => env_predicate,
1310             None => {
1311                 debug!("confirm_object_candidate: no env-predicate \
1312                         found in object type `{:?}`; ill-formed",
1313                        object_ty);
1314                 return Progress::error(selcx.tcx());
1315             }
1316         }
1317     };
1318
1319     confirm_param_env_candidate(selcx, obligation, env_predicate)
1320 }
1321
1322 fn confirm_generator_candidate<'cx, 'gcx, 'tcx>(
1323     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1324     obligation: &ProjectionTyObligation<'tcx>,
1325     vtable: VtableGeneratorData<'tcx, PredicateObligation<'tcx>>)
1326     -> Progress<'tcx>
1327 {
1328     let gen_sig = vtable.substs.poly_sig(vtable.generator_def_id, selcx.tcx());
1329     let Normalized {
1330         value: gen_sig,
1331         obligations
1332     } = normalize_with_depth(selcx,
1333                              obligation.param_env,
1334                              obligation.cause.clone(),
1335                              obligation.recursion_depth+1,
1336                              &gen_sig);
1337
1338     debug!("confirm_generator_candidate: obligation={:?},gen_sig={:?},obligations={:?}",
1339            obligation,
1340            gen_sig,
1341            obligations);
1342
1343     let tcx = selcx.tcx();
1344
1345     let gen_def_id = tcx.lang_items().gen_trait().unwrap();
1346
1347     let predicate =
1348         tcx.generator_trait_ref_and_outputs(gen_def_id,
1349                                             obligation.predicate.self_ty(),
1350                                             gen_sig)
1351         .map_bound(|(trait_ref, yield_ty, return_ty)| {
1352             let name = tcx.associated_item(obligation.predicate.item_def_id).ident.name;
1353             let ty = if name == "Return" {
1354                 return_ty
1355             } else if name == "Yield" {
1356                 yield_ty
1357             } else {
1358                 bug!()
1359             };
1360
1361             ty::ProjectionPredicate {
1362                 projection_ty: ty::ProjectionTy {
1363                     substs: trait_ref.substs,
1364                     item_def_id: obligation.predicate.item_def_id,
1365                 },
1366                 ty: ty
1367             }
1368         });
1369
1370     confirm_param_env_candidate(selcx, obligation, predicate)
1371         .with_addl_obligations(vtable.nested)
1372         .with_addl_obligations(obligations)
1373 }
1374
1375 fn confirm_fn_pointer_candidate<'cx, 'gcx, 'tcx>(
1376     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1377     obligation: &ProjectionTyObligation<'tcx>,
1378     fn_pointer_vtable: VtableFnPointerData<'tcx, PredicateObligation<'tcx>>)
1379     -> Progress<'tcx>
1380 {
1381     let fn_type = selcx.infcx().shallow_resolve(fn_pointer_vtable.fn_ty);
1382     let sig = fn_type.fn_sig(selcx.tcx());
1383     let Normalized {
1384         value: sig,
1385         obligations
1386     } = normalize_with_depth(selcx,
1387                              obligation.param_env,
1388                              obligation.cause.clone(),
1389                              obligation.recursion_depth+1,
1390                              &sig);
1391
1392     confirm_callable_candidate(selcx, obligation, sig, util::TupleArgumentsFlag::Yes)
1393         .with_addl_obligations(fn_pointer_vtable.nested)
1394         .with_addl_obligations(obligations)
1395 }
1396
1397 fn confirm_closure_candidate<'cx, 'gcx, 'tcx>(
1398     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1399     obligation: &ProjectionTyObligation<'tcx>,
1400     vtable: VtableClosureData<'tcx, PredicateObligation<'tcx>>)
1401     -> Progress<'tcx>
1402 {
1403     let tcx = selcx.tcx();
1404     let infcx = selcx.infcx();
1405     let closure_sig_ty = vtable.substs.closure_sig_ty(vtable.closure_def_id, tcx);
1406     let closure_sig = infcx.shallow_resolve(&closure_sig_ty).fn_sig(tcx);
1407     let Normalized {
1408         value: closure_sig,
1409         obligations
1410     } = normalize_with_depth(selcx,
1411                              obligation.param_env,
1412                              obligation.cause.clone(),
1413                              obligation.recursion_depth+1,
1414                              &closure_sig);
1415
1416     debug!("confirm_closure_candidate: obligation={:?},closure_sig={:?},obligations={:?}",
1417            obligation,
1418            closure_sig,
1419            obligations);
1420
1421     confirm_callable_candidate(selcx,
1422                                obligation,
1423                                closure_sig,
1424                                util::TupleArgumentsFlag::No)
1425         .with_addl_obligations(vtable.nested)
1426         .with_addl_obligations(obligations)
1427 }
1428
1429 fn confirm_callable_candidate<'cx, 'gcx, 'tcx>(
1430     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1431     obligation: &ProjectionTyObligation<'tcx>,
1432     fn_sig: ty::PolyFnSig<'tcx>,
1433     flag: util::TupleArgumentsFlag)
1434     -> Progress<'tcx>
1435 {
1436     let tcx = selcx.tcx();
1437
1438     debug!("confirm_callable_candidate({:?},{:?})",
1439            obligation,
1440            fn_sig);
1441
1442     // the `Output` associated type is declared on `FnOnce`
1443     let fn_once_def_id = tcx.lang_items().fn_once_trait().unwrap();
1444
1445     let predicate =
1446         tcx.closure_trait_ref_and_return_type(fn_once_def_id,
1447                                               obligation.predicate.self_ty(),
1448                                               fn_sig,
1449                                               flag)
1450         .map_bound(|(trait_ref, ret_type)| {
1451             ty::ProjectionPredicate {
1452                 projection_ty: ty::ProjectionTy::from_ref_and_name(
1453                     tcx,
1454                     trait_ref,
1455                     Ident::from_str(FN_OUTPUT_NAME),
1456                 ),
1457                 ty: ret_type
1458             }
1459         });
1460
1461     confirm_param_env_candidate(selcx, obligation, predicate)
1462 }
1463
1464 fn confirm_param_env_candidate<'cx, 'gcx, 'tcx>(
1465     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1466     obligation: &ProjectionTyObligation<'tcx>,
1467     poly_projection: ty::PolyProjectionPredicate<'tcx>)
1468     -> Progress<'tcx>
1469 {
1470     let infcx = selcx.infcx();
1471     let cause = obligation.cause.clone();
1472     let param_env = obligation.param_env;
1473     let trait_ref = obligation.predicate.trait_ref(infcx.tcx);
1474     match infcx.match_poly_projection_predicate(cause, param_env, poly_projection, trait_ref) {
1475         Ok(InferOk { value: ty_match, obligations }) => {
1476             Progress {
1477                 ty: ty_match.value,
1478                 obligations,
1479             }
1480         }
1481         Err(e) => {
1482             span_bug!(
1483                 obligation.cause.span,
1484                 "Failed to unify obligation `{:?}` \
1485                  with poly_projection `{:?}`: {:?}",
1486                 obligation,
1487                 poly_projection,
1488                 e);
1489         }
1490     }
1491 }
1492
1493 fn confirm_impl_candidate<'cx, 'gcx, 'tcx>(
1494     selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1495     obligation: &ProjectionTyObligation<'tcx>,
1496     impl_vtable: VtableImplData<'tcx, PredicateObligation<'tcx>>)
1497     -> Progress<'tcx>
1498 {
1499     let VtableImplData { substs, nested, impl_def_id } = impl_vtable;
1500
1501     let tcx = selcx.tcx();
1502     let param_env = obligation.param_env;
1503     let assoc_ty = assoc_ty_def(selcx, impl_def_id, obligation.predicate.item_def_id);
1504
1505     if !assoc_ty.item.defaultness.has_value() {
1506         // This means that the impl is missing a definition for the
1507         // associated type. This error will be reported by the type
1508         // checker method `check_impl_items_against_trait`, so here we
1509         // just return TyError.
1510         debug!("confirm_impl_candidate: no associated type {:?} for {:?}",
1511                assoc_ty.item.ident,
1512                obligation.predicate);
1513         return Progress {
1514             ty: tcx.types.err,
1515             obligations: nested,
1516         };
1517     }
1518     let substs = translate_substs(selcx.infcx(), param_env, impl_def_id, substs, assoc_ty.node);
1519     let ty = if let ty::AssociatedKind::Existential = assoc_ty.item.kind {
1520         let item_substs = Substs::identity_for_item(tcx, assoc_ty.item.def_id);
1521         tcx.mk_anon(assoc_ty.item.def_id, item_substs)
1522     } else {
1523         tcx.type_of(assoc_ty.item.def_id)
1524     };
1525     Progress {
1526         ty: ty.subst(tcx, substs),
1527         obligations: nested,
1528     }
1529 }
1530
1531 /// Locate the definition of an associated type in the specialization hierarchy,
1532 /// starting from the given impl.
1533 ///
1534 /// Based on the "projection mode", this lookup may in fact only examine the
1535 /// topmost impl. See the comments for `Reveal` for more details.
1536 fn assoc_ty_def<'cx, 'gcx, 'tcx>(
1537     selcx: &SelectionContext<'cx, 'gcx, 'tcx>,
1538     impl_def_id: DefId,
1539     assoc_ty_def_id: DefId)
1540     -> specialization_graph::NodeItem<ty::AssociatedItem>
1541 {
1542     let tcx = selcx.tcx();
1543     let assoc_ty_name = tcx.associated_item(assoc_ty_def_id).ident;
1544     let trait_def_id = tcx.impl_trait_ref(impl_def_id).unwrap().def_id;
1545     let trait_def = tcx.trait_def(trait_def_id);
1546
1547     // This function may be called while we are still building the
1548     // specialization graph that is queried below (via TraidDef::ancestors()),
1549     // so, in order to avoid unnecessary infinite recursion, we manually look
1550     // for the associated item at the given impl.
1551     // If there is no such item in that impl, this function will fail with a
1552     // cycle error if the specialization graph is currently being built.
1553     let impl_node = specialization_graph::Node::Impl(impl_def_id);
1554     for item in impl_node.items(tcx) {
1555         if item.kind == ty::AssociatedKind::Type &&
1556                 tcx.hygienic_eq(item.ident, assoc_ty_name, trait_def_id) {
1557             return specialization_graph::NodeItem {
1558                 node: specialization_graph::Node::Impl(impl_def_id),
1559                 item,
1560             };
1561         }
1562     }
1563
1564     if let Some(assoc_item) = trait_def
1565         .ancestors(tcx, impl_def_id)
1566         .defs(tcx, assoc_ty_name, ty::AssociatedKind::Type, trait_def_id)
1567         .next() {
1568         assoc_item
1569     } else {
1570         // This is saying that neither the trait nor
1571         // the impl contain a definition for this
1572         // associated type.  Normally this situation
1573         // could only arise through a compiler bug --
1574         // if the user wrote a bad item name, it
1575         // should have failed in astconv.
1576         bug!("No associated type `{}` for {}",
1577              assoc_ty_name,
1578              tcx.item_path_str(impl_def_id))
1579     }
1580 }
1581
1582 // # Cache
1583
1584 /// The projection cache. Unlike the standard caches, this can
1585 /// include infcx-dependent type variables - therefore, we have to roll
1586 /// the cache back each time we roll a snapshot back, to avoid assumptions
1587 /// on yet-unresolved inference variables. Types with skolemized regions
1588 /// also have to be removed when the respective snapshot ends.
1589 ///
1590 /// Because of that, projection cache entries can be "stranded" and left
1591 /// inaccessible when type variables inside the key are resolved. We make no
1592 /// attempt to recover or remove "stranded" entries, but rather let them be
1593 /// (for the lifetime of the infcx).
1594 ///
1595 /// Entries in the projection cache might contain inference variables
1596 /// that will be resolved by obligations on the projection cache entry - e.g.
1597 /// when a type parameter in the associated type is constrained through
1598 /// an "RFC 447" projection on the impl.
1599 ///
1600 /// When working with a fulfillment context, the derived obligations of each
1601 /// projection cache entry will be registered on the fulfillcx, so any users
1602 /// that can wait for a fulfillcx fixed point need not care about this. However,
1603 /// users that don't wait for a fixed point (e.g. trait evaluation) have to
1604 /// resolve the obligations themselves to make sure the projected result is
1605 /// ok and avoid issues like #43132.
1606 ///
1607 /// If that is done, after evaluation the obligations, it is a good idea to
1608 /// call `ProjectionCache::complete` to make sure the obligations won't be
1609 /// re-evaluated and avoid an exponential worst-case.
1610 ///
1611 /// FIXME: we probably also want some sort of cross-infcx cache here to
1612 /// reduce the amount of duplication. Let's see what we get with the Chalk
1613 /// reforms.
1614 pub struct ProjectionCache<'tcx> {
1615     map: SnapshotMap<ProjectionCacheKey<'tcx>, ProjectionCacheEntry<'tcx>>,
1616 }
1617
1618 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
1619 pub struct ProjectionCacheKey<'tcx> {
1620     ty: ty::ProjectionTy<'tcx>
1621 }
1622
1623 impl<'cx, 'gcx, 'tcx> ProjectionCacheKey<'tcx> {
1624     pub fn from_poly_projection_predicate(selcx: &mut SelectionContext<'cx, 'gcx, 'tcx>,
1625                                           predicate: &ty::PolyProjectionPredicate<'tcx>)
1626                                           -> Option<Self>
1627     {
1628         let infcx = selcx.infcx();
1629         // We don't do cross-snapshot caching of obligations with escaping regions,
1630         // so there's no cache key to use
1631         predicate.no_late_bound_regions()
1632             .map(|predicate| ProjectionCacheKey {
1633                 // We don't attempt to match up with a specific type-variable state
1634                 // from a specific call to `opt_normalize_projection_type` - if
1635                 // there's no precise match, the original cache entry is "stranded"
1636                 // anyway.
1637                 ty: infcx.resolve_type_vars_if_possible(&predicate.projection_ty)
1638             })
1639     }
1640 }
1641
1642 #[derive(Clone, Debug)]
1643 enum ProjectionCacheEntry<'tcx> {
1644     InProgress,
1645     Ambiguous,
1646     Error,
1647     NormalizedTy(NormalizedTy<'tcx>),
1648 }
1649
1650 // NB: intentionally not Clone
1651 pub struct ProjectionCacheSnapshot {
1652     snapshot: Snapshot,
1653 }
1654
1655 impl<'tcx> ProjectionCache<'tcx> {
1656     pub fn new() -> Self {
1657         ProjectionCache {
1658             map: SnapshotMap::new()
1659         }
1660     }
1661
1662     pub fn clear(&mut self) {
1663         self.map.clear();
1664     }
1665
1666     pub fn snapshot(&mut self) -> ProjectionCacheSnapshot {
1667         ProjectionCacheSnapshot { snapshot: self.map.snapshot() }
1668     }
1669
1670     pub fn rollback_to(&mut self, snapshot: ProjectionCacheSnapshot) {
1671         self.map.rollback_to(&snapshot.snapshot);
1672     }
1673
1674     pub fn rollback_skolemized(&mut self, snapshot: &ProjectionCacheSnapshot) {
1675         self.map.partial_rollback(&snapshot.snapshot, &|k| k.ty.has_re_skol());
1676     }
1677
1678     pub fn commit(&mut self, snapshot: &ProjectionCacheSnapshot) {
1679         self.map.commit(&snapshot.snapshot);
1680     }
1681
1682     /// Try to start normalize `key`; returns an error if
1683     /// normalization already occurred (this error corresponds to a
1684     /// cache hit, so it's actually a good thing).
1685     fn try_start(&mut self, key: ProjectionCacheKey<'tcx>)
1686                  -> Result<(), ProjectionCacheEntry<'tcx>> {
1687         if let Some(entry) = self.map.get(&key) {
1688             return Err(entry.clone());
1689         }
1690
1691         self.map.insert(key, ProjectionCacheEntry::InProgress);
1692         Ok(())
1693     }
1694
1695     /// Indicates that `key` was normalized to `value`.
1696     fn insert_ty(&mut self, key: ProjectionCacheKey<'tcx>, value: NormalizedTy<'tcx>) {
1697         debug!("ProjectionCacheEntry::insert_ty: adding cache entry: key={:?}, value={:?}",
1698                key, value);
1699         let fresh_key = self.map.insert(key, ProjectionCacheEntry::NormalizedTy(value));
1700         assert!(!fresh_key, "never started projecting `{:?}`", key);
1701     }
1702
1703     /// Mark the relevant projection cache key as having its derived obligations
1704     /// complete, so they won't have to be re-computed (this is OK to do in a
1705     /// snapshot - if the snapshot is rolled back, the obligations will be
1706     /// marked as incomplete again).
1707     pub fn complete(&mut self, key: ProjectionCacheKey<'tcx>) {
1708         let ty = match self.map.get(&key) {
1709             Some(&ProjectionCacheEntry::NormalizedTy(ref ty)) => {
1710                 debug!("ProjectionCacheEntry::complete({:?}) - completing {:?}",
1711                        key, ty);
1712                 ty.value
1713             }
1714             ref value => {
1715                 // Type inference could "strand behind" old cache entries. Leave
1716                 // them alone for now.
1717                 debug!("ProjectionCacheEntry::complete({:?}) - ignoring {:?}",
1718                        key, value);
1719                 return
1720             }
1721         };
1722
1723         self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1724             value: ty,
1725             obligations: vec![]
1726         }));
1727     }
1728
1729     /// A specialized version of `complete` for when the key's value is known
1730     /// to be a NormalizedTy.
1731     pub fn complete_normalized(&mut self, key: ProjectionCacheKey<'tcx>, ty: &NormalizedTy<'tcx>) {
1732         // We want to insert `ty` with no obligations. If the existing value
1733         // already has no obligations (as is common) we can use `insert_noop`
1734         // to do a minimal amount of work -- the HashMap insertion is skipped,
1735         // and minimal changes are made to the undo log.
1736         if ty.obligations.is_empty() {
1737             self.map.insert_noop();
1738         } else {
1739             self.map.insert(key, ProjectionCacheEntry::NormalizedTy(Normalized {
1740                 value: ty.value,
1741                 obligations: vec![]
1742             }));
1743         }
1744     }
1745
1746     /// Indicates that trying to normalize `key` resulted in
1747     /// ambiguity. No point in trying it again then until we gain more
1748     /// type information (in which case, the "fully resolved" key will
1749     /// be different).
1750     fn ambiguous(&mut self, key: ProjectionCacheKey<'tcx>) {
1751         let fresh = self.map.insert(key, ProjectionCacheEntry::Ambiguous);
1752         assert!(!fresh, "never started projecting `{:?}`", key);
1753     }
1754
1755     /// Indicates that trying to normalize `key` resulted in
1756     /// error.
1757     fn error(&mut self, key: ProjectionCacheKey<'tcx>) {
1758         let fresh = self.map.insert(key, ProjectionCacheEntry::Error);
1759         assert!(!fresh, "never started projecting `{:?}`", key);
1760     }
1761 }