src/librustc_trait_selection/traits/auto_trait.rs

   1 //! Support code for rustdoc and external tools.
   2 //! You really don't want to be using this unless you need to.
   3
   4 use super::*;
   5
   6 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
   7 use crate::infer::InferCtxt;
   8 use rustc_middle::ty::fold::TypeFolder;
   9 use rustc_middle::ty::{Region, RegionVid};
  10
  11 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  12
  13 use std::collections::hash_map::Entry;
  14 use std::collections::VecDeque;
  15
  16 // FIXME(twk): this is obviously not nice to duplicate like that
  17 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
  18 pub enum RegionTarget<'tcx> {
  19     Region(Region<'tcx>),
  20     RegionVid(RegionVid),
  21 }
  22
  23 #[derive(Default, Debug, Clone)]
  24 pub struct RegionDeps<'tcx> {
  25     larger: FxHashSet<RegionTarget<'tcx>>,
  26     smaller: FxHashSet<RegionTarget<'tcx>>,
  27 }
  28
  29 pub enum AutoTraitResult<A> {
  30     ExplicitImpl,
  31     PositiveImpl(A),
  32     NegativeImpl,
  33 }
  34
  35 impl<A> AutoTraitResult<A> {
  36     fn is_auto(&self) -> bool {
  37         match *self {
  38             AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl => true,
  39             _ => false,
  40         }
  41     }
  42 }
  43
  44 pub struct AutoTraitInfo<'cx> {
  45     pub full_user_env: ty::ParamEnv<'cx>,
  46     pub region_data: RegionConstraintData<'cx>,
  47     pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
  48 }
  49
  50 pub struct AutoTraitFinder<'tcx> {
  51     tcx: TyCtxt<'tcx>,
  52 }
  53
  54 impl<'tcx> AutoTraitFinder<'tcx> {
  55     pub fn new(tcx: TyCtxt<'tcx>) -> Self {
  56         AutoTraitFinder { tcx }
  57     }
  58
  59     /// Makes a best effort to determine whether and under which conditions an auto trait is
  60     /// implemented for a type. For example, if you have
  61     ///
  62     /// ```
  63     /// struct Foo<T> { data: Box<T> }
  64     /// ```
  65     ///
  66     /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
  67     /// The analysis attempts to account for custom impls as well as other complex cases. This
  68     /// result is intended for use by rustdoc and other such consumers.
  69     ///
  70     /// (Note that due to the coinductive nature of Send, the full and correct result is actually
  71     /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
  72     /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
  73     /// But this is often not the best way to present to the user.)
  74     ///
  75     /// Warning: The API should be considered highly unstable, and it may be refactored or removed
  76     /// in the future.
  77     pub fn find_auto_trait_generics<A>(
  78         &self,
  79         ty: Ty<'tcx>,
  80         orig_env: ty::ParamEnv<'tcx>,
  81         trait_did: DefId,
  82         auto_trait_callback: impl Fn(&InferCtxt<'_, 'tcx>, AutoTraitInfo<'tcx>) -> A,
  83     ) -> AutoTraitResult<A> {
  84         let tcx = self.tcx;
  85
  86         let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
  87
  88         let trait_pred = ty::Binder::bind(trait_ref);
  89
  90         let bail_out = tcx.infer_ctxt().enter(|infcx| {
  91             let mut selcx = SelectionContext::with_negative(&infcx, true);
  92             let result = selcx.select(&Obligation::new(
  93                 ObligationCause::dummy(),
  94                 orig_env,
  95                 trait_pred.to_poly_trait_predicate(),
  96             ));
  97
  98             match result {
  99                 Ok(Some(Vtable::VtableImpl(_))) => {
 100                     debug!(
 101                         "find_auto_trait_generics({:?}): \
 102                          manual impl found, bailing out",
 103                         trait_ref
 104                     );
 105                     true
 106                 }
 107                 _ => false,
 108             }
 109         });
 110
 111         // If an explicit impl exists, it always takes priority over an auto impl
 112         if bail_out {
 113             return AutoTraitResult::ExplicitImpl;
 114         }
 115
 116         tcx.infer_ctxt().enter(|infcx| {
 117             let mut fresh_preds = FxHashSet::default();
 118
 119             // Due to the way projections are handled by SelectionContext, we need to run
 120             // evaluate_predicates twice: once on the original param env, and once on the result of
 121             // the first evaluate_predicates call.
 122             //
 123             // The problem is this: most of rustc, including SelectionContext and traits::project,
 124             // are designed to work with a concrete usage of a type (e.g., Vec<u8>
 125             // fn<T>() { Vec<T> }. This information will generally never change - given
 126             // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
 127             // If we're unable to prove that 'T' implements a particular trait, we're done -
 128             // there's nothing left to do but error out.
 129             //
 130             // However, synthesizing an auto trait impl works differently. Here, we start out with
 131             // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
 132             // with - and progressively discover the conditions we need to fulfill for it to
 133             // implement a certain auto trait. This ends up breaking two assumptions made by trait
 134             // selection and projection:
 135             //
 136             // * We can always cache the result of a particular trait selection for the lifetime of
 137             // an InfCtxt
 138             // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
 139             // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
 140             //
 141             // We fix the first assumption by manually clearing out all of the InferCtxt's caches
 142             // in between calls to SelectionContext.select. This allows us to keep all of the
 143             // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
 144             // them between calls.
 145             //
 146             // We fix the second assumption by reprocessing the result of our first call to
 147             // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
 148             // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
 149             // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
 150             // SelectionContext to return it back to us.
 151
 152             let (new_env, user_env) = match self.evaluate_predicates(
 153                 &infcx,
 154                 trait_did,
 155                 ty,
 156                 orig_env,
 157                 orig_env,
 158                 &mut fresh_preds,
 159                 false,
 160             ) {
 161                 Some(e) => e,
 162                 None => return AutoTraitResult::NegativeImpl,
 163             };
 164
 165             let (full_env, full_user_env) = self
 166                 .evaluate_predicates(
 167                     &infcx,
 168                     trait_did,
 169                     ty,
 170                     new_env,
 171                     user_env,
 172                     &mut fresh_preds,
 173                     true,
 174                 )
 175                 .unwrap_or_else(|| {
 176                     panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
 177                 });
 178
 179             debug!(
 180                 "find_auto_trait_generics({:?}): fulfilling \
 181                  with {:?}",
 182                 trait_ref, full_env
 183             );
 184             infcx.clear_caches();
 185
 186             // At this point, we already have all of the bounds we need. FulfillmentContext is used
 187             // to store all of the necessary region/lifetime bounds in the InferContext, as well as
 188             // an additional sanity check.
 189             let mut fulfill = FulfillmentContext::new();
 190             fulfill.register_bound(&infcx, full_env, ty, trait_did, ObligationCause::dummy());
 191             fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
 192                 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, e)
 193             });
 194
 195             let body_id_map: FxHashMap<_, _> = infcx
 196                 .inner
 197                 .borrow()
 198                 .region_obligations
 199                 .iter()
 200                 .map(|&(id, _)| (id, vec![]))
 201                 .collect();
 202
 203             infcx.process_registered_region_obligations(&body_id_map, None, full_env);
 204
 205             let region_data = infcx
 206                 .inner
 207                 .borrow_mut()
 208                 .unwrap_region_constraints()
 209                 .region_constraint_data()
 210                 .clone();
 211
 212             let vid_to_region = self.map_vid_to_region(&region_data);
 213
 214             let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
 215
 216             AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info))
 217         })
 218     }
 219 }
 220
 221 impl AutoTraitFinder<'tcx> {
 222     /// The core logic responsible for computing the bounds for our synthesized impl.
 223     ///
 224     /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
 225     /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
 226     /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
 227     /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
 228     /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
 229     ///
 230     /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
 231     /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
 232     /// user code. According, it considers all possible ways that a `Predicate` could be met, which
 233     /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
 234     /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
 235     /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
 236     /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
 237     /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
 238     /// like this:
 239     ///
 240     ///     impl<T> Send for Foo<T> where T: IntoIterator
 241     ///
 242     /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
 243     /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
 244     ///
 245     /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
 246     /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
 247     /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
 248     /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
 249     /// needs to hold.
 250     ///
 251     /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
 252     /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
 253     /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
 254     /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
 255     /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
 256     /// else `SelectionContext` will choke on the missing predicates. However, this should never
 257     /// show up in the final synthesized generics: we don't want our generated docs page to contain
 258     /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
 259     /// separate `user_env`, which only holds the predicates that will actually be displayed to the
 260     /// user.
 261     fn evaluate_predicates(
 262         &self,
 263         infcx: &InferCtxt<'_, 'tcx>,
 264         trait_did: DefId,
 265         ty: Ty<'tcx>,
 266         param_env: ty::ParamEnv<'tcx>,
 267         user_env: ty::ParamEnv<'tcx>,
 268         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 269         only_projections: bool,
 270     ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
 271         let tcx = infcx.tcx;
 272
 273         let mut select = SelectionContext::with_negative(&infcx, true);
 274
 275         let mut already_visited = FxHashSet::default();
 276         let mut predicates = VecDeque::new();
 277         predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
 278             trait_ref: ty::TraitRef {
 279                 def_id: trait_did,
 280                 substs: infcx.tcx.mk_substs_trait(ty, &[]),
 281             },
 282         }));
 283
 284         let computed_preds = param_env.caller_bounds.iter().cloned();
 285         let mut user_computed_preds: FxHashSet<_> =
 286             user_env.caller_bounds.iter().cloned().collect();
 287
 288         let mut new_env = param_env;
 289         let dummy_cause = ObligationCause::dummy();
 290
 291         while let Some(pred) = predicates.pop_front() {
 292             infcx.clear_caches();
 293
 294             if !already_visited.insert(pred) {
 295                 continue;
 296             }
 297
 298             // Call `infcx.resolve_vars_if_possible` to see if we can
 299             // get rid of any inference variables.
 300             let obligation = infcx.resolve_vars_if_possible(&Obligation::new(
 301                 dummy_cause.clone(),
 302                 new_env,
 303                 pred,
 304             ));
 305             let result = select.select(&obligation);
 306
 307             match &result {
 308                 &Ok(Some(ref vtable)) => {
 309                     // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
 310                     // we immediately bail out, since it's impossible for us to continue.
 311
 312                     if let Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) = vtable {
 313                         // Blame 'tidy' for the weird bracket placement.
 314                         if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
 315                             debug!(
 316                                 "evaluate_nested_obligations: found explicit negative impl\
 317                                         {:?}, bailing out",
 318                                 impl_def_id
 319                             );
 320                             return None;
 321                         }
 322                     }
 323
 324                     let obligations = vtable.clone().nested_obligations().into_iter();
 325
 326                     if !self.evaluate_nested_obligations(
 327                         ty,
 328                         obligations,
 329                         &mut user_computed_preds,
 330                         fresh_preds,
 331                         &mut predicates,
 332                         &mut select,
 333                         only_projections,
 334                     ) {
 335                         return None;
 336                     }
 337                 }
 338                 &Ok(None) => {}
 339                 &Err(SelectionError::Unimplemented) => {
 340                     if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
 341                         already_visited.remove(&pred);
 342                         self.add_user_pred(
 343                             &mut user_computed_preds,
 344                             ty::Predicate::Trait(pred, hir::Constness::NotConst),
 345                         );
 346                         predicates.push_back(pred);
 347                     } else {
 348                         debug!(
 349                             "evaluate_nested_obligations: `Unimplemented` found, bailing: \
 350                              {:?} {:?} {:?}",
 351                             ty,
 352                             pred,
 353                             pred.skip_binder().trait_ref.substs
 354                         );
 355                         return None;
 356                     }
 357                 }
 358                 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
 359             };
 360
 361             let normalized_preds = elaborate_predicates(
 362                 tcx,
 363                 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
 364             )
 365             .map(|o| o.predicate);
 366             new_env =
 367                 ty::ParamEnv::new(tcx.mk_predicates(normalized_preds), param_env.reveal, None);
 368         }
 369
 370         let final_user_env = ty::ParamEnv::new(
 371             tcx.mk_predicates(user_computed_preds.into_iter()),
 372             user_env.reveal,
 373             None,
 374         );
 375         debug!(
 376             "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
 377              '{:?}'",
 378             ty, trait_did, new_env, final_user_env
 379         );
 380
 381         Some((new_env, final_user_env))
 382     }
 383
 384     /// This method is designed to work around the following issue:
 385     /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
 386     /// progressively building a `ParamEnv` based on the results we get.
 387     /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
 388     /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
 389     ///
 390     /// This can lead to a corner case when dealing with region parameters.
 391     /// During our selection loop in `evaluate_predicates`, we might end up with
 392     /// two trait predicates that differ only in their region parameters:
 393     /// one containing a HRTB lifetime parameter, and one containing a 'normal'
 394     /// lifetime parameter. For example:
 395     ///
 396     ///     T as MyTrait<'a>
 397     ///     T as MyTrait<'static>
 398     ///
 399     /// If we put both of these predicates in our computed `ParamEnv`, we'll
 400     /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
 401     ///
 402     /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
 403     /// Our end goal is to generate a user-visible description of the conditions
 404     /// under which a type implements an auto trait. A trait predicate involving
 405     /// a HRTB means that the type needs to work with any choice of lifetime,
 406     /// not just one specific lifetime (e.g., `'static`).
 407     fn add_user_pred<'c>(
 408         &self,
 409         user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>,
 410         new_pred: ty::Predicate<'c>,
 411     ) {
 412         let mut should_add_new = true;
 413         user_computed_preds.retain(|&old_pred| {
 414             if let (&ty::Predicate::Trait(new_trait, _), ty::Predicate::Trait(old_trait, _)) =
 415                 (&new_pred, old_pred)
 416             {
 417                 if new_trait.def_id() == old_trait.def_id() {
 418                     let new_substs = new_trait.skip_binder().trait_ref.substs;
 419                     let old_substs = old_trait.skip_binder().trait_ref.substs;
 420
 421                     if !new_substs.types().eq(old_substs.types()) {
 422                         // We can't compare lifetimes if the types are different,
 423                         // so skip checking `old_pred`.
 424                         return true;
 425                     }
 426
 427                     for (new_region, old_region) in new_substs.regions().zip(old_substs.regions()) {
 428                         match (new_region, old_region) {
 429                             // If both predicates have an `ReLateBound` (a HRTB) in the
 430                             // same spot, we do nothing.
 431                             (
 432                                 ty::RegionKind::ReLateBound(_, _),
 433                                 ty::RegionKind::ReLateBound(_, _),
 434                             ) => {}
 435
 436                             (ty::RegionKind::ReLateBound(_, _), _)
 437                             | (_, ty::RegionKind::ReVar(_)) => {
 438                                 // One of these is true:
 439                                 // The new predicate has a HRTB in a spot where the old
 440                                 // predicate does not (if they both had a HRTB, the previous
 441                                 // match arm would have executed). A HRBT is a 'stricter'
 442                                 // bound than anything else, so we want to keep the newer
 443                                 // predicate (with the HRBT) in place of the old predicate.
 444                                 //
 445                                 // OR
 446                                 //
 447                                 // The old predicate has a region variable where the new
 448                                 // predicate has some other kind of region. An region
 449                                 // variable isn't something we can actually display to a user,
 450                                 // so we choose their new predicate (which doesn't have a region
 451                                 // variable).
 452                                 //
 453                                 // In both cases, we want to remove the old predicate,
 454                                 // from `user_computed_preds`, and replace it with the new
 455                                 // one. Having both the old and the new
 456                                 // predicate in a `ParamEnv` would confuse `SelectionContext`.
 457                                 //
 458                                 // We're currently in the predicate passed to 'retain',
 459                                 // so we return `false` to remove the old predicate from
 460                                 // `user_computed_preds`.
 461                                 return false;
 462                             }
 463                             (_, ty::RegionKind::ReLateBound(_, _))
 464                             | (ty::RegionKind::ReVar(_), _) => {
 465                                 // This is the opposite situation as the previous arm.
 466                                 // One of these is true:
 467                                 //
 468                                 // The old predicate has a HRTB lifetime in a place where the
 469                                 // new predicate does not.
 470                                 //
 471                                 // OR
 472                                 //
 473                                 // The new predicate has a region variable where the old
 474                                 // predicate has some other type of region.
 475                                 //
 476                                 // We want to leave the old
 477                                 // predicate in `user_computed_preds`, and skip adding
 478                                 // new_pred to `user_computed_params`.
 479                                 should_add_new = false
 480                             }
 481                             _ => {}
 482                         }
 483                     }
 484                 }
 485             }
 486             true
 487         });
 488
 489         if should_add_new {
 490             user_computed_preds.insert(new_pred);
 491         }
 492     }
 493
 494     /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
 495     /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
 496     fn map_vid_to_region<'cx>(
 497         &self,
 498         regions: &RegionConstraintData<'cx>,
 499     ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
 500         let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
 501         let mut finished_map = FxHashMap::default();
 502
 503         for constraint in regions.constraints.keys() {
 504             match constraint {
 505                 &Constraint::VarSubVar(r1, r2) => {
 506                     {
 507                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 508                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 509                     }
 510
 511                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 512                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 513                 }
 514                 &Constraint::RegSubVar(region, vid) => {
 515                     {
 516                         let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
 517                         deps1.larger.insert(RegionTarget::RegionVid(vid));
 518                     }
 519
 520                     let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 521                     deps2.smaller.insert(RegionTarget::Region(region));
 522                 }
 523                 &Constraint::VarSubReg(vid, region) => {
 524                     finished_map.insert(vid, region);
 525                 }
 526                 &Constraint::RegSubReg(r1, r2) => {
 527                     {
 528                         let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
 529                         deps1.larger.insert(RegionTarget::Region(r2));
 530                     }
 531
 532                     let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
 533                     deps2.smaller.insert(RegionTarget::Region(r1));
 534                 }
 535             }
 536         }
 537
 538         while !vid_map.is_empty() {
 539             let target = *vid_map.keys().next().expect("Keys somehow empty");
 540             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 541
 542             for smaller in deps.smaller.iter() {
 543                 for larger in deps.larger.iter() {
 544                     match (smaller, larger) {
 545                         (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
 546                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 547                                 let smaller_deps = v.into_mut();
 548                                 smaller_deps.larger.insert(*larger);
 549                                 smaller_deps.larger.remove(&target);
 550                             }
 551
 552                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 553                                 let larger_deps = v.into_mut();
 554                                 larger_deps.smaller.insert(*smaller);
 555                                 larger_deps.smaller.remove(&target);
 556                             }
 557                         }
 558                         (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
 559                             finished_map.insert(v1, r1);
 560                         }
 561                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 562                             // Do nothing; we don't care about regions that are smaller than vids.
 563                         }
 564                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 565                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 566                                 let smaller_deps = v.into_mut();
 567                                 smaller_deps.larger.insert(*larger);
 568                                 smaller_deps.larger.remove(&target);
 569                             }
 570
 571                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 572                                 let larger_deps = v.into_mut();
 573                                 larger_deps.smaller.insert(*smaller);
 574                                 larger_deps.smaller.remove(&target);
 575                             }
 576                         }
 577                     }
 578                 }
 579             }
 580         }
 581         finished_map
 582     }
 583
 584     fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
 585         self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
 586     }
 587
 588     pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
 589         match ty.kind {
 590             ty::Param(_) => true,
 591             ty::Projection(p) => self.is_of_param(p.self_ty()),
 592             _ => false,
 593         }
 594     }
 595
 596     fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
 597         match p.ty().skip_binder().kind {
 598             ty::Projection(proj) if proj == p.skip_binder().projection_ty => true,
 599             _ => false,
 600         }
 601     }
 602
 603     fn evaluate_nested_obligations(
 604         &self,
 605         ty: Ty<'_>,
 606         nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
 607         computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 608         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 609         predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
 610         select: &mut SelectionContext<'_, 'tcx>,
 611         only_projections: bool,
 612     ) -> bool {
 613         let dummy_cause = ObligationCause::dummy();
 614
 615         for (obligation, mut predicate) in nested.map(|o| (o.clone(), o.predicate)) {
 616             let is_new_pred = fresh_preds.insert(self.clean_pred(select.infcx(), predicate));
 617
 618             // Resolve any inference variables that we can, to help selection succeed
 619             predicate = select.infcx().resolve_vars_if_possible(&predicate);
 620
 621             // We only add a predicate as a user-displayable bound if
 622             // it involves a generic parameter, and doesn't contain
 623             // any inference variables.
 624             //
 625             // Displaying a bound involving a concrete type (instead of a generic
 626             // parameter) would be pointless, since it's always true
 627             // (e.g. u8: Copy)
 628             // Displaying an inference variable is impossible, since they're
 629             // an internal compiler detail without a defined visual representation
 630             //
 631             // We check this by calling is_of_param on the relevant types
 632             // from the various possible predicates
 633             match &predicate {
 634                 &ty::Predicate::Trait(p, _) => {
 635                     if self.is_param_no_infer(p.skip_binder().trait_ref.substs)
 636                         && !only_projections
 637                         && is_new_pred
 638                     {
 639                         self.add_user_pred(computed_preds, predicate);
 640                     }
 641                     predicates.push_back(p);
 642                 }
 643                 &ty::Predicate::Projection(p) => {
 644                     debug!(
 645                         "evaluate_nested_obligations: examining projection predicate {:?}",
 646                         predicate
 647                     );
 648
 649                     // As described above, we only want to display
 650                     // bounds which include a generic parameter but don't include
 651                     // an inference variable.
 652                     // Additionally, we check if we've seen this predicate before,
 653                     // to avoid rendering duplicate bounds to the user.
 654                     if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
 655                         && !p.ty().skip_binder().has_infer_types()
 656                         && is_new_pred
 657                     {
 658                         debug!(
 659                             "evaluate_nested_obligations: adding projection predicate\
 660                             to computed_preds: {:?}",
 661                             predicate
 662                         );
 663
 664                         // Under unusual circumstances, we can end up with a self-refeential
 665                         // projection predicate. For example:
 666                         // <T as MyType>::Value == <T as MyType>::Value
 667                         // Not only is displaying this to the user pointless,
 668                         // having it in the ParamEnv will cause an issue if we try to call
 669                         // poly_project_and_unify_type on the predicate, since this kind of
 670                         // predicate will normally never end up in a ParamEnv.
 671                         //
 672                         // For these reasons, we ignore these weird predicates,
 673                         // ensuring that we're able to properly synthesize an auto trait impl
 674                         if self.is_self_referential_projection(p) {
 675                             debug!(
 676                                 "evaluate_nested_obligations: encountered a projection
 677                                  predicate equating a type with itself! Skipping"
 678                             );
 679                         } else {
 680                             self.add_user_pred(computed_preds, predicate);
 681                         }
 682                     }
 683
 684                     // There are three possible cases when we project a predicate:
 685                     //
 686                     // 1. We encounter an error. This means that it's impossible for
 687                     // our current type to implement the auto trait - there's bound
 688                     // that we could add to our ParamEnv that would 'fix' this kind
 689                     // of error, as it's not caused by an unimplemented type.
 690                     //
 691                     // 2. We successfully project the predicate (Ok(Some(_))), generating
 692                     //  some subobligations. We then process these subobligations
 693                     //  like any other generated sub-obligations.
 694                     //
 695                     // 3. We receive an 'ambiguous' result (Ok(None))
 696                     // If we were actually trying to compile a crate,
 697                     // we would need to re-process this obligation later.
 698                     // However, all we care about is finding out what bounds
 699                     // are needed for our type to implement a particular auto trait.
 700                     // We've already added this obligation to our computed ParamEnv
 701                     // above (if it was necessary). Therefore, we don't need
 702                     // to do any further processing of the obligation.
 703                     //
 704                     // Note that we *must* try to project *all* projection predicates
 705                     // we encounter, even ones without inference variable.
 706                     // This ensures that we detect any projection errors,
 707                     // which indicate that our type can *never* implement the given
 708                     // auto trait. In that case, we will generate an explicit negative
 709                     // impl (e.g. 'impl !Send for MyType'). However, we don't
 710                     // try to process any of the generated subobligations -
 711                     // they contain no new information, since we already know
 712                     // that our type implements the projected-through trait,
 713                     // and can lead to weird region issues.
 714                     //
 715                     // Normally, we'll generate a negative impl as a result of encountering
 716                     // a type with an explicit negative impl of an auto trait
 717                     // (for example, raw pointers have !Send and !Sync impls)
 718                     // However, through some **interesting** manipulations of the type
 719                     // system, it's actually possible to write a type that never
 720                     // implements an auto trait due to a projection error, not a normal
 721                     // negative impl error. To properly handle this case, we need
 722                     // to ensure that we catch any potential projection errors,
 723                     // and turn them into an explicit negative impl for our type.
 724                     debug!("Projecting and unifying projection predicate {:?}", predicate);
 725
 726                     match poly_project_and_unify_type(select, &obligation.with(p)) {
 727                         Err(e) => {
 728                             debug!(
 729                                 "evaluate_nested_obligations: Unable to unify predicate \
 730                                  '{:?}' '{:?}', bailing out",
 731                                 ty, e
 732                             );
 733                             return false;
 734                         }
 735                         Ok(Some(v)) => {
 736                             // We only care about sub-obligations
 737                             // when we started out trying to unify
 738                             // some inference variables. See the comment above
 739                             // for more infomration
 740                             if p.ty().skip_binder().has_infer_types() {
 741                                 if !self.evaluate_nested_obligations(
 742                                     ty,
 743                                     v.into_iter(),
 744                                     computed_preds,
 745                                     fresh_preds,
 746                                     predicates,
 747                                     select,
 748                                     only_projections,
 749                                 ) {
 750                                     return false;
 751                                 }
 752                             }
 753                         }
 754                         Ok(None) => {
 755                             // It's ok not to make progress when hvave no inference variables -
 756                             // in that case, we were only performing unifcation to check if an
 757                             // error occurred (which would indicate that it's impossible for our
 758                             // type to implement the auto trait).
 759                             // However, we should always make progress (either by generating
 760                             // subobligations or getting an error) when we started off with
 761                             // inference variables
 762                             if p.ty().skip_binder().has_infer_types() {
 763                                 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
 764                             }
 765                         }
 766                     }
 767                 }
 768                 &ty::Predicate::RegionOutlives(ref binder) => {
 769                     if select.infcx().region_outlives_predicate(&dummy_cause, binder).is_err() {
 770                         return false;
 771                     }
 772                 }
 773                 &ty::Predicate::TypeOutlives(ref binder) => {
 774                     match (
 775                         binder.no_bound_vars(),
 776                         binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
 777                     ) {
 778                         (None, Some(t_a)) => {
 779                             select.infcx().register_region_obligation_with_cause(
 780                                 t_a,
 781                                 select.infcx().tcx.lifetimes.re_static,
 782                                 &dummy_cause,
 783                             );
 784                         }
 785                         (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
 786                             select.infcx().register_region_obligation_with_cause(
 787                                 t_a,
 788                                 r_b,
 789                                 &dummy_cause,
 790                             );
 791                         }
 792                         _ => {}
 793                     };
 794                 }
 795                 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
 796             };
 797         }
 798         true
 799     }
 800
 801     pub fn clean_pred(
 802         &self,
 803         infcx: &InferCtxt<'_, 'tcx>,
 804         p: ty::Predicate<'tcx>,
 805     ) -> ty::Predicate<'tcx> {
 806         infcx.freshen(p)
 807     }
 808 }
 809
 810 // Replaces all ReVars in a type with ty::Region's, using the provided map
 811 pub struct RegionReplacer<'a, 'tcx> {
 812     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 813     tcx: TyCtxt<'tcx>,
 814 }
 815
 816 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 817     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 818         self.tcx
 819     }
 820
 821     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 822         (match r {
 823             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 824             _ => None,
 825         })
 826         .unwrap_or_else(|| r.super_fold_with(self))
 827     }
 828 }