src/librustc/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 crate::ty::fold::TypeFolder;
   9 use crate::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 {
  87             def_id: trait_did,
  88             substs: tcx.mk_substs_trait(ty, &[]),
  89         };
  90
  91         let trait_pred = ty::Binder::bind(trait_ref);
  92
  93         let bail_out = tcx.infer_ctxt().enter(|infcx| {
  94             let mut selcx = SelectionContext::with_negative(&infcx, true);
  95             let result = selcx.select(&Obligation::new(
  96                 ObligationCause::dummy(),
  97                 orig_env,
  98                 trait_pred.to_poly_trait_predicate(),
  99             ));
 100
 101             match result {
 102                 Ok(Some(Vtable::VtableImpl(_))) => {
 103                     debug!(
 104                         "find_auto_trait_generics({:?}): \
 105                          manual impl found, bailing out",
 106                         trait_ref
 107                     );
 108                     true
 109                 }
 110                 _ => false
 111             }
 112         });
 113
 114         // If an explicit impl exists, it always takes priority over an auto impl
 115         if bail_out {
 116             return AutoTraitResult::ExplicitImpl;
 117         }
 118
 119         return tcx.infer_ctxt().enter(|mut infcx| {
 120             let mut fresh_preds = FxHashSet::default();
 121
 122             // Due to the way projections are handled by SelectionContext, we need to run
 123             // evaluate_predicates twice: once on the original param env, and once on the result of
 124             // the first evaluate_predicates call.
 125             //
 126             // The problem is this: most of rustc, including SelectionContext and traits::project,
 127             // are designed to work with a concrete usage of a type (e.g., Vec<u8>
 128             // fn<T>() { Vec<T> }. This information will generally never change - given
 129             // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
 130             // If we're unable to prove that 'T' implements a particular trait, we're done -
 131             // there's nothing left to do but error out.
 132             //
 133             // However, synthesizing an auto trait impl works differently. Here, we start out with
 134             // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
 135             // with - and progressively discover the conditions we need to fulfill for it to
 136             // implement a certain auto trait. This ends up breaking two assumptions made by trait
 137             // selection and projection:
 138             //
 139             // * We can always cache the result of a particular trait selection for the lifetime of
 140             // an InfCtxt
 141             // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
 142             // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
 143             //
 144             // We fix the first assumption by manually clearing out all of the InferCtxt's caches
 145             // in between calls to SelectionContext.select. This allows us to keep all of the
 146             // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
 147             // them between calls.
 148             //
 149             // We fix the second assumption by reprocessing the result of our first call to
 150             // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
 151             // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
 152             // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
 153             // SelectionContext to return it back to us.
 154
 155             let (new_env, user_env) = match self.evaluate_predicates(
 156                 &mut infcx,
 157                 trait_did,
 158                 ty,
 159                 orig_env,
 160                 orig_env,
 161                 &mut fresh_preds,
 162                 false,
 163             ) {
 164                 Some(e) => e,
 165                 None => return AutoTraitResult::NegativeImpl,
 166             };
 167
 168             let (full_env, full_user_env) = self.evaluate_predicates(
 169                 &mut infcx,
 170                 trait_did,
 171                 ty,
 172                 new_env,
 173                 user_env,
 174                 &mut fresh_preds,
 175                 true,
 176             ).unwrap_or_else(|| {
 177                 panic!(
 178                     "Failed to fully process: {:?} {:?} {:?}",
 179                     ty, trait_did, orig_env
 180                 )
 181             });
 182
 183             debug!(
 184                 "find_auto_trait_generics({:?}): fulfilling \
 185                  with {:?}",
 186                 trait_ref, full_env
 187             );
 188             infcx.clear_caches();
 189
 190             // At this point, we already have all of the bounds we need. FulfillmentContext is used
 191             // to store all of the necessary region/lifetime bounds in the InferContext, as well as
 192             // an additional sanity check.
 193             let mut fulfill = FulfillmentContext::new();
 194             fulfill.register_bound(
 195                 &infcx,
 196                 full_env,
 197                 ty,
 198                 trait_did,
 199                 ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID),
 200             );
 201             fulfill.select_all_or_error(&infcx).unwrap_or_else(|e| {
 202                 panic!(
 203                     "Unable to fulfill trait {:?} for '{:?}': {:?}",
 204                     trait_did, ty, e
 205                 )
 206             });
 207
 208             let body_id_map: FxHashMap<_, _> = infcx
 209                 .region_obligations
 210                 .borrow()
 211                 .iter()
 212                 .map(|&(id, _)| (id, vec![]))
 213                 .collect();
 214
 215             infcx.process_registered_region_obligations(&body_id_map, None, full_env);
 216
 217             let region_data = infcx
 218                 .borrow_region_constraints()
 219                 .region_constraint_data()
 220                 .clone();
 221
 222             let vid_to_region = self.map_vid_to_region(&region_data);
 223
 224             let info = AutoTraitInfo {
 225                 full_user_env,
 226                 region_data,
 227                 vid_to_region,
 228             };
 229
 230             return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info));
 231         });
 232     }
 233 }
 234
 235 impl AutoTraitFinder<'tcx> {
 236     /// The core logic responsible for computing the bounds for our synthesized impl.
 237     ///
 238     /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
 239     /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
 240     /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
 241     /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
 242     /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
 243     ///
 244     /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
 245     /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
 246     /// user code. According, it considers all possible ways that a `Predicate` could be met, which
 247     /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
 248     /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
 249     /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
 250     /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
 251     /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
 252     /// like this:
 253     ///
 254     ///     impl<T> Send for Foo<T> where T: IntoIterator
 255     ///
 256     /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
 257     /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
 258     ///
 259     /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
 260     /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
 261     /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
 262     /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
 263     /// needs to hold.
 264     ///
 265     /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
 266     /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
 267     /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
 268     /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
 269     /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
 270     /// else `SelectionContext` will choke on the missing predicates. However, this should never
 271     /// show up in the final synthesized generics: we don't want our generated docs page to contain
 272     /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
 273     /// separate `user_env`, which only holds the predicates that will actually be displayed to the
 274     /// user.
 275     fn evaluate_predicates(
 276         &self,
 277         infcx: &InferCtxt<'_, 'tcx>,
 278         trait_did: DefId,
 279         ty: Ty<'tcx>,
 280         param_env: ty::ParamEnv<'tcx>,
 281         user_env: ty::ParamEnv<'tcx>,
 282         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 283         only_projections: bool,
 284     ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
 285         let tcx = infcx.tcx;
 286
 287         let mut select = SelectionContext::with_negative(&infcx, true);
 288
 289         let mut already_visited = FxHashSet::default();
 290         let mut predicates = VecDeque::new();
 291         predicates.push_back(ty::Binder::bind(ty::TraitPredicate {
 292             trait_ref: ty::TraitRef {
 293                 def_id: trait_did,
 294                 substs: infcx.tcx.mk_substs_trait(ty, &[]),
 295             },
 296         }));
 297
 298         let mut computed_preds: FxHashSet<_> = param_env.caller_bounds.iter().cloned().collect();
 299         let mut user_computed_preds: FxHashSet<_> =
 300             user_env.caller_bounds.iter().cloned().collect();
 301
 302         let mut new_env = param_env;
 303         let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID);
 304
 305         while let Some(pred) = predicates.pop_front() {
 306             infcx.clear_caches();
 307
 308             if !already_visited.insert(pred) {
 309                 continue;
 310             }
 311
 312             // Call `infcx.resolve_vars_if_possible` to see if we can
 313             // get rid of any inference variables.
 314             let obligation = infcx.resolve_vars_if_possible(
 315                 &Obligation::new(dummy_cause.clone(), new_env, pred)
 316             );
 317             let result = select.select(&obligation);
 318
 319             match &result {
 320                 &Ok(Some(ref vtable)) => {
 321                     // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
 322                     // we immediately bail out, since it's impossible for us to continue.
 323                     match vtable {
 324                         Vtable::VtableImpl(VtableImplData { impl_def_id, .. }) => {
 325                             // Blame 'tidy' for the weird bracket placement.
 326                             if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative
 327                             {
 328                                 debug!("evaluate_nested_obligations: found explicit negative impl\
 329                                         {:?}, bailing out", impl_def_id);
 330                                 return None;
 331                             }
 332                         },
 333                         _ => {}
 334                     }
 335
 336                     let obligations = vtable.clone().nested_obligations().into_iter();
 337
 338                     if !self.evaluate_nested_obligations(
 339                         ty,
 340                         obligations,
 341                         &mut user_computed_preds,
 342                         fresh_preds,
 343                         &mut predicates,
 344                         &mut select,
 345                         only_projections,
 346                     ) {
 347                         return None;
 348                     }
 349                 }
 350                 &Ok(None) => {}
 351                 &Err(SelectionError::Unimplemented) => {
 352                     if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
 353                         already_visited.remove(&pred);
 354                         self.add_user_pred(
 355                             &mut user_computed_preds,
 356                             ty::Predicate::Trait(pred),
 357                         );
 358                         predicates.push_back(pred);
 359                     } else {
 360                         debug!(
 361                             "evaluate_nested_obligations: `Unimplemented` found, bailing: \
 362                              {:?} {:?} {:?}",
 363                             ty,
 364                             pred,
 365                             pred.skip_binder().trait_ref.substs
 366                         );
 367                         return None;
 368                     }
 369                 }
 370                 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
 371             };
 372
 373             computed_preds.extend(user_computed_preds.iter().cloned());
 374             let normalized_preds =
 375                 elaborate_predicates(tcx, computed_preds.iter().cloned().collect());
 376             new_env = ty::ParamEnv::new(
 377                 tcx.mk_predicates(normalized_preds),
 378                 param_env.reveal,
 379                 None
 380             );
 381         }
 382
 383         let final_user_env = ty::ParamEnv::new(
 384             tcx.mk_predicates(user_computed_preds.into_iter()),
 385             user_env.reveal,
 386             None
 387         );
 388         debug!(
 389             "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
 390              '{:?}'",
 391             ty, trait_did, new_env, final_user_env
 392         );
 393
 394         return Some((new_env, final_user_env));
 395     }
 396
 397     /// This method is designed to work around the following issue:
 398     /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
 399     /// progressively building a `ParamEnv` based on the results we get.
 400     /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
 401     /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
 402     ///
 403     /// This can lead to a corner case when dealing with region parameters.
 404     /// During our selection loop in `evaluate_predicates`, we might end up with
 405     /// two trait predicates that differ only in their region parameters:
 406     /// one containing a HRTB lifetime parameter, and one containing a 'normal'
 407     /// lifetime parameter. For example:
 408     ///
 409     ///     T as MyTrait<'a>
 410     ///     T as MyTrait<'static>
 411     ///
 412     /// If we put both of these predicates in our computed `ParamEnv`, we'll
 413     /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
 414     ///
 415     /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
 416     /// Our end goal is to generate a user-visible description of the conditions
 417     /// under which a type implements an auto trait. A trait predicate involving
 418     /// a HRTB means that the type needs to work with any choice of lifetime,
 419     /// not just one specific lifetime (e.g., `'static`).
 420     fn add_user_pred<'c>(
 421         &self,
 422         user_computed_preds: &mut FxHashSet<ty::Predicate<'c>>,
 423         new_pred: ty::Predicate<'c>,
 424     ) {
 425         let mut should_add_new = true;
 426         user_computed_preds.retain(|&old_pred| {
 427             match (&new_pred, old_pred) {
 428                 (&ty::Predicate::Trait(new_trait), ty::Predicate::Trait(old_trait)) => {
 429                     if new_trait.def_id() == old_trait.def_id() {
 430                         let new_substs = new_trait.skip_binder().trait_ref.substs;
 431                         let old_substs = old_trait.skip_binder().trait_ref.substs;
 432
 433                         if !new_substs.types().eq(old_substs.types()) {
 434                             // We can't compare lifetimes if the types are different,
 435                             // so skip checking `old_pred`.
 436                             return true;
 437                         }
 438
 439                         for (new_region, old_region) in
 440                             new_substs.regions().zip(old_substs.regions())
 441                         {
 442                             match (new_region, old_region) {
 443                                 // If both predicates have an `ReLateBound` (a HRTB) in the
 444                                 // same spot, we do nothing.
 445                                 (
 446                                     ty::RegionKind::ReLateBound(_, _),
 447                                     ty::RegionKind::ReLateBound(_, _),
 448                                 ) => {}
 449
 450                                 (ty::RegionKind::ReLateBound(_, _), _) |
 451                                 (_, ty::RegionKind::ReVar(_)) => {
 452                                     // One of these is true:
 453                                     // The new predicate has a HRTB in a spot where the old
 454                                     // predicate does not (if they both had a HRTB, the previous
 455                                     // match arm would have executed). A HRBT is a 'stricter'
 456                                     // bound than anything else, so we want to keep the newer
 457                                     // predicate (with the HRBT) in place of the old predicate.
 458                                     //
 459                                     // OR
 460                                     //
 461                                     // The old predicate has a region variable where the new
 462                                     // predicate has some other kind of region. An region
 463                                     // variable isn't something we can actually display to a user,
 464                                     // so we choose their new predicate (which doesn't have a region
 465                                     // varaible).
 466                                     //
 467                                     // In both cases, we want to remove the old predicate,
 468                                     // from `user_computed_preds`, and replace it with the new
 469                                     // one. Having both the old and the new
 470                                     // predicate in a `ParamEnv` would confuse `SelectionContext`.
 471                                     //
 472                                     // We're currently in the predicate passed to 'retain',
 473                                     // so we return `false` to remove the old predicate from
 474                                     // `user_computed_preds`.
 475                                     return false;
 476                                 }
 477                                 (_, ty::RegionKind::ReLateBound(_, _)) |
 478                                 (ty::RegionKind::ReVar(_), _) => {
 479                                     // This is the opposite situation as the previous arm.
 480                                     // One of these is true:
 481                                     //
 482                                     // The old predicate has a HRTB lifetime in a place where the
 483                                     // new predicate does not.
 484                                     //
 485                                     // OR
 486                                     //
 487                                     // The new predicate has a region variable where the old
 488                                     // predicate has some other type of region.
 489                                     //
 490                                     // We want to leave the old
 491                                     // predicate in `user_computed_preds`, and skip adding
 492                                     // new_pred to `user_computed_params`.
 493                                     should_add_new = false
 494                                 },
 495                                 _ => {}
 496                             }
 497                         }
 498                     }
 499                 }
 500                 _ => {}
 501             }
 502             return true;
 503         });
 504
 505         if should_add_new {
 506             user_computed_preds.insert(new_pred);
 507         }
 508     }
 509
 510     /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
 511     /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
 512     fn map_vid_to_region<'cx>(
 513         &self,
 514         regions: &RegionConstraintData<'cx>,
 515     ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
 516         let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
 517         let mut finished_map = FxHashMap::default();
 518
 519         for constraint in regions.constraints.keys() {
 520             match constraint {
 521                 &Constraint::VarSubVar(r1, r2) => {
 522                     {
 523                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 524                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 525                     }
 526
 527                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 528                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 529                 }
 530                 &Constraint::RegSubVar(region, vid) => {
 531                     {
 532                         let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
 533                         deps1.larger.insert(RegionTarget::RegionVid(vid));
 534                     }
 535
 536                     let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 537                     deps2.smaller.insert(RegionTarget::Region(region));
 538                 }
 539                 &Constraint::VarSubReg(vid, region) => {
 540                     finished_map.insert(vid, region);
 541                 }
 542                 &Constraint::RegSubReg(r1, r2) => {
 543                     {
 544                         let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
 545                         deps1.larger.insert(RegionTarget::Region(r2));
 546                     }
 547
 548                     let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
 549                     deps2.smaller.insert(RegionTarget::Region(r1));
 550                 }
 551             }
 552         }
 553
 554         while !vid_map.is_empty() {
 555             let target = vid_map.keys().next().expect("Keys somehow empty").clone();
 556             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 557
 558             for smaller in deps.smaller.iter() {
 559                 for larger in deps.larger.iter() {
 560                     match (smaller, larger) {
 561                         (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
 562                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 563                                 let smaller_deps = v.into_mut();
 564                                 smaller_deps.larger.insert(*larger);
 565                                 smaller_deps.larger.remove(&target);
 566                             }
 567
 568                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 569                                 let larger_deps = v.into_mut();
 570                                 larger_deps.smaller.insert(*smaller);
 571                                 larger_deps.smaller.remove(&target);
 572                             }
 573                         }
 574                         (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
 575                             finished_map.insert(v1, r1);
 576                         }
 577                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 578                             // Do nothing; we don't care about regions that are smaller than vids.
 579                         }
 580                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 581                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 582                                 let smaller_deps = v.into_mut();
 583                                 smaller_deps.larger.insert(*larger);
 584                                 smaller_deps.larger.remove(&target);
 585                             }
 586
 587                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 588                                 let larger_deps = v.into_mut();
 589                                 larger_deps.smaller.insert(*smaller);
 590                                 larger_deps.smaller.remove(&target);
 591                             }
 592                         }
 593                     }
 594                 }
 595             }
 596         }
 597         finished_map
 598     }
 599
 600     fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
 601         return self.is_of_param(substs.type_at(0)) &&
 602             !substs.types().any(|t| t.has_infer_types());
 603     }
 604
 605     pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
 606         return match ty.kind {
 607             ty::Param(_) => true,
 608             ty::Projection(p) => self.is_of_param(p.self_ty()),
 609             _ => false,
 610         };
 611     }
 612
 613     fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
 614         match p.ty().skip_binder().kind {
 615             ty::Projection(proj) if proj == p.skip_binder().projection_ty => {
 616                 true
 617             },
 618             _ => false
 619         }
 620     }
 621
 622     fn evaluate_nested_obligations(
 623         &self,
 624         ty: Ty<'_>,
 625         nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
 626         computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 627         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 628         predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
 629         select: &mut SelectionContext<'_, 'tcx>,
 630         only_projections: bool,
 631     ) -> bool {
 632         let dummy_cause = ObligationCause::misc(DUMMY_SP, hir::DUMMY_HIR_ID);
 633
 634         for (obligation, mut predicate) in nested
 635             .map(|o| (o.clone(), o.predicate))
 636         {
 637             let is_new_pred =
 638                 fresh_preds.insert(self.clean_pred(select.infcx(), predicate));
 639
 640             // Resolve any inference variables that we can, to help selection succeed
 641             predicate = select.infcx().resolve_vars_if_possible(&predicate);
 642
 643             // We only add a predicate as a user-displayable bound if
 644             // it involves a generic parameter, and doesn't contain
 645             // any inference variables.
 646             //
 647             // Displaying a bound involving a concrete type (instead of a generic
 648             // parameter) would be pointless, since it's always true
 649             // (e.g. u8: Copy)
 650             // Displaying an inference variable is impossible, since they're
 651             // an internal compiler detail without a defined visual representation
 652             //
 653             // We check this by calling is_of_param on the relevant types
 654             // from the various possible predicates
 655             match &predicate {
 656                 &ty::Predicate::Trait(p) => {
 657                     if self.is_param_no_infer(p.skip_binder().trait_ref.substs)
 658                         && !only_projections
 659                         && is_new_pred {
 660
 661                         self.add_user_pred(computed_preds, predicate);
 662                     }
 663                     predicates.push_back(p);
 664                 }
 665                 &ty::Predicate::Projection(p) => {
 666                     debug!("evaluate_nested_obligations: examining projection predicate {:?}",
 667                            predicate);
 668
 669                     // As described above, we only want to display
 670                     // bounds which include a generic parameter but don't include
 671                     // an inference variable.
 672                     // Additionally, we check if we've seen this predicate before,
 673                     // to avoid rendering duplicate bounds to the user.
 674                     if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
 675                         && !p.ty().skip_binder().has_infer_types()
 676                         && is_new_pred {
 677                             debug!("evaluate_nested_obligations: adding projection predicate\
 678                             to computed_preds: {:?}", predicate);
 679
 680                             // Under unusual circumstances, we can end up with a self-refeential
 681                             // projection predicate. For example:
 682                             // <T as MyType>::Value == <T as MyType>::Value
 683                             // Not only is displaying this to the user pointless,
 684                             // having it in the ParamEnv will cause an issue if we try to call
 685                             // poly_project_and_unify_type on the predicate, since this kind of
 686                             // predicate will normally never end up in a ParamEnv.
 687                             //
 688                             // For these reasons, we ignore these weird predicates,
 689                             // ensuring that we're able to properly synthesize an auto trait impl
 690                             if self.is_self_referential_projection(p) {
 691                                 debug!("evaluate_nested_obligations: encountered a projection
 692                                  predicate equating a type with itself! Skipping");
 693
 694                             } else {
 695                                 self.add_user_pred(computed_preds, predicate);
 696                             }
 697                     }
 698
 699                     // There are three possible cases when we project a predicate:
 700                     //
 701                     // 1. We encounter an error. This means that it's impossible for
 702                     // our current type to implement the auto trait - there's bound
 703                     // that we could add to our ParamEnv that would 'fix' this kind
 704                     // of error, as it's not caused by an unimplemented type.
 705                     //
 706                     // 2. We successfully project the predicate (Ok(Some(_))), generating
 707                     //  some subobligations. We then process these subobligations
 708                     //  like any other generated sub-obligations.
 709                     //
 710                     // 3. We receieve an 'ambiguous' result (Ok(None))
 711                     // If we were actually trying to compile a crate,
 712                     // we would need to re-process this obligation later.
 713                     // However, all we care about is finding out what bounds
 714                     // are needed for our type to implement a particular auto trait.
 715                     // We've already added this obligation to our computed ParamEnv
 716                     // above (if it was necessary). Therefore, we don't need
 717                     // to do any further processing of the obligation.
 718                     //
 719                     // Note that we *must* try to project *all* projection predicates
 720                     // we encounter, even ones without inference variable.
 721                     // This ensures that we detect any projection errors,
 722                     // which indicate that our type can *never* implement the given
 723                     // auto trait. In that case, we will generate an explicit negative
 724                     // impl (e.g. 'impl !Send for MyType'). However, we don't
 725                     // try to process any of the generated subobligations -
 726                     // they contain no new information, since we already know
 727                     // that our type implements the projected-through trait,
 728                     // and can lead to weird region issues.
 729                     //
 730                     // Normally, we'll generate a negative impl as a result of encountering
 731                     // a type with an explicit negative impl of an auto trait
 732                     // (for example, raw pointers have !Send and !Sync impls)
 733                     // However, through some **interesting** manipulations of the type
 734                     // system, it's actually possible to write a type that never
 735                     // implements an auto trait due to a projection error, not a normal
 736                     // negative impl error. To properly handle this case, we need
 737                     // to ensure that we catch any potential projection errors,
 738                     // and turn them into an explicit negative impl for our type.
 739                     debug!("Projecting and unifying projection predicate {:?}",
 740                            predicate);
 741
 742                     match poly_project_and_unify_type(select, &obligation.with(p)) {
 743                         Err(e) => {
 744                             debug!(
 745                                 "evaluate_nested_obligations: Unable to unify predicate \
 746                                  '{:?}' '{:?}', bailing out",
 747                                 ty, e
 748                             );
 749                             return false;
 750                         }
 751                         Ok(Some(v)) => {
 752                             // We only care about sub-obligations
 753                             // when we started out trying to unify
 754                             // some inference variables. See the comment above
 755                             // for more infomration
 756                             if p.ty().skip_binder().has_infer_types() {
 757                                 if !self.evaluate_nested_obligations(
 758                                     ty,
 759                                     v.clone().iter().cloned(),
 760                                     computed_preds,
 761                                     fresh_preds,
 762                                     predicates,
 763                                     select,
 764                                     only_projections,
 765                                 ) {
 766                                     return false;
 767                                 }
 768                             }
 769                         }
 770                         Ok(None) => {
 771                             // It's ok not to make progress when hvave no inference variables -
 772                             // in that case, we were only performing unifcation to check if an
 773                             // error occurred (which would indicate that it's impossible for our
 774                             // type to implement the auto trait).
 775                             // However, we should always make progress (either by generating
 776                             // subobligations or getting an error) when we started off with
 777                             // inference variables
 778                             if p.ty().skip_binder().has_infer_types() {
 779                                 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
 780                             }
 781                         }
 782                     }
 783                 }
 784                 &ty::Predicate::RegionOutlives(ref binder) => {
 785                     if select
 786                         .infcx()
 787                         .region_outlives_predicate(&dummy_cause, binder)
 788                         .is_err()
 789                     {
 790                         return false;
 791                     }
 792                 }
 793                 &ty::Predicate::TypeOutlives(ref binder) => {
 794                     match (
 795                         binder.no_bound_vars(),
 796                         binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
 797                     ) {
 798                         (None, Some(t_a)) => {
 799                             select.infcx().register_region_obligation_with_cause(
 800                                 t_a,
 801                                 select.infcx().tcx.lifetimes.re_static,
 802                                 &dummy_cause,
 803                             );
 804                         }
 805                         (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
 806                             select.infcx().register_region_obligation_with_cause(
 807                                 t_a,
 808                                 r_b,
 809                                 &dummy_cause,
 810                             );
 811                         }
 812                         _ => {}
 813                     };
 814                 }
 815                 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
 816             };
 817         }
 818         return true;
 819     }
 820
 821     pub fn clean_pred(
 822         &self,
 823         infcx: &InferCtxt<'_, 'tcx>,
 824         p: ty::Predicate<'tcx>,
 825     ) -> ty::Predicate<'tcx> {
 826         infcx.freshen(p)
 827     }
 828 }
 829
 830 // Replaces all ReVars in a type with ty::Region's, using the provided map
 831 pub struct RegionReplacer<'a, 'tcx> {
 832     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 833     tcx: TyCtxt<'tcx>,
 834 }
 835
 836 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 837     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 838         self.tcx
 839     }
 840
 841     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 842         (match r {
 843             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 844             _ => None,
 845         }).unwrap_or_else(|| r.super_fold_with(self))
 846     }
 847 }