compiler/rustc_trait_selection/src/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::errors::UnableToConstructConstantValue;
   7 use crate::infer::region_constraints::{Constraint, RegionConstraintData};
   8 use crate::infer::InferCtxt;
   9 use crate::traits::project::ProjectAndUnifyResult;
  10 use rustc_middle::mir::interpret::ErrorHandled;
  11 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  12 use rustc_middle::ty::visit::TypeVisitable;
  13 use rustc_middle::ty::{PolyTraitRef, Region, RegionVid};
  14
  15 use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexSet};
  16
  17 use std::collections::hash_map::Entry;
  18 use std::collections::VecDeque;
  19 use std::iter;
  20
  21 // FIXME(twk): this is obviously not nice to duplicate like that
  22 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
  23 pub enum RegionTarget<'tcx> {
  24     Region(Region<'tcx>),
  25     RegionVid(RegionVid),
  26 }
  27
  28 #[derive(Default, Debug, Clone)]
  29 pub struct RegionDeps<'tcx> {
  30     larger: FxIndexSet<RegionTarget<'tcx>>,
  31     smaller: FxIndexSet<RegionTarget<'tcx>>,
  32 }
  33
  34 pub enum AutoTraitResult<A> {
  35     ExplicitImpl,
  36     PositiveImpl(A),
  37     NegativeImpl,
  38 }
  39
  40 #[allow(dead_code)]
  41 impl<A> AutoTraitResult<A> {
  42     fn is_auto(&self) -> bool {
  43         matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
  44     }
  45 }
  46
  47 pub struct AutoTraitInfo<'cx> {
  48     pub full_user_env: ty::ParamEnv<'cx>,
  49     pub region_data: RegionConstraintData<'cx>,
  50     pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
  51 }
  52
  53 pub struct AutoTraitFinder<'tcx> {
  54     tcx: TyCtxt<'tcx>,
  55 }
  56
  57 impl<'tcx> AutoTraitFinder<'tcx> {
  58     pub fn new(tcx: TyCtxt<'tcx>) -> Self {
  59         AutoTraitFinder { tcx }
  60     }
  61
  62     /// Makes a best effort to determine whether and under which conditions an auto trait is
  63     /// implemented for a type. For example, if you have
  64     ///
  65     /// ```
  66     /// struct Foo<T> { data: Box<T> }
  67     /// ```
  68     ///
  69     /// then this might return that `Foo<T>: Send` if `T: Send` (encoded in the AutoTraitResult
  70     /// type). The analysis attempts to account for custom impls as well as other complex cases.
  71     /// This result is intended for use by rustdoc and other such consumers.
  72     ///
  73     /// (Note that due to the coinductive nature of Send, the full and correct result is actually
  74     /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
  75     /// types are all Send. So, in our example, we might have that `Foo<T>: Send` if `Box<T>: Send`.
  76     /// But this is often not the best way to present to the user.)
  77     ///
  78     /// Warning: The API should be considered highly unstable, and it may be refactored or removed
  79     /// in the future.
  80     pub fn find_auto_trait_generics<A>(
  81         &self,
  82         ty: Ty<'tcx>,
  83         orig_env: ty::ParamEnv<'tcx>,
  84         trait_did: DefId,
  85         mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
  86     ) -> AutoTraitResult<A> {
  87         let tcx = self.tcx;
  88
  89         let trait_ref = tcx.mk_trait_ref(trait_did, [ty]);
  90
  91         let trait_pred = ty::Binder::dummy(trait_ref);
  92
  93         let infcx = tcx.infer_ctxt().build();
  94         let mut selcx = SelectionContext::new(&infcx);
  95         for f in [
  96             PolyTraitRef::to_poly_trait_predicate,
  97             PolyTraitRef::to_poly_trait_predicate_negative_polarity,
  98         ] {
  99             let result = selcx.select(&Obligation::new(
 100                 tcx,
 101                 ObligationCause::dummy(),
 102                 orig_env,
 103                 f(&trait_pred),
 104             ));
 105             if let Ok(Some(ImplSource::UserDefined(_))) = result {
 106                 debug!(
 107                     "find_auto_trait_generics({:?}): \
 108                  manual impl found, bailing out",
 109                     trait_ref
 110                 );
 111                 // If an explicit impl exists, it always takes priority over an auto impl
 112                 return AutoTraitResult::ExplicitImpl;
 113             }
 114         }
 115
 116         let infcx = tcx.infer_ctxt().build();
 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 Some((new_env, user_env)) = self.evaluate_predicates(
 153             &infcx,
 154             trait_did,
 155             ty,
 156             orig_env,
 157             orig_env,
 158             &mut fresh_preds,
 159             false,
 160         ) else {
 161             return AutoTraitResult::NegativeImpl;
 162         };
 163
 164         let (full_env, full_user_env) = self
 165             .evaluate_predicates(&infcx, trait_did, ty, new_env, user_env, &mut fresh_preds, true)
 166             .unwrap_or_else(|| {
 167                 panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
 168             });
 169
 170         debug!(
 171             "find_auto_trait_generics({:?}): fulfilling \
 172              with {:?}",
 173             trait_ref, full_env
 174         );
 175         infcx.clear_caches();
 176
 177         // At this point, we already have all of the bounds we need. FulfillmentContext is used
 178         // to store all of the necessary region/lifetime bounds in the InferContext, as well as
 179         // an additional sanity check.
 180         let errors =
 181             super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
 182         if !errors.is_empty() {
 183             panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
 184         }
 185
 186         infcx.process_registered_region_obligations(&Default::default(), full_env);
 187
 188         let region_data =
 189             infcx.inner.borrow_mut().unwrap_region_constraints().region_constraint_data().clone();
 190
 191         let vid_to_region = self.map_vid_to_region(&region_data);
 192
 193         let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
 194
 195         AutoTraitResult::PositiveImpl(auto_trait_callback(info))
 196     }
 197 }
 198
 199 impl<'tcx> AutoTraitFinder<'tcx> {
 200     /// The core logic responsible for computing the bounds for our synthesized impl.
 201     ///
 202     /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
 203     /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
 204     /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
 205     /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
 206     /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
 207     ///
 208     /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
 209     /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
 210     /// user code. According, it considers all possible ways that a `Predicate` could be met, which
 211     /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
 212     /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
 213     /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
 214     /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
 215     /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
 216     /// like this:
 217     /// ```ignore (illustrative)
 218     /// impl<T> Send for Foo<T> where T: IntoIterator
 219     /// ```
 220     /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
 221     /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
 222     ///
 223     /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
 224     /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
 225     /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
 226     /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
 227     /// needs to hold.
 228     ///
 229     /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
 230     /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
 231     /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
 232     /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
 233     /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
 234     /// else `SelectionContext` will choke on the missing predicates. However, this should never
 235     /// show up in the final synthesized generics: we don't want our generated docs page to contain
 236     /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
 237     /// separate `user_env`, which only holds the predicates that will actually be displayed to the
 238     /// user.
 239     fn evaluate_predicates(
 240         &self,
 241         infcx: &InferCtxt<'tcx>,
 242         trait_did: DefId,
 243         ty: Ty<'tcx>,
 244         param_env: ty::ParamEnv<'tcx>,
 245         user_env: ty::ParamEnv<'tcx>,
 246         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 247         only_projections: bool,
 248     ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
 249         let tcx = infcx.tcx;
 250
 251         // Don't try to process any nested obligations involving predicates
 252         // that are already in the `ParamEnv` (modulo regions): we already
 253         // know that they must hold.
 254         for predicate in param_env.caller_bounds() {
 255             fresh_preds.insert(self.clean_pred(infcx, predicate));
 256         }
 257
 258         let mut select = SelectionContext::new(&infcx);
 259
 260         let mut already_visited = FxHashSet::default();
 261         let mut predicates = VecDeque::new();
 262         predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
 263             trait_ref: infcx.tcx.mk_trait_ref(trait_did, [ty]),
 264
 265             constness: ty::BoundConstness::NotConst,
 266             // Auto traits are positive
 267             polarity: ty::ImplPolarity::Positive,
 268         }));
 269
 270         let computed_preds = param_env.caller_bounds().iter();
 271         let mut user_computed_preds: FxIndexSet<_> = user_env.caller_bounds().iter().collect();
 272
 273         let mut new_env = param_env;
 274         let dummy_cause = ObligationCause::dummy();
 275
 276         while let Some(pred) = predicates.pop_front() {
 277             infcx.clear_caches();
 278
 279             if !already_visited.insert(pred) {
 280                 continue;
 281             }
 282
 283             // Call `infcx.resolve_vars_if_possible` to see if we can
 284             // get rid of any inference variables.
 285             let obligation = infcx.resolve_vars_if_possible(Obligation::new(
 286                 tcx,
 287                 dummy_cause.clone(),
 288                 new_env,
 289                 pred,
 290             ));
 291             let result = select.select(&obligation);
 292
 293             match result {
 294                 Ok(Some(ref impl_source)) => {
 295                     // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
 296                     // we immediately bail out, since it's impossible for us to continue.
 297
 298                     if let ImplSource::UserDefined(ImplSourceUserDefinedData {
 299                         impl_def_id, ..
 300                     }) = impl_source
 301                     {
 302                         // Blame 'tidy' for the weird bracket placement.
 303                         if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
 304                             debug!(
 305                                 "evaluate_nested_obligations: found explicit negative impl\
 306                                         {:?}, bailing out",
 307                                 impl_def_id
 308                             );
 309                             return None;
 310                         }
 311                     }
 312
 313                     let obligations = impl_source.borrow_nested_obligations().iter().cloned();
 314
 315                     if !self.evaluate_nested_obligations(
 316                         ty,
 317                         obligations,
 318                         &mut user_computed_preds,
 319                         fresh_preds,
 320                         &mut predicates,
 321                         &mut select,
 322                         only_projections,
 323                     ) {
 324                         return None;
 325                     }
 326                 }
 327                 Ok(None) => {}
 328                 Err(SelectionError::Unimplemented) => {
 329                     if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
 330                         already_visited.remove(&pred);
 331                         self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
 332                         predicates.push_back(pred);
 333                     } else {
 334                         debug!(
 335                             "evaluate_nested_obligations: `Unimplemented` found, bailing: \
 336                              {:?} {:?} {:?}",
 337                             ty,
 338                             pred,
 339                             pred.skip_binder().trait_ref.substs
 340                         );
 341                         return None;
 342                     }
 343                 }
 344                 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
 345             };
 346
 347             let normalized_preds = elaborate_predicates(
 348                 tcx,
 349                 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
 350             )
 351             .map(|o| o.predicate);
 352             new_env = ty::ParamEnv::new(
 353                 tcx.mk_predicates(normalized_preds),
 354                 param_env.reveal(),
 355                 param_env.constness(),
 356             );
 357         }
 358
 359         let final_user_env = ty::ParamEnv::new(
 360             tcx.mk_predicates(user_computed_preds.into_iter()),
 361             user_env.reveal(),
 362             user_env.constness(),
 363         );
 364         debug!(
 365             "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
 366              '{:?}'",
 367             ty, trait_did, new_env, final_user_env
 368         );
 369
 370         Some((new_env, final_user_env))
 371     }
 372
 373     /// This method is designed to work around the following issue:
 374     /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
 375     /// progressively building a `ParamEnv` based on the results we get.
 376     /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
 377     /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
 378     ///
 379     /// This can lead to a corner case when dealing with region parameters.
 380     /// During our selection loop in `evaluate_predicates`, we might end up with
 381     /// two trait predicates that differ only in their region parameters:
 382     /// one containing a HRTB lifetime parameter, and one containing a 'normal'
 383     /// lifetime parameter. For example:
 384     /// ```ignore (illustrative)
 385     /// T as MyTrait<'a>
 386     /// T as MyTrait<'static>
 387     /// ```
 388     /// If we put both of these predicates in our computed `ParamEnv`, we'll
 389     /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
 390     ///
 391     /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
 392     /// Our end goal is to generate a user-visible description of the conditions
 393     /// under which a type implements an auto trait. A trait predicate involving
 394     /// a HRTB means that the type needs to work with any choice of lifetime,
 395     /// not just one specific lifetime (e.g., `'static`).
 396     fn add_user_pred(
 397         &self,
 398         user_computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
 399         new_pred: ty::Predicate<'tcx>,
 400     ) {
 401         let mut should_add_new = true;
 402         user_computed_preds.retain(|&old_pred| {
 403             if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
 404                 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
 405             {
 406                 if new_trait.def_id() == old_trait.def_id() {
 407                     let new_substs = new_trait.trait_ref.substs;
 408                     let old_substs = old_trait.trait_ref.substs;
 409
 410                     if !new_substs.types().eq(old_substs.types()) {
 411                         // We can't compare lifetimes if the types are different,
 412                         // so skip checking `old_pred`.
 413                         return true;
 414                     }
 415
 416                     for (new_region, old_region) in
 417                         iter::zip(new_substs.regions(), old_substs.regions())
 418                     {
 419                         match (*new_region, *old_region) {
 420                             // If both predicates have an `ReLateBound` (a HRTB) in the
 421                             // same spot, we do nothing.
 422                             (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
 423
 424                             (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
 425                                 // One of these is true:
 426                                 // The new predicate has a HRTB in a spot where the old
 427                                 // predicate does not (if they both had a HRTB, the previous
 428                                 // match arm would have executed). A HRBT is a 'stricter'
 429                                 // bound than anything else, so we want to keep the newer
 430                                 // predicate (with the HRBT) in place of the old predicate.
 431                                 //
 432                                 // OR
 433                                 //
 434                                 // The old predicate has a region variable where the new
 435                                 // predicate has some other kind of region. An region
 436                                 // variable isn't something we can actually display to a user,
 437                                 // so we choose their new predicate (which doesn't have a region
 438                                 // variable).
 439                                 //
 440                                 // In both cases, we want to remove the old predicate,
 441                                 // from `user_computed_preds`, and replace it with the new
 442                                 // one. Having both the old and the new
 443                                 // predicate in a `ParamEnv` would confuse `SelectionContext`.
 444                                 //
 445                                 // We're currently in the predicate passed to 'retain',
 446                                 // so we return `false` to remove the old predicate from
 447                                 // `user_computed_preds`.
 448                                 return false;
 449                             }
 450                             (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
 451                                 // This is the opposite situation as the previous arm.
 452                                 // One of these is true:
 453                                 //
 454                                 // The old predicate has a HRTB lifetime in a place where the
 455                                 // new predicate does not.
 456                                 //
 457                                 // OR
 458                                 //
 459                                 // The new predicate has a region variable where the old
 460                                 // predicate has some other type of region.
 461                                 //
 462                                 // We want to leave the old
 463                                 // predicate in `user_computed_preds`, and skip adding
 464                                 // new_pred to `user_computed_params`.
 465                                 should_add_new = false
 466                             }
 467                             _ => {}
 468                         }
 469                     }
 470                 }
 471             }
 472             true
 473         });
 474
 475         if should_add_new {
 476             user_computed_preds.insert(new_pred);
 477         }
 478     }
 479
 480     /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
 481     /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
 482     fn map_vid_to_region<'cx>(
 483         &self,
 484         regions: &RegionConstraintData<'cx>,
 485     ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
 486         let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
 487         let mut finished_map = FxHashMap::default();
 488
 489         for constraint in regions.constraints.keys() {
 490             match constraint {
 491                 &Constraint::VarSubVar(r1, r2) => {
 492                     {
 493                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 494                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 495                     }
 496
 497                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 498                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 499                 }
 500                 &Constraint::RegSubVar(region, vid) => {
 501                     {
 502                         let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
 503                         deps1.larger.insert(RegionTarget::RegionVid(vid));
 504                     }
 505
 506                     let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 507                     deps2.smaller.insert(RegionTarget::Region(region));
 508                 }
 509                 &Constraint::VarSubReg(vid, region) => {
 510                     finished_map.insert(vid, region);
 511                 }
 512                 &Constraint::RegSubReg(r1, r2) => {
 513                     {
 514                         let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
 515                         deps1.larger.insert(RegionTarget::Region(r2));
 516                     }
 517
 518                     let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
 519                     deps2.smaller.insert(RegionTarget::Region(r1));
 520                 }
 521             }
 522         }
 523
 524         while !vid_map.is_empty() {
 525             let target = *vid_map.keys().next().expect("Keys somehow empty");
 526             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 527
 528             for smaller in deps.smaller.iter() {
 529                 for larger in deps.larger.iter() {
 530                     match (smaller, larger) {
 531                         (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
 532                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 533                                 let smaller_deps = v.into_mut();
 534                                 smaller_deps.larger.insert(*larger);
 535                                 smaller_deps.larger.remove(&target);
 536                             }
 537
 538                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 539                                 let larger_deps = v.into_mut();
 540                                 larger_deps.smaller.insert(*smaller);
 541                                 larger_deps.smaller.remove(&target);
 542                             }
 543                         }
 544                         (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
 545                             finished_map.insert(v1, r1);
 546                         }
 547                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 548                             // Do nothing; we don't care about regions that are smaller than vids.
 549                         }
 550                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 551                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 552                                 let smaller_deps = v.into_mut();
 553                                 smaller_deps.larger.insert(*larger);
 554                                 smaller_deps.larger.remove(&target);
 555                             }
 556
 557                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 558                                 let larger_deps = v.into_mut();
 559                                 larger_deps.smaller.insert(*smaller);
 560                                 larger_deps.smaller.remove(&target);
 561                             }
 562                         }
 563                     }
 564                 }
 565             }
 566         }
 567         finished_map
 568     }
 569
 570     fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
 571         self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
 572     }
 573
 574     pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
 575         match ty.kind() {
 576             ty::Param(_) => true,
 577             ty::Projection(p) => self.is_of_param(p.self_ty()),
 578             _ => false,
 579         }
 580     }
 581
 582     fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
 583         if let Some(ty) = p.term().skip_binder().ty() {
 584             matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
 585         } else {
 586             false
 587         }
 588     }
 589
 590     fn evaluate_nested_obligations(
 591         &self,
 592         ty: Ty<'_>,
 593         nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
 594         computed_preds: &mut FxIndexSet<ty::Predicate<'tcx>>,
 595         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 596         predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
 597         select: &mut SelectionContext<'_, 'tcx>,
 598         only_projections: bool,
 599     ) -> bool {
 600         let dummy_cause = ObligationCause::dummy();
 601
 602         for obligation in nested {
 603             let is_new_pred =
 604                 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
 605
 606             // Resolve any inference variables that we can, to help selection succeed
 607             let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
 608
 609             // We only add a predicate as a user-displayable bound if
 610             // it involves a generic parameter, and doesn't contain
 611             // any inference variables.
 612             //
 613             // Displaying a bound involving a concrete type (instead of a generic
 614             // parameter) would be pointless, since it's always true
 615             // (e.g. u8: Copy)
 616             // Displaying an inference variable is impossible, since they're
 617             // an internal compiler detail without a defined visual representation
 618             //
 619             // We check this by calling is_of_param on the relevant types
 620             // from the various possible predicates
 621
 622             let bound_predicate = predicate.kind();
 623             match bound_predicate.skip_binder() {
 624                 ty::PredicateKind::Trait(p) => {
 625                     // Add this to `predicates` so that we end up calling `select`
 626                     // with it. If this predicate ends up being unimplemented,
 627                     // then `evaluate_predicates` will handle adding it the `ParamEnv`
 628                     // if possible.
 629                     predicates.push_back(bound_predicate.rebind(p));
 630                 }
 631                 ty::PredicateKind::Projection(p) => {
 632                     let p = bound_predicate.rebind(p);
 633                     debug!(
 634                         "evaluate_nested_obligations: examining projection predicate {:?}",
 635                         predicate
 636                     );
 637
 638                     // As described above, we only want to display
 639                     // bounds which include a generic parameter but don't include
 640                     // an inference variable.
 641                     // Additionally, we check if we've seen this predicate before,
 642                     // to avoid rendering duplicate bounds to the user.
 643                     if self.is_param_no_infer(p.skip_binder().projection_ty.substs)
 644                         && !p.term().skip_binder().has_infer_types()
 645                         && is_new_pred
 646                     {
 647                         debug!(
 648                             "evaluate_nested_obligations: adding projection predicate \
 649                             to computed_preds: {:?}",
 650                             predicate
 651                         );
 652
 653                         // Under unusual circumstances, we can end up with a self-referential
 654                         // projection predicate. For example:
 655                         // <T as MyType>::Value == <T as MyType>::Value
 656                         // Not only is displaying this to the user pointless,
 657                         // having it in the ParamEnv will cause an issue if we try to call
 658                         // poly_project_and_unify_type on the predicate, since this kind of
 659                         // predicate will normally never end up in a ParamEnv.
 660                         //
 661                         // For these reasons, we ignore these weird predicates,
 662                         // ensuring that we're able to properly synthesize an auto trait impl
 663                         if self.is_self_referential_projection(p) {
 664                             debug!(
 665                                 "evaluate_nested_obligations: encountered a projection
 666                                  predicate equating a type with itself! Skipping"
 667                             );
 668                         } else {
 669                             self.add_user_pred(computed_preds, predicate);
 670                         }
 671                     }
 672
 673                     // There are three possible cases when we project a predicate:
 674                     //
 675                     // 1. We encounter an error. This means that it's impossible for
 676                     // our current type to implement the auto trait - there's bound
 677                     // that we could add to our ParamEnv that would 'fix' this kind
 678                     // of error, as it's not caused by an unimplemented type.
 679                     //
 680                     // 2. We successfully project the predicate (Ok(Some(_))), generating
 681                     //  some subobligations. We then process these subobligations
 682                     //  like any other generated sub-obligations.
 683                     //
 684                     // 3. We receive an 'ambiguous' result (Ok(None))
 685                     // If we were actually trying to compile a crate,
 686                     // we would need to re-process this obligation later.
 687                     // However, all we care about is finding out what bounds
 688                     // are needed for our type to implement a particular auto trait.
 689                     // We've already added this obligation to our computed ParamEnv
 690                     // above (if it was necessary). Therefore, we don't need
 691                     // to do any further processing of the obligation.
 692                     //
 693                     // Note that we *must* try to project *all* projection predicates
 694                     // we encounter, even ones without inference variable.
 695                     // This ensures that we detect any projection errors,
 696                     // which indicate that our type can *never* implement the given
 697                     // auto trait. In that case, we will generate an explicit negative
 698                     // impl (e.g. 'impl !Send for MyType'). However, we don't
 699                     // try to process any of the generated subobligations -
 700                     // they contain no new information, since we already know
 701                     // that our type implements the projected-through trait,
 702                     // and can lead to weird region issues.
 703                     //
 704                     // Normally, we'll generate a negative impl as a result of encountering
 705                     // a type with an explicit negative impl of an auto trait
 706                     // (for example, raw pointers have !Send and !Sync impls)
 707                     // However, through some **interesting** manipulations of the type
 708                     // system, it's actually possible to write a type that never
 709                     // implements an auto trait due to a projection error, not a normal
 710                     // negative impl error. To properly handle this case, we need
 711                     // to ensure that we catch any potential projection errors,
 712                     // and turn them into an explicit negative impl for our type.
 713                     debug!("Projecting and unifying projection predicate {:?}", predicate);
 714
 715                     match project::poly_project_and_unify_type(
 716                         select,
 717                         &obligation.with(self.tcx, p),
 718                     ) {
 719                         ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 720                             debug!(
 721                                 "evaluate_nested_obligations: Unable to unify predicate \
 722                                  '{:?}' '{:?}', bailing out",
 723                                 ty, e
 724                             );
 725                             return false;
 726                         }
 727                         ProjectAndUnifyResult::Recursive => {
 728                             debug!("evaluate_nested_obligations: recursive projection predicate");
 729                             return false;
 730                         }
 731                         ProjectAndUnifyResult::Holds(v) => {
 732                             // We only care about sub-obligations
 733                             // when we started out trying to unify
 734                             // some inference variables. See the comment above
 735                             // for more information
 736                             if p.term().skip_binder().has_infer_types() {
 737                                 if !self.evaluate_nested_obligations(
 738                                     ty,
 739                                     v.into_iter(),
 740                                     computed_preds,
 741                                     fresh_preds,
 742                                     predicates,
 743                                     select,
 744                                     only_projections,
 745                                 ) {
 746                                     return false;
 747                                 }
 748                             }
 749                         }
 750                         ProjectAndUnifyResult::FailedNormalization => {
 751                             // It's ok not to make progress when have no inference variables -
 752                             // in that case, we were only performing unification to check if an
 753                             // error occurred (which would indicate that it's impossible for our
 754                             // type to implement the auto trait).
 755                             // However, we should always make progress (either by generating
 756                             // subobligations or getting an error) when we started off with
 757                             // inference variables
 758                             if p.term().skip_binder().has_infer_types() {
 759                                 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
 760                             }
 761                         }
 762                     }
 763                 }
 764                 ty::PredicateKind::RegionOutlives(binder) => {
 765                     let binder = bound_predicate.rebind(binder);
 766                     select.infcx().region_outlives_predicate(&dummy_cause, binder)
 767                 }
 768                 ty::PredicateKind::TypeOutlives(binder) => {
 769                     let binder = bound_predicate.rebind(binder);
 770                     match (
 771                         binder.no_bound_vars(),
 772                         binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
 773                     ) {
 774                         (None, Some(t_a)) => {
 775                             select.infcx().register_region_obligation_with_cause(
 776                                 t_a,
 777                                 select.infcx().tcx.lifetimes.re_static,
 778                                 &dummy_cause,
 779                             );
 780                         }
 781                         (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
 782                             select.infcx().register_region_obligation_with_cause(
 783                                 t_a,
 784                                 r_b,
 785                                 &dummy_cause,
 786                             );
 787                         }
 788                         _ => {}
 789                     };
 790                 }
 791                 ty::PredicateKind::ConstEquate(c1, c2) => {
 792                     let evaluate = |c: ty::Const<'tcx>| {
 793                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 794                             match select.infcx().const_eval_resolve(
 795                                 obligation.param_env,
 796                                 unevaluated,
 797                                 Some(obligation.cause.span),
 798                             ) {
 799                                 Ok(Some(valtree)) => {
 800                                     Ok(ty::Const::from_value(select.tcx(), valtree, c.ty()))
 801                                 }
 802                                 Ok(None) => {
 803                                     let tcx = self.tcx;
 804                                     let def_id = unevaluated.def.did;
 805                                     let reported =
 806                                         tcx.sess.emit_err(UnableToConstructConstantValue {
 807                                             span: tcx.def_span(def_id),
 808                                             unevaluated: unevaluated,
 809                                         });
 810                                     Err(ErrorHandled::Reported(reported))
 811                                 }
 812                                 Err(err) => Err(err),
 813                             }
 814                         } else {
 815                             Ok(c)
 816                         }
 817                     };
 818
 819                     match (evaluate(c1), evaluate(c2)) {
 820                         (Ok(c1), Ok(c2)) => {
 821                             match select
 822                                 .infcx()
 823                                 .at(&obligation.cause, obligation.param_env)
 824                                 .eq(c1, c2)
 825                             {
 826                                 Ok(_) => (),
 827                                 Err(_) => return false,
 828                             }
 829                         }
 830                         _ => return false,
 831                     }
 832                 }
 833                 // There's not really much we can do with these predicates -
 834                 // we start out with a `ParamEnv` with no inference variables,
 835                 // and these don't correspond to adding any new bounds to
 836                 // the `ParamEnv`.
 837                 ty::PredicateKind::WellFormed(..)
 838                 | ty::PredicateKind::ObjectSafe(..)
 839                 | ty::PredicateKind::ClosureKind(..)
 840                 | ty::PredicateKind::Subtype(..)
 841                 | ty::PredicateKind::ConstEvaluatable(..)
 842                 | ty::PredicateKind::Coerce(..)
 843                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
 844             };
 845         }
 846         true
 847     }
 848
 849     pub fn clean_pred(
 850         &self,
 851         infcx: &InferCtxt<'tcx>,
 852         p: ty::Predicate<'tcx>,
 853     ) -> ty::Predicate<'tcx> {
 854         infcx.freshen(p)
 855     }
 856 }
 857
 858 // Replaces all ReVars in a type with ty::Region's, using the provided map
 859 pub struct RegionReplacer<'a, 'tcx> {
 860     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 861     tcx: TyCtxt<'tcx>,
 862 }
 863
 864 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 865     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 866         self.tcx
 867     }
 868
 869     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 870         (match *r {
 871             ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 872             _ => None,
 873         })
 874         .unwrap_or_else(|| r.super_fold_with(self))
 875     }
 876 }