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::infer::region_constraints::{Constraint, RegionConstraintData};
   7 use crate::infer::InferCtxt;
   8 use crate::traits::project::ProjectAndUnifyResult;
   9 use rustc_middle::mir::interpret::ErrorHandled;
  10 use rustc_middle::ty::fold::{TypeFolder, TypeSuperFoldable};
  11 use rustc_middle::ty::visit::TypeVisitable;
  12 use rustc_middle::ty::{Region, RegionVid, Term};
  13
  14 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  15
  16 use std::collections::hash_map::Entry;
  17 use std::collections::VecDeque;
  18 use std::iter;
  19
  20 // FIXME(twk): this is obviously not nice to duplicate like that
  21 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
  22 pub enum RegionTarget<'tcx> {
  23     Region(Region<'tcx>),
  24     RegionVid(RegionVid),
  25 }
  26
  27 #[derive(Default, Debug, Clone)]
  28 pub struct RegionDeps<'tcx> {
  29     larger: FxHashSet<RegionTarget<'tcx>>,
  30     smaller: FxHashSet<RegionTarget<'tcx>>,
  31 }
  32
  33 pub enum AutoTraitResult<A> {
  34     ExplicitImpl,
  35     PositiveImpl(A),
  36     NegativeImpl,
  37 }
  38
  39 #[allow(dead_code)]
  40 impl<A> AutoTraitResult<A> {
  41     fn is_auto(&self) -> bool {
  42         matches!(self, AutoTraitResult::PositiveImpl(_) | AutoTraitResult::NegativeImpl)
  43     }
  44 }
  45
  46 pub struct AutoTraitInfo<'cx> {
  47     pub full_user_env: ty::ParamEnv<'cx>,
  48     pub region_data: RegionConstraintData<'cx>,
  49     pub vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'cx>>,
  50 }
  51
  52 pub struct AutoTraitFinder<'tcx> {
  53     tcx: TyCtxt<'tcx>,
  54 }
  55
  56 impl<'tcx> AutoTraitFinder<'tcx> {
  57     pub fn new(tcx: TyCtxt<'tcx>) -> Self {
  58         AutoTraitFinder { tcx }
  59     }
  60
  61     /// Makes a best effort to determine whether and under which conditions an auto trait is
  62     /// implemented for a type. For example, if you have
  63     ///
  64     /// ```
  65     /// struct Foo<T> { data: Box<T> }
  66     /// ```
  67     ///
  68     /// then this might return that Foo<T>: Send if T: Send (encoded in the AutoTraitResult type).
  69     /// The analysis attempts to account for custom impls as well as other complex cases. This
  70     /// result is intended for use by rustdoc and other such consumers.
  71     ///
  72     /// (Note that due to the coinductive nature of Send, the full and correct result is actually
  73     /// quite simple to generate. That is, when a type has no custom impl, it is Send iff its field
  74     /// types are all Send. So, in our example, we might have that Foo<T>: Send if Box<T>: Send.
  75     /// But this is often not the best way to present to the user.)
  76     ///
  77     /// Warning: The API should be considered highly unstable, and it may be refactored or removed
  78     /// in the future.
  79     pub fn find_auto_trait_generics<A>(
  80         &self,
  81         ty: Ty<'tcx>,
  82         orig_env: ty::ParamEnv<'tcx>,
  83         trait_did: DefId,
  84         mut auto_trait_callback: impl FnMut(AutoTraitInfo<'tcx>) -> A,
  85     ) -> AutoTraitResult<A> {
  86         let tcx = self.tcx;
  87
  88         let trait_ref = ty::TraitRef { def_id: trait_did, substs: tcx.mk_substs_trait(ty, &[]) };
  89
  90         let trait_pred = ty::Binder::dummy(trait_ref);
  91
  92         let bail_out = tcx.infer_ctxt().enter(|infcx| {
  93             let mut selcx = SelectionContext::new(&infcx);
  94             let result = selcx.select(&Obligation::new(
  95                 ObligationCause::dummy(),
  96                 orig_env,
  97                 trait_pred.to_poly_trait_predicate(),
  98             ));
  99
 100             match result {
 101                 Ok(Some(ImplSource::UserDefined(_))) => {
 102                     debug!(
 103                         "find_auto_trait_generics({:?}): \
 104                          manual impl found, bailing out",
 105                         trait_ref
 106                     );
 107                     return true;
 108                 }
 109                 _ => {}
 110             }
 111
 112             let result = selcx.select(&Obligation::new(
 113                 ObligationCause::dummy(),
 114                 orig_env,
 115                 trait_pred.to_poly_trait_predicate_negative_polarity(),
 116             ));
 117
 118             match result {
 119                 Ok(Some(ImplSource::UserDefined(_))) => {
 120                     debug!(
 121                         "find_auto_trait_generics({:?}): \
 122                          manual impl found, bailing out",
 123                         trait_ref
 124                     );
 125                     true
 126                 }
 127                 _ => false,
 128             }
 129         });
 130
 131         // If an explicit impl exists, it always takes priority over an auto impl
 132         if bail_out {
 133             return AutoTraitResult::ExplicitImpl;
 134         }
 135
 136         tcx.infer_ctxt().enter(|infcx| {
 137             let mut fresh_preds = FxHashSet::default();
 138
 139             // Due to the way projections are handled by SelectionContext, we need to run
 140             // evaluate_predicates twice: once on the original param env, and once on the result of
 141             // the first evaluate_predicates call.
 142             //
 143             // The problem is this: most of rustc, including SelectionContext and traits::project,
 144             // are designed to work with a concrete usage of a type (e.g., Vec<u8>
 145             // fn<T>() { Vec<T> }. This information will generally never change - given
 146             // the 'T' in fn<T>() { ... }, we'll never know anything else about 'T'.
 147             // If we're unable to prove that 'T' implements a particular trait, we're done -
 148             // there's nothing left to do but error out.
 149             //
 150             // However, synthesizing an auto trait impl works differently. Here, we start out with
 151             // a set of initial conditions - the ParamEnv of the struct/enum/union we're dealing
 152             // with - and progressively discover the conditions we need to fulfill for it to
 153             // implement a certain auto trait. This ends up breaking two assumptions made by trait
 154             // selection and projection:
 155             //
 156             // * We can always cache the result of a particular trait selection for the lifetime of
 157             // an InfCtxt
 158             // * Given a projection bound such as '<T as SomeTrait>::SomeItem = K', if 'T:
 159             // SomeTrait' doesn't hold, then we don't need to care about the 'SomeItem = K'
 160             //
 161             // We fix the first assumption by manually clearing out all of the InferCtxt's caches
 162             // in between calls to SelectionContext.select. This allows us to keep all of the
 163             // intermediate types we create bound to the 'tcx lifetime, rather than needing to lift
 164             // them between calls.
 165             //
 166             // We fix the second assumption by reprocessing the result of our first call to
 167             // evaluate_predicates. Using the example of '<T as SomeTrait>::SomeItem = K', our first
 168             // pass will pick up 'T: SomeTrait', but not 'SomeItem = K'. On our second pass,
 169             // traits::project will see that 'T: SomeTrait' is in our ParamEnv, allowing
 170             // SelectionContext to return it back to us.
 171
 172             let Some((new_env, user_env)) = self.evaluate_predicates(
 173                 &infcx,
 174                 trait_did,
 175                 ty,
 176                 orig_env,
 177                 orig_env,
 178                 &mut fresh_preds,
 179                 false,
 180             ) else {
 181                 return AutoTraitResult::NegativeImpl;
 182             };
 183
 184             let (full_env, full_user_env) = self
 185                 .evaluate_predicates(
 186                     &infcx,
 187                     trait_did,
 188                     ty,
 189                     new_env,
 190                     user_env,
 191                     &mut fresh_preds,
 192                     true,
 193                 )
 194                 .unwrap_or_else(|| {
 195                     panic!("Failed to fully process: {:?} {:?} {:?}", ty, trait_did, orig_env)
 196                 });
 197
 198             debug!(
 199                 "find_auto_trait_generics({:?}): fulfilling \
 200                  with {:?}",
 201                 trait_ref, full_env
 202             );
 203             infcx.clear_caches();
 204
 205             // At this point, we already have all of the bounds we need. FulfillmentContext is used
 206             // to store all of the necessary region/lifetime bounds in the InferContext, as well as
 207             // an additional sanity check.
 208             let errors =
 209                 super::fully_solve_bound(&infcx, ObligationCause::dummy(), full_env, ty, trait_did);
 210             if !errors.is_empty() {
 211                 panic!("Unable to fulfill trait {:?} for '{:?}': {:?}", trait_did, ty, errors);
 212             }
 213
 214             infcx.process_registered_region_obligations(&Default::default(), full_env);
 215
 216             let region_data = infcx
 217                 .inner
 218                 .borrow_mut()
 219                 .unwrap_region_constraints()
 220                 .region_constraint_data()
 221                 .clone();
 222
 223             let vid_to_region = self.map_vid_to_region(&region_data);
 224
 225             let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
 226
 227             AutoTraitResult::PositiveImpl(auto_trait_callback(info))
 228         })
 229     }
 230 }
 231
 232 impl<'tcx> AutoTraitFinder<'tcx> {
 233     /// The core logic responsible for computing the bounds for our synthesized impl.
 234     ///
 235     /// To calculate the bounds, we call `SelectionContext.select` in a loop. Like
 236     /// `FulfillmentContext`, we recursively select the nested obligations of predicates we
 237     /// encounter. However, whenever we encounter an `UnimplementedError` involving a type
 238     /// parameter, we add it to our `ParamEnv`. Since our goal is to determine when a particular
 239     /// type implements an auto trait, Unimplemented errors tell us what conditions need to be met.
 240     ///
 241     /// This method ends up working somewhat similarly to `FulfillmentContext`, but with a few key
 242     /// differences. `FulfillmentContext` works under the assumption that it's dealing with concrete
 243     /// user code. According, it considers all possible ways that a `Predicate` could be met, which
 244     /// isn't always what we want for a synthesized impl. For example, given the predicate `T:
 245     /// Iterator`, `FulfillmentContext` can end up reporting an Unimplemented error for `T:
 246     /// IntoIterator` -- since there's an implementation of `Iterator` where `T: IntoIterator`,
 247     /// `FulfillmentContext` will drive `SelectionContext` to consider that impl before giving up.
 248     /// If we were to rely on `FulfillmentContext`s decision, we might end up synthesizing an impl
 249     /// like this:
 250     /// ```ignore (illustrative)
 251     /// impl<T> Send for Foo<T> where T: IntoIterator
 252     /// ```
 253     /// While it might be technically true that Foo implements Send where `T: IntoIterator`,
 254     /// the bound is overly restrictive - it's really only necessary that `T: Iterator`.
 255     ///
 256     /// For this reason, `evaluate_predicates` handles predicates with type variables specially.
 257     /// When we encounter an `Unimplemented` error for a bound such as `T: Iterator`, we immediately
 258     /// add it to our `ParamEnv`, and add it to our stack for recursive evaluation. When we later
 259     /// select it, we'll pick up any nested bounds, without ever inferring that `T: IntoIterator`
 260     /// needs to hold.
 261     ///
 262     /// One additional consideration is supertrait bounds. Normally, a `ParamEnv` is only ever
 263     /// constructed once for a given type. As part of the construction process, the `ParamEnv` will
 264     /// have any supertrait bounds normalized -- e.g., if we have a type `struct Foo<T: Copy>`, the
 265     /// `ParamEnv` will contain `T: Copy` and `T: Clone`, since `Copy: Clone`. When we construct our
 266     /// own `ParamEnv`, we need to do this ourselves, through `traits::elaborate_predicates`, or
 267     /// else `SelectionContext` will choke on the missing predicates. However, this should never
 268     /// show up in the final synthesized generics: we don't want our generated docs page to contain
 269     /// something like `T: Copy + Clone`, as that's redundant. Therefore, we keep track of a
 270     /// separate `user_env`, which only holds the predicates that will actually be displayed to the
 271     /// user.
 272     fn evaluate_predicates(
 273         &self,
 274         infcx: &InferCtxt<'_, 'tcx>,
 275         trait_did: DefId,
 276         ty: Ty<'tcx>,
 277         param_env: ty::ParamEnv<'tcx>,
 278         user_env: ty::ParamEnv<'tcx>,
 279         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 280         only_projections: bool,
 281     ) -> Option<(ty::ParamEnv<'tcx>, ty::ParamEnv<'tcx>)> {
 282         let tcx = infcx.tcx;
 283
 284         // Don't try to process any nested obligations involving predicates
 285         // that are already in the `ParamEnv` (modulo regions): we already
 286         // know that they must hold.
 287         for predicate in param_env.caller_bounds() {
 288             fresh_preds.insert(self.clean_pred(infcx, predicate));
 289         }
 290
 291         let mut select = SelectionContext::new(&infcx);
 292
 293         let mut already_visited = FxHashSet::default();
 294         let mut predicates = VecDeque::new();
 295         predicates.push_back(ty::Binder::dummy(ty::TraitPredicate {
 296             trait_ref: ty::TraitRef {
 297                 def_id: trait_did,
 298                 substs: infcx.tcx.mk_substs_trait(ty, &[]),
 299             },
 300             constness: ty::BoundConstness::NotConst,
 301             // Auto traits are positive
 302             polarity: ty::ImplPolarity::Positive,
 303         }));
 304
 305         let computed_preds = param_env.caller_bounds().iter();
 306         let mut user_computed_preds: FxHashSet<_> = user_env.caller_bounds().iter().collect();
 307
 308         let mut new_env = param_env;
 309         let dummy_cause = ObligationCause::dummy();
 310
 311         while let Some(pred) = predicates.pop_front() {
 312             infcx.clear_caches();
 313
 314             if !already_visited.insert(pred) {
 315                 continue;
 316             }
 317
 318             // Call `infcx.resolve_vars_if_possible` to see if we can
 319             // get rid of any inference variables.
 320             let obligation =
 321                 infcx.resolve_vars_if_possible(Obligation::new(dummy_cause.clone(), new_env, pred));
 322             let result = select.select(&obligation);
 323
 324             match result {
 325                 Ok(Some(ref impl_source)) => {
 326                     // If we see an explicit negative impl (e.g., `impl !Send for MyStruct`),
 327                     // we immediately bail out, since it's impossible for us to continue.
 328
 329                     if let ImplSource::UserDefined(ImplSourceUserDefinedData {
 330                         impl_def_id, ..
 331                     }) = impl_source
 332                     {
 333                         // Blame 'tidy' for the weird bracket placement.
 334                         if infcx.tcx.impl_polarity(*impl_def_id) == ty::ImplPolarity::Negative {
 335                             debug!(
 336                                 "evaluate_nested_obligations: found explicit negative impl\
 337                                         {:?}, bailing out",
 338                                 impl_def_id
 339                             );
 340                             return None;
 341                         }
 342                     }
 343
 344                     let obligations = impl_source.clone().nested_obligations().into_iter();
 345
 346                     if !self.evaluate_nested_obligations(
 347                         ty,
 348                         obligations,
 349                         &mut user_computed_preds,
 350                         fresh_preds,
 351                         &mut predicates,
 352                         &mut select,
 353                         only_projections,
 354                     ) {
 355                         return None;
 356                     }
 357                 }
 358                 Ok(None) => {}
 359                 Err(SelectionError::Unimplemented) => {
 360                     if self.is_param_no_infer(pred.skip_binder().trait_ref.substs) {
 361                         already_visited.remove(&pred);
 362                         self.add_user_pred(&mut user_computed_preds, pred.to_predicate(self.tcx));
 363                         predicates.push_back(pred);
 364                     } else {
 365                         debug!(
 366                             "evaluate_nested_obligations: `Unimplemented` found, bailing: \
 367                              {:?} {:?} {:?}",
 368                             ty,
 369                             pred,
 370                             pred.skip_binder().trait_ref.substs
 371                         );
 372                         return None;
 373                     }
 374                 }
 375                 _ => panic!("Unexpected error for '{:?}': {:?}", ty, result),
 376             };
 377
 378             let normalized_preds = elaborate_predicates(
 379                 tcx,
 380                 computed_preds.clone().chain(user_computed_preds.iter().cloned()),
 381             )
 382             .map(|o| o.predicate);
 383             new_env = ty::ParamEnv::new(
 384                 tcx.mk_predicates(normalized_preds),
 385                 param_env.reveal(),
 386                 param_env.constness(),
 387             );
 388         }
 389
 390         let final_user_env = ty::ParamEnv::new(
 391             tcx.mk_predicates(user_computed_preds.into_iter()),
 392             user_env.reveal(),
 393             user_env.constness(),
 394         );
 395         debug!(
 396             "evaluate_nested_obligations(ty={:?}, trait_did={:?}): succeeded with '{:?}' \
 397              '{:?}'",
 398             ty, trait_did, new_env, final_user_env
 399         );
 400
 401         Some((new_env, final_user_env))
 402     }
 403
 404     /// This method is designed to work around the following issue:
 405     /// When we compute auto trait bounds, we repeatedly call `SelectionContext.select`,
 406     /// progressively building a `ParamEnv` based on the results we get.
 407     /// However, our usage of `SelectionContext` differs from its normal use within the compiler,
 408     /// in that we capture and re-reprocess predicates from `Unimplemented` errors.
 409     ///
 410     /// This can lead to a corner case when dealing with region parameters.
 411     /// During our selection loop in `evaluate_predicates`, we might end up with
 412     /// two trait predicates that differ only in their region parameters:
 413     /// one containing a HRTB lifetime parameter, and one containing a 'normal'
 414     /// lifetime parameter. For example:
 415     /// ```ignore (illustrative)
 416     /// T as MyTrait<'a>
 417     /// T as MyTrait<'static>
 418     /// ```
 419     /// If we put both of these predicates in our computed `ParamEnv`, we'll
 420     /// confuse `SelectionContext`, since it will (correctly) view both as being applicable.
 421     ///
 422     /// To solve this, we pick the 'more strict' lifetime bound -- i.e., the HRTB
 423     /// Our end goal is to generate a user-visible description of the conditions
 424     /// under which a type implements an auto trait. A trait predicate involving
 425     /// a HRTB means that the type needs to work with any choice of lifetime,
 426     /// not just one specific lifetime (e.g., `'static`).
 427     fn add_user_pred(
 428         &self,
 429         user_computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 430         new_pred: ty::Predicate<'tcx>,
 431     ) {
 432         let mut should_add_new = true;
 433         user_computed_preds.retain(|&old_pred| {
 434             if let (ty::PredicateKind::Trait(new_trait), ty::PredicateKind::Trait(old_trait)) =
 435                 (new_pred.kind().skip_binder(), old_pred.kind().skip_binder())
 436             {
 437                 if new_trait.def_id() == old_trait.def_id() {
 438                     let new_substs = new_trait.trait_ref.substs;
 439                     let old_substs = old_trait.trait_ref.substs;
 440
 441                     if !new_substs.types().eq(old_substs.types()) {
 442                         // We can't compare lifetimes if the types are different,
 443                         // so skip checking `old_pred`.
 444                         return true;
 445                     }
 446
 447                     for (new_region, old_region) in
 448                         iter::zip(new_substs.regions(), old_substs.regions())
 449                     {
 450                         match (*new_region, *old_region) {
 451                             // If both predicates have an `ReLateBound` (a HRTB) in the
 452                             // same spot, we do nothing.
 453                             (ty::ReLateBound(_, _), ty::ReLateBound(_, _)) => {}
 454
 455                             (ty::ReLateBound(_, _), _) | (_, ty::ReVar(_)) => {
 456                                 // One of these is true:
 457                                 // The new predicate has a HRTB in a spot where the old
 458                                 // predicate does not (if they both had a HRTB, the previous
 459                                 // match arm would have executed). A HRBT is a 'stricter'
 460                                 // bound than anything else, so we want to keep the newer
 461                                 // predicate (with the HRBT) in place of the old predicate.
 462                                 //
 463                                 // OR
 464                                 //
 465                                 // The old predicate has a region variable where the new
 466                                 // predicate has some other kind of region. An region
 467                                 // variable isn't something we can actually display to a user,
 468                                 // so we choose their new predicate (which doesn't have a region
 469                                 // variable).
 470                                 //
 471                                 // In both cases, we want to remove the old predicate,
 472                                 // from `user_computed_preds`, and replace it with the new
 473                                 // one. Having both the old and the new
 474                                 // predicate in a `ParamEnv` would confuse `SelectionContext`.
 475                                 //
 476                                 // We're currently in the predicate passed to 'retain',
 477                                 // so we return `false` to remove the old predicate from
 478                                 // `user_computed_preds`.
 479                                 return false;
 480                             }
 481                             (_, ty::ReLateBound(_, _)) | (ty::ReVar(_), _) => {
 482                                 // This is the opposite situation as the previous arm.
 483                                 // One of these is true:
 484                                 //
 485                                 // The old predicate has a HRTB lifetime in a place where the
 486                                 // new predicate does not.
 487                                 //
 488                                 // OR
 489                                 //
 490                                 // The new predicate has a region variable where the old
 491                                 // predicate has some other type of region.
 492                                 //
 493                                 // We want to leave the old
 494                                 // predicate in `user_computed_preds`, and skip adding
 495                                 // new_pred to `user_computed_params`.
 496                                 should_add_new = false
 497                             }
 498                             _ => {}
 499                         }
 500                     }
 501                 }
 502             }
 503             true
 504         });
 505
 506         if should_add_new {
 507             user_computed_preds.insert(new_pred);
 508         }
 509     }
 510
 511     /// This is very similar to `handle_lifetimes`. However, instead of matching `ty::Region`s
 512     /// to each other, we match `ty::RegionVid`s to `ty::Region`s.
 513     fn map_vid_to_region<'cx>(
 514         &self,
 515         regions: &RegionConstraintData<'cx>,
 516     ) -> FxHashMap<ty::RegionVid, ty::Region<'cx>> {
 517         let mut vid_map: FxHashMap<RegionTarget<'cx>, RegionDeps<'cx>> = FxHashMap::default();
 518         let mut finished_map = FxHashMap::default();
 519
 520         for constraint in regions.constraints.keys() {
 521             match constraint {
 522                 &Constraint::VarSubVar(r1, r2) => {
 523                     {
 524                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 525                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 526                     }
 527
 528                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 529                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 530                 }
 531                 &Constraint::RegSubVar(region, vid) => {
 532                     {
 533                         let deps1 = vid_map.entry(RegionTarget::Region(region)).or_default();
 534                         deps1.larger.insert(RegionTarget::RegionVid(vid));
 535                     }
 536
 537                     let deps2 = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 538                     deps2.smaller.insert(RegionTarget::Region(region));
 539                 }
 540                 &Constraint::VarSubReg(vid, region) => {
 541                     finished_map.insert(vid, region);
 542                 }
 543                 &Constraint::RegSubReg(r1, r2) => {
 544                     {
 545                         let deps1 = vid_map.entry(RegionTarget::Region(r1)).or_default();
 546                         deps1.larger.insert(RegionTarget::Region(r2));
 547                     }
 548
 549                     let deps2 = vid_map.entry(RegionTarget::Region(r2)).or_default();
 550                     deps2.smaller.insert(RegionTarget::Region(r1));
 551                 }
 552             }
 553         }
 554
 555         while !vid_map.is_empty() {
 556             let target = *vid_map.keys().next().expect("Keys somehow empty");
 557             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 558
 559             for smaller in deps.smaller.iter() {
 560                 for larger in deps.larger.iter() {
 561                     match (smaller, larger) {
 562                         (&RegionTarget::Region(_), &RegionTarget::Region(_)) => {
 563                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 564                                 let smaller_deps = v.into_mut();
 565                                 smaller_deps.larger.insert(*larger);
 566                                 smaller_deps.larger.remove(&target);
 567                             }
 568
 569                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 570                                 let larger_deps = v.into_mut();
 571                                 larger_deps.smaller.insert(*smaller);
 572                                 larger_deps.smaller.remove(&target);
 573                             }
 574                         }
 575                         (&RegionTarget::RegionVid(v1), &RegionTarget::Region(r1)) => {
 576                             finished_map.insert(v1, r1);
 577                         }
 578                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 579                             // Do nothing; we don't care about regions that are smaller than vids.
 580                         }
 581                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 582                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 583                                 let smaller_deps = v.into_mut();
 584                                 smaller_deps.larger.insert(*larger);
 585                                 smaller_deps.larger.remove(&target);
 586                             }
 587
 588                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 589                                 let larger_deps = v.into_mut();
 590                                 larger_deps.smaller.insert(*smaller);
 591                                 larger_deps.smaller.remove(&target);
 592                             }
 593                         }
 594                     }
 595                 }
 596             }
 597         }
 598         finished_map
 599     }
 600
 601     fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
 602         self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
 603     }
 604
 605     pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
 606         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         if let Term::Ty(ty) = p.term().skip_binder() {
 615             matches!(ty.kind(), ty::Projection(proj) if proj == &p.skip_binder().projection_ty)
 616         } else {
 617             false
 618         }
 619     }
 620
 621     fn evaluate_nested_obligations(
 622         &self,
 623         ty: Ty<'_>,
 624         nested: impl Iterator<Item = Obligation<'tcx, ty::Predicate<'tcx>>>,
 625         computed_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 626         fresh_preds: &mut FxHashSet<ty::Predicate<'tcx>>,
 627         predicates: &mut VecDeque<ty::PolyTraitPredicate<'tcx>>,
 628         select: &mut SelectionContext<'_, 'tcx>,
 629         only_projections: bool,
 630     ) -> bool {
 631         let dummy_cause = ObligationCause::dummy();
 632
 633         for obligation in nested {
 634             let is_new_pred =
 635                 fresh_preds.insert(self.clean_pred(select.infcx(), obligation.predicate));
 636
 637             // Resolve any inference variables that we can, to help selection succeed
 638             let predicate = select.infcx().resolve_vars_if_possible(obligation.predicate);
 639
 640             // We only add a predicate as a user-displayable bound if
 641             // it involves a generic parameter, and doesn't contain
 642             // any inference variables.
 643             //
 644             // Displaying a bound involving a concrete type (instead of a generic
 645             // parameter) would be pointless, since it's always true
 646             // (e.g. u8: Copy)
 647             // Displaying an inference variable is impossible, since they're
 648             // an internal compiler detail without a defined visual representation
 649             //
 650             // We check this by calling is_of_param on the relevant types
 651             // from the various possible predicates
 652
 653             let bound_predicate = predicate.kind();
 654             match bound_predicate.skip_binder() {
 655                 ty::PredicateKind::Trait(p) => {
 656                     // Add this to `predicates` so that we end up calling `select`
 657                     // with it. If this predicate ends up being unimplemented,
 658                     // then `evaluate_predicates` will handle adding it the `ParamEnv`
 659                     // if possible.
 660                     predicates.push_back(bound_predicate.rebind(p));
 661                 }
 662                 ty::PredicateKind::Projection(p) => {
 663                     let p = bound_predicate.rebind(p);
 664                     debug!(
 665                         "evaluate_nested_obligations: examining projection predicate {:?}",
 666                         predicate
 667                     );
 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.term().skip_binder().has_infer_types()
 676                         && is_new_pred
 677                     {
 678                         debug!(
 679                             "evaluate_nested_obligations: adding projection predicate \
 680                             to computed_preds: {:?}",
 681                             predicate
 682                         );
 683
 684                         // Under unusual circumstances, we can end up with a self-referential
 685                         // projection predicate. For example:
 686                         // <T as MyType>::Value == <T as MyType>::Value
 687                         // Not only is displaying this to the user pointless,
 688                         // having it in the ParamEnv will cause an issue if we try to call
 689                         // poly_project_and_unify_type on the predicate, since this kind of
 690                         // predicate will normally never end up in a ParamEnv.
 691                         //
 692                         // For these reasons, we ignore these weird predicates,
 693                         // ensuring that we're able to properly synthesize an auto trait impl
 694                         if self.is_self_referential_projection(p) {
 695                             debug!(
 696                                 "evaluate_nested_obligations: encountered a projection
 697                                  predicate equating a type with itself! Skipping"
 698                             );
 699                         } else {
 700                             self.add_user_pred(computed_preds, predicate);
 701                         }
 702                     }
 703
 704                     // There are three possible cases when we project a predicate:
 705                     //
 706                     // 1. We encounter an error. This means that it's impossible for
 707                     // our current type to implement the auto trait - there's bound
 708                     // that we could add to our ParamEnv that would 'fix' this kind
 709                     // of error, as it's not caused by an unimplemented type.
 710                     //
 711                     // 2. We successfully project the predicate (Ok(Some(_))), generating
 712                     //  some subobligations. We then process these subobligations
 713                     //  like any other generated sub-obligations.
 714                     //
 715                     // 3. We receive an 'ambiguous' result (Ok(None))
 716                     // If we were actually trying to compile a crate,
 717                     // we would need to re-process this obligation later.
 718                     // However, all we care about is finding out what bounds
 719                     // are needed for our type to implement a particular auto trait.
 720                     // We've already added this obligation to our computed ParamEnv
 721                     // above (if it was necessary). Therefore, we don't need
 722                     // to do any further processing of the obligation.
 723                     //
 724                     // Note that we *must* try to project *all* projection predicates
 725                     // we encounter, even ones without inference variable.
 726                     // This ensures that we detect any projection errors,
 727                     // which indicate that our type can *never* implement the given
 728                     // auto trait. In that case, we will generate an explicit negative
 729                     // impl (e.g. 'impl !Send for MyType'). However, we don't
 730                     // try to process any of the generated subobligations -
 731                     // they contain no new information, since we already know
 732                     // that our type implements the projected-through trait,
 733                     // and can lead to weird region issues.
 734                     //
 735                     // Normally, we'll generate a negative impl as a result of encountering
 736                     // a type with an explicit negative impl of an auto trait
 737                     // (for example, raw pointers have !Send and !Sync impls)
 738                     // However, through some **interesting** manipulations of the type
 739                     // system, it's actually possible to write a type that never
 740                     // implements an auto trait due to a projection error, not a normal
 741                     // negative impl error. To properly handle this case, we need
 742                     // to ensure that we catch any potential projection errors,
 743                     // and turn them into an explicit negative impl for our type.
 744                     debug!("Projecting and unifying projection predicate {:?}", predicate);
 745
 746                     match project::poly_project_and_unify_type(select, &obligation.with(p)) {
 747                         ProjectAndUnifyResult::MismatchedProjectionTypes(e) => {
 748                             debug!(
 749                                 "evaluate_nested_obligations: Unable to unify predicate \
 750                                  '{:?}' '{:?}', bailing out",
 751                                 ty, e
 752                             );
 753                             return false;
 754                         }
 755                         ProjectAndUnifyResult::Recursive => {
 756                             debug!("evaluate_nested_obligations: recursive projection predicate");
 757                             return false;
 758                         }
 759                         ProjectAndUnifyResult::Holds(v) => {
 760                             // We only care about sub-obligations
 761                             // when we started out trying to unify
 762                             // some inference variables. See the comment above
 763                             // for more information
 764                             if p.term().skip_binder().has_infer_types() {
 765                                 if !self.evaluate_nested_obligations(
 766                                     ty,
 767                                     v.into_iter(),
 768                                     computed_preds,
 769                                     fresh_preds,
 770                                     predicates,
 771                                     select,
 772                                     only_projections,
 773                                 ) {
 774                                     return false;
 775                                 }
 776                             }
 777                         }
 778                         ProjectAndUnifyResult::FailedNormalization => {
 779                             // It's ok not to make progress when have no inference variables -
 780                             // in that case, we were only performing unification to check if an
 781                             // error occurred (which would indicate that it's impossible for our
 782                             // type to implement the auto trait).
 783                             // However, we should always make progress (either by generating
 784                             // subobligations or getting an error) when we started off with
 785                             // inference variables
 786                             if p.term().skip_binder().has_infer_types() {
 787                                 panic!("Unexpected result when selecting {:?} {:?}", ty, obligation)
 788                             }
 789                         }
 790                     }
 791                 }
 792                 ty::PredicateKind::RegionOutlives(binder) => {
 793                     let binder = bound_predicate.rebind(binder);
 794                     select.infcx().region_outlives_predicate(&dummy_cause, binder)
 795                 }
 796                 ty::PredicateKind::TypeOutlives(binder) => {
 797                     let binder = bound_predicate.rebind(binder);
 798                     match (
 799                         binder.no_bound_vars(),
 800                         binder.map_bound_ref(|pred| pred.0).no_bound_vars(),
 801                     ) {
 802                         (None, Some(t_a)) => {
 803                             select.infcx().register_region_obligation_with_cause(
 804                                 t_a,
 805                                 select.infcx().tcx.lifetimes.re_static,
 806                                 &dummy_cause,
 807                             );
 808                         }
 809                         (Some(ty::OutlivesPredicate(t_a, r_b)), _) => {
 810                             select.infcx().register_region_obligation_with_cause(
 811                                 t_a,
 812                                 r_b,
 813                                 &dummy_cause,
 814                             );
 815                         }
 816                         _ => {}
 817                     };
 818                 }
 819                 ty::PredicateKind::ConstEquate(c1, c2) => {
 820                     let evaluate = |c: ty::Const<'tcx>| {
 821                         if let ty::ConstKind::Unevaluated(unevaluated) = c.kind() {
 822                             match select.infcx().const_eval_resolve(
 823                                 obligation.param_env,
 824                                 unevaluated,
 825                                 Some(obligation.cause.span),
 826                             ) {
 827                                 Ok(Some(valtree)) => {
 828                                     Ok(ty::Const::from_value(select.tcx(), valtree, c.ty()))
 829                                 }
 830                                 Ok(None) => {
 831                                     let tcx = self.tcx;
 832                                     let def_id = unevaluated.def.did;
 833                                     let reported = tcx.sess.struct_span_err(tcx.def_span(def_id), &format!("unable to construct a constant value for the unevaluated constant {:?}", unevaluated)).emit();
 834
 835                                     Err(ErrorHandled::Reported(reported))
 836                                 }
 837                                 Err(err) => Err(err),
 838                             }
 839                         } else {
 840                             Ok(c)
 841                         }
 842                     };
 843
 844                     match (evaluate(c1), evaluate(c2)) {
 845                         (Ok(c1), Ok(c2)) => {
 846                             match select
 847                                 .infcx()
 848                                 .at(&obligation.cause, obligation.param_env)
 849                                 .eq(c1, c2)
 850                             {
 851                                 Ok(_) => (),
 852                                 Err(_) => return false,
 853                             }
 854                         }
 855                         _ => return false,
 856                     }
 857                 }
 858                 // There's not really much we can do with these predicates -
 859                 // we start out with a `ParamEnv` with no inference variables,
 860                 // and these don't correspond to adding any new bounds to
 861                 // the `ParamEnv`.
 862                 ty::PredicateKind::WellFormed(..)
 863                 | ty::PredicateKind::ObjectSafe(..)
 864                 | ty::PredicateKind::ClosureKind(..)
 865                 | ty::PredicateKind::Subtype(..)
 866                 | ty::PredicateKind::ConstEvaluatable(..)
 867                 | ty::PredicateKind::Coerce(..)
 868                 | ty::PredicateKind::TypeWellFormedFromEnv(..) => {}
 869             };
 870         }
 871         true
 872     }
 873
 874     pub fn clean_pred(
 875         &self,
 876         infcx: &InferCtxt<'_, 'tcx>,
 877         p: ty::Predicate<'tcx>,
 878     ) -> ty::Predicate<'tcx> {
 879         infcx.freshen(p)
 880     }
 881 }
 882
 883 // Replaces all ReVars in a type with ty::Region's, using the provided map
 884 pub struct RegionReplacer<'a, 'tcx> {
 885     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 886     tcx: TyCtxt<'tcx>,
 887 }
 888
 889 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 890     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 891         self.tcx
 892     }
 893
 894     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 895         (match *r {
 896             ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 897             _ => None,
 898         })
 899         .unwrap_or_else(|| r.super_fold_with(self))
 900     }
 901 }