src/librustdoc/clean/auto_trait.rs

   1 use rustc_data_structures::fx::FxHashSet;
   2 use rustc_hir as hir;
   3 use rustc_hir::lang_items;
   4 use rustc_middle::ty::{self, Region, RegionVid, TypeFoldable};
   5 use rustc_trait_selection::traits::auto_trait::{self, AutoTraitResult};
   6
   7 use std::fmt::Debug;
   8
   9 use super::*;
  10
  11 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
  12 enum RegionTarget<'tcx> {
  13     Region(Region<'tcx>),
  14     RegionVid(RegionVid),
  15 }
  16
  17 #[derive(Default, Debug, Clone)]
  18 struct RegionDeps<'tcx> {
  19     larger: FxHashSet<RegionTarget<'tcx>>,
  20     smaller: FxHashSet<RegionTarget<'tcx>>,
  21 }
  22
  23 pub struct AutoTraitFinder<'a, 'tcx> {
  24     pub cx: &'a core::DocContext<'tcx>,
  25     pub f: auto_trait::AutoTraitFinder<'tcx>,
  26 }
  27
  28 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
  29     pub fn new(cx: &'a core::DocContext<'tcx>) -> Self {
  30         let f = auto_trait::AutoTraitFinder::new(cx.tcx);
  31
  32         AutoTraitFinder { cx, f }
  33     }
  34
  35     // FIXME(eddyb) figure out a better way to pass information about
  36     // parametrization of `ty` than `param_env_def_id`.
  37     pub fn get_auto_trait_impls(&self, ty: Ty<'tcx>, param_env_def_id: DefId) -> Vec<Item> {
  38         let param_env = self.cx.tcx.param_env(param_env_def_id);
  39
  40         debug!("get_auto_trait_impls({:?})", ty);
  41         let auto_traits = self.cx.auto_traits.iter().cloned();
  42         auto_traits
  43             .filter_map(|trait_def_id| {
  44                 let trait_ref = ty::TraitRef {
  45                     def_id: trait_def_id,
  46                     substs: self.cx.tcx.mk_substs_trait(ty, &[]),
  47                 };
  48                 if !self.cx.generated_synthetics.borrow_mut().insert((ty, trait_def_id)) {
  49                     debug!("get_auto_trait_impl_for({:?}): already generated, aborting", trait_ref);
  50                     return None;
  51                 }
  52
  53                 let result =
  54                     self.f.find_auto_trait_generics(ty, param_env, trait_def_id, |infcx, info| {
  55                         let region_data = info.region_data;
  56
  57                         let names_map = self
  58                             .cx
  59                             .tcx
  60                             .generics_of(param_env_def_id)
  61                             .params
  62                             .iter()
  63                             .filter_map(|param| match param.kind {
  64                                 ty::GenericParamDefKind::Lifetime => Some(param.name.to_string()),
  65                                 _ => None,
  66                             })
  67                             .map(|name| (name.clone(), Lifetime(name)))
  68                             .collect();
  69                         let lifetime_predicates = self.handle_lifetimes(&region_data, &names_map);
  70                         let new_generics = self.param_env_to_generics(
  71                             infcx.tcx,
  72                             param_env_def_id,
  73                             info.full_user_env,
  74                             lifetime_predicates,
  75                             info.vid_to_region,
  76                         );
  77
  78                         debug!(
  79                             "find_auto_trait_generics(param_env_def_id={:?}, trait_def_id={:?}): \
  80                             finished with {:?}",
  81                             param_env_def_id, trait_def_id, new_generics
  82                         );
  83
  84                         new_generics
  85                     });
  86
  87                 let polarity;
  88                 let new_generics = match result {
  89                     AutoTraitResult::PositiveImpl(new_generics) => {
  90                         polarity = None;
  91                         new_generics
  92                     }
  93                     AutoTraitResult::NegativeImpl => {
  94                         polarity = Some(ImplPolarity::Negative);
  95
  96                         // For negative impls, we use the generic params, but *not* the predicates,
  97                         // from the original type. Otherwise, the displayed impl appears to be a
  98                         // conditional negative impl, when it's really unconditional.
  99                         //
 100                         // For example, consider the struct Foo<T: Copy>(*mut T). Using
 101                         // the original predicates in our impl would cause us to generate
 102                         // `impl !Send for Foo<T: Copy>`, which makes it appear that Foo
 103                         // implements Send where T is not copy.
 104                         //
 105                         // Instead, we generate `impl !Send for Foo<T>`, which better
 106                         // expresses the fact that `Foo<T>` never implements `Send`,
 107                         // regardless of the choice of `T`.
 108                         let params = (
 109                             self.cx.tcx.generics_of(param_env_def_id),
 110                             ty::GenericPredicates::default(),
 111                         )
 112                             .clean(self.cx)
 113                             .params;
 114
 115                         Generics { params, where_predicates: Vec::new() }
 116                     }
 117                     AutoTraitResult::ExplicitImpl => return None,
 118                 };
 119
 120                 Some(Item {
 121                     source: Span::empty(),
 122                     name: None,
 123                     attrs: Default::default(),
 124                     visibility: Inherited,
 125                     def_id: self.cx.next_def_id(param_env_def_id.krate),
 126                     stability: None,
 127                     deprecation: None,
 128                     inner: ImplItem(Impl {
 129                         unsafety: hir::Unsafety::Normal,
 130                         generics: new_generics,
 131                         provided_trait_methods: Default::default(),
 132                         trait_: Some(trait_ref.clean(self.cx).get_trait_type().unwrap()),
 133                         for_: ty.clean(self.cx),
 134                         items: Vec::new(),
 135                         polarity,
 136                         synthetic: true,
 137                         blanket_impl: None,
 138                     }),
 139                 })
 140             })
 141             .collect()
 142     }
 143
 144     fn get_lifetime(
 145         &self,
 146         region: Region<'_>,
 147         names_map: &FxHashMap<String, Lifetime>,
 148     ) -> Lifetime {
 149         self.region_name(region)
 150             .map(|name| {
 151                 names_map.get(&name).unwrap_or_else(|| {
 152                     panic!("Missing lifetime with name {:?} for {:?}", name, region)
 153                 })
 154             })
 155             .unwrap_or(&Lifetime::statik())
 156             .clone()
 157     }
 158
 159     fn region_name(&self, region: Region<'_>) -> Option<String> {
 160         match region {
 161             &ty::ReEarlyBound(r) => Some(r.name.to_string()),
 162             _ => None,
 163         }
 164     }
 165
 166     // This method calculates two things: Lifetime constraints of the form 'a: 'b,
 167     // and region constraints of the form ReVar: 'a
 168     //
 169     // This is essentially a simplified version of lexical_region_resolve. However,
 170     // handle_lifetimes determines what *needs be* true in order for an impl to hold.
 171     // lexical_region_resolve, along with much of the rest of the compiler, is concerned
 172     // with determining if a given set up constraints/predicates *are* met, given some
 173     // starting conditions (e.g., user-provided code). For this reason, it's easier
 174     // to perform the calculations we need on our own, rather than trying to make
 175     // existing inference/solver code do what we want.
 176     fn handle_lifetimes<'cx>(
 177         &self,
 178         regions: &RegionConstraintData<'cx>,
 179         names_map: &FxHashMap<String, Lifetime>,
 180     ) -> Vec<WherePredicate> {
 181         // Our goal is to 'flatten' the list of constraints by eliminating
 182         // all intermediate RegionVids. At the end, all constraints should
 183         // be between Regions (aka region variables). This gives us the information
 184         // we need to create the Generics.
 185         let mut finished: FxHashMap<_, Vec<_>> = Default::default();
 186
 187         let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
 188
 189         // Flattening is done in two parts. First, we insert all of the constraints
 190         // into a map. Each RegionTarget (either a RegionVid or a Region) maps
 191         // to its smaller and larger regions. Note that 'larger' regions correspond
 192         // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
 193         for constraint in regions.constraints.keys() {
 194             match constraint {
 195                 &Constraint::VarSubVar(r1, r2) => {
 196                     {
 197                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 198                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 199                     }
 200
 201                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 202                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 203                 }
 204                 &Constraint::RegSubVar(region, vid) => {
 205                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 206                     deps.smaller.insert(RegionTarget::Region(region));
 207                 }
 208                 &Constraint::VarSubReg(vid, region) => {
 209                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 210                     deps.larger.insert(RegionTarget::Region(region));
 211                 }
 212                 &Constraint::RegSubReg(r1, r2) => {
 213                     // The constraint is already in the form that we want, so we're done with it
 214                     // Desired order is 'larger, smaller', so flip then
 215                     if self.region_name(r1) != self.region_name(r2) {
 216                         finished
 217                             .entry(self.region_name(r2).expect("no region_name found"))
 218                             .or_default()
 219                             .push(r1);
 220                     }
 221                 }
 222             }
 223         }
 224
 225         // Here, we 'flatten' the map one element at a time.
 226         // All of the element's sub and super regions are connected
 227         // to each other. For example, if we have a graph that looks like this:
 228         //
 229         // (A, B) - C - (D, E)
 230         // Where (A, B) are subregions, and (D,E) are super-regions
 231         //
 232         // then after deleting 'C', the graph will look like this:
 233         //  ... - A - (D, E ...)
 234         //  ... - B - (D, E, ...)
 235         //  (A, B, ...) - D - ...
 236         //  (A, B, ...) - E - ...
 237         //
 238         //  where '...' signifies the existing sub and super regions of an entry
 239         //  When two adjacent ty::Regions are encountered, we've computed a final
 240         //  constraint, and add it to our list. Since we make sure to never re-add
 241         //  deleted items, this process will always finish.
 242         while !vid_map.is_empty() {
 243             let target = *vid_map.keys().next().expect("Keys somehow empty");
 244             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 245
 246             for smaller in deps.smaller.iter() {
 247                 for larger in deps.larger.iter() {
 248                     match (smaller, larger) {
 249                         (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
 250                             if self.region_name(r1) != self.region_name(r2) {
 251                                 finished
 252                                     .entry(self.region_name(r2).expect("no region name found"))
 253                                     .or_default()
 254                                     .push(r1) // Larger, smaller
 255                             }
 256                         }
 257                         (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
 258                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 259                                 let smaller_deps = v.into_mut();
 260                                 smaller_deps.larger.insert(*larger);
 261                                 smaller_deps.larger.remove(&target);
 262                             }
 263                         }
 264                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 265                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 266                                 let deps = v.into_mut();
 267                                 deps.smaller.insert(*smaller);
 268                                 deps.smaller.remove(&target);
 269                             }
 270                         }
 271                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 272                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 273                                 let smaller_deps = v.into_mut();
 274                                 smaller_deps.larger.insert(*larger);
 275                                 smaller_deps.larger.remove(&target);
 276                             }
 277
 278                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 279                                 let larger_deps = v.into_mut();
 280                                 larger_deps.smaller.insert(*smaller);
 281                                 larger_deps.smaller.remove(&target);
 282                             }
 283                         }
 284                     }
 285                 }
 286             }
 287         }
 288
 289         let lifetime_predicates = names_map
 290             .iter()
 291             .flat_map(|(name, lifetime)| {
 292                 let empty = Vec::new();
 293                 let bounds: FxHashSet<GenericBound> = finished
 294                     .get(name)
 295                     .unwrap_or(&empty)
 296                     .iter()
 297                     .map(|region| GenericBound::Outlives(self.get_lifetime(region, names_map)))
 298                     .collect();
 299
 300                 if bounds.is_empty() {
 301                     return None;
 302                 }
 303                 Some(WherePredicate::RegionPredicate {
 304                     lifetime: lifetime.clone(),
 305                     bounds: bounds.into_iter().collect(),
 306                 })
 307             })
 308             .collect();
 309
 310         lifetime_predicates
 311     }
 312
 313     fn extract_for_generics(
 314         &self,
 315         tcx: TyCtxt<'tcx>,
 316         pred: ty::Predicate<'tcx>,
 317     ) -> FxHashSet<GenericParamDef> {
 318         let regions = match pred.ignore_quantifiers().skip_binder().kind() {
 319             &ty::PredicateKind::Trait(poly_trait_pred, _) => {
 320                 tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(poly_trait_pred))
 321             }
 322             &ty::PredicateKind::Projection(poly_proj_pred) => {
 323                 tcx.collect_referenced_late_bound_regions(&ty::Binder::bind(poly_proj_pred))
 324             }
 325             _ => return FxHashSet::default(),
 326         };
 327
 328         regions
 329             .into_iter()
 330             .filter_map(|br| {
 331                 match br {
 332                     // We only care about named late bound regions, as we need to add them
 333                     // to the 'for<>' section
 334                     ty::BrNamed(_, name) => Some(GenericParamDef {
 335                         name: name.to_string(),
 336                         kind: GenericParamDefKind::Lifetime,
 337                     }),
 338                     _ => None,
 339                 }
 340             })
 341             .collect()
 342     }
 343
 344     fn make_final_bounds(
 345         &self,
 346         ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
 347         ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)>,
 348         lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
 349     ) -> Vec<WherePredicate> {
 350         ty_to_bounds
 351             .into_iter()
 352             .flat_map(|(ty, mut bounds)| {
 353                 if let Some(data) = ty_to_fn.get(&ty) {
 354                     let (poly_trait, output) =
 355                         (data.0.as_ref().expect("as_ref failed").clone(), data.1.as_ref().cloned());
 356                     let new_ty = match &poly_trait.trait_ {
 357                         &Type::ResolvedPath {
 358                             ref path,
 359                             ref param_names,
 360                             ref did,
 361                             ref is_generic,
 362                         } => {
 363                             let mut new_path = path.clone();
 364                             let last_segment =
 365                                 new_path.segments.pop().expect("segments were empty");
 366
 367                             let (old_input, old_output) = match last_segment.args {
 368                                 GenericArgs::AngleBracketed { args, .. } => {
 369                                     let types = args
 370                                         .iter()
 371                                         .filter_map(|arg| match arg {
 372                                             GenericArg::Type(ty) => Some(ty.clone()),
 373                                             _ => None,
 374                                         })
 375                                         .collect();
 376                                     (types, None)
 377                                 }
 378                                 GenericArgs::Parenthesized { inputs, output, .. } => {
 379                                     (inputs, output)
 380                                 }
 381                             };
 382
 383                             if old_output.is_some() && old_output != output {
 384                                 panic!(
 385                                     "Output mismatch for {:?} {:?} {:?}",
 386                                     ty, old_output, data.1
 387                                 );
 388                             }
 389
 390                             let new_params =
 391                                 GenericArgs::Parenthesized { inputs: old_input, output };
 392
 393                             new_path
 394                                 .segments
 395                                 .push(PathSegment { name: last_segment.name, args: new_params });
 396
 397                             Type::ResolvedPath {
 398                                 path: new_path,
 399                                 param_names: param_names.clone(),
 400                                 did: *did,
 401                                 is_generic: *is_generic,
 402                             }
 403                         }
 404                         _ => panic!("Unexpected data: {:?}, {:?}", ty, data),
 405                     };
 406                     bounds.insert(GenericBound::TraitBound(
 407                         PolyTrait { trait_: new_ty, generic_params: poly_trait.generic_params },
 408                         hir::TraitBoundModifier::None,
 409                     ));
 410                 }
 411                 if bounds.is_empty() {
 412                     return None;
 413                 }
 414
 415                 let mut bounds_vec = bounds.into_iter().collect();
 416                 self.sort_where_bounds(&mut bounds_vec);
 417
 418                 Some(WherePredicate::BoundPredicate { ty, bounds: bounds_vec })
 419             })
 420             .chain(
 421                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 422                     |(lifetime, bounds)| {
 423                         let mut bounds_vec = bounds.into_iter().collect();
 424                         self.sort_where_bounds(&mut bounds_vec);
 425                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 426                     },
 427                 ),
 428             )
 429             .collect()
 430     }
 431
 432     // Converts the calculated ParamEnv and lifetime information to a clean::Generics, suitable for
 433     // display on the docs page. Cleaning the Predicates produces sub-optimal WherePredicate's,
 434     // so we fix them up:
 435     //
 436     // * Multiple bounds for the same type are coalesced into one: e.g., 'T: Copy', 'T: Debug'
 437     // becomes 'T: Copy + Debug'
 438     // * Fn bounds are handled specially - instead of leaving it as 'T: Fn(), <T as Fn::Output> =
 439     // K', we use the dedicated syntax 'T: Fn() -> K'
 440     // * We explcitly add a '?Sized' bound if we didn't find any 'Sized' predicates for a type
 441     fn param_env_to_generics(
 442         &self,
 443         tcx: TyCtxt<'tcx>,
 444         param_env_def_id: DefId,
 445         param_env: ty::ParamEnv<'tcx>,
 446         mut existing_predicates: Vec<WherePredicate>,
 447         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 448     ) -> Generics {
 449         debug!(
 450             "param_env_to_generics(param_env_def_id={:?}, param_env={:?}, \
 451              existing_predicates={:?})",
 452             param_env_def_id, param_env, existing_predicates
 453         );
 454
 455         // The `Sized` trait must be handled specially, since we only display it when
 456         // it is *not* required (i.e., '?Sized')
 457         let sized_trait = self.cx.tcx.require_lang_item(lang_items::SizedTraitLangItem, None);
 458
 459         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 460
 461         let orig_bounds: FxHashSet<_> =
 462             self.cx.tcx.param_env(param_env_def_id).caller_bounds().iter().collect();
 463         let clean_where_predicates = param_env
 464             .caller_bounds()
 465             .iter()
 466             .filter(|p| {
 467                 !orig_bounds.contains(p)
 468                     || match p.ignore_quantifiers().skip_binder().kind() {
 469                         ty::PredicateKind::Trait(pred, _) => pred.def_id() == sized_trait,
 470                         _ => false,
 471                     }
 472             })
 473             .map(|p| {
 474                 let replaced = p.fold_with(&mut replacer);
 475                 (replaced, replaced.clean(self.cx))
 476             });
 477
 478         let mut generic_params =
 479             (tcx.generics_of(param_env_def_id), tcx.explicit_predicates_of(param_env_def_id))
 480                 .clean(self.cx)
 481                 .params;
 482
 483         let mut has_sized = FxHashSet::default();
 484         let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 485         let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 486         let mut ty_to_traits: FxHashMap<Type, FxHashSet<Type>> = Default::default();
 487
 488         let mut ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)> = Default::default();
 489
 490         for (orig_p, p) in clean_where_predicates {
 491             if p.is_none() {
 492                 continue;
 493             }
 494             let p = p.unwrap();
 495             match p {
 496                 WherePredicate::BoundPredicate { ty, mut bounds } => {
 497                     // Writing a projection trait bound of the form
 498                     // <T as Trait>::Name : ?Sized
 499                     // is illegal, because ?Sized bounds can only
 500                     // be written in the (here, nonexistent) definition
 501                     // of the type.
 502                     // Therefore, we make sure that we never add a ?Sized
 503                     // bound for projections
 504                     if let Type::QPath { .. } = ty {
 505                         has_sized.insert(ty.clone());
 506                     }
 507
 508                     if bounds.is_empty() {
 509                         continue;
 510                     }
 511
 512                     let mut for_generics = self.extract_for_generics(tcx, orig_p);
 513
 514                     assert!(bounds.len() == 1);
 515                     let mut b = bounds.pop().expect("bounds were empty");
 516
 517                     if b.is_sized_bound(self.cx) {
 518                         has_sized.insert(ty.clone());
 519                     } else if !b
 520                         .get_trait_type()
 521                         .and_then(|t| {
 522                             ty_to_traits
 523                                 .get(&ty)
 524                                 .map(|bounds| bounds.contains(&strip_type(t.clone())))
 525                         })
 526                         .unwrap_or(false)
 527                     {
 528                         // If we've already added a projection bound for the same type, don't add
 529                         // this, as it would be a duplicate
 530
 531                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 532                         // as we want to combine them with any 'Output' qpaths
 533                         // later
 534
 535                         let is_fn = match &mut b {
 536                             &mut GenericBound::TraitBound(ref mut p, _) => {
 537                                 // Insert regions into the for_generics hash map first, to ensure
 538                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 539                                 for_generics.extend(p.generic_params.clone());
 540                                 p.generic_params = for_generics.into_iter().collect();
 541                                 self.is_fn_ty(tcx, &p.trait_)
 542                             }
 543                             _ => false,
 544                         };
 545
 546                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 547
 548                         if is_fn {
 549                             ty_to_fn
 550                                 .entry(ty.clone())
 551                                 .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
 552                                 .or_insert(((Some(poly_trait.clone())), None));
 553
 554                             ty_to_bounds.entry(ty.clone()).or_default();
 555                         } else {
 556                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 557                         }
 558                     }
 559                 }
 560                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 561                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 562                 }
 563                 WherePredicate::EqPredicate { lhs, rhs } => {
 564                     match lhs {
 565                         Type::QPath { name: ref left_name, ref self_type, ref trait_ } => {
 566                             let ty = &*self_type;
 567                             match **trait_ {
 568                                 Type::ResolvedPath {
 569                                     path: ref trait_path,
 570                                     ref param_names,
 571                                     ref did,
 572                                     ref is_generic,
 573                                 } => {
 574                                     let mut new_trait_path = trait_path.clone();
 575
 576                                     if self.is_fn_ty(tcx, trait_) && left_name == FN_OUTPUT_NAME {
 577                                         ty_to_fn
 578                                             .entry(*ty.clone())
 579                                             .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
 580                                             .or_insert((None, Some(rhs)));
 581                                         continue;
 582                                     }
 583
 584                                     let args = &mut new_trait_path
 585                                         .segments
 586                                         .last_mut()
 587                                         .expect("segments were empty")
 588                                         .args;
 589
 590                                     match args {
 591                                         // Convert somethiung like '<T as Iterator::Item> = u8'
 592                                         // to 'T: Iterator<Item=u8>'
 593                                         GenericArgs::AngleBracketed {
 594                                             ref mut bindings, ..
 595                                         } => {
 596                                             bindings.push(TypeBinding {
 597                                                 name: left_name.clone(),
 598                                                 kind: TypeBindingKind::Equality { ty: rhs },
 599                                             });
 600                                         }
 601                                         GenericArgs::Parenthesized { .. } => {
 602                                             existing_predicates.push(WherePredicate::EqPredicate {
 603                                                 lhs: lhs.clone(),
 604                                                 rhs,
 605                                             });
 606                                             continue; // If something other than a Fn ends up
 607                                             // with parenthesis, leave it alone
 608                                         }
 609                                     }
 610
 611                                     let bounds = ty_to_bounds.entry(*ty.clone()).or_default();
 612
 613                                     bounds.insert(GenericBound::TraitBound(
 614                                         PolyTrait {
 615                                             trait_: Type::ResolvedPath {
 616                                                 path: new_trait_path,
 617                                                 param_names: param_names.clone(),
 618                                                 did: *did,
 619                                                 is_generic: *is_generic,
 620                                             },
 621                                             generic_params: Vec::new(),
 622                                         },
 623                                         hir::TraitBoundModifier::None,
 624                                     ));
 625
 626                                     // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 627                                     // that we don't see a
 628                                     // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 629                                     // on the docs page.
 630                                     bounds.remove(&GenericBound::TraitBound(
 631                                         PolyTrait {
 632                                             trait_: *trait_.clone(),
 633                                             generic_params: Vec::new(),
 634                                         },
 635                                         hir::TraitBoundModifier::None,
 636                                     ));
 637                                     // Avoid creating any new duplicate bounds later in the outer
 638                                     // loop
 639                                     ty_to_traits
 640                                         .entry(*ty.clone())
 641                                         .or_default()
 642                                         .insert(*trait_.clone());
 643                                 }
 644                                 _ => panic!(
 645                                     "Unexpected trait {:?} for {:?}",
 646                                     trait_, param_env_def_id,
 647                                 ),
 648                             }
 649                         }
 650                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, param_env_def_id),
 651                     }
 652                 }
 653             };
 654         }
 655
 656         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 657
 658         existing_predicates.extend(final_bounds);
 659
 660         for param in generic_params.iter_mut() {
 661             match param.kind {
 662                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 663                     // We never want something like `impl<T=Foo>`.
 664                     default.take();
 665                     let generic_ty = Type::Generic(param.name.clone());
 666                     if !has_sized.contains(&generic_ty) {
 667                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 668                     }
 669                 }
 670                 GenericParamDefKind::Lifetime => {}
 671                 GenericParamDefKind::Const { .. } => {}
 672             }
 673         }
 674
 675         self.sort_where_predicates(&mut existing_predicates);
 676
 677         Generics { params: generic_params, where_predicates: existing_predicates }
 678     }
 679
 680     // Ensure that the predicates are in a consistent order. The precise
 681     // ordering doesn't actually matter, but it's important that
 682     // a given set of predicates always appears in the same order -
 683     // both for visual consistency between 'rustdoc' runs, and to
 684     // make writing tests much easier
 685     #[inline]
 686     fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
 687         // We should never have identical bounds - and if we do,
 688         // they're visually identical as well. Therefore, using
 689         // an unstable sort is fine.
 690         self.unstable_debug_sort(&mut predicates);
 691     }
 692
 693     // Ensure that the bounds are in a consistent order. The precise
 694     // ordering doesn't actually matter, but it's important that
 695     // a given set of bounds always appears in the same order -
 696     // both for visual consistency between 'rustdoc' runs, and to
 697     // make writing tests much easier
 698     #[inline]
 699     fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
 700         // We should never have identical bounds - and if we do,
 701         // they're visually identical as well. Therefore, using
 702         // an unstable sort is fine.
 703         self.unstable_debug_sort(&mut bounds);
 704     }
 705
 706     // This might look horrendously hacky, but it's actually not that bad.
 707     //
 708     // For performance reasons, we use several different FxHashMaps
 709     // in the process of computing the final set of where predicates.
 710     // However, the iteration order of a HashMap is completely unspecified.
 711     // In fact, the iteration of an FxHashMap can even vary between platforms,
 712     // since FxHasher has different behavior for 32-bit and 64-bit platforms.
 713     //
 714     // Obviously, it's extremely undesirable for documentation rendering
 715     // to be depndent on the platform it's run on. Apart from being confusing
 716     // to end users, it makes writing tests much more difficult, as predicates
 717     // can appear in any order in the final result.
 718     //
 719     // To solve this problem, we sort WherePredicates and GenericBounds
 720     // by their Debug string. The thing to keep in mind is that we don't really
 721     // care what the final order is - we're synthesizing an impl or bound
 722     // ourselves, so any order can be considered equally valid. By sorting the
 723     // predicates and bounds, however, we ensure that for a given codebase, all
 724     // auto-trait impls always render in exactly the same way.
 725     //
 726     // Using the Debug implementation for sorting prevents us from needing to
 727     // write quite a bit of almost entirely useless code (e.g., how should two
 728     // Types be sorted relative to each other). It also allows us to solve the
 729     // problem for both WherePredicates and GenericBounds at the same time. This
 730     // approach is probably somewhat slower, but the small number of items
 731     // involved (impls rarely have more than a few bounds) means that it
 732     // shouldn't matter in practice.
 733     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
 734         vec.sort_by_cached_key(|x| format!("{:?}", x))
 735     }
 736
 737     fn is_fn_ty(&self, tcx: TyCtxt<'_>, ty: &Type) -> bool {
 738         match &ty {
 739             &&Type::ResolvedPath { ref did, .. } => {
 740                 *did == tcx.require_lang_item(lang_items::FnTraitLangItem, None)
 741                     || *did == tcx.require_lang_item(lang_items::FnMutTraitLangItem, None)
 742                     || *did == tcx.require_lang_item(lang_items::FnOnceTraitLangItem, None)
 743             }
 744             _ => false,
 745         }
 746     }
 747 }
 748
 749 // Replaces all ReVars in a type with ty::Region's, using the provided map
 750 struct RegionReplacer<'a, 'tcx> {
 751     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 752     tcx: TyCtxt<'tcx>,
 753 }
 754
 755 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 756     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 757         self.tcx
 758     }
 759
 760     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 761         (match r {
 762             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 763             _ => None,
 764         })
 765         .unwrap_or_else(|| r.super_fold_with(self))
 766     }
 767 }