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::LangItem;
   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 crate struct AutoTraitFinder<'a, 'tcx> {
  24     crate cx: &'a core::DocContext<'tcx>,
  25     crate f: auto_trait::AutoTraitFinder<'tcx>,
  26 }
  27
  28 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
  29     crate 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     crate 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),
  65                                 _ => None,
  66                             })
  67                             .map(|name| (name, 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::dummy(),
 122                     name: None,
 123                     attrs: Default::default(),
 124                     visibility: Inherited,
 125                     def_id: self.cx.next_def_id(param_env_def_id.krate),
 126                     kind: box ImplItem(Impl {
 127                         unsafety: hir::Unsafety::Normal,
 128                         generics: new_generics,
 129                         provided_trait_methods: Default::default(),
 130                         trait_: Some(trait_ref.clean(self.cx).get_trait_type().unwrap()),
 131                         for_: ty.clean(self.cx),
 132                         items: Vec::new(),
 133                         polarity,
 134                         synthetic: true,
 135                         blanket_impl: None,
 136                     }),
 137                 })
 138             })
 139             .collect()
 140     }
 141
 142     fn get_lifetime(
 143         &self,
 144         region: Region<'_>,
 145         names_map: &FxHashMap<Symbol, Lifetime>,
 146     ) -> Lifetime {
 147         self.region_name(region)
 148             .map(|name| {
 149                 names_map.get(&name).unwrap_or_else(|| {
 150                     panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
 151                 })
 152             })
 153             .unwrap_or(&Lifetime::statik())
 154             .clone()
 155     }
 156
 157     fn region_name(&self, region: Region<'_>) -> Option<Symbol> {
 158         match region {
 159             &ty::ReEarlyBound(r) => Some(r.name),
 160             _ => None,
 161         }
 162     }
 163
 164     // This method calculates two things: Lifetime constraints of the form 'a: 'b,
 165     // and region constraints of the form ReVar: 'a
 166     //
 167     // This is essentially a simplified version of lexical_region_resolve. However,
 168     // handle_lifetimes determines what *needs be* true in order for an impl to hold.
 169     // lexical_region_resolve, along with much of the rest of the compiler, is concerned
 170     // with determining if a given set up constraints/predicates *are* met, given some
 171     // starting conditions (e.g., user-provided code). For this reason, it's easier
 172     // to perform the calculations we need on our own, rather than trying to make
 173     // existing inference/solver code do what we want.
 174     fn handle_lifetimes<'cx>(
 175         &self,
 176         regions: &RegionConstraintData<'cx>,
 177         names_map: &FxHashMap<Symbol, Lifetime>,
 178     ) -> Vec<WherePredicate> {
 179         // Our goal is to 'flatten' the list of constraints by eliminating
 180         // all intermediate RegionVids. At the end, all constraints should
 181         // be between Regions (aka region variables). This gives us the information
 182         // we need to create the Generics.
 183         let mut finished: FxHashMap<_, Vec<_>> = Default::default();
 184
 185         let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
 186
 187         // Flattening is done in two parts. First, we insert all of the constraints
 188         // into a map. Each RegionTarget (either a RegionVid or a Region) maps
 189         // to its smaller and larger regions. Note that 'larger' regions correspond
 190         // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
 191         for constraint in regions.constraints.keys() {
 192             match constraint {
 193                 &Constraint::VarSubVar(r1, r2) => {
 194                     {
 195                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 196                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 197                     }
 198
 199                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 200                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 201                 }
 202                 &Constraint::RegSubVar(region, vid) => {
 203                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 204                     deps.smaller.insert(RegionTarget::Region(region));
 205                 }
 206                 &Constraint::VarSubReg(vid, region) => {
 207                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 208                     deps.larger.insert(RegionTarget::Region(region));
 209                 }
 210                 &Constraint::RegSubReg(r1, r2) => {
 211                     // The constraint is already in the form that we want, so we're done with it
 212                     // Desired order is 'larger, smaller', so flip then
 213                     if self.region_name(r1) != self.region_name(r2) {
 214                         finished
 215                             .entry(self.region_name(r2).expect("no region_name found"))
 216                             .or_default()
 217                             .push(r1);
 218                     }
 219                 }
 220             }
 221         }
 222
 223         // Here, we 'flatten' the map one element at a time.
 224         // All of the element's sub and super regions are connected
 225         // to each other. For example, if we have a graph that looks like this:
 226         //
 227         // (A, B) - C - (D, E)
 228         // Where (A, B) are subregions, and (D,E) are super-regions
 229         //
 230         // then after deleting 'C', the graph will look like this:
 231         //  ... - A - (D, E ...)
 232         //  ... - B - (D, E, ...)
 233         //  (A, B, ...) - D - ...
 234         //  (A, B, ...) - E - ...
 235         //
 236         //  where '...' signifies the existing sub and super regions of an entry
 237         //  When two adjacent ty::Regions are encountered, we've computed a final
 238         //  constraint, and add it to our list. Since we make sure to never re-add
 239         //  deleted items, this process will always finish.
 240         while !vid_map.is_empty() {
 241             let target = *vid_map.keys().next().expect("Keys somehow empty");
 242             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 243
 244             for smaller in deps.smaller.iter() {
 245                 for larger in deps.larger.iter() {
 246                     match (smaller, larger) {
 247                         (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
 248                             if self.region_name(r1) != self.region_name(r2) {
 249                                 finished
 250                                     .entry(self.region_name(r2).expect("no region name found"))
 251                                     .or_default()
 252                                     .push(r1) // Larger, smaller
 253                             }
 254                         }
 255                         (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
 256                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 257                                 let smaller_deps = v.into_mut();
 258                                 smaller_deps.larger.insert(*larger);
 259                                 smaller_deps.larger.remove(&target);
 260                             }
 261                         }
 262                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 263                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 264                                 let deps = v.into_mut();
 265                                 deps.smaller.insert(*smaller);
 266                                 deps.smaller.remove(&target);
 267                             }
 268                         }
 269                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 270                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 271                                 let smaller_deps = v.into_mut();
 272                                 smaller_deps.larger.insert(*larger);
 273                                 smaller_deps.larger.remove(&target);
 274                             }
 275
 276                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 277                                 let larger_deps = v.into_mut();
 278                                 larger_deps.smaller.insert(*smaller);
 279                                 larger_deps.smaller.remove(&target);
 280                             }
 281                         }
 282                     }
 283                 }
 284             }
 285         }
 286
 287         let lifetime_predicates = names_map
 288             .iter()
 289             .flat_map(|(name, lifetime)| {
 290                 let empty = Vec::new();
 291                 let bounds: FxHashSet<GenericBound> = finished
 292                     .get(name)
 293                     .unwrap_or(&empty)
 294                     .iter()
 295                     .map(|region| GenericBound::Outlives(self.get_lifetime(region, names_map)))
 296                     .collect();
 297
 298                 if bounds.is_empty() {
 299                     return None;
 300                 }
 301                 Some(WherePredicate::RegionPredicate {
 302                     lifetime: lifetime.clone(),
 303                     bounds: bounds.into_iter().collect(),
 304                 })
 305             })
 306             .collect();
 307
 308         lifetime_predicates
 309     }
 310
 311     fn extract_for_generics(
 312         &self,
 313         tcx: TyCtxt<'tcx>,
 314         pred: ty::Predicate<'tcx>,
 315     ) -> FxHashSet<GenericParamDef> {
 316         let bound_predicate = pred.bound_atom();
 317         let regions = match bound_predicate.skip_binder() {
 318             ty::PredicateAtom::Trait(poly_trait_pred, _) => {
 319                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
 320             }
 321             ty::PredicateAtom::Projection(poly_proj_pred) => {
 322                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_proj_pred))
 323             }
 324             _ => return FxHashSet::default(),
 325         };
 326
 327         regions
 328             .into_iter()
 329             .filter_map(|br| {
 330                 match br {
 331                     // We only care about named late bound regions, as we need to add them
 332                     // to the 'for<>' section
 333                     ty::BrNamed(_, name) => {
 334                         Some(GenericParamDef { name, kind: GenericParamDefKind::Lifetime })
 335                     }
 336                     _ => None,
 337                 }
 338             })
 339             .collect()
 340     }
 341
 342     fn make_final_bounds(
 343         &self,
 344         ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
 345         ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)>,
 346         lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
 347     ) -> Vec<WherePredicate> {
 348         ty_to_bounds
 349             .into_iter()
 350             .flat_map(|(ty, mut bounds)| {
 351                 if let Some(data) = ty_to_fn.get(&ty) {
 352                     let (poly_trait, output) =
 353                         (data.0.as_ref().expect("as_ref failed").clone(), data.1.as_ref().cloned());
 354                     let new_ty = match poly_trait.trait_ {
 355                         Type::ResolvedPath {
 356                             ref path,
 357                             ref param_names,
 358                             ref did,
 359                             ref is_generic,
 360                         } => {
 361                             let mut new_path = path.clone();
 362                             let last_segment =
 363                                 new_path.segments.pop().expect("segments were empty");
 364
 365                             let (old_input, old_output) = match last_segment.args {
 366                                 GenericArgs::AngleBracketed { args, .. } => {
 367                                     let types = args
 368                                         .iter()
 369                                         .filter_map(|arg| match arg {
 370                                             GenericArg::Type(ty) => Some(ty.clone()),
 371                                             _ => None,
 372                                         })
 373                                         .collect();
 374                                     (types, None)
 375                                 }
 376                                 GenericArgs::Parenthesized { inputs, output, .. } => {
 377                                     (inputs, output)
 378                                 }
 379                             };
 380
 381                             if old_output.is_some() && old_output != output {
 382                                 panic!(
 383                                     "Output mismatch for {:?} {:?} {:?}",
 384                                     ty, old_output, data.1
 385                                 );
 386                             }
 387
 388                             let new_params =
 389                                 GenericArgs::Parenthesized { inputs: old_input, output };
 390
 391                             new_path
 392                                 .segments
 393                                 .push(PathSegment { name: last_segment.name, args: new_params });
 394
 395                             Type::ResolvedPath {
 396                                 path: new_path,
 397                                 param_names: param_names.clone(),
 398                                 did: *did,
 399                                 is_generic: *is_generic,
 400                             }
 401                         }
 402                         _ => panic!("Unexpected data: {:?}, {:?}", ty, data),
 403                     };
 404                     bounds.insert(GenericBound::TraitBound(
 405                         PolyTrait { trait_: new_ty, generic_params: poly_trait.generic_params },
 406                         hir::TraitBoundModifier::None,
 407                     ));
 408                 }
 409                 if bounds.is_empty() {
 410                     return None;
 411                 }
 412
 413                 let mut bounds_vec = bounds.into_iter().collect();
 414                 self.sort_where_bounds(&mut bounds_vec);
 415
 416                 Some(WherePredicate::BoundPredicate { ty, bounds: bounds_vec })
 417             })
 418             .chain(
 419                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 420                     |(lifetime, bounds)| {
 421                         let mut bounds_vec = bounds.into_iter().collect();
 422                         self.sort_where_bounds(&mut bounds_vec);
 423                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 424                     },
 425                 ),
 426             )
 427             .collect()
 428     }
 429
 430     // Converts the calculated ParamEnv and lifetime information to a clean::Generics, suitable for
 431     // display on the docs page. Cleaning the Predicates produces sub-optimal `WherePredicate`s,
 432     // so we fix them up:
 433     //
 434     // * Multiple bounds for the same type are coalesced into one: e.g., 'T: Copy', 'T: Debug'
 435     // becomes 'T: Copy + Debug'
 436     // * Fn bounds are handled specially - instead of leaving it as 'T: Fn(), <T as Fn::Output> =
 437     // K', we use the dedicated syntax 'T: Fn() -> K'
 438     // * We explicitly add a '?Sized' bound if we didn't find any 'Sized' predicates for a type
 439     fn param_env_to_generics(
 440         &self,
 441         tcx: TyCtxt<'tcx>,
 442         param_env_def_id: DefId,
 443         param_env: ty::ParamEnv<'tcx>,
 444         mut existing_predicates: Vec<WherePredicate>,
 445         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 446     ) -> Generics {
 447         debug!(
 448             "param_env_to_generics(param_env_def_id={:?}, param_env={:?}, \
 449              existing_predicates={:?})",
 450             param_env_def_id, param_env, existing_predicates
 451         );
 452
 453         // The `Sized` trait must be handled specially, since we only display it when
 454         // it is *not* required (i.e., '?Sized')
 455         let sized_trait = self.cx.tcx.require_lang_item(LangItem::Sized, None);
 456
 457         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 458
 459         let orig_bounds: FxHashSet<_> =
 460             self.cx.tcx.param_env(param_env_def_id).caller_bounds().iter().collect();
 461         let clean_where_predicates = param_env
 462             .caller_bounds()
 463             .iter()
 464             .filter(|p| {
 465                 !orig_bounds.contains(p)
 466                     || match p.skip_binders() {
 467                         ty::PredicateAtom::Trait(pred, _) => pred.def_id() == sized_trait,
 468                         _ => false,
 469                     }
 470             })
 471             .map(|p| {
 472                 let replaced = p.fold_with(&mut replacer);
 473                 (replaced, replaced.clean(self.cx))
 474             });
 475
 476         let mut generic_params =
 477             (tcx.generics_of(param_env_def_id), tcx.explicit_predicates_of(param_env_def_id))
 478                 .clean(self.cx)
 479                 .params;
 480
 481         debug!(
 482             "param_env_to_generics({:?}): generic_params={:?}",
 483             param_env_def_id, generic_params
 484         );
 485
 486         let mut has_sized = FxHashSet::default();
 487         let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 488         let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 489         let mut ty_to_traits: FxHashMap<Type, FxHashSet<Type>> = Default::default();
 490
 491         let mut ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)> = Default::default();
 492
 493         for (orig_p, p) in clean_where_predicates {
 494             if p.is_none() {
 495                 continue;
 496             }
 497             let p = p.unwrap();
 498             match p {
 499                 WherePredicate::BoundPredicate { ty, mut bounds } => {
 500                     // Writing a projection trait bound of the form
 501                     // <T as Trait>::Name : ?Sized
 502                     // is illegal, because ?Sized bounds can only
 503                     // be written in the (here, nonexistent) definition
 504                     // of the type.
 505                     // Therefore, we make sure that we never add a ?Sized
 506                     // bound for projections
 507                     if let Type::QPath { .. } = ty {
 508                         has_sized.insert(ty.clone());
 509                     }
 510
 511                     if bounds.is_empty() {
 512                         continue;
 513                     }
 514
 515                     let mut for_generics = self.extract_for_generics(tcx, orig_p);
 516
 517                     assert!(bounds.len() == 1);
 518                     let mut b = bounds.pop().expect("bounds were empty");
 519
 520                     if b.is_sized_bound(self.cx) {
 521                         has_sized.insert(ty.clone());
 522                     } else if !b
 523                         .get_trait_type()
 524                         .and_then(|t| {
 525                             ty_to_traits
 526                                 .get(&ty)
 527                                 .map(|bounds| bounds.contains(&strip_type(t.clone())))
 528                         })
 529                         .unwrap_or(false)
 530                     {
 531                         // If we've already added a projection bound for the same type, don't add
 532                         // this, as it would be a duplicate
 533
 534                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 535                         // as we want to combine them with any 'Output' qpaths
 536                         // later
 537
 538                         let is_fn = match &mut b {
 539                             &mut GenericBound::TraitBound(ref mut p, _) => {
 540                                 // Insert regions into the for_generics hash map first, to ensure
 541                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 542                                 for_generics.extend(p.generic_params.clone());
 543                                 p.generic_params = for_generics.into_iter().collect();
 544                                 self.is_fn_ty(tcx, &p.trait_)
 545                             }
 546                             _ => false,
 547                         };
 548
 549                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 550
 551                         if is_fn {
 552                             ty_to_fn
 553                                 .entry(ty.clone())
 554                                 .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
 555                                 .or_insert(((Some(poly_trait.clone())), None));
 556
 557                             ty_to_bounds.entry(ty.clone()).or_default();
 558                         } else {
 559                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 560                         }
 561                     }
 562                 }
 563                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 564                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 565                 }
 566                 WherePredicate::EqPredicate { lhs, rhs } => {
 567                     match lhs {
 568                         Type::QPath { name: left_name, ref self_type, ref trait_ } => {
 569                             let ty = &*self_type;
 570                             match **trait_ {
 571                                 Type::ResolvedPath {
 572                                     path: ref trait_path,
 573                                     ref param_names,
 574                                     ref did,
 575                                     ref is_generic,
 576                                 } => {
 577                                     let mut new_trait_path = trait_path.clone();
 578
 579                                     if self.is_fn_ty(tcx, trait_) && left_name == sym::Output {
 580                                         ty_to_fn
 581                                             .entry(*ty.clone())
 582                                             .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
 583                                             .or_insert((None, Some(rhs)));
 584                                         continue;
 585                                     }
 586
 587                                     let args = &mut new_trait_path
 588                                         .segments
 589                                         .last_mut()
 590                                         .expect("segments were empty")
 591                                         .args;
 592
 593                                     match args {
 594                                         // Convert something like '<T as Iterator::Item> = u8'
 595                                         // to 'T: Iterator<Item=u8>'
 596                                         GenericArgs::AngleBracketed {
 597                                             ref mut bindings, ..
 598                                         } => {
 599                                             bindings.push(TypeBinding {
 600                                                 name: left_name,
 601                                                 kind: TypeBindingKind::Equality { ty: rhs },
 602                                             });
 603                                         }
 604                                         GenericArgs::Parenthesized { .. } => {
 605                                             existing_predicates.push(WherePredicate::EqPredicate {
 606                                                 lhs: lhs.clone(),
 607                                                 rhs,
 608                                             });
 609                                             continue; // If something other than a Fn ends up
 610                                             // with parenthesis, leave it alone
 611                                         }
 612                                     }
 613
 614                                     let bounds = ty_to_bounds.entry(*ty.clone()).or_default();
 615
 616                                     bounds.insert(GenericBound::TraitBound(
 617                                         PolyTrait {
 618                                             trait_: Type::ResolvedPath {
 619                                                 path: new_trait_path,
 620                                                 param_names: param_names.clone(),
 621                                                 did: *did,
 622                                                 is_generic: *is_generic,
 623                                             },
 624                                             generic_params: Vec::new(),
 625                                         },
 626                                         hir::TraitBoundModifier::None,
 627                                     ));
 628
 629                                     // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 630                                     // that we don't see a
 631                                     // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 632                                     // on the docs page.
 633                                     bounds.remove(&GenericBound::TraitBound(
 634                                         PolyTrait {
 635                                             trait_: *trait_.clone(),
 636                                             generic_params: Vec::new(),
 637                                         },
 638                                         hir::TraitBoundModifier::None,
 639                                     ));
 640                                     // Avoid creating any new duplicate bounds later in the outer
 641                                     // loop
 642                                     ty_to_traits
 643                                         .entry(*ty.clone())
 644                                         .or_default()
 645                                         .insert(*trait_.clone());
 646                                 }
 647                                 _ => panic!(
 648                                     "Unexpected trait {:?} for {:?}",
 649                                     trait_, param_env_def_id,
 650                                 ),
 651                             }
 652                         }
 653                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, param_env_def_id),
 654                     }
 655                 }
 656             };
 657         }
 658
 659         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 660
 661         existing_predicates.extend(final_bounds);
 662
 663         for param in generic_params.iter_mut() {
 664             match param.kind {
 665                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 666                     // We never want something like `impl<T=Foo>`.
 667                     default.take();
 668                     let generic_ty = Type::Generic(param.name);
 669                     if !has_sized.contains(&generic_ty) {
 670                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 671                     }
 672                 }
 673                 GenericParamDefKind::Lifetime => {}
 674                 GenericParamDefKind::Const { .. } => {}
 675             }
 676         }
 677
 678         self.sort_where_predicates(&mut existing_predicates);
 679
 680         Generics { params: generic_params, where_predicates: existing_predicates }
 681     }
 682
 683     // Ensure that the predicates are in a consistent order. The precise
 684     // ordering doesn't actually matter, but it's important that
 685     // a given set of predicates always appears in the same order -
 686     // both for visual consistency between 'rustdoc' runs, and to
 687     // make writing tests much easier
 688     #[inline]
 689     fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
 690         // We should never have identical bounds - and if we do,
 691         // they're visually identical as well. Therefore, using
 692         // an unstable sort is fine.
 693         self.unstable_debug_sort(&mut predicates);
 694     }
 695
 696     // Ensure that the bounds are in a consistent order. The precise
 697     // ordering doesn't actually matter, but it's important that
 698     // a given set of bounds always appears in the same order -
 699     // both for visual consistency between 'rustdoc' runs, and to
 700     // make writing tests much easier
 701     #[inline]
 702     fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
 703         // We should never have identical bounds - and if we do,
 704         // they're visually identical as well. Therefore, using
 705         // an unstable sort is fine.
 706         self.unstable_debug_sort(&mut bounds);
 707     }
 708
 709     // This might look horrendously hacky, but it's actually not that bad.
 710     //
 711     // For performance reasons, we use several different FxHashMaps
 712     // in the process of computing the final set of where predicates.
 713     // However, the iteration order of a HashMap is completely unspecified.
 714     // In fact, the iteration of an FxHashMap can even vary between platforms,
 715     // since FxHasher has different behavior for 32-bit and 64-bit platforms.
 716     //
 717     // Obviously, it's extremely undesirable for documentation rendering
 718     // to be dependent on the platform it's run on. Apart from being confusing
 719     // to end users, it makes writing tests much more difficult, as predicates
 720     // can appear in any order in the final result.
 721     //
 722     // To solve this problem, we sort WherePredicates and GenericBounds
 723     // by their Debug string. The thing to keep in mind is that we don't really
 724     // care what the final order is - we're synthesizing an impl or bound
 725     // ourselves, so any order can be considered equally valid. By sorting the
 726     // predicates and bounds, however, we ensure that for a given codebase, all
 727     // auto-trait impls always render in exactly the same way.
 728     //
 729     // Using the Debug implementation for sorting prevents us from needing to
 730     // write quite a bit of almost entirely useless code (e.g., how should two
 731     // Types be sorted relative to each other). It also allows us to solve the
 732     // problem for both WherePredicates and GenericBounds at the same time. This
 733     // approach is probably somewhat slower, but the small number of items
 734     // involved (impls rarely have more than a few bounds) means that it
 735     // shouldn't matter in practice.
 736     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
 737         vec.sort_by_cached_key(|x| format!("{:?}", x))
 738     }
 739
 740     fn is_fn_ty(&self, tcx: TyCtxt<'_>, ty: &Type) -> bool {
 741         match ty {
 742             &Type::ResolvedPath { did, .. } => {
 743                 did == tcx.require_lang_item(LangItem::Fn, None)
 744                     || did == tcx.require_lang_item(LangItem::FnMut, None)
 745                     || did == tcx.require_lang_item(LangItem::FnOnce, None)
 746             }
 747             _ => false,
 748         }
 749     }
 750 }
 751
 752 // Replaces all ReVars in a type with ty::Region's, using the provided map
 753 struct RegionReplacer<'a, 'tcx> {
 754     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 755     tcx: TyCtxt<'tcx>,
 756 }
 757
 758 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 759     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 760         self.tcx
 761     }
 762
 763     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 764         (match r {
 765             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 766             _ => None,
 767         })
 768         .unwrap_or_else(|| r.super_fold_with(self))
 769     }
 770 }