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                     stability: None,
 127                     const_stability: None,
 128                     deprecation: None,
 129                     kind: ImplItem(Impl {
 130                         unsafety: hir::Unsafety::Normal,
 131                         generics: new_generics,
 132                         provided_trait_methods: Default::default(),
 133                         trait_: Some(trait_ref.clean(self.cx).get_trait_type().unwrap()),
 134                         for_: ty.clean(self.cx),
 135                         items: Vec::new(),
 136                         polarity,
 137                         synthetic: true,
 138                         blanket_impl: None,
 139                     }),
 140                 })
 141             })
 142             .collect()
 143     }
 144
 145     fn get_lifetime(
 146         &self,
 147         region: Region<'_>,
 148         names_map: &FxHashMap<Symbol, Lifetime>,
 149     ) -> Lifetime {
 150         self.region_name(region)
 151             .map(|name| {
 152                 names_map.get(&name).unwrap_or_else(|| {
 153                     panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
 154                 })
 155             })
 156             .unwrap_or(&Lifetime::statik())
 157             .clone()
 158     }
 159
 160     fn region_name(&self, region: Region<'_>) -> Option<Symbol> {
 161         match region {
 162             &ty::ReEarlyBound(r) => Some(r.name),
 163             _ => None,
 164         }
 165     }
 166
 167     // This method calculates two things: Lifetime constraints of the form 'a: 'b,
 168     // and region constraints of the form ReVar: 'a
 169     //
 170     // This is essentially a simplified version of lexical_region_resolve. However,
 171     // handle_lifetimes determines what *needs be* true in order for an impl to hold.
 172     // lexical_region_resolve, along with much of the rest of the compiler, is concerned
 173     // with determining if a given set up constraints/predicates *are* met, given some
 174     // starting conditions (e.g., user-provided code). For this reason, it's easier
 175     // to perform the calculations we need on our own, rather than trying to make
 176     // existing inference/solver code do what we want.
 177     fn handle_lifetimes<'cx>(
 178         &self,
 179         regions: &RegionConstraintData<'cx>,
 180         names_map: &FxHashMap<Symbol, Lifetime>,
 181     ) -> Vec<WherePredicate> {
 182         // Our goal is to 'flatten' the list of constraints by eliminating
 183         // all intermediate RegionVids. At the end, all constraints should
 184         // be between Regions (aka region variables). This gives us the information
 185         // we need to create the Generics.
 186         let mut finished: FxHashMap<_, Vec<_>> = Default::default();
 187
 188         let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
 189
 190         // Flattening is done in two parts. First, we insert all of the constraints
 191         // into a map. Each RegionTarget (either a RegionVid or a Region) maps
 192         // to its smaller and larger regions. Note that 'larger' regions correspond
 193         // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
 194         for constraint in regions.constraints.keys() {
 195             match constraint {
 196                 &Constraint::VarSubVar(r1, r2) => {
 197                     {
 198                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 199                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 200                     }
 201
 202                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 203                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 204                 }
 205                 &Constraint::RegSubVar(region, vid) => {
 206                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 207                     deps.smaller.insert(RegionTarget::Region(region));
 208                 }
 209                 &Constraint::VarSubReg(vid, region) => {
 210                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 211                     deps.larger.insert(RegionTarget::Region(region));
 212                 }
 213                 &Constraint::RegSubReg(r1, r2) => {
 214                     // The constraint is already in the form that we want, so we're done with it
 215                     // Desired order is 'larger, smaller', so flip then
 216                     if self.region_name(r1) != self.region_name(r2) {
 217                         finished
 218                             .entry(self.region_name(r2).expect("no region_name found"))
 219                             .or_default()
 220                             .push(r1);
 221                     }
 222                 }
 223             }
 224         }
 225
 226         // Here, we 'flatten' the map one element at a time.
 227         // All of the element's sub and super regions are connected
 228         // to each other. For example, if we have a graph that looks like this:
 229         //
 230         // (A, B) - C - (D, E)
 231         // Where (A, B) are subregions, and (D,E) are super-regions
 232         //
 233         // then after deleting 'C', the graph will look like this:
 234         //  ... - A - (D, E ...)
 235         //  ... - B - (D, E, ...)
 236         //  (A, B, ...) - D - ...
 237         //  (A, B, ...) - E - ...
 238         //
 239         //  where '...' signifies the existing sub and super regions of an entry
 240         //  When two adjacent ty::Regions are encountered, we've computed a final
 241         //  constraint, and add it to our list. Since we make sure to never re-add
 242         //  deleted items, this process will always finish.
 243         while !vid_map.is_empty() {
 244             let target = *vid_map.keys().next().expect("Keys somehow empty");
 245             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 246
 247             for smaller in deps.smaller.iter() {
 248                 for larger in deps.larger.iter() {
 249                     match (smaller, larger) {
 250                         (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
 251                             if self.region_name(r1) != self.region_name(r2) {
 252                                 finished
 253                                     .entry(self.region_name(r2).expect("no region name found"))
 254                                     .or_default()
 255                                     .push(r1) // Larger, smaller
 256                             }
 257                         }
 258                         (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
 259                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 260                                 let smaller_deps = v.into_mut();
 261                                 smaller_deps.larger.insert(*larger);
 262                                 smaller_deps.larger.remove(&target);
 263                             }
 264                         }
 265                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 266                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 267                                 let deps = v.into_mut();
 268                                 deps.smaller.insert(*smaller);
 269                                 deps.smaller.remove(&target);
 270                             }
 271                         }
 272                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 273                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 274                                 let smaller_deps = v.into_mut();
 275                                 smaller_deps.larger.insert(*larger);
 276                                 smaller_deps.larger.remove(&target);
 277                             }
 278
 279                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 280                                 let larger_deps = v.into_mut();
 281                                 larger_deps.smaller.insert(*smaller);
 282                                 larger_deps.smaller.remove(&target);
 283                             }
 284                         }
 285                     }
 286                 }
 287             }
 288         }
 289
 290         let lifetime_predicates = names_map
 291             .iter()
 292             .flat_map(|(name, lifetime)| {
 293                 let empty = Vec::new();
 294                 let bounds: FxHashSet<GenericBound> = finished
 295                     .get(name)
 296                     .unwrap_or(&empty)
 297                     .iter()
 298                     .map(|region| GenericBound::Outlives(self.get_lifetime(region, names_map)))
 299                     .collect();
 300
 301                 if bounds.is_empty() {
 302                     return None;
 303                 }
 304                 Some(WherePredicate::RegionPredicate {
 305                     lifetime: lifetime.clone(),
 306                     bounds: bounds.into_iter().collect(),
 307                 })
 308             })
 309             .collect();
 310
 311         lifetime_predicates
 312     }
 313
 314     fn extract_for_generics(
 315         &self,
 316         tcx: TyCtxt<'tcx>,
 317         pred: ty::Predicate<'tcx>,
 318     ) -> FxHashSet<GenericParamDef> {
 319         let bound_predicate = pred.bound_atom();
 320         let regions = match bound_predicate.skip_binder() {
 321             ty::PredicateAtom::Trait(poly_trait_pred, _) => {
 322                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
 323             }
 324             ty::PredicateAtom::Projection(poly_proj_pred) => {
 325                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_proj_pred))
 326             }
 327             _ => return FxHashSet::default(),
 328         };
 329
 330         regions
 331             .into_iter()
 332             .filter_map(|br| {
 333                 match br {
 334                     // We only care about named late bound regions, as we need to add them
 335                     // to the 'for<>' section
 336                     ty::BrNamed(_, name) => {
 337                         Some(GenericParamDef { name, kind: GenericParamDefKind::Lifetime })
 338                     }
 339                     _ => None,
 340                 }
 341             })
 342             .collect()
 343     }
 344
 345     fn make_final_bounds(
 346         &self,
 347         ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
 348         ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)>,
 349         lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
 350     ) -> Vec<WherePredicate> {
 351         ty_to_bounds
 352             .into_iter()
 353             .flat_map(|(ty, mut bounds)| {
 354                 if let Some(data) = ty_to_fn.get(&ty) {
 355                     let (poly_trait, output) =
 356                         (data.0.as_ref().expect("as_ref failed").clone(), data.1.as_ref().cloned());
 357                     let new_ty = match &poly_trait.trait_ {
 358                         &Type::ResolvedPath {
 359                             ref path,
 360                             ref param_names,
 361                             ref did,
 362                             ref is_generic,
 363                         } => {
 364                             let mut new_path = path.clone();
 365                             let last_segment =
 366                                 new_path.segments.pop().expect("segments were empty");
 367
 368                             let (old_input, old_output) = match last_segment.args {
 369                                 GenericArgs::AngleBracketed { args, .. } => {
 370                                     let types = args
 371                                         .iter()
 372                                         .filter_map(|arg| match arg {
 373                                             GenericArg::Type(ty) => Some(ty.clone()),
 374                                             _ => None,
 375                                         })
 376                                         .collect();
 377                                     (types, None)
 378                                 }
 379                                 GenericArgs::Parenthesized { inputs, output, .. } => {
 380                                     (inputs, output)
 381                                 }
 382                             };
 383
 384                             if old_output.is_some() && old_output != output {
 385                                 panic!(
 386                                     "Output mismatch for {:?} {:?} {:?}",
 387                                     ty, old_output, data.1
 388                                 );
 389                             }
 390
 391                             let new_params =
 392                                 GenericArgs::Parenthesized { inputs: old_input, output };
 393
 394                             new_path
 395                                 .segments
 396                                 .push(PathSegment { name: last_segment.name, args: new_params });
 397
 398                             Type::ResolvedPath {
 399                                 path: new_path,
 400                                 param_names: param_names.clone(),
 401                                 did: *did,
 402                                 is_generic: *is_generic,
 403                             }
 404                         }
 405                         _ => panic!("Unexpected data: {:?}, {:?}", ty, data),
 406                     };
 407                     bounds.insert(GenericBound::TraitBound(
 408                         PolyTrait { trait_: new_ty, generic_params: poly_trait.generic_params },
 409                         hir::TraitBoundModifier::None,
 410                     ));
 411                 }
 412                 if bounds.is_empty() {
 413                     return None;
 414                 }
 415
 416                 let mut bounds_vec = bounds.into_iter().collect();
 417                 self.sort_where_bounds(&mut bounds_vec);
 418
 419                 Some(WherePredicate::BoundPredicate { ty, bounds: bounds_vec })
 420             })
 421             .chain(
 422                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 423                     |(lifetime, bounds)| {
 424                         let mut bounds_vec = bounds.into_iter().collect();
 425                         self.sort_where_bounds(&mut bounds_vec);
 426                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 427                     },
 428                 ),
 429             )
 430             .collect()
 431     }
 432
 433     // Converts the calculated ParamEnv and lifetime information to a clean::Generics, suitable for
 434     // display on the docs page. Cleaning the Predicates produces sub-optimal `WherePredicate`s,
 435     // so we fix them up:
 436     //
 437     // * Multiple bounds for the same type are coalesced into one: e.g., 'T: Copy', 'T: Debug'
 438     // becomes 'T: Copy + Debug'
 439     // * Fn bounds are handled specially - instead of leaving it as 'T: Fn(), <T as Fn::Output> =
 440     // K', we use the dedicated syntax 'T: Fn() -> K'
 441     // * We explicitly add a '?Sized' bound if we didn't find any 'Sized' predicates for a type
 442     fn param_env_to_generics(
 443         &self,
 444         tcx: TyCtxt<'tcx>,
 445         param_env_def_id: DefId,
 446         param_env: ty::ParamEnv<'tcx>,
 447         mut existing_predicates: Vec<WherePredicate>,
 448         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 449     ) -> Generics {
 450         debug!(
 451             "param_env_to_generics(param_env_def_id={:?}, param_env={:?}, \
 452              existing_predicates={:?})",
 453             param_env_def_id, param_env, existing_predicates
 454         );
 455
 456         // The `Sized` trait must be handled specially, since we only display it when
 457         // it is *not* required (i.e., '?Sized')
 458         let sized_trait = self.cx.tcx.require_lang_item(LangItem::Sized, None);
 459
 460         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 461
 462         let orig_bounds: FxHashSet<_> =
 463             self.cx.tcx.param_env(param_env_def_id).caller_bounds().iter().collect();
 464         let clean_where_predicates = param_env
 465             .caller_bounds()
 466             .iter()
 467             .filter(|p| {
 468                 !orig_bounds.contains(p)
 469                     || match p.skip_binders() {
 470                         ty::PredicateAtom::Trait(pred, _) => pred.def_id() == sized_trait,
 471                         _ => false,
 472                     }
 473             })
 474             .map(|p| {
 475                 let replaced = p.fold_with(&mut replacer);
 476                 (replaced, replaced.clean(self.cx))
 477             });
 478
 479         let mut generic_params =
 480             (tcx.generics_of(param_env_def_id), tcx.explicit_predicates_of(param_env_def_id))
 481                 .clean(self.cx)
 482                 .params;
 483
 484         debug!(
 485             "param_env_to_generics({:?}): generic_params={:?}",
 486             param_env_def_id, generic_params
 487         );
 488
 489         let mut has_sized = FxHashSet::default();
 490         let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 491         let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 492         let mut ty_to_traits: FxHashMap<Type, FxHashSet<Type>> = Default::default();
 493
 494         let mut ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)> = Default::default();
 495
 496         for (orig_p, p) in clean_where_predicates {
 497             if p.is_none() {
 498                 continue;
 499             }
 500             let p = p.unwrap();
 501             match p {
 502                 WherePredicate::BoundPredicate { ty, mut bounds } => {
 503                     // Writing a projection trait bound of the form
 504                     // <T as Trait>::Name : ?Sized
 505                     // is illegal, because ?Sized bounds can only
 506                     // be written in the (here, nonexistent) definition
 507                     // of the type.
 508                     // Therefore, we make sure that we never add a ?Sized
 509                     // bound for projections
 510                     if let Type::QPath { .. } = ty {
 511                         has_sized.insert(ty.clone());
 512                     }
 513
 514                     if bounds.is_empty() {
 515                         continue;
 516                     }
 517
 518                     let mut for_generics = self.extract_for_generics(tcx, orig_p);
 519
 520                     assert!(bounds.len() == 1);
 521                     let mut b = bounds.pop().expect("bounds were empty");
 522
 523                     if b.is_sized_bound(self.cx) {
 524                         has_sized.insert(ty.clone());
 525                     } else if !b
 526                         .get_trait_type()
 527                         .and_then(|t| {
 528                             ty_to_traits
 529                                 .get(&ty)
 530                                 .map(|bounds| bounds.contains(&strip_type(t.clone())))
 531                         })
 532                         .unwrap_or(false)
 533                     {
 534                         // If we've already added a projection bound for the same type, don't add
 535                         // this, as it would be a duplicate
 536
 537                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 538                         // as we want to combine them with any 'Output' qpaths
 539                         // later
 540
 541                         let is_fn = match &mut b {
 542                             &mut GenericBound::TraitBound(ref mut p, _) => {
 543                                 // Insert regions into the for_generics hash map first, to ensure
 544                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 545                                 for_generics.extend(p.generic_params.clone());
 546                                 p.generic_params = for_generics.into_iter().collect();
 547                                 self.is_fn_ty(tcx, &p.trait_)
 548                             }
 549                             _ => false,
 550                         };
 551
 552                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 553
 554                         if is_fn {
 555                             ty_to_fn
 556                                 .entry(ty.clone())
 557                                 .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
 558                                 .or_insert(((Some(poly_trait.clone())), None));
 559
 560                             ty_to_bounds.entry(ty.clone()).or_default();
 561                         } else {
 562                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 563                         }
 564                     }
 565                 }
 566                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 567                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 568                 }
 569                 WherePredicate::EqPredicate { lhs, rhs } => {
 570                     match lhs {
 571                         Type::QPath { name: left_name, ref self_type, ref trait_ } => {
 572                             let ty = &*self_type;
 573                             match **trait_ {
 574                                 Type::ResolvedPath {
 575                                     path: ref trait_path,
 576                                     ref param_names,
 577                                     ref did,
 578                                     ref is_generic,
 579                                 } => {
 580                                     let mut new_trait_path = trait_path.clone();
 581
 582                                     if self.is_fn_ty(tcx, trait_) && left_name == sym::Output {
 583                                         ty_to_fn
 584                                             .entry(*ty.clone())
 585                                             .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
 586                                             .or_insert((None, Some(rhs)));
 587                                         continue;
 588                                     }
 589
 590                                     let args = &mut new_trait_path
 591                                         .segments
 592                                         .last_mut()
 593                                         .expect("segments were empty")
 594                                         .args;
 595
 596                                     match args {
 597                                         // Convert something like '<T as Iterator::Item> = u8'
 598                                         // to 'T: Iterator<Item=u8>'
 599                                         GenericArgs::AngleBracketed {
 600                                             ref mut bindings, ..
 601                                         } => {
 602                                             bindings.push(TypeBinding {
 603                                                 name: left_name.clone(),
 604                                                 kind: TypeBindingKind::Equality { ty: rhs },
 605                                             });
 606                                         }
 607                                         GenericArgs::Parenthesized { .. } => {
 608                                             existing_predicates.push(WherePredicate::EqPredicate {
 609                                                 lhs: lhs.clone(),
 610                                                 rhs,
 611                                             });
 612                                             continue; // If something other than a Fn ends up
 613                                             // with parenthesis, leave it alone
 614                                         }
 615                                     }
 616
 617                                     let bounds = ty_to_bounds.entry(*ty.clone()).or_default();
 618
 619                                     bounds.insert(GenericBound::TraitBound(
 620                                         PolyTrait {
 621                                             trait_: Type::ResolvedPath {
 622                                                 path: new_trait_path,
 623                                                 param_names: param_names.clone(),
 624                                                 did: *did,
 625                                                 is_generic: *is_generic,
 626                                             },
 627                                             generic_params: Vec::new(),
 628                                         },
 629                                         hir::TraitBoundModifier::None,
 630                                     ));
 631
 632                                     // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 633                                     // that we don't see a
 634                                     // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 635                                     // on the docs page.
 636                                     bounds.remove(&GenericBound::TraitBound(
 637                                         PolyTrait {
 638                                             trait_: *trait_.clone(),
 639                                             generic_params: Vec::new(),
 640                                         },
 641                                         hir::TraitBoundModifier::None,
 642                                     ));
 643                                     // Avoid creating any new duplicate bounds later in the outer
 644                                     // loop
 645                                     ty_to_traits
 646                                         .entry(*ty.clone())
 647                                         .or_default()
 648                                         .insert(*trait_.clone());
 649                                 }
 650                                 _ => panic!(
 651                                     "Unexpected trait {:?} for {:?}",
 652                                     trait_, param_env_def_id,
 653                                 ),
 654                             }
 655                         }
 656                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, param_env_def_id),
 657                     }
 658                 }
 659             };
 660         }
 661
 662         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 663
 664         existing_predicates.extend(final_bounds);
 665
 666         for param in generic_params.iter_mut() {
 667             match param.kind {
 668                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 669                     // We never want something like `impl<T=Foo>`.
 670                     default.take();
 671                     let generic_ty = Type::Generic(param.name.clone());
 672                     if !has_sized.contains(&generic_ty) {
 673                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 674                     }
 675                 }
 676                 GenericParamDefKind::Lifetime => {}
 677                 GenericParamDefKind::Const { .. } => {}
 678             }
 679         }
 680
 681         self.sort_where_predicates(&mut existing_predicates);
 682
 683         Generics { params: generic_params, where_predicates: existing_predicates }
 684     }
 685
 686     // Ensure that the predicates are in a consistent order. The precise
 687     // ordering doesn't actually matter, but it's important that
 688     // a given set of predicates always appears in the same order -
 689     // both for visual consistency between 'rustdoc' runs, and to
 690     // make writing tests much easier
 691     #[inline]
 692     fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
 693         // We should never have identical bounds - and if we do,
 694         // they're visually identical as well. Therefore, using
 695         // an unstable sort is fine.
 696         self.unstable_debug_sort(&mut predicates);
 697     }
 698
 699     // Ensure that the bounds are in a consistent order. The precise
 700     // ordering doesn't actually matter, but it's important that
 701     // a given set of bounds always appears in the same order -
 702     // both for visual consistency between 'rustdoc' runs, and to
 703     // make writing tests much easier
 704     #[inline]
 705     fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
 706         // We should never have identical bounds - and if we do,
 707         // they're visually identical as well. Therefore, using
 708         // an unstable sort is fine.
 709         self.unstable_debug_sort(&mut bounds);
 710     }
 711
 712     // This might look horrendously hacky, but it's actually not that bad.
 713     //
 714     // For performance reasons, we use several different FxHashMaps
 715     // in the process of computing the final set of where predicates.
 716     // However, the iteration order of a HashMap is completely unspecified.
 717     // In fact, the iteration of an FxHashMap can even vary between platforms,
 718     // since FxHasher has different behavior for 32-bit and 64-bit platforms.
 719     //
 720     // Obviously, it's extremely undesirable for documentation rendering
 721     // to be dependent on the platform it's run on. Apart from being confusing
 722     // to end users, it makes writing tests much more difficult, as predicates
 723     // can appear in any order in the final result.
 724     //
 725     // To solve this problem, we sort WherePredicates and GenericBounds
 726     // by their Debug string. The thing to keep in mind is that we don't really
 727     // care what the final order is - we're synthesizing an impl or bound
 728     // ourselves, so any order can be considered equally valid. By sorting the
 729     // predicates and bounds, however, we ensure that for a given codebase, all
 730     // auto-trait impls always render in exactly the same way.
 731     //
 732     // Using the Debug implementation for sorting prevents us from needing to
 733     // write quite a bit of almost entirely useless code (e.g., how should two
 734     // Types be sorted relative to each other). It also allows us to solve the
 735     // problem for both WherePredicates and GenericBounds at the same time. This
 736     // approach is probably somewhat slower, but the small number of items
 737     // involved (impls rarely have more than a few bounds) means that it
 738     // shouldn't matter in practice.
 739     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
 740         vec.sort_by_cached_key(|x| format!("{:?}", x))
 741     }
 742
 743     fn is_fn_ty(&self, tcx: TyCtxt<'_>, ty: &Type) -> bool {
 744         match &ty {
 745             &&Type::ResolvedPath { ref did, .. } => {
 746                 *did == tcx.require_lang_item(LangItem::Fn, None)
 747                     || *did == tcx.require_lang_item(LangItem::FnMut, None)
 748                     || *did == tcx.require_lang_item(LangItem::FnOnce, None)
 749             }
 750             _ => false,
 751         }
 752     }
 753 }
 754
 755 // Replaces all ReVars in a type with ty::Region's, using the provided map
 756 struct RegionReplacer<'a, 'tcx> {
 757     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 758     tcx: TyCtxt<'tcx>,
 759 }
 760
 761 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 762     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 763         self.tcx
 764     }
 765
 766     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 767         (match r {
 768             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 769             _ => None,
 770         })
 771         .unwrap_or_else(|| r.super_fold_with(self))
 772     }
 773 }