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 mut core::DocContext<'tcx>,
  25 }
  26
  27 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx> {
  28     crate fn new(cx: &'a mut core::DocContext<'tcx>) -> Self {
  29         AutoTraitFinder { cx }
  30     }
  31
  32     fn generate_for_trait(
  33         &mut self,
  34         ty: Ty<'tcx>,
  35         trait_def_id: DefId,
  36         param_env: ty::ParamEnv<'tcx>,
  37         item_def_id: DefId,
  38         f: &auto_trait::AutoTraitFinder<'tcx>,
  39         // If this is set, show only negative trait implementations, not positive ones.
  40         discard_positive_impl: bool,
  41     ) -> Option<Item> {
  42         let tcx = self.cx.tcx;
  43         let trait_ref = ty::TraitRef { def_id: trait_def_id, substs: tcx.mk_substs_trait(ty, &[]) };
  44         if !self.cx.generated_synthetics.insert((ty, trait_def_id)) {
  45             debug!("get_auto_trait_impl_for({:?}): already generated, aborting", trait_ref);
  46             return None;
  47         }
  48
  49         let result = f.find_auto_trait_generics(ty, param_env, trait_def_id, |info| {
  50             let region_data = info.region_data;
  51
  52             let names_map = tcx
  53                 .generics_of(item_def_id)
  54                 .params
  55                 .iter()
  56                 .filter_map(|param| match param.kind {
  57                     ty::GenericParamDefKind::Lifetime => Some(param.name),
  58                     _ => None,
  59                 })
  60                 .map(|name| (name, Lifetime(name)))
  61                 .collect();
  62             let lifetime_predicates = Self::handle_lifetimes(&region_data, &names_map);
  63             let new_generics = self.param_env_to_generics(
  64                 item_def_id,
  65                 info.full_user_env,
  66                 lifetime_predicates,
  67                 info.vid_to_region,
  68             );
  69
  70             debug!(
  71                 "find_auto_trait_generics(item_def_id={:?}, trait_def_id={:?}): \
  72                     finished with {:?}",
  73                 item_def_id, trait_def_id, new_generics
  74             );
  75
  76             new_generics
  77         });
  78
  79         let polarity;
  80         let new_generics = match result {
  81             AutoTraitResult::PositiveImpl(new_generics) => {
  82                 polarity = ty::ImplPolarity::Positive;
  83                 if discard_positive_impl {
  84                     return None;
  85                 }
  86                 new_generics
  87             }
  88             AutoTraitResult::NegativeImpl => {
  89                 polarity = ty::ImplPolarity::Negative;
  90
  91                 // For negative impls, we use the generic params, but *not* the predicates,
  92                 // from the original type. Otherwise, the displayed impl appears to be a
  93                 // conditional negative impl, when it's really unconditional.
  94                 //
  95                 // For example, consider the struct Foo<T: Copy>(*mut T). Using
  96                 // the original predicates in our impl would cause us to generate
  97                 // `impl !Send for Foo<T: Copy>`, which makes it appear that Foo
  98                 // implements Send where T is not copy.
  99                 //
 100                 // Instead, we generate `impl !Send for Foo<T>`, which better
 101                 // expresses the fact that `Foo<T>` never implements `Send`,
 102                 // regardless of the choice of `T`.
 103                 let params = (tcx.generics_of(item_def_id), ty::GenericPredicates::default())
 104                     .clean(self.cx)
 105                     .params;
 106
 107                 Generics { params, where_predicates: Vec::new() }
 108             }
 109             AutoTraitResult::ExplicitImpl => return None,
 110         };
 111
 112         Some(Item {
 113             name: None,
 114             attrs: Default::default(),
 115             visibility: Inherited,
 116             def_id: ItemId::Auto { trait_: trait_def_id, for_: item_def_id },
 117             kind: box ImplItem(Impl {
 118                 unsafety: hir::Unsafety::Normal,
 119                 generics: new_generics,
 120                 trait_: Some(trait_ref.clean(self.cx)),
 121                 for_: ty.clean(self.cx),
 122                 items: Vec::new(),
 123                 polarity,
 124                 kind: ImplKind::Auto,
 125             }),
 126             cfg: None,
 127         })
 128     }
 129
 130     crate fn get_auto_trait_impls(&mut self, item_def_id: DefId) -> Vec<Item> {
 131         let tcx = self.cx.tcx;
 132         let param_env = tcx.param_env(item_def_id);
 133         let ty = tcx.type_of(item_def_id);
 134         let f = auto_trait::AutoTraitFinder::new(tcx);
 135
 136         debug!("get_auto_trait_impls({:?})", ty);
 137         let auto_traits: Vec<_> = self.cx.auto_traits.iter().copied().collect();
 138         let mut auto_traits: Vec<Item> = auto_traits
 139             .into_iter()
 140             .filter_map(|trait_def_id| {
 141                 self.generate_for_trait(ty, trait_def_id, param_env, item_def_id, &f, false)
 142             })
 143             .collect();
 144         // We are only interested in case the type *doesn't* implement the Sized trait.
 145         if !ty.is_sized(tcx.at(rustc_span::DUMMY_SP), param_env) {
 146             // In case `#![no_core]` is used, `sized_trait` returns nothing.
 147             if let Some(item) = tcx.lang_items().sized_trait().and_then(|sized_trait_did| {
 148                 self.generate_for_trait(ty, sized_trait_did, param_env, item_def_id, &f, true)
 149             }) {
 150                 auto_traits.push(item);
 151             }
 152         }
 153         auto_traits
 154     }
 155
 156     fn get_lifetime(region: Region<'_>, names_map: &FxHashMap<Symbol, Lifetime>) -> Lifetime {
 157         region_name(region)
 158             .map(|name| {
 159                 names_map.get(&name).unwrap_or_else(|| {
 160                     panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
 161                 })
 162             })
 163             .unwrap_or(&Lifetime::statik())
 164             .clone()
 165     }
 166
 167     /// This method calculates two things: Lifetime constraints of the form `'a: 'b`,
 168     /// and region constraints of the form `RegionVid: '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         regions: &RegionConstraintData<'cx>,
 179         names_map: &FxHashMap<Symbol, 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 region_name(r1) != region_name(r2) {
 216                         finished
 217                             .entry(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 region_name(r1) != region_name(r2) {
 251                                 finished
 252                                     .entry(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(&self, pred: ty::Predicate<'tcx>) -> FxHashSet<GenericParamDef> {
 314         let bound_predicate = pred.kind();
 315         let tcx = self.cx.tcx;
 316         let regions = match bound_predicate.skip_binder() {
 317             ty::PredicateKind::Trait(poly_trait_pred) => {
 318                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
 319             }
 320             ty::PredicateKind::Projection(poly_proj_pred) => {
 321                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_proj_pred))
 322             }
 323             _ => return FxHashSet::default(),
 324         };
 325
 326         regions
 327             .into_iter()
 328             .filter_map(|br| {
 329                 match br {
 330                     // We only care about named late bound regions, as we need to add them
 331                     // to the 'for<>' section
 332                     ty::BrNamed(_, name) => Some(GenericParamDef {
 333                         name,
 334                         kind: GenericParamDefKind::Lifetime { outlives: vec![] },
 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().unwrap().clone(), data.1.as_ref().cloned().map(Box::new));
 354                     let mut new_path = poly_trait.trait_.clone();
 355                     let last_segment = new_path.segments.pop().expect("segments were empty");
 356
 357                     let (old_input, old_output) = match last_segment.args {
 358                         GenericArgs::AngleBracketed { args, .. } => {
 359                             let types = args
 360                                 .iter()
 361                                 .filter_map(|arg| match arg {
 362                                     GenericArg::Type(ty) => Some(ty.clone()),
 363                                     _ => None,
 364                                 })
 365                                 .collect();
 366                             (types, None)
 367                         }
 368                         GenericArgs::Parenthesized { inputs, output } => (inputs, output),
 369                     };
 370
 371                     if old_output.is_some() && old_output != output {
 372                         panic!("Output mismatch for {:?} {:?} {:?}", ty, old_output, data.1);
 373                     }
 374
 375                     let new_params = GenericArgs::Parenthesized { inputs: old_input, output };
 376
 377                     new_path
 378                         .segments
 379                         .push(PathSegment { name: last_segment.name, args: new_params });
 380
 381                     bounds.insert(GenericBound::TraitBound(
 382                         PolyTrait { trait_: new_path, generic_params: poly_trait.generic_params },
 383                         hir::TraitBoundModifier::None,
 384                     ));
 385                 }
 386                 if bounds.is_empty() {
 387                     return None;
 388                 }
 389
 390                 let mut bounds_vec = bounds.into_iter().collect();
 391                 self.sort_where_bounds(&mut bounds_vec);
 392
 393                 Some(WherePredicate::BoundPredicate {
 394                     ty,
 395                     bounds: bounds_vec,
 396                     bound_params: Vec::new(),
 397                 })
 398             })
 399             .chain(
 400                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 401                     |(lifetime, bounds)| {
 402                         let mut bounds_vec = bounds.into_iter().collect();
 403                         self.sort_where_bounds(&mut bounds_vec);
 404                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 405                     },
 406                 ),
 407             )
 408             .collect()
 409     }
 410
 411     /// Converts the calculated `ParamEnv` and lifetime information to a [`clean::Generics`](Generics), suitable for
 412     /// display on the docs page. Cleaning the `Predicates` produces sub-optimal [`WherePredicate`]s,
 413     /// so we fix them up:
 414     ///
 415     /// * Multiple bounds for the same type are coalesced into one: e.g., `T: Copy`, `T: Debug`
 416     /// becomes `T: Copy + Debug`
 417     /// * `Fn` bounds are handled specially - instead of leaving it as `T: Fn(), <T as Fn::Output> =
 418     /// K`, we use the dedicated syntax `T: Fn() -> K`
 419     /// * We explicitly add a `?Sized` bound if we didn't find any `Sized` predicates for a type
 420     fn param_env_to_generics(
 421         &mut self,
 422         item_def_id: DefId,
 423         param_env: ty::ParamEnv<'tcx>,
 424         mut existing_predicates: Vec<WherePredicate>,
 425         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 426     ) -> Generics {
 427         debug!(
 428             "param_env_to_generics(item_def_id={:?}, param_env={:?}, \
 429              existing_predicates={:?})",
 430             item_def_id, param_env, existing_predicates
 431         );
 432
 433         let tcx = self.cx.tcx;
 434
 435         // The `Sized` trait must be handled specially, since we only display it when
 436         // it is *not* required (i.e., '?Sized')
 437         let sized_trait = tcx.require_lang_item(LangItem::Sized, None);
 438
 439         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 440
 441         let orig_bounds: FxHashSet<_> = tcx.param_env(item_def_id).caller_bounds().iter().collect();
 442         let clean_where_predicates = param_env
 443             .caller_bounds()
 444             .iter()
 445             .filter(|p| {
 446                 !orig_bounds.contains(p)
 447                     || match p.kind().skip_binder() {
 448                         ty::PredicateKind::Trait(pred) => pred.def_id() == sized_trait,
 449                         _ => false,
 450                     }
 451             })
 452             .map(|p| p.fold_with(&mut replacer));
 453
 454         let mut generic_params =
 455             (tcx.generics_of(item_def_id), tcx.explicit_predicates_of(item_def_id))
 456                 .clean(self.cx)
 457                 .params;
 458
 459         debug!("param_env_to_generics({:?}): generic_params={:?}", item_def_id, generic_params);
 460
 461         let mut has_sized = FxHashSet::default();
 462         let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 463         let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 464         let mut ty_to_traits: FxHashMap<Type, FxHashSet<Path>> = Default::default();
 465
 466         let mut ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)> = Default::default();
 467
 468         for p in clean_where_predicates {
 469             let (orig_p, p) = (p, p.clean(self.cx));
 470             if p.is_none() {
 471                 continue;
 472             }
 473             let p = p.unwrap();
 474             match p {
 475                 WherePredicate::BoundPredicate { ty, mut bounds, .. } => {
 476                     // Writing a projection trait bound of the form
 477                     // <T as Trait>::Name : ?Sized
 478                     // is illegal, because ?Sized bounds can only
 479                     // be written in the (here, nonexistent) definition
 480                     // of the type.
 481                     // Therefore, we make sure that we never add a ?Sized
 482                     // bound for projections
 483                     if let Type::QPath { .. } = ty {
 484                         has_sized.insert(ty.clone());
 485                     }
 486
 487                     if bounds.is_empty() {
 488                         continue;
 489                     }
 490
 491                     let mut for_generics = self.extract_for_generics(orig_p);
 492
 493                     assert!(bounds.len() == 1);
 494                     let mut b = bounds.pop().expect("bounds were empty");
 495
 496                     if b.is_sized_bound(self.cx) {
 497                         has_sized.insert(ty.clone());
 498                     } else if !b
 499                         .get_trait_path()
 500                         .and_then(|trait_| {
 501                             ty_to_traits
 502                                 .get(&ty)
 503                                 .map(|bounds| bounds.contains(&strip_path_generics(trait_.clone())))
 504                         })
 505                         .unwrap_or(false)
 506                     {
 507                         // If we've already added a projection bound for the same type, don't add
 508                         // this, as it would be a duplicate
 509
 510                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 511                         // as we want to combine them with any 'Output' qpaths
 512                         // later
 513
 514                         let is_fn = match b {
 515                             GenericBound::TraitBound(ref mut p, _) => {
 516                                 // Insert regions into the for_generics hash map first, to ensure
 517                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 518                                 for_generics.extend(p.generic_params.clone());
 519                                 p.generic_params = for_generics.into_iter().collect();
 520                                 self.is_fn_trait(&p.trait_)
 521                             }
 522                             _ => false,
 523                         };
 524
 525                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 526
 527                         if is_fn {
 528                             ty_to_fn
 529                                 .entry(ty.clone())
 530                                 .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
 531                                 .or_insert(((Some(poly_trait.clone())), None));
 532
 533                             ty_to_bounds.entry(ty.clone()).or_default();
 534                         } else {
 535                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 536                         }
 537                     }
 538                 }
 539                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 540                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 541                 }
 542                 WherePredicate::EqPredicate { lhs, rhs } => {
 543                     match lhs {
 544                         Type::QPath { name: left_name, ref self_type, ref trait_, .. } => {
 545                             let ty = &*self_type;
 546                             let mut new_trait = trait_.clone();
 547
 548                             if self.is_fn_trait(trait_) && left_name == sym::Output {
 549                                 ty_to_fn
 550                                     .entry(*ty.clone())
 551                                     .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
 552                                     .or_insert((None, Some(rhs)));
 553                                 continue;
 554                             }
 555
 556                             let args = &mut new_trait
 557                                 .segments
 558                                 .last_mut()
 559                                 .expect("segments were empty")
 560                                 .args;
 561
 562                             match args {
 563                                 // Convert something like '<T as Iterator::Item> = u8'
 564                                 // to 'T: Iterator<Item=u8>'
 565                                 GenericArgs::AngleBracketed { ref mut bindings, .. } => {
 566                                     bindings.push(TypeBinding {
 567                                         name: left_name,
 568                                         kind: TypeBindingKind::Equality { ty: rhs },
 569                                     });
 570                                 }
 571                                 GenericArgs::Parenthesized { .. } => {
 572                                     existing_predicates.push(WherePredicate::EqPredicate {
 573                                         lhs: lhs.clone(),
 574                                         rhs,
 575                                     });
 576                                     continue; // If something other than a Fn ends up
 577                                     // with parentheses, leave it alone
 578                                 }
 579                             }
 580
 581                             let bounds = ty_to_bounds.entry(*ty.clone()).or_default();
 582
 583                             bounds.insert(GenericBound::TraitBound(
 584                                 PolyTrait { trait_: new_trait, generic_params: Vec::new() },
 585                                 hir::TraitBoundModifier::None,
 586                             ));
 587
 588                             // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 589                             // that we don't see a
 590                             // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 591                             // on the docs page.
 592                             bounds.remove(&GenericBound::TraitBound(
 593                                 PolyTrait { trait_: trait_.clone(), generic_params: Vec::new() },
 594                                 hir::TraitBoundModifier::None,
 595                             ));
 596                             // Avoid creating any new duplicate bounds later in the outer
 597                             // loop
 598                             ty_to_traits.entry(*ty.clone()).or_default().insert(trait_.clone());
 599                         }
 600                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, item_def_id),
 601                     }
 602                 }
 603             };
 604         }
 605
 606         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 607
 608         existing_predicates.extend(final_bounds);
 609
 610         for param in generic_params.iter_mut() {
 611             match param.kind {
 612                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 613                     // We never want something like `impl<T=Foo>`.
 614                     default.take();
 615                     let generic_ty = Type::Generic(param.name);
 616                     if !has_sized.contains(&generic_ty) {
 617                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 618                     }
 619                 }
 620                 GenericParamDefKind::Lifetime { .. } => {}
 621                 GenericParamDefKind::Const { ref mut default, .. } => {
 622                     // We never want something like `impl<const N: usize = 10>`
 623                     default.take();
 624                 }
 625             }
 626         }
 627
 628         self.sort_where_predicates(&mut existing_predicates);
 629
 630         Generics { params: generic_params, where_predicates: existing_predicates }
 631     }
 632
 633     /// Ensure that the predicates are in a consistent order. The precise
 634     /// ordering doesn't actually matter, but it's important that
 635     /// a given set of predicates always appears in the same order -
 636     /// both for visual consistency between 'rustdoc' runs, and to
 637     /// make writing tests much easier
 638     #[inline]
 639     fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
 640         // We should never have identical bounds - and if we do,
 641         // they're visually identical as well. Therefore, using
 642         // an unstable sort is fine.
 643         self.unstable_debug_sort(&mut predicates);
 644     }
 645
 646     /// Ensure that the bounds are in a consistent order. The precise
 647     /// ordering doesn't actually matter, but it's important that
 648     /// a given set of bounds always appears in the same order -
 649     /// both for visual consistency between 'rustdoc' runs, and to
 650     /// make writing tests much easier
 651     #[inline]
 652     fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
 653         // We should never have identical bounds - and if we do,
 654         // they're visually identical as well. Therefore, using
 655         // an unstable sort is fine.
 656         self.unstable_debug_sort(&mut bounds);
 657     }
 658
 659     /// This might look horrendously hacky, but it's actually not that bad.
 660     ///
 661     /// For performance reasons, we use several different FxHashMaps
 662     /// in the process of computing the final set of where predicates.
 663     /// However, the iteration order of a HashMap is completely unspecified.
 664     /// In fact, the iteration of an FxHashMap can even vary between platforms,
 665     /// since FxHasher has different behavior for 32-bit and 64-bit platforms.
 666     ///
 667     /// Obviously, it's extremely undesirable for documentation rendering
 668     /// to be dependent on the platform it's run on. Apart from being confusing
 669     /// to end users, it makes writing tests much more difficult, as predicates
 670     /// can appear in any order in the final result.
 671     ///
 672     /// To solve this problem, we sort WherePredicates and GenericBounds
 673     /// by their Debug string. The thing to keep in mind is that we don't really
 674     /// care what the final order is - we're synthesizing an impl or bound
 675     /// ourselves, so any order can be considered equally valid. By sorting the
 676     /// predicates and bounds, however, we ensure that for a given codebase, all
 677     /// auto-trait impls always render in exactly the same way.
 678     ///
 679     /// Using the Debug implementation for sorting prevents us from needing to
 680     /// write quite a bit of almost entirely useless code (e.g., how should two
 681     /// Types be sorted relative to each other). It also allows us to solve the
 682     /// problem for both WherePredicates and GenericBounds at the same time. This
 683     /// approach is probably somewhat slower, but the small number of items
 684     /// involved (impls rarely have more than a few bounds) means that it
 685     /// shouldn't matter in practice.
 686     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
 687         vec.sort_by_cached_key(|x| format!("{:?}", x))
 688     }
 689
 690     fn is_fn_trait(&self, path: &Path) -> bool {
 691         let tcx = self.cx.tcx;
 692         let did = path.def_id();
 693         did == tcx.require_lang_item(LangItem::Fn, None)
 694             || did == tcx.require_lang_item(LangItem::FnMut, None)
 695             || did == tcx.require_lang_item(LangItem::FnOnce, None)
 696     }
 697 }
 698
 699 fn region_name(region: Region<'_>) -> Option<Symbol> {
 700     match *region {
 701         ty::ReEarlyBound(r) => Some(r.name),
 702         _ => None,
 703     }
 704 }
 705
 706 /// Replaces all [`ty::RegionVid`]s in a type with [`ty::Region`]s, using the provided map.
 707 struct RegionReplacer<'a, 'tcx> {
 708     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 709     tcx: TyCtxt<'tcx>,
 710 }
 711
 712 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 713     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 714         self.tcx
 715     }
 716
 717     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 718         (match *r {
 719             ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 720             _ => None,
 721         })
 722         .unwrap_or_else(|| r.super_fold_with(self))
 723     }
 724 }