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, TypeSuperFoldable};
   5 use rustc_trait_selection::traits::auto_trait::{self, AutoTraitResult};
   6 use thin_vec::ThinVec;
   7
   8 use std::fmt::Debug;
   9
  10 use super::*;
  11
  12 #[derive(Eq, PartialEq, Hash, Copy, Clone, Debug)]
  13 enum RegionTarget<'tcx> {
  14     Region(Region<'tcx>),
  15     RegionVid(RegionVid),
  16 }
  17
  18 #[derive(Default, Debug, Clone)]
  19 struct RegionDeps<'tcx> {
  20     larger: FxHashSet<RegionTarget<'tcx>>,
  21     smaller: FxHashSet<RegionTarget<'tcx>>,
  22 }
  23
  24 pub(crate) struct AutoTraitFinder<'a, 'tcx> {
  25     pub(crate) cx: &'a mut core::DocContext<'tcx>,
  26 }
  27
  28 impl<'a, 'tcx> AutoTraitFinder<'a, 'tcx>
  29 where
  30     'tcx: 'a, // should be an implied bound; rustc bug #98852.
  31 {
  32     pub(crate) fn new(cx: &'a mut core::DocContext<'tcx>) -> Self {
  33         AutoTraitFinder { cx }
  34     }
  35
  36     fn generate_for_trait(
  37         &mut self,
  38         ty: Ty<'tcx>,
  39         trait_def_id: DefId,
  40         param_env: ty::ParamEnv<'tcx>,
  41         item_def_id: DefId,
  42         f: &auto_trait::AutoTraitFinder<'tcx>,
  43         // If this is set, show only negative trait implementations, not positive ones.
  44         discard_positive_impl: bool,
  45     ) -> Option<Item> {
  46         let tcx = self.cx.tcx;
  47         let trait_ref = ty::Binder::dummy(tcx.mk_trait_ref(trait_def_id, [ty]));
  48         if !self.cx.generated_synthetics.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 = f.find_auto_trait_generics(ty, param_env, trait_def_id, |info| {
  54             let region_data = info.region_data;
  55
  56             let names_map = tcx
  57                 .generics_of(item_def_id)
  58                 .params
  59                 .iter()
  60                 .filter_map(|param| match param.kind {
  61                     ty::GenericParamDefKind::Lifetime => Some(param.name),
  62                     _ => None,
  63                 })
  64                 .map(|name| (name, Lifetime(name)))
  65                 .collect();
  66             let lifetime_predicates = Self::handle_lifetimes(&region_data, &names_map);
  67             let new_generics = self.param_env_to_generics(
  68                 item_def_id,
  69                 info.full_user_env,
  70                 lifetime_predicates,
  71                 info.vid_to_region,
  72             );
  73
  74             debug!(
  75                 "find_auto_trait_generics(item_def_id={:?}, trait_def_id={:?}): \
  76                     finished with {:?}",
  77                 item_def_id, trait_def_id, new_generics
  78             );
  79
  80             new_generics
  81         });
  82
  83         let polarity;
  84         let new_generics = match result {
  85             AutoTraitResult::PositiveImpl(new_generics) => {
  86                 polarity = ty::ImplPolarity::Positive;
  87                 if discard_positive_impl {
  88                     return None;
  89                 }
  90                 new_generics
  91             }
  92             AutoTraitResult::NegativeImpl => {
  93                 polarity = ty::ImplPolarity::Negative;
  94
  95                 // For negative impls, we use the generic params, but *not* the predicates,
  96                 // from the original type. Otherwise, the displayed impl appears to be a
  97                 // conditional negative impl, when it's really unconditional.
  98                 //
  99                 // For example, consider the struct Foo<T: Copy>(*mut T). Using
 100                 // the original predicates in our impl would cause us to generate
 101                 // `impl !Send for Foo<T: Copy>`, which makes it appear that Foo
 102                 // implements Send where T is not copy.
 103                 //
 104                 // Instead, we generate `impl !Send for Foo<T>`, which better
 105                 // expresses the fact that `Foo<T>` never implements `Send`,
 106                 // regardless of the choice of `T`.
 107                 let raw_generics = clean_ty_generics(
 108                     self.cx,
 109                     tcx.generics_of(item_def_id),
 110                     ty::GenericPredicates::default(),
 111                 );
 112                 let params = raw_generics.params;
 113
 114                 Generics { params, where_predicates: ThinVec::new() }
 115             }
 116             AutoTraitResult::ExplicitImpl => return None,
 117         };
 118
 119         Some(Item {
 120             name: None,
 121             attrs: Default::default(),
 122             item_id: ItemId::Auto { trait_: trait_def_id, for_: item_def_id },
 123             kind: Box::new(ImplItem(Box::new(Impl {
 124                 unsafety: hir::Unsafety::Normal,
 125                 generics: new_generics,
 126                 trait_: Some(clean_trait_ref_with_bindings(self.cx, trait_ref, ThinVec::new())),
 127                 for_: clean_middle_ty(ty::Binder::dummy(ty), self.cx, None),
 128                 items: Vec::new(),
 129                 polarity,
 130                 kind: ImplKind::Auto,
 131             }))),
 132             cfg: None,
 133             inline_stmt_id: None,
 134         })
 135     }
 136
 137     pub(crate) fn get_auto_trait_impls(&mut self, item_def_id: DefId) -> Vec<Item> {
 138         let tcx = self.cx.tcx;
 139         let param_env = tcx.param_env(item_def_id);
 140         let ty = tcx.type_of(item_def_id);
 141         let f = auto_trait::AutoTraitFinder::new(tcx);
 142
 143         debug!("get_auto_trait_impls({:?})", ty);
 144         let auto_traits: Vec<_> = self.cx.auto_traits.iter().copied().collect();
 145         let mut auto_traits: Vec<Item> = auto_traits
 146             .into_iter()
 147             .filter_map(|trait_def_id| {
 148                 self.generate_for_trait(ty, trait_def_id, param_env, item_def_id, &f, false)
 149             })
 150             .collect();
 151         // We are only interested in case the type *doesn't* implement the Sized trait.
 152         if !ty.is_sized(tcx, param_env) {
 153             // In case `#![no_core]` is used, `sized_trait` returns nothing.
 154             if let Some(item) = tcx.lang_items().sized_trait().and_then(|sized_trait_did| {
 155                 self.generate_for_trait(ty, sized_trait_did, param_env, item_def_id, &f, true)
 156             }) {
 157                 auto_traits.push(item);
 158             }
 159         }
 160         auto_traits
 161     }
 162
 163     fn get_lifetime(region: Region<'_>, names_map: &FxHashMap<Symbol, Lifetime>) -> Lifetime {
 164         region_name(region)
 165             .map(|name| {
 166                 names_map.get(&name).unwrap_or_else(|| {
 167                     panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
 168                 })
 169             })
 170             .unwrap_or(&Lifetime::statik())
 171             .clone()
 172     }
 173
 174     /// This method calculates two things: Lifetime constraints of the form `'a: 'b`,
 175     /// and region constraints of the form `RegionVid: 'a`
 176     ///
 177     /// This is essentially a simplified version of lexical_region_resolve. However,
 178     /// handle_lifetimes determines what *needs be* true in order for an impl to hold.
 179     /// lexical_region_resolve, along with much of the rest of the compiler, is concerned
 180     /// with determining if a given set up constraints/predicates *are* met, given some
 181     /// starting conditions (e.g., user-provided code). For this reason, it's easier
 182     /// to perform the calculations we need on our own, rather than trying to make
 183     /// existing inference/solver code do what we want.
 184     fn handle_lifetimes<'cx>(
 185         regions: &RegionConstraintData<'cx>,
 186         names_map: &FxHashMap<Symbol, Lifetime>,
 187     ) -> ThinVec<WherePredicate> {
 188         // Our goal is to 'flatten' the list of constraints by eliminating
 189         // all intermediate RegionVids. At the end, all constraints should
 190         // be between Regions (aka region variables). This gives us the information
 191         // we need to create the Generics.
 192         let mut finished: FxHashMap<_, Vec<_>> = Default::default();
 193
 194         let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
 195
 196         // Flattening is done in two parts. First, we insert all of the constraints
 197         // into a map. Each RegionTarget (either a RegionVid or a Region) maps
 198         // to its smaller and larger regions. Note that 'larger' regions correspond
 199         // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
 200         for constraint in regions.constraints.keys() {
 201             match *constraint {
 202                 Constraint::VarSubVar(r1, r2) => {
 203                     {
 204                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 205                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 206                     }
 207
 208                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 209                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 210                 }
 211                 Constraint::RegSubVar(region, vid) => {
 212                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 213                     deps.smaller.insert(RegionTarget::Region(region));
 214                 }
 215                 Constraint::VarSubReg(vid, region) => {
 216                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 217                     deps.larger.insert(RegionTarget::Region(region));
 218                 }
 219                 Constraint::RegSubReg(r1, r2) => {
 220                     // The constraint is already in the form that we want, so we're done with it
 221                     // Desired order is 'larger, smaller', so flip then
 222                     if region_name(r1) != region_name(r2) {
 223                         finished
 224                             .entry(region_name(r2).expect("no region_name found"))
 225                             .or_default()
 226                             .push(r1);
 227                     }
 228                 }
 229             }
 230         }
 231
 232         // Here, we 'flatten' the map one element at a time.
 233         // All of the element's sub and super regions are connected
 234         // to each other. For example, if we have a graph that looks like this:
 235         //
 236         // (A, B) - C - (D, E)
 237         // Where (A, B) are subregions, and (D,E) are super-regions
 238         //
 239         // then after deleting 'C', the graph will look like this:
 240         //  ... - A - (D, E ...)
 241         //  ... - B - (D, E, ...)
 242         //  (A, B, ...) - D - ...
 243         //  (A, B, ...) - E - ...
 244         //
 245         //  where '...' signifies the existing sub and super regions of an entry
 246         //  When two adjacent ty::Regions are encountered, we've computed a final
 247         //  constraint, and add it to our list. Since we make sure to never re-add
 248         //  deleted items, this process will always finish.
 249         while !vid_map.is_empty() {
 250             let target = *vid_map.keys().next().expect("Keys somehow empty");
 251             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 252
 253             for smaller in deps.smaller.iter() {
 254                 for larger in deps.larger.iter() {
 255                     match (smaller, larger) {
 256                         (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
 257                             if region_name(r1) != region_name(r2) {
 258                                 finished
 259                                     .entry(region_name(r2).expect("no region name found"))
 260                                     .or_default()
 261                                     .push(r1) // Larger, smaller
 262                             }
 263                         }
 264                         (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
 265                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 266                                 let smaller_deps = v.into_mut();
 267                                 smaller_deps.larger.insert(*larger);
 268                                 smaller_deps.larger.remove(&target);
 269                             }
 270                         }
 271                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 272                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 273                                 let deps = v.into_mut();
 274                                 deps.smaller.insert(*smaller);
 275                                 deps.smaller.remove(&target);
 276                             }
 277                         }
 278                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 279                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 280                                 let smaller_deps = v.into_mut();
 281                                 smaller_deps.larger.insert(*larger);
 282                                 smaller_deps.larger.remove(&target);
 283                             }
 284
 285                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 286                                 let larger_deps = v.into_mut();
 287                                 larger_deps.smaller.insert(*smaller);
 288                                 larger_deps.smaller.remove(&target);
 289                             }
 290                         }
 291                     }
 292                 }
 293             }
 294         }
 295
 296         let lifetime_predicates = names_map
 297             .iter()
 298             .flat_map(|(name, lifetime)| {
 299                 let empty = Vec::new();
 300                 let bounds: FxHashSet<GenericBound> = finished
 301                     .get(name)
 302                     .unwrap_or(&empty)
 303                     .iter()
 304                     .map(|region| GenericBound::Outlives(Self::get_lifetime(*region, names_map)))
 305                     .collect();
 306
 307                 if bounds.is_empty() {
 308                     return None;
 309                 }
 310                 Some(WherePredicate::RegionPredicate {
 311                     lifetime: lifetime.clone(),
 312                     bounds: bounds.into_iter().collect(),
 313                 })
 314             })
 315             .collect();
 316
 317         lifetime_predicates
 318     }
 319
 320     fn extract_for_generics(&self, pred: ty::Predicate<'tcx>) -> FxHashSet<GenericParamDef> {
 321         let bound_predicate = pred.kind();
 322         let tcx = self.cx.tcx;
 323         let regions = match bound_predicate.skip_binder() {
 324             ty::PredicateKind::Clause(ty::Clause::Trait(poly_trait_pred)) => {
 325                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
 326             }
 327             ty::PredicateKind::Clause(ty::Clause::Projection(poly_proj_pred)) => {
 328                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_proj_pred))
 329             }
 330             _ => return FxHashSet::default(),
 331         };
 332
 333         regions
 334             .into_iter()
 335             .filter_map(|br| {
 336                 match br {
 337                     // We only care about named late bound regions, as we need to add them
 338                     // to the 'for<>' section
 339                     ty::BrNamed(_, name) => Some(GenericParamDef::lifetime(name)),
 340                     _ => None,
 341                 }
 342             })
 343             .collect()
 344     }
 345
 346     fn make_final_bounds(
 347         &self,
 348         ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
 349         ty_to_fn: FxHashMap<Type, (PolyTrait, Option<Type>)>,
 350         lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
 351     ) -> Vec<WherePredicate> {
 352         ty_to_bounds
 353             .into_iter()
 354             .flat_map(|(ty, mut bounds)| {
 355                 if let Some((ref poly_trait, ref output)) = ty_to_fn.get(&ty) {
 356                     let mut new_path = poly_trait.trait_.clone();
 357                     let last_segment = new_path.segments.pop().expect("segments were empty");
 358
 359                     let (old_input, old_output) = match last_segment.args {
 360                         GenericArgs::AngleBracketed { args, .. } => {
 361                             let types = args
 362                                 .iter()
 363                                 .filter_map(|arg| match arg {
 364                                     GenericArg::Type(ty) => Some(ty.clone()),
 365                                     _ => None,
 366                                 })
 367                                 .collect();
 368                             (types, None)
 369                         }
 370                         GenericArgs::Parenthesized { inputs, output } => (inputs, output),
 371                     };
 372
 373                     let output = output.as_ref().cloned().map(Box::new);
 374                     if old_output.is_some() && old_output != output {
 375                         panic!("Output mismatch for {:?} {:?} {:?}", ty, old_output, output);
 376                     }
 377
 378                     let new_params = GenericArgs::Parenthesized { inputs: old_input, output };
 379
 380                     new_path
 381                         .segments
 382                         .push(PathSegment { name: last_segment.name, args: new_params });
 383
 384                     bounds.insert(GenericBound::TraitBound(
 385                         PolyTrait {
 386                             trait_: new_path,
 387                             generic_params: poly_trait.generic_params.clone(),
 388                         },
 389                         hir::TraitBoundModifier::None,
 390                     ));
 391                 }
 392                 if bounds.is_empty() {
 393                     return None;
 394                 }
 395
 396                 let mut bounds_vec = bounds.into_iter().collect();
 397                 self.sort_where_bounds(&mut bounds_vec);
 398
 399                 Some(WherePredicate::BoundPredicate {
 400                     ty,
 401                     bounds: bounds_vec,
 402                     bound_params: Vec::new(),
 403                 })
 404             })
 405             .chain(
 406                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 407                     |(lifetime, bounds)| {
 408                         let mut bounds_vec = bounds.into_iter().collect();
 409                         self.sort_where_bounds(&mut bounds_vec);
 410                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 411                     },
 412                 ),
 413             )
 414             .collect()
 415     }
 416
 417     /// Converts the calculated `ParamEnv` and lifetime information to a [`clean::Generics`](Generics), suitable for
 418     /// display on the docs page. Cleaning the `Predicates` produces sub-optimal [`WherePredicate`]s,
 419     /// so we fix them up:
 420     ///
 421     /// * Multiple bounds for the same type are coalesced into one: e.g., `T: Copy`, `T: Debug`
 422     /// becomes `T: Copy + Debug`
 423     /// * `Fn` bounds are handled specially - instead of leaving it as `T: Fn(), <T as Fn::Output> =
 424     /// K`, we use the dedicated syntax `T: Fn() -> K`
 425     /// * We explicitly add a `?Sized` bound if we didn't find any `Sized` predicates for a type
 426     fn param_env_to_generics(
 427         &mut self,
 428         item_def_id: DefId,
 429         param_env: ty::ParamEnv<'tcx>,
 430         mut existing_predicates: ThinVec<WherePredicate>,
 431         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 432     ) -> Generics {
 433         debug!(
 434             "param_env_to_generics(item_def_id={:?}, param_env={:?}, \
 435              existing_predicates={:?})",
 436             item_def_id, param_env, existing_predicates
 437         );
 438
 439         let tcx = self.cx.tcx;
 440
 441         // The `Sized` trait must be handled specially, since we only display it when
 442         // it is *not* required (i.e., '?Sized')
 443         let sized_trait = tcx.require_lang_item(LangItem::Sized, None);
 444
 445         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 446
 447         let orig_bounds: FxHashSet<_> = tcx.param_env(item_def_id).caller_bounds().iter().collect();
 448         let clean_where_predicates = param_env
 449             .caller_bounds()
 450             .iter()
 451             .filter(|p| {
 452                 !orig_bounds.contains(p)
 453                     || match p.kind().skip_binder() {
 454                         ty::PredicateKind::Clause(ty::Clause::Trait(pred)) => {
 455                             pred.def_id() == sized_trait
 456                         }
 457                         _ => false,
 458                     }
 459             })
 460             .map(|p| p.fold_with(&mut replacer));
 461
 462         let raw_generics = clean_ty_generics(
 463             self.cx,
 464             tcx.generics_of(item_def_id),
 465             tcx.explicit_predicates_of(item_def_id),
 466         );
 467         let mut generic_params = raw_generics.params;
 468
 469         debug!("param_env_to_generics({:?}): generic_params={:?}", item_def_id, generic_params);
 470
 471         let mut has_sized = FxHashSet::default();
 472         let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 473         let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 474         let mut ty_to_traits: FxHashMap<Type, FxHashSet<Path>> = Default::default();
 475
 476         let mut ty_to_fn: FxHashMap<Type, (PolyTrait, Option<Type>)> = Default::default();
 477
 478         // FIXME: This code shares much of the logic found in `clean_ty_generics` and
 479         //        `simplify::where_clause`. Consider deduplicating it to avoid diverging
 480         //        implementations.
 481         //        Further, the code below does not merge (partially re-sugared) bounds like
 482         //        `Tr<A = T>` & `Tr<B = U>` and it does not render higher-ranked parameters
 483         //        originating from equality predicates.
 484         for p in clean_where_predicates {
 485             let (orig_p, p) = (p, clean_predicate(p, self.cx));
 486             if p.is_none() {
 487                 continue;
 488             }
 489             let p = p.unwrap();
 490             match p {
 491                 WherePredicate::BoundPredicate { ty, mut bounds, .. } => {
 492                     // Writing a projection trait bound of the form
 493                     // <T as Trait>::Name : ?Sized
 494                     // is illegal, because ?Sized bounds can only
 495                     // be written in the (here, nonexistent) definition
 496                     // of the type.
 497                     // Therefore, we make sure that we never add a ?Sized
 498                     // bound for projections
 499                     if let Type::QPath { .. } = ty {
 500                         has_sized.insert(ty.clone());
 501                     }
 502
 503                     if bounds.is_empty() {
 504                         continue;
 505                     }
 506
 507                     let mut for_generics = self.extract_for_generics(orig_p);
 508
 509                     assert!(bounds.len() == 1);
 510                     let mut b = bounds.pop().expect("bounds were empty");
 511
 512                     if b.is_sized_bound(self.cx) {
 513                         has_sized.insert(ty.clone());
 514                     } else if !b
 515                         .get_trait_path()
 516                         .and_then(|trait_| {
 517                             ty_to_traits
 518                                 .get(&ty)
 519                                 .map(|bounds| bounds.contains(&strip_path_generics(trait_)))
 520                         })
 521                         .unwrap_or(false)
 522                     {
 523                         // If we've already added a projection bound for the same type, don't add
 524                         // this, as it would be a duplicate
 525
 526                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 527                         // as we want to combine them with any 'Output' qpaths
 528                         // later
 529
 530                         let is_fn = match b {
 531                             GenericBound::TraitBound(ref mut p, _) => {
 532                                 // Insert regions into the for_generics hash map first, to ensure
 533                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 534                                 for_generics.extend(p.generic_params.drain(..));
 535                                 p.generic_params.extend(for_generics);
 536                                 self.is_fn_trait(&p.trait_)
 537                             }
 538                             _ => false,
 539                         };
 540
 541                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 542
 543                         if is_fn {
 544                             ty_to_fn
 545                                 .entry(ty.clone())
 546                                 .and_modify(|e| *e = (poly_trait.clone(), e.1.clone()))
 547                                 .or_insert(((poly_trait.clone()), None));
 548
 549                             ty_to_bounds.entry(ty.clone()).or_default();
 550                         } else {
 551                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 552                         }
 553                     }
 554                 }
 555                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 556                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 557                 }
 558                 WherePredicate::EqPredicate { lhs, rhs, bound_params } => {
 559                     match *lhs {
 560                         Type::QPath(box QPathData {
 561                             ref assoc, ref self_type, ref trait_, ..
 562                         }) => {
 563                             let ty = &*self_type;
 564                             let mut new_trait = trait_.clone();
 565
 566                             if self.is_fn_trait(trait_) && assoc.name == sym::Output {
 567                                 ty_to_fn
 568                                     .entry(ty.clone())
 569                                     .and_modify(|e| {
 570                                         *e = (e.0.clone(), Some(rhs.ty().unwrap().clone()))
 571                                     })
 572                                     .or_insert((
 573                                         PolyTrait {
 574                                             trait_: trait_.clone(),
 575                                             generic_params: Vec::new(),
 576                                         },
 577                                         Some(rhs.ty().unwrap().clone()),
 578                                     ));
 579                                 continue;
 580                             }
 581
 582                             let args = &mut new_trait
 583                                 .segments
 584                                 .last_mut()
 585                                 .expect("segments were empty")
 586                                 .args;
 587
 588                             match args {
 589                                 // Convert something like '<T as Iterator::Item> = u8'
 590                                 // to 'T: Iterator<Item=u8>'
 591                                 GenericArgs::AngleBracketed { ref mut bindings, .. } => {
 592                                     bindings.push(TypeBinding {
 593                                         assoc: assoc.clone(),
 594                                         kind: TypeBindingKind::Equality { term: *rhs },
 595                                     });
 596                                 }
 597                                 GenericArgs::Parenthesized { .. } => {
 598                                     existing_predicates.push(WherePredicate::EqPredicate {
 599                                         lhs: lhs.clone(),
 600                                         rhs,
 601                                         bound_params,
 602                                     });
 603                                     continue; // If something other than a Fn ends up
 604                                     // with parentheses, leave it alone
 605                                 }
 606                             }
 607
 608                             let bounds = ty_to_bounds.entry(ty.clone()).or_default();
 609
 610                             bounds.insert(GenericBound::TraitBound(
 611                                 PolyTrait { trait_: new_trait, generic_params: Vec::new() },
 612                                 hir::TraitBoundModifier::None,
 613                             ));
 614
 615                             // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 616                             // that we don't see a
 617                             // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 618                             // on the docs page.
 619                             bounds.remove(&GenericBound::TraitBound(
 620                                 PolyTrait { trait_: trait_.clone(), generic_params: Vec::new() },
 621                                 hir::TraitBoundModifier::None,
 622                             ));
 623                             // Avoid creating any new duplicate bounds later in the outer
 624                             // loop
 625                             ty_to_traits.entry(ty.clone()).or_default().insert(trait_.clone());
 626                         }
 627                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, item_def_id),
 628                     }
 629                 }
 630             };
 631         }
 632
 633         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 634
 635         existing_predicates.extend(final_bounds);
 636
 637         for param in generic_params.iter_mut() {
 638             match param.kind {
 639                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 640                     // We never want something like `impl<T=Foo>`.
 641                     default.take();
 642                     let generic_ty = Type::Generic(param.name);
 643                     if !has_sized.contains(&generic_ty) {
 644                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 645                     }
 646                 }
 647                 GenericParamDefKind::Lifetime { .. } => {}
 648                 GenericParamDefKind::Const { ref mut default, .. } => {
 649                     // We never want something like `impl<const N: usize = 10>`
 650                     default.take();
 651                 }
 652             }
 653         }
 654
 655         self.sort_where_predicates(&mut existing_predicates);
 656
 657         Generics { params: generic_params, where_predicates: existing_predicates }
 658     }
 659
 660     /// Ensure that the predicates are in a consistent order. The precise
 661     /// ordering doesn't actually matter, but it's important that
 662     /// a given set of predicates always appears in the same order -
 663     /// both for visual consistency between 'rustdoc' runs, and to
 664     /// make writing tests much easier
 665     #[inline]
 666     fn sort_where_predicates(&self, predicates: &mut [WherePredicate]) {
 667         // We should never have identical bounds - and if we do,
 668         // they're visually identical as well. Therefore, using
 669         // an unstable sort is fine.
 670         self.unstable_debug_sort(predicates);
 671     }
 672
 673     /// Ensure that the bounds are in a consistent order. The precise
 674     /// ordering doesn't actually matter, but it's important that
 675     /// a given set of bounds always appears in the same order -
 676     /// both for visual consistency between 'rustdoc' runs, and to
 677     /// make writing tests much easier
 678     #[inline]
 679     fn sort_where_bounds(&self, bounds: &mut Vec<GenericBound>) {
 680         // We should never have identical bounds - and if we do,
 681         // they're visually identical as well. Therefore, using
 682         // an unstable sort is fine.
 683         self.unstable_debug_sort(bounds);
 684     }
 685
 686     /// This might look horrendously hacky, but it's actually not that bad.
 687     ///
 688     /// For performance reasons, we use several different FxHashMaps
 689     /// in the process of computing the final set of where predicates.
 690     /// However, the iteration order of a HashMap is completely unspecified.
 691     /// In fact, the iteration of an FxHashMap can even vary between platforms,
 692     /// since FxHasher has different behavior for 32-bit and 64-bit platforms.
 693     ///
 694     /// Obviously, it's extremely undesirable for documentation rendering
 695     /// to be dependent on the platform it's run on. Apart from being confusing
 696     /// to end users, it makes writing tests much more difficult, as predicates
 697     /// can appear in any order in the final result.
 698     ///
 699     /// To solve this problem, we sort WherePredicates and GenericBounds
 700     /// by their Debug string. The thing to keep in mind is that we don't really
 701     /// care what the final order is - we're synthesizing an impl or bound
 702     /// ourselves, so any order can be considered equally valid. By sorting the
 703     /// predicates and bounds, however, we ensure that for a given codebase, all
 704     /// auto-trait impls always render in exactly the same way.
 705     ///
 706     /// Using the Debug implementation for sorting prevents us from needing to
 707     /// write quite a bit of almost entirely useless code (e.g., how should two
 708     /// Types be sorted relative to each other). It also allows us to solve the
 709     /// problem for both WherePredicates and GenericBounds at the same time. This
 710     /// approach is probably somewhat slower, but the small number of items
 711     /// involved (impls rarely have more than a few bounds) means that it
 712     /// shouldn't matter in practice.
 713     fn unstable_debug_sort<T: Debug>(&self, vec: &mut [T]) {
 714         vec.sort_by_cached_key(|x| format!("{:?}", x))
 715     }
 716
 717     fn is_fn_trait(&self, path: &Path) -> bool {
 718         let tcx = self.cx.tcx;
 719         let did = path.def_id();
 720         did == tcx.require_lang_item(LangItem::Fn, None)
 721             || did == tcx.require_lang_item(LangItem::FnMut, None)
 722             || did == tcx.require_lang_item(LangItem::FnOnce, None)
 723     }
 724 }
 725
 726 fn region_name(region: Region<'_>) -> Option<Symbol> {
 727     match *region {
 728         ty::ReEarlyBound(r) => Some(r.name),
 729         _ => None,
 730     }
 731 }
 732
 733 /// Replaces all [`ty::RegionVid`]s in a type with [`ty::Region`]s, using the provided map.
 734 struct RegionReplacer<'a, 'tcx> {
 735     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 736     tcx: TyCtxt<'tcx>,
 737 }
 738
 739 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 740     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 741         self.tcx
 742     }
 743
 744     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 745         (match *r {
 746             ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 747             _ => None,
 748         })
 749         .unwrap_or_else(|| r.super_fold_with(self))
 750     }
 751 }