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         param_env_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, |infcx, info| {
  50             let region_data = info.region_data;
  51
  52             let names_map = tcx
  53                 .generics_of(param_env_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                 infcx.tcx,
  65                 param_env_def_id,
  66                 info.full_user_env,
  67                 lifetime_predicates,
  68                 info.vid_to_region,
  69             );
  70
  71             debug!(
  72                 "find_auto_trait_generics(param_env_def_id={:?}, trait_def_id={:?}): \
  73                     finished with {:?}",
  74                 param_env_def_id, trait_def_id, new_generics
  75             );
  76
  77             new_generics
  78         });
  79
  80         let negative_polarity;
  81         let new_generics = match result {
  82             AutoTraitResult::PositiveImpl(new_generics) => {
  83                 negative_polarity = false;
  84                 if discard_positive_impl {
  85                     return None;
  86                 }
  87                 new_generics
  88             }
  89             AutoTraitResult::NegativeImpl => {
  90                 negative_polarity = true;
  91
  92                 // For negative impls, we use the generic params, but *not* the predicates,
  93                 // from the original type. Otherwise, the displayed impl appears to be a
  94                 // conditional negative impl, when it's really unconditional.
  95                 //
  96                 // For example, consider the struct Foo<T: Copy>(*mut T). Using
  97                 // the original predicates in our impl would cause us to generate
  98                 // `impl !Send for Foo<T: Copy>`, which makes it appear that Foo
  99                 // implements Send where T is not copy.
 100                 //
 101                 // Instead, we generate `impl !Send for Foo<T>`, which better
 102                 // expresses the fact that `Foo<T>` never implements `Send`,
 103                 // regardless of the choice of `T`.
 104                 let params = (tcx.generics_of(param_env_def_id), ty::GenericPredicates::default())
 105                     .clean(self.cx)
 106                     .params;
 107
 108                 Generics { params, where_predicates: Vec::new() }
 109             }
 110             AutoTraitResult::ExplicitImpl => return None,
 111         };
 112
 113         Some(Item {
 114             source: Span::dummy(),
 115             name: None,
 116             attrs: Default::default(),
 117             visibility: Inherited,
 118             def_id: self.cx.next_def_id(param_env_def_id.krate),
 119             kind: box ImplItem(Impl {
 120                 unsafety: hir::Unsafety::Normal,
 121                 generics: new_generics,
 122                 provided_trait_methods: Default::default(),
 123                 trait_: Some(trait_ref.clean(self.cx).get_trait_type().unwrap()),
 124                 for_: ty.clean(self.cx),
 125                 items: Vec::new(),
 126                 negative_polarity,
 127                 synthetic: true,
 128                 blanket_impl: None,
 129             }),
 130         })
 131     }
 132
 133     // FIXME(eddyb) figure out a better way to pass information about
 134     // parametrization of `ty` than `param_env_def_id`.
 135     crate fn get_auto_trait_impls(&mut self, ty: Ty<'tcx>, param_env_def_id: DefId) -> Vec<Item> {
 136         let tcx = self.cx.tcx;
 137         let param_env = tcx.param_env(param_env_def_id);
 138         let f = auto_trait::AutoTraitFinder::new(self.cx.tcx);
 139
 140         debug!("get_auto_trait_impls({:?})", ty);
 141         let auto_traits: Vec<_> = self.cx.auto_traits.iter().cloned().collect();
 142         let mut auto_traits: Vec<Item> = auto_traits
 143             .into_iter()
 144             .filter_map(|trait_def_id| {
 145                 self.generate_for_trait(ty, trait_def_id, param_env, param_env_def_id, &f, false)
 146             })
 147             .collect();
 148         // We are only interested in case the type *doesn't* implement the Sized trait.
 149         if !ty.is_sized(self.cx.tcx.at(rustc_span::DUMMY_SP), param_env) {
 150             // In case `#![no_core]` is used, `sized_trait` returns nothing.
 151             if let Some(item) = self.cx.tcx.lang_items().sized_trait().and_then(|sized_trait_did| {
 152                 self.generate_for_trait(ty, sized_trait_did, param_env, param_env_def_id, &f, true)
 153             }) {
 154                 auto_traits.push(item);
 155             }
 156         }
 157         auto_traits
 158     }
 159
 160     fn get_lifetime(region: Region<'_>, names_map: &FxHashMap<Symbol, Lifetime>) -> Lifetime {
 161         region_name(region)
 162             .map(|name| {
 163                 names_map.get(&name).unwrap_or_else(|| {
 164                     panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
 165                 })
 166             })
 167             .unwrap_or(&Lifetime::statik())
 168             .clone()
 169     }
 170
 171     // This method calculates two things: Lifetime constraints of the form 'a: 'b,
 172     // and region constraints of the form ReVar: 'a
 173     //
 174     // This is essentially a simplified version of lexical_region_resolve. However,
 175     // handle_lifetimes determines what *needs be* true in order for an impl to hold.
 176     // lexical_region_resolve, along with much of the rest of the compiler, is concerned
 177     // with determining if a given set up constraints/predicates *are* met, given some
 178     // starting conditions (e.g., user-provided code). For this reason, it's easier
 179     // to perform the calculations we need on our own, rather than trying to make
 180     // existing inference/solver code do what we want.
 181     fn handle_lifetimes<'cx>(
 182         regions: &RegionConstraintData<'cx>,
 183         names_map: &FxHashMap<Symbol, Lifetime>,
 184     ) -> Vec<WherePredicate> {
 185         // Our goal is to 'flatten' the list of constraints by eliminating
 186         // all intermediate RegionVids. At the end, all constraints should
 187         // be between Regions (aka region variables). This gives us the information
 188         // we need to create the Generics.
 189         let mut finished: FxHashMap<_, Vec<_>> = Default::default();
 190
 191         let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
 192
 193         // Flattening is done in two parts. First, we insert all of the constraints
 194         // into a map. Each RegionTarget (either a RegionVid or a Region) maps
 195         // to its smaller and larger regions. Note that 'larger' regions correspond
 196         // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
 197         for constraint in regions.constraints.keys() {
 198             match constraint {
 199                 &Constraint::VarSubVar(r1, r2) => {
 200                     {
 201                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 202                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 203                     }
 204
 205                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 206                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 207                 }
 208                 &Constraint::RegSubVar(region, vid) => {
 209                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 210                     deps.smaller.insert(RegionTarget::Region(region));
 211                 }
 212                 &Constraint::VarSubReg(vid, region) => {
 213                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 214                     deps.larger.insert(RegionTarget::Region(region));
 215                 }
 216                 &Constraint::RegSubReg(r1, r2) => {
 217                     // The constraint is already in the form that we want, so we're done with it
 218                     // Desired order is 'larger, smaller', so flip then
 219                     if region_name(r1) != region_name(r2) {
 220                         finished
 221                             .entry(region_name(r2).expect("no region_name found"))
 222                             .or_default()
 223                             .push(r1);
 224                     }
 225                 }
 226             }
 227         }
 228
 229         // Here, we 'flatten' the map one element at a time.
 230         // All of the element's sub and super regions are connected
 231         // to each other. For example, if we have a graph that looks like this:
 232         //
 233         // (A, B) - C - (D, E)
 234         // Where (A, B) are subregions, and (D,E) are super-regions
 235         //
 236         // then after deleting 'C', the graph will look like this:
 237         //  ... - A - (D, E ...)
 238         //  ... - B - (D, E, ...)
 239         //  (A, B, ...) - D - ...
 240         //  (A, B, ...) - E - ...
 241         //
 242         //  where '...' signifies the existing sub and super regions of an entry
 243         //  When two adjacent ty::Regions are encountered, we've computed a final
 244         //  constraint, and add it to our list. Since we make sure to never re-add
 245         //  deleted items, this process will always finish.
 246         while !vid_map.is_empty() {
 247             let target = *vid_map.keys().next().expect("Keys somehow empty");
 248             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 249
 250             for smaller in deps.smaller.iter() {
 251                 for larger in deps.larger.iter() {
 252                     match (smaller, larger) {
 253                         (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
 254                             if region_name(r1) != region_name(r2) {
 255                                 finished
 256                                     .entry(region_name(r2).expect("no region name found"))
 257                                     .or_default()
 258                                     .push(r1) // Larger, smaller
 259                             }
 260                         }
 261                         (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
 262                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 263                                 let smaller_deps = v.into_mut();
 264                                 smaller_deps.larger.insert(*larger);
 265                                 smaller_deps.larger.remove(&target);
 266                             }
 267                         }
 268                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 269                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 270                                 let deps = v.into_mut();
 271                                 deps.smaller.insert(*smaller);
 272                                 deps.smaller.remove(&target);
 273                             }
 274                         }
 275                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 276                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 277                                 let smaller_deps = v.into_mut();
 278                                 smaller_deps.larger.insert(*larger);
 279                                 smaller_deps.larger.remove(&target);
 280                             }
 281
 282                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 283                                 let larger_deps = v.into_mut();
 284                                 larger_deps.smaller.insert(*smaller);
 285                                 larger_deps.smaller.remove(&target);
 286                             }
 287                         }
 288                     }
 289                 }
 290             }
 291         }
 292
 293         let lifetime_predicates = names_map
 294             .iter()
 295             .flat_map(|(name, lifetime)| {
 296                 let empty = Vec::new();
 297                 let bounds: FxHashSet<GenericBound> = finished
 298                     .get(name)
 299                     .unwrap_or(&empty)
 300                     .iter()
 301                     .map(|region| GenericBound::Outlives(Self::get_lifetime(region, names_map)))
 302                     .collect();
 303
 304                 if bounds.is_empty() {
 305                     return None;
 306                 }
 307                 Some(WherePredicate::RegionPredicate {
 308                     lifetime: lifetime.clone(),
 309                     bounds: bounds.into_iter().collect(),
 310                 })
 311             })
 312             .collect();
 313
 314         lifetime_predicates
 315     }
 316
 317     fn extract_for_generics(
 318         &self,
 319         tcx: TyCtxt<'tcx>,
 320         pred: ty::Predicate<'tcx>,
 321     ) -> FxHashSet<GenericParamDef> {
 322         let bound_predicate = pred.kind();
 323         let regions = match bound_predicate.skip_binder() {
 324             ty::PredicateKind::Trait(poly_trait_pred, _) => {
 325                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
 326             }
 327             ty::PredicateKind::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) => {
 340                         Some(GenericParamDef { name, kind: GenericParamDefKind::Lifetime })
 341                     }
 342                     _ => None,
 343                 }
 344             })
 345             .collect()
 346     }
 347
 348     fn make_final_bounds(
 349         &self,
 350         ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
 351         ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)>,
 352         lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
 353     ) -> Vec<WherePredicate> {
 354         ty_to_bounds
 355             .into_iter()
 356             .flat_map(|(ty, mut bounds)| {
 357                 if let Some(data) = ty_to_fn.get(&ty) {
 358                     let (poly_trait, output) =
 359                         (data.0.as_ref().expect("as_ref failed").clone(), data.1.as_ref().cloned());
 360                     let new_ty = match poly_trait.trait_ {
 361                         Type::ResolvedPath {
 362                             ref path,
 363                             ref param_names,
 364                             ref did,
 365                             ref is_generic,
 366                         } => {
 367                             let mut new_path = path.clone();
 368                             let last_segment =
 369                                 new_path.segments.pop().expect("segments were empty");
 370
 371                             let (old_input, old_output) = match last_segment.args {
 372                                 GenericArgs::AngleBracketed { args, .. } => {
 373                                     let types = args
 374                                         .iter()
 375                                         .filter_map(|arg| match arg {
 376                                             GenericArg::Type(ty) => Some(ty.clone()),
 377                                             _ => None,
 378                                         })
 379                                         .collect();
 380                                     (types, None)
 381                                 }
 382                                 GenericArgs::Parenthesized { inputs, output, .. } => {
 383                                     (inputs, output)
 384                                 }
 385                             };
 386
 387                             if old_output.is_some() && old_output != output {
 388                                 panic!(
 389                                     "Output mismatch for {:?} {:?} {:?}",
 390                                     ty, old_output, data.1
 391                                 );
 392                             }
 393
 394                             let new_params =
 395                                 GenericArgs::Parenthesized { inputs: old_input, output };
 396
 397                             new_path
 398                                 .segments
 399                                 .push(PathSegment { name: last_segment.name, args: new_params });
 400
 401                             Type::ResolvedPath {
 402                                 path: new_path,
 403                                 param_names: param_names.clone(),
 404                                 did: *did,
 405                                 is_generic: *is_generic,
 406                             }
 407                         }
 408                         _ => panic!("Unexpected data: {:?}, {:?}", ty, data),
 409                     };
 410                     bounds.insert(GenericBound::TraitBound(
 411                         PolyTrait { trait_: new_ty, generic_params: poly_trait.generic_params },
 412                         hir::TraitBoundModifier::None,
 413                     ));
 414                 }
 415                 if bounds.is_empty() {
 416                     return None;
 417                 }
 418
 419                 let mut bounds_vec = bounds.into_iter().collect();
 420                 self.sort_where_bounds(&mut bounds_vec);
 421
 422                 Some(WherePredicate::BoundPredicate { ty, bounds: bounds_vec })
 423             })
 424             .chain(
 425                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 426                     |(lifetime, bounds)| {
 427                         let mut bounds_vec = bounds.into_iter().collect();
 428                         self.sort_where_bounds(&mut bounds_vec);
 429                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 430                     },
 431                 ),
 432             )
 433             .collect()
 434     }
 435
 436     // Converts the calculated ParamEnv and lifetime information to a clean::Generics, suitable for
 437     // display on the docs page. Cleaning the Predicates produces sub-optimal `WherePredicate`s,
 438     // so we fix them up:
 439     //
 440     // * Multiple bounds for the same type are coalesced into one: e.g., 'T: Copy', 'T: Debug'
 441     // becomes 'T: Copy + Debug'
 442     // * Fn bounds are handled specially - instead of leaving it as 'T: Fn(), <T as Fn::Output> =
 443     // K', we use the dedicated syntax 'T: Fn() -> K'
 444     // * We explicitly add a '?Sized' bound if we didn't find any 'Sized' predicates for a type
 445     fn param_env_to_generics(
 446         &mut self,
 447         tcx: TyCtxt<'tcx>,
 448         param_env_def_id: DefId,
 449         param_env: ty::ParamEnv<'tcx>,
 450         mut existing_predicates: Vec<WherePredicate>,
 451         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 452     ) -> Generics {
 453         debug!(
 454             "param_env_to_generics(param_env_def_id={:?}, param_env={:?}, \
 455              existing_predicates={:?})",
 456             param_env_def_id, param_env, existing_predicates
 457         );
 458
 459         // The `Sized` trait must be handled specially, since we only display it when
 460         // it is *not* required (i.e., '?Sized')
 461         let sized_trait = self.cx.tcx.require_lang_item(LangItem::Sized, None);
 462
 463         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 464
 465         let orig_bounds: FxHashSet<_> =
 466             self.cx.tcx.param_env(param_env_def_id).caller_bounds().iter().collect();
 467         let clean_where_predicates = param_env
 468             .caller_bounds()
 469             .iter()
 470             .filter(|p| {
 471                 !orig_bounds.contains(p)
 472                     || match p.kind().skip_binder() {
 473                         ty::PredicateKind::Trait(pred, _) => pred.def_id() == sized_trait,
 474                         _ => false,
 475                     }
 476             })
 477             .map(|p| p.fold_with(&mut replacer));
 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 p in clean_where_predicates {
 497             let (orig_p, p) = (p, p.clean(self.cx));
 498             if p.is_none() {
 499                 continue;
 500             }
 501             let p = p.unwrap();
 502             match p {
 503                 WherePredicate::BoundPredicate { ty, mut bounds } => {
 504                     // Writing a projection trait bound of the form
 505                     // <T as Trait>::Name : ?Sized
 506                     // is illegal, because ?Sized bounds can only
 507                     // be written in the (here, nonexistent) definition
 508                     // of the type.
 509                     // Therefore, we make sure that we never add a ?Sized
 510                     // bound for projections
 511                     if let Type::QPath { .. } = ty {
 512                         has_sized.insert(ty.clone());
 513                     }
 514
 515                     if bounds.is_empty() {
 516                         continue;
 517                     }
 518
 519                     let mut for_generics = self.extract_for_generics(tcx, orig_p);
 520
 521                     assert!(bounds.len() == 1);
 522                     let mut b = bounds.pop().expect("bounds were empty");
 523
 524                     if b.is_sized_bound(self.cx) {
 525                         has_sized.insert(ty.clone());
 526                     } else if !b
 527                         .get_trait_type()
 528                         .and_then(|t| {
 529                             ty_to_traits
 530                                 .get(&ty)
 531                                 .map(|bounds| bounds.contains(&strip_type(t.clone())))
 532                         })
 533                         .unwrap_or(false)
 534                     {
 535                         // If we've already added a projection bound for the same type, don't add
 536                         // this, as it would be a duplicate
 537
 538                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 539                         // as we want to combine them with any 'Output' qpaths
 540                         // later
 541
 542                         let is_fn = match &mut b {
 543                             &mut GenericBound::TraitBound(ref mut p, _) => {
 544                                 // Insert regions into the for_generics hash map first, to ensure
 545                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 546                                 for_generics.extend(p.generic_params.clone());
 547                                 p.generic_params = for_generics.into_iter().collect();
 548                                 self.is_fn_ty(tcx, &p.trait_)
 549                             }
 550                             _ => false,
 551                         };
 552
 553                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 554
 555                         if is_fn {
 556                             ty_to_fn
 557                                 .entry(ty.clone())
 558                                 .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
 559                                 .or_insert(((Some(poly_trait.clone())), None));
 560
 561                             ty_to_bounds.entry(ty.clone()).or_default();
 562                         } else {
 563                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 564                         }
 565                     }
 566                 }
 567                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 568                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 569                 }
 570                 WherePredicate::EqPredicate { lhs, rhs } => {
 571                     match lhs {
 572                         Type::QPath { name: left_name, ref self_type, ref trait_ } => {
 573                             let ty = &*self_type;
 574                             match **trait_ {
 575                                 Type::ResolvedPath {
 576                                     path: ref trait_path,
 577                                     ref param_names,
 578                                     ref did,
 579                                     ref is_generic,
 580                                 } => {
 581                                     let mut new_trait_path = trait_path.clone();
 582
 583                                     if self.is_fn_ty(tcx, trait_) && left_name == sym::Output {
 584                                         ty_to_fn
 585                                             .entry(*ty.clone())
 586                                             .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
 587                                             .or_insert((None, Some(rhs)));
 588                                         continue;
 589                                     }
 590
 591                                     let args = &mut new_trait_path
 592                                         .segments
 593                                         .last_mut()
 594                                         .expect("segments were empty")
 595                                         .args;
 596
 597                                     match args {
 598                                         // Convert something like '<T as Iterator::Item> = u8'
 599                                         // to 'T: Iterator<Item=u8>'
 600                                         GenericArgs::AngleBracketed {
 601                                             ref mut bindings, ..
 602                                         } => {
 603                                             bindings.push(TypeBinding {
 604                                                 name: left_name,
 605                                                 kind: TypeBindingKind::Equality { ty: rhs },
 606                                             });
 607                                         }
 608                                         GenericArgs::Parenthesized { .. } => {
 609                                             existing_predicates.push(WherePredicate::EqPredicate {
 610                                                 lhs: lhs.clone(),
 611                                                 rhs,
 612                                             });
 613                                             continue; // If something other than a Fn ends up
 614                                             // with parenthesis, leave it alone
 615                                         }
 616                                     }
 617
 618                                     let bounds = ty_to_bounds.entry(*ty.clone()).or_default();
 619
 620                                     bounds.insert(GenericBound::TraitBound(
 621                                         PolyTrait {
 622                                             trait_: Type::ResolvedPath {
 623                                                 path: new_trait_path,
 624                                                 param_names: param_names.clone(),
 625                                                 did: *did,
 626                                                 is_generic: *is_generic,
 627                                             },
 628                                             generic_params: Vec::new(),
 629                                         },
 630                                         hir::TraitBoundModifier::None,
 631                                     ));
 632
 633                                     // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 634                                     // that we don't see a
 635                                     // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 636                                     // on the docs page.
 637                                     bounds.remove(&GenericBound::TraitBound(
 638                                         PolyTrait {
 639                                             trait_: *trait_.clone(),
 640                                             generic_params: Vec::new(),
 641                                         },
 642                                         hir::TraitBoundModifier::None,
 643                                     ));
 644                                     // Avoid creating any new duplicate bounds later in the outer
 645                                     // loop
 646                                     ty_to_traits
 647                                         .entry(*ty.clone())
 648                                         .or_default()
 649                                         .insert(*trait_.clone());
 650                                 }
 651                                 _ => panic!(
 652                                     "Unexpected trait {:?} for {:?}",
 653                                     trait_, param_env_def_id,
 654                                 ),
 655                             }
 656                         }
 657                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, param_env_def_id),
 658                     }
 659                 }
 660             };
 661         }
 662
 663         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 664
 665         existing_predicates.extend(final_bounds);
 666
 667         for param in generic_params.iter_mut() {
 668             match param.kind {
 669                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 670                     // We never want something like `impl<T=Foo>`.
 671                     default.take();
 672                     let generic_ty = Type::Generic(param.name);
 673                     if !has_sized.contains(&generic_ty) {
 674                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 675                     }
 676                 }
 677                 GenericParamDefKind::Lifetime => {}
 678                 GenericParamDefKind::Const { .. } => {}
 679             }
 680         }
 681
 682         self.sort_where_predicates(&mut existing_predicates);
 683
 684         Generics { params: generic_params, where_predicates: existing_predicates }
 685     }
 686
 687     // Ensure that the predicates are in a consistent order. The precise
 688     // ordering doesn't actually matter, but it's important that
 689     // a given set of predicates always appears in the same order -
 690     // both for visual consistency between 'rustdoc' runs, and to
 691     // make writing tests much easier
 692     #[inline]
 693     fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
 694         // We should never have identical bounds - and if we do,
 695         // they're visually identical as well. Therefore, using
 696         // an unstable sort is fine.
 697         self.unstable_debug_sort(&mut predicates);
 698     }
 699
 700     // Ensure that the bounds are in a consistent order. The precise
 701     // ordering doesn't actually matter, but it's important that
 702     // a given set of bounds always appears in the same order -
 703     // both for visual consistency between 'rustdoc' runs, and to
 704     // make writing tests much easier
 705     #[inline]
 706     fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
 707         // We should never have identical bounds - and if we do,
 708         // they're visually identical as well. Therefore, using
 709         // an unstable sort is fine.
 710         self.unstable_debug_sort(&mut bounds);
 711     }
 712
 713     // This might look horrendously hacky, but it's actually not that bad.
 714     //
 715     // For performance reasons, we use several different FxHashMaps
 716     // in the process of computing the final set of where predicates.
 717     // However, the iteration order of a HashMap is completely unspecified.
 718     // In fact, the iteration of an FxHashMap can even vary between platforms,
 719     // since FxHasher has different behavior for 32-bit and 64-bit platforms.
 720     //
 721     // Obviously, it's extremely undesirable for documentation rendering
 722     // to be dependent on the platform it's run on. Apart from being confusing
 723     // to end users, it makes writing tests much more difficult, as predicates
 724     // can appear in any order in the final result.
 725     //
 726     // To solve this problem, we sort WherePredicates and GenericBounds
 727     // by their Debug string. The thing to keep in mind is that we don't really
 728     // care what the final order is - we're synthesizing an impl or bound
 729     // ourselves, so any order can be considered equally valid. By sorting the
 730     // predicates and bounds, however, we ensure that for a given codebase, all
 731     // auto-trait impls always render in exactly the same way.
 732     //
 733     // Using the Debug implementation for sorting prevents us from needing to
 734     // write quite a bit of almost entirely useless code (e.g., how should two
 735     // Types be sorted relative to each other). It also allows us to solve the
 736     // problem for both WherePredicates and GenericBounds at the same time. This
 737     // approach is probably somewhat slower, but the small number of items
 738     // involved (impls rarely have more than a few bounds) means that it
 739     // shouldn't matter in practice.
 740     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
 741         vec.sort_by_cached_key(|x| format!("{:?}", x))
 742     }
 743
 744     fn is_fn_ty(&self, tcx: TyCtxt<'_>, ty: &Type) -> bool {
 745         match ty {
 746             &Type::ResolvedPath { did, .. } => {
 747                 did == tcx.require_lang_item(LangItem::Fn, None)
 748                     || did == tcx.require_lang_item(LangItem::FnMut, None)
 749                     || did == tcx.require_lang_item(LangItem::FnOnce, None)
 750             }
 751             _ => false,
 752         }
 753     }
 754 }
 755
 756 fn region_name(region: Region<'_>) -> Option<Symbol> {
 757     match region {
 758         &ty::ReEarlyBound(r) => Some(r.name),
 759         _ => None,
 760     }
 761 }
 762
 763 // Replaces all ReVars in a type with ty::Region's, using the provided map
 764 struct RegionReplacer<'a, 'tcx> {
 765     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 766     tcx: TyCtxt<'tcx>,
 767 }
 768
 769 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 770     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 771         self.tcx
 772     }
 773
 774     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 775         (match r {
 776             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 777             _ => None,
 778         })
 779         .unwrap_or_else(|| r.super_fold_with(self))
 780     }
 781 }