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 negative_polarity;
  80         let new_generics = match result {
  81             AutoTraitResult::PositiveImpl(new_generics) => {
  82                 negative_polarity = false;
  83                 if discard_positive_impl {
  84                     return None;
  85                 }
  86                 new_generics
  87             }
  88             AutoTraitResult::NegativeImpl => {
  89                 negative_polarity = true;
  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                 span: Span::dummy(),
 119                 unsafety: hir::Unsafety::Normal,
 120                 generics: new_generics,
 121                 trait_: Some(trait_ref.clean(self.cx).get_trait_type().unwrap()),
 122                 for_: ty.clean(self.cx),
 123                 items: Vec::new(),
 124                 negative_polarity,
 125                 synthetic: true,
 126                 blanket_impl: None,
 127             }),
 128             cfg: None,
 129         })
 130     }
 131
 132     crate fn get_auto_trait_impls(&mut self, item_def_id: DefId) -> Vec<Item> {
 133         let tcx = self.cx.tcx;
 134         let param_env = tcx.param_env(item_def_id);
 135         let ty = tcx.type_of(item_def_id);
 136         let f = auto_trait::AutoTraitFinder::new(tcx);
 137
 138         debug!("get_auto_trait_impls({:?})", ty);
 139         let auto_traits: Vec<_> = self.cx.auto_traits.iter().cloned().collect();
 140         let mut auto_traits: Vec<Item> = auto_traits
 141             .into_iter()
 142             .filter_map(|trait_def_id| {
 143                 self.generate_for_trait(ty, trait_def_id, param_env, item_def_id, &f, false)
 144             })
 145             .collect();
 146         // We are only interested in case the type *doesn't* implement the Sized trait.
 147         if !ty.is_sized(tcx.at(rustc_span::DUMMY_SP), param_env) {
 148             // In case `#![no_core]` is used, `sized_trait` returns nothing.
 149             if let Some(item) = tcx.lang_items().sized_trait().and_then(|sized_trait_did| {
 150                 self.generate_for_trait(ty, sized_trait_did, param_env, item_def_id, &f, true)
 151             }) {
 152                 auto_traits.push(item);
 153             }
 154         }
 155         auto_traits
 156     }
 157
 158     fn get_lifetime(region: Region<'_>, names_map: &FxHashMap<Symbol, Lifetime>) -> Lifetime {
 159         region_name(region)
 160             .map(|name| {
 161                 names_map.get(&name).unwrap_or_else(|| {
 162                     panic!("Missing lifetime with name {:?} for {:?}", name.as_str(), region)
 163                 })
 164             })
 165             .unwrap_or(&Lifetime::statik())
 166             .clone()
 167     }
 168
 169     // This method calculates two things: Lifetime constraints of the form 'a: 'b,
 170     // and region constraints of the form ReVar: 'a
 171     //
 172     // This is essentially a simplified version of lexical_region_resolve. However,
 173     // handle_lifetimes determines what *needs be* true in order for an impl to hold.
 174     // lexical_region_resolve, along with much of the rest of the compiler, is concerned
 175     // with determining if a given set up constraints/predicates *are* met, given some
 176     // starting conditions (e.g., user-provided code). For this reason, it's easier
 177     // to perform the calculations we need on our own, rather than trying to make
 178     // existing inference/solver code do what we want.
 179     fn handle_lifetimes<'cx>(
 180         regions: &RegionConstraintData<'cx>,
 181         names_map: &FxHashMap<Symbol, Lifetime>,
 182     ) -> Vec<WherePredicate> {
 183         // Our goal is to 'flatten' the list of constraints by eliminating
 184         // all intermediate RegionVids. At the end, all constraints should
 185         // be between Regions (aka region variables). This gives us the information
 186         // we need to create the Generics.
 187         let mut finished: FxHashMap<_, Vec<_>> = Default::default();
 188
 189         let mut vid_map: FxHashMap<RegionTarget<'_>, RegionDeps<'_>> = Default::default();
 190
 191         // Flattening is done in two parts. First, we insert all of the constraints
 192         // into a map. Each RegionTarget (either a RegionVid or a Region) maps
 193         // to its smaller and larger regions. Note that 'larger' regions correspond
 194         // to sub-regions in Rust code (e.g., in 'a: 'b, 'a is the larger region).
 195         for constraint in regions.constraints.keys() {
 196             match constraint {
 197                 &Constraint::VarSubVar(r1, r2) => {
 198                     {
 199                         let deps1 = vid_map.entry(RegionTarget::RegionVid(r1)).or_default();
 200                         deps1.larger.insert(RegionTarget::RegionVid(r2));
 201                     }
 202
 203                     let deps2 = vid_map.entry(RegionTarget::RegionVid(r2)).or_default();
 204                     deps2.smaller.insert(RegionTarget::RegionVid(r1));
 205                 }
 206                 &Constraint::RegSubVar(region, vid) => {
 207                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 208                     deps.smaller.insert(RegionTarget::Region(region));
 209                 }
 210                 &Constraint::VarSubReg(vid, region) => {
 211                     let deps = vid_map.entry(RegionTarget::RegionVid(vid)).or_default();
 212                     deps.larger.insert(RegionTarget::Region(region));
 213                 }
 214                 &Constraint::RegSubReg(r1, r2) => {
 215                     // The constraint is already in the form that we want, so we're done with it
 216                     // Desired order is 'larger, smaller', so flip then
 217                     if region_name(r1) != region_name(r2) {
 218                         finished
 219                             .entry(region_name(r2).expect("no region_name found"))
 220                             .or_default()
 221                             .push(r1);
 222                     }
 223                 }
 224             }
 225         }
 226
 227         // Here, we 'flatten' the map one element at a time.
 228         // All of the element's sub and super regions are connected
 229         // to each other. For example, if we have a graph that looks like this:
 230         //
 231         // (A, B) - C - (D, E)
 232         // Where (A, B) are subregions, and (D,E) are super-regions
 233         //
 234         // then after deleting 'C', the graph will look like this:
 235         //  ... - A - (D, E ...)
 236         //  ... - B - (D, E, ...)
 237         //  (A, B, ...) - D - ...
 238         //  (A, B, ...) - E - ...
 239         //
 240         //  where '...' signifies the existing sub and super regions of an entry
 241         //  When two adjacent ty::Regions are encountered, we've computed a final
 242         //  constraint, and add it to our list. Since we make sure to never re-add
 243         //  deleted items, this process will always finish.
 244         while !vid_map.is_empty() {
 245             let target = *vid_map.keys().next().expect("Keys somehow empty");
 246             let deps = vid_map.remove(&target).expect("Entry somehow missing");
 247
 248             for smaller in deps.smaller.iter() {
 249                 for larger in deps.larger.iter() {
 250                     match (smaller, larger) {
 251                         (&RegionTarget::Region(r1), &RegionTarget::Region(r2)) => {
 252                             if region_name(r1) != region_name(r2) {
 253                                 finished
 254                                     .entry(region_name(r2).expect("no region name found"))
 255                                     .or_default()
 256                                     .push(r1) // Larger, smaller
 257                             }
 258                         }
 259                         (&RegionTarget::RegionVid(_), &RegionTarget::Region(_)) => {
 260                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 261                                 let smaller_deps = v.into_mut();
 262                                 smaller_deps.larger.insert(*larger);
 263                                 smaller_deps.larger.remove(&target);
 264                             }
 265                         }
 266                         (&RegionTarget::Region(_), &RegionTarget::RegionVid(_)) => {
 267                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 268                                 let deps = v.into_mut();
 269                                 deps.smaller.insert(*smaller);
 270                                 deps.smaller.remove(&target);
 271                             }
 272                         }
 273                         (&RegionTarget::RegionVid(_), &RegionTarget::RegionVid(_)) => {
 274                             if let Entry::Occupied(v) = vid_map.entry(*smaller) {
 275                                 let smaller_deps = v.into_mut();
 276                                 smaller_deps.larger.insert(*larger);
 277                                 smaller_deps.larger.remove(&target);
 278                             }
 279
 280                             if let Entry::Occupied(v) = vid_map.entry(*larger) {
 281                                 let larger_deps = v.into_mut();
 282                                 larger_deps.smaller.insert(*smaller);
 283                                 larger_deps.smaller.remove(&target);
 284                             }
 285                         }
 286                     }
 287                 }
 288             }
 289         }
 290
 291         let lifetime_predicates = names_map
 292             .iter()
 293             .flat_map(|(name, lifetime)| {
 294                 let empty = Vec::new();
 295                 let bounds: FxHashSet<GenericBound> = finished
 296                     .get(name)
 297                     .unwrap_or(&empty)
 298                     .iter()
 299                     .map(|region| GenericBound::Outlives(Self::get_lifetime(region, names_map)))
 300                     .collect();
 301
 302                 if bounds.is_empty() {
 303                     return None;
 304                 }
 305                 Some(WherePredicate::RegionPredicate {
 306                     lifetime: lifetime.clone(),
 307                     bounds: bounds.into_iter().collect(),
 308                 })
 309             })
 310             .collect();
 311
 312         lifetime_predicates
 313     }
 314
 315     fn extract_for_generics(&self, pred: ty::Predicate<'tcx>) -> FxHashSet<GenericParamDef> {
 316         let bound_predicate = pred.kind();
 317         let tcx = self.cx.tcx;
 318         let regions = match bound_predicate.skip_binder() {
 319             ty::PredicateKind::Trait(poly_trait_pred) => {
 320                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_trait_pred))
 321             }
 322             ty::PredicateKind::Projection(poly_proj_pred) => {
 323                 tcx.collect_referenced_late_bound_regions(&bound_predicate.rebind(poly_proj_pred))
 324             }
 325             _ => return FxHashSet::default(),
 326         };
 327
 328         regions
 329             .into_iter()
 330             .filter_map(|br| {
 331                 match br {
 332                     // We only care about named late bound regions, as we need to add them
 333                     // to the 'for<>' section
 334                     ty::BrNamed(_, name) => Some(GenericParamDef {
 335                         name,
 336                         kind: GenericParamDefKind::Lifetime { outlives: vec![] },
 337                     }),
 338                     _ => None,
 339                 }
 340             })
 341             .collect()
 342     }
 343
 344     fn make_final_bounds(
 345         &self,
 346         ty_to_bounds: FxHashMap<Type, FxHashSet<GenericBound>>,
 347         ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)>,
 348         lifetime_to_bounds: FxHashMap<Lifetime, FxHashSet<GenericBound>>,
 349     ) -> Vec<WherePredicate> {
 350         ty_to_bounds
 351             .into_iter()
 352             .flat_map(|(ty, mut bounds)| {
 353                 if let Some(data) = ty_to_fn.get(&ty) {
 354                     let (poly_trait, output) =
 355                         (data.0.as_ref().unwrap().clone(), data.1.as_ref().cloned().map(Box::new));
 356                     let new_ty = match poly_trait.trait_ {
 357                         Type::ResolvedPath { ref path, ref did, ref is_generic } => {
 358                             let mut new_path = path.clone();
 359                             let last_segment =
 360                                 new_path.segments.pop().expect("segments were empty");
 361
 362                             let (old_input, old_output) = match last_segment.args {
 363                                 GenericArgs::AngleBracketed { args, .. } => {
 364                                     let types = args
 365                                         .iter()
 366                                         .filter_map(|arg| match arg {
 367                                             GenericArg::Type(ty) => Some(ty.clone()),
 368                                             _ => None,
 369                                         })
 370                                         .collect();
 371                                     (types, None)
 372                                 }
 373                                 GenericArgs::Parenthesized { inputs, output, .. } => {
 374                                     (inputs, output)
 375                                 }
 376                             };
 377
 378                             if old_output.is_some() && old_output != output {
 379                                 panic!(
 380                                     "Output mismatch for {:?} {:?} {:?}",
 381                                     ty, old_output, data.1
 382                                 );
 383                             }
 384
 385                             let new_params =
 386                                 GenericArgs::Parenthesized { inputs: old_input, output };
 387
 388                             new_path
 389                                 .segments
 390                                 .push(PathSegment { name: last_segment.name, args: new_params });
 391
 392                             Type::ResolvedPath {
 393                                 path: new_path,
 394                                 did: *did,
 395                                 is_generic: *is_generic,
 396                             }
 397                         }
 398                         _ => panic!("Unexpected data: {:?}, {:?}", ty, data),
 399                     };
 400                     bounds.insert(GenericBound::TraitBound(
 401                         PolyTrait { trait_: new_ty, generic_params: poly_trait.generic_params },
 402                         hir::TraitBoundModifier::None,
 403                     ));
 404                 }
 405                 if bounds.is_empty() {
 406                     return None;
 407                 }
 408
 409                 let mut bounds_vec = bounds.into_iter().collect();
 410                 self.sort_where_bounds(&mut bounds_vec);
 411
 412                 Some(WherePredicate::BoundPredicate {
 413                     ty,
 414                     bounds: bounds_vec,
 415                     bound_params: Vec::new(),
 416                 })
 417             })
 418             .chain(
 419                 lifetime_to_bounds.into_iter().filter(|&(_, ref bounds)| !bounds.is_empty()).map(
 420                     |(lifetime, bounds)| {
 421                         let mut bounds_vec = bounds.into_iter().collect();
 422                         self.sort_where_bounds(&mut bounds_vec);
 423                         WherePredicate::RegionPredicate { lifetime, bounds: bounds_vec }
 424                     },
 425                 ),
 426             )
 427             .collect()
 428     }
 429
 430     // Converts the calculated ParamEnv and lifetime information to a clean::Generics, suitable for
 431     // display on the docs page. Cleaning the Predicates produces sub-optimal `WherePredicate`s,
 432     // so we fix them up:
 433     //
 434     // * Multiple bounds for the same type are coalesced into one: e.g., 'T: Copy', 'T: Debug'
 435     // becomes 'T: Copy + Debug'
 436     // * Fn bounds are handled specially - instead of leaving it as 'T: Fn(), <T as Fn::Output> =
 437     // K', we use the dedicated syntax 'T: Fn() -> K'
 438     // * We explicitly add a '?Sized' bound if we didn't find any 'Sized' predicates for a type
 439     fn param_env_to_generics(
 440         &mut self,
 441         item_def_id: DefId,
 442         param_env: ty::ParamEnv<'tcx>,
 443         mut existing_predicates: Vec<WherePredicate>,
 444         vid_to_region: FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 445     ) -> Generics {
 446         debug!(
 447             "param_env_to_generics(item_def_id={:?}, param_env={:?}, \
 448              existing_predicates={:?})",
 449             item_def_id, param_env, existing_predicates
 450         );
 451
 452         let tcx = self.cx.tcx;
 453
 454         // The `Sized` trait must be handled specially, since we only display it when
 455         // it is *not* required (i.e., '?Sized')
 456         let sized_trait = tcx.require_lang_item(LangItem::Sized, None);
 457
 458         let mut replacer = RegionReplacer { vid_to_region: &vid_to_region, tcx };
 459
 460         let orig_bounds: FxHashSet<_> = tcx.param_env(item_def_id).caller_bounds().iter().collect();
 461         let clean_where_predicates = param_env
 462             .caller_bounds()
 463             .iter()
 464             .filter(|p| {
 465                 !orig_bounds.contains(p)
 466                     || match p.kind().skip_binder() {
 467                         ty::PredicateKind::Trait(pred) => pred.def_id() == sized_trait,
 468                         _ => false,
 469                     }
 470             })
 471             .map(|p| p.fold_with(&mut replacer));
 472
 473         let mut generic_params =
 474             (tcx.generics_of(item_def_id), tcx.explicit_predicates_of(item_def_id))
 475                 .clean(self.cx)
 476                 .params;
 477
 478         debug!("param_env_to_generics({:?}): generic_params={:?}", item_def_id, generic_params);
 479
 480         let mut has_sized = FxHashSet::default();
 481         let mut ty_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 482         let mut lifetime_to_bounds: FxHashMap<_, FxHashSet<_>> = Default::default();
 483         let mut ty_to_traits: FxHashMap<Type, FxHashSet<Type>> = Default::default();
 484
 485         let mut ty_to_fn: FxHashMap<Type, (Option<PolyTrait>, Option<Type>)> = Default::default();
 486
 487         for p in clean_where_predicates {
 488             let (orig_p, p) = (p, p.clean(self.cx));
 489             if p.is_none() {
 490                 continue;
 491             }
 492             let p = p.unwrap();
 493             match p {
 494                 WherePredicate::BoundPredicate { ty, mut bounds, .. } => {
 495                     // Writing a projection trait bound of the form
 496                     // <T as Trait>::Name : ?Sized
 497                     // is illegal, because ?Sized bounds can only
 498                     // be written in the (here, nonexistent) definition
 499                     // of the type.
 500                     // Therefore, we make sure that we never add a ?Sized
 501                     // bound for projections
 502                     if let Type::QPath { .. } = ty {
 503                         has_sized.insert(ty.clone());
 504                     }
 505
 506                     if bounds.is_empty() {
 507                         continue;
 508                     }
 509
 510                     let mut for_generics = self.extract_for_generics(orig_p);
 511
 512                     assert!(bounds.len() == 1);
 513                     let mut b = bounds.pop().expect("bounds were empty");
 514
 515                     if b.is_sized_bound(self.cx) {
 516                         has_sized.insert(ty.clone());
 517                     } else if !b
 518                         .get_trait_type()
 519                         .and_then(|t| {
 520                             ty_to_traits
 521                                 .get(&ty)
 522                                 .map(|bounds| bounds.contains(&strip_type(t.clone())))
 523                         })
 524                         .unwrap_or(false)
 525                     {
 526                         // If we've already added a projection bound for the same type, don't add
 527                         // this, as it would be a duplicate
 528
 529                         // Handle any 'Fn/FnOnce/FnMut' bounds specially,
 530                         // as we want to combine them with any 'Output' qpaths
 531                         // later
 532
 533                         let is_fn = match &mut b {
 534                             &mut GenericBound::TraitBound(ref mut p, _) => {
 535                                 // Insert regions into the for_generics hash map first, to ensure
 536                                 // that we don't end up with duplicate bounds (e.g., for<'b, 'b>)
 537                                 for_generics.extend(p.generic_params.clone());
 538                                 p.generic_params = for_generics.into_iter().collect();
 539                                 self.is_fn_ty(&p.trait_)
 540                             }
 541                             _ => false,
 542                         };
 543
 544                         let poly_trait = b.get_poly_trait().expect("Cannot get poly trait");
 545
 546                         if is_fn {
 547                             ty_to_fn
 548                                 .entry(ty.clone())
 549                                 .and_modify(|e| *e = (Some(poly_trait.clone()), e.1.clone()))
 550                                 .or_insert(((Some(poly_trait.clone())), None));
 551
 552                             ty_to_bounds.entry(ty.clone()).or_default();
 553                         } else {
 554                             ty_to_bounds.entry(ty.clone()).or_default().insert(b.clone());
 555                         }
 556                     }
 557                 }
 558                 WherePredicate::RegionPredicate { lifetime, bounds } => {
 559                     lifetime_to_bounds.entry(lifetime).or_default().extend(bounds);
 560                 }
 561                 WherePredicate::EqPredicate { lhs, rhs } => {
 562                     match lhs {
 563                         Type::QPath { name: left_name, ref self_type, ref trait_, .. } => {
 564                             let ty = &*self_type;
 565                             match **trait_ {
 566                                 Type::ResolvedPath {
 567                                     path: ref trait_path,
 568                                     ref did,
 569                                     ref is_generic,
 570                                 } => {
 571                                     let mut new_trait_path = trait_path.clone();
 572
 573                                     if self.is_fn_ty(trait_) && left_name == sym::Output {
 574                                         ty_to_fn
 575                                             .entry(*ty.clone())
 576                                             .and_modify(|e| *e = (e.0.clone(), Some(rhs.clone())))
 577                                             .or_insert((None, Some(rhs)));
 578                                         continue;
 579                                     }
 580
 581                                     let args = &mut new_trait_path
 582                                         .segments
 583                                         .last_mut()
 584                                         .expect("segments were empty")
 585                                         .args;
 586
 587                                     match args {
 588                                         // Convert something like '<T as Iterator::Item> = u8'
 589                                         // to 'T: Iterator<Item=u8>'
 590                                         GenericArgs::AngleBracketed {
 591                                             ref mut bindings, ..
 592                                         } => {
 593                                             bindings.push(TypeBinding {
 594                                                 name: left_name,
 595                                                 kind: TypeBindingKind::Equality { ty: rhs },
 596                                             });
 597                                         }
 598                                         GenericArgs::Parenthesized { .. } => {
 599                                             existing_predicates.push(WherePredicate::EqPredicate {
 600                                                 lhs: lhs.clone(),
 601                                                 rhs,
 602                                             });
 603                                             continue; // If something other than a Fn ends up
 604                                             // with parenthesis, 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 {
 612                                             trait_: Type::ResolvedPath {
 613                                                 path: new_trait_path,
 614                                                 did: *did,
 615                                                 is_generic: *is_generic,
 616                                             },
 617                                             generic_params: Vec::new(),
 618                                         },
 619                                         hir::TraitBoundModifier::None,
 620                                     ));
 621
 622                                     // Remove any existing 'plain' bound (e.g., 'T: Iterator`) so
 623                                     // that we don't see a
 624                                     // duplicate bound like `T: Iterator + Iterator<Item=u8>`
 625                                     // on the docs page.
 626                                     bounds.remove(&GenericBound::TraitBound(
 627                                         PolyTrait {
 628                                             trait_: *trait_.clone(),
 629                                             generic_params: Vec::new(),
 630                                         },
 631                                         hir::TraitBoundModifier::None,
 632                                     ));
 633                                     // Avoid creating any new duplicate bounds later in the outer
 634                                     // loop
 635                                     ty_to_traits
 636                                         .entry(*ty.clone())
 637                                         .or_default()
 638                                         .insert(*trait_.clone());
 639                                 }
 640                                 _ => panic!("Unexpected trait {:?} for {:?}", trait_, item_def_id),
 641                             }
 642                         }
 643                         _ => panic!("Unexpected LHS {:?} for {:?}", lhs, item_def_id),
 644                     }
 645                 }
 646             };
 647         }
 648
 649         let final_bounds = self.make_final_bounds(ty_to_bounds, ty_to_fn, lifetime_to_bounds);
 650
 651         existing_predicates.extend(final_bounds);
 652
 653         for param in generic_params.iter_mut() {
 654             match param.kind {
 655                 GenericParamDefKind::Type { ref mut default, ref mut bounds, .. } => {
 656                     // We never want something like `impl<T=Foo>`.
 657                     default.take();
 658                     let generic_ty = Type::Generic(param.name);
 659                     if !has_sized.contains(&generic_ty) {
 660                         bounds.insert(0, GenericBound::maybe_sized(self.cx));
 661                     }
 662                 }
 663                 GenericParamDefKind::Lifetime { .. } => {}
 664                 GenericParamDefKind::Const { ref mut default, .. } => {
 665                     // We never want something like `impl<const N: usize = 10>`
 666                     default.take();
 667                 }
 668             }
 669         }
 670
 671         self.sort_where_predicates(&mut existing_predicates);
 672
 673         Generics { params: generic_params, where_predicates: existing_predicates }
 674     }
 675
 676     // Ensure that the predicates are in a consistent order. The precise
 677     // ordering doesn't actually matter, but it's important that
 678     // a given set of predicates always appears in the same order -
 679     // both for visual consistency between 'rustdoc' runs, and to
 680     // make writing tests much easier
 681     #[inline]
 682     fn sort_where_predicates(&self, mut predicates: &mut Vec<WherePredicate>) {
 683         // We should never have identical bounds - and if we do,
 684         // they're visually identical as well. Therefore, using
 685         // an unstable sort is fine.
 686         self.unstable_debug_sort(&mut predicates);
 687     }
 688
 689     // Ensure that the bounds are in a consistent order. The precise
 690     // ordering doesn't actually matter, but it's important that
 691     // a given set of bounds always appears in the same order -
 692     // both for visual consistency between 'rustdoc' runs, and to
 693     // make writing tests much easier
 694     #[inline]
 695     fn sort_where_bounds(&self, mut bounds: &mut Vec<GenericBound>) {
 696         // We should never have identical bounds - and if we do,
 697         // they're visually identical as well. Therefore, using
 698         // an unstable sort is fine.
 699         self.unstable_debug_sort(&mut bounds);
 700     }
 701
 702     // This might look horrendously hacky, but it's actually not that bad.
 703     //
 704     // For performance reasons, we use several different FxHashMaps
 705     // in the process of computing the final set of where predicates.
 706     // However, the iteration order of a HashMap is completely unspecified.
 707     // In fact, the iteration of an FxHashMap can even vary between platforms,
 708     // since FxHasher has different behavior for 32-bit and 64-bit platforms.
 709     //
 710     // Obviously, it's extremely undesirable for documentation rendering
 711     // to be dependent on the platform it's run on. Apart from being confusing
 712     // to end users, it makes writing tests much more difficult, as predicates
 713     // can appear in any order in the final result.
 714     //
 715     // To solve this problem, we sort WherePredicates and GenericBounds
 716     // by their Debug string. The thing to keep in mind is that we don't really
 717     // care what the final order is - we're synthesizing an impl or bound
 718     // ourselves, so any order can be considered equally valid. By sorting the
 719     // predicates and bounds, however, we ensure that for a given codebase, all
 720     // auto-trait impls always render in exactly the same way.
 721     //
 722     // Using the Debug implementation for sorting prevents us from needing to
 723     // write quite a bit of almost entirely useless code (e.g., how should two
 724     // Types be sorted relative to each other). It also allows us to solve the
 725     // problem for both WherePredicates and GenericBounds at the same time. This
 726     // approach is probably somewhat slower, but the small number of items
 727     // involved (impls rarely have more than a few bounds) means that it
 728     // shouldn't matter in practice.
 729     fn unstable_debug_sort<T: Debug>(&self, vec: &mut Vec<T>) {
 730         vec.sort_by_cached_key(|x| format!("{:?}", x))
 731     }
 732
 733     fn is_fn_ty(&self, ty: &Type) -> bool {
 734         let tcx = self.cx.tcx;
 735         match ty {
 736             &Type::ResolvedPath { did, .. } => {
 737                 did == tcx.require_lang_item(LangItem::Fn, None)
 738                     || did == tcx.require_lang_item(LangItem::FnMut, None)
 739                     || did == tcx.require_lang_item(LangItem::FnOnce, None)
 740             }
 741             _ => false,
 742         }
 743     }
 744 }
 745
 746 fn region_name(region: Region<'_>) -> Option<Symbol> {
 747     match region {
 748         &ty::ReEarlyBound(r) => Some(r.name),
 749         _ => None,
 750     }
 751 }
 752
 753 // Replaces all ReVars in a type with ty::Region's, using the provided map
 754 struct RegionReplacer<'a, 'tcx> {
 755     vid_to_region: &'a FxHashMap<ty::RegionVid, ty::Region<'tcx>>,
 756     tcx: TyCtxt<'tcx>,
 757 }
 758
 759 impl<'a, 'tcx> TypeFolder<'tcx> for RegionReplacer<'a, 'tcx> {
 760     fn tcx<'b>(&'b self) -> TyCtxt<'tcx> {
 761         self.tcx
 762     }
 763
 764     fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
 765         (match r {
 766             &ty::ReVar(vid) => self.vid_to_region.get(&vid).cloned(),
 767             _ => None,
 768         })
 769         .unwrap_or_else(|| r.super_fold_with(self))
 770     }
 771 }