src/librustc_resolve/late/lifetimes.rs

   1 //! Name resolution for lifetimes.
   2 //!
   3 //! Name resolution for lifetimes follows *much* simpler rules than the
   4 //! full resolve. For example, lifetime names are never exported or
   5 //! used between functions, and they operate in a purely top-down
   6 //! way. Therefore, we break lifetime name resolution into a separate pass.
   7
   8 use crate::late::diagnostics::{ForLifetimeSpanType, MissingLifetimeSpot};
   9 use rustc_ast::walk_list;
  10 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
  11 use rustc_errors::{struct_span_err, Applicability, DiagnosticBuilder};
  12 use rustc_hir as hir;
  13 use rustc_hir::def::{DefKind, Res};
  14 use rustc_hir::def_id::{CrateNum, DefId, DefIdMap, LOCAL_CRATE};
  15 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
  16 use rustc_hir::{GenericArg, GenericParam, LifetimeName, Node, ParamName, QPath};
  17 use rustc_hir::{GenericParamKind, HirIdMap, HirIdSet, LifetimeParamKind};
  18 use rustc_middle::hir::map::Map;
  19 use rustc_middle::middle::resolve_lifetime::*;
  20 use rustc_middle::ty::{self, DefIdTree, GenericParamDefKind, TyCtxt};
  21 use rustc_middle::{bug, span_bug};
  22 use rustc_session::lint;
  23 use rustc_span::symbol::{kw, sym, Ident, Symbol};
  24 use rustc_span::Span;
  25 use std::borrow::Cow;
  26 use std::cell::Cell;
  27 use std::mem::take;
  28
  29 use log::debug;
  30
  31 // This counts the no of times a lifetime is used
  32 #[derive(Clone, Copy, Debug)]
  33 pub enum LifetimeUseSet<'tcx> {
  34     One(&'tcx hir::Lifetime),
  35     Many,
  36 }
  37
  38 trait RegionExt {
  39     fn early(hir_map: &Map<'_>, index: &mut u32, param: &GenericParam<'_>) -> (ParamName, Region);
  40
  41     fn late(hir_map: &Map<'_>, param: &GenericParam<'_>) -> (ParamName, Region);
  42
  43     fn late_anon(index: &Cell<u32>) -> Region;
  44
  45     fn id(&self) -> Option<DefId>;
  46
  47     fn shifted(self, amount: u32) -> Region;
  48
  49     fn shifted_out_to_binder(self, binder: ty::DebruijnIndex) -> Region;
  50
  51     fn subst<'a, L>(self, params: L, map: &NamedRegionMap) -> Option<Region>
  52     where
  53         L: Iterator<Item = &'a hir::Lifetime>;
  54 }
  55
  56 impl RegionExt for Region {
  57     fn early(hir_map: &Map<'_>, index: &mut u32, param: &GenericParam<'_>) -> (ParamName, Region) {
  58         let i = *index;
  59         *index += 1;
  60         let def_id = hir_map.local_def_id(param.hir_id);
  61         let origin = LifetimeDefOrigin::from_param(param);
  62         debug!("Region::early: index={} def_id={:?}", i, def_id);
  63         (param.name.normalize_to_macros_2_0(), Region::EarlyBound(i, def_id.to_def_id(), origin))
  64     }
  65
  66     fn late(hir_map: &Map<'_>, param: &GenericParam<'_>) -> (ParamName, Region) {
  67         let depth = ty::INNERMOST;
  68         let def_id = hir_map.local_def_id(param.hir_id);
  69         let origin = LifetimeDefOrigin::from_param(param);
  70         debug!(
  71             "Region::late: param={:?} depth={:?} def_id={:?} origin={:?}",
  72             param, depth, def_id, origin,
  73         );
  74         (param.name.normalize_to_macros_2_0(), Region::LateBound(depth, def_id.to_def_id(), origin))
  75     }
  76
  77     fn late_anon(index: &Cell<u32>) -> Region {
  78         let i = index.get();
  79         index.set(i + 1);
  80         let depth = ty::INNERMOST;
  81         Region::LateBoundAnon(depth, i)
  82     }
  83
  84     fn id(&self) -> Option<DefId> {
  85         match *self {
  86             Region::Static | Region::LateBoundAnon(..) => None,
  87
  88             Region::EarlyBound(_, id, _) | Region::LateBound(_, id, _) | Region::Free(_, id) => {
  89                 Some(id)
  90             }
  91         }
  92     }
  93
  94     fn shifted(self, amount: u32) -> Region {
  95         match self {
  96             Region::LateBound(debruijn, id, origin) => {
  97                 Region::LateBound(debruijn.shifted_in(amount), id, origin)
  98             }
  99             Region::LateBoundAnon(debruijn, index) => {
 100                 Region::LateBoundAnon(debruijn.shifted_in(amount), index)
 101             }
 102             _ => self,
 103         }
 104     }
 105
 106     fn shifted_out_to_binder(self, binder: ty::DebruijnIndex) -> Region {
 107         match self {
 108             Region::LateBound(debruijn, id, origin) => {
 109                 Region::LateBound(debruijn.shifted_out_to_binder(binder), id, origin)
 110             }
 111             Region::LateBoundAnon(debruijn, index) => {
 112                 Region::LateBoundAnon(debruijn.shifted_out_to_binder(binder), index)
 113             }
 114             _ => self,
 115         }
 116     }
 117
 118     fn subst<'a, L>(self, mut params: L, map: &NamedRegionMap) -> Option<Region>
 119     where
 120         L: Iterator<Item = &'a hir::Lifetime>,
 121     {
 122         if let Region::EarlyBound(index, _, _) = self {
 123             params.nth(index as usize).and_then(|lifetime| map.defs.get(&lifetime.hir_id).cloned())
 124         } else {
 125             Some(self)
 126         }
 127     }
 128 }
 129
 130 /// Maps the id of each lifetime reference to the lifetime decl
 131 /// that it corresponds to.
 132 ///
 133 /// FIXME. This struct gets converted to a `ResolveLifetimes` for
 134 /// actual use. It has the same data, but indexed by `LocalDefId`.  This
 135 /// is silly.
 136 #[derive(Default)]
 137 struct NamedRegionMap {
 138     // maps from every use of a named (not anonymous) lifetime to a
 139     // `Region` describing how that region is bound
 140     defs: HirIdMap<Region>,
 141
 142     // the set of lifetime def ids that are late-bound; a region can
 143     // be late-bound if (a) it does NOT appear in a where-clause and
 144     // (b) it DOES appear in the arguments.
 145     late_bound: HirIdSet,
 146
 147     // For each type and trait definition, maps type parameters
 148     // to the trait object lifetime defaults computed from them.
 149     object_lifetime_defaults: HirIdMap<Vec<ObjectLifetimeDefault>>,
 150 }
 151
 152 crate struct LifetimeContext<'a, 'tcx> {
 153     crate tcx: TyCtxt<'tcx>,
 154     map: &'a mut NamedRegionMap,
 155     scope: ScopeRef<'a>,
 156
 157     /// This is slightly complicated. Our representation for poly-trait-refs contains a single
 158     /// binder and thus we only allow a single level of quantification. However,
 159     /// the syntax of Rust permits quantification in two places, e.g., `T: for <'a> Foo<'a>`
 160     /// and `for <'a, 'b> &'b T: Foo<'a>`. In order to get the De Bruijn indices
 161     /// correct when representing these constraints, we should only introduce one
 162     /// scope. However, we want to support both locations for the quantifier and
 163     /// during lifetime resolution we want precise information (so we can't
 164     /// desugar in an earlier phase).
 165     ///
 166     /// So, if we encounter a quantifier at the outer scope, we set
 167     /// `trait_ref_hack` to `true` (and introduce a scope), and then if we encounter
 168     /// a quantifier at the inner scope, we error. If `trait_ref_hack` is `false`,
 169     /// then we introduce the scope at the inner quantifier.
 170     trait_ref_hack: bool,
 171
 172     /// Used to disallow the use of in-band lifetimes in `fn` or `Fn` syntax.
 173     is_in_fn_syntax: bool,
 174
 175     is_in_const_generic: bool,
 176
 177     /// List of labels in the function/method currently under analysis.
 178     labels_in_fn: Vec<Ident>,
 179
 180     /// Cache for cross-crate per-definition object lifetime defaults.
 181     xcrate_object_lifetime_defaults: DefIdMap<Vec<ObjectLifetimeDefault>>,
 182
 183     lifetime_uses: &'a mut DefIdMap<LifetimeUseSet<'tcx>>,
 184
 185     /// When encountering an undefined named lifetime, we will suggest introducing it in these
 186     /// places.
 187     crate missing_named_lifetime_spots: Vec<MissingLifetimeSpot<'tcx>>,
 188 }
 189
 190 #[derive(Debug)]
 191 enum Scope<'a> {
 192     /// Declares lifetimes, and each can be early-bound or late-bound.
 193     /// The `DebruijnIndex` of late-bound lifetimes starts at `1` and
 194     /// it should be shifted by the number of `Binder`s in between the
 195     /// declaration `Binder` and the location it's referenced from.
 196     Binder {
 197         lifetimes: FxHashMap<hir::ParamName, Region>,
 198
 199         /// if we extend this scope with another scope, what is the next index
 200         /// we should use for an early-bound region?
 201         next_early_index: u32,
 202
 203         /// Flag is set to true if, in this binder, `'_` would be
 204         /// equivalent to a "single-use region". This is true on
 205         /// impls, but not other kinds of items.
 206         track_lifetime_uses: bool,
 207
 208         /// Whether or not this binder would serve as the parent
 209         /// binder for opaque types introduced within. For example:
 210         ///
 211         /// ```text
 212         ///     fn foo<'a>() -> impl for<'b> Trait<Item = impl Trait2<'a>>
 213         /// ```
 214         ///
 215         /// Here, the opaque types we create for the `impl Trait`
 216         /// and `impl Trait2` references will both have the `foo` item
 217         /// as their parent. When we get to `impl Trait2`, we find
 218         /// that it is nested within the `for<>` binder -- this flag
 219         /// allows us to skip that when looking for the parent binder
 220         /// of the resulting opaque type.
 221         opaque_type_parent: bool,
 222
 223         s: ScopeRef<'a>,
 224     },
 225
 226     /// Lifetimes introduced by a fn are scoped to the call-site for that fn,
 227     /// if this is a fn body, otherwise the original definitions are used.
 228     /// Unspecified lifetimes are inferred, unless an elision scope is nested,
 229     /// e.g., `(&T, fn(&T) -> &T);` becomes `(&'_ T, for<'a> fn(&'a T) -> &'a T)`.
 230     Body {
 231         id: hir::BodyId,
 232         s: ScopeRef<'a>,
 233     },
 234
 235     /// A scope which either determines unspecified lifetimes or errors
 236     /// on them (e.g., due to ambiguity). For more details, see `Elide`.
 237     Elision {
 238         elide: Elide,
 239         s: ScopeRef<'a>,
 240     },
 241
 242     /// Use a specific lifetime (if `Some`) or leave it unset (to be
 243     /// inferred in a function body or potentially error outside one),
 244     /// for the default choice of lifetime in a trait object type.
 245     ObjectLifetimeDefault {
 246         lifetime: Option<Region>,
 247         s: ScopeRef<'a>,
 248     },
 249
 250     Root,
 251 }
 252
 253 #[derive(Clone, Debug)]
 254 enum Elide {
 255     /// Use a fresh anonymous late-bound lifetime each time, by
 256     /// incrementing the counter to generate sequential indices.
 257     FreshLateAnon(Cell<u32>),
 258     /// Always use this one lifetime.
 259     Exact(Region),
 260     /// Less or more than one lifetime were found, error on unspecified.
 261     Error(Vec<ElisionFailureInfo>),
 262     /// Forbid lifetime elision inside of a larger scope where it would be
 263     /// permitted. For example, in let position impl trait.
 264     Forbid,
 265 }
 266
 267 #[derive(Clone, Debug)]
 268 crate struct ElisionFailureInfo {
 269     /// Where we can find the argument pattern.
 270     parent: Option<hir::BodyId>,
 271     /// The index of the argument in the original definition.
 272     index: usize,
 273     lifetime_count: usize,
 274     have_bound_regions: bool,
 275     crate span: Span,
 276 }
 277
 278 type ScopeRef<'a> = &'a Scope<'a>;
 279
 280 const ROOT_SCOPE: ScopeRef<'static> = &Scope::Root;
 281
 282 pub fn provide(providers: &mut ty::query::Providers) {
 283     *providers = ty::query::Providers {
 284         resolve_lifetimes,
 285
 286         named_region_map: |tcx, id| tcx.resolve_lifetimes(LOCAL_CRATE).defs.get(&id),
 287         is_late_bound_map: |tcx, id| tcx.resolve_lifetimes(LOCAL_CRATE).late_bound.get(&id),
 288         object_lifetime_defaults_map: |tcx, id| {
 289             tcx.resolve_lifetimes(LOCAL_CRATE).object_lifetime_defaults.get(&id)
 290         },
 291
 292         ..*providers
 293     };
 294 }
 295
 296 /// Computes the `ResolveLifetimes` map that contains data for the
 297 /// entire crate. You should not read the result of this query
 298 /// directly, but rather use `named_region_map`, `is_late_bound_map`,
 299 /// etc.
 300 fn resolve_lifetimes(tcx: TyCtxt<'_>, for_krate: CrateNum) -> ResolveLifetimes {
 301     assert_eq!(for_krate, LOCAL_CRATE);
 302
 303     let named_region_map = krate(tcx);
 304
 305     let mut rl = ResolveLifetimes::default();
 306
 307     for (hir_id, v) in named_region_map.defs {
 308         let map = rl.defs.entry(hir_id.owner).or_default();
 309         map.insert(hir_id.local_id, v);
 310     }
 311     for hir_id in named_region_map.late_bound {
 312         let map = rl.late_bound.entry(hir_id.owner).or_default();
 313         map.insert(hir_id.local_id);
 314     }
 315     for (hir_id, v) in named_region_map.object_lifetime_defaults {
 316         let map = rl.object_lifetime_defaults.entry(hir_id.owner).or_default();
 317         map.insert(hir_id.local_id, v);
 318     }
 319
 320     rl
 321 }
 322
 323 fn krate(tcx: TyCtxt<'_>) -> NamedRegionMap {
 324     let krate = tcx.hir().krate();
 325     let mut map = NamedRegionMap {
 326         defs: Default::default(),
 327         late_bound: Default::default(),
 328         object_lifetime_defaults: compute_object_lifetime_defaults(tcx),
 329     };
 330     {
 331         let mut visitor = LifetimeContext {
 332             tcx,
 333             map: &mut map,
 334             scope: ROOT_SCOPE,
 335             trait_ref_hack: false,
 336             is_in_fn_syntax: false,
 337             is_in_const_generic: false,
 338             labels_in_fn: vec![],
 339             xcrate_object_lifetime_defaults: Default::default(),
 340             lifetime_uses: &mut Default::default(),
 341             missing_named_lifetime_spots: vec![],
 342         };
 343         for item in krate.items.values() {
 344             visitor.visit_item(item);
 345         }
 346     }
 347     map
 348 }
 349
 350 /// In traits, there is an implicit `Self` type parameter which comes before the generics.
 351 /// We have to account for this when computing the index of the other generic parameters.
 352 /// This function returns whether there is such an implicit parameter defined on the given item.
 353 fn sub_items_have_self_param(node: &hir::ItemKind<'_>) -> bool {
 354     match *node {
 355         hir::ItemKind::Trait(..) | hir::ItemKind::TraitAlias(..) => true,
 356         _ => false,
 357     }
 358 }
 359
 360 impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 361     type Map = Map<'tcx>;
 362
 363     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 364         NestedVisitorMap::All(self.tcx.hir())
 365     }
 366
 367     // We want to nest trait/impl items in their parent, but nothing else.
 368     fn visit_nested_item(&mut self, _: hir::ItemId) {}
 369
 370     fn visit_nested_body(&mut self, body: hir::BodyId) {
 371         // Each body has their own set of labels, save labels.
 372         let saved = take(&mut self.labels_in_fn);
 373         let body = self.tcx.hir().body(body);
 374         extract_labels(self, body);
 375         self.with(Scope::Body { id: body.id(), s: self.scope }, |_, this| {
 376             this.visit_body(body);
 377         });
 378         self.labels_in_fn = saved;
 379     }
 380
 381     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
 382         match item.kind {
 383             hir::ItemKind::Fn(ref sig, ref generics, _) => {
 384                 self.missing_named_lifetime_spots.push(generics.into());
 385                 self.visit_early_late(None, &sig.decl, generics, |this| {
 386                     intravisit::walk_item(this, item);
 387                 });
 388                 self.missing_named_lifetime_spots.pop();
 389             }
 390
 391             hir::ItemKind::ExternCrate(_)
 392             | hir::ItemKind::Use(..)
 393             | hir::ItemKind::Mod(..)
 394             | hir::ItemKind::ForeignMod(..)
 395             | hir::ItemKind::GlobalAsm(..) => {
 396                 // These sorts of items have no lifetime parameters at all.
 397                 intravisit::walk_item(self, item);
 398             }
 399             hir::ItemKind::Static(..) | hir::ItemKind::Const(..) => {
 400                 // No lifetime parameters, but implied 'static.
 401                 let scope = Scope::Elision { elide: Elide::Exact(Region::Static), s: ROOT_SCOPE };
 402                 self.with(scope, |_, this| intravisit::walk_item(this, item));
 403             }
 404             hir::ItemKind::OpaqueTy(hir::OpaqueTy { .. }) => {
 405                 // Opaque types are visited when we visit the
 406                 // `TyKind::OpaqueDef`, so that they have the lifetimes from
 407                 // their parent opaque_ty in scope.
 408             }
 409             hir::ItemKind::TyAlias(_, ref generics)
 410             | hir::ItemKind::Enum(_, ref generics)
 411             | hir::ItemKind::Struct(_, ref generics)
 412             | hir::ItemKind::Union(_, ref generics)
 413             | hir::ItemKind::Trait(_, _, ref generics, ..)
 414             | hir::ItemKind::TraitAlias(ref generics, ..)
 415             | hir::ItemKind::Impl { ref generics, .. } => {
 416                 self.missing_named_lifetime_spots.push(generics.into());
 417
 418                 // Impls permit `'_` to be used and it is equivalent to "some fresh lifetime name".
 419                 // This is not true for other kinds of items.x
 420                 let track_lifetime_uses = match item.kind {
 421                     hir::ItemKind::Impl { .. } => true,
 422                     _ => false,
 423                 };
 424                 // These kinds of items have only early-bound lifetime parameters.
 425                 let mut index = if sub_items_have_self_param(&item.kind) {
 426                     1 // Self comes before lifetimes
 427                 } else {
 428                     0
 429                 };
 430                 let mut non_lifetime_count = 0;
 431                 let lifetimes = generics
 432                     .params
 433                     .iter()
 434                     .filter_map(|param| match param.kind {
 435                         GenericParamKind::Lifetime { .. } => {
 436                             Some(Region::early(&self.tcx.hir(), &mut index, param))
 437                         }
 438                         GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
 439                             non_lifetime_count += 1;
 440                             None
 441                         }
 442                     })
 443                     .collect();
 444                 let scope = Scope::Binder {
 445                     lifetimes,
 446                     next_early_index: index + non_lifetime_count,
 447                     opaque_type_parent: true,
 448                     track_lifetime_uses,
 449                     s: ROOT_SCOPE,
 450                 };
 451                 self.with(scope, |old_scope, this| {
 452                     this.check_lifetime_params(old_scope, &generics.params);
 453                     intravisit::walk_item(this, item);
 454                 });
 455                 self.missing_named_lifetime_spots.pop();
 456             }
 457         }
 458     }
 459
 460     fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem<'tcx>) {
 461         match item.kind {
 462             hir::ForeignItemKind::Fn(ref decl, _, ref generics) => {
 463                 self.visit_early_late(None, decl, generics, |this| {
 464                     intravisit::walk_foreign_item(this, item);
 465                 })
 466             }
 467             hir::ForeignItemKind::Static(..) => {
 468                 intravisit::walk_foreign_item(self, item);
 469             }
 470             hir::ForeignItemKind::Type => {
 471                 intravisit::walk_foreign_item(self, item);
 472             }
 473         }
 474     }
 475
 476     fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
 477         debug!("visit_ty: id={:?} ty={:?}", ty.hir_id, ty);
 478         debug!("visit_ty: ty.kind={:?}", ty.kind);
 479         match ty.kind {
 480             hir::TyKind::BareFn(ref c) => {
 481                 let next_early_index = self.next_early_index();
 482                 let was_in_fn_syntax = self.is_in_fn_syntax;
 483                 self.is_in_fn_syntax = true;
 484                 let lifetime_span: Option<Span> =
 485                     c.generic_params.iter().rev().find_map(|param| match param.kind {
 486                         GenericParamKind::Lifetime { .. } => Some(param.span),
 487                         _ => None,
 488                     });
 489                 let (span, span_type) = if let Some(span) = lifetime_span {
 490                     (span.shrink_to_hi(), ForLifetimeSpanType::TypeTail)
 491                 } else {
 492                     (ty.span.shrink_to_lo(), ForLifetimeSpanType::TypeEmpty)
 493                 };
 494                 self.missing_named_lifetime_spots
 495                     .push(MissingLifetimeSpot::HigherRanked { span, span_type });
 496                 let scope = Scope::Binder {
 497                     lifetimes: c
 498                         .generic_params
 499                         .iter()
 500                         .filter_map(|param| match param.kind {
 501                             GenericParamKind::Lifetime { .. } => {
 502                                 Some(Region::late(&self.tcx.hir(), param))
 503                             }
 504                             _ => None,
 505                         })
 506                         .collect(),
 507                     s: self.scope,
 508                     next_early_index,
 509                     track_lifetime_uses: true,
 510                     opaque_type_parent: false,
 511                 };
 512                 self.with(scope, |old_scope, this| {
 513                     // a bare fn has no bounds, so everything
 514                     // contained within is scoped within its binder.
 515                     this.check_lifetime_params(old_scope, &c.generic_params);
 516                     intravisit::walk_ty(this, ty);
 517                 });
 518                 self.missing_named_lifetime_spots.pop();
 519                 self.is_in_fn_syntax = was_in_fn_syntax;
 520             }
 521             hir::TyKind::TraitObject(bounds, ref lifetime) => {
 522                 debug!("visit_ty: TraitObject(bounds={:?}, lifetime={:?})", bounds, lifetime);
 523                 for bound in bounds {
 524                     self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
 525                 }
 526                 match lifetime.name {
 527                     LifetimeName::Implicit => {
 528                         // For types like `dyn Foo`, we should
 529                         // generate a special form of elided.
 530                         span_bug!(ty.span, "object-lifetime-default expected, not implicit",);
 531                     }
 532                     LifetimeName::ImplicitObjectLifetimeDefault => {
 533                         // If the user does not write *anything*, we
 534                         // use the object lifetime defaulting
 535                         // rules. So e.g., `Box<dyn Debug>` becomes
 536                         // `Box<dyn Debug + 'static>`.
 537                         self.resolve_object_lifetime_default(lifetime)
 538                     }
 539                     LifetimeName::Underscore => {
 540                         // If the user writes `'_`, we use the *ordinary* elision
 541                         // rules. So the `'_` in e.g., `Box<dyn Debug + '_>` will be
 542                         // resolved the same as the `'_` in `&'_ Foo`.
 543                         //
 544                         // cc #48468
 545                         self.resolve_elided_lifetimes(vec![lifetime])
 546                     }
 547                     LifetimeName::Param(_) | LifetimeName::Static => {
 548                         // If the user wrote an explicit name, use that.
 549                         self.visit_lifetime(lifetime);
 550                     }
 551                     LifetimeName::Error => {}
 552                 }
 553             }
 554             hir::TyKind::Rptr(ref lifetime_ref, ref mt) => {
 555                 self.visit_lifetime(lifetime_ref);
 556                 let scope = Scope::ObjectLifetimeDefault {
 557                     lifetime: self.map.defs.get(&lifetime_ref.hir_id).cloned(),
 558                     s: self.scope,
 559                 };
 560                 self.with(scope, |_, this| this.visit_ty(&mt.ty));
 561             }
 562             hir::TyKind::OpaqueDef(item_id, lifetimes) => {
 563                 // Resolve the lifetimes in the bounds to the lifetime defs in the generics.
 564                 // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
 565                 // `type MyAnonTy<'b> = impl MyTrait<'b>;`
 566                 //                 ^                  ^ this gets resolved in the scope of
 567                 //                                      the opaque_ty generics
 568                 let opaque_ty = self.tcx.hir().expect_item(item_id.id);
 569                 let (generics, bounds) = match opaque_ty.kind {
 570                     // Named opaque `impl Trait` types are reached via `TyKind::Path`.
 571                     // This arm is for `impl Trait` in the types of statics, constants and locals.
 572                     hir::ItemKind::OpaqueTy(hir::OpaqueTy { impl_trait_fn: None, .. }) => {
 573                         intravisit::walk_ty(self, ty);
 574
 575                         // Elided lifetimes are not allowed in non-return
 576                         // position impl Trait
 577                         let scope = Scope::Elision { elide: Elide::Forbid, s: self.scope };
 578                         self.with(scope, |_, this| {
 579                             intravisit::walk_item(this, opaque_ty);
 580                         });
 581
 582                         return;
 583                     }
 584                     // RPIT (return position impl trait)
 585                     hir::ItemKind::OpaqueTy(hir::OpaqueTy {
 586                         impl_trait_fn: Some(_),
 587                         ref generics,
 588                         bounds,
 589                         ..
 590                     }) => (generics, bounds),
 591                     ref i => bug!("`impl Trait` pointed to non-opaque type?? {:#?}", i),
 592                 };
 593
 594                 // Resolve the lifetimes that are applied to the opaque type.
 595                 // These are resolved in the current scope.
 596                 // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
 597                 // `fn foo<'a>() -> MyAnonTy<'a> { ... }`
 598                 //          ^                 ^this gets resolved in the current scope
 599                 for lifetime in lifetimes {
 600                     if let hir::GenericArg::Lifetime(lifetime) = lifetime {
 601                         self.visit_lifetime(lifetime);
 602
 603                         // Check for predicates like `impl for<'a> Trait<impl OtherTrait<'a>>`
 604                         // and ban them. Type variables instantiated inside binders aren't
 605                         // well-supported at the moment, so this doesn't work.
 606                         // In the future, this should be fixed and this error should be removed.
 607                         let def = self.map.defs.get(&lifetime.hir_id).cloned();
 608                         if let Some(Region::LateBound(_, def_id, _)) = def {
 609                             if let Some(def_id) = def_id.as_local() {
 610                                 let hir_id = self.tcx.hir().as_local_hir_id(def_id);
 611                                 // Ensure that the parent of the def is an item, not HRTB
 612                                 let parent_id = self.tcx.hir().get_parent_node(hir_id);
 613                                 let parent_impl_id = hir::ImplItemId { hir_id: parent_id };
 614                                 let parent_trait_id = hir::TraitItemId { hir_id: parent_id };
 615                                 let krate = self.tcx.hir().krate();
 616
 617                                 if !(krate.items.contains_key(&parent_id)
 618                                     || krate.impl_items.contains_key(&parent_impl_id)
 619                                     || krate.trait_items.contains_key(&parent_trait_id))
 620                                 {
 621                                     struct_span_err!(
 622                                         self.tcx.sess,
 623                                         lifetime.span,
 624                                         E0657,
 625                                         "`impl Trait` can only capture lifetimes \
 626                                          bound at the fn or impl level"
 627                                     )
 628                                     .emit();
 629                                     self.uninsert_lifetime_on_error(lifetime, def.unwrap());
 630                                 }
 631                             }
 632                         }
 633                     }
 634                 }
 635
 636                 // We want to start our early-bound indices at the end of the parent scope,
 637                 // not including any parent `impl Trait`s.
 638                 let mut index = self.next_early_index_for_opaque_type();
 639                 debug!("visit_ty: index = {}", index);
 640
 641                 let mut elision = None;
 642                 let mut lifetimes = FxHashMap::default();
 643                 let mut non_lifetime_count = 0;
 644                 for param in generics.params {
 645                     match param.kind {
 646                         GenericParamKind::Lifetime { .. } => {
 647                             let (name, reg) = Region::early(&self.tcx.hir(), &mut index, &param);
 648                             let def_id = if let Region::EarlyBound(_, def_id, _) = reg {
 649                                 def_id
 650                             } else {
 651                                 bug!();
 652                             };
 653                             if let hir::ParamName::Plain(param_name) = name {
 654                                 if param_name.name == kw::UnderscoreLifetime {
 655                                     // Pick the elided lifetime "definition" if one exists
 656                                     // and use it to make an elision scope.
 657                                     self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
 658                                     elision = Some(reg);
 659                                 } else {
 660                                     lifetimes.insert(name, reg);
 661                                 }
 662                             } else {
 663                                 self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
 664                                 lifetimes.insert(name, reg);
 665                             }
 666                         }
 667                         GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
 668                             non_lifetime_count += 1;
 669                         }
 670                     }
 671                 }
 672                 let next_early_index = index + non_lifetime_count;
 673
 674                 if let Some(elision_region) = elision {
 675                     let scope =
 676                         Scope::Elision { elide: Elide::Exact(elision_region), s: self.scope };
 677                     self.with(scope, |_old_scope, this| {
 678                         let scope = Scope::Binder {
 679                             lifetimes,
 680                             next_early_index,
 681                             s: this.scope,
 682                             track_lifetime_uses: true,
 683                             opaque_type_parent: false,
 684                         };
 685                         this.with(scope, |_old_scope, this| {
 686                             this.visit_generics(generics);
 687                             for bound in bounds {
 688                                 this.visit_param_bound(bound);
 689                             }
 690                         });
 691                     });
 692                 } else {
 693                     let scope = Scope::Binder {
 694                         lifetimes,
 695                         next_early_index,
 696                         s: self.scope,
 697                         track_lifetime_uses: true,
 698                         opaque_type_parent: false,
 699                     };
 700                     self.with(scope, |_old_scope, this| {
 701                         this.visit_generics(generics);
 702                         for bound in bounds {
 703                             this.visit_param_bound(bound);
 704                         }
 705                     });
 706                 }
 707             }
 708             _ => intravisit::walk_ty(self, ty),
 709         }
 710     }
 711
 712     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem<'tcx>) {
 713         use self::hir::TraitItemKind::*;
 714         match trait_item.kind {
 715             Fn(ref sig, _) => {
 716                 self.missing_named_lifetime_spots.push((&trait_item.generics).into());
 717                 let tcx = self.tcx;
 718                 self.visit_early_late(
 719                     Some(tcx.hir().get_parent_item(trait_item.hir_id)),
 720                     &sig.decl,
 721                     &trait_item.generics,
 722                     |this| intravisit::walk_trait_item(this, trait_item),
 723                 );
 724                 self.missing_named_lifetime_spots.pop();
 725             }
 726             Type(bounds, ref ty) => {
 727                 self.missing_named_lifetime_spots.push((&trait_item.generics).into());
 728                 let generics = &trait_item.generics;
 729                 let mut index = self.next_early_index();
 730                 debug!("visit_ty: index = {}", index);
 731                 let mut non_lifetime_count = 0;
 732                 let lifetimes = generics
 733                     .params
 734                     .iter()
 735                     .filter_map(|param| match param.kind {
 736                         GenericParamKind::Lifetime { .. } => {
 737                             Some(Region::early(&self.tcx.hir(), &mut index, param))
 738                         }
 739                         GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
 740                             non_lifetime_count += 1;
 741                             None
 742                         }
 743                     })
 744                     .collect();
 745                 let scope = Scope::Binder {
 746                     lifetimes,
 747                     next_early_index: index + non_lifetime_count,
 748                     s: self.scope,
 749                     track_lifetime_uses: true,
 750                     opaque_type_parent: true,
 751                 };
 752                 self.with(scope, |old_scope, this| {
 753                     this.check_lifetime_params(old_scope, &generics.params);
 754                     this.visit_generics(generics);
 755                     for bound in bounds {
 756                         this.visit_param_bound(bound);
 757                     }
 758                     if let Some(ty) = ty {
 759                         this.visit_ty(ty);
 760                     }
 761                 });
 762                 self.missing_named_lifetime_spots.pop();
 763             }
 764             Const(_, _) => {
 765                 // Only methods and types support generics.
 766                 assert!(trait_item.generics.params.is_empty());
 767                 self.missing_named_lifetime_spots.push(MissingLifetimeSpot::Static);
 768                 intravisit::walk_trait_item(self, trait_item);
 769                 self.missing_named_lifetime_spots.pop();
 770             }
 771         }
 772     }
 773
 774     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem<'tcx>) {
 775         use self::hir::ImplItemKind::*;
 776         match impl_item.kind {
 777             Fn(ref sig, _) => {
 778                 self.missing_named_lifetime_spots.push((&impl_item.generics).into());
 779                 let tcx = self.tcx;
 780                 self.visit_early_late(
 781                     Some(tcx.hir().get_parent_item(impl_item.hir_id)),
 782                     &sig.decl,
 783                     &impl_item.generics,
 784                     |this| intravisit::walk_impl_item(this, impl_item),
 785                 );
 786                 self.missing_named_lifetime_spots.pop();
 787             }
 788             TyAlias(ref ty) => {
 789                 let generics = &impl_item.generics;
 790                 self.missing_named_lifetime_spots.push(generics.into());
 791                 let mut index = self.next_early_index();
 792                 let mut non_lifetime_count = 0;
 793                 debug!("visit_ty: index = {}", index);
 794                 let lifetimes = generics
 795                     .params
 796                     .iter()
 797                     .filter_map(|param| match param.kind {
 798                         GenericParamKind::Lifetime { .. } => {
 799                             Some(Region::early(&self.tcx.hir(), &mut index, param))
 800                         }
 801                         GenericParamKind::Const { .. } | GenericParamKind::Type { .. } => {
 802                             non_lifetime_count += 1;
 803                             None
 804                         }
 805                     })
 806                     .collect();
 807                 let scope = Scope::Binder {
 808                     lifetimes,
 809                     next_early_index: index + non_lifetime_count,
 810                     s: self.scope,
 811                     track_lifetime_uses: true,
 812                     opaque_type_parent: true,
 813                 };
 814                 self.with(scope, |old_scope, this| {
 815                     this.check_lifetime_params(old_scope, &generics.params);
 816                     this.visit_generics(generics);
 817                     this.visit_ty(ty);
 818                 });
 819                 self.missing_named_lifetime_spots.pop();
 820             }
 821             Const(_, _) => {
 822                 // Only methods and types support generics.
 823                 assert!(impl_item.generics.params.is_empty());
 824                 self.missing_named_lifetime_spots.push(MissingLifetimeSpot::Static);
 825                 intravisit::walk_impl_item(self, impl_item);
 826                 self.missing_named_lifetime_spots.pop();
 827             }
 828         }
 829     }
 830
 831     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
 832         debug!("visit_lifetime(lifetime_ref={:?})", lifetime_ref);
 833         if lifetime_ref.is_elided() {
 834             self.resolve_elided_lifetimes(vec![lifetime_ref]);
 835             return;
 836         }
 837         if lifetime_ref.is_static() {
 838             self.insert_lifetime(lifetime_ref, Region::Static);
 839             return;
 840         }
 841         if self.is_in_const_generic && lifetime_ref.name != LifetimeName::Error {
 842             self.emit_non_static_lt_in_const_generic_error(lifetime_ref);
 843             return;
 844         }
 845         self.resolve_lifetime_ref(lifetime_ref);
 846     }
 847
 848     fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
 849         for (i, segment) in path.segments.iter().enumerate() {
 850             let depth = path.segments.len() - i - 1;
 851             if let Some(ref args) = segment.args {
 852                 self.visit_segment_args(path.res, depth, args);
 853             }
 854         }
 855     }
 856
 857     fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl<'tcx>) {
 858         let output = match fd.output {
 859             hir::FnRetTy::DefaultReturn(_) => None,
 860             hir::FnRetTy::Return(ref ty) => Some(&**ty),
 861         };
 862         self.visit_fn_like_elision(&fd.inputs, output);
 863     }
 864
 865     fn visit_generics(&mut self, generics: &'tcx hir::Generics<'tcx>) {
 866         check_mixed_explicit_and_in_band_defs(self.tcx, &generics.params);
 867         for param in generics.params {
 868             match param.kind {
 869                 GenericParamKind::Lifetime { .. } => {}
 870                 GenericParamKind::Type { ref default, .. } => {
 871                     walk_list!(self, visit_param_bound, param.bounds);
 872                     if let Some(ref ty) = default {
 873                         self.visit_ty(&ty);
 874                     }
 875                 }
 876                 GenericParamKind::Const { ref ty, .. } => {
 877                     let was_in_const_generic = self.is_in_const_generic;
 878                     self.is_in_const_generic = true;
 879                     walk_list!(self, visit_param_bound, param.bounds);
 880                     self.visit_ty(&ty);
 881                     self.is_in_const_generic = was_in_const_generic;
 882                 }
 883             }
 884         }
 885         for predicate in generics.where_clause.predicates {
 886             match predicate {
 887                 &hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
 888                     ref bounded_ty,
 889                     bounds,
 890                     ref bound_generic_params,
 891                     ..
 892                 }) => {
 893                     let lifetimes: FxHashMap<_, _> = bound_generic_params
 894                         .iter()
 895                         .filter_map(|param| match param.kind {
 896                             GenericParamKind::Lifetime { .. } => {
 897                                 Some(Region::late(&self.tcx.hir(), param))
 898                             }
 899                             _ => None,
 900                         })
 901                         .collect();
 902                     if !lifetimes.is_empty() {
 903                         let next_early_index = self.next_early_index();
 904                         let scope = Scope::Binder {
 905                             lifetimes,
 906                             s: self.scope,
 907                             next_early_index,
 908                             track_lifetime_uses: true,
 909                             opaque_type_parent: false,
 910                         };
 911                         let result = self.with(scope, |old_scope, this| {
 912                             this.check_lifetime_params(old_scope, &bound_generic_params);
 913                             this.visit_ty(&bounded_ty);
 914                             this.trait_ref_hack = true;
 915                             walk_list!(this, visit_param_bound, bounds);
 916                             this.trait_ref_hack = false;
 917                         });
 918                         result
 919                     } else {
 920                         self.visit_ty(&bounded_ty);
 921                         walk_list!(self, visit_param_bound, bounds);
 922                     }
 923                 }
 924                 &hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
 925                     ref lifetime,
 926                     bounds,
 927                     ..
 928                 }) => {
 929                     self.visit_lifetime(lifetime);
 930                     walk_list!(self, visit_param_bound, bounds);
 931                 }
 932                 &hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
 933                     ref lhs_ty,
 934                     ref rhs_ty,
 935                     ..
 936                 }) => {
 937                     self.visit_ty(lhs_ty);
 938                     self.visit_ty(rhs_ty);
 939                 }
 940             }
 941         }
 942     }
 943
 944     fn visit_poly_trait_ref(
 945         &mut self,
 946         trait_ref: &'tcx hir::PolyTraitRef<'tcx>,
 947         _modifier: hir::TraitBoundModifier,
 948     ) {
 949         debug!("visit_poly_trait_ref(trait_ref={:?})", trait_ref);
 950
 951         let should_pop_missing_lt = self.is_trait_ref_fn_scope(trait_ref);
 952
 953         let trait_ref_hack = take(&mut self.trait_ref_hack);
 954         if !trait_ref_hack
 955             || trait_ref.bound_generic_params.iter().any(|param| match param.kind {
 956                 GenericParamKind::Lifetime { .. } => true,
 957                 _ => false,
 958             })
 959         {
 960             if trait_ref_hack {
 961                 struct_span_err!(
 962                     self.tcx.sess,
 963                     trait_ref.span,
 964                     E0316,
 965                     "nested quantification of lifetimes"
 966                 )
 967                 .emit();
 968             }
 969             let next_early_index = self.next_early_index();
 970             let scope = Scope::Binder {
 971                 lifetimes: trait_ref
 972                     .bound_generic_params
 973                     .iter()
 974                     .filter_map(|param| match param.kind {
 975                         GenericParamKind::Lifetime { .. } => {
 976                             Some(Region::late(&self.tcx.hir(), param))
 977                         }
 978                         _ => None,
 979                     })
 980                     .collect(),
 981                 s: self.scope,
 982                 next_early_index,
 983                 track_lifetime_uses: true,
 984                 opaque_type_parent: false,
 985             };
 986             self.with(scope, |old_scope, this| {
 987                 this.check_lifetime_params(old_scope, &trait_ref.bound_generic_params);
 988                 walk_list!(this, visit_generic_param, trait_ref.bound_generic_params);
 989                 this.visit_trait_ref(&trait_ref.trait_ref);
 990             });
 991         } else {
 992             self.visit_trait_ref(&trait_ref.trait_ref);
 993         }
 994         self.trait_ref_hack = trait_ref_hack;
 995         if should_pop_missing_lt {
 996             self.missing_named_lifetime_spots.pop();
 997         }
 998     }
 999 }
1000
1001 #[derive(Copy, Clone, PartialEq)]
1002 enum ShadowKind {
1003     Label,
1004     Lifetime,
1005 }
1006 struct Original {
1007     kind: ShadowKind,
1008     span: Span,
1009 }
1010 struct Shadower {
1011     kind: ShadowKind,
1012     span: Span,
1013 }
1014
1015 fn original_label(span: Span) -> Original {
1016     Original { kind: ShadowKind::Label, span }
1017 }
1018 fn shadower_label(span: Span) -> Shadower {
1019     Shadower { kind: ShadowKind::Label, span }
1020 }
1021 fn original_lifetime(span: Span) -> Original {
1022     Original { kind: ShadowKind::Lifetime, span }
1023 }
1024 fn shadower_lifetime(param: &hir::GenericParam<'_>) -> Shadower {
1025     Shadower { kind: ShadowKind::Lifetime, span: param.span }
1026 }
1027
1028 impl ShadowKind {
1029     fn desc(&self) -> &'static str {
1030         match *self {
1031             ShadowKind::Label => "label",
1032             ShadowKind::Lifetime => "lifetime",
1033         }
1034     }
1035 }
1036
1037 fn check_mixed_explicit_and_in_band_defs(tcx: TyCtxt<'_>, params: &[hir::GenericParam<'_>]) {
1038     let lifetime_params: Vec<_> = params
1039         .iter()
1040         .filter_map(|param| match param.kind {
1041             GenericParamKind::Lifetime { kind, .. } => Some((kind, param.span)),
1042             _ => None,
1043         })
1044         .collect();
1045     let explicit = lifetime_params.iter().find(|(kind, _)| *kind == LifetimeParamKind::Explicit);
1046     let in_band = lifetime_params.iter().find(|(kind, _)| *kind == LifetimeParamKind::InBand);
1047
1048     if let (Some((_, explicit_span)), Some((_, in_band_span))) = (explicit, in_band) {
1049         struct_span_err!(
1050             tcx.sess,
1051             *in_band_span,
1052             E0688,
1053             "cannot mix in-band and explicit lifetime definitions"
1054         )
1055         .span_label(*in_band_span, "in-band lifetime definition here")
1056         .span_label(*explicit_span, "explicit lifetime definition here")
1057         .emit();
1058     }
1059 }
1060
1061 fn signal_shadowing_problem(tcx: TyCtxt<'_>, name: Symbol, orig: Original, shadower: Shadower) {
1062     let mut err = if let (ShadowKind::Lifetime, ShadowKind::Lifetime) = (orig.kind, shadower.kind) {
1063         // lifetime/lifetime shadowing is an error
1064         struct_span_err!(
1065             tcx.sess,
1066             shadower.span,
1067             E0496,
1068             "{} name `{}` shadows a \
1069              {} name that is already in scope",
1070             shadower.kind.desc(),
1071             name,
1072             orig.kind.desc()
1073         )
1074     } else {
1075         // shadowing involving a label is only a warning, due to issues with
1076         // labels and lifetimes not being macro-hygienic.
1077         tcx.sess.struct_span_warn(
1078             shadower.span,
1079             &format!(
1080                 "{} name `{}` shadows a \
1081                  {} name that is already in scope",
1082                 shadower.kind.desc(),
1083                 name,
1084                 orig.kind.desc()
1085             ),
1086         )
1087     };
1088     err.span_label(orig.span, "first declared here");
1089     err.span_label(shadower.span, format!("lifetime {} already in scope", name));
1090     err.emit();
1091 }
1092
1093 // Adds all labels in `b` to `ctxt.labels_in_fn`, signalling a warning
1094 // if one of the label shadows a lifetime or another label.
1095 fn extract_labels(ctxt: &mut LifetimeContext<'_, '_>, body: &hir::Body<'_>) {
1096     struct GatherLabels<'a, 'tcx> {
1097         tcx: TyCtxt<'tcx>,
1098         scope: ScopeRef<'a>,
1099         labels_in_fn: &'a mut Vec<Ident>,
1100     }
1101
1102     let mut gather =
1103         GatherLabels { tcx: ctxt.tcx, scope: ctxt.scope, labels_in_fn: &mut ctxt.labels_in_fn };
1104     gather.visit_body(body);
1105
1106     impl<'v, 'a, 'tcx> Visitor<'v> for GatherLabels<'a, 'tcx> {
1107         type Map = intravisit::ErasedMap<'v>;
1108
1109         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
1110             NestedVisitorMap::None
1111         }
1112
1113         fn visit_expr(&mut self, ex: &hir::Expr<'_>) {
1114             if let Some(label) = expression_label(ex) {
1115                 for prior_label in &self.labels_in_fn[..] {
1116                     // FIXME (#24278): non-hygienic comparison
1117                     if label.name == prior_label.name {
1118                         signal_shadowing_problem(
1119                             self.tcx,
1120                             label.name,
1121                             original_label(prior_label.span),
1122                             shadower_label(label.span),
1123                         );
1124                     }
1125                 }
1126
1127                 check_if_label_shadows_lifetime(self.tcx, self.scope, label);
1128
1129                 self.labels_in_fn.push(label);
1130             }
1131             intravisit::walk_expr(self, ex)
1132         }
1133     }
1134
1135     fn expression_label(ex: &hir::Expr<'_>) -> Option<Ident> {
1136         if let hir::ExprKind::Loop(_, Some(label), _) = ex.kind { Some(label.ident) } else { None }
1137     }
1138
1139     fn check_if_label_shadows_lifetime(tcx: TyCtxt<'_>, mut scope: ScopeRef<'_>, label: Ident) {
1140         loop {
1141             match *scope {
1142                 Scope::Body { s, .. }
1143                 | Scope::Elision { s, .. }
1144                 | Scope::ObjectLifetimeDefault { s, .. } => {
1145                     scope = s;
1146                 }
1147
1148                 Scope::Root => {
1149                     return;
1150                 }
1151
1152                 Scope::Binder { ref lifetimes, s, .. } => {
1153                     // FIXME (#24278): non-hygienic comparison
1154                     if let Some(def) =
1155                         lifetimes.get(&hir::ParamName::Plain(label.normalize_to_macros_2_0()))
1156                     {
1157                         let hir_id = tcx.hir().as_local_hir_id(def.id().unwrap().expect_local());
1158
1159                         signal_shadowing_problem(
1160                             tcx,
1161                             label.name,
1162                             original_lifetime(tcx.hir().span(hir_id)),
1163                             shadower_label(label.span),
1164                         );
1165                         return;
1166                     }
1167                     scope = s;
1168                 }
1169             }
1170         }
1171     }
1172 }
1173
1174 fn compute_object_lifetime_defaults(tcx: TyCtxt<'_>) -> HirIdMap<Vec<ObjectLifetimeDefault>> {
1175     let mut map = HirIdMap::default();
1176     for item in tcx.hir().krate().items.values() {
1177         match item.kind {
1178             hir::ItemKind::Struct(_, ref generics)
1179             | hir::ItemKind::Union(_, ref generics)
1180             | hir::ItemKind::Enum(_, ref generics)
1181             | hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1182                 ref generics, impl_trait_fn: None, ..
1183             })
1184             | hir::ItemKind::TyAlias(_, ref generics)
1185             | hir::ItemKind::Trait(_, _, ref generics, ..) => {
1186                 let result = object_lifetime_defaults_for_item(tcx, generics);
1187
1188                 // Debugging aid.
1189                 if tcx.sess.contains_name(&item.attrs, sym::rustc_object_lifetime_default) {
1190                     let object_lifetime_default_reprs: String = result
1191                         .iter()
1192                         .map(|set| match *set {
1193                             Set1::Empty => "BaseDefault".into(),
1194                             Set1::One(Region::Static) => "'static".into(),
1195                             Set1::One(Region::EarlyBound(mut i, _, _)) => generics
1196                                 .params
1197                                 .iter()
1198                                 .find_map(|param| match param.kind {
1199                                     GenericParamKind::Lifetime { .. } => {
1200                                         if i == 0 {
1201                                             return Some(param.name.ident().to_string().into());
1202                                         }
1203                                         i -= 1;
1204                                         None
1205                                     }
1206                                     _ => None,
1207                                 })
1208                                 .unwrap(),
1209                             Set1::One(_) => bug!(),
1210                             Set1::Many => "Ambiguous".into(),
1211                         })
1212                         .collect::<Vec<Cow<'static, str>>>()
1213                         .join(",");
1214                     tcx.sess.span_err(item.span, &object_lifetime_default_reprs);
1215                 }
1216
1217                 map.insert(item.hir_id, result);
1218             }
1219             _ => {}
1220         }
1221     }
1222     map
1223 }
1224
1225 /// Scan the bounds and where-clauses on parameters to extract bounds
1226 /// of the form `T:'a` so as to determine the `ObjectLifetimeDefault`
1227 /// for each type parameter.
1228 fn object_lifetime_defaults_for_item(
1229     tcx: TyCtxt<'_>,
1230     generics: &hir::Generics<'_>,
1231 ) -> Vec<ObjectLifetimeDefault> {
1232     fn add_bounds(set: &mut Set1<hir::LifetimeName>, bounds: &[hir::GenericBound<'_>]) {
1233         for bound in bounds {
1234             if let hir::GenericBound::Outlives(ref lifetime) = *bound {
1235                 set.insert(lifetime.name.normalize_to_macros_2_0());
1236             }
1237         }
1238     }
1239
1240     generics
1241         .params
1242         .iter()
1243         .filter_map(|param| match param.kind {
1244             GenericParamKind::Lifetime { .. } => None,
1245             GenericParamKind::Type { .. } => {
1246                 let mut set = Set1::Empty;
1247
1248                 add_bounds(&mut set, &param.bounds);
1249
1250                 let param_def_id = tcx.hir().local_def_id(param.hir_id);
1251                 for predicate in generics.where_clause.predicates {
1252                     // Look for `type: ...` where clauses.
1253                     let data = match *predicate {
1254                         hir::WherePredicate::BoundPredicate(ref data) => data,
1255                         _ => continue,
1256                     };
1257
1258                     // Ignore `for<'a> type: ...` as they can change what
1259                     // lifetimes mean (although we could "just" handle it).
1260                     if !data.bound_generic_params.is_empty() {
1261                         continue;
1262                     }
1263
1264                     let res = match data.bounded_ty.kind {
1265                         hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => path.res,
1266                         _ => continue,
1267                     };
1268
1269                     if res == Res::Def(DefKind::TyParam, param_def_id.to_def_id()) {
1270                         add_bounds(&mut set, &data.bounds);
1271                     }
1272                 }
1273
1274                 Some(match set {
1275                     Set1::Empty => Set1::Empty,
1276                     Set1::One(name) => {
1277                         if name == hir::LifetimeName::Static {
1278                             Set1::One(Region::Static)
1279                         } else {
1280                             generics
1281                                 .params
1282                                 .iter()
1283                                 .filter_map(|param| match param.kind {
1284                                     GenericParamKind::Lifetime { .. } => Some((
1285                                         param.hir_id,
1286                                         hir::LifetimeName::Param(param.name),
1287                                         LifetimeDefOrigin::from_param(param),
1288                                     )),
1289                                     _ => None,
1290                                 })
1291                                 .enumerate()
1292                                 .find(|&(_, (_, lt_name, _))| lt_name == name)
1293                                 .map_or(Set1::Many, |(i, (id, _, origin))| {
1294                                     let def_id = tcx.hir().local_def_id(id);
1295                                     Set1::One(Region::EarlyBound(
1296                                         i as u32,
1297                                         def_id.to_def_id(),
1298                                         origin,
1299                                     ))
1300                                 })
1301                         }
1302                     }
1303                     Set1::Many => Set1::Many,
1304                 })
1305             }
1306             GenericParamKind::Const { .. } => {
1307                 // Generic consts don't impose any constraints.
1308                 //
1309                 // We still store a dummy value here to allow generic parameters
1310                 // in an arbitrary order.
1311                 Some(Set1::Empty)
1312             }
1313         })
1314         .collect()
1315 }
1316
1317 impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
1318     // FIXME(#37666) this works around a limitation in the region inferencer
1319     fn hack<F>(&mut self, f: F)
1320     where
1321         F: for<'b> FnOnce(&mut LifetimeContext<'b, 'tcx>),
1322     {
1323         f(self)
1324     }
1325
1326     fn with<F>(&mut self, wrap_scope: Scope<'_>, f: F)
1327     where
1328         F: for<'b> FnOnce(ScopeRef<'_>, &mut LifetimeContext<'b, 'tcx>),
1329     {
1330         let LifetimeContext { tcx, map, lifetime_uses, .. } = self;
1331         let labels_in_fn = take(&mut self.labels_in_fn);
1332         let xcrate_object_lifetime_defaults = take(&mut self.xcrate_object_lifetime_defaults);
1333         let missing_named_lifetime_spots = take(&mut self.missing_named_lifetime_spots);
1334         let mut this = LifetimeContext {
1335             tcx: *tcx,
1336             map,
1337             scope: &wrap_scope,
1338             trait_ref_hack: self.trait_ref_hack,
1339             is_in_fn_syntax: self.is_in_fn_syntax,
1340             is_in_const_generic: self.is_in_const_generic,
1341             labels_in_fn,
1342             xcrate_object_lifetime_defaults,
1343             lifetime_uses,
1344             missing_named_lifetime_spots,
1345         };
1346         debug!("entering scope {:?}", this.scope);
1347         f(self.scope, &mut this);
1348         this.check_uses_for_lifetimes_defined_by_scope();
1349         debug!("exiting scope {:?}", this.scope);
1350         self.labels_in_fn = this.labels_in_fn;
1351         self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;
1352         self.missing_named_lifetime_spots = this.missing_named_lifetime_spots;
1353     }
1354
1355     /// helper method to determine the span to remove when suggesting the
1356     /// deletion of a lifetime
1357     fn lifetime_deletion_span(&self, name: Ident, generics: &hir::Generics<'_>) -> Option<Span> {
1358         generics.params.iter().enumerate().find_map(|(i, param)| {
1359             if param.name.ident() == name {
1360                 let mut in_band = false;
1361                 if let hir::GenericParamKind::Lifetime { kind } = param.kind {
1362                     if let hir::LifetimeParamKind::InBand = kind {
1363                         in_band = true;
1364                     }
1365                 }
1366                 if in_band {
1367                     Some(param.span)
1368                 } else {
1369                     if generics.params.len() == 1 {
1370                         // if sole lifetime, remove the entire `<>` brackets
1371                         Some(generics.span)
1372                     } else {
1373                         // if removing within `<>` brackets, we also want to
1374                         // delete a leading or trailing comma as appropriate
1375                         if i >= generics.params.len() - 1 {
1376                             Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
1377                         } else {
1378                             Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
1379                         }
1380                     }
1381                 }
1382             } else {
1383                 None
1384             }
1385         })
1386     }
1387
1388     // helper method to issue suggestions from `fn rah<'a>(&'a T)` to `fn rah(&T)`
1389     // or from `fn rah<'a>(T<'a>)` to `fn rah(T<'_>)`
1390     fn suggest_eliding_single_use_lifetime(
1391         &self,
1392         err: &mut DiagnosticBuilder<'_>,
1393         def_id: DefId,
1394         lifetime: &hir::Lifetime,
1395     ) {
1396         let name = lifetime.name.ident();
1397         let mut remove_decl = None;
1398         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1399             if let Some(generics) = self.tcx.hir().get_generics(parent_def_id) {
1400                 remove_decl = self.lifetime_deletion_span(name, generics);
1401             }
1402         }
1403
1404         let mut remove_use = None;
1405         let mut elide_use = None;
1406         let mut find_arg_use_span = |inputs: &[hir::Ty<'_>]| {
1407             for input in inputs {
1408                 match input.kind {
1409                     hir::TyKind::Rptr(lt, _) => {
1410                         if lt.name.ident() == name {
1411                             // include the trailing whitespace between the lifetime and type names
1412                             let lt_through_ty_span = lifetime.span.to(input.span.shrink_to_hi());
1413                             remove_use = Some(
1414                                 self.tcx
1415                                     .sess
1416                                     .source_map()
1417                                     .span_until_non_whitespace(lt_through_ty_span),
1418                             );
1419                             break;
1420                         }
1421                     }
1422                     hir::TyKind::Path(ref qpath) => {
1423                         if let QPath::Resolved(_, path) = qpath {
1424                             let last_segment = &path.segments[path.segments.len() - 1];
1425                             let generics = last_segment.generic_args();
1426                             for arg in generics.args.iter() {
1427                                 if let GenericArg::Lifetime(lt) = arg {
1428                                     if lt.name.ident() == name {
1429                                         elide_use = Some(lt.span);
1430                                         break;
1431                                     }
1432                                 }
1433                             }
1434                             break;
1435                         }
1436                     }
1437                     _ => {}
1438                 }
1439             }
1440         };
1441         if let Node::Lifetime(hir_lifetime) = self.tcx.hir().get(lifetime.hir_id) {
1442             if let Some(parent) =
1443                 self.tcx.hir().find(self.tcx.hir().get_parent_item(hir_lifetime.hir_id))
1444             {
1445                 match parent {
1446                     Node::Item(item) => {
1447                         if let hir::ItemKind::Fn(sig, _, _) = &item.kind {
1448                             find_arg_use_span(sig.decl.inputs);
1449                         }
1450                     }
1451                     Node::ImplItem(impl_item) => {
1452                         if let hir::ImplItemKind::Fn(sig, _) = &impl_item.kind {
1453                             find_arg_use_span(sig.decl.inputs);
1454                         }
1455                     }
1456                     _ => {}
1457                 }
1458             }
1459         }
1460
1461         let msg = "elide the single-use lifetime";
1462         match (remove_decl, remove_use, elide_use) {
1463             (Some(decl_span), Some(use_span), None) => {
1464                 // if both declaration and use deletion spans start at the same
1465                 // place ("start at" because the latter includes trailing
1466                 // whitespace), then this is an in-band lifetime
1467                 if decl_span.shrink_to_lo() == use_span.shrink_to_lo() {
1468                     err.span_suggestion(
1469                         use_span,
1470                         msg,
1471                         String::new(),
1472                         Applicability::MachineApplicable,
1473                     );
1474                 } else {
1475                     err.multipart_suggestion(
1476                         msg,
1477                         vec![(decl_span, String::new()), (use_span, String::new())],
1478                         Applicability::MachineApplicable,
1479                     );
1480                 }
1481             }
1482             (Some(decl_span), None, Some(use_span)) => {
1483                 err.multipart_suggestion(
1484                     msg,
1485                     vec![(decl_span, String::new()), (use_span, "'_".to_owned())],
1486                     Applicability::MachineApplicable,
1487                 );
1488             }
1489             _ => {}
1490         }
1491     }
1492
1493     fn check_uses_for_lifetimes_defined_by_scope(&mut self) {
1494         let defined_by = match self.scope {
1495             Scope::Binder { lifetimes, .. } => lifetimes,
1496             _ => {
1497                 debug!("check_uses_for_lifetimes_defined_by_scope: not in a binder scope");
1498                 return;
1499             }
1500         };
1501
1502         let mut def_ids: Vec<_> = defined_by
1503             .values()
1504             .flat_map(|region| match region {
1505                 Region::EarlyBound(_, def_id, _)
1506                 | Region::LateBound(_, def_id, _)
1507                 | Region::Free(_, def_id) => Some(*def_id),
1508
1509                 Region::LateBoundAnon(..) | Region::Static => None,
1510             })
1511             .collect();
1512
1513         // ensure that we issue lints in a repeatable order
1514         def_ids.sort_by_cached_key(|&def_id| self.tcx.def_path_hash(def_id));
1515
1516         for def_id in def_ids {
1517             debug!("check_uses_for_lifetimes_defined_by_scope: def_id = {:?}", def_id);
1518
1519             let lifetimeuseset = self.lifetime_uses.remove(&def_id);
1520
1521             debug!(
1522                 "check_uses_for_lifetimes_defined_by_scope: lifetimeuseset = {:?}",
1523                 lifetimeuseset
1524             );
1525
1526             match lifetimeuseset {
1527                 Some(LifetimeUseSet::One(lifetime)) => {
1528                     let hir_id = self.tcx.hir().as_local_hir_id(def_id.expect_local());
1529                     debug!("hir id first={:?}", hir_id);
1530                     if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
1531                         Node::Lifetime(hir_lifetime) => Some((
1532                             hir_lifetime.hir_id,
1533                             hir_lifetime.span,
1534                             hir_lifetime.name.ident(),
1535                         )),
1536                         Node::GenericParam(param) => {
1537                             Some((param.hir_id, param.span, param.name.ident()))
1538                         }
1539                         _ => None,
1540                     } {
1541                         debug!("id = {:?} span = {:?} name = {:?}", id, span, name);
1542                         if name.name == kw::UnderscoreLifetime {
1543                             continue;
1544                         }
1545
1546                         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1547                             if let Some(def_id) = parent_def_id.as_local() {
1548                                 let parent_hir_id = self.tcx.hir().as_local_hir_id(def_id);
1549                                 // lifetimes in `derive` expansions don't count (Issue #53738)
1550                                 if self.tcx.hir().attrs(parent_hir_id).iter().any(|attr| {
1551                                     self.tcx.sess.check_name(attr, sym::automatically_derived)
1552                                 }) {
1553                                     continue;
1554                                 }
1555                             }
1556                         }
1557
1558                         self.tcx.struct_span_lint_hir(
1559                             lint::builtin::SINGLE_USE_LIFETIMES,
1560                             id,
1561                             span,
1562                             |lint| {
1563                                 let mut err = lint.build(&format!(
1564                                     "lifetime parameter `{}` only used once",
1565                                     name
1566                                 ));
1567                                 if span == lifetime.span {
1568                                     // spans are the same for in-band lifetime declarations
1569                                     err.span_label(span, "this lifetime is only used here");
1570                                 } else {
1571                                     err.span_label(span, "this lifetime...");
1572                                     err.span_label(lifetime.span, "...is used only here");
1573                                 }
1574                                 self.suggest_eliding_single_use_lifetime(
1575                                     &mut err, def_id, lifetime,
1576                                 );
1577                                 err.emit();
1578                             },
1579                         );
1580                     }
1581                 }
1582                 Some(LifetimeUseSet::Many) => {
1583                     debug!("not one use lifetime");
1584                 }
1585                 None => {
1586                     let hir_id = self.tcx.hir().as_local_hir_id(def_id.expect_local());
1587                     if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
1588                         Node::Lifetime(hir_lifetime) => Some((
1589                             hir_lifetime.hir_id,
1590                             hir_lifetime.span,
1591                             hir_lifetime.name.ident(),
1592                         )),
1593                         Node::GenericParam(param) => {
1594                             Some((param.hir_id, param.span, param.name.ident()))
1595                         }
1596                         _ => None,
1597                     } {
1598                         debug!("id ={:?} span = {:?} name = {:?}", id, span, name);
1599                         self.tcx.struct_span_lint_hir(
1600                             lint::builtin::UNUSED_LIFETIMES,
1601                             id,
1602                             span,
1603                             |lint| {
1604                                 let mut err = lint
1605                                     .build(&format!("lifetime parameter `{}` never used", name));
1606                                 if let Some(parent_def_id) = self.tcx.parent(def_id) {
1607                                     if let Some(generics) =
1608                                         self.tcx.hir().get_generics(parent_def_id)
1609                                     {
1610                                         let unused_lt_span =
1611                                             self.lifetime_deletion_span(name, generics);
1612                                         if let Some(span) = unused_lt_span {
1613                                             err.span_suggestion(
1614                                                 span,
1615                                                 "elide the unused lifetime",
1616                                                 String::new(),
1617                                                 Applicability::MachineApplicable,
1618                                             );
1619                                         }
1620                                     }
1621                                 }
1622                                 err.emit();
1623                             },
1624                         );
1625                     }
1626                 }
1627             }
1628         }
1629     }
1630
1631     /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
1632     ///
1633     /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
1634     /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
1635     /// within type bounds; those are early bound lifetimes, and the rest are late bound.
1636     ///
1637     /// For example:
1638     ///
1639     ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
1640     ///
1641     /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
1642     /// lifetimes may be interspersed together.
1643     ///
1644     /// If early bound lifetimes are present, we separate them into their own list (and likewise
1645     /// for late bound). They will be numbered sequentially, starting from the lowest index that is
1646     /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
1647     /// bound lifetimes are resolved by name and associated with a binder ID (`binder_id`), so the
1648     /// ordering is not important there.
1649     fn visit_early_late<F>(
1650         &mut self,
1651         parent_id: Option<hir::HirId>,
1652         decl: &'tcx hir::FnDecl<'tcx>,
1653         generics: &'tcx hir::Generics<'tcx>,
1654         walk: F,
1655     ) where
1656         F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
1657     {
1658         insert_late_bound_lifetimes(self.map, decl, generics);
1659
1660         // Find the start of nested early scopes, e.g., in methods.
1661         let mut index = 0;
1662         if let Some(parent_id) = parent_id {
1663             let parent = self.tcx.hir().expect_item(parent_id);
1664             if sub_items_have_self_param(&parent.kind) {
1665                 index += 1; // Self comes before lifetimes
1666             }
1667             match parent.kind {
1668                 hir::ItemKind::Trait(_, _, ref generics, ..)
1669                 | hir::ItemKind::Impl { ref generics, .. } => {
1670                     index += generics.params.len() as u32;
1671                 }
1672                 _ => {}
1673             }
1674         }
1675
1676         let mut non_lifetime_count = 0;
1677         let lifetimes = generics
1678             .params
1679             .iter()
1680             .filter_map(|param| match param.kind {
1681                 GenericParamKind::Lifetime { .. } => {
1682                     if self.map.late_bound.contains(&param.hir_id) {
1683                         Some(Region::late(&self.tcx.hir(), param))
1684                     } else {
1685                         Some(Region::early(&self.tcx.hir(), &mut index, param))
1686                     }
1687                 }
1688                 GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
1689                     non_lifetime_count += 1;
1690                     None
1691                 }
1692             })
1693             .collect();
1694         let next_early_index = index + non_lifetime_count;
1695
1696         let scope = Scope::Binder {
1697             lifetimes,
1698             next_early_index,
1699             s: self.scope,
1700             opaque_type_parent: true,
1701             track_lifetime_uses: false,
1702         };
1703         self.with(scope, move |old_scope, this| {
1704             this.check_lifetime_params(old_scope, &generics.params);
1705             this.hack(walk); // FIXME(#37666) workaround in place of `walk(this)`
1706         });
1707     }
1708
1709     fn next_early_index_helper(&self, only_opaque_type_parent: bool) -> u32 {
1710         let mut scope = self.scope;
1711         loop {
1712             match *scope {
1713                 Scope::Root => return 0,
1714
1715                 Scope::Binder { next_early_index, opaque_type_parent, .. }
1716                     if (!only_opaque_type_parent || opaque_type_parent) =>
1717                 {
1718                     return next_early_index;
1719                 }
1720
1721                 Scope::Binder { s, .. }
1722                 | Scope::Body { s, .. }
1723                 | Scope::Elision { s, .. }
1724                 | Scope::ObjectLifetimeDefault { s, .. } => scope = s,
1725             }
1726         }
1727     }
1728
1729     /// Returns the next index one would use for an early-bound-region
1730     /// if extending the current scope.
1731     fn next_early_index(&self) -> u32 {
1732         self.next_early_index_helper(true)
1733     }
1734
1735     /// Returns the next index one would use for an `impl Trait` that
1736     /// is being converted into an opaque type alias `impl Trait`. This will be the
1737     /// next early index from the enclosing item, for the most
1738     /// part. See the `opaque_type_parent` field for more info.
1739     fn next_early_index_for_opaque_type(&self) -> u32 {
1740         self.next_early_index_helper(false)
1741     }
1742
1743     fn resolve_lifetime_ref(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
1744         debug!("resolve_lifetime_ref(lifetime_ref={:?})", lifetime_ref);
1745
1746         // If we've already reported an error, just ignore `lifetime_ref`.
1747         if let LifetimeName::Error = lifetime_ref.name {
1748             return;
1749         }
1750
1751         // Walk up the scope chain, tracking the number of fn scopes
1752         // that we pass through, until we find a lifetime with the
1753         // given name or we run out of scopes.
1754         // search.
1755         let mut late_depth = 0;
1756         let mut scope = self.scope;
1757         let mut outermost_body = None;
1758         let result = loop {
1759             match *scope {
1760                 Scope::Body { id, s } => {
1761                     outermost_body = Some(id);
1762                     scope = s;
1763                 }
1764
1765                 Scope::Root => {
1766                     break None;
1767                 }
1768
1769                 Scope::Binder { ref lifetimes, s, .. } => {
1770                     match lifetime_ref.name {
1771                         LifetimeName::Param(param_name) => {
1772                             if let Some(&def) = lifetimes.get(&param_name.normalize_to_macros_2_0())
1773                             {
1774                                 break Some(def.shifted(late_depth));
1775                             }
1776                         }
1777                         _ => bug!("expected LifetimeName::Param"),
1778                     }
1779
1780                     late_depth += 1;
1781                     scope = s;
1782                 }
1783
1784                 Scope::Elision { s, .. } | Scope::ObjectLifetimeDefault { s, .. } => {
1785                     scope = s;
1786                 }
1787             }
1788         };
1789
1790         if let Some(mut def) = result {
1791             if let Region::EarlyBound(..) = def {
1792                 // Do not free early-bound regions, only late-bound ones.
1793             } else if let Some(body_id) = outermost_body {
1794                 let fn_id = self.tcx.hir().body_owner(body_id);
1795                 match self.tcx.hir().get(fn_id) {
1796                     Node::Item(&hir::Item { kind: hir::ItemKind::Fn(..), .. })
1797                     | Node::TraitItem(&hir::TraitItem {
1798                         kind: hir::TraitItemKind::Fn(..), ..
1799                     })
1800                     | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }) => {
1801                         let scope = self.tcx.hir().local_def_id(fn_id);
1802                         def = Region::Free(scope.to_def_id(), def.id().unwrap());
1803                     }
1804                     _ => {}
1805                 }
1806             }
1807
1808             // Check for fn-syntax conflicts with in-band lifetime definitions
1809             if self.is_in_fn_syntax {
1810                 match def {
1811                     Region::EarlyBound(_, _, LifetimeDefOrigin::InBand)
1812                     | Region::LateBound(_, _, LifetimeDefOrigin::InBand) => {
1813                         struct_span_err!(
1814                             self.tcx.sess,
1815                             lifetime_ref.span,
1816                             E0687,
1817                             "lifetimes used in `fn` or `Fn` syntax must be \
1818                              explicitly declared using `<...>` binders"
1819                         )
1820                         .span_label(lifetime_ref.span, "in-band lifetime definition")
1821                         .emit();
1822                     }
1823
1824                     Region::Static
1825                     | Region::EarlyBound(
1826                         _,
1827                         _,
1828                         LifetimeDefOrigin::ExplicitOrElided | LifetimeDefOrigin::Error,
1829                     )
1830                     | Region::LateBound(
1831                         _,
1832                         _,
1833                         LifetimeDefOrigin::ExplicitOrElided | LifetimeDefOrigin::Error,
1834                     )
1835                     | Region::LateBoundAnon(..)
1836                     | Region::Free(..) => {}
1837                 }
1838             }
1839
1840             self.insert_lifetime(lifetime_ref, def);
1841         } else {
1842             self.emit_undeclared_lifetime_error(lifetime_ref);
1843         }
1844     }
1845
1846     fn visit_segment_args(
1847         &mut self,
1848         res: Res,
1849         depth: usize,
1850         generic_args: &'tcx hir::GenericArgs<'tcx>,
1851     ) {
1852         debug!(
1853             "visit_segment_args(res={:?}, depth={:?}, generic_args={:?})",
1854             res, depth, generic_args,
1855         );
1856
1857         if generic_args.parenthesized {
1858             let was_in_fn_syntax = self.is_in_fn_syntax;
1859             self.is_in_fn_syntax = true;
1860             self.visit_fn_like_elision(generic_args.inputs(), Some(generic_args.bindings[0].ty()));
1861             self.is_in_fn_syntax = was_in_fn_syntax;
1862             return;
1863         }
1864
1865         let mut elide_lifetimes = true;
1866         let lifetimes = generic_args
1867             .args
1868             .iter()
1869             .filter_map(|arg| match arg {
1870                 hir::GenericArg::Lifetime(lt) => {
1871                     if !lt.is_elided() {
1872                         elide_lifetimes = false;
1873                     }
1874                     Some(lt)
1875                 }
1876                 _ => None,
1877             })
1878             .collect();
1879         if elide_lifetimes {
1880             self.resolve_elided_lifetimes(lifetimes);
1881         } else {
1882             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
1883         }
1884
1885         // Figure out if this is a type/trait segment,
1886         // which requires object lifetime defaults.
1887         let parent_def_id = |this: &mut Self, def_id: DefId| {
1888             let def_key = this.tcx.def_key(def_id);
1889             DefId { krate: def_id.krate, index: def_key.parent.expect("missing parent") }
1890         };
1891         let type_def_id = match res {
1892             Res::Def(DefKind::AssocTy, def_id) if depth == 1 => Some(parent_def_id(self, def_id)),
1893             Res::Def(DefKind::Variant, def_id) if depth == 0 => Some(parent_def_id(self, def_id)),
1894             Res::Def(
1895                 DefKind::Struct
1896                 | DefKind::Union
1897                 | DefKind::Enum
1898                 | DefKind::TyAlias
1899                 | DefKind::Trait,
1900                 def_id,
1901             ) if depth == 0 => Some(def_id),
1902             _ => None,
1903         };
1904
1905         debug!("visit_segment_args: type_def_id={:?}", type_def_id);
1906
1907         // Compute a vector of defaults, one for each type parameter,
1908         // per the rules given in RFCs 599 and 1156. Example:
1909         //
1910         // ```rust
1911         // struct Foo<'a, T: 'a, U> { }
1912         // ```
1913         //
1914         // If you have `Foo<'x, dyn Bar, dyn Baz>`, we want to default
1915         // `dyn Bar` to `dyn Bar + 'x` (because of the `T: 'a` bound)
1916         // and `dyn Baz` to `dyn Baz + 'static` (because there is no
1917         // such bound).
1918         //
1919         // Therefore, we would compute `object_lifetime_defaults` to a
1920         // vector like `['x, 'static]`. Note that the vector only
1921         // includes type parameters.
1922         let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
1923             let in_body = {
1924                 let mut scope = self.scope;
1925                 loop {
1926                     match *scope {
1927                         Scope::Root => break false,
1928
1929                         Scope::Body { .. } => break true,
1930
1931                         Scope::Binder { s, .. }
1932                         | Scope::Elision { s, .. }
1933                         | Scope::ObjectLifetimeDefault { s, .. } => {
1934                             scope = s;
1935                         }
1936                     }
1937                 }
1938             };
1939
1940             let map = &self.map;
1941             let unsubst = if let Some(def_id) = def_id.as_local() {
1942                 let id = self.tcx.hir().as_local_hir_id(def_id);
1943                 &map.object_lifetime_defaults[&id]
1944             } else {
1945                 let tcx = self.tcx;
1946                 self.xcrate_object_lifetime_defaults.entry(def_id).or_insert_with(|| {
1947                     tcx.generics_of(def_id)
1948                         .params
1949                         .iter()
1950                         .filter_map(|param| match param.kind {
1951                             GenericParamDefKind::Type { object_lifetime_default, .. } => {
1952                                 Some(object_lifetime_default)
1953                             }
1954                             GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
1955                         })
1956                         .collect()
1957                 })
1958             };
1959             debug!("visit_segment_args: unsubst={:?}", unsubst);
1960             unsubst
1961                 .iter()
1962                 .map(|set| match *set {
1963                     Set1::Empty => {
1964                         if in_body {
1965                             None
1966                         } else {
1967                             Some(Region::Static)
1968                         }
1969                     }
1970                     Set1::One(r) => {
1971                         let lifetimes = generic_args.args.iter().filter_map(|arg| match arg {
1972                             GenericArg::Lifetime(lt) => Some(lt),
1973                             _ => None,
1974                         });
1975                         r.subst(lifetimes, map)
1976                     }
1977                     Set1::Many => None,
1978                 })
1979                 .collect()
1980         });
1981
1982         debug!("visit_segment_args: object_lifetime_defaults={:?}", object_lifetime_defaults);
1983
1984         let mut i = 0;
1985         for arg in generic_args.args {
1986             match arg {
1987                 GenericArg::Lifetime(_) => {}
1988                 GenericArg::Type(ty) => {
1989                     if let Some(&lt) = object_lifetime_defaults.get(i) {
1990                         let scope = Scope::ObjectLifetimeDefault { lifetime: lt, s: self.scope };
1991                         self.with(scope, |_, this| this.visit_ty(ty));
1992                     } else {
1993                         self.visit_ty(ty);
1994                     }
1995                     i += 1;
1996                 }
1997                 GenericArg::Const(ct) => {
1998                     self.visit_anon_const(&ct.value);
1999                 }
2000             }
2001         }
2002
2003         // Hack: when resolving the type `XX` in binding like `dyn
2004         // Foo<'b, Item = XX>`, the current object-lifetime default
2005         // would be to examine the trait `Foo` to check whether it has
2006         // a lifetime bound declared on `Item`. e.g., if `Foo` is
2007         // declared like so, then the default object lifetime bound in
2008         // `XX` should be `'b`:
2009         //
2010         // ```rust
2011         // trait Foo<'a> {
2012         //   type Item: 'a;
2013         // }
2014         // ```
2015         //
2016         // but if we just have `type Item;`, then it would be
2017         // `'static`. However, we don't get all of this logic correct.
2018         //
2019         // Instead, we do something hacky: if there are no lifetime parameters
2020         // to the trait, then we simply use a default object lifetime
2021         // bound of `'static`, because there is no other possibility. On the other hand,
2022         // if there ARE lifetime parameters, then we require the user to give an
2023         // explicit bound for now.
2024         //
2025         // This is intended to leave room for us to implement the
2026         // correct behavior in the future.
2027         let has_lifetime_parameter = generic_args.args.iter().any(|arg| match arg {
2028             GenericArg::Lifetime(_) => true,
2029             _ => false,
2030         });
2031
2032         // Resolve lifetimes found in the type `XX` from `Item = XX` bindings.
2033         for b in generic_args.bindings {
2034             let scope = Scope::ObjectLifetimeDefault {
2035                 lifetime: if has_lifetime_parameter { None } else { Some(Region::Static) },
2036                 s: self.scope,
2037             };
2038             self.with(scope, |_, this| this.visit_assoc_type_binding(b));
2039         }
2040     }
2041
2042     fn visit_fn_like_elision(
2043         &mut self,
2044         inputs: &'tcx [hir::Ty<'tcx>],
2045         output: Option<&'tcx hir::Ty<'tcx>>,
2046     ) {
2047         debug!("visit_fn_like_elision: enter");
2048         let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
2049         let arg_scope = Scope::Elision { elide: arg_elide.clone(), s: self.scope };
2050         self.with(arg_scope, |_, this| {
2051             for input in inputs {
2052                 this.visit_ty(input);
2053             }
2054             match *this.scope {
2055                 Scope::Elision { ref elide, .. } => {
2056                     arg_elide = elide.clone();
2057                 }
2058                 _ => bug!(),
2059             }
2060         });
2061
2062         let output = match output {
2063             Some(ty) => ty,
2064             None => return,
2065         };
2066
2067         debug!("visit_fn_like_elision: determine output");
2068
2069         // Figure out if there's a body we can get argument names from,
2070         // and whether there's a `self` argument (treated specially).
2071         let mut assoc_item_kind = None;
2072         let mut impl_self = None;
2073         let parent = self.tcx.hir().get_parent_node(output.hir_id);
2074         let body = match self.tcx.hir().get(parent) {
2075             // `fn` definitions and methods.
2076             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(.., body), .. }) => Some(body),
2077
2078             Node::TraitItem(&hir::TraitItem { kind: hir::TraitItemKind::Fn(_, ref m), .. }) => {
2079                 if let hir::ItemKind::Trait(.., ref trait_items) =
2080                     self.tcx.hir().expect_item(self.tcx.hir().get_parent_item(parent)).kind
2081                 {
2082                     assoc_item_kind =
2083                         trait_items.iter().find(|ti| ti.id.hir_id == parent).map(|ti| ti.kind);
2084                 }
2085                 match *m {
2086                     hir::TraitFn::Required(_) => None,
2087                     hir::TraitFn::Provided(body) => Some(body),
2088                 }
2089             }
2090
2091             Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(_, body), .. }) => {
2092                 if let hir::ItemKind::Impl { ref self_ty, ref items, .. } =
2093                     self.tcx.hir().expect_item(self.tcx.hir().get_parent_item(parent)).kind
2094                 {
2095                     impl_self = Some(self_ty);
2096                     assoc_item_kind =
2097                         items.iter().find(|ii| ii.id.hir_id == parent).map(|ii| ii.kind);
2098                 }
2099                 Some(body)
2100             }
2101
2102             // Foreign functions, `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
2103             Node::ForeignItem(_) | Node::Ty(_) | Node::TraitRef(_) => None,
2104             // Everything else (only closures?) doesn't
2105             // actually enjoy elision in return types.
2106             _ => {
2107                 self.visit_ty(output);
2108                 return;
2109             }
2110         };
2111
2112         let has_self = match assoc_item_kind {
2113             Some(hir::AssocItemKind::Fn { has_self }) => has_self,
2114             _ => false,
2115         };
2116
2117         // In accordance with the rules for lifetime elision, we can determine
2118         // what region to use for elision in the output type in two ways.
2119         // First (determined here), if `self` is by-reference, then the
2120         // implied output region is the region of the self parameter.
2121         if has_self {
2122             struct SelfVisitor<'a> {
2123                 map: &'a NamedRegionMap,
2124                 impl_self: Option<&'a hir::TyKind<'a>>,
2125                 lifetime: Set1<Region>,
2126             }
2127
2128             impl SelfVisitor<'_> {
2129                 // Look for `self: &'a Self` - also desugared from `&'a self`,
2130                 // and if that matches, use it for elision and return early.
2131                 fn is_self_ty(&self, res: Res) -> bool {
2132                     if let Res::SelfTy(..) = res {
2133                         return true;
2134                     }
2135
2136                     // Can't always rely on literal (or implied) `Self` due
2137                     // to the way elision rules were originally specified.
2138                     if let Some(&hir::TyKind::Path(hir::QPath::Resolved(None, ref path))) =
2139                         self.impl_self
2140                     {
2141                         match path.res {
2142                             // Permit the types that unambiguously always
2143                             // result in the same type constructor being used
2144                             // (it can't differ between `Self` and `self`).
2145                             Res::Def(DefKind::Struct | DefKind::Union | DefKind::Enum, _)
2146                             | Res::PrimTy(_) => return res == path.res,
2147                             _ => {}
2148                         }
2149                     }
2150
2151                     false
2152                 }
2153             }
2154
2155             impl<'a> Visitor<'a> for SelfVisitor<'a> {
2156                 type Map = intravisit::ErasedMap<'a>;
2157
2158                 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2159                     NestedVisitorMap::None
2160                 }
2161
2162                 fn visit_ty(&mut self, ty: &'a hir::Ty<'a>) {
2163                     if let hir::TyKind::Rptr(lifetime_ref, ref mt) = ty.kind {
2164                         if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.kind
2165                         {
2166                             if self.is_self_ty(path.res) {
2167                                 if let Some(lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
2168                                     self.lifetime.insert(*lifetime);
2169                                 }
2170                             }
2171                         }
2172                     }
2173                     intravisit::walk_ty(self, ty)
2174                 }
2175             }
2176
2177             let mut visitor = SelfVisitor {
2178                 map: self.map,
2179                 impl_self: impl_self.map(|ty| &ty.kind),
2180                 lifetime: Set1::Empty,
2181             };
2182             visitor.visit_ty(&inputs[0]);
2183             if let Set1::One(lifetime) = visitor.lifetime {
2184                 let scope = Scope::Elision { elide: Elide::Exact(lifetime), s: self.scope };
2185                 self.with(scope, |_, this| this.visit_ty(output));
2186                 return;
2187             }
2188         }
2189
2190         // Second, if there was exactly one lifetime (either a substitution or a
2191         // reference) in the arguments, then any anonymous regions in the output
2192         // have that lifetime.
2193         let mut possible_implied_output_region = None;
2194         let mut lifetime_count = 0;
2195         let arg_lifetimes = inputs
2196             .iter()
2197             .enumerate()
2198             .skip(has_self as usize)
2199             .map(|(i, input)| {
2200                 let mut gather = GatherLifetimes {
2201                     map: self.map,
2202                     outer_index: ty::INNERMOST,
2203                     have_bound_regions: false,
2204                     lifetimes: Default::default(),
2205                 };
2206                 gather.visit_ty(input);
2207
2208                 lifetime_count += gather.lifetimes.len();
2209
2210                 if lifetime_count == 1 && gather.lifetimes.len() == 1 {
2211                     // there's a chance that the unique lifetime of this
2212                     // iteration will be the appropriate lifetime for output
2213                     // parameters, so lets store it.
2214                     possible_implied_output_region = gather.lifetimes.iter().cloned().next();
2215                 }
2216
2217                 ElisionFailureInfo {
2218                     parent: body,
2219                     index: i,
2220                     lifetime_count: gather.lifetimes.len(),
2221                     have_bound_regions: gather.have_bound_regions,
2222                     span: input.span,
2223                 }
2224             })
2225             .collect();
2226
2227         let elide = if lifetime_count == 1 {
2228             Elide::Exact(possible_implied_output_region.unwrap())
2229         } else {
2230             Elide::Error(arg_lifetimes)
2231         };
2232
2233         debug!("visit_fn_like_elision: elide={:?}", elide);
2234
2235         let scope = Scope::Elision { elide, s: self.scope };
2236         self.with(scope, |_, this| this.visit_ty(output));
2237         debug!("visit_fn_like_elision: exit");
2238
2239         struct GatherLifetimes<'a> {
2240             map: &'a NamedRegionMap,
2241             outer_index: ty::DebruijnIndex,
2242             have_bound_regions: bool,
2243             lifetimes: FxHashSet<Region>,
2244         }
2245
2246         impl<'v, 'a> Visitor<'v> for GatherLifetimes<'a> {
2247             type Map = intravisit::ErasedMap<'v>;
2248
2249             fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2250                 NestedVisitorMap::None
2251             }
2252
2253             fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2254                 if let hir::TyKind::BareFn(_) = ty.kind {
2255                     self.outer_index.shift_in(1);
2256                 }
2257                 match ty.kind {
2258                     hir::TyKind::TraitObject(bounds, ref lifetime) => {
2259                         for bound in bounds {
2260                             self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
2261                         }
2262
2263                         // Stay on the safe side and don't include the object
2264                         // lifetime default (which may not end up being used).
2265                         if !lifetime.is_elided() {
2266                             self.visit_lifetime(lifetime);
2267                         }
2268                     }
2269                     _ => {
2270                         intravisit::walk_ty(self, ty);
2271                     }
2272                 }
2273                 if let hir::TyKind::BareFn(_) = ty.kind {
2274                     self.outer_index.shift_out(1);
2275                 }
2276             }
2277
2278             fn visit_generic_param(&mut self, param: &hir::GenericParam<'_>) {
2279                 if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2280                     // FIXME(eddyb) Do we want this? It only makes a difference
2281                     // if this `for<'a>` lifetime parameter is never used.
2282                     self.have_bound_regions = true;
2283                 }
2284
2285                 intravisit::walk_generic_param(self, param);
2286             }
2287
2288             fn visit_poly_trait_ref(
2289                 &mut self,
2290                 trait_ref: &hir::PolyTraitRef<'_>,
2291                 modifier: hir::TraitBoundModifier,
2292             ) {
2293                 self.outer_index.shift_in(1);
2294                 intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
2295                 self.outer_index.shift_out(1);
2296             }
2297
2298             fn visit_lifetime(&mut self, lifetime_ref: &hir::Lifetime) {
2299                 if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
2300                     match lifetime {
2301                         Region::LateBound(debruijn, _, _) | Region::LateBoundAnon(debruijn, _)
2302                             if debruijn < self.outer_index =>
2303                         {
2304                             self.have_bound_regions = true;
2305                         }
2306                         _ => {
2307                             self.lifetimes.insert(lifetime.shifted_out_to_binder(self.outer_index));
2308                         }
2309                     }
2310                 }
2311             }
2312         }
2313     }
2314
2315     fn resolve_elided_lifetimes(&mut self, lifetime_refs: Vec<&'tcx hir::Lifetime>) {
2316         debug!("resolve_elided_lifetimes(lifetime_refs={:?})", lifetime_refs);
2317
2318         if lifetime_refs.is_empty() {
2319             return;
2320         }
2321
2322         let span = lifetime_refs[0].span;
2323         let mut late_depth = 0;
2324         let mut scope = self.scope;
2325         let mut lifetime_names = FxHashSet::default();
2326         let mut lifetime_spans = vec![];
2327         let error = loop {
2328             match *scope {
2329                 // Do not assign any resolution, it will be inferred.
2330                 Scope::Body { .. } => return,
2331
2332                 Scope::Root => break None,
2333
2334                 Scope::Binder { s, ref lifetimes, .. } => {
2335                     // collect named lifetimes for suggestions
2336                     for name in lifetimes.keys() {
2337                         if let hir::ParamName::Plain(name) = name {
2338                             lifetime_names.insert(name.name);
2339                             lifetime_spans.push(name.span);
2340                         }
2341                     }
2342                     late_depth += 1;
2343                     scope = s;
2344                 }
2345
2346                 Scope::Elision { ref elide, ref s, .. } => {
2347                     let lifetime = match *elide {
2348                         Elide::FreshLateAnon(ref counter) => {
2349                             for lifetime_ref in lifetime_refs {
2350                                 let lifetime = Region::late_anon(counter).shifted(late_depth);
2351                                 self.insert_lifetime(lifetime_ref, lifetime);
2352                             }
2353                             return;
2354                         }
2355                         Elide::Exact(l) => l.shifted(late_depth),
2356                         Elide::Error(ref e) => {
2357                             let mut scope = s;
2358                             loop {
2359                                 match scope {
2360                                     Scope::Binder { ref lifetimes, s, .. } => {
2361                                         // Collect named lifetimes for suggestions.
2362                                         for name in lifetimes.keys() {
2363                                             if let hir::ParamName::Plain(name) = name {
2364                                                 lifetime_names.insert(name.name);
2365                                                 lifetime_spans.push(name.span);
2366                                             }
2367                                         }
2368                                         scope = s;
2369                                     }
2370                                     Scope::ObjectLifetimeDefault { ref s, .. }
2371                                     | Scope::Elision { ref s, .. } => {
2372                                         scope = s;
2373                                     }
2374                                     _ => break,
2375                                 }
2376                             }
2377                             break Some(e);
2378                         }
2379                         Elide::Forbid => break None,
2380                     };
2381                     for lifetime_ref in lifetime_refs {
2382                         self.insert_lifetime(lifetime_ref, lifetime);
2383                     }
2384                     return;
2385                 }
2386
2387                 Scope::ObjectLifetimeDefault { s, .. } => {
2388                     scope = s;
2389                 }
2390             }
2391         };
2392
2393         let mut err = self.report_missing_lifetime_specifiers(span, lifetime_refs.len());
2394
2395         if let Some(params) = error {
2396             // If there's no lifetime available, suggest `'static`.
2397             if self.report_elision_failure(&mut err, params) && lifetime_names.is_empty() {
2398                 lifetime_names.insert(kw::StaticLifetime);
2399             }
2400         }
2401         self.add_missing_lifetime_specifiers_label(
2402             &mut err,
2403             span,
2404             lifetime_refs.len(),
2405             &lifetime_names,
2406             lifetime_spans,
2407             error.map(|p| &p[..]).unwrap_or(&[]),
2408         );
2409         err.emit();
2410     }
2411
2412     fn report_elision_failure(
2413         &mut self,
2414         db: &mut DiagnosticBuilder<'_>,
2415         params: &[ElisionFailureInfo],
2416     ) -> bool /* add `'static` lifetime to lifetime list */ {
2417         let mut m = String::new();
2418         let len = params.len();
2419
2420         let elided_params: Vec<_> =
2421             params.iter().cloned().filter(|info| info.lifetime_count > 0).collect();
2422
2423         let elided_len = elided_params.len();
2424
2425         for (i, info) in elided_params.into_iter().enumerate() {
2426             let ElisionFailureInfo { parent, index, lifetime_count: n, have_bound_regions, span } =
2427                 info;
2428
2429             db.span_label(span, "");
2430             let help_name = if let Some(ident) =
2431                 parent.and_then(|body| self.tcx.hir().body(body).params[index].pat.simple_ident())
2432             {
2433                 format!("`{}`", ident)
2434             } else {
2435                 format!("argument {}", index + 1)
2436             };
2437
2438             m.push_str(
2439                 &(if n == 1 {
2440                     help_name
2441                 } else {
2442                     format!(
2443                         "one of {}'s {} {}lifetimes",
2444                         help_name,
2445                         n,
2446                         if have_bound_regions { "free " } else { "" }
2447                     )
2448                 })[..],
2449             );
2450
2451             if elided_len == 2 && i == 0 {
2452                 m.push_str(" or ");
2453             } else if i + 2 == elided_len {
2454                 m.push_str(", or ");
2455             } else if i != elided_len - 1 {
2456                 m.push_str(", ");
2457             }
2458         }
2459
2460         if len == 0 {
2461             db.help(
2462                 "this function's return type contains a borrowed value, \
2463                  but there is no value for it to be borrowed from",
2464             );
2465             true
2466         } else if elided_len == 0 {
2467             db.help(
2468                 "this function's return type contains a borrowed value with \
2469                  an elided lifetime, but the lifetime cannot be derived from \
2470                  the arguments",
2471             );
2472             true
2473         } else if elided_len == 1 {
2474             db.help(&format!(
2475                 "this function's return type contains a borrowed value, \
2476                  but the signature does not say which {} it is borrowed from",
2477                 m
2478             ));
2479             false
2480         } else {
2481             db.help(&format!(
2482                 "this function's return type contains a borrowed value, \
2483                  but the signature does not say whether it is borrowed from {}",
2484                 m
2485             ));
2486             false
2487         }
2488     }
2489
2490     fn resolve_object_lifetime_default(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
2491         debug!("resolve_object_lifetime_default(lifetime_ref={:?})", lifetime_ref);
2492         let mut late_depth = 0;
2493         let mut scope = self.scope;
2494         let lifetime = loop {
2495             match *scope {
2496                 Scope::Binder { s, .. } => {
2497                     late_depth += 1;
2498                     scope = s;
2499                 }
2500
2501                 Scope::Root | Scope::Elision { .. } => break Region::Static,
2502
2503                 Scope::Body { .. } | Scope::ObjectLifetimeDefault { lifetime: None, .. } => return,
2504
2505                 Scope::ObjectLifetimeDefault { lifetime: Some(l), .. } => break l,
2506             }
2507         };
2508         self.insert_lifetime(lifetime_ref, lifetime.shifted(late_depth));
2509     }
2510
2511     fn check_lifetime_params(
2512         &mut self,
2513         old_scope: ScopeRef<'_>,
2514         params: &'tcx [hir::GenericParam<'tcx>],
2515     ) {
2516         let lifetimes: Vec<_> = params
2517             .iter()
2518             .filter_map(|param| match param.kind {
2519                 GenericParamKind::Lifetime { .. } => {
2520                     Some((param, param.name.normalize_to_macros_2_0()))
2521                 }
2522                 _ => None,
2523             })
2524             .collect();
2525         for (i, (lifetime_i, lifetime_i_name)) in lifetimes.iter().enumerate() {
2526             if let hir::ParamName::Plain(_) = lifetime_i_name {
2527                 let name = lifetime_i_name.ident().name;
2528                 if name == kw::UnderscoreLifetime || name == kw::StaticLifetime {
2529                     let mut err = struct_span_err!(
2530                         self.tcx.sess,
2531                         lifetime_i.span,
2532                         E0262,
2533                         "invalid lifetime parameter name: `{}`",
2534                         lifetime_i.name.ident(),
2535                     );
2536                     err.span_label(
2537                         lifetime_i.span,
2538                         format!("{} is a reserved lifetime name", name),
2539                     );
2540                     err.emit();
2541                 }
2542             }
2543
2544             // It is a hard error to shadow a lifetime within the same scope.
2545             for (lifetime_j, lifetime_j_name) in lifetimes.iter().skip(i + 1) {
2546                 if lifetime_i_name == lifetime_j_name {
2547                     struct_span_err!(
2548                         self.tcx.sess,
2549                         lifetime_j.span,
2550                         E0263,
2551                         "lifetime name `{}` declared twice in the same scope",
2552                         lifetime_j.name.ident()
2553                     )
2554                     .span_label(lifetime_j.span, "declared twice")
2555                     .span_label(lifetime_i.span, "previous declaration here")
2556                     .emit();
2557                 }
2558             }
2559
2560             // It is a soft error to shadow a lifetime within a parent scope.
2561             self.check_lifetime_param_for_shadowing(old_scope, &lifetime_i);
2562
2563             for bound in lifetime_i.bounds {
2564                 match bound {
2565                     hir::GenericBound::Outlives(ref lt) => match lt.name {
2566                         hir::LifetimeName::Underscore => self.tcx.sess.delay_span_bug(
2567                             lt.span,
2568                             "use of `'_` in illegal place, but not caught by lowering",
2569                         ),
2570                         hir::LifetimeName::Static => {
2571                             self.insert_lifetime(lt, Region::Static);
2572                             self.tcx
2573                                 .sess
2574                                 .struct_span_warn(
2575                                     lifetime_i.span.to(lt.span),
2576                                     &format!(
2577                                         "unnecessary lifetime parameter `{}`",
2578                                         lifetime_i.name.ident(),
2579                                     ),
2580                                 )
2581                                 .help(&format!(
2582                                     "you can use the `'static` lifetime directly, in place of `{}`",
2583                                     lifetime_i.name.ident(),
2584                                 ))
2585                                 .emit();
2586                         }
2587                         hir::LifetimeName::Param(_) | hir::LifetimeName::Implicit => {
2588                             self.resolve_lifetime_ref(lt);
2589                         }
2590                         hir::LifetimeName::ImplicitObjectLifetimeDefault => {
2591                             self.tcx.sess.delay_span_bug(
2592                                 lt.span,
2593                                 "lowering generated `ImplicitObjectLifetimeDefault` \
2594                                  outside of an object type",
2595                             )
2596                         }
2597                         hir::LifetimeName::Error => {
2598                             // No need to do anything, error already reported.
2599                         }
2600                     },
2601                     _ => bug!(),
2602                 }
2603             }
2604         }
2605     }
2606
2607     fn check_lifetime_param_for_shadowing(
2608         &self,
2609         mut old_scope: ScopeRef<'_>,
2610         param: &'tcx hir::GenericParam<'tcx>,
2611     ) {
2612         for label in &self.labels_in_fn {
2613             // FIXME (#24278): non-hygienic comparison
2614             if param.name.ident().name == label.name {
2615                 signal_shadowing_problem(
2616                     self.tcx,
2617                     label.name,
2618                     original_label(label.span),
2619                     shadower_lifetime(&param),
2620                 );
2621                 return;
2622             }
2623         }
2624
2625         loop {
2626             match *old_scope {
2627                 Scope::Body { s, .. }
2628                 | Scope::Elision { s, .. }
2629                 | Scope::ObjectLifetimeDefault { s, .. } => {
2630                     old_scope = s;
2631                 }
2632
2633                 Scope::Root => {
2634                     return;
2635                 }
2636
2637                 Scope::Binder { ref lifetimes, s, .. } => {
2638                     if let Some(&def) = lifetimes.get(&param.name.normalize_to_macros_2_0()) {
2639                         let hir_id =
2640                             self.tcx.hir().as_local_hir_id(def.id().unwrap().expect_local());
2641
2642                         signal_shadowing_problem(
2643                             self.tcx,
2644                             param.name.ident().name,
2645                             original_lifetime(self.tcx.hir().span(hir_id)),
2646                             shadower_lifetime(&param),
2647                         );
2648                         return;
2649                     }
2650
2651                     old_scope = s;
2652                 }
2653             }
2654         }
2655     }
2656
2657     /// Returns `true` if, in the current scope, replacing `'_` would be
2658     /// equivalent to a single-use lifetime.
2659     fn track_lifetime_uses(&self) -> bool {
2660         let mut scope = self.scope;
2661         loop {
2662             match *scope {
2663                 Scope::Root => break false,
2664
2665                 // Inside of items, it depends on the kind of item.
2666                 Scope::Binder { track_lifetime_uses, .. } => break track_lifetime_uses,
2667
2668                 // Inside a body, `'_` will use an inference variable,
2669                 // should be fine.
2670                 Scope::Body { .. } => break true,
2671
2672                 // A lifetime only used in a fn argument could as well
2673                 // be replaced with `'_`, as that would generate a
2674                 // fresh name, too.
2675                 Scope::Elision { elide: Elide::FreshLateAnon(_), .. } => break true,
2676
2677                 // In the return type or other such place, `'_` is not
2678                 // going to make a fresh name, so we cannot
2679                 // necessarily replace a single-use lifetime with
2680                 // `'_`.
2681                 Scope::Elision {
2682                     elide: Elide::Exact(_) | Elide::Error(_) | Elide::Forbid, ..
2683                 } => break false,
2684
2685                 Scope::ObjectLifetimeDefault { s, .. } => scope = s,
2686             }
2687         }
2688     }
2689
2690     fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
2691         debug!(
2692             "insert_lifetime: {} resolved to {:?} span={:?}",
2693             self.tcx.hir().node_to_string(lifetime_ref.hir_id),
2694             def,
2695             self.tcx.sess.source_map().span_to_string(lifetime_ref.span)
2696         );
2697         self.map.defs.insert(lifetime_ref.hir_id, def);
2698
2699         match def {
2700             Region::LateBoundAnon(..) | Region::Static => {
2701                 // These are anonymous lifetimes or lifetimes that are not declared.
2702             }
2703
2704             Region::Free(_, def_id)
2705             | Region::LateBound(_, def_id, _)
2706             | Region::EarlyBound(_, def_id, _) => {
2707                 // A lifetime declared by the user.
2708                 let track_lifetime_uses = self.track_lifetime_uses();
2709                 debug!("insert_lifetime: track_lifetime_uses={}", track_lifetime_uses);
2710                 if track_lifetime_uses && !self.lifetime_uses.contains_key(&def_id) {
2711                     debug!("insert_lifetime: first use of {:?}", def_id);
2712                     self.lifetime_uses.insert(def_id, LifetimeUseSet::One(lifetime_ref));
2713                 } else {
2714                     debug!("insert_lifetime: many uses of {:?}", def_id);
2715                     self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
2716                 }
2717             }
2718         }
2719     }
2720
2721     /// Sometimes we resolve a lifetime, but later find that it is an
2722     /// error (esp. around impl trait). In that case, we remove the
2723     /// entry into `map.defs` so as not to confuse later code.
2724     fn uninsert_lifetime_on_error(&mut self, lifetime_ref: &'tcx hir::Lifetime, bad_def: Region) {
2725         let old_value = self.map.defs.remove(&lifetime_ref.hir_id);
2726         assert_eq!(old_value, Some(bad_def));
2727     }
2728 }
2729
2730 /// Detects late-bound lifetimes and inserts them into
2731 /// `map.late_bound`.
2732 ///
2733 /// A region declared on a fn is **late-bound** if:
2734 /// - it is constrained by an argument type;
2735 /// - it does not appear in a where-clause.
2736 ///
2737 /// "Constrained" basically means that it appears in any type but
2738 /// not amongst the inputs to a projection. In other words, `<&'a
2739 /// T as Trait<''b>>::Foo` does not constrain `'a` or `'b`.
2740 fn insert_late_bound_lifetimes(
2741     map: &mut NamedRegionMap,
2742     decl: &hir::FnDecl<'_>,
2743     generics: &hir::Generics<'_>,
2744 ) {
2745     debug!("insert_late_bound_lifetimes(decl={:?}, generics={:?})", decl, generics);
2746
2747     let mut constrained_by_input = ConstrainedCollector::default();
2748     for arg_ty in decl.inputs {
2749         constrained_by_input.visit_ty(arg_ty);
2750     }
2751
2752     let mut appears_in_output = AllCollector::default();
2753     intravisit::walk_fn_ret_ty(&mut appears_in_output, &decl.output);
2754
2755     debug!("insert_late_bound_lifetimes: constrained_by_input={:?}", constrained_by_input.regions);
2756
2757     // Walk the lifetimes that appear in where clauses.
2758     //
2759     // Subtle point: because we disallow nested bindings, we can just
2760     // ignore binders here and scrape up all names we see.
2761     let mut appears_in_where_clause = AllCollector::default();
2762     appears_in_where_clause.visit_generics(generics);
2763
2764     for param in generics.params {
2765         if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2766             if !param.bounds.is_empty() {
2767                 // `'a: 'b` means both `'a` and `'b` are referenced
2768                 appears_in_where_clause
2769                     .regions
2770                     .insert(hir::LifetimeName::Param(param.name.normalize_to_macros_2_0()));
2771             }
2772         }
2773     }
2774
2775     debug!(
2776         "insert_late_bound_lifetimes: appears_in_where_clause={:?}",
2777         appears_in_where_clause.regions
2778     );
2779
2780     // Late bound regions are those that:
2781     // - appear in the inputs
2782     // - do not appear in the where-clauses
2783     // - are not implicitly captured by `impl Trait`
2784     for param in generics.params {
2785         match param.kind {
2786             hir::GenericParamKind::Lifetime { .. } => { /* fall through */ }
2787
2788             // Neither types nor consts are late-bound.
2789             hir::GenericParamKind::Type { .. } | hir::GenericParamKind::Const { .. } => continue,
2790         }
2791
2792         let lt_name = hir::LifetimeName::Param(param.name.normalize_to_macros_2_0());
2793         // appears in the where clauses? early-bound.
2794         if appears_in_where_clause.regions.contains(&lt_name) {
2795             continue;
2796         }
2797
2798         // does not appear in the inputs, but appears in the return type? early-bound.
2799         if !constrained_by_input.regions.contains(&lt_name)
2800             && appears_in_output.regions.contains(&lt_name)
2801         {
2802             continue;
2803         }
2804
2805         debug!(
2806             "insert_late_bound_lifetimes: lifetime {:?} with id {:?} is late-bound",
2807             param.name.ident(),
2808             param.hir_id
2809         );
2810
2811         let inserted = map.late_bound.insert(param.hir_id);
2812         assert!(inserted, "visited lifetime {:?} twice", param.hir_id);
2813     }
2814
2815     return;
2816
2817     #[derive(Default)]
2818     struct ConstrainedCollector {
2819         regions: FxHashSet<hir::LifetimeName>,
2820     }
2821
2822     impl<'v> Visitor<'v> for ConstrainedCollector {
2823         type Map = intravisit::ErasedMap<'v>;
2824
2825         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2826             NestedVisitorMap::None
2827         }
2828
2829         fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
2830             match ty.kind {
2831                 hir::TyKind::Path(
2832                     hir::QPath::Resolved(Some(_), _) | hir::QPath::TypeRelative(..),
2833                 ) => {
2834                     // ignore lifetimes appearing in associated type
2835                     // projections, as they are not *constrained*
2836                     // (defined above)
2837                 }
2838
2839                 hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => {
2840                     // consider only the lifetimes on the final
2841                     // segment; I am not sure it's even currently
2842                     // valid to have them elsewhere, but even if it
2843                     // is, those would be potentially inputs to
2844                     // projections
2845                     if let Some(last_segment) = path.segments.last() {
2846                         self.visit_path_segment(path.span, last_segment);
2847                     }
2848                 }
2849
2850                 _ => {
2851                     intravisit::walk_ty(self, ty);
2852                 }
2853             }
2854         }
2855
2856         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2857             self.regions.insert(lifetime_ref.name.normalize_to_macros_2_0());
2858         }
2859     }
2860
2861     #[derive(Default)]
2862     struct AllCollector {
2863         regions: FxHashSet<hir::LifetimeName>,
2864     }
2865
2866     impl<'v> Visitor<'v> for AllCollector {
2867         type Map = intravisit::ErasedMap<'v>;
2868
2869         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2870             NestedVisitorMap::None
2871         }
2872
2873         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2874             self.regions.insert(lifetime_ref.name.normalize_to_macros_2_0());
2875         }
2876     }
2877 }