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 tracing::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().local_def_id_to_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_param_bound(&mut self, bound: &'tcx hir::GenericBound<'tcx>) {
 945         match bound {
 946             hir::GenericBound::LangItemTrait { .. } if !self.trait_ref_hack => {
 947                 let scope = Scope::Binder {
 948                     lifetimes: FxHashMap::default(),
 949                     s: self.scope,
 950                     next_early_index: self.next_early_index(),
 951                     track_lifetime_uses: true,
 952                     opaque_type_parent: false,
 953                 };
 954                 self.with(scope, |_, this| {
 955                     intravisit::walk_param_bound(this, bound);
 956                 });
 957             }
 958             _ => intravisit::walk_param_bound(self, bound),
 959         }
 960     }
 961
 962     fn visit_poly_trait_ref(
 963         &mut self,
 964         trait_ref: &'tcx hir::PolyTraitRef<'tcx>,
 965         _modifier: hir::TraitBoundModifier,
 966     ) {
 967         debug!("visit_poly_trait_ref(trait_ref={:?})", trait_ref);
 968
 969         let should_pop_missing_lt = self.is_trait_ref_fn_scope(trait_ref);
 970
 971         let trait_ref_hack = take(&mut self.trait_ref_hack);
 972         if !trait_ref_hack
 973             || trait_ref.bound_generic_params.iter().any(|param| match param.kind {
 974                 GenericParamKind::Lifetime { .. } => true,
 975                 _ => false,
 976             })
 977         {
 978             if trait_ref_hack {
 979                 struct_span_err!(
 980                     self.tcx.sess,
 981                     trait_ref.span,
 982                     E0316,
 983                     "nested quantification of lifetimes"
 984                 )
 985                 .emit();
 986             }
 987             let next_early_index = self.next_early_index();
 988             let scope = Scope::Binder {
 989                 lifetimes: trait_ref
 990                     .bound_generic_params
 991                     .iter()
 992                     .filter_map(|param| match param.kind {
 993                         GenericParamKind::Lifetime { .. } => {
 994                             Some(Region::late(&self.tcx.hir(), param))
 995                         }
 996                         _ => None,
 997                     })
 998                     .collect(),
 999                 s: self.scope,
1000                 next_early_index,
1001                 track_lifetime_uses: true,
1002                 opaque_type_parent: false,
1003             };
1004             self.with(scope, |old_scope, this| {
1005                 this.check_lifetime_params(old_scope, &trait_ref.bound_generic_params);
1006                 walk_list!(this, visit_generic_param, trait_ref.bound_generic_params);
1007                 this.visit_trait_ref(&trait_ref.trait_ref);
1008             });
1009         } else {
1010             self.visit_trait_ref(&trait_ref.trait_ref);
1011         }
1012         self.trait_ref_hack = trait_ref_hack;
1013         if should_pop_missing_lt {
1014             self.missing_named_lifetime_spots.pop();
1015         }
1016     }
1017 }
1018
1019 #[derive(Copy, Clone, PartialEq)]
1020 enum ShadowKind {
1021     Label,
1022     Lifetime,
1023 }
1024 struct Original {
1025     kind: ShadowKind,
1026     span: Span,
1027 }
1028 struct Shadower {
1029     kind: ShadowKind,
1030     span: Span,
1031 }
1032
1033 fn original_label(span: Span) -> Original {
1034     Original { kind: ShadowKind::Label, span }
1035 }
1036 fn shadower_label(span: Span) -> Shadower {
1037     Shadower { kind: ShadowKind::Label, span }
1038 }
1039 fn original_lifetime(span: Span) -> Original {
1040     Original { kind: ShadowKind::Lifetime, span }
1041 }
1042 fn shadower_lifetime(param: &hir::GenericParam<'_>) -> Shadower {
1043     Shadower { kind: ShadowKind::Lifetime, span: param.span }
1044 }
1045
1046 impl ShadowKind {
1047     fn desc(&self) -> &'static str {
1048         match *self {
1049             ShadowKind::Label => "label",
1050             ShadowKind::Lifetime => "lifetime",
1051         }
1052     }
1053 }
1054
1055 fn check_mixed_explicit_and_in_band_defs(tcx: TyCtxt<'_>, params: &[hir::GenericParam<'_>]) {
1056     let lifetime_params: Vec<_> = params
1057         .iter()
1058         .filter_map(|param| match param.kind {
1059             GenericParamKind::Lifetime { kind, .. } => Some((kind, param.span)),
1060             _ => None,
1061         })
1062         .collect();
1063     let explicit = lifetime_params.iter().find(|(kind, _)| *kind == LifetimeParamKind::Explicit);
1064     let in_band = lifetime_params.iter().find(|(kind, _)| *kind == LifetimeParamKind::InBand);
1065
1066     if let (Some((_, explicit_span)), Some((_, in_band_span))) = (explicit, in_band) {
1067         struct_span_err!(
1068             tcx.sess,
1069             *in_band_span,
1070             E0688,
1071             "cannot mix in-band and explicit lifetime definitions"
1072         )
1073         .span_label(*in_band_span, "in-band lifetime definition here")
1074         .span_label(*explicit_span, "explicit lifetime definition here")
1075         .emit();
1076     }
1077 }
1078
1079 fn signal_shadowing_problem(tcx: TyCtxt<'_>, name: Symbol, orig: Original, shadower: Shadower) {
1080     let mut err = if let (ShadowKind::Lifetime, ShadowKind::Lifetime) = (orig.kind, shadower.kind) {
1081         // lifetime/lifetime shadowing is an error
1082         struct_span_err!(
1083             tcx.sess,
1084             shadower.span,
1085             E0496,
1086             "{} name `{}` shadows a \
1087              {} name that is already in scope",
1088             shadower.kind.desc(),
1089             name,
1090             orig.kind.desc()
1091         )
1092     } else {
1093         // shadowing involving a label is only a warning, due to issues with
1094         // labels and lifetimes not being macro-hygienic.
1095         tcx.sess.struct_span_warn(
1096             shadower.span,
1097             &format!(
1098                 "{} name `{}` shadows a \
1099                  {} name that is already in scope",
1100                 shadower.kind.desc(),
1101                 name,
1102                 orig.kind.desc()
1103             ),
1104         )
1105     };
1106     err.span_label(orig.span, "first declared here");
1107     err.span_label(shadower.span, format!("lifetime {} already in scope", name));
1108     err.emit();
1109 }
1110
1111 // Adds all labels in `b` to `ctxt.labels_in_fn`, signalling a warning
1112 // if one of the label shadows a lifetime or another label.
1113 fn extract_labels(ctxt: &mut LifetimeContext<'_, '_>, body: &hir::Body<'_>) {
1114     struct GatherLabels<'a, 'tcx> {
1115         tcx: TyCtxt<'tcx>,
1116         scope: ScopeRef<'a>,
1117         labels_in_fn: &'a mut Vec<Ident>,
1118     }
1119
1120     let mut gather =
1121         GatherLabels { tcx: ctxt.tcx, scope: ctxt.scope, labels_in_fn: &mut ctxt.labels_in_fn };
1122     gather.visit_body(body);
1123
1124     impl<'v, 'a, 'tcx> Visitor<'v> for GatherLabels<'a, 'tcx> {
1125         type Map = intravisit::ErasedMap<'v>;
1126
1127         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
1128             NestedVisitorMap::None
1129         }
1130
1131         fn visit_expr(&mut self, ex: &hir::Expr<'_>) {
1132             if let Some(label) = expression_label(ex) {
1133                 for prior_label in &self.labels_in_fn[..] {
1134                     // FIXME (#24278): non-hygienic comparison
1135                     if label.name == prior_label.name {
1136                         signal_shadowing_problem(
1137                             self.tcx,
1138                             label.name,
1139                             original_label(prior_label.span),
1140                             shadower_label(label.span),
1141                         );
1142                     }
1143                 }
1144
1145                 check_if_label_shadows_lifetime(self.tcx, self.scope, label);
1146
1147                 self.labels_in_fn.push(label);
1148             }
1149             intravisit::walk_expr(self, ex)
1150         }
1151     }
1152
1153     fn expression_label(ex: &hir::Expr<'_>) -> Option<Ident> {
1154         if let hir::ExprKind::Loop(_, Some(label), _) = ex.kind { Some(label.ident) } else { None }
1155     }
1156
1157     fn check_if_label_shadows_lifetime(tcx: TyCtxt<'_>, mut scope: ScopeRef<'_>, label: Ident) {
1158         loop {
1159             match *scope {
1160                 Scope::Body { s, .. }
1161                 | Scope::Elision { s, .. }
1162                 | Scope::ObjectLifetimeDefault { s, .. } => {
1163                     scope = s;
1164                 }
1165
1166                 Scope::Root => {
1167                     return;
1168                 }
1169
1170                 Scope::Binder { ref lifetimes, s, .. } => {
1171                     // FIXME (#24278): non-hygienic comparison
1172                     if let Some(def) =
1173                         lifetimes.get(&hir::ParamName::Plain(label.normalize_to_macros_2_0()))
1174                     {
1175                         let hir_id =
1176                             tcx.hir().local_def_id_to_hir_id(def.id().unwrap().expect_local());
1177
1178                         signal_shadowing_problem(
1179                             tcx,
1180                             label.name,
1181                             original_lifetime(tcx.hir().span(hir_id)),
1182                             shadower_label(label.span),
1183                         );
1184                         return;
1185                     }
1186                     scope = s;
1187                 }
1188             }
1189         }
1190     }
1191 }
1192
1193 fn compute_object_lifetime_defaults(tcx: TyCtxt<'_>) -> HirIdMap<Vec<ObjectLifetimeDefault>> {
1194     let mut map = HirIdMap::default();
1195     for item in tcx.hir().krate().items.values() {
1196         match item.kind {
1197             hir::ItemKind::Struct(_, ref generics)
1198             | hir::ItemKind::Union(_, ref generics)
1199             | hir::ItemKind::Enum(_, ref generics)
1200             | hir::ItemKind::OpaqueTy(hir::OpaqueTy {
1201                 ref generics, impl_trait_fn: None, ..
1202             })
1203             | hir::ItemKind::TyAlias(_, ref generics)
1204             | hir::ItemKind::Trait(_, _, ref generics, ..) => {
1205                 let result = object_lifetime_defaults_for_item(tcx, generics);
1206
1207                 // Debugging aid.
1208                 if tcx.sess.contains_name(&item.attrs, sym::rustc_object_lifetime_default) {
1209                     let object_lifetime_default_reprs: String = result
1210                         .iter()
1211                         .map(|set| match *set {
1212                             Set1::Empty => "BaseDefault".into(),
1213                             Set1::One(Region::Static) => "'static".into(),
1214                             Set1::One(Region::EarlyBound(mut i, _, _)) => generics
1215                                 .params
1216                                 .iter()
1217                                 .find_map(|param| match param.kind {
1218                                     GenericParamKind::Lifetime { .. } => {
1219                                         if i == 0 {
1220                                             return Some(param.name.ident().to_string().into());
1221                                         }
1222                                         i -= 1;
1223                                         None
1224                                     }
1225                                     _ => None,
1226                                 })
1227                                 .unwrap(),
1228                             Set1::One(_) => bug!(),
1229                             Set1::Many => "Ambiguous".into(),
1230                         })
1231                         .collect::<Vec<Cow<'static, str>>>()
1232                         .join(",");
1233                     tcx.sess.span_err(item.span, &object_lifetime_default_reprs);
1234                 }
1235
1236                 map.insert(item.hir_id, result);
1237             }
1238             _ => {}
1239         }
1240     }
1241     map
1242 }
1243
1244 /// Scan the bounds and where-clauses on parameters to extract bounds
1245 /// of the form `T:'a` so as to determine the `ObjectLifetimeDefault`
1246 /// for each type parameter.
1247 fn object_lifetime_defaults_for_item(
1248     tcx: TyCtxt<'_>,
1249     generics: &hir::Generics<'_>,
1250 ) -> Vec<ObjectLifetimeDefault> {
1251     fn add_bounds(set: &mut Set1<hir::LifetimeName>, bounds: &[hir::GenericBound<'_>]) {
1252         for bound in bounds {
1253             if let hir::GenericBound::Outlives(ref lifetime) = *bound {
1254                 set.insert(lifetime.name.normalize_to_macros_2_0());
1255             }
1256         }
1257     }
1258
1259     generics
1260         .params
1261         .iter()
1262         .filter_map(|param| match param.kind {
1263             GenericParamKind::Lifetime { .. } => None,
1264             GenericParamKind::Type { .. } => {
1265                 let mut set = Set1::Empty;
1266
1267                 add_bounds(&mut set, &param.bounds);
1268
1269                 let param_def_id = tcx.hir().local_def_id(param.hir_id);
1270                 for predicate in generics.where_clause.predicates {
1271                     // Look for `type: ...` where clauses.
1272                     let data = match *predicate {
1273                         hir::WherePredicate::BoundPredicate(ref data) => data,
1274                         _ => continue,
1275                     };
1276
1277                     // Ignore `for<'a> type: ...` as they can change what
1278                     // lifetimes mean (although we could "just" handle it).
1279                     if !data.bound_generic_params.is_empty() {
1280                         continue;
1281                     }
1282
1283                     let res = match data.bounded_ty.kind {
1284                         hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => path.res,
1285                         _ => continue,
1286                     };
1287
1288                     if res == Res::Def(DefKind::TyParam, param_def_id.to_def_id()) {
1289                         add_bounds(&mut set, &data.bounds);
1290                     }
1291                 }
1292
1293                 Some(match set {
1294                     Set1::Empty => Set1::Empty,
1295                     Set1::One(name) => {
1296                         if name == hir::LifetimeName::Static {
1297                             Set1::One(Region::Static)
1298                         } else {
1299                             generics
1300                                 .params
1301                                 .iter()
1302                                 .filter_map(|param| match param.kind {
1303                                     GenericParamKind::Lifetime { .. } => Some((
1304                                         param.hir_id,
1305                                         hir::LifetimeName::Param(param.name),
1306                                         LifetimeDefOrigin::from_param(param),
1307                                     )),
1308                                     _ => None,
1309                                 })
1310                                 .enumerate()
1311                                 .find(|&(_, (_, lt_name, _))| lt_name == name)
1312                                 .map_or(Set1::Many, |(i, (id, _, origin))| {
1313                                     let def_id = tcx.hir().local_def_id(id);
1314                                     Set1::One(Region::EarlyBound(
1315                                         i as u32,
1316                                         def_id.to_def_id(),
1317                                         origin,
1318                                     ))
1319                                 })
1320                         }
1321                     }
1322                     Set1::Many => Set1::Many,
1323                 })
1324             }
1325             GenericParamKind::Const { .. } => {
1326                 // Generic consts don't impose any constraints.
1327                 //
1328                 // We still store a dummy value here to allow generic parameters
1329                 // in an arbitrary order.
1330                 Some(Set1::Empty)
1331             }
1332         })
1333         .collect()
1334 }
1335
1336 impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
1337     // FIXME(#37666) this works around a limitation in the region inferencer
1338     fn hack<F>(&mut self, f: F)
1339     where
1340         F: for<'b> FnOnce(&mut LifetimeContext<'b, 'tcx>),
1341     {
1342         f(self)
1343     }
1344
1345     fn with<F>(&mut self, wrap_scope: Scope<'_>, f: F)
1346     where
1347         F: for<'b> FnOnce(ScopeRef<'_>, &mut LifetimeContext<'b, 'tcx>),
1348     {
1349         let LifetimeContext { tcx, map, lifetime_uses, .. } = self;
1350         let labels_in_fn = take(&mut self.labels_in_fn);
1351         let xcrate_object_lifetime_defaults = take(&mut self.xcrate_object_lifetime_defaults);
1352         let missing_named_lifetime_spots = take(&mut self.missing_named_lifetime_spots);
1353         let mut this = LifetimeContext {
1354             tcx: *tcx,
1355             map,
1356             scope: &wrap_scope,
1357             trait_ref_hack: self.trait_ref_hack,
1358             is_in_fn_syntax: self.is_in_fn_syntax,
1359             is_in_const_generic: self.is_in_const_generic,
1360             labels_in_fn,
1361             xcrate_object_lifetime_defaults,
1362             lifetime_uses,
1363             missing_named_lifetime_spots,
1364         };
1365         debug!("entering scope {:?}", this.scope);
1366         f(self.scope, &mut this);
1367         this.check_uses_for_lifetimes_defined_by_scope();
1368         debug!("exiting scope {:?}", this.scope);
1369         self.labels_in_fn = this.labels_in_fn;
1370         self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;
1371         self.missing_named_lifetime_spots = this.missing_named_lifetime_spots;
1372     }
1373
1374     /// helper method to determine the span to remove when suggesting the
1375     /// deletion of a lifetime
1376     fn lifetime_deletion_span(&self, name: Ident, generics: &hir::Generics<'_>) -> Option<Span> {
1377         generics.params.iter().enumerate().find_map(|(i, param)| {
1378             if param.name.ident() == name {
1379                 let mut in_band = false;
1380                 if let hir::GenericParamKind::Lifetime { kind } = param.kind {
1381                     if let hir::LifetimeParamKind::InBand = kind {
1382                         in_band = true;
1383                     }
1384                 }
1385                 if in_band {
1386                     Some(param.span)
1387                 } else {
1388                     if generics.params.len() == 1 {
1389                         // if sole lifetime, remove the entire `<>` brackets
1390                         Some(generics.span)
1391                     } else {
1392                         // if removing within `<>` brackets, we also want to
1393                         // delete a leading or trailing comma as appropriate
1394                         if i >= generics.params.len() - 1 {
1395                             Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
1396                         } else {
1397                             Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
1398                         }
1399                     }
1400                 }
1401             } else {
1402                 None
1403             }
1404         })
1405     }
1406
1407     // helper method to issue suggestions from `fn rah<'a>(&'a T)` to `fn rah(&T)`
1408     // or from `fn rah<'a>(T<'a>)` to `fn rah(T<'_>)`
1409     fn suggest_eliding_single_use_lifetime(
1410         &self,
1411         err: &mut DiagnosticBuilder<'_>,
1412         def_id: DefId,
1413         lifetime: &hir::Lifetime,
1414     ) {
1415         let name = lifetime.name.ident();
1416         let mut remove_decl = None;
1417         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1418             if let Some(generics) = self.tcx.hir().get_generics(parent_def_id) {
1419                 remove_decl = self.lifetime_deletion_span(name, generics);
1420             }
1421         }
1422
1423         let mut remove_use = None;
1424         let mut elide_use = None;
1425         let mut find_arg_use_span = |inputs: &[hir::Ty<'_>]| {
1426             for input in inputs {
1427                 match input.kind {
1428                     hir::TyKind::Rptr(lt, _) => {
1429                         if lt.name.ident() == name {
1430                             // include the trailing whitespace between the lifetime and type names
1431                             let lt_through_ty_span = lifetime.span.to(input.span.shrink_to_hi());
1432                             remove_use = Some(
1433                                 self.tcx
1434                                     .sess
1435                                     .source_map()
1436                                     .span_until_non_whitespace(lt_through_ty_span),
1437                             );
1438                             break;
1439                         }
1440                     }
1441                     hir::TyKind::Path(ref qpath) => {
1442                         if let QPath::Resolved(_, path) = qpath {
1443                             let last_segment = &path.segments[path.segments.len() - 1];
1444                             let generics = last_segment.generic_args();
1445                             for arg in generics.args.iter() {
1446                                 if let GenericArg::Lifetime(lt) = arg {
1447                                     if lt.name.ident() == name {
1448                                         elide_use = Some(lt.span);
1449                                         break;
1450                                     }
1451                                 }
1452                             }
1453                             break;
1454                         }
1455                     }
1456                     _ => {}
1457                 }
1458             }
1459         };
1460         if let Node::Lifetime(hir_lifetime) = self.tcx.hir().get(lifetime.hir_id) {
1461             if let Some(parent) =
1462                 self.tcx.hir().find(self.tcx.hir().get_parent_item(hir_lifetime.hir_id))
1463             {
1464                 match parent {
1465                     Node::Item(item) => {
1466                         if let hir::ItemKind::Fn(sig, _, _) = &item.kind {
1467                             find_arg_use_span(sig.decl.inputs);
1468                         }
1469                     }
1470                     Node::ImplItem(impl_item) => {
1471                         if let hir::ImplItemKind::Fn(sig, _) = &impl_item.kind {
1472                             find_arg_use_span(sig.decl.inputs);
1473                         }
1474                     }
1475                     _ => {}
1476                 }
1477             }
1478         }
1479
1480         let msg = "elide the single-use lifetime";
1481         match (remove_decl, remove_use, elide_use) {
1482             (Some(decl_span), Some(use_span), None) => {
1483                 // if both declaration and use deletion spans start at the same
1484                 // place ("start at" because the latter includes trailing
1485                 // whitespace), then this is an in-band lifetime
1486                 if decl_span.shrink_to_lo() == use_span.shrink_to_lo() {
1487                     err.span_suggestion(
1488                         use_span,
1489                         msg,
1490                         String::new(),
1491                         Applicability::MachineApplicable,
1492                     );
1493                 } else {
1494                     err.multipart_suggestion(
1495                         msg,
1496                         vec![(decl_span, String::new()), (use_span, String::new())],
1497                         Applicability::MachineApplicable,
1498                     );
1499                 }
1500             }
1501             (Some(decl_span), None, Some(use_span)) => {
1502                 err.multipart_suggestion(
1503                     msg,
1504                     vec![(decl_span, String::new()), (use_span, "'_".to_owned())],
1505                     Applicability::MachineApplicable,
1506                 );
1507             }
1508             _ => {}
1509         }
1510     }
1511
1512     fn check_uses_for_lifetimes_defined_by_scope(&mut self) {
1513         let defined_by = match self.scope {
1514             Scope::Binder { lifetimes, .. } => lifetimes,
1515             _ => {
1516                 debug!("check_uses_for_lifetimes_defined_by_scope: not in a binder scope");
1517                 return;
1518             }
1519         };
1520
1521         let mut def_ids: Vec<_> = defined_by
1522             .values()
1523             .flat_map(|region| match region {
1524                 Region::EarlyBound(_, def_id, _)
1525                 | Region::LateBound(_, def_id, _)
1526                 | Region::Free(_, def_id) => Some(*def_id),
1527
1528                 Region::LateBoundAnon(..) | Region::Static => None,
1529             })
1530             .collect();
1531
1532         // ensure that we issue lints in a repeatable order
1533         def_ids.sort_by_cached_key(|&def_id| self.tcx.def_path_hash(def_id));
1534
1535         for def_id in def_ids {
1536             debug!("check_uses_for_lifetimes_defined_by_scope: def_id = {:?}", def_id);
1537
1538             let lifetimeuseset = self.lifetime_uses.remove(&def_id);
1539
1540             debug!(
1541                 "check_uses_for_lifetimes_defined_by_scope: lifetimeuseset = {:?}",
1542                 lifetimeuseset
1543             );
1544
1545             match lifetimeuseset {
1546                 Some(LifetimeUseSet::One(lifetime)) => {
1547                     let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
1548                     debug!("hir id first={:?}", hir_id);
1549                     if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
1550                         Node::Lifetime(hir_lifetime) => Some((
1551                             hir_lifetime.hir_id,
1552                             hir_lifetime.span,
1553                             hir_lifetime.name.ident(),
1554                         )),
1555                         Node::GenericParam(param) => {
1556                             Some((param.hir_id, param.span, param.name.ident()))
1557                         }
1558                         _ => None,
1559                     } {
1560                         debug!("id = {:?} span = {:?} name = {:?}", id, span, name);
1561                         if name.name == kw::UnderscoreLifetime {
1562                             continue;
1563                         }
1564
1565                         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1566                             if let Some(def_id) = parent_def_id.as_local() {
1567                                 let parent_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
1568                                 // lifetimes in `derive` expansions don't count (Issue #53738)
1569                                 if self.tcx.hir().attrs(parent_hir_id).iter().any(|attr| {
1570                                     self.tcx.sess.check_name(attr, sym::automatically_derived)
1571                                 }) {
1572                                     continue;
1573                                 }
1574                             }
1575                         }
1576
1577                         self.tcx.struct_span_lint_hir(
1578                             lint::builtin::SINGLE_USE_LIFETIMES,
1579                             id,
1580                             span,
1581                             |lint| {
1582                                 let mut err = lint.build(&format!(
1583                                     "lifetime parameter `{}` only used once",
1584                                     name
1585                                 ));
1586                                 if span == lifetime.span {
1587                                     // spans are the same for in-band lifetime declarations
1588                                     err.span_label(span, "this lifetime is only used here");
1589                                 } else {
1590                                     err.span_label(span, "this lifetime...");
1591                                     err.span_label(lifetime.span, "...is used only here");
1592                                 }
1593                                 self.suggest_eliding_single_use_lifetime(
1594                                     &mut err, def_id, lifetime,
1595                                 );
1596                                 err.emit();
1597                             },
1598                         );
1599                     }
1600                 }
1601                 Some(LifetimeUseSet::Many) => {
1602                     debug!("not one use lifetime");
1603                 }
1604                 None => {
1605                     let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
1606                     if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
1607                         Node::Lifetime(hir_lifetime) => Some((
1608                             hir_lifetime.hir_id,
1609                             hir_lifetime.span,
1610                             hir_lifetime.name.ident(),
1611                         )),
1612                         Node::GenericParam(param) => {
1613                             Some((param.hir_id, param.span, param.name.ident()))
1614                         }
1615                         _ => None,
1616                     } {
1617                         debug!("id ={:?} span = {:?} name = {:?}", id, span, name);
1618                         self.tcx.struct_span_lint_hir(
1619                             lint::builtin::UNUSED_LIFETIMES,
1620                             id,
1621                             span,
1622                             |lint| {
1623                                 let mut err = lint
1624                                     .build(&format!("lifetime parameter `{}` never used", name));
1625                                 if let Some(parent_def_id) = self.tcx.parent(def_id) {
1626                                     if let Some(generics) =
1627                                         self.tcx.hir().get_generics(parent_def_id)
1628                                     {
1629                                         let unused_lt_span =
1630                                             self.lifetime_deletion_span(name, generics);
1631                                         if let Some(span) = unused_lt_span {
1632                                             err.span_suggestion(
1633                                                 span,
1634                                                 "elide the unused lifetime",
1635                                                 String::new(),
1636                                                 Applicability::MachineApplicable,
1637                                             );
1638                                         }
1639                                     }
1640                                 }
1641                                 err.emit();
1642                             },
1643                         );
1644                     }
1645                 }
1646             }
1647         }
1648     }
1649
1650     /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
1651     ///
1652     /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
1653     /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
1654     /// within type bounds; those are early bound lifetimes, and the rest are late bound.
1655     ///
1656     /// For example:
1657     ///
1658     ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
1659     ///
1660     /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
1661     /// lifetimes may be interspersed together.
1662     ///
1663     /// If early bound lifetimes are present, we separate them into their own list (and likewise
1664     /// for late bound). They will be numbered sequentially, starting from the lowest index that is
1665     /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
1666     /// bound lifetimes are resolved by name and associated with a binder ID (`binder_id`), so the
1667     /// ordering is not important there.
1668     fn visit_early_late<F>(
1669         &mut self,
1670         parent_id: Option<hir::HirId>,
1671         decl: &'tcx hir::FnDecl<'tcx>,
1672         generics: &'tcx hir::Generics<'tcx>,
1673         walk: F,
1674     ) where
1675         F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
1676     {
1677         insert_late_bound_lifetimes(self.map, decl, generics);
1678
1679         // Find the start of nested early scopes, e.g., in methods.
1680         let mut index = 0;
1681         if let Some(parent_id) = parent_id {
1682             let parent = self.tcx.hir().expect_item(parent_id);
1683             if sub_items_have_self_param(&parent.kind) {
1684                 index += 1; // Self comes before lifetimes
1685             }
1686             match parent.kind {
1687                 hir::ItemKind::Trait(_, _, ref generics, ..)
1688                 | hir::ItemKind::Impl { ref generics, .. } => {
1689                     index += generics.params.len() as u32;
1690                 }
1691                 _ => {}
1692             }
1693         }
1694
1695         let mut non_lifetime_count = 0;
1696         let lifetimes = generics
1697             .params
1698             .iter()
1699             .filter_map(|param| match param.kind {
1700                 GenericParamKind::Lifetime { .. } => {
1701                     if self.map.late_bound.contains(&param.hir_id) {
1702                         Some(Region::late(&self.tcx.hir(), param))
1703                     } else {
1704                         Some(Region::early(&self.tcx.hir(), &mut index, param))
1705                     }
1706                 }
1707                 GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
1708                     non_lifetime_count += 1;
1709                     None
1710                 }
1711             })
1712             .collect();
1713         let next_early_index = index + non_lifetime_count;
1714
1715         let scope = Scope::Binder {
1716             lifetimes,
1717             next_early_index,
1718             s: self.scope,
1719             opaque_type_parent: true,
1720             track_lifetime_uses: false,
1721         };
1722         self.with(scope, move |old_scope, this| {
1723             this.check_lifetime_params(old_scope, &generics.params);
1724             this.hack(walk); // FIXME(#37666) workaround in place of `walk(this)`
1725         });
1726     }
1727
1728     fn next_early_index_helper(&self, only_opaque_type_parent: bool) -> u32 {
1729         let mut scope = self.scope;
1730         loop {
1731             match *scope {
1732                 Scope::Root => return 0,
1733
1734                 Scope::Binder { next_early_index, opaque_type_parent, .. }
1735                     if (!only_opaque_type_parent || opaque_type_parent) =>
1736                 {
1737                     return next_early_index;
1738                 }
1739
1740                 Scope::Binder { s, .. }
1741                 | Scope::Body { s, .. }
1742                 | Scope::Elision { s, .. }
1743                 | Scope::ObjectLifetimeDefault { s, .. } => scope = s,
1744             }
1745         }
1746     }
1747
1748     /// Returns the next index one would use for an early-bound-region
1749     /// if extending the current scope.
1750     fn next_early_index(&self) -> u32 {
1751         self.next_early_index_helper(true)
1752     }
1753
1754     /// Returns the next index one would use for an `impl Trait` that
1755     /// is being converted into an opaque type alias `impl Trait`. This will be the
1756     /// next early index from the enclosing item, for the most
1757     /// part. See the `opaque_type_parent` field for more info.
1758     fn next_early_index_for_opaque_type(&self) -> u32 {
1759         self.next_early_index_helper(false)
1760     }
1761
1762     fn resolve_lifetime_ref(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
1763         debug!("resolve_lifetime_ref(lifetime_ref={:?})", lifetime_ref);
1764
1765         // If we've already reported an error, just ignore `lifetime_ref`.
1766         if let LifetimeName::Error = lifetime_ref.name {
1767             return;
1768         }
1769
1770         // Walk up the scope chain, tracking the number of fn scopes
1771         // that we pass through, until we find a lifetime with the
1772         // given name or we run out of scopes.
1773         // search.
1774         let mut late_depth = 0;
1775         let mut scope = self.scope;
1776         let mut outermost_body = None;
1777         let result = loop {
1778             match *scope {
1779                 Scope::Body { id, s } => {
1780                     outermost_body = Some(id);
1781                     scope = s;
1782                 }
1783
1784                 Scope::Root => {
1785                     break None;
1786                 }
1787
1788                 Scope::Binder { ref lifetimes, s, .. } => {
1789                     match lifetime_ref.name {
1790                         LifetimeName::Param(param_name) => {
1791                             if let Some(&def) = lifetimes.get(&param_name.normalize_to_macros_2_0())
1792                             {
1793                                 break Some(def.shifted(late_depth));
1794                             }
1795                         }
1796                         _ => bug!("expected LifetimeName::Param"),
1797                     }
1798
1799                     late_depth += 1;
1800                     scope = s;
1801                 }
1802
1803                 Scope::Elision { s, .. } | Scope::ObjectLifetimeDefault { s, .. } => {
1804                     scope = s;
1805                 }
1806             }
1807         };
1808
1809         if let Some(mut def) = result {
1810             if let Region::EarlyBound(..) = def {
1811                 // Do not free early-bound regions, only late-bound ones.
1812             } else if let Some(body_id) = outermost_body {
1813                 let fn_id = self.tcx.hir().body_owner(body_id);
1814                 match self.tcx.hir().get(fn_id) {
1815                     Node::Item(&hir::Item { kind: hir::ItemKind::Fn(..), .. })
1816                     | Node::TraitItem(&hir::TraitItem {
1817                         kind: hir::TraitItemKind::Fn(..), ..
1818                     })
1819                     | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(..), .. }) => {
1820                         let scope = self.tcx.hir().local_def_id(fn_id);
1821                         def = Region::Free(scope.to_def_id(), def.id().unwrap());
1822                     }
1823                     _ => {}
1824                 }
1825             }
1826
1827             // Check for fn-syntax conflicts with in-band lifetime definitions
1828             if self.is_in_fn_syntax {
1829                 match def {
1830                     Region::EarlyBound(_, _, LifetimeDefOrigin::InBand)
1831                     | Region::LateBound(_, _, LifetimeDefOrigin::InBand) => {
1832                         struct_span_err!(
1833                             self.tcx.sess,
1834                             lifetime_ref.span,
1835                             E0687,
1836                             "lifetimes used in `fn` or `Fn` syntax must be \
1837                              explicitly declared using `<...>` binders"
1838                         )
1839                         .span_label(lifetime_ref.span, "in-band lifetime definition")
1840                         .emit();
1841                     }
1842
1843                     Region::Static
1844                     | Region::EarlyBound(
1845                         _,
1846                         _,
1847                         LifetimeDefOrigin::ExplicitOrElided | LifetimeDefOrigin::Error,
1848                     )
1849                     | Region::LateBound(
1850                         _,
1851                         _,
1852                         LifetimeDefOrigin::ExplicitOrElided | LifetimeDefOrigin::Error,
1853                     )
1854                     | Region::LateBoundAnon(..)
1855                     | Region::Free(..) => {}
1856                 }
1857             }
1858
1859             self.insert_lifetime(lifetime_ref, def);
1860         } else {
1861             self.emit_undeclared_lifetime_error(lifetime_ref);
1862         }
1863     }
1864
1865     fn visit_segment_args(
1866         &mut self,
1867         res: Res,
1868         depth: usize,
1869         generic_args: &'tcx hir::GenericArgs<'tcx>,
1870     ) {
1871         debug!(
1872             "visit_segment_args(res={:?}, depth={:?}, generic_args={:?})",
1873             res, depth, generic_args,
1874         );
1875
1876         if generic_args.parenthesized {
1877             let was_in_fn_syntax = self.is_in_fn_syntax;
1878             self.is_in_fn_syntax = true;
1879             self.visit_fn_like_elision(generic_args.inputs(), Some(generic_args.bindings[0].ty()));
1880             self.is_in_fn_syntax = was_in_fn_syntax;
1881             return;
1882         }
1883
1884         let mut elide_lifetimes = true;
1885         let lifetimes = generic_args
1886             .args
1887             .iter()
1888             .filter_map(|arg| match arg {
1889                 hir::GenericArg::Lifetime(lt) => {
1890                     if !lt.is_elided() {
1891                         elide_lifetimes = false;
1892                     }
1893                     Some(lt)
1894                 }
1895                 _ => None,
1896             })
1897             .collect();
1898         if elide_lifetimes {
1899             self.resolve_elided_lifetimes(lifetimes);
1900         } else {
1901             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
1902         }
1903
1904         // Figure out if this is a type/trait segment,
1905         // which requires object lifetime defaults.
1906         let parent_def_id = |this: &mut Self, def_id: DefId| {
1907             let def_key = this.tcx.def_key(def_id);
1908             DefId { krate: def_id.krate, index: def_key.parent.expect("missing parent") }
1909         };
1910         let type_def_id = match res {
1911             Res::Def(DefKind::AssocTy, def_id) if depth == 1 => Some(parent_def_id(self, def_id)),
1912             Res::Def(DefKind::Variant, def_id) if depth == 0 => Some(parent_def_id(self, def_id)),
1913             Res::Def(
1914                 DefKind::Struct
1915                 | DefKind::Union
1916                 | DefKind::Enum
1917                 | DefKind::TyAlias
1918                 | DefKind::Trait,
1919                 def_id,
1920             ) if depth == 0 => Some(def_id),
1921             _ => None,
1922         };
1923
1924         debug!("visit_segment_args: type_def_id={:?}", type_def_id);
1925
1926         // Compute a vector of defaults, one for each type parameter,
1927         // per the rules given in RFCs 599 and 1156. Example:
1928         //
1929         // ```rust
1930         // struct Foo<'a, T: 'a, U> { }
1931         // ```
1932         //
1933         // If you have `Foo<'x, dyn Bar, dyn Baz>`, we want to default
1934         // `dyn Bar` to `dyn Bar + 'x` (because of the `T: 'a` bound)
1935         // and `dyn Baz` to `dyn Baz + 'static` (because there is no
1936         // such bound).
1937         //
1938         // Therefore, we would compute `object_lifetime_defaults` to a
1939         // vector like `['x, 'static]`. Note that the vector only
1940         // includes type parameters.
1941         let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
1942             let in_body = {
1943                 let mut scope = self.scope;
1944                 loop {
1945                     match *scope {
1946                         Scope::Root => break false,
1947
1948                         Scope::Body { .. } => break true,
1949
1950                         Scope::Binder { s, .. }
1951                         | Scope::Elision { s, .. }
1952                         | Scope::ObjectLifetimeDefault { s, .. } => {
1953                             scope = s;
1954                         }
1955                     }
1956                 }
1957             };
1958
1959             let map = &self.map;
1960             let unsubst = if let Some(def_id) = def_id.as_local() {
1961                 let id = self.tcx.hir().local_def_id_to_hir_id(def_id);
1962                 &map.object_lifetime_defaults[&id]
1963             } else {
1964                 let tcx = self.tcx;
1965                 self.xcrate_object_lifetime_defaults.entry(def_id).or_insert_with(|| {
1966                     tcx.generics_of(def_id)
1967                         .params
1968                         .iter()
1969                         .filter_map(|param| match param.kind {
1970                             GenericParamDefKind::Type { object_lifetime_default, .. } => {
1971                                 Some(object_lifetime_default)
1972                             }
1973                             GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
1974                         })
1975                         .collect()
1976                 })
1977             };
1978             debug!("visit_segment_args: unsubst={:?}", unsubst);
1979             unsubst
1980                 .iter()
1981                 .map(|set| match *set {
1982                     Set1::Empty => {
1983                         if in_body {
1984                             None
1985                         } else {
1986                             Some(Region::Static)
1987                         }
1988                     }
1989                     Set1::One(r) => {
1990                         let lifetimes = generic_args.args.iter().filter_map(|arg| match arg {
1991                             GenericArg::Lifetime(lt) => Some(lt),
1992                             _ => None,
1993                         });
1994                         r.subst(lifetimes, map)
1995                     }
1996                     Set1::Many => None,
1997                 })
1998                 .collect()
1999         });
2000
2001         debug!("visit_segment_args: object_lifetime_defaults={:?}", object_lifetime_defaults);
2002
2003         let mut i = 0;
2004         for arg in generic_args.args {
2005             match arg {
2006                 GenericArg::Lifetime(_) => {}
2007                 GenericArg::Type(ty) => {
2008                     if let Some(&lt) = object_lifetime_defaults.get(i) {
2009                         let scope = Scope::ObjectLifetimeDefault { lifetime: lt, s: self.scope };
2010                         self.with(scope, |_, this| this.visit_ty(ty));
2011                     } else {
2012                         self.visit_ty(ty);
2013                     }
2014                     i += 1;
2015                 }
2016                 GenericArg::Const(ct) => {
2017                     self.visit_anon_const(&ct.value);
2018                 }
2019             }
2020         }
2021
2022         // Hack: when resolving the type `XX` in binding like `dyn
2023         // Foo<'b, Item = XX>`, the current object-lifetime default
2024         // would be to examine the trait `Foo` to check whether it has
2025         // a lifetime bound declared on `Item`. e.g., if `Foo` is
2026         // declared like so, then the default object lifetime bound in
2027         // `XX` should be `'b`:
2028         //
2029         // ```rust
2030         // trait Foo<'a> {
2031         //   type Item: 'a;
2032         // }
2033         // ```
2034         //
2035         // but if we just have `type Item;`, then it would be
2036         // `'static`. However, we don't get all of this logic correct.
2037         //
2038         // Instead, we do something hacky: if there are no lifetime parameters
2039         // to the trait, then we simply use a default object lifetime
2040         // bound of `'static`, because there is no other possibility. On the other hand,
2041         // if there ARE lifetime parameters, then we require the user to give an
2042         // explicit bound for now.
2043         //
2044         // This is intended to leave room for us to implement the
2045         // correct behavior in the future.
2046         let has_lifetime_parameter = generic_args.args.iter().any(|arg| match arg {
2047             GenericArg::Lifetime(_) => true,
2048             _ => false,
2049         });
2050
2051         // Resolve lifetimes found in the type `XX` from `Item = XX` bindings.
2052         for b in generic_args.bindings {
2053             let scope = Scope::ObjectLifetimeDefault {
2054                 lifetime: if has_lifetime_parameter { None } else { Some(Region::Static) },
2055                 s: self.scope,
2056             };
2057             self.with(scope, |_, this| this.visit_assoc_type_binding(b));
2058         }
2059     }
2060
2061     fn visit_fn_like_elision(
2062         &mut self,
2063         inputs: &'tcx [hir::Ty<'tcx>],
2064         output: Option<&'tcx hir::Ty<'tcx>>,
2065     ) {
2066         debug!("visit_fn_like_elision: enter");
2067         let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
2068         let arg_scope = Scope::Elision { elide: arg_elide.clone(), s: self.scope };
2069         self.with(arg_scope, |_, this| {
2070             for input in inputs {
2071                 this.visit_ty(input);
2072             }
2073             match *this.scope {
2074                 Scope::Elision { ref elide, .. } => {
2075                     arg_elide = elide.clone();
2076                 }
2077                 _ => bug!(),
2078             }
2079         });
2080
2081         let output = match output {
2082             Some(ty) => ty,
2083             None => return,
2084         };
2085
2086         debug!("visit_fn_like_elision: determine output");
2087
2088         // Figure out if there's a body we can get argument names from,
2089         // and whether there's a `self` argument (treated specially).
2090         let mut assoc_item_kind = None;
2091         let mut impl_self = None;
2092         let parent = self.tcx.hir().get_parent_node(output.hir_id);
2093         let body = match self.tcx.hir().get(parent) {
2094             // `fn` definitions and methods.
2095             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(.., body), .. }) => Some(body),
2096
2097             Node::TraitItem(&hir::TraitItem { kind: hir::TraitItemKind::Fn(_, ref m), .. }) => {
2098                 if let hir::ItemKind::Trait(.., ref trait_items) =
2099                     self.tcx.hir().expect_item(self.tcx.hir().get_parent_item(parent)).kind
2100                 {
2101                     assoc_item_kind =
2102                         trait_items.iter().find(|ti| ti.id.hir_id == parent).map(|ti| ti.kind);
2103                 }
2104                 match *m {
2105                     hir::TraitFn::Required(_) => None,
2106                     hir::TraitFn::Provided(body) => Some(body),
2107                 }
2108             }
2109
2110             Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(_, body), .. }) => {
2111                 if let hir::ItemKind::Impl { ref self_ty, ref items, .. } =
2112                     self.tcx.hir().expect_item(self.tcx.hir().get_parent_item(parent)).kind
2113                 {
2114                     impl_self = Some(self_ty);
2115                     assoc_item_kind =
2116                         items.iter().find(|ii| ii.id.hir_id == parent).map(|ii| ii.kind);
2117                 }
2118                 Some(body)
2119             }
2120
2121             // Foreign functions, `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
2122             Node::ForeignItem(_) | Node::Ty(_) | Node::TraitRef(_) => None,
2123             // Everything else (only closures?) doesn't
2124             // actually enjoy elision in return types.
2125             _ => {
2126                 self.visit_ty(output);
2127                 return;
2128             }
2129         };
2130
2131         let has_self = match assoc_item_kind {
2132             Some(hir::AssocItemKind::Fn { has_self }) => has_self,
2133             _ => false,
2134         };
2135
2136         // In accordance with the rules for lifetime elision, we can determine
2137         // what region to use for elision in the output type in two ways.
2138         // First (determined here), if `self` is by-reference, then the
2139         // implied output region is the region of the self parameter.
2140         if has_self {
2141             struct SelfVisitor<'a> {
2142                 map: &'a NamedRegionMap,
2143                 impl_self: Option<&'a hir::TyKind<'a>>,
2144                 lifetime: Set1<Region>,
2145             }
2146
2147             impl SelfVisitor<'_> {
2148                 // Look for `self: &'a Self` - also desugared from `&'a self`,
2149                 // and if that matches, use it for elision and return early.
2150                 fn is_self_ty(&self, res: Res) -> bool {
2151                     if let Res::SelfTy(..) = res {
2152                         return true;
2153                     }
2154
2155                     // Can't always rely on literal (or implied) `Self` due
2156                     // to the way elision rules were originally specified.
2157                     if let Some(&hir::TyKind::Path(hir::QPath::Resolved(None, ref path))) =
2158                         self.impl_self
2159                     {
2160                         match path.res {
2161                             // Permit the types that unambiguously always
2162                             // result in the same type constructor being used
2163                             // (it can't differ between `Self` and `self`).
2164                             Res::Def(DefKind::Struct | DefKind::Union | DefKind::Enum, _)
2165                             | Res::PrimTy(_) => return res == path.res,
2166                             _ => {}
2167                         }
2168                     }
2169
2170                     false
2171                 }
2172             }
2173
2174             impl<'a> Visitor<'a> for SelfVisitor<'a> {
2175                 type Map = intravisit::ErasedMap<'a>;
2176
2177                 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2178                     NestedVisitorMap::None
2179                 }
2180
2181                 fn visit_ty(&mut self, ty: &'a hir::Ty<'a>) {
2182                     if let hir::TyKind::Rptr(lifetime_ref, ref mt) = ty.kind {
2183                         if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.kind
2184                         {
2185                             if self.is_self_ty(path.res) {
2186                                 if let Some(lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
2187                                     self.lifetime.insert(*lifetime);
2188                                 }
2189                             }
2190                         }
2191                     }
2192                     intravisit::walk_ty(self, ty)
2193                 }
2194             }
2195
2196             let mut visitor = SelfVisitor {
2197                 map: self.map,
2198                 impl_self: impl_self.map(|ty| &ty.kind),
2199                 lifetime: Set1::Empty,
2200             };
2201             visitor.visit_ty(&inputs[0]);
2202             if let Set1::One(lifetime) = visitor.lifetime {
2203                 let scope = Scope::Elision { elide: Elide::Exact(lifetime), s: self.scope };
2204                 self.with(scope, |_, this| this.visit_ty(output));
2205                 return;
2206             }
2207         }
2208
2209         // Second, if there was exactly one lifetime (either a substitution or a
2210         // reference) in the arguments, then any anonymous regions in the output
2211         // have that lifetime.
2212         let mut possible_implied_output_region = None;
2213         let mut lifetime_count = 0;
2214         let arg_lifetimes = inputs
2215             .iter()
2216             .enumerate()
2217             .skip(has_self as usize)
2218             .map(|(i, input)| {
2219                 let mut gather = GatherLifetimes {
2220                     map: self.map,
2221                     outer_index: ty::INNERMOST,
2222                     have_bound_regions: false,
2223                     lifetimes: Default::default(),
2224                 };
2225                 gather.visit_ty(input);
2226
2227                 lifetime_count += gather.lifetimes.len();
2228
2229                 if lifetime_count == 1 && gather.lifetimes.len() == 1 {
2230                     // there's a chance that the unique lifetime of this
2231                     // iteration will be the appropriate lifetime for output
2232                     // parameters, so lets store it.
2233                     possible_implied_output_region = gather.lifetimes.iter().cloned().next();
2234                 }
2235
2236                 ElisionFailureInfo {
2237                     parent: body,
2238                     index: i,
2239                     lifetime_count: gather.lifetimes.len(),
2240                     have_bound_regions: gather.have_bound_regions,
2241                     span: input.span,
2242                 }
2243             })
2244             .collect();
2245
2246         let elide = if lifetime_count == 1 {
2247             Elide::Exact(possible_implied_output_region.unwrap())
2248         } else {
2249             Elide::Error(arg_lifetimes)
2250         };
2251
2252         debug!("visit_fn_like_elision: elide={:?}", elide);
2253
2254         let scope = Scope::Elision { elide, s: self.scope };
2255         self.with(scope, |_, this| this.visit_ty(output));
2256         debug!("visit_fn_like_elision: exit");
2257
2258         struct GatherLifetimes<'a> {
2259             map: &'a NamedRegionMap,
2260             outer_index: ty::DebruijnIndex,
2261             have_bound_regions: bool,
2262             lifetimes: FxHashSet<Region>,
2263         }
2264
2265         impl<'v, 'a> Visitor<'v> for GatherLifetimes<'a> {
2266             type Map = intravisit::ErasedMap<'v>;
2267
2268             fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2269                 NestedVisitorMap::None
2270             }
2271
2272             fn visit_ty(&mut self, ty: &hir::Ty<'_>) {
2273                 if let hir::TyKind::BareFn(_) = ty.kind {
2274                     self.outer_index.shift_in(1);
2275                 }
2276                 match ty.kind {
2277                     hir::TyKind::TraitObject(bounds, ref lifetime) => {
2278                         for bound in bounds {
2279                             self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
2280                         }
2281
2282                         // Stay on the safe side and don't include the object
2283                         // lifetime default (which may not end up being used).
2284                         if !lifetime.is_elided() {
2285                             self.visit_lifetime(lifetime);
2286                         }
2287                     }
2288                     _ => {
2289                         intravisit::walk_ty(self, ty);
2290                     }
2291                 }
2292                 if let hir::TyKind::BareFn(_) = ty.kind {
2293                     self.outer_index.shift_out(1);
2294                 }
2295             }
2296
2297             fn visit_generic_param(&mut self, param: &hir::GenericParam<'_>) {
2298                 if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2299                     // FIXME(eddyb) Do we want this? It only makes a difference
2300                     // if this `for<'a>` lifetime parameter is never used.
2301                     self.have_bound_regions = true;
2302                 }
2303
2304                 intravisit::walk_generic_param(self, param);
2305             }
2306
2307             fn visit_poly_trait_ref(
2308                 &mut self,
2309                 trait_ref: &hir::PolyTraitRef<'_>,
2310                 modifier: hir::TraitBoundModifier,
2311             ) {
2312                 self.outer_index.shift_in(1);
2313                 intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
2314                 self.outer_index.shift_out(1);
2315             }
2316
2317             fn visit_param_bound(&mut self, bound: &hir::GenericBound<'_>) {
2318                 if let hir::GenericBound::LangItemTrait { .. } = bound {
2319                     self.outer_index.shift_in(1);
2320                     intravisit::walk_param_bound(self, bound);
2321                     self.outer_index.shift_out(1);
2322                 } else {
2323                     intravisit::walk_param_bound(self, bound);
2324                 }
2325             }
2326
2327             fn visit_lifetime(&mut self, lifetime_ref: &hir::Lifetime) {
2328                 if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
2329                     match lifetime {
2330                         Region::LateBound(debruijn, _, _) | Region::LateBoundAnon(debruijn, _)
2331                             if debruijn < self.outer_index =>
2332                         {
2333                             self.have_bound_regions = true;
2334                         }
2335                         _ => {
2336                             self.lifetimes.insert(lifetime.shifted_out_to_binder(self.outer_index));
2337                         }
2338                     }
2339                 }
2340             }
2341         }
2342     }
2343
2344     fn resolve_elided_lifetimes(&mut self, lifetime_refs: Vec<&'tcx hir::Lifetime>) {
2345         debug!("resolve_elided_lifetimes(lifetime_refs={:?})", lifetime_refs);
2346
2347         if lifetime_refs.is_empty() {
2348             return;
2349         }
2350
2351         let span = lifetime_refs[0].span;
2352         let mut late_depth = 0;
2353         let mut scope = self.scope;
2354         let mut lifetime_names = FxHashSet::default();
2355         let mut lifetime_spans = vec![];
2356         let error = loop {
2357             match *scope {
2358                 // Do not assign any resolution, it will be inferred.
2359                 Scope::Body { .. } => return,
2360
2361                 Scope::Root => break None,
2362
2363                 Scope::Binder { s, ref lifetimes, .. } => {
2364                     // collect named lifetimes for suggestions
2365                     for name in lifetimes.keys() {
2366                         if let hir::ParamName::Plain(name) = name {
2367                             lifetime_names.insert(name.name);
2368                             lifetime_spans.push(name.span);
2369                         }
2370                     }
2371                     late_depth += 1;
2372                     scope = s;
2373                 }
2374
2375                 Scope::Elision { ref elide, ref s, .. } => {
2376                     let lifetime = match *elide {
2377                         Elide::FreshLateAnon(ref counter) => {
2378                             for lifetime_ref in lifetime_refs {
2379                                 let lifetime = Region::late_anon(counter).shifted(late_depth);
2380                                 self.insert_lifetime(lifetime_ref, lifetime);
2381                             }
2382                             return;
2383                         }
2384                         Elide::Exact(l) => l.shifted(late_depth),
2385                         Elide::Error(ref e) => {
2386                             let mut scope = s;
2387                             loop {
2388                                 match scope {
2389                                     Scope::Binder { ref lifetimes, s, .. } => {
2390                                         // Collect named lifetimes for suggestions.
2391                                         for name in lifetimes.keys() {
2392                                             if let hir::ParamName::Plain(name) = name {
2393                                                 lifetime_names.insert(name.name);
2394                                                 lifetime_spans.push(name.span);
2395                                             }
2396                                         }
2397                                         scope = s;
2398                                     }
2399                                     Scope::ObjectLifetimeDefault { ref s, .. }
2400                                     | Scope::Elision { ref s, .. } => {
2401                                         scope = s;
2402                                     }
2403                                     _ => break,
2404                                 }
2405                             }
2406                             break Some(e);
2407                         }
2408                         Elide::Forbid => break None,
2409                     };
2410                     for lifetime_ref in lifetime_refs {
2411                         self.insert_lifetime(lifetime_ref, lifetime);
2412                     }
2413                     return;
2414                 }
2415
2416                 Scope::ObjectLifetimeDefault { s, .. } => {
2417                     scope = s;
2418                 }
2419             }
2420         };
2421
2422         let mut err = self.report_missing_lifetime_specifiers(span, lifetime_refs.len());
2423
2424         if let Some(params) = error {
2425             // If there's no lifetime available, suggest `'static`.
2426             if self.report_elision_failure(&mut err, params) && lifetime_names.is_empty() {
2427                 lifetime_names.insert(kw::StaticLifetime);
2428             }
2429         }
2430         self.add_missing_lifetime_specifiers_label(
2431             &mut err,
2432             span,
2433             lifetime_refs.len(),
2434             &lifetime_names,
2435             lifetime_spans,
2436             error.map(|p| &p[..]).unwrap_or(&[]),
2437         );
2438         err.emit();
2439     }
2440
2441     fn report_elision_failure(
2442         &mut self,
2443         db: &mut DiagnosticBuilder<'_>,
2444         params: &[ElisionFailureInfo],
2445     ) -> bool /* add `'static` lifetime to lifetime list */ {
2446         let mut m = String::new();
2447         let len = params.len();
2448
2449         let elided_params: Vec<_> =
2450             params.iter().cloned().filter(|info| info.lifetime_count > 0).collect();
2451
2452         let elided_len = elided_params.len();
2453
2454         for (i, info) in elided_params.into_iter().enumerate() {
2455             let ElisionFailureInfo { parent, index, lifetime_count: n, have_bound_regions, span } =
2456                 info;
2457
2458             db.span_label(span, "");
2459             let help_name = if let Some(ident) =
2460                 parent.and_then(|body| self.tcx.hir().body(body).params[index].pat.simple_ident())
2461             {
2462                 format!("`{}`", ident)
2463             } else {
2464                 format!("argument {}", index + 1)
2465             };
2466
2467             m.push_str(
2468                 &(if n == 1 {
2469                     help_name
2470                 } else {
2471                     format!(
2472                         "one of {}'s {} {}lifetimes",
2473                         help_name,
2474                         n,
2475                         if have_bound_regions { "free " } else { "" }
2476                     )
2477                 })[..],
2478             );
2479
2480             if elided_len == 2 && i == 0 {
2481                 m.push_str(" or ");
2482             } else if i + 2 == elided_len {
2483                 m.push_str(", or ");
2484             } else if i != elided_len - 1 {
2485                 m.push_str(", ");
2486             }
2487         }
2488
2489         if len == 0 {
2490             db.help(
2491                 "this function's return type contains a borrowed value, \
2492                  but there is no value for it to be borrowed from",
2493             );
2494             true
2495         } else if elided_len == 0 {
2496             db.help(
2497                 "this function's return type contains a borrowed value with \
2498                  an elided lifetime, but the lifetime cannot be derived from \
2499                  the arguments",
2500             );
2501             true
2502         } else if elided_len == 1 {
2503             db.help(&format!(
2504                 "this function's return type contains a borrowed value, \
2505                  but the signature does not say which {} it is borrowed from",
2506                 m
2507             ));
2508             false
2509         } else {
2510             db.help(&format!(
2511                 "this function's return type contains a borrowed value, \
2512                  but the signature does not say whether it is borrowed from {}",
2513                 m
2514             ));
2515             false
2516         }
2517     }
2518
2519     fn resolve_object_lifetime_default(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
2520         debug!("resolve_object_lifetime_default(lifetime_ref={:?})", lifetime_ref);
2521         let mut late_depth = 0;
2522         let mut scope = self.scope;
2523         let lifetime = loop {
2524             match *scope {
2525                 Scope::Binder { s, .. } => {
2526                     late_depth += 1;
2527                     scope = s;
2528                 }
2529
2530                 Scope::Root | Scope::Elision { .. } => break Region::Static,
2531
2532                 Scope::Body { .. } | Scope::ObjectLifetimeDefault { lifetime: None, .. } => return,
2533
2534                 Scope::ObjectLifetimeDefault { lifetime: Some(l), .. } => break l,
2535             }
2536         };
2537         self.insert_lifetime(lifetime_ref, lifetime.shifted(late_depth));
2538     }
2539
2540     fn check_lifetime_params(
2541         &mut self,
2542         old_scope: ScopeRef<'_>,
2543         params: &'tcx [hir::GenericParam<'tcx>],
2544     ) {
2545         let lifetimes: Vec<_> = params
2546             .iter()
2547             .filter_map(|param| match param.kind {
2548                 GenericParamKind::Lifetime { .. } => {
2549                     Some((param, param.name.normalize_to_macros_2_0()))
2550                 }
2551                 _ => None,
2552             })
2553             .collect();
2554         for (i, (lifetime_i, lifetime_i_name)) in lifetimes.iter().enumerate() {
2555             if let hir::ParamName::Plain(_) = lifetime_i_name {
2556                 let name = lifetime_i_name.ident().name;
2557                 if name == kw::UnderscoreLifetime || name == kw::StaticLifetime {
2558                     let mut err = struct_span_err!(
2559                         self.tcx.sess,
2560                         lifetime_i.span,
2561                         E0262,
2562                         "invalid lifetime parameter name: `{}`",
2563                         lifetime_i.name.ident(),
2564                     );
2565                     err.span_label(
2566                         lifetime_i.span,
2567                         format!("{} is a reserved lifetime name", name),
2568                     );
2569                     err.emit();
2570                 }
2571             }
2572
2573             // It is a hard error to shadow a lifetime within the same scope.
2574             for (lifetime_j, lifetime_j_name) in lifetimes.iter().skip(i + 1) {
2575                 if lifetime_i_name == lifetime_j_name {
2576                     struct_span_err!(
2577                         self.tcx.sess,
2578                         lifetime_j.span,
2579                         E0263,
2580                         "lifetime name `{}` declared twice in the same scope",
2581                         lifetime_j.name.ident()
2582                     )
2583                     .span_label(lifetime_j.span, "declared twice")
2584                     .span_label(lifetime_i.span, "previous declaration here")
2585                     .emit();
2586                 }
2587             }
2588
2589             // It is a soft error to shadow a lifetime within a parent scope.
2590             self.check_lifetime_param_for_shadowing(old_scope, &lifetime_i);
2591
2592             for bound in lifetime_i.bounds {
2593                 match bound {
2594                     hir::GenericBound::Outlives(ref lt) => match lt.name {
2595                         hir::LifetimeName::Underscore => self.tcx.sess.delay_span_bug(
2596                             lt.span,
2597                             "use of `'_` in illegal place, but not caught by lowering",
2598                         ),
2599                         hir::LifetimeName::Static => {
2600                             self.insert_lifetime(lt, Region::Static);
2601                             self.tcx
2602                                 .sess
2603                                 .struct_span_warn(
2604                                     lifetime_i.span.to(lt.span),
2605                                     &format!(
2606                                         "unnecessary lifetime parameter `{}`",
2607                                         lifetime_i.name.ident(),
2608                                     ),
2609                                 )
2610                                 .help(&format!(
2611                                     "you can use the `'static` lifetime directly, in place of `{}`",
2612                                     lifetime_i.name.ident(),
2613                                 ))
2614                                 .emit();
2615                         }
2616                         hir::LifetimeName::Param(_) | hir::LifetimeName::Implicit => {
2617                             self.resolve_lifetime_ref(lt);
2618                         }
2619                         hir::LifetimeName::ImplicitObjectLifetimeDefault => {
2620                             self.tcx.sess.delay_span_bug(
2621                                 lt.span,
2622                                 "lowering generated `ImplicitObjectLifetimeDefault` \
2623                                  outside of an object type",
2624                             )
2625                         }
2626                         hir::LifetimeName::Error => {
2627                             // No need to do anything, error already reported.
2628                         }
2629                     },
2630                     _ => bug!(),
2631                 }
2632             }
2633         }
2634     }
2635
2636     fn check_lifetime_param_for_shadowing(
2637         &self,
2638         mut old_scope: ScopeRef<'_>,
2639         param: &'tcx hir::GenericParam<'tcx>,
2640     ) {
2641         for label in &self.labels_in_fn {
2642             // FIXME (#24278): non-hygienic comparison
2643             if param.name.ident().name == label.name {
2644                 signal_shadowing_problem(
2645                     self.tcx,
2646                     label.name,
2647                     original_label(label.span),
2648                     shadower_lifetime(&param),
2649                 );
2650                 return;
2651             }
2652         }
2653
2654         loop {
2655             match *old_scope {
2656                 Scope::Body { s, .. }
2657                 | Scope::Elision { s, .. }
2658                 | Scope::ObjectLifetimeDefault { s, .. } => {
2659                     old_scope = s;
2660                 }
2661
2662                 Scope::Root => {
2663                     return;
2664                 }
2665
2666                 Scope::Binder { ref lifetimes, s, .. } => {
2667                     if let Some(&def) = lifetimes.get(&param.name.normalize_to_macros_2_0()) {
2668                         let hir_id =
2669                             self.tcx.hir().local_def_id_to_hir_id(def.id().unwrap().expect_local());
2670
2671                         signal_shadowing_problem(
2672                             self.tcx,
2673                             param.name.ident().name,
2674                             original_lifetime(self.tcx.hir().span(hir_id)),
2675                             shadower_lifetime(&param),
2676                         );
2677                         return;
2678                     }
2679
2680                     old_scope = s;
2681                 }
2682             }
2683         }
2684     }
2685
2686     /// Returns `true` if, in the current scope, replacing `'_` would be
2687     /// equivalent to a single-use lifetime.
2688     fn track_lifetime_uses(&self) -> bool {
2689         let mut scope = self.scope;
2690         loop {
2691             match *scope {
2692                 Scope::Root => break false,
2693
2694                 // Inside of items, it depends on the kind of item.
2695                 Scope::Binder { track_lifetime_uses, .. } => break track_lifetime_uses,
2696
2697                 // Inside a body, `'_` will use an inference variable,
2698                 // should be fine.
2699                 Scope::Body { .. } => break true,
2700
2701                 // A lifetime only used in a fn argument could as well
2702                 // be replaced with `'_`, as that would generate a
2703                 // fresh name, too.
2704                 Scope::Elision { elide: Elide::FreshLateAnon(_), .. } => break true,
2705
2706                 // In the return type or other such place, `'_` is not
2707                 // going to make a fresh name, so we cannot
2708                 // necessarily replace a single-use lifetime with
2709                 // `'_`.
2710                 Scope::Elision {
2711                     elide: Elide::Exact(_) | Elide::Error(_) | Elide::Forbid, ..
2712                 } => break false,
2713
2714                 Scope::ObjectLifetimeDefault { s, .. } => scope = s,
2715             }
2716         }
2717     }
2718
2719     fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
2720         debug!(
2721             "insert_lifetime: {} resolved to {:?} span={:?}",
2722             self.tcx.hir().node_to_string(lifetime_ref.hir_id),
2723             def,
2724             self.tcx.sess.source_map().span_to_string(lifetime_ref.span)
2725         );
2726         self.map.defs.insert(lifetime_ref.hir_id, def);
2727
2728         match def {
2729             Region::LateBoundAnon(..) | Region::Static => {
2730                 // These are anonymous lifetimes or lifetimes that are not declared.
2731             }
2732
2733             Region::Free(_, def_id)
2734             | Region::LateBound(_, def_id, _)
2735             | Region::EarlyBound(_, def_id, _) => {
2736                 // A lifetime declared by the user.
2737                 let track_lifetime_uses = self.track_lifetime_uses();
2738                 debug!("insert_lifetime: track_lifetime_uses={}", track_lifetime_uses);
2739                 if track_lifetime_uses && !self.lifetime_uses.contains_key(&def_id) {
2740                     debug!("insert_lifetime: first use of {:?}", def_id);
2741                     self.lifetime_uses.insert(def_id, LifetimeUseSet::One(lifetime_ref));
2742                 } else {
2743                     debug!("insert_lifetime: many uses of {:?}", def_id);
2744                     self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
2745                 }
2746             }
2747         }
2748     }
2749
2750     /// Sometimes we resolve a lifetime, but later find that it is an
2751     /// error (esp. around impl trait). In that case, we remove the
2752     /// entry into `map.defs` so as not to confuse later code.
2753     fn uninsert_lifetime_on_error(&mut self, lifetime_ref: &'tcx hir::Lifetime, bad_def: Region) {
2754         let old_value = self.map.defs.remove(&lifetime_ref.hir_id);
2755         assert_eq!(old_value, Some(bad_def));
2756     }
2757 }
2758
2759 /// Detects late-bound lifetimes and inserts them into
2760 /// `map.late_bound`.
2761 ///
2762 /// A region declared on a fn is **late-bound** if:
2763 /// - it is constrained by an argument type;
2764 /// - it does not appear in a where-clause.
2765 ///
2766 /// "Constrained" basically means that it appears in any type but
2767 /// not amongst the inputs to a projection. In other words, `<&'a
2768 /// T as Trait<''b>>::Foo` does not constrain `'a` or `'b`.
2769 fn insert_late_bound_lifetimes(
2770     map: &mut NamedRegionMap,
2771     decl: &hir::FnDecl<'_>,
2772     generics: &hir::Generics<'_>,
2773 ) {
2774     debug!("insert_late_bound_lifetimes(decl={:?}, generics={:?})", decl, generics);
2775
2776     let mut constrained_by_input = ConstrainedCollector::default();
2777     for arg_ty in decl.inputs {
2778         constrained_by_input.visit_ty(arg_ty);
2779     }
2780
2781     let mut appears_in_output = AllCollector::default();
2782     intravisit::walk_fn_ret_ty(&mut appears_in_output, &decl.output);
2783
2784     debug!("insert_late_bound_lifetimes: constrained_by_input={:?}", constrained_by_input.regions);
2785
2786     // Walk the lifetimes that appear in where clauses.
2787     //
2788     // Subtle point: because we disallow nested bindings, we can just
2789     // ignore binders here and scrape up all names we see.
2790     let mut appears_in_where_clause = AllCollector::default();
2791     appears_in_where_clause.visit_generics(generics);
2792
2793     for param in generics.params {
2794         if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2795             if !param.bounds.is_empty() {
2796                 // `'a: 'b` means both `'a` and `'b` are referenced
2797                 appears_in_where_clause
2798                     .regions
2799                     .insert(hir::LifetimeName::Param(param.name.normalize_to_macros_2_0()));
2800             }
2801         }
2802     }
2803
2804     debug!(
2805         "insert_late_bound_lifetimes: appears_in_where_clause={:?}",
2806         appears_in_where_clause.regions
2807     );
2808
2809     // Late bound regions are those that:
2810     // - appear in the inputs
2811     // - do not appear in the where-clauses
2812     // - are not implicitly captured by `impl Trait`
2813     for param in generics.params {
2814         match param.kind {
2815             hir::GenericParamKind::Lifetime { .. } => { /* fall through */ }
2816
2817             // Neither types nor consts are late-bound.
2818             hir::GenericParamKind::Type { .. } | hir::GenericParamKind::Const { .. } => continue,
2819         }
2820
2821         let lt_name = hir::LifetimeName::Param(param.name.normalize_to_macros_2_0());
2822         // appears in the where clauses? early-bound.
2823         if appears_in_where_clause.regions.contains(&lt_name) {
2824             continue;
2825         }
2826
2827         // does not appear in the inputs, but appears in the return type? early-bound.
2828         if !constrained_by_input.regions.contains(&lt_name)
2829             && appears_in_output.regions.contains(&lt_name)
2830         {
2831             continue;
2832         }
2833
2834         debug!(
2835             "insert_late_bound_lifetimes: lifetime {:?} with id {:?} is late-bound",
2836             param.name.ident(),
2837             param.hir_id
2838         );
2839
2840         let inserted = map.late_bound.insert(param.hir_id);
2841         assert!(inserted, "visited lifetime {:?} twice", param.hir_id);
2842     }
2843
2844     return;
2845
2846     #[derive(Default)]
2847     struct ConstrainedCollector {
2848         regions: FxHashSet<hir::LifetimeName>,
2849     }
2850
2851     impl<'v> Visitor<'v> for ConstrainedCollector {
2852         type Map = intravisit::ErasedMap<'v>;
2853
2854         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2855             NestedVisitorMap::None
2856         }
2857
2858         fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
2859             match ty.kind {
2860                 hir::TyKind::Path(
2861                     hir::QPath::Resolved(Some(_), _) | hir::QPath::TypeRelative(..),
2862                 ) => {
2863                     // ignore lifetimes appearing in associated type
2864                     // projections, as they are not *constrained*
2865                     // (defined above)
2866                 }
2867
2868                 hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => {
2869                     // consider only the lifetimes on the final
2870                     // segment; I am not sure it's even currently
2871                     // valid to have them elsewhere, but even if it
2872                     // is, those would be potentially inputs to
2873                     // projections
2874                     if let Some(last_segment) = path.segments.last() {
2875                         self.visit_path_segment(path.span, last_segment);
2876                     }
2877                 }
2878
2879                 _ => {
2880                     intravisit::walk_ty(self, ty);
2881                 }
2882             }
2883         }
2884
2885         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2886             self.regions.insert(lifetime_ref.name.normalize_to_macros_2_0());
2887         }
2888     }
2889
2890     #[derive(Default)]
2891     struct AllCollector {
2892         regions: FxHashSet<hir::LifetimeName>,
2893     }
2894
2895     impl<'v> Visitor<'v> for AllCollector {
2896         type Map = intravisit::ErasedMap<'v>;
2897
2898         fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
2899             NestedVisitorMap::None
2900         }
2901
2902         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2903             self.regions.insert(lifetime_ref.name.normalize_to_macros_2_0());
2904         }
2905     }
2906 }