src/librustc/middle/resolve_lifetime.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Name resolution for lifetimes.
  12 //!
  13 //! Name resolution for lifetimes follows MUCH simpler rules than the
  14 //! full resolve. For example, lifetime names are never exported or
  15 //! used between functions, and they operate in a purely top-down
  16 //! way. Therefore we break lifetime name resolution into a separate pass.
  17
  18 use hir::def::Def;
  19 use hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
  20 use hir::map::Map;
  21 use hir::{GenericArg, GenericParam, ItemLocalId, LifetimeName, Node, ParamName};
  22 use ty::{self, DefIdTree, GenericParamDefKind, TyCtxt};
  23
  24 use errors::{Applicability, DiagnosticBuilder};
  25 use rustc::lint;
  26 use rustc_data_structures::sync::Lrc;
  27 use session::Session;
  28 use std::borrow::Cow;
  29 use std::cell::Cell;
  30 use std::mem::replace;
  31 use syntax::ast;
  32 use syntax::attr;
  33 use syntax::ptr::P;
  34 use syntax::symbol::keywords;
  35 use syntax_pos::Span;
  36 use util::nodemap::{DefIdMap, FxHashMap, FxHashSet, NodeMap, NodeSet};
  37
  38 use hir::intravisit::{self, NestedVisitorMap, Visitor};
  39 use hir::{self, GenericParamKind, LifetimeParamKind};
  40
  41 /// The origin of a named lifetime definition.
  42 ///
  43 /// This is used to prevent the usage of in-band lifetimes in `Fn`/`fn` syntax.
  44 #[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
  45 pub enum LifetimeDefOrigin {
  46     // Explicit binders like `fn foo<'a>(x: &'a u8)` or elided like `impl Foo<&u32>`
  47     ExplicitOrElided,
  48     // In-band declarations like `fn foo(x: &'a u8)`
  49     InBand,
  50     // Some kind of erroneous origin
  51     Error,
  52 }
  53
  54 impl LifetimeDefOrigin {
  55     fn from_param(param: &GenericParam) -> Self {
  56         match param.kind {
  57             GenericParamKind::Lifetime { kind } => match kind {
  58                 LifetimeParamKind::InBand => LifetimeDefOrigin::InBand,
  59                 LifetimeParamKind::Explicit => LifetimeDefOrigin::ExplicitOrElided,
  60                 LifetimeParamKind::Elided => LifetimeDefOrigin::ExplicitOrElided,
  61                 LifetimeParamKind::Error => LifetimeDefOrigin::Error,
  62             },
  63             _ => bug!("expected a lifetime param"),
  64         }
  65     }
  66 }
  67
  68 // This counts the no of times a lifetime is used
  69 #[derive(Clone, Copy, Debug)]
  70 pub enum LifetimeUseSet<'tcx> {
  71     One(&'tcx hir::Lifetime),
  72     Many,
  73 }
  74
  75 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
  76 pub enum Region {
  77     Static,
  78     EarlyBound(
  79         /* index */ u32,
  80         /* lifetime decl */ DefId,
  81         LifetimeDefOrigin,
  82     ),
  83     LateBound(
  84         ty::DebruijnIndex,
  85         /* lifetime decl */ DefId,
  86         LifetimeDefOrigin,
  87     ),
  88     LateBoundAnon(ty::DebruijnIndex, /* anon index */ u32),
  89     Free(DefId, /* lifetime decl */ DefId),
  90 }
  91
  92 impl Region {
  93     fn early(hir_map: &Map<'_>, index: &mut u32, param: &GenericParam) -> (ParamName, Region) {
  94         let i = *index;
  95         *index += 1;
  96         let def_id = hir_map.local_def_id(param.id);
  97         let origin = LifetimeDefOrigin::from_param(param);
  98         debug!("Region::early: index={} def_id={:?}", i, def_id);
  99         (param.name.modern(), Region::EarlyBound(i, def_id, origin))
 100     }
 101
 102     fn late(hir_map: &Map<'_>, param: &GenericParam) -> (ParamName, Region) {
 103         let depth = ty::INNERMOST;
 104         let def_id = hir_map.local_def_id(param.id);
 105         let origin = LifetimeDefOrigin::from_param(param);
 106         debug!(
 107             "Region::late: param={:?} depth={:?} def_id={:?} origin={:?}",
 108             param, depth, def_id, origin,
 109         );
 110         (
 111             param.name.modern(),
 112             Region::LateBound(depth, def_id, origin),
 113         )
 114     }
 115
 116     fn late_anon(index: &Cell<u32>) -> Region {
 117         let i = index.get();
 118         index.set(i + 1);
 119         let depth = ty::INNERMOST;
 120         Region::LateBoundAnon(depth, i)
 121     }
 122
 123     fn id(&self) -> Option<DefId> {
 124         match *self {
 125             Region::Static | Region::LateBoundAnon(..) => None,
 126
 127             Region::EarlyBound(_, id, _) | Region::LateBound(_, id, _) | Region::Free(_, id) => {
 128                 Some(id)
 129             }
 130         }
 131     }
 132
 133     fn shifted(self, amount: u32) -> Region {
 134         match self {
 135             Region::LateBound(debruijn, id, origin) => {
 136                 Region::LateBound(debruijn.shifted_in(amount), id, origin)
 137             }
 138             Region::LateBoundAnon(debruijn, index) => {
 139                 Region::LateBoundAnon(debruijn.shifted_in(amount), index)
 140             }
 141             _ => self,
 142         }
 143     }
 144
 145     fn shifted_out_to_binder(self, binder: ty::DebruijnIndex) -> Region {
 146         match self {
 147             Region::LateBound(debruijn, id, origin) => {
 148                 Region::LateBound(debruijn.shifted_out_to_binder(binder), id, origin)
 149             }
 150             Region::LateBoundAnon(debruijn, index) => {
 151                 Region::LateBoundAnon(debruijn.shifted_out_to_binder(binder), index)
 152             }
 153             _ => self,
 154         }
 155     }
 156
 157     fn subst<'a, L>(self, mut params: L, map: &NamedRegionMap) -> Option<Region>
 158     where
 159         L: Iterator<Item = &'a hir::Lifetime>,
 160     {
 161         if let Region::EarlyBound(index, _, _) = self {
 162             params
 163                 .nth(index as usize)
 164                 .and_then(|lifetime| map.defs.get(&lifetime.id).cloned())
 165         } else {
 166             Some(self)
 167         }
 168     }
 169 }
 170
 171 /// A set containing, at most, one known element.
 172 /// If two distinct values are inserted into a set, then it
 173 /// becomes `Many`, which can be used to detect ambiguities.
 174 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug)]
 175 pub enum Set1<T> {
 176     Empty,
 177     One(T),
 178     Many,
 179 }
 180
 181 impl<T: PartialEq> Set1<T> {
 182     pub fn insert(&mut self, value: T) {
 183         if let Set1::Empty = *self {
 184             *self = Set1::One(value);
 185             return;
 186         }
 187         if let Set1::One(ref old) = *self {
 188             if *old == value {
 189                 return;
 190             }
 191         }
 192         *self = Set1::Many;
 193     }
 194 }
 195
 196 pub type ObjectLifetimeDefault = Set1<Region>;
 197
 198 /// Maps the id of each lifetime reference to the lifetime decl
 199 /// that it corresponds to.
 200 ///
 201 /// FIXME. This struct gets converted to a `ResolveLifetimes` for
 202 /// actual use. It has the same data, but indexed by `DefIndex`.  This
 203 /// is silly.
 204 #[derive(Default)]
 205 struct NamedRegionMap {
 206     // maps from every use of a named (not anonymous) lifetime to a
 207     // `Region` describing how that region is bound
 208     pub defs: NodeMap<Region>,
 209
 210     // the set of lifetime def ids that are late-bound; a region can
 211     // be late-bound if (a) it does NOT appear in a where-clause and
 212     // (b) it DOES appear in the arguments.
 213     pub late_bound: NodeSet,
 214
 215     // For each type and trait definition, maps type parameters
 216     // to the trait object lifetime defaults computed from them.
 217     pub object_lifetime_defaults: NodeMap<Vec<ObjectLifetimeDefault>>,
 218 }
 219
 220 /// See `NamedRegionMap`.
 221 #[derive(Default)]
 222 pub struct ResolveLifetimes {
 223     defs: FxHashMap<LocalDefId, Lrc<FxHashMap<ItemLocalId, Region>>>,
 224     late_bound: FxHashMap<LocalDefId, Lrc<FxHashSet<ItemLocalId>>>,
 225     object_lifetime_defaults:
 226         FxHashMap<LocalDefId, Lrc<FxHashMap<ItemLocalId, Lrc<Vec<ObjectLifetimeDefault>>>>>,
 227 }
 228
 229 impl_stable_hash_for!(struct ::middle::resolve_lifetime::ResolveLifetimes {
 230     defs,
 231     late_bound,
 232     object_lifetime_defaults
 233 });
 234
 235 struct LifetimeContext<'a, 'tcx: 'a> {
 236     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 237     map: &'a mut NamedRegionMap,
 238     scope: ScopeRef<'a>,
 239
 240     /// Deep breath. Our representation for poly trait refs contains a single
 241     /// binder and thus we only allow a single level of quantification. However,
 242     /// the syntax of Rust permits quantification in two places, e.g., `T: for <'a> Foo<'a>`
 243     /// and `for <'a, 'b> &'b T: Foo<'a>`. In order to get the de Bruijn indices
 244     /// correct when representing these constraints, we should only introduce one
 245     /// scope. However, we want to support both locations for the quantifier and
 246     /// during lifetime resolution we want precise information (so we can't
 247     /// desugar in an earlier phase).
 248     ///
 249     /// SO, if we encounter a quantifier at the outer scope, we set
 250     /// trait_ref_hack to true (and introduce a scope), and then if we encounter
 251     /// a quantifier at the inner scope, we error. If trait_ref_hack is false,
 252     /// then we introduce the scope at the inner quantifier.
 253     ///
 254     /// I'm sorry.
 255     trait_ref_hack: bool,
 256
 257     /// Used to disallow the use of in-band lifetimes in `fn` or `Fn` syntax.
 258     is_in_fn_syntax: bool,
 259
 260     /// List of labels in the function/method currently under analysis.
 261     labels_in_fn: Vec<ast::Ident>,
 262
 263     /// Cache for cross-crate per-definition object lifetime defaults.
 264     xcrate_object_lifetime_defaults: DefIdMap<Vec<ObjectLifetimeDefault>>,
 265
 266     lifetime_uses: &'a mut DefIdMap<LifetimeUseSet<'tcx>>,
 267 }
 268
 269 #[derive(Debug)]
 270 enum Scope<'a> {
 271     /// Declares lifetimes, and each can be early-bound or late-bound.
 272     /// The `DebruijnIndex` of late-bound lifetimes starts at `1` and
 273     /// it should be shifted by the number of `Binder`s in between the
 274     /// declaration `Binder` and the location it's referenced from.
 275     Binder {
 276         lifetimes: FxHashMap<hir::ParamName, Region>,
 277
 278         /// if we extend this scope with another scope, what is the next index
 279         /// we should use for an early-bound region?
 280         next_early_index: u32,
 281
 282         /// Flag is set to true if, in this binder, `'_` would be
 283         /// equivalent to a "single-use region". This is true on
 284         /// impls, but not other kinds of items.
 285         track_lifetime_uses: bool,
 286
 287         /// Whether or not this binder would serve as the parent
 288         /// binder for abstract types introduced within. For example:
 289         ///
 290         ///     fn foo<'a>() -> impl for<'b> Trait<Item = impl Trait2<'a>>
 291         ///
 292         /// Here, the abstract types we create for the `impl Trait`
 293         /// and `impl Trait2` references will both have the `foo` item
 294         /// as their parent. When we get to `impl Trait2`, we find
 295         /// that it is nested within the `for<>` binder -- this flag
 296         /// allows us to skip that when looking for the parent binder
 297         /// of the resulting abstract type.
 298         abstract_type_parent: bool,
 299
 300         s: ScopeRef<'a>,
 301     },
 302
 303     /// Lifetimes introduced by a fn are scoped to the call-site for that fn,
 304     /// if this is a fn body, otherwise the original definitions are used.
 305     /// Unspecified lifetimes are inferred, unless an elision scope is nested,
 306     /// e.g. `(&T, fn(&T) -> &T);` becomes `(&'_ T, for<'a> fn(&'a T) -> &'a T)`.
 307     Body {
 308         id: hir::BodyId,
 309         s: ScopeRef<'a>,
 310     },
 311
 312     /// A scope which either determines unspecified lifetimes or errors
 313     /// on them (e.g. due to ambiguity). For more details, see `Elide`.
 314     Elision {
 315         elide: Elide,
 316         s: ScopeRef<'a>,
 317     },
 318
 319     /// Use a specific lifetime (if `Some`) or leave it unset (to be
 320     /// inferred in a function body or potentially error outside one),
 321     /// for the default choice of lifetime in a trait object type.
 322     ObjectLifetimeDefault {
 323         lifetime: Option<Region>,
 324         s: ScopeRef<'a>,
 325     },
 326
 327     Root,
 328 }
 329
 330 #[derive(Clone, Debug)]
 331 enum Elide {
 332     /// Use a fresh anonymous late-bound lifetime each time, by
 333     /// incrementing the counter to generate sequential indices.
 334     FreshLateAnon(Cell<u32>),
 335     /// Always use this one lifetime.
 336     Exact(Region),
 337     /// Less or more than one lifetime were found, error on unspecified.
 338     Error(Vec<ElisionFailureInfo>),
 339 }
 340
 341 #[derive(Clone, Debug)]
 342 struct ElisionFailureInfo {
 343     /// Where we can find the argument pattern.
 344     parent: Option<hir::BodyId>,
 345     /// The index of the argument in the original definition.
 346     index: usize,
 347     lifetime_count: usize,
 348     have_bound_regions: bool,
 349 }
 350
 351 type ScopeRef<'a> = &'a Scope<'a>;
 352
 353 const ROOT_SCOPE: ScopeRef<'static> = &Scope::Root;
 354
 355 pub fn provide(providers: &mut ty::query::Providers<'_>) {
 356     *providers = ty::query::Providers {
 357         resolve_lifetimes,
 358
 359         named_region_map: |tcx, id| {
 360             let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
 361             tcx.resolve_lifetimes(LOCAL_CRATE).defs.get(&id).cloned()
 362         },
 363
 364         is_late_bound_map: |tcx, id| {
 365             let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
 366             tcx.resolve_lifetimes(LOCAL_CRATE)
 367                 .late_bound
 368                 .get(&id)
 369                 .cloned()
 370         },
 371
 372         object_lifetime_defaults_map: |tcx, id| {
 373             let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
 374             tcx.resolve_lifetimes(LOCAL_CRATE)
 375                 .object_lifetime_defaults
 376                 .get(&id)
 377                 .cloned()
 378         },
 379
 380         ..*providers
 381     };
 382
 383     // (*) FIXME the query should be defined to take a LocalDefId
 384 }
 385
 386 /// Computes the `ResolveLifetimes` map that contains data for the
 387 /// entire crate. You should not read the result of this query
 388 /// directly, but rather use `named_region_map`, `is_late_bound_map`,
 389 /// etc.
 390 fn resolve_lifetimes<'tcx>(
 391     tcx: TyCtxt<'_, 'tcx, 'tcx>,
 392     for_krate: CrateNum,
 393 ) -> Lrc<ResolveLifetimes> {
 394     assert_eq!(for_krate, LOCAL_CRATE);
 395
 396     let named_region_map = krate(tcx);
 397
 398     let mut rl = ResolveLifetimes::default();
 399
 400     for (k, v) in named_region_map.defs {
 401         let hir_id = tcx.hir.node_to_hir_id(k);
 402         let map = rl.defs.entry(hir_id.owner_local_def_id()).or_default();
 403         Lrc::get_mut(map).unwrap().insert(hir_id.local_id, v);
 404     }
 405     for k in named_region_map.late_bound {
 406         let hir_id = tcx.hir.node_to_hir_id(k);
 407         let map = rl.late_bound
 408             .entry(hir_id.owner_local_def_id())
 409             .or_default();
 410         Lrc::get_mut(map).unwrap().insert(hir_id.local_id);
 411     }
 412     for (k, v) in named_region_map.object_lifetime_defaults {
 413         let hir_id = tcx.hir.node_to_hir_id(k);
 414         let map = rl.object_lifetime_defaults
 415             .entry(hir_id.owner_local_def_id())
 416             .or_default();
 417         Lrc::get_mut(map)
 418             .unwrap()
 419             .insert(hir_id.local_id, Lrc::new(v));
 420     }
 421
 422     Lrc::new(rl)
 423 }
 424
 425 fn krate<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) -> NamedRegionMap {
 426     let krate = tcx.hir.krate();
 427     let mut map = NamedRegionMap {
 428         defs: NodeMap(),
 429         late_bound: NodeSet(),
 430         object_lifetime_defaults: compute_object_lifetime_defaults(tcx),
 431     };
 432     {
 433         let mut visitor = LifetimeContext {
 434             tcx,
 435             map: &mut map,
 436             scope: ROOT_SCOPE,
 437             trait_ref_hack: false,
 438             is_in_fn_syntax: false,
 439             labels_in_fn: vec![],
 440             xcrate_object_lifetime_defaults: DefIdMap(),
 441             lifetime_uses: &mut DefIdMap(),
 442         };
 443         for (_, item) in &krate.items {
 444             visitor.visit_item(item);
 445         }
 446     }
 447     map
 448 }
 449
 450 impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 451     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 452         NestedVisitorMap::All(&self.tcx.hir)
 453     }
 454
 455     // We want to nest trait/impl items in their parent, but nothing else.
 456     fn visit_nested_item(&mut self, _: hir::ItemId) {}
 457
 458     fn visit_nested_body(&mut self, body: hir::BodyId) {
 459         // Each body has their own set of labels, save labels.
 460         let saved = replace(&mut self.labels_in_fn, vec![]);
 461         let body = self.tcx.hir.body(body);
 462         extract_labels(self, body);
 463         self.with(
 464             Scope::Body {
 465                 id: body.id(),
 466                 s: self.scope,
 467             },
 468             |_, this| {
 469                 this.visit_body(body);
 470             },
 471         );
 472         replace(&mut self.labels_in_fn, saved);
 473     }
 474
 475     fn visit_item(&mut self, item: &'tcx hir::Item) {
 476         match item.node {
 477             hir::ItemKind::Fn(ref decl, _, ref generics, _) => {
 478                 self.visit_early_late(None, decl, generics, |this| {
 479                     intravisit::walk_item(this, item);
 480                 });
 481             }
 482
 483             hir::ItemKind::ExternCrate(_)
 484             | hir::ItemKind::Use(..)
 485             | hir::ItemKind::Mod(..)
 486             | hir::ItemKind::ForeignMod(..)
 487             | hir::ItemKind::GlobalAsm(..) => {
 488                 // These sorts of items have no lifetime parameters at all.
 489                 intravisit::walk_item(self, item);
 490             }
 491             hir::ItemKind::Static(..) | hir::ItemKind::Const(..) => {
 492                 // No lifetime parameters, but implied 'static.
 493                 let scope = Scope::Elision {
 494                     elide: Elide::Exact(Region::Static),
 495                     s: ROOT_SCOPE,
 496                 };
 497                 self.with(scope, |_, this| intravisit::walk_item(this, item));
 498             }
 499             hir::ItemKind::Existential(hir::ExistTy {
 500                 impl_trait_fn: Some(_),
 501                 ..
 502             }) => {
 503                 // currently existential type declarations are just generated from impl Trait
 504                 // items. doing anything on this node is irrelevant, as we currently don't need
 505                 // it.
 506             }
 507             hir::ItemKind::Ty(_, ref generics)
 508             | hir::ItemKind::Existential(hir::ExistTy {
 509                 impl_trait_fn: None,
 510                 ref generics,
 511                 ..
 512             })
 513             | hir::ItemKind::Enum(_, ref generics)
 514             | hir::ItemKind::Struct(_, ref generics)
 515             | hir::ItemKind::Union(_, ref generics)
 516             | hir::ItemKind::Trait(_, _, ref generics, ..)
 517             | hir::ItemKind::TraitAlias(ref generics, ..)
 518             | hir::ItemKind::Impl(_, _, _, ref generics, ..) => {
 519                 // Impls permit `'_` to be used and it is equivalent to "some fresh lifetime name".
 520                 // This is not true for other kinds of items.x
 521                 let track_lifetime_uses = match item.node {
 522                     hir::ItemKind::Impl(..) => true,
 523                     _ => false,
 524                 };
 525                 // These kinds of items have only early bound lifetime parameters.
 526                 let mut index = if let hir::ItemKind::Trait(..) = item.node {
 527                     1 // Self comes before lifetimes
 528                 } else {
 529                     0
 530                 };
 531                 let mut type_count = 0;
 532                 let lifetimes = generics
 533                     .params
 534                     .iter()
 535                     .filter_map(|param| match param.kind {
 536                         GenericParamKind::Lifetime { .. } => {
 537                             Some(Region::early(&self.tcx.hir, &mut index, param))
 538                         }
 539                         GenericParamKind::Type { .. } => {
 540                             type_count += 1;
 541                             None
 542                         }
 543                     })
 544                     .collect();
 545                 let scope = Scope::Binder {
 546                     lifetimes,
 547                     next_early_index: index + type_count,
 548                     abstract_type_parent: true,
 549                     track_lifetime_uses,
 550                     s: ROOT_SCOPE,
 551                 };
 552                 self.with(scope, |old_scope, this| {
 553                     this.check_lifetime_params(old_scope, &generics.params);
 554                     intravisit::walk_item(this, item);
 555                 });
 556             }
 557         }
 558     }
 559
 560     fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem) {
 561         match item.node {
 562             hir::ForeignItemKind::Fn(ref decl, _, ref generics) => {
 563                 self.visit_early_late(None, decl, generics, |this| {
 564                     intravisit::walk_foreign_item(this, item);
 565                 })
 566             }
 567             hir::ForeignItemKind::Static(..) => {
 568                 intravisit::walk_foreign_item(self, item);
 569             }
 570             hir::ForeignItemKind::Type => {
 571                 intravisit::walk_foreign_item(self, item);
 572             }
 573         }
 574     }
 575
 576     fn visit_ty(&mut self, ty: &'tcx hir::Ty) {
 577         debug!("visit_ty: id={:?} ty={:?}", ty.id, ty);
 578         match ty.node {
 579             hir::TyKind::BareFn(ref c) => {
 580                 let next_early_index = self.next_early_index();
 581                 let was_in_fn_syntax = self.is_in_fn_syntax;
 582                 self.is_in_fn_syntax = true;
 583                 let scope = Scope::Binder {
 584                     lifetimes: c.generic_params
 585                         .iter()
 586                         .filter_map(|param| match param.kind {
 587                             GenericParamKind::Lifetime { .. } => {
 588                                 Some(Region::late(&self.tcx.hir, param))
 589                             }
 590                             _ => None,
 591                         })
 592                         .collect(),
 593                     s: self.scope,
 594                     next_early_index,
 595                     track_lifetime_uses: true,
 596                     abstract_type_parent: false,
 597                 };
 598                 self.with(scope, |old_scope, this| {
 599                     // a bare fn has no bounds, so everything
 600                     // contained within is scoped within its binder.
 601                     this.check_lifetime_params(old_scope, &c.generic_params);
 602                     intravisit::walk_ty(this, ty);
 603                 });
 604                 self.is_in_fn_syntax = was_in_fn_syntax;
 605             }
 606             hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
 607                 for bound in bounds {
 608                     self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
 609                 }
 610                 match lifetime.name {
 611                     LifetimeName::Implicit => {
 612                         // If the user does not write *anything*, we
 613                         // use the object lifetime defaulting
 614                         // rules. So e.g. `Box<dyn Debug>` becomes
 615                         // `Box<dyn Debug + 'static>`.
 616                         self.resolve_object_lifetime_default(lifetime)
 617                     }
 618                     LifetimeName::Underscore => {
 619                         // If the user writes `'_`, we use the *ordinary* elision
 620                         // rules. So the `'_` in e.g. `Box<dyn Debug + '_>` will be
 621                         // resolved the same as the `'_` in `&'_ Foo`.
 622                         //
 623                         // cc #48468
 624                         self.resolve_elided_lifetimes(vec![lifetime])
 625                     }
 626                     LifetimeName::Param(_) | LifetimeName::Static => {
 627                         // If the user wrote an explicit name, use that.
 628                         self.visit_lifetime(lifetime);
 629                     }
 630                     LifetimeName::Error => {}
 631                 }
 632             }
 633             hir::TyKind::Rptr(ref lifetime_ref, ref mt) => {
 634                 self.visit_lifetime(lifetime_ref);
 635                 let scope = Scope::ObjectLifetimeDefault {
 636                     lifetime: self.map.defs.get(&lifetime_ref.id).cloned(),
 637                     s: self.scope,
 638                 };
 639                 self.with(scope, |_, this| this.visit_ty(&mt.ty));
 640             }
 641             hir::TyKind::Def(item_id, ref lifetimes) => {
 642                 // Resolve the lifetimes in the bounds to the lifetime defs in the generics.
 643                 // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
 644                 // `abstract type MyAnonTy<'b>: MyTrait<'b>;`
 645                 //                          ^            ^ this gets resolved in the scope of
 646                 //                                         the exist_ty generics
 647                 let (generics, bounds) = match self.tcx.hir.expect_item(item_id.id).node {
 648                     // named existential types are reached via TyKind::Path
 649                     // this arm is for `impl Trait` in the types of statics, constants and locals
 650                     hir::ItemKind::Existential(hir::ExistTy {
 651                         impl_trait_fn: None,
 652                         ..
 653                     }) => {
 654                         intravisit::walk_ty(self, ty);
 655                         return;
 656                     }
 657                     // RPIT (return position impl trait)
 658                     hir::ItemKind::Existential(hir::ExistTy {
 659                         ref generics,
 660                         ref bounds,
 661                         ..
 662                     }) => (generics, bounds),
 663                     ref i => bug!("impl Trait pointed to non-existential type?? {:#?}", i),
 664                 };
 665
 666                 // Resolve the lifetimes that are applied to the existential type.
 667                 // These are resolved in the current scope.
 668                 // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
 669                 // `fn foo<'a>() -> MyAnonTy<'a> { ... }`
 670                 //          ^                 ^this gets resolved in the current scope
 671                 for lifetime in lifetimes {
 672                     if let hir::GenericArg::Lifetime(lifetime) = lifetime {
 673                         self.visit_lifetime(lifetime);
 674
 675                         // Check for predicates like `impl for<'a> Trait<impl OtherTrait<'a>>`
 676                         // and ban them. Type variables instantiated inside binders aren't
 677                         // well-supported at the moment, so this doesn't work.
 678                         // In the future, this should be fixed and this error should be removed.
 679                         let def = self.map.defs.get(&lifetime.id).cloned();
 680                         if let Some(Region::LateBound(_, def_id, _)) = def {
 681                             if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {
 682                                 // Ensure that the parent of the def is an item, not HRTB
 683                                 let parent_id = self.tcx.hir.get_parent_node(node_id);
 684                                 let parent_impl_id = hir::ImplItemId { node_id: parent_id };
 685                                 let parent_trait_id = hir::TraitItemId { node_id: parent_id };
 686                                 let krate = self.tcx.hir.forest.krate();
 687                                 if !(krate.items.contains_key(&parent_id)
 688                                     || krate.impl_items.contains_key(&parent_impl_id)
 689                                     || krate.trait_items.contains_key(&parent_trait_id))
 690                                 {
 691                                     span_err!(
 692                                         self.tcx.sess,
 693                                         lifetime.span,
 694                                         E0657,
 695                                         "`impl Trait` can only capture lifetimes \
 696                                          bound at the fn or impl level"
 697                                     );
 698                                     self.uninsert_lifetime_on_error(lifetime, def.unwrap());
 699                                 }
 700                             }
 701                         }
 702                     }
 703                 }
 704
 705                 // We want to start our early-bound indices at the end of the parent scope,
 706                 // not including any parent `impl Trait`s.
 707                 let mut index = self.next_early_index_for_abstract_type();
 708                 debug!("visit_ty: index = {}", index);
 709
 710                 let mut elision = None;
 711                 let mut lifetimes = FxHashMap::default();
 712                 let mut type_count = 0;
 713                 for param in &generics.params {
 714                     match param.kind {
 715                         GenericParamKind::Lifetime { .. } => {
 716                             let (name, reg) = Region::early(&self.tcx.hir, &mut index, &param);
 717                             if let hir::ParamName::Plain(param_name) = name {
 718                                 if param_name.name == keywords::UnderscoreLifetime.name() {
 719                                     // Pick the elided lifetime "definition" if one exists
 720                                     // and use it to make an elision scope.
 721                                     elision = Some(reg);
 722                                 } else {
 723                                     lifetimes.insert(name, reg);
 724                                 }
 725                             } else {
 726                                 lifetimes.insert(name, reg);
 727                             }
 728                         }
 729                         GenericParamKind::Type { .. } => {
 730                             type_count += 1;
 731                         }
 732                     }
 733                 }
 734                 let next_early_index = index + type_count;
 735
 736                 if let Some(elision_region) = elision {
 737                     let scope = Scope::Elision {
 738                         elide: Elide::Exact(elision_region),
 739                         s: self.scope,
 740                     };
 741                     self.with(scope, |_old_scope, this| {
 742                         let scope = Scope::Binder {
 743                             lifetimes,
 744                             next_early_index,
 745                             s: this.scope,
 746                             track_lifetime_uses: true,
 747                             abstract_type_parent: false,
 748                         };
 749                         this.with(scope, |_old_scope, this| {
 750                             this.visit_generics(generics);
 751                             for bound in bounds {
 752                                 this.visit_param_bound(bound);
 753                             }
 754                         });
 755                     });
 756                 } else {
 757                     let scope = Scope::Binder {
 758                         lifetimes,
 759                         next_early_index,
 760                         s: self.scope,
 761                         track_lifetime_uses: true,
 762                         abstract_type_parent: false,
 763                     };
 764                     self.with(scope, |_old_scope, this| {
 765                         this.visit_generics(generics);
 766                         for bound in bounds {
 767                             this.visit_param_bound(bound);
 768                         }
 769                     });
 770                 }
 771             }
 772             _ => intravisit::walk_ty(self, ty),
 773         }
 774     }
 775
 776     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
 777         use self::hir::TraitItemKind::*;
 778         match trait_item.node {
 779             Method(ref sig, _) => {
 780                 let tcx = self.tcx;
 781                 self.visit_early_late(
 782                     Some(tcx.hir.get_parent(trait_item.id)),
 783                     &sig.decl,
 784                     &trait_item.generics,
 785                     |this| intravisit::walk_trait_item(this, trait_item),
 786                 );
 787             }
 788             Type(ref bounds, ref ty) => {
 789                 let generics = &trait_item.generics;
 790                 let mut index = self.next_early_index();
 791                 debug!("visit_ty: index = {}", index);
 792                 let mut type_count = 0;
 793                 let lifetimes = generics
 794                     .params
 795                     .iter()
 796                     .filter_map(|param| match param.kind {
 797                         GenericParamKind::Lifetime { .. } => {
 798                             Some(Region::early(&self.tcx.hir, &mut index, param))
 799                         }
 800                         GenericParamKind::Type { .. } => {
 801                             type_count += 1;
 802                             None
 803                         }
 804                     })
 805                     .collect();
 806                 let scope = Scope::Binder {
 807                     lifetimes,
 808                     next_early_index: index + type_count,
 809                     s: self.scope,
 810                     track_lifetime_uses: true,
 811                     abstract_type_parent: true,
 812                 };
 813                 self.with(scope, |_old_scope, this| {
 814                     this.visit_generics(generics);
 815                     for bound in bounds {
 816                         this.visit_param_bound(bound);
 817                     }
 818                     if let Some(ty) = ty {
 819                         this.visit_ty(ty);
 820                     }
 821                 });
 822             }
 823             Const(_, _) => {
 824                 // Only methods and types support generics.
 825                 assert!(trait_item.generics.params.is_empty());
 826                 intravisit::walk_trait_item(self, trait_item);
 827             }
 828         }
 829     }
 830
 831     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
 832         use self::hir::ImplItemKind::*;
 833         match impl_item.node {
 834             Method(ref sig, _) => {
 835                 let tcx = self.tcx;
 836                 self.visit_early_late(
 837                     Some(tcx.hir.get_parent(impl_item.id)),
 838                     &sig.decl,
 839                     &impl_item.generics,
 840                     |this| intravisit::walk_impl_item(this, impl_item),
 841                 )
 842             }
 843             Type(ref ty) => {
 844                 let generics = &impl_item.generics;
 845                 let mut index = self.next_early_index();
 846                 let mut next_early_index = index;
 847                 debug!("visit_ty: index = {}", index);
 848                 let lifetimes = generics
 849                     .params
 850                     .iter()
 851                     .filter_map(|param| match param.kind {
 852                         GenericParamKind::Lifetime { .. } => {
 853                             Some(Region::early(&self.tcx.hir, &mut index, param))
 854                         }
 855                         GenericParamKind::Type { .. } => {
 856                             next_early_index += 1;
 857                             None
 858                         }
 859                     })
 860                     .collect();
 861                 let scope = Scope::Binder {
 862                     lifetimes,
 863                     next_early_index,
 864                     s: self.scope,
 865                     track_lifetime_uses: true,
 866                     abstract_type_parent: true,
 867                 };
 868                 self.with(scope, |_old_scope, this| {
 869                     this.visit_generics(generics);
 870                     this.visit_ty(ty);
 871                 });
 872             }
 873             Existential(ref bounds) => {
 874                 let generics = &impl_item.generics;
 875                 let mut index = self.next_early_index();
 876                 let mut next_early_index = index;
 877                 debug!("visit_ty: index = {}", index);
 878                 let lifetimes = generics
 879                     .params
 880                     .iter()
 881                     .filter_map(|param| match param.kind {
 882                         GenericParamKind::Lifetime { .. } => {
 883                             Some(Region::early(&self.tcx.hir, &mut index, param))
 884                         }
 885                         GenericParamKind::Type { .. } => {
 886                             next_early_index += 1;
 887                             None
 888                         }
 889                     })
 890                     .collect();
 891
 892                 let scope = Scope::Binder {
 893                     lifetimes,
 894                     next_early_index,
 895                     s: self.scope,
 896                     track_lifetime_uses: true,
 897                     abstract_type_parent: true,
 898                 };
 899                 self.with(scope, |_old_scope, this| {
 900                     this.visit_generics(generics);
 901                     for bound in bounds {
 902                         this.visit_param_bound(bound);
 903                     }
 904                 });
 905             }
 906             Const(_, _) => {
 907                 // Only methods and types support generics.
 908                 assert!(impl_item.generics.params.is_empty());
 909                 intravisit::walk_impl_item(self, impl_item);
 910             }
 911         }
 912     }
 913
 914     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
 915         if lifetime_ref.is_elided() {
 916             self.resolve_elided_lifetimes(vec![lifetime_ref]);
 917             return;
 918         }
 919         if lifetime_ref.is_static() {
 920             self.insert_lifetime(lifetime_ref, Region::Static);
 921             return;
 922         }
 923         self.resolve_lifetime_ref(lifetime_ref);
 924     }
 925
 926     fn visit_path(&mut self, path: &'tcx hir::Path, _: hir::HirId) {
 927         for (i, segment) in path.segments.iter().enumerate() {
 928             let depth = path.segments.len() - i - 1;
 929             if let Some(ref args) = segment.args {
 930                 self.visit_segment_args(path.def, depth, args);
 931             }
 932         }
 933     }
 934
 935     fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl) {
 936         let output = match fd.output {
 937             hir::DefaultReturn(_) => None,
 938             hir::Return(ref ty) => Some(ty),
 939         };
 940         self.visit_fn_like_elision(&fd.inputs, output);
 941     }
 942
 943     fn visit_generics(&mut self, generics: &'tcx hir::Generics) {
 944         check_mixed_explicit_and_in_band_defs(self.tcx, &generics.params);
 945         for param in &generics.params {
 946             match param.kind {
 947                 GenericParamKind::Lifetime { .. } => {}
 948                 GenericParamKind::Type { ref default, .. } => {
 949                     walk_list!(self, visit_param_bound, &param.bounds);
 950                     if let Some(ref ty) = default {
 951                         self.visit_ty(&ty);
 952                     }
 953                 }
 954             }
 955         }
 956         for predicate in &generics.where_clause.predicates {
 957             match predicate {
 958                 &hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
 959                     ref bounded_ty,
 960                     ref bounds,
 961                     ref bound_generic_params,
 962                     ..
 963                 }) => {
 964                     let lifetimes: FxHashMap<_, _> = bound_generic_params
 965                         .iter()
 966                         .filter_map(|param| match param.kind {
 967                             GenericParamKind::Lifetime { .. } => {
 968                                 Some(Region::late(&self.tcx.hir, param))
 969                             }
 970                             _ => None,
 971                         })
 972                         .collect();
 973                     if !lifetimes.is_empty() {
 974                         self.trait_ref_hack = true;
 975                         let next_early_index = self.next_early_index();
 976                         let scope = Scope::Binder {
 977                             lifetimes,
 978                             s: self.scope,
 979                             next_early_index,
 980                             track_lifetime_uses: true,
 981                             abstract_type_parent: false,
 982                         };
 983                         let result = self.with(scope, |old_scope, this| {
 984                             this.check_lifetime_params(old_scope, &bound_generic_params);
 985                             this.visit_ty(&bounded_ty);
 986                             walk_list!(this, visit_param_bound, bounds);
 987                         });
 988                         self.trait_ref_hack = false;
 989                         result
 990                     } else {
 991                         self.visit_ty(&bounded_ty);
 992                         walk_list!(self, visit_param_bound, bounds);
 993                     }
 994                 }
 995                 &hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
 996                     ref lifetime,
 997                     ref bounds,
 998                     ..
 999                 }) => {
1000                     self.visit_lifetime(lifetime);
1001                     walk_list!(self, visit_param_bound, bounds);
1002                 }
1003                 &hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
1004                     ref lhs_ty,
1005                     ref rhs_ty,
1006                     ..
1007                 }) => {
1008                     self.visit_ty(lhs_ty);
1009                     self.visit_ty(rhs_ty);
1010                 }
1011             }
1012         }
1013     }
1014
1015     fn visit_poly_trait_ref(
1016         &mut self,
1017         trait_ref: &'tcx hir::PolyTraitRef,
1018         _modifier: hir::TraitBoundModifier,
1019     ) {
1020         debug!("visit_poly_trait_ref trait_ref={:?}", trait_ref);
1021
1022         if !self.trait_ref_hack || trait_ref.bound_generic_params.iter().any(|param| {
1023             match param.kind {
1024                 GenericParamKind::Lifetime { .. } => true,
1025                 _ => false,
1026             }
1027         }) {
1028             if self.trait_ref_hack {
1029                 span_err!(
1030                     self.tcx.sess,
1031                     trait_ref.span,
1032                     E0316,
1033                     "nested quantification of lifetimes"
1034                 );
1035             }
1036             let next_early_index = self.next_early_index();
1037             let scope = Scope::Binder {
1038                 lifetimes: trait_ref
1039                     .bound_generic_params
1040                     .iter()
1041                     .filter_map(|param| match param.kind {
1042                         GenericParamKind::Lifetime { .. } => {
1043                             Some(Region::late(&self.tcx.hir, param))
1044                         }
1045                         _ => None,
1046                     })
1047                     .collect(),
1048                 s: self.scope,
1049                 next_early_index,
1050                 track_lifetime_uses: true,
1051                 abstract_type_parent: false,
1052             };
1053             self.with(scope, |old_scope, this| {
1054                 this.check_lifetime_params(old_scope, &trait_ref.bound_generic_params);
1055                 walk_list!(this, visit_generic_param, &trait_ref.bound_generic_params);
1056                 this.visit_trait_ref(&trait_ref.trait_ref)
1057             })
1058         } else {
1059             self.visit_trait_ref(&trait_ref.trait_ref)
1060         }
1061     }
1062 }
1063
1064 #[derive(Copy, Clone, PartialEq)]
1065 enum ShadowKind {
1066     Label,
1067     Lifetime,
1068 }
1069 struct Original {
1070     kind: ShadowKind,
1071     span: Span,
1072 }
1073 struct Shadower {
1074     kind: ShadowKind,
1075     span: Span,
1076 }
1077
1078 fn original_label(span: Span) -> Original {
1079     Original {
1080         kind: ShadowKind::Label,
1081         span: span,
1082     }
1083 }
1084 fn shadower_label(span: Span) -> Shadower {
1085     Shadower {
1086         kind: ShadowKind::Label,
1087         span: span,
1088     }
1089 }
1090 fn original_lifetime(span: Span) -> Original {
1091     Original {
1092         kind: ShadowKind::Lifetime,
1093         span: span,
1094     }
1095 }
1096 fn shadower_lifetime(param: &hir::GenericParam) -> Shadower {
1097     Shadower {
1098         kind: ShadowKind::Lifetime,
1099         span: param.span,
1100     }
1101 }
1102
1103 impl ShadowKind {
1104     fn desc(&self) -> &'static str {
1105         match *self {
1106             ShadowKind::Label => "label",
1107             ShadowKind::Lifetime => "lifetime",
1108         }
1109     }
1110 }
1111
1112 fn check_mixed_explicit_and_in_band_defs(tcx: TyCtxt<'_, '_, '_>, params: &P<[hir::GenericParam]>) {
1113     let lifetime_params: Vec<_> = params
1114         .iter()
1115         .filter_map(|param| match param.kind {
1116             GenericParamKind::Lifetime { kind, .. } => Some((kind, param.span)),
1117             _ => None,
1118         })
1119         .collect();
1120     let explicit = lifetime_params
1121         .iter()
1122         .find(|(kind, _)| *kind == LifetimeParamKind::Explicit);
1123     let in_band = lifetime_params
1124         .iter()
1125         .find(|(kind, _)| *kind == LifetimeParamKind::InBand);
1126
1127     if let (Some((_, explicit_span)), Some((_, in_band_span))) = (explicit, in_band) {
1128         struct_span_err!(
1129             tcx.sess,
1130             *in_band_span,
1131             E0688,
1132             "cannot mix in-band and explicit lifetime definitions"
1133         ).span_label(*in_band_span, "in-band lifetime definition here")
1134             .span_label(*explicit_span, "explicit lifetime definition here")
1135             .emit();
1136     }
1137 }
1138
1139 fn signal_shadowing_problem(
1140     tcx: TyCtxt<'_, '_, '_>,
1141     name: ast::Name,
1142     orig: Original,
1143     shadower: Shadower,
1144 ) {
1145     let mut err = if let (ShadowKind::Lifetime, ShadowKind::Lifetime) = (orig.kind, shadower.kind) {
1146         // lifetime/lifetime shadowing is an error
1147         struct_span_err!(
1148             tcx.sess,
1149             shadower.span,
1150             E0496,
1151             "{} name `{}` shadows a \
1152              {} name that is already in scope",
1153             shadower.kind.desc(),
1154             name,
1155             orig.kind.desc()
1156         )
1157     } else {
1158         // shadowing involving a label is only a warning, due to issues with
1159         // labels and lifetimes not being macro-hygienic.
1160         tcx.sess.struct_span_warn(
1161             shadower.span,
1162             &format!(
1163                 "{} name `{}` shadows a \
1164                  {} name that is already in scope",
1165                 shadower.kind.desc(),
1166                 name,
1167                 orig.kind.desc()
1168             ),
1169         )
1170     };
1171     err.span_label(orig.span, "first declared here");
1172     err.span_label(shadower.span, format!("lifetime {} already in scope", name));
1173     err.emit();
1174 }
1175
1176 // Adds all labels in `b` to `ctxt.labels_in_fn`, signalling a warning
1177 // if one of the label shadows a lifetime or another label.
1178 fn extract_labels(ctxt: &mut LifetimeContext<'_, '_>, body: &hir::Body) {
1179     struct GatherLabels<'a, 'tcx: 'a> {
1180         tcx: TyCtxt<'a, 'tcx, 'tcx>,
1181         scope: ScopeRef<'a>,
1182         labels_in_fn: &'a mut Vec<ast::Ident>,
1183     }
1184
1185     let mut gather = GatherLabels {
1186         tcx: ctxt.tcx,
1187         scope: ctxt.scope,
1188         labels_in_fn: &mut ctxt.labels_in_fn,
1189     };
1190     gather.visit_body(body);
1191
1192     impl<'v, 'a, 'tcx> Visitor<'v> for GatherLabels<'a, 'tcx> {
1193         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
1194             NestedVisitorMap::None
1195         }
1196
1197         fn visit_expr(&mut self, ex: &hir::Expr) {
1198             if let Some(label) = expression_label(ex) {
1199                 for prior_label in &self.labels_in_fn[..] {
1200                     // FIXME (#24278): non-hygienic comparison
1201                     if label.name == prior_label.name {
1202                         signal_shadowing_problem(
1203                             self.tcx,
1204                             label.name,
1205                             original_label(prior_label.span),
1206                             shadower_label(label.span),
1207                         );
1208                     }
1209                 }
1210
1211                 check_if_label_shadows_lifetime(self.tcx, self.scope, label);
1212
1213                 self.labels_in_fn.push(label);
1214             }
1215             intravisit::walk_expr(self, ex)
1216         }
1217     }
1218
1219     fn expression_label(ex: &hir::Expr) -> Option<ast::Ident> {
1220         match ex.node {
1221             hir::ExprKind::While(.., Some(label)) | hir::ExprKind::Loop(_, Some(label), _) => {
1222                 Some(label.ident)
1223             }
1224             _ => None,
1225         }
1226     }
1227
1228     fn check_if_label_shadows_lifetime(
1229         tcx: TyCtxt<'_, '_, '_>,
1230         mut scope: ScopeRef<'_>,
1231         label: ast::Ident,
1232     ) {
1233         loop {
1234             match *scope {
1235                 Scope::Body { s, .. }
1236                 | Scope::Elision { s, .. }
1237                 | Scope::ObjectLifetimeDefault { s, .. } => {
1238                     scope = s;
1239                 }
1240
1241                 Scope::Root => {
1242                     return;
1243                 }
1244
1245                 Scope::Binder {
1246                     ref lifetimes, s, ..
1247                 } => {
1248                     // FIXME (#24278): non-hygienic comparison
1249                     if let Some(def) = lifetimes.get(&hir::ParamName::Plain(label.modern())) {
1250                         let node_id = tcx.hir.as_local_node_id(def.id().unwrap()).unwrap();
1251
1252                         signal_shadowing_problem(
1253                             tcx,
1254                             label.name,
1255                             original_lifetime(tcx.hir.span(node_id)),
1256                             shadower_label(label.span),
1257                         );
1258                         return;
1259                     }
1260                     scope = s;
1261                 }
1262             }
1263         }
1264     }
1265 }
1266
1267 fn compute_object_lifetime_defaults(
1268     tcx: TyCtxt<'_, '_, '_>,
1269 ) -> NodeMap<Vec<ObjectLifetimeDefault>> {
1270     let mut map = NodeMap();
1271     for item in tcx.hir.krate().items.values() {
1272         match item.node {
1273             hir::ItemKind::Struct(_, ref generics)
1274             | hir::ItemKind::Union(_, ref generics)
1275             | hir::ItemKind::Enum(_, ref generics)
1276             | hir::ItemKind::Existential(hir::ExistTy {
1277                 ref generics,
1278                 impl_trait_fn: None,
1279                 ..
1280             })
1281             | hir::ItemKind::Ty(_, ref generics)
1282             | hir::ItemKind::Trait(_, _, ref generics, ..) => {
1283                 let result = object_lifetime_defaults_for_item(tcx, generics);
1284
1285                 // Debugging aid.
1286                 if attr::contains_name(&item.attrs, "rustc_object_lifetime_default") {
1287                     let object_lifetime_default_reprs: String = result
1288                         .iter()
1289                         .map(|set| match *set {
1290                             Set1::Empty => "BaseDefault".into(),
1291                             Set1::One(Region::Static) => "'static".into(),
1292                             Set1::One(Region::EarlyBound(mut i, _, _)) => generics
1293                                 .params
1294                                 .iter()
1295                                 .find_map(|param| match param.kind {
1296                                     GenericParamKind::Lifetime { .. } => {
1297                                         if i == 0 {
1298                                             return Some(param.name.ident().to_string().into());
1299                                         }
1300                                         i -= 1;
1301                                         None
1302                                     }
1303                                     _ => None,
1304                                 })
1305                                 .unwrap(),
1306                             Set1::One(_) => bug!(),
1307                             Set1::Many => "Ambiguous".into(),
1308                         })
1309                         .collect::<Vec<Cow<'static, str>>>()
1310                         .join(",");
1311                     tcx.sess.span_err(item.span, &object_lifetime_default_reprs);
1312                 }
1313
1314                 map.insert(item.id, result);
1315             }
1316             _ => {}
1317         }
1318     }
1319     map
1320 }
1321
1322 /// Scan the bounds and where-clauses on parameters to extract bounds
1323 /// of the form `T:'a` so as to determine the `ObjectLifetimeDefault`
1324 /// for each type parameter.
1325 fn object_lifetime_defaults_for_item(
1326     tcx: TyCtxt<'_, '_, '_>,
1327     generics: &hir::Generics,
1328 ) -> Vec<ObjectLifetimeDefault> {
1329     fn add_bounds(set: &mut Set1<hir::LifetimeName>, bounds: &[hir::GenericBound]) {
1330         for bound in bounds {
1331             if let hir::GenericBound::Outlives(ref lifetime) = *bound {
1332                 set.insert(lifetime.name.modern());
1333             }
1334         }
1335     }
1336
1337     generics
1338         .params
1339         .iter()
1340         .filter_map(|param| match param.kind {
1341             GenericParamKind::Lifetime { .. } => None,
1342             GenericParamKind::Type { .. } => {
1343                 let mut set = Set1::Empty;
1344
1345                 add_bounds(&mut set, &param.bounds);
1346
1347                 let param_def_id = tcx.hir.local_def_id(param.id);
1348                 for predicate in &generics.where_clause.predicates {
1349                     // Look for `type: ...` where clauses.
1350                     let data = match *predicate {
1351                         hir::WherePredicate::BoundPredicate(ref data) => data,
1352                         _ => continue,
1353                     };
1354
1355                     // Ignore `for<'a> type: ...` as they can change what
1356                     // lifetimes mean (although we could "just" handle it).
1357                     if !data.bound_generic_params.is_empty() {
1358                         continue;
1359                     }
1360
1361                     let def = match data.bounded_ty.node {
1362                         hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => path.def,
1363                         _ => continue,
1364                     };
1365
1366                     if def == Def::TyParam(param_def_id) {
1367                         add_bounds(&mut set, &data.bounds);
1368                     }
1369                 }
1370
1371                 Some(match set {
1372                     Set1::Empty => Set1::Empty,
1373                     Set1::One(name) => {
1374                         if name == hir::LifetimeName::Static {
1375                             Set1::One(Region::Static)
1376                         } else {
1377                             generics
1378                                 .params
1379                                 .iter()
1380                                 .filter_map(|param| match param.kind {
1381                                     GenericParamKind::Lifetime { .. } => Some((
1382                                         param.id,
1383                                         hir::LifetimeName::Param(param.name),
1384                                         LifetimeDefOrigin::from_param(param),
1385                                     )),
1386                                     _ => None,
1387                                 })
1388                                 .enumerate()
1389                                 .find(|&(_, (_, lt_name, _))| lt_name == name)
1390                                 .map_or(Set1::Many, |(i, (id, _, origin))| {
1391                                     let def_id = tcx.hir.local_def_id(id);
1392                                     Set1::One(Region::EarlyBound(i as u32, def_id, origin))
1393                                 })
1394                         }
1395                     }
1396                     Set1::Many => Set1::Many,
1397                 })
1398             }
1399         })
1400         .collect()
1401 }
1402
1403 impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
1404     // FIXME(#37666) this works around a limitation in the region inferencer
1405     fn hack<F>(&mut self, f: F)
1406     where
1407         F: for<'b> FnOnce(&mut LifetimeContext<'b, 'tcx>),
1408     {
1409         f(self)
1410     }
1411
1412     fn with<F>(&mut self, wrap_scope: Scope<'_>, f: F)
1413     where
1414         F: for<'b> FnOnce(ScopeRef<'_>, &mut LifetimeContext<'b, 'tcx>),
1415     {
1416         let LifetimeContext {
1417             tcx,
1418             map,
1419             lifetime_uses,
1420             ..
1421         } = self;
1422         let labels_in_fn = replace(&mut self.labels_in_fn, vec![]);
1423         let xcrate_object_lifetime_defaults =
1424             replace(&mut self.xcrate_object_lifetime_defaults, DefIdMap());
1425         let mut this = LifetimeContext {
1426             tcx: *tcx,
1427             map: map,
1428             scope: &wrap_scope,
1429             trait_ref_hack: self.trait_ref_hack,
1430             is_in_fn_syntax: self.is_in_fn_syntax,
1431             labels_in_fn,
1432             xcrate_object_lifetime_defaults,
1433             lifetime_uses: lifetime_uses,
1434         };
1435         debug!("entering scope {:?}", this.scope);
1436         f(self.scope, &mut this);
1437         this.check_uses_for_lifetimes_defined_by_scope();
1438         debug!("exiting scope {:?}", this.scope);
1439         self.labels_in_fn = this.labels_in_fn;
1440         self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;
1441     }
1442
1443     /// helper method to determine the span to remove when suggesting the
1444     /// deletion of a lifetime
1445     fn lifetime_deletion_span(&self, name: ast::Ident, generics: &hir::Generics) -> Option<Span> {
1446         if generics.params.len() == 1 {
1447             // if sole lifetime, remove the `<>` brackets
1448             Some(generics.span)
1449         } else {
1450             generics.params.iter().enumerate().find_map(|(i, param)| {
1451                 if param.name.ident() == name {
1452                     // We also want to delete a leading or trailing comma
1453                     // as appropriate
1454                     if i >= generics.params.len() - 1 {
1455                         Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
1456                     } else {
1457                         Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
1458                     }
1459                 } else {
1460                     None
1461                 }
1462             })
1463         }
1464     }
1465
1466     fn check_uses_for_lifetimes_defined_by_scope(&mut self) {
1467         let defined_by = match self.scope {
1468             Scope::Binder { lifetimes, .. } => lifetimes,
1469             _ => {
1470                 debug!("check_uses_for_lifetimes_defined_by_scope: not in a binder scope");
1471                 return;
1472             }
1473         };
1474
1475         let mut def_ids: Vec<_> = defined_by
1476             .values()
1477             .flat_map(|region| match region {
1478                 Region::EarlyBound(_, def_id, _)
1479                 | Region::LateBound(_, def_id, _)
1480                 | Region::Free(_, def_id) => Some(*def_id),
1481
1482                 Region::LateBoundAnon(..) | Region::Static => None,
1483             })
1484             .collect();
1485
1486         // ensure that we issue lints in a repeatable order
1487         def_ids.sort_by_key(|&def_id| self.tcx.def_path_hash(def_id));
1488
1489         for def_id in def_ids {
1490             debug!(
1491                 "check_uses_for_lifetimes_defined_by_scope: def_id = {:?}",
1492                 def_id
1493             );
1494
1495             let lifetimeuseset = self.lifetime_uses.remove(&def_id);
1496
1497             debug!(
1498                 "check_uses_for_lifetimes_defined_by_scope: lifetimeuseset = {:?}",
1499                 lifetimeuseset
1500             );
1501
1502             match lifetimeuseset {
1503                 Some(LifetimeUseSet::One(lifetime)) => {
1504                     let node_id = self.tcx.hir.as_local_node_id(def_id).unwrap();
1505                     debug!("node id first={:?}", node_id);
1506                     if let Some((id, span, name)) = match self.tcx.hir.get(node_id) {
1507                         Node::Lifetime(hir_lifetime) => Some((
1508                             hir_lifetime.id,
1509                             hir_lifetime.span,
1510                             hir_lifetime.name.ident(),
1511                         )),
1512                         Node::GenericParam(param) => {
1513                             Some((param.id, param.span, param.name.ident()))
1514                         }
1515                         _ => None,
1516                     } {
1517                         debug!("id = {:?} span = {:?} name = {:?}", node_id, span, name);
1518                         let mut err = self.tcx.struct_span_lint_node(
1519                             lint::builtin::SINGLE_USE_LIFETIMES,
1520                             id,
1521                             span,
1522                             &format!("lifetime parameter `{}` only used once", name),
1523                         );
1524                         err.span_label(span, "this lifetime...");
1525                         err.span_label(lifetime.span, "...is used only here");
1526                         err.emit();
1527                     }
1528                 }
1529                 Some(LifetimeUseSet::Many) => {
1530                     debug!("Not one use lifetime");
1531                 }
1532                 None => {
1533                     let node_id = self.tcx.hir.as_local_node_id(def_id).unwrap();
1534                     if let Some((id, span, name)) = match self.tcx.hir.get(node_id) {
1535                         Node::Lifetime(hir_lifetime) => Some((
1536                             hir_lifetime.id,
1537                             hir_lifetime.span,
1538                             hir_lifetime.name.ident(),
1539                         )),
1540                         Node::GenericParam(param) => {
1541                             Some((param.id, param.span, param.name.ident()))
1542                         }
1543                         _ => None,
1544                     } {
1545                         debug!("id ={:?} span = {:?} name = {:?}", node_id, span, name);
1546                         let mut err = self.tcx.struct_span_lint_node(
1547                             lint::builtin::UNUSED_LIFETIMES,
1548                             id,
1549                             span,
1550                             &format!("lifetime parameter `{}` never used", name),
1551                         );
1552                         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1553                             if let Some(generics) = self.tcx.hir.get_generics(parent_def_id) {
1554                                 let unused_lt_span = self.lifetime_deletion_span(name, generics);
1555                                 if let Some(span) = unused_lt_span {
1556                                     err.span_suggestion_with_applicability(
1557                                         span,
1558                                         "remove it",
1559                                         String::new(),
1560                                         Applicability::MachineApplicable,
1561                                     );
1562                                 }
1563                             }
1564                         }
1565                         err.emit();
1566                     }
1567                 }
1568             }
1569         }
1570     }
1571
1572     /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
1573     ///
1574     /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
1575     /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
1576     /// within type bounds; those are early bound lifetimes, and the rest are late bound.
1577     ///
1578     /// For example:
1579     ///
1580     ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
1581     ///
1582     /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
1583     /// lifetimes may be interspersed together.
1584     ///
1585     /// If early bound lifetimes are present, we separate them into their own list (and likewise
1586     /// for late bound). They will be numbered sequentially, starting from the lowest index that is
1587     /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
1588     /// bound lifetimes are resolved by name and associated with a binder id (`binder_id`), so the
1589     /// ordering is not important there.
1590     fn visit_early_late<F>(
1591         &mut self,
1592         parent_id: Option<ast::NodeId>,
1593         decl: &'tcx hir::FnDecl,
1594         generics: &'tcx hir::Generics,
1595         walk: F,
1596     ) where
1597         F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
1598     {
1599         insert_late_bound_lifetimes(self.map, decl, generics);
1600
1601         // Find the start of nested early scopes, e.g. in methods.
1602         let mut index = 0;
1603         if let Some(parent_id) = parent_id {
1604             let parent = self.tcx.hir.expect_item(parent_id);
1605             if let hir::ItemKind::Trait(..) = parent.node {
1606                 index += 1; // Self comes first.
1607             }
1608             match parent.node {
1609                 hir::ItemKind::Trait(_, _, ref generics, ..)
1610                 | hir::ItemKind::Impl(_, _, _, ref generics, ..) => {
1611                     index += generics.params.len() as u32;
1612                 }
1613                 _ => {}
1614             }
1615         }
1616
1617         let mut type_count = 0;
1618         let lifetimes = generics
1619             .params
1620             .iter()
1621             .filter_map(|param| match param.kind {
1622                 GenericParamKind::Lifetime { .. } => {
1623                     if self.map.late_bound.contains(&param.id) {
1624                         Some(Region::late(&self.tcx.hir, param))
1625                     } else {
1626                         Some(Region::early(&self.tcx.hir, &mut index, param))
1627                     }
1628                 }
1629                 GenericParamKind::Type { .. } => {
1630                     type_count += 1;
1631                     None
1632                 }
1633             })
1634             .collect();
1635         let next_early_index = index + type_count;
1636
1637         let scope = Scope::Binder {
1638             lifetimes,
1639             next_early_index,
1640             s: self.scope,
1641             abstract_type_parent: true,
1642             track_lifetime_uses: false,
1643         };
1644         self.with(scope, move |old_scope, this| {
1645             this.check_lifetime_params(old_scope, &generics.params);
1646             this.hack(walk); // FIXME(#37666) workaround in place of `walk(this)`
1647         });
1648     }
1649
1650     fn next_early_index_helper(&self, only_abstract_type_parent: bool) -> u32 {
1651         let mut scope = self.scope;
1652         loop {
1653             match *scope {
1654                 Scope::Root => return 0,
1655
1656                 Scope::Binder {
1657                     next_early_index,
1658                     abstract_type_parent,
1659                     ..
1660                 } if (!only_abstract_type_parent || abstract_type_parent) =>
1661                 {
1662                     return next_early_index
1663                 }
1664
1665                 Scope::Binder { s, .. }
1666                 | Scope::Body { s, .. }
1667                 | Scope::Elision { s, .. }
1668                 | Scope::ObjectLifetimeDefault { s, .. } => scope = s,
1669             }
1670         }
1671     }
1672
1673     /// Returns the next index one would use for an early-bound-region
1674     /// if extending the current scope.
1675     fn next_early_index(&self) -> u32 {
1676         self.next_early_index_helper(true)
1677     }
1678
1679     /// Returns the next index one would use for an `impl Trait` that
1680     /// is being converted into an `abstract type`. This will be the
1681     /// next early index from the enclosing item, for the most
1682     /// part. See the `abstract_type_parent` field for more info.
1683     fn next_early_index_for_abstract_type(&self) -> u32 {
1684         self.next_early_index_helper(false)
1685     }
1686
1687     fn resolve_lifetime_ref(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
1688         debug!("resolve_lifetime_ref(lifetime_ref={:?})", lifetime_ref);
1689
1690         // If we've already reported an error, just ignore `lifetime_ref`.
1691         if let LifetimeName::Error = lifetime_ref.name {
1692             return;
1693         }
1694
1695         // Walk up the scope chain, tracking the number of fn scopes
1696         // that we pass through, until we find a lifetime with the
1697         // given name or we run out of scopes.
1698         // search.
1699         let mut late_depth = 0;
1700         let mut scope = self.scope;
1701         let mut outermost_body = None;
1702         let result = loop {
1703             match *scope {
1704                 Scope::Body { id, s } => {
1705                     outermost_body = Some(id);
1706                     scope = s;
1707                 }
1708
1709                 Scope::Root => {
1710                     break None;
1711                 }
1712
1713                 Scope::Binder {
1714                     ref lifetimes, s, ..
1715                 } => {
1716                     match lifetime_ref.name {
1717                         LifetimeName::Param(param_name) => {
1718                             if let Some(&def) = lifetimes.get(&param_name.modern()) {
1719                                 break Some(def.shifted(late_depth));
1720                             }
1721                         }
1722                         _ => bug!("expected LifetimeName::Param"),
1723                     }
1724
1725                     late_depth += 1;
1726                     scope = s;
1727                 }
1728
1729                 Scope::Elision { s, .. } | Scope::ObjectLifetimeDefault { s, .. } => {
1730                     scope = s;
1731                 }
1732             }
1733         };
1734
1735         if let Some(mut def) = result {
1736             if let Region::EarlyBound(..) = def {
1737                 // Do not free early-bound regions, only late-bound ones.
1738             } else if let Some(body_id) = outermost_body {
1739                 let fn_id = self.tcx.hir.body_owner(body_id);
1740                 match self.tcx.hir.get(fn_id) {
1741                     Node::Item(&hir::Item {
1742                         node: hir::ItemKind::Fn(..),
1743                         ..
1744                     })
1745                     | Node::TraitItem(&hir::TraitItem {
1746                         node: hir::TraitItemKind::Method(..),
1747                         ..
1748                     })
1749                     | Node::ImplItem(&hir::ImplItem {
1750                         node: hir::ImplItemKind::Method(..),
1751                         ..
1752                     }) => {
1753                         let scope = self.tcx.hir.local_def_id(fn_id);
1754                         def = Region::Free(scope, def.id().unwrap());
1755                     }
1756                     _ => {}
1757                 }
1758             }
1759
1760             // Check for fn-syntax conflicts with in-band lifetime definitions
1761             if self.is_in_fn_syntax {
1762                 match def {
1763                     Region::EarlyBound(_, _, LifetimeDefOrigin::InBand)
1764                     | Region::LateBound(_, _, LifetimeDefOrigin::InBand) => {
1765                         struct_span_err!(
1766                             self.tcx.sess,
1767                             lifetime_ref.span,
1768                             E0687,
1769                             "lifetimes used in `fn` or `Fn` syntax must be \
1770                              explicitly declared using `<...>` binders"
1771                         ).span_label(lifetime_ref.span, "in-band lifetime definition")
1772                             .emit();
1773                     }
1774
1775                     Region::Static
1776                     | Region::EarlyBound(_, _, LifetimeDefOrigin::ExplicitOrElided)
1777                     | Region::LateBound(_, _, LifetimeDefOrigin::ExplicitOrElided)
1778                     | Region::EarlyBound(_, _, LifetimeDefOrigin::Error)
1779                     | Region::LateBound(_, _, LifetimeDefOrigin::Error)
1780                     | Region::LateBoundAnon(..)
1781                     | Region::Free(..) => {}
1782                 }
1783             }
1784
1785             self.insert_lifetime(lifetime_ref, def);
1786         } else {
1787             struct_span_err!(
1788                 self.tcx.sess,
1789                 lifetime_ref.span,
1790                 E0261,
1791                 "use of undeclared lifetime name `{}`",
1792                 lifetime_ref
1793             ).span_label(lifetime_ref.span, "undeclared lifetime")
1794                 .emit();
1795         }
1796     }
1797
1798     fn visit_segment_args(&mut self, def: Def, depth: usize, generic_args: &'tcx hir::GenericArgs) {
1799         if generic_args.parenthesized {
1800             let was_in_fn_syntax = self.is_in_fn_syntax;
1801             self.is_in_fn_syntax = true;
1802             self.visit_fn_like_elision(generic_args.inputs(), Some(&generic_args.bindings[0].ty));
1803             self.is_in_fn_syntax = was_in_fn_syntax;
1804             return;
1805         }
1806
1807         let mut elide_lifetimes = true;
1808         let lifetimes = generic_args
1809             .args
1810             .iter()
1811             .filter_map(|arg| match arg {
1812                 hir::GenericArg::Lifetime(lt) => {
1813                     if !lt.is_elided() {
1814                         elide_lifetimes = false;
1815                     }
1816                     Some(lt)
1817                 }
1818                 _ => None,
1819             })
1820             .collect();
1821         if elide_lifetimes {
1822             self.resolve_elided_lifetimes(lifetimes);
1823         } else {
1824             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
1825         }
1826
1827         // Figure out if this is a type/trait segment,
1828         // which requires object lifetime defaults.
1829         let parent_def_id = |this: &mut Self, def_id: DefId| {
1830             let def_key = this.tcx.def_key(def_id);
1831             DefId {
1832                 krate: def_id.krate,
1833                 index: def_key.parent.expect("missing parent"),
1834             }
1835         };
1836         let type_def_id = match def {
1837             Def::AssociatedTy(def_id) if depth == 1 => Some(parent_def_id(self, def_id)),
1838             Def::Variant(def_id) if depth == 0 => Some(parent_def_id(self, def_id)),
1839             Def::Struct(def_id)
1840             | Def::Union(def_id)
1841             | Def::Enum(def_id)
1842             | Def::TyAlias(def_id)
1843             | Def::Trait(def_id) if depth == 0 =>
1844             {
1845                 Some(def_id)
1846             }
1847             _ => None,
1848         };
1849
1850         let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
1851             let in_body = {
1852                 let mut scope = self.scope;
1853                 loop {
1854                     match *scope {
1855                         Scope::Root => break false,
1856
1857                         Scope::Body { .. } => break true,
1858
1859                         Scope::Binder { s, .. }
1860                         | Scope::Elision { s, .. }
1861                         | Scope::ObjectLifetimeDefault { s, .. } => {
1862                             scope = s;
1863                         }
1864                     }
1865                 }
1866             };
1867
1868             let map = &self.map;
1869             let unsubst = if let Some(id) = self.tcx.hir.as_local_node_id(def_id) {
1870                 &map.object_lifetime_defaults[&id]
1871             } else {
1872                 let tcx = self.tcx;
1873                 self.xcrate_object_lifetime_defaults
1874                     .entry(def_id)
1875                     .or_insert_with(|| {
1876                         tcx.generics_of(def_id)
1877                             .params
1878                             .iter()
1879                             .filter_map(|param| match param.kind {
1880                                 GenericParamDefKind::Type {
1881                                     object_lifetime_default,
1882                                     ..
1883                                 } => Some(object_lifetime_default),
1884                                 GenericParamDefKind::Lifetime => None,
1885                             })
1886                             .collect()
1887                     })
1888             };
1889             unsubst
1890                 .iter()
1891                 .map(|set| match *set {
1892                     Set1::Empty => if in_body {
1893                         None
1894                     } else {
1895                         Some(Region::Static)
1896                     },
1897                     Set1::One(r) => {
1898                         let lifetimes = generic_args.args.iter().filter_map(|arg| match arg {
1899                             GenericArg::Lifetime(lt) => Some(lt),
1900                             _ => None,
1901                         });
1902                         r.subst(lifetimes, map)
1903                     }
1904                     Set1::Many => None,
1905                 })
1906                 .collect()
1907         });
1908
1909         let mut i = 0;
1910         for arg in &generic_args.args {
1911             match arg {
1912                 GenericArg::Lifetime(_) => {}
1913                 GenericArg::Type(ty) => {
1914                     if let Some(&lt) = object_lifetime_defaults.get(i) {
1915                         let scope = Scope::ObjectLifetimeDefault {
1916                             lifetime: lt,
1917                             s: self.scope,
1918                         };
1919                         self.with(scope, |_, this| this.visit_ty(ty));
1920                     } else {
1921                         self.visit_ty(ty);
1922                     }
1923                     i += 1;
1924                 }
1925             }
1926         }
1927
1928         for b in &generic_args.bindings {
1929             self.visit_assoc_type_binding(b);
1930         }
1931     }
1932
1933     fn visit_fn_like_elision(&mut self, inputs: &'tcx [hir::Ty], output: Option<&'tcx P<hir::Ty>>) {
1934         debug!("visit_fn_like_elision: enter");
1935         let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
1936         let arg_scope = Scope::Elision {
1937             elide: arg_elide.clone(),
1938             s: self.scope,
1939         };
1940         self.with(arg_scope, |_, this| {
1941             for input in inputs {
1942                 this.visit_ty(input);
1943             }
1944             match *this.scope {
1945                 Scope::Elision { ref elide, .. } => {
1946                     arg_elide = elide.clone();
1947                 }
1948                 _ => bug!(),
1949             }
1950         });
1951
1952         let output = match output {
1953             Some(ty) => ty,
1954             None => return,
1955         };
1956
1957         debug!("visit_fn_like_elision: determine output");
1958
1959         // Figure out if there's a body we can get argument names from,
1960         // and whether there's a `self` argument (treated specially).
1961         let mut assoc_item_kind = None;
1962         let mut impl_self = None;
1963         let parent = self.tcx.hir.get_parent_node(output.id);
1964         let body = match self.tcx.hir.get(parent) {
1965             // `fn` definitions and methods.
1966             Node::Item(&hir::Item {
1967                 node: hir::ItemKind::Fn(.., body),
1968                 ..
1969             }) => Some(body),
1970
1971             Node::TraitItem(&hir::TraitItem {
1972                 node: hir::TraitItemKind::Method(_, ref m),
1973                 ..
1974             }) => {
1975                 if let hir::ItemKind::Trait(.., ref trait_items) = self.tcx
1976                     .hir
1977                     .expect_item(self.tcx.hir.get_parent(parent))
1978                     .node
1979                 {
1980                     assoc_item_kind = trait_items
1981                         .iter()
1982                         .find(|ti| ti.id.node_id == parent)
1983                         .map(|ti| ti.kind);
1984                 }
1985                 match *m {
1986                     hir::TraitMethod::Required(_) => None,
1987                     hir::TraitMethod::Provided(body) => Some(body),
1988                 }
1989             }
1990
1991             Node::ImplItem(&hir::ImplItem {
1992                 node: hir::ImplItemKind::Method(_, body),
1993                 ..
1994             }) => {
1995                 if let hir::ItemKind::Impl(.., ref self_ty, ref impl_items) = self.tcx
1996                     .hir
1997                     .expect_item(self.tcx.hir.get_parent(parent))
1998                     .node
1999                 {
2000                     impl_self = Some(self_ty);
2001                     assoc_item_kind = impl_items
2002                         .iter()
2003                         .find(|ii| ii.id.node_id == parent)
2004                         .map(|ii| ii.kind);
2005                 }
2006                 Some(body)
2007             }
2008
2009             // Foreign functions, `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
2010             Node::ForeignItem(_) | Node::Ty(_) | Node::TraitRef(_) => None,
2011             // Everything else (only closures?) doesn't
2012             // actually enjoy elision in return types.
2013             _ => {
2014                 self.visit_ty(output);
2015                 return;
2016             }
2017         };
2018
2019         let has_self = match assoc_item_kind {
2020             Some(hir::AssociatedItemKind::Method { has_self }) => has_self,
2021             _ => false,
2022         };
2023
2024         // In accordance with the rules for lifetime elision, we can determine
2025         // what region to use for elision in the output type in two ways.
2026         // First (determined here), if `self` is by-reference, then the
2027         // implied output region is the region of the self parameter.
2028         if has_self {
2029             // Look for `self: &'a Self` - also desugared from `&'a self`,
2030             // and if that matches, use it for elision and return early.
2031             let is_self_ty = |def: Def| {
2032                 if let Def::SelfTy(..) = def {
2033                     return true;
2034                 }
2035
2036                 // Can't always rely on literal (or implied) `Self` due
2037                 // to the way elision rules were originally specified.
2038                 let impl_self = impl_self.map(|ty| &ty.node);
2039                 if let Some(&hir::TyKind::Path(hir::QPath::Resolved(None, ref path))) = impl_self {
2040                     match path.def {
2041                         // Whitelist the types that unambiguously always
2042                         // result in the same type constructor being used
2043                         // (it can't differ between `Self` and `self`).
2044                         Def::Struct(_) | Def::Union(_) | Def::Enum(_) | Def::PrimTy(_) => {
2045                             return def == path.def
2046                         }
2047                         _ => {}
2048                     }
2049                 }
2050
2051                 false
2052             };
2053
2054             if let hir::TyKind::Rptr(lifetime_ref, ref mt) = inputs[0].node {
2055                 if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.node {
2056                     if is_self_ty(path.def) {
2057                         if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.id) {
2058                             let scope = Scope::Elision {
2059                                 elide: Elide::Exact(lifetime),
2060                                 s: self.scope,
2061                             };
2062                             self.with(scope, |_, this| this.visit_ty(output));
2063                             return;
2064                         }
2065                     }
2066                 }
2067             }
2068         }
2069
2070         // Second, if there was exactly one lifetime (either a substitution or a
2071         // reference) in the arguments, then any anonymous regions in the output
2072         // have that lifetime.
2073         let mut possible_implied_output_region = None;
2074         let mut lifetime_count = 0;
2075         let arg_lifetimes = inputs
2076             .iter()
2077             .enumerate()
2078             .skip(has_self as usize)
2079             .map(|(i, input)| {
2080                 let mut gather = GatherLifetimes {
2081                     map: self.map,
2082                     outer_index: ty::INNERMOST,
2083                     have_bound_regions: false,
2084                     lifetimes: Default::default(),
2085                 };
2086                 gather.visit_ty(input);
2087
2088                 lifetime_count += gather.lifetimes.len();
2089
2090                 if lifetime_count == 1 && gather.lifetimes.len() == 1 {
2091                     // there's a chance that the unique lifetime of this
2092                     // iteration will be the appropriate lifetime for output
2093                     // parameters, so lets store it.
2094                     possible_implied_output_region = gather.lifetimes.iter().cloned().next();
2095                 }
2096
2097                 ElisionFailureInfo {
2098                     parent: body,
2099                     index: i,
2100                     lifetime_count: gather.lifetimes.len(),
2101                     have_bound_regions: gather.have_bound_regions,
2102                 }
2103             })
2104             .collect();
2105
2106         let elide = if lifetime_count == 1 {
2107             Elide::Exact(possible_implied_output_region.unwrap())
2108         } else {
2109             Elide::Error(arg_lifetimes)
2110         };
2111
2112         debug!("visit_fn_like_elision: elide={:?}", elide);
2113
2114         let scope = Scope::Elision {
2115             elide,
2116             s: self.scope,
2117         };
2118         self.with(scope, |_, this| this.visit_ty(output));
2119         debug!("visit_fn_like_elision: exit");
2120
2121         struct GatherLifetimes<'a> {
2122             map: &'a NamedRegionMap,
2123             outer_index: ty::DebruijnIndex,
2124             have_bound_regions: bool,
2125             lifetimes: FxHashSet<Region>,
2126         }
2127
2128         impl<'v, 'a> Visitor<'v> for GatherLifetimes<'a> {
2129             fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
2130                 NestedVisitorMap::None
2131             }
2132
2133             fn visit_ty(&mut self, ty: &hir::Ty) {
2134                 if let hir::TyKind::BareFn(_) = ty.node {
2135                     self.outer_index.shift_in(1);
2136                 }
2137                 if let hir::TyKind::TraitObject(ref bounds, ref lifetime) = ty.node {
2138                     for bound in bounds {
2139                         self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
2140                     }
2141
2142                     // Stay on the safe side and don't include the object
2143                     // lifetime default (which may not end up being used).
2144                     if !lifetime.is_elided() {
2145                         self.visit_lifetime(lifetime);
2146                     }
2147                 } else {
2148                     intravisit::walk_ty(self, ty);
2149                 }
2150                 if let hir::TyKind::BareFn(_) = ty.node {
2151                     self.outer_index.shift_out(1);
2152                 }
2153             }
2154
2155             fn visit_generic_param(&mut self, param: &hir::GenericParam) {
2156                 if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2157                     // FIXME(eddyb) Do we want this? It only makes a difference
2158                     // if this `for<'a>` lifetime parameter is never used.
2159                     self.have_bound_regions = true;
2160                 }
2161
2162                 intravisit::walk_generic_param(self, param);
2163             }
2164
2165             fn visit_poly_trait_ref(
2166                 &mut self,
2167                 trait_ref: &hir::PolyTraitRef,
2168                 modifier: hir::TraitBoundModifier,
2169             ) {
2170                 self.outer_index.shift_in(1);
2171                 intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
2172                 self.outer_index.shift_out(1);
2173             }
2174
2175             fn visit_lifetime(&mut self, lifetime_ref: &hir::Lifetime) {
2176                 if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.id) {
2177                     match lifetime {
2178                         Region::LateBound(debruijn, _, _) | Region::LateBoundAnon(debruijn, _)
2179                             if debruijn < self.outer_index =>
2180                         {
2181                             self.have_bound_regions = true;
2182                         }
2183                         _ => {
2184                             self.lifetimes
2185                                 .insert(lifetime.shifted_out_to_binder(self.outer_index));
2186                         }
2187                     }
2188                 }
2189             }
2190         }
2191     }
2192
2193     fn resolve_elided_lifetimes(&mut self, lifetime_refs: Vec<&'tcx hir::Lifetime>) {
2194         if lifetime_refs.is_empty() {
2195             return;
2196         }
2197
2198         let span = lifetime_refs[0].span;
2199         let mut late_depth = 0;
2200         let mut scope = self.scope;
2201         let error = loop {
2202             match *scope {
2203                 // Do not assign any resolution, it will be inferred.
2204                 Scope::Body { .. } => return,
2205
2206                 Scope::Root => break None,
2207
2208                 Scope::Binder { s, .. } => {
2209                     late_depth += 1;
2210                     scope = s;
2211                 }
2212
2213                 Scope::Elision { ref elide, .. } => {
2214                     let lifetime = match *elide {
2215                         Elide::FreshLateAnon(ref counter) => {
2216                             for lifetime_ref in lifetime_refs {
2217                                 let lifetime = Region::late_anon(counter).shifted(late_depth);
2218                                 self.insert_lifetime(lifetime_ref, lifetime);
2219                             }
2220                             return;
2221                         }
2222                         Elide::Exact(l) => l.shifted(late_depth),
2223                         Elide::Error(ref e) => break Some(e),
2224                     };
2225                     for lifetime_ref in lifetime_refs {
2226                         self.insert_lifetime(lifetime_ref, lifetime);
2227                     }
2228                     return;
2229                 }
2230
2231                 Scope::ObjectLifetimeDefault { s, .. } => {
2232                     scope = s;
2233                 }
2234             }
2235         };
2236
2237         let mut err = report_missing_lifetime_specifiers(self.tcx.sess, span, lifetime_refs.len());
2238         let mut add_label = true;
2239
2240         if let Some(params) = error {
2241             if lifetime_refs.len() == 1 {
2242                 add_label = add_label && self.report_elision_failure(&mut err, params, span);
2243             }
2244         }
2245         if add_label {
2246             add_missing_lifetime_specifiers_label(&mut err, span, lifetime_refs.len());
2247         }
2248
2249         err.emit();
2250     }
2251
2252     fn suggest_lifetime(&self, db: &mut DiagnosticBuilder<'_>, span: Span, msg: &str) -> bool {
2253         match self.tcx.sess.source_map().span_to_snippet(span) {
2254             Ok(ref snippet) => {
2255                 let (sugg, applicability) = if snippet == "&" {
2256                     ("&'static ".to_owned(), Applicability::MachineApplicable)
2257                 } else if snippet == "'_" {
2258                     ("'static".to_owned(), Applicability::MachineApplicable)
2259                 } else {
2260                     (format!("{} + 'static", snippet), Applicability::MaybeIncorrect)
2261                 };
2262                 db.span_suggestion_with_applicability(span, msg, sugg, applicability);
2263                 false
2264             }
2265             Err(_) => {
2266                 db.help(msg);
2267                 true
2268             }
2269         }
2270     }
2271
2272     fn report_elision_failure(
2273         &mut self,
2274         db: &mut DiagnosticBuilder<'_>,
2275         params: &[ElisionFailureInfo],
2276         span: Span,
2277     ) -> bool {
2278         let mut m = String::new();
2279         let len = params.len();
2280
2281         let elided_params: Vec<_> = params
2282             .iter()
2283             .cloned()
2284             .filter(|info| info.lifetime_count > 0)
2285             .collect();
2286
2287         let elided_len = elided_params.len();
2288
2289         for (i, info) in elided_params.into_iter().enumerate() {
2290             let ElisionFailureInfo {
2291                 parent,
2292                 index,
2293                 lifetime_count: n,
2294                 have_bound_regions,
2295             } = info;
2296
2297             let help_name = if let Some(body) = parent {
2298                 let arg = &self.tcx.hir.body(body).arguments[index];
2299                 format!("`{}`", self.tcx.hir.node_to_pretty_string(arg.pat.id))
2300             } else {
2301                 format!("argument {}", index + 1)
2302             };
2303
2304             m.push_str(
2305                 &(if n == 1 {
2306                     help_name
2307                 } else {
2308                     format!(
2309                         "one of {}'s {} {}lifetimes",
2310                         help_name,
2311                         n,
2312                         if have_bound_regions { "free " } else { "" }
2313                     )
2314                 })[..],
2315             );
2316
2317             if elided_len == 2 && i == 0 {
2318                 m.push_str(" or ");
2319             } else if i + 2 == elided_len {
2320                 m.push_str(", or ");
2321             } else if i != elided_len - 1 {
2322                 m.push_str(", ");
2323             }
2324         }
2325
2326         if len == 0 {
2327             help!(
2328                 db,
2329                 "this function's return type contains a borrowed value, but \
2330                  there is no value for it to be borrowed from"
2331             );
2332             self.suggest_lifetime(db, span, "consider giving it a 'static lifetime")
2333         } else if elided_len == 0 {
2334             help!(
2335                 db,
2336                 "this function's return type contains a borrowed value with \
2337                  an elided lifetime, but the lifetime cannot be derived from \
2338                  the arguments"
2339             );
2340             let msg = "consider giving it an explicit bounded or 'static lifetime";
2341             self.suggest_lifetime(db, span, msg)
2342         } else if elided_len == 1 {
2343             help!(
2344                 db,
2345                 "this function's return type contains a borrowed value, but \
2346                  the signature does not say which {} it is borrowed from",
2347                 m
2348             );
2349             true
2350         } else {
2351             help!(
2352                 db,
2353                 "this function's return type contains a borrowed value, but \
2354                  the signature does not say whether it is borrowed from {}",
2355                 m
2356             );
2357             true
2358         }
2359     }
2360
2361     fn resolve_object_lifetime_default(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
2362         let mut late_depth = 0;
2363         let mut scope = self.scope;
2364         let lifetime = loop {
2365             match *scope {
2366                 Scope::Binder { s, .. } => {
2367                     late_depth += 1;
2368                     scope = s;
2369                 }
2370
2371                 Scope::Root | Scope::Elision { .. } => break Region::Static,
2372
2373                 Scope::Body { .. } | Scope::ObjectLifetimeDefault { lifetime: None, .. } => return,
2374
2375                 Scope::ObjectLifetimeDefault {
2376                     lifetime: Some(l), ..
2377                 } => break l,
2378             }
2379         };
2380         self.insert_lifetime(lifetime_ref, lifetime.shifted(late_depth));
2381     }
2382
2383     fn check_lifetime_params(
2384         &mut self,
2385         old_scope: ScopeRef<'_>,
2386         params: &'tcx [hir::GenericParam],
2387     ) {
2388         let lifetimes: Vec<_> = params
2389             .iter()
2390             .filter_map(|param| match param.kind {
2391                 GenericParamKind::Lifetime { .. } => Some((param, param.name)),
2392                 _ => None,
2393             })
2394             .collect();
2395         for (i, (lifetime_i, lifetime_i_name)) in lifetimes.iter().enumerate() {
2396             if let hir::ParamName::Plain(_) = lifetime_i_name {
2397                 let name = lifetime_i_name.ident().name;
2398                 if name == keywords::UnderscoreLifetime.name()
2399                     || name == keywords::StaticLifetime.name()
2400                 {
2401                     let mut err = struct_span_err!(
2402                         self.tcx.sess,
2403                         lifetime_i.span,
2404                         E0262,
2405                         "invalid lifetime parameter name: `{}`",
2406                         lifetime_i.name.ident(),
2407                     );
2408                     err.span_label(
2409                         lifetime_i.span,
2410                         format!("{} is a reserved lifetime name", name),
2411                     );
2412                     err.emit();
2413                 }
2414             }
2415
2416             // It is a hard error to shadow a lifetime within the same scope.
2417             for (lifetime_j, lifetime_j_name) in lifetimes.iter().skip(i + 1) {
2418                 if lifetime_i_name == lifetime_j_name {
2419                     struct_span_err!(
2420                         self.tcx.sess,
2421                         lifetime_j.span,
2422                         E0263,
2423                         "lifetime name `{}` declared twice in the same scope",
2424                         lifetime_j.name.ident()
2425                     ).span_label(lifetime_j.span, "declared twice")
2426                         .span_label(lifetime_i.span, "previous declaration here")
2427                         .emit();
2428                 }
2429             }
2430
2431             // It is a soft error to shadow a lifetime within a parent scope.
2432             self.check_lifetime_param_for_shadowing(old_scope, &lifetime_i);
2433
2434             for bound in &lifetime_i.bounds {
2435                 match bound {
2436                     hir::GenericBound::Outlives(lt) => match lt.name {
2437                         hir::LifetimeName::Underscore => self.tcx.sess.delay_span_bug(
2438                             lt.span,
2439                             "use of `'_` in illegal place, but not caught by lowering",
2440                         ),
2441                         hir::LifetimeName::Static => {
2442                             self.insert_lifetime(lt, Region::Static);
2443                             self.tcx
2444                                 .sess
2445                                 .struct_span_warn(
2446                                     lifetime_i.span.to(lt.span),
2447                                     &format!(
2448                                         "unnecessary lifetime parameter `{}`",
2449                                         lifetime_i.name.ident(),
2450                                     ),
2451                                 )
2452                                 .help(&format!(
2453                                     "you can use the `'static` lifetime directly, in place of `{}`",
2454                                     lifetime_i.name.ident(),
2455                                 ))
2456                                 .emit();
2457                         }
2458                         hir::LifetimeName::Param(_) | hir::LifetimeName::Implicit => {
2459                             self.resolve_lifetime_ref(lt);
2460                         }
2461                         hir::LifetimeName::Error => {
2462                             // No need to do anything, error already reported.
2463                         }
2464                     },
2465                     _ => bug!(),
2466                 }
2467             }
2468         }
2469     }
2470
2471     fn check_lifetime_param_for_shadowing(
2472         &self,
2473         mut old_scope: ScopeRef<'_>,
2474         param: &'tcx hir::GenericParam,
2475     ) {
2476         for label in &self.labels_in_fn {
2477             // FIXME (#24278): non-hygienic comparison
2478             if param.name.ident().name == label.name {
2479                 signal_shadowing_problem(
2480                     self.tcx,
2481                     label.name,
2482                     original_label(label.span),
2483                     shadower_lifetime(&param),
2484                 );
2485                 return;
2486             }
2487         }
2488
2489         loop {
2490             match *old_scope {
2491                 Scope::Body { s, .. }
2492                 | Scope::Elision { s, .. }
2493                 | Scope::ObjectLifetimeDefault { s, .. } => {
2494                     old_scope = s;
2495                 }
2496
2497                 Scope::Root => {
2498                     return;
2499                 }
2500
2501                 Scope::Binder {
2502                     ref lifetimes, s, ..
2503                 } => {
2504                     if let Some(&def) = lifetimes.get(&param.name.modern()) {
2505                         let node_id = self.tcx.hir.as_local_node_id(def.id().unwrap()).unwrap();
2506
2507                         signal_shadowing_problem(
2508                             self.tcx,
2509                             param.name.ident().name,
2510                             original_lifetime(self.tcx.hir.span(node_id)),
2511                             shadower_lifetime(&param),
2512                         );
2513                         return;
2514                     }
2515
2516                     old_scope = s;
2517                 }
2518             }
2519         }
2520     }
2521
2522     /// Returns true if, in the current scope, replacing `'_` would be
2523     /// equivalent to a single-use lifetime.
2524     fn track_lifetime_uses(&self) -> bool {
2525         let mut scope = self.scope;
2526         loop {
2527             match *scope {
2528                 Scope::Root => break false,
2529
2530                 // Inside of items, it depends on the kind of item.
2531                 Scope::Binder {
2532                     track_lifetime_uses,
2533                     ..
2534                 } => break track_lifetime_uses,
2535
2536                 // Inside a body, `'_` will use an inference variable,
2537                 // should be fine.
2538                 Scope::Body { .. } => break true,
2539
2540                 // A lifetime only used in a fn argument could as well
2541                 // be replaced with `'_`, as that would generate a
2542                 // fresh name, too.
2543                 Scope::Elision {
2544                     elide: Elide::FreshLateAnon(_),
2545                     ..
2546                 } => break true,
2547
2548                 // In the return type or other such place, `'_` is not
2549                 // going to make a fresh name, so we cannot
2550                 // necessarily replace a single-use lifetime with
2551                 // `'_`.
2552                 Scope::Elision {
2553                     elide: Elide::Exact(_),
2554                     ..
2555                 } => break false,
2556                 Scope::Elision {
2557                     elide: Elide::Error(_),
2558                     ..
2559                 } => break false,
2560
2561                 Scope::ObjectLifetimeDefault { s, .. } => scope = s,
2562             }
2563         }
2564     }
2565
2566     fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
2567         if lifetime_ref.id == ast::DUMMY_NODE_ID {
2568             span_bug!(
2569                 lifetime_ref.span,
2570                 "lifetime reference not renumbered, \
2571                  probably a bug in syntax::fold"
2572             );
2573         }
2574
2575         debug!(
2576             "insert_lifetime: {} resolved to {:?} span={:?}",
2577             self.tcx.hir.node_to_string(lifetime_ref.id),
2578             def,
2579             self.tcx.sess.source_map().span_to_string(lifetime_ref.span)
2580         );
2581         self.map.defs.insert(lifetime_ref.id, def);
2582
2583         match def {
2584             Region::LateBoundAnon(..) | Region::Static => {
2585                 // These are anonymous lifetimes or lifetimes that are not declared.
2586             }
2587
2588             Region::Free(_, def_id)
2589             | Region::LateBound(_, def_id, _)
2590             | Region::EarlyBound(_, def_id, _) => {
2591                 // A lifetime declared by the user.
2592                 let track_lifetime_uses = self.track_lifetime_uses();
2593                 debug!(
2594                     "insert_lifetime: track_lifetime_uses={}",
2595                     track_lifetime_uses
2596                 );
2597                 if track_lifetime_uses && !self.lifetime_uses.contains_key(&def_id) {
2598                     debug!("insert_lifetime: first use of {:?}", def_id);
2599                     self.lifetime_uses
2600                         .insert(def_id, LifetimeUseSet::One(lifetime_ref));
2601                 } else {
2602                     debug!("insert_lifetime: many uses of {:?}", def_id);
2603                     self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
2604                 }
2605             }
2606         }
2607     }
2608
2609     /// Sometimes we resolve a lifetime, but later find that it is an
2610     /// error (esp. around impl trait). In that case, we remove the
2611     /// entry into `map.defs` so as not to confuse later code.
2612     fn uninsert_lifetime_on_error(&mut self, lifetime_ref: &'tcx hir::Lifetime, bad_def: Region) {
2613         let old_value = self.map.defs.remove(&lifetime_ref.id);
2614         assert_eq!(old_value, Some(bad_def));
2615     }
2616 }
2617
2618 /// Detects late-bound lifetimes and inserts them into
2619 /// `map.late_bound`.
2620 ///
2621 /// A region declared on a fn is **late-bound** if:
2622 /// - it is constrained by an argument type;
2623 /// - it does not appear in a where-clause.
2624 ///
2625 /// "Constrained" basically means that it appears in any type but
2626 /// not amongst the inputs to a projection.  In other words, `<&'a
2627 /// T as Trait<''b>>::Foo` does not constrain `'a` or `'b`.
2628 fn insert_late_bound_lifetimes(
2629     map: &mut NamedRegionMap,
2630     decl: &hir::FnDecl,
2631     generics: &hir::Generics,
2632 ) {
2633     debug!(
2634         "insert_late_bound_lifetimes(decl={:?}, generics={:?})",
2635         decl, generics
2636     );
2637
2638     let mut constrained_by_input = ConstrainedCollector::default();
2639     for arg_ty in &decl.inputs {
2640         constrained_by_input.visit_ty(arg_ty);
2641     }
2642
2643     let mut appears_in_output = AllCollector {
2644         regions: Default::default(),
2645     };
2646     intravisit::walk_fn_ret_ty(&mut appears_in_output, &decl.output);
2647
2648     debug!(
2649         "insert_late_bound_lifetimes: constrained_by_input={:?}",
2650         constrained_by_input.regions
2651     );
2652
2653     // Walk the lifetimes that appear in where clauses.
2654     //
2655     // Subtle point: because we disallow nested bindings, we can just
2656     // ignore binders here and scrape up all names we see.
2657     let mut appears_in_where_clause = AllCollector {
2658         regions: Default::default(),
2659     };
2660     appears_in_where_clause.visit_generics(generics);
2661
2662     for param in &generics.params {
2663         if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2664             if !param.bounds.is_empty() {
2665                 // `'a: 'b` means both `'a` and `'b` are referenced
2666                 appears_in_where_clause
2667                     .regions
2668                     .insert(hir::LifetimeName::Param(param.name.modern()));
2669             }
2670         }
2671     }
2672
2673     debug!(
2674         "insert_late_bound_lifetimes: appears_in_where_clause={:?}",
2675         appears_in_where_clause.regions
2676     );
2677
2678     // Late bound regions are those that:
2679     // - appear in the inputs
2680     // - do not appear in the where-clauses
2681     // - are not implicitly captured by `impl Trait`
2682     for param in &generics.params {
2683         match param.kind {
2684             hir::GenericParamKind::Lifetime { .. } => { /* fall through */ }
2685
2686             // Types are not late-bound.
2687             hir::GenericParamKind::Type { .. } => continue,
2688         }
2689
2690         let lt_name = hir::LifetimeName::Param(param.name.modern());
2691         // appears in the where clauses? early-bound.
2692         if appears_in_where_clause.regions.contains(&lt_name) {
2693             continue;
2694         }
2695
2696         // does not appear in the inputs, but appears in the return type? early-bound.
2697         if !constrained_by_input.regions.contains(&lt_name)
2698             && appears_in_output.regions.contains(&lt_name)
2699         {
2700             continue;
2701         }
2702
2703         debug!(
2704             "insert_late_bound_lifetimes: lifetime {:?} with id {:?} is late-bound",
2705             param.name.ident(),
2706             param.id
2707         );
2708
2709         let inserted = map.late_bound.insert(param.id);
2710         assert!(inserted, "visited lifetime {:?} twice", param.id);
2711     }
2712
2713     return;
2714
2715     #[derive(Default)]
2716     struct ConstrainedCollector {
2717         regions: FxHashSet<hir::LifetimeName>,
2718     }
2719
2720     impl<'v> Visitor<'v> for ConstrainedCollector {
2721         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
2722             NestedVisitorMap::None
2723         }
2724
2725         fn visit_ty(&mut self, ty: &'v hir::Ty) {
2726             match ty.node {
2727                 hir::TyKind::Path(hir::QPath::Resolved(Some(_), _))
2728                 | hir::TyKind::Path(hir::QPath::TypeRelative(..)) => {
2729                     // ignore lifetimes appearing in associated type
2730                     // projections, as they are not *constrained*
2731                     // (defined above)
2732                 }
2733
2734                 hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => {
2735                     // consider only the lifetimes on the final
2736                     // segment; I am not sure it's even currently
2737                     // valid to have them elsewhere, but even if it
2738                     // is, those would be potentially inputs to
2739                     // projections
2740                     if let Some(last_segment) = path.segments.last() {
2741                         self.visit_path_segment(path.span, last_segment);
2742                     }
2743                 }
2744
2745                 _ => {
2746                     intravisit::walk_ty(self, ty);
2747                 }
2748             }
2749         }
2750
2751         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2752             self.regions.insert(lifetime_ref.name.modern());
2753         }
2754     }
2755
2756     struct AllCollector {
2757         regions: FxHashSet<hir::LifetimeName>,
2758     }
2759
2760     impl<'v> Visitor<'v> for AllCollector {
2761         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
2762             NestedVisitorMap::None
2763         }
2764
2765         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2766             self.regions.insert(lifetime_ref.name.modern());
2767         }
2768     }
2769 }
2770
2771 fn report_missing_lifetime_specifiers(
2772     sess: &Session,
2773     span: Span,
2774     count: usize,
2775 ) -> DiagnosticBuilder<'_> {
2776     struct_span_err!(
2777         sess,
2778         span,
2779         E0106,
2780         "missing lifetime specifier{}",
2781         if count > 1 { "s" } else { "" }
2782     )
2783 }
2784
2785 fn add_missing_lifetime_specifiers_label(
2786     err: &mut DiagnosticBuilder<'_>,
2787     span: Span,
2788     count: usize,
2789 ) {
2790     if count > 1 {
2791         err.span_label(span, format!("expected {} lifetime parameters", count));
2792     } else {
2793         err.span_label(span, "expected lifetime parameter");
2794     };
2795 }