src/librustc/middle/resolve_lifetime.rs

   1 //! Name resolution for lifetimes.
   2 //!
   3 //! Name resolution for lifetimes follows *much* simpler rules than the
   4 //! full resolve. For example, lifetime names are never exported or
   5 //! used between functions, and they operate in a purely top-down
   6 //! way. Therefore, we break lifetime name resolution into a separate pass.
   7
   8 use crate::hir::def::{Res, DefKind};
   9 use crate::hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
  10 use crate::hir::map::Map;
  11 use crate::hir::{GenericArg, GenericParam, ItemLocalId, LifetimeName, Node, ParamName};
  12 use crate::ty::{self, DefIdTree, GenericParamDefKind, TyCtxt};
  13
  14 use crate::rustc::lint;
  15 use crate::session::Session;
  16 use crate::util::nodemap::{DefIdMap, FxHashMap, FxHashSet, HirIdMap, HirIdSet};
  17 use errors::{Applicability, DiagnosticBuilder};
  18 use rustc_macros::HashStable;
  19 use std::borrow::Cow;
  20 use std::cell::Cell;
  21 use std::mem::replace;
  22 use syntax::ast;
  23 use syntax::attr;
  24 use syntax::ptr::P;
  25 use syntax::symbol::{kw, sym};
  26 use syntax_pos::Span;
  27
  28 use crate::hir::intravisit::{self, NestedVisitorMap, Visitor};
  29 use crate::hir::{self, GenericParamKind, LifetimeParamKind};
  30
  31 /// The origin of a named lifetime definition.
  32 ///
  33 /// This is used to prevent the usage of in-band lifetimes in `Fn`/`fn` syntax.
  34 #[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug, HashStable)]
  35 pub enum LifetimeDefOrigin {
  36     // Explicit binders like `fn foo<'a>(x: &'a u8)` or elided like `impl Foo<&u32>`
  37     ExplicitOrElided,
  38     // In-band declarations like `fn foo(x: &'a u8)`
  39     InBand,
  40     // Some kind of erroneous origin
  41     Error,
  42 }
  43
  44 impl LifetimeDefOrigin {
  45     fn from_param(param: &GenericParam) -> Self {
  46         match param.kind {
  47             GenericParamKind::Lifetime { kind } => match kind {
  48                 LifetimeParamKind::InBand => LifetimeDefOrigin::InBand,
  49                 LifetimeParamKind::Explicit => LifetimeDefOrigin::ExplicitOrElided,
  50                 LifetimeParamKind::Elided => LifetimeDefOrigin::ExplicitOrElided,
  51                 LifetimeParamKind::Error => LifetimeDefOrigin::Error,
  52             },
  53             _ => bug!("expected a lifetime param"),
  54         }
  55     }
  56 }
  57
  58 // This counts the no of times a lifetime is used
  59 #[derive(Clone, Copy, Debug)]
  60 pub enum LifetimeUseSet<'tcx> {
  61     One(&'tcx hir::Lifetime),
  62     Many,
  63 }
  64
  65 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug, HashStable)]
  66 pub enum Region {
  67     Static,
  68     EarlyBound(
  69         /* index */ u32,
  70         /* lifetime decl */ DefId,
  71         LifetimeDefOrigin,
  72     ),
  73     LateBound(
  74         ty::DebruijnIndex,
  75         /* lifetime decl */ DefId,
  76         LifetimeDefOrigin,
  77     ),
  78     LateBoundAnon(ty::DebruijnIndex, /* anon index */ u32),
  79     Free(DefId, /* lifetime decl */ DefId),
  80 }
  81
  82 impl Region {
  83     fn early(hir_map: &Map<'_>, index: &mut u32, param: &GenericParam) -> (ParamName, Region) {
  84         let i = *index;
  85         *index += 1;
  86         let def_id = hir_map.local_def_id_from_hir_id(param.hir_id);
  87         let origin = LifetimeDefOrigin::from_param(param);
  88         debug!("Region::early: index={} def_id={:?}", i, def_id);
  89         (param.name.modern(), Region::EarlyBound(i, def_id, origin))
  90     }
  91
  92     fn late(hir_map: &Map<'_>, param: &GenericParam) -> (ParamName, Region) {
  93         let depth = ty::INNERMOST;
  94         let def_id = hir_map.local_def_id_from_hir_id(param.hir_id);
  95         let origin = LifetimeDefOrigin::from_param(param);
  96         debug!(
  97             "Region::late: param={:?} depth={:?} def_id={:?} origin={:?}",
  98             param, depth, def_id, origin,
  99         );
 100         (
 101             param.name.modern(),
 102             Region::LateBound(depth, def_id, origin),
 103         )
 104     }
 105
 106     fn late_anon(index: &Cell<u32>) -> Region {
 107         let i = index.get();
 108         index.set(i + 1);
 109         let depth = ty::INNERMOST;
 110         Region::LateBoundAnon(depth, i)
 111     }
 112
 113     fn id(&self) -> Option<DefId> {
 114         match *self {
 115             Region::Static | Region::LateBoundAnon(..) => None,
 116
 117             Region::EarlyBound(_, id, _) | Region::LateBound(_, id, _) | Region::Free(_, id) => {
 118                 Some(id)
 119             }
 120         }
 121     }
 122
 123     fn shifted(self, amount: u32) -> Region {
 124         match self {
 125             Region::LateBound(debruijn, id, origin) => {
 126                 Region::LateBound(debruijn.shifted_in(amount), id, origin)
 127             }
 128             Region::LateBoundAnon(debruijn, index) => {
 129                 Region::LateBoundAnon(debruijn.shifted_in(amount), index)
 130             }
 131             _ => self,
 132         }
 133     }
 134
 135     fn shifted_out_to_binder(self, binder: ty::DebruijnIndex) -> Region {
 136         match self {
 137             Region::LateBound(debruijn, id, origin) => {
 138                 Region::LateBound(debruijn.shifted_out_to_binder(binder), id, origin)
 139             }
 140             Region::LateBoundAnon(debruijn, index) => {
 141                 Region::LateBoundAnon(debruijn.shifted_out_to_binder(binder), index)
 142             }
 143             _ => self,
 144         }
 145     }
 146
 147     fn subst<'a, L>(self, mut params: L, map: &NamedRegionMap) -> Option<Region>
 148     where
 149         L: Iterator<Item = &'a hir::Lifetime>,
 150     {
 151         if let Region::EarlyBound(index, _, _) = self {
 152             params
 153                 .nth(index as usize)
 154                 .and_then(|lifetime| map.defs.get(&lifetime.hir_id).cloned())
 155         } else {
 156             Some(self)
 157         }
 158     }
 159 }
 160
 161 /// A set containing, at most, one known element.
 162 /// If two distinct values are inserted into a set, then it
 163 /// becomes `Many`, which can be used to detect ambiguities.
 164 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug, HashStable)]
 165 pub enum Set1<T> {
 166     Empty,
 167     One(T),
 168     Many,
 169 }
 170
 171 impl<T: PartialEq> Set1<T> {
 172     pub fn insert(&mut self, value: T) {
 173         *self = match self {
 174             Set1::Empty => Set1::One(value),
 175             Set1::One(old) if *old == value => return,
 176             _ => Set1::Many,
 177         };
 178     }
 179 }
 180
 181 pub type ObjectLifetimeDefault = Set1<Region>;
 182
 183 /// Maps the id of each lifetime reference to the lifetime decl
 184 /// that it corresponds to.
 185 ///
 186 /// FIXME. This struct gets converted to a `ResolveLifetimes` for
 187 /// actual use. It has the same data, but indexed by `DefIndex`.  This
 188 /// is silly.
 189 #[derive(Default)]
 190 struct NamedRegionMap {
 191     // maps from every use of a named (not anonymous) lifetime to a
 192     // `Region` describing how that region is bound
 193     pub defs: HirIdMap<Region>,
 194
 195     // the set of lifetime def ids that are late-bound; a region can
 196     // be late-bound if (a) it does NOT appear in a where-clause and
 197     // (b) it DOES appear in the arguments.
 198     pub late_bound: HirIdSet,
 199
 200     // For each type and trait definition, maps type parameters
 201     // to the trait object lifetime defaults computed from them.
 202     pub object_lifetime_defaults: HirIdMap<Vec<ObjectLifetimeDefault>>,
 203 }
 204
 205 /// See [`NamedRegionMap`].
 206 #[derive(Default)]
 207 pub struct ResolveLifetimes {
 208     defs: FxHashMap<LocalDefId, FxHashMap<ItemLocalId, Region>>,
 209     late_bound: FxHashMap<LocalDefId, FxHashSet<ItemLocalId>>,
 210     object_lifetime_defaults:
 211         FxHashMap<LocalDefId, FxHashMap<ItemLocalId, Vec<ObjectLifetimeDefault>>>,
 212 }
 213
 214 impl_stable_hash_for!(struct crate::middle::resolve_lifetime::ResolveLifetimes {
 215     defs,
 216     late_bound,
 217     object_lifetime_defaults
 218 });
 219
 220 struct LifetimeContext<'a, 'tcx> {
 221     tcx: TyCtxt<'tcx>,
 222     map: &'a mut NamedRegionMap,
 223     scope: ScopeRef<'a>,
 224
 225     /// This is slightly complicated. Our representation for poly-trait-refs contains a single
 226     /// binder and thus we only allow a single level of quantification. However,
 227     /// the syntax of Rust permits quantification in two places, e.g., `T: for <'a> Foo<'a>`
 228     /// and `for <'a, 'b> &'b T: Foo<'a>`. In order to get the De Bruijn indices
 229     /// correct when representing these constraints, we should only introduce one
 230     /// scope. However, we want to support both locations for the quantifier and
 231     /// during lifetime resolution we want precise information (so we can't
 232     /// desugar in an earlier phase).
 233     ///
 234     /// So, if we encounter a quantifier at the outer scope, we set
 235     /// `trait_ref_hack` to `true` (and introduce a scope), and then if we encounter
 236     /// a quantifier at the inner scope, we error. If `trait_ref_hack` is `false`,
 237     /// then we introduce the scope at the inner quantifier.
 238     trait_ref_hack: bool,
 239
 240     /// Used to disallow the use of in-band lifetimes in `fn` or `Fn` syntax.
 241     is_in_fn_syntax: bool,
 242
 243     /// List of labels in the function/method currently under analysis.
 244     labels_in_fn: Vec<ast::Ident>,
 245
 246     /// Cache for cross-crate per-definition object lifetime defaults.
 247     xcrate_object_lifetime_defaults: DefIdMap<Vec<ObjectLifetimeDefault>>,
 248
 249     lifetime_uses: &'a mut DefIdMap<LifetimeUseSet<'tcx>>,
 250 }
 251
 252 #[derive(Debug)]
 253 enum Scope<'a> {
 254     /// Declares lifetimes, and each can be early-bound or late-bound.
 255     /// The `DebruijnIndex` of late-bound lifetimes starts at `1` and
 256     /// it should be shifted by the number of `Binder`s in between the
 257     /// declaration `Binder` and the location it's referenced from.
 258     Binder {
 259         lifetimes: FxHashMap<hir::ParamName, Region>,
 260
 261         /// if we extend this scope with another scope, what is the next index
 262         /// we should use for an early-bound region?
 263         next_early_index: u32,
 264
 265         /// Flag is set to true if, in this binder, `'_` would be
 266         /// equivalent to a "single-use region". This is true on
 267         /// impls, but not other kinds of items.
 268         track_lifetime_uses: bool,
 269
 270         /// Whether or not this binder would serve as the parent
 271         /// binder for abstract types introduced within. For example:
 272         ///
 273         ///     fn foo<'a>() -> impl for<'b> Trait<Item = impl Trait2<'a>>
 274         ///
 275         /// Here, the abstract types we create for the `impl Trait`
 276         /// and `impl Trait2` references will both have the `foo` item
 277         /// as their parent. When we get to `impl Trait2`, we find
 278         /// that it is nested within the `for<>` binder -- this flag
 279         /// allows us to skip that when looking for the parent binder
 280         /// of the resulting abstract type.
 281         abstract_type_parent: bool,
 282
 283         s: ScopeRef<'a>,
 284     },
 285
 286     /// Lifetimes introduced by a fn are scoped to the call-site for that fn,
 287     /// if this is a fn body, otherwise the original definitions are used.
 288     /// Unspecified lifetimes are inferred, unless an elision scope is nested,
 289     /// e.g., `(&T, fn(&T) -> &T);` becomes `(&'_ T, for<'a> fn(&'a T) -> &'a T)`.
 290     Body {
 291         id: hir::BodyId,
 292         s: ScopeRef<'a>,
 293     },
 294
 295     /// A scope which either determines unspecified lifetimes or errors
 296     /// on them (e.g., due to ambiguity). For more details, see `Elide`.
 297     Elision {
 298         elide: Elide,
 299         s: ScopeRef<'a>,
 300     },
 301
 302     /// Use a specific lifetime (if `Some`) or leave it unset (to be
 303     /// inferred in a function body or potentially error outside one),
 304     /// for the default choice of lifetime in a trait object type.
 305     ObjectLifetimeDefault {
 306         lifetime: Option<Region>,
 307         s: ScopeRef<'a>,
 308     },
 309
 310     Root,
 311 }
 312
 313 #[derive(Clone, Debug)]
 314 enum Elide {
 315     /// Use a fresh anonymous late-bound lifetime each time, by
 316     /// incrementing the counter to generate sequential indices.
 317     FreshLateAnon(Cell<u32>),
 318     /// Always use this one lifetime.
 319     Exact(Region),
 320     /// Less or more than one lifetime were found, error on unspecified.
 321     Error(Vec<ElisionFailureInfo>),
 322 }
 323
 324 #[derive(Clone, Debug)]
 325 struct ElisionFailureInfo {
 326     /// Where we can find the argument pattern.
 327     parent: Option<hir::BodyId>,
 328     /// The index of the argument in the original definition.
 329     index: usize,
 330     lifetime_count: usize,
 331     have_bound_regions: bool,
 332 }
 333
 334 type ScopeRef<'a> = &'a Scope<'a>;
 335
 336 const ROOT_SCOPE: ScopeRef<'static> = &Scope::Root;
 337
 338 pub fn provide(providers: &mut ty::query::Providers<'_>) {
 339     *providers = ty::query::Providers {
 340         resolve_lifetimes,
 341
 342         named_region_map: |tcx, id| {
 343             let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
 344             tcx.resolve_lifetimes(LOCAL_CRATE).defs.get(&id)
 345         },
 346
 347         is_late_bound_map: |tcx, id| {
 348             let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
 349             tcx.resolve_lifetimes(LOCAL_CRATE)
 350                 .late_bound
 351                 .get(&id)
 352         },
 353
 354         object_lifetime_defaults_map: |tcx, id| {
 355             let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
 356             tcx.resolve_lifetimes(LOCAL_CRATE)
 357                 .object_lifetime_defaults
 358                 .get(&id)
 359         },
 360
 361         ..*providers
 362     };
 363
 364     // (*) FIXME the query should be defined to take a LocalDefId
 365 }
 366
 367 /// Computes the `ResolveLifetimes` map that contains data for the
 368 /// entire crate. You should not read the result of this query
 369 /// directly, but rather use `named_region_map`, `is_late_bound_map`,
 370 /// etc.
 371 fn resolve_lifetimes<'tcx>(tcx: TyCtxt<'tcx>, for_krate: CrateNum) -> &'tcx ResolveLifetimes {
 372     assert_eq!(for_krate, LOCAL_CRATE);
 373
 374     let named_region_map = krate(tcx);
 375
 376     let mut rl = ResolveLifetimes::default();
 377
 378     for (hir_id, v) in named_region_map.defs {
 379         let map = rl.defs.entry(hir_id.owner_local_def_id()).or_default();
 380         map.insert(hir_id.local_id, v);
 381     }
 382     for hir_id in named_region_map.late_bound {
 383         let map = rl.late_bound
 384             .entry(hir_id.owner_local_def_id())
 385             .or_default();
 386         map.insert(hir_id.local_id);
 387     }
 388     for (hir_id, v) in named_region_map.object_lifetime_defaults {
 389         let map = rl.object_lifetime_defaults
 390             .entry(hir_id.owner_local_def_id())
 391             .or_default();
 392         map.insert(hir_id.local_id, v);
 393     }
 394
 395     tcx.arena.alloc(rl)
 396 }
 397
 398 fn krate<'tcx>(tcx: TyCtxt<'tcx>) -> NamedRegionMap {
 399     let krate = tcx.hir().krate();
 400     let mut map = NamedRegionMap {
 401         defs: Default::default(),
 402         late_bound: Default::default(),
 403         object_lifetime_defaults: compute_object_lifetime_defaults(tcx),
 404     };
 405     {
 406         let mut visitor = LifetimeContext {
 407             tcx,
 408             map: &mut map,
 409             scope: ROOT_SCOPE,
 410             trait_ref_hack: false,
 411             is_in_fn_syntax: false,
 412             labels_in_fn: vec![],
 413             xcrate_object_lifetime_defaults: Default::default(),
 414             lifetime_uses: &mut Default::default(),
 415         };
 416         for (_, item) in &krate.items {
 417             visitor.visit_item(item);
 418         }
 419     }
 420     map
 421 }
 422
 423 /// In traits, there is an implicit `Self` type parameter which comes before the generics.
 424 /// We have to account for this when computing the index of the other generic parameters.
 425 /// This function returns whether there is such an implicit parameter defined on the given item.
 426 fn sub_items_have_self_param(node: &hir::ItemKind) -> bool {
 427     match *node {
 428         hir::ItemKind::Trait(..) |
 429         hir::ItemKind::TraitAlias(..) => true,
 430         _ => false,
 431     }
 432 }
 433
 434 impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 435     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 436         NestedVisitorMap::All(&self.tcx.hir())
 437     }
 438
 439     // We want to nest trait/impl items in their parent, but nothing else.
 440     fn visit_nested_item(&mut self, _: hir::ItemId) {}
 441
 442     fn visit_nested_body(&mut self, body: hir::BodyId) {
 443         // Each body has their own set of labels, save labels.
 444         let saved = replace(&mut self.labels_in_fn, vec![]);
 445         let body = self.tcx.hir().body(body);
 446         extract_labels(self, body);
 447         self.with(
 448             Scope::Body {
 449                 id: body.id(),
 450                 s: self.scope,
 451             },
 452             |_, this| {
 453                 this.visit_body(body);
 454             },
 455         );
 456         replace(&mut self.labels_in_fn, saved);
 457     }
 458
 459     fn visit_item(&mut self, item: &'tcx hir::Item) {
 460         match item.node {
 461             hir::ItemKind::Fn(ref decl, _, ref generics, _) => {
 462                 self.visit_early_late(None, decl, generics, |this| {
 463                     intravisit::walk_item(this, item);
 464                 });
 465             }
 466
 467             hir::ItemKind::ExternCrate(_)
 468             | hir::ItemKind::Use(..)
 469             | hir::ItemKind::Mod(..)
 470             | hir::ItemKind::ForeignMod(..)
 471             | hir::ItemKind::GlobalAsm(..) => {
 472                 // These sorts of items have no lifetime parameters at all.
 473                 intravisit::walk_item(self, item);
 474             }
 475             hir::ItemKind::Static(..) | hir::ItemKind::Const(..) => {
 476                 // No lifetime parameters, but implied 'static.
 477                 let scope = Scope::Elision {
 478                     elide: Elide::Exact(Region::Static),
 479                     s: ROOT_SCOPE,
 480                 };
 481                 self.with(scope, |_, this| intravisit::walk_item(this, item));
 482             }
 483             hir::ItemKind::Existential(hir::ExistTy {
 484                 impl_trait_fn: Some(_),
 485                 ..
 486             }) => {
 487                 // currently existential type declarations are just generated from impl Trait
 488                 // items. doing anything on this node is irrelevant, as we currently don't need
 489                 // it.
 490             }
 491             hir::ItemKind::Ty(_, ref generics)
 492             | hir::ItemKind::Existential(hir::ExistTy {
 493                 impl_trait_fn: None,
 494                 ref generics,
 495                 ..
 496             })
 497             | hir::ItemKind::Enum(_, ref generics)
 498             | hir::ItemKind::Struct(_, ref generics)
 499             | hir::ItemKind::Union(_, ref generics)
 500             | hir::ItemKind::Trait(_, _, ref generics, ..)
 501             | hir::ItemKind::TraitAlias(ref generics, ..)
 502             | hir::ItemKind::Impl(_, _, _, ref generics, ..) => {
 503                 // Impls permit `'_` to be used and it is equivalent to "some fresh lifetime name".
 504                 // This is not true for other kinds of items.x
 505                 let track_lifetime_uses = match item.node {
 506                     hir::ItemKind::Impl(..) => true,
 507                     _ => false,
 508                 };
 509                 // These kinds of items have only early-bound lifetime parameters.
 510                 let mut index = if sub_items_have_self_param(&item.node) {
 511                     1 // Self comes before lifetimes
 512                 } else {
 513                     0
 514                 };
 515                 let mut non_lifetime_count = 0;
 516                 let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
 517                     GenericParamKind::Lifetime { .. } => {
 518                         Some(Region::early(&self.tcx.hir(), &mut index, param))
 519                     }
 520                     GenericParamKind::Type { .. } |
 521                     GenericParamKind::Const { .. } => {
 522                         non_lifetime_count += 1;
 523                         None
 524                     }
 525                 }).collect();
 526                 let scope = Scope::Binder {
 527                     lifetimes,
 528                     next_early_index: index + non_lifetime_count,
 529                     abstract_type_parent: true,
 530                     track_lifetime_uses,
 531                     s: ROOT_SCOPE,
 532                 };
 533                 self.with(scope, |old_scope, this| {
 534                     this.check_lifetime_params(old_scope, &generics.params);
 535                     intravisit::walk_item(this, item);
 536                 });
 537             }
 538         }
 539     }
 540
 541     fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem) {
 542         match item.node {
 543             hir::ForeignItemKind::Fn(ref decl, _, ref generics) => {
 544                 self.visit_early_late(None, decl, generics, |this| {
 545                     intravisit::walk_foreign_item(this, item);
 546                 })
 547             }
 548             hir::ForeignItemKind::Static(..) => {
 549                 intravisit::walk_foreign_item(self, item);
 550             }
 551             hir::ForeignItemKind::Type => {
 552                 intravisit::walk_foreign_item(self, item);
 553             }
 554         }
 555     }
 556
 557     fn visit_ty(&mut self, ty: &'tcx hir::Ty) {
 558         debug!("visit_ty: id={:?} ty={:?}", ty.hir_id, ty);
 559         match ty.node {
 560             hir::TyKind::BareFn(ref c) => {
 561                 let next_early_index = self.next_early_index();
 562                 let was_in_fn_syntax = self.is_in_fn_syntax;
 563                 self.is_in_fn_syntax = true;
 564                 let scope = Scope::Binder {
 565                     lifetimes: c.generic_params
 566                         .iter()
 567                         .filter_map(|param| match param.kind {
 568                             GenericParamKind::Lifetime { .. } => {
 569                                 Some(Region::late(&self.tcx.hir(), param))
 570                             }
 571                             _ => None,
 572                         })
 573                         .collect(),
 574                     s: self.scope,
 575                     next_early_index,
 576                     track_lifetime_uses: true,
 577                     abstract_type_parent: false,
 578                 };
 579                 self.with(scope, |old_scope, this| {
 580                     // a bare fn has no bounds, so everything
 581                     // contained within is scoped within its binder.
 582                     this.check_lifetime_params(old_scope, &c.generic_params);
 583                     intravisit::walk_ty(this, ty);
 584                 });
 585                 self.is_in_fn_syntax = was_in_fn_syntax;
 586             }
 587             hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
 588                 for bound in bounds {
 589                     self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
 590                 }
 591                 match lifetime.name {
 592                     LifetimeName::Implicit => {
 593                         // If the user does not write *anything*, we
 594                         // use the object lifetime defaulting
 595                         // rules. So e.g., `Box<dyn Debug>` becomes
 596                         // `Box<dyn Debug + 'static>`.
 597                         self.resolve_object_lifetime_default(lifetime)
 598                     }
 599                     LifetimeName::Underscore => {
 600                         // If the user writes `'_`, we use the *ordinary* elision
 601                         // rules. So the `'_` in e.g., `Box<dyn Debug + '_>` will be
 602                         // resolved the same as the `'_` in `&'_ Foo`.
 603                         //
 604                         // cc #48468
 605                         self.resolve_elided_lifetimes(vec![lifetime])
 606                     }
 607                     LifetimeName::Param(_) | LifetimeName::Static => {
 608                         // If the user wrote an explicit name, use that.
 609                         self.visit_lifetime(lifetime);
 610                     }
 611                     LifetimeName::Error => {}
 612                 }
 613             }
 614             hir::TyKind::Rptr(ref lifetime_ref, ref mt) => {
 615                 self.visit_lifetime(lifetime_ref);
 616                 let scope = Scope::ObjectLifetimeDefault {
 617                     lifetime: self.map.defs.get(&lifetime_ref.hir_id).cloned(),
 618                     s: self.scope,
 619                 };
 620                 self.with(scope, |_, this| this.visit_ty(&mt.ty));
 621             }
 622             hir::TyKind::Def(item_id, ref lifetimes) => {
 623                 // Resolve the lifetimes in the bounds to the lifetime defs in the generics.
 624                 // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
 625                 // `abstract type MyAnonTy<'b>: MyTrait<'b>;`
 626                 //                          ^            ^ this gets resolved in the scope of
 627                 //                                         the exist_ty generics
 628                 let (generics, bounds) = match self.tcx.hir().expect_item(item_id.id).node
 629                 {
 630                     // named existential types are reached via TyKind::Path
 631                     // this arm is for `impl Trait` in the types of statics, constants and locals
 632                     hir::ItemKind::Existential(hir::ExistTy {
 633                         impl_trait_fn: None,
 634                         ..
 635                     }) => {
 636                         intravisit::walk_ty(self, ty);
 637                         return;
 638                     }
 639                     // RPIT (return position impl trait)
 640                     hir::ItemKind::Existential(hir::ExistTy {
 641                         ref generics,
 642                         ref bounds,
 643                         ..
 644                     }) => (generics, bounds),
 645                     ref i => bug!("impl Trait pointed to non-existential type?? {:#?}", i),
 646                 };
 647
 648                 // Resolve the lifetimes that are applied to the existential type.
 649                 // These are resolved in the current scope.
 650                 // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
 651                 // `fn foo<'a>() -> MyAnonTy<'a> { ... }`
 652                 //          ^                 ^this gets resolved in the current scope
 653                 for lifetime in lifetimes {
 654                     if let hir::GenericArg::Lifetime(lifetime) = lifetime {
 655                         self.visit_lifetime(lifetime);
 656
 657                         // Check for predicates like `impl for<'a> Trait<impl OtherTrait<'a>>`
 658                         // and ban them. Type variables instantiated inside binders aren't
 659                         // well-supported at the moment, so this doesn't work.
 660                         // In the future, this should be fixed and this error should be removed.
 661                         let def = self.map.defs.get(&lifetime.hir_id).cloned();
 662                         if let Some(Region::LateBound(_, def_id, _)) = def {
 663                             if let Some(hir_id) = self.tcx.hir().as_local_hir_id(def_id) {
 664                                 // Ensure that the parent of the def is an item, not HRTB
 665                                 let parent_id = self.tcx.hir().get_parent_node(hir_id);
 666                                 let parent_impl_id = hir::ImplItemId { hir_id: parent_id };
 667                                 let parent_trait_id = hir::TraitItemId { hir_id: parent_id };
 668                                 let krate = self.tcx.hir().forest.krate();
 669
 670                                 if !(krate.items.contains_key(&parent_id)
 671                                     || krate.impl_items.contains_key(&parent_impl_id)
 672                                     || krate.trait_items.contains_key(&parent_trait_id))
 673                                 {
 674                                     span_err!(
 675                                         self.tcx.sess,
 676                                         lifetime.span,
 677                                         E0657,
 678                                         "`impl Trait` can only capture lifetimes \
 679                                          bound at the fn or impl level"
 680                                     );
 681                                     self.uninsert_lifetime_on_error(lifetime, def.unwrap());
 682                                 }
 683                             }
 684                         }
 685                     }
 686                 }
 687
 688                 // We want to start our early-bound indices at the end of the parent scope,
 689                 // not including any parent `impl Trait`s.
 690                 let mut index = self.next_early_index_for_abstract_type();
 691                 debug!("visit_ty: index = {}", index);
 692
 693                 let mut elision = None;
 694                 let mut lifetimes = FxHashMap::default();
 695                 let mut non_lifetime_count = 0;
 696                 for param in &generics.params {
 697                     match param.kind {
 698                         GenericParamKind::Lifetime { .. } => {
 699                             let (name, reg) = Region::early(&self.tcx.hir(), &mut index, &param);
 700                             if let hir::ParamName::Plain(param_name) = name {
 701                                 if param_name.name == kw::UnderscoreLifetime {
 702                                     // Pick the elided lifetime "definition" if one exists
 703                                     // and use it to make an elision scope.
 704                                     elision = Some(reg);
 705                                 } else {
 706                                     lifetimes.insert(name, reg);
 707                                 }
 708                             } else {
 709                                 lifetimes.insert(name, reg);
 710                             }
 711                         }
 712                         GenericParamKind::Type { .. } |
 713                         GenericParamKind::Const { .. } => {
 714                             non_lifetime_count += 1;
 715                         }
 716                     }
 717                 }
 718                 let next_early_index = index + non_lifetime_count;
 719
 720                 if let Some(elision_region) = elision {
 721                     let scope = Scope::Elision {
 722                         elide: Elide::Exact(elision_region),
 723                         s: self.scope,
 724                     };
 725                     self.with(scope, |_old_scope, this| {
 726                         let scope = Scope::Binder {
 727                             lifetimes,
 728                             next_early_index,
 729                             s: this.scope,
 730                             track_lifetime_uses: true,
 731                             abstract_type_parent: false,
 732                         };
 733                         this.with(scope, |_old_scope, this| {
 734                             this.visit_generics(generics);
 735                             for bound in bounds {
 736                                 this.visit_param_bound(bound);
 737                             }
 738                         });
 739                     });
 740                 } else {
 741                     let scope = Scope::Binder {
 742                         lifetimes,
 743                         next_early_index,
 744                         s: self.scope,
 745                         track_lifetime_uses: true,
 746                         abstract_type_parent: false,
 747                     };
 748                     self.with(scope, |_old_scope, this| {
 749                         this.visit_generics(generics);
 750                         for bound in bounds {
 751                             this.visit_param_bound(bound);
 752                         }
 753                     });
 754                 }
 755             }
 756             hir::TyKind::CVarArgs(ref lt) => {
 757                 // Resolve the generated lifetime for the C-variadic arguments.
 758                 // The lifetime is generated in AST -> HIR lowering.
 759                 if lt.name.is_elided() {
 760                     self.resolve_elided_lifetimes(vec![lt])
 761                 }
 762             }
 763             _ => intravisit::walk_ty(self, ty),
 764         }
 765     }
 766
 767     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
 768         use self::hir::TraitItemKind::*;
 769         match trait_item.node {
 770             Method(ref sig, _) => {
 771                 let tcx = self.tcx;
 772                 self.visit_early_late(
 773                     Some(tcx.hir().get_parent_item(trait_item.hir_id)),
 774                     &sig.decl,
 775                     &trait_item.generics,
 776                     |this| intravisit::walk_trait_item(this, trait_item),
 777                 );
 778             }
 779             Type(ref bounds, ref ty) => {
 780                 let generics = &trait_item.generics;
 781                 let mut index = self.next_early_index();
 782                 debug!("visit_ty: index = {}", index);
 783                 let mut non_lifetime_count = 0;
 784                 let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
 785                     GenericParamKind::Lifetime { .. } => {
 786                         Some(Region::early(&self.tcx.hir(), &mut index, param))
 787                     }
 788                     GenericParamKind::Type { .. } |
 789                     GenericParamKind::Const { .. } => {
 790                         non_lifetime_count += 1;
 791                         None
 792                     }
 793                 }).collect();
 794                 let scope = Scope::Binder {
 795                     lifetimes,
 796                     next_early_index: index + non_lifetime_count,
 797                     s: self.scope,
 798                     track_lifetime_uses: true,
 799                     abstract_type_parent: true,
 800                 };
 801                 self.with(scope, |_old_scope, this| {
 802                     this.visit_generics(generics);
 803                     for bound in bounds {
 804                         this.visit_param_bound(bound);
 805                     }
 806                     if let Some(ty) = ty {
 807                         this.visit_ty(ty);
 808                     }
 809                 });
 810             }
 811             Const(_, _) => {
 812                 // Only methods and types support generics.
 813                 assert!(trait_item.generics.params.is_empty());
 814                 intravisit::walk_trait_item(self, trait_item);
 815             }
 816         }
 817     }
 818
 819     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
 820         use self::hir::ImplItemKind::*;
 821         match impl_item.node {
 822             Method(ref sig, _) => {
 823                 let tcx = self.tcx;
 824                 self.visit_early_late(
 825                     Some(tcx.hir().get_parent_item(impl_item.hir_id)),
 826                     &sig.decl,
 827                     &impl_item.generics,
 828                     |this| intravisit::walk_impl_item(this, impl_item),
 829                 )
 830             }
 831             Type(ref ty) => {
 832                 let generics = &impl_item.generics;
 833                 let mut index = self.next_early_index();
 834                 let mut non_lifetime_count = 0;
 835                 debug!("visit_ty: index = {}", index);
 836                 let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
 837                     GenericParamKind::Lifetime { .. } => {
 838                         Some(Region::early(&self.tcx.hir(), &mut index, param))
 839                     }
 840                     GenericParamKind::Const { .. } |
 841                     GenericParamKind::Type { .. } => {
 842                         non_lifetime_count += 1;
 843                         None
 844                     }
 845                 }).collect();
 846                 let scope = Scope::Binder {
 847                     lifetimes,
 848                     next_early_index: index + non_lifetime_count,
 849                     s: self.scope,
 850                     track_lifetime_uses: true,
 851                     abstract_type_parent: true,
 852                 };
 853                 self.with(scope, |_old_scope, this| {
 854                     this.visit_generics(generics);
 855                     this.visit_ty(ty);
 856                 });
 857             }
 858             Existential(ref bounds) => {
 859                 let generics = &impl_item.generics;
 860                 let mut index = self.next_early_index();
 861                 let mut next_early_index = index;
 862                 debug!("visit_ty: index = {}", index);
 863                 let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
 864                     GenericParamKind::Lifetime { .. } => {
 865                         Some(Region::early(&self.tcx.hir(), &mut index, param))
 866                     }
 867                     GenericParamKind::Type { .. } => {
 868                         next_early_index += 1;
 869                         None
 870                     }
 871                     GenericParamKind::Const { .. } => {
 872                         next_early_index += 1;
 873                         None
 874                     }
 875                 }).collect();
 876
 877                 let scope = Scope::Binder {
 878                     lifetimes,
 879                     next_early_index,
 880                     s: self.scope,
 881                     track_lifetime_uses: true,
 882                     abstract_type_parent: true,
 883                 };
 884                 self.with(scope, |_old_scope, this| {
 885                     this.visit_generics(generics);
 886                     for bound in bounds {
 887                         this.visit_param_bound(bound);
 888                     }
 889                 });
 890             }
 891             Const(_, _) => {
 892                 // Only methods and types support generics.
 893                 assert!(impl_item.generics.params.is_empty());
 894                 intravisit::walk_impl_item(self, impl_item);
 895             }
 896         }
 897     }
 898
 899     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
 900         if lifetime_ref.is_elided() {
 901             self.resolve_elided_lifetimes(vec![lifetime_ref]);
 902             return;
 903         }
 904         if lifetime_ref.is_static() {
 905             self.insert_lifetime(lifetime_ref, Region::Static);
 906             return;
 907         }
 908         self.resolve_lifetime_ref(lifetime_ref);
 909     }
 910
 911     fn visit_path(&mut self, path: &'tcx hir::Path, _: hir::HirId) {
 912         for (i, segment) in path.segments.iter().enumerate() {
 913             let depth = path.segments.len() - i - 1;
 914             if let Some(ref args) = segment.args {
 915                 self.visit_segment_args(path.res, depth, args);
 916             }
 917         }
 918     }
 919
 920     fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl) {
 921         let output = match fd.output {
 922             hir::DefaultReturn(_) => None,
 923             hir::Return(ref ty) => Some(&**ty),
 924         };
 925         self.visit_fn_like_elision(&fd.inputs, output);
 926     }
 927
 928     fn visit_generics(&mut self, generics: &'tcx hir::Generics) {
 929         check_mixed_explicit_and_in_band_defs(self.tcx, &generics.params);
 930         for param in &generics.params {
 931             match param.kind {
 932                 GenericParamKind::Lifetime { .. } => {}
 933                 GenericParamKind::Type { ref default, .. } => {
 934                     walk_list!(self, visit_param_bound, &param.bounds);
 935                     if let Some(ref ty) = default {
 936                         self.visit_ty(&ty);
 937                     }
 938                 }
 939                 GenericParamKind::Const { ref ty, .. } => {
 940                     walk_list!(self, visit_param_bound, &param.bounds);
 941                     self.visit_ty(&ty);
 942                 }
 943             }
 944         }
 945         for predicate in &generics.where_clause.predicates {
 946             match predicate {
 947                 &hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
 948                     ref bounded_ty,
 949                     ref bounds,
 950                     ref bound_generic_params,
 951                     ..
 952                 }) => {
 953                     let lifetimes: FxHashMap<_, _> = bound_generic_params
 954                         .iter()
 955                         .filter_map(|param| match param.kind {
 956                             GenericParamKind::Lifetime { .. } => {
 957                                 Some(Region::late(&self.tcx.hir(), param))
 958                             }
 959                             _ => None,
 960                         })
 961                         .collect();
 962                     if !lifetimes.is_empty() {
 963                         self.trait_ref_hack = true;
 964                         let next_early_index = self.next_early_index();
 965                         let scope = Scope::Binder {
 966                             lifetimes,
 967                             s: self.scope,
 968                             next_early_index,
 969                             track_lifetime_uses: true,
 970                             abstract_type_parent: false,
 971                         };
 972                         let result = self.with(scope, |old_scope, this| {
 973                             this.check_lifetime_params(old_scope, &bound_generic_params);
 974                             this.visit_ty(&bounded_ty);
 975                             walk_list!(this, visit_param_bound, bounds);
 976                         });
 977                         self.trait_ref_hack = false;
 978                         result
 979                     } else {
 980                         self.visit_ty(&bounded_ty);
 981                         walk_list!(self, visit_param_bound, bounds);
 982                     }
 983                 }
 984                 &hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
 985                     ref lifetime,
 986                     ref bounds,
 987                     ..
 988                 }) => {
 989                     self.visit_lifetime(lifetime);
 990                     walk_list!(self, visit_param_bound, bounds);
 991                 }
 992                 &hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
 993                     ref lhs_ty,
 994                     ref rhs_ty,
 995                     ..
 996                 }) => {
 997                     self.visit_ty(lhs_ty);
 998                     self.visit_ty(rhs_ty);
 999                 }
1000             }
1001         }
1002     }
1003
1004     fn visit_poly_trait_ref(
1005         &mut self,
1006         trait_ref: &'tcx hir::PolyTraitRef,
1007         _modifier: hir::TraitBoundModifier,
1008     ) {
1009         debug!("visit_poly_trait_ref(trait_ref={:?})", trait_ref);
1010
1011         if !self.trait_ref_hack || trait_ref.bound_generic_params.iter().any(|param| {
1012             match param.kind {
1013                 GenericParamKind::Lifetime { .. } => true,
1014                 _ => false,
1015             }
1016         }) {
1017             if self.trait_ref_hack {
1018                 span_err!(
1019                     self.tcx.sess,
1020                     trait_ref.span,
1021                     E0316,
1022                     "nested quantification of lifetimes"
1023                 );
1024             }
1025             let next_early_index = self.next_early_index();
1026             let scope = Scope::Binder {
1027                 lifetimes: trait_ref
1028                     .bound_generic_params
1029                     .iter()
1030                     .filter_map(|param| match param.kind {
1031                         GenericParamKind::Lifetime { .. } => {
1032                             Some(Region::late(&self.tcx.hir(), param))
1033                         }
1034                         _ => None,
1035                     })
1036                     .collect(),
1037                 s: self.scope,
1038                 next_early_index,
1039                 track_lifetime_uses: true,
1040                 abstract_type_parent: false,
1041             };
1042             self.with(scope, |old_scope, this| {
1043                 this.check_lifetime_params(old_scope, &trait_ref.bound_generic_params);
1044                 walk_list!(this, visit_generic_param, &trait_ref.bound_generic_params);
1045                 this.visit_trait_ref(&trait_ref.trait_ref)
1046             })
1047         } else {
1048             self.visit_trait_ref(&trait_ref.trait_ref)
1049         }
1050     }
1051 }
1052
1053 #[derive(Copy, Clone, PartialEq)]
1054 enum ShadowKind {
1055     Label,
1056     Lifetime,
1057 }
1058 struct Original {
1059     kind: ShadowKind,
1060     span: Span,
1061 }
1062 struct Shadower {
1063     kind: ShadowKind,
1064     span: Span,
1065 }
1066
1067 fn original_label(span: Span) -> Original {
1068     Original {
1069         kind: ShadowKind::Label,
1070         span: span,
1071     }
1072 }
1073 fn shadower_label(span: Span) -> Shadower {
1074     Shadower {
1075         kind: ShadowKind::Label,
1076         span: span,
1077     }
1078 }
1079 fn original_lifetime(span: Span) -> Original {
1080     Original {
1081         kind: ShadowKind::Lifetime,
1082         span: span,
1083     }
1084 }
1085 fn shadower_lifetime(param: &hir::GenericParam) -> Shadower {
1086     Shadower {
1087         kind: ShadowKind::Lifetime,
1088         span: param.span,
1089     }
1090 }
1091
1092 impl ShadowKind {
1093     fn desc(&self) -> &'static str {
1094         match *self {
1095             ShadowKind::Label => "label",
1096             ShadowKind::Lifetime => "lifetime",
1097         }
1098     }
1099 }
1100
1101 fn check_mixed_explicit_and_in_band_defs(tcx: TyCtxt<'_>, params: &P<[hir::GenericParam]>) {
1102     let lifetime_params: Vec<_> = params
1103         .iter()
1104         .filter_map(|param| match param.kind {
1105             GenericParamKind::Lifetime { kind, .. } => Some((kind, param.span)),
1106             _ => None,
1107         })
1108         .collect();
1109     let explicit = lifetime_params
1110         .iter()
1111         .find(|(kind, _)| *kind == LifetimeParamKind::Explicit);
1112     let in_band = lifetime_params
1113         .iter()
1114         .find(|(kind, _)| *kind == LifetimeParamKind::InBand);
1115
1116     if let (Some((_, explicit_span)), Some((_, in_band_span))) = (explicit, in_band) {
1117         struct_span_err!(
1118             tcx.sess,
1119             *in_band_span,
1120             E0688,
1121             "cannot mix in-band and explicit lifetime definitions"
1122         ).span_label(*in_band_span, "in-band lifetime definition here")
1123             .span_label(*explicit_span, "explicit lifetime definition here")
1124             .emit();
1125     }
1126 }
1127
1128 fn signal_shadowing_problem(tcx: TyCtxt<'_>, name: ast::Name, orig: Original, shadower: Shadower) {
1129     let mut err = if let (ShadowKind::Lifetime, ShadowKind::Lifetime) = (orig.kind, shadower.kind) {
1130         // lifetime/lifetime shadowing is an error
1131         struct_span_err!(
1132             tcx.sess,
1133             shadower.span,
1134             E0496,
1135             "{} name `{}` shadows a \
1136              {} name that is already in scope",
1137             shadower.kind.desc(),
1138             name,
1139             orig.kind.desc()
1140         )
1141     } else {
1142         // shadowing involving a label is only a warning, due to issues with
1143         // labels and lifetimes not being macro-hygienic.
1144         tcx.sess.struct_span_warn(
1145             shadower.span,
1146             &format!(
1147                 "{} name `{}` shadows a \
1148                  {} name that is already in scope",
1149                 shadower.kind.desc(),
1150                 name,
1151                 orig.kind.desc()
1152             ),
1153         )
1154     };
1155     err.span_label(orig.span, "first declared here");
1156     err.span_label(shadower.span, format!("lifetime {} already in scope", name));
1157     err.emit();
1158 }
1159
1160 // Adds all labels in `b` to `ctxt.labels_in_fn`, signalling a warning
1161 // if one of the label shadows a lifetime or another label.
1162 fn extract_labels(ctxt: &mut LifetimeContext<'_, '_>, body: &hir::Body) {
1163     struct GatherLabels<'a, 'tcx> {
1164         tcx: TyCtxt<'tcx>,
1165         scope: ScopeRef<'a>,
1166         labels_in_fn: &'a mut Vec<ast::Ident>,
1167     }
1168
1169     let mut gather = GatherLabels {
1170         tcx: ctxt.tcx,
1171         scope: ctxt.scope,
1172         labels_in_fn: &mut ctxt.labels_in_fn,
1173     };
1174     gather.visit_body(body);
1175
1176     impl<'v, 'a, 'tcx> Visitor<'v> for GatherLabels<'a, 'tcx> {
1177         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
1178             NestedVisitorMap::None
1179         }
1180
1181         fn visit_expr(&mut self, ex: &hir::Expr) {
1182             if let Some(label) = expression_label(ex) {
1183                 for prior_label in &self.labels_in_fn[..] {
1184                     // FIXME (#24278): non-hygienic comparison
1185                     if label.name == prior_label.name {
1186                         signal_shadowing_problem(
1187                             self.tcx,
1188                             label.name,
1189                             original_label(prior_label.span),
1190                             shadower_label(label.span),
1191                         );
1192                     }
1193                 }
1194
1195                 check_if_label_shadows_lifetime(self.tcx, self.scope, label);
1196
1197                 self.labels_in_fn.push(label);
1198             }
1199             intravisit::walk_expr(self, ex)
1200         }
1201     }
1202
1203     fn expression_label(ex: &hir::Expr) -> Option<ast::Ident> {
1204         match ex.node {
1205             hir::ExprKind::While(.., Some(label)) | hir::ExprKind::Loop(_, Some(label), _) => {
1206                 Some(label.ident)
1207             }
1208             _ => None,
1209         }
1210     }
1211
1212     fn check_if_label_shadows_lifetime(
1213         tcx: TyCtxt<'_>,
1214         mut scope: ScopeRef<'_>,
1215         label: ast::Ident,
1216     ) {
1217         loop {
1218             match *scope {
1219                 Scope::Body { s, .. }
1220                 | Scope::Elision { s, .. }
1221                 | Scope::ObjectLifetimeDefault { s, .. } => {
1222                     scope = s;
1223                 }
1224
1225                 Scope::Root => {
1226                     return;
1227                 }
1228
1229                 Scope::Binder {
1230                     ref lifetimes, s, ..
1231                 } => {
1232                     // FIXME (#24278): non-hygienic comparison
1233                     if let Some(def) = lifetimes.get(&hir::ParamName::Plain(label.modern())) {
1234                         let hir_id = tcx.hir().as_local_hir_id(def.id().unwrap()).unwrap();
1235
1236                         signal_shadowing_problem(
1237                             tcx,
1238                             label.name,
1239                             original_lifetime(tcx.hir().span(hir_id)),
1240                             shadower_label(label.span),
1241                         );
1242                         return;
1243                     }
1244                     scope = s;
1245                 }
1246             }
1247         }
1248     }
1249 }
1250
1251 fn compute_object_lifetime_defaults(tcx: TyCtxt<'_>) -> HirIdMap<Vec<ObjectLifetimeDefault>> {
1252     let mut map = HirIdMap::default();
1253     for item in tcx.hir().krate().items.values() {
1254         match item.node {
1255             hir::ItemKind::Struct(_, ref generics)
1256             | hir::ItemKind::Union(_, ref generics)
1257             | hir::ItemKind::Enum(_, ref generics)
1258             | hir::ItemKind::Existential(hir::ExistTy {
1259                 ref generics,
1260                 impl_trait_fn: None,
1261                 ..
1262             })
1263             | hir::ItemKind::Ty(_, ref generics)
1264             | hir::ItemKind::Trait(_, _, ref generics, ..) => {
1265                 let result = object_lifetime_defaults_for_item(tcx, generics);
1266
1267                 // Debugging aid.
1268                 if attr::contains_name(&item.attrs, sym::rustc_object_lifetime_default) {
1269                     let object_lifetime_default_reprs: String = result
1270                         .iter()
1271                         .map(|set| match *set {
1272                             Set1::Empty => "BaseDefault".into(),
1273                             Set1::One(Region::Static) => "'static".into(),
1274                             Set1::One(Region::EarlyBound(mut i, _, _)) => generics
1275                                 .params
1276                                 .iter()
1277                                 .find_map(|param| match param.kind {
1278                                     GenericParamKind::Lifetime { .. } => {
1279                                         if i == 0 {
1280                                             return Some(param.name.ident().to_string().into());
1281                                         }
1282                                         i -= 1;
1283                                         None
1284                                     }
1285                                     _ => None,
1286                                 })
1287                                 .unwrap(),
1288                             Set1::One(_) => bug!(),
1289                             Set1::Many => "Ambiguous".into(),
1290                         })
1291                         .collect::<Vec<Cow<'static, str>>>()
1292                         .join(",");
1293                     tcx.sess.span_err(item.span, &object_lifetime_default_reprs);
1294                 }
1295
1296                 map.insert(item.hir_id, result);
1297             }
1298             _ => {}
1299         }
1300     }
1301     map
1302 }
1303
1304 /// Scan the bounds and where-clauses on parameters to extract bounds
1305 /// of the form `T:'a` so as to determine the `ObjectLifetimeDefault`
1306 /// for each type parameter.
1307 fn object_lifetime_defaults_for_item(
1308     tcx: TyCtxt<'_>,
1309     generics: &hir::Generics,
1310 ) -> Vec<ObjectLifetimeDefault> {
1311     fn add_bounds(set: &mut Set1<hir::LifetimeName>, bounds: &[hir::GenericBound]) {
1312         for bound in bounds {
1313             if let hir::GenericBound::Outlives(ref lifetime) = *bound {
1314                 set.insert(lifetime.name.modern());
1315             }
1316         }
1317     }
1318
1319     generics
1320         .params
1321         .iter()
1322         .filter_map(|param| match param.kind {
1323             GenericParamKind::Lifetime { .. } => None,
1324             GenericParamKind::Type { .. } => {
1325                 let mut set = Set1::Empty;
1326
1327                 add_bounds(&mut set, &param.bounds);
1328
1329                 let param_def_id = tcx.hir().local_def_id_from_hir_id(param.hir_id);
1330                 for predicate in &generics.where_clause.predicates {
1331                     // Look for `type: ...` where clauses.
1332                     let data = match *predicate {
1333                         hir::WherePredicate::BoundPredicate(ref data) => data,
1334                         _ => continue,
1335                     };
1336
1337                     // Ignore `for<'a> type: ...` as they can change what
1338                     // lifetimes mean (although we could "just" handle it).
1339                     if !data.bound_generic_params.is_empty() {
1340                         continue;
1341                     }
1342
1343                     let res = match data.bounded_ty.node {
1344                         hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => path.res,
1345                         _ => continue,
1346                     };
1347
1348                     if res == Res::Def(DefKind::TyParam, param_def_id) {
1349                         add_bounds(&mut set, &data.bounds);
1350                     }
1351                 }
1352
1353                 Some(match set {
1354                     Set1::Empty => Set1::Empty,
1355                     Set1::One(name) => {
1356                         if name == hir::LifetimeName::Static {
1357                             Set1::One(Region::Static)
1358                         } else {
1359                             generics
1360                                 .params
1361                                 .iter()
1362                                 .filter_map(|param| match param.kind {
1363                                     GenericParamKind::Lifetime { .. } => Some((
1364                                         param.hir_id,
1365                                         hir::LifetimeName::Param(param.name),
1366                                         LifetimeDefOrigin::from_param(param),
1367                                     )),
1368                                     _ => None,
1369                                 })
1370                                 .enumerate()
1371                                 .find(|&(_, (_, lt_name, _))| lt_name == name)
1372                                 .map_or(Set1::Many, |(i, (id, _, origin))| {
1373                                     let def_id = tcx.hir().local_def_id_from_hir_id(id);
1374                                     Set1::One(Region::EarlyBound(i as u32, def_id, origin))
1375                                 })
1376                         }
1377                     }
1378                     Set1::Many => Set1::Many,
1379                 })
1380             }
1381             GenericParamKind::Const { .. } => {
1382                 // Generic consts don't impose any constraints.
1383                 None
1384             }
1385         })
1386         .collect()
1387 }
1388
1389 impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
1390     // FIXME(#37666) this works around a limitation in the region inferencer
1391     fn hack<F>(&mut self, f: F)
1392     where
1393         F: for<'b> FnOnce(&mut LifetimeContext<'b, 'tcx>),
1394     {
1395         f(self)
1396     }
1397
1398     fn with<F>(&mut self, wrap_scope: Scope<'_>, f: F)
1399     where
1400         F: for<'b> FnOnce(ScopeRef<'_>, &mut LifetimeContext<'b, 'tcx>),
1401     {
1402         let LifetimeContext {
1403             tcx,
1404             map,
1405             lifetime_uses,
1406             ..
1407         } = self;
1408         let labels_in_fn = replace(&mut self.labels_in_fn, vec![]);
1409         let xcrate_object_lifetime_defaults =
1410             replace(&mut self.xcrate_object_lifetime_defaults, DefIdMap::default());
1411         let mut this = LifetimeContext {
1412             tcx: *tcx,
1413             map: map,
1414             scope: &wrap_scope,
1415             trait_ref_hack: self.trait_ref_hack,
1416             is_in_fn_syntax: self.is_in_fn_syntax,
1417             labels_in_fn,
1418             xcrate_object_lifetime_defaults,
1419             lifetime_uses: lifetime_uses,
1420         };
1421         debug!("entering scope {:?}", this.scope);
1422         f(self.scope, &mut this);
1423         this.check_uses_for_lifetimes_defined_by_scope();
1424         debug!("exiting scope {:?}", this.scope);
1425         self.labels_in_fn = this.labels_in_fn;
1426         self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;
1427     }
1428
1429     /// helper method to determine the span to remove when suggesting the
1430     /// deletion of a lifetime
1431     fn lifetime_deletion_span(&self, name: ast::Ident, generics: &hir::Generics) -> Option<Span> {
1432         generics.params.iter().enumerate().find_map(|(i, param)| {
1433             if param.name.ident() == name {
1434                 let mut in_band = false;
1435                 if let hir::GenericParamKind::Lifetime { kind } = param.kind {
1436                     if let hir::LifetimeParamKind::InBand = kind {
1437                         in_band = true;
1438                     }
1439                 }
1440                 if in_band {
1441                     Some(param.span)
1442                 } else {
1443                     if generics.params.len() == 1 {
1444                         // if sole lifetime, remove the entire `<>` brackets
1445                         Some(generics.span)
1446                     } else {
1447                         // if removing within `<>` brackets, we also want to
1448                         // delete a leading or trailing comma as appropriate
1449                         if i >= generics.params.len() - 1 {
1450                             Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
1451                         } else {
1452                             Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
1453                         }
1454                     }
1455                 }
1456             } else {
1457                 None
1458             }
1459         })
1460     }
1461
1462     // helper method to issue suggestions from `fn rah<'a>(&'a T)` to `fn rah(&T)`
1463     fn suggest_eliding_single_use_lifetime(
1464         &self, err: &mut DiagnosticBuilder<'_>, def_id: DefId, lifetime: &hir::Lifetime
1465     ) {
1466         // FIXME: future work: also suggest `impl Foo<'_>` for `impl<'a> Foo<'a>`
1467         let name = lifetime.name.ident();
1468         let mut remove_decl = None;
1469         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1470             if let Some(generics) = self.tcx.hir().get_generics(parent_def_id) {
1471                 remove_decl = self.lifetime_deletion_span(name, generics);
1472             }
1473         }
1474
1475         let mut remove_use = None;
1476         let mut find_arg_use_span = |inputs: &hir::HirVec<hir::Ty>| {
1477             for input in inputs {
1478                 if let hir::TyKind::Rptr(lt, _) = input.node {
1479                     if lt.name.ident() == name {
1480                         // include the trailing whitespace between the ampersand and the type name
1481                         let lt_through_ty_span = lifetime.span.to(input.span.shrink_to_hi());
1482                         remove_use = Some(
1483                             self.tcx.sess.source_map()
1484                                 .span_until_non_whitespace(lt_through_ty_span)
1485                         );
1486                         break;
1487                     }
1488                 }
1489             }
1490         };
1491         if let Node::Lifetime(hir_lifetime) = self.tcx.hir().get(lifetime.hir_id) {
1492             if let Some(parent) = self.tcx.hir().find(
1493                 self.tcx.hir().get_parent_item(hir_lifetime.hir_id))
1494             {
1495                 match parent {
1496                     Node::Item(item) => {
1497                         if let hir::ItemKind::Fn(decl, _, _, _) = &item.node {
1498                             find_arg_use_span(&decl.inputs);
1499                         }
1500                     },
1501                     Node::ImplItem(impl_item) => {
1502                         if let hir::ImplItemKind::Method(sig, _) = &impl_item.node {
1503                             find_arg_use_span(&sig.decl.inputs);
1504                         }
1505                     }
1506                     _ => {}
1507                 }
1508             }
1509         }
1510
1511         if let (Some(decl_span), Some(use_span)) = (remove_decl, remove_use) {
1512             // if both declaration and use deletion spans start at the same
1513             // place ("start at" because the latter includes trailing
1514             // whitespace), then this is an in-band lifetime
1515             if decl_span.shrink_to_lo() == use_span.shrink_to_lo() {
1516                 err.span_suggestion(
1517                     use_span,
1518                     "elide the single-use lifetime",
1519                     String::new(),
1520                     Applicability::MachineApplicable,
1521                 );
1522             } else {
1523                 err.multipart_suggestion(
1524                     "elide the single-use lifetime",
1525                     vec![(decl_span, String::new()), (use_span, String::new())],
1526                     Applicability::MachineApplicable,
1527                 );
1528             }
1529         }
1530     }
1531
1532     fn check_uses_for_lifetimes_defined_by_scope(&mut self) {
1533         let defined_by = match self.scope {
1534             Scope::Binder { lifetimes, .. } => lifetimes,
1535             _ => {
1536                 debug!("check_uses_for_lifetimes_defined_by_scope: not in a binder scope");
1537                 return;
1538             }
1539         };
1540
1541         let mut def_ids: Vec<_> = defined_by
1542             .values()
1543             .flat_map(|region| match region {
1544                 Region::EarlyBound(_, def_id, _)
1545                 | Region::LateBound(_, def_id, _)
1546                 | Region::Free(_, def_id) => Some(*def_id),
1547
1548                 Region::LateBoundAnon(..) | Region::Static => None,
1549             })
1550             .collect();
1551
1552         // ensure that we issue lints in a repeatable order
1553         def_ids.sort_by_cached_key(|&def_id| self.tcx.def_path_hash(def_id));
1554
1555         for def_id in def_ids {
1556             debug!(
1557                 "check_uses_for_lifetimes_defined_by_scope: def_id = {:?}",
1558                 def_id
1559             );
1560
1561             let lifetimeuseset = self.lifetime_uses.remove(&def_id);
1562
1563             debug!(
1564                 "check_uses_for_lifetimes_defined_by_scope: lifetimeuseset = {:?}",
1565                 lifetimeuseset
1566             );
1567
1568             match lifetimeuseset {
1569                 Some(LifetimeUseSet::One(lifetime)) => {
1570                     let hir_id = self.tcx.hir().as_local_hir_id(def_id).unwrap();
1571                     debug!("hir id first={:?}", hir_id);
1572                     if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
1573                         Node::Lifetime(hir_lifetime) => Some((
1574                             hir_lifetime.hir_id,
1575                             hir_lifetime.span,
1576                             hir_lifetime.name.ident(),
1577                         )),
1578                         Node::GenericParam(param) => {
1579                             Some((param.hir_id, param.span, param.name.ident()))
1580                         }
1581                         _ => None,
1582                     } {
1583                         debug!("id = {:?} span = {:?} name = {:?}", id, span, name);
1584
1585                         if name.name == kw::UnderscoreLifetime {
1586                             continue;
1587                         }
1588
1589                         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1590                             if let Some(parent_hir_id) = self.tcx.hir()
1591                                 .as_local_hir_id(parent_def_id) {
1592                                     // lifetimes in `derive` expansions don't count (Issue #53738)
1593                                     if self.tcx.hir().attrs(parent_hir_id).iter()
1594                                         .any(|attr| attr.check_name(sym::automatically_derived)) {
1595                                             continue;
1596                                         }
1597                                 }
1598                         }
1599
1600                         let mut err = self.tcx.struct_span_lint_hir(
1601                             lint::builtin::SINGLE_USE_LIFETIMES,
1602                             id,
1603                             span,
1604                             &format!("lifetime parameter `{}` only used once", name),
1605                         );
1606
1607                         if span == lifetime.span {
1608                             // spans are the same for in-band lifetime declarations
1609                             err.span_label(span, "this lifetime is only used here");
1610                         } else {
1611                             err.span_label(span, "this lifetime...");
1612                             err.span_label(lifetime.span, "...is used only here");
1613                         }
1614                         self.suggest_eliding_single_use_lifetime(&mut err, def_id, lifetime);
1615                         err.emit();
1616                     }
1617                 }
1618                 Some(LifetimeUseSet::Many) => {
1619                     debug!("Not one use lifetime");
1620                 }
1621                 None => {
1622                     let hir_id = self.tcx.hir().as_local_hir_id(def_id).unwrap();
1623                     if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
1624                         Node::Lifetime(hir_lifetime) => Some((
1625                             hir_lifetime.hir_id,
1626                             hir_lifetime.span,
1627                             hir_lifetime.name.ident(),
1628                         )),
1629                         Node::GenericParam(param) => {
1630                             Some((param.hir_id, param.span, param.name.ident()))
1631                         }
1632                         _ => None,
1633                     } {
1634                         debug!("id ={:?} span = {:?} name = {:?}", id, span, name);
1635                         let mut err = self.tcx.struct_span_lint_hir(
1636                             lint::builtin::UNUSED_LIFETIMES,
1637                             id,
1638                             span,
1639                             &format!("lifetime parameter `{}` never used", name),
1640                         );
1641                         if let Some(parent_def_id) = self.tcx.parent(def_id) {
1642                             if let Some(generics) = self.tcx.hir().get_generics(parent_def_id) {
1643                                 let unused_lt_span = self.lifetime_deletion_span(name, generics);
1644                                 if let Some(span) = unused_lt_span {
1645                                     err.span_suggestion(
1646                                         span,
1647                                         "elide the unused lifetime",
1648                                         String::new(),
1649                                         Applicability::MachineApplicable,
1650                                     );
1651                                 }
1652                             }
1653                         }
1654                         err.emit();
1655                     }
1656                 }
1657             }
1658         }
1659     }
1660
1661     /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
1662     ///
1663     /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
1664     /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
1665     /// within type bounds; those are early bound lifetimes, and the rest are late bound.
1666     ///
1667     /// For example:
1668     ///
1669     ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
1670     ///
1671     /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
1672     /// lifetimes may be interspersed together.
1673     ///
1674     /// If early bound lifetimes are present, we separate them into their own list (and likewise
1675     /// for late bound). They will be numbered sequentially, starting from the lowest index that is
1676     /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
1677     /// bound lifetimes are resolved by name and associated with a binder ID (`binder_id`), so the
1678     /// ordering is not important there.
1679     fn visit_early_late<F>(
1680         &mut self,
1681         parent_id: Option<hir::HirId>,
1682         decl: &'tcx hir::FnDecl,
1683         generics: &'tcx hir::Generics,
1684         walk: F,
1685     ) where
1686         F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
1687     {
1688         insert_late_bound_lifetimes(self.map, decl, generics);
1689
1690         // Find the start of nested early scopes, e.g., in methods.
1691         let mut index = 0;
1692         if let Some(parent_id) = parent_id {
1693             let parent = self.tcx.hir().expect_item(parent_id);
1694             if sub_items_have_self_param(&parent.node) {
1695                 index += 1; // Self comes before lifetimes
1696             }
1697             match parent.node {
1698                 hir::ItemKind::Trait(_, _, ref generics, ..)
1699                 | hir::ItemKind::Impl(_, _, _, ref generics, ..) => {
1700                     index += generics.params.len() as u32;
1701                 }
1702                 _ => {}
1703             }
1704         }
1705
1706         let mut non_lifetime_count = 0;
1707         let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
1708             GenericParamKind::Lifetime { .. } => {
1709                 if self.map.late_bound.contains(&param.hir_id) {
1710                     Some(Region::late(&self.tcx.hir(), param))
1711                 } else {
1712                     Some(Region::early(&self.tcx.hir(), &mut index, param))
1713                 }
1714             }
1715             GenericParamKind::Type { .. } |
1716             GenericParamKind::Const { .. } => {
1717                 non_lifetime_count += 1;
1718                 None
1719             }
1720         }).collect();
1721         let next_early_index = index + non_lifetime_count;
1722
1723         let scope = Scope::Binder {
1724             lifetimes,
1725             next_early_index,
1726             s: self.scope,
1727             abstract_type_parent: true,
1728             track_lifetime_uses: false,
1729         };
1730         self.with(scope, move |old_scope, this| {
1731             this.check_lifetime_params(old_scope, &generics.params);
1732             this.hack(walk); // FIXME(#37666) workaround in place of `walk(this)`
1733         });
1734     }
1735
1736     fn next_early_index_helper(&self, only_abstract_type_parent: bool) -> u32 {
1737         let mut scope = self.scope;
1738         loop {
1739             match *scope {
1740                 Scope::Root => return 0,
1741
1742                 Scope::Binder {
1743                     next_early_index,
1744                     abstract_type_parent,
1745                     ..
1746                 } if (!only_abstract_type_parent || abstract_type_parent) =>
1747                 {
1748                     return next_early_index
1749                 }
1750
1751                 Scope::Binder { s, .. }
1752                 | Scope::Body { s, .. }
1753                 | Scope::Elision { s, .. }
1754                 | Scope::ObjectLifetimeDefault { s, .. } => scope = s,
1755             }
1756         }
1757     }
1758
1759     /// Returns the next index one would use for an early-bound-region
1760     /// if extending the current scope.
1761     fn next_early_index(&self) -> u32 {
1762         self.next_early_index_helper(true)
1763     }
1764
1765     /// Returns the next index one would use for an `impl Trait` that
1766     /// is being converted into an `abstract type`. This will be the
1767     /// next early index from the enclosing item, for the most
1768     /// part. See the `abstract_type_parent` field for more info.
1769     fn next_early_index_for_abstract_type(&self) -> u32 {
1770         self.next_early_index_helper(false)
1771     }
1772
1773     fn resolve_lifetime_ref(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
1774         debug!("resolve_lifetime_ref(lifetime_ref={:?})", lifetime_ref);
1775
1776         // If we've already reported an error, just ignore `lifetime_ref`.
1777         if let LifetimeName::Error = lifetime_ref.name {
1778             return;
1779         }
1780
1781         // Walk up the scope chain, tracking the number of fn scopes
1782         // that we pass through, until we find a lifetime with the
1783         // given name or we run out of scopes.
1784         // search.
1785         let mut late_depth = 0;
1786         let mut scope = self.scope;
1787         let mut outermost_body = None;
1788         let result = loop {
1789             match *scope {
1790                 Scope::Body { id, s } => {
1791                     outermost_body = Some(id);
1792                     scope = s;
1793                 }
1794
1795                 Scope::Root => {
1796                     break None;
1797                 }
1798
1799                 Scope::Binder {
1800                     ref lifetimes, s, ..
1801                 } => {
1802                     match lifetime_ref.name {
1803                         LifetimeName::Param(param_name) => {
1804                             if let Some(&def) = lifetimes.get(&param_name.modern()) {
1805                                 break Some(def.shifted(late_depth));
1806                             }
1807                         }
1808                         _ => bug!("expected LifetimeName::Param"),
1809                     }
1810
1811                     late_depth += 1;
1812                     scope = s;
1813                 }
1814
1815                 Scope::Elision { s, .. } | Scope::ObjectLifetimeDefault { s, .. } => {
1816                     scope = s;
1817                 }
1818             }
1819         };
1820
1821         if let Some(mut def) = result {
1822             if let Region::EarlyBound(..) = def {
1823                 // Do not free early-bound regions, only late-bound ones.
1824             } else if let Some(body_id) = outermost_body {
1825                 let fn_id = self.tcx.hir().body_owner(body_id);
1826                 match self.tcx.hir().get(fn_id) {
1827                     Node::Item(&hir::Item {
1828                         node: hir::ItemKind::Fn(..),
1829                         ..
1830                     })
1831                     | Node::TraitItem(&hir::TraitItem {
1832                         node: hir::TraitItemKind::Method(..),
1833                         ..
1834                     })
1835                     | Node::ImplItem(&hir::ImplItem {
1836                         node: hir::ImplItemKind::Method(..),
1837                         ..
1838                     }) => {
1839                         let scope = self.tcx.hir().local_def_id_from_hir_id(fn_id);
1840                         def = Region::Free(scope, def.id().unwrap());
1841                     }
1842                     _ => {}
1843                 }
1844             }
1845
1846             // Check for fn-syntax conflicts with in-band lifetime definitions
1847             if self.is_in_fn_syntax {
1848                 match def {
1849                     Region::EarlyBound(_, _, LifetimeDefOrigin::InBand)
1850                     | Region::LateBound(_, _, LifetimeDefOrigin::InBand) => {
1851                         struct_span_err!(
1852                             self.tcx.sess,
1853                             lifetime_ref.span,
1854                             E0687,
1855                             "lifetimes used in `fn` or `Fn` syntax must be \
1856                              explicitly declared using `<...>` binders"
1857                         ).span_label(lifetime_ref.span, "in-band lifetime definition")
1858                             .emit();
1859                     }
1860
1861                     Region::Static
1862                     | Region::EarlyBound(_, _, LifetimeDefOrigin::ExplicitOrElided)
1863                     | Region::LateBound(_, _, LifetimeDefOrigin::ExplicitOrElided)
1864                     | Region::EarlyBound(_, _, LifetimeDefOrigin::Error)
1865                     | Region::LateBound(_, _, LifetimeDefOrigin::Error)
1866                     | Region::LateBoundAnon(..)
1867                     | Region::Free(..) => {}
1868                 }
1869             }
1870
1871             self.insert_lifetime(lifetime_ref, def);
1872         } else {
1873             struct_span_err!(
1874                 self.tcx.sess,
1875                 lifetime_ref.span,
1876                 E0261,
1877                 "use of undeclared lifetime name `{}`",
1878                 lifetime_ref
1879             ).span_label(lifetime_ref.span, "undeclared lifetime")
1880                 .emit();
1881         }
1882     }
1883
1884     fn visit_segment_args(&mut self, res: Res, depth: usize, generic_args: &'tcx hir::GenericArgs) {
1885         if generic_args.parenthesized {
1886             let was_in_fn_syntax = self.is_in_fn_syntax;
1887             self.is_in_fn_syntax = true;
1888             self.visit_fn_like_elision(generic_args.inputs(), Some(generic_args.bindings[0].ty()));
1889             self.is_in_fn_syntax = was_in_fn_syntax;
1890             return;
1891         }
1892
1893         let mut elide_lifetimes = true;
1894         let lifetimes = generic_args
1895             .args
1896             .iter()
1897             .filter_map(|arg| match arg {
1898                 hir::GenericArg::Lifetime(lt) => {
1899                     if !lt.is_elided() {
1900                         elide_lifetimes = false;
1901                     }
1902                     Some(lt)
1903                 }
1904                 _ => None,
1905             })
1906             .collect();
1907         if elide_lifetimes {
1908             self.resolve_elided_lifetimes(lifetimes);
1909         } else {
1910             lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
1911         }
1912
1913         // Figure out if this is a type/trait segment,
1914         // which requires object lifetime defaults.
1915         let parent_def_id = |this: &mut Self, def_id: DefId| {
1916             let def_key = this.tcx.def_key(def_id);
1917             DefId {
1918                 krate: def_id.krate,
1919                 index: def_key.parent.expect("missing parent"),
1920             }
1921         };
1922         let type_def_id = match res {
1923             Res::Def(DefKind::AssocTy, def_id)
1924                 if depth == 1 => Some(parent_def_id(self, def_id)),
1925             Res::Def(DefKind::Variant, def_id)
1926                 if depth == 0 => Some(parent_def_id(self, def_id)),
1927             Res::Def(DefKind::Struct, def_id)
1928             | Res::Def(DefKind::Union, def_id)
1929             | Res::Def(DefKind::Enum, def_id)
1930             | Res::Def(DefKind::TyAlias, def_id)
1931             | Res::Def(DefKind::Trait, def_id) if depth == 0 =>
1932             {
1933                 Some(def_id)
1934             }
1935             _ => None,
1936         };
1937
1938         let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
1939             let in_body = {
1940                 let mut scope = self.scope;
1941                 loop {
1942                     match *scope {
1943                         Scope::Root => break false,
1944
1945                         Scope::Body { .. } => break true,
1946
1947                         Scope::Binder { s, .. }
1948                         | Scope::Elision { s, .. }
1949                         | Scope::ObjectLifetimeDefault { s, .. } => {
1950                             scope = s;
1951                         }
1952                     }
1953                 }
1954             };
1955
1956             let map = &self.map;
1957             let unsubst = if let Some(id) = self.tcx.hir().as_local_hir_id(def_id) {
1958                 &map.object_lifetime_defaults[&id]
1959             } else {
1960                 let tcx = self.tcx;
1961                 self.xcrate_object_lifetime_defaults
1962                     .entry(def_id)
1963                     .or_insert_with(|| {
1964                         tcx.generics_of(def_id)
1965                             .params
1966                             .iter()
1967                             .filter_map(|param| match param.kind {
1968                                 GenericParamDefKind::Type {
1969                                     object_lifetime_default,
1970                                     ..
1971                                 } => Some(object_lifetime_default),
1972                                 GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
1973                             })
1974                             .collect()
1975                     })
1976             };
1977             unsubst
1978                 .iter()
1979                 .map(|set| match *set {
1980                     Set1::Empty => if in_body {
1981                         None
1982                     } else {
1983                         Some(Region::Static)
1984                     },
1985                     Set1::One(r) => {
1986                         let lifetimes = generic_args.args.iter().filter_map(|arg| match arg {
1987                             GenericArg::Lifetime(lt) => Some(lt),
1988                             _ => None,
1989                         });
1990                         r.subst(lifetimes, map)
1991                     }
1992                     Set1::Many => None,
1993                 })
1994                 .collect()
1995         });
1996
1997         let mut i = 0;
1998         for arg in &generic_args.args {
1999             match arg {
2000                 GenericArg::Lifetime(_) => {}
2001                 GenericArg::Type(ty) => {
2002                     if let Some(&lt) = object_lifetime_defaults.get(i) {
2003                         let scope = Scope::ObjectLifetimeDefault {
2004                             lifetime: lt,
2005                             s: self.scope,
2006                         };
2007                         self.with(scope, |_, this| this.visit_ty(ty));
2008                     } else {
2009                         self.visit_ty(ty);
2010                     }
2011                     i += 1;
2012                 }
2013                 GenericArg::Const(ct) => {
2014                     self.visit_anon_const(&ct.value);
2015                 }
2016             }
2017         }
2018
2019         for b in &generic_args.bindings {
2020             self.visit_assoc_type_binding(b);
2021         }
2022     }
2023
2024     fn visit_fn_like_elision(&mut self, inputs: &'tcx [hir::Ty], output: Option<&'tcx hir::Ty>) {
2025         debug!("visit_fn_like_elision: enter");
2026         let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
2027         let arg_scope = Scope::Elision {
2028             elide: arg_elide.clone(),
2029             s: self.scope,
2030         };
2031         self.with(arg_scope, |_, this| {
2032             for input in inputs {
2033                 this.visit_ty(input);
2034             }
2035             match *this.scope {
2036                 Scope::Elision { ref elide, .. } => {
2037                     arg_elide = elide.clone();
2038                 }
2039                 _ => bug!(),
2040             }
2041         });
2042
2043         let output = match output {
2044             Some(ty) => ty,
2045             None => return,
2046         };
2047
2048         debug!("visit_fn_like_elision: determine output");
2049
2050         // Figure out if there's a body we can get argument names from,
2051         // and whether there's a `self` argument (treated specially).
2052         let mut assoc_item_kind = None;
2053         let mut impl_self = None;
2054         let parent = self.tcx.hir().get_parent_node(output.hir_id);
2055         let body = match self.tcx.hir().get(parent) {
2056             // `fn` definitions and methods.
2057             Node::Item(&hir::Item {
2058                 node: hir::ItemKind::Fn(.., body),
2059                 ..
2060             }) => Some(body),
2061
2062             Node::TraitItem(&hir::TraitItem {
2063                 node: hir::TraitItemKind::Method(_, ref m),
2064                 ..
2065             }) => {
2066                 if let hir::ItemKind::Trait(.., ref trait_items) = self.tcx
2067                     .hir()
2068                     .expect_item(self.tcx.hir().get_parent_item(parent))
2069                     .node
2070                 {
2071                     assoc_item_kind = trait_items
2072                         .iter()
2073                         .find(|ti| ti.id.hir_id == parent)
2074                         .map(|ti| ti.kind);
2075                 }
2076                 match *m {
2077                     hir::TraitMethod::Required(_) => None,
2078                     hir::TraitMethod::Provided(body) => Some(body),
2079                 }
2080             }
2081
2082             Node::ImplItem(&hir::ImplItem {
2083                 node: hir::ImplItemKind::Method(_, body),
2084                 ..
2085             }) => {
2086                 if let hir::ItemKind::Impl(.., ref self_ty, ref impl_items) = self.tcx
2087                     .hir()
2088                     .expect_item(self.tcx.hir().get_parent_item(parent))
2089                     .node
2090                 {
2091                     impl_self = Some(self_ty);
2092                     assoc_item_kind = impl_items
2093                         .iter()
2094                         .find(|ii| ii.id.hir_id == parent)
2095                         .map(|ii| ii.kind);
2096                 }
2097                 Some(body)
2098             }
2099
2100             // Foreign functions, `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
2101             Node::ForeignItem(_) | Node::Ty(_) | Node::TraitRef(_) => None,
2102             // Everything else (only closures?) doesn't
2103             // actually enjoy elision in return types.
2104             _ => {
2105                 self.visit_ty(output);
2106                 return;
2107             }
2108         };
2109
2110         let has_self = match assoc_item_kind {
2111             Some(hir::AssocItemKind::Method { has_self }) => has_self,
2112             _ => false,
2113         };
2114
2115         // In accordance with the rules for lifetime elision, we can determine
2116         // what region to use for elision in the output type in two ways.
2117         // First (determined here), if `self` is by-reference, then the
2118         // implied output region is the region of the self parameter.
2119         if has_self {
2120             // Look for `self: &'a Self` - also desugared from `&'a self`,
2121             // and if that matches, use it for elision and return early.
2122             let is_self_ty = |res: Res| {
2123                 if let Res::SelfTy(..) = res {
2124                     return true;
2125                 }
2126
2127                 // Can't always rely on literal (or implied) `Self` due
2128                 // to the way elision rules were originally specified.
2129                 let impl_self = impl_self.map(|ty| &ty.node);
2130                 if let Some(&hir::TyKind::Path(hir::QPath::Resolved(None, ref path))) = impl_self {
2131                     match path.res {
2132                         // Whitelist the types that unambiguously always
2133                         // result in the same type constructor being used
2134                         // (it can't differ between `Self` and `self`).
2135                         Res::Def(DefKind::Struct, _)
2136                         | Res::Def(DefKind::Union, _)
2137                         | Res::Def(DefKind::Enum, _)
2138                         | Res::PrimTy(_) => {
2139                             return res == path.res
2140                         }
2141                         _ => {}
2142                     }
2143                 }
2144
2145                 false
2146             };
2147
2148             if let hir::TyKind::Rptr(lifetime_ref, ref mt) = inputs[0].node {
2149                 if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.node {
2150                     if is_self_ty(path.res) {
2151                         if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
2152                             let scope = Scope::Elision {
2153                                 elide: Elide::Exact(lifetime),
2154                                 s: self.scope,
2155                             };
2156                             self.with(scope, |_, this| this.visit_ty(output));
2157                             return;
2158                         }
2159                     }
2160                 }
2161             }
2162         }
2163
2164         // Second, if there was exactly one lifetime (either a substitution or a
2165         // reference) in the arguments, then any anonymous regions in the output
2166         // have that lifetime.
2167         let mut possible_implied_output_region = None;
2168         let mut lifetime_count = 0;
2169         let arg_lifetimes = inputs
2170             .iter()
2171             .enumerate()
2172             .skip(has_self as usize)
2173             .map(|(i, input)| {
2174                 let mut gather = GatherLifetimes {
2175                     map: self.map,
2176                     outer_index: ty::INNERMOST,
2177                     have_bound_regions: false,
2178                     lifetimes: Default::default(),
2179                 };
2180                 gather.visit_ty(input);
2181
2182                 lifetime_count += gather.lifetimes.len();
2183
2184                 if lifetime_count == 1 && gather.lifetimes.len() == 1 {
2185                     // there's a chance that the unique lifetime of this
2186                     // iteration will be the appropriate lifetime for output
2187                     // parameters, so lets store it.
2188                     possible_implied_output_region = gather.lifetimes.iter().cloned().next();
2189                 }
2190
2191                 ElisionFailureInfo {
2192                     parent: body,
2193                     index: i,
2194                     lifetime_count: gather.lifetimes.len(),
2195                     have_bound_regions: gather.have_bound_regions,
2196                 }
2197             })
2198             .collect();
2199
2200         let elide = if lifetime_count == 1 {
2201             Elide::Exact(possible_implied_output_region.unwrap())
2202         } else {
2203             Elide::Error(arg_lifetimes)
2204         };
2205
2206         debug!("visit_fn_like_elision: elide={:?}", elide);
2207
2208         let scope = Scope::Elision {
2209             elide,
2210             s: self.scope,
2211         };
2212         self.with(scope, |_, this| this.visit_ty(output));
2213         debug!("visit_fn_like_elision: exit");
2214
2215         struct GatherLifetimes<'a> {
2216             map: &'a NamedRegionMap,
2217             outer_index: ty::DebruijnIndex,
2218             have_bound_regions: bool,
2219             lifetimes: FxHashSet<Region>,
2220         }
2221
2222         impl<'v, 'a> Visitor<'v> for GatherLifetimes<'a> {
2223             fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
2224                 NestedVisitorMap::None
2225             }
2226
2227             fn visit_ty(&mut self, ty: &hir::Ty) {
2228                 if let hir::TyKind::BareFn(_) = ty.node {
2229                     self.outer_index.shift_in(1);
2230                 }
2231                 match ty.node {
2232                     hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
2233                         for bound in bounds {
2234                             self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
2235                         }
2236
2237                         // Stay on the safe side and don't include the object
2238                         // lifetime default (which may not end up being used).
2239                         if !lifetime.is_elided() {
2240                             self.visit_lifetime(lifetime);
2241                         }
2242                     }
2243                     hir::TyKind::CVarArgs(_) => {}
2244                     _ => {
2245                         intravisit::walk_ty(self, ty);
2246                     }
2247                 }
2248                 if let hir::TyKind::BareFn(_) = ty.node {
2249                     self.outer_index.shift_out(1);
2250                 }
2251             }
2252
2253             fn visit_generic_param(&mut self, param: &hir::GenericParam) {
2254                 if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2255                     // FIXME(eddyb) Do we want this? It only makes a difference
2256                     // if this `for<'a>` lifetime parameter is never used.
2257                     self.have_bound_regions = true;
2258                 }
2259
2260                 intravisit::walk_generic_param(self, param);
2261             }
2262
2263             fn visit_poly_trait_ref(
2264                 &mut self,
2265                 trait_ref: &hir::PolyTraitRef,
2266                 modifier: hir::TraitBoundModifier,
2267             ) {
2268                 self.outer_index.shift_in(1);
2269                 intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
2270                 self.outer_index.shift_out(1);
2271             }
2272
2273             fn visit_lifetime(&mut self, lifetime_ref: &hir::Lifetime) {
2274                 if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
2275                     match lifetime {
2276                         Region::LateBound(debruijn, _, _) | Region::LateBoundAnon(debruijn, _)
2277                             if debruijn < self.outer_index =>
2278                         {
2279                             self.have_bound_regions = true;
2280                         }
2281                         _ => {
2282                             self.lifetimes
2283                                 .insert(lifetime.shifted_out_to_binder(self.outer_index));
2284                         }
2285                     }
2286                 }
2287             }
2288         }
2289     }
2290
2291     fn resolve_elided_lifetimes(&mut self, lifetime_refs: Vec<&'tcx hir::Lifetime>) {
2292         if lifetime_refs.is_empty() {
2293             return;
2294         }
2295
2296         let span = lifetime_refs[0].span;
2297         let mut late_depth = 0;
2298         let mut scope = self.scope;
2299         let mut lifetime_names = FxHashSet::default();
2300         let error = loop {
2301             match *scope {
2302                 // Do not assign any resolution, it will be inferred.
2303                 Scope::Body { .. } => return,
2304
2305                 Scope::Root => break None,
2306
2307                 Scope::Binder { s, ref lifetimes, .. } => {
2308                     // collect named lifetimes for suggestions
2309                     for name in lifetimes.keys() {
2310                         if let hir::ParamName::Plain(name) = name {
2311                             lifetime_names.insert(*name);
2312                         }
2313                     }
2314                     late_depth += 1;
2315                     scope = s;
2316                 }
2317
2318                 Scope::Elision { ref elide, ref s, .. } => {
2319                     let lifetime = match *elide {
2320                         Elide::FreshLateAnon(ref counter) => {
2321                             for lifetime_ref in lifetime_refs {
2322                                 let lifetime = Region::late_anon(counter).shifted(late_depth);
2323                                 self.insert_lifetime(lifetime_ref, lifetime);
2324                             }
2325                             return;
2326                         }
2327                         Elide::Exact(l) => l.shifted(late_depth),
2328                         Elide::Error(ref e) => {
2329                             if let Scope::Binder { ref lifetimes, .. } = s {
2330                                 // collect named lifetimes for suggestions
2331                                 for name in lifetimes.keys() {
2332                                     if let hir::ParamName::Plain(name) = name {
2333                                         lifetime_names.insert(*name);
2334                                     }
2335                                 }
2336                             }
2337                             break Some(e);
2338                         }
2339                     };
2340                     for lifetime_ref in lifetime_refs {
2341                         self.insert_lifetime(lifetime_ref, lifetime);
2342                     }
2343                     return;
2344                 }
2345
2346                 Scope::ObjectLifetimeDefault { s, .. } => {
2347                     scope = s;
2348                 }
2349             }
2350         };
2351
2352         let mut err = report_missing_lifetime_specifiers(self.tcx.sess, span, lifetime_refs.len());
2353         let mut add_label = true;
2354
2355         if let Some(params) = error {
2356             if lifetime_refs.len() == 1 {
2357                 add_label = add_label && self.report_elision_failure(&mut err, params, span);
2358             }
2359         }
2360         if add_label {
2361             add_missing_lifetime_specifiers_label(
2362                 &mut err,
2363                 span,
2364                 lifetime_refs.len(),
2365                 &lifetime_names,
2366                 self.tcx.sess.source_map().span_to_snippet(span).ok().as_ref().map(|s| s.as_str()),
2367             );
2368         }
2369
2370         err.emit();
2371     }
2372
2373     fn suggest_lifetime(&self, db: &mut DiagnosticBuilder<'_>, span: Span, msg: &str) -> bool {
2374         match self.tcx.sess.source_map().span_to_snippet(span) {
2375             Ok(ref snippet) => {
2376                 let (sugg, applicability) = if snippet == "&" {
2377                     ("&'static ".to_owned(), Applicability::MachineApplicable)
2378                 } else if snippet == "'_" {
2379                     ("'static".to_owned(), Applicability::MachineApplicable)
2380                 } else {
2381                     (format!("{} + 'static", snippet), Applicability::MaybeIncorrect)
2382                 };
2383                 db.span_suggestion(span, msg, sugg, applicability);
2384                 false
2385             }
2386             Err(_) => {
2387                 db.help(msg);
2388                 true
2389             }
2390         }
2391     }
2392
2393     fn report_elision_failure(
2394         &mut self,
2395         db: &mut DiagnosticBuilder<'_>,
2396         params: &[ElisionFailureInfo],
2397         span: Span,
2398     ) -> bool {
2399         let mut m = String::new();
2400         let len = params.len();
2401
2402         let elided_params: Vec<_> = params
2403             .iter()
2404             .cloned()
2405             .filter(|info| info.lifetime_count > 0)
2406             .collect();
2407
2408         let elided_len = elided_params.len();
2409
2410         for (i, info) in elided_params.into_iter().enumerate() {
2411             let ElisionFailureInfo {
2412                 parent,
2413                 index,
2414                 lifetime_count: n,
2415                 have_bound_regions,
2416             } = info;
2417
2418             let help_name = if let Some(ident) = parent.and_then(|body| {
2419                 self.tcx.hir().body(body).arguments[index].pat.simple_ident()
2420             }) {
2421                 format!("`{}`", ident)
2422             } else {
2423                 format!("argument {}", index + 1)
2424             };
2425
2426             m.push_str(
2427                 &(if n == 1 {
2428                     help_name
2429                 } else {
2430                     format!(
2431                         "one of {}'s {} {}lifetimes",
2432                         help_name,
2433                         n,
2434                         if have_bound_regions { "free " } else { "" }
2435                     )
2436                 })[..],
2437             );
2438
2439             if elided_len == 2 && i == 0 {
2440                 m.push_str(" or ");
2441             } else if i + 2 == elided_len {
2442                 m.push_str(", or ");
2443             } else if i != elided_len - 1 {
2444                 m.push_str(", ");
2445             }
2446         }
2447
2448         if len == 0 {
2449             help!(
2450                 db,
2451                 "this function's return type contains a borrowed value, but \
2452                  there is no value for it to be borrowed from"
2453             );
2454             self.suggest_lifetime(db, span, "consider giving it a 'static lifetime")
2455         } else if elided_len == 0 {
2456             help!(
2457                 db,
2458                 "this function's return type contains a borrowed value with \
2459                  an elided lifetime, but the lifetime cannot be derived from \
2460                  the arguments"
2461             );
2462             let msg = "consider giving it an explicit bounded or 'static lifetime";
2463             self.suggest_lifetime(db, span, msg)
2464         } else if elided_len == 1 {
2465             help!(
2466                 db,
2467                 "this function's return type contains a borrowed value, but \
2468                  the signature does not say which {} it is borrowed from",
2469                 m
2470             );
2471             true
2472         } else {
2473             help!(
2474                 db,
2475                 "this function's return type contains a borrowed value, but \
2476                  the signature does not say whether it is borrowed from {}",
2477                 m
2478             );
2479             true
2480         }
2481     }
2482
2483     fn resolve_object_lifetime_default(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
2484         let mut late_depth = 0;
2485         let mut scope = self.scope;
2486         let lifetime = loop {
2487             match *scope {
2488                 Scope::Binder { s, .. } => {
2489                     late_depth += 1;
2490                     scope = s;
2491                 }
2492
2493                 Scope::Root | Scope::Elision { .. } => break Region::Static,
2494
2495                 Scope::Body { .. } | Scope::ObjectLifetimeDefault { lifetime: None, .. } => return,
2496
2497                 Scope::ObjectLifetimeDefault {
2498                     lifetime: Some(l), ..
2499                 } => break l,
2500             }
2501         };
2502         self.insert_lifetime(lifetime_ref, lifetime.shifted(late_depth));
2503     }
2504
2505     fn check_lifetime_params(
2506         &mut self,
2507         old_scope: ScopeRef<'_>,
2508         params: &'tcx [hir::GenericParam],
2509     ) {
2510         let lifetimes: Vec<_> = params
2511             .iter()
2512             .filter_map(|param| match param.kind {
2513                 GenericParamKind::Lifetime { .. } => Some((param, param.name)),
2514                 _ => None,
2515             })
2516             .collect();
2517         for (i, (lifetime_i, lifetime_i_name)) in lifetimes.iter().enumerate() {
2518             if let hir::ParamName::Plain(_) = lifetime_i_name {
2519                 let name = lifetime_i_name.ident().name;
2520                 if name == kw::UnderscoreLifetime
2521                     || name == kw::StaticLifetime
2522                 {
2523                     let mut err = struct_span_err!(
2524                         self.tcx.sess,
2525                         lifetime_i.span,
2526                         E0262,
2527                         "invalid lifetime parameter name: `{}`",
2528                         lifetime_i.name.ident(),
2529                     );
2530                     err.span_label(
2531                         lifetime_i.span,
2532                         format!("{} is a reserved lifetime name", name),
2533                     );
2534                     err.emit();
2535                 }
2536             }
2537
2538             // It is a hard error to shadow a lifetime within the same scope.
2539             for (lifetime_j, lifetime_j_name) in lifetimes.iter().skip(i + 1) {
2540                 if lifetime_i_name == lifetime_j_name {
2541                     struct_span_err!(
2542                         self.tcx.sess,
2543                         lifetime_j.span,
2544                         E0263,
2545                         "lifetime name `{}` declared twice in the same scope",
2546                         lifetime_j.name.ident()
2547                     ).span_label(lifetime_j.span, "declared twice")
2548                         .span_label(lifetime_i.span, "previous declaration here")
2549                         .emit();
2550                 }
2551             }
2552
2553             // It is a soft error to shadow a lifetime within a parent scope.
2554             self.check_lifetime_param_for_shadowing(old_scope, &lifetime_i);
2555
2556             for bound in &lifetime_i.bounds {
2557                 match bound {
2558                     hir::GenericBound::Outlives(lt) => match lt.name {
2559                         hir::LifetimeName::Underscore => self.tcx.sess.delay_span_bug(
2560                             lt.span,
2561                             "use of `'_` in illegal place, but not caught by lowering",
2562                         ),
2563                         hir::LifetimeName::Static => {
2564                             self.insert_lifetime(lt, Region::Static);
2565                             self.tcx
2566                                 .sess
2567                                 .struct_span_warn(
2568                                     lifetime_i.span.to(lt.span),
2569                                     &format!(
2570                                         "unnecessary lifetime parameter `{}`",
2571                                         lifetime_i.name.ident(),
2572                                     ),
2573                                 )
2574                                 .help(&format!(
2575                                     "you can use the `'static` lifetime directly, in place of `{}`",
2576                                     lifetime_i.name.ident(),
2577                                 ))
2578                                 .emit();
2579                         }
2580                         hir::LifetimeName::Param(_) | hir::LifetimeName::Implicit => {
2581                             self.resolve_lifetime_ref(lt);
2582                         }
2583                         hir::LifetimeName::Error => {
2584                             // No need to do anything, error already reported.
2585                         }
2586                     },
2587                     _ => bug!(),
2588                 }
2589             }
2590         }
2591     }
2592
2593     fn check_lifetime_param_for_shadowing(
2594         &self,
2595         mut old_scope: ScopeRef<'_>,
2596         param: &'tcx hir::GenericParam,
2597     ) {
2598         for label in &self.labels_in_fn {
2599             // FIXME (#24278): non-hygienic comparison
2600             if param.name.ident().name == label.name {
2601                 signal_shadowing_problem(
2602                     self.tcx,
2603                     label.name,
2604                     original_label(label.span),
2605                     shadower_lifetime(&param),
2606                 );
2607                 return;
2608             }
2609         }
2610
2611         loop {
2612             match *old_scope {
2613                 Scope::Body { s, .. }
2614                 | Scope::Elision { s, .. }
2615                 | Scope::ObjectLifetimeDefault { s, .. } => {
2616                     old_scope = s;
2617                 }
2618
2619                 Scope::Root => {
2620                     return;
2621                 }
2622
2623                 Scope::Binder {
2624                     ref lifetimes, s, ..
2625                 } => {
2626                     if let Some(&def) = lifetimes.get(&param.name.modern()) {
2627                         let hir_id = self.tcx.hir().as_local_hir_id(def.id().unwrap()).unwrap();
2628
2629                         signal_shadowing_problem(
2630                             self.tcx,
2631                             param.name.ident().name,
2632                             original_lifetime(self.tcx.hir().span(hir_id)),
2633                             shadower_lifetime(&param),
2634                         );
2635                         return;
2636                     }
2637
2638                     old_scope = s;
2639                 }
2640             }
2641         }
2642     }
2643
2644     /// Returns `true` if, in the current scope, replacing `'_` would be
2645     /// equivalent to a single-use lifetime.
2646     fn track_lifetime_uses(&self) -> bool {
2647         let mut scope = self.scope;
2648         loop {
2649             match *scope {
2650                 Scope::Root => break false,
2651
2652                 // Inside of items, it depends on the kind of item.
2653                 Scope::Binder {
2654                     track_lifetime_uses,
2655                     ..
2656                 } => break track_lifetime_uses,
2657
2658                 // Inside a body, `'_` will use an inference variable,
2659                 // should be fine.
2660                 Scope::Body { .. } => break true,
2661
2662                 // A lifetime only used in a fn argument could as well
2663                 // be replaced with `'_`, as that would generate a
2664                 // fresh name, too.
2665                 Scope::Elision {
2666                     elide: Elide::FreshLateAnon(_),
2667                     ..
2668                 } => break true,
2669
2670                 // In the return type or other such place, `'_` is not
2671                 // going to make a fresh name, so we cannot
2672                 // necessarily replace a single-use lifetime with
2673                 // `'_`.
2674                 Scope::Elision {
2675                     elide: Elide::Exact(_),
2676                     ..
2677                 } => break false,
2678                 Scope::Elision {
2679                     elide: Elide::Error(_),
2680                     ..
2681                 } => break false,
2682
2683                 Scope::ObjectLifetimeDefault { s, .. } => scope = s,
2684             }
2685         }
2686     }
2687
2688     fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
2689         if lifetime_ref.hir_id == hir::DUMMY_HIR_ID {
2690             span_bug!(
2691                 lifetime_ref.span,
2692                 "lifetime reference not renumbered, \
2693                  probably a bug in syntax::fold"
2694             );
2695         }
2696
2697         debug!(
2698             "insert_lifetime: {} resolved to {:?} span={:?}",
2699             self.tcx.hir().node_to_string(lifetime_ref.hir_id),
2700             def,
2701             self.tcx.sess.source_map().span_to_string(lifetime_ref.span)
2702         );
2703         self.map.defs.insert(lifetime_ref.hir_id, def);
2704
2705         match def {
2706             Region::LateBoundAnon(..) | Region::Static => {
2707                 // These are anonymous lifetimes or lifetimes that are not declared.
2708             }
2709
2710             Region::Free(_, def_id)
2711             | Region::LateBound(_, def_id, _)
2712             | Region::EarlyBound(_, def_id, _) => {
2713                 // A lifetime declared by the user.
2714                 let track_lifetime_uses = self.track_lifetime_uses();
2715                 debug!(
2716                     "insert_lifetime: track_lifetime_uses={}",
2717                     track_lifetime_uses
2718                 );
2719                 if track_lifetime_uses && !self.lifetime_uses.contains_key(&def_id) {
2720                     debug!("insert_lifetime: first use of {:?}", def_id);
2721                     self.lifetime_uses
2722                         .insert(def_id, LifetimeUseSet::One(lifetime_ref));
2723                 } else {
2724                     debug!("insert_lifetime: many uses of {:?}", def_id);
2725                     self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
2726                 }
2727             }
2728         }
2729     }
2730
2731     /// Sometimes we resolve a lifetime, but later find that it is an
2732     /// error (esp. around impl trait). In that case, we remove the
2733     /// entry into `map.defs` so as not to confuse later code.
2734     fn uninsert_lifetime_on_error(&mut self, lifetime_ref: &'tcx hir::Lifetime, bad_def: Region) {
2735         let old_value = self.map.defs.remove(&lifetime_ref.hir_id);
2736         assert_eq!(old_value, Some(bad_def));
2737     }
2738 }
2739
2740 /// Detects late-bound lifetimes and inserts them into
2741 /// `map.late_bound`.
2742 ///
2743 /// A region declared on a fn is **late-bound** if:
2744 /// - it is constrained by an argument type;
2745 /// - it does not appear in a where-clause.
2746 ///
2747 /// "Constrained" basically means that it appears in any type but
2748 /// not amongst the inputs to a projection. In other words, `<&'a
2749 /// T as Trait<''b>>::Foo` does not constrain `'a` or `'b`.
2750 fn insert_late_bound_lifetimes(
2751     map: &mut NamedRegionMap,
2752     decl: &hir::FnDecl,
2753     generics: &hir::Generics,
2754 ) {
2755     debug!(
2756         "insert_late_bound_lifetimes(decl={:?}, generics={:?})",
2757         decl, generics
2758     );
2759
2760     let mut constrained_by_input = ConstrainedCollector::default();
2761     for arg_ty in &decl.inputs {
2762         constrained_by_input.visit_ty(arg_ty);
2763     }
2764
2765     let mut appears_in_output = AllCollector::default();
2766     intravisit::walk_fn_ret_ty(&mut appears_in_output, &decl.output);
2767
2768     debug!(
2769         "insert_late_bound_lifetimes: constrained_by_input={:?}",
2770         constrained_by_input.regions
2771     );
2772
2773     // Walk the lifetimes that appear in where clauses.
2774     //
2775     // Subtle point: because we disallow nested bindings, we can just
2776     // ignore binders here and scrape up all names we see.
2777     let mut appears_in_where_clause = AllCollector::default();
2778     appears_in_where_clause.visit_generics(generics);
2779
2780     for param in &generics.params {
2781         if let hir::GenericParamKind::Lifetime { .. } = param.kind {
2782             if !param.bounds.is_empty() {
2783                 // `'a: 'b` means both `'a` and `'b` are referenced
2784                 appears_in_where_clause
2785                     .regions
2786                     .insert(hir::LifetimeName::Param(param.name.modern()));
2787             }
2788         }
2789     }
2790
2791     debug!(
2792         "insert_late_bound_lifetimes: appears_in_where_clause={:?}",
2793         appears_in_where_clause.regions
2794     );
2795
2796     // Late bound regions are those that:
2797     // - appear in the inputs
2798     // - do not appear in the where-clauses
2799     // - are not implicitly captured by `impl Trait`
2800     for param in &generics.params {
2801         match param.kind {
2802             hir::GenericParamKind::Lifetime { .. } => { /* fall through */ }
2803
2804             // Neither types nor consts are late-bound.
2805             hir::GenericParamKind::Type { .. }
2806             | hir::GenericParamKind::Const { .. } => continue,
2807         }
2808
2809         let lt_name = hir::LifetimeName::Param(param.name.modern());
2810         // appears in the where clauses? early-bound.
2811         if appears_in_where_clause.regions.contains(&lt_name) {
2812             continue;
2813         }
2814
2815         // does not appear in the inputs, but appears in the return type? early-bound.
2816         if !constrained_by_input.regions.contains(&lt_name)
2817             && appears_in_output.regions.contains(&lt_name)
2818         {
2819             continue;
2820         }
2821
2822         debug!(
2823             "insert_late_bound_lifetimes: lifetime {:?} with id {:?} is late-bound",
2824             param.name.ident(),
2825             param.hir_id
2826         );
2827
2828         let inserted = map.late_bound.insert(param.hir_id);
2829         assert!(inserted, "visited lifetime {:?} twice", param.hir_id);
2830     }
2831
2832     return;
2833
2834     #[derive(Default)]
2835     struct ConstrainedCollector {
2836         regions: FxHashSet<hir::LifetimeName>,
2837     }
2838
2839     impl<'v> Visitor<'v> for ConstrainedCollector {
2840         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
2841             NestedVisitorMap::None
2842         }
2843
2844         fn visit_ty(&mut self, ty: &'v hir::Ty) {
2845             match ty.node {
2846                 hir::TyKind::Path(hir::QPath::Resolved(Some(_), _))
2847                 | hir::TyKind::Path(hir::QPath::TypeRelative(..)) => {
2848                     // ignore lifetimes appearing in associated type
2849                     // projections, as they are not *constrained*
2850                     // (defined above)
2851                 }
2852
2853                 hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => {
2854                     // consider only the lifetimes on the final
2855                     // segment; I am not sure it's even currently
2856                     // valid to have them elsewhere, but even if it
2857                     // is, those would be potentially inputs to
2858                     // projections
2859                     if let Some(last_segment) = path.segments.last() {
2860                         self.visit_path_segment(path.span, last_segment);
2861                     }
2862                 }
2863
2864                 _ => {
2865                     intravisit::walk_ty(self, ty);
2866                 }
2867             }
2868         }
2869
2870         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2871             self.regions.insert(lifetime_ref.name.modern());
2872         }
2873     }
2874
2875     #[derive(Default)]
2876     struct AllCollector {
2877         regions: FxHashSet<hir::LifetimeName>,
2878     }
2879
2880     impl<'v> Visitor<'v> for AllCollector {
2881         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
2882             NestedVisitorMap::None
2883         }
2884
2885         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
2886             self.regions.insert(lifetime_ref.name.modern());
2887         }
2888     }
2889 }
2890
2891 pub fn report_missing_lifetime_specifiers(
2892     sess: &Session,
2893     span: Span,
2894     count: usize,
2895 ) -> DiagnosticBuilder<'_> {
2896     struct_span_err!(
2897         sess,
2898         span,
2899         E0106,
2900         "missing lifetime specifier{}",
2901         if count > 1 { "s" } else { "" }
2902     )
2903 }
2904
2905 fn add_missing_lifetime_specifiers_label(
2906     err: &mut DiagnosticBuilder<'_>,
2907     span: Span,
2908     count: usize,
2909     lifetime_names: &FxHashSet<ast::Ident>,
2910     snippet: Option<&str>,
2911 ) {
2912     if count > 1 {
2913         err.span_label(span, format!("expected {} lifetime parameters", count));
2914     } else if let (1, Some(name), Some("&")) = (
2915         lifetime_names.len(),
2916         lifetime_names.iter().next(),
2917         snippet,
2918     ) {
2919         err.span_suggestion(
2920             span,
2921             "consider using the named lifetime",
2922             format!("&{} ", name),
2923             Applicability::MaybeIncorrect,
2924         );
2925     } else {
2926         err.span_label(span, "expected lifetime parameter");
2927     }
2928 }