src/librustc_resolve/lifetimes.rs

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