src/librustc/middle/resolve_lifetime.rs

   1 // Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Name resolution for lifetimes.
  12 //!
  13 //! Name resolution for lifetimes follows MUCH simpler rules than the
  14 //! full resolve. For example, lifetime names are never exported or
  15 //! used between functions, and they operate in a purely top-down
  16 //! way. Therefore we break lifetime name resolution into a separate pass.
  17
  18 use dep_graph::DepNode;
  19 use hir::map::Map;
  20 use session::Session;
  21 use hir::def::Def;
  22 use hir::def_id::DefId;
  23 use middle::region;
  24 use ty;
  25
  26 use std::cell::Cell;
  27 use std::mem::replace;
  28 use syntax::ast;
  29 use syntax::attr;
  30 use syntax::ptr::P;
  31 use syntax::symbol::keywords;
  32 use syntax_pos::Span;
  33 use errors::DiagnosticBuilder;
  34 use util::nodemap::{NodeMap, NodeSet, FxHashSet, FxHashMap, DefIdMap};
  35 use rustc_back::slice;
  36
  37 use hir;
  38 use hir::intravisit::{self, Visitor, NestedVisitorMap};
  39
  40 #[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
  41 pub enum Region {
  42     Static,
  43     EarlyBound(/* index */ u32, /* lifetime decl */ ast::NodeId),
  44     LateBound(ty::DebruijnIndex, /* lifetime decl */ ast::NodeId),
  45     LateBoundAnon(ty::DebruijnIndex, /* anon index */ u32),
  46     Free(region::CallSiteScopeData, /* lifetime decl */ ast::NodeId),
  47 }
  48
  49 impl Region {
  50     fn early(index: &mut u32, def: &hir::LifetimeDef) -> (ast::Name, Region) {
  51         let i = *index;
  52         *index += 1;
  53         (def.lifetime.name, Region::EarlyBound(i, def.lifetime.id))
  54     }
  55
  56     fn late(def: &hir::LifetimeDef) -> (ast::Name, Region) {
  57         let depth = ty::DebruijnIndex::new(1);
  58         (def.lifetime.name, Region::LateBound(depth, def.lifetime.id))
  59     }
  60
  61     fn late_anon(index: &Cell<u32>) -> Region {
  62         let i = index.get();
  63         index.set(i + 1);
  64         let depth = ty::DebruijnIndex::new(1);
  65         Region::LateBoundAnon(depth, i)
  66     }
  67
  68     fn id(&self) -> Option<ast::NodeId> {
  69         match *self {
  70             Region::Static |
  71             Region::LateBoundAnon(..) => None,
  72
  73             Region::EarlyBound(_, id) |
  74             Region::LateBound(_, id) |
  75             Region::Free(_, id) => Some(id)
  76         }
  77     }
  78
  79     fn shifted(self, amount: u32) -> Region {
  80         match self {
  81             Region::LateBound(depth, id) => {
  82                 Region::LateBound(depth.shifted(amount), id)
  83             }
  84             Region::LateBoundAnon(depth, index) => {
  85                 Region::LateBoundAnon(depth.shifted(amount), index)
  86             }
  87             _ => self
  88         }
  89     }
  90
  91     fn from_depth(self, depth: u32) -> Region {
  92         match self {
  93             Region::LateBound(debruijn, id) => {
  94                 Region::LateBound(ty::DebruijnIndex {
  95                     depth: debruijn.depth - (depth - 1)
  96                 }, id)
  97             }
  98             Region::LateBoundAnon(debruijn, index) => {
  99                 Region::LateBoundAnon(ty::DebruijnIndex {
 100                     depth: debruijn.depth - (depth - 1)
 101                 }, index)
 102             }
 103             _ => self
 104         }
 105     }
 106
 107     fn subst(self, params: &[hir::Lifetime], map: &NamedRegionMap)
 108              -> Option<Region> {
 109         if let Region::EarlyBound(index, _) = self {
 110             params.get(index as usize).and_then(|lifetime| {
 111                 map.defs.get(&lifetime.id).cloned()
 112             })
 113         } else {
 114             Some(self)
 115         }
 116     }
 117 }
 118
 119 /// A set containing, at most, one known element.
 120 /// If two distinct values are inserted into a set, then it
 121 /// becomes `Many`, which can be used to detect ambiguities.
 122 #[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug)]
 123 pub enum Set1<T> {
 124     Empty,
 125     One(T),
 126     Many
 127 }
 128
 129 impl<T: PartialEq> Set1<T> {
 130     pub fn insert(&mut self, value: T) {
 131         if let Set1::Empty = *self {
 132             *self = Set1::One(value);
 133             return;
 134         }
 135         if let Set1::One(ref old) = *self {
 136             if *old == value {
 137                 return;
 138             }
 139         }
 140         *self = Set1::Many;
 141     }
 142 }
 143
 144 pub type ObjectLifetimeDefault = Set1<Region>;
 145
 146 // Maps the id of each lifetime reference to the lifetime decl
 147 // that it corresponds to.
 148 pub struct NamedRegionMap {
 149     // maps from every use of a named (not anonymous) lifetime to a
 150     // `Region` describing how that region is bound
 151     pub defs: NodeMap<Region>,
 152
 153     // the set of lifetime def ids that are late-bound; a region can
 154     // be late-bound if (a) it does NOT appear in a where-clause and
 155     // (b) it DOES appear in the arguments.
 156     pub late_bound: NodeSet,
 157
 158     // Contains the node-ids for lifetimes that were (incorrectly) categorized
 159     // as late-bound, until #32330 was fixed.
 160     pub issue_32330: NodeMap<ty::Issue32330>,
 161
 162     // For each type and trait definition, maps type parameters
 163     // to the trait object lifetime defaults computed from them.
 164     pub object_lifetime_defaults: NodeMap<Vec<ObjectLifetimeDefault>>,
 165 }
 166
 167 struct LifetimeContext<'a, 'tcx: 'a> {
 168     sess: &'a Session,
 169     hir_map: &'a Map<'tcx>,
 170     map: &'a mut NamedRegionMap,
 171     scope: ScopeRef<'a>,
 172     // Deep breath. Our representation for poly trait refs contains a single
 173     // binder and thus we only allow a single level of quantification. However,
 174     // the syntax of Rust permits quantification in two places, e.g., `T: for <'a> Foo<'a>`
 175     // and `for <'a, 'b> &'b T: Foo<'a>`. In order to get the de Bruijn indices
 176     // correct when representing these constraints, we should only introduce one
 177     // scope. However, we want to support both locations for the quantifier and
 178     // during lifetime resolution we want precise information (so we can't
 179     // desugar in an earlier phase).
 180
 181     // SO, if we encounter a quantifier at the outer scope, we set
 182     // trait_ref_hack to true (and introduce a scope), and then if we encounter
 183     // a quantifier at the inner scope, we error. If trait_ref_hack is false,
 184     // then we introduce the scope at the inner quantifier.
 185
 186     // I'm sorry.
 187     trait_ref_hack: bool,
 188
 189     // List of labels in the function/method currently under analysis.
 190     labels_in_fn: Vec<(ast::Name, Span)>,
 191
 192     // Cache for cross-crate per-definition object lifetime defaults.
 193     xcrate_object_lifetime_defaults: DefIdMap<Vec<ObjectLifetimeDefault>>,
 194 }
 195
 196 #[derive(Debug)]
 197 enum Scope<'a> {
 198     /// Declares lifetimes, and each can be early-bound or late-bound.
 199     /// The `DebruijnIndex` of late-bound lifetimes starts at `1` and
 200     /// it should be shifted by the number of `Binder`s in between the
 201     /// declaration `Binder` and the location it's referenced from.
 202     Binder {
 203         lifetimes: FxHashMap<ast::Name, Region>,
 204         s: ScopeRef<'a>
 205     },
 206
 207     /// Lifetimes introduced by a fn are scoped to the call-site for that fn,
 208     /// if this is a fn body, otherwise the original definitions are used.
 209     /// Unspecified lifetimes are inferred, unless an elision scope is nested,
 210     /// e.g. `(&T, fn(&T) -> &T);` becomes `(&'_ T, for<'a> fn(&'a T) -> &'a T)`.
 211     Body {
 212         id: hir::BodyId,
 213         s: ScopeRef<'a>
 214     },
 215
 216     /// A scope which either determines unspecified lifetimes or errors
 217     /// on them (e.g. due to ambiguity). For more details, see `Elide`.
 218     Elision {
 219         elide: Elide,
 220         s: ScopeRef<'a>
 221     },
 222
 223     /// Use a specific lifetime (if `Some`) or leave it unset (to be
 224     /// inferred in a function body or potentially error outside one),
 225     /// for the default choice of lifetime in a trait object type.
 226     ObjectLifetimeDefault {
 227         lifetime: Option<Region>,
 228         s: ScopeRef<'a>
 229     },
 230
 231     Root
 232 }
 233
 234 #[derive(Clone, Debug)]
 235 enum Elide {
 236     /// Use a fresh anonymous late-bound lifetime each time, by
 237     /// incrementing the counter to generate sequential indices.
 238     FreshLateAnon(Cell<u32>),
 239     /// Always use this one lifetime.
 240     Exact(Region),
 241     /// Less or more than one lifetime were found, error on unspecified.
 242     Error(Vec<ElisionFailureInfo>)
 243 }
 244
 245 #[derive(Clone, Debug)]
 246 struct ElisionFailureInfo {
 247     /// Where we can find the argument pattern.
 248     parent: Option<hir::BodyId>,
 249     /// The index of the argument in the original definition.
 250     index: usize,
 251     lifetime_count: usize,
 252     have_bound_regions: bool
 253 }
 254
 255 type ScopeRef<'a> = &'a Scope<'a>;
 256
 257 const ROOT_SCOPE: ScopeRef<'static> = &Scope::Root;
 258
 259 pub fn krate(sess: &Session,
 260              hir_map: &Map)
 261              -> Result<NamedRegionMap, usize> {
 262     let _task = hir_map.dep_graph.in_task(DepNode::ResolveLifetimes);
 263     let krate = hir_map.krate();
 264     let mut map = NamedRegionMap {
 265         defs: NodeMap(),
 266         late_bound: NodeSet(),
 267         issue_32330: NodeMap(),
 268         object_lifetime_defaults: compute_object_lifetime_defaults(sess, hir_map),
 269     };
 270     sess.track_errors(|| {
 271         let mut visitor = LifetimeContext {
 272             sess: sess,
 273             hir_map: hir_map,
 274             map: &mut map,
 275             scope: ROOT_SCOPE,
 276             trait_ref_hack: false,
 277             labels_in_fn: vec![],
 278             xcrate_object_lifetime_defaults: DefIdMap(),
 279         };
 280         for (_, item) in &krate.items {
 281             visitor.visit_item(item);
 282         }
 283     })?;
 284     Ok(map)
 285 }
 286
 287 impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
 288     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 289         NestedVisitorMap::All(self.hir_map)
 290     }
 291
 292     // We want to nest trait/impl items in their parent, but nothing else.
 293     fn visit_nested_item(&mut self, _: hir::ItemId) {}
 294
 295     fn visit_nested_body(&mut self, body: hir::BodyId) {
 296         // Each body has their own set of labels, save labels.
 297         let saved = replace(&mut self.labels_in_fn, vec![]);
 298         let body = self.hir_map.body(body);
 299         extract_labels(self, body);
 300         self.with(Scope::Body { id: body.id(), s: self.scope }, |_, this| {
 301             this.visit_body(body);
 302         });
 303         replace(&mut self.labels_in_fn, saved);
 304     }
 305
 306     fn visit_item(&mut self, item: &'tcx hir::Item) {
 307         match item.node {
 308             hir::ItemFn(ref decl, _, _, _, ref generics, _) => {
 309                 self.visit_early_late(item.id, None, decl, generics, |this| {
 310                     intravisit::walk_item(this, item);
 311                 });
 312             }
 313             hir::ItemExternCrate(_) |
 314             hir::ItemUse(..) |
 315             hir::ItemMod(..) |
 316             hir::ItemDefaultImpl(..) |
 317             hir::ItemForeignMod(..) => {
 318                 // These sorts of items have no lifetime parameters at all.
 319                 intravisit::walk_item(self, item);
 320             }
 321             hir::ItemStatic(..) |
 322             hir::ItemConst(..) => {
 323                 // No lifetime parameters, but implied 'static.
 324                 let scope = Scope::Elision {
 325                     elide: Elide::Exact(Region::Static),
 326                     s: ROOT_SCOPE
 327                 };
 328                 self.with(scope, |_, this| intravisit::walk_item(this, item));
 329             }
 330             hir::ItemTy(_, ref generics) |
 331             hir::ItemEnum(_, ref generics) |
 332             hir::ItemStruct(_, ref generics) |
 333             hir::ItemUnion(_, ref generics) |
 334             hir::ItemTrait(_, ref generics, ..) |
 335             hir::ItemImpl(_, _, ref generics, ..) => {
 336                 // These kinds of items have only early bound lifetime parameters.
 337                 let mut index = if let hir::ItemTrait(..) = item.node {
 338                     1 // Self comes before lifetimes
 339                 } else {
 340                     0
 341                 };
 342                 let lifetimes = generics.lifetimes.iter().map(|def| {
 343                     Region::early(&mut index, def)
 344                 }).collect();
 345                 let scope = Scope::Binder {
 346                     lifetimes: lifetimes,
 347                     s: ROOT_SCOPE
 348                 };
 349                 self.with(scope, |old_scope, this| {
 350                     this.check_lifetime_defs(old_scope, &generics.lifetimes);
 351                     intravisit::walk_item(this, item);
 352                 });
 353             }
 354         }
 355     }
 356
 357     fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem) {
 358         match item.node {
 359             hir::ForeignItemFn(ref decl, _, ref generics) => {
 360                 self.visit_early_late(item.id, None, decl, generics, |this| {
 361                     intravisit::walk_foreign_item(this, item);
 362                 })
 363             }
 364             hir::ForeignItemStatic(..) => {
 365                 intravisit::walk_foreign_item(self, item);
 366             }
 367         }
 368     }
 369
 370     fn visit_ty(&mut self, ty: &'tcx hir::Ty) {
 371         match ty.node {
 372             hir::TyBareFn(ref c) => {
 373                 let scope = Scope::Binder {
 374                     lifetimes: c.lifetimes.iter().map(Region::late).collect(),
 375                     s: self.scope
 376                 };
 377                 self.with(scope, |old_scope, this| {
 378                     // a bare fn has no bounds, so everything
 379                     // contained within is scoped within its binder.
 380                     this.check_lifetime_defs(old_scope, &c.lifetimes);
 381                     intravisit::walk_ty(this, ty);
 382                 });
 383             }
 384             hir::TyTraitObject(ref bounds, ref lifetime) => {
 385                 for bound in bounds {
 386                     self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
 387                 }
 388                 if lifetime.is_elided() {
 389                     self.resolve_object_lifetime_default(lifetime)
 390                 } else {
 391                     self.visit_lifetime(lifetime);
 392                 }
 393             }
 394             hir::TyRptr(ref lifetime_ref, ref mt) => {
 395                 self.visit_lifetime(lifetime_ref);
 396                 let scope = Scope::ObjectLifetimeDefault {
 397                     lifetime: self.map.defs.get(&lifetime_ref.id).cloned(),
 398                     s: self.scope
 399                 };
 400                 self.with(scope, |_, this| this.visit_ty(&mt.ty));
 401             }
 402             _ => {
 403                 intravisit::walk_ty(self, ty)
 404             }
 405         }
 406     }
 407
 408     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
 409         if let hir::TraitItemKind::Method(ref sig, _) = trait_item.node {
 410             self.visit_early_late(
 411                 trait_item.id,
 412                 Some(self.hir_map.get_parent(trait_item.id)),
 413                 &sig.decl, &sig.generics,
 414                 |this| intravisit::walk_trait_item(this, trait_item))
 415         } else {
 416             intravisit::walk_trait_item(self, trait_item);
 417         }
 418     }
 419
 420     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
 421         if let hir::ImplItemKind::Method(ref sig, _) = impl_item.node {
 422             self.visit_early_late(
 423                 impl_item.id,
 424                 Some(self.hir_map.get_parent(impl_item.id)),
 425                 &sig.decl, &sig.generics,
 426                 |this| intravisit::walk_impl_item(this, impl_item))
 427         } else {
 428             intravisit::walk_impl_item(self, impl_item);
 429         }
 430     }
 431
 432     fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
 433         if lifetime_ref.is_elided() {
 434             self.resolve_elided_lifetimes(slice::ref_slice(lifetime_ref));
 435             return;
 436         }
 437         if lifetime_ref.is_static() {
 438             self.insert_lifetime(lifetime_ref, Region::Static);
 439             return;
 440         }
 441         self.resolve_lifetime_ref(lifetime_ref);
 442     }
 443
 444     fn visit_path(&mut self, path: &'tcx hir::Path, _: ast::NodeId) {
 445         for (i, segment) in path.segments.iter().enumerate() {
 446             let depth = path.segments.len() - i - 1;
 447             self.visit_segment_parameters(path.def, depth, &segment.parameters);
 448         }
 449     }
 450
 451     fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl) {
 452         let output = match fd.output {
 453             hir::DefaultReturn(_) => None,
 454             hir::Return(ref ty) => Some(ty)
 455         };
 456         self.visit_fn_like_elision(&fd.inputs, output);
 457     }
 458
 459     fn visit_generics(&mut self, generics: &'tcx hir::Generics) {
 460         for ty_param in generics.ty_params.iter() {
 461             walk_list!(self, visit_ty_param_bound, &ty_param.bounds);
 462             if let Some(ref ty) = ty_param.default {
 463                 self.visit_ty(&ty);
 464             }
 465         }
 466         for predicate in &generics.where_clause.predicates {
 467             match predicate {
 468                 &hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate{ ref bounded_ty,
 469                                                                                ref bounds,
 470                                                                                ref bound_lifetimes,
 471                                                                                .. }) => {
 472                     if !bound_lifetimes.is_empty() {
 473                         self.trait_ref_hack = true;
 474                         let scope = Scope::Binder {
 475                             lifetimes: bound_lifetimes.iter().map(Region::late).collect(),
 476                             s: self.scope
 477                         };
 478                         let result = self.with(scope, |old_scope, this| {
 479                             this.check_lifetime_defs(old_scope, bound_lifetimes);
 480                             this.visit_ty(&bounded_ty);
 481                             walk_list!(this, visit_ty_param_bound, bounds);
 482                         });
 483                         self.trait_ref_hack = false;
 484                         result
 485                     } else {
 486                         self.visit_ty(&bounded_ty);
 487                         walk_list!(self, visit_ty_param_bound, bounds);
 488                     }
 489                 }
 490                 &hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate{ref lifetime,
 491                                                                                 ref bounds,
 492                                                                                 .. }) => {
 493
 494                     self.visit_lifetime(lifetime);
 495                     for bound in bounds {
 496                         self.visit_lifetime(bound);
 497                     }
 498                 }
 499                 &hir::WherePredicate::EqPredicate(hir::WhereEqPredicate{ref lhs_ty,
 500                                                                         ref rhs_ty,
 501                                                                         .. }) => {
 502                     self.visit_ty(lhs_ty);
 503                     self.visit_ty(rhs_ty);
 504                 }
 505             }
 506         }
 507     }
 508
 509     fn visit_poly_trait_ref(&mut self,
 510                             trait_ref: &'tcx hir::PolyTraitRef,
 511                             _modifier: hir::TraitBoundModifier) {
 512         debug!("visit_poly_trait_ref trait_ref={:?}", trait_ref);
 513
 514         if !self.trait_ref_hack || !trait_ref.bound_lifetimes.is_empty() {
 515             if self.trait_ref_hack {
 516                 span_err!(self.sess, trait_ref.span, E0316,
 517                           "nested quantification of lifetimes");
 518             }
 519             let scope = Scope::Binder {
 520                 lifetimes: trait_ref.bound_lifetimes.iter().map(Region::late).collect(),
 521                 s: self.scope
 522             };
 523             self.with(scope, |old_scope, this| {
 524                 this.check_lifetime_defs(old_scope, &trait_ref.bound_lifetimes);
 525                 for lifetime in &trait_ref.bound_lifetimes {
 526                     this.visit_lifetime_def(lifetime);
 527                 }
 528                 this.visit_trait_ref(&trait_ref.trait_ref)
 529             })
 530         } else {
 531             self.visit_trait_ref(&trait_ref.trait_ref)
 532         }
 533     }
 534 }
 535
 536 #[derive(Copy, Clone, PartialEq)]
 537 enum ShadowKind { Label, Lifetime }
 538 struct Original { kind: ShadowKind, span: Span }
 539 struct Shadower { kind: ShadowKind, span: Span }
 540
 541 fn original_label(span: Span) -> Original {
 542     Original { kind: ShadowKind::Label, span: span }
 543 }
 544 fn shadower_label(span: Span) -> Shadower {
 545     Shadower { kind: ShadowKind::Label, span: span }
 546 }
 547 fn original_lifetime(span: Span) -> Original {
 548     Original { kind: ShadowKind::Lifetime, span: span }
 549 }
 550 fn shadower_lifetime(l: &hir::Lifetime) -> Shadower {
 551     Shadower { kind: ShadowKind::Lifetime, span: l.span }
 552 }
 553
 554 impl ShadowKind {
 555     fn desc(&self) -> &'static str {
 556         match *self {
 557             ShadowKind::Label => "label",
 558             ShadowKind::Lifetime => "lifetime",
 559         }
 560     }
 561 }
 562
 563 fn signal_shadowing_problem(sess: &Session, name: ast::Name, orig: Original, shadower: Shadower) {
 564     let mut err = if let (ShadowKind::Lifetime, ShadowKind::Lifetime) = (orig.kind, shadower.kind) {
 565         // lifetime/lifetime shadowing is an error
 566         struct_span_err!(sess, shadower.span, E0496,
 567                          "{} name `{}` shadows a \
 568                           {} name that is already in scope",
 569                          shadower.kind.desc(), name, orig.kind.desc())
 570     } else {
 571         // shadowing involving a label is only a warning, due to issues with
 572         // labels and lifetimes not being macro-hygienic.
 573         sess.struct_span_warn(shadower.span,
 574                               &format!("{} name `{}` shadows a \
 575                                         {} name that is already in scope",
 576                                        shadower.kind.desc(), name, orig.kind.desc()))
 577     };
 578     err.span_label(orig.span, &"first declared here");
 579     err.span_label(shadower.span,
 580                    &format!("lifetime {} already in scope", name));
 581     err.emit();
 582 }
 583
 584 // Adds all labels in `b` to `ctxt.labels_in_fn`, signalling a warning
 585 // if one of the label shadows a lifetime or another label.
 586 fn extract_labels(ctxt: &mut LifetimeContext, body: &hir::Body) {
 587     struct GatherLabels<'a, 'tcx: 'a> {
 588         sess: &'a Session,
 589         hir_map: &'a Map<'tcx>,
 590         scope: ScopeRef<'a>,
 591         labels_in_fn: &'a mut Vec<(ast::Name, Span)>,
 592     }
 593
 594     let mut gather = GatherLabels {
 595         sess: ctxt.sess,
 596         hir_map: ctxt.hir_map,
 597         scope: ctxt.scope,
 598         labels_in_fn: &mut ctxt.labels_in_fn,
 599     };
 600     gather.visit_body(body);
 601
 602     impl<'v, 'a, 'tcx> Visitor<'v> for GatherLabels<'a, 'tcx> {
 603         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 604             NestedVisitorMap::None
 605         }
 606
 607         fn visit_expr(&mut self, ex: &hir::Expr) {
 608             if let Some((label, label_span)) = expression_label(ex) {
 609                 for &(prior, prior_span) in &self.labels_in_fn[..] {
 610                     // FIXME (#24278): non-hygienic comparison
 611                     if label == prior {
 612                         signal_shadowing_problem(self.sess,
 613                                                  label,
 614                                                  original_label(prior_span),
 615                                                  shadower_label(label_span));
 616                     }
 617                 }
 618
 619                 check_if_label_shadows_lifetime(self.sess,
 620                                                 self.hir_map,
 621                                                 self.scope,
 622                                                 label,
 623                                                 label_span);
 624
 625                 self.labels_in_fn.push((label, label_span));
 626             }
 627             intravisit::walk_expr(self, ex)
 628         }
 629     }
 630
 631     fn expression_label(ex: &hir::Expr) -> Option<(ast::Name, Span)> {
 632         match ex.node {
 633             hir::ExprWhile(.., Some(label)) |
 634             hir::ExprLoop(_, Some(label), _) => Some((label.node, label.span)),
 635             _ => None,
 636         }
 637     }
 638
 639     fn check_if_label_shadows_lifetime<'a>(sess: &'a Session,
 640                                            hir_map: &Map,
 641                                            mut scope: ScopeRef<'a>,
 642                                            label: ast::Name,
 643                                            label_span: Span) {
 644         loop {
 645             match *scope {
 646                 Scope::Body { s, .. } |
 647                 Scope::Elision { s, .. } |
 648                 Scope::ObjectLifetimeDefault { s, .. } => { scope = s; }
 649
 650                 Scope::Root => { return; }
 651
 652                 Scope::Binder { ref lifetimes, s } => {
 653                     // FIXME (#24278): non-hygienic comparison
 654                     if let Some(def) = lifetimes.get(&label) {
 655                         signal_shadowing_problem(
 656                             sess,
 657                             label,
 658                             original_lifetime(hir_map.span(def.id().unwrap())),
 659                             shadower_label(label_span));
 660                         return;
 661                     }
 662                     scope = s;
 663                 }
 664             }
 665         }
 666     }
 667 }
 668
 669 fn compute_object_lifetime_defaults(sess: &Session, hir_map: &Map)
 670                                     -> NodeMap<Vec<ObjectLifetimeDefault>> {
 671     let mut map = NodeMap();
 672     for item in hir_map.krate().items.values() {
 673         match item.node {
 674             hir::ItemStruct(_, ref generics) |
 675             hir::ItemUnion(_, ref generics) |
 676             hir::ItemEnum(_, ref generics) |
 677             hir::ItemTy(_, ref generics) |
 678             hir::ItemTrait(_, ref generics, ..) => {
 679                 let result = object_lifetime_defaults_for_item(hir_map, generics);
 680
 681                 // Debugging aid.
 682                 if attr::contains_name(&item.attrs, "rustc_object_lifetime_default") {
 683                     let object_lifetime_default_reprs: String =
 684                         result.iter().map(|set| {
 685                             match *set {
 686                                 Set1::Empty => "BaseDefault".to_string(),
 687                                 Set1::One(Region::Static) => "'static".to_string(),
 688                                 Set1::One(Region::EarlyBound(i, _)) => {
 689                                     generics.lifetimes[i as usize].lifetime.name.to_string()
 690                                 }
 691                                 Set1::One(_) => bug!(),
 692                                 Set1::Many => "Ambiguous".to_string(),
 693                             }
 694                         }).collect::<Vec<String>>().join(",");
 695                     sess.span_err(item.span, &object_lifetime_default_reprs);
 696                 }
 697
 698                 map.insert(item.id, result);
 699             }
 700             _ => {}
 701         }
 702     }
 703     map
 704 }
 705
 706 /// Scan the bounds and where-clauses on parameters to extract bounds
 707 /// of the form `T:'a` so as to determine the `ObjectLifetimeDefault`
 708 /// for each type parameter.
 709 fn object_lifetime_defaults_for_item(hir_map: &Map, generics: &hir::Generics)
 710                                      -> Vec<ObjectLifetimeDefault> {
 711     fn add_bounds(set: &mut Set1<ast::Name>, bounds: &[hir::TyParamBound]) {
 712         for bound in bounds {
 713             if let hir::RegionTyParamBound(ref lifetime) = *bound {
 714                 set.insert(lifetime.name);
 715             }
 716         }
 717     }
 718
 719     generics.ty_params.iter().map(|param| {
 720         let mut set = Set1::Empty;
 721
 722         add_bounds(&mut set, &param.bounds);
 723
 724         let param_def_id = hir_map.local_def_id(param.id);
 725         for predicate in &generics.where_clause.predicates {
 726             // Look for `type: ...` where clauses.
 727             let data = match *predicate {
 728                 hir::WherePredicate::BoundPredicate(ref data) => data,
 729                 _ => continue
 730             };
 731
 732             // Ignore `for<'a> type: ...` as they can change what
 733             // lifetimes mean (although we could "just" handle it).
 734             if !data.bound_lifetimes.is_empty() {
 735                 continue;
 736             }
 737
 738             let def = match data.bounded_ty.node {
 739                 hir::TyPath(hir::QPath::Resolved(None, ref path)) => path.def,
 740                 _ => continue
 741             };
 742
 743             if def == Def::TyParam(param_def_id) {
 744                 add_bounds(&mut set, &data.bounds);
 745             }
 746         }
 747
 748         match set {
 749             Set1::Empty => Set1::Empty,
 750             Set1::One(name) => {
 751                 if name == keywords::StaticLifetime.name() {
 752                     Set1::One(Region::Static)
 753                 } else {
 754                     generics.lifetimes.iter().enumerate().find(|&(_, def)| {
 755                         def.lifetime.name == name
 756                     }).map_or(Set1::Many, |(i, def)| {
 757                         Set1::One(Region::EarlyBound(i as u32, def.lifetime.id))
 758                     })
 759                 }
 760             }
 761             Set1::Many => Set1::Many
 762         }
 763     }).collect()
 764 }
 765
 766 impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
 767     // FIXME(#37666) this works around a limitation in the region inferencer
 768     fn hack<F>(&mut self, f: F) where
 769         F: for<'b> FnOnce(&mut LifetimeContext<'b, 'tcx>),
 770     {
 771         f(self)
 772     }
 773
 774     fn with<F>(&mut self, wrap_scope: Scope, f: F) where
 775         F: for<'b> FnOnce(ScopeRef, &mut LifetimeContext<'b, 'tcx>),
 776     {
 777         let LifetimeContext {sess, hir_map, ref mut map, ..} = *self;
 778         let labels_in_fn = replace(&mut self.labels_in_fn, vec![]);
 779         let xcrate_object_lifetime_defaults =
 780             replace(&mut self.xcrate_object_lifetime_defaults, DefIdMap());
 781         let mut this = LifetimeContext {
 782             sess: sess,
 783             hir_map: hir_map,
 784             map: *map,
 785             scope: &wrap_scope,
 786             trait_ref_hack: self.trait_ref_hack,
 787             labels_in_fn: labels_in_fn,
 788             xcrate_object_lifetime_defaults: xcrate_object_lifetime_defaults,
 789         };
 790         debug!("entering scope {:?}", this.scope);
 791         f(self.scope, &mut this);
 792         debug!("exiting scope {:?}", this.scope);
 793         self.labels_in_fn = this.labels_in_fn;
 794         self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;
 795     }
 796
 797     /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
 798     ///
 799     /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
 800     /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
 801     /// within type bounds; those are early bound lifetimes, and the rest are late bound.
 802     ///
 803     /// For example:
 804     ///
 805     ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
 806     ///
 807     /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
 808     /// lifetimes may be interspersed together.
 809     ///
 810     /// If early bound lifetimes are present, we separate them into their own list (and likewise
 811     /// for late bound). They will be numbered sequentially, starting from the lowest index that is
 812     /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
 813     /// bound lifetimes are resolved by name and associated with a binder id (`binder_id`), so the
 814     /// ordering is not important there.
 815     fn visit_early_late<F>(&mut self,
 816                            fn_id: ast::NodeId,
 817                            parent_id: Option<ast::NodeId>,
 818                            decl: &'tcx hir::FnDecl,
 819                            generics: &'tcx hir::Generics,
 820                            walk: F) where
 821         F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
 822     {
 823         let fn_def_id = self.hir_map.local_def_id(fn_id);
 824         insert_late_bound_lifetimes(self.map,
 825                                     fn_def_id,
 826                                     decl,
 827                                     generics);
 828
 829         // Find the start of nested early scopes, e.g. in methods.
 830         let mut index = 0;
 831         if let Some(parent_id) = parent_id {
 832             let parent = self.hir_map.expect_item(parent_id);
 833             if let hir::ItemTrait(..) = parent.node {
 834                 index += 1; // Self comes first.
 835             }
 836             match parent.node {
 837                 hir::ItemTrait(_, ref generics, ..) |
 838                 hir::ItemImpl(_, _, ref generics, ..) => {
 839                     index += (generics.lifetimes.len() + generics.ty_params.len()) as u32;
 840                 }
 841                 _ => {}
 842             }
 843         }
 844
 845         let lifetimes = generics.lifetimes.iter().map(|def| {
 846             if self.map.late_bound.contains(&def.lifetime.id) {
 847                 Region::late(def)
 848             } else {
 849                 Region::early(&mut index, def)
 850             }
 851         }).collect();
 852
 853         let scope = Scope::Binder {
 854             lifetimes: lifetimes,
 855             s: self.scope
 856         };
 857         self.with(scope, move |old_scope, this| {
 858             this.check_lifetime_defs(old_scope, &generics.lifetimes);
 859             this.hack(walk); // FIXME(#37666) workaround in place of `walk(this)`
 860         });
 861     }
 862
 863     fn resolve_lifetime_ref(&mut self, lifetime_ref: &hir::Lifetime) {
 864         // Walk up the scope chain, tracking the number of fn scopes
 865         // that we pass through, until we find a lifetime with the
 866         // given name or we run out of scopes.
 867         // search.
 868         let mut late_depth = 0;
 869         let mut scope = self.scope;
 870         let mut outermost_body = None;
 871         let result = loop {
 872             match *scope {
 873                 Scope::Body { id, s } => {
 874                     outermost_body = Some(id);
 875                     scope = s;
 876                 }
 877
 878                 Scope::Root => {
 879                     break None;
 880                 }
 881
 882                 Scope::Binder { ref lifetimes, s } => {
 883                     if let Some(&def) = lifetimes.get(&lifetime_ref.name) {
 884                         break Some(def.shifted(late_depth));
 885                     } else {
 886                         late_depth += 1;
 887                         scope = s;
 888                     }
 889                 }
 890
 891                 Scope::Elision { s, .. } |
 892                 Scope::ObjectLifetimeDefault { s, .. } => {
 893                     scope = s;
 894                 }
 895             }
 896         };
 897
 898         if let Some(mut def) = result {
 899             if let Some(body_id) = outermost_body {
 900                 let fn_id = self.hir_map.body_owner(body_id);
 901                 let scope_data = region::CallSiteScopeData {
 902                     fn_id: fn_id, body_id: body_id.node_id
 903                 };
 904                 match self.hir_map.get(fn_id) {
 905                     hir::map::NodeItem(&hir::Item {
 906                         node: hir::ItemFn(..), ..
 907                     }) |
 908                     hir::map::NodeTraitItem(&hir::TraitItem {
 909                         node: hir::TraitItemKind::Method(..), ..
 910                     }) |
 911                     hir::map::NodeImplItem(&hir::ImplItem {
 912                         node: hir::ImplItemKind::Method(..), ..
 913                     }) => {
 914                         def = Region::Free(scope_data, def.id().unwrap());
 915                     }
 916                     _ => {}
 917                 }
 918             }
 919             self.insert_lifetime(lifetime_ref, def);
 920         } else {
 921             struct_span_err!(self.sess, lifetime_ref.span, E0261,
 922                 "use of undeclared lifetime name `{}`", lifetime_ref.name)
 923                 .span_label(lifetime_ref.span, &format!("undeclared lifetime"))
 924                 .emit();
 925         }
 926     }
 927
 928     fn visit_segment_parameters(&mut self,
 929                                 def: Def,
 930                                 depth: usize,
 931                                 params: &'tcx hir::PathParameters) {
 932         let data = match *params {
 933             hir::ParenthesizedParameters(ref data) => {
 934                 self.visit_fn_like_elision(&data.inputs, data.output.as_ref());
 935                 return;
 936             }
 937             hir::AngleBracketedParameters(ref data) => data
 938         };
 939
 940         if data.lifetimes.iter().all(|l| l.is_elided()) {
 941             self.resolve_elided_lifetimes(&data.lifetimes);
 942         } else {
 943             for l in &data.lifetimes { self.visit_lifetime(l); }
 944         }
 945
 946         // Figure out if this is a type/trait segment,
 947         // which requires object lifetime defaults.
 948         let parent_def_id = |this: &mut Self, def_id: DefId| {
 949             let def_key = if def_id.is_local() {
 950                 this.hir_map.def_key(def_id)
 951             } else {
 952                 this.sess.cstore.def_key(def_id)
 953             };
 954             DefId {
 955                 krate: def_id.krate,
 956                 index: def_key.parent.expect("missing parent")
 957             }
 958         };
 959         let type_def_id = match def {
 960             Def::AssociatedTy(def_id) if depth == 1 => {
 961                 Some(parent_def_id(self, def_id))
 962             }
 963             Def::Variant(def_id) if depth == 0 => {
 964                 Some(parent_def_id(self, def_id))
 965             }
 966             Def::Struct(def_id) |
 967             Def::Union(def_id) |
 968             Def::Enum(def_id) |
 969             Def::TyAlias(def_id) |
 970             Def::Trait(def_id) if depth == 0 => Some(def_id),
 971             _ => None
 972         };
 973
 974         let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
 975             let in_body = {
 976                 let mut scope = self.scope;
 977                 loop {
 978                     match *scope {
 979                         Scope::Root => break false,
 980
 981                         Scope::Body { .. } => break true,
 982
 983                         Scope::Binder { s, .. } |
 984                         Scope::Elision { s, .. } |
 985                         Scope::ObjectLifetimeDefault { s, .. } => {
 986                             scope = s;
 987                         }
 988                     }
 989                 }
 990             };
 991
 992             let map = &self.map;
 993             let unsubst = if let Some(id) = self.hir_map.as_local_node_id(def_id) {
 994                 &map.object_lifetime_defaults[&id]
 995             } else {
 996                 let cstore = &self.sess.cstore;
 997                 self.xcrate_object_lifetime_defaults.entry(def_id).or_insert_with(|| {
 998                     cstore.item_generics_cloned(def_id).types.into_iter().map(|def| {
 999                         def.object_lifetime_default
1000                     }).collect()
1001                 })
1002             };
1003             unsubst.iter().map(|set| {
1004                 match *set {
1005                     Set1::Empty => {
1006                         if in_body {
1007                             None
1008                         } else {
1009                             Some(Region::Static)
1010                         }
1011                     }
1012                     Set1::One(r) => r.subst(&data.lifetimes, map),
1013                     Set1::Many => None
1014                 }
1015             }).collect()
1016         });
1017
1018         for (i, ty) in data.types.iter().enumerate() {
1019             if let Some(&lt) = object_lifetime_defaults.get(i) {
1020                 let scope = Scope::ObjectLifetimeDefault {
1021                     lifetime: lt,
1022                     s: self.scope
1023                 };
1024                 self.with(scope, |_, this| this.visit_ty(ty));
1025             } else {
1026                 self.visit_ty(ty);
1027             }
1028         }
1029
1030         for b in &data.bindings { self.visit_assoc_type_binding(b); }
1031     }
1032
1033     fn visit_fn_like_elision(&mut self, inputs: &'tcx [P<hir::Ty>],
1034                              output: Option<&'tcx P<hir::Ty>>) {
1035         let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
1036         let arg_scope = Scope::Elision {
1037             elide: arg_elide.clone(),
1038             s: self.scope
1039         };
1040         self.with(arg_scope, |_, this| {
1041             for input in inputs {
1042                 this.visit_ty(input);
1043             }
1044             match *this.scope {
1045                 Scope::Elision { ref elide, .. } => {
1046                     arg_elide = elide.clone();
1047                 }
1048                 _ => bug!()
1049             }
1050         });
1051
1052         let output = match output {
1053             Some(ty) => ty,
1054             None => return
1055         };
1056
1057         // Figure out if there's a body we can get argument names from,
1058         // and whether there's a `self` argument (treated specially).
1059         let mut assoc_item_kind = None;
1060         let mut impl_self = None;
1061         let parent = self.hir_map.get_parent_node(output.id);
1062         let body = match self.hir_map.get(parent) {
1063             // `fn` definitions and methods.
1064             hir::map::NodeItem(&hir::Item {
1065                 node: hir::ItemFn(.., body), ..
1066             })  => Some(body),
1067
1068             hir::map::NodeTraitItem(&hir::TraitItem {
1069                 node: hir::TraitItemKind::Method(_, ref m), ..
1070             }) => {
1071                 match self.hir_map.expect_item(self.hir_map.get_parent(parent)).node {
1072                     hir::ItemTrait(.., ref trait_items) => {
1073                         assoc_item_kind = trait_items.iter().find(|ti| ti.id.node_id == parent)
1074                                                             .map(|ti| ti.kind);
1075                     }
1076                     _ => {}
1077                 }
1078                 match *m {
1079                     hir::TraitMethod::Required(_) => None,
1080                     hir::TraitMethod::Provided(body) => Some(body),
1081                 }
1082             }
1083
1084             hir::map::NodeImplItem(&hir::ImplItem {
1085                 node: hir::ImplItemKind::Method(_, body), ..
1086             }) => {
1087                 match self.hir_map.expect_item(self.hir_map.get_parent(parent)).node {
1088                     hir::ItemImpl(.., ref self_ty, ref impl_items) => {
1089                         impl_self = Some(self_ty);
1090                         assoc_item_kind = impl_items.iter().find(|ii| ii.id.node_id == parent)
1091                                                            .map(|ii| ii.kind);
1092                     }
1093                     _ => {}
1094                 }
1095                 Some(body)
1096             }
1097
1098             // `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
1099             hir::map::NodeTy(_) | hir::map::NodeTraitRef(_) => None,
1100
1101             // Foreign `fn` decls are terrible because we messed up,
1102             // and their return types get argument type elision.
1103             // And now too much code out there is abusing this rule.
1104             hir::map::NodeForeignItem(_) => {
1105                 let arg_scope = Scope::Elision {
1106                     elide: arg_elide,
1107                     s: self.scope
1108                 };
1109                 self.with(arg_scope, |_, this| this.visit_ty(output));
1110                 return;
1111             }
1112
1113             // Everything else (only closures?) doesn't
1114             // actually enjoy elision in return types.
1115             _ => {
1116                 self.visit_ty(output);
1117                 return;
1118             }
1119         };
1120
1121         let has_self = match assoc_item_kind {
1122             Some(hir::AssociatedItemKind::Method { has_self }) => has_self,
1123             _ => false
1124         };
1125
1126         // In accordance with the rules for lifetime elision, we can determine
1127         // what region to use for elision in the output type in two ways.
1128         // First (determined here), if `self` is by-reference, then the
1129         // implied output region is the region of the self parameter.
1130         if has_self {
1131             // Look for `self: &'a Self` - also desugared from `&'a self`,
1132             // and if that matches, use it for elision and return early.
1133             let is_self_ty = |def: Def| {
1134                 if let Def::SelfTy(..) = def {
1135                     return true;
1136                 }
1137
1138                 // Can't always rely on literal (or implied) `Self` due
1139                 // to the way elision rules were originally specified.
1140                 let impl_self = impl_self.map(|ty| &ty.node);
1141                 if let Some(&hir::TyPath(hir::QPath::Resolved(None, ref path))) = impl_self {
1142                     match path.def {
1143                         // Whitelist the types that unambiguously always
1144                         // result in the same type constructor being used
1145                         // (it can't differ between `Self` and `self`).
1146                         Def::Struct(_) |
1147                         Def::Union(_) |
1148                         Def::Enum(_) |
1149                         Def::PrimTy(_) => return def == path.def,
1150                         _ => {}
1151                     }
1152                 }
1153
1154                 false
1155             };
1156
1157             if let hir::TyRptr(lifetime_ref, ref mt) = inputs[0].node {
1158                 if let hir::TyPath(hir::QPath::Resolved(None, ref path)) = mt.ty.node {
1159                     if is_self_ty(path.def) {
1160                         if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.id) {
1161                             let scope = Scope::Elision {
1162                                 elide: Elide::Exact(lifetime),
1163                                 s: self.scope
1164                             };
1165                             self.with(scope, |_, this| this.visit_ty(output));
1166                             return;
1167                         }
1168                     }
1169                 }
1170             }
1171         }
1172
1173         // Second, if there was exactly one lifetime (either a substitution or a
1174         // reference) in the arguments, then any anonymous regions in the output
1175         // have that lifetime.
1176         let mut possible_implied_output_region = None;
1177         let mut lifetime_count = 0;
1178         let arg_lifetimes = inputs.iter().enumerate().skip(has_self as usize).map(|(i, input)| {
1179             let mut gather = GatherLifetimes {
1180                 map: self.map,
1181                 binder_depth: 1,
1182                 have_bound_regions: false,
1183                 lifetimes: FxHashSet()
1184             };
1185             gather.visit_ty(input);
1186
1187             lifetime_count += gather.lifetimes.len();
1188
1189             if lifetime_count == 1 && gather.lifetimes.len() == 1 {
1190                 // there's a chance that the unique lifetime of this
1191                 // iteration will be the appropriate lifetime for output
1192                 // parameters, so lets store it.
1193                 possible_implied_output_region = gather.lifetimes.iter().cloned().next();
1194             }
1195
1196             ElisionFailureInfo {
1197                 parent: body,
1198                 index: i,
1199                 lifetime_count: gather.lifetimes.len(),
1200                 have_bound_regions: gather.have_bound_regions
1201             }
1202         }).collect();
1203
1204         let elide = if lifetime_count == 1 {
1205             Elide::Exact(possible_implied_output_region.unwrap())
1206         } else {
1207             Elide::Error(arg_lifetimes)
1208         };
1209
1210         let scope = Scope::Elision {
1211             elide: elide,
1212             s: self.scope
1213         };
1214         self.with(scope, |_, this| this.visit_ty(output));
1215
1216         struct GatherLifetimes<'a> {
1217             map: &'a NamedRegionMap,
1218             binder_depth: u32,
1219             have_bound_regions: bool,
1220             lifetimes: FxHashSet<Region>,
1221         }
1222
1223         impl<'v, 'a> Visitor<'v> for GatherLifetimes<'a> {
1224             fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
1225                 NestedVisitorMap::None
1226             }
1227
1228             fn visit_ty(&mut self, ty: &hir::Ty) {
1229                 if let hir::TyBareFn(_) = ty.node {
1230                     self.binder_depth += 1;
1231                 }
1232                 if let hir::TyTraitObject(ref bounds, ref lifetime) = ty.node {
1233                     for bound in bounds {
1234                         self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
1235                     }
1236
1237                     // Stay on the safe side and don't include the object
1238                     // lifetime default (which may not end up being used).
1239                     if !lifetime.is_elided() {
1240                         self.visit_lifetime(lifetime);
1241                     }
1242                 } else {
1243                     intravisit::walk_ty(self, ty);
1244                 }
1245                 if let hir::TyBareFn(_) = ty.node {
1246                     self.binder_depth -= 1;
1247                 }
1248             }
1249
1250             fn visit_poly_trait_ref(&mut self,
1251                                     trait_ref: &hir::PolyTraitRef,
1252                                     modifier: hir::TraitBoundModifier) {
1253                 self.binder_depth += 1;
1254                 intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
1255                 self.binder_depth -= 1;
1256             }
1257
1258             fn visit_lifetime_def(&mut self, lifetime_def: &hir::LifetimeDef) {
1259                 for l in &lifetime_def.bounds { self.visit_lifetime(l); }
1260             }
1261
1262             fn visit_lifetime(&mut self, lifetime_ref: &hir::Lifetime) {
1263                 if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.id) {
1264                     match lifetime {
1265                         Region::LateBound(debruijn, _) |
1266                         Region::LateBoundAnon(debruijn, _)
1267                                 if debruijn.depth < self.binder_depth => {
1268                             self.have_bound_regions = true;
1269                         }
1270                         _ => {
1271                             self.lifetimes.insert(lifetime.from_depth(self.binder_depth));
1272                         }
1273                     }
1274                 }
1275             }
1276         }
1277
1278     }
1279
1280     fn resolve_elided_lifetimes(&mut self, lifetime_refs: &[hir::Lifetime]) {
1281         if lifetime_refs.is_empty() {
1282             return;
1283         }
1284
1285         let span = lifetime_refs[0].span;
1286         let mut late_depth = 0;
1287         let mut scope = self.scope;
1288         let error = loop {
1289             match *scope {
1290                 // Do not assign any resolution, it will be inferred.
1291                 Scope::Body { .. } => return,
1292
1293                 Scope::Root => break None,
1294
1295                 Scope::Binder { s, .. } => {
1296                     late_depth += 1;
1297                     scope = s;
1298                 }
1299
1300                 Scope::Elision { ref elide, .. } => {
1301                     let lifetime = match *elide {
1302                         Elide::FreshLateAnon(ref counter) => {
1303                             for lifetime_ref in lifetime_refs {
1304                                 let lifetime = Region::late_anon(counter).shifted(late_depth);
1305                                 self.insert_lifetime(lifetime_ref, lifetime);
1306                             }
1307                             return;
1308                         }
1309                         Elide::Exact(l) => l.shifted(late_depth),
1310                         Elide::Error(ref e) => break Some(e)
1311                     };
1312                     for lifetime_ref in lifetime_refs {
1313                         self.insert_lifetime(lifetime_ref, lifetime);
1314                     }
1315                     return;
1316                 }
1317
1318                 Scope::ObjectLifetimeDefault { s, .. } => {
1319                     scope = s;
1320                 }
1321             }
1322         };
1323
1324         let mut err = struct_span_err!(self.sess, span, E0106,
1325             "missing lifetime specifier{}",
1326             if lifetime_refs.len() > 1 { "s" } else { "" });
1327         let msg = if lifetime_refs.len() > 1 {
1328             format!("expected {} lifetime parameters", lifetime_refs.len())
1329         } else {
1330             format!("expected lifetime parameter")
1331         };
1332         err.span_label(span, &msg);
1333
1334         if let Some(params) = error {
1335             if lifetime_refs.len() == 1 {
1336                 self.report_elision_failure(&mut err, params);
1337             }
1338         }
1339         err.emit();
1340     }
1341
1342     fn report_elision_failure(&mut self,
1343                               db: &mut DiagnosticBuilder,
1344                               params: &[ElisionFailureInfo]) {
1345         let mut m = String::new();
1346         let len = params.len();
1347
1348         let elided_params: Vec<_> = params.iter().cloned()
1349                                           .filter(|info| info.lifetime_count > 0)
1350                                           .collect();
1351
1352         let elided_len = elided_params.len();
1353
1354         for (i, info) in elided_params.into_iter().enumerate() {
1355             let ElisionFailureInfo {
1356                 parent, index, lifetime_count: n, have_bound_regions
1357             } = info;
1358
1359             let help_name = if let Some(body) = parent {
1360                 let arg = &self.hir_map.body(body).arguments[index];
1361                 format!("`{}`", self.hir_map.node_to_pretty_string(arg.pat.id))
1362             } else {
1363                 format!("argument {}", index + 1)
1364             };
1365
1366             m.push_str(&(if n == 1 {
1367                 help_name
1368             } else {
1369                 format!("one of {}'s {} elided {}lifetimes", help_name, n,
1370                         if have_bound_regions { "free " } else { "" } )
1371             })[..]);
1372
1373             if elided_len == 2 && i == 0 {
1374                 m.push_str(" or ");
1375             } else if i + 2 == elided_len {
1376                 m.push_str(", or ");
1377             } else if i != elided_len - 1 {
1378                 m.push_str(", ");
1379             }
1380
1381         }
1382
1383         if len == 0 {
1384             help!(db,
1385                   "this function's return type contains a borrowed value, but \
1386                    there is no value for it to be borrowed from");
1387             help!(db,
1388                   "consider giving it a 'static lifetime");
1389         } else if elided_len == 0 {
1390             help!(db,
1391                   "this function's return type contains a borrowed value with \
1392                    an elided lifetime, but the lifetime cannot be derived from \
1393                    the arguments");
1394             help!(db,
1395                   "consider giving it an explicit bounded or 'static \
1396                    lifetime");
1397         } else if elided_len == 1 {
1398             help!(db,
1399                   "this function's return type contains a borrowed value, but \
1400                    the signature does not say which {} it is borrowed from",
1401                   m);
1402         } else {
1403             help!(db,
1404                   "this function's return type contains a borrowed value, but \
1405                    the signature does not say whether it is borrowed from {}",
1406                   m);
1407         }
1408     }
1409
1410     fn resolve_object_lifetime_default(&mut self, lifetime_ref: &hir::Lifetime) {
1411         let mut late_depth = 0;
1412         let mut scope = self.scope;
1413         let lifetime = loop {
1414             match *scope {
1415                 Scope::Binder { s, .. } => {
1416                     late_depth += 1;
1417                     scope = s;
1418                 }
1419
1420                 Scope::Root |
1421                 Scope::Elision { .. } => break Region::Static,
1422
1423                 Scope::Body { .. } |
1424                 Scope::ObjectLifetimeDefault { lifetime: None, .. } => return,
1425
1426                 Scope::ObjectLifetimeDefault { lifetime: Some(l), .. } => break l
1427             }
1428         };
1429         self.insert_lifetime(lifetime_ref, lifetime.shifted(late_depth));
1430     }
1431
1432     fn check_lifetime_defs(&mut self, old_scope: ScopeRef, lifetimes: &[hir::LifetimeDef]) {
1433         for i in 0..lifetimes.len() {
1434             let lifetime_i = &lifetimes[i];
1435
1436             for lifetime in lifetimes {
1437                 if lifetime.lifetime.is_static() {
1438                     let lifetime = lifetime.lifetime;
1439                     let mut err = struct_span_err!(self.sess, lifetime.span, E0262,
1440                                   "invalid lifetime parameter name: `{}`", lifetime.name);
1441                     err.span_label(lifetime.span,
1442                                    &format!("{} is a reserved lifetime name", lifetime.name));
1443                     err.emit();
1444                 }
1445             }
1446
1447             // It is a hard error to shadow a lifetime within the same scope.
1448             for j in i + 1..lifetimes.len() {
1449                 let lifetime_j = &lifetimes[j];
1450
1451                 if lifetime_i.lifetime.name == lifetime_j.lifetime.name {
1452                     struct_span_err!(self.sess, lifetime_j.lifetime.span, E0263,
1453                                      "lifetime name `{}` declared twice in the same scope",
1454                                      lifetime_j.lifetime.name)
1455                         .span_label(lifetime_j.lifetime.span,
1456                                     &format!("declared twice"))
1457                         .span_label(lifetime_i.lifetime.span,
1458                                    &format!("previous declaration here"))
1459                         .emit();
1460                 }
1461             }
1462
1463             // It is a soft error to shadow a lifetime within a parent scope.
1464             self.check_lifetime_def_for_shadowing(old_scope, &lifetime_i.lifetime);
1465
1466             for bound in &lifetime_i.bounds {
1467                 if !bound.is_static() {
1468                     self.resolve_lifetime_ref(bound);
1469                 } else {
1470                     self.insert_lifetime(bound, Region::Static);
1471                     self.sess.struct_span_warn(lifetime_i.lifetime.span.to(bound.span),
1472                         &format!("unnecessary lifetime parameter `{}`", lifetime_i.lifetime.name))
1473                         .help(&format!("you can use the `'static` lifetime directly, in place \
1474                                         of `{}`", lifetime_i.lifetime.name))
1475                         .emit();
1476                 }
1477             }
1478         }
1479     }
1480
1481     fn check_lifetime_def_for_shadowing(&self,
1482                                         mut old_scope: ScopeRef,
1483                                         lifetime: &hir::Lifetime)
1484     {
1485         for &(label, label_span) in &self.labels_in_fn {
1486             // FIXME (#24278): non-hygienic comparison
1487             if lifetime.name == label {
1488                 signal_shadowing_problem(self.sess,
1489                                          lifetime.name,
1490                                          original_label(label_span),
1491                                          shadower_lifetime(&lifetime));
1492                 return;
1493             }
1494         }
1495
1496         loop {
1497             match *old_scope {
1498                 Scope::Body { s, .. } |
1499                 Scope::Elision { s, .. } |
1500                 Scope::ObjectLifetimeDefault { s, .. } => {
1501                     old_scope = s;
1502                 }
1503
1504                 Scope::Root => {
1505                     return;
1506                 }
1507
1508                 Scope::Binder { ref lifetimes, s } => {
1509                     if let Some(&def) = lifetimes.get(&lifetime.name) {
1510                         signal_shadowing_problem(
1511                             self.sess,
1512                             lifetime.name,
1513                             original_lifetime(self.hir_map.span(def.id().unwrap())),
1514                             shadower_lifetime(&lifetime));
1515                         return;
1516                     }
1517
1518                     old_scope = s;
1519                 }
1520             }
1521         }
1522     }
1523
1524     fn insert_lifetime(&mut self,
1525                        lifetime_ref: &hir::Lifetime,
1526                        def: Region) {
1527         if lifetime_ref.id == ast::DUMMY_NODE_ID {
1528             span_bug!(lifetime_ref.span,
1529                       "lifetime reference not renumbered, \
1530                        probably a bug in syntax::fold");
1531         }
1532
1533         debug!("{} resolved to {:?} span={:?}",
1534                self.hir_map.node_to_string(lifetime_ref.id),
1535                def,
1536                self.sess.codemap().span_to_string(lifetime_ref.span));
1537         self.map.defs.insert(lifetime_ref.id, def);
1538     }
1539 }
1540
1541 ///////////////////////////////////////////////////////////////////////////
1542
1543 /// Detects late-bound lifetimes and inserts them into
1544 /// `map.late_bound`.
1545 ///
1546 /// A region declared on a fn is **late-bound** if:
1547 /// - it is constrained by an argument type;
1548 /// - it does not appear in a where-clause.
1549 ///
1550 /// "Constrained" basically means that it appears in any type but
1551 /// not amongst the inputs to a projection.  In other words, `<&'a
1552 /// T as Trait<''b>>::Foo` does not constrain `'a` or `'b`.
1553 fn insert_late_bound_lifetimes(map: &mut NamedRegionMap,
1554                                fn_def_id: DefId,
1555                                decl: &hir::FnDecl,
1556                                generics: &hir::Generics) {
1557     debug!("insert_late_bound_lifetimes(decl={:?}, generics={:?})", decl, generics);
1558
1559     let mut constrained_by_input = ConstrainedCollector { regions: FxHashSet() };
1560     for arg_ty in &decl.inputs {
1561         constrained_by_input.visit_ty(arg_ty);
1562     }
1563
1564     let mut appears_in_output = AllCollector {
1565         regions: FxHashSet(),
1566         impl_trait: false
1567     };
1568     intravisit::walk_fn_ret_ty(&mut appears_in_output, &decl.output);
1569
1570     debug!("insert_late_bound_lifetimes: constrained_by_input={:?}",
1571            constrained_by_input.regions);
1572
1573     // Walk the lifetimes that appear in where clauses.
1574     //
1575     // Subtle point: because we disallow nested bindings, we can just
1576     // ignore binders here and scrape up all names we see.
1577     let mut appears_in_where_clause = AllCollector {
1578         regions: FxHashSet(),
1579         impl_trait: false
1580     };
1581     for ty_param in generics.ty_params.iter() {
1582         walk_list!(&mut appears_in_where_clause,
1583                    visit_ty_param_bound,
1584                    &ty_param.bounds);
1585     }
1586     walk_list!(&mut appears_in_where_clause,
1587                visit_where_predicate,
1588                &generics.where_clause.predicates);
1589     for lifetime_def in &generics.lifetimes {
1590         if !lifetime_def.bounds.is_empty() {
1591             // `'a: 'b` means both `'a` and `'b` are referenced
1592             appears_in_where_clause.visit_lifetime_def(lifetime_def);
1593         }
1594     }
1595
1596     debug!("insert_late_bound_lifetimes: appears_in_where_clause={:?}",
1597            appears_in_where_clause.regions);
1598
1599     // Late bound regions are those that:
1600     // - appear in the inputs
1601     // - do not appear in the where-clauses
1602     // - are not implicitly captured by `impl Trait`
1603     for lifetime in &generics.lifetimes {
1604         let name = lifetime.lifetime.name;
1605
1606         // appears in the where clauses? early-bound.
1607         if appears_in_where_clause.regions.contains(&name) { continue; }
1608
1609         // any `impl Trait` in the return type? early-bound.
1610         if appears_in_output.impl_trait { continue; }
1611
1612         // does not appear in the inputs, but appears in the return
1613         // type? eventually this will be early-bound, but for now we
1614         // just mark it so we can issue warnings.
1615         let constrained_by_input = constrained_by_input.regions.contains(&name);
1616         let appears_in_output = appears_in_output.regions.contains(&name);
1617         if !constrained_by_input && appears_in_output {
1618             debug!("inserting issue_32330 entry for {:?}, {:?} on {:?}",
1619                    lifetime.lifetime.id,
1620                    name,
1621                    fn_def_id);
1622             map.issue_32330.insert(
1623                 lifetime.lifetime.id,
1624                 ty::Issue32330 {
1625                     fn_def_id: fn_def_id,
1626                     region_name: name,
1627                 });
1628             continue;
1629         }
1630
1631         debug!("insert_late_bound_lifetimes: \
1632                 lifetime {:?} with id {:?} is late-bound",
1633                lifetime.lifetime.name, lifetime.lifetime.id);
1634
1635         let inserted = map.late_bound.insert(lifetime.lifetime.id);
1636         assert!(inserted, "visited lifetime {:?} twice", lifetime.lifetime.id);
1637     }
1638
1639     return;
1640
1641     struct ConstrainedCollector {
1642         regions: FxHashSet<ast::Name>,
1643     }
1644
1645     impl<'v> Visitor<'v> for ConstrainedCollector {
1646         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
1647             NestedVisitorMap::None
1648         }
1649
1650         fn visit_ty(&mut self, ty: &'v hir::Ty) {
1651             match ty.node {
1652                 hir::TyPath(hir::QPath::Resolved(Some(_), _)) |
1653                 hir::TyPath(hir::QPath::TypeRelative(..)) => {
1654                     // ignore lifetimes appearing in associated type
1655                     // projections, as they are not *constrained*
1656                     // (defined above)
1657                 }
1658
1659                 hir::TyPath(hir::QPath::Resolved(None, ref path)) => {
1660                     // consider only the lifetimes on the final
1661                     // segment; I am not sure it's even currently
1662                     // valid to have them elsewhere, but even if it
1663                     // is, those would be potentially inputs to
1664                     // projections
1665                     if let Some(last_segment) = path.segments.last() {
1666                         self.visit_path_segment(path.span, last_segment);
1667                     }
1668                 }
1669
1670                 _ => {
1671                     intravisit::walk_ty(self, ty);
1672                 }
1673             }
1674         }
1675
1676         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
1677             self.regions.insert(lifetime_ref.name);
1678         }
1679     }
1680
1681     struct AllCollector {
1682         regions: FxHashSet<ast::Name>,
1683         impl_trait: bool
1684     }
1685
1686     impl<'v> Visitor<'v> for AllCollector {
1687         fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
1688             NestedVisitorMap::None
1689         }
1690
1691         fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
1692             self.regions.insert(lifetime_ref.name);
1693         }
1694
1695         fn visit_ty(&mut self, ty: &hir::Ty) {
1696             if let hir::TyImplTrait(_) = ty.node {
1697                 self.impl_trait = true;
1698             }
1699             intravisit::walk_ty(self, ty);
1700         }
1701     }
1702 }