rust/src/librustc/middle/resolve_lifetime.rs

// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! Name resolution for lifetimes.
//!
//! Name resolution for lifetimes follows MUCH simpler rules than the
//! full resolve. For example, lifetime names are never exported or
//! used between functions, and they operate in a purely top-down
//! way. Therefore we break lifetime name resolution into a separate pass.

use hir::map::Map;
use hir::def::Def;
use hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
use hir::ItemLocalId;
use ty::{self, TyCtxt};

use std::cell::Cell;
use std::mem::replace;
use std::rc::Rc;
use syntax::ast;
use syntax::attr;
use syntax::ptr::P;
use syntax_pos::Span;
use errors::DiagnosticBuilder;
use util::nodemap::{DefIdMap, FxHashMap, FxHashSet, NodeMap, NodeSet};
use std::slice;
use rustc::lint;

use hir::{self, GenericParamsExt};
use hir::intravisit::{self, NestedVisitorMap, Visitor};

/// The origin of a named lifetime definition.
///
/// This is used to prevent the usage of in-band lifetimes in `Fn`/`fn` syntax.
#[derive(Copy, Clone, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
pub enum LifetimeDefOrigin {
    // Explicit binders like `fn foo<'a>(x: &'a u8)`
    Explicit,
    // In-band declarations like `fn foo(x: &'a u8)`
    InBand,
}

impl LifetimeDefOrigin {
    fn from_is_in_band(is_in_band: bool) -> Self {
        if is_in_band {
            LifetimeDefOrigin::InBand
        } else {
            LifetimeDefOrigin::Explicit
        }
    }
}

// This counts the no of times a lifetime is used
#[derive(Clone, Copy, Debug)]
pub enum LifetimeUseSet<'tcx> {
    One(&'tcx hir::Lifetime),
    Many,
}

#[derive(Clone, Copy, PartialEq, Eq, Hash, RustcEncodable, RustcDecodable, Debug)]
pub enum Region {
    Static,
    EarlyBound(
        /* index */ u32,
        /* lifetime decl */ DefId,
        LifetimeDefOrigin,
    ),
    LateBound(
        ty::DebruijnIndex,
        /* lifetime decl */ DefId,
        LifetimeDefOrigin,
    ),
    LateBoundAnon(ty::DebruijnIndex, /* anon index */ u32),
    Free(DefId, /* lifetime decl */ DefId),
}

impl Region {
    fn early(
        hir_map: &Map,
        index: &mut u32,
        def: &hir::LifetimeDef,
    ) -> (hir::LifetimeName, Region) {
        let i = *index;
        *index += 1;
        let def_id = hir_map.local_def_id(def.lifetime.id);
        let origin = LifetimeDefOrigin::from_is_in_band(def.in_band);
        debug!("Region::early: index={} def_id={:?}", i, def_id);
        (def.lifetime.name, Region::EarlyBound(i, def_id, origin))
    }

    fn late(hir_map: &Map, def: &hir::LifetimeDef) -> (hir::LifetimeName, Region) {
        let depth = ty::DebruijnIndex::new(1);
        let def_id = hir_map.local_def_id(def.lifetime.id);
        let origin = LifetimeDefOrigin::from_is_in_band(def.in_band);
        (def.lifetime.name, Region::LateBound(depth, def_id, origin))
    }

    fn late_anon(index: &Cell<u32>) -> Region {
        let i = index.get();
        index.set(i + 1);
        let depth = ty::DebruijnIndex::new(1);
        Region::LateBoundAnon(depth, i)
    }

    fn id(&self) -> Option<DefId> {
        match *self {
            Region::Static | Region::LateBoundAnon(..) => None,

            Region::EarlyBound(_, id, _) | Region::LateBound(_, id, _) | Region::Free(_, id) => {
                Some(id)
            }
        }
    }

    fn shifted(self, amount: u32) -> Region {
        match self {
            Region::LateBound(depth, id, origin) => {
                Region::LateBound(depth.shifted(amount), id, origin)
            }
            Region::LateBoundAnon(depth, index) => {
                Region::LateBoundAnon(depth.shifted(amount), index)
            }
            _ => self,
        }
    }

    fn from_depth(self, depth: u32) -> Region {
        match self {
            Region::LateBound(debruijn, id, origin) => Region::LateBound(
                ty::DebruijnIndex {
                    depth: debruijn.depth - (depth - 1),
                },
                id,
                origin,
            ),
            Region::LateBoundAnon(debruijn, index) => Region::LateBoundAnon(
                ty::DebruijnIndex {
                    depth: debruijn.depth - (depth - 1),
                },
                index,
            ),
            _ => self,
        }
    }

    fn subst(self, params: &[hir::Lifetime], map: &NamedRegionMap) -> Option<Region> {
        if let Region::EarlyBound(index, _, _) = self {
            params
                .get(index as usize)
                .and_then(|lifetime| map.defs.get(&lifetime.id).cloned())
        } else {
            Some(self)
        }
    }
}

/// A set containing, at most, one known element.
/// If two distinct values are inserted into a set, then it
/// becomes `Many`, which can be used to detect ambiguities.
#[derive(Copy, Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Debug)]
pub enum Set1<T> {
    Empty,
    One(T),
    Many,
}

impl<T: PartialEq> Set1<T> {
    pub fn insert(&mut self, value: T) {
        if let Set1::Empty = *self {
            *self = Set1::One(value);
            return;
        }
        if let Set1::One(ref old) = *self {
            if *old == value {
                return;
            }
        }
        *self = Set1::Many;
    }
}

pub type ObjectLifetimeDefault = Set1<Region>;

/// Maps the id of each lifetime reference to the lifetime decl
/// that it corresponds to.
///
/// FIXME. This struct gets converted to a `ResolveLifetimes` for
/// actual use. It has the same data, but indexed by `DefIndex`.  This
/// is silly.
#[derive(Default)]
struct NamedRegionMap {
    // maps from every use of a named (not anonymous) lifetime to a
    // `Region` describing how that region is bound
    pub defs: NodeMap<Region>,

    // the set of lifetime def ids that are late-bound; a region can
    // be late-bound if (a) it does NOT appear in a where-clause and
    // (b) it DOES appear in the arguments.
    pub late_bound: NodeSet,

    // For each type and trait definition, maps type parameters
    // to the trait object lifetime defaults computed from them.
    pub object_lifetime_defaults: NodeMap<Vec<ObjectLifetimeDefault>>,
}

/// See `NamedRegionMap`.
pub struct ResolveLifetimes {
    defs: FxHashMap<LocalDefId, Rc<FxHashMap<ItemLocalId, Region>>>,
    late_bound: FxHashMap<LocalDefId, Rc<FxHashSet<ItemLocalId>>>,
    object_lifetime_defaults:
        FxHashMap<LocalDefId, Rc<FxHashMap<ItemLocalId, Rc<Vec<ObjectLifetimeDefault>>>>>,
}

impl_stable_hash_for!(struct ::middle::resolve_lifetime::ResolveLifetimes {
    defs,
    late_bound,
    object_lifetime_defaults
});

struct LifetimeContext<'a, 'tcx: 'a> {
    tcx: TyCtxt<'a, 'tcx, 'tcx>,
    map: &'a mut NamedRegionMap,
    scope: ScopeRef<'a>,
    // Deep breath. Our representation for poly trait refs contains a single
    // binder and thus we only allow a single level of quantification. However,
    // the syntax of Rust permits quantification in two places, e.g., `T: for <'a> Foo<'a>`
    // and `for <'a, 'b> &'b T: Foo<'a>`. In order to get the de Bruijn indices
    // correct when representing these constraints, we should only introduce one
    // scope. However, we want to support both locations for the quantifier and
    // during lifetime resolution we want precise information (so we can't
    // desugar in an earlier phase).

    // SO, if we encounter a quantifier at the outer scope, we set
    // trait_ref_hack to true (and introduce a scope), and then if we encounter
    // a quantifier at the inner scope, we error. If trait_ref_hack is false,
    // then we introduce the scope at the inner quantifier.

    // I'm sorry.
    trait_ref_hack: bool,

    // Used to disallow the use of in-band lifetimes in `fn` or `Fn` syntax.
    is_in_fn_syntax: bool,

    // List of labels in the function/method currently under analysis.
    labels_in_fn: Vec<(ast::Name, Span)>,

    // Cache for cross-crate per-definition object lifetime defaults.
    xcrate_object_lifetime_defaults: DefIdMap<Vec<ObjectLifetimeDefault>>,

    lifetime_uses: DefIdMap<LifetimeUseSet<'tcx>>,
}

#[derive(Debug)]
enum Scope<'a> {
    /// Declares lifetimes, and each can be early-bound or late-bound.
    /// The `DebruijnIndex` of late-bound lifetimes starts at `1` and
    /// it should be shifted by the number of `Binder`s in between the
    /// declaration `Binder` and the location it's referenced from.
    Binder {
        lifetimes: FxHashMap<hir::LifetimeName, Region>,

        /// if we extend this scope with another scope, what is the next index
        /// we should use for an early-bound region?
        next_early_index: u32,

        s: ScopeRef<'a>,
    },

    /// Lifetimes introduced by a fn are scoped to the call-site for that fn,
    /// if this is a fn body, otherwise the original definitions are used.
    /// Unspecified lifetimes are inferred, unless an elision scope is nested,
    /// e.g. `(&T, fn(&T) -> &T);` becomes `(&'_ T, for<'a> fn(&'a T) -> &'a T)`.
    Body {
        id: hir::BodyId,
        s: ScopeRef<'a>,
    },

    /// A scope which either determines unspecified lifetimes or errors
    /// on them (e.g. due to ambiguity). For more details, see `Elide`.
    Elision {
        elide: Elide,
        s: ScopeRef<'a>,
    },

    /// Use a specific lifetime (if `Some`) or leave it unset (to be
    /// inferred in a function body or potentially error outside one),
    /// for the default choice of lifetime in a trait object type.
    ObjectLifetimeDefault {
        lifetime: Option<Region>,
        s: ScopeRef<'a>,
    },

    Root,
}

#[derive(Clone, Debug)]
enum Elide {
    /// Use a fresh anonymous late-bound lifetime each time, by
    /// incrementing the counter to generate sequential indices.
    FreshLateAnon(Cell<u32>),
    /// Always use this one lifetime.
    Exact(Region),
    /// Less or more than one lifetime were found, error on unspecified.
    Error(Vec<ElisionFailureInfo>),
}

#[derive(Clone, Debug)]
struct ElisionFailureInfo {
    /// Where we can find the argument pattern.
    parent: Option<hir::BodyId>,
    /// The index of the argument in the original definition.
    index: usize,
    lifetime_count: usize,
    have_bound_regions: bool,
}

type ScopeRef<'a> = &'a Scope<'a>;

const ROOT_SCOPE: ScopeRef<'static> = &Scope::Root;

pub fn provide(providers: &mut ty::maps::Providers) {
    *providers = ty::maps::Providers {
        resolve_lifetimes,

        named_region_map: |tcx, id| {
            let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
            tcx.resolve_lifetimes(LOCAL_CRATE).defs.get(&id).cloned()
        },

        is_late_bound_map: |tcx, id| {
            let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
            tcx.resolve_lifetimes(LOCAL_CRATE)
                .late_bound
                .get(&id)
                .cloned()
        },

        object_lifetime_defaults_map: |tcx, id| {
            let id = LocalDefId::from_def_id(DefId::local(id)); // (*)
            tcx.resolve_lifetimes(LOCAL_CRATE)
                .object_lifetime_defaults
                .get(&id)
                .cloned()
        },

        ..*providers
    };

    // (*) FIXME the query should be defined to take a LocalDefId
}

/// Computes the `ResolveLifetimes` map that contains data for the
/// entire crate. You should not read the result of this query
/// directly, but rather use `named_region_map`, `is_late_bound_map`,
/// etc.
fn resolve_lifetimes<'tcx>(
    tcx: TyCtxt<'_, 'tcx, 'tcx>,
    for_krate: CrateNum,
) -> Rc<ResolveLifetimes> {
    assert_eq!(for_krate, LOCAL_CRATE);

    let named_region_map = krate(tcx);

    let mut defs = FxHashMap();
    for (k, v) in named_region_map.defs {
        let hir_id = tcx.hir.node_to_hir_id(k);
        let map = defs.entry(hir_id.owner_local_def_id())
            .or_insert_with(|| Rc::new(FxHashMap()));
        Rc::get_mut(map).unwrap().insert(hir_id.local_id, v);
    }
    let mut late_bound = FxHashMap();
    for k in named_region_map.late_bound {
        let hir_id = tcx.hir.node_to_hir_id(k);
        let map = late_bound
            .entry(hir_id.owner_local_def_id())
            .or_insert_with(|| Rc::new(FxHashSet()));
        Rc::get_mut(map).unwrap().insert(hir_id.local_id);
    }
    let mut object_lifetime_defaults = FxHashMap();
    for (k, v) in named_region_map.object_lifetime_defaults {
        let hir_id = tcx.hir.node_to_hir_id(k);
        let map = object_lifetime_defaults
            .entry(hir_id.owner_local_def_id())
            .or_insert_with(|| Rc::new(FxHashMap()));
        Rc::get_mut(map)
            .unwrap()
            .insert(hir_id.local_id, Rc::new(v));
    }

    Rc::new(ResolveLifetimes {
        defs,
        late_bound,
        object_lifetime_defaults,
    })
}

fn krate<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>) -> NamedRegionMap {
    let krate = tcx.hir.krate();
    let mut map = NamedRegionMap {
        defs: NodeMap(),
        late_bound: NodeSet(),
        object_lifetime_defaults: compute_object_lifetime_defaults(tcx),
    };
    {
        let mut visitor = LifetimeContext {
            tcx,
            map: &mut map,
            scope: ROOT_SCOPE,
            trait_ref_hack: false,
            is_in_fn_syntax: false,
            labels_in_fn: vec![],
            xcrate_object_lifetime_defaults: DefIdMap(),
            lifetime_uses: DefIdMap(),
        };
        for (_, item) in &krate.items {
            visitor.visit_item(item);
        }
    }
    map
}

impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
        NestedVisitorMap::All(&self.tcx.hir)
    }

    // We want to nest trait/impl items in their parent, but nothing else.
    fn visit_nested_item(&mut self, _: hir::ItemId) {}

    fn visit_nested_body(&mut self, body: hir::BodyId) {
        // Each body has their own set of labels, save labels.
        let saved = replace(&mut self.labels_in_fn, vec![]);
        let body = self.tcx.hir.body(body);
        extract_labels(self, body);
        self.with(
            Scope::Body {
                id: body.id(),
                s: self.scope,
            },
            |_, this| {
                this.visit_body(body);
            },
        );
        replace(&mut self.labels_in_fn, saved);
    }

    fn visit_item(&mut self, item: &'tcx hir::Item) {
        match item.node {
            hir::ItemFn(ref decl, _, _, _, ref generics, _) => {
                self.visit_early_late(None,
                                      decl,
                                      generics,
                                      |this| {
                                          intravisit::walk_item(this, item);
                });
            }
            hir::ItemExternCrate(_)
            | hir::ItemUse(..)
            | hir::ItemMod(..)
            | hir::ItemAutoImpl(..)
            | hir::ItemForeignMod(..)
            | hir::ItemGlobalAsm(..) => {
                // These sorts of items have no lifetime parameters at all.
                intravisit::walk_item(self, item);
            }
            hir::ItemStatic(..) | hir::ItemConst(..) => {
                // No lifetime parameters, but implied 'static.
                let scope = Scope::Elision {
                    elide: Elide::Exact(Region::Static),
                    s: ROOT_SCOPE,
                };
                self.with(scope, |_, this| intravisit::walk_item(this, item));
            }
            hir::ItemTy(_, ref generics)
            | hir::ItemEnum(_, ref generics)
            | hir::ItemStruct(_, ref generics)
            | hir::ItemUnion(_, ref generics)
            | hir::ItemTrait(_, _, ref generics, ..)
            | hir::ItemTraitAlias(ref generics, ..)
            | hir::ItemImpl(_, _, _, ref generics, ..) => {
                // These kinds of items have only early bound lifetime parameters.
                let mut index = if let hir::ItemTrait(..) = item.node {
                    1 // Self comes before lifetimes
                } else {
                    0
                };
                let lifetimes = generics.lifetimes()
                    .map(|def| Region::early(&self.tcx.hir, &mut index, def))
                    .collect();
                let next_early_index = index + generics.ty_params().count() as u32;
                let scope = Scope::Binder {
                    lifetimes,
                    next_early_index,
                    s: ROOT_SCOPE,
                };
                self.with(scope, |old_scope, this| {
                    this.check_lifetime_params(old_scope, &generics.params);
                    intravisit::walk_item(this, item);
                });
            }
        }
    }

    fn visit_foreign_item(&mut self, item: &'tcx hir::ForeignItem) {
        match item.node {
            hir::ForeignItemFn(ref decl, _, ref generics) => {
                self.visit_early_late(None,
                                      decl,
                                      generics,
                                      |this| {
                                          intravisit::walk_foreign_item(this, item);
                                      })
            }
            hir::ForeignItemStatic(..) => {
                intravisit::walk_foreign_item(self, item);
            }
            hir::ForeignItemType => {
                intravisit::walk_foreign_item(self, item);
            }
        }
    }

    fn visit_ty(&mut self, ty: &'tcx hir::Ty) {
        debug!("visit_ty: ty={:?}", ty);
        match ty.node {
            hir::TyBareFn(ref c) => {
                let next_early_index = self.next_early_index();
                let was_in_fn_syntax = self.is_in_fn_syntax;
                self.is_in_fn_syntax = true;
                let scope = Scope::Binder {
                    lifetimes: c.generic_params
                        .lifetimes()
                        .map(|def| Region::late(&self.tcx.hir, def))
                        .collect(),
                    s: self.scope,
                    next_early_index,
                };
                self.with(scope, |old_scope, this| {
                    // a bare fn has no bounds, so everything
                    // contained within is scoped within its binder.
                    this.check_lifetime_params(old_scope, &c.generic_params);
                    intravisit::walk_ty(this, ty);
                });
                self.is_in_fn_syntax = was_in_fn_syntax;
            }
            hir::TyTraitObject(ref bounds, ref lifetime) => {
                for bound in bounds {
                    self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
                }
                if lifetime.is_elided() {
                    self.resolve_object_lifetime_default(lifetime)
                } else {
                    self.visit_lifetime(lifetime);
                }
            }
            hir::TyRptr(ref lifetime_ref, ref mt) => {
                self.visit_lifetime(lifetime_ref);
                let scope = Scope::ObjectLifetimeDefault {
                    lifetime: self.map.defs.get(&lifetime_ref.id).cloned(),
                    s: self.scope,
                };
                self.with(scope, |_, this| this.visit_ty(&mt.ty));
            }
            hir::TyImplTraitExistential(ref exist_ty, ref lifetimes) => {
                // Resolve the lifetimes that are applied to the existential type.
                // These are resolved in the current scope.
                // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
                // `fn foo<'a>() -> MyAnonTy<'a> { ... }`
                //          ^                 ^this gets resolved in the current scope
                for lifetime in lifetimes {
                    self.visit_lifetime(lifetime);

                    // Check for predicates like `impl for<'a> SomeTrait<impl OtherTrait<'a>>`
                    // and ban them. Type variables instantiated inside binders aren't
                    // well-supported at the moment, so this doesn't work.
                    // In the future, this should be fixed and this error should be removed.
                    let def = self.map.defs.get(&lifetime.id);
                    if let Some(&Region::LateBound(_, def_id, _)) = def {
                        if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {
                            // Ensure that the parent of the def is an item, not HRTB
                            let parent_id = self.tcx.hir.get_parent_node(node_id);
                            let parent_impl_id = hir::ImplItemId { node_id: parent_id };
                            let parent_trait_id = hir::TraitItemId { node_id: parent_id };
                            let krate = self.tcx.hir.forest.krate();
                            if !(krate.items.contains_key(&parent_id)
                                || krate.impl_items.contains_key(&parent_impl_id)
                                || krate.trait_items.contains_key(&parent_trait_id))
                            {
                                span_err!(
                                    self.tcx.sess,
                                    lifetime.span,
                                    E0657,
                                    "`impl Trait` can only capture lifetimes \
                                     bound at the fn or impl level"
                                );
                            }
                        }
                    }
                }

                // Resolve the lifetimes in the bounds to the lifetime defs in the generics.
                // `fn foo<'a>() -> impl MyTrait<'a> { ... }` desugars to
                // `abstract type MyAnonTy<'b>: MyTrait<'b>;`
                //                          ^            ^ this gets resolved in the scope of
                //                                         the exist_ty generics
                let hir::ExistTy {
                    ref generics,
                    ref bounds,
                } = *exist_ty;
                let mut index = self.next_early_index();
                debug!("visit_ty: index = {}", index);

                let mut elision = None;
                let mut lifetimes = FxHashMap();
                for lt_def in generics.lifetimes() {
                    let (lt_name, region) = Region::early(&self.tcx.hir, &mut index, &lt_def);
                    if let hir::LifetimeName::Underscore = lt_name {
                        // Pick the elided lifetime "definition" if one exists and use it to make an
                        // elision scope.
                        elision = Some(region);
                    } else {
                        lifetimes.insert(lt_name, region);
                    }
                }

                let next_early_index = index + generics.ty_params().count() as u32;

                if let Some(elision_region) = elision {
                    let scope = Scope::Elision {
                        elide: Elide::Exact(elision_region),
                        s: self.scope
                    };
                    self.with(scope, |_old_scope, this| {
                        let scope = Scope::Binder { lifetimes, next_early_index, s: this.scope };
                        this.with(scope, |_old_scope, this| {
                            this.visit_generics(generics);
                            for bound in bounds {
                                this.visit_ty_param_bound(bound);
                            }
                        });
                    });
                } else {
                    let scope = Scope::Binder { lifetimes, next_early_index, s: self.scope };
                    self.with(scope, |_old_scope, this| {
                        this.visit_generics(generics);
                        for bound in bounds {
                            this.visit_ty_param_bound(bound);
                        }
                    });
                }
            }
            _ => intravisit::walk_ty(self, ty),
        }
    }

    fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
        use self::hir::TraitItemKind::*;
        match trait_item.node {
            Method(ref sig, _) => {
                let tcx = self.tcx;
                self.visit_early_late(
                    Some(tcx.hir.get_parent(trait_item.id)),
                    &sig.decl,
                    &trait_item.generics,
                    |this| intravisit::walk_trait_item(this, trait_item),
                );
            },
            Type(ref bounds, ref ty) => {
                let generics = &trait_item.generics;
                let mut index = self.next_early_index();
                debug!("visit_ty: index = {}", index);
                let lifetimes = generics.lifetimes()
                    .map(|lt_def| Region::early(&self.tcx.hir, &mut index, lt_def))
                    .collect();

                let next_early_index = index + generics.ty_params().count() as u32;
                let scope = Scope::Binder { lifetimes, next_early_index, s: self.scope };
                self.with(scope, |_old_scope, this| {
                    this.visit_generics(generics);
                    for bound in bounds {
                        this.visit_ty_param_bound(bound);
                    }
                    if let Some(ty) = ty {
                        this.visit_ty(ty);
                    }
                });
            },
            Const(_, _) => {
                // Only methods and types support generics.
                assert!(trait_item.generics.params.is_empty());
                intravisit::walk_trait_item(self, trait_item);
            },
        }
    }

    fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
        use self::hir::ImplItemKind::*;
        match impl_item.node {
            Method(ref sig, _) => {
                let tcx = self.tcx;
                self.visit_early_late(
                    Some(tcx.hir.get_parent(impl_item.id)),
                    &sig.decl,
                    &impl_item.generics,
                    |this| intravisit::walk_impl_item(this, impl_item),
                )
            },
            Type(ref ty) => {
                let generics = &impl_item.generics;
                let mut index = self.next_early_index();
                debug!("visit_ty: index = {}", index);
                let lifetimes = generics.lifetimes()
                    .map(|lt_def| Region::early(&self.tcx.hir, &mut index, lt_def))
                    .collect();

                let next_early_index = index + generics.ty_params().count() as u32;
                let scope = Scope::Binder { lifetimes, next_early_index, s: self.scope };
                self.with(scope, |_old_scope, this| {
                    this.visit_generics(generics);
                    this.visit_ty(ty);
                });
            },
            Const(_, _) => {
                // Only methods and types support generics.
                assert!(impl_item.generics.params.is_empty());
                intravisit::walk_impl_item(self, impl_item);
            },
        }
    }

    fn visit_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
        if lifetime_ref.is_elided() {
            self.resolve_elided_lifetimes(slice::from_ref(lifetime_ref));
            return;
        }
        if lifetime_ref.is_static() {
            self.insert_lifetime(lifetime_ref, Region::Static);
            return;
        }
        self.resolve_lifetime_ref(lifetime_ref);
    }

    fn visit_path(&mut self, path: &'tcx hir::Path, _: ast::NodeId) {
        for (i, segment) in path.segments.iter().enumerate() {
            let depth = path.segments.len() - i - 1;
            if let Some(ref parameters) = segment.parameters {
                self.visit_segment_parameters(path.def, depth, parameters);
            }
        }
    }

    fn visit_fn_decl(&mut self, fd: &'tcx hir::FnDecl) {
        let output = match fd.output {
            hir::DefaultReturn(_) => None,
            hir::Return(ref ty) => Some(ty),
        };
        self.visit_fn_like_elision(&fd.inputs, output);
    }

    fn visit_generics(&mut self, generics: &'tcx hir::Generics) {
        check_mixed_explicit_and_in_band_defs(
            self.tcx,
            &generics.lifetimes().cloned().collect::<Vec<_>>()
        );
        for ty_param in generics.ty_params() {
            walk_list!(self, visit_ty_param_bound, &ty_param.bounds);
            if let Some(ref ty) = ty_param.default {
                self.visit_ty(&ty);
            }
        }
        for predicate in &generics.where_clause.predicates {
            match predicate {
                &hir::WherePredicate::BoundPredicate(hir::WhereBoundPredicate {
                    ref bounded_ty,
                    ref bounds,
                    ref bound_generic_params,
                    ..
                }) => {
                    if bound_generic_params.iter().any(|p| p.is_lifetime_param()) {
                        self.trait_ref_hack = true;
                        let next_early_index = self.next_early_index();
                        let scope = Scope::Binder {
                            lifetimes: bound_generic_params.lifetimes()
                                .map(|def| Region::late(&self.tcx.hir, def))
                                .collect(),
                            s: self.scope,
                            next_early_index,
                        };
                        let result = self.with(scope, |old_scope, this| {
                            this.check_lifetime_params(old_scope, &bound_generic_params);
                            this.visit_ty(&bounded_ty);
                            walk_list!(this, visit_ty_param_bound, bounds);
                        });
                        self.trait_ref_hack = false;
                        result
                    } else {
                        self.visit_ty(&bounded_ty);
                        walk_list!(self, visit_ty_param_bound, bounds);
                    }
                }
                &hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
                    ref lifetime,
                    ref bounds,
                    ..
                }) => {
                    self.visit_lifetime(lifetime);
                    for bound in bounds {
                        self.visit_lifetime(bound);
                    }
                }
                &hir::WherePredicate::EqPredicate(hir::WhereEqPredicate {
                    ref lhs_ty,
                    ref rhs_ty,
                    ..
                }) => {
                    self.visit_ty(lhs_ty);
                    self.visit_ty(rhs_ty);
                }
            }
        }
    }

    fn visit_poly_trait_ref(
        &mut self,
        trait_ref: &'tcx hir::PolyTraitRef,
        _modifier: hir::TraitBoundModifier,
    ) {
        debug!("visit_poly_trait_ref trait_ref={:?}", trait_ref);

        if !self.trait_ref_hack ||
            trait_ref.bound_generic_params.iter().any(|p| p.is_lifetime_param())
        {
            if self.trait_ref_hack {
                span_err!(
                    self.tcx.sess,
                    trait_ref.span,
                    E0316,
                    "nested quantification of lifetimes"
                );
            }
            let next_early_index = self.next_early_index();
            let scope = Scope::Binder {
                lifetimes: trait_ref.bound_generic_params
                    .lifetimes()
                    .map(|def| Region::late(&self.tcx.hir, def))
                    .collect(),
                s: self.scope,
                next_early_index,
            };
            self.with(scope, |old_scope, this| {
                this.check_lifetime_params(old_scope, &trait_ref.bound_generic_params);
                walk_list!(this, visit_generic_param, &trait_ref.bound_generic_params);
                this.visit_trait_ref(&trait_ref.trait_ref)
            })
        } else {
            self.visit_trait_ref(&trait_ref.trait_ref)
        }
    }
}

#[derive(Copy, Clone, PartialEq)]
enum ShadowKind {
    Label,
    Lifetime,
}
struct Original {
    kind: ShadowKind,
    span: Span,
}
struct Shadower {
    kind: ShadowKind,
    span: Span,
}

fn original_label(span: Span) -> Original {
    Original {
        kind: ShadowKind::Label,
        span: span,
    }
}
fn shadower_label(span: Span) -> Shadower {
    Shadower {
        kind: ShadowKind::Label,
        span: span,
    }
}
fn original_lifetime(span: Span) -> Original {
    Original {
        kind: ShadowKind::Lifetime,
        span: span,
    }
}
fn shadower_lifetime(l: &hir::Lifetime) -> Shadower {
    Shadower {
        kind: ShadowKind::Lifetime,
        span: l.span,
    }
}

impl ShadowKind {
    fn desc(&self) -> &'static str {
        match *self {
            ShadowKind::Label => "label",
            ShadowKind::Lifetime => "lifetime",
        }
    }
}

fn check_mixed_explicit_and_in_band_defs(
    tcx: TyCtxt<'_, '_, '_>,
    lifetime_defs: &[hir::LifetimeDef],
) {
    let oob_def = lifetime_defs.iter().find(|lt| !lt.in_band);
    let in_band_def = lifetime_defs.iter().find(|lt| lt.in_band);

    if let (Some(oob_def), Some(in_band_def)) = (oob_def, in_band_def) {
        struct_span_err!(
            tcx.sess,
            in_band_def.lifetime.span,
            E0688,
            "cannot mix in-band and explicit lifetime definitions"
        ).span_label(
            in_band_def.lifetime.span,
            "in-band lifetime definition here",
        )
            .span_label(oob_def.lifetime.span, "explicit lifetime definition here")
            .emit();
    }
}

fn signal_shadowing_problem(
    tcx: TyCtxt<'_, '_, '_>,
    name: ast::Name,
    orig: Original,
    shadower: Shadower,
) {
    let mut err = if let (ShadowKind::Lifetime, ShadowKind::Lifetime) = (orig.kind, shadower.kind) {
        // lifetime/lifetime shadowing is an error
        struct_span_err!(
            tcx.sess,
            shadower.span,
            E0496,
            "{} name `{}` shadows a \
             {} name that is already in scope",
            shadower.kind.desc(),
            name,
            orig.kind.desc()
        )
    } else {
        // shadowing involving a label is only a warning, due to issues with
        // labels and lifetimes not being macro-hygienic.
        tcx.sess.struct_span_warn(
            shadower.span,
            &format!(
                "{} name `{}` shadows a \
                 {} name that is already in scope",
                shadower.kind.desc(),
                name,
                orig.kind.desc()
            ),
        )
    };
    err.span_label(orig.span, "first declared here");
    err.span_label(shadower.span, format!("lifetime {} already in scope", name));
    err.emit();
}

// Adds all labels in `b` to `ctxt.labels_in_fn`, signalling a warning
// if one of the label shadows a lifetime or another label.
fn extract_labels(ctxt: &mut LifetimeContext<'_, '_>, body: &hir::Body) {
    struct GatherLabels<'a, 'tcx: 'a> {
        tcx: TyCtxt<'a, 'tcx, 'tcx>,
        scope: ScopeRef<'a>,
        labels_in_fn: &'a mut Vec<(ast::Name, Span)>,
    }

    let mut gather = GatherLabels {
        tcx: ctxt.tcx,
        scope: ctxt.scope,
        labels_in_fn: &mut ctxt.labels_in_fn,
    };
    gather.visit_body(body);

    impl<'v, 'a, 'tcx> Visitor<'v> for GatherLabels<'a, 'tcx> {
        fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
            NestedVisitorMap::None
        }

        fn visit_expr(&mut self, ex: &hir::Expr) {
            if let Some((label, label_span)) = expression_label(ex) {
                for &(prior, prior_span) in &self.labels_in_fn[..] {
                    // FIXME (#24278): non-hygienic comparison
                    if label == prior {
                        signal_shadowing_problem(
                            self.tcx,
                            label,
                            original_label(prior_span),
                            shadower_label(label_span),
                        );
                    }
                }

                check_if_label_shadows_lifetime(self.tcx, self.scope, label, label_span);

                self.labels_in_fn.push((label, label_span));
            }
            intravisit::walk_expr(self, ex)
        }
    }

    fn expression_label(ex: &hir::Expr) -> Option<(ast::Name, Span)> {
        match ex.node {
            hir::ExprWhile(.., Some(label)) | hir::ExprLoop(_, Some(label), _) => {
                Some((label.node, label.span))
            }
            _ => None,
        }
    }

    fn check_if_label_shadows_lifetime(
        tcx: TyCtxt<'_, '_, '_>,
        mut scope: ScopeRef<'_>,
        label: ast::Name,
        label_span: Span,
    ) {
        loop {
            match *scope {
                Scope::Body { s, .. }
                | Scope::Elision { s, .. }
                | Scope::ObjectLifetimeDefault { s, .. } => {
                    scope = s;
                }

                Scope::Root => {
                    return;
                }

                Scope::Binder {
                    ref lifetimes,
                    s,
                    next_early_index: _,
                } => {
                    // FIXME (#24278): non-hygienic comparison
                    if let Some(def) = lifetimes.get(&hir::LifetimeName::Name(label)) {
                        let node_id = tcx.hir.as_local_node_id(def.id().unwrap()).unwrap();

                        signal_shadowing_problem(
                            tcx,
                            label,
                            original_lifetime(tcx.hir.span(node_id)),
                            shadower_label(label_span),
                        );
                        return;
                    }
                    scope = s;
                }
            }
        }
    }
}

fn compute_object_lifetime_defaults(
    tcx: TyCtxt<'_, '_, '_>,
) -> NodeMap<Vec<ObjectLifetimeDefault>> {
    let mut map = NodeMap();
    for item in tcx.hir.krate().items.values() {
        match item.node {
            hir::ItemStruct(_, ref generics)
            | hir::ItemUnion(_, ref generics)
            | hir::ItemEnum(_, ref generics)
            | hir::ItemTy(_, ref generics)
            | hir::ItemTrait(_, _, ref generics, ..) => {
                let result = object_lifetime_defaults_for_item(tcx, generics);

                // Debugging aid.
                if attr::contains_name(&item.attrs, "rustc_object_lifetime_default") {
                    let object_lifetime_default_reprs: String = result
                        .iter()
                        .map(|set| match *set {
                            Set1::Empty => "BaseDefault".to_string(),
                            Set1::One(Region::Static) => "'static".to_string(),
                            Set1::One(Region::EarlyBound(i, _, _)) => generics.lifetimes()
                                .nth(i as usize)
                                .unwrap()
                                .lifetime
                                .name
                                .name()
                                .to_string(),
                            Set1::One(_) => bug!(),
                            Set1::Many => "Ambiguous".to_string(),
                        })
                        .collect::<Vec<String>>()
                        .join(",");
                    tcx.sess.span_err(item.span, &object_lifetime_default_reprs);
                }

                map.insert(item.id, result);
            }
            _ => {}
        }
    }
    map
}

/// Scan the bounds and where-clauses on parameters to extract bounds
/// of the form `T:'a` so as to determine the `ObjectLifetimeDefault`
/// for each type parameter.
fn object_lifetime_defaults_for_item(
    tcx: TyCtxt<'_, '_, '_>,
    generics: &hir::Generics,
) -> Vec<ObjectLifetimeDefault> {
    fn add_bounds(set: &mut Set1<hir::LifetimeName>, bounds: &[hir::TyParamBound]) {
        for bound in bounds {
            if let hir::RegionTyParamBound(ref lifetime) = *bound {
                set.insert(lifetime.name);
            }
        }
    }

    generics.ty_params()
        .map(|param| {
            let mut set = Set1::Empty;

            add_bounds(&mut set, &param.bounds);

            let param_def_id = tcx.hir.local_def_id(param.id);
            for predicate in &generics.where_clause.predicates {
                // Look for `type: ...` where clauses.
                let data = match *predicate {
                    hir::WherePredicate::BoundPredicate(ref data) => data,
                    _ => continue,
                };

                // Ignore `for<'a> type: ...` as they can change what
                // lifetimes mean (although we could "just" handle it).
                if !data.bound_generic_params.is_empty() {
                    continue;
                }

                let def = match data.bounded_ty.node {
                    hir::TyPath(hir::QPath::Resolved(None, ref path)) => path.def,
                    _ => continue,
                };

                if def == Def::TyParam(param_def_id) {
                    add_bounds(&mut set, &data.bounds);
                }
            }

            match set {
                Set1::Empty => Set1::Empty,
                Set1::One(name) => {
                    if name == hir::LifetimeName::Static {
                        Set1::One(Region::Static)
                    } else {
                        generics
                            .lifetimes()
                            .enumerate()
                            .find(|&(_, def)| def.lifetime.name == name)
                            .map_or(Set1::Many, |(i, def)| {
                                let def_id = tcx.hir.local_def_id(def.lifetime.id);
                                let origin = LifetimeDefOrigin::from_is_in_band(def.in_band);
                                Set1::One(Region::EarlyBound(i as u32, def_id, origin))
                            })
                    }
                }
                Set1::Many => Set1::Many,
            }
        })
        .collect()
}

impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
    // FIXME(#37666) this works around a limitation in the region inferencer
    fn hack<F>(&mut self, f: F)
    where
        F: for<'b> FnOnce(&mut LifetimeContext<'b, 'tcx>),
    {
        f(self)
    }

    fn with<F>(&mut self, wrap_scope: Scope, f: F)
    where
        F: for<'b> FnOnce(ScopeRef, &mut LifetimeContext<'b, 'tcx>),
    {
        let LifetimeContext {
            tcx, ref mut map, ..
        } = *self;
        let labels_in_fn = replace(&mut self.labels_in_fn, vec![]);
        let xcrate_object_lifetime_defaults =
            replace(&mut self.xcrate_object_lifetime_defaults, DefIdMap());
        let mut this = LifetimeContext {
            tcx,
            map: *map,
            scope: &wrap_scope,
            trait_ref_hack: self.trait_ref_hack,
            is_in_fn_syntax: self.is_in_fn_syntax,
            labels_in_fn,
            xcrate_object_lifetime_defaults,
            lifetime_uses: DefIdMap(),
        };
        debug!("entering scope {:?}", this.scope);
        f(self.scope, &mut this);
        debug!("exiting scope {:?}", this.scope);
        self.labels_in_fn = this.labels_in_fn;
        self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;

        for (def_id, lifetimeuseset) in &this.lifetime_uses {
            match lifetimeuseset {
                &LifetimeUseSet::One(_) => {
                    let node_id = this.tcx.hir.as_local_node_id(*def_id).unwrap();
                    debug!("node id first={:?}", node_id);
                    if let hir::map::NodeLifetime(hir_lifetime) = this.tcx.hir.get(node_id) {
                        let span = hir_lifetime.span;
                        let id = hir_lifetime.id;
                        debug!("id ={:?} span = {:?} hir_lifetime = {:?}",
                            node_id,
                            span,
                            hir_lifetime);

                        this.tcx
                            .struct_span_lint_node(lint::builtin::SINGLE_USE_LIFETIME,
                                                   id,
                                                   span,
                                                   &format!("lifetime name `{}` only used once",
                                                   hir_lifetime.name.name()))
                            .emit();
                    }
                }
                _ => {
                    debug!("Not one use lifetime");
                }
            }
        }
    }

    /// Visits self by adding a scope and handling recursive walk over the contents with `walk`.
    ///
    /// Handles visiting fns and methods. These are a bit complicated because we must distinguish
    /// early- vs late-bound lifetime parameters. We do this by checking which lifetimes appear
    /// within type bounds; those are early bound lifetimes, and the rest are late bound.
    ///
    /// For example:
    ///
    ///    fn foo<'a,'b,'c,T:Trait<'b>>(...)
    ///
    /// Here `'a` and `'c` are late bound but `'b` is early bound. Note that early- and late-bound
    /// lifetimes may be interspersed together.
    ///
    /// If early bound lifetimes are present, we separate them into their own list (and likewise
    /// for late bound). They will be numbered sequentially, starting from the lowest index that is
    /// already in scope (for a fn item, that will be 0, but for a method it might not be). Late
    /// bound lifetimes are resolved by name and associated with a binder id (`binder_id`), so the
    /// ordering is not important there.
    fn visit_early_late<F>(
        &mut self,
        parent_id: Option<ast::NodeId>,
        decl: &'tcx hir::FnDecl,
        generics: &'tcx hir::Generics,
        walk: F,
    ) where
        F: for<'b, 'c> FnOnce(&'b mut LifetimeContext<'c, 'tcx>),
    {
        insert_late_bound_lifetimes(self.map, decl, generics);

        // Find the start of nested early scopes, e.g. in methods.
        let mut index = 0;
        if let Some(parent_id) = parent_id {
            let parent = self.tcx.hir.expect_item(parent_id);
            if let hir::ItemTrait(..) = parent.node {
                index += 1; // Self comes first.
            }
            match parent.node {
                hir::ItemTrait(_, _, ref generics, ..)
                | hir::ItemImpl(_, _, _, ref generics, ..) => {
                    index += generics.params.len() as u32;
                }
                _ => {}
            }
        }

        let lifetimes = generics
            .lifetimes()
            .map(|def| {
                if self.map.late_bound.contains(&def.lifetime.id) {
                    Region::late(&self.tcx.hir, def)
                } else {
                    Region::early(&self.tcx.hir, &mut index, def)
                }
            })
            .collect();

        let next_early_index = index + generics.ty_params().count() as u32;

        let scope = Scope::Binder {
            lifetimes,
            next_early_index,
            s: self.scope,
        };
        self.with(scope, move |old_scope, this| {
            this.check_lifetime_params(old_scope, &generics.params);
            this.hack(walk); // FIXME(#37666) workaround in place of `walk(this)`
        });
    }

    /// Returns the next index one would use for an early-bound-region
    /// if extending the current scope.
    fn next_early_index(&self) -> u32 {
        let mut scope = self.scope;
        loop {
            match *scope {
                Scope::Root => return 0,

                Scope::Binder {
                    next_early_index, ..
                } => return next_early_index,

                Scope::Body { s, .. }
                | Scope::Elision { s, .. }
                | Scope::ObjectLifetimeDefault { s, .. } => scope = s,
            }
        }
    }

    fn resolve_lifetime_ref(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
        debug!("resolve_lifetime_ref(lifetime_ref={:?})", lifetime_ref);
        // Walk up the scope chain, tracking the number of fn scopes
        // that we pass through, until we find a lifetime with the
        // given name or we run out of scopes.
        // search.
        let mut late_depth = 0;
        let mut scope = self.scope;
        let mut outermost_body = None;
        let result = loop {
            match *scope {
                Scope::Body { id, s } => {
                    outermost_body = Some(id);
                    scope = s;
                }

                Scope::Root => {
                    break None;
                }

                Scope::Binder {
                    ref lifetimes,
                    s,
                    next_early_index: _,
                } => {
                    if let Some(&def) = lifetimes.get(&lifetime_ref.name) {
                        break Some(def.shifted(late_depth));
                    } else {
                        late_depth += 1;
                        scope = s;
                    }
                }

                Scope::Elision { s, .. } | Scope::ObjectLifetimeDefault { s, .. } => {
                    scope = s;
                }
            }
        };

        if let Some(mut def) = result {
            if let Region::EarlyBound(..) = def {
                // Do not free early-bound regions, only late-bound ones.
            } else if let Some(body_id) = outermost_body {
                let fn_id = self.tcx.hir.body_owner(body_id);
                match self.tcx.hir.get(fn_id) {
                    hir::map::NodeItem(&hir::Item {
                        node: hir::ItemFn(..),
                        ..
                    })
                    | hir::map::NodeTraitItem(&hir::TraitItem {
                        node: hir::TraitItemKind::Method(..),
                        ..
                    })
                    | hir::map::NodeImplItem(&hir::ImplItem {
                        node: hir::ImplItemKind::Method(..),
                        ..
                    }) => {
                        let scope = self.tcx.hir.local_def_id(fn_id);
                        def = Region::Free(scope, def.id().unwrap());
                    }
                    _ => {}
                }
            }

            // Check for fn-syntax conflicts with in-band lifetime definitions
            if self.is_in_fn_syntax {
                match def {
                    Region::EarlyBound(_, _, LifetimeDefOrigin::InBand)
                    | Region::LateBound(_, _, LifetimeDefOrigin::InBand) => {
                        struct_span_err!(
                            self.tcx.sess,
                            lifetime_ref.span,
                            E0687,
                            "lifetimes used in `fn` or `Fn` syntax must be \
                             explicitly declared using `<...>` binders"
                        ).span_label(lifetime_ref.span, "in-band lifetime definition")
                            .emit();
                    }

                    Region::Static
                    | Region::EarlyBound(_, _, LifetimeDefOrigin::Explicit)
                    | Region::LateBound(_, _, LifetimeDefOrigin::Explicit)
                    | Region::LateBoundAnon(..)
                    | Region::Free(..) => {}
                }
            }

            self.insert_lifetime(lifetime_ref, def);
        } else {
            struct_span_err!(
                self.tcx.sess,
                lifetime_ref.span,
                E0261,
                "use of undeclared lifetime name `{}`",
                lifetime_ref.name.name()
            ).span_label(lifetime_ref.span, "undeclared lifetime")
                .emit();
        }
    }

    fn visit_segment_parameters(
        &mut self,
        def: Def,
        depth: usize,
        params: &'tcx hir::PathParameters,
    ) {
        if params.parenthesized {
            let was_in_fn_syntax = self.is_in_fn_syntax;
            self.is_in_fn_syntax = true;
            self.visit_fn_like_elision(params.inputs(), Some(&params.bindings[0].ty));
            self.is_in_fn_syntax = was_in_fn_syntax;
            return;
        }

        if params.lifetimes.iter().all(|l| l.is_elided()) {
            self.resolve_elided_lifetimes(&params.lifetimes);
        } else {
            for l in &params.lifetimes {
                self.visit_lifetime(l);
            }
        }

        // Figure out if this is a type/trait segment,
        // which requires object lifetime defaults.
        let parent_def_id = |this: &mut Self, def_id: DefId| {
            let def_key = this.tcx.def_key(def_id);
            DefId {
                krate: def_id.krate,
                index: def_key.parent.expect("missing parent"),
            }
        };
        let type_def_id = match def {
            Def::AssociatedTy(def_id) if depth == 1 => Some(parent_def_id(self, def_id)),
            Def::Variant(def_id) if depth == 0 => Some(parent_def_id(self, def_id)),
            Def::Struct(def_id)
            | Def::Union(def_id)
            | Def::Enum(def_id)
            | Def::TyAlias(def_id)
            | Def::Trait(def_id) if depth == 0 =>
            {
                Some(def_id)
            }
            _ => None,
        };

        let object_lifetime_defaults = type_def_id.map_or(vec![], |def_id| {
            let in_body = {
                let mut scope = self.scope;
                loop {
                    match *scope {
                        Scope::Root => break false,

                        Scope::Body { .. } => break true,

                        Scope::Binder { s, .. }
                        | Scope::Elision { s, .. }
                        | Scope::ObjectLifetimeDefault { s, .. } => {
                            scope = s;
                        }
                    }
                }
            };

            let map = &self.map;
            let unsubst = if let Some(id) = self.tcx.hir.as_local_node_id(def_id) {
                &map.object_lifetime_defaults[&id]
            } else {
                let tcx = self.tcx;
                self.xcrate_object_lifetime_defaults
                    .entry(def_id)
                    .or_insert_with(|| {
                        tcx.generics_of(def_id)
                            .types
                            .iter()
                            .map(|def| def.object_lifetime_default)
                            .collect()
                    })
            };
            unsubst
                .iter()
                .map(|set| match *set {
                    Set1::Empty => if in_body {
                        None
                    } else {
                        Some(Region::Static)
                    },
                    Set1::One(r) => r.subst(&params.lifetimes, map),
                    Set1::Many => None,
                })
                .collect()
        });

        for (i, ty) in params.types.iter().enumerate() {
            if let Some(&lt) = object_lifetime_defaults.get(i) {
                let scope = Scope::ObjectLifetimeDefault {
                    lifetime: lt,
                    s: self.scope,
                };
                self.with(scope, |_, this| this.visit_ty(ty));
            } else {
                self.visit_ty(ty);
            }
        }

        for b in &params.bindings {
            self.visit_assoc_type_binding(b);
        }
    }

    fn visit_fn_like_elision(
        &mut self,
        inputs: &'tcx [P<hir::Ty>],
        output: Option<&'tcx P<hir::Ty>>,
    ) {
        let mut arg_elide = Elide::FreshLateAnon(Cell::new(0));
        let arg_scope = Scope::Elision {
            elide: arg_elide.clone(),
            s: self.scope,
        };
        self.with(arg_scope, |_, this| {
            for input in inputs {
                this.visit_ty(input);
            }
            match *this.scope {
                Scope::Elision { ref elide, .. } => {
                    arg_elide = elide.clone();
                }
                _ => bug!(),
            }
        });

        let output = match output {
            Some(ty) => ty,
            None => return,
        };

        // Figure out if there's a body we can get argument names from,
        // and whether there's a `self` argument (treated specially).
        let mut assoc_item_kind = None;
        let mut impl_self = None;
        let parent = self.tcx.hir.get_parent_node(output.id);
        let body = match self.tcx.hir.get(parent) {
            // `fn` definitions and methods.
            hir::map::NodeItem(&hir::Item {
                node: hir::ItemFn(.., body),
                ..
            }) => Some(body),

            hir::map::NodeTraitItem(&hir::TraitItem {
                node: hir::TraitItemKind::Method(_, ref m),
                ..
            }) => {
                match self.tcx
                    .hir
                    .expect_item(self.tcx.hir.get_parent(parent))
                    .node
                {
                    hir::ItemTrait(.., ref trait_items) => {
                        assoc_item_kind = trait_items
                            .iter()
                            .find(|ti| ti.id.node_id == parent)
                            .map(|ti| ti.kind);
                    }
                    _ => {}
                }
                match *m {
                    hir::TraitMethod::Required(_) => None,
                    hir::TraitMethod::Provided(body) => Some(body),
                }
            }

            hir::map::NodeImplItem(&hir::ImplItem {
                node: hir::ImplItemKind::Method(_, body),
                ..
            }) => {
                match self.tcx
                    .hir
                    .expect_item(self.tcx.hir.get_parent(parent))
                    .node
                {
                    hir::ItemImpl(.., ref self_ty, ref impl_items) => {
                        impl_self = Some(self_ty);
                        assoc_item_kind = impl_items
                            .iter()
                            .find(|ii| ii.id.node_id == parent)
                            .map(|ii| ii.kind);
                    }
                    _ => {}
                }
                Some(body)
            }

            // Foreign functions, `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
            hir::map::NodeForeignItem(_) | hir::map::NodeTy(_) | hir::map::NodeTraitRef(_) =>
                None,
            // Everything else (only closures?) doesn't
            // actually enjoy elision in return types.
            _ => {
                self.visit_ty(output);
                return;
            }
        };

        let has_self = match assoc_item_kind {
            Some(hir::AssociatedItemKind::Method { has_self }) => has_self,
            _ => false,
        };

        // In accordance with the rules for lifetime elision, we can determine
        // what region to use for elision in the output type in two ways.
        // First (determined here), if `self` is by-reference, then the
        // implied output region is the region of the self parameter.
        if has_self {
            // Look for `self: &'a Self` - also desugared from `&'a self`,
            // and if that matches, use it for elision and return early.
            let is_self_ty = |def: Def| {
                if let Def::SelfTy(..) = def {
                    return true;
                }

                // Can't always rely on literal (or implied) `Self` due
                // to the way elision rules were originally specified.
                let impl_self = impl_self.map(|ty| &ty.node);
                if let Some(&hir::TyPath(hir::QPath::Resolved(None, ref path))) = impl_self {
                    match path.def {
                        // Whitelist the types that unambiguously always
                        // result in the same type constructor being used
                        // (it can't differ between `Self` and `self`).
                        Def::Struct(_) | Def::Union(_) | Def::Enum(_) | Def::PrimTy(_) => {
                            return def == path.def
                        }
                        _ => {}
                    }
                }

                false
            };

            if let hir::TyRptr(lifetime_ref, ref mt) = inputs[0].node {
                if let hir::TyPath(hir::QPath::Resolved(None, ref path)) = mt.ty.node {
                    if is_self_ty(path.def) {
                        if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.id) {
                            let scope = Scope::Elision {
                                elide: Elide::Exact(lifetime),
                                s: self.scope,
                            };
                            self.with(scope, |_, this| this.visit_ty(output));
                            return;
                        }
                    }
                }
            }
        }

        // Second, if there was exactly one lifetime (either a substitution or a
        // reference) in the arguments, then any anonymous regions in the output
        // have that lifetime.
        let mut possible_implied_output_region = None;
        let mut lifetime_count = 0;
        let arg_lifetimes = inputs
            .iter()
            .enumerate()
            .skip(has_self as usize)
            .map(|(i, input)| {
                let mut gather = GatherLifetimes {
                    map: self.map,
                    binder_depth: 1,
                    have_bound_regions: false,
                    lifetimes: FxHashSet(),
                };
                gather.visit_ty(input);

                lifetime_count += gather.lifetimes.len();

                if lifetime_count == 1 && gather.lifetimes.len() == 1 {
                    // there's a chance that the unique lifetime of this
                    // iteration will be the appropriate lifetime for output
                    // parameters, so lets store it.
                    possible_implied_output_region = gather.lifetimes.iter().cloned().next();
                }

                ElisionFailureInfo {
                    parent: body,
                    index: i,
                    lifetime_count: gather.lifetimes.len(),
                    have_bound_regions: gather.have_bound_regions,
                }
            })
            .collect();

        let elide = if lifetime_count == 1 {
            Elide::Exact(possible_implied_output_region.unwrap())
        } else {
            Elide::Error(arg_lifetimes)
        };

        let scope = Scope::Elision {
            elide,
            s: self.scope,
        };
        self.with(scope, |_, this| this.visit_ty(output));

        struct GatherLifetimes<'a> {
            map: &'a NamedRegionMap,
            binder_depth: u32,
            have_bound_regions: bool,
            lifetimes: FxHashSet<Region>,
        }

        impl<'v, 'a> Visitor<'v> for GatherLifetimes<'a> {
            fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
                NestedVisitorMap::None
            }

            fn visit_ty(&mut self, ty: &hir::Ty) {
                if let hir::TyBareFn(_) = ty.node {
                    self.binder_depth += 1;
                }
                if let hir::TyTraitObject(ref bounds, ref lifetime) = ty.node {
                    for bound in bounds {
                        self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
                    }

                    // Stay on the safe side and don't include the object
                    // lifetime default (which may not end up being used).
                    if !lifetime.is_elided() {
                        self.visit_lifetime(lifetime);
                    }
                } else {
                    intravisit::walk_ty(self, ty);
                }
                if let hir::TyBareFn(_) = ty.node {
                    self.binder_depth -= 1;
                }
            }

            fn visit_generic_param(&mut self, param: &hir::GenericParam) {
                if let hir::GenericParam::Lifetime(ref lifetime_def) = *param {
                    for l in &lifetime_def.bounds {
                        self.visit_lifetime(l);
                    }
                }

                intravisit::walk_generic_param(self, param);
            }

            fn visit_poly_trait_ref(
                &mut self,
                trait_ref: &hir::PolyTraitRef,
                modifier: hir::TraitBoundModifier,
            ) {
                self.binder_depth += 1;
                intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
                self.binder_depth -= 1;
            }

            fn visit_lifetime(&mut self, lifetime_ref: &hir::Lifetime) {
                if let Some(&lifetime) = self.map.defs.get(&lifetime_ref.id) {
                    match lifetime {
                        Region::LateBound(debruijn, _, _) | Region::LateBoundAnon(debruijn, _)
                            if debruijn.depth < self.binder_depth =>
                        {
                            self.have_bound_regions = true;
                        }
                        _ => {
                            self.lifetimes
                                .insert(lifetime.from_depth(self.binder_depth));
                        }
                    }
                }
            }
        }
    }

    fn resolve_elided_lifetimes(&mut self, lifetime_refs: &'tcx [hir::Lifetime]) {
        if lifetime_refs.is_empty() {
            return;
        }

        let span = lifetime_refs[0].span;
        let mut late_depth = 0;
        let mut scope = self.scope;
        let error = loop {
            match *scope {
                // Do not assign any resolution, it will be inferred.
                Scope::Body { .. } => return,

                Scope::Root => break None,

                Scope::Binder { s, .. } => {
                    late_depth += 1;
                    scope = s;
                }

                Scope::Elision { ref elide, .. } => {
                    let lifetime = match *elide {
                        Elide::FreshLateAnon(ref counter) => {
                            for lifetime_ref in lifetime_refs {
                                let lifetime = Region::late_anon(counter).shifted(late_depth);
                                self.insert_lifetime(lifetime_ref, lifetime);
                            }
                            return;
                        }
                        Elide::Exact(l) => l.shifted(late_depth),
                        Elide::Error(ref e) => break Some(e),
                    };
                    for lifetime_ref in lifetime_refs {
                        self.insert_lifetime(lifetime_ref, lifetime);
                    }
                    return;
                }

                Scope::ObjectLifetimeDefault { s, .. } => {
                    scope = s;
                }
            }
        };

        let mut err = struct_span_err!(
            self.tcx.sess,
            span,
            E0106,
            "missing lifetime specifier{}",
            if lifetime_refs.len() > 1 { "s" } else { "" }
        );
        let msg = if lifetime_refs.len() > 1 {
            format!("expected {} lifetime parameters", lifetime_refs.len())
        } else {
            format!("expected lifetime parameter")
        };
        err.span_label(span, msg);

        if let Some(params) = error {
            if lifetime_refs.len() == 1 {
                self.report_elision_failure(&mut err, params);
            }
        }
        err.emit();
    }

    fn report_elision_failure(
        &mut self,
        db: &mut DiagnosticBuilder,
        params: &[ElisionFailureInfo],
    ) {
        let mut m = String::new();
        let len = params.len();

        let elided_params: Vec<_> = params
            .iter()
            .cloned()
            .filter(|info| info.lifetime_count > 0)
            .collect();

        let elided_len = elided_params.len();

        for (i, info) in elided_params.into_iter().enumerate() {
            let ElisionFailureInfo {
                parent,
                index,
                lifetime_count: n,
                have_bound_regions,
            } = info;

            let help_name = if let Some(body) = parent {
                let arg = &self.tcx.hir.body(body).arguments[index];
                format!("`{}`", self.tcx.hir.node_to_pretty_string(arg.pat.id))
            } else {
                format!("argument {}", index + 1)
            };

            m.push_str(
                &(if n == 1 {
                    help_name
                } else {
                    format!(
                        "one of {}'s {} {}lifetimes",
                        help_name,
                        n,
                        if have_bound_regions { "free " } else { "" }
                    )
                })[..],
            );

            if elided_len == 2 && i == 0 {
                m.push_str(" or ");
            } else if i + 2 == elided_len {
                m.push_str(", or ");
            } else if i != elided_len - 1 {
                m.push_str(", ");
            }
        }

        if len == 0 {
            help!(
                db,
                "this function's return type contains a borrowed value, but \
                 there is no value for it to be borrowed from"
            );
            help!(db, "consider giving it a 'static lifetime");
        } else if elided_len == 0 {
            help!(
                db,
                "this function's return type contains a borrowed value with \
                 an elided lifetime, but the lifetime cannot be derived from \
                 the arguments"
            );
            help!(
                db,
                "consider giving it an explicit bounded or 'static \
                 lifetime"
            );
        } else if elided_len == 1 {
            help!(
                db,
                "this function's return type contains a borrowed value, but \
                 the signature does not say which {} it is borrowed from",
                m
            );
        } else {
            help!(
                db,
                "this function's return type contains a borrowed value, but \
                 the signature does not say whether it is borrowed from {}",
                m
            );
        }
    }

    fn resolve_object_lifetime_default(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
        let mut late_depth = 0;
        let mut scope = self.scope;
        let lifetime = loop {
            match *scope {
                Scope::Binder { s, .. } => {
                    late_depth += 1;
                    scope = s;
                }

                Scope::Root | Scope::Elision { .. } => break Region::Static,

                Scope::Body { .. } | Scope::ObjectLifetimeDefault { lifetime: None, .. } => return,

                Scope::ObjectLifetimeDefault {
                    lifetime: Some(l), ..
                } => break l,
            }
        };
        self.insert_lifetime(lifetime_ref, lifetime.shifted(late_depth));
    }

    fn check_lifetime_params(&mut self, old_scope: ScopeRef, params: &'tcx [hir::GenericParam]) {
        for (i, lifetime_i) in params.lifetimes().enumerate() {
            for lifetime in params.lifetimes() {
                match lifetime.lifetime.name {
                    hir::LifetimeName::Static | hir::LifetimeName::Underscore => {
                        let lifetime = lifetime.lifetime;
                        let name = lifetime.name.name();
                        let mut err = struct_span_err!(
                            self.tcx.sess,
                            lifetime.span,
                            E0262,
                            "invalid lifetime parameter name: `{}`",
                            name
                        );
                        err.span_label(
                            lifetime.span,
                            format!("{} is a reserved lifetime name", name),
                        );
                        err.emit();
                    }
                    hir::LifetimeName::Implicit | hir::LifetimeName::Name(_) => {}
                }
            }

            // It is a hard error to shadow a lifetime within the same scope.
            for lifetime_j in params.lifetimes().skip(i + 1) {
                if lifetime_i.lifetime.name == lifetime_j.lifetime.name {
                    struct_span_err!(
                        self.tcx.sess,
                        lifetime_j.lifetime.span,
                        E0263,
                        "lifetime name `{}` declared twice in the same scope",
                        lifetime_j.lifetime.name.name()
                    ).span_label(lifetime_j.lifetime.span, "declared twice")
                        .span_label(lifetime_i.lifetime.span, "previous declaration here")
                        .emit();
                }
            }

            // It is a soft error to shadow a lifetime within a parent scope.
            self.check_lifetime_def_for_shadowing(old_scope, &lifetime_i.lifetime);

            for bound in &lifetime_i.bounds {
                match bound.name {
                    hir::LifetimeName::Underscore => {
                        let mut err = struct_span_err!(
                            self.tcx.sess,
                            bound.span,
                            E0637,
                            "invalid lifetime bound name: `'_`"
                        );
                        err.span_label(bound.span, "`'_` is a reserved lifetime name");
                        err.emit();
                    }
                    hir::LifetimeName::Static => {
                        self.insert_lifetime(bound, Region::Static);
                        self.tcx
                            .sess
                            .struct_span_warn(
                                lifetime_i.lifetime.span.to(bound.span),
                                &format!(
                                    "unnecessary lifetime parameter `{}`",
                                    lifetime_i.lifetime.name.name()
                                ),
                            )
                            .help(&format!(
                                "you can use the `'static` lifetime directly, in place \
                                 of `{}`",
                                lifetime_i.lifetime.name.name()
                            ))
                            .emit();
                    }
                    hir::LifetimeName::Implicit | hir::LifetimeName::Name(_) => {
                        self.resolve_lifetime_ref(bound);
                    }
                }
            }
        }
    }

    fn check_lifetime_def_for_shadowing(&self,
                                        mut old_scope: ScopeRef,
                                        lifetime: &'tcx hir::Lifetime) {
        for &(label, label_span) in &self.labels_in_fn {
            // FIXME (#24278): non-hygienic comparison
            if lifetime.name.name() == label {
                signal_shadowing_problem(
                    self.tcx,
                    label,
                    original_label(label_span),
                    shadower_lifetime(&lifetime),
                );
                return;
            }
        }

        loop {
            match *old_scope {
                Scope::Body { s, .. }
                | Scope::Elision { s, .. }
                | Scope::ObjectLifetimeDefault { s, .. } => {
                    old_scope = s;
                }

                Scope::Root => {
                    return;
                }

                Scope::Binder {
                    ref lifetimes,
                    s,
                    next_early_index: _,
                } => {
                    if let Some(&def) = lifetimes.get(&lifetime.name) {
                        let node_id = self.tcx.hir.as_local_node_id(def.id().unwrap()).unwrap();

                        signal_shadowing_problem(
                            self.tcx,
                            lifetime.name.name(),
                            original_lifetime(self.tcx.hir.span(node_id)),
                            shadower_lifetime(&lifetime),
                        );
                        return;
                    }

                    old_scope = s;
                }
            }
        }
    }

    fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
        if lifetime_ref.id == ast::DUMMY_NODE_ID {
            span_bug!(
                lifetime_ref.span,
                "lifetime reference not renumbered, \
                 probably a bug in syntax::fold"
            );
        }

        debug!(
            "insert_lifetime: {} resolved to {:?} span={:?}",
            self.tcx.hir.node_to_string(lifetime_ref.id),
            def,
            self.tcx.sess.codemap().span_to_string(lifetime_ref.span)
        );
        self.map.defs.insert(lifetime_ref.id, def);

        match def {
            Region::LateBoundAnon(..) |
            Region::Static => {
                // These are anonymous lifetimes or lifetimes that are not declared.
            }

            Region::Free(_, def_id) |
            Region::LateBound(_, def_id, _) |
            Region::EarlyBound(_, def_id, _) => {
                // A lifetime declared by the user.
                if !self.lifetime_uses.contains_key(&def_id) {
                    self.lifetime_uses
                        .insert(def_id, LifetimeUseSet::One(lifetime_ref));
                } else {
                    self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
                }
            }
        }
    }
}

///////////////////////////////////////////////////////////////////////////

/// Detects late-bound lifetimes and inserts them into
/// `map.late_bound`.
///
/// A region declared on a fn is **late-bound** if:
/// - it is constrained by an argument type;
/// - it does not appear in a where-clause.
///
/// "Constrained" basically means that it appears in any type but
/// not amongst the inputs to a projection.  In other words, `<&'a
/// T as Trait<''b>>::Foo` does not constrain `'a` or `'b`.
fn insert_late_bound_lifetimes(
    map: &mut NamedRegionMap,
    decl: &hir::FnDecl,
    generics: &hir::Generics,
) {
    debug!(
        "insert_late_bound_lifetimes(decl={:?}, generics={:?})",
        decl,
        generics
    );

    let mut constrained_by_input = ConstrainedCollector {
        regions: FxHashSet(),
    };
    for arg_ty in &decl.inputs {
        constrained_by_input.visit_ty(arg_ty);
    }

    let mut appears_in_output = AllCollector {
        regions: FxHashSet(),
    };
    intravisit::walk_fn_ret_ty(&mut appears_in_output, &decl.output);

    debug!(
        "insert_late_bound_lifetimes: constrained_by_input={:?}",
        constrained_by_input.regions
    );

    // Walk the lifetimes that appear in where clauses.
    //
    // Subtle point: because we disallow nested bindings, we can just
    // ignore binders here and scrape up all names we see.
    let mut appears_in_where_clause = AllCollector {
        regions: FxHashSet(),
    };

    for param in &generics.params {
        match *param {
            hir::GenericParam::Lifetime(ref lifetime_def) => {
                if !lifetime_def.bounds.is_empty() {
                    // `'a: 'b` means both `'a` and `'b` are referenced
                    appears_in_where_clause.visit_generic_param(param);
                }
            }
            hir::GenericParam::Type(ref ty_param) => {
                walk_list!(
                    &mut appears_in_where_clause,
                    visit_ty_param_bound,
                    &ty_param.bounds
                );
            }
        }
    }

    walk_list!(
        &mut appears_in_where_clause,
        visit_where_predicate,
        &generics.where_clause.predicates
    );

    debug!(
        "insert_late_bound_lifetimes: appears_in_where_clause={:?}",
        appears_in_where_clause.regions
    );

    // Late bound regions are those that:
    // - appear in the inputs
    // - do not appear in the where-clauses
    // - are not implicitly captured by `impl Trait`
    for lifetime in generics.lifetimes() {
        let name = lifetime.lifetime.name;

        // appears in the where clauses? early-bound.
        if appears_in_where_clause.regions.contains(&name) {
            continue;
        }

        // does not appear in the inputs, but appears in the return type? early-bound.
        if !constrained_by_input.regions.contains(&name)
            && appears_in_output.regions.contains(&name)
        {
            continue;
        }

        debug!(
            "insert_late_bound_lifetimes: \
             lifetime {:?} with id {:?} is late-bound",
            lifetime.lifetime.name,
            lifetime.lifetime.id
        );

        let inserted = map.late_bound.insert(lifetime.lifetime.id);
        assert!(
            inserted,
            "visited lifetime {:?} twice",
            lifetime.lifetime.id
        );
    }

    return;

    struct ConstrainedCollector {
        regions: FxHashSet<hir::LifetimeName>,
    }

    impl<'v> Visitor<'v> for ConstrainedCollector {
        fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
            NestedVisitorMap::None
        }

        fn visit_ty(&mut self, ty: &'v hir::Ty) {
            match ty.node {
                hir::TyPath(hir::QPath::Resolved(Some(_), _))
                | hir::TyPath(hir::QPath::TypeRelative(..)) => {
                    // ignore lifetimes appearing in associated type
                    // projections, as they are not *constrained*
                    // (defined above)
                }

                hir::TyPath(hir::QPath::Resolved(None, ref path)) => {
                    // consider only the lifetimes on the final
                    // segment; I am not sure it's even currently
                    // valid to have them elsewhere, but even if it
                    // is, those would be potentially inputs to
                    // projections
                    if let Some(last_segment) = path.segments.last() {
                        self.visit_path_segment(path.span, last_segment);
                    }
                }

                _ => {
                    intravisit::walk_ty(self, ty);
                }
            }
        }

        fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
            self.regions.insert(lifetime_ref.name);
        }
    }

    struct AllCollector {
        regions: FxHashSet<hir::LifetimeName>,
    }

    impl<'v> Visitor<'v> for AllCollector {
        fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
            NestedVisitorMap::None
        }

        fn visit_lifetime(&mut self, lifetime_ref: &'v hir::Lifetime) {
            self.regions.insert(lifetime_ref.name);
        }
    }
}