resolve_lifetime.rs

//! 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 crate::hir::def::{Res, DefKind};
use crate::hir::def_id::{CrateNum, DefId, LocalDefId, LOCAL_CRATE};
use crate::hir::map::Map;
use crate::hir::ptr::P;
use crate::hir::{GenericArg, GenericParam, ItemLocalId, LifetimeName, Node, ParamName, QPath};
use crate::ty::{self, DefIdTree, GenericParamDefKind, TyCtxt};

use crate::rustc::lint;
use crate::session::Session;
use crate::util::nodemap::{DefIdMap, FxHashMap, FxHashSet, HirIdMap, HirIdSet};
use errors::{Applicability, DiagnosticBuilder};
use rustc_macros::HashStable;
use std::borrow::Cow;
use std::cell::Cell;
use std::mem::{replace, take};
use syntax::ast;
use syntax::attr;
use syntax::symbol::{kw, sym};
use syntax_pos::Span;

use crate::hir::intravisit::{self, NestedVisitorMap, Visitor};
use crate::hir::{self, GenericParamKind, LifetimeParamKind};

/// 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, HashStable)]
pub enum LifetimeDefOrigin {
    // Explicit binders like `fn foo<'a>(x: &'a u8)` or elided like `impl Foo<&u32>`
    ExplicitOrElided,
    // In-band declarations like `fn foo(x: &'a u8)`
    InBand,
    // Some kind of erroneous origin
    Error,
}

impl LifetimeDefOrigin {
    fn from_param(param: &GenericParam) -> Self {
        match param.kind {
            GenericParamKind::Lifetime { kind } => match kind {
                LifetimeParamKind::InBand => LifetimeDefOrigin::InBand,
                LifetimeParamKind::Explicit => LifetimeDefOrigin::ExplicitOrElided,
                LifetimeParamKind::Elided => LifetimeDefOrigin::ExplicitOrElided,
                LifetimeParamKind::Error => LifetimeDefOrigin::Error,
            },
            _ => bug!("expected a lifetime param"),
        }
    }
}

// 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, HashStable)]
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, param: &GenericParam) -> (ParamName, Region) {
        let i = *index;
        *index += 1;
        let def_id = hir_map.local_def_id(param.hir_id);
        let origin = LifetimeDefOrigin::from_param(param);
        debug!("Region::early: index={} def_id={:?}", i, def_id);
        (param.name.modern(), Region::EarlyBound(i, def_id, origin))
    }

    fn late(hir_map: &Map<'_>, param: &GenericParam) -> (ParamName, Region) {
        let depth = ty::INNERMOST;
        let def_id = hir_map.local_def_id(param.hir_id);
        let origin = LifetimeDefOrigin::from_param(param);
        debug!(
            "Region::late: param={:?} depth={:?} def_id={:?} origin={:?}",
            param, depth, def_id, origin,
        );
        (
            param.name.modern(),
            Region::LateBound(depth, def_id, origin),
        )
    }

    fn late_anon(index: &Cell<u32>) -> Region {
        let i = index.get();
        index.set(i + 1);
        let depth = ty::INNERMOST;
        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(debruijn, id, origin) => {
                Region::LateBound(debruijn.shifted_in(amount), id, origin)
            }
            Region::LateBoundAnon(debruijn, index) => {
                Region::LateBoundAnon(debruijn.shifted_in(amount), index)
            }
            _ => self,
        }
    }

    fn shifted_out_to_binder(self, binder: ty::DebruijnIndex) -> Region {
        match self {
            Region::LateBound(debruijn, id, origin) => {
                Region::LateBound(debruijn.shifted_out_to_binder(binder), id, origin)
            }
            Region::LateBoundAnon(debruijn, index) => {
                Region::LateBoundAnon(debruijn.shifted_out_to_binder(binder), index)
            }
            _ => self,
        }
    }

    fn subst<'a, L>(self, mut params: L, map: &NamedRegionMap) -> Option<Region>
    where
        L: Iterator<Item = &'a hir::Lifetime>,
    {
        if let Region::EarlyBound(index, _, _) = self {
            params
                .nth(index as usize)
                .and_then(|lifetime| map.defs.get(&lifetime.hir_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, HashStable)]
pub enum Set1<T> {
    Empty,
    One(T),
    Many,
}

impl<T: PartialEq> Set1<T> {
    pub fn insert(&mut self, value: T) {
        *self = match self {
            Set1::Empty => Set1::One(value),
            Set1::One(old) if *old == value => return,
            _ => 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: HirIdMap<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: HirIdSet,

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

/// See [`NamedRegionMap`].
#[derive(Default)]
pub struct ResolveLifetimes {
    defs: FxHashMap<LocalDefId, FxHashMap<ItemLocalId, Region>>,
    late_bound: FxHashMap<LocalDefId, FxHashSet<ItemLocalId>>,
    object_lifetime_defaults:
        FxHashMap<LocalDefId, FxHashMap<ItemLocalId, Vec<ObjectLifetimeDefault>>>,
}

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

struct LifetimeContext<'a, 'tcx> {
    tcx: TyCtxt<'tcx>,
    map: &'a mut NamedRegionMap,
    scope: ScopeRef<'a>,

    /// This is slightly complicated. 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.
    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::Ident>,

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

    lifetime_uses: &'a mut 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::ParamName, 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,

        /// Flag is set to true if, in this binder, `'_` would be
        /// equivalent to a "single-use region". This is true on
        /// impls, but not other kinds of items.
        track_lifetime_uses: bool,

        /// Whether or not this binder would serve as the parent
        /// binder for abstract types introduced within. For example:
        ///
        ///     fn foo<'a>() -> impl for<'b> Trait<Item = impl Trait2<'a>>
        ///
        /// Here, the abstract types we create for the `impl Trait`
        /// and `impl Trait2` references will both have the `foo` item
        /// as their parent. When we get to `impl Trait2`, we find
        /// that it is nested within the `for<>` binder -- this flag
        /// allows us to skip that when looking for the parent binder
        /// of the resulting abstract type.
        abstract_type_parent: bool,

        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::query::Providers<'_>) {
    *providers = ty::query::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)
        },

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

        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)
        },

        ..*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: TyCtxt<'_>, for_krate: CrateNum) -> &ResolveLifetimes {
    assert_eq!(for_krate, LOCAL_CRATE);

    let named_region_map = krate(tcx);

    let mut rl = ResolveLifetimes::default();

    for (hir_id, v) in named_region_map.defs {
        let map = rl.defs.entry(hir_id.owner_local_def_id()).or_default();
        map.insert(hir_id.local_id, v);
    }
    for hir_id in named_region_map.late_bound {
        let map = rl.late_bound
            .entry(hir_id.owner_local_def_id())
            .or_default();
        map.insert(hir_id.local_id);
    }
    for (hir_id, v) in named_region_map.object_lifetime_defaults {
        let map = rl.object_lifetime_defaults
            .entry(hir_id.owner_local_def_id())
            .or_default();
        map.insert(hir_id.local_id, v);
    }

    tcx.arena.alloc(rl)
}

fn krate(tcx: TyCtxt<'_>) -> NamedRegionMap {
    let krate = tcx.hir().krate();
    let mut map = NamedRegionMap {
        defs: Default::default(),
        late_bound: Default::default(),
        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: Default::default(),
            lifetime_uses: &mut Default::default(),
        };
        for (_, item) in &krate.items {
            visitor.visit_item(item);
        }
    }
    map
}

/// In traits, there is an implicit `Self` type parameter which comes before the generics.
/// We have to account for this when computing the index of the other generic parameters.
/// This function returns whether there is such an implicit parameter defined on the given item.
fn sub_items_have_self_param(node: &hir::ItemKind) -> bool {
    match *node {
        hir::ItemKind::Trait(..) |
        hir::ItemKind::TraitAlias(..) => true,
        _ => false,
    }
}

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 = take(&mut self.labels_in_fn);
        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::ItemKind::Fn(ref decl, _, ref generics, _) => {
                self.visit_early_late(None, decl, generics, |this| {
                    intravisit::walk_item(this, item);
                });
            }

            hir::ItemKind::ExternCrate(_)
            | hir::ItemKind::Use(..)
            | hir::ItemKind::Mod(..)
            | hir::ItemKind::ForeignMod(..)
            | hir::ItemKind::GlobalAsm(..) => {
                // These sorts of items have no lifetime parameters at all.
                intravisit::walk_item(self, item);
            }
            hir::ItemKind::Static(..) | hir::ItemKind::Const(..) => {
                // 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::ItemKind::Existential(hir::ExistTy {
                impl_trait_fn: Some(_),
                ..
            }) => {
                // currently existential type declarations are just generated from impl Trait
                // items. doing anything on this node is irrelevant, as we currently don't need
                // it.
            }
            hir::ItemKind::Ty(_, ref generics)
            | hir::ItemKind::Existential(hir::ExistTy {
                impl_trait_fn: None,
                ref generics,
                ..
            })
            | hir::ItemKind::Enum(_, ref generics)
            | hir::ItemKind::Struct(_, ref generics)
            | hir::ItemKind::Union(_, ref generics)
            | hir::ItemKind::Trait(_, _, ref generics, ..)
            | hir::ItemKind::TraitAlias(ref generics, ..)
            | hir::ItemKind::Impl(_, _, _, ref generics, ..) => {
                // Impls permit `'_` to be used and it is equivalent to "some fresh lifetime name".
                // This is not true for other kinds of items.x
                let track_lifetime_uses = match item.node {
                    hir::ItemKind::Impl(..) => true,
                    _ => false,
                };
                // These kinds of items have only early-bound lifetime parameters.
                let mut index = if sub_items_have_self_param(&item.node) {
                    1 // Self comes before lifetimes
                } else {
                    0
                };
                let mut non_lifetime_count = 0;
                let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
                    GenericParamKind::Lifetime { .. } => {
                        Some(Region::early(&self.tcx.hir(), &mut index, param))
                    }
                    GenericParamKind::Type { .. } |
                    GenericParamKind::Const { .. } => {
                        non_lifetime_count += 1;
                        None
                    }
                }).collect();
                let scope = Scope::Binder {
                    lifetimes,
                    next_early_index: index + non_lifetime_count,
                    abstract_type_parent: true,
                    track_lifetime_uses,
                    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::ForeignItemKind::Fn(ref decl, _, ref generics) => {
                self.visit_early_late(None, decl, generics, |this| {
                    intravisit::walk_foreign_item(this, item);
                })
            }
            hir::ForeignItemKind::Static(..) => {
                intravisit::walk_foreign_item(self, item);
            }
            hir::ForeignItemKind::Type => {
                intravisit::walk_foreign_item(self, item);
            }
        }
    }

    fn visit_ty(&mut self, ty: &'tcx hir::Ty) {
        debug!("visit_ty: id={:?} ty={:?}", ty.hir_id, ty);
        match ty.node {
            hir::TyKind::BareFn(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
                        .iter()
                        .filter_map(|param| match param.kind {
                            GenericParamKind::Lifetime { .. } => {
                                Some(Region::late(&self.tcx.hir(), param))
                            }
                            _ => None,
                        })
                        .collect(),
                    s: self.scope,
                    next_early_index,
                    track_lifetime_uses: true,
                    abstract_type_parent: false,
                };
                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::TyKind::TraitObject(ref bounds, ref lifetime) => {
                for bound in bounds {
                    self.visit_poly_trait_ref(bound, hir::TraitBoundModifier::None);
                }
                match lifetime.name {
                    LifetimeName::Implicit => {
                        // If the user does not write *anything*, we
                        // use the object lifetime defaulting
                        // rules. So e.g., `Box<dyn Debug>` becomes
                        // `Box<dyn Debug + 'static>`.
                        self.resolve_object_lifetime_default(lifetime)
                    }
                    LifetimeName::Underscore => {
                        // If the user writes `'_`, we use the *ordinary* elision
                        // rules. So the `'_` in e.g., `Box<dyn Debug + '_>` will be
                        // resolved the same as the `'_` in `&'_ Foo`.
                        //
                        // cc #48468
                        self.resolve_elided_lifetimes(vec![lifetime])
                    }
                    LifetimeName::Param(_) | LifetimeName::Static => {
                        // If the user wrote an explicit name, use that.
                        self.visit_lifetime(lifetime);
                    }
                    LifetimeName::Error => {}
                }
            }
            hir::TyKind::Rptr(ref lifetime_ref, ref mt) => {
                self.visit_lifetime(lifetime_ref);
                let scope = Scope::ObjectLifetimeDefault {
                    lifetime: self.map.defs.get(&lifetime_ref.hir_id).cloned(),
                    s: self.scope,
                };
                self.with(scope, |_, this| this.visit_ty(&mt.ty));
            }
            hir::TyKind::Def(item_id, ref lifetimes) => {
                // 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 (generics, bounds) = match self.tcx.hir().expect_item(item_id.id).node
                {
                    // named existential types are reached via TyKind::Path
                    // this arm is for `impl Trait` in the types of statics, constants and locals
                    hir::ItemKind::Existential(hir::ExistTy {
                        impl_trait_fn: None,
                        ..
                    }) => {
                        intravisit::walk_ty(self, ty);
                        return;
                    }
                    // RPIT (return position impl trait)
                    hir::ItemKind::Existential(hir::ExistTy {
                        ref generics,
                        ref bounds,
                        ..
                    }) => (generics, bounds),
                    ref i => bug!("impl Trait pointed to non-existential type?? {:#?}", i),
                };

                // 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 {
                    if let hir::GenericArg::Lifetime(lifetime) = lifetime {
                        self.visit_lifetime(lifetime);

                        // Check for predicates like `impl for<'a> Trait<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.hir_id).cloned();
                        if let Some(Region::LateBound(_, def_id, _)) = def {
                            if let Some(hir_id) = self.tcx.hir().as_local_hir_id(def_id) {
                                // Ensure that the parent of the def is an item, not HRTB
                                let parent_id = self.tcx.hir().get_parent_node(hir_id);
                                let parent_impl_id = hir::ImplItemId { hir_id: parent_id };
                                let parent_trait_id = hir::TraitItemId { hir_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"
                                    );
                                    self.uninsert_lifetime_on_error(lifetime, def.unwrap());
                                }
                            }
                        }
                    }
                }

                // We want to start our early-bound indices at the end of the parent scope,
                // not including any parent `impl Trait`s.
                let mut index = self.next_early_index_for_abstract_type();
                debug!("visit_ty: index = {}", index);

                let mut elision = None;
                let mut lifetimes = FxHashMap::default();
                let mut non_lifetime_count = 0;
                for param in &generics.params {
                    match param.kind {
                        GenericParamKind::Lifetime { .. } => {
                            let (name, reg) = Region::early(&self.tcx.hir(), &mut index, &param);
                            if let hir::ParamName::Plain(param_name) = name {
                                if param_name.name == kw::UnderscoreLifetime {
                                    // Pick the elided lifetime "definition" if one exists
                                    // and use it to make an elision scope.
                                    elision = Some(reg);
                                } else {
                                    lifetimes.insert(name, reg);
                                }
                            } else {
                                lifetimes.insert(name, reg);
                            }
                        }
                        GenericParamKind::Type { .. } |
                        GenericParamKind::Const { .. } => {
                            non_lifetime_count += 1;
                        }
                    }
                }
                let next_early_index = index + non_lifetime_count;

                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,
                            track_lifetime_uses: true,
                            abstract_type_parent: false,
                        };
                        this.with(scope, |_old_scope, this| {
                            this.visit_generics(generics);
                            for bound in bounds {
                                this.visit_param_bound(bound);
                            }
                        });
                    });
                } else {
                    let scope = Scope::Binder {
                        lifetimes,
                        next_early_index,
                        s: self.scope,
                        track_lifetime_uses: true,
                        abstract_type_parent: false,
                    };
                    self.with(scope, |_old_scope, this| {
                        this.visit_generics(generics);
                        for bound in bounds {
                            this.visit_param_bound(bound);
                        }
                    });
                }
            }
            hir::TyKind::CVarArgs(ref lt) => {
                // Resolve the generated lifetime for the C-variadic arguments.
                // The lifetime is generated in AST -> HIR lowering.
                if lt.name.is_elided() {
                    self.resolve_elided_lifetimes(vec![lt])
                }
            }
            _ => 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_item(trait_item.hir_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 mut non_lifetime_count = 0;
                let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
                    GenericParamKind::Lifetime { .. } => {
                        Some(Region::early(&self.tcx.hir(), &mut index, param))
                    }
                    GenericParamKind::Type { .. } |
                    GenericParamKind::Const { .. } => {
                        non_lifetime_count += 1;
                        None
                    }
                }).collect();
                let scope = Scope::Binder {
                    lifetimes,
                    next_early_index: index + non_lifetime_count,
                    s: self.scope,
                    track_lifetime_uses: true,
                    abstract_type_parent: true,
                };
                self.with(scope, |_old_scope, this| {
                    this.visit_generics(generics);
                    for bound in bounds {
                        this.visit_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_item(impl_item.hir_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();
                let mut non_lifetime_count = 0;
                debug!("visit_ty: index = {}", index);
                let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
                    GenericParamKind::Lifetime { .. } => {
                        Some(Region::early(&self.tcx.hir(), &mut index, param))
                    }
                    GenericParamKind::Const { .. } |
                    GenericParamKind::Type { .. } => {
                        non_lifetime_count += 1;
                        None
                    }
                }).collect();
                let scope = Scope::Binder {
                    lifetimes,
                    next_early_index: index + non_lifetime_count,
                    s: self.scope,
                    track_lifetime_uses: true,
                    abstract_type_parent: true,
                };
                self.with(scope, |_old_scope, this| {
                    this.visit_generics(generics);
                    this.visit_ty(ty);
                });
            }
            Existential(ref bounds) => {
                let generics = &impl_item.generics;
                let mut index = self.next_early_index();
                let mut next_early_index = index;
                debug!("visit_ty: index = {}", index);
                let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
                    GenericParamKind::Lifetime { .. } => {
                        Some(Region::early(&self.tcx.hir(), &mut index, param))
                    }
                    GenericParamKind::Type { .. } => {
                        next_early_index += 1;
                        None
                    }
                    GenericParamKind::Const { .. } => {
                        next_early_index += 1;
                        None
                    }
                }).collect();

                let scope = Scope::Binder {
                    lifetimes,
                    next_early_index,
                    s: self.scope,
                    track_lifetime_uses: true,
                    abstract_type_parent: true,
                };
                self.with(scope, |_old_scope, this| {
                    this.visit_generics(generics);
                    for bound in bounds {
                        this.visit_param_bound(bound);
                    }
                });
            }
            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(vec![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, _: hir::HirId) {
        for (i, segment) in path.segments.iter().enumerate() {
            let depth = path.segments.len() - i - 1;
            if let Some(ref args) = segment.args {
                self.visit_segment_args(path.res, depth, args);
            }
        }
    }

    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.params);
        for param in &generics.params {
            match param.kind {
                GenericParamKind::Lifetime { .. } => {}
                GenericParamKind::Type { ref default, .. } => {
                    walk_list!(self, visit_param_bound, &param.bounds);
                    if let Some(ref ty) = default {
                        self.visit_ty(&ty);
                    }
                }
                GenericParamKind::Const { ref ty, .. } => {
                    walk_list!(self, visit_param_bound, &param.bounds);
                    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,
                    ..
                }) => {
                    let lifetimes: FxHashMap<_, _> = bound_generic_params
                        .iter()
                        .filter_map(|param| match param.kind {
                            GenericParamKind::Lifetime { .. } => {
                                Some(Region::late(&self.tcx.hir(), param))
                            }
                            _ => None,
                        })
                        .collect();
                    if !lifetimes.is_empty() {
                        self.trait_ref_hack = true;
                        let next_early_index = self.next_early_index();
                        let scope = Scope::Binder {
                            lifetimes,
                            s: self.scope,
                            next_early_index,
                            track_lifetime_uses: true,
                            abstract_type_parent: false,
                        };
                        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_param_bound, bounds);
                        });
                        self.trait_ref_hack = false;
                        result
                    } else {
                        self.visit_ty(&bounded_ty);
                        walk_list!(self, visit_param_bound, bounds);
                    }
                }
                &hir::WherePredicate::RegionPredicate(hir::WhereRegionPredicate {
                    ref lifetime,
                    ref bounds,
                    ..
                }) => {
                    self.visit_lifetime(lifetime);
                    walk_list!(self, visit_param_bound, bounds);
                }
                &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(|param| {
            match param.kind {
                GenericParamKind::Lifetime { .. } => true,
                _ => false,
            }
        }) {
            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
                    .iter()
                    .filter_map(|param| match param.kind {
                        GenericParamKind::Lifetime { .. } => {
                            Some(Region::late(&self.tcx.hir(), param))
                        }
                        _ => None,
                    })
                    .collect(),
                s: self.scope,
                next_early_index,
                track_lifetime_uses: true,
                abstract_type_parent: false,
            };
            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(param: &hir::GenericParam) -> Shadower {
    Shadower {
        kind: ShadowKind::Lifetime,
        span: param.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<'_>, params: &P<[hir::GenericParam]>) {
    let lifetime_params: Vec<_> = params
        .iter()
        .filter_map(|param| match param.kind {
            GenericParamKind::Lifetime { kind, .. } => Some((kind, param.span)),
            _ => None,
        })
        .collect();
    let explicit = lifetime_params
        .iter()
        .find(|(kind, _)| *kind == LifetimeParamKind::Explicit);
    let in_band = lifetime_params
        .iter()
        .find(|(kind, _)| *kind == LifetimeParamKind::InBand);

    if let (Some((_, explicit_span)), Some((_, in_band_span))) = (explicit, in_band) {
        struct_span_err!(
            tcx.sess,
            *in_band_span,
            E0688,
            "cannot mix in-band and explicit lifetime definitions"
        ).span_label(*in_band_span, "in-band lifetime definition here")
            .span_label(*explicit_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> {
        tcx: TyCtxt<'tcx>,
        scope: ScopeRef<'a>,
        labels_in_fn: &'a mut Vec<ast::Ident>,
    }

    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) = expression_label(ex) {
                for prior_label in &self.labels_in_fn[..] {
                    // FIXME (#24278): non-hygienic comparison
                    if label.name == prior_label.name {
                        signal_shadowing_problem(
                            self.tcx,
                            label.name,
                            original_label(prior_label.span),
                            shadower_label(label.span),
                        );
                    }
                }

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

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

    fn expression_label(ex: &hir::Expr) -> Option<ast::Ident> {
        if let hir::ExprKind::Loop(_, Some(label), _) = ex.node {
            Some(label.ident)
        } else {
            None
        }
    }

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

                Scope::Root => {
                    return;
                }

                Scope::Binder {
                    ref lifetimes, s, ..
                } => {
                    // FIXME (#24278): non-hygienic comparison
                    if let Some(def) = lifetimes.get(&hir::ParamName::Plain(label.modern())) {
                        let hir_id = tcx.hir().as_local_hir_id(def.id().unwrap()).unwrap();

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

fn compute_object_lifetime_defaults(tcx: TyCtxt<'_>) -> HirIdMap<Vec<ObjectLifetimeDefault>> {
    let mut map = HirIdMap::default();
    for item in tcx.hir().krate().items.values() {
        match item.node {
            hir::ItemKind::Struct(_, ref generics)
            | hir::ItemKind::Union(_, ref generics)
            | hir::ItemKind::Enum(_, ref generics)
            | hir::ItemKind::Existential(hir::ExistTy {
                ref generics,
                impl_trait_fn: None,
                ..
            })
            | hir::ItemKind::Ty(_, ref generics)
            | hir::ItemKind::Trait(_, _, ref generics, ..) => {
                let result = object_lifetime_defaults_for_item(tcx, generics);

                // Debugging aid.
                if attr::contains_name(&item.attrs, sym::rustc_object_lifetime_default) {
                    let object_lifetime_default_reprs: String = result
                        .iter()
                        .map(|set| match *set {
                            Set1::Empty => "BaseDefault".into(),
                            Set1::One(Region::Static) => "'static".into(),
                            Set1::One(Region::EarlyBound(mut i, _, _)) => generics
                                .params
                                .iter()
                                .find_map(|param| match param.kind {
                                    GenericParamKind::Lifetime { .. } => {
                                        if i == 0 {
                                            return Some(param.name.ident().to_string().into());
                                        }
                                        i -= 1;
                                        None
                                    }
                                    _ => None,
                                })
                                .unwrap(),
                            Set1::One(_) => bug!(),
                            Set1::Many => "Ambiguous".into(),
                        })
                        .collect::<Vec<Cow<'static, str>>>()
                        .join(",");
                    tcx.sess.span_err(item.span, &object_lifetime_default_reprs);
                }

                map.insert(item.hir_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::GenericBound]) {
        for bound in bounds {
            if let hir::GenericBound::Outlives(ref lifetime) = *bound {
                set.insert(lifetime.name.modern());
            }
        }
    }

    generics
        .params
        .iter()
        .filter_map(|param| match param.kind {
            GenericParamKind::Lifetime { .. } => None,
            GenericParamKind::Type { .. } => {
                let mut set = Set1::Empty;

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

                let param_def_id = tcx.hir().local_def_id(param.hir_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 res = match data.bounded_ty.node {
                        hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) => path.res,
                        _ => continue,
                    };

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

                Some(match set {
                    Set1::Empty => Set1::Empty,
                    Set1::One(name) => {
                        if name == hir::LifetimeName::Static {
                            Set1::One(Region::Static)
                        } else {
                            generics
                                .params
                                .iter()
                                .filter_map(|param| match param.kind {
                                    GenericParamKind::Lifetime { .. } => Some((
                                        param.hir_id,
                                        hir::LifetimeName::Param(param.name),
                                        LifetimeDefOrigin::from_param(param),
                                    )),
                                    _ => None,
                                })
                                .enumerate()
                                .find(|&(_, (_, lt_name, _))| lt_name == name)
                                .map_or(Set1::Many, |(i, (id, _, origin))| {
                                    let def_id = tcx.hir().local_def_id(id);
                                    Set1::One(Region::EarlyBound(i as u32, def_id, origin))
                                })
                        }
                    }
                    Set1::Many => Set1::Many,
                })
            }
            GenericParamKind::Const { .. } => {
                // Generic consts don't impose any constraints.
                None
            }
        })
        .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,
            map,
            lifetime_uses,
            ..
        } = self;
        let labels_in_fn = take(&mut self.labels_in_fn);
        let xcrate_object_lifetime_defaults = take(&mut self.xcrate_object_lifetime_defaults);
        let mut this = LifetimeContext {
            tcx: *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: lifetime_uses,
        };
        debug!("entering scope {:?}", this.scope);
        f(self.scope, &mut this);
        this.check_uses_for_lifetimes_defined_by_scope();
        debug!("exiting scope {:?}", this.scope);
        self.labels_in_fn = this.labels_in_fn;
        self.xcrate_object_lifetime_defaults = this.xcrate_object_lifetime_defaults;
    }

    /// helper method to determine the span to remove when suggesting the
    /// deletion of a lifetime
    fn lifetime_deletion_span(&self, name: ast::Ident, generics: &hir::Generics) -> Option<Span> {
        generics.params.iter().enumerate().find_map(|(i, param)| {
            if param.name.ident() == name {
                let mut in_band = false;
                if let hir::GenericParamKind::Lifetime { kind } = param.kind {
                    if let hir::LifetimeParamKind::InBand = kind {
                        in_band = true;
                    }
                }
                if in_band {
                    Some(param.span)
                } else {
                    if generics.params.len() == 1 {
                        // if sole lifetime, remove the entire `<>` brackets
                        Some(generics.span)
                    } else {
                        // if removing within `<>` brackets, we also want to
                        // delete a leading or trailing comma as appropriate
                        if i >= generics.params.len() - 1 {
                            Some(generics.params[i - 1].span.shrink_to_hi().to(param.span))
                        } else {
                            Some(param.span.to(generics.params[i + 1].span.shrink_to_lo()))
                        }
                    }
                }
            } else {
                None
            }
        })
    }

    // helper method to issue suggestions from `fn rah<'a>(&'a T)` to `fn rah(&T)`
    // or from `fn rah<'a>(T<'a>)` to `fn rah(T<'_>)`
    fn suggest_eliding_single_use_lifetime(
        &self, err: &mut DiagnosticBuilder<'_>, def_id: DefId, lifetime: &hir::Lifetime
    ) {
        let name = lifetime.name.ident();
        let mut remove_decl = None;
        if let Some(parent_def_id) = self.tcx.parent(def_id) {
            if let Some(generics) = self.tcx.hir().get_generics(parent_def_id) {
                remove_decl = self.lifetime_deletion_span(name, generics);
            }
        }

        let mut remove_use = None;
        let mut elide_use = None;
        let mut find_arg_use_span = |inputs: &hir::HirVec<hir::Ty>| {
            for input in inputs {
                match input.node {
                    hir::TyKind::Rptr(lt, _) => {
                        if lt.name.ident() == name {
                            // include the trailing whitespace between the lifetime and type names
                            let lt_through_ty_span = lifetime.span.to(input.span.shrink_to_hi());
                            remove_use = Some(
                                self.tcx.sess.source_map()
                                    .span_until_non_whitespace(lt_through_ty_span)
                            );
                            break;
                        }
                    }
                    hir::TyKind::Path(ref qpath) => {
                        if let QPath::Resolved(_, path) = qpath {

                            let last_segment = &path.segments[path.segments.len()-1];
                            let generics = last_segment.generic_args();
                            for arg in generics.args.iter() {
                                if let GenericArg::Lifetime(lt) = arg {
                                    if lt.name.ident() == name {
                                        elide_use = Some(lt.span);
                                        break;
                                    }
                                }
                            }
                            break;
                        }
                    },
                    _ => {}
                }
            }
        };
        if let Node::Lifetime(hir_lifetime) = self.tcx.hir().get(lifetime.hir_id) {
            if let Some(parent) = self.tcx.hir().find(
                self.tcx.hir().get_parent_item(hir_lifetime.hir_id))
            {
                match parent {
                    Node::Item(item) => {
                        if let hir::ItemKind::Fn(decl, _, _, _) = &item.node {
                            find_arg_use_span(&decl.inputs);
                        }
                    },
                    Node::ImplItem(impl_item) => {
                        if let hir::ImplItemKind::Method(sig, _) = &impl_item.node {
                            find_arg_use_span(&sig.decl.inputs);
                        }
                    }
                    _ => {}
                }
            }
        }

        let msg = "elide the single-use lifetime";
        match (remove_decl, remove_use, elide_use) {
            (Some(decl_span), Some(use_span), None) => {
                // if both declaration and use deletion spans start at the same
                // place ("start at" because the latter includes trailing
                // whitespace), then this is an in-band lifetime
                if decl_span.shrink_to_lo() == use_span.shrink_to_lo() {
                    err.span_suggestion(
                        use_span,
                        msg,
                        String::new(),
                        Applicability::MachineApplicable,
                    );
                } else {
                    err.multipart_suggestion(
                        msg,
                        vec![(decl_span, String::new()), (use_span, String::new())],
                        Applicability::MachineApplicable,
                    );
                }
            }
            (Some(decl_span), None, Some(use_span)) => {
                err.multipart_suggestion(
                    msg,
                    vec![(decl_span, String::new()), (use_span, "'_".to_owned())],
                    Applicability::MachineApplicable,
                );
            }
            _ => {}
        }
    }

    fn check_uses_for_lifetimes_defined_by_scope(&mut self) {
        let defined_by = match self.scope {
            Scope::Binder { lifetimes, .. } => lifetimes,
            _ => {
                debug!("check_uses_for_lifetimes_defined_by_scope: not in a binder scope");
                return;
            }
        };

        let mut def_ids: Vec<_> = defined_by
            .values()
            .flat_map(|region| match region {
                Region::EarlyBound(_, def_id, _)
                | Region::LateBound(_, def_id, _)
                | Region::Free(_, def_id) => Some(*def_id),

                Region::LateBoundAnon(..) | Region::Static => None,
            })
            .collect();

        // ensure that we issue lints in a repeatable order
        def_ids.sort_by_cached_key(|&def_id| self.tcx.def_path_hash(def_id));

        for def_id in def_ids {
            debug!(
                "check_uses_for_lifetimes_defined_by_scope: def_id = {:?}",
                def_id
            );

            let lifetimeuseset = self.lifetime_uses.remove(&def_id);

            debug!(
                "check_uses_for_lifetimes_defined_by_scope: lifetimeuseset = {:?}",
                lifetimeuseset
            );

            match lifetimeuseset {
                Some(LifetimeUseSet::One(lifetime)) => {
                    let hir_id = self.tcx.hir().as_local_hir_id(def_id).unwrap();
                    debug!("hir id first={:?}", hir_id);
                    if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
                        Node::Lifetime(hir_lifetime) => Some((
                            hir_lifetime.hir_id,
                            hir_lifetime.span,
                            hir_lifetime.name.ident(),
                        )),
                        Node::GenericParam(param) => {
                            Some((param.hir_id, param.span, param.name.ident()))
                        }
                        _ => None,
                    } {
                        debug!("id = {:?} span = {:?} name = {:?}", id, span, name);

                        if name.name == kw::UnderscoreLifetime {
                            continue;
                        }

                        if let Some(parent_def_id) = self.tcx.parent(def_id) {
                            if let Some(parent_hir_id) = self.tcx.hir()
                                .as_local_hir_id(parent_def_id) {
                                    // lifetimes in `derive` expansions don't count (Issue #53738)
                                    if self.tcx.hir().attrs(parent_hir_id).iter()
                                        .any(|attr| attr.check_name(sym::automatically_derived)) {
                                            continue;
                                        }
                                }
                        }

                        let mut err = self.tcx.struct_span_lint_hir(
                            lint::builtin::SINGLE_USE_LIFETIMES,
                            id,
                            span,
                            &format!("lifetime parameter `{}` only used once", name),
                        );

                        if span == lifetime.span {
                            // spans are the same for in-band lifetime declarations
                            err.span_label(span, "this lifetime is only used here");
                        } else {
                            err.span_label(span, "this lifetime...");
                            err.span_label(lifetime.span, "...is used only here");
                        }
                        self.suggest_eliding_single_use_lifetime(&mut err, def_id, lifetime);
                        err.emit();
                    }
                }
                Some(LifetimeUseSet::Many) => {
                    debug!("not one use lifetime");
                }
                None => {
                    let hir_id = self.tcx.hir().as_local_hir_id(def_id).unwrap();
                    if let Some((id, span, name)) = match self.tcx.hir().get(hir_id) {
                        Node::Lifetime(hir_lifetime) => Some((
                            hir_lifetime.hir_id,
                            hir_lifetime.span,
                            hir_lifetime.name.ident(),
                        )),
                        Node::GenericParam(param) => {
                            Some((param.hir_id, param.span, param.name.ident()))
                        }
                        _ => None,
                    } {
                        debug!("id ={:?} span = {:?} name = {:?}", id, span, name);
                        let mut err = self.tcx.struct_span_lint_hir(
                            lint::builtin::UNUSED_LIFETIMES,
                            id,
                            span,
                            &format!("lifetime parameter `{}` never used", name),
                        );
                        if let Some(parent_def_id) = self.tcx.parent(def_id) {
                            if let Some(generics) = self.tcx.hir().get_generics(parent_def_id) {
                                let unused_lt_span = self.lifetime_deletion_span(name, generics);
                                if let Some(span) = unused_lt_span {
                                    err.span_suggestion(
                                        span,
                                        "elide the unused lifetime",
                                        String::new(),
                                        Applicability::MachineApplicable,
                                    );
                                }
                            }
                        }
                        err.emit();
                    }
                }
            }
        }
    }

    /// 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<hir::HirId>,
        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 sub_items_have_self_param(&parent.node) {
                index += 1; // Self comes before lifetimes
            }
            match parent.node {
                hir::ItemKind::Trait(_, _, ref generics, ..)
                | hir::ItemKind::Impl(_, _, _, ref generics, ..) => {
                    index += generics.params.len() as u32;
                }
                _ => {}
            }
        }

        let mut non_lifetime_count = 0;
        let lifetimes = generics.params.iter().filter_map(|param| match param.kind {
            GenericParamKind::Lifetime { .. } => {
                if self.map.late_bound.contains(&param.hir_id) {
                    Some(Region::late(&self.tcx.hir(), param))
                } else {
                    Some(Region::early(&self.tcx.hir(), &mut index, param))
                }
            }
            GenericParamKind::Type { .. } |
            GenericParamKind::Const { .. } => {
                non_lifetime_count += 1;
                None
            }
        }).collect();
        let next_early_index = index + non_lifetime_count;

        let scope = Scope::Binder {
            lifetimes,
            next_early_index,
            s: self.scope,
            abstract_type_parent: true,
            track_lifetime_uses: false,
        };
        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)`
        });
    }

    fn next_early_index_helper(&self, only_abstract_type_parent: bool) -> u32 {
        let mut scope = self.scope;
        loop {
            match *scope {
                Scope::Root => return 0,

                Scope::Binder {
                    next_early_index,
                    abstract_type_parent,
                    ..
                } if (!only_abstract_type_parent || abstract_type_parent) =>
                {
                    return next_early_index
                }

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

    /// Returns the next index one would use for an early-bound-region
    /// if extending the current scope.
    fn next_early_index(&self) -> u32 {
        self.next_early_index_helper(true)
    }

    /// Returns the next index one would use for an `impl Trait` that
    /// is being converted into an `abstract type`. This will be the
    /// next early index from the enclosing item, for the most
    /// part. See the `abstract_type_parent` field for more info.
    fn next_early_index_for_abstract_type(&self) -> u32 {
        self.next_early_index_helper(false)
    }

    fn resolve_lifetime_ref(&mut self, lifetime_ref: &'tcx hir::Lifetime) {
        debug!("resolve_lifetime_ref(lifetime_ref={:?})", lifetime_ref);

        // If we've already reported an error, just ignore `lifetime_ref`.
        if let LifetimeName::Error = lifetime_ref.name {
            return;
        }

        // 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, ..
                } => {
                    match lifetime_ref.name {
                        LifetimeName::Param(param_name) => {
                            if let Some(&def) = lifetimes.get(&param_name.modern()) {
                                break Some(def.shifted(late_depth));
                            }
                        }
                        _ => bug!("expected LifetimeName::Param"),
                    }

                    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) {
                    Node::Item(&hir::Item {
                        node: hir::ItemKind::Fn(..),
                        ..
                    })
                    | Node::TraitItem(&hir::TraitItem {
                        node: hir::TraitItemKind::Method(..),
                        ..
                    })
                    | Node::ImplItem(&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::ExplicitOrElided)
                    | Region::LateBound(_, _, LifetimeDefOrigin::ExplicitOrElided)
                    | Region::EarlyBound(_, _, LifetimeDefOrigin::Error)
                    | Region::LateBound(_, _, LifetimeDefOrigin::Error)
                    | 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
            ).span_label(lifetime_ref.span, "undeclared lifetime")
                .emit();
        }
    }

    fn visit_segment_args(&mut self, res: Res, depth: usize, generic_args: &'tcx hir::GenericArgs) {
        if generic_args.parenthesized {
            let was_in_fn_syntax = self.is_in_fn_syntax;
            self.is_in_fn_syntax = true;
            self.visit_fn_like_elision(generic_args.inputs(), Some(generic_args.bindings[0].ty()));
            self.is_in_fn_syntax = was_in_fn_syntax;
            return;
        }

        let mut elide_lifetimes = true;
        let lifetimes = generic_args
            .args
            .iter()
            .filter_map(|arg| match arg {
                hir::GenericArg::Lifetime(lt) => {
                    if !lt.is_elided() {
                        elide_lifetimes = false;
                    }
                    Some(lt)
                }
                _ => None,
            })
            .collect();
        if elide_lifetimes {
            self.resolve_elided_lifetimes(lifetimes);
        } else {
            lifetimes.iter().for_each(|lt| self.visit_lifetime(lt));
        }

        // 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 res {
            Res::Def(DefKind::AssocTy, def_id)
                if depth == 1 => Some(parent_def_id(self, def_id)),
            Res::Def(DefKind::Variant, def_id)
                if depth == 0 => Some(parent_def_id(self, def_id)),
            Res::Def(DefKind::Struct, def_id)
            | Res::Def(DefKind::Union, def_id)
            | Res::Def(DefKind::Enum, def_id)
            | Res::Def(DefKind::TyAlias, def_id)
            | Res::Def(DefKind::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_hir_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)
                            .params
                            .iter()
                            .filter_map(|param| match param.kind {
                                GenericParamDefKind::Type {
                                    object_lifetime_default,
                                    ..
                                } => Some(object_lifetime_default),
                                GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
                            })
                            .collect()
                    })
            };
            unsubst
                .iter()
                .map(|set| match *set {
                    Set1::Empty => if in_body {
                        None
                    } else {
                        Some(Region::Static)
                    },
                    Set1::One(r) => {
                        let lifetimes = generic_args.args.iter().filter_map(|arg| match arg {
                            GenericArg::Lifetime(lt) => Some(lt),
                            _ => None,
                        });
                        r.subst(lifetimes, map)
                    }
                    Set1::Many => None,
                })
                .collect()
        });

        let mut i = 0;
        for arg in &generic_args.args {
            match arg {
                GenericArg::Lifetime(_) => {}
                GenericArg::Type(ty) => {
                    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);
                    }
                    i += 1;
                }
                GenericArg::Const(ct) => {
                    self.visit_anon_const(&ct.value);
                }
            }
        }

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

    fn visit_fn_like_elision(&mut self, inputs: &'tcx [hir::Ty], output: Option<&'tcx hir::Ty>) {
        debug!("visit_fn_like_elision: enter");
        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,
        };

        debug!("visit_fn_like_elision: determine output");

        // 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.hir_id);
        let body = match self.tcx.hir().get(parent) {
            // `fn` definitions and methods.
            Node::Item(&hir::Item {
                node: hir::ItemKind::Fn(.., body),
                ..
            }) => Some(body),

            Node::TraitItem(&hir::TraitItem {
                node: hir::TraitItemKind::Method(_, ref m),
                ..
            }) => {
                if let hir::ItemKind::Trait(.., ref trait_items) = self.tcx
                    .hir()
                    .expect_item(self.tcx.hir().get_parent_item(parent))
                    .node
                {
                    assoc_item_kind = trait_items
                        .iter()
                        .find(|ti| ti.id.hir_id == parent)
                        .map(|ti| ti.kind);
                }
                match *m {
                    hir::TraitMethod::Required(_) => None,
                    hir::TraitMethod::Provided(body) => Some(body),
                }
            }

            Node::ImplItem(&hir::ImplItem {
                node: hir::ImplItemKind::Method(_, body),
                ..
            }) => {
                if let hir::ItemKind::Impl(.., ref self_ty, ref impl_items) = self.tcx
                    .hir()
                    .expect_item(self.tcx.hir().get_parent_item(parent))
                    .node
                {
                    impl_self = Some(self_ty);
                    assoc_item_kind = impl_items
                        .iter()
                        .find(|ii| ii.id.hir_id == parent)
                        .map(|ii| ii.kind);
                }
                Some(body)
            }

            // Foreign functions, `fn(...) -> R` and `Trait(...) -> R` (both types and bounds).
            Node::ForeignItem(_) | Node::Ty(_) | Node::TraitRef(_) => 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::AssocItemKind::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 {
            struct SelfVisitor<'a> {
                map: &'a NamedRegionMap,
                impl_self: Option<&'a hir::TyKind>,
                lifetime: Set1<Region>,
            }

            impl SelfVisitor<'_> {
                // Look for `self: &'a Self` - also desugared from `&'a self`,
                // and if that matches, use it for elision and return early.
                fn is_self_ty(&self, res: Res) -> bool {
                    if let Res::SelfTy(..) = res {
                        return true;
                    }

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

                    false
                }
            }

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

                fn visit_ty(&mut self, ty: &'a hir::Ty) {
                    if let hir::TyKind::Rptr(lifetime_ref, ref mt) = ty.node {
                        if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) = mt.ty.node
                        {
                            if self.is_self_ty(path.res) {
                                if let Some(lifetime) = self.map.defs.get(&lifetime_ref.hir_id) {
                                    self.lifetime.insert(*lifetime);
                                }
                            }
                        }
                    }
                    intravisit::walk_ty(self, ty)
                }
            }

            let mut visitor = SelfVisitor {
                map: self.map,
                impl_self: impl_self.map(|ty| &ty.node),
                lifetime: Set1::Empty,
            };
            visitor.visit_ty(&inputs[0]);
            if let Set1::One(lifetime) = visitor.lifetime {
                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,
                    outer_index: ty::INNERMOST,
                    have_bound_regions: false,
                    lifetimes: Default::default(),
                };
                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)
        };

        debug!("visit_fn_like_elision: elide={:?}", elide);

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

        struct GatherLifetimes<'a> {
            map: &'a NamedRegionMap,
            outer_index: ty::DebruijnIndex,
            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::TyKind::BareFn(_) = ty.node {
                    self.outer_index.shift_in(1);
                }
                match ty.node {
                    hir::TyKind::TraitObject(ref bounds, ref lifetime) => {
                        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);
                        }
                    }
                    hir::TyKind::CVarArgs(_) => {}
                    _ => {
                        intravisit::walk_ty(self, ty);
                    }
                }
                if let hir::TyKind::BareFn(_) = ty.node {
                    self.outer_index.shift_out(1);
                }
            }

            fn visit_generic_param(&mut self, param: &hir::GenericParam) {
                if let hir::GenericParamKind::Lifetime { .. } = param.kind {
                    // FIXME(eddyb) Do we want this? It only makes a difference
                    // if this `for<'a>` lifetime parameter is never used.
                    self.have_bound_regions = true;
                }

                intravisit::walk_generic_param(self, param);
            }

            fn visit_poly_trait_ref(
                &mut self,
                trait_ref: &hir::PolyTraitRef,
                modifier: hir::TraitBoundModifier,
            ) {
                self.outer_index.shift_in(1);
                intravisit::walk_poly_trait_ref(self, trait_ref, modifier);
                self.outer_index.shift_out(1);
            }

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

    fn resolve_elided_lifetimes(&mut self, lifetime_refs: Vec<&'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 mut lifetime_names = FxHashSet::default();
        let error = loop {
            match *scope {
                // Do not assign any resolution, it will be inferred.
                Scope::Body { .. } => return,

                Scope::Root => break None,

                Scope::Binder { s, ref lifetimes, .. } => {
                    // collect named lifetimes for suggestions
                    for name in lifetimes.keys() {
                        if let hir::ParamName::Plain(name) = name {
                            lifetime_names.insert(*name);
                        }
                    }
                    late_depth += 1;
                    scope = s;
                }

                Scope::Elision { ref elide, ref s, .. } => {
                    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) => {
                            if let Scope::Binder { ref lifetimes, .. } = s {
                                // collect named lifetimes for suggestions
                                for name in lifetimes.keys() {
                                    if let hir::ParamName::Plain(name) = name {
                                        lifetime_names.insert(*name);
                                    }
                                }
                            }
                            break Some(e);
                        }
                    };
                    for lifetime_ref in lifetime_refs {
                        self.insert_lifetime(lifetime_ref, lifetime);
                    }
                    return;
                }

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

        let mut err = report_missing_lifetime_specifiers(self.tcx.sess, span, lifetime_refs.len());
        let mut add_label = true;

        if let Some(params) = error {
            if lifetime_refs.len() == 1 {
                add_label = add_label && self.report_elision_failure(&mut err, params, span);
            }
        }
        if add_label {
            add_missing_lifetime_specifiers_label(
                &mut err,
                span,
                lifetime_refs.len(),
                &lifetime_names,
                self.tcx.sess.source_map().span_to_snippet(span).ok().as_ref().map(|s| s.as_str()),
            );
        }

        err.emit();
    }

    fn suggest_lifetime(&self, db: &mut DiagnosticBuilder<'_>, span: Span, msg: &str) -> bool {
        match self.tcx.sess.source_map().span_to_snippet(span) {
            Ok(ref snippet) => {
                let (sugg, applicability) = if snippet == "&" {
                    ("&'static ".to_owned(), Applicability::MachineApplicable)
                } else if snippet == "'_" {
                    ("'static".to_owned(), Applicability::MachineApplicable)
                } else {
                    (format!("{} + 'static", snippet), Applicability::MaybeIncorrect)
                };
                db.span_suggestion(span, msg, sugg, applicability);
                false
            }
            Err(_) => {
                db.help(msg);
                true
            }
        }
    }

    fn report_elision_failure(
        &mut self,
        db: &mut DiagnosticBuilder<'_>,
        params: &[ElisionFailureInfo],
        span: Span,
    ) -> bool {
        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(ident) = parent.and_then(|body| {
                self.tcx.hir().body(body).arguments[index].pat.simple_ident()
            }) {
                format!("`{}`", ident)
            } 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"
            );
            self.suggest_lifetime(db, span, "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"
            );
            let msg = "consider giving it an explicit bounded or 'static lifetime";
            self.suggest_lifetime(db, span, msg)
        } 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
            );
            true
        } else {
            help!(
                db,
                "this function's return type contains a borrowed value, but \
                 the signature does not say whether it is borrowed from {}",
                m
            );
            true
        }
    }

    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],
    ) {
        let lifetimes: Vec<_> = params
            .iter()
            .filter_map(|param| match param.kind {
                GenericParamKind::Lifetime { .. } => Some((param, param.name)),
                _ => None,
            })
            .collect();
        for (i, (lifetime_i, lifetime_i_name)) in lifetimes.iter().enumerate() {
            if let hir::ParamName::Plain(_) = lifetime_i_name {
                let name = lifetime_i_name.ident().name;
                if name == kw::UnderscoreLifetime
                    || name == kw::StaticLifetime
                {
                    let mut err = struct_span_err!(
                        self.tcx.sess,
                        lifetime_i.span,
                        E0262,
                        "invalid lifetime parameter name: `{}`",
                        lifetime_i.name.ident(),
                    );
                    err.span_label(
                        lifetime_i.span,
                        format!("{} is a reserved lifetime name", name),
                    );
                    err.emit();
                }
            }

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

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

            for bound in &lifetime_i.bounds {
                match bound {
                    hir::GenericBound::Outlives(lt) => match lt.name {
                        hir::LifetimeName::Underscore => self.tcx.sess.delay_span_bug(
                            lt.span,
                            "use of `'_` in illegal place, but not caught by lowering",
                        ),
                        hir::LifetimeName::Static => {
                            self.insert_lifetime(lt, Region::Static);
                            self.tcx
                                .sess
                                .struct_span_warn(
                                    lifetime_i.span.to(lt.span),
                                    &format!(
                                        "unnecessary lifetime parameter `{}`",
                                        lifetime_i.name.ident(),
                                    ),
                                )
                                .help(&format!(
                                    "you can use the `'static` lifetime directly, in place of `{}`",
                                    lifetime_i.name.ident(),
                                ))
                                .emit();
                        }
                        hir::LifetimeName::Param(_) | hir::LifetimeName::Implicit => {
                            self.resolve_lifetime_ref(lt);
                        }
                        hir::LifetimeName::Error => {
                            // No need to do anything, error already reported.
                        }
                    },
                    _ => bug!(),
                }
            }
        }
    }

    fn check_lifetime_param_for_shadowing(
        &self,
        mut old_scope: ScopeRef<'_>,
        param: &'tcx hir::GenericParam,
    ) {
        for label in &self.labels_in_fn {
            // FIXME (#24278): non-hygienic comparison
            if param.name.ident().name == label.name {
                signal_shadowing_problem(
                    self.tcx,
                    label.name,
                    original_label(label.span),
                    shadower_lifetime(&param),
                );
                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, ..
                } => {
                    if let Some(&def) = lifetimes.get(&param.name.modern()) {
                        let hir_id = self.tcx.hir().as_local_hir_id(def.id().unwrap()).unwrap();

                        signal_shadowing_problem(
                            self.tcx,
                            param.name.ident().name,
                            original_lifetime(self.tcx.hir().span(hir_id)),
                            shadower_lifetime(&param),
                        );
                        return;
                    }

                    old_scope = s;
                }
            }
        }
    }

    /// Returns `true` if, in the current scope, replacing `'_` would be
    /// equivalent to a single-use lifetime.
    fn track_lifetime_uses(&self) -> bool {
        let mut scope = self.scope;
        loop {
            match *scope {
                Scope::Root => break false,

                // Inside of items, it depends on the kind of item.
                Scope::Binder {
                    track_lifetime_uses,
                    ..
                } => break track_lifetime_uses,

                // Inside a body, `'_` will use an inference variable,
                // should be fine.
                Scope::Body { .. } => break true,

                // A lifetime only used in a fn argument could as well
                // be replaced with `'_`, as that would generate a
                // fresh name, too.
                Scope::Elision {
                    elide: Elide::FreshLateAnon(_),
                    ..
                } => break true,

                // In the return type or other such place, `'_` is not
                // going to make a fresh name, so we cannot
                // necessarily replace a single-use lifetime with
                // `'_`.
                Scope::Elision {
                    elide: Elide::Exact(_),
                    ..
                } => break false,
                Scope::Elision {
                    elide: Elide::Error(_),
                    ..
                } => break false,

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

    fn insert_lifetime(&mut self, lifetime_ref: &'tcx hir::Lifetime, def: Region) {
        if lifetime_ref.hir_id == hir::DUMMY_HIR_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.hir_id),
            def,
            self.tcx.sess.source_map().span_to_string(lifetime_ref.span)
        );
        self.map.defs.insert(lifetime_ref.hir_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.
                let track_lifetime_uses = self.track_lifetime_uses();
                debug!(
                    "insert_lifetime: track_lifetime_uses={}",
                    track_lifetime_uses
                );
                if track_lifetime_uses && !self.lifetime_uses.contains_key(&def_id) {
                    debug!("insert_lifetime: first use of {:?}", def_id);
                    self.lifetime_uses
                        .insert(def_id, LifetimeUseSet::One(lifetime_ref));
                } else {
                    debug!("insert_lifetime: many uses of {:?}", def_id);
                    self.lifetime_uses.insert(def_id, LifetimeUseSet::Many);
                }
            }
        }
    }

    /// Sometimes we resolve a lifetime, but later find that it is an
    /// error (esp. around impl trait). In that case, we remove the
    /// entry into `map.defs` so as not to confuse later code.
    fn uninsert_lifetime_on_error(&mut self, lifetime_ref: &'tcx hir::Lifetime, bad_def: Region) {
        let old_value = self.map.defs.remove(&lifetime_ref.hir_id);
        assert_eq!(old_value, Some(bad_def));
    }
}

/// 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::default();
    for arg_ty in &decl.inputs {
        constrained_by_input.visit_ty(arg_ty);
    }

    let mut appears_in_output = AllCollector::default();
    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::default();
    appears_in_where_clause.visit_generics(generics);

    for param in &generics.params {
        if let hir::GenericParamKind::Lifetime { .. } = param.kind {
            if !param.bounds.is_empty() {
                // `'a: 'b` means both `'a` and `'b` are referenced
                appears_in_where_clause
                    .regions
                    .insert(hir::LifetimeName::Param(param.name.modern()));
            }
        }
    }

    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 param in &generics.params {
        match param.kind {
            hir::GenericParamKind::Lifetime { .. } => { /* fall through */ }

            // Neither types nor consts are late-bound.
            hir::GenericParamKind::Type { .. }
            | hir::GenericParamKind::Const { .. } => continue,
        }

        let lt_name = hir::LifetimeName::Param(param.name.modern());
        // appears in the where clauses? early-bound.
        if appears_in_where_clause.regions.contains(&lt_name) {
            continue;
        }

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

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

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

    return;

    #[derive(Default)]
    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::TyKind::Path(hir::QPath::Resolved(Some(_), _))
                | hir::TyKind::Path(hir::QPath::TypeRelative(..)) => {
                    // ignore lifetimes appearing in associated type
                    // projections, as they are not *constrained*
                    // (defined above)
                }

                hir::TyKind::Path(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.modern());
        }
    }

    #[derive(Default)]
    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.modern());
        }
    }
}

pub fn report_missing_lifetime_specifiers(
    sess: &Session,
    span: Span,
    count: usize,
) -> DiagnosticBuilder<'_> {
    struct_span_err!(
        sess,
        span,
        E0106,
        "missing lifetime specifier{}",
        if count > 1 { "s" } else { "" }
    )
}

fn add_missing_lifetime_specifiers_label(
    err: &mut DiagnosticBuilder<'_>,
    span: Span,
    count: usize,
    lifetime_names: &FxHashSet<ast::Ident>,
    snippet: Option<&str>,
) {
    if count > 1 {
        err.span_label(span, format!("expected {} lifetime parameters", count));
    } else if let (1, Some(name), Some("&")) = (
        lifetime_names.len(),
        lifetime_names.iter().next(),
        snippet,
    ) {
        err.span_suggestion(
            span,
            "consider using the named lifetime",
            format!("&{} ", name),
            Applicability::MaybeIncorrect,
        );
    } else {
        err.span_label(span, "expected lifetime parameter");
    }
}