rustc: remove temporary lifetime extension by borrow hint

[rust.git] / src / librustc_typeck / check / regionck.rs

// Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

//! The region check is a final pass that runs over the AST after we have
//! inferred the type constraints but before we have actually finalized
//! the types.  Its purpose is to embed a variety of region constraints.
//! Inserting these constraints as a separate pass is good because (1) it
//! localizes the code that has to do with region inference and (2) often
//! we cannot know what constraints are needed until the basic types have
//! been inferred.
//!
//! ### Interaction with the borrow checker
//!
//! In general, the job of the borrowck module (which runs later) is to
//! check that all soundness criteria are met, given a particular set of
//! regions. The job of *this* module is to anticipate the needs of the
//! borrow checker and infer regions that will satisfy its requirements.
//! It is generally true that the inference doesn't need to be sound,
//! meaning that if there is a bug and we inferred bad regions, the borrow
//! checker should catch it. This is not entirely true though; for
//! example, the borrow checker doesn't check subtyping, and it doesn't
//! check that region pointers are always live when they are used. It
//! might be worthwhile to fix this so that borrowck serves as a kind of
//! verification step -- that would add confidence in the overall
//! correctness of the compiler, at the cost of duplicating some type
//! checks and effort.
//!
//! ### Inferring the duration of borrows, automatic and otherwise
//!
//! Whenever we introduce a borrowed pointer, for example as the result of
//! a borrow expression `let x = &data`, the lifetime of the pointer `x`
//! is always specified as a region inference variable. `regionck` has the
//! job of adding constraints such that this inference variable is as
//! narrow as possible while still accommodating all uses (that is, every
//! dereference of the resulting pointer must be within the lifetime).
//!
//! #### Reborrows
//!
//! Generally speaking, `regionck` does NOT try to ensure that the data
//! `data` will outlive the pointer `x`. That is the job of borrowck.  The
//! one exception is when "re-borrowing" the contents of another borrowed
//! pointer. For example, imagine you have a borrowed pointer `b` with
//! lifetime L1 and you have an expression `&*b`. The result of this
//! expression will be another borrowed pointer with lifetime L2 (which is
//! an inference variable). The borrow checker is going to enforce the
//! constraint that L2 < L1, because otherwise you are re-borrowing data
//! for a lifetime larger than the original loan.  However, without the
//! routines in this module, the region inferencer would not know of this
//! dependency and thus it might infer the lifetime of L2 to be greater
//! than L1 (issue #3148).
//!
//! There are a number of troublesome scenarios in the tests
//! `region-dependent-*.rs`, but here is one example:
//!
//!     struct Foo { i: i32 }
//!     struct Bar { foo: Foo  }
//!     fn get_i<'a>(x: &'a Bar) -> &'a i32 {
//!        let foo = &x.foo; // Lifetime L1
//!        &foo.i            // Lifetime L2
//!     }
//!
//! Note that this comes up either with `&` expressions, `ref`
//! bindings, and `autorefs`, which are the three ways to introduce
//! a borrow.
//!
//! The key point here is that when you are borrowing a value that
//! is "guaranteed" by a borrowed pointer, you must link the
//! lifetime of that borrowed pointer (L1, here) to the lifetime of
//! the borrow itself (L2).  What do I mean by "guaranteed" by a
//! borrowed pointer? I mean any data that is reached by first
//! dereferencing a borrowed pointer and then either traversing
//! interior offsets or boxes.  We say that the guarantor
//! of such data is the region of the borrowed pointer that was
//! traversed.  This is essentially the same as the ownership
//! relation, except that a borrowed pointer never owns its
//! contents.

use check::dropck;
use check::FnCtxt;
use middle::free_region::FreeRegionMap;
use middle::mem_categorization as mc;
use middle::mem_categorization::Categorization;
use middle::region::{CodeExtent, RegionMaps};
use rustc::hir::def_id::DefId;
use rustc::ty::subst::Substs;
use rustc::traits;
use rustc::ty::{self, Ty, TypeFoldable};
use rustc::infer::{self, GenericKind, SubregionOrigin, VerifyBound};
use rustc::ty::adjustment;
use rustc::ty::wf::ImpliedBound;

use std::mem;
use std::ops::Deref;
use std::rc::Rc;
use syntax::ast;
use syntax_pos::Span;
use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
use rustc::hir::{self, PatKind};

// a variation on try that just returns unit
macro_rules! ignore_err {
    ($e:expr) => (match $e { Ok(e) => e, Err(_) => return () })
}

///////////////////////////////////////////////////////////////////////////
// PUBLIC ENTRY POINTS

impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
    pub fn regionck_expr(&self, body: &'gcx hir::Body) {
        let subject = self.tcx.hir.body_owner_def_id(body.id());
        let id = body.value.id;
        let mut rcx = RegionCtxt::new(self, RepeatingScope(id), id, Subject(subject));
        if self.err_count_since_creation() == 0 {
            // regionck assumes typeck succeeded
            rcx.visit_body(body);
            rcx.visit_region_obligations(id);
        }
        rcx.resolve_regions_and_report_errors();

        assert!(self.tables.borrow().free_region_map.is_empty());
        self.tables.borrow_mut().free_region_map = rcx.free_region_map;
    }

    /// Region checking during the WF phase for items. `wf_tys` are the
    /// types from which we should derive implied bounds, if any.
    pub fn regionck_item(&self,
                         item_id: ast::NodeId,
                         span: Span,
                         wf_tys: &[Ty<'tcx>]) {
        debug!("regionck_item(item.id={:?}, wf_tys={:?}", item_id, wf_tys);
        let subject = self.tcx.hir.local_def_id(item_id);
        let mut rcx = RegionCtxt::new(self, RepeatingScope(item_id), item_id, Subject(subject));
        rcx.free_region_map.relate_free_regions_from_predicates(
            &self.param_env.caller_bounds);
        rcx.relate_free_regions(wf_tys, item_id, span);
        rcx.visit_region_obligations(item_id);
        rcx.resolve_regions_and_report_errors();
    }

    pub fn regionck_fn(&self,
                       fn_id: ast::NodeId,
                       body: &'gcx hir::Body) {
        debug!("regionck_fn(id={})", fn_id);
        let subject = self.tcx.hir.body_owner_def_id(body.id());
        let node_id = body.value.id;
        let mut rcx = RegionCtxt::new(self, RepeatingScope(node_id), node_id, Subject(subject));

        if self.err_count_since_creation() == 0 {
            // regionck assumes typeck succeeded
            rcx.visit_fn_body(fn_id, body, self.tcx.hir.span(fn_id));
        }

        rcx.free_region_map.relate_free_regions_from_predicates(
            &self.param_env.caller_bounds);

        rcx.resolve_regions_and_report_errors();

        // In this mode, we also copy the free-region-map into the
        // tables of the enclosing fcx. In the other regionck modes
        // (e.g., `regionck_item`), we don't have an enclosing tables.
        assert!(self.tables.borrow().free_region_map.is_empty());
        self.tables.borrow_mut().free_region_map = rcx.free_region_map;
    }
}

///////////////////////////////////////////////////////////////////////////
// INTERNALS

pub struct RegionCtxt<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
    pub fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,

    region_bound_pairs: Vec<(ty::Region<'tcx>, GenericKind<'tcx>)>,

    pub region_maps: Rc<RegionMaps>,

    free_region_map: FreeRegionMap<'tcx>,

    // id of innermost fn body id
    body_id: ast::NodeId,

    // call_site scope of innermost fn
    call_site_scope: Option<CodeExtent>,

    // id of innermost fn or loop
    repeating_scope: ast::NodeId,

    // id of AST node being analyzed (the subject of the analysis).
    subject_def_id: DefId,

}

impl<'a, 'gcx, 'tcx> Deref for RegionCtxt<'a, 'gcx, 'tcx> {
    type Target = FnCtxt<'a, 'gcx, 'tcx>;
    fn deref(&self) -> &Self::Target {
        &self.fcx
    }
}

pub struct RepeatingScope(ast::NodeId);
pub struct Subject(DefId);

impl<'a, 'gcx, 'tcx> RegionCtxt<'a, 'gcx, 'tcx> {
    pub fn new(fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
               RepeatingScope(initial_repeating_scope): RepeatingScope,
               initial_body_id: ast::NodeId,
               Subject(subject): Subject) -> RegionCtxt<'a, 'gcx, 'tcx> {
        let region_maps = fcx.tcx.region_maps(subject);
        RegionCtxt {
            fcx: fcx,
            region_maps: region_maps,
            repeating_scope: initial_repeating_scope,
            body_id: initial_body_id,
            call_site_scope: None,
            subject_def_id: subject,
            region_bound_pairs: Vec::new(),
            free_region_map: FreeRegionMap::new(),
        }
    }

    fn set_call_site_scope(&mut self, call_site_scope: Option<CodeExtent>)
                           -> Option<CodeExtent> {
        mem::replace(&mut self.call_site_scope, call_site_scope)
    }

    fn set_body_id(&mut self, body_id: ast::NodeId) -> ast::NodeId {
        mem::replace(&mut self.body_id, body_id)
    }

    fn set_repeating_scope(&mut self, scope: ast::NodeId) -> ast::NodeId {
        mem::replace(&mut self.repeating_scope, scope)
    }

    /// Try to resolve the type for the given node, returning t_err if an error results.  Note that
    /// we never care about the details of the error, the same error will be detected and reported
    /// in the writeback phase.
    ///
    /// Note one important point: we do not attempt to resolve *region variables* here.  This is
    /// because regionck is essentially adding constraints to those region variables and so may yet
    /// influence how they are resolved.
    ///
    /// Consider this silly example:
    ///
    /// ```
    /// fn borrow(x: &i32) -> &i32 {x}
    /// fn foo(x: @i32) -> i32 {  // block: B
    ///     let b = borrow(x);    // region: <R0>
    ///     *b
    /// }
    /// ```
    ///
    /// Here, the region of `b` will be `<R0>`.  `<R0>` is constrained to be some subregion of the
    /// block B and some superregion of the call.  If we forced it now, we'd choose the smaller
    /// region (the call).  But that would make the *b illegal.  Since we don't resolve, the type
    /// of b will be `&<R0>.i32` and then `*b` will require that `<R0>` be bigger than the let and
    /// the `*b` expression, so we will effectively resolve `<R0>` to be the block B.
    pub fn resolve_type(&self, unresolved_ty: Ty<'tcx>) -> Ty<'tcx> {
        self.resolve_type_vars_if_possible(&unresolved_ty)
    }

    /// Try to resolve the type for the given node.
    fn resolve_node_type(&self, id: ast::NodeId) -> Ty<'tcx> {
        let t = self.node_ty(id);
        self.resolve_type(t)
    }

    /// Try to resolve the type for the given node.
    pub fn resolve_expr_type_adjusted(&mut self, expr: &hir::Expr) -> Ty<'tcx> {
        let ty = self.tables.borrow().expr_ty_adjusted(expr);
        self.resolve_type(ty)
    }

    fn visit_fn_body(&mut self,
                     id: ast::NodeId, // the id of the fn itself
                     body: &'gcx hir::Body,
                     span: Span)
    {
        // When we enter a function, we can derive
        debug!("visit_fn_body(id={})", id);

        let body_id = body.id();

        let call_site = CodeExtent::CallSiteScope(body_id);
        let old_call_site_scope = self.set_call_site_scope(Some(call_site));

        let fn_sig = {
            let fn_sig_map = &self.tables.borrow().liberated_fn_sigs;
            match fn_sig_map.get(&id) {
                Some(f) => f.clone(),
                None => {
                    bug!("No fn-sig entry for id={}", id);
                }
            }
        };

        let old_region_bounds_pairs_len = self.region_bound_pairs.len();

        // Collect the types from which we create inferred bounds.
        // For the return type, if diverging, substitute `bool` just
        // because it will have no effect.
        //
        // FIXME(#27579) return types should not be implied bounds
        let fn_sig_tys: Vec<_> =
            fn_sig.inputs().iter().cloned().chain(Some(fn_sig.output())).collect();

        let old_body_id = self.set_body_id(body_id.node_id);
        self.relate_free_regions(&fn_sig_tys[..], body_id.node_id, span);
        self.link_fn_args(CodeExtent::Misc(body_id.node_id), &body.arguments);
        self.visit_body(body);
        self.visit_region_obligations(body_id.node_id);

        let call_site_scope = self.call_site_scope.unwrap();
        debug!("visit_fn_body body.id {:?} call_site_scope: {:?}",
               body.id(), call_site_scope);
        let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope));
        self.type_of_node_must_outlive(infer::CallReturn(span),
                                       body_id.node_id,
                                       call_site_region);

        self.region_bound_pairs.truncate(old_region_bounds_pairs_len);

        self.set_body_id(old_body_id);
        self.set_call_site_scope(old_call_site_scope);
    }

    fn visit_region_obligations(&mut self, node_id: ast::NodeId)
    {
        debug!("visit_region_obligations: node_id={}", node_id);

        // region checking can introduce new pending obligations
        // which, when processed, might generate new region
        // obligations. So make sure we process those.
        self.select_all_obligations_or_error();

        // Make a copy of the region obligations vec because we'll need
        // to be able to borrow the fulfillment-cx below when projecting.
        let region_obligations =
            self.fulfillment_cx
                .borrow()
                .region_obligations(node_id)
                .to_vec();

        for r_o in &region_obligations {
            debug!("visit_region_obligations: r_o={:?} cause={:?}",
                   r_o, r_o.cause);
            let sup_type = self.resolve_type(r_o.sup_type);
            let origin = self.code_to_origin(&r_o.cause, sup_type);
            self.type_must_outlive(origin, sup_type, r_o.sub_region);
        }

        // Processing the region obligations should not cause the list to grow further:
        assert_eq!(region_obligations.len(),
                   self.fulfillment_cx.borrow().region_obligations(node_id).len());
    }

    fn code_to_origin(&self,
                      cause: &traits::ObligationCause<'tcx>,
                      sup_type: Ty<'tcx>)
                      -> SubregionOrigin<'tcx> {
        SubregionOrigin::from_obligation_cause(cause,
                                               || infer::RelateParamBound(cause.span, sup_type))
    }

    /// This method populates the region map's `free_region_map`. It walks over the transformed
    /// argument and return types for each function just before we check the body of that function,
    /// looking for types where you have a borrowed pointer to other borrowed data (e.g., `&'a &'b
    /// [usize]`.  We do not allow references to outlive the things they point at, so we can assume
    /// that `'a <= 'b`. This holds for both the argument and return types, basically because, on
    /// the caller side, the caller is responsible for checking that the type of every expression
    /// (including the actual values for the arguments, as well as the return type of the fn call)
    /// is well-formed.
    ///
    /// Tests: `src/test/compile-fail/regions-free-region-ordering-*.rs`
    fn relate_free_regions(&mut self,
                           fn_sig_tys: &[Ty<'tcx>],
                           body_id: ast::NodeId,
                           span: Span) {
        debug!("relate_free_regions >>");

        for &ty in fn_sig_tys {
            let ty = self.resolve_type(ty);
            debug!("relate_free_regions(t={:?})", ty);
            let implied_bounds = ty::wf::implied_bounds(self, body_id, ty, span);

            // Record any relations between free regions that we observe into the free-region-map.
            self.free_region_map.relate_free_regions_from_implied_bounds(&implied_bounds);

            // But also record other relationships, such as `T:'x`,
            // that don't go into the free-region-map but which we use
            // here.
            for implication in implied_bounds {
                debug!("implication: {:?}", implication);
                match implication {
                    ImpliedBound::RegionSubRegion(r_a @ &ty::ReEarlyBound(_),
                                                  &ty::ReVar(vid_b)) |
                    ImpliedBound::RegionSubRegion(r_a @ &ty::ReFree(_),
                                                  &ty::ReVar(vid_b)) => {
                        self.add_given(r_a, vid_b);
                    }
                    ImpliedBound::RegionSubParam(r_a, param_b) => {
                        self.region_bound_pairs.push((r_a, GenericKind::Param(param_b)));
                    }
                    ImpliedBound::RegionSubProjection(r_a, projection_b) => {
                        self.region_bound_pairs.push((r_a, GenericKind::Projection(projection_b)));
                    }
                    ImpliedBound::RegionSubRegion(..) => {
                        // In principle, we could record (and take
                        // advantage of) every relationship here, but
                        // we are also free not to -- it simply means
                        // strictly less that we can successfully type
                        // check. (It may also be that we should
                        // revise our inference system to be more
                        // general and to make use of *every*
                        // relationship that arises here, but
                        // presently we do not.)
                    }
                }
            }
        }

        debug!("<< relate_free_regions");
    }

    fn resolve_regions_and_report_errors(&self) {
        self.fcx.resolve_regions_and_report_errors(self.subject_def_id,
                                                   &self.region_maps,
                                                   &self.free_region_map);
    }

    fn constrain_bindings_in_pat(&mut self, pat: &hir::Pat) {
        debug!("regionck::visit_pat(pat={:?})", pat);
        pat.each_binding(|_, id, span, _| {
            // If we have a variable that contains region'd data, that
            // data will be accessible from anywhere that the variable is
            // accessed. We must be wary of loops like this:
            //
            //     // from src/test/compile-fail/borrowck-lend-flow.rs
            //     let mut v = box 3, w = box 4;
            //     let mut x = &mut w;
            //     loop {
            //         **x += 1;   // (2)
            //         borrow(v);  //~ ERROR cannot borrow
            //         x = &mut v; // (1)
            //     }
            //
            // Typically, we try to determine the region of a borrow from
            // those points where it is dereferenced. In this case, one
            // might imagine that the lifetime of `x` need only be the
            // body of the loop. But of course this is incorrect because
            // the pointer that is created at point (1) is consumed at
            // point (2), meaning that it must be live across the loop
            // iteration. The easiest way to guarantee this is to require
            // that the lifetime of any regions that appear in a
            // variable's type enclose at least the variable's scope.

            let var_scope = self.region_maps.var_scope(id);
            let var_region = self.tcx.mk_region(ty::ReScope(var_scope));

            let origin = infer::BindingTypeIsNotValidAtDecl(span);
            self.type_of_node_must_outlive(origin, id, var_region);

            let typ = self.resolve_node_type(id);
            let _ = dropck::check_safety_of_destructor_if_necessary(
                self, typ, span, var_scope);
        })
    }
}

impl<'a, 'gcx, 'tcx> Visitor<'gcx> for RegionCtxt<'a, 'gcx, 'tcx> {
    // (..) FIXME(#3238) should use visit_pat, not visit_arm/visit_local,
    // However, right now we run into an issue whereby some free
    // regions are not properly related if they appear within the
    // types of arguments that must be inferred. This could be
    // addressed by deferring the construction of the region
    // hierarchy, and in particular the relationships between free
    // regions, until regionck, as described in #3238.

    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'gcx> {
        NestedVisitorMap::None
    }

    fn visit_fn(&mut self, _fk: intravisit::FnKind<'gcx>, _: &'gcx hir::FnDecl,
                b: hir::BodyId, span: Span, id: ast::NodeId) {
        let body = self.tcx.hir.body(b);
        self.visit_fn_body(id, body, span)
    }

    //visit_pat: visit_pat, // (..) see above

    fn visit_arm(&mut self, arm: &'gcx hir::Arm) {
        // see above
        for p in &arm.pats {
            self.constrain_bindings_in_pat(p);
        }
        intravisit::walk_arm(self, arm);
    }

    fn visit_local(&mut self, l: &'gcx hir::Local) {
        // see above
        self.constrain_bindings_in_pat(&l.pat);
        self.link_local(l);
        intravisit::walk_local(self, l);
    }

    fn visit_expr(&mut self, expr: &'gcx hir::Expr) {
        debug!("regionck::visit_expr(e={:?}, repeating_scope={})",
               expr, self.repeating_scope);

        // No matter what, the type of each expression must outlive the
        // scope of that expression. This also guarantees basic WF.
        let expr_ty = self.resolve_node_type(expr.id);
        // the region corresponding to this expression
        let expr_region = self.tcx.node_scope_region(expr.id);
        self.type_must_outlive(infer::ExprTypeIsNotInScope(expr_ty, expr.span),
                               expr_ty, expr_region);

        let is_method_call = self.tables.borrow().is_method_call(expr);

        // If we are calling a method (either explicitly or via an
        // overloaded operator), check that all of the types provided as
        // arguments for its type parameters are well-formed, and all the regions
        // provided as arguments outlive the call.
        if is_method_call {
            let origin = match expr.node {
                hir::ExprMethodCall(..) =>
                    infer::ParameterOrigin::MethodCall,
                hir::ExprUnary(op, _) if op == hir::UnDeref =>
                    infer::ParameterOrigin::OverloadedDeref,
                _ =>
                    infer::ParameterOrigin::OverloadedOperator
            };

            let substs = self.tables.borrow().node_substs(expr.id);
            self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
            // Arguments (sub-expressions) are checked via `constrain_call`, below.
        }

        // Check any autoderefs or autorefs that appear.
        let cmt_result = self.constrain_adjustments(expr);

        // If necessary, constrain destructors in this expression. This will be
        // the adjusted form if there is an adjustment.
        match cmt_result {
            Ok(head_cmt) => {
                self.check_safety_of_rvalue_destructor_if_necessary(head_cmt, expr.span);
            }
            Err(..) => {
                self.tcx.sess.delay_span_bug(expr.span, "cat_expr Errd");
            }
        }

        debug!("regionck::visit_expr(e={:?}, repeating_scope={}) - visiting subexprs",
               expr, self.repeating_scope);
        match expr.node {
            hir::ExprPath(_) => {
                let substs = self.tables.borrow().node_substs(expr.id);
                let origin = infer::ParameterOrigin::Path;
                self.substs_wf_in_scope(origin, substs, expr.span, expr_region);
            }

            hir::ExprCall(ref callee, ref args) => {
                if is_method_call {
                    self.constrain_call(expr, Some(&callee), args.iter().map(|e| &*e));
                } else {
                    self.constrain_callee(callee.id, expr, &callee);
                    self.constrain_call(expr, None, args.iter().map(|e| &*e));
                }

                intravisit::walk_expr(self, expr);
            }

            hir::ExprMethodCall(.., ref args) => {
                self.constrain_call(expr, Some(&args[0]), args[1..].iter().map(|e| &*e));

                intravisit::walk_expr(self, expr);
            }

            hir::ExprAssignOp(_, ref lhs, ref rhs) => {
                if is_method_call {
                    self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());
                }

                intravisit::walk_expr(self, expr);
            }

            hir::ExprIndex(ref lhs, ref rhs) if is_method_call => {
                self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());

                intravisit::walk_expr(self, expr);
            },

            hir::ExprBinary(_, ref lhs, ref rhs) if is_method_call => {
                // As `ExprMethodCall`, but the call is via an overloaded op.
                self.constrain_call(expr, Some(&lhs), Some(&**rhs).into_iter());

                intravisit::walk_expr(self, expr);
            }

            hir::ExprBinary(_, ref lhs, ref rhs) => {
                // If you do `x OP y`, then the types of `x` and `y` must
                // outlive the operation you are performing.
                let lhs_ty = self.resolve_expr_type_adjusted(&lhs);
                let rhs_ty = self.resolve_expr_type_adjusted(&rhs);
                for &ty in &[lhs_ty, rhs_ty] {
                    self.type_must_outlive(infer::Operand(expr.span),
                                           ty, expr_region);
                }
                intravisit::walk_expr(self, expr);
            }

            hir::ExprUnary(hir::UnDeref, ref base) => {
                // For *a, the lifetime of a must enclose the deref
                if is_method_call {
                    self.constrain_call(expr, Some(base), None::<hir::Expr>.iter());
                }
                // For overloaded derefs, base_ty is the input to `Deref::deref`,
                // but it's a reference type uing the same region as the output.
                let base_ty = self.resolve_expr_type_adjusted(base);
                if let ty::TyRef(r_ptr, _) = base_ty.sty {
                    self.mk_subregion_due_to_dereference(expr.span, expr_region, r_ptr);
                }

                intravisit::walk_expr(self, expr);
            }

            hir::ExprUnary(_, ref lhs) if is_method_call => {
                // As above.
                self.constrain_call(expr, Some(&lhs), None::<hir::Expr>.iter());

                intravisit::walk_expr(self, expr);
            }

            hir::ExprIndex(ref vec_expr, _) => {
                // For a[b], the lifetime of a must enclose the deref
                let vec_type = self.resolve_expr_type_adjusted(&vec_expr);
                self.constrain_index(expr, vec_type);

                intravisit::walk_expr(self, expr);
            }

            hir::ExprCast(ref source, _) => {
                // Determine if we are casting `source` to a trait
                // instance.  If so, we have to be sure that the type of
                // the source obeys the trait's region bound.
                self.constrain_cast(expr, &source);
                intravisit::walk_expr(self, expr);
            }

            hir::ExprAddrOf(m, ref base) => {
                self.link_addr_of(expr, m, &base);

                // Require that when you write a `&expr` expression, the
                // resulting pointer has a lifetime that encompasses the
                // `&expr` expression itself. Note that we constraining
                // the type of the node expr.id here *before applying
                // adjustments*.
                //
                // FIXME(#6268) nested method calls requires that this rule change
                let ty0 = self.resolve_node_type(expr.id);
                self.type_must_outlive(infer::AddrOf(expr.span), ty0, expr_region);
                intravisit::walk_expr(self, expr);
            }

            hir::ExprMatch(ref discr, ref arms, _) => {
                self.link_match(&discr, &arms[..]);

                intravisit::walk_expr(self, expr);
            }

            hir::ExprClosure(.., body_id, _) => {
                self.check_expr_fn_block(expr, body_id);
            }

            hir::ExprLoop(ref body, _, _) => {
                let repeating_scope = self.set_repeating_scope(body.id);
                intravisit::walk_expr(self, expr);
                self.set_repeating_scope(repeating_scope);
            }

            hir::ExprWhile(ref cond, ref body, _) => {
                let repeating_scope = self.set_repeating_scope(cond.id);
                self.visit_expr(&cond);

                self.set_repeating_scope(body.id);
                self.visit_block(&body);

                self.set_repeating_scope(repeating_scope);
            }

            hir::ExprRet(Some(ref ret_expr)) => {
                let call_site_scope = self.call_site_scope;
                debug!("visit_expr ExprRet ret_expr.id {} call_site_scope: {:?}",
                       ret_expr.id, call_site_scope);
                let call_site_region = self.tcx.mk_region(ty::ReScope(call_site_scope.unwrap()));
                self.type_of_node_must_outlive(infer::CallReturn(ret_expr.span),
                                               ret_expr.id,
                                               call_site_region);
                intravisit::walk_expr(self, expr);
            }

            _ => {
                intravisit::walk_expr(self, expr);
            }
        }
    }
}

impl<'a, 'gcx, 'tcx> RegionCtxt<'a, 'gcx, 'tcx> {
    fn constrain_cast(&mut self,
                      cast_expr: &hir::Expr,
                      source_expr: &hir::Expr)
    {
        debug!("constrain_cast(cast_expr={:?}, source_expr={:?})",
               cast_expr,
               source_expr);

        let source_ty = self.resolve_node_type(source_expr.id);
        let target_ty = self.resolve_node_type(cast_expr.id);

        self.walk_cast(cast_expr, source_ty, target_ty);
    }

    fn walk_cast(&mut self,
                 cast_expr: &hir::Expr,
                 from_ty: Ty<'tcx>,
                 to_ty: Ty<'tcx>) {
        debug!("walk_cast(from_ty={:?}, to_ty={:?})",
               from_ty,
               to_ty);
        match (&from_ty.sty, &to_ty.sty) {
            /*From:*/ (&ty::TyRef(from_r, ref from_mt),
            /*To:  */  &ty::TyRef(to_r, ref to_mt)) => {
                // Target cannot outlive source, naturally.
                self.sub_regions(infer::Reborrow(cast_expr.span), to_r, from_r);
                self.walk_cast(cast_expr, from_mt.ty, to_mt.ty);
            }

            /*From:*/ (_,
            /*To:  */  &ty::TyDynamic(.., r)) => {
                // When T is existentially quantified as a trait
                // `Foo+'to`, it must outlive the region bound `'to`.
                self.type_must_outlive(infer::RelateObjectBound(cast_expr.span), from_ty, r);
            }

            /*From:*/ (&ty::TyAdt(from_def, _),
            /*To:  */  &ty::TyAdt(to_def, _)) if from_def.is_box() && to_def.is_box() => {
                self.walk_cast(cast_expr, from_ty.boxed_ty(), to_ty.boxed_ty());
            }

            _ => { }
        }
    }

    fn check_expr_fn_block(&mut self,
                           expr: &'gcx hir::Expr,
                           body_id: hir::BodyId) {
        let repeating_scope = self.set_repeating_scope(body_id.node_id);
        intravisit::walk_expr(self, expr);
        self.set_repeating_scope(repeating_scope);
    }

    fn constrain_callee(&mut self,
                        callee_id: ast::NodeId,
                        _call_expr: &hir::Expr,
                        _callee_expr: &hir::Expr) {
        let callee_ty = self.resolve_node_type(callee_id);
        match callee_ty.sty {
            ty::TyFnDef(..) | ty::TyFnPtr(_) => { }
            _ => {
                // this should not happen, but it does if the program is
                // erroneous
                //
                // bug!(
                //     callee_expr.span,
                //     "Calling non-function: {}",
                //     callee_ty);
            }
        }
    }

    fn constrain_call<'b, I: Iterator<Item=&'b hir::Expr>>(&mut self,
                                                           call_expr: &hir::Expr,
                                                           receiver: Option<&hir::Expr>,
                                                           arg_exprs: I) {
        //! Invoked on every call site (i.e., normal calls, method calls,
        //! and overloaded operators). Constrains the regions which appear
        //! in the type of the function. Also constrains the regions that
        //! appear in the arguments appropriately.

        debug!("constrain_call(call_expr={:?}, receiver={:?})",
                call_expr,
                receiver);

        // `callee_region` is the scope representing the time in which the
        // call occurs.
        //
        // FIXME(#6268) to support nested method calls, should be callee_id
        let callee_scope = CodeExtent::Misc(call_expr.id);
        let callee_region = self.tcx.mk_region(ty::ReScope(callee_scope));

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

        for arg_expr in arg_exprs {
            debug!("Argument: {:?}", arg_expr);

            // ensure that any regions appearing in the argument type are
            // valid for at least the lifetime of the function:
            self.type_of_node_must_outlive(infer::CallArg(arg_expr.span),
                                           arg_expr.id, callee_region);
        }

        // as loop above, but for receiver
        if let Some(r) = receiver {
            debug!("receiver: {:?}", r);
            self.type_of_node_must_outlive(infer::CallRcvr(r.span),
                                           r.id, callee_region);
        }
    }

    /// Invoked on any adjustments that occur. Checks that if this is a region pointer being
    /// dereferenced, the lifetime of the pointer includes the deref expr.
    fn constrain_adjustments(&mut self, expr: &hir::Expr) -> mc::McResult<mc::cmt<'tcx>> {
        debug!("constrain_adjustments(expr={:?})", expr);

        let mut cmt = {
            let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
            mc.cat_expr_unadjusted(expr)?
        };

        //NOTE(@jroesch): mixed RefCell borrow causes crash
        let adjustments = self.tables.borrow().expr_adjustments(&expr).to_vec();
        if adjustments.is_empty() {
            return Ok(cmt);
        }

        debug!("constrain_adjustments: adjustments={:?}", adjustments);

        // If necessary, constrain destructors in the unadjusted form of this
        // expression.
        self.check_safety_of_rvalue_destructor_if_necessary(cmt.clone(), expr.span);

        let expr_region = self.tcx.node_scope_region(expr.id);
        for adjustment in adjustments {
            debug!("constrain_adjustments: adjustment={:?}, cmt={:?}",
                   adjustment, cmt);

            if let adjustment::Adjust::Deref(Some(deref)) = adjustment.kind {
                debug!("constrain_adjustments: overloaded deref: {:?}", deref);

                // Treat overloaded autoderefs as if an AutoBorrow adjustment
                // was applied on the base type, as that is always the case.
                let input = self.tcx.mk_ref(deref.region, ty::TypeAndMut {
                    ty: cmt.ty,
                    mutbl: deref.mutbl,
                });
                let output = self.tcx.mk_ref(deref.region, ty::TypeAndMut {
                    ty: adjustment.target,
                    mutbl: deref.mutbl,
                });

                self.link_region(expr.span, deref.region,
                                 ty::BorrowKind::from_mutbl(deref.mutbl), cmt.clone());

                // Specialized version of constrain_call.
                self.type_must_outlive(infer::CallRcvr(expr.span),
                                       input, expr_region);
                self.type_must_outlive(infer::CallReturn(expr.span),
                                       output, expr_region);
            }

            if let adjustment::Adjust::Borrow(ref autoref) = adjustment.kind {
                self.link_autoref(expr, cmt.clone(), autoref);

                // Require that the resulting region encompasses
                // the current node.
                //
                // FIXME(#6268) remove to support nested method calls
                self.type_of_node_must_outlive(infer::AutoBorrow(expr.span),
                                               expr.id, expr_region);
            }

            {
                let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
                cmt = mc.cat_expr_adjusted(expr, cmt, &adjustment)?;
            }

            if let Categorization::Deref(_, mc::BorrowedPtr(_, r_ptr)) = cmt.cat {
                self.mk_subregion_due_to_dereference(expr.span,
                                                     expr_region, r_ptr);
            }
        }

        Ok(cmt)
    }

    pub fn mk_subregion_due_to_dereference(&mut self,
                                           deref_span: Span,
                                           minimum_lifetime: ty::Region<'tcx>,
                                           maximum_lifetime: ty::Region<'tcx>) {
        self.sub_regions(infer::DerefPointer(deref_span),
                         minimum_lifetime, maximum_lifetime)
    }

    fn check_safety_of_rvalue_destructor_if_necessary(&mut self,
                                                     cmt: mc::cmt<'tcx>,
                                                     span: Span) {
        match cmt.cat {
            Categorization::Rvalue(region) => {
                match *region {
                    ty::ReScope(rvalue_scope) => {
                        let typ = self.resolve_type(cmt.ty);
                        let _ = dropck::check_safety_of_destructor_if_necessary(
                            self, typ, span, rvalue_scope);
                    }
                    ty::ReStatic => {}
                    _ => {
                        span_bug!(span,
                                  "unexpected rvalue region in rvalue \
                                   destructor safety checking: `{:?}`",
                                  region);
                    }
                }
            }
            _ => {}
        }
    }

    /// Invoked on any index expression that occurs. Checks that if this is a slice
    /// being indexed, the lifetime of the pointer includes the deref expr.
    fn constrain_index(&mut self,
                       index_expr: &hir::Expr,
                       indexed_ty: Ty<'tcx>)
    {
        debug!("constrain_index(index_expr=?, indexed_ty={}",
               self.ty_to_string(indexed_ty));

        let r_index_expr = ty::ReScope(CodeExtent::Misc(index_expr.id));
        if let ty::TyRef(r_ptr, mt) = indexed_ty.sty {
            match mt.ty.sty {
                ty::TySlice(_) | ty::TyStr => {
                    self.sub_regions(infer::IndexSlice(index_expr.span),
                                     self.tcx.mk_region(r_index_expr), r_ptr);
                }
                _ => {}
            }
        }
    }

    /// Guarantees that any lifetimes which appear in the type of the node `id` (after applying
    /// adjustments) are valid for at least `minimum_lifetime`
    fn type_of_node_must_outlive(&mut self,
        origin: infer::SubregionOrigin<'tcx>,
        id: ast::NodeId,
        minimum_lifetime: ty::Region<'tcx>)
    {
        // Try to resolve the type.  If we encounter an error, then typeck
        // is going to fail anyway, so just stop here and let typeck
        // report errors later on in the writeback phase.
        let ty0 = self.resolve_node_type(id);
        let ty = self.tables.borrow().adjustments.get(&id)
            .and_then(|adj| adj.last())
            .map_or(ty0, |adj| adj.target);
        let ty = self.resolve_type(ty);
        debug!("constrain_regions_in_type_of_node(\
                ty={}, ty0={}, id={}, minimum_lifetime={:?})",
                ty,  ty0,
               id, minimum_lifetime);
        self.type_must_outlive(origin, ty, minimum_lifetime);
    }

    /// Computes the guarantor for an expression `&base` and then ensures that the lifetime of the
    /// resulting pointer is linked to the lifetime of its guarantor (if any).
    fn link_addr_of(&mut self, expr: &hir::Expr,
                    mutability: hir::Mutability, base: &hir::Expr) {
        debug!("link_addr_of(expr={:?}, base={:?})", expr, base);

        let cmt = {
            let mc = mc::MemCategorizationContext::new(self, &self.region_maps);
            ignore_err!(mc.cat_expr(base))
        };

        debug!("link_addr_of: cmt={:?}", cmt);

        self.link_region_from_node_type(expr.span, expr.id, mutability, cmt);
    }

    /// Computes the guarantors for any ref bindings in a `let` and
    /// then ensures that the lifetime of the resulting pointer is
    /// linked to the lifetime of the initialization expression.
    fn link_local(&self, local: &hir::Local) {
        debug!("regionck::for_local()");
        let init_expr = match local.init {
            None => { return; }
            Some(ref expr) => &**expr,
        };
        let mc = &mc::MemCategorizationContext::new(self, &self.region_maps);
        let discr_cmt = ignore_err!(mc.cat_expr(init_expr));
        self.link_pattern(mc, discr_cmt, &local.pat);
    }

    /// Computes the guarantors for any ref bindings in a match and
    /// then ensures that the lifetime of the resulting pointer is
    /// linked to the lifetime of its guarantor (if any).
    fn link_match(&self, discr: &hir::Expr, arms: &[hir::Arm]) {
        debug!("regionck::for_match()");
        let mc = &mc::MemCategorizationContext::new(self, &self.region_maps);
        let discr_cmt = ignore_err!(mc.cat_expr(discr));
        debug!("discr_cmt={:?}", discr_cmt);
        for arm in arms {
            for root_pat in &arm.pats {
                self.link_pattern(mc, discr_cmt.clone(), &root_pat);
            }
        }
    }

    /// Computes the guarantors for any ref bindings in a match and
    /// then ensures that the lifetime of the resulting pointer is
    /// linked to the lifetime of its guarantor (if any).
    fn link_fn_args(&self, body_scope: CodeExtent, args: &[hir::Arg]) {
        debug!("regionck::link_fn_args(body_scope={:?})", body_scope);
        let mc = &mc::MemCategorizationContext::new(self, &self.region_maps);
        for arg in args {
            let arg_ty = self.node_ty(arg.id);
            let re_scope = self.tcx.mk_region(ty::ReScope(body_scope));
            let arg_cmt = mc.cat_rvalue(
                arg.id, arg.pat.span, re_scope, arg_ty);
            debug!("arg_ty={:?} arg_cmt={:?} arg={:?}",
                   arg_ty,
                   arg_cmt,
                   arg);
            self.link_pattern(mc, arg_cmt, &arg.pat);
        }
    }

    /// Link lifetimes of any ref bindings in `root_pat` to the pointers found
    /// in the discriminant, if needed.
    fn link_pattern<'t>(&self,
                        mc: &mc::MemCategorizationContext<'a, 'gcx, 'tcx>,
                        discr_cmt: mc::cmt<'tcx>,
                        root_pat: &hir::Pat) {
        debug!("link_pattern(discr_cmt={:?}, root_pat={:?})",
               discr_cmt,
               root_pat);
        let _ = mc.cat_pattern(discr_cmt, root_pat, |_, sub_cmt, sub_pat| {
                match sub_pat.node {
                    // `ref x` pattern
                    PatKind::Binding(hir::BindByRef(mutbl), ..) => {
                        self.link_region_from_node_type(sub_pat.span, sub_pat.id,
                                                        mutbl, sub_cmt);
                    }
                    _ => {}
                }
            });
    }

    /// Link lifetime of borrowed pointer resulting from autoref to lifetimes in the value being
    /// autoref'd.
    fn link_autoref(&self,
                    expr: &hir::Expr,
                    expr_cmt: mc::cmt<'tcx>,
                    autoref: &adjustment::AutoBorrow<'tcx>)
    {
        debug!("link_autoref(autoref={:?}, expr_cmt={:?})", autoref, expr_cmt);

        match *autoref {
            adjustment::AutoBorrow::Ref(r, m) => {
                self.link_region(expr.span, r,
                                 ty::BorrowKind::from_mutbl(m), expr_cmt);
            }

            adjustment::AutoBorrow::RawPtr(m) => {
                let r = self.tcx.node_scope_region(expr.id);
                self.link_region(expr.span, r, ty::BorrowKind::from_mutbl(m), expr_cmt);
            }
        }
    }

    /// Like `link_region()`, except that the region is extracted from the type of `id`,
    /// which must be some reference (`&T`, `&str`, etc).
    fn link_region_from_node_type(&self,
                                  span: Span,
                                  id: ast::NodeId,
                                  mutbl: hir::Mutability,
                                  cmt_borrowed: mc::cmt<'tcx>) {
        debug!("link_region_from_node_type(id={:?}, mutbl={:?}, cmt_borrowed={:?})",
               id, mutbl, cmt_borrowed);

        let rptr_ty = self.resolve_node_type(id);
        if let ty::TyRef(r, _) = rptr_ty.sty {
            debug!("rptr_ty={}",  rptr_ty);
            self.link_region(span, r, ty::BorrowKind::from_mutbl(mutbl),
                             cmt_borrowed);
        }
    }

    /// Informs the inference engine that `borrow_cmt` is being borrowed with
    /// kind `borrow_kind` and lifetime `borrow_region`.
    /// In order to ensure borrowck is satisfied, this may create constraints
    /// between regions, as explained in `link_reborrowed_region()`.
    fn link_region(&self,
                   span: Span,
                   borrow_region: ty::Region<'tcx>,
                   borrow_kind: ty::BorrowKind,
                   borrow_cmt: mc::cmt<'tcx>) {
        let mut borrow_cmt = borrow_cmt;
        let mut borrow_kind = borrow_kind;

        let origin = infer::DataBorrowed(borrow_cmt.ty, span);
        self.type_must_outlive(origin, borrow_cmt.ty, borrow_region);

        loop {
            debug!("link_region(borrow_region={:?}, borrow_kind={:?}, borrow_cmt={:?})",
                   borrow_region,
                   borrow_kind,
                   borrow_cmt);
            match borrow_cmt.cat.clone() {
                Categorization::Deref(ref_cmt, mc::Implicit(ref_kind, ref_region)) |
                Categorization::Deref(ref_cmt, mc::BorrowedPtr(ref_kind, ref_region)) => {
                    match self.link_reborrowed_region(span,
                                                      borrow_region, borrow_kind,
                                                      ref_cmt, ref_region, ref_kind,
                                                      borrow_cmt.note) {
                        Some((c, k)) => {
                            borrow_cmt = c;
                            borrow_kind = k;
                        }
                        None => {
                            return;
                        }
                    }
                }

                Categorization::Downcast(cmt_base, _) |
                Categorization::Deref(cmt_base, mc::Unique) |
                Categorization::Interior(cmt_base, _) => {
                    // Borrowing interior or owned data requires the base
                    // to be valid and borrowable in the same fashion.
                    borrow_cmt = cmt_base;
                    borrow_kind = borrow_kind;
                }

                Categorization::Deref(_, mc::UnsafePtr(..)) |
                Categorization::StaticItem |
                Categorization::Upvar(..) |
                Categorization::Local(..) |
                Categorization::Rvalue(..) => {
                    // These are all "base cases" with independent lifetimes
                    // that are not subject to inference
                    return;
                }
            }
        }
    }

    /// This is the most complicated case: the path being borrowed is
    /// itself the referent of a borrowed pointer. Let me give an
    /// example fragment of code to make clear(er) the situation:
    ///
    ///    let r: &'a mut T = ...;  // the original reference "r" has lifetime 'a
    ///    ...
    ///    &'z *r                   // the reborrow has lifetime 'z
    ///
    /// Now, in this case, our primary job is to add the inference
    /// constraint that `'z <= 'a`. Given this setup, let's clarify the
    /// parameters in (roughly) terms of the example:
    ///
    ///     A borrow of: `& 'z bk * r` where `r` has type `& 'a bk T`
    ///     borrow_region   ^~                 ref_region    ^~
    ///     borrow_kind        ^~               ref_kind        ^~
    ///     ref_cmt                 ^
    ///
    /// Here `bk` stands for some borrow-kind (e.g., `mut`, `uniq`, etc).
    ///
    /// Unfortunately, there are some complications beyond the simple
    /// scenario I just painted:
    ///
    /// 1. The reference `r` might in fact be a "by-ref" upvar. In that
    ///    case, we have two jobs. First, we are inferring whether this reference
    ///    should be an `&T`, `&mut T`, or `&uniq T` reference, and we must
    ///    adjust that based on this borrow (e.g., if this is an `&mut` borrow,
    ///    then `r` must be an `&mut` reference). Second, whenever we link
    ///    two regions (here, `'z <= 'a`), we supply a *cause*, and in this
    ///    case we adjust the cause to indicate that the reference being
    ///    "reborrowed" is itself an upvar. This provides a nicer error message
    ///    should something go wrong.
    ///
    /// 2. There may in fact be more levels of reborrowing. In the
    ///    example, I said the borrow was like `&'z *r`, but it might
    ///    in fact be a borrow like `&'z **q` where `q` has type `&'a
    ///    &'b mut T`. In that case, we want to ensure that `'z <= 'a`
    ///    and `'z <= 'b`. This is explained more below.
    ///
    /// The return value of this function indicates whether we need to
    /// recurse and process `ref_cmt` (see case 2 above).
    fn link_reborrowed_region(&self,
                              span: Span,
                              borrow_region: ty::Region<'tcx>,
                              borrow_kind: ty::BorrowKind,
                              ref_cmt: mc::cmt<'tcx>,
                              ref_region: ty::Region<'tcx>,
                              mut ref_kind: ty::BorrowKind,
                              note: mc::Note)
                              -> Option<(mc::cmt<'tcx>, ty::BorrowKind)>
    {
        // Possible upvar ID we may need later to create an entry in the
        // maybe link map.

        // Detect by-ref upvar `x`:
        let cause = match note {
            mc::NoteUpvarRef(ref upvar_id) => {
                let upvar_capture_map = &self.tables.borrow_mut().upvar_capture_map;
                match upvar_capture_map.get(upvar_id) {
                    Some(&ty::UpvarCapture::ByRef(ref upvar_borrow)) => {
                        // The mutability of the upvar may have been modified
                        // by the above adjustment, so update our local variable.
                        ref_kind = upvar_borrow.kind;

                        infer::ReborrowUpvar(span, *upvar_id)
                    }
                    _ => {
                        span_bug!( span, "Illegal upvar id: {:?}", upvar_id);
                    }
                }
            }
            mc::NoteClosureEnv(ref upvar_id) => {
                // We don't have any mutability changes to propagate, but
                // we do want to note that an upvar reborrow caused this
                // link
                infer::ReborrowUpvar(span, *upvar_id)
            }
            _ => {
                infer::Reborrow(span)
            }
        };

        debug!("link_reborrowed_region: {:?} <= {:?}",
               borrow_region,
               ref_region);
        self.sub_regions(cause, borrow_region, ref_region);

        // If we end up needing to recurse and establish a region link
        // with `ref_cmt`, calculate what borrow kind we will end up
        // needing. This will be used below.
        //
        // One interesting twist is that we can weaken the borrow kind
        // when we recurse: to reborrow an `&mut` referent as mutable,
        // borrowck requires a unique path to the `&mut` reference but not
        // necessarily a *mutable* path.
        let new_borrow_kind = match borrow_kind {
            ty::ImmBorrow =>
                ty::ImmBorrow,
            ty::MutBorrow | ty::UniqueImmBorrow =>
                ty::UniqueImmBorrow
        };

        // Decide whether we need to recurse and link any regions within
        // the `ref_cmt`. This is concerned for the case where the value
        // being reborrowed is in fact a borrowed pointer found within
        // another borrowed pointer. For example:
        //
        //    let p: &'b &'a mut T = ...;
        //    ...
        //    &'z **p
        //
        // What makes this case particularly tricky is that, if the data
        // being borrowed is a `&mut` or `&uniq` borrow, borrowck requires
        // not only that `'z <= 'a`, (as before) but also `'z <= 'b`
        // (otherwise the user might mutate through the `&mut T` reference
        // after `'b` expires and invalidate the borrow we are looking at
        // now).
        //
        // So let's re-examine our parameters in light of this more
        // complicated (possible) scenario:
        //
        //     A borrow of: `& 'z bk * * p` where `p` has type `&'b bk & 'a bk T`
        //     borrow_region   ^~                 ref_region             ^~
        //     borrow_kind        ^~               ref_kind                 ^~
        //     ref_cmt                 ^~~
        //
        // (Note that since we have not examined `ref_cmt.cat`, we don't
        // know whether this scenario has occurred; but I wanted to show
        // how all the types get adjusted.)
        match ref_kind {
            ty::ImmBorrow => {
                // The reference being reborrowed is a sharable ref of
                // type `&'a T`. In this case, it doesn't matter where we
                // *found* the `&T` pointer, the memory it references will
                // be valid and immutable for `'a`. So we can stop here.
                //
                // (Note that the `borrow_kind` must also be ImmBorrow or
                // else the user is borrowed imm memory as mut memory,
                // which means they'll get an error downstream in borrowck
                // anyhow.)
                return None;
            }

            ty::MutBorrow | ty::UniqueImmBorrow => {
                // The reference being reborrowed is either an `&mut T` or
                // `&uniq T`. This is the case where recursion is needed.
                return Some((ref_cmt, new_borrow_kind));
            }
        }
    }

    /// Checks that the values provided for type/region arguments in a given
    /// expression are well-formed and in-scope.
    fn substs_wf_in_scope(&mut self,
                          origin: infer::ParameterOrigin,
                          substs: &Substs<'tcx>,
                          expr_span: Span,
                          expr_region: ty::Region<'tcx>) {
        debug!("substs_wf_in_scope(substs={:?}, \
                expr_region={:?}, \
                origin={:?}, \
                expr_span={:?})",
               substs, expr_region, origin, expr_span);

        let origin = infer::ParameterInScope(origin, expr_span);

        for region in substs.regions() {
            self.sub_regions(origin.clone(), expr_region, region);
        }

        for ty in substs.types() {
            let ty = self.resolve_type(ty);
            self.type_must_outlive(origin.clone(), ty, expr_region);
        }
    }

    /// Ensures that type is well-formed in `region`, which implies (among
    /// other things) that all borrowed data reachable via `ty` outlives
    /// `region`.
    pub fn type_must_outlive(&self,
                             origin: infer::SubregionOrigin<'tcx>,
                             ty: Ty<'tcx>,
                             region: ty::Region<'tcx>)
    {
        let ty = self.resolve_type(ty);

        debug!("type_must_outlive(ty={:?}, region={:?}, origin={:?})",
               ty,
               region,
               origin);

        assert!(!ty.has_escaping_regions());

        let components = self.tcx.outlives_components(ty);
        self.components_must_outlive(origin, components, region);
    }

    fn components_must_outlive(&self,
                               origin: infer::SubregionOrigin<'tcx>,
                               components: Vec<ty::outlives::Component<'tcx>>,
                               region: ty::Region<'tcx>)
    {
        for component in components {
            let origin = origin.clone();
            match component {
                ty::outlives::Component::Region(region1) => {
                    self.sub_regions(origin, region, region1);
                }
                ty::outlives::Component::Param(param_ty) => {
                    self.param_ty_must_outlive(origin, region, param_ty);
                }
                ty::outlives::Component::Projection(projection_ty) => {
                    self.projection_must_outlive(origin, region, projection_ty);
                }
                ty::outlives::Component::EscapingProjection(subcomponents) => {
                    self.components_must_outlive(origin, subcomponents, region);
                }
                ty::outlives::Component::UnresolvedInferenceVariable(v) => {
                    // ignore this, we presume it will yield an error
                    // later, since if a type variable is not resolved by
                    // this point it never will be
                    self.tcx.sess.delay_span_bug(
                        origin.span(),
                        &format!("unresolved inference variable in outlives: {:?}", v));
                }
            }
        }
    }

    fn param_ty_must_outlive(&self,
                             origin: infer::SubregionOrigin<'tcx>,
                             region: ty::Region<'tcx>,
                             param_ty: ty::ParamTy) {
        debug!("param_ty_must_outlive(region={:?}, param_ty={:?}, origin={:?})",
               region, param_ty, origin);

        let verify_bound = self.param_bound(param_ty);
        let generic = GenericKind::Param(param_ty);
        self.verify_generic_bound(origin, generic, region, verify_bound);
    }

    fn projection_must_outlive(&self,
                               origin: infer::SubregionOrigin<'tcx>,
                               region: ty::Region<'tcx>,
                               projection_ty: ty::ProjectionTy<'tcx>)
    {
        debug!("projection_must_outlive(region={:?}, projection_ty={:?}, origin={:?})",
               region, projection_ty, origin);

        // This case is thorny for inference. The fundamental problem is
        // that there are many cases where we have choice, and inference
        // doesn't like choice (the current region inference in
        // particular). :) First off, we have to choose between using the
        // OutlivesProjectionEnv, OutlivesProjectionTraitDef, and
        // OutlivesProjectionComponent rules, any one of which is
        // sufficient.  If there are no inference variables involved, it's
        // not hard to pick the right rule, but if there are, we're in a
        // bit of a catch 22: if we picked which rule we were going to
        // use, we could add constraints to the region inference graph
        // that make it apply, but if we don't add those constraints, the
        // rule might not apply (but another rule might). For now, we err
        // on the side of adding too few edges into the graph.

        // Compute the bounds we can derive from the environment or trait
        // definition.  We know that the projection outlives all the
        // regions in this list.
        let env_bounds = self.projection_declared_bounds(origin.span(), projection_ty);

        debug!("projection_must_outlive: env_bounds={:?}",
               env_bounds);

        // If we know that the projection outlives 'static, then we're
        // done here.
        if env_bounds.contains(&&ty::ReStatic) {
            debug!("projection_must_outlive: 'static as declared bound");
            return;
        }

        // If declared bounds list is empty, the only applicable rule is
        // OutlivesProjectionComponent. If there are inference variables,
        // then, we can break down the outlives into more primitive
        // components without adding unnecessary edges.
        //
        // If there are *no* inference variables, however, we COULD do
        // this, but we choose not to, because the error messages are less
        // good. For example, a requirement like `T::Item: 'r` would be
        // translated to a requirement that `T: 'r`; when this is reported
        // to the user, it will thus say "T: 'r must hold so that T::Item:
        // 'r holds". But that makes it sound like the only way to fix
        // the problem is to add `T: 'r`, which isn't true. So, if there are no
        // inference variables, we use a verify constraint instead of adding
        // edges, which winds up enforcing the same condition.
        let needs_infer = projection_ty.trait_ref.needs_infer();
        if env_bounds.is_empty() && needs_infer {
            debug!("projection_must_outlive: no declared bounds");

            for component_ty in projection_ty.trait_ref.substs.types() {
                self.type_must_outlive(origin.clone(), component_ty, region);
            }

            for r in projection_ty.trait_ref.substs.regions() {
                self.sub_regions(origin.clone(), region, r);
            }

            return;
        }

        // If we find that there is a unique declared bound `'b`, and this bound
        // appears in the trait reference, then the best action is to require that `'b:'r`,
        // so do that. This is best no matter what rule we use:
        //
        // - OutlivesProjectionEnv or OutlivesProjectionTraitDef: these would translate to
        // the requirement that `'b:'r`
        // - OutlivesProjectionComponent: this would require `'b:'r` in addition to
        // other conditions
        if !env_bounds.is_empty() && env_bounds[1..].iter().all(|b| *b == env_bounds[0]) {
            let unique_bound = env_bounds[0];
            debug!("projection_must_outlive: unique declared bound = {:?}", unique_bound);
            if projection_ty.trait_ref.substs.regions().any(|r| env_bounds.contains(&r)) {
                debug!("projection_must_outlive: unique declared bound appears in trait ref");
                self.sub_regions(origin.clone(), region, unique_bound);
                return;
            }
        }

        // Fallback to verifying after the fact that there exists a
        // declared bound, or that all the components appearing in the
        // projection outlive; in some cases, this may add insufficient
        // edges into the inference graph, leading to inference failures
        // even though a satisfactory solution exists.
        let verify_bound = self.projection_bound(origin.span(), env_bounds, projection_ty);
        let generic = GenericKind::Projection(projection_ty);
        self.verify_generic_bound(origin, generic.clone(), region, verify_bound);
    }

    fn type_bound(&self, span: Span, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
        match ty.sty {
            ty::TyParam(p) => {
                self.param_bound(p)
            }
            ty::TyProjection(data) => {
                let declared_bounds = self.projection_declared_bounds(span, data);
                self.projection_bound(span, declared_bounds, data)
            }
            _ => {
                self.recursive_type_bound(span, ty)
            }
        }
    }

    fn param_bound(&self, param_ty: ty::ParamTy) -> VerifyBound<'tcx> {
        debug!("param_bound(param_ty={:?})",
               param_ty);

        let mut param_bounds = self.declared_generic_bounds_from_env(GenericKind::Param(param_ty));

        // Add in the default bound of fn body that applies to all in
        // scope type parameters:
        param_bounds.extend(self.implicit_region_bound);

        VerifyBound::AnyRegion(param_bounds)
    }

    fn projection_declared_bounds(&self,
                                  span: Span,
                                  projection_ty: ty::ProjectionTy<'tcx>)
                                  -> Vec<ty::Region<'tcx>>
    {
        // First assemble bounds from where clauses and traits.

        let mut declared_bounds =
            self.declared_generic_bounds_from_env(GenericKind::Projection(projection_ty));

        declared_bounds.extend_from_slice(
            &self.declared_projection_bounds_from_trait(span, projection_ty));

        declared_bounds
    }

    fn projection_bound(&self,
                        span: Span,
                        declared_bounds: Vec<ty::Region<'tcx>>,
                        projection_ty: ty::ProjectionTy<'tcx>)
                        -> VerifyBound<'tcx> {
        debug!("projection_bound(declared_bounds={:?}, projection_ty={:?})",
               declared_bounds, projection_ty);

        // see the extensive comment in projection_must_outlive
        let item_name = projection_ty.item_name(self.tcx);
        let ty = self.tcx.mk_projection(projection_ty.trait_ref, item_name);
        let recursive_bound = self.recursive_type_bound(span, ty);

        VerifyBound::AnyRegion(declared_bounds).or(recursive_bound)
    }

    fn recursive_type_bound(&self, span: Span, ty: Ty<'tcx>) -> VerifyBound<'tcx> {
        let mut bounds = vec![];

        for subty in ty.walk_shallow() {
            bounds.push(self.type_bound(span, subty));
        }

        let mut regions = ty.regions();
        regions.retain(|r| !r.is_late_bound()); // ignore late-bound regions
        bounds.push(VerifyBound::AllRegions(regions));

        // remove bounds that must hold, since they are not interesting
        bounds.retain(|b| !b.must_hold());

        if bounds.len() == 1 {
            bounds.pop().unwrap()
        } else {
            VerifyBound::AllBounds(bounds)
        }
    }

    fn declared_generic_bounds_from_env(&self, generic: GenericKind<'tcx>)
                                        -> Vec<ty::Region<'tcx>>
    {
        let param_env = &self.param_env;

        // To start, collect bounds from user:
        let mut param_bounds = self.tcx.required_region_bounds(generic.to_ty(self.tcx),
                                                               param_env.caller_bounds.to_vec());

        // Next, collect regions we scraped from the well-formedness
        // constraints in the fn signature. To do that, we walk the list
        // of known relations from the fn ctxt.
        //
        // This is crucial because otherwise code like this fails:
        //
        //     fn foo<'a, A>(x: &'a A) { x.bar() }
        //
        // The problem is that the type of `x` is `&'a A`. To be
        // well-formed, then, A must be lower-generic by `'a`, but we
        // don't know that this holds from first principles.
        for &(r, p) in &self.region_bound_pairs {
            debug!("generic={:?} p={:?}",
                   generic,
                   p);
            if generic == p {
                param_bounds.push(r);
            }
        }

        param_bounds
    }

    fn declared_projection_bounds_from_trait(&self,
                                             span: Span,
                                             projection_ty: ty::ProjectionTy<'tcx>)
                                             -> Vec<ty::Region<'tcx>>
    {
        debug!("projection_bounds(projection_ty={:?})",
               projection_ty);
        let item_name = projection_ty.item_name(self.tcx);
        let ty = self.tcx.mk_projection(projection_ty.trait_ref.clone(),
                                        item_name);

        // Say we have a projection `<T as SomeTrait<'a>>::SomeType`. We are interested
        // in looking for a trait definition like:
        //
        // ```
        // trait SomeTrait<'a> {
        //     type SomeType : 'a;
        // }
        // ```
        //
        // we can thus deduce that `<T as SomeTrait<'a>>::SomeType : 'a`.
        let trait_predicates = self.tcx.predicates_of(projection_ty.trait_ref.def_id);
        assert_eq!(trait_predicates.parent, None);
        let predicates = trait_predicates.predicates.as_slice().to_vec();
        traits::elaborate_predicates(self.tcx, predicates)
            .filter_map(|predicate| {
                // we're only interesting in `T : 'a` style predicates:
                let outlives = match predicate {
                    ty::Predicate::TypeOutlives(data) => data,
                    _ => { return None; }
                };

                debug!("projection_bounds: outlives={:?} (1)",
                       outlives);

                // apply the substitutions (and normalize any projected types)
                let outlives = self.instantiate_type_scheme(span,
                                                            projection_ty.trait_ref.substs,
                                                            &outlives);

                debug!("projection_bounds: outlives={:?} (2)",
                       outlives);

                let region_result = self.commit_if_ok(|_| {
                    let (outlives, _) =
                        self.replace_late_bound_regions_with_fresh_var(
                            span,
                            infer::AssocTypeProjection(projection_ty.item_name(self.tcx)),
                            &outlives);

                    debug!("projection_bounds: outlives={:?} (3)",
                           outlives);

                    // check whether this predicate applies to our current projection
                    let cause = self.fcx.misc(span);
                    match self.eq_types(false, &cause, ty, outlives.0) {
                        Ok(ok) => {
                            self.register_infer_ok_obligations(ok);
                            Ok(outlives.1)
                        }
                        Err(_) => { Err(()) }
                    }
                });

                debug!("projection_bounds: region_result={:?}",
                       region_result);

                region_result.ok()
            })
            .collect()
    }
}