/
check_loans.rs
827 lines (737 loc) · 30.9 KB
/
check_loans.rs
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// Copyright 2012-2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
// ----------------------------------------------------------------------
// Checking loans
//
// Phase 2 of check: we walk down the tree and check that:
// 1. assignments are always made to mutable locations;
// 2. loans made in overlapping scopes do not conflict
// 3. assignments do not affect things loaned out as immutable
// 4. moves do not affect things loaned out in any way
use std::hashmap::HashSet;
use mc = middle::mem_categorization;
use middle::borrowck::*;
use middle::moves;
use middle::ty;
use syntax::ast::{MutImmutable, MutMutable};
use syntax::ast;
use syntax::ast_util;
use syntax::codemap::Span;
use syntax::visit::Visitor;
use syntax::visit;
use util::ppaux::Repr;
#[deriving(Clone)]
struct CheckLoanCtxt<'a> {
bccx: &'a BorrowckCtxt,
dfcx_loans: &'a LoanDataFlow,
move_data: @move_data::FlowedMoveData,
all_loans: &'a [Loan],
reported: @mut HashSet<ast::NodeId>,
}
impl<'a> Visitor<()> for CheckLoanCtxt<'a> {
fn visit_expr(&mut self, ex:@ast::Expr, _:()) {
check_loans_in_expr(self, ex);
}
fn visit_local(&mut self, l:@ast::Local, _:()) {
check_loans_in_local(self, l);
}
fn visit_block(&mut self, b:ast::P<ast::Block>, _:()) {
check_loans_in_block(self, b);
}
fn visit_pat(&mut self, p:&ast::Pat, _:()) {
check_loans_in_pat(self, p);
}
fn visit_fn(&mut self, fk:&visit::fn_kind, fd:&ast::fn_decl,
b:ast::P<ast::Block>, s:Span, n:ast::NodeId, _:()) {
check_loans_in_fn(self, fk, fd, b, s, n);
}
// FIXME(#10894) should continue recursing
fn visit_ty(&mut self, _t: &ast::Ty, _: ()) {}
}
pub fn check_loans(bccx: &BorrowckCtxt,
dfcx_loans: &LoanDataFlow,
move_data: move_data::FlowedMoveData,
all_loans: &[Loan],
body: ast::P<ast::Block>) {
debug!("check_loans(body id={:?})", body.id);
let mut clcx = CheckLoanCtxt {
bccx: bccx,
dfcx_loans: dfcx_loans,
move_data: @move_data,
all_loans: all_loans,
reported: @mut HashSet::new(),
};
clcx.visit_block(body, ());
}
enum MoveError {
MoveOk,
MoveWhileBorrowed(/*loan*/@LoanPath, /*loan*/Span)
}
impl<'a> CheckLoanCtxt<'a> {
pub fn tcx(&self) -> ty::ctxt { self.bccx.tcx }
pub fn each_issued_loan(&self, scope_id: ast::NodeId, op: |&Loan| -> bool)
-> bool {
//! Iterates over each loan that has been issued
//! on entrance to `scope_id`, regardless of whether it is
//! actually *in scope* at that point. Sometimes loans
//! are issued for future scopes and thus they may have been
//! *issued* but not yet be in effect.
self.dfcx_loans.each_bit_on_entry_frozen(scope_id, |loan_index| {
let loan = &self.all_loans[loan_index];
op(loan)
})
}
pub fn each_in_scope_loan(&self,
scope_id: ast::NodeId,
op: |&Loan| -> bool)
-> bool {
//! Like `each_issued_loan()`, but only considers loans that are
//! currently in scope.
let region_maps = self.tcx().region_maps;
self.each_issued_loan(scope_id, |loan| {
if region_maps.is_subscope_of(scope_id, loan.kill_scope) {
op(loan)
} else {
true
}
})
}
pub fn each_in_scope_restriction(&self,
scope_id: ast::NodeId,
loan_path: @LoanPath,
op: |&Loan, &Restriction| -> bool)
-> bool {
//! Iterates through all the in-scope restrictions for the
//! given `loan_path`
self.each_in_scope_loan(scope_id, |loan| {
let mut ret = true;
for restr in loan.restrictions.iter() {
if restr.loan_path == loan_path {
if !op(loan, restr) {
ret = false;
break;
}
}
}
ret
})
}
pub fn loans_generated_by(&self, scope_id: ast::NodeId) -> ~[uint] {
//! Returns a vector of the loans that are generated as
//! we encounter `scope_id`.
let mut result = ~[];
self.dfcx_loans.each_gen_bit_frozen(scope_id, |loan_index| {
result.push(loan_index);
true
});
return result;
}
pub fn check_for_conflicting_loans(&self, scope_id: ast::NodeId) {
//! Checks to see whether any of the loans that are issued
//! by `scope_id` conflict with loans that have already been
//! issued when we enter `scope_id` (for example, we do not
//! permit two `&mut` borrows of the same variable).
debug!("check_for_conflicting_loans(scope_id={:?})", scope_id);
let new_loan_indices = self.loans_generated_by(scope_id);
debug!("new_loan_indices = {:?}", new_loan_indices);
self.each_issued_loan(scope_id, |issued_loan| {
for &new_loan_index in new_loan_indices.iter() {
let new_loan = &self.all_loans[new_loan_index];
self.report_error_if_loans_conflict(issued_loan, new_loan);
}
true
});
for (i, &x) in new_loan_indices.iter().enumerate() {
let old_loan = &self.all_loans[x];
for &y in new_loan_indices.slice_from(i+1).iter() {
let new_loan = &self.all_loans[y];
self.report_error_if_loans_conflict(old_loan, new_loan);
}
}
}
pub fn report_error_if_loans_conflict(&self,
old_loan: &Loan,
new_loan: &Loan) {
//! Checks whether `old_loan` and `new_loan` can safely be issued
//! simultaneously.
debug!("report_error_if_loans_conflict(old_loan={}, new_loan={})",
old_loan.repr(self.tcx()),
new_loan.repr(self.tcx()));
// Should only be called for loans that are in scope at the same time.
let region_maps = self.tcx().region_maps;
assert!(region_maps.scopes_intersect(old_loan.kill_scope,
new_loan.kill_scope));
self.report_error_if_loan_conflicts_with_restriction(
old_loan, new_loan, old_loan, new_loan) &&
self.report_error_if_loan_conflicts_with_restriction(
new_loan, old_loan, old_loan, new_loan);
}
pub fn report_error_if_loan_conflicts_with_restriction(&self,
loan1: &Loan,
loan2: &Loan,
old_loan: &Loan,
new_loan: &Loan)
-> bool {
//! Checks whether the restrictions introduced by `loan1` would
//! prohibit `loan2`. Returns false if an error is reported.
debug!("report_error_if_loan_conflicts_with_restriction(\
loan1={}, loan2={})",
loan1.repr(self.tcx()),
loan2.repr(self.tcx()));
// Restrictions that would cause the new loan to be illegal:
let illegal_if = match loan2.mutbl {
MutableMutability => RESTR_ALIAS | RESTR_FREEZE | RESTR_CLAIM,
ImmutableMutability => RESTR_ALIAS | RESTR_FREEZE,
ConstMutability => RESTR_ALIAS,
};
debug!("illegal_if={:?}", illegal_if);
for restr in loan1.restrictions.iter() {
if !restr.set.intersects(illegal_if) { continue; }
if restr.loan_path != loan2.loan_path { continue; }
match (new_loan.mutbl, old_loan.mutbl) {
(MutableMutability, MutableMutability) => {
self.bccx.span_err(
new_loan.span,
format!("cannot borrow `{}` as mutable \
more than once at a time",
self.bccx.loan_path_to_str(new_loan.loan_path)));
self.bccx.span_note(
old_loan.span,
format!("previous borrow of `{}` as mutable occurs here",
self.bccx.loan_path_to_str(new_loan.loan_path)));
return false;
}
_ => {
self.bccx.span_err(
new_loan.span,
format!("cannot borrow `{}` as {} because \
it is also borrowed as {}",
self.bccx.loan_path_to_str(new_loan.loan_path),
self.bccx.mut_to_str(new_loan.mutbl),
self.bccx.mut_to_str(old_loan.mutbl)));
self.bccx.span_note(
old_loan.span,
format!("previous borrow of `{}` occurs here",
self.bccx.loan_path_to_str(new_loan.loan_path)));
return false;
}
}
}
true
}
pub fn is_local_variable(&self, cmt: mc::cmt) -> bool {
match cmt.cat {
mc::cat_local(_) => true,
_ => false
}
}
pub fn check_if_path_is_moved(&self,
id: ast::NodeId,
span: Span,
use_kind: MovedValueUseKind,
lp: @LoanPath) {
/*!
* Reports an error if `expr` (which should be a path)
* is using a moved/uninitialized value
*/
debug!("check_if_path_is_moved(id={:?}, use_kind={:?}, lp={})",
id, use_kind, lp.repr(self.bccx.tcx));
self.move_data.each_move_of(id, lp, |move, moved_lp| {
self.bccx.report_use_of_moved_value(
span,
use_kind,
lp,
move,
moved_lp);
false
});
}
pub fn check_assignment(&self, expr: @ast::Expr) {
// We don't use cat_expr() here because we don't want to treat
// auto-ref'd parameters in overloaded operators as rvalues.
let cmt = match self.bccx.tcx.adjustments.find(&expr.id) {
None => self.bccx.cat_expr_unadjusted(expr),
Some(&adj) => self.bccx.cat_expr_autoderefd(expr, adj)
};
debug!("check_assignment(cmt={})", cmt.repr(self.tcx()));
// Mutable values can be assigned, as long as they obey loans
// and aliasing restrictions:
if cmt.mutbl.is_mutable() {
if check_for_aliasable_mutable_writes(self, expr, cmt) {
if check_for_assignment_to_restricted_or_frozen_location(
self, expr, cmt)
{
// Safe, but record for lint pass later:
mark_variable_as_used_mut(self, cmt);
}
}
return;
}
// For immutable local variables, assignments are legal
// if they cannot already have been assigned
if self.is_local_variable(cmt) {
assert!(cmt.mutbl.is_immutable()); // no "const" locals
let lp = opt_loan_path(cmt).unwrap();
self.move_data.each_assignment_of(expr.id, lp, |assign| {
self.bccx.report_reassigned_immutable_variable(
expr.span,
lp,
assign);
false
});
return;
}
// Otherwise, just a plain error.
self.bccx.span_err(
expr.span,
format!("cannot assign to {} {}",
cmt.mutbl.to_user_str(),
self.bccx.cmt_to_str(cmt)));
return;
fn mark_variable_as_used_mut(this: &CheckLoanCtxt,
cmt: mc::cmt) {
//! If the mutability of the `cmt` being written is inherited
//! from a local variable, liveness will
//! not have been able to detect that this variable's mutability
//! is important, so we must add the variable to the
//! `used_mut_nodes` table here.
let mut cmt = cmt;
loop {
debug!("mark_writes_through_upvars_as_used_mut(cmt={})",
cmt.repr(this.tcx()));
match cmt.cat {
mc::cat_local(id) |
mc::cat_arg(id) |
mc::cat_self(id) => {
this.tcx().used_mut_nodes.insert(id);
return;
}
mc::cat_stack_upvar(b) => {
cmt = b;
}
mc::cat_rvalue(..) |
mc::cat_static_item |
mc::cat_copied_upvar(..) |
mc::cat_deref(_, _, mc::unsafe_ptr(..)) |
mc::cat_deref(_, _, mc::gc_ptr(..)) |
mc::cat_deref(_, _, mc::region_ptr(..)) => {
assert_eq!(cmt.mutbl, mc::McDeclared);
return;
}
mc::cat_discr(b, _) |
mc::cat_deref(b, _, mc::uniq_ptr) => {
assert_eq!(cmt.mutbl, mc::McInherited);
cmt = b;
}
mc::cat_downcast(b) |
mc::cat_interior(b, _) => {
if cmt.mutbl == mc::McInherited {
cmt = b;
} else {
return; // field declared as mutable or some such
}
}
}
}
}
fn check_for_aliasable_mutable_writes(this: &CheckLoanCtxt,
expr: @ast::Expr,
cmt: mc::cmt) -> bool {
//! Safety checks related to writes to aliasable, mutable locations
let guarantor = cmt.guarantor();
debug!("check_for_aliasable_mutable_writes(cmt={}, guarantor={})",
cmt.repr(this.tcx()), guarantor.repr(this.tcx()));
match guarantor.cat {
mc::cat_deref(b, _, mc::region_ptr(MutMutable, _)) => {
// Statically prohibit writes to `&mut` when aliasable
check_for_aliasability_violation(this, expr, b);
}
mc::cat_deref(_, deref_count, mc::gc_ptr(ast::MutMutable)) => {
// Dynamically check writes to `@mut`
let key = root_map_key {
id: guarantor.id,
derefs: deref_count
};
debug!("Inserting write guard at {:?}", key);
this.bccx.write_guard_map.insert(key);
}
_ => {}
}
return true; // no errors reported
}
fn check_for_aliasability_violation(this: &CheckLoanCtxt,
expr: @ast::Expr,
cmt: mc::cmt) -> bool {
let mut cmt = cmt;
loop {
match cmt.cat {
mc::cat_deref(b, _, mc::region_ptr(MutMutable, _)) |
mc::cat_downcast(b) |
mc::cat_stack_upvar(b) |
mc::cat_deref(b, _, mc::uniq_ptr) |
mc::cat_interior(b, _) |
mc::cat_discr(b, _) => {
// Aliasability depends on base cmt
cmt = b;
}
mc::cat_copied_upvar(_) |
mc::cat_rvalue(..) |
mc::cat_local(..) |
mc::cat_arg(_) |
mc::cat_self(..) |
mc::cat_deref(_, _, mc::unsafe_ptr(..)) |
mc::cat_static_item(..) |
mc::cat_deref(_, _, mc::gc_ptr(_)) |
mc::cat_deref(_, _, mc::region_ptr(MutImmutable, _)) => {
// Aliasability is independent of base cmt
match cmt.freely_aliasable() {
None => {
return true;
}
Some(cause) => {
this.bccx.report_aliasability_violation(
expr.span,
MutabilityViolation,
cause);
return false;
}
}
}
}
}
}
fn check_for_assignment_to_restricted_or_frozen_location(
this: &CheckLoanCtxt,
expr: @ast::Expr,
cmt: mc::cmt) -> bool
{
//! Check for assignments that violate the terms of an
//! outstanding loan.
let loan_path = match opt_loan_path(cmt) {
Some(lp) => lp,
None => { return true; /* no loan path, can't be any loans */ }
};
// Start by searching for an assignment to a *restricted*
// location. Here is one example of the kind of error caught
// by this check:
//
// let mut v = ~[1, 2, 3];
// let p = &v;
// v = ~[4];
//
// In this case, creating `p` triggers a RESTR_MUTATE
// restriction on the path `v`.
//
// Here is a second, more subtle example:
//
// let mut v = ~[1, 2, 3];
// let p = &const v[0];
// v[0] = 4; // OK
// v[1] = 5; // OK
// v = ~[4, 5, 3]; // Error
//
// In this case, `p` is pointing to `v[0]`, and it is a
// `const` pointer in any case. So the first two
// assignments are legal (and would be permitted by this
// check). However, the final assignment (which is
// logically equivalent) is forbidden, because it would
// cause the existing `v` array to be freed, thus
// invalidating `p`. In the code, this error results
// because `gather_loans::restrictions` adds a
// `RESTR_MUTATE` restriction whenever the contents of an
// owned pointer are borrowed, and hence while `v[*]` is not
// restricted from being written, `v` is.
let cont = this.each_in_scope_restriction(expr.id,
loan_path,
|loan, restr| {
if restr.set.intersects(RESTR_MUTATE) {
this.report_illegal_mutation(expr, loan_path, loan);
false
} else {
true
}
});
if !cont { return false }
// The previous code handled assignments to paths that
// have been restricted. This covers paths that have been
// directly lent out and their base paths, but does not
// cover random extensions of those paths. For example,
// the following program is not declared illegal by the
// previous check:
//
// let mut v = ~[1, 2, 3];
// let p = &v;
// v[0] = 4; // declared error by loop below, not code above
//
// The reason that this passes the previous check whereas
// an assignment like `v = ~[4]` fails is because the assignment
// here is to `v[*]`, and the existing restrictions were issued
// for `v`, not `v[*]`.
//
// So in this loop, we walk back up the loan path so long
// as the mutability of the path is dependent on a super
// path, and check that the super path was not lent out as
// mutable or immutable (a const loan is ok).
//
// Mutability of a path can be dependent on the super path
// in two ways. First, it might be inherited mutability.
// Second, the pointee of an `&mut` pointer can only be
// mutated if it is found in an unaliased location, so we
// have to check that the owner location is not borrowed.
//
// Note that we are *not* checking for any and all
// restrictions. We are only interested in the pointers
// that the user created, whereas we add restrictions for
// all kinds of paths that are not directly aliased. If we checked
// for all restrictions, and not just loans, then the following
// valid program would be considered illegal:
//
// let mut v = ~[1, 2, 3];
// let p = &const v[0];
// v[1] = 5; // ok
//
// Here the restriction that `v` not be mutated would be misapplied
// to block the subpath `v[1]`.
let full_loan_path = loan_path;
let mut loan_path = loan_path;
loop {
match *loan_path {
// Peel back one layer if, for `loan_path` to be
// mutable, `lp_base` must be mutable. This occurs
// with inherited mutability and with `&mut`
// pointers.
LpExtend(lp_base, mc::McInherited, _) |
LpExtend(lp_base, _, LpDeref(mc::region_ptr(ast::MutMutable, _))) => {
loan_path = lp_base;
}
// Otherwise stop iterating
LpExtend(_, mc::McDeclared, _) |
LpExtend(_, mc::McImmutable, _) |
LpVar(_) => {
return true;
}
}
// Check for a non-const loan of `loan_path`
let cont = this.each_in_scope_loan(expr.id, |loan| {
if loan.loan_path == loan_path &&
loan.mutbl != ConstMutability {
this.report_illegal_mutation(expr,
full_loan_path,
loan);
false
} else {
true
}
});
if !cont { return false }
}
}
}
pub fn report_illegal_mutation(&self,
expr: @ast::Expr,
loan_path: &LoanPath,
loan: &Loan) {
self.bccx.span_err(
expr.span,
format!("cannot assign to `{}` because it is borrowed",
self.bccx.loan_path_to_str(loan_path)));
self.bccx.span_note(
loan.span,
format!("borrow of `{}` occurs here",
self.bccx.loan_path_to_str(loan_path)));
}
fn check_move_out_from_expr(&self, expr: @ast::Expr) {
match expr.node {
ast::ExprFnBlock(..) | ast::ExprProc(..) => {
// moves due to capture clauses are checked
// in `check_loans_in_fn`, so that we can
// give a better error message
}
_ => {
self.check_move_out_from_id(expr.id, expr.span)
}
}
}
fn check_move_out_from_id(&self, id: ast::NodeId, span: Span) {
self.move_data.each_path_moved_by(id, |_, move_path| {
match self.analyze_move_out_from(id, move_path) {
MoveOk => {}
MoveWhileBorrowed(loan_path, loan_span) => {
self.bccx.span_err(
span,
format!("cannot move out of `{}` \
because it is borrowed",
self.bccx.loan_path_to_str(move_path)));
self.bccx.span_note(
loan_span,
format!("borrow of `{}` occurs here",
self.bccx.loan_path_to_str(loan_path)));
}
}
true
});
}
pub fn analyze_move_out_from(&self,
expr_id: ast::NodeId,
move_path: @LoanPath) -> MoveError {
debug!("analyze_move_out_from(expr_id={:?}, move_path={})",
expr_id, move_path.repr(self.tcx()));
// FIXME(#4384) inadequare if/when we permit `move a.b`
let mut ret = MoveOk;
// check for a conflicting loan:
self.each_in_scope_restriction(expr_id, move_path, |loan, _| {
// Any restriction prevents moves.
ret = MoveWhileBorrowed(loan.loan_path, loan.span);
false
});
ret
}
pub fn check_call(&self,
_expr: @ast::Expr,
_callee: Option<@ast::Expr>,
_callee_id: ast::NodeId,
_callee_span: Span,
_args: &[@ast::Expr]) {
// NB: This call to check for conflicting loans is not truly
// necessary, because the callee_id never issues new loans.
// However, I added it for consistency and lest the system
// should change in the future.
//
// FIXME(#6268) nested method calls
// self.check_for_conflicting_loans(callee_id);
}
}
fn check_loans_in_fn<'a>(this: &mut CheckLoanCtxt<'a>,
fk: &visit::fn_kind,
decl: &ast::fn_decl,
body: ast::P<ast::Block>,
sp: Span,
id: ast::NodeId) {
match *fk {
visit::fk_item_fn(..) |
visit::fk_method(..) => {
// Don't process nested items.
return;
}
visit::fk_fn_block(..) => {
check_captured_variables(this, id, sp);
}
}
visit::walk_fn(this, fk, decl, body, sp, id, ());
fn check_captured_variables(this: &CheckLoanCtxt,
closure_id: ast::NodeId,
span: Span) {
let cap_vars = this.bccx.capture_map.get(&closure_id);
for cap_var in cap_vars.iter() {
let var_id = ast_util::def_id_of_def(cap_var.def).node;
let var_path = @LpVar(var_id);
this.check_if_path_is_moved(closure_id, span,
MovedInCapture, var_path);
match cap_var.mode {
moves::CapRef | moves::CapCopy => {}
moves::CapMove => {
check_by_move_capture(this, closure_id, cap_var, var_path);
}
}
}
return;
fn check_by_move_capture(this: &CheckLoanCtxt,
closure_id: ast::NodeId,
cap_var: &moves::CaptureVar,
move_path: @LoanPath) {
let move_err = this.analyze_move_out_from(closure_id, move_path);
match move_err {
MoveOk => {}
MoveWhileBorrowed(loan_path, loan_span) => {
this.bccx.span_err(
cap_var.span,
format!("cannot move `{}` into closure \
because it is borrowed",
this.bccx.loan_path_to_str(move_path)));
this.bccx.span_note(
loan_span,
format!("borrow of `{}` occurs here",
this.bccx.loan_path_to_str(loan_path)));
}
}
}
}
}
fn check_loans_in_local<'a>(this: &mut CheckLoanCtxt<'a>,
local: @ast::Local) {
visit::walk_local(this, local, ());
}
fn check_loans_in_expr<'a>(this: &mut CheckLoanCtxt<'a>,
expr: @ast::Expr) {
visit::walk_expr(this, expr, ());
debug!("check_loans_in_expr(expr={})",
expr.repr(this.tcx()));
this.check_for_conflicting_loans(expr.id);
this.check_move_out_from_expr(expr);
match expr.node {
ast::ExprSelf |
ast::ExprPath(..) => {
if !this.move_data.is_assignee(expr.id) {
let cmt = this.bccx.cat_expr_unadjusted(expr);
debug!("path cmt={}", cmt.repr(this.tcx()));
let r = opt_loan_path(cmt);
for &lp in r.iter() {
this.check_if_path_is_moved(expr.id, expr.span, MovedInUse, lp);
}
}
}
ast::ExprAssign(dest, _) |
ast::ExprAssignOp(_, _, dest, _) => {
this.check_assignment(dest);
}
ast::ExprCall(f, ref args, _) => {
this.check_call(expr, Some(f), f.id, f.span, *args);
}
ast::ExprMethodCall(callee_id, _, _, _, ref args, _) => {
this.check_call(expr, None, callee_id, expr.span, *args);
}
ast::ExprIndex(callee_id, _, rval) |
ast::ExprBinary(callee_id, _, _, rval)
if this.bccx.method_map.contains_key(&expr.id) => {
this.check_call(expr,
None,
callee_id,
expr.span,
[rval]);
}
ast::ExprUnary(callee_id, _, _) | ast::ExprIndex(callee_id, _, _)
if this.bccx.method_map.contains_key(&expr.id) => {
this.check_call(expr,
None,
callee_id,
expr.span,
[]);
}
ast::ExprInlineAsm(ref ia) => {
for &(_, out) in ia.outputs.iter() {
this.check_assignment(out);
}
}
_ => { }
}
}
fn check_loans_in_pat<'a>(this: &mut CheckLoanCtxt<'a>,
pat: &ast::Pat)
{
this.check_for_conflicting_loans(pat.id);
this.check_move_out_from_id(pat.id, pat.span);
visit::walk_pat(this, pat, ());
}
fn check_loans_in_block<'a>(this: &mut CheckLoanCtxt<'a>,
blk: ast::P<ast::Block>)
{
visit::walk_block(this, blk, ());
this.check_for_conflicting_loans(blk.id);
}