rust/src/librustc/middle/expr_use_visitor.rs

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// Copyright 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.
/*!
* A different sort of visitor for walking fn bodies. Unlike the
* normal visitor, which just walks the entire body in one shot, the
* `ExprUseVisitor` determines how expressions are being used.
*/
use mc = middle::mem_categorization;
use middle::freevars;
use middle::pat_util;
use middle::ty;
use middle::typeck;
use syntax::ast;
use syntax::ast_util;
use syntax::codemap::{Span};
use util::ppaux::Repr;
///////////////////////////////////////////////////////////////////////////
// The Delegate trait
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/// This trait defines the callbacks you can expect to receive when
/// employing the ExprUseVisitor.
pub trait Delegate {
// The value found at `cmt` is either copied or moved, depending
// on mode.
fn consume(&mut self,
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consume_id: ast::NodeId,
consume_span: Span,
cmt: mc::cmt,
mode: ConsumeMode);
// The value found at `cmt` is either copied or moved via the
// pattern binding `consume_pat`, depending on mode.
fn consume_pat(&mut self,
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consume_pat: &ast::Pat,
cmt: mc::cmt,
mode: ConsumeMode);
// The value found at `borrow` is being borrowed at the point
// `borrow_id` for the region `loan_region` with kind `bk`.
fn borrow(&mut self,
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borrow_id: ast::NodeId,
borrow_span: Span,
cmt: mc::cmt,
loan_region: ty::Region,
bk: ty::BorrowKind,
loan_cause: LoanCause);
// The local variable `id` is declared but not initialized.
fn decl_without_init(&mut self,
id: ast::NodeId,
span: Span);
// The path at `cmt` is being assigned to.
fn mutate(&mut self,
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assignment_id: ast::NodeId,
assignment_span: Span,
assignee_cmt: mc::cmt,
mode: MutateMode);
}
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#[deriving(Eq)]
pub enum LoanCause {
ClosureCapture(Span),
AddrOf,
AutoRef,
RefBinding,
OverloadedOperator,
ClosureInvocation
}
#[deriving(Eq,Show)]
pub enum ConsumeMode {
Copy, // reference to x where x has a type that copies
Move, // reference to x where x has a type that moves
}
#[deriving(Eq,Show)]
pub enum MutateMode {
JustWrite, // x = y
WriteAndRead, // x += y
}
///////////////////////////////////////////////////////////////////////////
// The ExprUseVisitor type
//
// This is the code that actually walks the tree. Like
// mem_categorization, it requires a TYPER, which is a type that
// supplies types from the tree. After type checking is complete, you
// can just use the tcx as the typer.
pub struct ExprUseVisitor<'d,'t,TYPER> {
typer: &'t TYPER,
mc: mc::MemCategorizationContext<'t,TYPER>,
delegate: &'d mut Delegate,
}
// If the TYPER results in an error, it's because the type check
// failed (or will fail, when the error is uncovered and reported
// during writeback). In this case, we just ignore this part of the
// code.
//
// Note that this macro appears similar to try!(), but, unlike try!(),
// it does not propagate the error.
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macro_rules! return_if_err(
($inp: expr) => (
match $inp {
Ok(v) => v,
Err(()) => return
}
)
)
impl<'d,'t,TYPER:mc::Typer> ExprUseVisitor<'d,'t,TYPER> {
pub fn new(delegate: &'d mut Delegate,
typer: &'t TYPER)
-> ExprUseVisitor<'d,'t,TYPER> {
ExprUseVisitor { typer: typer,
mc: mc::MemCategorizationContext::new(typer),
delegate: delegate }
}
pub fn walk_fn(&mut self,
decl: &ast::FnDecl,
body: &ast::Block) {
self.walk_arg_patterns(decl, body);
self.walk_block(body);
}
fn walk_arg_patterns(&mut self,
decl: &ast::FnDecl,
body: &ast::Block) {
for arg in decl.inputs.iter() {
let arg_ty = ty::node_id_to_type(self.tcx(), arg.pat.id);
let arg_cmt = self.mc.cat_rvalue(
arg.id,
arg.pat.span,
ty::ReScope(body.id), // Args live only as long as the fn body.
arg_ty);
self.walk_pat(arg_cmt, arg.pat);
}
}
fn tcx<'a>(&'a self) -> &'a ty::ctxt {
self.typer.tcx()
}
fn delegate_consume(&mut self,
consume_id: ast::NodeId,
consume_span: Span,
cmt: mc::cmt) {
let mode = copy_or_move(self.tcx(), cmt.ty);
self.delegate.consume(consume_id, consume_span, cmt, mode);
}
fn consume_exprs(&mut self, exprs: &Vec<@ast::Expr>) {
for &expr in exprs.iter() {
self.consume_expr(expr);
}
}
fn consume_expr(&mut self, expr: &ast::Expr) {
debug!("consume_expr(expr={})", expr.repr(self.tcx()));
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let cmt = return_if_err!(self.mc.cat_expr(expr));
self.delegate_consume(expr.id, expr.span, cmt);
match expr.node {
ast::ExprParen(subexpr) => {
// Argh but is ExprParen horrible. So, if we consume
// `(x)`, that generally is also consuming `x`, UNLESS
// there are adjustments on the `(x)` expression
// (e.g., autoderefs and autorefs).
if self.typer.adjustments().borrow().contains_key(&expr.id) {
self.walk_expr(expr);
} else {
self.consume_expr(subexpr);
}
}
_ => {
self.walk_expr(expr)
}
}
}
fn mutate_expr(&mut self,
assignment_expr: &ast::Expr,
expr: &ast::Expr,
mode: MutateMode) {
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let cmt = return_if_err!(self.mc.cat_expr(expr));
self.delegate.mutate(assignment_expr.id, assignment_expr.span, cmt, mode);
self.walk_expr(expr);
}
fn borrow_expr(&mut self,
expr: &ast::Expr,
r: ty::Region,
bk: ty::BorrowKind,
cause: LoanCause) {
debug!("borrow_expr(expr={}, r={}, bk={})",
expr.repr(self.tcx()), r.repr(self.tcx()), bk.repr(self.tcx()));
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let cmt = return_if_err!(self.mc.cat_expr(expr));
self.delegate.borrow(expr.id, expr.span, cmt, r, bk, cause);
// Note: Unlike consume, we can ignore ExprParen. cat_expr
// already skips over them, and walk will uncover any
// attachments or whatever.
self.walk_expr(expr)
}
fn select_from_expr(&mut self, expr: &ast::Expr) {
self.walk_expr(expr)
}
fn walk_expr(&mut self, expr: &ast::Expr) {
debug!("walk_expr(expr={})", expr.repr(self.tcx()));
self.walk_adjustment(expr);
match expr.node {
ast::ExprParen(subexpr) => {
self.walk_expr(subexpr)
}
ast::ExprPath(..) => { }
ast::ExprUnary(ast::UnDeref, base) => { // *base
if !self.walk_overloaded_operator(expr, base, []) {
self.select_from_expr(base);
}
}
ast::ExprField(base, _, _) => { // base.f
self.select_from_expr(base);
}
ast::ExprIndex(lhs, rhs) => { // lhs[rhs]
if !self.walk_overloaded_operator(expr, lhs, [rhs]) {
self.select_from_expr(lhs);
self.consume_expr(rhs);
}
}
ast::ExprCall(callee, ref args) => { // callee(args)
self.walk_callee(expr, callee);
self.consume_exprs(args);
}
ast::ExprMethodCall(_, _, ref args) => { // callee.m(args)
self.consume_exprs(args);
}
ast::ExprStruct(_, ref fields, opt_with) => {
self.walk_struct_expr(expr, fields, opt_with);
}
ast::ExprTup(ref exprs) => {
self.consume_exprs(exprs);
}
ast::ExprIf(cond_expr, then_blk, opt_else_expr) => {
self.consume_expr(cond_expr);
self.walk_block(then_blk);
for else_expr in opt_else_expr.iter() {
self.consume_expr(*else_expr);
}
}
ast::ExprMatch(discr, ref arms) => {
// treatment of the discriminant is handled while
// walking the arms:
self.walk_expr(discr);
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let discr_cmt = return_if_err!(self.mc.cat_expr(discr));
for arm in arms.iter() {
self.walk_arm(discr_cmt.clone(), arm);
}
}
ast::ExprVec(ref exprs) => {
self.consume_exprs(exprs);
}
ast::ExprAddrOf(m, base) => { // &base
// make sure that the thing we are pointing out stays valid
// for the lifetime `scope_r` of the resulting ptr:
let expr_ty = ty::expr_ty(self.tcx(), expr);
if !ty::type_is_bot(expr_ty) {
let r = ty::ty_region(self.tcx(), expr.span, expr_ty);
let bk = ty::BorrowKind::from_mutbl(m);
self.borrow_expr(base, r, bk, AddrOf);
} else {
self.walk_expr(base);
}
}
ast::ExprInlineAsm(ref ia) => {
for &(_, input) in ia.inputs.iter() {
self.consume_expr(input);
}
for &(_, output) in ia.outputs.iter() {
self.mutate_expr(expr, output, JustWrite);
}
}
ast::ExprBreak(..) |
ast::ExprAgain(..) |
ast::ExprLit(..) => {}
ast::ExprLoop(blk, _) => {
self.walk_block(blk);
}
ast::ExprWhile(cond_expr, blk) => {
self.consume_expr(cond_expr);
self.walk_block(blk);
}
ast::ExprForLoop(..) => fail!("non-desugared expr_for_loop"),
ast::ExprUnary(_, lhs) => {
if !self.walk_overloaded_operator(expr, lhs, []) {
self.consume_expr(lhs);
}
}
ast::ExprBinary(_, lhs, rhs) => {
if !self.walk_overloaded_operator(expr, lhs, [rhs]) {
self.consume_expr(lhs);
self.consume_expr(rhs);
}
}
ast::ExprBlock(blk) => {
self.walk_block(blk);
}
ast::ExprRet(ref opt_expr) => {
for expr in opt_expr.iter() {
self.consume_expr(*expr);
}
}
ast::ExprAssign(lhs, rhs) => {
self.mutate_expr(expr, lhs, JustWrite);
self.consume_expr(rhs);
}
ast::ExprCast(base, _) => {
self.consume_expr(base);
}
ast::ExprAssignOp(_, lhs, rhs) => {
// This will have to change if/when we support
// overloaded operators for `+=` and so forth.
self.mutate_expr(expr, lhs, WriteAndRead);
self.consume_expr(rhs);
}
ast::ExprRepeat(base, count) => {
self.consume_expr(base);
self.consume_expr(count);
}
ast::ExprFnBlock(..) |
ast::ExprProc(..) => {
self.walk_captures(expr)
}
ast::ExprVstore(base, _) => {
self.consume_expr(base);
}
ast::ExprBox(place, base) => {
self.consume_expr(place);
self.consume_expr(base);
}
ast::ExprMac(..) => {
self.tcx().sess.span_bug(
expr.span,
"macro expression remains after expansion");
}
}
}
fn walk_callee(&mut self, call: &ast::Expr, callee: &ast::Expr) {
let callee_ty = ty::expr_ty_adjusted(self.tcx(), callee);
debug!("walk_callee: callee={} callee_ty={}",
callee.repr(self.tcx()), callee_ty.repr(self.tcx()));
match ty::get(callee_ty).sty {
ty::ty_bare_fn(..) => {
self.consume_expr(callee);
}
ty::ty_closure(ref f) => {
match f.onceness {
ast::Many => {
self.borrow_expr(callee,
ty::ReScope(call.id),
ty::UniqueImmBorrow,
ClosureInvocation);
}
ast::Once => {
self.consume_expr(callee);
}
}
}
_ => {
self.tcx().sess.span_bug(
callee.span,
format!("unxpected callee type {}",
callee_ty.repr(self.tcx())));
}
}
}
fn walk_stmt(&mut self, stmt: &ast::Stmt) {
match stmt.node {
ast::StmtDecl(decl, _) => {
match decl.node {
ast::DeclLocal(local) => {
self.walk_local(local);
}
ast::DeclItem(_) => {
// we don't visit nested items in this visitor,
// only the fn body we were given.
}
}
}
ast::StmtExpr(expr, _) |
ast::StmtSemi(expr, _) => {
self.consume_expr(expr);
}
ast::StmtMac(..) => {
self.tcx().sess.span_bug(
stmt.span,
format!("unexpanded stmt macro"));
}
}
}
fn walk_local(&mut self, local: @ast::Local) {
match local.init {
None => {
let delegate = &mut self.delegate;
pat_util::pat_bindings(&self.typer.tcx().def_map, local.pat, |_, id, span, _| {
delegate.decl_without_init(id, span);
})
}
Some(expr) => {
// Variable declarations with
// initializers are considered
// "assigns", which is handled by
// `walk_pat`:
self.walk_expr(expr);
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let init_cmt = return_if_err!(self.mc.cat_expr(expr));
self.walk_pat(init_cmt, local.pat);
}
}
}
fn walk_block(&mut self, blk: &ast::Block) {
/*!
* Indicates that the value of `blk` will be consumed,
* meaning either copied or moved depending on its type.
*/
debug!("walk_block(blk.id={:?})", blk.id);
for stmt in blk.stmts.iter() {
self.walk_stmt(*stmt);
}
for tail_expr in blk.expr.iter() {
self.consume_expr(*tail_expr);
}
}
fn walk_struct_expr(&mut self,
_expr: &ast::Expr,
fields: &Vec<ast::Field>,
opt_with: Option<@ast::Expr>) {
// Consume the expressions supplying values for each field.
for field in fields.iter() {
self.consume_expr(field.expr);
}
let with_expr = match opt_with {
Some(w) => { w }
None => { return; }
};
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let with_cmt = return_if_err!(self.mc.cat_expr(with_expr));
// Select just those fields of the `with`
// expression that will actually be used
let with_fields = match ty::get(with_cmt.ty).sty {
ty::ty_struct(did, ref substs) => {
ty::struct_fields(self.tcx(), did, substs)
}
_ => {
self.tcx().sess.span_bug(
with_expr.span,
format!("with expression doesn't evaluate to a struct"));
}
};
// Consume those fields of the with expression that are needed.
for with_field in with_fields.iter() {
if !contains_field_named(with_field, fields) {
let cmt_field = self.mc.cat_field(with_expr,
with_cmt.clone(),
with_field.ident,
with_field.mt.ty);
self.delegate_consume(with_expr.id, with_expr.span, cmt_field);
}
}
fn contains_field_named(field: &ty::field,
fields: &Vec<ast::Field>)
-> bool
{
fields.iter().any(
|f| f.ident.node.name == field.ident.name)
}
}
// Invoke the appropriate delegate calls for anything that gets
// consumed or borrowed as part of the automatic adjustment
// process.
fn walk_adjustment(&mut self, expr: &ast::Expr) {
let typer = self.typer;
match typer.adjustments().borrow().find(&expr.id) {
None => { }
Some(adjustment) => {
match *adjustment {
ty::AutoAddEnv(..) |
ty::AutoObject(..) => {
// Creating an object or closure consumes the
// input and stores it into the resulting rvalue.
debug!("walk_adjustment(AutoAddEnv|AutoObject)");
let cmt_unadjusted =
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return_if_err!(self.mc.cat_expr_unadjusted(expr));
self.delegate_consume(expr.id, expr.span, cmt_unadjusted);
}
ty::AutoDerefRef(ty::AutoDerefRef {
autoref: ref opt_autoref,
autoderefs: n
}) => {
self.walk_autoderefs(expr, n);
match *opt_autoref {
None => { }
Some(ref r) => {
self.walk_autoref(expr, r, n);
}
}
}
}
}
}
}
fn walk_autoderefs(&mut self,
expr: &ast::Expr,
autoderefs: uint) {
/*!
* Autoderefs for overloaded Deref calls in fact reference
* their receiver. That is, if we have `(*x)` where `x` is of
* type `Rc<T>`, then this in fact is equivalent to
* `x.deref()`. Since `deref()` is declared with `&self`, this
* is an autoref of `x`.
*/
debug!("walk_autoderefs expr={} autoderefs={}", expr.repr(self.tcx()), autoderefs);
for i in range(0, autoderefs) {
let deref_id = typeck::MethodCall::autoderef(expr.id, i as u32);
match self.typer.node_method_ty(deref_id) {
None => {}
Some(method_ty) => {
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let cmt = return_if_err!(self.mc.cat_expr_autoderefd(expr, i));
let self_ty = *ty::ty_fn_args(method_ty).get(0);
let (m, r) = match ty::get(self_ty).sty {
ty::ty_rptr(r, ref m) => (m.mutbl, r),
_ => self.tcx().sess.span_bug(expr.span,
format!("bad overloaded deref type {}",
method_ty.repr(self.tcx())))
};
let bk = ty::BorrowKind::from_mutbl(m);
self.delegate.borrow(expr.id, expr.span, cmt,
r, bk, AutoRef);
}
}
}
}
fn walk_autoref(&mut self,
expr: &ast::Expr,
autoref: &ty::AutoRef,
autoderefs: uint) {
debug!("walk_autoref expr={} autoderefs={}", expr.repr(self.tcx()), autoderefs);
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let cmt_derefd = return_if_err!(
self.mc.cat_expr_autoderefd(expr, autoderefs));
debug!("walk_autoref: cmt_derefd={}", cmt_derefd.repr(self.tcx()));
match *autoref {
ty::AutoPtr(r, m) => {
self.delegate.borrow(expr.id,
expr.span,
cmt_derefd,
r,
ty::BorrowKind::from_mutbl(m),
AutoRef)
}
ty::AutoBorrowVec(r, m) | ty::AutoBorrowVecRef(r, m) => {
let cmt_index = self.mc.cat_index(expr, cmt_derefd, autoderefs+1);
self.delegate.borrow(expr.id,
expr.span,
cmt_index,
r,
ty::BorrowKind::from_mutbl(m),
AutoRef)
}
ty::AutoBorrowObj(r, m) => {
let cmt_deref = self.mc.cat_deref_obj(expr, cmt_derefd);
self.delegate.borrow(expr.id,
expr.span,
cmt_deref,
r,
ty::BorrowKind::from_mutbl(m),
AutoRef)
}
ty::AutoUnsafe(_) => {}
}
}
fn walk_overloaded_operator(&mut self,
expr: &ast::Expr,
receiver: &ast::Expr,
args: &[@ast::Expr])
-> bool
{
if !self.typer.is_method_call(expr.id) {
return false;
}
self.walk_expr(receiver);
// Arguments (but not receivers) to overloaded operator
// methods are implicitly autoref'd which sadly does not use
// adjustments, so we must hardcode the borrow here.
let r = ty::ReScope(expr.id);
let bk = ty::ImmBorrow;
for &arg in args.iter() {
self.borrow_expr(arg, r, bk, OverloadedOperator);
}
return true;
}
fn walk_arm(&mut self, discr_cmt: mc::cmt, arm: &ast::Arm) {
for &pat in arm.pats.iter() {
self.walk_pat(discr_cmt.clone(), pat);
}
for guard in arm.guard.iter() {
self.consume_expr(*guard);
}
self.consume_expr(arm.body);
}
fn walk_pat(&mut self, cmt_discr: mc::cmt, pat: @ast::Pat) {
debug!("walk_pat cmt_discr={} pat={}", cmt_discr.repr(self.tcx()),
pat.repr(self.tcx()));
let mc = &self.mc;
let typer = self.typer;
let tcx = typer.tcx();
let def_map = &self.typer.tcx().def_map;
let delegate = &mut self.delegate;
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return_if_err!(mc.cat_pattern(cmt_discr, pat, |mc, cmt_pat, pat| {
if pat_util::pat_is_binding(def_map, pat) {
let tcx = typer.tcx();
debug!("binding cmt_pat={} pat={}",
cmt_pat.repr(tcx),
pat.repr(tcx));
// pat_ty: the type of the binding being produced.
let pat_ty = ty::node_id_to_type(tcx, pat.id);
// Each match binding is effectively an assignment to the
// binding being produced.
let def = def_map.borrow().get_copy(&pat.id);
match mc.cat_def(pat.id, pat.span, pat_ty, def) {
Ok(binding_cmt) => {
delegate.mutate(pat.id, pat.span, binding_cmt, JustWrite);
}
Err(_) => { }
}
// It is also a borrow or copy/move of the value being matched.
match pat.node {
ast::PatIdent(ast::BindByRef(m), _, _) => {
let (r, bk) = {
(ty::ty_region(tcx, pat.span, pat_ty),
ty::BorrowKind::from_mutbl(m))
};
delegate.borrow(pat.id, pat.span, cmt_pat,
r, bk, RefBinding);
}
ast::PatIdent(ast::BindByValue(_), _, _) => {
let mode = copy_or_move(typer.tcx(), cmt_pat.ty);
delegate.consume_pat(pat, cmt_pat, mode);
}
_ => {
typer.tcx().sess.span_bug(
pat.span,
"binding pattern not an identifier");
}
}
} else {
match pat.node {
ast::PatVec(_, Some(slice_pat), _) => {
// The `slice_pat` here creates a slice into
// the original vector. This is effectively a
// borrow of the elements of the vector being
// matched.
let (slice_cmt, slice_mutbl, slice_r) = {
match mc.cat_slice_pattern(cmt_pat, slice_pat) {
Ok(v) => v,
Err(()) => {
tcx.sess.span_bug(slice_pat.span,
"Err from mc")
}
}
};
// Note: We declare here that the borrow
// occurs upon entering the `[...]`
// pattern. This implies that something like
// `[a, ..b]` where `a` is a move is illegal,
// because the borrow is already in effect.
// In fact such a move would be safe-ish, but
// it effectively *requires* that we use the
// nulling out semantics to indicate when a
// value has been moved, which we are trying
// to move away from. Otherwise, how can we
// indicate that the first element in the
// vector has been moved? Eventually, we
// could perhaps modify this rule to permit
// `[..a, b]` where `b` is a move, because in
// that case we can adjust the length of the
// original vec accordingly, but we'd have to
// make trans do the right thing, and it would
// only work for `~` vectors. It seems simpler
// to just require that people call
// `vec.pop()` or `vec.unshift()`.
let slice_bk = ty::BorrowKind::from_mutbl(slice_mutbl);
delegate.borrow(pat.id, pat.span,
slice_cmt, slice_r,
slice_bk, RefBinding);
}
_ => { }
}
}
}));
}
fn walk_captures(&mut self, closure_expr: &ast::Expr) {
debug!("walk_captures({})", closure_expr.repr(self.tcx()));
let tcx = self.typer.tcx();
freevars::with_freevars(tcx, closure_expr.id, |freevars| {
match freevars::get_capture_mode(self.tcx(), closure_expr.id) {
freevars::CaptureByRef => {
self.walk_by_ref_captures(closure_expr, freevars);
}
freevars::CaptureByValue => {
self.walk_by_value_captures(closure_expr, freevars);
}
}
});
}
fn walk_by_ref_captures(&mut self,
closure_expr: &ast::Expr,
freevars: &[freevars::freevar_entry]) {
for freevar in freevars.iter() {
let id_var = ast_util::def_id_of_def(freevar.def).node;
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let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.id,
closure_expr.span,
freevar.def));
// Lookup the kind of borrow the callee requires, as
// inferred by regionbk
let upvar_id = ty::UpvarId { var_id: id_var,
closure_expr_id: closure_expr.id };
let upvar_borrow = self.tcx().upvar_borrow_map.borrow()
.get_copy(&upvar_id);
self.delegate.borrow(closure_expr.id,
closure_expr.span,
cmt_var,
upvar_borrow.region,
upvar_borrow.kind,
ClosureCapture(freevar.span));
}
}
fn walk_by_value_captures(&mut self,
closure_expr: &ast::Expr,
freevars: &[freevars::freevar_entry]) {
for freevar in freevars.iter() {
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let cmt_var = return_if_err!(self.cat_captured_var(closure_expr.id,
closure_expr.span,
freevar.def));
self.delegate_consume(closure_expr.id, freevar.span, cmt_var);
}
}
fn cat_captured_var(&mut self,
closure_id: ast::NodeId,
closure_span: Span,
upvar_def: ast::Def)
-> mc::McResult<mc::cmt> {
// Create the cmt for the variable being borrowed, from the
// caller's perspective
let var_id = ast_util::def_id_of_def(upvar_def).node;
let var_ty = ty::node_id_to_type(self.tcx(), var_id);
self.mc.cat_def(closure_id, closure_span, var_ty, upvar_def)
}
}
fn copy_or_move(tcx: &ty::ctxt, ty: ty::t) -> ConsumeMode {
if ty::type_moves_by_default(tcx, ty) { Move } else { Copy }
}