1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
12 * A classic liveness analysis based on dataflow over the AST. Computes,
13 * for each local variable in a function, whether that variable is live
14 * at a given point. Program execution points are identified by their
19 * The basic model is that each local variable is assigned an index. We
20 * represent sets of local variables using a vector indexed by this
21 * index. The value in the vector is either 0, indicating the variable
22 * is dead, or the id of an expression that uses the variable.
24 * We conceptually walk over the AST in reverse execution order. If we
25 * find a use of a variable, we add it to the set of live variables. If
26 * we find an assignment to a variable, we remove it from the set of live
27 * variables. When we have to merge two flows, we take the union of
28 * those two flows---if the variable is live on both paths, we simply
29 * pick one id. In the event of loops, we continue doing this until a
30 * fixed point is reached.
32 * ## Checking initialization
34 * At the function entry point, all variables must be dead. If this is
35 * not the case, we can report an error using the id found in the set of
36 * live variables, which identifies a use of the variable which is not
37 * dominated by an assignment.
41 * After each explicit move, the variable must be dead.
43 * ## Computing last uses
45 * Any use of the variable where the variable is dead afterwards is a
48 * # Implementation details
50 * The actual implementation contains two (nested) walks over the AST.
51 * The outer walk has the job of building up the ir_maps instance for the
52 * enclosing function. On the way down the tree, it identifies those AST
53 * nodes and variable IDs that will be needed for the liveness analysis
54 * and assigns them contiguous IDs. The liveness id for an AST node is
55 * called a `live_node` (it's a newtype'd uint) and the id for a variable
56 * is called a `variable` (another newtype'd uint).
58 * On the way back up the tree, as we are about to exit from a function
59 * declaration we allocate a `liveness` instance. Now that we know
60 * precisely how many nodes and variables we need, we can allocate all
61 * the various arrays that we will need to precisely the right size. We then
62 * perform the actual propagation on the `liveness` instance.
64 * This propagation is encoded in the various `propagate_through_*()`
65 * methods. It effectively does a reverse walk of the AST; whenever we
66 * reach a loop node, we iterate until a fixed point is reached.
68 * ## The `Users` struct
70 * At each live node `N`, we track three pieces of information for each
71 * variable `V` (these are encapsulated in the `Users` struct):
73 * - `reader`: the `LiveNode` ID of some node which will read the value
74 * that `V` holds on entry to `N`. Formally: a node `M` such
75 * that there exists a path `P` from `N` to `M` where `P` does not
76 * write `V`. If the `reader` is `invalid_node()`, then the current
77 * value will never be read (the variable is dead, essentially).
79 * - `writer`: the `LiveNode` ID of some node which will write the
80 * variable `V` and which is reachable from `N`. Formally: a node `M`
81 * such that there exists a path `P` from `N` to `M` and `M` writes
82 * `V`. If the `writer` is `invalid_node()`, then there is no writer
83 * of `V` that follows `N`.
85 * - `used`: a boolean value indicating whether `V` is *used*. We
86 * distinguish a *read* from a *use* in that a *use* is some read that
87 * is not just used to generate a new value. For example, `x += 1` is
88 * a read but not a use. This is used to generate better warnings.
90 * ## Special Variables
92 * We generate various special variables for various, well, special purposes.
93 * These are described in the `specials` struct:
95 * - `exit_ln`: a live node that is generated to represent every 'exit' from
96 * the function, whether it be by explicit return, panic, or other means.
98 * - `fallthrough_ln`: a live node that represents a fallthrough
100 * - `no_ret_var`: a synthetic variable that is only 'read' from, the
101 * fallthrough node. This allows us to detect functions where we fail
102 * to return explicitly.
103 * - `clean_exit_var`: a synthetic variable that is only 'read' from the
104 * fallthrough node. It is only live if the function could converge
105 * via means other than an explicit `return` expression. That is, it is
106 * only dead if the end of the function's block can never be reached.
107 * It is the responsibility of typeck to ensure that there are no
108 * `return` expressions in a function declared as diverging.
112 use middle::mem_categorization::Typer;
113 use middle::pat_util;
117 use util::nodemap::NodeMap;
121 use std::mem::transmute;
127 use syntax::codemap::{BytePos, original_sp, Span};
128 use syntax::parse::token::special_idents;
129 use syntax::parse::token;
130 use syntax::print::pprust::{expr_to_string, block_to_string};
132 use syntax::{visit, ast_util};
133 use syntax::visit::{Visitor, FnKind};
135 /// For use with `propagate_through_loop`.
137 /// An endless `loop` loop.
139 /// A `while` loop, with the given expression as condition.
141 /// A `for` loop, with the given pattern to bind.
145 #[deriving(PartialEq)]
146 struct Variable(uint);
147 #[deriving(PartialEq)]
148 struct LiveNode(uint);
151 fn get(&self) -> uint { let Variable(v) = *self; v }
155 fn get(&self) -> uint { let LiveNode(v) = *self; v }
158 impl Clone for LiveNode {
159 fn clone(&self) -> LiveNode {
164 #[deriving(PartialEq, Show)]
172 fn live_node_kind_to_string(lnk: LiveNodeKind, cx: &ty::ctxt) -> String {
173 let cm = cx.sess.codemap();
176 format!("Free var node [{}]", cm.span_to_string(s))
179 format!("Expr node [{}]", cm.span_to_string(s))
182 format!("Var def node [{}]", cm.span_to_string(s))
184 ExitNode => "Exit node".to_string(),
188 impl<'a, 'tcx, 'v> Visitor<'v> for IrMaps<'a, 'tcx> {
189 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl,
190 b: &'v Block, s: Span, n: NodeId) {
191 visit_fn(self, fk, fd, b, s, n);
193 fn visit_local(&mut self, l: &ast::Local) { visit_local(self, l); }
194 fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); }
195 fn visit_arm(&mut self, a: &Arm) { visit_arm(self, a); }
198 pub fn check_crate(tcx: &ty::ctxt) {
199 visit::walk_crate(&mut IrMaps::new(tcx), tcx.map.krate());
200 tcx.sess.abort_if_errors();
203 impl fmt::Show for LiveNode {
204 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
205 write!(f, "ln({})", self.get())
209 impl fmt::Show for Variable {
210 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
211 write!(f, "v({})", self.get())
215 // ______________________________________________________________________
218 // This is the first pass and the one that drives the main
219 // computation. It walks up and down the IR once. On the way down,
220 // we count for each function the number of variables as well as
221 // liveness nodes. A liveness node is basically an expression or
222 // capture clause that does something of interest: either it has
223 // interesting control flow or it uses/defines a local variable.
225 // On the way back up, at each function node we create liveness sets
226 // (we now know precisely how big to make our various vectors and so
227 // forth) and then do the data-flow propagation to compute the set
228 // of live variables at each program point.
230 // Finally, we run back over the IR one last time and, using the
231 // computed liveness, check various safety conditions. For example,
232 // there must be no live nodes at the definition site for a variable
233 // unless it has an initializer. Similarly, each non-mutable local
234 // variable must not be assigned if there is some successor
235 // assignment. And so forth.
238 fn is_valid(&self) -> bool {
239 self.get() != uint::MAX
243 fn invalid_node() -> LiveNode { LiveNode(uint::MAX) }
264 struct IrMaps<'a, 'tcx: 'a> {
265 tcx: &'a ty::ctxt<'tcx>,
267 num_live_nodes: uint,
269 live_node_map: NodeMap<LiveNode>,
270 variable_map: NodeMap<Variable>,
271 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
272 var_kinds: Vec<VarKind>,
273 lnks: Vec<LiveNodeKind>,
276 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
277 fn new(tcx: &'a ty::ctxt<'tcx>) -> IrMaps<'a, 'tcx> {
282 live_node_map: NodeMap::new(),
283 variable_map: NodeMap::new(),
284 capture_info_map: NodeMap::new(),
285 var_kinds: Vec::new(),
290 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
291 let ln = LiveNode(self.num_live_nodes);
293 self.num_live_nodes += 1;
295 debug!("{} is of kind {}", ln.to_string(),
296 live_node_kind_to_string(lnk, self.tcx));
301 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
302 let ln = self.add_live_node(lnk);
303 self.live_node_map.insert(node_id, ln);
305 debug!("{} is node {}", ln.to_string(), node_id);
308 fn add_variable(&mut self, vk: VarKind) -> Variable {
309 let v = Variable(self.num_vars);
310 self.var_kinds.push(vk);
314 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
315 self.variable_map.insert(node_id, v);
317 ImplicitRet | CleanExit => {}
320 debug!("{} is {}", v.to_string(), vk);
325 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
326 match self.variable_map.find(&node_id) {
331 .span_bug(span, format!("no variable registered for id {}",
332 node_id).as_slice());
337 fn variable_name(&self, var: Variable) -> String {
338 match self.var_kinds[var.get()] {
339 Local(LocalInfo { ident: nm, .. }) | Arg(_, nm) => {
340 token::get_ident(nm).get().to_string()
342 ImplicitRet => "<implicit-ret>".to_string(),
343 CleanExit => "<clean-exit>".to_string()
347 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
348 self.capture_info_map.insert(node_id, Rc::new(cs));
351 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
356 impl<'a, 'tcx, 'v> Visitor<'v> for Liveness<'a, 'tcx> {
357 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl, b: &'v Block, s: Span, n: NodeId) {
358 check_fn(self, fk, fd, b, s, n);
360 fn visit_local(&mut self, l: &ast::Local) {
361 check_local(self, l);
363 fn visit_expr(&mut self, ex: &Expr) {
364 check_expr(self, ex);
366 fn visit_arm(&mut self, a: &Arm) {
371 fn visit_fn(ir: &mut IrMaps,
377 debug!("visit_fn: id={}", id);
378 let _i = ::util::common::indenter();
380 // swap in a new set of IR maps for this function body:
381 let mut fn_maps = IrMaps::new(ir.tcx);
384 debug!("creating fn_maps: {}",
385 transmute::<&IrMaps, *const IrMaps>(&fn_maps));
388 for arg in decl.inputs.iter() {
389 pat_util::pat_bindings(&ir.tcx.def_map,
391 |_bm, arg_id, _x, path1| {
392 debug!("adding argument {}", arg_id);
393 let ident = path1.node;
394 fn_maps.add_variable(Arg(arg_id, ident));
398 // gather up the various local variables, significant expressions,
400 visit::walk_fn(&mut fn_maps, fk, decl, body, sp);
402 // Special nodes and variables:
403 // - exit_ln represents the end of the fn, either by return or panic
404 // - implicit_ret_var is a pseudo-variable that represents
405 // an implicit return
406 let specials = Specials {
407 exit_ln: fn_maps.add_live_node(ExitNode),
408 fallthrough_ln: fn_maps.add_live_node(ExitNode),
409 no_ret_var: fn_maps.add_variable(ImplicitRet),
410 clean_exit_var: fn_maps.add_variable(CleanExit)
414 let mut lsets = Liveness::new(&mut fn_maps, specials);
415 let entry_ln = lsets.compute(decl, body);
417 // check for various error conditions
418 lsets.visit_block(body);
419 lsets.check_ret(id, sp, fk, entry_ln, body);
420 lsets.warn_about_unused_args(decl, entry_ln);
423 fn visit_local(ir: &mut IrMaps, local: &ast::Local) {
424 pat_util::pat_bindings(&ir.tcx.def_map, &*local.pat, |_, p_id, sp, path1| {
425 debug!("adding local variable {}", p_id);
426 let name = path1.node;
427 ir.add_live_node_for_node(p_id, VarDefNode(sp));
428 ir.add_variable(Local(LocalInfo {
433 visit::walk_local(ir, local);
436 fn visit_arm(ir: &mut IrMaps, arm: &Arm) {
437 for pat in arm.pats.iter() {
438 pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
439 debug!("adding local variable {} from match with bm {}",
441 let name = path1.node;
442 ir.add_live_node_for_node(p_id, VarDefNode(sp));
443 ir.add_variable(Local(LocalInfo {
449 visit::walk_arm(ir, arm);
452 fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
454 // live nodes required for uses or definitions of variables:
456 let def = ir.tcx.def_map.borrow().get_copy(&expr.id);
457 debug!("expr {}: path that leads to {}", expr.id, def);
459 DefLocal(..) => ir.add_live_node_for_node(expr.id, ExprNode(expr.span)),
462 visit::walk_expr(ir, expr);
464 ExprFnBlock(..) | ExprProc(..) | ExprUnboxedFn(..) => {
465 // Interesting control flow (for loops can contain labeled
466 // breaks or continues)
467 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
469 // Make a live_node for each captured variable, with the span
470 // being the location that the variable is used. This results
471 // in better error messages than just pointing at the closure
472 // construction site.
473 let mut call_caps = Vec::new();
474 ty::with_freevars(ir.tcx, expr.id, |freevars| {
475 for fv in freevars.iter() {
478 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
479 call_caps.push(CaptureInfo {ln: fv_ln,
486 ir.set_captures(expr.id, call_caps);
488 visit::walk_expr(ir, expr);
491 // live nodes required for interesting control flow:
492 ExprIf(..) | ExprMatch(..) | ExprWhile(..) | ExprLoop(..) => {
493 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
494 visit::walk_expr(ir, expr);
497 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
499 ExprWhileLet(..) => {
500 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
502 ExprForLoop(ref pat, _, _, _) => {
503 pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
504 debug!("adding local variable {} from for loop with bm {}",
506 let name = path1.node;
507 ir.add_live_node_for_node(p_id, VarDefNode(sp));
508 ir.add_variable(Local(LocalInfo {
513 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
514 visit::walk_expr(ir, expr);
516 ExprBinary(op, _, _) if ast_util::lazy_binop(op) => {
517 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
518 visit::walk_expr(ir, expr);
521 // otherwise, live nodes are not required:
522 ExprIndex(..) | ExprField(..) | ExprTupField(..) | ExprVec(..) |
523 ExprCall(..) | ExprMethodCall(..) | ExprTup(..) | ExprSlice(..) |
524 ExprBinary(..) | ExprAddrOf(..) |
525 ExprCast(..) | ExprUnary(..) | ExprBreak(_) |
526 ExprAgain(_) | ExprLit(_) | ExprRet(..) | ExprBlock(..) |
527 ExprAssign(..) | ExprAssignOp(..) | ExprMac(..) |
528 ExprStruct(..) | ExprRepeat(..) | ExprParen(..) |
529 ExprInlineAsm(..) | ExprBox(..) => {
530 visit::walk_expr(ir, expr);
535 // ______________________________________________________________________
536 // Computing liveness sets
538 // Actually we compute just a bit more than just liveness, but we use
539 // the same basic propagation framework in all cases.
548 fn invalid_users() -> Users {
550 reader: invalid_node(),
551 writer: invalid_node(),
558 fallthrough_ln: LiveNode,
559 no_ret_var: Variable,
560 clean_exit_var: Variable
563 static ACC_READ: uint = 1u;
564 static ACC_WRITE: uint = 2u;
565 static ACC_USE: uint = 4u;
567 struct Liveness<'a, 'tcx: 'a> {
568 ir: &'a mut IrMaps<'a, 'tcx>,
570 successors: Vec<LiveNode>,
572 // The list of node IDs for the nested loop scopes
574 loop_scope: Vec<NodeId>,
575 // mappings from loop node ID to LiveNode
576 // ("break" label should map to loop node ID,
577 // it probably doesn't now)
578 break_ln: NodeMap<LiveNode>,
579 cont_ln: NodeMap<LiveNode>
582 impl<'a, 'tcx> Liveness<'a, 'tcx> {
583 fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> {
584 let num_live_nodes = ir.num_live_nodes;
585 let num_vars = ir.num_vars;
589 successors: Vec::from_elem(num_live_nodes, invalid_node()),
590 users: Vec::from_elem(num_live_nodes * num_vars, invalid_users()),
591 loop_scope: Vec::new(),
592 break_ln: NodeMap::new(),
593 cont_ln: NodeMap::new(),
597 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
598 match self.ir.live_node_map.find(&node_id) {
601 // This must be a mismatch between the ir_map construction
602 // above and the propagation code below; the two sets of
603 // code have to agree about which AST nodes are worth
604 // creating liveness nodes for.
605 self.ir.tcx.sess.span_bug(
607 format!("no live node registered for node {}",
608 node_id).as_slice());
613 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
614 self.ir.variable(node_id, span)
617 fn pat_bindings(&mut self,
619 f: |&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId|) {
620 pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| {
621 let ln = self.live_node(p_id, sp);
622 let var = self.variable(p_id, sp);
623 f(self, ln, var, sp, p_id);
627 fn arm_pats_bindings(&mut self,
629 f: |&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId|) {
632 self.pat_bindings(pat, f);
638 fn define_bindings_in_pat(&mut self, pat: &Pat, succ: LiveNode)
640 self.define_bindings_in_arm_pats(Some(pat), succ)
643 fn define_bindings_in_arm_pats(&mut self, pat: Option<&Pat>, succ: LiveNode)
646 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
647 this.init_from_succ(ln, succ);
648 this.define(ln, var);
654 fn idx(&self, ln: LiveNode, var: Variable) -> uint {
655 ln.get() * self.ir.num_vars + var.get()
658 fn live_on_entry(&self, ln: LiveNode, var: Variable)
659 -> Option<LiveNodeKind> {
660 assert!(ln.is_valid());
661 let reader = self.users[self.idx(ln, var)].reader;
662 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
666 Is this variable live on entry to any of its successor nodes?
668 fn live_on_exit(&self, ln: LiveNode, var: Variable)
669 -> Option<LiveNodeKind> {
670 let successor = self.successors[ln.get()];
671 self.live_on_entry(successor, var)
674 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
675 assert!(ln.is_valid());
676 self.users[self.idx(ln, var)].used
679 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
680 -> Option<LiveNodeKind> {
681 assert!(ln.is_valid());
682 let writer = self.users[self.idx(ln, var)].writer;
683 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
686 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
687 -> Option<LiveNodeKind> {
688 let successor = self.successors[ln.get()];
689 self.assigned_on_entry(successor, var)
692 fn indices2(&mut self,
695 op: |&mut Liveness<'a, 'tcx>, uint, uint|) {
696 let node_base_idx = self.idx(ln, Variable(0u));
697 let succ_base_idx = self.idx(succ_ln, Variable(0u));
698 for var_idx in range(0u, self.ir.num_vars) {
699 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
706 test: |uint| -> LiveNode) -> io::IoResult<()> {
707 let node_base_idx = self.idx(ln, Variable(0));
708 for var_idx in range(0u, self.ir.num_vars) {
709 let idx = node_base_idx + var_idx;
710 if test(idx).is_valid() {
711 try!(write!(wr, " {}", Variable(var_idx).to_string()));
717 fn find_loop_scope(&self,
718 opt_label: Option<Ident>,
724 // Refers to a labeled loop. Use the results of resolve
726 match self.ir.tcx.def_map.borrow().find(&id) {
727 Some(&DefLabel(loop_id)) => loop_id,
728 _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \
729 doesn't refer to a loop")
733 // Vanilla 'break' or 'loop', so use the enclosing
735 if self.loop_scope.len() == 0 {
736 self.ir.tcx.sess.span_bug(sp, "break outside loop");
738 *self.loop_scope.last().unwrap()
744 #[allow(unused_must_use)]
745 fn ln_str(&self, ln: LiveNode) -> String {
746 let mut wr = io::MemWriter::new();
748 let wr = &mut wr as &mut io::Writer;
749 write!(wr, "[ln({}) of kind {} reads", ln.get(), self.ir.lnk(ln));
750 self.write_vars(wr, ln, |idx| self.users[idx].reader);
751 write!(wr, " writes");
752 self.write_vars(wr, ln, |idx| self.users[idx].writer);
753 write!(wr, " precedes {}]", self.successors[ln.get()].to_string());
755 str::from_utf8(wr.unwrap().as_slice()).unwrap().to_string()
758 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
759 *self.successors.get_mut(ln.get()) = succ_ln;
761 // It is not necessary to initialize the
762 // values to empty because this is the value
763 // they have when they are created, and the sets
764 // only grow during iterations.
766 // self.indices(ln) { |idx|
767 // self.users[idx] = invalid_users();
771 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
772 // more efficient version of init_empty() / merge_from_succ()
773 *self.successors.get_mut(ln.get()) = succ_ln;
775 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
776 *this.users.get_mut(idx) = this.users[succ_idx]
778 debug!("init_from_succ(ln={}, succ={})",
779 self.ln_str(ln), self.ln_str(succ_ln));
782 fn merge_from_succ(&mut self,
787 if ln == succ_ln { return false; }
789 let mut changed = false;
790 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
791 changed |= copy_if_invalid(this.users[succ_idx].reader,
792 &mut this.users.get_mut(idx).reader);
793 changed |= copy_if_invalid(this.users[succ_idx].writer,
794 &mut this.users.get_mut(idx).writer);
795 if this.users[succ_idx].used && !this.users[idx].used {
796 this.users.get_mut(idx).used = true;
801 debug!("merge_from_succ(ln={}, succ={}, first_merge={}, changed={})",
802 ln.to_string(), self.ln_str(succ_ln), first_merge, changed);
805 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
806 if src.is_valid() && !dst.is_valid() {
815 // Indicates that a local variable was *defined*; we know that no
816 // uses of the variable can precede the definition (resolve checks
817 // this) so we just clear out all the data.
818 fn define(&mut self, writer: LiveNode, var: Variable) {
819 let idx = self.idx(writer, var);
820 self.users.get_mut(idx).reader = invalid_node();
821 self.users.get_mut(idx).writer = invalid_node();
823 debug!("{} defines {} (idx={}): {}", writer.to_string(), var.to_string(),
824 idx, self.ln_str(writer));
827 // Either read, write, or both depending on the acc bitset
828 fn acc(&mut self, ln: LiveNode, var: Variable, acc: uint) {
829 debug!("{} accesses[{:x}] {}: {}",
830 ln.to_string(), acc, var.to_string(), self.ln_str(ln));
832 let idx = self.idx(ln, var);
833 let user = self.users.get_mut(idx);
835 if (acc & ACC_WRITE) != 0 {
836 user.reader = invalid_node();
840 // Important: if we both read/write, must do read second
841 // or else the write will override.
842 if (acc & ACC_READ) != 0 {
846 if (acc & ACC_USE) != 0 {
851 // _______________________________________________________________________
853 fn compute(&mut self, decl: &FnDecl, body: &Block) -> LiveNode {
854 // if there is a `break` or `again` at the top level, then it's
855 // effectively a return---this only occurs in `for` loops,
856 // where the body is really a closure.
858 debug!("compute: using id for block, {}", block_to_string(body));
860 let exit_ln = self.s.exit_ln;
861 let entry_ln: LiveNode =
862 self.with_loop_nodes(body.id, exit_ln, exit_ln,
863 |this| this.propagate_through_fn_block(decl, body));
865 // hack to skip the loop unless debug! is enabled:
866 debug!("^^ liveness computation results for body {} (entry={})",
868 for ln_idx in range(0u, self.ir.num_live_nodes) {
869 debug!("{}", self.ln_str(LiveNode(ln_idx)));
873 entry_ln.to_string());
878 fn propagate_through_fn_block(&mut self, _: &FnDecl, blk: &Block)
880 // the fallthrough exit is only for those cases where we do not
881 // explicitly return:
883 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
884 if blk.expr.is_none() {
885 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
887 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
889 self.propagate_through_block(blk, s.fallthrough_ln)
892 fn propagate_through_block(&mut self, blk: &Block, succ: LiveNode)
894 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
895 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
896 self.propagate_through_stmt(&**stmt, succ)
900 fn propagate_through_stmt(&mut self, stmt: &Stmt, succ: LiveNode)
903 StmtDecl(ref decl, _) => {
904 self.propagate_through_decl(&**decl, succ)
907 StmtExpr(ref expr, _) | StmtSemi(ref expr, _) => {
908 self.propagate_through_expr(&**expr, succ)
912 self.ir.tcx.sess.span_bug(stmt.span, "unexpanded macro");
917 fn propagate_through_decl(&mut self, decl: &Decl, succ: LiveNode)
920 DeclLocal(ref local) => {
921 self.propagate_through_local(&**local, succ)
927 fn propagate_through_local(&mut self, local: &ast::Local, succ: LiveNode)
929 // Note: we mark the variable as defined regardless of whether
930 // there is an initializer. Initially I had thought to only mark
931 // the live variable as defined if it was initialized, and then we
932 // could check for uninit variables just by scanning what is live
933 // at the start of the function. But that doesn't work so well for
934 // immutable variables defined in a loop:
935 // loop { let x; x = 5; }
936 // because the "assignment" loops back around and generates an error.
938 // So now we just check that variables defined w/o an
939 // initializer are not live at the point of their
940 // initialization, which is mildly more complex than checking
941 // once at the func header but otherwise equivalent.
943 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
944 self.define_bindings_in_pat(&*local.pat, succ)
947 fn propagate_through_exprs(&mut self, exprs: &[P<Expr>], succ: LiveNode)
949 exprs.iter().rev().fold(succ, |succ, expr| {
950 self.propagate_through_expr(&**expr, succ)
954 fn propagate_through_opt_expr(&mut self,
955 opt_expr: Option<&Expr>,
958 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
961 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
963 debug!("propagate_through_expr: {}", expr_to_string(expr));
966 // Interesting cases with control flow or which gen/kill
969 self.access_path(expr, succ, ACC_READ | ACC_USE)
972 ExprField(ref e, _, _) => {
973 self.propagate_through_expr(&**e, succ)
976 ExprTupField(ref e, _, _) => {
977 self.propagate_through_expr(&**e, succ)
980 ExprFnBlock(_, _, ref blk) |
981 ExprProc(_, ref blk) |
982 ExprUnboxedFn(_, _, _, ref blk) => {
983 debug!("{} is an ExprFnBlock, ExprProc, or ExprUnboxedFn",
984 expr_to_string(expr));
987 The next-node for a break is the successor of the entire
988 loop. The next-node for a continue is the top of this loop.
990 let node = self.live_node(expr.id, expr.span);
991 self.with_loop_nodes(blk.id, succ, node, |this| {
993 // the construction of a closure itself is not important,
994 // but we have to consider the closed over variables.
995 let caps = match this.ir.capture_info_map.find(&expr.id) {
996 Some(caps) => caps.clone(),
998 this.ir.tcx.sess.span_bug(expr.span, "no registered caps");
1001 caps.iter().rev().fold(succ, |succ, cap| {
1002 this.init_from_succ(cap.ln, succ);
1003 let var = this.variable(cap.var_nid, expr.span);
1004 this.acc(cap.ln, var, ACC_READ | ACC_USE);
1010 ExprIf(ref cond, ref then, ref els) => {
1024 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
1025 let then_ln = self.propagate_through_block(&**then, succ);
1026 let ln = self.live_node(expr.id, expr.span);
1027 self.init_from_succ(ln, else_ln);
1028 self.merge_from_succ(ln, then_ln, false);
1029 self.propagate_through_expr(&**cond, ln)
1033 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
1036 ExprWhile(ref cond, ref blk, _) => {
1037 self.propagate_through_loop(expr, WhileLoop(&**cond), &**blk, succ)
1040 ExprWhileLet(..) => {
1041 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
1044 ExprForLoop(ref pat, ref head, ref blk, _) => {
1045 let ln = self.propagate_through_loop(expr, ForLoop(&**pat), &**blk, succ);
1046 self.propagate_through_expr(&**head, ln)
1049 // Note that labels have been resolved, so we don't need to look
1050 // at the label ident
1051 ExprLoop(ref blk, _) => {
1052 self.propagate_through_loop(expr, LoopLoop, &**blk, succ)
1055 ExprMatch(ref e, ref arms, _) => {
1070 let ln = self.live_node(expr.id, expr.span);
1071 self.init_empty(ln, succ);
1072 let mut first_merge = true;
1073 for arm in arms.iter() {
1075 self.propagate_through_expr(&*arm.body, succ);
1077 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1078 // only consider the first pattern; any later patterns must have
1079 // the same bindings, and we also consider the first pattern to be
1080 // the "authoritative" set of ids
1082 self.define_bindings_in_arm_pats(arm.pats.as_slice().head().map(|p| &**p),
1084 self.merge_from_succ(ln, arm_succ, first_merge);
1085 first_merge = false;
1087 self.propagate_through_expr(&**e, ln)
1090 ExprRet(ref o_e) => {
1091 // ignore succ and subst exit_ln:
1092 let exit_ln = self.s.exit_ln;
1093 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1096 ExprBreak(opt_label) => {
1097 // Find which label this break jumps to
1098 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1100 // Now that we know the label we're going to,
1101 // look it up in the break loop nodes table
1103 match self.break_ln.find(&sc) {
1105 None => self.ir.tcx.sess.span_bug(expr.span,
1106 "break to unknown label")
1110 ExprAgain(opt_label) => {
1111 // Find which label this expr continues to
1112 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1114 // Now that we know the label we're going to,
1115 // look it up in the continue loop nodes table
1117 match self.cont_ln.find(&sc) {
1119 None => self.ir.tcx.sess.span_bug(expr.span,
1120 "loop to unknown label")
1124 ExprAssign(ref l, ref r) => {
1125 // see comment on lvalues in
1126 // propagate_through_lvalue_components()
1127 let succ = self.write_lvalue(&**l, succ, ACC_WRITE);
1128 let succ = self.propagate_through_lvalue_components(&**l, succ);
1129 self.propagate_through_expr(&**r, succ)
1132 ExprAssignOp(_, ref l, ref r) => {
1133 // see comment on lvalues in
1134 // propagate_through_lvalue_components()
1135 let succ = self.write_lvalue(&**l, succ, ACC_WRITE|ACC_READ);
1136 let succ = self.propagate_through_expr(&**r, succ);
1137 self.propagate_through_lvalue_components(&**l, succ)
1140 // Uninteresting cases: just propagate in rev exec order
1142 ExprVec(ref exprs) => {
1143 self.propagate_through_exprs(exprs.as_slice(), succ)
1146 ExprRepeat(ref element, ref count) => {
1147 let succ = self.propagate_through_expr(&**count, succ);
1148 self.propagate_through_expr(&**element, succ)
1151 ExprStruct(_, ref fields, ref with_expr) => {
1152 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1153 fields.iter().rev().fold(succ, |succ, field| {
1154 self.propagate_through_expr(&*field.expr, succ)
1158 ExprCall(ref f, ref args) => {
1159 let diverges = !self.ir.tcx.is_method_call(expr.id) && {
1160 let t_ret = ty::ty_fn_ret(ty::expr_ty(self.ir.tcx, &**f));
1161 t_ret == ty::FnDiverging
1163 let succ = if diverges {
1168 let succ = self.propagate_through_exprs(args.as_slice(), succ);
1169 self.propagate_through_expr(&**f, succ)
1172 ExprMethodCall(_, _, ref args) => {
1173 let method_call = typeck::MethodCall::expr(expr.id);
1174 let method_ty = self.ir.tcx.method_map.borrow().find(&method_call).unwrap().ty;
1175 let diverges = ty::ty_fn_ret(method_ty) == ty::FnDiverging;
1176 let succ = if diverges {
1181 self.propagate_through_exprs(args.as_slice(), succ)
1184 ExprTup(ref exprs) => {
1185 self.propagate_through_exprs(exprs.as_slice(), succ)
1188 ExprBinary(op, ref l, ref r) if ast_util::lazy_binop(op) => {
1189 let r_succ = self.propagate_through_expr(&**r, succ);
1191 let ln = self.live_node(expr.id, expr.span);
1192 self.init_from_succ(ln, succ);
1193 self.merge_from_succ(ln, r_succ, false);
1195 self.propagate_through_expr(&**l, ln)
1198 ExprIndex(ref l, ref r) |
1199 ExprBinary(_, ref l, ref r) |
1200 ExprBox(ref l, ref r) => {
1201 let r_succ = self.propagate_through_expr(&**r, succ);
1202 self.propagate_through_expr(&**l, r_succ)
1205 ExprSlice(ref e1, ref e2, ref e3, _) => {
1206 let succ = e3.as_ref().map_or(succ, |e| self.propagate_through_expr(&**e, succ));
1207 let succ = e2.as_ref().map_or(succ, |e| self.propagate_through_expr(&**e, succ));
1208 self.propagate_through_expr(&**e1, succ)
1211 ExprAddrOf(_, ref e) |
1212 ExprCast(ref e, _) |
1213 ExprUnary(_, ref e) |
1214 ExprParen(ref e) => {
1215 self.propagate_through_expr(&**e, succ)
1218 ExprInlineAsm(ref ia) => {
1220 let succ = ia.outputs.iter().rev().fold(succ, |succ, &(_, ref expr, _)| {
1221 // see comment on lvalues
1222 // in propagate_through_lvalue_components()
1223 let succ = self.write_lvalue(&**expr, succ, ACC_WRITE);
1224 self.propagate_through_lvalue_components(&**expr, succ)
1226 // Inputs are executed first. Propagate last because of rev order
1227 ia.inputs.iter().rev().fold(succ, |succ, &(_, ref expr)| {
1228 self.propagate_through_expr(&**expr, succ)
1236 ExprBlock(ref blk) => {
1237 self.propagate_through_block(&**blk, succ)
1241 self.ir.tcx.sess.span_bug(expr.span, "unexpanded macro");
1246 fn propagate_through_lvalue_components(&mut self,
1252 // In general, the full flow graph structure for an
1253 // assignment/move/etc can be handled in one of two ways,
1254 // depending on whether what is being assigned is a "tracked
1255 // value" or not. A tracked value is basically a local
1256 // variable or argument.
1258 // The two kinds of graphs are:
1260 // Tracked lvalue Untracked lvalue
1261 // ----------------------++-----------------------
1265 // (rvalue) || (rvalue)
1268 // (write of lvalue) || (lvalue components)
1273 // ----------------------++-----------------------
1275 // I will cover the two cases in turn:
1277 // # Tracked lvalues
1279 // A tracked lvalue is a local variable/argument `x`. In
1280 // these cases, the link_node where the write occurs is linked
1281 // to node id of `x`. The `write_lvalue()` routine generates
1282 // the contents of this node. There are no subcomponents to
1285 // # Non-tracked lvalues
1287 // These are lvalues like `x[5]` or `x.f`. In that case, we
1288 // basically ignore the value which is written to but generate
1289 // reads for the components---`x` in these two examples. The
1290 // components reads are generated by
1291 // `propagate_through_lvalue_components()` (this fn).
1293 // # Illegal lvalues
1295 // It is still possible to observe assignments to non-lvalues;
1296 // these errors are detected in the later pass borrowck. We
1297 // just ignore such cases and treat them as reads.
1300 ExprPath(_) => succ,
1301 ExprField(ref e, _, _) => self.propagate_through_expr(&**e, succ),
1302 ExprTupField(ref e, _, _) => self.propagate_through_expr(&**e, succ),
1303 _ => self.propagate_through_expr(expr, succ)
1307 // see comment on propagate_through_lvalue()
1308 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: uint)
1311 ExprPath(_) => self.access_path(expr, succ, acc),
1313 // We do not track other lvalues, so just propagate through
1314 // to their subcomponents. Also, it may happen that
1315 // non-lvalues occur here, because those are detected in the
1316 // later pass borrowck.
1321 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: uint)
1323 match self.ir.tcx.def_map.borrow().get_copy(&expr.id) {
1325 let ln = self.live_node(expr.id, expr.span);
1327 self.init_from_succ(ln, succ);
1328 let var = self.variable(nid, expr.span);
1329 self.acc(ln, var, acc);
1337 fn propagate_through_loop(&mut self,
1346 We model control flow like this:
1364 let mut first_merge = true;
1365 let ln = self.live_node(expr.id, expr.span);
1366 self.init_empty(ln, succ);
1370 // If this is not a `loop` loop, then it's possible we bypass
1371 // the body altogether. Otherwise, the only way is via a `break`
1372 // in the loop body.
1373 self.merge_from_succ(ln, succ, first_merge);
1374 first_merge = false;
1377 debug!("propagate_through_loop: using id for loop body {} {}",
1378 expr.id, block_to_string(body));
1380 let cond_ln = match kind {
1382 ForLoop(ref pat) => self.define_bindings_in_pat(*pat, ln),
1383 WhileLoop(ref cond) => self.propagate_through_expr(&**cond, ln),
1385 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1386 this.propagate_through_block(body, cond_ln)
1389 // repeat until fixed point is reached:
1390 while self.merge_from_succ(ln, body_ln, first_merge) {
1391 first_merge = false;
1393 let new_cond_ln = match kind {
1395 ForLoop(ref pat) => {
1396 self.define_bindings_in_pat(*pat, ln)
1398 WhileLoop(ref cond) => {
1399 self.propagate_through_expr(&**cond, ln)
1402 assert!(cond_ln == new_cond_ln);
1403 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1404 |this| this.propagate_through_block(body, cond_ln)));
1410 fn with_loop_nodes<R>(&mut self,
1411 loop_node_id: NodeId,
1414 f: |&mut Liveness<'a, 'tcx>| -> R)
1416 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1417 self.loop_scope.push(loop_node_id);
1418 self.break_ln.insert(loop_node_id, break_ln);
1419 self.cont_ln.insert(loop_node_id, cont_ln);
1421 self.loop_scope.pop();
1426 // _______________________________________________________________________
1427 // Checking for error conditions
1429 fn check_local(this: &mut Liveness, local: &ast::Local) {
1432 this.warn_about_unused_or_dead_vars_in_pat(&*local.pat);
1435 this.pat_bindings(&*local.pat, |this, ln, var, sp, id| {
1436 this.warn_about_unused(sp, id, ln, var);
1441 visit::walk_local(this, local);
1444 fn check_arm(this: &mut Liveness, arm: &Arm) {
1445 // only consider the first pattern; any later patterns must have
1446 // the same bindings, and we also consider the first pattern to be
1447 // the "authoritative" set of ids
1448 this.arm_pats_bindings(arm.pats.as_slice().head().map(|p| &**p), |this, ln, var, sp, id| {
1449 this.warn_about_unused(sp, id, ln, var);
1451 visit::walk_arm(this, arm);
1454 fn check_expr(this: &mut Liveness, expr: &Expr) {
1456 ExprAssign(ref l, ref r) => {
1457 this.check_lvalue(&**l);
1458 this.visit_expr(&**r);
1460 visit::walk_expr(this, expr);
1463 ExprAssignOp(_, ref l, _) => {
1464 this.check_lvalue(&**l);
1466 visit::walk_expr(this, expr);
1469 ExprInlineAsm(ref ia) => {
1470 for &(_, ref input) in ia.inputs.iter() {
1471 this.visit_expr(&**input);
1474 // Output operands must be lvalues
1475 for &(_, ref out, _) in ia.outputs.iter() {
1476 this.check_lvalue(&**out);
1477 this.visit_expr(&**out);
1480 visit::walk_expr(this, expr);
1483 ExprForLoop(ref pat, _, _, _) => {
1484 this.pat_bindings(&**pat, |this, ln, var, sp, id| {
1485 this.warn_about_unused(sp, id, ln, var);
1488 visit::walk_expr(this, expr);
1491 // no correctness conditions related to liveness
1492 ExprCall(..) | ExprMethodCall(..) | ExprIf(..) | ExprMatch(..) |
1493 ExprWhile(..) | ExprLoop(..) | ExprIndex(..) | ExprField(..) |
1494 ExprTupField(..) | ExprVec(..) | ExprTup(..) | ExprBinary(..) |
1495 ExprCast(..) | ExprUnary(..) | ExprRet(..) | ExprBreak(..) |
1496 ExprAgain(..) | ExprLit(_) | ExprBlock(..) | ExprSlice(..) |
1497 ExprMac(..) | ExprAddrOf(..) | ExprStruct(..) | ExprRepeat(..) |
1498 ExprParen(..) | ExprFnBlock(..) | ExprProc(..) | ExprUnboxedFn(..) |
1499 ExprPath(..) | ExprBox(..) => {
1500 visit::walk_expr(this, expr);
1503 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
1505 ExprWhileLet(..) => {
1506 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
1511 fn check_fn(_v: &Liveness,
1517 // do not check contents of nested fns
1520 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1521 fn fn_ret(&self, id: NodeId) -> ty::FnOutput {
1522 let fn_ty = ty::node_id_to_type(self.ir.tcx, id);
1523 match ty::get(fn_ty).sty {
1524 ty::ty_unboxed_closure(closure_def_id, _, _) =>
1525 self.ir.tcx.unboxed_closures()
1527 .find(&closure_def_id)
1532 _ => ty::ty_fn_ret(fn_ty)
1542 match self.fn_ret(id) {
1543 ty::FnConverging(t_ret)
1544 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
1546 if ty::type_is_nil(t_ret) {
1547 // for nil return types, it is ok to not return a value expl.
1549 let ends_with_stmt = match body.expr {
1550 None if body.stmts.len() > 0 =>
1551 match body.stmts.last().unwrap().node {
1552 StmtSemi(ref e, _) => {
1553 let t_stmt = ty::expr_ty(self.ir.tcx, &**e);
1554 ty::get(t_stmt).sty == ty::get(t_ret).sty
1560 self.ir.tcx.sess.span_err(
1561 sp, "not all control paths return a value");
1563 let last_stmt = body.stmts.last().unwrap();
1564 let original_span = original_sp(self.ir.tcx.sess.codemap(),
1565 last_stmt.span, sp);
1566 let span_semicolon = Span {
1567 lo: original_span.hi - BytePos(1),
1568 hi: original_span.hi,
1569 expn_id: original_span.expn_id
1571 self.ir.tcx.sess.span_note(
1572 span_semicolon, "consider removing this semicolon:");
1577 if self.live_on_entry(entry_ln, self.s.clean_exit_var).is_some() => {
1578 self.ir.tcx.sess.span_err(sp,
1579 "computation may converge in a function marked as diverging");
1586 fn check_lvalue(&mut self, expr: &Expr) {
1589 match self.ir.tcx.def_map.borrow().get_copy(&expr.id) {
1591 // Assignment to an immutable variable or argument: only legal
1592 // if there is no later assignment. If this local is actually
1593 // mutable, then check for a reassignment to flag the mutability
1595 let ln = self.live_node(expr.id, expr.span);
1596 let var = self.variable(nid, expr.span);
1597 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1604 // For other kinds of lvalues, no checks are required,
1605 // and any embedded expressions are actually rvalues
1606 visit::walk_expr(self, expr);
1611 fn should_warn(&self, var: Variable) -> Option<String> {
1612 let name = self.ir.variable_name(var);
1613 if name.len() == 0 || name.as_bytes()[0] == ('_' as u8) {
1620 fn warn_about_unused_args(&self, decl: &FnDecl, entry_ln: LiveNode) {
1621 for arg in decl.inputs.iter() {
1622 pat_util::pat_bindings(&self.ir.tcx.def_map,
1624 |_bm, p_id, sp, path1| {
1625 let var = self.variable(p_id, sp);
1626 // Ignore unused self.
1627 let ident = path1.node;
1628 if ident.name != special_idents::self_.name {
1629 self.warn_about_unused(sp, p_id, entry_ln, var);
1635 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &Pat) {
1636 self.pat_bindings(pat, |this, ln, var, sp, id| {
1637 if !this.warn_about_unused(sp, id, ln, var) {
1638 this.warn_about_dead_assign(sp, id, ln, var);
1643 fn warn_about_unused(&self,
1649 if !self.used_on_entry(ln, var) {
1650 let r = self.should_warn(var);
1651 for name in r.iter() {
1653 // annoying: for parameters in funcs like `fn(x: int)
1654 // {ret}`, there is only one node, so asking about
1655 // assigned_on_exit() is not meaningful.
1656 let is_assigned = if ln == self.s.exit_ln {
1659 self.assigned_on_exit(ln, var).is_some()
1663 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1664 format!("variable `{}` is assigned to, but never used",
1667 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1668 format!("unused variable: `{}`", *name));
1677 fn warn_about_dead_assign(&self,
1682 if self.live_on_exit(ln, var).is_none() {
1683 let r = self.should_warn(var);
1684 for name in r.iter() {
1685 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1686 format!("value assigned to `{}` is never read", *name));