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.
11 //! A classic liveness analysis based on dataflow over the AST. Computes,
12 //! for each local variable in a function, whether that variable is live
13 //! at a given point. Program execution points are identified by their
18 //! The basic model is that each local variable is assigned an index. We
19 //! represent sets of local variables using a vector indexed by this
20 //! index. The value in the vector is either 0, indicating the variable
21 //! is dead, or the id of an expression that uses the variable.
23 //! We conceptually walk over the AST in reverse execution order. If we
24 //! find a use of a variable, we add it to the set of live variables. If
25 //! we find an assignment to a variable, we remove it from the set of live
26 //! variables. When we have to merge two flows, we take the union of
27 //! those two flows---if the variable is live on both paths, we simply
28 //! pick one id. In the event of loops, we continue doing this until a
29 //! fixed point is reached.
31 //! ## Checking initialization
33 //! At the function entry point, all variables must be dead. If this is
34 //! not the case, we can report an error using the id found in the set of
35 //! live variables, which identifies a use of the variable which is not
36 //! dominated by an assignment.
40 //! After each explicit move, the variable must be dead.
42 //! ## Computing last uses
44 //! Any use of the variable where the variable is dead afterwards is a
47 //! # Implementation details
49 //! The actual implementation contains two (nested) walks over the AST.
50 //! The outer walk has the job of building up the ir_maps instance for the
51 //! enclosing function. On the way down the tree, it identifies those AST
52 //! nodes and variable IDs that will be needed for the liveness analysis
53 //! and assigns them contiguous IDs. The liveness id for an AST node is
54 //! called a `live_node` (it's a newtype'd usize) and the id for a variable
55 //! is called a `variable` (another newtype'd usize).
57 //! On the way back up the tree, as we are about to exit from a function
58 //! declaration we allocate a `liveness` instance. Now that we know
59 //! precisely how many nodes and variables we need, we can allocate all
60 //! the various arrays that we will need to precisely the right size. We then
61 //! perform the actual propagation on the `liveness` instance.
63 //! This propagation is encoded in the various `propagate_through_*()`
64 //! methods. It effectively does a reverse walk of the AST; whenever we
65 //! reach a loop node, we iterate until a fixed point is reached.
67 //! ## The `Users` struct
69 //! At each live node `N`, we track three pieces of information for each
70 //! variable `V` (these are encapsulated in the `Users` struct):
72 //! - `reader`: the `LiveNode` ID of some node which will read the value
73 //! that `V` holds on entry to `N`. Formally: a node `M` such
74 //! that there exists a path `P` from `N` to `M` where `P` does not
75 //! write `V`. If the `reader` is `invalid_node()`, then the current
76 //! value will never be read (the variable is dead, essentially).
78 //! - `writer`: the `LiveNode` ID of some node which will write the
79 //! variable `V` and which is reachable from `N`. Formally: a node `M`
80 //! such that there exists a path `P` from `N` to `M` and `M` writes
81 //! `V`. If the `writer` is `invalid_node()`, then there is no writer
82 //! of `V` that follows `N`.
84 //! - `used`: a boolean value indicating whether `V` is *used*. We
85 //! distinguish a *read* from a *use* in that a *use* is some read that
86 //! is not just used to generate a new value. For example, `x += 1` is
87 //! a read but not a use. This is used to generate better warnings.
89 //! ## Special Variables
91 //! We generate various special variables for various, well, special purposes.
92 //! These are described in the `specials` struct:
94 //! - `exit_ln`: a live node that is generated to represent every 'exit' from
95 //! the function, whether it be by explicit return, panic, or other means.
97 //! - `fallthrough_ln`: a live node that represents a fallthrough
99 //! - `no_ret_var`: a synthetic variable that is only 'read' from, the
100 //! fallthrough node. This allows us to detect functions where we fail
101 //! to return explicitly.
102 //! - `clean_exit_var`: a synthetic variable that is only 'read' from the
103 //! fallthrough node. It is only live if the function could converge
104 //! via means other than an explicit `return` expression. That is, it is
105 //! only dead if the end of the function's block can never be reached.
106 //! It is the responsibility of typeck to ensure that there are no
107 //! `return` expressions in a function declared as diverging.
108 use self::LoopKind::*;
109 use self::LiveNodeKind::*;
110 use self::VarKind::*;
113 use middle::pat_util;
117 use util::nodemap::NodeMap;
119 use std::{fmt, usize};
120 use std::io::prelude::*;
123 use syntax::ast::{self, NodeId, Expr};
124 use syntax::codemap::{BytePos, original_sp, Span};
125 use syntax::parse::token::special_idents;
126 use syntax::print::pprust::{expr_to_string, block_to_string};
128 use syntax::ast_util;
129 use syntax::visit::{self, Visitor, FnKind};
131 /// For use with `propagate_through_loop`.
133 /// An endless `loop` loop.
135 /// A `while` loop, with the given expression as condition.
139 #[derive(Copy, Clone, PartialEq)]
140 struct Variable(usize);
142 #[derive(Copy, PartialEq)]
143 struct LiveNode(usize);
146 fn get(&self) -> usize { let Variable(v) = *self; v }
150 fn get(&self) -> usize { let LiveNode(v) = *self; v }
153 impl Clone for LiveNode {
154 fn clone(&self) -> LiveNode {
159 #[derive(Copy, Clone, PartialEq, Debug)]
167 fn live_node_kind_to_string(lnk: LiveNodeKind, cx: &ty::ctxt) -> String {
168 let cm = cx.sess.codemap();
171 format!("Free var node [{}]", cm.span_to_string(s))
174 format!("Expr node [{}]", cm.span_to_string(s))
177 format!("Var def node [{}]", cm.span_to_string(s))
179 ExitNode => "Exit node".to_string(),
183 impl<'a, 'tcx, 'v> Visitor<'v> for IrMaps<'a, 'tcx> {
184 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v ast::FnDecl,
185 b: &'v ast::Block, s: Span, id: NodeId) {
186 visit_fn(self, fk, fd, b, s, id);
188 fn visit_local(&mut self, l: &ast::Local) { visit_local(self, l); }
189 fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); }
190 fn visit_arm(&mut self, a: &ast::Arm) { visit_arm(self, a); }
193 pub fn check_crate(tcx: &ty::ctxt) {
194 visit::walk_crate(&mut IrMaps::new(tcx), tcx.map.krate());
195 tcx.sess.abort_if_errors();
198 impl fmt::Debug for LiveNode {
199 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
200 write!(f, "ln({})", self.get())
204 impl fmt::Debug for Variable {
205 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
206 write!(f, "v({})", self.get())
210 // ______________________________________________________________________
213 // This is the first pass and the one that drives the main
214 // computation. It walks up and down the IR once. On the way down,
215 // we count for each function the number of variables as well as
216 // liveness nodes. A liveness node is basically an expression or
217 // capture clause that does something of interest: either it has
218 // interesting control flow or it uses/defines a local variable.
220 // On the way back up, at each function node we create liveness sets
221 // (we now know precisely how big to make our various vectors and so
222 // forth) and then do the data-flow propagation to compute the set
223 // of live variables at each program point.
225 // Finally, we run back over the IR one last time and, using the
226 // computed liveness, check various safety conditions. For example,
227 // there must be no live nodes at the definition site for a variable
228 // unless it has an initializer. Similarly, each non-mutable local
229 // variable must not be assigned if there is some successor
230 // assignment. And so forth.
233 fn is_valid(&self) -> bool {
234 self.get() != usize::MAX
238 fn invalid_node() -> LiveNode { LiveNode(usize::MAX) }
245 #[derive(Copy, Clone, Debug)]
251 #[derive(Copy, Clone, Debug)]
253 Arg(NodeId, ast::Name),
259 struct IrMaps<'a, 'tcx: 'a> {
260 tcx: &'a ty::ctxt<'tcx>,
262 num_live_nodes: usize,
264 live_node_map: NodeMap<LiveNode>,
265 variable_map: NodeMap<Variable>,
266 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
267 var_kinds: Vec<VarKind>,
268 lnks: Vec<LiveNodeKind>,
271 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
272 fn new(tcx: &'a ty::ctxt<'tcx>) -> IrMaps<'a, 'tcx> {
277 live_node_map: NodeMap(),
278 variable_map: NodeMap(),
279 capture_info_map: NodeMap(),
280 var_kinds: Vec::new(),
285 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
286 let ln = LiveNode(self.num_live_nodes);
288 self.num_live_nodes += 1;
290 debug!("{:?} is of kind {}", ln,
291 live_node_kind_to_string(lnk, self.tcx));
296 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
297 let ln = self.add_live_node(lnk);
298 self.live_node_map.insert(node_id, ln);
300 debug!("{:?} is node {}", ln, node_id);
303 fn add_variable(&mut self, vk: VarKind) -> Variable {
304 let v = Variable(self.num_vars);
305 self.var_kinds.push(vk);
309 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
310 self.variable_map.insert(node_id, v);
312 ImplicitRet | CleanExit => {}
315 debug!("{:?} is {:?}", v, vk);
320 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
321 match self.variable_map.get(&node_id) {
326 .span_bug(span, &format!("no variable registered for id {}",
332 fn variable_name(&self, var: Variable) -> String {
333 match self.var_kinds[var.get()] {
334 Local(LocalInfo { name, .. }) | Arg(_, name) => {
337 ImplicitRet => "<implicit-ret>".to_string(),
338 CleanExit => "<clean-exit>".to_string()
342 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
343 self.capture_info_map.insert(node_id, Rc::new(cs));
346 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
351 impl<'a, 'tcx, 'v> Visitor<'v> for Liveness<'a, 'tcx> {
352 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v ast::FnDecl,
353 b: &'v ast::Block, s: Span, n: NodeId) {
354 check_fn(self, fk, fd, b, s, n);
356 fn visit_local(&mut self, l: &ast::Local) {
357 check_local(self, l);
359 fn visit_expr(&mut self, ex: &Expr) {
360 check_expr(self, ex);
362 fn visit_arm(&mut self, a: &ast::Arm) {
367 fn visit_fn(ir: &mut IrMaps,
375 // swap in a new set of IR maps for this function body:
376 let mut fn_maps = IrMaps::new(ir.tcx);
378 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
380 for arg in &decl.inputs {
381 pat_util::pat_bindings(&ir.tcx.def_map,
383 |_bm, arg_id, _x, path1| {
384 debug!("adding argument {}", arg_id);
385 let name = path1.node.name;
386 fn_maps.add_variable(Arg(arg_id, name));
390 // gather up the various local variables, significant expressions,
392 visit::walk_fn(&mut fn_maps, fk, decl, body, sp);
394 // Special nodes and variables:
395 // - exit_ln represents the end of the fn, either by return or panic
396 // - implicit_ret_var is a pseudo-variable that represents
397 // an implicit return
398 let specials = Specials {
399 exit_ln: fn_maps.add_live_node(ExitNode),
400 fallthrough_ln: fn_maps.add_live_node(ExitNode),
401 no_ret_var: fn_maps.add_variable(ImplicitRet),
402 clean_exit_var: fn_maps.add_variable(CleanExit)
406 let mut lsets = Liveness::new(&mut fn_maps, specials);
407 let entry_ln = lsets.compute(decl, body);
409 // check for various error conditions
410 lsets.visit_block(body);
411 lsets.check_ret(id, sp, fk, entry_ln, body);
412 lsets.warn_about_unused_args(decl, entry_ln);
415 fn visit_local(ir: &mut IrMaps, local: &ast::Local) {
416 pat_util::pat_bindings(&ir.tcx.def_map, &*local.pat, |_, p_id, sp, path1| {
417 debug!("adding local variable {}", p_id);
418 let name = path1.node.name;
419 ir.add_live_node_for_node(p_id, VarDefNode(sp));
420 ir.add_variable(Local(LocalInfo {
425 visit::walk_local(ir, local);
428 fn visit_arm(ir: &mut IrMaps, arm: &ast::Arm) {
429 for pat in &arm.pats {
430 pat_util::pat_bindings(&ir.tcx.def_map, &**pat, |bm, p_id, sp, path1| {
431 debug!("adding local variable {} from match with bm {:?}",
433 let name = path1.node.name;
434 ir.add_live_node_for_node(p_id, VarDefNode(sp));
435 ir.add_variable(Local(LocalInfo {
441 visit::walk_arm(ir, arm);
444 fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
446 // live nodes required for uses or definitions of variables:
447 ast::ExprPath(..) => {
448 let def = ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def();
449 debug!("expr {}: path that leads to {:?}", expr.id, def);
450 if let DefLocal(..) = def {
451 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
453 visit::walk_expr(ir, expr);
455 ast::ExprClosure(..) => {
456 // Interesting control flow (for loops can contain labeled
457 // breaks or continues)
458 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
460 // Make a live_node for each captured variable, with the span
461 // being the location that the variable is used. This results
462 // in better error messages than just pointing at the closure
463 // construction site.
464 let mut call_caps = Vec::new();
465 ir.tcx.with_freevars(expr.id, |freevars| {
467 if let DefLocal(rv) = fv.def {
468 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
469 call_caps.push(CaptureInfo {ln: fv_ln,
474 ir.set_captures(expr.id, call_caps);
476 visit::walk_expr(ir, expr);
479 // live nodes required for interesting control flow:
480 ast::ExprIf(..) | ast::ExprMatch(..) | ast::ExprWhile(..) | ast::ExprLoop(..) => {
481 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
482 visit::walk_expr(ir, expr);
484 ast::ExprIfLet(..) => {
485 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
487 ast::ExprWhileLet(..) => {
488 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
490 ast::ExprForLoop(..) => {
491 ir.tcx.sess.span_bug(expr.span, "non-desugared ExprForLoop");
493 ast::ExprBinary(op, _, _) if ast_util::lazy_binop(op.node) => {
494 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
495 visit::walk_expr(ir, expr);
498 // otherwise, live nodes are not required:
499 ast::ExprIndex(..) | ast::ExprField(..) | ast::ExprTupField(..) |
500 ast::ExprVec(..) | ast::ExprCall(..) | ast::ExprMethodCall(..) |
501 ast::ExprTup(..) | ast::ExprBinary(..) | ast::ExprAddrOf(..) |
502 ast::ExprCast(..) | ast::ExprUnary(..) | ast::ExprBreak(_) |
503 ast::ExprAgain(_) | ast::ExprLit(_) | ast::ExprRet(..) |
504 ast::ExprBlock(..) | ast::ExprAssign(..) | ast::ExprAssignOp(..) |
505 ast::ExprMac(..) | ast::ExprStruct(..) | ast::ExprRepeat(..) |
506 ast::ExprParen(..) | ast::ExprInlineAsm(..) | ast::ExprBox(..) |
507 ast::ExprRange(..) => {
508 visit::walk_expr(ir, expr);
513 // ______________________________________________________________________
514 // Computing liveness sets
516 // Actually we compute just a bit more than just liveness, but we use
517 // the same basic propagation framework in all cases.
519 #[derive(Clone, Copy)]
526 fn invalid_users() -> Users {
528 reader: invalid_node(),
529 writer: invalid_node(),
534 #[derive(Copy, Clone)]
537 fallthrough_ln: LiveNode,
538 no_ret_var: Variable,
539 clean_exit_var: Variable
542 const ACC_READ: u32 = 1;
543 const ACC_WRITE: u32 = 2;
544 const ACC_USE: u32 = 4;
546 struct Liveness<'a, 'tcx: 'a> {
547 ir: &'a mut IrMaps<'a, 'tcx>,
549 successors: Vec<LiveNode>,
551 // The list of node IDs for the nested loop scopes
553 loop_scope: Vec<NodeId>,
554 // mappings from loop node ID to LiveNode
555 // ("break" label should map to loop node ID,
556 // it probably doesn't now)
557 break_ln: NodeMap<LiveNode>,
558 cont_ln: NodeMap<LiveNode>
561 impl<'a, 'tcx> Liveness<'a, 'tcx> {
562 fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> {
563 let num_live_nodes = ir.num_live_nodes;
564 let num_vars = ir.num_vars;
568 successors: vec![invalid_node(); num_live_nodes],
569 users: vec![invalid_users(); num_live_nodes * num_vars],
570 loop_scope: Vec::new(),
576 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
577 match self.ir.live_node_map.get(&node_id) {
580 // This must be a mismatch between the ir_map construction
581 // above and the propagation code below; the two sets of
582 // code have to agree about which AST nodes are worth
583 // creating liveness nodes for.
584 self.ir.tcx.sess.span_bug(
586 &format!("no live node registered for node {}",
592 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
593 self.ir.variable(node_id, span)
596 fn pat_bindings<F>(&mut self, pat: &ast::Pat, mut f: F) where
597 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
599 pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| {
600 let ln = self.live_node(p_id, sp);
601 let var = self.variable(p_id, sp);
602 f(self, ln, var, sp, p_id);
606 fn arm_pats_bindings<F>(&mut self, pat: Option<&ast::Pat>, f: F) where
607 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
611 self.pat_bindings(pat, f);
617 fn define_bindings_in_pat(&mut self, pat: &ast::Pat, succ: LiveNode)
619 self.define_bindings_in_arm_pats(Some(pat), succ)
622 fn define_bindings_in_arm_pats(&mut self, pat: Option<&ast::Pat>, succ: LiveNode)
625 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
626 this.init_from_succ(ln, succ);
627 this.define(ln, var);
633 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
634 ln.get() * self.ir.num_vars + var.get()
637 fn live_on_entry(&self, ln: LiveNode, var: Variable)
638 -> Option<LiveNodeKind> {
639 assert!(ln.is_valid());
640 let reader = self.users[self.idx(ln, var)].reader;
641 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
645 Is this variable live on entry to any of its successor nodes?
647 fn live_on_exit(&self, ln: LiveNode, var: Variable)
648 -> Option<LiveNodeKind> {
649 let successor = self.successors[ln.get()];
650 self.live_on_entry(successor, var)
653 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
654 assert!(ln.is_valid());
655 self.users[self.idx(ln, var)].used
658 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
659 -> Option<LiveNodeKind> {
660 assert!(ln.is_valid());
661 let writer = self.users[self.idx(ln, var)].writer;
662 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
665 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
666 -> Option<LiveNodeKind> {
667 let successor = self.successors[ln.get()];
668 self.assigned_on_entry(successor, var)
671 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
672 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
674 let node_base_idx = self.idx(ln, Variable(0));
675 let succ_base_idx = self.idx(succ_ln, Variable(0));
676 for var_idx in 0..self.ir.num_vars {
677 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
681 fn write_vars<F>(&self,
685 -> io::Result<()> where
686 F: FnMut(usize) -> LiveNode,
688 let node_base_idx = self.idx(ln, Variable(0));
689 for var_idx in 0..self.ir.num_vars {
690 let idx = node_base_idx + var_idx;
691 if test(idx).is_valid() {
692 try!(write!(wr, " {:?}", Variable(var_idx)));
698 fn find_loop_scope(&self,
699 opt_label: Option<ast::Ident>,
705 // Refers to a labeled loop. Use the results of resolve
707 match self.ir.tcx.def_map.borrow().get(&id).map(|d| d.full_def()) {
708 Some(DefLabel(loop_id)) => loop_id,
709 _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \
710 doesn't refer to a loop")
714 // Vanilla 'break' or 'loop', so use the enclosing
716 if self.loop_scope.is_empty() {
717 self.ir.tcx.sess.span_bug(sp, "break outside loop");
719 *self.loop_scope.last().unwrap()
725 #[allow(unused_must_use)]
726 fn ln_str(&self, ln: LiveNode) -> String {
727 let mut wr = Vec::new();
729 let wr = &mut wr as &mut Write;
730 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
731 self.write_vars(wr, ln, |idx| self.users[idx].reader);
732 write!(wr, " writes");
733 self.write_vars(wr, ln, |idx| self.users[idx].writer);
734 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
736 String::from_utf8(wr).unwrap()
739 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
740 self.successors[ln.get()] = succ_ln;
742 // It is not necessary to initialize the
743 // values to empty because this is the value
744 // they have when they are created, and the sets
745 // only grow during iterations.
747 // self.indices(ln) { |idx|
748 // self.users[idx] = invalid_users();
752 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
753 // more efficient version of init_empty() / merge_from_succ()
754 self.successors[ln.get()] = succ_ln;
756 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
757 this.users[idx] = this.users[succ_idx]
759 debug!("init_from_succ(ln={}, succ={})",
760 self.ln_str(ln), self.ln_str(succ_ln));
763 fn merge_from_succ(&mut self,
768 if ln == succ_ln { return false; }
770 let mut changed = false;
771 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
772 changed |= copy_if_invalid(this.users[succ_idx].reader,
773 &mut this.users[idx].reader);
774 changed |= copy_if_invalid(this.users[succ_idx].writer,
775 &mut this.users[idx].writer);
776 if this.users[succ_idx].used && !this.users[idx].used {
777 this.users[idx].used = true;
782 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
783 ln, self.ln_str(succ_ln), first_merge, changed);
786 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
787 if src.is_valid() && !dst.is_valid() {
796 // Indicates that a local variable was *defined*; we know that no
797 // uses of the variable can precede the definition (resolve checks
798 // this) so we just clear out all the data.
799 fn define(&mut self, writer: LiveNode, var: Variable) {
800 let idx = self.idx(writer, var);
801 self.users[idx].reader = invalid_node();
802 self.users[idx].writer = invalid_node();
804 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
805 idx, self.ln_str(writer));
808 // Either read, write, or both depending on the acc bitset
809 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
810 debug!("{:?} accesses[{:x}] {:?}: {}",
811 ln, acc, var, self.ln_str(ln));
813 let idx = self.idx(ln, var);
814 let user = &mut self.users[idx];
816 if (acc & ACC_WRITE) != 0 {
817 user.reader = invalid_node();
821 // Important: if we both read/write, must do read second
822 // or else the write will override.
823 if (acc & ACC_READ) != 0 {
827 if (acc & ACC_USE) != 0 {
832 // _______________________________________________________________________
834 fn compute(&mut self, decl: &ast::FnDecl, body: &ast::Block) -> LiveNode {
835 // if there is a `break` or `again` at the top level, then it's
836 // effectively a return---this only occurs in `for` loops,
837 // where the body is really a closure.
839 debug!("compute: using id for block, {}", block_to_string(body));
841 let exit_ln = self.s.exit_ln;
842 let entry_ln: LiveNode =
843 self.with_loop_nodes(body.id, exit_ln, exit_ln,
844 |this| this.propagate_through_fn_block(decl, body));
846 // hack to skip the loop unless debug! is enabled:
847 debug!("^^ liveness computation results for body {} (entry={:?})",
849 for ln_idx in 0..self.ir.num_live_nodes {
850 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
859 fn propagate_through_fn_block(&mut self, _: &ast::FnDecl, blk: &ast::Block)
861 // the fallthrough exit is only for those cases where we do not
862 // explicitly return:
864 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
865 if blk.expr.is_none() {
866 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
868 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
870 self.propagate_through_block(blk, s.fallthrough_ln)
873 fn propagate_through_block(&mut self, blk: &ast::Block, succ: LiveNode)
875 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
876 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
877 self.propagate_through_stmt(&**stmt, succ)
881 fn propagate_through_stmt(&mut self, stmt: &ast::Stmt, succ: LiveNode)
884 ast::StmtDecl(ref decl, _) => {
885 self.propagate_through_decl(&**decl, succ)
888 ast::StmtExpr(ref expr, _) | ast::StmtSemi(ref expr, _) => {
889 self.propagate_through_expr(&**expr, succ)
892 ast::StmtMac(..) => {
893 self.ir.tcx.sess.span_bug(stmt.span, "unexpanded macro");
898 fn propagate_through_decl(&mut self, decl: &ast::Decl, succ: LiveNode)
901 ast::DeclLocal(ref local) => {
902 self.propagate_through_local(&**local, succ)
904 ast::DeclItem(_) => succ,
908 fn propagate_through_local(&mut self, local: &ast::Local, succ: LiveNode)
910 // Note: we mark the variable as defined regardless of whether
911 // there is an initializer. Initially I had thought to only mark
912 // the live variable as defined if it was initialized, and then we
913 // could check for uninit variables just by scanning what is live
914 // at the start of the function. But that doesn't work so well for
915 // immutable variables defined in a loop:
916 // loop { let x; x = 5; }
917 // because the "assignment" loops back around and generates an error.
919 // So now we just check that variables defined w/o an
920 // initializer are not live at the point of their
921 // initialization, which is mildly more complex than checking
922 // once at the func header but otherwise equivalent.
924 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
925 self.define_bindings_in_pat(&*local.pat, succ)
928 fn propagate_through_exprs(&mut self, exprs: &[P<Expr>], succ: LiveNode)
930 exprs.iter().rev().fold(succ, |succ, expr| {
931 self.propagate_through_expr(&**expr, succ)
935 fn propagate_through_opt_expr(&mut self,
936 opt_expr: Option<&Expr>,
939 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
942 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
944 debug!("propagate_through_expr: {}", expr_to_string(expr));
947 // Interesting cases with control flow or which gen/kill
949 ast::ExprPath(..) => {
950 self.access_path(expr, succ, ACC_READ | ACC_USE)
953 ast::ExprField(ref e, _) => {
954 self.propagate_through_expr(&**e, succ)
957 ast::ExprTupField(ref e, _) => {
958 self.propagate_through_expr(&**e, succ)
961 ast::ExprClosure(_, _, ref blk) => {
962 debug!("{} is an ExprClosure",
963 expr_to_string(expr));
966 The next-node for a break is the successor of the entire
967 loop. The next-node for a continue is the top of this loop.
969 let node = self.live_node(expr.id, expr.span);
970 self.with_loop_nodes(blk.id, succ, node, |this| {
972 // the construction of a closure itself is not important,
973 // but we have to consider the closed over variables.
974 let caps = match this.ir.capture_info_map.get(&expr.id) {
975 Some(caps) => caps.clone(),
977 this.ir.tcx.sess.span_bug(expr.span, "no registered caps");
980 caps.iter().rev().fold(succ, |succ, cap| {
981 this.init_from_succ(cap.ln, succ);
982 let var = this.variable(cap.var_nid, expr.span);
983 this.acc(cap.ln, var, ACC_READ | ACC_USE);
989 ast::ExprIf(ref cond, ref then, ref els) => {
1003 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
1004 let then_ln = self.propagate_through_block(&**then, succ);
1005 let ln = self.live_node(expr.id, expr.span);
1006 self.init_from_succ(ln, else_ln);
1007 self.merge_from_succ(ln, then_ln, false);
1008 self.propagate_through_expr(&**cond, ln)
1011 ast::ExprIfLet(..) => {
1012 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
1015 ast::ExprWhile(ref cond, ref blk, _) => {
1016 self.propagate_through_loop(expr, WhileLoop(&**cond), &**blk, succ)
1019 ast::ExprWhileLet(..) => {
1020 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
1023 ast::ExprForLoop(..) => {
1024 self.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprForLoop");
1027 // Note that labels have been resolved, so we don't need to look
1028 // at the label ident
1029 ast::ExprLoop(ref blk, _) => {
1030 self.propagate_through_loop(expr, LoopLoop, &**blk, succ)
1033 ast::ExprMatch(ref e, ref arms, _) => {
1048 let ln = self.live_node(expr.id, expr.span);
1049 self.init_empty(ln, succ);
1050 let mut first_merge = true;
1053 self.propagate_through_expr(&*arm.body, succ);
1055 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1056 // only consider the first pattern; any later patterns must have
1057 // the same bindings, and we also consider the first pattern to be
1058 // the "authoritative" set of ids
1060 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1062 self.merge_from_succ(ln, arm_succ, first_merge);
1063 first_merge = false;
1065 self.propagate_through_expr(&**e, ln)
1068 ast::ExprRet(ref o_e) => {
1069 // ignore succ and subst exit_ln:
1070 let exit_ln = self.s.exit_ln;
1071 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1074 ast::ExprBreak(opt_label) => {
1075 // Find which label this break jumps to
1076 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1078 // Now that we know the label we're going to,
1079 // look it up in the break loop nodes table
1081 match self.break_ln.get(&sc) {
1083 None => self.ir.tcx.sess.span_bug(expr.span,
1084 "break to unknown label")
1088 ast::ExprAgain(opt_label) => {
1089 // Find which label this expr continues to
1090 let sc = self.find_loop_scope(opt_label, expr.id, expr.span);
1092 // Now that we know the label we're going to,
1093 // look it up in the continue loop nodes table
1095 match self.cont_ln.get(&sc) {
1097 None => self.ir.tcx.sess.span_bug(expr.span,
1098 "loop to unknown label")
1102 ast::ExprAssign(ref l, ref r) => {
1103 // see comment on lvalues in
1104 // propagate_through_lvalue_components()
1105 let succ = self.write_lvalue(&**l, succ, ACC_WRITE);
1106 let succ = self.propagate_through_lvalue_components(&**l, succ);
1107 self.propagate_through_expr(&**r, succ)
1110 ast::ExprAssignOp(_, ref l, ref r) => {
1111 // see comment on lvalues in
1112 // propagate_through_lvalue_components()
1113 let succ = self.write_lvalue(&**l, succ, ACC_WRITE|ACC_READ);
1114 let succ = self.propagate_through_expr(&**r, succ);
1115 self.propagate_through_lvalue_components(&**l, succ)
1118 // Uninteresting cases: just propagate in rev exec order
1120 ast::ExprVec(ref exprs) => {
1121 self.propagate_through_exprs(&exprs[..], succ)
1124 ast::ExprRepeat(ref element, ref count) => {
1125 let succ = self.propagate_through_expr(&**count, succ);
1126 self.propagate_through_expr(&**element, succ)
1129 ast::ExprStruct(_, ref fields, ref with_expr) => {
1130 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1131 fields.iter().rev().fold(succ, |succ, field| {
1132 self.propagate_through_expr(&*field.expr, succ)
1136 ast::ExprCall(ref f, ref args) => {
1137 let diverges = !self.ir.tcx.is_method_call(expr.id) &&
1138 self.ir.tcx.expr_ty_adjusted(&**f).fn_ret().diverges();
1139 let succ = if diverges {
1144 let succ = self.propagate_through_exprs(&args[..], succ);
1145 self.propagate_through_expr(&**f, succ)
1148 ast::ExprMethodCall(_, _, ref args) => {
1149 let method_call = ty::MethodCall::expr(expr.id);
1150 let method_ty = self.ir.tcx.tables.borrow().method_map[&method_call].ty;
1151 let succ = if method_ty.fn_ret().diverges() {
1156 self.propagate_through_exprs(&args[..], succ)
1159 ast::ExprTup(ref exprs) => {
1160 self.propagate_through_exprs(&exprs[..], succ)
1163 ast::ExprBinary(op, ref l, ref r) if ast_util::lazy_binop(op.node) => {
1164 let r_succ = self.propagate_through_expr(&**r, succ);
1166 let ln = self.live_node(expr.id, expr.span);
1167 self.init_from_succ(ln, succ);
1168 self.merge_from_succ(ln, r_succ, false);
1170 self.propagate_through_expr(&**l, ln)
1173 ast::ExprIndex(ref l, ref r) |
1174 ast::ExprBinary(_, ref l, ref r) |
1175 ast::ExprBox(Some(ref l), ref r) => {
1176 let r_succ = self.propagate_through_expr(&**r, succ);
1177 self.propagate_through_expr(&**l, r_succ)
1180 ast::ExprRange(ref e1, ref e2) => {
1181 let succ = e2.as_ref().map_or(succ, |e| self.propagate_through_expr(&**e, succ));
1182 e1.as_ref().map_or(succ, |e| self.propagate_through_expr(&**e, succ))
1185 ast::ExprBox(None, ref e) |
1186 ast::ExprAddrOf(_, ref e) |
1187 ast::ExprCast(ref e, _) |
1188 ast::ExprUnary(_, ref e) |
1189 ast::ExprParen(ref e) => {
1190 self.propagate_through_expr(&**e, succ)
1193 ast::ExprInlineAsm(ref ia) => {
1195 let succ = ia.outputs.iter().rev().fold(succ, |succ, &(_, ref expr, _)| {
1196 // see comment on lvalues
1197 // in propagate_through_lvalue_components()
1198 let succ = self.write_lvalue(&**expr, succ, ACC_WRITE);
1199 self.propagate_through_lvalue_components(&**expr, succ)
1201 // Inputs are executed first. Propagate last because of rev order
1202 ia.inputs.iter().rev().fold(succ, |succ, &(_, ref expr)| {
1203 self.propagate_through_expr(&**expr, succ)
1207 ast::ExprLit(..) => {
1211 ast::ExprBlock(ref blk) => {
1212 self.propagate_through_block(&**blk, succ)
1215 ast::ExprMac(..) => {
1216 self.ir.tcx.sess.span_bug(expr.span, "unexpanded macro");
1221 fn propagate_through_lvalue_components(&mut self,
1227 // In general, the full flow graph structure for an
1228 // assignment/move/etc can be handled in one of two ways,
1229 // depending on whether what is being assigned is a "tracked
1230 // value" or not. A tracked value is basically a local
1231 // variable or argument.
1233 // The two kinds of graphs are:
1235 // Tracked lvalue Untracked lvalue
1236 // ----------------------++-----------------------
1240 // (rvalue) || (rvalue)
1243 // (write of lvalue) || (lvalue components)
1248 // ----------------------++-----------------------
1250 // I will cover the two cases in turn:
1252 // # Tracked lvalues
1254 // A tracked lvalue is a local variable/argument `x`. In
1255 // these cases, the link_node where the write occurs is linked
1256 // to node id of `x`. The `write_lvalue()` routine generates
1257 // the contents of this node. There are no subcomponents to
1260 // # Non-tracked lvalues
1262 // These are lvalues like `x[5]` or `x.f`. In that case, we
1263 // basically ignore the value which is written to but generate
1264 // reads for the components---`x` in these two examples. The
1265 // components reads are generated by
1266 // `propagate_through_lvalue_components()` (this fn).
1268 // # Illegal lvalues
1270 // It is still possible to observe assignments to non-lvalues;
1271 // these errors are detected in the later pass borrowck. We
1272 // just ignore such cases and treat them as reads.
1275 ast::ExprPath(..) => succ,
1276 ast::ExprField(ref e, _) => self.propagate_through_expr(&**e, succ),
1277 ast::ExprTupField(ref e, _) => self.propagate_through_expr(&**e, succ),
1278 _ => self.propagate_through_expr(expr, succ)
1282 // see comment on propagate_through_lvalue()
1283 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1286 ast::ExprPath(..) => {
1287 self.access_path(expr, succ, acc)
1290 // We do not track other lvalues, so just propagate through
1291 // to their subcomponents. Also, it may happen that
1292 // non-lvalues occur here, because those are detected in the
1293 // later pass borrowck.
1298 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1300 match self.ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def() {
1302 let ln = self.live_node(expr.id, expr.span);
1304 self.init_from_succ(ln, succ);
1305 let var = self.variable(nid, expr.span);
1306 self.acc(ln, var, acc);
1314 fn propagate_through_loop(&mut self,
1323 We model control flow like this:
1341 let mut first_merge = true;
1342 let ln = self.live_node(expr.id, expr.span);
1343 self.init_empty(ln, succ);
1347 // If this is not a `loop` loop, then it's possible we bypass
1348 // the body altogether. Otherwise, the only way is via a `break`
1349 // in the loop body.
1350 self.merge_from_succ(ln, succ, first_merge);
1351 first_merge = false;
1354 debug!("propagate_through_loop: using id for loop body {} {}",
1355 expr.id, block_to_string(body));
1357 let cond_ln = match kind {
1359 WhileLoop(ref cond) => self.propagate_through_expr(&**cond, ln),
1361 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1362 this.propagate_through_block(body, cond_ln)
1365 // repeat until fixed point is reached:
1366 while self.merge_from_succ(ln, body_ln, first_merge) {
1367 first_merge = false;
1369 let new_cond_ln = match kind {
1371 WhileLoop(ref cond) => {
1372 self.propagate_through_expr(&**cond, ln)
1375 assert!(cond_ln == new_cond_ln);
1376 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1377 |this| this.propagate_through_block(body, cond_ln)));
1383 fn with_loop_nodes<R, F>(&mut self,
1384 loop_node_id: NodeId,
1389 F: FnOnce(&mut Liveness<'a, 'tcx>) -> R,
1391 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1392 self.loop_scope.push(loop_node_id);
1393 self.break_ln.insert(loop_node_id, break_ln);
1394 self.cont_ln.insert(loop_node_id, cont_ln);
1396 self.loop_scope.pop();
1401 // _______________________________________________________________________
1402 // Checking for error conditions
1404 fn check_local(this: &mut Liveness, local: &ast::Local) {
1407 this.warn_about_unused_or_dead_vars_in_pat(&*local.pat);
1410 this.pat_bindings(&*local.pat, |this, ln, var, sp, id| {
1411 this.warn_about_unused(sp, id, ln, var);
1416 visit::walk_local(this, local);
1419 fn check_arm(this: &mut Liveness, arm: &ast::Arm) {
1420 // only consider the first pattern; any later patterns must have
1421 // the same bindings, and we also consider the first pattern to be
1422 // the "authoritative" set of ids
1423 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1424 this.warn_about_unused(sp, id, ln, var);
1426 visit::walk_arm(this, arm);
1429 fn check_expr(this: &mut Liveness, expr: &Expr) {
1431 ast::ExprAssign(ref l, ref r) => {
1432 this.check_lvalue(&**l);
1433 this.visit_expr(&**r);
1435 visit::walk_expr(this, expr);
1438 ast::ExprAssignOp(_, ref l, _) => {
1439 this.check_lvalue(&**l);
1441 visit::walk_expr(this, expr);
1444 ast::ExprInlineAsm(ref ia) => {
1445 for &(_, ref input) in &ia.inputs {
1446 this.visit_expr(&**input);
1449 // Output operands must be lvalues
1450 for &(_, ref out, _) in &ia.outputs {
1451 this.check_lvalue(&**out);
1452 this.visit_expr(&**out);
1455 visit::walk_expr(this, expr);
1458 // no correctness conditions related to liveness
1459 ast::ExprCall(..) | ast::ExprMethodCall(..) | ast::ExprIf(..) |
1460 ast::ExprMatch(..) | ast::ExprWhile(..) | ast::ExprLoop(..) |
1461 ast::ExprIndex(..) | ast::ExprField(..) | ast::ExprTupField(..) |
1462 ast::ExprVec(..) | ast::ExprTup(..) | ast::ExprBinary(..) |
1463 ast::ExprCast(..) | ast::ExprUnary(..) | ast::ExprRet(..) |
1464 ast::ExprBreak(..) | ast::ExprAgain(..) | ast::ExprLit(_) |
1465 ast::ExprBlock(..) | ast::ExprMac(..) | ast::ExprAddrOf(..) |
1466 ast::ExprStruct(..) | ast::ExprRepeat(..) | ast::ExprParen(..) |
1467 ast::ExprClosure(..) | ast::ExprPath(..) | ast::ExprBox(..) |
1468 ast::ExprRange(..) => {
1469 visit::walk_expr(this, expr);
1471 ast::ExprIfLet(..) => {
1472 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprIfLet");
1474 ast::ExprWhileLet(..) => {
1475 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprWhileLet");
1477 ast::ExprForLoop(..) => {
1478 this.ir.tcx.sess.span_bug(expr.span, "non-desugared ExprForLoop");
1483 fn check_fn(_v: &Liveness,
1485 _decl: &ast::FnDecl,
1489 // do not check contents of nested fns
1492 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1493 fn fn_ret(&self, id: NodeId) -> ty::PolyFnOutput<'tcx> {
1494 let fn_ty = self.ir.tcx.node_id_to_type(id);
1496 ty::TyClosure(closure_def_id, ref substs) =>
1497 self.ir.tcx.closure_type(closure_def_id, substs).sig.output(),
1509 // within the fn body, late-bound regions are liberated:
1511 self.ir.tcx.liberate_late_bound_regions(
1512 region::DestructionScopeData::new(body.id),
1516 ty::FnConverging(t_ret)
1517 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
1520 // for nil return types, it is ok to not return a value expl.
1522 let ends_with_stmt = match body.expr {
1523 None if !body.stmts.is_empty() =>
1524 match body.stmts.first().unwrap().node {
1525 ast::StmtSemi(ref e, _) => {
1526 self.ir.tcx.expr_ty(&**e) == t_ret
1532 span_err!(self.ir.tcx.sess, sp, E0269, "not all control paths return a value");
1534 let last_stmt = body.stmts.first().unwrap();
1535 let original_span = original_sp(self.ir.tcx.sess.codemap(),
1536 last_stmt.span, sp);
1537 let span_semicolon = Span {
1538 lo: original_span.hi - BytePos(1),
1539 hi: original_span.hi,
1540 expn_id: original_span.expn_id
1542 self.ir.tcx.sess.span_help(
1543 span_semicolon, "consider removing this semicolon:");
1548 if self.live_on_entry(entry_ln, self.s.clean_exit_var).is_some() => {
1549 span_err!(self.ir.tcx.sess, sp, E0270,
1550 "computation may converge in a function marked as diverging");
1557 fn check_lvalue(&mut self, expr: &Expr) {
1559 ast::ExprPath(..) => {
1560 if let DefLocal(nid) = self.ir.tcx.def_map.borrow().get(&expr.id)
1563 // Assignment to an immutable variable or argument: only legal
1564 // if there is no later assignment. If this local is actually
1565 // mutable, then check for a reassignment to flag the mutability
1567 let ln = self.live_node(expr.id, expr.span);
1568 let var = self.variable(nid, expr.span);
1569 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1573 // For other kinds of lvalues, no checks are required,
1574 // and any embedded expressions are actually rvalues
1575 visit::walk_expr(self, expr);
1580 fn should_warn(&self, var: Variable) -> Option<String> {
1581 let name = self.ir.variable_name(var);
1582 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1589 fn warn_about_unused_args(&self, decl: &ast::FnDecl, entry_ln: LiveNode) {
1590 for arg in &decl.inputs {
1591 pat_util::pat_bindings(&self.ir.tcx.def_map,
1593 |_bm, p_id, sp, path1| {
1594 let var = self.variable(p_id, sp);
1595 // Ignore unused self.
1596 let ident = path1.node;
1597 if ident.name != special_idents::self_.name {
1598 self.warn_about_unused(sp, p_id, entry_ln, var);
1604 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &ast::Pat) {
1605 self.pat_bindings(pat, |this, ln, var, sp, id| {
1606 if !this.warn_about_unused(sp, id, ln, var) {
1607 this.warn_about_dead_assign(sp, id, ln, var);
1612 fn warn_about_unused(&self,
1618 if !self.used_on_entry(ln, var) {
1619 let r = self.should_warn(var);
1620 if let Some(name) = r {
1622 // annoying: for parameters in funcs like `fn(x: int)
1623 // {ret}`, there is only one node, so asking about
1624 // assigned_on_exit() is not meaningful.
1625 let is_assigned = if ln == self.s.exit_ln {
1628 self.assigned_on_exit(ln, var).is_some()
1632 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1633 format!("variable `{}` is assigned to, but never used",
1636 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1637 format!("unused variable: `{}`", name));
1646 fn warn_about_dead_assign(&self,
1651 if self.live_on_exit(ln, var).is_none() {
1652 let r = self.should_warn(var);
1653 if let Some(name) = r {
1654 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1655 format!("value assigned to `{}` is never read", name));