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::*;
112 use dep_graph::DepNode;
114 use middle::pat_util;
115 use middle::ty::{self, TyCtxt, ParameterEnvironment};
116 use middle::traits::{self, ProjectionMode};
119 use util::nodemap::NodeMap;
121 use std::{fmt, usize};
122 use std::io::prelude::*;
125 use syntax::ast::{self, NodeId};
126 use syntax::codemap::{BytePos, original_sp, Span};
127 use syntax::parse::token::special_idents;
130 use rustc_front::hir::Expr;
131 use rustc_front::hir;
132 use rustc_front::print::pprust::{expr_to_string, block_to_string};
133 use rustc_front::intravisit::{self, Visitor, FnKind};
135 /// For use with `propagate_through_loop`.
137 /// An endless `loop` loop.
139 /// A `while` loop, with the given expression as condition.
143 #[derive(Copy, Clone, PartialEq)]
144 struct Variable(usize);
146 #[derive(Copy, PartialEq)]
147 struct LiveNode(usize);
150 fn get(&self) -> usize { let Variable(v) = *self; v }
154 fn get(&self) -> usize { let LiveNode(v) = *self; v }
157 impl Clone for LiveNode {
158 fn clone(&self) -> LiveNode {
163 #[derive(Copy, Clone, PartialEq, Debug)]
171 fn live_node_kind_to_string(lnk: LiveNodeKind, cx: &TyCtxt) -> String {
172 let cm = cx.sess.codemap();
175 format!("Free var node [{}]", cm.span_to_string(s))
178 format!("Expr node [{}]", cm.span_to_string(s))
181 format!("Var def node [{}]", cm.span_to_string(s))
183 ExitNode => "Exit node".to_string(),
187 impl<'a, 'tcx, 'v> Visitor<'v> for IrMaps<'a, 'tcx> {
188 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl,
189 b: &'v hir::Block, s: Span, id: NodeId) {
190 visit_fn(self, fk, fd, b, s, id);
192 fn visit_local(&mut self, l: &hir::Local) { visit_local(self, l); }
193 fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); }
194 fn visit_arm(&mut self, a: &hir::Arm) { visit_arm(self, a); }
197 pub fn check_crate(tcx: &TyCtxt) {
198 let _task = tcx.dep_graph.in_task(DepNode::Liveness);
199 tcx.map.krate().visit_all_items(&mut IrMaps::new(tcx));
200 tcx.sess.abort_if_errors();
203 impl fmt::Debug for LiveNode {
204 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
205 write!(f, "ln({})", self.get())
209 impl fmt::Debug 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() != usize::MAX
243 fn invalid_node() -> LiveNode { LiveNode(usize::MAX) }
250 #[derive(Copy, Clone, Debug)]
256 #[derive(Copy, Clone, Debug)]
258 Arg(NodeId, ast::Name),
264 struct IrMaps<'a, 'tcx: 'a> {
265 tcx: &'a TyCtxt<'tcx>,
267 num_live_nodes: usize,
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 TyCtxt<'tcx>) -> IrMaps<'a, 'tcx> {
282 live_node_map: NodeMap(),
283 variable_map: NodeMap(),
284 capture_info_map: NodeMap(),
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,
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, 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, vk);
325 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
326 match self.variable_map.get(&node_id) {
331 .span_bug(span, &format!("no variable registered for id {}",
337 fn variable_name(&self, var: Variable) -> String {
338 match self.var_kinds[var.get()] {
339 Local(LocalInfo { name, .. }) | Arg(_, name) => {
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 hir::FnDecl,
358 b: &'v hir::Block, s: Span, n: NodeId) {
359 check_fn(self, fk, fd, b, s, n);
361 fn visit_local(&mut self, l: &hir::Local) {
362 check_local(self, l);
364 fn visit_expr(&mut self, ex: &Expr) {
365 check_expr(self, ex);
367 fn visit_arm(&mut self, a: &hir::Arm) {
372 fn visit_fn(ir: &mut IrMaps,
380 // swap in a new set of IR maps for this function body:
381 let mut fn_maps = IrMaps::new(ir.tcx);
383 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
385 for arg in &decl.inputs {
386 pat_util::pat_bindings(&ir.tcx.def_map,
388 |_bm, arg_id, _x, path1| {
389 debug!("adding argument {}", arg_id);
390 let name = path1.node;
391 fn_maps.add_variable(Arg(arg_id, name));
395 // gather up the various local variables, significant expressions,
397 intravisit::walk_fn(&mut fn_maps, fk, decl, body, sp);
399 // Special nodes and variables:
400 // - exit_ln represents the end of the fn, either by return or panic
401 // - implicit_ret_var is a pseudo-variable that represents
402 // an implicit return
403 let specials = Specials {
404 exit_ln: fn_maps.add_live_node(ExitNode),
405 fallthrough_ln: fn_maps.add_live_node(ExitNode),
406 no_ret_var: fn_maps.add_variable(ImplicitRet),
407 clean_exit_var: fn_maps.add_variable(CleanExit)
411 let mut lsets = Liveness::new(&mut fn_maps, specials);
412 let entry_ln = lsets.compute(decl, body);
414 // check for various error conditions
415 lsets.visit_block(body);
416 lsets.check_ret(id, sp, fk, entry_ln, body);
417 lsets.warn_about_unused_args(decl, entry_ln);
420 fn visit_local(ir: &mut IrMaps, local: &hir::Local) {
421 pat_util::pat_bindings(&ir.tcx.def_map, &local.pat, |_, p_id, sp, path1| {
422 debug!("adding local variable {}", p_id);
423 let name = path1.node;
424 ir.add_live_node_for_node(p_id, VarDefNode(sp));
425 ir.add_variable(Local(LocalInfo {
430 intravisit::walk_local(ir, local);
433 fn visit_arm(ir: &mut IrMaps, arm: &hir::Arm) {
434 for pat in &arm.pats {
435 pat_util::pat_bindings(&ir.tcx.def_map, &pat, |bm, p_id, sp, path1| {
436 debug!("adding local variable {} from match with bm {:?}",
438 let name = path1.node;
439 ir.add_live_node_for_node(p_id, VarDefNode(sp));
440 ir.add_variable(Local(LocalInfo {
446 intravisit::walk_arm(ir, arm);
449 fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
451 // live nodes required for uses or definitions of variables:
452 hir::ExprPath(..) => {
453 let def = ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def();
454 debug!("expr {}: path that leads to {:?}", expr.id, def);
455 if let Def::Local(..) = def {
456 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
458 intravisit::walk_expr(ir, expr);
460 hir::ExprClosure(..) => {
461 // Interesting control flow (for loops can contain labeled
462 // breaks or continues)
463 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
465 // Make a live_node for each captured variable, with the span
466 // being the location that the variable is used. This results
467 // in better error messages than just pointing at the closure
468 // construction site.
469 let mut call_caps = Vec::new();
470 ir.tcx.with_freevars(expr.id, |freevars| {
472 if let Def::Local(_, rv) = fv.def {
473 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
474 call_caps.push(CaptureInfo {ln: fv_ln,
479 ir.set_captures(expr.id, call_caps);
481 intravisit::walk_expr(ir, expr);
484 // live nodes required for interesting control flow:
485 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
486 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
487 intravisit::walk_expr(ir, expr);
489 hir::ExprBinary(op, _, _) if ::rustc_front::util::lazy_binop(op.node) => {
490 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
491 intravisit::walk_expr(ir, expr);
494 // otherwise, live nodes are not required:
495 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
496 hir::ExprVec(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
497 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
498 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(_) |
499 hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) |
500 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
501 hir::ExprStruct(..) | hir::ExprRepeat(..) |
502 hir::ExprInlineAsm(..) | hir::ExprBox(..) |
503 hir::ExprType(..) => {
504 intravisit::walk_expr(ir, expr);
509 // ______________________________________________________________________
510 // Computing liveness sets
512 // Actually we compute just a bit more than just liveness, but we use
513 // the same basic propagation framework in all cases.
515 #[derive(Clone, Copy)]
522 fn invalid_users() -> Users {
524 reader: invalid_node(),
525 writer: invalid_node(),
530 #[derive(Copy, Clone)]
533 fallthrough_ln: LiveNode,
534 no_ret_var: Variable,
535 clean_exit_var: Variable
538 const ACC_READ: u32 = 1;
539 const ACC_WRITE: u32 = 2;
540 const ACC_USE: u32 = 4;
542 struct Liveness<'a, 'tcx: 'a> {
543 ir: &'a mut IrMaps<'a, 'tcx>,
545 successors: Vec<LiveNode>,
547 // The list of node IDs for the nested loop scopes
549 loop_scope: Vec<NodeId>,
550 // mappings from loop node ID to LiveNode
551 // ("break" label should map to loop node ID,
552 // it probably doesn't now)
553 break_ln: NodeMap<LiveNode>,
554 cont_ln: NodeMap<LiveNode>
557 impl<'a, 'tcx> Liveness<'a, 'tcx> {
558 fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> {
559 let num_live_nodes = ir.num_live_nodes;
560 let num_vars = ir.num_vars;
564 successors: vec![invalid_node(); num_live_nodes],
565 users: vec![invalid_users(); num_live_nodes * num_vars],
566 loop_scope: Vec::new(),
572 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
573 match self.ir.live_node_map.get(&node_id) {
576 // This must be a mismatch between the ir_map construction
577 // above and the propagation code below; the two sets of
578 // code have to agree about which AST nodes are worth
579 // creating liveness nodes for.
580 self.ir.tcx.sess.span_bug(
582 &format!("no live node registered for node {}",
588 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
589 self.ir.variable(node_id, span)
592 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
593 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
595 pat_util::pat_bindings(&self.ir.tcx.def_map, pat, |_bm, p_id, sp, _n| {
596 let ln = self.live_node(p_id, sp);
597 let var = self.variable(p_id, sp);
598 f(self, ln, var, sp, p_id);
602 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
603 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
607 self.pat_bindings(pat, f);
613 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
615 self.define_bindings_in_arm_pats(Some(pat), succ)
618 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
621 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
622 this.init_from_succ(ln, succ);
623 this.define(ln, var);
629 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
630 ln.get() * self.ir.num_vars + var.get()
633 fn live_on_entry(&self, ln: LiveNode, var: Variable)
634 -> Option<LiveNodeKind> {
635 assert!(ln.is_valid());
636 let reader = self.users[self.idx(ln, var)].reader;
637 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
641 Is this variable live on entry to any of its successor nodes?
643 fn live_on_exit(&self, ln: LiveNode, var: Variable)
644 -> Option<LiveNodeKind> {
645 let successor = self.successors[ln.get()];
646 self.live_on_entry(successor, var)
649 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
650 assert!(ln.is_valid());
651 self.users[self.idx(ln, var)].used
654 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
655 -> Option<LiveNodeKind> {
656 assert!(ln.is_valid());
657 let writer = self.users[self.idx(ln, var)].writer;
658 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
661 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
662 -> Option<LiveNodeKind> {
663 let successor = self.successors[ln.get()];
664 self.assigned_on_entry(successor, var)
667 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
668 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
670 let node_base_idx = self.idx(ln, Variable(0));
671 let succ_base_idx = self.idx(succ_ln, Variable(0));
672 for var_idx in 0..self.ir.num_vars {
673 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
677 fn write_vars<F>(&self,
681 -> io::Result<()> where
682 F: FnMut(usize) -> LiveNode,
684 let node_base_idx = self.idx(ln, Variable(0));
685 for var_idx in 0..self.ir.num_vars {
686 let idx = node_base_idx + var_idx;
687 if test(idx).is_valid() {
688 try!(write!(wr, " {:?}", Variable(var_idx)));
694 fn find_loop_scope(&self,
695 opt_label: Option<ast::Name>,
701 // Refers to a labeled loop. Use the results of resolve
703 match self.ir.tcx.def_map.borrow().get(&id).map(|d| d.full_def()) {
704 Some(Def::Label(loop_id)) => loop_id,
705 _ => self.ir.tcx.sess.span_bug(sp, "label on break/loop \
706 doesn't refer to a loop")
710 // Vanilla 'break' or 'loop', so use the enclosing
712 if self.loop_scope.is_empty() {
713 self.ir.tcx.sess.span_bug(sp, "break outside loop");
715 *self.loop_scope.last().unwrap()
721 #[allow(unused_must_use)]
722 fn ln_str(&self, ln: LiveNode) -> String {
723 let mut wr = Vec::new();
725 let wr = &mut wr as &mut Write;
726 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
727 self.write_vars(wr, ln, |idx| self.users[idx].reader);
728 write!(wr, " writes");
729 self.write_vars(wr, ln, |idx| self.users[idx].writer);
730 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
732 String::from_utf8(wr).unwrap()
735 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
736 self.successors[ln.get()] = succ_ln;
738 // It is not necessary to initialize the
739 // values to empty because this is the value
740 // they have when they are created, and the sets
741 // only grow during iterations.
743 // self.indices(ln) { |idx|
744 // self.users[idx] = invalid_users();
748 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
749 // more efficient version of init_empty() / merge_from_succ()
750 self.successors[ln.get()] = succ_ln;
752 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
753 this.users[idx] = this.users[succ_idx]
755 debug!("init_from_succ(ln={}, succ={})",
756 self.ln_str(ln), self.ln_str(succ_ln));
759 fn merge_from_succ(&mut self,
764 if ln == succ_ln { return false; }
766 let mut changed = false;
767 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
768 changed |= copy_if_invalid(this.users[succ_idx].reader,
769 &mut this.users[idx].reader);
770 changed |= copy_if_invalid(this.users[succ_idx].writer,
771 &mut this.users[idx].writer);
772 if this.users[succ_idx].used && !this.users[idx].used {
773 this.users[idx].used = true;
778 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
779 ln, self.ln_str(succ_ln), first_merge, changed);
782 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
783 if src.is_valid() && !dst.is_valid() {
792 // Indicates that a local variable was *defined*; we know that no
793 // uses of the variable can precede the definition (resolve checks
794 // this) so we just clear out all the data.
795 fn define(&mut self, writer: LiveNode, var: Variable) {
796 let idx = self.idx(writer, var);
797 self.users[idx].reader = invalid_node();
798 self.users[idx].writer = invalid_node();
800 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
801 idx, self.ln_str(writer));
804 // Either read, write, or both depending on the acc bitset
805 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
806 debug!("{:?} accesses[{:x}] {:?}: {}",
807 ln, acc, var, self.ln_str(ln));
809 let idx = self.idx(ln, var);
810 let user = &mut self.users[idx];
812 if (acc & ACC_WRITE) != 0 {
813 user.reader = invalid_node();
817 // Important: if we both read/write, must do read second
818 // or else the write will override.
819 if (acc & ACC_READ) != 0 {
823 if (acc & ACC_USE) != 0 {
828 // _______________________________________________________________________
830 fn compute(&mut self, decl: &hir::FnDecl, body: &hir::Block) -> LiveNode {
831 // if there is a `break` or `again` at the top level, then it's
832 // effectively a return---this only occurs in `for` loops,
833 // where the body is really a closure.
835 debug!("compute: using id for block, {}", block_to_string(body));
837 let exit_ln = self.s.exit_ln;
838 let entry_ln: LiveNode =
839 self.with_loop_nodes(body.id, exit_ln, exit_ln,
840 |this| this.propagate_through_fn_block(decl, body));
842 // hack to skip the loop unless debug! is enabled:
843 debug!("^^ liveness computation results for body {} (entry={:?})",
845 for ln_idx in 0..self.ir.num_live_nodes {
846 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
855 fn propagate_through_fn_block(&mut self, _: &hir::FnDecl, blk: &hir::Block)
857 // the fallthrough exit is only for those cases where we do not
858 // explicitly return:
860 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
861 if blk.expr.is_none() {
862 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
864 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
866 self.propagate_through_block(blk, s.fallthrough_ln)
869 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
871 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
872 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
873 self.propagate_through_stmt(stmt, succ)
877 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
880 hir::StmtDecl(ref decl, _) => {
881 self.propagate_through_decl(&decl, succ)
884 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
885 self.propagate_through_expr(&expr, succ)
890 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
893 hir::DeclLocal(ref local) => {
894 self.propagate_through_local(&local, succ)
896 hir::DeclItem(_) => succ,
900 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
902 // Note: we mark the variable as defined regardless of whether
903 // there is an initializer. Initially I had thought to only mark
904 // the live variable as defined if it was initialized, and then we
905 // could check for uninit variables just by scanning what is live
906 // at the start of the function. But that doesn't work so well for
907 // immutable variables defined in a loop:
908 // loop { let x; x = 5; }
909 // because the "assignment" loops back around and generates an error.
911 // So now we just check that variables defined w/o an
912 // initializer are not live at the point of their
913 // initialization, which is mildly more complex than checking
914 // once at the func header but otherwise equivalent.
916 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
917 self.define_bindings_in_pat(&local.pat, succ)
920 fn propagate_through_exprs(&mut self, exprs: &[P<Expr>], succ: LiveNode)
922 exprs.iter().rev().fold(succ, |succ, expr| {
923 self.propagate_through_expr(&expr, succ)
927 fn propagate_through_opt_expr(&mut self,
928 opt_expr: Option<&Expr>,
931 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
934 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
936 debug!("propagate_through_expr: {}", expr_to_string(expr));
939 // Interesting cases with control flow or which gen/kill
941 hir::ExprPath(..) => {
942 self.access_path(expr, succ, ACC_READ | ACC_USE)
945 hir::ExprField(ref e, _) => {
946 self.propagate_through_expr(&e, succ)
949 hir::ExprTupField(ref e, _) => {
950 self.propagate_through_expr(&e, succ)
953 hir::ExprClosure(_, _, ref blk) => {
954 debug!("{} is an ExprClosure",
955 expr_to_string(expr));
958 The next-node for a break is the successor of the entire
959 loop. The next-node for a continue is the top of this loop.
961 let node = self.live_node(expr.id, expr.span);
962 self.with_loop_nodes(blk.id, succ, node, |this| {
964 // the construction of a closure itself is not important,
965 // but we have to consider the closed over variables.
966 let caps = match this.ir.capture_info_map.get(&expr.id) {
967 Some(caps) => caps.clone(),
969 this.ir.tcx.sess.span_bug(expr.span, "no registered caps");
972 caps.iter().rev().fold(succ, |succ, cap| {
973 this.init_from_succ(cap.ln, succ);
974 let var = this.variable(cap.var_nid, expr.span);
975 this.acc(cap.ln, var, ACC_READ | ACC_USE);
981 hir::ExprIf(ref cond, ref then, ref els) => {
995 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
996 let then_ln = self.propagate_through_block(&then, succ);
997 let ln = self.live_node(expr.id, expr.span);
998 self.init_from_succ(ln, else_ln);
999 self.merge_from_succ(ln, then_ln, false);
1000 self.propagate_through_expr(&cond, ln)
1003 hir::ExprWhile(ref cond, ref blk, _) => {
1004 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1007 // Note that labels have been resolved, so we don't need to look
1008 // at the label ident
1009 hir::ExprLoop(ref blk, _) => {
1010 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1013 hir::ExprMatch(ref e, ref arms, _) => {
1028 let ln = self.live_node(expr.id, expr.span);
1029 self.init_empty(ln, succ);
1030 let mut first_merge = true;
1033 self.propagate_through_expr(&arm.body, succ);
1035 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1036 // only consider the first pattern; any later patterns must have
1037 // the same bindings, and we also consider the first pattern to be
1038 // the "authoritative" set of ids
1040 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1042 self.merge_from_succ(ln, arm_succ, first_merge);
1043 first_merge = false;
1045 self.propagate_through_expr(&e, ln)
1048 hir::ExprRet(ref o_e) => {
1049 // ignore succ and subst exit_ln:
1050 let exit_ln = self.s.exit_ln;
1051 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1054 hir::ExprBreak(opt_label) => {
1055 // Find which label this break jumps to
1056 let sc = self.find_loop_scope(opt_label.map(|l| l.node.name), expr.id, expr.span);
1058 // Now that we know the label we're going to,
1059 // look it up in the break loop nodes table
1061 match self.break_ln.get(&sc) {
1063 None => self.ir.tcx.sess.span_bug(expr.span,
1064 "break to unknown label")
1068 hir::ExprAgain(opt_label) => {
1069 // Find which label this expr continues to
1070 let sc = self.find_loop_scope(opt_label.map(|l| l.node.name), expr.id, expr.span);
1072 // Now that we know the label we're going to,
1073 // look it up in the continue loop nodes table
1075 match self.cont_ln.get(&sc) {
1077 None => self.ir.tcx.sess.span_bug(expr.span,
1078 "loop to unknown label")
1082 hir::ExprAssign(ref l, ref r) => {
1083 // see comment on lvalues in
1084 // propagate_through_lvalue_components()
1085 let succ = self.write_lvalue(&l, succ, ACC_WRITE);
1086 let succ = self.propagate_through_lvalue_components(&l, succ);
1087 self.propagate_through_expr(&r, succ)
1090 hir::ExprAssignOp(_, ref l, ref r) => {
1091 // an overloaded assign op is like a method call
1092 if self.ir.tcx.is_method_call(expr.id) {
1093 let succ = self.propagate_through_expr(&l, succ);
1094 self.propagate_through_expr(&r, succ)
1096 // see comment on lvalues in
1097 // propagate_through_lvalue_components()
1098 let succ = self.write_lvalue(&l, succ, ACC_WRITE|ACC_READ);
1099 let succ = self.propagate_through_expr(&r, succ);
1100 self.propagate_through_lvalue_components(&l, succ)
1104 // Uninteresting cases: just propagate in rev exec order
1106 hir::ExprVec(ref exprs) => {
1107 self.propagate_through_exprs(&exprs[..], succ)
1110 hir::ExprRepeat(ref element, ref count) => {
1111 let succ = self.propagate_through_expr(&count, succ);
1112 self.propagate_through_expr(&element, succ)
1115 hir::ExprStruct(_, ref fields, ref with_expr) => {
1116 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1117 fields.iter().rev().fold(succ, |succ, field| {
1118 self.propagate_through_expr(&field.expr, succ)
1122 hir::ExprCall(ref f, ref args) => {
1123 let diverges = !self.ir.tcx.is_method_call(expr.id) &&
1124 self.ir.tcx.expr_ty_adjusted(&f).fn_ret().diverges();
1125 let succ = if diverges {
1130 let succ = self.propagate_through_exprs(&args[..], succ);
1131 self.propagate_through_expr(&f, succ)
1134 hir::ExprMethodCall(_, _, ref args) => {
1135 let method_call = ty::MethodCall::expr(expr.id);
1136 let method_ty = self.ir.tcx.tables.borrow().method_map[&method_call].ty;
1137 let succ = if method_ty.fn_ret().diverges() {
1142 self.propagate_through_exprs(&args[..], succ)
1145 hir::ExprTup(ref exprs) => {
1146 self.propagate_through_exprs(&exprs[..], succ)
1149 hir::ExprBinary(op, ref l, ref r) if ::rustc_front::util::lazy_binop(op.node) => {
1150 let r_succ = self.propagate_through_expr(&r, succ);
1152 let ln = self.live_node(expr.id, expr.span);
1153 self.init_from_succ(ln, succ);
1154 self.merge_from_succ(ln, r_succ, false);
1156 self.propagate_through_expr(&l, ln)
1159 hir::ExprIndex(ref l, ref r) |
1160 hir::ExprBinary(_, ref l, ref r) => {
1161 let r_succ = self.propagate_through_expr(&r, succ);
1162 self.propagate_through_expr(&l, r_succ)
1165 hir::ExprBox(ref e) |
1166 hir::ExprAddrOf(_, ref e) |
1167 hir::ExprCast(ref e, _) |
1168 hir::ExprType(ref e, _) |
1169 hir::ExprUnary(_, ref e) => {
1170 self.propagate_through_expr(&e, succ)
1173 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1174 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1175 // see comment on lvalues
1176 // in propagate_through_lvalue_components()
1178 self.propagate_through_expr(output, succ)
1180 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1181 let succ = self.write_lvalue(output, succ, acc);
1182 self.propagate_through_lvalue_components(output, succ)
1186 // Inputs are executed first. Propagate last because of rev order
1187 self.propagate_through_exprs(inputs, succ)
1190 hir::ExprLit(..) => {
1194 hir::ExprBlock(ref blk) => {
1195 self.propagate_through_block(&blk, succ)
1200 fn propagate_through_lvalue_components(&mut self,
1206 // In general, the full flow graph structure for an
1207 // assignment/move/etc can be handled in one of two ways,
1208 // depending on whether what is being assigned is a "tracked
1209 // value" or not. A tracked value is basically a local
1210 // variable or argument.
1212 // The two kinds of graphs are:
1214 // Tracked lvalue Untracked lvalue
1215 // ----------------------++-----------------------
1219 // (rvalue) || (rvalue)
1222 // (write of lvalue) || (lvalue components)
1227 // ----------------------++-----------------------
1229 // I will cover the two cases in turn:
1231 // # Tracked lvalues
1233 // A tracked lvalue is a local variable/argument `x`. In
1234 // these cases, the link_node where the write occurs is linked
1235 // to node id of `x`. The `write_lvalue()` routine generates
1236 // the contents of this node. There are no subcomponents to
1239 // # Non-tracked lvalues
1241 // These are lvalues like `x[5]` or `x.f`. In that case, we
1242 // basically ignore the value which is written to but generate
1243 // reads for the components---`x` in these two examples. The
1244 // components reads are generated by
1245 // `propagate_through_lvalue_components()` (this fn).
1247 // # Illegal lvalues
1249 // It is still possible to observe assignments to non-lvalues;
1250 // these errors are detected in the later pass borrowck. We
1251 // just ignore such cases and treat them as reads.
1254 hir::ExprPath(..) => succ,
1255 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1256 hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ),
1257 _ => self.propagate_through_expr(expr, succ)
1261 // see comment on propagate_through_lvalue()
1262 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1265 hir::ExprPath(..) => {
1266 self.access_path(expr, succ, acc)
1269 // We do not track other lvalues, so just propagate through
1270 // to their subcomponents. Also, it may happen that
1271 // non-lvalues occur here, because those are detected in the
1272 // later pass borrowck.
1277 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1279 match self.ir.tcx.def_map.borrow().get(&expr.id).unwrap().full_def() {
1280 Def::Local(_, nid) => {
1281 let ln = self.live_node(expr.id, expr.span);
1283 self.init_from_succ(ln, succ);
1284 let var = self.variable(nid, expr.span);
1285 self.acc(ln, var, acc);
1293 fn propagate_through_loop(&mut self,
1302 We model control flow like this:
1320 let mut first_merge = true;
1321 let ln = self.live_node(expr.id, expr.span);
1322 self.init_empty(ln, succ);
1326 // If this is not a `loop` loop, then it's possible we bypass
1327 // the body altogether. Otherwise, the only way is via a `break`
1328 // in the loop body.
1329 self.merge_from_succ(ln, succ, first_merge);
1330 first_merge = false;
1333 debug!("propagate_through_loop: using id for loop body {} {}",
1334 expr.id, block_to_string(body));
1336 let cond_ln = match kind {
1338 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1340 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1341 this.propagate_through_block(body, cond_ln)
1344 // repeat until fixed point is reached:
1345 while self.merge_from_succ(ln, body_ln, first_merge) {
1346 first_merge = false;
1348 let new_cond_ln = match kind {
1350 WhileLoop(ref cond) => {
1351 self.propagate_through_expr(&cond, ln)
1354 assert!(cond_ln == new_cond_ln);
1355 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1356 |this| this.propagate_through_block(body, cond_ln)));
1362 fn with_loop_nodes<R, F>(&mut self,
1363 loop_node_id: NodeId,
1368 F: FnOnce(&mut Liveness<'a, 'tcx>) -> R,
1370 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1371 self.loop_scope.push(loop_node_id);
1372 self.break_ln.insert(loop_node_id, break_ln);
1373 self.cont_ln.insert(loop_node_id, cont_ln);
1375 self.loop_scope.pop();
1380 // _______________________________________________________________________
1381 // Checking for error conditions
1383 fn check_local(this: &mut Liveness, local: &hir::Local) {
1386 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1389 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1390 this.warn_about_unused(sp, id, ln, var);
1395 intravisit::walk_local(this, local);
1398 fn check_arm(this: &mut Liveness, arm: &hir::Arm) {
1399 // only consider the first pattern; any later patterns must have
1400 // the same bindings, and we also consider the first pattern to be
1401 // the "authoritative" set of ids
1402 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1403 this.warn_about_unused(sp, id, ln, var);
1405 intravisit::walk_arm(this, arm);
1408 fn check_expr(this: &mut Liveness, expr: &Expr) {
1410 hir::ExprAssign(ref l, _) => {
1411 this.check_lvalue(&l);
1413 intravisit::walk_expr(this, expr);
1416 hir::ExprAssignOp(_, ref l, _) => {
1417 if !this.ir.tcx.is_method_call(expr.id) {
1418 this.check_lvalue(&l);
1421 intravisit::walk_expr(this, expr);
1424 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1425 for input in inputs {
1426 this.visit_expr(input);
1429 // Output operands must be lvalues
1430 for (o, output) in ia.outputs.iter().zip(outputs) {
1432 this.check_lvalue(output);
1434 this.visit_expr(output);
1437 intravisit::walk_expr(this, expr);
1440 // no correctness conditions related to liveness
1441 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1442 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1443 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
1444 hir::ExprVec(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1445 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1446 hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) |
1447 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1448 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1449 hir::ExprClosure(..) | hir::ExprPath(..) | hir::ExprBox(..) |
1450 hir::ExprType(..) => {
1451 intravisit::walk_expr(this, expr);
1456 fn check_fn(_v: &Liveness,
1458 _decl: &hir::FnDecl,
1462 // do not check contents of nested fns
1465 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1466 fn fn_ret(&self, id: NodeId) -> ty::PolyFnOutput<'tcx> {
1467 let fn_ty = self.ir.tcx.node_id_to_type(id);
1469 ty::TyClosure(closure_def_id, ref substs) =>
1470 self.ir.tcx.closure_type(closure_def_id, substs).sig.output(),
1482 // within the fn body, late-bound regions are liberated
1483 // and must outlive the *call-site* of the function.
1485 self.ir.tcx.liberate_late_bound_regions(
1486 self.ir.tcx.region_maps.call_site_extent(id, body.id),
1490 ty::FnConverging(t_ret)
1491 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() => {
1493 let param_env = ParameterEnvironment::for_item(&self.ir.tcx, id);
1494 let infcx = infer::new_infer_ctxt(&self.ir.tcx,
1495 &self.ir.tcx.tables,
1497 ProjectionMode::Any);
1498 let cause = traits::ObligationCause::dummy();
1499 let norm = traits::fully_normalize(&infcx,
1503 if norm.unwrap().is_nil() {
1504 // for nil return types, it is ok to not return a value expl.
1506 let ends_with_stmt = match body.expr {
1507 None if !body.stmts.is_empty() =>
1508 match body.stmts.first().unwrap().node {
1509 hir::StmtSemi(ref e, _) => {
1510 self.ir.tcx.expr_ty(&e) == t_ret
1516 let mut err = struct_span_err!(self.ir.tcx.sess,
1519 "not all control paths return a value");
1521 let last_stmt = body.stmts.first().unwrap();
1522 let original_span = original_sp(self.ir.tcx.sess.codemap(),
1523 last_stmt.span, sp);
1524 let span_semicolon = Span {
1525 lo: original_span.hi - BytePos(1),
1526 hi: original_span.hi,
1527 expn_id: original_span.expn_id
1529 err.span_help(span_semicolon, "consider removing this semicolon:");
1535 if self.live_on_entry(entry_ln, self.s.clean_exit_var).is_some() => {
1536 span_err!(self.ir.tcx.sess, sp, E0270,
1537 "computation may converge in a function marked as diverging");
1544 fn check_lvalue(&mut self, expr: &Expr) {
1546 hir::ExprPath(..) => {
1547 if let Def::Local(_, nid) = self.ir.tcx.def_map.borrow().get(&expr.id)
1550 // Assignment to an immutable variable or argument: only legal
1551 // if there is no later assignment. If this local is actually
1552 // mutable, then check for a reassignment to flag the mutability
1554 let ln = self.live_node(expr.id, expr.span);
1555 let var = self.variable(nid, expr.span);
1556 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1560 // For other kinds of lvalues, no checks are required,
1561 // and any embedded expressions are actually rvalues
1562 intravisit::walk_expr(self, expr);
1567 fn should_warn(&self, var: Variable) -> Option<String> {
1568 let name = self.ir.variable_name(var);
1569 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1576 fn warn_about_unused_args(&self, decl: &hir::FnDecl, entry_ln: LiveNode) {
1577 for arg in &decl.inputs {
1578 pat_util::pat_bindings(&self.ir.tcx.def_map,
1580 |_bm, p_id, sp, path1| {
1581 let var = self.variable(p_id, sp);
1582 // Ignore unused self.
1583 let name = path1.node;
1584 if name != special_idents::self_.name {
1585 if !self.warn_about_unused(sp, p_id, entry_ln, var) {
1586 if self.live_on_entry(entry_ln, var).is_none() {
1587 self.report_dead_assign(p_id, sp, var, true);
1595 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1596 self.pat_bindings(pat, |this, ln, var, sp, id| {
1597 if !this.warn_about_unused(sp, id, ln, var) {
1598 this.warn_about_dead_assign(sp, id, ln, var);
1603 fn warn_about_unused(&self,
1609 if !self.used_on_entry(ln, var) {
1610 let r = self.should_warn(var);
1611 if let Some(name) = r {
1613 // annoying: for parameters in funcs like `fn(x: i32)
1614 // {ret}`, there is only one node, so asking about
1615 // assigned_on_exit() is not meaningful.
1616 let is_assigned = if ln == self.s.exit_ln {
1619 self.assigned_on_exit(ln, var).is_some()
1623 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1624 format!("variable `{}` is assigned to, but never used",
1626 } else if name != "self" {
1627 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1628 format!("unused variable: `{}`", name));
1637 fn warn_about_dead_assign(&self,
1642 if self.live_on_exit(ln, var).is_none() {
1643 self.report_dead_assign(id, sp, var, false);
1647 fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) {
1648 if let Some(name) = self.should_warn(var) {
1650 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1651 format!("value passed to `{}` is never read", name));
1653 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1654 format!("value assigned to `{}` is never read", name));