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;
115 use ty::{self, Ty, TyCtxt, ParameterEnvironment};
116 use traits::{self, Reveal};
117 use ty::subst::Subst;
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::original_sp;
127 use syntax::parse::token::keywords;
129 use syntax_pos::{BytePos, Span};
133 use hir::print::{expr_to_string, block_to_string};
134 use hir::intravisit::{self, Visitor, FnKind};
136 /// For use with `propagate_through_loop`.
138 /// An endless `loop` loop.
140 /// A `while` loop, with the given expression as condition.
144 #[derive(Copy, Clone, PartialEq)]
145 struct Variable(usize);
147 #[derive(Copy, PartialEq)]
148 struct LiveNode(usize);
151 fn get(&self) -> usize { let Variable(v) = *self; v }
155 fn get(&self) -> usize { let LiveNode(v) = *self; v }
158 impl Clone for LiveNode {
159 fn clone(&self) -> LiveNode {
164 #[derive(Copy, Clone, PartialEq, Debug)]
172 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt) -> String {
173 let cm = tcx.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 hir::FnDecl,
190 b: &'v hir::Block, s: Span, id: NodeId) {
191 visit_fn(self, fk, fd, b, s, id);
193 fn visit_local(&mut self, l: &hir::Local) { visit_local(self, l); }
194 fn visit_expr(&mut self, ex: &Expr) { visit_expr(self, ex); }
195 fn visit_arm(&mut self, a: &hir::Arm) { visit_arm(self, a); }
198 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
199 let _task = tcx.dep_graph.in_task(DepNode::Liveness);
200 tcx.map.krate().visit_all_items(&mut IrMaps::new(tcx));
201 tcx.sess.abort_if_errors();
204 impl fmt::Debug for LiveNode {
205 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
206 write!(f, "ln({})", self.get())
210 impl fmt::Debug for Variable {
211 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
212 write!(f, "v({})", self.get())
216 // ______________________________________________________________________
219 // This is the first pass and the one that drives the main
220 // computation. It walks up and down the IR once. On the way down,
221 // we count for each function the number of variables as well as
222 // liveness nodes. A liveness node is basically an expression or
223 // capture clause that does something of interest: either it has
224 // interesting control flow or it uses/defines a local variable.
226 // On the way back up, at each function node we create liveness sets
227 // (we now know precisely how big to make our various vectors and so
228 // forth) and then do the data-flow propagation to compute the set
229 // of live variables at each program point.
231 // Finally, we run back over the IR one last time and, using the
232 // computed liveness, check various safety conditions. For example,
233 // there must be no live nodes at the definition site for a variable
234 // unless it has an initializer. Similarly, each non-mutable local
235 // variable must not be assigned if there is some successor
236 // assignment. And so forth.
239 fn is_valid(&self) -> bool {
240 self.get() != usize::MAX
244 fn invalid_node() -> LiveNode { LiveNode(usize::MAX) }
251 #[derive(Copy, Clone, Debug)]
257 #[derive(Copy, Clone, Debug)]
259 Arg(NodeId, ast::Name),
265 struct IrMaps<'a, 'tcx: 'a> {
266 tcx: TyCtxt<'a, 'tcx, 'tcx>,
268 num_live_nodes: usize,
270 live_node_map: NodeMap<LiveNode>,
271 variable_map: NodeMap<Variable>,
272 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
273 var_kinds: Vec<VarKind>,
274 lnks: Vec<LiveNodeKind>,
277 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
278 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
283 live_node_map: NodeMap(),
284 variable_map: NodeMap(),
285 capture_info_map: NodeMap(),
286 var_kinds: Vec::new(),
291 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
292 let ln = LiveNode(self.num_live_nodes);
294 self.num_live_nodes += 1;
296 debug!("{:?} is of kind {}", ln,
297 live_node_kind_to_string(lnk, self.tcx));
302 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
303 let ln = self.add_live_node(lnk);
304 self.live_node_map.insert(node_id, ln);
306 debug!("{:?} is node {}", ln, node_id);
309 fn add_variable(&mut self, vk: VarKind) -> Variable {
310 let v = Variable(self.num_vars);
311 self.var_kinds.push(vk);
315 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
316 self.variable_map.insert(node_id, v);
318 ImplicitRet | CleanExit => {}
321 debug!("{:?} is {:?}", v, vk);
326 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
327 match self.variable_map.get(&node_id) {
330 span_bug!(span, "no variable registered for id {}", node_id);
335 fn variable_name(&self, var: Variable) -> String {
336 match self.var_kinds[var.get()] {
337 Local(LocalInfo { name, .. }) | Arg(_, name) => {
340 ImplicitRet => "<implicit-ret>".to_string(),
341 CleanExit => "<clean-exit>".to_string()
345 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
346 self.capture_info_map.insert(node_id, Rc::new(cs));
349 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
354 impl<'a, 'tcx, 'v> Visitor<'v> for Liveness<'a, 'tcx> {
355 fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl,
356 b: &'v hir::Block, s: Span, n: NodeId) {
357 check_fn(self, fk, fd, b, s, n);
359 fn visit_local(&mut self, l: &hir::Local) {
360 check_local(self, l);
362 fn visit_expr(&mut self, ex: &Expr) {
363 check_expr(self, ex);
365 fn visit_arm(&mut self, a: &hir::Arm) {
370 fn visit_fn(ir: &mut IrMaps,
378 // swap in a new set of IR maps for this function body:
379 let mut fn_maps = IrMaps::new(ir.tcx);
381 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
383 for arg in &decl.inputs {
384 pat_util::pat_bindings(&arg.pat, |_bm, arg_id, _x, path1| {
385 debug!("adding argument {}", arg_id);
386 let name = path1.node;
387 fn_maps.add_variable(Arg(arg_id, name));
391 // gather up the various local variables, significant expressions,
393 intravisit::walk_fn(&mut fn_maps, fk, decl, body, sp, id);
395 // Special nodes and variables:
396 // - exit_ln represents the end of the fn, either by return or panic
397 // - implicit_ret_var is a pseudo-variable that represents
398 // an implicit return
399 let specials = Specials {
400 exit_ln: fn_maps.add_live_node(ExitNode),
401 fallthrough_ln: fn_maps.add_live_node(ExitNode),
402 no_ret_var: fn_maps.add_variable(ImplicitRet),
403 clean_exit_var: fn_maps.add_variable(CleanExit)
407 let mut lsets = Liveness::new(&mut fn_maps, specials);
408 let entry_ln = lsets.compute(decl, body);
410 // check for various error conditions
411 lsets.visit_block(body);
412 lsets.check_ret(id, sp, fk, entry_ln, body);
413 lsets.warn_about_unused_args(decl, entry_ln);
416 fn visit_local(ir: &mut IrMaps, local: &hir::Local) {
417 pat_util::pat_bindings(&local.pat, |_, p_id, sp, path1| {
418 debug!("adding local variable {}", p_id);
419 let name = path1.node;
420 ir.add_live_node_for_node(p_id, VarDefNode(sp));
421 ir.add_variable(Local(LocalInfo {
426 intravisit::walk_local(ir, local);
429 fn visit_arm(ir: &mut IrMaps, arm: &hir::Arm) {
430 for pat in &arm.pats {
431 pat_util::pat_bindings(&pat, |bm, p_id, sp, path1| {
432 debug!("adding local variable {} from match with bm {:?}",
434 let name = path1.node;
435 ir.add_live_node_for_node(p_id, VarDefNode(sp));
436 ir.add_variable(Local(LocalInfo {
442 intravisit::walk_arm(ir, arm);
445 fn visit_expr(ir: &mut IrMaps, expr: &Expr) {
447 // live nodes required for uses or definitions of variables:
448 hir::ExprPath(..) => {
449 let def = ir.tcx.expect_def(expr.id);
450 debug!("expr {}: path that leads to {:?}", expr.id, def);
451 if let Def::Local(..) = def {
452 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
454 intravisit::walk_expr(ir, expr);
456 hir::ExprClosure(..) => {
457 // Interesting control flow (for loops can contain labeled
458 // breaks or continues)
459 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
461 // Make a live_node for each captured variable, with the span
462 // being the location that the variable is used. This results
463 // in better error messages than just pointing at the closure
464 // construction site.
465 let mut call_caps = Vec::new();
466 ir.tcx.with_freevars(expr.id, |freevars| {
468 if let Def::Local(_, rv) = fv.def {
469 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
470 call_caps.push(CaptureInfo {ln: fv_ln,
475 ir.set_captures(expr.id, call_caps);
477 intravisit::walk_expr(ir, expr);
480 // live nodes required for interesting control flow:
481 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
482 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
483 intravisit::walk_expr(ir, expr);
485 hir::ExprBinary(op, _, _) if op.node.is_lazy() => {
486 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
487 intravisit::walk_expr(ir, expr);
490 // otherwise, live nodes are not required:
491 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
492 hir::ExprVec(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
493 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
494 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(_) |
495 hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) |
496 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
497 hir::ExprStruct(..) | hir::ExprRepeat(..) |
498 hir::ExprInlineAsm(..) | hir::ExprBox(..) |
499 hir::ExprType(..) => {
500 intravisit::walk_expr(ir, expr);
505 // ______________________________________________________________________
506 // Computing liveness sets
508 // Actually we compute just a bit more than just liveness, but we use
509 // the same basic propagation framework in all cases.
511 #[derive(Clone, Copy)]
518 fn invalid_users() -> Users {
520 reader: invalid_node(),
521 writer: invalid_node(),
526 #[derive(Copy, Clone)]
529 fallthrough_ln: LiveNode,
530 no_ret_var: Variable,
531 clean_exit_var: Variable
534 const ACC_READ: u32 = 1;
535 const ACC_WRITE: u32 = 2;
536 const ACC_USE: u32 = 4;
538 struct Liveness<'a, 'tcx: 'a> {
539 ir: &'a mut IrMaps<'a, 'tcx>,
541 successors: Vec<LiveNode>,
543 // The list of node IDs for the nested loop scopes
545 loop_scope: Vec<NodeId>,
546 // mappings from loop node ID to LiveNode
547 // ("break" label should map to loop node ID,
548 // it probably doesn't now)
549 break_ln: NodeMap<LiveNode>,
550 cont_ln: NodeMap<LiveNode>
553 impl<'a, 'tcx> Liveness<'a, 'tcx> {
554 fn new(ir: &'a mut IrMaps<'a, 'tcx>, specials: Specials) -> Liveness<'a, 'tcx> {
555 let num_live_nodes = ir.num_live_nodes;
556 let num_vars = ir.num_vars;
560 successors: vec![invalid_node(); num_live_nodes],
561 users: vec![invalid_users(); num_live_nodes * num_vars],
562 loop_scope: Vec::new(),
568 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
569 match self.ir.live_node_map.get(&node_id) {
572 // This must be a mismatch between the ir_map construction
573 // above and the propagation code below; the two sets of
574 // code have to agree about which AST nodes are worth
575 // creating liveness nodes for.
578 "no live node registered for node {}",
584 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
585 self.ir.variable(node_id, span)
588 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
589 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
591 pat_util::pat_bindings(pat, |_bm, p_id, sp, _n| {
592 let ln = self.live_node(p_id, sp);
593 let var = self.variable(p_id, sp);
594 f(self, ln, var, sp, p_id);
598 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
599 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
601 if let Some(pat) = pat {
602 self.pat_bindings(pat, f);
606 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
608 self.define_bindings_in_arm_pats(Some(pat), succ)
611 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
614 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
615 this.init_from_succ(ln, succ);
616 this.define(ln, var);
622 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
623 ln.get() * self.ir.num_vars + var.get()
626 fn live_on_entry(&self, ln: LiveNode, var: Variable)
627 -> Option<LiveNodeKind> {
628 assert!(ln.is_valid());
629 let reader = self.users[self.idx(ln, var)].reader;
630 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
634 Is this variable live on entry to any of its successor nodes?
636 fn live_on_exit(&self, ln: LiveNode, var: Variable)
637 -> Option<LiveNodeKind> {
638 let successor = self.successors[ln.get()];
639 self.live_on_entry(successor, var)
642 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
643 assert!(ln.is_valid());
644 self.users[self.idx(ln, var)].used
647 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
648 -> Option<LiveNodeKind> {
649 assert!(ln.is_valid());
650 let writer = self.users[self.idx(ln, var)].writer;
651 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
654 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
655 -> Option<LiveNodeKind> {
656 let successor = self.successors[ln.get()];
657 self.assigned_on_entry(successor, var)
660 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
661 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
663 let node_base_idx = self.idx(ln, Variable(0));
664 let succ_base_idx = self.idx(succ_ln, Variable(0));
665 for var_idx in 0..self.ir.num_vars {
666 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
670 fn write_vars<F>(&self,
674 -> io::Result<()> where
675 F: FnMut(usize) -> LiveNode,
677 let node_base_idx = self.idx(ln, Variable(0));
678 for var_idx in 0..self.ir.num_vars {
679 let idx = node_base_idx + var_idx;
680 if test(idx).is_valid() {
681 write!(wr, " {:?}", Variable(var_idx))?;
687 fn find_loop_scope(&self,
688 opt_label: Option<ast::Name>,
694 // Refers to a labeled loop. Use the results of resolve
696 match self.ir.tcx.expect_def(id) {
697 Def::Label(loop_id) => loop_id,
698 _ => span_bug!(sp, "label on break/loop \
699 doesn't refer to a loop")
703 // Vanilla 'break' or 'loop', so use the enclosing
705 if self.loop_scope.is_empty() {
706 span_bug!(sp, "break outside loop");
708 *self.loop_scope.last().unwrap()
714 #[allow(unused_must_use)]
715 fn ln_str(&self, ln: LiveNode) -> String {
716 let mut wr = Vec::new();
718 let wr = &mut wr as &mut Write;
719 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
720 self.write_vars(wr, ln, |idx| self.users[idx].reader);
721 write!(wr, " writes");
722 self.write_vars(wr, ln, |idx| self.users[idx].writer);
723 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
725 String::from_utf8(wr).unwrap()
728 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
729 self.successors[ln.get()] = succ_ln;
731 // It is not necessary to initialize the
732 // values to empty because this is the value
733 // they have when they are created, and the sets
734 // only grow during iterations.
736 // self.indices(ln) { |idx|
737 // self.users[idx] = invalid_users();
741 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
742 // more efficient version of init_empty() / merge_from_succ()
743 self.successors[ln.get()] = succ_ln;
745 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
746 this.users[idx] = this.users[succ_idx]
748 debug!("init_from_succ(ln={}, succ={})",
749 self.ln_str(ln), self.ln_str(succ_ln));
752 fn merge_from_succ(&mut self,
757 if ln == succ_ln { return false; }
759 let mut changed = false;
760 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
761 changed |= copy_if_invalid(this.users[succ_idx].reader,
762 &mut this.users[idx].reader);
763 changed |= copy_if_invalid(this.users[succ_idx].writer,
764 &mut this.users[idx].writer);
765 if this.users[succ_idx].used && !this.users[idx].used {
766 this.users[idx].used = true;
771 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
772 ln, self.ln_str(succ_ln), first_merge, changed);
775 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
776 if src.is_valid() && !dst.is_valid() {
785 // Indicates that a local variable was *defined*; we know that no
786 // uses of the variable can precede the definition (resolve checks
787 // this) so we just clear out all the data.
788 fn define(&mut self, writer: LiveNode, var: Variable) {
789 let idx = self.idx(writer, var);
790 self.users[idx].reader = invalid_node();
791 self.users[idx].writer = invalid_node();
793 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
794 idx, self.ln_str(writer));
797 // Either read, write, or both depending on the acc bitset
798 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
799 debug!("{:?} accesses[{:x}] {:?}: {}",
800 ln, acc, var, self.ln_str(ln));
802 let idx = self.idx(ln, var);
803 let user = &mut self.users[idx];
805 if (acc & ACC_WRITE) != 0 {
806 user.reader = invalid_node();
810 // Important: if we both read/write, must do read second
811 // or else the write will override.
812 if (acc & ACC_READ) != 0 {
816 if (acc & ACC_USE) != 0 {
821 // _______________________________________________________________________
823 fn compute(&mut self, decl: &hir::FnDecl, body: &hir::Block) -> LiveNode {
824 // if there is a `break` or `again` at the top level, then it's
825 // effectively a return---this only occurs in `for` loops,
826 // where the body is really a closure.
828 debug!("compute: using id for block, {}", block_to_string(body));
830 let exit_ln = self.s.exit_ln;
831 let entry_ln: LiveNode =
832 self.with_loop_nodes(body.id, exit_ln, exit_ln,
833 |this| this.propagate_through_fn_block(decl, body));
835 // hack to skip the loop unless debug! is enabled:
836 debug!("^^ liveness computation results for body {} (entry={:?})",
838 for ln_idx in 0..self.ir.num_live_nodes {
839 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
848 fn propagate_through_fn_block(&mut self, _: &hir::FnDecl, blk: &hir::Block)
850 // the fallthrough exit is only for those cases where we do not
851 // explicitly return:
853 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
854 if blk.expr.is_none() {
855 self.acc(s.fallthrough_ln, s.no_ret_var, ACC_READ)
857 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
859 self.propagate_through_block(blk, s.fallthrough_ln)
862 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
864 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
865 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
866 self.propagate_through_stmt(stmt, succ)
870 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
873 hir::StmtDecl(ref decl, _) => {
874 self.propagate_through_decl(&decl, succ)
877 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
878 self.propagate_through_expr(&expr, succ)
883 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
886 hir::DeclLocal(ref local) => {
887 self.propagate_through_local(&local, succ)
889 hir::DeclItem(_) => succ,
893 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
895 // Note: we mark the variable as defined regardless of whether
896 // there is an initializer. Initially I had thought to only mark
897 // the live variable as defined if it was initialized, and then we
898 // could check for uninit variables just by scanning what is live
899 // at the start of the function. But that doesn't work so well for
900 // immutable variables defined in a loop:
901 // loop { let x; x = 5; }
902 // because the "assignment" loops back around and generates an error.
904 // So now we just check that variables defined w/o an
905 // initializer are not live at the point of their
906 // initialization, which is mildly more complex than checking
907 // once at the func header but otherwise equivalent.
909 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
910 self.define_bindings_in_pat(&local.pat, succ)
913 fn propagate_through_exprs(&mut self, exprs: &[P<Expr>], succ: LiveNode)
915 exprs.iter().rev().fold(succ, |succ, expr| {
916 self.propagate_through_expr(&expr, succ)
920 fn propagate_through_opt_expr(&mut self,
921 opt_expr: Option<&Expr>,
924 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
927 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
929 debug!("propagate_through_expr: {}", expr_to_string(expr));
932 // Interesting cases with control flow or which gen/kill
934 hir::ExprPath(..) => {
935 self.access_path(expr, succ, ACC_READ | ACC_USE)
938 hir::ExprField(ref e, _) => {
939 self.propagate_through_expr(&e, succ)
942 hir::ExprTupField(ref e, _) => {
943 self.propagate_through_expr(&e, succ)
946 hir::ExprClosure(_, _, ref blk, _) => {
947 debug!("{} is an ExprClosure",
948 expr_to_string(expr));
951 The next-node for a break is the successor of the entire
952 loop. The next-node for a continue is the top of this loop.
954 let node = self.live_node(expr.id, expr.span);
955 self.with_loop_nodes(blk.id, succ, node, |this| {
957 // the construction of a closure itself is not important,
958 // but we have to consider the closed over variables.
959 let caps = match this.ir.capture_info_map.get(&expr.id) {
960 Some(caps) => caps.clone(),
962 span_bug!(expr.span, "no registered caps");
965 caps.iter().rev().fold(succ, |succ, cap| {
966 this.init_from_succ(cap.ln, succ);
967 let var = this.variable(cap.var_nid, expr.span);
968 this.acc(cap.ln, var, ACC_READ | ACC_USE);
974 hir::ExprIf(ref cond, ref then, ref els) => {
988 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
989 let then_ln = self.propagate_through_block(&then, succ);
990 let ln = self.live_node(expr.id, expr.span);
991 self.init_from_succ(ln, else_ln);
992 self.merge_from_succ(ln, then_ln, false);
993 self.propagate_through_expr(&cond, ln)
996 hir::ExprWhile(ref cond, ref blk, _) => {
997 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1000 // Note that labels have been resolved, so we don't need to look
1001 // at the label ident
1002 hir::ExprLoop(ref blk, _) => {
1003 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1006 hir::ExprMatch(ref e, ref arms, _) => {
1021 let ln = self.live_node(expr.id, expr.span);
1022 self.init_empty(ln, succ);
1023 let mut first_merge = true;
1026 self.propagate_through_expr(&arm.body, succ);
1028 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1029 // only consider the first pattern; any later patterns must have
1030 // the same bindings, and we also consider the first pattern to be
1031 // the "authoritative" set of ids
1033 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1035 self.merge_from_succ(ln, arm_succ, first_merge);
1036 first_merge = false;
1038 self.propagate_through_expr(&e, ln)
1041 hir::ExprRet(ref o_e) => {
1042 // ignore succ and subst exit_ln:
1043 let exit_ln = self.s.exit_ln;
1044 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1047 hir::ExprBreak(opt_label) => {
1048 // Find which label this break jumps to
1049 let sc = self.find_loop_scope(opt_label.map(|l| l.node), expr.id, expr.span);
1051 // Now that we know the label we're going to,
1052 // look it up in the break loop nodes table
1054 match self.break_ln.get(&sc) {
1056 None => span_bug!(expr.span, "break to unknown label")
1060 hir::ExprAgain(opt_label) => {
1061 // Find which label this expr continues to
1062 let sc = self.find_loop_scope(opt_label.map(|l| l.node), expr.id, expr.span);
1064 // Now that we know the label we're going to,
1065 // look it up in the continue loop nodes table
1067 match self.cont_ln.get(&sc) {
1069 None => span_bug!(expr.span, "loop to unknown label")
1073 hir::ExprAssign(ref l, ref r) => {
1074 // see comment on lvalues in
1075 // propagate_through_lvalue_components()
1076 let succ = self.write_lvalue(&l, succ, ACC_WRITE);
1077 let succ = self.propagate_through_lvalue_components(&l, succ);
1078 self.propagate_through_expr(&r, succ)
1081 hir::ExprAssignOp(_, ref l, ref r) => {
1082 // an overloaded assign op is like a method call
1083 if self.ir.tcx.is_method_call(expr.id) {
1084 let succ = self.propagate_through_expr(&l, succ);
1085 self.propagate_through_expr(&r, succ)
1087 // see comment on lvalues in
1088 // propagate_through_lvalue_components()
1089 let succ = self.write_lvalue(&l, succ, ACC_WRITE|ACC_READ);
1090 let succ = self.propagate_through_expr(&r, succ);
1091 self.propagate_through_lvalue_components(&l, succ)
1095 // Uninteresting cases: just propagate in rev exec order
1097 hir::ExprVec(ref exprs) => {
1098 self.propagate_through_exprs(&exprs[..], succ)
1101 hir::ExprRepeat(ref element, ref count) => {
1102 let succ = self.propagate_through_expr(&count, succ);
1103 self.propagate_through_expr(&element, succ)
1106 hir::ExprStruct(_, ref fields, ref with_expr) => {
1107 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1108 fields.iter().rev().fold(succ, |succ, field| {
1109 self.propagate_through_expr(&field.expr, succ)
1113 hir::ExprCall(ref f, ref args) => {
1114 // FIXME(canndrew): This is_never should really be an is_uninhabited
1115 let diverges = !self.ir.tcx.is_method_call(expr.id) &&
1116 self.ir.tcx.expr_ty_adjusted(&f).fn_ret().0.is_never();
1117 let succ = if diverges {
1122 let succ = self.propagate_through_exprs(&args[..], succ);
1123 self.propagate_through_expr(&f, succ)
1126 hir::ExprMethodCall(_, _, ref args) => {
1127 let method_call = ty::MethodCall::expr(expr.id);
1128 let method_ty = self.ir.tcx.tables.borrow().method_map[&method_call].ty;
1129 // FIXME(canndrew): This is_never should really be an is_uninhabited
1130 let succ = if method_ty.fn_ret().0.is_never() {
1135 self.propagate_through_exprs(&args[..], succ)
1138 hir::ExprTup(ref exprs) => {
1139 self.propagate_through_exprs(&exprs[..], succ)
1142 hir::ExprBinary(op, ref l, ref r) if op.node.is_lazy() => {
1143 let r_succ = self.propagate_through_expr(&r, succ);
1145 let ln = self.live_node(expr.id, expr.span);
1146 self.init_from_succ(ln, succ);
1147 self.merge_from_succ(ln, r_succ, false);
1149 self.propagate_through_expr(&l, ln)
1152 hir::ExprIndex(ref l, ref r) |
1153 hir::ExprBinary(_, ref l, ref r) => {
1154 let r_succ = self.propagate_through_expr(&r, succ);
1155 self.propagate_through_expr(&l, r_succ)
1158 hir::ExprBox(ref e) |
1159 hir::ExprAddrOf(_, ref e) |
1160 hir::ExprCast(ref e, _) |
1161 hir::ExprType(ref e, _) |
1162 hir::ExprUnary(_, ref e) => {
1163 self.propagate_through_expr(&e, succ)
1166 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1167 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1168 // see comment on lvalues
1169 // in propagate_through_lvalue_components()
1171 self.propagate_through_expr(output, succ)
1173 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1174 let succ = self.write_lvalue(output, succ, acc);
1175 self.propagate_through_lvalue_components(output, succ)
1179 // Inputs are executed first. Propagate last because of rev order
1180 self.propagate_through_exprs(inputs, succ)
1183 hir::ExprLit(..) => {
1187 hir::ExprBlock(ref blk) => {
1188 self.propagate_through_block(&blk, succ)
1193 fn propagate_through_lvalue_components(&mut self,
1199 // In general, the full flow graph structure for an
1200 // assignment/move/etc can be handled in one of two ways,
1201 // depending on whether what is being assigned is a "tracked
1202 // value" or not. A tracked value is basically a local
1203 // variable or argument.
1205 // The two kinds of graphs are:
1207 // Tracked lvalue Untracked lvalue
1208 // ----------------------++-----------------------
1212 // (rvalue) || (rvalue)
1215 // (write of lvalue) || (lvalue components)
1220 // ----------------------++-----------------------
1222 // I will cover the two cases in turn:
1224 // # Tracked lvalues
1226 // A tracked lvalue is a local variable/argument `x`. In
1227 // these cases, the link_node where the write occurs is linked
1228 // to node id of `x`. The `write_lvalue()` routine generates
1229 // the contents of this node. There are no subcomponents to
1232 // # Non-tracked lvalues
1234 // These are lvalues like `x[5]` or `x.f`. In that case, we
1235 // basically ignore the value which is written to but generate
1236 // reads for the components---`x` in these two examples. The
1237 // components reads are generated by
1238 // `propagate_through_lvalue_components()` (this fn).
1240 // # Illegal lvalues
1242 // It is still possible to observe assignments to non-lvalues;
1243 // these errors are detected in the later pass borrowck. We
1244 // just ignore such cases and treat them as reads.
1247 hir::ExprPath(..) => succ,
1248 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1249 hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ),
1250 _ => self.propagate_through_expr(expr, succ)
1254 // see comment on propagate_through_lvalue()
1255 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1258 hir::ExprPath(..) => {
1259 self.access_path(expr, succ, acc)
1262 // We do not track other lvalues, so just propagate through
1263 // to their subcomponents. Also, it may happen that
1264 // non-lvalues occur here, because those are detected in the
1265 // later pass borrowck.
1270 fn access_path(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1272 match self.ir.tcx.expect_def(expr.id) {
1273 Def::Local(_, nid) => {
1274 let ln = self.live_node(expr.id, expr.span);
1276 self.init_from_succ(ln, succ);
1277 let var = self.variable(nid, expr.span);
1278 self.acc(ln, var, acc);
1286 fn propagate_through_loop(&mut self,
1295 We model control flow like this:
1313 let mut first_merge = true;
1314 let ln = self.live_node(expr.id, expr.span);
1315 self.init_empty(ln, succ);
1319 // If this is not a `loop` loop, then it's possible we bypass
1320 // the body altogether. Otherwise, the only way is via a `break`
1321 // in the loop body.
1322 self.merge_from_succ(ln, succ, first_merge);
1323 first_merge = false;
1326 debug!("propagate_through_loop: using id for loop body {} {}",
1327 expr.id, block_to_string(body));
1329 let cond_ln = match kind {
1331 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1333 let body_ln = self.with_loop_nodes(expr.id, succ, ln, |this| {
1334 this.propagate_through_block(body, cond_ln)
1337 // repeat until fixed point is reached:
1338 while self.merge_from_succ(ln, body_ln, first_merge) {
1339 first_merge = false;
1341 let new_cond_ln = match kind {
1343 WhileLoop(ref cond) => {
1344 self.propagate_through_expr(&cond, ln)
1347 assert!(cond_ln == new_cond_ln);
1348 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1349 |this| this.propagate_through_block(body, cond_ln)));
1355 fn with_loop_nodes<R, F>(&mut self,
1356 loop_node_id: NodeId,
1361 F: FnOnce(&mut Liveness<'a, 'tcx>) -> R,
1363 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1364 self.loop_scope.push(loop_node_id);
1365 self.break_ln.insert(loop_node_id, break_ln);
1366 self.cont_ln.insert(loop_node_id, cont_ln);
1368 self.loop_scope.pop();
1373 // _______________________________________________________________________
1374 // Checking for error conditions
1376 fn check_local(this: &mut Liveness, local: &hir::Local) {
1379 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1382 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1383 this.warn_about_unused(sp, id, ln, var);
1388 intravisit::walk_local(this, local);
1391 fn check_arm(this: &mut Liveness, arm: &hir::Arm) {
1392 // only consider the first pattern; any later patterns must have
1393 // the same bindings, and we also consider the first pattern to be
1394 // the "authoritative" set of ids
1395 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1396 this.warn_about_unused(sp, id, ln, var);
1398 intravisit::walk_arm(this, arm);
1401 fn check_expr(this: &mut Liveness, expr: &Expr) {
1403 hir::ExprAssign(ref l, _) => {
1404 this.check_lvalue(&l);
1406 intravisit::walk_expr(this, expr);
1409 hir::ExprAssignOp(_, ref l, _) => {
1410 if !this.ir.tcx.is_method_call(expr.id) {
1411 this.check_lvalue(&l);
1414 intravisit::walk_expr(this, expr);
1417 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1418 for input in inputs {
1419 this.visit_expr(input);
1422 // Output operands must be lvalues
1423 for (o, output) in ia.outputs.iter().zip(outputs) {
1425 this.check_lvalue(output);
1427 this.visit_expr(output);
1430 intravisit::walk_expr(this, expr);
1433 // no correctness conditions related to liveness
1434 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1435 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1436 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
1437 hir::ExprVec(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1438 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1439 hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) |
1440 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1441 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1442 hir::ExprClosure(..) | hir::ExprPath(..) | hir::ExprBox(..) |
1443 hir::ExprType(..) => {
1444 intravisit::walk_expr(this, expr);
1449 fn check_fn(_v: &Liveness,
1451 _decl: &hir::FnDecl,
1455 // do not check contents of nested fns
1458 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1459 fn fn_ret(&self, id: NodeId) -> ty::Binder<Ty<'tcx>> {
1460 let fn_ty = self.ir.tcx.node_id_to_type(id);
1462 ty::TyClosure(closure_def_id, substs) =>
1463 self.ir.tcx.closure_type(closure_def_id, substs).sig.output(),
1475 // within the fn body, late-bound regions are liberated
1476 // and must outlive the *call-site* of the function.
1478 self.ir.tcx.liberate_late_bound_regions(
1479 self.ir.tcx.region_maps.call_site_extent(id, body.id),
1482 if self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() {
1483 let param_env = ParameterEnvironment::for_item(self.ir.tcx, id);
1484 let t_ret_subst = fn_ret.subst(self.ir.tcx, ¶m_env.free_substs);
1485 let is_nil = self.ir.tcx.infer_ctxt(None, Some(param_env),
1486 Reveal::All).enter(|infcx| {
1487 let cause = traits::ObligationCause::dummy();
1488 traits::fully_normalize(&infcx, cause, &t_ret_subst).unwrap().is_nil()
1491 // for nil return types, it is ok to not return a value expl.
1493 let ends_with_stmt = match body.expr {
1494 None if !body.stmts.is_empty() =>
1495 match body.stmts.last().unwrap().node {
1496 hir::StmtSemi(ref e, _) => {
1497 self.ir.tcx.expr_ty(&e) == fn_ret
1503 let mut err = struct_span_err!(self.ir.tcx.sess,
1506 "not all control paths return a value");
1508 let last_stmt = body.stmts.last().unwrap();
1509 let original_span = original_sp(self.ir.tcx.sess.codemap(),
1510 last_stmt.span, sp);
1511 let span_semicolon = Span {
1512 lo: original_span.hi - BytePos(1),
1513 hi: original_span.hi,
1514 expn_id: original_span.expn_id
1516 err.span_help(span_semicolon, "consider removing this semicolon:");
1523 fn check_lvalue(&mut self, expr: &Expr) {
1525 hir::ExprPath(..) => {
1526 if let Def::Local(_, nid) = self.ir.tcx.expect_def(expr.id) {
1527 // Assignment to an immutable variable or argument: only legal
1528 // if there is no later assignment. If this local is actually
1529 // mutable, then check for a reassignment to flag the mutability
1531 let ln = self.live_node(expr.id, expr.span);
1532 let var = self.variable(nid, expr.span);
1533 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1537 // For other kinds of lvalues, no checks are required,
1538 // and any embedded expressions are actually rvalues
1539 intravisit::walk_expr(self, expr);
1544 fn should_warn(&self, var: Variable) -> Option<String> {
1545 let name = self.ir.variable_name(var);
1546 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1553 fn warn_about_unused_args(&self, decl: &hir::FnDecl, entry_ln: LiveNode) {
1554 for arg in &decl.inputs {
1555 pat_util::pat_bindings(&arg.pat, |_bm, p_id, sp, path1| {
1556 let var = self.variable(p_id, sp);
1557 // Ignore unused self.
1558 let name = path1.node;
1559 if name != keywords::SelfValue.name() {
1560 if !self.warn_about_unused(sp, p_id, entry_ln, var) {
1561 if self.live_on_entry(entry_ln, var).is_none() {
1562 self.report_dead_assign(p_id, sp, var, true);
1570 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1571 self.pat_bindings(pat, |this, ln, var, sp, id| {
1572 if !this.warn_about_unused(sp, id, ln, var) {
1573 this.warn_about_dead_assign(sp, id, ln, var);
1578 fn warn_about_unused(&self,
1584 if !self.used_on_entry(ln, var) {
1585 let r = self.should_warn(var);
1586 if let Some(name) = r {
1588 // annoying: for parameters in funcs like `fn(x: i32)
1589 // {ret}`, there is only one node, so asking about
1590 // assigned_on_exit() is not meaningful.
1591 let is_assigned = if ln == self.s.exit_ln {
1594 self.assigned_on_exit(ln, var).is_some()
1598 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1599 format!("variable `{}` is assigned to, but never used",
1601 } else if name != "self" {
1602 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1603 format!("unused variable: `{}`", name));
1612 fn warn_about_dead_assign(&self,
1617 if self.live_on_exit(ln, var).is_none() {
1618 self.report_dead_assign(id, sp, var, false);
1622 fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) {
1623 if let Some(name) = self.should_warn(var) {
1625 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1626 format!("value passed to `{}` is never read", name));
1628 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1629 format!("value assigned to `{}` is never read", name));