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 ty::{self, TyCtxt, ParameterEnvironment};
115 use traits::{self, Reveal};
116 use ty::subst::Subst;
118 use util::nodemap::NodeMap;
120 use std::{fmt, usize};
121 use std::io::prelude::*;
124 use syntax::ast::{self, NodeId};
125 use syntax::symbol::keywords;
126 use syntax_pos::Span;
130 use hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
132 /// For use with `propagate_through_loop`.
134 /// An endless `loop` loop.
136 /// A `while` loop, with the given expression as condition.
140 #[derive(Copy, Clone, PartialEq)]
141 struct Variable(usize);
143 #[derive(Copy, PartialEq)]
144 struct LiveNode(usize);
147 fn get(&self) -> usize { let Variable(v) = *self; v }
151 fn get(&self) -> usize { let LiveNode(v) = *self; v }
154 impl Clone for LiveNode {
155 fn clone(&self) -> LiveNode {
160 #[derive(Copy, Clone, PartialEq, Debug)]
168 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt) -> String {
169 let cm = tcx.sess.codemap();
172 format!("Free var node [{}]", cm.span_to_string(s))
175 format!("Expr node [{}]", cm.span_to_string(s))
178 format!("Var def node [{}]", cm.span_to_string(s))
180 ExitNode => "Exit node".to_string(),
184 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
185 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
186 NestedVisitorMap::OnlyBodies(&self.tcx.hir)
189 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
190 b: hir::BodyId, s: Span, id: NodeId) {
191 visit_fn(self, fk, fd, b, s, id);
193 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
194 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
195 fn visit_arm(&mut self, a: &'tcx 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.hir.krate().visit_all_item_likes(&mut IrMaps::new(tcx).as_deep_visitor());
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 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
356 decl: &'tcx hir::FnDecl,
357 body_id: hir::BodyId,
362 // swap in a new set of IR maps for this function body:
363 let mut fn_maps = IrMaps::new(ir.tcx);
365 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
367 let body = ir.tcx.hir.body(body_id);
369 for arg in &body.arguments {
370 arg.pat.each_binding(|_bm, arg_id, _x, path1| {
371 debug!("adding argument {}", arg_id);
372 let name = path1.node;
373 fn_maps.add_variable(Arg(arg_id, name));
377 // gather up the various local variables, significant expressions,
379 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
382 let mut lsets = Liveness::new(&mut fn_maps, body_id);
383 let entry_ln = lsets.compute(&body.value);
385 // check for various error conditions
386 lsets.visit_body(body);
387 lsets.check_ret(id, sp, entry_ln, body);
388 lsets.warn_about_unused_args(body, entry_ln);
391 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
392 local.pat.each_binding(|_, p_id, sp, path1| {
393 debug!("adding local variable {}", p_id);
394 let name = path1.node;
395 ir.add_live_node_for_node(p_id, VarDefNode(sp));
396 ir.add_variable(Local(LocalInfo {
401 intravisit::walk_local(ir, local);
404 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
405 for pat in &arm.pats {
406 pat.each_binding(|bm, p_id, sp, path1| {
407 debug!("adding local variable {} from match with bm {:?}",
409 let name = path1.node;
410 ir.add_live_node_for_node(p_id, VarDefNode(sp));
411 ir.add_variable(Local(LocalInfo {
417 intravisit::walk_arm(ir, arm);
420 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
422 // live nodes required for uses or definitions of variables:
423 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
424 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
425 if let Def::Local(..) = path.def {
426 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
428 intravisit::walk_expr(ir, expr);
430 hir::ExprClosure(..) => {
431 // Interesting control flow (for loops can contain labeled
432 // breaks or continues)
433 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
435 // Make a live_node for each captured variable, with the span
436 // being the location that the variable is used. This results
437 // in better error messages than just pointing at the closure
438 // construction site.
439 let mut call_caps = Vec::new();
440 ir.tcx.with_freevars(expr.id, |freevars| {
442 if let Def::Local(def_id) = fv.def {
443 let rv = ir.tcx.hir.as_local_node_id(def_id).unwrap();
444 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
445 call_caps.push(CaptureInfo {ln: fv_ln,
450 ir.set_captures(expr.id, call_caps);
452 intravisit::walk_expr(ir, expr);
455 // live nodes required for interesting control flow:
456 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
457 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
458 intravisit::walk_expr(ir, expr);
460 hir::ExprBinary(op, ..) if op.node.is_lazy() => {
461 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
462 intravisit::walk_expr(ir, expr);
465 // otherwise, live nodes are not required:
466 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
467 hir::ExprArray(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
468 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
469 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(..) |
470 hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) |
471 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
472 hir::ExprStruct(..) | hir::ExprRepeat(..) |
473 hir::ExprInlineAsm(..) | hir::ExprBox(..) |
474 hir::ExprType(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
475 intravisit::walk_expr(ir, expr);
480 // ______________________________________________________________________
481 // Computing liveness sets
483 // Actually we compute just a bit more than just liveness, but we use
484 // the same basic propagation framework in all cases.
486 #[derive(Clone, Copy)]
493 fn invalid_users() -> Users {
495 reader: invalid_node(),
496 writer: invalid_node(),
501 #[derive(Copy, Clone)]
504 fallthrough_ln: LiveNode,
505 no_ret_var: Variable,
506 clean_exit_var: Variable
509 const ACC_READ: u32 = 1;
510 const ACC_WRITE: u32 = 2;
511 const ACC_USE: u32 = 4;
513 struct Liveness<'a, 'tcx: 'a> {
514 ir: &'a mut IrMaps<'a, 'tcx>,
515 tables: &'a ty::TypeckTables<'tcx>,
517 successors: Vec<LiveNode>,
519 // The list of node IDs for the nested loop scopes
521 loop_scope: Vec<NodeId>,
522 // mappings from loop node ID to LiveNode
523 // ("break" label should map to loop node ID,
524 // it probably doesn't now)
525 break_ln: NodeMap<LiveNode>,
526 cont_ln: NodeMap<LiveNode>
529 impl<'a, 'tcx> Liveness<'a, 'tcx> {
530 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
531 // Special nodes and variables:
532 // - exit_ln represents the end of the fn, either by return or panic
533 // - implicit_ret_var is a pseudo-variable that represents
534 // an implicit return
535 let specials = Specials {
536 exit_ln: ir.add_live_node(ExitNode),
537 fallthrough_ln: ir.add_live_node(ExitNode),
538 no_ret_var: ir.add_variable(ImplicitRet),
539 clean_exit_var: ir.add_variable(CleanExit)
542 let tables = ir.tcx.body_tables(body);
544 let num_live_nodes = ir.num_live_nodes;
545 let num_vars = ir.num_vars;
551 successors: vec![invalid_node(); num_live_nodes],
552 users: vec![invalid_users(); num_live_nodes * num_vars],
553 loop_scope: Vec::new(),
559 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
560 match self.ir.live_node_map.get(&node_id) {
563 // This must be a mismatch between the ir_map construction
564 // above and the propagation code below; the two sets of
565 // code have to agree about which AST nodes are worth
566 // creating liveness nodes for.
569 "no live node registered for node {}",
575 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
576 self.ir.variable(node_id, span)
579 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
580 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
582 pat.each_binding(|_bm, p_id, sp, _n| {
583 let ln = self.live_node(p_id, sp);
584 let var = self.variable(p_id, sp);
585 f(self, ln, var, sp, p_id);
589 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
590 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
592 if let Some(pat) = pat {
593 self.pat_bindings(pat, f);
597 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
599 self.define_bindings_in_arm_pats(Some(pat), succ)
602 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
605 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
606 this.init_from_succ(ln, succ);
607 this.define(ln, var);
613 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
614 ln.get() * self.ir.num_vars + var.get()
617 fn live_on_entry(&self, ln: LiveNode, var: Variable)
618 -> Option<LiveNodeKind> {
619 assert!(ln.is_valid());
620 let reader = self.users[self.idx(ln, var)].reader;
621 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
625 Is this variable live on entry to any of its successor nodes?
627 fn live_on_exit(&self, ln: LiveNode, var: Variable)
628 -> Option<LiveNodeKind> {
629 let successor = self.successors[ln.get()];
630 self.live_on_entry(successor, var)
633 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
634 assert!(ln.is_valid());
635 self.users[self.idx(ln, var)].used
638 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
639 -> Option<LiveNodeKind> {
640 assert!(ln.is_valid());
641 let writer = self.users[self.idx(ln, var)].writer;
642 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
645 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
646 -> Option<LiveNodeKind> {
647 let successor = self.successors[ln.get()];
648 self.assigned_on_entry(successor, var)
651 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
652 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
654 let node_base_idx = self.idx(ln, Variable(0));
655 let succ_base_idx = self.idx(succ_ln, Variable(0));
656 for var_idx in 0..self.ir.num_vars {
657 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
661 fn write_vars<F>(&self,
665 -> io::Result<()> where
666 F: FnMut(usize) -> LiveNode,
668 let node_base_idx = self.idx(ln, Variable(0));
669 for var_idx in 0..self.ir.num_vars {
670 let idx = node_base_idx + var_idx;
671 if test(idx).is_valid() {
672 write!(wr, " {:?}", Variable(var_idx))?;
679 #[allow(unused_must_use)]
680 fn ln_str(&self, ln: LiveNode) -> String {
681 let mut wr = Vec::new();
683 let wr = &mut wr as &mut Write;
684 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
685 self.write_vars(wr, ln, |idx| self.users[idx].reader);
686 write!(wr, " writes");
687 self.write_vars(wr, ln, |idx| self.users[idx].writer);
688 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
690 String::from_utf8(wr).unwrap()
693 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
694 self.successors[ln.get()] = succ_ln;
696 // It is not necessary to initialize the
697 // values to empty because this is the value
698 // they have when they are created, and the sets
699 // only grow during iterations.
701 // self.indices(ln) { |idx|
702 // self.users[idx] = invalid_users();
706 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
707 // more efficient version of init_empty() / merge_from_succ()
708 self.successors[ln.get()] = succ_ln;
710 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
711 this.users[idx] = this.users[succ_idx]
713 debug!("init_from_succ(ln={}, succ={})",
714 self.ln_str(ln), self.ln_str(succ_ln));
717 fn merge_from_succ(&mut self,
722 if ln == succ_ln { return false; }
724 let mut changed = false;
725 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
726 changed |= copy_if_invalid(this.users[succ_idx].reader,
727 &mut this.users[idx].reader);
728 changed |= copy_if_invalid(this.users[succ_idx].writer,
729 &mut this.users[idx].writer);
730 if this.users[succ_idx].used && !this.users[idx].used {
731 this.users[idx].used = true;
736 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
737 ln, self.ln_str(succ_ln), first_merge, changed);
740 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
741 if src.is_valid() && !dst.is_valid() {
750 // Indicates that a local variable was *defined*; we know that no
751 // uses of the variable can precede the definition (resolve checks
752 // this) so we just clear out all the data.
753 fn define(&mut self, writer: LiveNode, var: Variable) {
754 let idx = self.idx(writer, var);
755 self.users[idx].reader = invalid_node();
756 self.users[idx].writer = invalid_node();
758 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
759 idx, self.ln_str(writer));
762 // Either read, write, or both depending on the acc bitset
763 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
764 debug!("{:?} accesses[{:x}] {:?}: {}",
765 ln, acc, var, self.ln_str(ln));
767 let idx = self.idx(ln, var);
768 let user = &mut self.users[idx];
770 if (acc & ACC_WRITE) != 0 {
771 user.reader = invalid_node();
775 // Important: if we both read/write, must do read second
776 // or else the write will override.
777 if (acc & ACC_READ) != 0 {
781 if (acc & ACC_USE) != 0 {
786 // _______________________________________________________________________
788 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
789 // if there is a `break` or `again` at the top level, then it's
790 // effectively a return---this only occurs in `for` loops,
791 // where the body is really a closure.
793 debug!("compute: using id for body, {}", self.ir.tcx.hir.node_to_pretty_string(body.id));
795 let exit_ln = self.s.exit_ln;
796 let entry_ln: LiveNode = self.with_loop_nodes(body.id, exit_ln, exit_ln, |this| {
797 // the fallthrough exit is only for those cases where we do not
798 // explicitly return:
800 this.init_from_succ(s.fallthrough_ln, s.exit_ln);
801 this.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
803 this.propagate_through_expr(body, s.fallthrough_ln)
806 // hack to skip the loop unless debug! is enabled:
807 debug!("^^ liveness computation results for body {} (entry={:?})",
809 for ln_idx in 0..self.ir.num_live_nodes {
810 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
819 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
821 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
822 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
823 self.propagate_through_stmt(stmt, succ)
827 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
830 hir::StmtDecl(ref decl, _) => {
831 self.propagate_through_decl(&decl, succ)
834 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
835 self.propagate_through_expr(&expr, succ)
840 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
843 hir::DeclLocal(ref local) => {
844 self.propagate_through_local(&local, succ)
846 hir::DeclItem(_) => succ,
850 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
852 // Note: we mark the variable as defined regardless of whether
853 // there is an initializer. Initially I had thought to only mark
854 // the live variable as defined if it was initialized, and then we
855 // could check for uninit variables just by scanning what is live
856 // at the start of the function. But that doesn't work so well for
857 // immutable variables defined in a loop:
858 // loop { let x; x = 5; }
859 // because the "assignment" loops back around and generates an error.
861 // So now we just check that variables defined w/o an
862 // initializer are not live at the point of their
863 // initialization, which is mildly more complex than checking
864 // once at the func header but otherwise equivalent.
866 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
867 self.define_bindings_in_pat(&local.pat, succ)
870 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
872 exprs.iter().rev().fold(succ, |succ, expr| {
873 self.propagate_through_expr(&expr, succ)
877 fn propagate_through_opt_expr(&mut self,
878 opt_expr: Option<&Expr>,
881 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
884 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
886 debug!("propagate_through_expr: {}", self.ir.tcx.hir.node_to_pretty_string(expr.id));
889 // Interesting cases with control flow or which gen/kill
891 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
892 self.access_path(expr.id, path, succ, ACC_READ | ACC_USE)
895 hir::ExprField(ref e, _) => {
896 self.propagate_through_expr(&e, succ)
899 hir::ExprTupField(ref e, _) => {
900 self.propagate_through_expr(&e, succ)
903 hir::ExprClosure(.., blk_id, _) => {
904 debug!("{} is an ExprClosure",
905 self.ir.tcx.hir.node_to_pretty_string(expr.id));
908 The next-node for a break is the successor of the entire
909 loop. The next-node for a continue is the top of this loop.
911 let node = self.live_node(expr.id, expr.span);
912 self.with_loop_nodes(blk_id.node_id, succ, node, |this| {
914 // the construction of a closure itself is not important,
915 // but we have to consider the closed over variables.
916 let caps = match this.ir.capture_info_map.get(&expr.id) {
917 Some(caps) => caps.clone(),
919 span_bug!(expr.span, "no registered caps");
922 caps.iter().rev().fold(succ, |succ, cap| {
923 this.init_from_succ(cap.ln, succ);
924 let var = this.variable(cap.var_nid, expr.span);
925 this.acc(cap.ln, var, ACC_READ | ACC_USE);
931 hir::ExprIf(ref cond, ref then, ref els) => {
945 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
946 let then_ln = self.propagate_through_block(&then, succ);
947 let ln = self.live_node(expr.id, expr.span);
948 self.init_from_succ(ln, else_ln);
949 self.merge_from_succ(ln, then_ln, false);
950 self.propagate_through_expr(&cond, ln)
953 hir::ExprWhile(ref cond, ref blk, _) => {
954 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
957 // Note that labels have been resolved, so we don't need to look
958 // at the label ident
959 hir::ExprLoop(ref blk, _, _) => {
960 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
963 hir::ExprMatch(ref e, ref arms, _) => {
978 let ln = self.live_node(expr.id, expr.span);
979 self.init_empty(ln, succ);
980 let mut first_merge = true;
983 self.propagate_through_expr(&arm.body, succ);
985 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
986 // only consider the first pattern; any later patterns must have
987 // the same bindings, and we also consider the first pattern to be
988 // the "authoritative" set of ids
990 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
992 self.merge_from_succ(ln, arm_succ, first_merge);
995 self.propagate_through_expr(&e, ln)
998 hir::ExprRet(ref o_e) => {
999 // ignore succ and subst exit_ln:
1000 let exit_ln = self.s.exit_ln;
1001 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1004 hir::ExprBreak(label, ref opt_expr) => {
1005 // Find which label this break jumps to
1006 let sc = match label.loop_id.into() {
1007 Ok(loop_id) => loop_id,
1008 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1011 // Now that we know the label we're going to,
1012 // look it up in the break loop nodes table
1014 match self.break_ln.get(&sc) {
1015 Some(&b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1016 None => span_bug!(expr.span, "break to unknown label")
1020 hir::ExprAgain(label) => {
1021 // Find which label this expr continues to
1022 let sc = match label.loop_id.into() {
1023 Ok(loop_id) => loop_id,
1024 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1028 // Now that we know the label we're going to,
1029 // look it up in the continue loop nodes table
1031 match self.cont_ln.get(&sc) {
1033 None => span_bug!(expr.span, "continue to unknown label")
1037 hir::ExprAssign(ref l, ref r) => {
1038 // see comment on lvalues in
1039 // propagate_through_lvalue_components()
1040 let succ = self.write_lvalue(&l, succ, ACC_WRITE);
1041 let succ = self.propagate_through_lvalue_components(&l, succ);
1042 self.propagate_through_expr(&r, succ)
1045 hir::ExprAssignOp(_, ref l, ref r) => {
1046 // an overloaded assign op is like a method call
1047 if self.tables.is_method_call(expr.id) {
1048 let succ = self.propagate_through_expr(&l, succ);
1049 self.propagate_through_expr(&r, succ)
1051 // see comment on lvalues in
1052 // propagate_through_lvalue_components()
1053 let succ = self.write_lvalue(&l, succ, ACC_WRITE|ACC_READ);
1054 let succ = self.propagate_through_expr(&r, succ);
1055 self.propagate_through_lvalue_components(&l, succ)
1059 // Uninteresting cases: just propagate in rev exec order
1061 hir::ExprArray(ref exprs) => {
1062 self.propagate_through_exprs(exprs, succ)
1065 hir::ExprStruct(_, ref fields, ref with_expr) => {
1066 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1067 fields.iter().rev().fold(succ, |succ, field| {
1068 self.propagate_through_expr(&field.expr, succ)
1072 hir::ExprCall(ref f, ref args) => {
1073 // FIXME(canndrew): This is_never should really be an is_uninhabited
1074 let diverges = !self.tables.is_method_call(expr.id) &&
1075 self.tables.expr_ty_adjusted(&f).fn_ret().0.is_never();
1076 let succ = if diverges {
1081 let succ = self.propagate_through_exprs(args, succ);
1082 self.propagate_through_expr(&f, succ)
1085 hir::ExprMethodCall(.., ref args) => {
1086 let method_call = ty::MethodCall::expr(expr.id);
1087 let method_ty = self.tables.method_map[&method_call].ty;
1088 // FIXME(canndrew): This is_never should really be an is_uninhabited
1089 let succ = if method_ty.fn_ret().0.is_never() {
1094 self.propagate_through_exprs(args, succ)
1097 hir::ExprTup(ref exprs) => {
1098 self.propagate_through_exprs(exprs, succ)
1101 hir::ExprBinary(op, ref l, ref r) if op.node.is_lazy() => {
1102 let r_succ = self.propagate_through_expr(&r, succ);
1104 let ln = self.live_node(expr.id, expr.span);
1105 self.init_from_succ(ln, succ);
1106 self.merge_from_succ(ln, r_succ, false);
1108 self.propagate_through_expr(&l, ln)
1111 hir::ExprIndex(ref l, ref r) |
1112 hir::ExprBinary(_, ref l, ref r) => {
1113 let r_succ = self.propagate_through_expr(&r, succ);
1114 self.propagate_through_expr(&l, r_succ)
1117 hir::ExprBox(ref e) |
1118 hir::ExprAddrOf(_, ref e) |
1119 hir::ExprCast(ref e, _) |
1120 hir::ExprType(ref e, _) |
1121 hir::ExprUnary(_, ref e) |
1122 hir::ExprRepeat(ref e, _) => {
1123 self.propagate_through_expr(&e, succ)
1126 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1127 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1128 // see comment on lvalues
1129 // in propagate_through_lvalue_components()
1131 self.propagate_through_expr(output, succ)
1133 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1134 let succ = self.write_lvalue(output, succ, acc);
1135 self.propagate_through_lvalue_components(output, succ)
1139 // Inputs are executed first. Propagate last because of rev order
1140 self.propagate_through_exprs(inputs, succ)
1143 hir::ExprLit(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
1147 hir::ExprBlock(ref blk) => {
1148 self.propagate_through_block(&blk, succ)
1153 fn propagate_through_lvalue_components(&mut self,
1159 // In general, the full flow graph structure for an
1160 // assignment/move/etc can be handled in one of two ways,
1161 // depending on whether what is being assigned is a "tracked
1162 // value" or not. A tracked value is basically a local
1163 // variable or argument.
1165 // The two kinds of graphs are:
1167 // Tracked lvalue Untracked lvalue
1168 // ----------------------++-----------------------
1172 // (rvalue) || (rvalue)
1175 // (write of lvalue) || (lvalue components)
1180 // ----------------------++-----------------------
1182 // I will cover the two cases in turn:
1184 // # Tracked lvalues
1186 // A tracked lvalue is a local variable/argument `x`. In
1187 // these cases, the link_node where the write occurs is linked
1188 // to node id of `x`. The `write_lvalue()` routine generates
1189 // the contents of this node. There are no subcomponents to
1192 // # Non-tracked lvalues
1194 // These are lvalues like `x[5]` or `x.f`. In that case, we
1195 // basically ignore the value which is written to but generate
1196 // reads for the components---`x` in these two examples. The
1197 // components reads are generated by
1198 // `propagate_through_lvalue_components()` (this fn).
1200 // # Illegal lvalues
1202 // It is still possible to observe assignments to non-lvalues;
1203 // these errors are detected in the later pass borrowck. We
1204 // just ignore such cases and treat them as reads.
1207 hir::ExprPath(_) => succ,
1208 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1209 hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ),
1210 _ => self.propagate_through_expr(expr, succ)
1214 // see comment on propagate_through_lvalue()
1215 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1218 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1219 self.access_path(expr.id, path, succ, acc)
1222 // We do not track other lvalues, so just propagate through
1223 // to their subcomponents. Also, it may happen that
1224 // non-lvalues occur here, because those are detected in the
1225 // later pass borrowck.
1230 fn access_path(&mut self, id: NodeId, path: &hir::Path, succ: LiveNode, acc: u32)
1233 Def::Local(def_id) => {
1234 let nid = self.ir.tcx.hir.as_local_node_id(def_id).unwrap();
1235 let ln = self.live_node(id, path.span);
1237 self.init_from_succ(ln, succ);
1238 let var = self.variable(nid, path.span);
1239 self.acc(ln, var, acc);
1247 fn propagate_through_loop(&mut self,
1256 We model control flow like this:
1274 let mut first_merge = true;
1275 let ln = self.live_node(expr.id, expr.span);
1276 self.init_empty(ln, succ);
1280 // If this is not a `loop` loop, then it's possible we bypass
1281 // the body altogether. Otherwise, the only way is via a `break`
1282 // in the loop body.
1283 self.merge_from_succ(ln, succ, first_merge);
1284 first_merge = false;
1287 debug!("propagate_through_loop: using id for loop body {} {}",
1288 expr.id, self.ir.tcx.hir.node_to_pretty_string(body.id));
1290 let (cond_ln, body_ln) = self.with_loop_nodes(expr.id, succ, ln, |this| {
1291 let cond_ln = match kind {
1293 WhileLoop(ref cond) => this.propagate_through_expr(&cond, ln),
1295 let body_ln = this.propagate_through_block(body, cond_ln);
1299 // repeat until fixed point is reached:
1300 while self.merge_from_succ(ln, body_ln, first_merge) {
1301 first_merge = false;
1303 let new_cond_ln = match kind {
1305 WhileLoop(ref cond) => {
1306 self.propagate_through_expr(&cond, ln)
1309 assert!(cond_ln == new_cond_ln);
1310 assert!(body_ln == self.with_loop_nodes(expr.id, succ, ln,
1311 |this| this.propagate_through_block(body, cond_ln)));
1317 fn with_loop_nodes<R, F>(&mut self,
1318 loop_node_id: NodeId,
1323 F: FnOnce(&mut Liveness<'a, 'tcx>) -> R,
1325 debug!("with_loop_nodes: {} {}", loop_node_id, break_ln.get());
1326 self.loop_scope.push(loop_node_id);
1327 self.break_ln.insert(loop_node_id, break_ln);
1328 self.cont_ln.insert(loop_node_id, cont_ln);
1330 self.loop_scope.pop();
1335 // _______________________________________________________________________
1336 // Checking for error conditions
1338 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1339 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1340 NestedVisitorMap::None
1343 fn visit_local(&mut self, l: &'tcx hir::Local) {
1344 check_local(self, l);
1346 fn visit_expr(&mut self, ex: &'tcx Expr) {
1347 check_expr(self, ex);
1349 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1354 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1357 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1360 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1361 this.warn_about_unused(sp, id, ln, var);
1366 intravisit::walk_local(this, local);
1369 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1370 // only consider the first pattern; any later patterns must have
1371 // the same bindings, and we also consider the first pattern to be
1372 // the "authoritative" set of ids
1373 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1374 this.warn_about_unused(sp, id, ln, var);
1376 intravisit::walk_arm(this, arm);
1379 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1381 hir::ExprAssign(ref l, _) => {
1382 this.check_lvalue(&l);
1384 intravisit::walk_expr(this, expr);
1387 hir::ExprAssignOp(_, ref l, _) => {
1388 if !this.tables.is_method_call(expr.id) {
1389 this.check_lvalue(&l);
1392 intravisit::walk_expr(this, expr);
1395 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1396 for input in inputs {
1397 this.visit_expr(input);
1400 // Output operands must be lvalues
1401 for (o, output) in ia.outputs.iter().zip(outputs) {
1403 this.check_lvalue(output);
1405 this.visit_expr(output);
1408 intravisit::walk_expr(this, expr);
1411 // no correctness conditions related to liveness
1412 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1413 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1414 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
1415 hir::ExprArray(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1416 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1417 hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) |
1418 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1419 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1420 hir::ExprClosure(..) | hir::ExprPath(_) |
1421 hir::ExprBox(..) | hir::ExprType(..) => {
1422 intravisit::walk_expr(this, expr);
1427 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1434 let fn_ty = self.ir.tcx.item_type(self.ir.tcx.hir.local_def_id(id));
1435 let fn_sig = match fn_ty.sty {
1436 ty::TyClosure(closure_def_id, substs) => {
1437 self.ir.tcx.closure_type(closure_def_id)
1438 .subst(self.ir.tcx, substs.substs)
1443 let fn_ret = fn_sig.output();
1445 // within the fn body, late-bound regions are liberated
1446 // and must outlive the *call-site* of the function.
1448 self.ir.tcx.liberate_late_bound_regions(
1449 self.ir.tcx.region_maps.call_site_extent(id, body.value.id),
1452 if !fn_ret.is_never() && self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() {
1453 let param_env = ParameterEnvironment::for_item(self.ir.tcx, id);
1454 let t_ret_subst = fn_ret.subst(self.ir.tcx, ¶m_env.free_substs);
1455 let is_nil = self.ir.tcx.infer_ctxt(param_env, Reveal::All).enter(|infcx| {
1456 let cause = traits::ObligationCause::dummy();
1457 traits::fully_normalize(&infcx, cause, &t_ret_subst).unwrap().is_nil()
1460 // for nil return types, it is ok to not return a value expl.
1462 span_bug!(sp, "not all control paths return a value");
1467 fn check_lvalue(&mut self, expr: &'tcx Expr) {
1469 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1470 if let Def::Local(def_id) = path.def {
1471 // Assignment to an immutable variable or argument: only legal
1472 // if there is no later assignment. If this local is actually
1473 // mutable, then check for a reassignment to flag the mutability
1475 let nid = self.ir.tcx.hir.as_local_node_id(def_id).unwrap();
1476 let ln = self.live_node(expr.id, expr.span);
1477 let var = self.variable(nid, expr.span);
1478 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1482 // For other kinds of lvalues, no checks are required,
1483 // and any embedded expressions are actually rvalues
1484 intravisit::walk_expr(self, expr);
1489 fn should_warn(&self, var: Variable) -> Option<String> {
1490 let name = self.ir.variable_name(var);
1491 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1498 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1499 for arg in &body.arguments {
1500 arg.pat.each_binding(|_bm, p_id, sp, path1| {
1501 let var = self.variable(p_id, sp);
1502 // Ignore unused self.
1503 let name = path1.node;
1504 if name != keywords::SelfValue.name() {
1505 if !self.warn_about_unused(sp, p_id, entry_ln, var) {
1506 if self.live_on_entry(entry_ln, var).is_none() {
1507 self.report_dead_assign(p_id, sp, var, true);
1515 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1516 self.pat_bindings(pat, |this, ln, var, sp, id| {
1517 if !this.warn_about_unused(sp, id, ln, var) {
1518 this.warn_about_dead_assign(sp, id, ln, var);
1523 fn warn_about_unused(&self,
1529 if !self.used_on_entry(ln, var) {
1530 let r = self.should_warn(var);
1531 if let Some(name) = r {
1533 // annoying: for parameters in funcs like `fn(x: i32)
1534 // {ret}`, there is only one node, so asking about
1535 // assigned_on_exit() is not meaningful.
1536 let is_assigned = if ln == self.s.exit_ln {
1539 self.assigned_on_exit(ln, var).is_some()
1543 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1544 format!("variable `{}` is assigned to, but never used",
1546 } else if name != "self" {
1547 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1548 format!("unused variable: `{}`", name));
1557 fn warn_about_dead_assign(&self,
1562 if self.live_on_exit(ln, var).is_none() {
1563 self.report_dead_assign(id, sp, var, false);
1567 fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) {
1568 if let Some(name) = self.should_warn(var) {
1570 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1571 format!("value passed to `{}` is never read", name));
1573 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1574 format!("value assigned to `{}` is never read", name));