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 u32) and the id for a variable
55 //! is called a `variable` (another newtype'd u32).
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 //! - `clean_exit_var`: a synthetic variable that is only 'read' from the
100 //! fallthrough node. It is only live if the function could converge
101 //! via means other than an explicit `return` expression. That is, it is
102 //! only dead if the end of the function's block can never be reached.
103 //! It is the responsibility of typeck to ensure that there are no
104 //! `return` expressions in a function declared as diverging.
105 use self::LoopKind::*;
106 use self::LiveNodeKind::*;
107 use self::VarKind::*;
110 use ty::{self, TyCtxt};
112 use errors::Applicability;
113 use util::nodemap::{NodeMap, HirIdMap, HirIdSet};
115 use std::collections::VecDeque;
117 use std::io::prelude::*;
120 use syntax::ast::{self, NodeId};
122 use syntax::symbol::keywords;
123 use syntax_pos::Span;
125 use hir::{Expr, HirId};
127 use hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
129 /// For use with `propagate_through_loop`.
131 /// An endless `loop` loop.
133 /// A `while` loop, with the given expression as condition.
137 #[derive(Copy, Clone, PartialEq)]
138 struct Variable(u32);
140 #[derive(Copy, Clone, PartialEq)]
141 struct LiveNode(u32);
144 fn get(&self) -> usize { self.0 as usize }
148 fn get(&self) -> usize { self.0 as usize }
151 #[derive(Copy, Clone, PartialEq, Debug)]
159 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt) -> String {
160 let cm = tcx.sess.codemap();
163 format!("Free var node [{}]", cm.span_to_string(s))
166 format!("Expr node [{}]", cm.span_to_string(s))
169 format!("Var def node [{}]", cm.span_to_string(s))
171 ExitNode => "Exit node".to_string(),
175 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
176 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
177 NestedVisitorMap::OnlyBodies(&self.tcx.hir)
180 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
181 b: hir::BodyId, s: Span, id: NodeId) {
182 visit_fn(self, fk, fd, b, s, id);
185 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
186 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
187 fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
190 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
191 tcx.hir.krate().visit_all_item_likes(&mut IrMaps::new(tcx).as_deep_visitor());
192 tcx.sess.abort_if_errors();
195 impl fmt::Debug for LiveNode {
196 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
197 write!(f, "ln({})", self.get())
201 impl fmt::Debug for Variable {
202 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
203 write!(f, "v({})", self.get())
207 // ______________________________________________________________________
210 // This is the first pass and the one that drives the main
211 // computation. It walks up and down the IR once. On the way down,
212 // we count for each function the number of variables as well as
213 // liveness nodes. A liveness node is basically an expression or
214 // capture clause that does something of interest: either it has
215 // interesting control flow or it uses/defines a local variable.
217 // On the way back up, at each function node we create liveness sets
218 // (we now know precisely how big to make our various vectors and so
219 // forth) and then do the data-flow propagation to compute the set
220 // of live variables at each program point.
222 // Finally, we run back over the IR one last time and, using the
223 // computed liveness, check various safety conditions. For example,
224 // there must be no live nodes at the definition site for a variable
225 // unless it has an initializer. Similarly, each non-mutable local
226 // variable must not be assigned if there is some successor
227 // assignment. And so forth.
230 fn is_valid(&self) -> bool {
235 fn invalid_node() -> LiveNode { LiveNode(u32::MAX) }
242 #[derive(Copy, Clone, Debug)]
249 #[derive(Copy, Clone, Debug)]
251 Arg(HirId, ast::Name),
256 struct IrMaps<'a, 'tcx: 'a> {
257 tcx: TyCtxt<'a, 'tcx, 'tcx>,
259 num_live_nodes: usize,
261 live_node_map: HirIdMap<LiveNode>,
262 variable_map: HirIdMap<Variable>,
263 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
264 var_kinds: Vec<VarKind>,
265 lnks: Vec<LiveNodeKind>,
268 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
269 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
274 live_node_map: HirIdMap(),
275 variable_map: HirIdMap(),
276 capture_info_map: NodeMap(),
277 var_kinds: Vec::new(),
282 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
283 let ln = LiveNode(self.num_live_nodes as u32);
285 self.num_live_nodes += 1;
287 debug!("{:?} is of kind {}", ln,
288 live_node_kind_to_string(lnk, self.tcx));
293 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
294 let ln = self.add_live_node(lnk);
295 self.live_node_map.insert(hir_id, ln);
297 debug!("{:?} is node {:?}", ln, hir_id);
300 fn add_variable(&mut self, vk: VarKind) -> Variable {
301 let v = Variable(self.num_vars as u32);
302 self.var_kinds.push(vk);
306 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
307 self.variable_map.insert(node_id, v);
312 debug!("{:?} is {:?}", v, vk);
317 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
318 match self.variable_map.get(&hir_id) {
321 span_bug!(span, "no variable registered for id {:?}", hir_id);
326 fn variable_name(&self, var: Variable) -> String {
327 match self.var_kinds[var.get()] {
328 Local(LocalInfo { name, .. }) | Arg(_, name) => {
331 CleanExit => "<clean-exit>".to_string()
335 fn variable_is_shorthand(&self, var: Variable) -> bool {
336 match self.var_kinds[var.get()] {
337 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
338 Arg(..) | CleanExit => false
342 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
343 self.capture_info_map.insert(node_id, Rc::new(cs));
346 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
351 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
353 decl: &'tcx hir::FnDecl,
354 body_id: hir::BodyId,
359 // swap in a new set of IR maps for this function body:
360 let mut fn_maps = IrMaps::new(ir.tcx);
362 // Don't run unused pass for #[derive()]
363 if let FnKind::Method(..) = fk {
364 let parent = ir.tcx.hir.get_parent(id);
365 if let Some(hir::map::Node::NodeItem(i)) = ir.tcx.hir.find(parent) {
366 if i.attrs.iter().any(|a| a.check_name("automatically_derived")) {
372 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
374 let body = ir.tcx.hir.body(body_id);
376 for arg in &body.arguments {
377 arg.pat.each_binding(|_bm, hir_id, _x, ident| {
378 debug!("adding argument {:?}", hir_id);
379 fn_maps.add_variable(Arg(hir_id, ident.name));
383 // gather up the various local variables, significant expressions,
385 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
388 let mut lsets = Liveness::new(&mut fn_maps, body_id);
389 let entry_ln = lsets.compute(&body.value);
391 // check for various error conditions
392 lsets.visit_body(body);
393 lsets.warn_about_unused_args(body, entry_ln);
396 fn add_from_pat<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, pat: &P<hir::Pat>) {
397 // For struct patterns, take note of which fields used shorthand
398 // (`x` rather than `x: x`).
399 let mut shorthand_field_ids = HirIdSet();
400 let mut pats = VecDeque::new();
402 while let Some(pat) = pats.pop_front() {
405 Binding(_, _, _, ref inner_pat) => {
406 pats.extend(inner_pat.iter());
408 Struct(_, ref fields, _) => {
409 for field in fields {
410 if field.node.is_shorthand {
411 shorthand_field_ids.insert(field.node.pat.hir_id);
415 Ref(ref inner_pat, _) |
416 Box(ref inner_pat) => {
417 pats.push_back(inner_pat);
419 TupleStruct(_, ref inner_pats, _) |
420 Tuple(ref inner_pats, _) => {
421 pats.extend(inner_pats.iter());
423 Slice(ref pre_pats, ref inner_pat, ref post_pats) => {
424 pats.extend(pre_pats.iter());
425 pats.extend(inner_pat.iter());
426 pats.extend(post_pats.iter());
432 pat.each_binding(|_bm, hir_id, _sp, ident| {
433 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
434 ir.add_variable(Local(LocalInfo {
437 is_shorthand: shorthand_field_ids.contains(&hir_id)
442 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
443 add_from_pat(ir, &local.pat);
444 intravisit::walk_local(ir, local);
447 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
448 for pat in &arm.pats {
449 add_from_pat(ir, pat);
451 intravisit::walk_arm(ir, arm);
454 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
456 // live nodes required for uses or definitions of variables:
457 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
458 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
459 if let Def::Local(..) = path.def {
460 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
462 intravisit::walk_expr(ir, expr);
464 hir::ExprClosure(..) => {
465 // Interesting control flow (for loops can contain labeled
466 // breaks or continues)
467 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
469 // Make a live_node for each captured variable, with the span
470 // being the location that the variable is used. This results
471 // in better error messages than just pointing at the closure
472 // construction site.
473 let mut call_caps = Vec::new();
474 ir.tcx.with_freevars(expr.id, |freevars| {
476 if let Def::Local(rv) = fv.def {
477 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
478 let var_hid = ir.tcx.hir.node_to_hir_id(rv);
479 call_caps.push(CaptureInfo { ln: fv_ln, var_hid });
483 ir.set_captures(expr.id, call_caps);
485 intravisit::walk_expr(ir, expr);
488 // live nodes required for interesting control flow:
489 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
490 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
491 intravisit::walk_expr(ir, expr);
493 hir::ExprBinary(op, ..) if op.node.is_lazy() => {
494 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
495 intravisit::walk_expr(ir, expr);
498 // otherwise, live nodes are not required:
499 hir::ExprIndex(..) | hir::ExprField(..) |
500 hir::ExprArray(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
501 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
502 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(..) |
503 hir::ExprContinue(_) | hir::ExprLit(_) | hir::ExprRet(..) |
504 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
505 hir::ExprStruct(..) | hir::ExprRepeat(..) |
506 hir::ExprInlineAsm(..) | hir::ExprBox(..) | hir::ExprYield(..) |
507 hir::ExprType(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
508 intravisit::walk_expr(ir, expr);
513 // ______________________________________________________________________
514 // Computing liveness sets
516 // Actually we compute just a bit more than just liveness, but we use
517 // the same basic propagation framework in all cases.
519 #[derive(Clone, Copy)]
526 fn invalid_users() -> Users {
528 reader: invalid_node(),
529 writer: invalid_node(),
534 #[derive(Copy, Clone)]
537 fallthrough_ln: LiveNode,
538 clean_exit_var: Variable
541 const ACC_READ: u32 = 1;
542 const ACC_WRITE: u32 = 2;
543 const ACC_USE: u32 = 4;
545 struct Liveness<'a, 'tcx: 'a> {
546 ir: &'a mut IrMaps<'a, 'tcx>,
547 tables: &'a ty::TypeckTables<'tcx>,
549 successors: Vec<LiveNode>,
552 // mappings from loop node ID to LiveNode
553 // ("break" label should map to loop node ID,
554 // it probably doesn't now)
555 break_ln: NodeMap<LiveNode>,
556 cont_ln: NodeMap<LiveNode>,
559 impl<'a, 'tcx> Liveness<'a, 'tcx> {
560 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
561 // Special nodes and variables:
562 // - exit_ln represents the end of the fn, either by return or panic
563 // - implicit_ret_var is a pseudo-variable that represents
564 // an implicit return
565 let specials = Specials {
566 exit_ln: ir.add_live_node(ExitNode),
567 fallthrough_ln: ir.add_live_node(ExitNode),
568 clean_exit_var: ir.add_variable(CleanExit)
571 let tables = ir.tcx.body_tables(body);
573 let num_live_nodes = ir.num_live_nodes;
574 let num_vars = ir.num_vars;
580 successors: vec![invalid_node(); num_live_nodes],
581 users: vec![invalid_users(); num_live_nodes * num_vars],
587 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
588 match self.ir.live_node_map.get(&hir_id) {
591 // This must be a mismatch between the ir_map construction
592 // above and the propagation code below; the two sets of
593 // code have to agree about which AST nodes are worth
594 // creating liveness nodes for.
597 "no live node registered for node {:?}",
603 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
604 self.ir.variable(hir_id, span)
607 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
608 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
610 pat.each_binding(|_bm, hir_id, sp, n| {
611 let ln = self.live_node(hir_id, sp);
612 let var = self.variable(hir_id, n.span);
613 f(self, ln, var, n.span, hir_id);
617 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
618 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
620 if let Some(pat) = pat {
621 self.pat_bindings(pat, f);
625 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
627 self.define_bindings_in_arm_pats(Some(pat), succ)
630 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
633 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
634 this.init_from_succ(ln, succ);
635 this.define(ln, var);
641 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
642 ln.get() * self.ir.num_vars + var.get()
645 fn live_on_entry(&self, ln: LiveNode, var: Variable)
646 -> Option<LiveNodeKind> {
647 assert!(ln.is_valid());
648 let reader = self.users[self.idx(ln, var)].reader;
649 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
653 Is this variable live on entry to any of its successor nodes?
655 fn live_on_exit(&self, ln: LiveNode, var: Variable)
656 -> Option<LiveNodeKind> {
657 let successor = self.successors[ln.get()];
658 self.live_on_entry(successor, var)
661 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
662 assert!(ln.is_valid());
663 self.users[self.idx(ln, var)].used
666 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
667 -> Option<LiveNodeKind> {
668 assert!(ln.is_valid());
669 let writer = self.users[self.idx(ln, var)].writer;
670 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
673 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
674 -> Option<LiveNodeKind> {
675 let successor = self.successors[ln.get()];
676 self.assigned_on_entry(successor, var)
679 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
680 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
682 let node_base_idx = self.idx(ln, Variable(0));
683 let succ_base_idx = self.idx(succ_ln, Variable(0));
684 for var_idx in 0..self.ir.num_vars {
685 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
689 fn write_vars<F>(&self,
693 -> io::Result<()> where
694 F: FnMut(usize) -> LiveNode,
696 let node_base_idx = self.idx(ln, Variable(0));
697 for var_idx in 0..self.ir.num_vars {
698 let idx = node_base_idx + var_idx;
699 if test(idx).is_valid() {
700 write!(wr, " {:?}", Variable(var_idx as u32))?;
707 #[allow(unused_must_use)]
708 fn ln_str(&self, ln: LiveNode) -> String {
709 let mut wr = Vec::new();
711 let wr = &mut wr as &mut dyn Write;
712 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
713 self.write_vars(wr, ln, |idx| self.users[idx].reader);
714 write!(wr, " writes");
715 self.write_vars(wr, ln, |idx| self.users[idx].writer);
716 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
718 String::from_utf8(wr).unwrap()
721 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
722 self.successors[ln.get()] = succ_ln;
724 // It is not necessary to initialize the
725 // values to empty because this is the value
726 // they have when they are created, and the sets
727 // only grow during iterations.
729 // self.indices(ln) { |idx|
730 // self.users[idx] = invalid_users();
734 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
735 // more efficient version of init_empty() / merge_from_succ()
736 self.successors[ln.get()] = succ_ln;
738 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
739 this.users[idx] = this.users[succ_idx]
741 debug!("init_from_succ(ln={}, succ={})",
742 self.ln_str(ln), self.ln_str(succ_ln));
745 fn merge_from_succ(&mut self,
750 if ln == succ_ln { return false; }
752 let mut changed = false;
753 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
754 changed |= copy_if_invalid(this.users[succ_idx].reader,
755 &mut this.users[idx].reader);
756 changed |= copy_if_invalid(this.users[succ_idx].writer,
757 &mut this.users[idx].writer);
758 if this.users[succ_idx].used && !this.users[idx].used {
759 this.users[idx].used = true;
764 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
765 ln, self.ln_str(succ_ln), first_merge, changed);
768 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
769 if src.is_valid() && !dst.is_valid() {
778 // Indicates that a local variable was *defined*; we know that no
779 // uses of the variable can precede the definition (resolve checks
780 // this) so we just clear out all the data.
781 fn define(&mut self, writer: LiveNode, var: Variable) {
782 let idx = self.idx(writer, var);
783 self.users[idx].reader = invalid_node();
784 self.users[idx].writer = invalid_node();
786 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
787 idx, self.ln_str(writer));
790 // Either read, write, or both depending on the acc bitset
791 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
792 debug!("{:?} accesses[{:x}] {:?}: {}",
793 ln, acc, var, self.ln_str(ln));
795 let idx = self.idx(ln, var);
796 let user = &mut self.users[idx];
798 if (acc & ACC_WRITE) != 0 {
799 user.reader = invalid_node();
803 // Important: if we both read/write, must do read second
804 // or else the write will override.
805 if (acc & ACC_READ) != 0 {
809 if (acc & ACC_USE) != 0 {
814 // _______________________________________________________________________
816 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
817 // if there is a `break` or `again` at the top level, then it's
818 // effectively a return---this only occurs in `for` loops,
819 // where the body is really a closure.
821 debug!("compute: using id for body, {}", self.ir.tcx.hir.node_to_pretty_string(body.id));
823 let exit_ln = self.s.exit_ln;
825 self.break_ln.insert(body.id, exit_ln);
826 self.cont_ln.insert(body.id, exit_ln);
828 // the fallthrough exit is only for those cases where we do not
829 // explicitly return:
831 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
832 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
834 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
836 // hack to skip the loop unless debug! is enabled:
837 debug!("^^ liveness computation results for body {} (entry={:?})",
839 for ln_idx in 0..self.ir.num_live_nodes {
840 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
849 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
851 if blk.targeted_by_break {
852 self.break_ln.insert(blk.id, succ);
854 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
855 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
856 self.propagate_through_stmt(stmt, succ)
860 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
863 hir::StmtDecl(ref decl, _) => {
864 self.propagate_through_decl(&decl, succ)
867 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
868 self.propagate_through_expr(&expr, succ)
873 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
876 hir::DeclLocal(ref local) => {
877 self.propagate_through_local(&local, succ)
879 hir::DeclItem(_) => succ,
883 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
885 // Note: we mark the variable as defined regardless of whether
886 // there is an initializer. Initially I had thought to only mark
887 // the live variable as defined if it was initialized, and then we
888 // could check for uninit variables just by scanning what is live
889 // at the start of the function. But that doesn't work so well for
890 // immutable variables defined in a loop:
891 // loop { let x; x = 5; }
892 // because the "assignment" loops back around and generates an error.
894 // So now we just check that variables defined w/o an
895 // initializer are not live at the point of their
896 // initialization, which is mildly more complex than checking
897 // once at the func header but otherwise equivalent.
899 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
900 self.define_bindings_in_pat(&local.pat, succ)
903 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
905 exprs.iter().rev().fold(succ, |succ, expr| {
906 self.propagate_through_expr(&expr, succ)
910 fn propagate_through_opt_expr(&mut self,
911 opt_expr: Option<&Expr>,
914 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
917 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
919 debug!("propagate_through_expr: {}", self.ir.tcx.hir.node_to_pretty_string(expr.id));
922 // Interesting cases with control flow or which gen/kill
923 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
924 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
927 hir::ExprField(ref e, _) => {
928 self.propagate_through_expr(&e, succ)
931 hir::ExprClosure(.., blk_id, _, _) => {
932 debug!("{} is an ExprClosure", self.ir.tcx.hir.node_to_pretty_string(expr.id));
934 // The next-node for a break is the successor of the entire
935 // loop. The next-node for a continue is the top of this loop.
936 let node = self.live_node(expr.hir_id, expr.span);
940 self.break_ln.insert(blk_id.node_id, break_ln);
941 self.cont_ln.insert(blk_id.node_id, cont_ln);
943 // the construction of a closure itself is not important,
944 // but we have to consider the closed over variables.
945 let caps = match self.ir.capture_info_map.get(&expr.id) {
946 Some(caps) => caps.clone(),
948 span_bug!(expr.span, "no registered caps");
951 caps.iter().rev().fold(succ, |succ, cap| {
952 self.init_from_succ(cap.ln, succ);
953 let var = self.variable(cap.var_hid, expr.span);
954 self.acc(cap.ln, var, ACC_READ | ACC_USE);
959 hir::ExprIf(ref cond, ref then, ref els) => {
973 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
974 let then_ln = self.propagate_through_expr(&then, succ);
975 let ln = self.live_node(expr.hir_id, expr.span);
976 self.init_from_succ(ln, else_ln);
977 self.merge_from_succ(ln, then_ln, false);
978 self.propagate_through_expr(&cond, ln)
981 hir::ExprWhile(ref cond, ref blk, _) => {
982 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
985 // Note that labels have been resolved, so we don't need to look
986 // at the label ident
987 hir::ExprLoop(ref blk, _, _) => {
988 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
991 hir::ExprMatch(ref e, ref arms, _) => {
1006 let ln = self.live_node(expr.hir_id, expr.span);
1007 self.init_empty(ln, succ);
1008 let mut first_merge = true;
1011 self.propagate_through_expr(&arm.body, succ);
1013 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
1014 // only consider the first pattern; any later patterns must have
1015 // the same bindings, and we also consider the first pattern to be
1016 // the "authoritative" set of ids
1018 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1020 self.merge_from_succ(ln, arm_succ, first_merge);
1021 first_merge = false;
1023 self.propagate_through_expr(&e, ln)
1026 hir::ExprRet(ref o_e) => {
1027 // ignore succ and subst exit_ln:
1028 let exit_ln = self.s.exit_ln;
1029 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1032 hir::ExprBreak(label, ref opt_expr) => {
1033 // Find which label this break jumps to
1034 let target = match label.target_id {
1035 Ok(node_id) => self.break_ln.get(&node_id),
1036 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1039 // Now that we know the label we're going to,
1040 // look it up in the break loop nodes table
1043 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1044 None => span_bug!(expr.span, "break to unknown label")
1048 hir::ExprContinue(label) => {
1049 // Find which label this expr continues to
1050 let sc = match label.target_id {
1051 Ok(node_id) => node_id,
1052 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1055 // Now that we know the label we're going to,
1056 // look it up in the continue loop nodes table
1058 match self.cont_ln.get(&sc) {
1060 None => span_bug!(expr.span, "continue to unknown label")
1064 hir::ExprAssign(ref l, ref r) => {
1065 // see comment on places in
1066 // propagate_through_place_components()
1067 let succ = self.write_place(&l, succ, ACC_WRITE);
1068 let succ = self.propagate_through_place_components(&l, succ);
1069 self.propagate_through_expr(&r, succ)
1072 hir::ExprAssignOp(_, ref l, ref r) => {
1073 // an overloaded assign op is like a method call
1074 if self.tables.is_method_call(expr) {
1075 let succ = self.propagate_through_expr(&l, succ);
1076 self.propagate_through_expr(&r, succ)
1078 // see comment on places in
1079 // propagate_through_place_components()
1080 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1081 let succ = self.propagate_through_expr(&r, succ);
1082 self.propagate_through_place_components(&l, succ)
1086 // Uninteresting cases: just propagate in rev exec order
1088 hir::ExprArray(ref exprs) => {
1089 self.propagate_through_exprs(exprs, succ)
1092 hir::ExprStruct(_, ref fields, ref with_expr) => {
1093 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1094 fields.iter().rev().fold(succ, |succ, field| {
1095 self.propagate_through_expr(&field.expr, succ)
1099 hir::ExprCall(ref f, ref args) => {
1100 // FIXME(canndrew): This is_never should really be an is_uninhabited
1101 let succ = if self.tables.expr_ty(expr).is_never() {
1106 let succ = self.propagate_through_exprs(args, succ);
1107 self.propagate_through_expr(&f, succ)
1110 hir::ExprMethodCall(.., ref args) => {
1111 // FIXME(canndrew): This is_never should really be an is_uninhabited
1112 let succ = if self.tables.expr_ty(expr).is_never() {
1117 self.propagate_through_exprs(args, succ)
1120 hir::ExprTup(ref exprs) => {
1121 self.propagate_through_exprs(exprs, succ)
1124 hir::ExprBinary(op, ref l, ref r) if op.node.is_lazy() => {
1125 let r_succ = self.propagate_through_expr(&r, succ);
1127 let ln = self.live_node(expr.hir_id, expr.span);
1128 self.init_from_succ(ln, succ);
1129 self.merge_from_succ(ln, r_succ, false);
1131 self.propagate_through_expr(&l, ln)
1134 hir::ExprIndex(ref l, ref r) |
1135 hir::ExprBinary(_, ref l, ref r) => {
1136 let r_succ = self.propagate_through_expr(&r, succ);
1137 self.propagate_through_expr(&l, r_succ)
1140 hir::ExprBox(ref e) |
1141 hir::ExprAddrOf(_, ref e) |
1142 hir::ExprCast(ref e, _) |
1143 hir::ExprType(ref e, _) |
1144 hir::ExprUnary(_, ref e) |
1145 hir::ExprYield(ref e) |
1146 hir::ExprRepeat(ref e, _) => {
1147 self.propagate_through_expr(&e, succ)
1150 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1151 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1152 // see comment on places
1153 // in propagate_through_place_components()
1155 self.propagate_through_expr(output, succ)
1157 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1158 let succ = self.write_place(output, succ, acc);
1159 self.propagate_through_place_components(output, succ)
1163 // Inputs are executed first. Propagate last because of rev order
1164 self.propagate_through_exprs(inputs, succ)
1167 hir::ExprLit(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
1171 // Note that labels have been resolved, so we don't need to look
1172 // at the label ident
1173 hir::ExprBlock(ref blk, _) => {
1174 self.propagate_through_block(&blk, succ)
1179 fn propagate_through_place_components(&mut self,
1185 // In general, the full flow graph structure for an
1186 // assignment/move/etc can be handled in one of two ways,
1187 // depending on whether what is being assigned is a "tracked
1188 // value" or not. A tracked value is basically a local
1189 // variable or argument.
1191 // The two kinds of graphs are:
1193 // Tracked place Untracked place
1194 // ----------------------++-----------------------
1198 // (rvalue) || (rvalue)
1201 // (write of place) || (place components)
1206 // ----------------------++-----------------------
1208 // I will cover the two cases in turn:
1212 // A tracked place is a local variable/argument `x`. In
1213 // these cases, the link_node where the write occurs is linked
1214 // to node id of `x`. The `write_place()` routine generates
1215 // the contents of this node. There are no subcomponents to
1218 // # Non-tracked places
1220 // These are places like `x[5]` or `x.f`. In that case, we
1221 // basically ignore the value which is written to but generate
1222 // reads for the components---`x` in these two examples. The
1223 // components reads are generated by
1224 // `propagate_through_place_components()` (this fn).
1228 // It is still possible to observe assignments to non-places;
1229 // these errors are detected in the later pass borrowck. We
1230 // just ignore such cases and treat them as reads.
1233 hir::ExprPath(_) => succ,
1234 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1235 _ => self.propagate_through_expr(expr, succ)
1239 // see comment on propagate_through_place()
1240 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1243 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1244 self.access_path(expr.hir_id, path, succ, acc)
1247 // We do not track other places, so just propagate through
1248 // to their subcomponents. Also, it may happen that
1249 // non-places occur here, because those are detected in the
1250 // later pass borrowck.
1255 fn access_var(&mut self, hir_id: HirId, nid: NodeId, succ: LiveNode, acc: u32, span: Span)
1257 let ln = self.live_node(hir_id, span);
1259 self.init_from_succ(ln, succ);
1260 let var_hid = self.ir.tcx.hir.node_to_hir_id(nid);
1261 let var = self.variable(var_hid, span);
1262 self.acc(ln, var, acc);
1267 fn access_path(&mut self, hir_id: HirId, path: &hir::Path, succ: LiveNode, acc: u32)
1270 Def::Local(nid) => {
1271 self.access_var(hir_id, nid, succ, acc, path.span)
1277 fn propagate_through_loop(&mut self,
1286 We model control flow like this:
1304 let mut first_merge = true;
1305 let ln = self.live_node(expr.hir_id, expr.span);
1306 self.init_empty(ln, succ);
1310 // If this is not a `loop` loop, then it's possible we bypass
1311 // the body altogether. Otherwise, the only way is via a `break`
1312 // in the loop body.
1313 self.merge_from_succ(ln, succ, first_merge);
1314 first_merge = false;
1317 debug!("propagate_through_loop: using id for loop body {} {}",
1318 expr.id, self.ir.tcx.hir.node_to_pretty_string(body.id));
1320 let break_ln = succ;
1322 self.break_ln.insert(expr.id, break_ln);
1323 self.cont_ln.insert(expr.id, cont_ln);
1325 let cond_ln = match kind {
1327 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1329 let body_ln = self.propagate_through_block(body, cond_ln);
1331 // repeat until fixed point is reached:
1332 while self.merge_from_succ(ln, body_ln, first_merge) {
1333 first_merge = false;
1335 let new_cond_ln = match kind {
1337 WhileLoop(ref cond) => {
1338 self.propagate_through_expr(&cond, ln)
1341 assert!(cond_ln == new_cond_ln);
1342 assert!(body_ln == self.propagate_through_block(body, cond_ln));
1349 // _______________________________________________________________________
1350 // Checking for error conditions
1352 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1353 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1354 NestedVisitorMap::None
1357 fn visit_local(&mut self, l: &'tcx hir::Local) {
1358 check_local(self, l);
1360 fn visit_expr(&mut self, ex: &'tcx Expr) {
1361 check_expr(self, ex);
1363 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1368 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1371 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1374 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1375 let span = local.pat.simple_ident().map_or(sp, |ident| ident.span);
1376 this.warn_about_unused(span, id, ln, var);
1381 intravisit::walk_local(this, local);
1384 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1385 // only consider the first pattern; any later patterns must have
1386 // the same bindings, and we also consider the first pattern to be
1387 // the "authoritative" set of ids
1388 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1389 this.warn_about_unused(sp, id, ln, var);
1391 intravisit::walk_arm(this, arm);
1394 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1396 hir::ExprAssign(ref l, _) => {
1397 this.check_place(&l);
1399 intravisit::walk_expr(this, expr);
1402 hir::ExprAssignOp(_, ref l, _) => {
1403 if !this.tables.is_method_call(expr) {
1404 this.check_place(&l);
1407 intravisit::walk_expr(this, expr);
1410 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1411 for input in inputs {
1412 this.visit_expr(input);
1415 // Output operands must be places
1416 for (o, output) in ia.outputs.iter().zip(outputs) {
1418 this.check_place(output);
1420 this.visit_expr(output);
1423 intravisit::walk_expr(this, expr);
1426 // no correctness conditions related to liveness
1427 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1428 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1429 hir::ExprIndex(..) | hir::ExprField(..) |
1430 hir::ExprArray(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1431 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1432 hir::ExprBreak(..) | hir::ExprContinue(..) | hir::ExprLit(_) |
1433 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1434 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1435 hir::ExprClosure(..) | hir::ExprPath(_) | hir::ExprYield(..) |
1436 hir::ExprBox(..) | hir::ExprType(..) => {
1437 intravisit::walk_expr(this, expr);
1442 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1443 fn check_place(&mut self, expr: &'tcx Expr) {
1445 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1446 if let Def::Local(nid) = path.def {
1447 // Assignment to an immutable variable or argument: only legal
1448 // if there is no later assignment. If this local is actually
1449 // mutable, then check for a reassignment to flag the mutability
1451 let ln = self.live_node(expr.hir_id, expr.span);
1452 let var_hid = self.ir.tcx.hir.node_to_hir_id(nid);
1453 let var = self.variable(var_hid, expr.span);
1454 self.warn_about_dead_assign(expr.span, expr.hir_id, ln, var);
1458 // For other kinds of places, no checks are required,
1459 // and any embedded expressions are actually rvalues
1460 intravisit::walk_expr(self, expr);
1465 fn should_warn(&self, var: Variable) -> Option<String> {
1466 let name = self.ir.variable_name(var);
1467 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1474 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1475 for arg in &body.arguments {
1476 arg.pat.each_binding(|_bm, hir_id, _, ident| {
1477 let sp = ident.span;
1478 let var = self.variable(hir_id, sp);
1479 // Ignore unused self.
1480 if ident.name != keywords::SelfValue.name() {
1481 if !self.warn_about_unused(sp, hir_id, entry_ln, var) {
1482 if self.live_on_entry(entry_ln, var).is_none() {
1483 self.report_dead_assign(hir_id, sp, var, true);
1491 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1492 self.pat_bindings(pat, |this, ln, var, sp, id| {
1493 if !this.warn_about_unused(sp, id, ln, var) {
1494 this.warn_about_dead_assign(sp, id, ln, var);
1499 fn warn_about_unused(&self,
1505 if !self.used_on_entry(ln, var) {
1506 let r = self.should_warn(var);
1507 if let Some(name) = r {
1509 // annoying: for parameters in funcs like `fn(x: i32)
1510 // {ret}`, there is only one node, so asking about
1511 // assigned_on_exit() is not meaningful.
1512 let is_assigned = if ln == self.s.exit_ln {
1515 self.assigned_on_exit(ln, var).is_some()
1518 let suggest_underscore_msg = format!("consider using `_{}` instead",
1523 .lint_hir_note(lint::builtin::UNUSED_VARIABLES, hir_id, sp,
1524 &format!("variable `{}` is assigned to, but never used",
1526 &suggest_underscore_msg);
1527 } else if name != "self" {
1528 let msg = format!("unused variable: `{}`", name);
1529 let mut err = self.ir.tcx
1530 .struct_span_lint_hir(lint::builtin::UNUSED_VARIABLES, hir_id, sp, &msg);
1531 if self.ir.variable_is_shorthand(var) {
1532 err.span_suggestion_with_applicability(sp, "try ignoring the field",
1533 format!("{}: _", name),
1534 Applicability::MachineApplicable);
1536 err.span_suggestion_short_with_applicability(
1537 sp, &suggest_underscore_msg,
1538 format!("_{}", name),
1539 Applicability::MachineApplicable,
1551 fn warn_about_dead_assign(&self,
1556 if self.live_on_exit(ln, var).is_none() {
1557 self.report_dead_assign(hir_id, sp, var, false);
1561 fn report_dead_assign(&self, hir_id: HirId, sp: Span, var: Variable, is_argument: bool) {
1562 if let Some(name) = self.should_warn(var) {
1564 self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1565 &format!("value passed to `{}` is never read", name));
1567 self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1568 &format!("value assigned to `{}` is never read", name));