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.
106 use self::LoopKind::*;
107 use self::LiveNodeKind::*;
108 use self::VarKind::*;
112 use ty::{self, TyCtxt};
114 use errors::Applicability;
115 use util::nodemap::{NodeMap, HirIdMap, HirIdSet};
117 use std::collections::VecDeque;
119 use std::io::prelude::*;
122 use syntax::ast::{self, NodeId};
124 use syntax::symbol::keywords;
125 use syntax_pos::Span;
127 use hir::{Expr, HirId};
129 use hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
131 /// For use with `propagate_through_loop`.
133 /// An endless `loop` loop.
135 /// A `while` loop, with the given expression as condition.
139 #[derive(Copy, Clone, PartialEq)]
140 struct Variable(u32);
142 #[derive(Copy, Clone, PartialEq)]
143 struct LiveNode(u32);
146 fn get(&self) -> usize { self.0 as usize }
150 fn get(&self) -> usize { self.0 as usize }
153 #[derive(Copy, Clone, PartialEq, Debug)]
161 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt) -> String {
162 let cm = tcx.sess.source_map();
165 format!("Free var node [{}]", cm.span_to_string(s))
168 format!("Expr node [{}]", cm.span_to_string(s))
171 format!("Var def node [{}]", cm.span_to_string(s))
173 ExitNode => "Exit node".to_string(),
177 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
178 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
179 NestedVisitorMap::OnlyBodies(&self.tcx.hir)
182 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
183 b: hir::BodyId, s: Span, id: NodeId) {
184 visit_fn(self, fk, fd, b, s, id);
187 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
188 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
189 fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
192 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
193 tcx.hir.krate().visit_all_item_likes(&mut IrMaps::new(tcx).as_deep_visitor());
194 tcx.sess.abort_if_errors();
197 impl fmt::Debug for LiveNode {
198 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
199 write!(f, "ln({})", self.get())
203 impl fmt::Debug for Variable {
204 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
205 write!(f, "v({})", self.get())
209 // ______________________________________________________________________
212 // This is the first pass and the one that drives the main
213 // computation. It walks up and down the IR once. On the way down,
214 // we count for each function the number of variables as well as
215 // liveness nodes. A liveness node is basically an expression or
216 // capture clause that does something of interest: either it has
217 // interesting control flow or it uses/defines a local variable.
219 // On the way back up, at each function node we create liveness sets
220 // (we now know precisely how big to make our various vectors and so
221 // forth) and then do the data-flow propagation to compute the set
222 // of live variables at each program point.
224 // Finally, we run back over the IR one last time and, using the
225 // computed liveness, check various safety conditions. For example,
226 // there must be no live nodes at the definition site for a variable
227 // unless it has an initializer. Similarly, each non-mutable local
228 // variable must not be assigned if there is some successor
229 // assignment. And so forth.
232 fn is_valid(&self) -> bool {
237 fn invalid_node() -> LiveNode { LiveNode(u32::MAX) }
244 #[derive(Copy, Clone, Debug)]
251 #[derive(Copy, Clone, Debug)]
253 Arg(HirId, ast::Name),
258 struct IrMaps<'a, 'tcx: 'a> {
259 tcx: TyCtxt<'a, 'tcx, 'tcx>,
261 num_live_nodes: usize,
263 live_node_map: HirIdMap<LiveNode>,
264 variable_map: HirIdMap<Variable>,
265 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
266 var_kinds: Vec<VarKind>,
267 lnks: Vec<LiveNodeKind>,
270 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
271 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
276 live_node_map: HirIdMap(),
277 variable_map: HirIdMap(),
278 capture_info_map: NodeMap(),
279 var_kinds: Vec::new(),
284 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
285 let ln = LiveNode(self.num_live_nodes as u32);
287 self.num_live_nodes += 1;
289 debug!("{:?} is of kind {}", ln,
290 live_node_kind_to_string(lnk, self.tcx));
295 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
296 let ln = self.add_live_node(lnk);
297 self.live_node_map.insert(hir_id, ln);
299 debug!("{:?} is node {:?}", ln, hir_id);
302 fn add_variable(&mut self, vk: VarKind) -> Variable {
303 let v = Variable(self.num_vars as u32);
304 self.var_kinds.push(vk);
308 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
309 self.variable_map.insert(node_id, v);
314 debug!("{:?} is {:?}", v, vk);
319 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
320 match self.variable_map.get(&hir_id) {
323 span_bug!(span, "no variable registered for id {:?}", hir_id);
328 fn variable_name(&self, var: Variable) -> String {
329 match self.var_kinds[var.get()] {
330 Local(LocalInfo { name, .. }) | Arg(_, name) => {
333 CleanExit => "<clean-exit>".to_string()
337 fn variable_is_shorthand(&self, var: Variable) -> bool {
338 match self.var_kinds[var.get()] {
339 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
340 Arg(..) | CleanExit => false
344 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
345 self.capture_info_map.insert(node_id, Rc::new(cs));
348 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
353 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
355 decl: &'tcx hir::FnDecl,
356 body_id: hir::BodyId,
361 // swap in a new set of IR maps for this function body:
362 let mut fn_maps = IrMaps::new(ir.tcx);
364 // Don't run unused pass for #[derive()]
365 if let FnKind::Method(..) = fk {
366 let parent = ir.tcx.hir.get_parent(id);
367 if let Some(Node::Item(i)) = ir.tcx.hir.find(parent) {
368 if i.attrs.iter().any(|a| a.check_name("automatically_derived")) {
374 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
376 let body = ir.tcx.hir.body(body_id);
378 for arg in &body.arguments {
379 arg.pat.each_binding(|_bm, hir_id, _x, ident| {
380 debug!("adding argument {:?}", hir_id);
381 fn_maps.add_variable(Arg(hir_id, ident.name));
385 // gather up the various local variables, significant expressions,
387 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
390 let mut lsets = Liveness::new(&mut fn_maps, body_id);
391 let entry_ln = lsets.compute(&body.value);
393 // check for various error conditions
394 lsets.visit_body(body);
395 lsets.warn_about_unused_args(body, entry_ln);
398 fn add_from_pat<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, pat: &P<hir::Pat>) {
399 // For struct patterns, take note of which fields used shorthand
400 // (`x` rather than `x: x`).
401 let mut shorthand_field_ids = HirIdSet();
402 let mut pats = VecDeque::new();
404 while let Some(pat) = pats.pop_front() {
407 Binding(_, _, _, ref inner_pat) => {
408 pats.extend(inner_pat.iter());
410 Struct(_, ref fields, _) => {
411 for field in fields {
412 if field.node.is_shorthand {
413 shorthand_field_ids.insert(field.node.pat.hir_id);
417 Ref(ref inner_pat, _) |
418 Box(ref inner_pat) => {
419 pats.push_back(inner_pat);
421 TupleStruct(_, ref inner_pats, _) |
422 Tuple(ref inner_pats, _) => {
423 pats.extend(inner_pats.iter());
425 Slice(ref pre_pats, ref inner_pat, ref post_pats) => {
426 pats.extend(pre_pats.iter());
427 pats.extend(inner_pat.iter());
428 pats.extend(post_pats.iter());
434 pat.each_binding(|_bm, hir_id, _sp, ident| {
435 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
436 ir.add_variable(Local(LocalInfo {
439 is_shorthand: shorthand_field_ids.contains(&hir_id)
444 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
445 add_from_pat(ir, &local.pat);
446 intravisit::walk_local(ir, local);
449 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
450 for pat in &arm.pats {
451 add_from_pat(ir, pat);
453 intravisit::walk_arm(ir, arm);
456 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
458 // live nodes required for uses or definitions of variables:
459 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
460 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
461 if let Def::Local(..) = path.def {
462 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
464 intravisit::walk_expr(ir, expr);
466 hir::ExprKind::Closure(..) => {
467 // Interesting control flow (for loops can contain labeled
468 // breaks or continues)
469 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
471 // Make a live_node for each captured variable, with the span
472 // being the location that the variable is used. This results
473 // in better error messages than just pointing at the closure
474 // construction site.
475 let mut call_caps = Vec::new();
476 ir.tcx.with_freevars(expr.id, |freevars| {
478 if let Def::Local(rv) = fv.def {
479 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
480 let var_hid = ir.tcx.hir.node_to_hir_id(rv);
481 call_caps.push(CaptureInfo { ln: fv_ln, var_hid });
485 ir.set_captures(expr.id, call_caps);
487 intravisit::walk_expr(ir, expr);
490 // live nodes required for interesting control flow:
491 hir::ExprKind::If(..) |
492 hir::ExprKind::Match(..) |
493 hir::ExprKind::While(..) |
494 hir::ExprKind::Loop(..) => {
495 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
496 intravisit::walk_expr(ir, expr);
498 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
499 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
500 intravisit::walk_expr(ir, expr);
503 // otherwise, live nodes are not required:
504 hir::ExprKind::Index(..) |
505 hir::ExprKind::Field(..) |
506 hir::ExprKind::Array(..) |
507 hir::ExprKind::Call(..) |
508 hir::ExprKind::MethodCall(..) |
509 hir::ExprKind::Tup(..) |
510 hir::ExprKind::Binary(..) |
511 hir::ExprKind::AddrOf(..) |
512 hir::ExprKind::Cast(..) |
513 hir::ExprKind::Unary(..) |
514 hir::ExprKind::Break(..) |
515 hir::ExprKind::Continue(_) |
516 hir::ExprKind::Lit(_) |
517 hir::ExprKind::Ret(..) |
518 hir::ExprKind::Block(..) |
519 hir::ExprKind::Assign(..) |
520 hir::ExprKind::AssignOp(..) |
521 hir::ExprKind::Struct(..) |
522 hir::ExprKind::Repeat(..) |
523 hir::ExprKind::InlineAsm(..) |
524 hir::ExprKind::Box(..) |
525 hir::ExprKind::Yield(..) |
526 hir::ExprKind::Type(..) |
527 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
528 intravisit::walk_expr(ir, expr);
533 // ______________________________________________________________________
534 // Computing liveness sets
536 // Actually we compute just a bit more than just liveness, but we use
537 // the same basic propagation framework in all cases.
539 #[derive(Clone, Copy)]
546 fn invalid_users() -> Users {
548 reader: invalid_node(),
549 writer: invalid_node(),
554 #[derive(Copy, Clone)]
557 fallthrough_ln: LiveNode,
558 clean_exit_var: Variable
561 const ACC_READ: u32 = 1;
562 const ACC_WRITE: u32 = 2;
563 const ACC_USE: u32 = 4;
565 struct Liveness<'a, 'tcx: 'a> {
566 ir: &'a mut IrMaps<'a, 'tcx>,
567 tables: &'a ty::TypeckTables<'tcx>,
569 successors: Vec<LiveNode>,
572 // mappings from loop node ID to LiveNode
573 // ("break" label should map to loop node ID,
574 // it probably doesn't now)
575 break_ln: NodeMap<LiveNode>,
576 cont_ln: NodeMap<LiveNode>,
579 impl<'a, 'tcx> Liveness<'a, 'tcx> {
580 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
581 // Special nodes and variables:
582 // - exit_ln represents the end of the fn, either by return or panic
583 // - implicit_ret_var is a pseudo-variable that represents
584 // an implicit return
585 let specials = Specials {
586 exit_ln: ir.add_live_node(ExitNode),
587 fallthrough_ln: ir.add_live_node(ExitNode),
588 clean_exit_var: ir.add_variable(CleanExit)
591 let tables = ir.tcx.body_tables(body);
593 let num_live_nodes = ir.num_live_nodes;
594 let num_vars = ir.num_vars;
600 successors: vec![invalid_node(); num_live_nodes],
601 users: vec![invalid_users(); num_live_nodes * num_vars],
607 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
608 match self.ir.live_node_map.get(&hir_id) {
611 // This must be a mismatch between the ir_map construction
612 // above and the propagation code below; the two sets of
613 // code have to agree about which AST nodes are worth
614 // creating liveness nodes for.
617 "no live node registered for node {:?}",
623 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
624 self.ir.variable(hir_id, span)
627 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
628 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
630 pat.each_binding(|_bm, hir_id, sp, n| {
631 let ln = self.live_node(hir_id, sp);
632 let var = self.variable(hir_id, n.span);
633 f(self, ln, var, n.span, hir_id);
637 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
638 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
640 if let Some(pat) = pat {
641 self.pat_bindings(pat, f);
645 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
647 self.define_bindings_in_arm_pats(Some(pat), succ)
650 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
653 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
654 this.init_from_succ(ln, succ);
655 this.define(ln, var);
661 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
662 ln.get() * self.ir.num_vars + var.get()
665 fn live_on_entry(&self, ln: LiveNode, var: Variable)
666 -> Option<LiveNodeKind> {
667 assert!(ln.is_valid());
668 let reader = self.users[self.idx(ln, var)].reader;
669 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
673 Is this variable live on entry to any of its successor nodes?
675 fn live_on_exit(&self, ln: LiveNode, var: Variable)
676 -> Option<LiveNodeKind> {
677 let successor = self.successors[ln.get()];
678 self.live_on_entry(successor, var)
681 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
682 assert!(ln.is_valid());
683 self.users[self.idx(ln, var)].used
686 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
687 -> Option<LiveNodeKind> {
688 assert!(ln.is_valid());
689 let writer = self.users[self.idx(ln, var)].writer;
690 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
693 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
694 -> Option<LiveNodeKind> {
695 let successor = self.successors[ln.get()];
696 self.assigned_on_entry(successor, var)
699 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
700 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
702 let node_base_idx = self.idx(ln, Variable(0));
703 let succ_base_idx = self.idx(succ_ln, Variable(0));
704 for var_idx in 0..self.ir.num_vars {
705 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
709 fn write_vars<F>(&self,
713 -> io::Result<()> where
714 F: FnMut(usize) -> LiveNode,
716 let node_base_idx = self.idx(ln, Variable(0));
717 for var_idx in 0..self.ir.num_vars {
718 let idx = node_base_idx + var_idx;
719 if test(idx).is_valid() {
720 write!(wr, " {:?}", Variable(var_idx as u32))?;
727 #[allow(unused_must_use)]
728 fn ln_str(&self, ln: LiveNode) -> String {
729 let mut wr = Vec::new();
731 let wr = &mut wr as &mut dyn Write;
732 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
733 self.write_vars(wr, ln, |idx| self.users[idx].reader);
734 write!(wr, " writes");
735 self.write_vars(wr, ln, |idx| self.users[idx].writer);
736 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
738 String::from_utf8(wr).unwrap()
741 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
742 self.successors[ln.get()] = succ_ln;
744 // It is not necessary to initialize the
745 // values to empty because this is the value
746 // they have when they are created, and the sets
747 // only grow during iterations.
749 // self.indices(ln) { |idx|
750 // self.users[idx] = invalid_users();
754 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
755 // more efficient version of init_empty() / merge_from_succ()
756 self.successors[ln.get()] = succ_ln;
758 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
759 this.users[idx] = this.users[succ_idx]
761 debug!("init_from_succ(ln={}, succ={})",
762 self.ln_str(ln), self.ln_str(succ_ln));
765 fn merge_from_succ(&mut self,
770 if ln == succ_ln { return false; }
772 let mut changed = false;
773 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
774 changed |= copy_if_invalid(this.users[succ_idx].reader,
775 &mut this.users[idx].reader);
776 changed |= copy_if_invalid(this.users[succ_idx].writer,
777 &mut this.users[idx].writer);
778 if this.users[succ_idx].used && !this.users[idx].used {
779 this.users[idx].used = true;
784 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
785 ln, self.ln_str(succ_ln), first_merge, changed);
788 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
789 if src.is_valid() && !dst.is_valid() {
798 // Indicates that a local variable was *defined*; we know that no
799 // uses of the variable can precede the definition (resolve checks
800 // this) so we just clear out all the data.
801 fn define(&mut self, writer: LiveNode, var: Variable) {
802 let idx = self.idx(writer, var);
803 self.users[idx].reader = invalid_node();
804 self.users[idx].writer = invalid_node();
806 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
807 idx, self.ln_str(writer));
810 // Either read, write, or both depending on the acc bitset
811 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
812 debug!("{:?} accesses[{:x}] {:?}: {}",
813 ln, acc, var, self.ln_str(ln));
815 let idx = self.idx(ln, var);
816 let user = &mut self.users[idx];
818 if (acc & ACC_WRITE) != 0 {
819 user.reader = invalid_node();
823 // Important: if we both read/write, must do read second
824 // or else the write will override.
825 if (acc & ACC_READ) != 0 {
829 if (acc & ACC_USE) != 0 {
834 // _______________________________________________________________________
836 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
837 // if there is a `break` or `again` at the top level, then it's
838 // effectively a return---this only occurs in `for` loops,
839 // where the body is really a closure.
841 debug!("compute: using id for body, {}", self.ir.tcx.hir.node_to_pretty_string(body.id));
843 let exit_ln = self.s.exit_ln;
845 self.break_ln.insert(body.id, exit_ln);
846 self.cont_ln.insert(body.id, exit_ln);
848 // the fallthrough exit is only for those cases where we do not
849 // explicitly return:
851 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
852 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
854 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
856 // hack to skip the loop unless debug! is enabled:
857 debug!("^^ liveness computation results for body {} (entry={:?})",
859 for ln_idx in 0..self.ir.num_live_nodes {
860 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
869 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
871 if blk.targeted_by_break {
872 self.break_ln.insert(blk.id, succ);
874 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
875 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
876 self.propagate_through_stmt(stmt, succ)
880 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
883 hir::StmtKind::Decl(ref decl, _) => {
884 self.propagate_through_decl(&decl, succ)
887 hir::StmtKind::Expr(ref expr, _) | hir::StmtKind::Semi(ref expr, _) => {
888 self.propagate_through_expr(&expr, succ)
893 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
896 hir::DeclKind::Local(ref local) => {
897 self.propagate_through_local(&local, succ)
899 hir::DeclKind::Item(_) => succ,
903 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
905 // Note: we mark the variable as defined regardless of whether
906 // there is an initializer. Initially I had thought to only mark
907 // the live variable as defined if it was initialized, and then we
908 // could check for uninit variables just by scanning what is live
909 // at the start of the function. But that doesn't work so well for
910 // immutable variables defined in a loop:
911 // loop { let x; x = 5; }
912 // because the "assignment" loops back around and generates an error.
914 // So now we just check that variables defined w/o an
915 // initializer are not live at the point of their
916 // initialization, which is mildly more complex than checking
917 // once at the func header but otherwise equivalent.
919 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
920 self.define_bindings_in_pat(&local.pat, succ)
923 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
925 exprs.iter().rev().fold(succ, |succ, expr| {
926 self.propagate_through_expr(&expr, succ)
930 fn propagate_through_opt_expr(&mut self,
931 opt_expr: Option<&Expr>,
934 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
937 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
939 debug!("propagate_through_expr: {}", self.ir.tcx.hir.node_to_pretty_string(expr.id));
942 // Interesting cases with control flow or which gen/kill
943 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
944 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
947 hir::ExprKind::Field(ref e, _) => {
948 self.propagate_through_expr(&e, succ)
951 hir::ExprKind::Closure(.., blk_id, _, _) => {
952 debug!("{} is an ExprKind::Closure", self.ir.tcx.hir.node_to_pretty_string(expr.id));
954 // The next-node for a break is the successor of the entire
955 // loop. The next-node for a continue is the top of this loop.
956 let node = self.live_node(expr.hir_id, expr.span);
960 self.break_ln.insert(blk_id.node_id, break_ln);
961 self.cont_ln.insert(blk_id.node_id, cont_ln);
963 // the construction of a closure itself is not important,
964 // but we have to consider the closed over variables.
965 let caps = match self.ir.capture_info_map.get(&expr.id) {
966 Some(caps) => caps.clone(),
968 span_bug!(expr.span, "no registered caps");
971 caps.iter().rev().fold(succ, |succ, cap| {
972 self.init_from_succ(cap.ln, succ);
973 let var = self.variable(cap.var_hid, expr.span);
974 self.acc(cap.ln, var, ACC_READ | ACC_USE);
979 hir::ExprKind::If(ref cond, ref then, ref els) => {
993 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
994 let then_ln = self.propagate_through_expr(&then, succ);
995 let ln = self.live_node(expr.hir_id, expr.span);
996 self.init_from_succ(ln, else_ln);
997 self.merge_from_succ(ln, then_ln, false);
998 self.propagate_through_expr(&cond, ln)
1001 hir::ExprKind::While(ref cond, ref blk, _) => {
1002 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1005 // Note that labels have been resolved, so we don't need to look
1006 // at the label ident
1007 hir::ExprKind::Loop(ref blk, _, _) => {
1008 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1011 hir::ExprKind::Match(ref e, ref arms, _) => {
1026 let ln = self.live_node(expr.hir_id, expr.span);
1027 self.init_empty(ln, succ);
1028 let mut first_merge = true;
1031 self.propagate_through_expr(&arm.body, succ);
1033 self.propagate_through_opt_expr(
1034 arm.guard.as_ref().map(|g|
1036 hir::Guard::If(e) => &**e,
1039 // only consider the first pattern; any later patterns must have
1040 // the same bindings, and we also consider the first pattern to be
1041 // the "authoritative" set of ids
1043 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1045 self.merge_from_succ(ln, arm_succ, first_merge);
1046 first_merge = false;
1048 self.propagate_through_expr(&e, ln)
1051 hir::ExprKind::Ret(ref o_e) => {
1052 // ignore succ and subst exit_ln:
1053 let exit_ln = self.s.exit_ln;
1054 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1057 hir::ExprKind::Break(label, ref opt_expr) => {
1058 // Find which label this break jumps to
1059 let target = match label.target_id {
1060 Ok(node_id) => self.break_ln.get(&node_id),
1061 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1064 // Now that we know the label we're going to,
1065 // look it up in the break loop nodes table
1068 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1069 None => span_bug!(expr.span, "break to unknown label")
1073 hir::ExprKind::Continue(label) => {
1074 // Find which label this expr continues to
1075 let sc = match label.target_id {
1076 Ok(node_id) => node_id,
1077 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1080 // Now that we know the label we're going to,
1081 // look it up in the continue loop nodes table
1083 match self.cont_ln.get(&sc) {
1085 None => span_bug!(expr.span, "continue to unknown label")
1089 hir::ExprKind::Assign(ref l, ref r) => {
1090 // see comment on places in
1091 // propagate_through_place_components()
1092 let succ = self.write_place(&l, succ, ACC_WRITE);
1093 let succ = self.propagate_through_place_components(&l, succ);
1094 self.propagate_through_expr(&r, succ)
1097 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1098 // an overloaded assign op is like a method call
1099 if self.tables.is_method_call(expr) {
1100 let succ = self.propagate_through_expr(&l, succ);
1101 self.propagate_through_expr(&r, succ)
1103 // see comment on places in
1104 // propagate_through_place_components()
1105 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1106 let succ = self.propagate_through_expr(&r, succ);
1107 self.propagate_through_place_components(&l, succ)
1111 // Uninteresting cases: just propagate in rev exec order
1113 hir::ExprKind::Array(ref exprs) => {
1114 self.propagate_through_exprs(exprs, succ)
1117 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1118 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1119 fields.iter().rev().fold(succ, |succ, field| {
1120 self.propagate_through_expr(&field.expr, succ)
1124 hir::ExprKind::Call(ref f, ref args) => {
1125 // FIXME(canndrew): This is_never should really be an is_uninhabited
1126 let succ = if self.tables.expr_ty(expr).is_never() {
1131 let succ = self.propagate_through_exprs(args, succ);
1132 self.propagate_through_expr(&f, succ)
1135 hir::ExprKind::MethodCall(.., ref args) => {
1136 // FIXME(canndrew): This is_never should really be an is_uninhabited
1137 let succ = if self.tables.expr_ty(expr).is_never() {
1142 self.propagate_through_exprs(args, succ)
1145 hir::ExprKind::Tup(ref exprs) => {
1146 self.propagate_through_exprs(exprs, succ)
1149 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1150 let r_succ = self.propagate_through_expr(&r, succ);
1152 let ln = self.live_node(expr.hir_id, expr.span);
1153 self.init_from_succ(ln, succ);
1154 self.merge_from_succ(ln, r_succ, false);
1156 self.propagate_through_expr(&l, ln)
1159 hir::ExprKind::Index(ref l, ref r) |
1160 hir::ExprKind::Binary(_, ref l, ref r) => {
1161 let r_succ = self.propagate_through_expr(&r, succ);
1162 self.propagate_through_expr(&l, r_succ)
1165 hir::ExprKind::Box(ref e) |
1166 hir::ExprKind::AddrOf(_, ref e) |
1167 hir::ExprKind::Cast(ref e, _) |
1168 hir::ExprKind::Type(ref e, _) |
1169 hir::ExprKind::Unary(_, ref e) |
1170 hir::ExprKind::Yield(ref e) |
1171 hir::ExprKind::Repeat(ref e, _) => {
1172 self.propagate_through_expr(&e, succ)
1175 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1176 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1177 // see comment on places
1178 // in propagate_through_place_components()
1180 self.propagate_through_expr(output, succ)
1182 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1183 let succ = self.write_place(output, succ, acc);
1184 self.propagate_through_place_components(output, succ)
1188 // Inputs are executed first. Propagate last because of rev order
1189 self.propagate_through_exprs(inputs, succ)
1192 hir::ExprKind::Lit(..) | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
1196 // Note that labels have been resolved, so we don't need to look
1197 // at the label ident
1198 hir::ExprKind::Block(ref blk, _) => {
1199 self.propagate_through_block(&blk, succ)
1204 fn propagate_through_place_components(&mut self,
1210 // In general, the full flow graph structure for an
1211 // assignment/move/etc can be handled in one of two ways,
1212 // depending on whether what is being assigned is a "tracked
1213 // value" or not. A tracked value is basically a local
1214 // variable or argument.
1216 // The two kinds of graphs are:
1218 // Tracked place Untracked place
1219 // ----------------------++-----------------------
1223 // (rvalue) || (rvalue)
1226 // (write of place) || (place components)
1231 // ----------------------++-----------------------
1233 // I will cover the two cases in turn:
1237 // A tracked place is a local variable/argument `x`. In
1238 // these cases, the link_node where the write occurs is linked
1239 // to node id of `x`. The `write_place()` routine generates
1240 // the contents of this node. There are no subcomponents to
1243 // # Non-tracked places
1245 // These are places like `x[5]` or `x.f`. In that case, we
1246 // basically ignore the value which is written to but generate
1247 // reads for the components---`x` in these two examples. The
1248 // components reads are generated by
1249 // `propagate_through_place_components()` (this fn).
1253 // It is still possible to observe assignments to non-places;
1254 // these errors are detected in the later pass borrowck. We
1255 // just ignore such cases and treat them as reads.
1258 hir::ExprKind::Path(_) => succ,
1259 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1260 _ => self.propagate_through_expr(expr, succ)
1264 // see comment on propagate_through_place()
1265 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1268 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1269 self.access_path(expr.hir_id, path, succ, acc)
1272 // We do not track other places, so just propagate through
1273 // to their subcomponents. Also, it may happen that
1274 // non-places occur here, because those are detected in the
1275 // later pass borrowck.
1280 fn access_var(&mut self, hir_id: HirId, nid: NodeId, succ: LiveNode, acc: u32, span: Span)
1282 let ln = self.live_node(hir_id, span);
1284 self.init_from_succ(ln, succ);
1285 let var_hid = self.ir.tcx.hir.node_to_hir_id(nid);
1286 let var = self.variable(var_hid, span);
1287 self.acc(ln, var, acc);
1292 fn access_path(&mut self, hir_id: HirId, path: &hir::Path, succ: LiveNode, acc: u32)
1295 Def::Local(nid) => {
1296 self.access_var(hir_id, nid, succ, acc, path.span)
1302 fn propagate_through_loop(&mut self,
1311 We model control flow like this:
1329 let mut first_merge = true;
1330 let ln = self.live_node(expr.hir_id, expr.span);
1331 self.init_empty(ln, succ);
1335 // If this is not a `loop` loop, then it's possible we bypass
1336 // the body altogether. Otherwise, the only way is via a `break`
1337 // in the loop body.
1338 self.merge_from_succ(ln, succ, first_merge);
1339 first_merge = false;
1342 debug!("propagate_through_loop: using id for loop body {} {}",
1343 expr.id, self.ir.tcx.hir.node_to_pretty_string(body.id));
1345 let break_ln = succ;
1347 self.break_ln.insert(expr.id, break_ln);
1348 self.cont_ln.insert(expr.id, cont_ln);
1350 let cond_ln = match kind {
1352 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1354 let body_ln = self.propagate_through_block(body, cond_ln);
1356 // repeat until fixed point is reached:
1357 while self.merge_from_succ(ln, body_ln, first_merge) {
1358 first_merge = false;
1360 let new_cond_ln = match kind {
1362 WhileLoop(ref cond) => {
1363 self.propagate_through_expr(&cond, ln)
1366 assert!(cond_ln == new_cond_ln);
1367 assert!(body_ln == self.propagate_through_block(body, cond_ln));
1374 // _______________________________________________________________________
1375 // Checking for error conditions
1377 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1378 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1379 NestedVisitorMap::None
1382 fn visit_local(&mut self, l: &'tcx hir::Local) {
1383 check_local(self, l);
1385 fn visit_expr(&mut self, ex: &'tcx Expr) {
1386 check_expr(self, ex);
1388 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1393 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1396 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1399 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1400 let span = local.pat.simple_ident().map_or(sp, |ident| ident.span);
1401 this.warn_about_unused(span, id, ln, var);
1406 intravisit::walk_local(this, local);
1409 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1410 // only consider the first pattern; any later patterns must have
1411 // the same bindings, and we also consider the first pattern to be
1412 // the "authoritative" set of ids
1413 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1414 this.warn_about_unused(sp, id, ln, var);
1416 intravisit::walk_arm(this, arm);
1419 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1421 hir::ExprKind::Assign(ref l, _) => {
1422 this.check_place(&l);
1424 intravisit::walk_expr(this, expr);
1427 hir::ExprKind::AssignOp(_, ref l, _) => {
1428 if !this.tables.is_method_call(expr) {
1429 this.check_place(&l);
1432 intravisit::walk_expr(this, expr);
1435 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1436 for input in inputs {
1437 this.visit_expr(input);
1440 // Output operands must be places
1441 for (o, output) in ia.outputs.iter().zip(outputs) {
1443 this.check_place(output);
1445 this.visit_expr(output);
1448 intravisit::walk_expr(this, expr);
1451 // no correctness conditions related to liveness
1452 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) | hir::ExprKind::If(..) |
1453 hir::ExprKind::Match(..) | hir::ExprKind::While(..) | hir::ExprKind::Loop(..) |
1454 hir::ExprKind::Index(..) | hir::ExprKind::Field(..) |
1455 hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) |
1456 hir::ExprKind::Cast(..) | hir::ExprKind::Unary(..) | hir::ExprKind::Ret(..) |
1457 hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) | hir::ExprKind::Lit(_) |
1458 hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) |
1459 hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) |
1460 hir::ExprKind::Closure(..) | hir::ExprKind::Path(_) | hir::ExprKind::Yield(..) |
1461 hir::ExprKind::Box(..) | hir::ExprKind::Type(..) => {
1462 intravisit::walk_expr(this, expr);
1467 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1468 fn check_place(&mut self, expr: &'tcx Expr) {
1470 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1471 if let Def::Local(nid) = path.def {
1472 // Assignment to an immutable variable or argument: only legal
1473 // if there is no later assignment. If this local is actually
1474 // mutable, then check for a reassignment to flag the mutability
1476 let ln = self.live_node(expr.hir_id, expr.span);
1477 let var_hid = self.ir.tcx.hir.node_to_hir_id(nid);
1478 let var = self.variable(var_hid, expr.span);
1479 self.warn_about_dead_assign(expr.span, expr.hir_id, ln, var);
1483 // For other kinds of places, no checks are required,
1484 // and any embedded expressions are actually rvalues
1485 intravisit::walk_expr(self, expr);
1490 fn should_warn(&self, var: Variable) -> Option<String> {
1491 let name = self.ir.variable_name(var);
1492 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1499 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1500 for arg in &body.arguments {
1501 arg.pat.each_binding(|_bm, hir_id, _, ident| {
1502 let sp = ident.span;
1503 let var = self.variable(hir_id, sp);
1504 // Ignore unused self.
1505 if ident.name != keywords::SelfValue.name() {
1506 if !self.warn_about_unused(sp, hir_id, entry_ln, var) {
1507 if self.live_on_entry(entry_ln, var).is_none() {
1508 self.report_dead_assign(hir_id, sp, var, true);
1516 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1517 self.pat_bindings(pat, |this, ln, var, sp, id| {
1518 if !this.warn_about_unused(sp, id, ln, var) {
1519 this.warn_about_dead_assign(sp, id, ln, var);
1524 fn warn_about_unused(&self,
1530 if !self.used_on_entry(ln, var) {
1531 let r = self.should_warn(var);
1532 if let Some(name) = r {
1534 // annoying: for parameters in funcs like `fn(x: i32)
1535 // {ret}`, there is only one node, so asking about
1536 // assigned_on_exit() is not meaningful.
1537 let is_assigned = if ln == self.s.exit_ln {
1540 self.assigned_on_exit(ln, var).is_some()
1543 let suggest_underscore_msg = format!("consider using `_{}` instead",
1548 .lint_hir_note(lint::builtin::UNUSED_VARIABLES, hir_id, sp,
1549 &format!("variable `{}` is assigned to, but never used",
1551 &suggest_underscore_msg);
1552 } else if name != "self" {
1553 let msg = format!("unused variable: `{}`", name);
1554 let mut err = self.ir.tcx
1555 .struct_span_lint_hir(lint::builtin::UNUSED_VARIABLES, hir_id, sp, &msg);
1556 if self.ir.variable_is_shorthand(var) {
1557 err.span_suggestion_with_applicability(sp, "try ignoring the field",
1558 format!("{}: _", name),
1559 Applicability::MachineApplicable);
1561 err.span_suggestion_short_with_applicability(
1562 sp, &suggest_underscore_msg,
1563 format!("_{}", name),
1564 Applicability::MachineApplicable,
1576 fn warn_about_dead_assign(&self,
1581 if self.live_on_exit(ln, var).is_none() {
1582 self.report_dead_assign(hir_id, sp, var, false);
1586 fn report_dead_assign(&self, hir_id: HirId, sp: Span, var: Variable, is_argument: bool) {
1587 if let Some(name) = self.should_warn(var) {
1589 self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1590 &format!("value passed to `{}` is never read", name));
1592 self.ir.tcx.lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1593 &format!("value assigned to `{}` is never read", name));