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::*;
113 use ty::{self, TyCtxt, ParameterEnvironment};
114 use traits::{self, Reveal};
115 use ty::subst::Subst;
117 use util::nodemap::NodeMap;
119 use std::{fmt, usize};
120 use std::io::prelude::*;
123 use syntax::ast::{self, NodeId};
124 use syntax::symbol::keywords;
125 use syntax_pos::Span;
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(usize);
142 #[derive(Copy, PartialEq)]
143 struct LiveNode(usize);
146 fn get(&self) -> usize { let Variable(v) = *self; v }
150 fn get(&self) -> usize { let LiveNode(v) = *self; v }
153 impl Clone for LiveNode {
154 fn clone(&self) -> LiveNode {
159 #[derive(Copy, Clone, PartialEq, Debug)]
167 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt) -> String {
168 let cm = tcx.sess.codemap();
171 format!("Free var node [{}]", cm.span_to_string(s))
174 format!("Expr node [{}]", cm.span_to_string(s))
177 format!("Var def node [{}]", cm.span_to_string(s))
179 ExitNode => "Exit node".to_string(),
183 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
184 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
185 NestedVisitorMap::OnlyBodies(&self.tcx.hir)
188 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
189 b: hir::BodyId, s: Span, id: NodeId) {
190 visit_fn(self, fk, fd, b, s, id);
192 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
193 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
194 fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
197 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
198 tcx.hir.krate().visit_all_item_likes(&mut IrMaps::new(tcx).as_deep_visitor());
199 tcx.sess.abort_if_errors();
202 impl fmt::Debug for LiveNode {
203 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
204 write!(f, "ln({})", self.get())
208 impl fmt::Debug for Variable {
209 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
210 write!(f, "v({})", self.get())
214 // ______________________________________________________________________
217 // This is the first pass and the one that drives the main
218 // computation. It walks up and down the IR once. On the way down,
219 // we count for each function the number of variables as well as
220 // liveness nodes. A liveness node is basically an expression or
221 // capture clause that does something of interest: either it has
222 // interesting control flow or it uses/defines a local variable.
224 // On the way back up, at each function node we create liveness sets
225 // (we now know precisely how big to make our various vectors and so
226 // forth) and then do the data-flow propagation to compute the set
227 // of live variables at each program point.
229 // Finally, we run back over the IR one last time and, using the
230 // computed liveness, check various safety conditions. For example,
231 // there must be no live nodes at the definition site for a variable
232 // unless it has an initializer. Similarly, each non-mutable local
233 // variable must not be assigned if there is some successor
234 // assignment. And so forth.
237 fn is_valid(&self) -> bool {
238 self.get() != usize::MAX
242 fn invalid_node() -> LiveNode { LiveNode(usize::MAX) }
249 #[derive(Copy, Clone, Debug)]
255 #[derive(Copy, Clone, Debug)]
257 Arg(NodeId, ast::Name),
263 struct IrMaps<'a, 'tcx: 'a> {
264 tcx: TyCtxt<'a, 'tcx, 'tcx>,
266 num_live_nodes: usize,
268 live_node_map: NodeMap<LiveNode>,
269 variable_map: NodeMap<Variable>,
270 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
271 var_kinds: Vec<VarKind>,
272 lnks: Vec<LiveNodeKind>,
275 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
276 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
281 live_node_map: NodeMap(),
282 variable_map: NodeMap(),
283 capture_info_map: NodeMap(),
284 var_kinds: Vec::new(),
289 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
290 let ln = LiveNode(self.num_live_nodes);
292 self.num_live_nodes += 1;
294 debug!("{:?} is of kind {}", ln,
295 live_node_kind_to_string(lnk, self.tcx));
300 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
301 let ln = self.add_live_node(lnk);
302 self.live_node_map.insert(node_id, ln);
304 debug!("{:?} is node {}", ln, node_id);
307 fn add_variable(&mut self, vk: VarKind) -> Variable {
308 let v = Variable(self.num_vars);
309 self.var_kinds.push(vk);
313 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
314 self.variable_map.insert(node_id, v);
316 ImplicitRet | CleanExit => {}
319 debug!("{:?} is {:?}", v, vk);
324 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
325 match self.variable_map.get(&node_id) {
328 span_bug!(span, "no variable registered for id {}", node_id);
333 fn variable_name(&self, var: Variable) -> String {
334 match self.var_kinds[var.get()] {
335 Local(LocalInfo { name, .. }) | Arg(_, name) => {
338 ImplicitRet => "<implicit-ret>".to_string(),
339 CleanExit => "<clean-exit>".to_string()
343 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
344 self.capture_info_map.insert(node_id, Rc::new(cs));
347 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
352 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
354 decl: &'tcx hir::FnDecl,
355 body_id: hir::BodyId,
360 // swap in a new set of IR maps for this function body:
361 let mut fn_maps = IrMaps::new(ir.tcx);
363 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
365 let body = ir.tcx.hir.body(body_id);
367 for arg in &body.arguments {
368 arg.pat.each_binding(|_bm, arg_id, _x, path1| {
369 debug!("adding argument {}", arg_id);
370 let name = path1.node;
371 fn_maps.add_variable(Arg(arg_id, name));
375 // gather up the various local variables, significant expressions,
377 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
380 let mut lsets = Liveness::new(&mut fn_maps, body_id);
381 let entry_ln = lsets.compute(&body.value);
383 // check for various error conditions
384 lsets.visit_body(body);
385 lsets.check_ret(id, sp, entry_ln, body);
386 lsets.warn_about_unused_args(body, entry_ln);
389 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
390 local.pat.each_binding(|_, p_id, sp, path1| {
391 debug!("adding local variable {}", p_id);
392 let name = path1.node;
393 ir.add_live_node_for_node(p_id, VarDefNode(sp));
394 ir.add_variable(Local(LocalInfo {
399 intravisit::walk_local(ir, local);
402 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
403 for pat in &arm.pats {
404 pat.each_binding(|bm, p_id, sp, path1| {
405 debug!("adding local variable {} from match with bm {:?}",
407 let name = path1.node;
408 ir.add_live_node_for_node(p_id, VarDefNode(sp));
409 ir.add_variable(Local(LocalInfo {
415 intravisit::walk_arm(ir, arm);
418 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
420 // live nodes required for uses or definitions of variables:
421 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
422 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
423 if let Def::Local(..) = path.def {
424 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
426 intravisit::walk_expr(ir, expr);
428 hir::ExprClosure(..) => {
429 // Interesting control flow (for loops can contain labeled
430 // breaks or continues)
431 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
433 // Make a live_node for each captured variable, with the span
434 // being the location that the variable is used. This results
435 // in better error messages than just pointing at the closure
436 // construction site.
437 let mut call_caps = Vec::new();
438 ir.tcx.with_freevars(expr.id, |freevars| {
440 if let Def::Local(def_id) = fv.def {
441 let rv = ir.tcx.hir.as_local_node_id(def_id).unwrap();
442 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
443 call_caps.push(CaptureInfo {ln: fv_ln,
448 ir.set_captures(expr.id, call_caps);
450 intravisit::walk_expr(ir, expr);
453 // live nodes required for interesting control flow:
454 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
455 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
456 intravisit::walk_expr(ir, expr);
458 hir::ExprBinary(op, ..) if op.node.is_lazy() => {
459 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
460 intravisit::walk_expr(ir, expr);
463 // otherwise, live nodes are not required:
464 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
465 hir::ExprArray(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
466 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
467 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(..) |
468 hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) |
469 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
470 hir::ExprStruct(..) | hir::ExprRepeat(..) |
471 hir::ExprInlineAsm(..) | hir::ExprBox(..) |
472 hir::ExprType(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
473 intravisit::walk_expr(ir, expr);
478 // ______________________________________________________________________
479 // Computing liveness sets
481 // Actually we compute just a bit more than just liveness, but we use
482 // the same basic propagation framework in all cases.
484 #[derive(Clone, Copy)]
491 fn invalid_users() -> Users {
493 reader: invalid_node(),
494 writer: invalid_node(),
499 #[derive(Copy, Clone)]
502 fallthrough_ln: LiveNode,
503 no_ret_var: Variable,
504 clean_exit_var: Variable
507 const ACC_READ: u32 = 1;
508 const ACC_WRITE: u32 = 2;
509 const ACC_USE: u32 = 4;
511 struct Liveness<'a, 'tcx: 'a> {
512 ir: &'a mut IrMaps<'a, 'tcx>,
513 tables: &'a ty::TypeckTables<'tcx>,
515 successors: Vec<LiveNode>,
518 // mappings from loop node ID to LiveNode
519 // ("break" label should map to loop node ID,
520 // it probably doesn't now)
521 break_ln: NodeMap<LiveNode>,
522 cont_ln: NodeMap<LiveNode>,
524 // mappings from node ID to LiveNode for "breakable" blocks-- currently only `catch {...}`
525 breakable_block_ln: NodeMap<LiveNode>,
528 impl<'a, 'tcx> Liveness<'a, 'tcx> {
529 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
530 // Special nodes and variables:
531 // - exit_ln represents the end of the fn, either by return or panic
532 // - implicit_ret_var is a pseudo-variable that represents
533 // an implicit return
534 let specials = Specials {
535 exit_ln: ir.add_live_node(ExitNode),
536 fallthrough_ln: ir.add_live_node(ExitNode),
537 no_ret_var: ir.add_variable(ImplicitRet),
538 clean_exit_var: ir.add_variable(CleanExit)
541 let tables = ir.tcx.body_tables(body);
543 let num_live_nodes = ir.num_live_nodes;
544 let num_vars = ir.num_vars;
550 successors: vec![invalid_node(); num_live_nodes],
551 users: vec![invalid_users(); num_live_nodes * num_vars],
554 breakable_block_ln: NodeMap(),
558 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
559 match self.ir.live_node_map.get(&node_id) {
562 // This must be a mismatch between the ir_map construction
563 // above and the propagation code below; the two sets of
564 // code have to agree about which AST nodes are worth
565 // creating liveness nodes for.
568 "no live node registered for node {}",
574 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
575 self.ir.variable(node_id, span)
578 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
579 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
581 pat.each_binding(|_bm, p_id, sp, _n| {
582 let ln = self.live_node(p_id, sp);
583 let var = self.variable(p_id, sp);
584 f(self, ln, var, sp, p_id);
588 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
589 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
591 if let Some(pat) = pat {
592 self.pat_bindings(pat, f);
596 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
598 self.define_bindings_in_arm_pats(Some(pat), succ)
601 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
604 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
605 this.init_from_succ(ln, succ);
606 this.define(ln, var);
612 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
613 ln.get() * self.ir.num_vars + var.get()
616 fn live_on_entry(&self, ln: LiveNode, var: Variable)
617 -> Option<LiveNodeKind> {
618 assert!(ln.is_valid());
619 let reader = self.users[self.idx(ln, var)].reader;
620 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
624 Is this variable live on entry to any of its successor nodes?
626 fn live_on_exit(&self, ln: LiveNode, var: Variable)
627 -> Option<LiveNodeKind> {
628 let successor = self.successors[ln.get()];
629 self.live_on_entry(successor, var)
632 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
633 assert!(ln.is_valid());
634 self.users[self.idx(ln, var)].used
637 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
638 -> Option<LiveNodeKind> {
639 assert!(ln.is_valid());
640 let writer = self.users[self.idx(ln, var)].writer;
641 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
644 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
645 -> Option<LiveNodeKind> {
646 let successor = self.successors[ln.get()];
647 self.assigned_on_entry(successor, var)
650 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
651 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
653 let node_base_idx = self.idx(ln, Variable(0));
654 let succ_base_idx = self.idx(succ_ln, Variable(0));
655 for var_idx in 0..self.ir.num_vars {
656 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
660 fn write_vars<F>(&self,
664 -> io::Result<()> where
665 F: FnMut(usize) -> LiveNode,
667 let node_base_idx = self.idx(ln, Variable(0));
668 for var_idx in 0..self.ir.num_vars {
669 let idx = node_base_idx + var_idx;
670 if test(idx).is_valid() {
671 write!(wr, " {:?}", Variable(var_idx))?;
678 #[allow(unused_must_use)]
679 fn ln_str(&self, ln: LiveNode) -> String {
680 let mut wr = Vec::new();
682 let wr = &mut wr as &mut Write;
683 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
684 self.write_vars(wr, ln, |idx| self.users[idx].reader);
685 write!(wr, " writes");
686 self.write_vars(wr, ln, |idx| self.users[idx].writer);
687 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
689 String::from_utf8(wr).unwrap()
692 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
693 self.successors[ln.get()] = succ_ln;
695 // It is not necessary to initialize the
696 // values to empty because this is the value
697 // they have when they are created, and the sets
698 // only grow during iterations.
700 // self.indices(ln) { |idx|
701 // self.users[idx] = invalid_users();
705 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
706 // more efficient version of init_empty() / merge_from_succ()
707 self.successors[ln.get()] = succ_ln;
709 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
710 this.users[idx] = this.users[succ_idx]
712 debug!("init_from_succ(ln={}, succ={})",
713 self.ln_str(ln), self.ln_str(succ_ln));
716 fn merge_from_succ(&mut self,
721 if ln == succ_ln { return false; }
723 let mut changed = false;
724 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
725 changed |= copy_if_invalid(this.users[succ_idx].reader,
726 &mut this.users[idx].reader);
727 changed |= copy_if_invalid(this.users[succ_idx].writer,
728 &mut this.users[idx].writer);
729 if this.users[succ_idx].used && !this.users[idx].used {
730 this.users[idx].used = true;
735 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
736 ln, self.ln_str(succ_ln), first_merge, changed);
739 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
740 if src.is_valid() && !dst.is_valid() {
749 // Indicates that a local variable was *defined*; we know that no
750 // uses of the variable can precede the definition (resolve checks
751 // this) so we just clear out all the data.
752 fn define(&mut self, writer: LiveNode, var: Variable) {
753 let idx = self.idx(writer, var);
754 self.users[idx].reader = invalid_node();
755 self.users[idx].writer = invalid_node();
757 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
758 idx, self.ln_str(writer));
761 // Either read, write, or both depending on the acc bitset
762 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
763 debug!("{:?} accesses[{:x}] {:?}: {}",
764 ln, acc, var, self.ln_str(ln));
766 let idx = self.idx(ln, var);
767 let user = &mut self.users[idx];
769 if (acc & ACC_WRITE) != 0 {
770 user.reader = invalid_node();
774 // Important: if we both read/write, must do read second
775 // or else the write will override.
776 if (acc & ACC_READ) != 0 {
780 if (acc & ACC_USE) != 0 {
785 // _______________________________________________________________________
787 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
788 // if there is a `break` or `again` at the top level, then it's
789 // effectively a return---this only occurs in `for` loops,
790 // where the body is really a closure.
792 debug!("compute: using id for body, {}", self.ir.tcx.hir.node_to_pretty_string(body.id));
794 let exit_ln = self.s.exit_ln;
796 self.break_ln.insert(body.id, exit_ln);
797 self.cont_ln.insert(body.id, exit_ln);
799 // the fallthrough exit is only for those cases where we do not
800 // explicitly return:
802 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
803 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
805 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
807 // hack to skip the loop unless debug! is enabled:
808 debug!("^^ liveness computation results for body {} (entry={:?})",
810 for ln_idx in 0..self.ir.num_live_nodes {
811 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
820 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
822 if blk.targeted_by_break {
823 self.breakable_block_ln.insert(blk.id, succ);
825 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
826 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
827 self.propagate_through_stmt(stmt, succ)
831 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
834 hir::StmtDecl(ref decl, _) => {
835 self.propagate_through_decl(&decl, succ)
838 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
839 self.propagate_through_expr(&expr, succ)
844 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
847 hir::DeclLocal(ref local) => {
848 self.propagate_through_local(&local, succ)
850 hir::DeclItem(_) => succ,
854 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
856 // Note: we mark the variable as defined regardless of whether
857 // there is an initializer. Initially I had thought to only mark
858 // the live variable as defined if it was initialized, and then we
859 // could check for uninit variables just by scanning what is live
860 // at the start of the function. But that doesn't work so well for
861 // immutable variables defined in a loop:
862 // loop { let x; x = 5; }
863 // because the "assignment" loops back around and generates an error.
865 // So now we just check that variables defined w/o an
866 // initializer are not live at the point of their
867 // initialization, which is mildly more complex than checking
868 // once at the func header but otherwise equivalent.
870 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
871 self.define_bindings_in_pat(&local.pat, succ)
874 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
876 exprs.iter().rev().fold(succ, |succ, expr| {
877 self.propagate_through_expr(&expr, succ)
881 fn propagate_through_opt_expr(&mut self,
882 opt_expr: Option<&Expr>,
885 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
888 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
890 debug!("propagate_through_expr: {}", self.ir.tcx.hir.node_to_pretty_string(expr.id));
893 // Interesting cases with control flow or which gen/kill
895 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
896 self.access_path(expr.id, path, succ, ACC_READ | ACC_USE)
899 hir::ExprField(ref e, _) => {
900 self.propagate_through_expr(&e, succ)
903 hir::ExprTupField(ref e, _) => {
904 self.propagate_through_expr(&e, succ)
907 hir::ExprClosure(.., blk_id, _) => {
908 debug!("{} is an ExprClosure", self.ir.tcx.hir.node_to_pretty_string(expr.id));
911 The next-node for a break is the successor of the entire
912 loop. The next-node for a continue is the top of this loop.
914 let node = self.live_node(expr.id, expr.span);
918 self.break_ln.insert(blk_id.node_id, break_ln);
919 self.cont_ln.insert(blk_id.node_id, cont_ln);
921 // the construction of a closure itself is not important,
922 // but we have to consider the closed over variables.
923 let caps = match self.ir.capture_info_map.get(&expr.id) {
924 Some(caps) => caps.clone(),
926 span_bug!(expr.span, "no registered caps");
929 caps.iter().rev().fold(succ, |succ, cap| {
930 self.init_from_succ(cap.ln, succ);
931 let var = self.variable(cap.var_nid, expr.span);
932 self.acc(cap.ln, var, ACC_READ | ACC_USE);
937 hir::ExprIf(ref cond, ref then, ref els) => {
951 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
952 let then_ln = self.propagate_through_expr(&then, succ);
953 let ln = self.live_node(expr.id, expr.span);
954 self.init_from_succ(ln, else_ln);
955 self.merge_from_succ(ln, then_ln, false);
956 self.propagate_through_expr(&cond, ln)
959 hir::ExprWhile(ref cond, ref blk, _) => {
960 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
963 // Note that labels have been resolved, so we don't need to look
964 // at the label ident
965 hir::ExprLoop(ref blk, _, _) => {
966 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
969 hir::ExprMatch(ref e, ref arms, _) => {
984 let ln = self.live_node(expr.id, expr.span);
985 self.init_empty(ln, succ);
986 let mut first_merge = true;
989 self.propagate_through_expr(&arm.body, succ);
991 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
992 // only consider the first pattern; any later patterns must have
993 // the same bindings, and we also consider the first pattern to be
994 // the "authoritative" set of ids
996 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
998 self.merge_from_succ(ln, arm_succ, first_merge);
1001 self.propagate_through_expr(&e, ln)
1004 hir::ExprRet(ref o_e) => {
1005 // ignore succ and subst exit_ln:
1006 let exit_ln = self.s.exit_ln;
1007 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1010 hir::ExprBreak(label, ref opt_expr) => {
1011 // Find which label this break jumps to
1012 let target = match label.target_id {
1013 hir::ScopeTarget::Block(node_id) =>
1014 self.breakable_block_ln.get(&node_id),
1015 hir::ScopeTarget::Loop(hir::LoopIdResult::Ok(node_id)) =>
1016 self.break_ln.get(&node_id),
1017 hir::ScopeTarget::Loop(hir::LoopIdResult::Err(err)) =>
1018 span_bug!(expr.span, "loop scope error: {}", err),
1021 // Now that we know the label we're going to,
1022 // look it up in the break loop nodes table
1025 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1026 None => span_bug!(expr.span, "break to unknown label")
1030 hir::ExprAgain(label) => {
1031 // Find which label this expr continues to
1032 let sc = match label.target_id {
1033 hir::ScopeTarget::Block(_) => bug!("can't `continue` to a non-loop block"),
1034 hir::ScopeTarget::Loop(hir::LoopIdResult::Ok(node_id)) => node_id,
1035 hir::ScopeTarget::Loop(hir::LoopIdResult::Err(err)) =>
1036 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 continue loop nodes table
1042 match self.cont_ln.get(&sc) {
1044 None => span_bug!(expr.span, "continue to unknown label")
1048 hir::ExprAssign(ref l, ref r) => {
1049 // see comment on lvalues in
1050 // propagate_through_lvalue_components()
1051 let succ = self.write_lvalue(&l, succ, ACC_WRITE);
1052 let succ = self.propagate_through_lvalue_components(&l, succ);
1053 self.propagate_through_expr(&r, succ)
1056 hir::ExprAssignOp(_, ref l, ref r) => {
1057 // an overloaded assign op is like a method call
1058 if self.tables.is_method_call(expr.id) {
1059 let succ = self.propagate_through_expr(&l, succ);
1060 self.propagate_through_expr(&r, succ)
1062 // see comment on lvalues in
1063 // propagate_through_lvalue_components()
1064 let succ = self.write_lvalue(&l, succ, ACC_WRITE|ACC_READ);
1065 let succ = self.propagate_through_expr(&r, succ);
1066 self.propagate_through_lvalue_components(&l, succ)
1070 // Uninteresting cases: just propagate in rev exec order
1072 hir::ExprArray(ref exprs) => {
1073 self.propagate_through_exprs(exprs, succ)
1076 hir::ExprStruct(_, ref fields, ref with_expr) => {
1077 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1078 fields.iter().rev().fold(succ, |succ, field| {
1079 self.propagate_through_expr(&field.expr, succ)
1083 hir::ExprCall(ref f, ref args) => {
1084 // FIXME(canndrew): This is_never should really be an is_uninhabited
1085 let diverges = !self.tables.is_method_call(expr.id) &&
1086 self.tables.expr_ty_adjusted(&f).fn_ret().0.is_never();
1087 let succ = if diverges {
1092 let succ = self.propagate_through_exprs(args, succ);
1093 self.propagate_through_expr(&f, succ)
1096 hir::ExprMethodCall(.., ref args) => {
1097 let method_call = ty::MethodCall::expr(expr.id);
1098 let method_ty = self.tables.method_map[&method_call].ty;
1099 // FIXME(canndrew): This is_never should really be an is_uninhabited
1100 let succ = if method_ty.fn_ret().0.is_never() {
1105 self.propagate_through_exprs(args, succ)
1108 hir::ExprTup(ref exprs) => {
1109 self.propagate_through_exprs(exprs, succ)
1112 hir::ExprBinary(op, ref l, ref r) if op.node.is_lazy() => {
1113 let r_succ = self.propagate_through_expr(&r, succ);
1115 let ln = self.live_node(expr.id, expr.span);
1116 self.init_from_succ(ln, succ);
1117 self.merge_from_succ(ln, r_succ, false);
1119 self.propagate_through_expr(&l, ln)
1122 hir::ExprIndex(ref l, ref r) |
1123 hir::ExprBinary(_, ref l, ref r) => {
1124 let r_succ = self.propagate_through_expr(&r, succ);
1125 self.propagate_through_expr(&l, r_succ)
1128 hir::ExprBox(ref e) |
1129 hir::ExprAddrOf(_, ref e) |
1130 hir::ExprCast(ref e, _) |
1131 hir::ExprType(ref e, _) |
1132 hir::ExprUnary(_, ref e) |
1133 hir::ExprRepeat(ref e, _) => {
1134 self.propagate_through_expr(&e, succ)
1137 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1138 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1139 // see comment on lvalues
1140 // in propagate_through_lvalue_components()
1142 self.propagate_through_expr(output, succ)
1144 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1145 let succ = self.write_lvalue(output, succ, acc);
1146 self.propagate_through_lvalue_components(output, succ)
1150 // Inputs are executed first. Propagate last because of rev order
1151 self.propagate_through_exprs(inputs, succ)
1154 hir::ExprLit(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
1158 hir::ExprBlock(ref blk) => {
1159 self.propagate_through_block(&blk, succ)
1164 fn propagate_through_lvalue_components(&mut self,
1170 // In general, the full flow graph structure for an
1171 // assignment/move/etc can be handled in one of two ways,
1172 // depending on whether what is being assigned is a "tracked
1173 // value" or not. A tracked value is basically a local
1174 // variable or argument.
1176 // The two kinds of graphs are:
1178 // Tracked lvalue Untracked lvalue
1179 // ----------------------++-----------------------
1183 // (rvalue) || (rvalue)
1186 // (write of lvalue) || (lvalue components)
1191 // ----------------------++-----------------------
1193 // I will cover the two cases in turn:
1195 // # Tracked lvalues
1197 // A tracked lvalue is a local variable/argument `x`. In
1198 // these cases, the link_node where the write occurs is linked
1199 // to node id of `x`. The `write_lvalue()` routine generates
1200 // the contents of this node. There are no subcomponents to
1203 // # Non-tracked lvalues
1205 // These are lvalues like `x[5]` or `x.f`. In that case, we
1206 // basically ignore the value which is written to but generate
1207 // reads for the components---`x` in these two examples. The
1208 // components reads are generated by
1209 // `propagate_through_lvalue_components()` (this fn).
1211 // # Illegal lvalues
1213 // It is still possible to observe assignments to non-lvalues;
1214 // these errors are detected in the later pass borrowck. We
1215 // just ignore such cases and treat them as reads.
1218 hir::ExprPath(_) => succ,
1219 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1220 hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ),
1221 _ => self.propagate_through_expr(expr, succ)
1225 // see comment on propagate_through_lvalue()
1226 fn write_lvalue(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1229 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1230 self.access_path(expr.id, path, succ, acc)
1233 // We do not track other lvalues, so just propagate through
1234 // to their subcomponents. Also, it may happen that
1235 // non-lvalues occur here, because those are detected in the
1236 // later pass borrowck.
1241 fn access_path(&mut self, id: NodeId, path: &hir::Path, succ: LiveNode, acc: u32)
1244 Def::Local(def_id) => {
1245 let nid = self.ir.tcx.hir.as_local_node_id(def_id).unwrap();
1246 let ln = self.live_node(id, path.span);
1248 self.init_from_succ(ln, succ);
1249 let var = self.variable(nid, path.span);
1250 self.acc(ln, var, acc);
1258 fn propagate_through_loop(&mut self,
1267 We model control flow like this:
1285 let mut first_merge = true;
1286 let ln = self.live_node(expr.id, expr.span);
1287 self.init_empty(ln, succ);
1291 // If this is not a `loop` loop, then it's possible we bypass
1292 // the body altogether. Otherwise, the only way is via a `break`
1293 // in the loop body.
1294 self.merge_from_succ(ln, succ, first_merge);
1295 first_merge = false;
1298 debug!("propagate_through_loop: using id for loop body {} {}",
1299 expr.id, self.ir.tcx.hir.node_to_pretty_string(body.id));
1301 let break_ln = succ;
1303 self.break_ln.insert(expr.id, break_ln);
1304 self.cont_ln.insert(expr.id, cont_ln);
1306 let cond_ln = match kind {
1308 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1310 let body_ln = self.propagate_through_block(body, cond_ln);
1312 // repeat until fixed point is reached:
1313 while self.merge_from_succ(ln, body_ln, first_merge) {
1314 first_merge = false;
1316 let new_cond_ln = match kind {
1318 WhileLoop(ref cond) => {
1319 self.propagate_through_expr(&cond, ln)
1322 assert!(cond_ln == new_cond_ln);
1323 assert!(body_ln == self.propagate_through_block(body, cond_ln));
1330 // _______________________________________________________________________
1331 // Checking for error conditions
1333 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1334 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1335 NestedVisitorMap::None
1338 fn visit_local(&mut self, l: &'tcx hir::Local) {
1339 check_local(self, l);
1341 fn visit_expr(&mut self, ex: &'tcx Expr) {
1342 check_expr(self, ex);
1344 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1349 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1352 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1355 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1356 this.warn_about_unused(sp, id, ln, var);
1361 intravisit::walk_local(this, local);
1364 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1365 // only consider the first pattern; any later patterns must have
1366 // the same bindings, and we also consider the first pattern to be
1367 // the "authoritative" set of ids
1368 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1369 this.warn_about_unused(sp, id, ln, var);
1371 intravisit::walk_arm(this, arm);
1374 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1376 hir::ExprAssign(ref l, _) => {
1377 this.check_lvalue(&l);
1379 intravisit::walk_expr(this, expr);
1382 hir::ExprAssignOp(_, ref l, _) => {
1383 if !this.tables.is_method_call(expr.id) {
1384 this.check_lvalue(&l);
1387 intravisit::walk_expr(this, expr);
1390 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1391 for input in inputs {
1392 this.visit_expr(input);
1395 // Output operands must be lvalues
1396 for (o, output) in ia.outputs.iter().zip(outputs) {
1398 this.check_lvalue(output);
1400 this.visit_expr(output);
1403 intravisit::walk_expr(this, expr);
1406 // no correctness conditions related to liveness
1407 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1408 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1409 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
1410 hir::ExprArray(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1411 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1412 hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) |
1413 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1414 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1415 hir::ExprClosure(..) | hir::ExprPath(_) |
1416 hir::ExprBox(..) | hir::ExprType(..) => {
1417 intravisit::walk_expr(this, expr);
1422 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1429 let fn_ty = self.ir.tcx.type_of(self.ir.tcx.hir.local_def_id(id));
1430 let fn_sig = match fn_ty.sty {
1431 ty::TyClosure(closure_def_id, substs) => {
1432 self.ir.tcx.closure_type(closure_def_id)
1433 .subst(self.ir.tcx, substs.substs)
1438 let fn_ret = fn_sig.output();
1440 // within the fn body, late-bound regions are liberated
1441 // and must outlive the *call-site* of the function.
1443 self.ir.tcx.liberate_late_bound_regions(
1444 Some(self.ir.tcx.region_maps().call_site_extent(id, body.value.id)),
1447 if !fn_ret.is_never() && self.live_on_entry(entry_ln, self.s.no_ret_var).is_some() {
1448 let param_env = ParameterEnvironment::for_item(self.ir.tcx, id);
1449 let t_ret_subst = fn_ret.subst(self.ir.tcx, ¶m_env.free_substs);
1450 let is_nil = self.ir.tcx.infer_ctxt(param_env, Reveal::All).enter(|infcx| {
1451 let cause = traits::ObligationCause::dummy();
1452 traits::fully_normalize(&infcx, cause, &t_ret_subst).unwrap().is_nil()
1455 // for nil return types, it is ok to not return a value expl.
1457 span_bug!(sp, "not all control paths return a value");
1462 fn check_lvalue(&mut self, expr: &'tcx Expr) {
1464 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1465 if let Def::Local(def_id) = path.def {
1466 // Assignment to an immutable variable or argument: only legal
1467 // if there is no later assignment. If this local is actually
1468 // mutable, then check for a reassignment to flag the mutability
1470 let nid = self.ir.tcx.hir.as_local_node_id(def_id).unwrap();
1471 let ln = self.live_node(expr.id, expr.span);
1472 let var = self.variable(nid, expr.span);
1473 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1477 // For other kinds of lvalues, no checks are required,
1478 // and any embedded expressions are actually rvalues
1479 intravisit::walk_expr(self, expr);
1484 fn should_warn(&self, var: Variable) -> Option<String> {
1485 let name = self.ir.variable_name(var);
1486 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1493 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1494 for arg in &body.arguments {
1495 arg.pat.each_binding(|_bm, p_id, sp, path1| {
1496 let var = self.variable(p_id, sp);
1497 // Ignore unused self.
1498 let name = path1.node;
1499 if name != keywords::SelfValue.name() {
1500 if !self.warn_about_unused(sp, p_id, entry_ln, var) {
1501 if self.live_on_entry(entry_ln, var).is_none() {
1502 self.report_dead_assign(p_id, sp, var, true);
1510 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1511 self.pat_bindings(pat, |this, ln, var, sp, id| {
1512 if !this.warn_about_unused(sp, id, ln, var) {
1513 this.warn_about_dead_assign(sp, id, ln, var);
1518 fn warn_about_unused(&self,
1524 if !self.used_on_entry(ln, var) {
1525 let r = self.should_warn(var);
1526 if let Some(name) = r {
1528 // annoying: for parameters in funcs like `fn(x: i32)
1529 // {ret}`, there is only one node, so asking about
1530 // assigned_on_exit() is not meaningful.
1531 let is_assigned = if ln == self.s.exit_ln {
1534 self.assigned_on_exit(ln, var).is_some()
1538 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1539 format!("variable `{}` is assigned to, but never used",
1541 } else if name != "self" {
1542 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_VARIABLES, id, sp,
1543 format!("unused variable: `{}`", name));
1552 fn warn_about_dead_assign(&self,
1557 if self.live_on_exit(ln, var).is_none() {
1558 self.report_dead_assign(id, sp, var, false);
1562 fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) {
1563 if let Some(name) = self.should_warn(var) {
1565 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1566 format!("value passed to `{}` is never read", name));
1568 self.ir.tcx.sess.add_lint(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1569 format!("value assigned to `{}` is never read", name));