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 //! - `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 util::nodemap::NodeMap;
114 use std::{fmt, usize};
115 use std::io::prelude::*;
118 use syntax::ast::{self, NodeId};
119 use syntax::symbol::keywords;
120 use syntax_pos::Span;
124 use hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
126 /// For use with `propagate_through_loop`.
128 /// An endless `loop` loop.
130 /// A `while` loop, with the given expression as condition.
134 #[derive(Copy, Clone, PartialEq)]
135 struct Variable(usize);
137 #[derive(Copy, PartialEq)]
138 struct LiveNode(usize);
141 fn get(&self) -> usize { let Variable(v) = *self; v }
145 fn get(&self) -> usize { let LiveNode(v) = *self; v }
148 impl Clone for LiveNode {
149 fn clone(&self) -> LiveNode {
154 #[derive(Copy, Clone, PartialEq, Debug)]
162 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt) -> String {
163 let cm = tcx.sess.codemap();
166 format!("Free var node [{}]", cm.span_to_string(s))
169 format!("Expr node [{}]", cm.span_to_string(s))
172 format!("Var def node [{}]", cm.span_to_string(s))
174 ExitNode => "Exit node".to_string(),
178 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
179 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
180 NestedVisitorMap::OnlyBodies(&self.tcx.hir)
183 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
184 b: hir::BodyId, s: Span, id: NodeId) {
185 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 {
233 self.get() != usize::MAX
237 fn invalid_node() -> LiveNode { LiveNode(usize::MAX) }
244 #[derive(Copy, Clone, Debug)]
250 #[derive(Copy, Clone, Debug)]
252 Arg(NodeId, ast::Name),
257 struct IrMaps<'a, 'tcx: 'a> {
258 tcx: TyCtxt<'a, 'tcx, 'tcx>,
260 num_live_nodes: usize,
262 live_node_map: NodeMap<LiveNode>,
263 variable_map: NodeMap<Variable>,
264 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
265 var_kinds: Vec<VarKind>,
266 lnks: Vec<LiveNodeKind>,
269 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
270 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
275 live_node_map: NodeMap(),
276 variable_map: NodeMap(),
277 capture_info_map: NodeMap(),
278 var_kinds: Vec::new(),
283 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
284 let ln = LiveNode(self.num_live_nodes);
286 self.num_live_nodes += 1;
288 debug!("{:?} is of kind {}", ln,
289 live_node_kind_to_string(lnk, self.tcx));
294 fn add_live_node_for_node(&mut self, node_id: NodeId, lnk: LiveNodeKind) {
295 let ln = self.add_live_node(lnk);
296 self.live_node_map.insert(node_id, ln);
298 debug!("{:?} is node {}", ln, node_id);
301 fn add_variable(&mut self, vk: VarKind) -> Variable {
302 let v = Variable(self.num_vars);
303 self.var_kinds.push(vk);
307 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
308 self.variable_map.insert(node_id, v);
313 debug!("{:?} is {:?}", v, vk);
318 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
319 match self.variable_map.get(&node_id) {
322 span_bug!(span, "no variable registered for id {}", node_id);
327 fn variable_name(&self, var: Variable) -> String {
328 match self.var_kinds[var.get()] {
329 Local(LocalInfo { name, .. }) | Arg(_, name) => {
332 CleanExit => "<clean-exit>".to_string()
336 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
337 self.capture_info_map.insert(node_id, Rc::new(cs));
340 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
345 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
347 decl: &'tcx hir::FnDecl,
348 body_id: hir::BodyId,
353 // swap in a new set of IR maps for this function body:
354 let mut fn_maps = IrMaps::new(ir.tcx);
356 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps);
358 let body = ir.tcx.hir.body(body_id);
360 for arg in &body.arguments {
361 arg.pat.each_binding(|_bm, arg_id, _x, path1| {
362 debug!("adding argument {}", arg_id);
363 let name = path1.node;
364 fn_maps.add_variable(Arg(arg_id, name));
368 // gather up the various local variables, significant expressions,
370 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
373 let mut lsets = Liveness::new(&mut fn_maps, body_id);
374 let entry_ln = lsets.compute(&body.value);
376 // check for various error conditions
377 lsets.visit_body(body);
378 lsets.warn_about_unused_args(body, entry_ln);
381 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
382 local.pat.each_binding(|_, p_id, sp, path1| {
383 debug!("adding local variable {}", p_id);
384 let name = path1.node;
385 ir.add_live_node_for_node(p_id, VarDefNode(sp));
386 ir.add_variable(Local(LocalInfo {
391 intravisit::walk_local(ir, local);
394 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
395 for pat in &arm.pats {
396 pat.each_binding(|bm, p_id, sp, path1| {
397 debug!("adding local variable {} from match with bm {:?}",
399 let name = path1.node;
400 ir.add_live_node_for_node(p_id, VarDefNode(sp));
401 ir.add_variable(Local(LocalInfo {
407 intravisit::walk_arm(ir, arm);
410 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
412 // live nodes required for uses or definitions of variables:
413 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
414 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
415 if let Def::Local(..) = path.def {
416 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
418 intravisit::walk_expr(ir, expr);
420 hir::ExprClosure(..) => {
421 // Interesting control flow (for loops can contain labeled
422 // breaks or continues)
423 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
425 // Make a live_node for each captured variable, with the span
426 // being the location that the variable is used. This results
427 // in better error messages than just pointing at the closure
428 // construction site.
429 let mut call_caps = Vec::new();
430 ir.tcx.with_freevars(expr.id, |freevars| {
432 if let Def::Local(rv) = fv.def {
433 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
434 call_caps.push(CaptureInfo {ln: fv_ln,
439 ir.set_captures(expr.id, call_caps);
441 intravisit::walk_expr(ir, expr);
444 // live nodes required for interesting control flow:
445 hir::ExprIf(..) | hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) => {
446 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
447 intravisit::walk_expr(ir, expr);
449 hir::ExprBinary(op, ..) if op.node.is_lazy() => {
450 ir.add_live_node_for_node(expr.id, ExprNode(expr.span));
451 intravisit::walk_expr(ir, expr);
454 // otherwise, live nodes are not required:
455 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
456 hir::ExprArray(..) | hir::ExprCall(..) | hir::ExprMethodCall(..) |
457 hir::ExprTup(..) | hir::ExprBinary(..) | hir::ExprAddrOf(..) |
458 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprBreak(..) |
459 hir::ExprAgain(_) | hir::ExprLit(_) | hir::ExprRet(..) |
460 hir::ExprBlock(..) | hir::ExprAssign(..) | hir::ExprAssignOp(..) |
461 hir::ExprStruct(..) | hir::ExprRepeat(..) |
462 hir::ExprInlineAsm(..) | hir::ExprBox(..) | hir::ExprYield(..) |
463 hir::ExprType(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
464 intravisit::walk_expr(ir, expr);
469 // ______________________________________________________________________
470 // Computing liveness sets
472 // Actually we compute just a bit more than just liveness, but we use
473 // the same basic propagation framework in all cases.
475 #[derive(Clone, Copy)]
482 fn invalid_users() -> Users {
484 reader: invalid_node(),
485 writer: invalid_node(),
490 #[derive(Copy, Clone)]
493 fallthrough_ln: LiveNode,
494 clean_exit_var: Variable
497 const ACC_READ: u32 = 1;
498 const ACC_WRITE: u32 = 2;
499 const ACC_USE: u32 = 4;
501 struct Liveness<'a, 'tcx: 'a> {
502 ir: &'a mut IrMaps<'a, 'tcx>,
503 tables: &'a ty::TypeckTables<'tcx>,
505 successors: Vec<LiveNode>,
508 // mappings from loop node ID to LiveNode
509 // ("break" label should map to loop node ID,
510 // it probably doesn't now)
511 break_ln: NodeMap<LiveNode>,
512 cont_ln: NodeMap<LiveNode>,
514 // mappings from node ID to LiveNode for "breakable" blocks-- currently only `catch {...}`
515 breakable_block_ln: NodeMap<LiveNode>,
518 impl<'a, 'tcx> Liveness<'a, 'tcx> {
519 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
520 // Special nodes and variables:
521 // - exit_ln represents the end of the fn, either by return or panic
522 // - implicit_ret_var is a pseudo-variable that represents
523 // an implicit return
524 let specials = Specials {
525 exit_ln: ir.add_live_node(ExitNode),
526 fallthrough_ln: ir.add_live_node(ExitNode),
527 clean_exit_var: ir.add_variable(CleanExit)
530 let tables = ir.tcx.body_tables(body);
532 let num_live_nodes = ir.num_live_nodes;
533 let num_vars = ir.num_vars;
539 successors: vec![invalid_node(); num_live_nodes],
540 users: vec![invalid_users(); num_live_nodes * num_vars],
543 breakable_block_ln: NodeMap(),
547 fn live_node(&self, node_id: NodeId, span: Span) -> LiveNode {
548 match self.ir.live_node_map.get(&node_id) {
551 // This must be a mismatch between the ir_map construction
552 // above and the propagation code below; the two sets of
553 // code have to agree about which AST nodes are worth
554 // creating liveness nodes for.
557 "no live node registered for node {}",
563 fn variable(&self, node_id: NodeId, span: Span) -> Variable {
564 self.ir.variable(node_id, span)
567 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
568 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
570 pat.each_binding(|_bm, p_id, sp, _n| {
571 let ln = self.live_node(p_id, sp);
572 let var = self.variable(p_id, sp);
573 f(self, ln, var, sp, p_id);
577 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
578 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, NodeId),
580 if let Some(pat) = pat {
581 self.pat_bindings(pat, f);
585 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
587 self.define_bindings_in_arm_pats(Some(pat), succ)
590 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
593 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
594 this.init_from_succ(ln, succ);
595 this.define(ln, var);
601 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
602 ln.get() * self.ir.num_vars + var.get()
605 fn live_on_entry(&self, ln: LiveNode, var: Variable)
606 -> Option<LiveNodeKind> {
607 assert!(ln.is_valid());
608 let reader = self.users[self.idx(ln, var)].reader;
609 if reader.is_valid() {Some(self.ir.lnk(reader))} else {None}
613 Is this variable live on entry to any of its successor nodes?
615 fn live_on_exit(&self, ln: LiveNode, var: Variable)
616 -> Option<LiveNodeKind> {
617 let successor = self.successors[ln.get()];
618 self.live_on_entry(successor, var)
621 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
622 assert!(ln.is_valid());
623 self.users[self.idx(ln, var)].used
626 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
627 -> Option<LiveNodeKind> {
628 assert!(ln.is_valid());
629 let writer = self.users[self.idx(ln, var)].writer;
630 if writer.is_valid() {Some(self.ir.lnk(writer))} else {None}
633 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
634 -> Option<LiveNodeKind> {
635 let successor = self.successors[ln.get()];
636 self.assigned_on_entry(successor, var)
639 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
640 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
642 let node_base_idx = self.idx(ln, Variable(0));
643 let succ_base_idx = self.idx(succ_ln, Variable(0));
644 for var_idx in 0..self.ir.num_vars {
645 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
649 fn write_vars<F>(&self,
653 -> io::Result<()> where
654 F: FnMut(usize) -> LiveNode,
656 let node_base_idx = self.idx(ln, Variable(0));
657 for var_idx in 0..self.ir.num_vars {
658 let idx = node_base_idx + var_idx;
659 if test(idx).is_valid() {
660 write!(wr, " {:?}", Variable(var_idx))?;
667 #[allow(unused_must_use)]
668 fn ln_str(&self, ln: LiveNode) -> String {
669 let mut wr = Vec::new();
671 let wr = &mut wr as &mut Write;
672 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
673 self.write_vars(wr, ln, |idx| self.users[idx].reader);
674 write!(wr, " writes");
675 self.write_vars(wr, ln, |idx| self.users[idx].writer);
676 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
678 String::from_utf8(wr).unwrap()
681 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
682 self.successors[ln.get()] = succ_ln;
684 // It is not necessary to initialize the
685 // values to empty because this is the value
686 // they have when they are created, and the sets
687 // only grow during iterations.
689 // self.indices(ln) { |idx|
690 // self.users[idx] = invalid_users();
694 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
695 // more efficient version of init_empty() / merge_from_succ()
696 self.successors[ln.get()] = succ_ln;
698 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
699 this.users[idx] = this.users[succ_idx]
701 debug!("init_from_succ(ln={}, succ={})",
702 self.ln_str(ln), self.ln_str(succ_ln));
705 fn merge_from_succ(&mut self,
710 if ln == succ_ln { return false; }
712 let mut changed = false;
713 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
714 changed |= copy_if_invalid(this.users[succ_idx].reader,
715 &mut this.users[idx].reader);
716 changed |= copy_if_invalid(this.users[succ_idx].writer,
717 &mut this.users[idx].writer);
718 if this.users[succ_idx].used && !this.users[idx].used {
719 this.users[idx].used = true;
724 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
725 ln, self.ln_str(succ_ln), first_merge, changed);
728 fn copy_if_invalid(src: LiveNode, dst: &mut LiveNode) -> bool {
729 if src.is_valid() && !dst.is_valid() {
738 // Indicates that a local variable was *defined*; we know that no
739 // uses of the variable can precede the definition (resolve checks
740 // this) so we just clear out all the data.
741 fn define(&mut self, writer: LiveNode, var: Variable) {
742 let idx = self.idx(writer, var);
743 self.users[idx].reader = invalid_node();
744 self.users[idx].writer = invalid_node();
746 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
747 idx, self.ln_str(writer));
750 // Either read, write, or both depending on the acc bitset
751 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
752 debug!("{:?} accesses[{:x}] {:?}: {}",
753 ln, acc, var, self.ln_str(ln));
755 let idx = self.idx(ln, var);
756 let user = &mut self.users[idx];
758 if (acc & ACC_WRITE) != 0 {
759 user.reader = invalid_node();
763 // Important: if we both read/write, must do read second
764 // or else the write will override.
765 if (acc & ACC_READ) != 0 {
769 if (acc & ACC_USE) != 0 {
774 // _______________________________________________________________________
776 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
777 // if there is a `break` or `again` at the top level, then it's
778 // effectively a return---this only occurs in `for` loops,
779 // where the body is really a closure.
781 debug!("compute: using id for body, {}", self.ir.tcx.hir.node_to_pretty_string(body.id));
783 let exit_ln = self.s.exit_ln;
785 self.break_ln.insert(body.id, exit_ln);
786 self.cont_ln.insert(body.id, exit_ln);
788 // the fallthrough exit is only for those cases where we do not
789 // explicitly return:
791 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
792 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
794 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
796 // hack to skip the loop unless debug! is enabled:
797 debug!("^^ liveness computation results for body {} (entry={:?})",
799 for ln_idx in 0..self.ir.num_live_nodes {
800 debug!("{:?}", self.ln_str(LiveNode(ln_idx)));
809 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
811 if blk.targeted_by_break {
812 self.breakable_block_ln.insert(blk.id, succ);
814 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
815 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
816 self.propagate_through_stmt(stmt, succ)
820 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
823 hir::StmtDecl(ref decl, _) => {
824 self.propagate_through_decl(&decl, succ)
827 hir::StmtExpr(ref expr, _) | hir::StmtSemi(ref expr, _) => {
828 self.propagate_through_expr(&expr, succ)
833 fn propagate_through_decl(&mut self, decl: &hir::Decl, succ: LiveNode)
836 hir::DeclLocal(ref local) => {
837 self.propagate_through_local(&local, succ)
839 hir::DeclItem(_) => succ,
843 fn propagate_through_local(&mut self, local: &hir::Local, succ: LiveNode)
845 // Note: we mark the variable as defined regardless of whether
846 // there is an initializer. Initially I had thought to only mark
847 // the live variable as defined if it was initialized, and then we
848 // could check for uninit variables just by scanning what is live
849 // at the start of the function. But that doesn't work so well for
850 // immutable variables defined in a loop:
851 // loop { let x; x = 5; }
852 // because the "assignment" loops back around and generates an error.
854 // So now we just check that variables defined w/o an
855 // initializer are not live at the point of their
856 // initialization, which is mildly more complex than checking
857 // once at the func header but otherwise equivalent.
859 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
860 self.define_bindings_in_pat(&local.pat, succ)
863 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
865 exprs.iter().rev().fold(succ, |succ, expr| {
866 self.propagate_through_expr(&expr, succ)
870 fn propagate_through_opt_expr(&mut self,
871 opt_expr: Option<&Expr>,
874 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
877 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
879 debug!("propagate_through_expr: {}", self.ir.tcx.hir.node_to_pretty_string(expr.id));
882 // Interesting cases with control flow or which gen/kill
883 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
884 self.access_path(expr.id, path, succ, ACC_READ | ACC_USE)
887 hir::ExprField(ref e, _) => {
888 self.propagate_through_expr(&e, succ)
891 hir::ExprTupField(ref e, _) => {
892 self.propagate_through_expr(&e, succ)
895 hir::ExprClosure(.., blk_id, _, _) => {
896 debug!("{} is an ExprClosure", self.ir.tcx.hir.node_to_pretty_string(expr.id));
899 The next-node for a break is the successor of the entire
900 loop. The next-node for a continue is the top of this loop.
902 let node = self.live_node(expr.id, expr.span);
906 self.break_ln.insert(blk_id.node_id, break_ln);
907 self.cont_ln.insert(blk_id.node_id, cont_ln);
909 // the construction of a closure itself is not important,
910 // but we have to consider the closed over variables.
911 let caps = match self.ir.capture_info_map.get(&expr.id) {
912 Some(caps) => caps.clone(),
914 span_bug!(expr.span, "no registered caps");
917 caps.iter().rev().fold(succ, |succ, cap| {
918 self.init_from_succ(cap.ln, succ);
919 let var = self.variable(cap.var_nid, expr.span);
920 self.acc(cap.ln, var, ACC_READ | ACC_USE);
925 hir::ExprIf(ref cond, ref then, ref els) => {
939 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
940 let then_ln = self.propagate_through_expr(&then, succ);
941 let ln = self.live_node(expr.id, expr.span);
942 self.init_from_succ(ln, else_ln);
943 self.merge_from_succ(ln, then_ln, false);
944 self.propagate_through_expr(&cond, ln)
947 hir::ExprWhile(ref cond, ref blk, _) => {
948 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
951 // Note that labels have been resolved, so we don't need to look
952 // at the label ident
953 hir::ExprLoop(ref blk, _, _) => {
954 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
957 hir::ExprMatch(ref e, ref arms, _) => {
972 let ln = self.live_node(expr.id, expr.span);
973 self.init_empty(ln, succ);
974 let mut first_merge = true;
977 self.propagate_through_expr(&arm.body, succ);
979 self.propagate_through_opt_expr(arm.guard.as_ref().map(|e| &**e), body_succ);
980 // only consider the first pattern; any later patterns must have
981 // the same bindings, and we also consider the first pattern to be
982 // the "authoritative" set of ids
984 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
986 self.merge_from_succ(ln, arm_succ, first_merge);
989 self.propagate_through_expr(&e, ln)
992 hir::ExprRet(ref o_e) => {
993 // ignore succ and subst exit_ln:
994 let exit_ln = self.s.exit_ln;
995 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
998 hir::ExprBreak(label, ref opt_expr) => {
999 // Find which label this break jumps to
1000 let target = match label.target_id {
1001 hir::ScopeTarget::Block(node_id) =>
1002 self.breakable_block_ln.get(&node_id),
1003 hir::ScopeTarget::Loop(hir::LoopIdResult::Ok(node_id)) =>
1004 self.break_ln.get(&node_id),
1005 hir::ScopeTarget::Loop(hir::LoopIdResult::Err(err)) =>
1006 span_bug!(expr.span, "loop scope error: {}", err),
1009 // Now that we know the label we're going to,
1010 // look it up in the break loop nodes table
1013 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1014 None => span_bug!(expr.span, "break to unknown label")
1018 hir::ExprAgain(label) => {
1019 // Find which label this expr continues to
1020 let sc = match label.target_id {
1021 hir::ScopeTarget::Block(_) => bug!("can't `continue` to a non-loop block"),
1022 hir::ScopeTarget::Loop(hir::LoopIdResult::Ok(node_id)) => node_id,
1023 hir::ScopeTarget::Loop(hir::LoopIdResult::Err(err)) =>
1024 span_bug!(expr.span, "loop scope error: {}", err),
1027 // Now that we know the label we're going to,
1028 // look it up in the continue loop nodes table
1030 match self.cont_ln.get(&sc) {
1032 None => span_bug!(expr.span, "continue to unknown label")
1036 hir::ExprAssign(ref l, ref r) => {
1037 // see comment on places in
1038 // propagate_through_place_components()
1039 let succ = self.write_place(&l, succ, ACC_WRITE);
1040 let succ = self.propagate_through_place_components(&l, succ);
1041 self.propagate_through_expr(&r, succ)
1044 hir::ExprAssignOp(_, ref l, ref r) => {
1045 // an overloaded assign op is like a method call
1046 if self.tables.is_method_call(expr) {
1047 let succ = self.propagate_through_expr(&l, succ);
1048 self.propagate_through_expr(&r, succ)
1050 // see comment on places in
1051 // propagate_through_place_components()
1052 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1053 let succ = self.propagate_through_expr(&r, succ);
1054 self.propagate_through_place_components(&l, succ)
1058 // Uninteresting cases: just propagate in rev exec order
1060 hir::ExprArray(ref exprs) => {
1061 self.propagate_through_exprs(exprs, succ)
1064 hir::ExprStruct(_, ref fields, ref with_expr) => {
1065 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1066 fields.iter().rev().fold(succ, |succ, field| {
1067 self.propagate_through_expr(&field.expr, succ)
1071 hir::ExprCall(ref f, ref args) => {
1072 // FIXME(canndrew): This is_never should really be an is_uninhabited
1073 let succ = if self.tables.expr_ty(expr).is_never() {
1078 let succ = self.propagate_through_exprs(args, succ);
1079 self.propagate_through_expr(&f, succ)
1082 hir::ExprMethodCall(.., ref args) => {
1083 // FIXME(canndrew): This is_never should really be an is_uninhabited
1084 let succ = if self.tables.expr_ty(expr).is_never() {
1089 self.propagate_through_exprs(args, succ)
1092 hir::ExprTup(ref exprs) => {
1093 self.propagate_through_exprs(exprs, succ)
1096 hir::ExprBinary(op, ref l, ref r) if op.node.is_lazy() => {
1097 let r_succ = self.propagate_through_expr(&r, succ);
1099 let ln = self.live_node(expr.id, expr.span);
1100 self.init_from_succ(ln, succ);
1101 self.merge_from_succ(ln, r_succ, false);
1103 self.propagate_through_expr(&l, ln)
1106 hir::ExprIndex(ref l, ref r) |
1107 hir::ExprBinary(_, ref l, ref r) => {
1108 let r_succ = self.propagate_through_expr(&r, succ);
1109 self.propagate_through_expr(&l, r_succ)
1112 hir::ExprBox(ref e) |
1113 hir::ExprAddrOf(_, ref e) |
1114 hir::ExprCast(ref e, _) |
1115 hir::ExprType(ref e, _) |
1116 hir::ExprUnary(_, ref e) |
1117 hir::ExprYield(ref e) |
1118 hir::ExprRepeat(ref e, _) => {
1119 self.propagate_through_expr(&e, succ)
1122 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1123 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1124 // see comment on places
1125 // in propagate_through_place_components()
1127 self.propagate_through_expr(output, succ)
1129 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1130 let succ = self.write_place(output, succ, acc);
1131 self.propagate_through_place_components(output, succ)
1135 // Inputs are executed first. Propagate last because of rev order
1136 self.propagate_through_exprs(inputs, succ)
1139 hir::ExprLit(..) | hir::ExprPath(hir::QPath::TypeRelative(..)) => {
1143 hir::ExprBlock(ref blk) => {
1144 self.propagate_through_block(&blk, succ)
1149 fn propagate_through_place_components(&mut self,
1155 // In general, the full flow graph structure for an
1156 // assignment/move/etc can be handled in one of two ways,
1157 // depending on whether what is being assigned is a "tracked
1158 // value" or not. A tracked value is basically a local
1159 // variable or argument.
1161 // The two kinds of graphs are:
1163 // Tracked place Untracked place
1164 // ----------------------++-----------------------
1168 // (rvalue) || (rvalue)
1171 // (write of place) || (place components)
1176 // ----------------------++-----------------------
1178 // I will cover the two cases in turn:
1182 // A tracked place is a local variable/argument `x`. In
1183 // these cases, the link_node where the write occurs is linked
1184 // to node id of `x`. The `write_place()` routine generates
1185 // the contents of this node. There are no subcomponents to
1188 // # Non-tracked places
1190 // These are places like `x[5]` or `x.f`. In that case, we
1191 // basically ignore the value which is written to but generate
1192 // reads for the components---`x` in these two examples. The
1193 // components reads are generated by
1194 // `propagate_through_place_components()` (this fn).
1198 // It is still possible to observe assignments to non-places;
1199 // these errors are detected in the later pass borrowck. We
1200 // just ignore such cases and treat them as reads.
1203 hir::ExprPath(_) => succ,
1204 hir::ExprField(ref e, _) => self.propagate_through_expr(&e, succ),
1205 hir::ExprTupField(ref e, _) => self.propagate_through_expr(&e, succ),
1206 _ => self.propagate_through_expr(expr, succ)
1210 // see comment on propagate_through_place()
1211 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32)
1214 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1215 self.access_path(expr.id, path, succ, acc)
1218 // We do not track other places, so just propagate through
1219 // to their subcomponents. Also, it may happen that
1220 // non-places occur here, because those are detected in the
1221 // later pass borrowck.
1226 fn access_var(&mut self, id: NodeId, nid: NodeId, succ: LiveNode, acc: u32, span: Span)
1228 let ln = self.live_node(id, span);
1230 self.init_from_succ(ln, succ);
1231 let var = self.variable(nid, span);
1232 self.acc(ln, var, acc);
1237 fn access_path(&mut self, id: NodeId, path: &hir::Path, succ: LiveNode, acc: u32)
1240 Def::Local(nid) => {
1241 self.access_var(id, nid, succ, acc, path.span)
1247 fn propagate_through_loop(&mut self,
1256 We model control flow like this:
1274 let mut first_merge = true;
1275 let ln = self.live_node(expr.id, expr.span);
1276 self.init_empty(ln, succ);
1280 // If this is not a `loop` loop, then it's possible we bypass
1281 // the body altogether. Otherwise, the only way is via a `break`
1282 // in the loop body.
1283 self.merge_from_succ(ln, succ, first_merge);
1284 first_merge = false;
1287 debug!("propagate_through_loop: using id for loop body {} {}",
1288 expr.id, self.ir.tcx.hir.node_to_pretty_string(body.id));
1290 let break_ln = succ;
1292 self.break_ln.insert(expr.id, break_ln);
1293 self.cont_ln.insert(expr.id, cont_ln);
1295 let cond_ln = match kind {
1297 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1299 let body_ln = self.propagate_through_block(body, cond_ln);
1301 // repeat until fixed point is reached:
1302 while self.merge_from_succ(ln, body_ln, first_merge) {
1303 first_merge = false;
1305 let new_cond_ln = match kind {
1307 WhileLoop(ref cond) => {
1308 self.propagate_through_expr(&cond, ln)
1311 assert!(cond_ln == new_cond_ln);
1312 assert!(body_ln == self.propagate_through_block(body, cond_ln));
1319 // _______________________________________________________________________
1320 // Checking for error conditions
1322 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1323 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1324 NestedVisitorMap::None
1327 fn visit_local(&mut self, l: &'tcx hir::Local) {
1328 check_local(self, l);
1330 fn visit_expr(&mut self, ex: &'tcx Expr) {
1331 check_expr(self, ex);
1333 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1338 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1341 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1344 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1345 this.warn_about_unused(sp, id, ln, var);
1350 intravisit::walk_local(this, local);
1353 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1354 // only consider the first pattern; any later patterns must have
1355 // the same bindings, and we also consider the first pattern to be
1356 // the "authoritative" set of ids
1357 this.arm_pats_bindings(arm.pats.first().map(|p| &**p), |this, ln, var, sp, id| {
1358 this.warn_about_unused(sp, id, ln, var);
1360 intravisit::walk_arm(this, arm);
1363 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1365 hir::ExprAssign(ref l, _) => {
1366 this.check_place(&l);
1368 intravisit::walk_expr(this, expr);
1371 hir::ExprAssignOp(_, ref l, _) => {
1372 if !this.tables.is_method_call(expr) {
1373 this.check_place(&l);
1376 intravisit::walk_expr(this, expr);
1379 hir::ExprInlineAsm(ref ia, ref outputs, ref inputs) => {
1380 for input in inputs {
1381 this.visit_expr(input);
1384 // Output operands must be places
1385 for (o, output) in ia.outputs.iter().zip(outputs) {
1387 this.check_place(output);
1389 this.visit_expr(output);
1392 intravisit::walk_expr(this, expr);
1395 // no correctness conditions related to liveness
1396 hir::ExprCall(..) | hir::ExprMethodCall(..) | hir::ExprIf(..) |
1397 hir::ExprMatch(..) | hir::ExprWhile(..) | hir::ExprLoop(..) |
1398 hir::ExprIndex(..) | hir::ExprField(..) | hir::ExprTupField(..) |
1399 hir::ExprArray(..) | hir::ExprTup(..) | hir::ExprBinary(..) |
1400 hir::ExprCast(..) | hir::ExprUnary(..) | hir::ExprRet(..) |
1401 hir::ExprBreak(..) | hir::ExprAgain(..) | hir::ExprLit(_) |
1402 hir::ExprBlock(..) | hir::ExprAddrOf(..) |
1403 hir::ExprStruct(..) | hir::ExprRepeat(..) |
1404 hir::ExprClosure(..) | hir::ExprPath(_) | hir::ExprYield(..) |
1405 hir::ExprBox(..) | hir::ExprType(..) => {
1406 intravisit::walk_expr(this, expr);
1411 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1412 fn check_place(&mut self, expr: &'tcx Expr) {
1414 hir::ExprPath(hir::QPath::Resolved(_, ref path)) => {
1415 if let Def::Local(nid) = path.def {
1416 // Assignment to an immutable variable or argument: only legal
1417 // if there is no later assignment. If this local is actually
1418 // mutable, then check for a reassignment to flag the mutability
1420 let ln = self.live_node(expr.id, expr.span);
1421 let var = self.variable(nid, expr.span);
1422 self.warn_about_dead_assign(expr.span, expr.id, ln, var);
1426 // For other kinds of places, no checks are required,
1427 // and any embedded expressions are actually rvalues
1428 intravisit::walk_expr(self, expr);
1433 fn should_warn(&self, var: Variable) -> Option<String> {
1434 let name = self.ir.variable_name(var);
1435 if name.is_empty() || name.as_bytes()[0] == ('_' as u8) {
1442 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1443 for arg in &body.arguments {
1444 arg.pat.each_binding(|_bm, p_id, sp, path1| {
1445 let var = self.variable(p_id, sp);
1446 // Ignore unused self.
1447 let name = path1.node;
1448 if name != keywords::SelfValue.name() {
1449 if !self.warn_about_unused(sp, p_id, entry_ln, var) {
1450 if self.live_on_entry(entry_ln, var).is_none() {
1451 self.report_dead_assign(p_id, sp, var, true);
1459 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1460 self.pat_bindings(pat, |this, ln, var, sp, id| {
1461 if !this.warn_about_unused(sp, id, ln, var) {
1462 this.warn_about_dead_assign(sp, id, ln, var);
1467 fn warn_about_unused(&self,
1473 if !self.used_on_entry(ln, var) {
1474 let r = self.should_warn(var);
1475 if let Some(name) = r {
1477 // annoying: for parameters in funcs like `fn(x: i32)
1478 // {ret}`, there is only one node, so asking about
1479 // assigned_on_exit() is not meaningful.
1480 let is_assigned = if ln == self.s.exit_ln {
1483 self.assigned_on_exit(ln, var).is_some()
1487 self.ir.tcx.lint_node_note(lint::builtin::UNUSED_VARIABLES, id, sp,
1488 &format!("variable `{}` is assigned to, but never used",
1490 &format!("to avoid this warning, consider using `_{}` instead",
1492 } else if name != "self" {
1493 self.ir.tcx.lint_node_note(lint::builtin::UNUSED_VARIABLES, id, sp,
1494 &format!("unused variable: `{}`", name),
1495 &format!("to avoid this warning, consider using `_{}` instead",
1505 fn warn_about_dead_assign(&self,
1510 if self.live_on_exit(ln, var).is_none() {
1511 self.report_dead_assign(id, sp, var, false);
1515 fn report_dead_assign(&self, id: NodeId, sp: Span, var: Variable, is_argument: bool) {
1516 if let Some(name) = self.should_warn(var) {
1518 self.ir.tcx.lint_node(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1519 &format!("value passed to `{}` is never read", name));
1521 self.ir.tcx.lint_node(lint::builtin::UNUSED_ASSIGNMENTS, id, sp,
1522 &format!("value assigned to `{}` is never read", name));