1 //! A classic liveness analysis based on dataflow over the AST. Computes,
2 //! for each local variable in a function, whether that variable is live
3 //! at a given point. Program execution points are identified by their
8 //! The basic model is that each local variable is assigned an index. We
9 //! represent sets of local variables using a vector indexed by this
10 //! index. The value in the vector is either 0, indicating the variable
11 //! is dead, or the id of an expression that uses the variable.
13 //! We conceptually walk over the AST in reverse execution order. If we
14 //! find a use of a variable, we add it to the set of live variables. If
15 //! we find an assignment to a variable, we remove it from the set of live
16 //! variables. When we have to merge two flows, we take the union of
17 //! those two flows---if the variable is live on both paths, we simply
18 //! pick one id. In the event of loops, we continue doing this until a
19 //! fixed point is reached.
21 //! ## Checking initialization
23 //! At the function entry point, all variables must be dead. If this is
24 //! not the case, we can report an error using the id found in the set of
25 //! live variables, which identifies a use of the variable which is not
26 //! dominated by an assignment.
30 //! After each explicit move, the variable must be dead.
32 //! ## Computing last uses
34 //! Any use of the variable where the variable is dead afterwards is a
37 //! # Implementation details
39 //! The actual implementation contains two (nested) walks over the AST.
40 //! The outer walk has the job of building up the ir_maps instance for the
41 //! enclosing function. On the way down the tree, it identifies those AST
42 //! nodes and variable IDs that will be needed for the liveness analysis
43 //! and assigns them contiguous IDs. The liveness id for an AST node is
44 //! called a `live_node` (it's a newtype'd u32) and the id for a variable
45 //! is called a `variable` (another newtype'd u32).
47 //! On the way back up the tree, as we are about to exit from a function
48 //! declaration we allocate a `liveness` instance. Now that we know
49 //! precisely how many nodes and variables we need, we can allocate all
50 //! the various arrays that we will need to precisely the right size. We then
51 //! perform the actual propagation on the `liveness` instance.
53 //! This propagation is encoded in the various `propagate_through_*()`
54 //! methods. It effectively does a reverse walk of the AST; whenever we
55 //! reach a loop node, we iterate until a fixed point is reached.
57 //! ## The `RWU` struct
59 //! At each live node `N`, we track three pieces of information for each
60 //! variable `V` (these are encapsulated in the `RWU` struct):
62 //! - `reader`: the `LiveNode` ID of some node which will read the value
63 //! that `V` holds on entry to `N`. Formally: a node `M` such
64 //! that there exists a path `P` from `N` to `M` where `P` does not
65 //! write `V`. If the `reader` is `invalid_node()`, then the current
66 //! value will never be read (the variable is dead, essentially).
68 //! - `writer`: the `LiveNode` ID of some node which will write the
69 //! variable `V` and which is reachable from `N`. Formally: a node `M`
70 //! such that there exists a path `P` from `N` to `M` and `M` writes
71 //! `V`. If the `writer` is `invalid_node()`, then there is no writer
72 //! of `V` that follows `N`.
74 //! - `used`: a boolean value indicating whether `V` is *used*. We
75 //! distinguish a *read* from a *use* in that a *use* is some read that
76 //! is not just used to generate a new value. For example, `x += 1` is
77 //! a read but not a use. This is used to generate better warnings.
79 //! ## Special Variables
81 //! We generate various special variables for various, well, special purposes.
82 //! These are described in the `specials` struct:
84 //! - `exit_ln`: a live node that is generated to represent every 'exit' from
85 //! the function, whether it be by explicit return, panic, or other means.
87 //! - `fallthrough_ln`: a live node that represents a fallthrough
89 //! - `clean_exit_var`: a synthetic variable that is only 'read' from the
90 //! fallthrough node. It is only live if the function could converge
91 //! via means other than an explicit `return` expression. That is, it is
92 //! only dead if the end of the function's block can never be reached.
93 //! It is the responsibility of typeck to ensure that there are no
94 //! `return` expressions in a function declared as diverging.
96 use self::LoopKind::*;
97 use self::LiveNodeKind::*;
102 use ty::{self, TyCtxt};
103 use ty::query::Providers;
105 use errors::Applicability;
106 use util::nodemap::{NodeMap, HirIdMap, HirIdSet};
108 use std::collections::{BTreeMap, VecDeque};
110 use std::io::prelude::*;
113 use syntax::ast::{self, NodeId};
115 use syntax::symbol::keywords;
116 use syntax_pos::Span;
119 use hir::{Expr, HirId};
120 use hir::def_id::DefId;
121 use hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
123 /// For use with `propagate_through_loop`.
125 /// An endless `loop` loop.
127 /// A `while` loop, with the given expression as condition.
131 #[derive(Copy, Clone, PartialEq)]
132 struct Variable(u32);
134 #[derive(Copy, Clone, PartialEq)]
135 struct LiveNode(u32);
138 fn get(&self) -> usize { self.0 as usize }
142 fn get(&self) -> usize { self.0 as usize }
145 #[derive(Copy, Clone, PartialEq, Debug)]
153 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_, '_, '_>) -> String {
154 let cm = tcx.sess.source_map();
157 format!("Free var node [{}]", cm.span_to_string(s))
160 format!("Expr node [{}]", cm.span_to_string(s))
163 format!("Var def node [{}]", cm.span_to_string(s))
165 ExitNode => "Exit node".to_owned(),
169 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
170 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
171 NestedVisitorMap::OnlyBodies(&self.tcx.hir())
174 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
175 b: hir::BodyId, s: Span, id: NodeId) {
176 visit_fn(self, fk, fd, b, s, id);
179 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
180 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
181 fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
184 fn check_mod_liveness<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, module_def_id: DefId) {
185 tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor());
188 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
189 for &module in tcx.hir().krate().modules.keys() {
190 tcx.ensure().check_mod_liveness(tcx.hir().local_def_id(module));
192 tcx.sess.abort_if_errors();
195 pub fn provide(providers: &mut Providers<'_>) {
196 *providers = Providers {
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 {
242 fn invalid_node() -> LiveNode { LiveNode(u32::MAX) }
249 #[derive(Copy, Clone, Debug)]
256 #[derive(Copy, Clone, Debug)]
258 Arg(HirId, ast::Name),
263 struct IrMaps<'a, 'tcx: 'a> {
264 tcx: TyCtxt<'a, 'tcx, 'tcx>,
265 num_live_nodes: usize,
267 live_node_map: HirIdMap<LiveNode>,
268 variable_map: HirIdMap<Variable>,
269 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
270 var_kinds: Vec<VarKind>,
271 lnks: Vec<LiveNodeKind>,
274 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
275 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
280 live_node_map: HirIdMap::default(),
281 variable_map: HirIdMap::default(),
282 capture_info_map: Default::default(),
283 var_kinds: Vec::new(),
288 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
289 let ln = LiveNode(self.num_live_nodes as u32);
291 self.num_live_nodes += 1;
293 debug!("{:?} is of kind {}", ln,
294 live_node_kind_to_string(lnk, self.tcx));
299 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
300 let ln = self.add_live_node(lnk);
301 self.live_node_map.insert(hir_id, ln);
303 debug!("{:?} is node {:?}", ln, hir_id);
306 fn add_variable(&mut self, vk: VarKind) -> Variable {
307 let v = Variable(self.num_vars as u32);
308 self.var_kinds.push(vk);
312 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
313 self.variable_map.insert(node_id, v);
318 debug!("{:?} is {:?}", v, vk);
323 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
324 match self.variable_map.get(&hir_id) {
327 span_bug!(span, "no variable registered for id {:?}", hir_id);
332 fn variable_name(&self, var: Variable) -> String {
333 match self.var_kinds[var.get()] {
334 Local(LocalInfo { name, .. }) | Arg(_, name) => {
337 CleanExit => "<clean-exit>".to_owned()
341 fn variable_is_shorthand(&self, var: Variable) -> bool {
342 match self.var_kinds[var.get()] {
343 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
344 Arg(..) | CleanExit => false
348 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
349 self.capture_info_map.insert(node_id, Rc::new(cs));
352 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
357 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
359 decl: &'tcx hir::FnDecl,
360 body_id: hir::BodyId,
365 // swap in a new set of IR maps for this function body:
366 let mut fn_maps = IrMaps::new(ir.tcx);
368 // Don't run unused pass for #[derive()]
369 if let FnKind::Method(..) = fk {
370 let parent = ir.tcx.hir().get_parent(id);
371 if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
372 if i.attrs.iter().any(|a| a.check_name("automatically_derived")) {
378 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps<'_, '_>);
380 let body = ir.tcx.hir().body(body_id);
382 for arg in &body.arguments {
383 arg.pat.each_binding(|_bm, hir_id, _x, ident| {
384 debug!("adding argument {:?}", hir_id);
385 fn_maps.add_variable(Arg(hir_id, ident.name));
389 // gather up the various local variables, significant expressions,
391 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
394 let mut lsets = Liveness::new(&mut fn_maps, body_id);
395 let entry_ln = lsets.compute(&body.value);
397 // check for various error conditions
398 lsets.visit_body(body);
399 lsets.warn_about_unused_args(body, entry_ln);
402 fn add_from_pat<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, pat: &P<hir::Pat>) {
403 // For struct patterns, take note of which fields used shorthand
404 // (`x` rather than `x: x`).
405 let mut shorthand_field_ids = HirIdSet::default();
406 let mut pats = VecDeque::new();
408 while let Some(pat) = pats.pop_front() {
411 Binding(_, _, _, _, ref inner_pat) => {
412 pats.extend(inner_pat.iter());
414 Struct(_, ref fields, _) => {
415 for field in fields {
416 if field.node.is_shorthand {
417 shorthand_field_ids.insert(field.node.pat.hir_id);
421 Ref(ref inner_pat, _) |
422 Box(ref inner_pat) => {
423 pats.push_back(inner_pat);
425 TupleStruct(_, ref inner_pats, _) |
426 Tuple(ref inner_pats, _) => {
427 pats.extend(inner_pats.iter());
429 Slice(ref pre_pats, ref inner_pat, ref post_pats) => {
430 pats.extend(pre_pats.iter());
431 pats.extend(inner_pat.iter());
432 pats.extend(post_pats.iter());
438 pat.each_binding(|_bm, hir_id, _sp, ident| {
439 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
440 ir.add_variable(Local(LocalInfo {
443 is_shorthand: shorthand_field_ids.contains(&hir_id)
448 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
449 add_from_pat(ir, &local.pat);
450 intravisit::walk_local(ir, local);
453 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
454 for pat in &arm.pats {
455 add_from_pat(ir, pat);
457 intravisit::walk_arm(ir, arm);
460 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
462 // live nodes required for uses or definitions of variables:
463 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
464 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
465 if let Def::Local(..) = path.def {
466 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
468 intravisit::walk_expr(ir, expr);
470 hir::ExprKind::Closure(..) => {
471 // Interesting control flow (for loops can contain labeled
472 // breaks or continues)
473 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
475 // Make a live_node for each captured variable, with the span
476 // being the location that the variable is used. This results
477 // in better error messages than just pointing at the closure
478 // construction site.
479 let mut call_caps = Vec::new();
480 ir.tcx.with_freevars(expr.id, |freevars| {
481 call_caps.extend(freevars.iter().filter_map(|fv| {
482 if let Def::Local(rv) = fv.def {
483 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
484 let var_hid = ir.tcx.hir().node_to_hir_id(rv);
485 Some(CaptureInfo { ln: fv_ln, var_hid })
491 ir.set_captures(expr.id, call_caps);
493 intravisit::walk_expr(ir, expr);
496 // live nodes required for interesting control flow:
497 hir::ExprKind::If(..) |
498 hir::ExprKind::Match(..) |
499 hir::ExprKind::While(..) |
500 hir::ExprKind::Loop(..) => {
501 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
502 intravisit::walk_expr(ir, expr);
504 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
505 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
506 intravisit::walk_expr(ir, expr);
509 // otherwise, live nodes are not required:
510 hir::ExprKind::Index(..) |
511 hir::ExprKind::Field(..) |
512 hir::ExprKind::Array(..) |
513 hir::ExprKind::Call(..) |
514 hir::ExprKind::MethodCall(..) |
515 hir::ExprKind::Tup(..) |
516 hir::ExprKind::Binary(..) |
517 hir::ExprKind::AddrOf(..) |
518 hir::ExprKind::Cast(..) |
519 hir::ExprKind::Unary(..) |
520 hir::ExprKind::Break(..) |
521 hir::ExprKind::Continue(_) |
522 hir::ExprKind::Lit(_) |
523 hir::ExprKind::Ret(..) |
524 hir::ExprKind::Block(..) |
525 hir::ExprKind::Assign(..) |
526 hir::ExprKind::AssignOp(..) |
527 hir::ExprKind::Struct(..) |
528 hir::ExprKind::Repeat(..) |
529 hir::ExprKind::InlineAsm(..) |
530 hir::ExprKind::Box(..) |
531 hir::ExprKind::Yield(..) |
532 hir::ExprKind::Type(..) |
534 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
535 intravisit::walk_expr(ir, expr);
540 // ______________________________________________________________________
541 // Computing liveness sets
543 // Actually we compute just a bit more than just liveness, but we use
544 // the same basic propagation framework in all cases.
546 #[derive(Clone, Copy)]
553 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
554 /// very large, so it uses a more compact representation that takes advantage
555 /// of the fact that when the number of `RWU`s is large, most of them have an
556 /// invalid reader and an invalid writer.
558 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
559 /// an index into `unpacked_rwus`. In the common cases, this compacts the
560 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
563 /// More compact representations are possible -- e.g., use only 2 bits per
564 /// packed `RWU` and make the secondary table a HashMap that maps from
565 /// indices to `RWU`s -- but this one strikes a good balance between size
567 packed_rwus: Vec<u32>,
568 unpacked_rwus: Vec<RWU>,
571 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
572 const INV_INV_FALSE: u32 = u32::MAX;
574 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
575 const INV_INV_TRUE: u32 = u32::MAX - 1;
578 fn new(num_rwus: usize) -> RWUTable {
580 packed_rwus: vec![INV_INV_FALSE; num_rwus],
581 unpacked_rwus: vec![],
585 fn get(&self, idx: usize) -> RWU {
586 let packed_rwu = self.packed_rwus[idx];
588 INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false },
589 INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true },
590 _ => self.unpacked_rwus[packed_rwu as usize],
594 fn get_reader(&self, idx: usize) -> LiveNode {
595 let packed_rwu = self.packed_rwus[idx];
597 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
598 _ => self.unpacked_rwus[packed_rwu as usize].reader,
602 fn get_writer(&self, idx: usize) -> LiveNode {
603 let packed_rwu = self.packed_rwus[idx];
605 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
606 _ => self.unpacked_rwus[packed_rwu as usize].writer,
610 fn get_used(&self, idx: usize) -> bool {
611 let packed_rwu = self.packed_rwus[idx];
613 INV_INV_FALSE => false,
614 INV_INV_TRUE => true,
615 _ => self.unpacked_rwus[packed_rwu as usize].used,
620 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
621 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
624 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
625 if rwu.reader == invalid_node() && rwu.writer == invalid_node() {
626 // When we overwrite an indexing entry in `self.packed_rwus` with
627 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
628 // from `self.unpacked_rwus`; it's not worth the effort, and we
629 // can't have entries shifting around anyway.
630 self.packed_rwus[idx] = if rwu.used {
636 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
638 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
639 self.unpacked_rwus.push(rwu);
643 fn assign_inv_inv(&mut self, idx: usize) {
644 self.packed_rwus[idx] = if self.get_used(idx) {
652 #[derive(Copy, Clone)]
655 fallthrough_ln: LiveNode,
656 clean_exit_var: Variable
659 const ACC_READ: u32 = 1;
660 const ACC_WRITE: u32 = 2;
661 const ACC_USE: u32 = 4;
663 struct Liveness<'a, 'tcx: 'a> {
664 ir: &'a mut IrMaps<'a, 'tcx>,
665 tables: &'a ty::TypeckTables<'tcx>,
667 successors: Vec<LiveNode>,
670 // mappings from loop node ID to LiveNode
671 // ("break" label should map to loop node ID,
672 // it probably doesn't now)
673 break_ln: NodeMap<LiveNode>,
674 cont_ln: NodeMap<LiveNode>,
677 impl<'a, 'tcx> Liveness<'a, 'tcx> {
678 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
679 // Special nodes and variables:
680 // - exit_ln represents the end of the fn, either by return or panic
681 // - implicit_ret_var is a pseudo-variable that represents
682 // an implicit return
683 let specials = Specials {
684 exit_ln: ir.add_live_node(ExitNode),
685 fallthrough_ln: ir.add_live_node(ExitNode),
686 clean_exit_var: ir.add_variable(CleanExit)
689 let tables = ir.tcx.body_tables(body);
691 let num_live_nodes = ir.num_live_nodes;
692 let num_vars = ir.num_vars;
698 successors: vec![invalid_node(); num_live_nodes],
699 rwu_table: RWUTable::new(num_live_nodes * num_vars),
700 break_ln: Default::default(),
701 cont_ln: Default::default(),
705 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
706 match self.ir.live_node_map.get(&hir_id) {
709 // This must be a mismatch between the ir_map construction
710 // above and the propagation code below; the two sets of
711 // code have to agree about which AST nodes are worth
712 // creating liveness nodes for.
715 "no live node registered for node {:?}",
721 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
722 self.ir.variable(hir_id, span)
725 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
726 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
728 pat.each_binding(|_bm, hir_id, sp, n| {
729 let ln = self.live_node(hir_id, sp);
730 let var = self.variable(hir_id, n.span);
731 f(self, ln, var, n.span, hir_id);
735 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
736 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
738 if let Some(pat) = pat {
739 self.pat_bindings(pat, f);
743 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
745 self.define_bindings_in_arm_pats(Some(pat), succ)
748 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
751 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
752 this.init_from_succ(ln, succ);
753 this.define(ln, var);
759 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
760 ln.get() * self.ir.num_vars + var.get()
763 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
764 assert!(ln.is_valid());
765 let reader = self.rwu_table.get_reader(self.idx(ln, var));
766 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
769 // Is this variable live on entry to any of its successor nodes?
770 fn live_on_exit(&self, ln: LiveNode, var: Variable)
771 -> Option<LiveNodeKind> {
772 let successor = self.successors[ln.get()];
773 self.live_on_entry(successor, var)
776 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
777 assert!(ln.is_valid());
778 self.rwu_table.get_used(self.idx(ln, var))
781 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
782 -> Option<LiveNodeKind> {
783 assert!(ln.is_valid());
784 let writer = self.rwu_table.get_writer(self.idx(ln, var));
785 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
788 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
789 -> Option<LiveNodeKind> {
790 let successor = self.successors[ln.get()];
791 self.assigned_on_entry(successor, var)
794 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
795 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
797 let node_base_idx = self.idx(ln, Variable(0));
798 let succ_base_idx = self.idx(succ_ln, Variable(0));
799 for var_idx in 0..self.ir.num_vars {
800 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
804 fn write_vars<F>(&self,
808 -> io::Result<()> where
809 F: FnMut(usize) -> LiveNode,
811 let node_base_idx = self.idx(ln, Variable(0));
812 for var_idx in 0..self.ir.num_vars {
813 let idx = node_base_idx + var_idx;
814 if test(idx).is_valid() {
815 write!(wr, " {:?}", Variable(var_idx as u32))?;
822 #[allow(unused_must_use)]
823 fn ln_str(&self, ln: LiveNode) -> String {
824 let mut wr = Vec::new();
826 let wr = &mut wr as &mut dyn Write;
827 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
828 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx));
829 write!(wr, " writes");
830 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx));
831 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
833 String::from_utf8(wr).unwrap()
836 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
837 self.successors[ln.get()] = succ_ln;
839 // It is not necessary to initialize the RWUs here because they are all
840 // set to INV_INV_FALSE when they are created, and the sets only grow
841 // during iterations.
844 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
845 // more efficient version of init_empty() / merge_from_succ()
846 self.successors[ln.get()] = succ_ln;
848 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
849 this.rwu_table.copy_packed(idx, succ_idx);
851 debug!("init_from_succ(ln={}, succ={})",
852 self.ln_str(ln), self.ln_str(succ_ln));
855 fn merge_from_succ(&mut self,
860 if ln == succ_ln { return false; }
862 let mut changed = false;
863 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
864 let mut rwu = this.rwu_table.get(idx);
865 let succ_rwu = this.rwu_table.get(succ_idx);
866 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
867 rwu.reader = succ_rwu.reader;
871 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
872 rwu.writer = succ_rwu.writer;
876 if succ_rwu.used && !rwu.used {
882 this.rwu_table.assign_unpacked(idx, rwu);
886 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
887 ln, self.ln_str(succ_ln), first_merge, changed);
891 // Indicates that a local variable was *defined*; we know that no
892 // uses of the variable can precede the definition (resolve checks
893 // this) so we just clear out all the data.
894 fn define(&mut self, writer: LiveNode, var: Variable) {
895 let idx = self.idx(writer, var);
896 self.rwu_table.assign_inv_inv(idx);
898 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
899 idx, self.ln_str(writer));
902 // Either read, write, or both depending on the acc bitset
903 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
904 debug!("{:?} accesses[{:x}] {:?}: {}",
905 ln, acc, var, self.ln_str(ln));
907 let idx = self.idx(ln, var);
908 let mut rwu = self.rwu_table.get(idx);
910 if (acc & ACC_WRITE) != 0 {
911 rwu.reader = invalid_node();
915 // Important: if we both read/write, must do read second
916 // or else the write will override.
917 if (acc & ACC_READ) != 0 {
921 if (acc & ACC_USE) != 0 {
925 self.rwu_table.assign_unpacked(idx, rwu);
928 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
929 debug!("compute: using id for body, {}", self.ir.tcx.hir().node_to_pretty_string(body.id));
931 // the fallthrough exit is only for those cases where we do not
932 // explicitly return:
934 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
935 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
937 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
939 // hack to skip the loop unless debug! is enabled:
940 debug!("^^ liveness computation results for body {} (entry={:?})", {
941 for ln_idx in 0..self.ir.num_live_nodes {
942 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
951 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
953 if blk.targeted_by_break {
954 self.break_ln.insert(blk.id, succ);
956 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
957 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
958 self.propagate_through_stmt(stmt, succ)
962 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
965 hir::StmtKind::Local(ref local) => {
966 // Note: we mark the variable as defined regardless of whether
967 // there is an initializer. Initially I had thought to only mark
968 // the live variable as defined if it was initialized, and then we
969 // could check for uninit variables just by scanning what is live
970 // at the start of the function. But that doesn't work so well for
971 // immutable variables defined in a loop:
972 // loop { let x; x = 5; }
973 // because the "assignment" loops back around and generates an error.
975 // So now we just check that variables defined w/o an
976 // initializer are not live at the point of their
977 // initialization, which is mildly more complex than checking
978 // once at the func header but otherwise equivalent.
980 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
981 self.define_bindings_in_pat(&local.pat, succ)
983 hir::StmtKind::Item(..) => succ,
984 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
985 self.propagate_through_expr(&expr, succ)
990 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
992 exprs.iter().rev().fold(succ, |succ, expr| {
993 self.propagate_through_expr(&expr, succ)
997 fn propagate_through_opt_expr(&mut self,
998 opt_expr: Option<&Expr>,
1001 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
1004 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
1006 debug!("propagate_through_expr: {}", self.ir.tcx.hir().node_to_pretty_string(expr.id));
1009 // Interesting cases with control flow or which gen/kill
1010 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1011 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
1014 hir::ExprKind::Field(ref e, _) => {
1015 self.propagate_through_expr(&e, succ)
1018 hir::ExprKind::Closure(..) => {
1019 debug!("{} is an ExprKind::Closure",
1020 self.ir.tcx.hir().node_to_pretty_string(expr.id));
1022 // the construction of a closure itself is not important,
1023 // but we have to consider the closed over variables.
1024 let caps = self.ir.capture_info_map.get(&expr.id).cloned().unwrap_or_else(||
1025 span_bug!(expr.span, "no registered caps"));
1027 caps.iter().rev().fold(succ, |succ, cap| {
1028 self.init_from_succ(cap.ln, succ);
1029 let var = self.variable(cap.var_hid, expr.span);
1030 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1035 hir::ExprKind::If(ref cond, ref then, ref els) => {
1049 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
1050 let then_ln = self.propagate_through_expr(&then, succ);
1051 let ln = self.live_node(expr.hir_id, expr.span);
1052 self.init_from_succ(ln, else_ln);
1053 self.merge_from_succ(ln, then_ln, false);
1054 self.propagate_through_expr(&cond, ln)
1057 hir::ExprKind::While(ref cond, ref blk, _) => {
1058 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1061 // Note that labels have been resolved, so we don't need to look
1062 // at the label ident
1063 hir::ExprKind::Loop(ref blk, _, _) => {
1064 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1067 hir::ExprKind::Match(ref e, ref arms, _) => {
1082 let ln = self.live_node(expr.hir_id, expr.span);
1083 self.init_empty(ln, succ);
1084 let mut first_merge = true;
1086 let body_succ = self.propagate_through_expr(&arm.body, succ);
1088 let guard_succ = self.propagate_through_opt_expr(
1089 arm.guard.as_ref().map(|hir::Guard::If(e)| &**e),
1092 // only consider the first pattern; any later patterns must have
1093 // the same bindings, and we also consider the first pattern to be
1094 // the "authoritative" set of ids
1096 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1098 self.merge_from_succ(ln, arm_succ, first_merge);
1099 first_merge = false;
1101 self.propagate_through_expr(&e, ln)
1104 hir::ExprKind::Ret(ref o_e) => {
1105 // ignore succ and subst exit_ln:
1106 let exit_ln = self.s.exit_ln;
1107 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1110 hir::ExprKind::Break(label, ref opt_expr) => {
1111 // Find which label this break jumps to
1112 let target = match label.target_id {
1113 Ok(node_id) => self.break_ln.get(&node_id),
1114 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1117 // Now that we know the label we're going to,
1118 // look it up in the break loop nodes table
1121 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1122 None => span_bug!(expr.span, "break to unknown label")
1126 hir::ExprKind::Continue(label) => {
1127 // Find which label this expr continues to
1128 let sc = label.target_id.unwrap_or_else(|err|
1129 span_bug!(expr.span, "loop scope error: {}", err));
1131 // Now that we know the label we're going to,
1132 // look it up in the continue loop nodes table
1133 self.cont_ln.get(&sc).cloned().unwrap_or_else(||
1134 span_bug!(expr.span, "continue to unknown label"))
1137 hir::ExprKind::Assign(ref l, ref r) => {
1138 // see comment on places in
1139 // propagate_through_place_components()
1140 let succ = self.write_place(&l, succ, ACC_WRITE);
1141 let succ = self.propagate_through_place_components(&l, succ);
1142 self.propagate_through_expr(&r, succ)
1145 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1146 // an overloaded assign op is like a method call
1147 if self.tables.is_method_call(expr) {
1148 let succ = self.propagate_through_expr(&l, succ);
1149 self.propagate_through_expr(&r, succ)
1151 // see comment on places in
1152 // propagate_through_place_components()
1153 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1154 let succ = self.propagate_through_expr(&r, succ);
1155 self.propagate_through_place_components(&l, succ)
1159 // Uninteresting cases: just propagate in rev exec order
1161 hir::ExprKind::Array(ref exprs) => {
1162 self.propagate_through_exprs(exprs, succ)
1165 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1166 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1167 fields.iter().rev().fold(succ, |succ, field| {
1168 self.propagate_through_expr(&field.expr, succ)
1172 hir::ExprKind::Call(ref f, ref args) => {
1173 let m = self.ir.tcx.hir().get_module_parent(expr.id);
1174 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1179 let succ = self.propagate_through_exprs(args, succ);
1180 self.propagate_through_expr(&f, succ)
1183 hir::ExprKind::MethodCall(.., ref args) => {
1184 let m = self.ir.tcx.hir().get_module_parent(expr.id);
1185 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1191 self.propagate_through_exprs(args, succ)
1194 hir::ExprKind::Tup(ref exprs) => {
1195 self.propagate_through_exprs(exprs, succ)
1198 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1199 let r_succ = self.propagate_through_expr(&r, succ);
1201 let ln = self.live_node(expr.hir_id, expr.span);
1202 self.init_from_succ(ln, succ);
1203 self.merge_from_succ(ln, r_succ, false);
1205 self.propagate_through_expr(&l, ln)
1208 hir::ExprKind::Index(ref l, ref r) |
1209 hir::ExprKind::Binary(_, ref l, ref r) => {
1210 let r_succ = self.propagate_through_expr(&r, succ);
1211 self.propagate_through_expr(&l, r_succ)
1214 hir::ExprKind::Box(ref e) |
1215 hir::ExprKind::AddrOf(_, ref e) |
1216 hir::ExprKind::Cast(ref e, _) |
1217 hir::ExprKind::Type(ref e, _) |
1218 hir::ExprKind::Unary(_, ref e) |
1219 hir::ExprKind::Yield(ref e) |
1220 hir::ExprKind::Repeat(ref e, _) => {
1221 self.propagate_through_expr(&e, succ)
1224 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1225 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1226 // see comment on places
1227 // in propagate_through_place_components()
1229 self.propagate_through_expr(output, succ)
1231 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1232 let succ = self.write_place(output, succ, acc);
1233 self.propagate_through_place_components(output, succ)
1236 // Inputs are executed first. Propagate last because of rev order
1237 self.propagate_through_exprs(inputs, succ)
1240 hir::ExprKind::Lit(..) | hir::ExprKind::Err |
1241 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
1245 // Note that labels have been resolved, so we don't need to look
1246 // at the label ident
1247 hir::ExprKind::Block(ref blk, _) => {
1248 self.propagate_through_block(&blk, succ)
1253 fn propagate_through_place_components(&mut self,
1259 // In general, the full flow graph structure for an
1260 // assignment/move/etc can be handled in one of two ways,
1261 // depending on whether what is being assigned is a "tracked
1262 // value" or not. A tracked value is basically a local
1263 // variable or argument.
1265 // The two kinds of graphs are:
1267 // Tracked place Untracked place
1268 // ----------------------++-----------------------
1272 // (rvalue) || (rvalue)
1275 // (write of place) || (place components)
1280 // ----------------------++-----------------------
1282 // I will cover the two cases in turn:
1286 // A tracked place is a local variable/argument `x`. In
1287 // these cases, the link_node where the write occurs is linked
1288 // to node id of `x`. The `write_place()` routine generates
1289 // the contents of this node. There are no subcomponents to
1292 // # Non-tracked places
1294 // These are places like `x[5]` or `x.f`. In that case, we
1295 // basically ignore the value which is written to but generate
1296 // reads for the components---`x` in these two examples. The
1297 // components reads are generated by
1298 // `propagate_through_place_components()` (this fn).
1302 // It is still possible to observe assignments to non-places;
1303 // these errors are detected in the later pass borrowck. We
1304 // just ignore such cases and treat them as reads.
1307 hir::ExprKind::Path(_) => succ,
1308 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1309 _ => self.propagate_through_expr(expr, succ)
1313 // see comment on propagate_through_place()
1314 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32) -> LiveNode {
1316 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1317 self.access_path(expr.hir_id, path, succ, acc)
1320 // We do not track other places, so just propagate through
1321 // to their subcomponents. Also, it may happen that
1322 // non-places occur here, because those are detected in the
1323 // later pass borrowck.
1328 fn access_var(&mut self, hir_id: HirId, nid: NodeId, succ: LiveNode, acc: u32, span: Span)
1330 let ln = self.live_node(hir_id, span);
1332 self.init_from_succ(ln, succ);
1333 let var_hid = self.ir.tcx.hir().node_to_hir_id(nid);
1334 let var = self.variable(var_hid, span);
1335 self.acc(ln, var, acc);
1340 fn access_path(&mut self, hir_id: HirId, path: &hir::Path, succ: LiveNode, acc: u32)
1343 Def::Local(nid) => {
1344 self.access_var(hir_id, nid, succ, acc, path.span)
1350 fn propagate_through_loop(&mut self,
1358 We model control flow like this:
1376 let mut first_merge = true;
1377 let ln = self.live_node(expr.hir_id, expr.span);
1378 self.init_empty(ln, succ);
1382 // If this is not a `loop` loop, then it's possible we bypass
1383 // the body altogether. Otherwise, the only way is via a `break`
1384 // in the loop body.
1385 self.merge_from_succ(ln, succ, first_merge);
1386 first_merge = false;
1389 debug!("propagate_through_loop: using id for loop body {} {}",
1390 expr.id, self.ir.tcx.hir().node_to_pretty_string(body.id));
1393 self.break_ln.insert(expr.id, succ);
1395 let cond_ln = match kind {
1397 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1400 self.cont_ln.insert(expr.id, cond_ln);
1402 let body_ln = self.propagate_through_block(body, cond_ln);
1404 // repeat until fixed point is reached:
1405 while self.merge_from_succ(ln, body_ln, first_merge) {
1406 first_merge = false;
1408 let new_cond_ln = match kind {
1410 WhileLoop(ref cond) => {
1411 self.propagate_through_expr(&cond, ln)
1414 assert_eq!(cond_ln, new_cond_ln);
1415 assert_eq!(body_ln, self.propagate_through_block(body, cond_ln));
1422 // _______________________________________________________________________
1423 // Checking for error conditions
1425 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1426 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1427 NestedVisitorMap::None
1430 fn visit_local(&mut self, l: &'tcx hir::Local) {
1431 check_local(self, l);
1433 fn visit_expr(&mut self, ex: &'tcx Expr) {
1434 check_expr(self, ex);
1436 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1441 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1444 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1447 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1448 let span = local.pat.simple_ident().map_or(sp, |ident| ident.span);
1449 this.warn_about_unused(vec![span], id, ln, var);
1454 intravisit::walk_local(this, local);
1457 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1458 // Only consider the variable from the first pattern; any later patterns must have
1459 // the same bindings, and we also consider the first pattern to be the "authoritative" set of
1460 // ids. However, we should take the spans of variables with the same name from the later
1461 // patterns so the suggestions to prefix with underscores will apply to those too.
1462 let mut vars: BTreeMap<String, (LiveNode, Variable, HirId, Vec<Span>)> = Default::default();
1464 for pat in &arm.pats {
1465 this.arm_pats_bindings(Some(&*pat), |this, ln, var, sp, id| {
1466 let name = this.ir.variable_name(var);
1468 .and_modify(|(.., spans)| {
1471 .or_insert_with(|| {
1472 (ln, var, id, vec![sp])
1477 for (_, (ln, var, id, spans)) in vars {
1478 this.warn_about_unused(spans, id, ln, var);
1481 intravisit::walk_arm(this, arm);
1484 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1486 hir::ExprKind::Assign(ref l, _) => {
1487 this.check_place(&l);
1489 intravisit::walk_expr(this, expr);
1492 hir::ExprKind::AssignOp(_, ref l, _) => {
1493 if !this.tables.is_method_call(expr) {
1494 this.check_place(&l);
1497 intravisit::walk_expr(this, expr);
1500 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1501 for input in inputs {
1502 this.visit_expr(input);
1505 // Output operands must be places
1506 for (o, output) in ia.outputs.iter().zip(outputs) {
1508 this.check_place(output);
1510 this.visit_expr(output);
1513 intravisit::walk_expr(this, expr);
1516 // no correctness conditions related to liveness
1517 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) | hir::ExprKind::If(..) |
1518 hir::ExprKind::Match(..) | hir::ExprKind::While(..) | hir::ExprKind::Loop(..) |
1519 hir::ExprKind::Index(..) | hir::ExprKind::Field(..) |
1520 hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) |
1521 hir::ExprKind::Cast(..) | hir::ExprKind::Unary(..) | hir::ExprKind::Ret(..) |
1522 hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) | hir::ExprKind::Lit(_) |
1523 hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) |
1524 hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) |
1525 hir::ExprKind::Closure(..) | hir::ExprKind::Path(_) | hir::ExprKind::Yield(..) |
1526 hir::ExprKind::Box(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err => {
1527 intravisit::walk_expr(this, expr);
1532 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1533 fn check_place(&mut self, expr: &'tcx Expr) {
1535 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1536 if let Def::Local(nid) = path.def {
1537 // Assignment to an immutable variable or argument: only legal
1538 // if there is no later assignment. If this local is actually
1539 // mutable, then check for a reassignment to flag the mutability
1541 let ln = self.live_node(expr.hir_id, expr.span);
1542 let var_hid = self.ir.tcx.hir().node_to_hir_id(nid);
1543 let var = self.variable(var_hid, expr.span);
1544 self.warn_about_dead_assign(expr.span, expr.hir_id, ln, var);
1548 // For other kinds of places, no checks are required,
1549 // and any embedded expressions are actually rvalues
1550 intravisit::walk_expr(self, expr);
1555 fn should_warn(&self, var: Variable) -> Option<String> {
1556 let name = self.ir.variable_name(var);
1557 if name.is_empty() || name.as_bytes()[0] == b'_' {
1564 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1565 for arg in &body.arguments {
1566 arg.pat.each_binding(|_bm, hir_id, _, ident| {
1567 let sp = ident.span;
1568 let var = self.variable(hir_id, sp);
1569 // Ignore unused self.
1570 if ident.name != keywords::SelfLower.name() {
1571 if !self.warn_about_unused(vec![sp], hir_id, entry_ln, var) {
1572 if self.live_on_entry(entry_ln, var).is_none() {
1573 self.report_dead_assign(hir_id, sp, var, true);
1581 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1582 self.pat_bindings(pat, |this, ln, var, sp, id| {
1583 if !this.warn_about_unused(vec![sp], id, ln, var) {
1584 this.warn_about_dead_assign(sp, id, ln, var);
1589 fn warn_about_unused(&self,
1595 if !self.used_on_entry(ln, var) {
1596 let r = self.should_warn(var);
1597 if let Some(name) = r {
1598 // annoying: for parameters in funcs like `fn(x: i32)
1599 // {ret}`, there is only one node, so asking about
1600 // assigned_on_exit() is not meaningful.
1601 let is_assigned = if ln == self.s.exit_ln {
1604 self.assigned_on_exit(ln, var).is_some()
1608 self.ir.tcx.lint_hir_note(
1609 lint::builtin::UNUSED_VARIABLES,
1612 &format!("variable `{}` is assigned to, but never used", name),
1613 &format!("consider using `_{}` instead", name),
1615 } else if name != "self" {
1616 let mut err = self.ir.tcx.struct_span_lint_hir(
1617 lint::builtin::UNUSED_VARIABLES,
1620 &format!("unused variable: `{}`", name),
1623 if self.ir.variable_is_shorthand(var) {
1624 err.multipart_suggestion(
1625 "try ignoring the field",
1626 spans.iter().map(|span| (*span, format!("{}: _", name))).collect(),
1627 Applicability::MachineApplicable
1630 err.multipart_suggestion(
1631 "consider prefixing with an underscore",
1632 spans.iter().map(|span| (*span, format!("_{}", name))).collect(),
1633 Applicability::MachineApplicable,
1646 fn warn_about_dead_assign(&self, sp: Span, hir_id: HirId, ln: LiveNode, var: Variable) {
1647 if self.live_on_exit(ln, var).is_none() {
1648 self.report_dead_assign(hir_id, sp, var, false);
1652 fn report_dead_assign(&self, hir_id: HirId, sp: Span, var: Variable, is_argument: bool) {
1653 if let Some(name) = self.should_warn(var) {
1655 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1656 &format!("value passed to `{}` is never read", name))
1657 .help("maybe it is overwritten before being read?")
1660 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1661 &format!("value assigned to `{}` is never read", name))
1662 .help("maybe it is overwritten before being read?")