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::LiveNodeKind::*;
100 use rustc::hir::{Expr, HirId};
101 use rustc::hir::def::*;
102 use rustc::hir::def_id::DefId;
103 use rustc::hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
104 use rustc::hir::Node;
105 use rustc::hir::ptr::P;
106 use rustc::ty::{self, TyCtxt};
107 use rustc::ty::query::Providers;
109 use rustc::util::nodemap::{HirIdMap, HirIdSet};
111 use errors::Applicability;
112 use rustc_data_structures::fx::FxIndexMap;
113 use std::collections::VecDeque;
115 use std::io::prelude::*;
119 use syntax::symbol::sym;
120 use syntax_pos::Span;
122 #[derive(Copy, Clone, PartialEq)]
123 struct Variable(u32);
125 #[derive(Copy, Clone, PartialEq)]
126 struct LiveNode(u32);
129 fn get(&self) -> usize { self.0 as usize }
133 fn get(&self) -> usize { self.0 as usize }
136 #[derive(Copy, Clone, PartialEq, Debug)]
144 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
145 let cm = tcx.sess.source_map();
148 format!("Upvar node [{}]", cm.span_to_string(s))
151 format!("Expr node [{}]", cm.span_to_string(s))
154 format!("Var def node [{}]", cm.span_to_string(s))
156 ExitNode => "Exit node".to_owned(),
160 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
161 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
162 NestedVisitorMap::OnlyBodies(&self.tcx.hir())
165 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
166 b: hir::BodyId, s: Span, id: HirId) {
167 visit_fn(self, fk, fd, b, s, id);
170 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
171 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
172 fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
175 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: DefId) {
176 tcx.hir().visit_item_likes_in_module(
178 &mut IrMaps::new(tcx, module_def_id).as_deep_visitor(),
182 pub fn provide(providers: &mut Providers<'_>) {
183 *providers = Providers {
189 impl fmt::Debug for LiveNode {
190 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
191 write!(f, "ln({})", self.get())
195 impl fmt::Debug for Variable {
196 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
197 write!(f, "v({})", self.get())
201 // ______________________________________________________________________
204 // This is the first pass and the one that drives the main
205 // computation. It walks up and down the IR once. On the way down,
206 // we count for each function the number of variables as well as
207 // liveness nodes. A liveness node is basically an expression or
208 // capture clause that does something of interest: either it has
209 // interesting control flow or it uses/defines a local variable.
211 // On the way back up, at each function node we create liveness sets
212 // (we now know precisely how big to make our various vectors and so
213 // forth) and then do the data-flow propagation to compute the set
214 // of live variables at each program point.
216 // Finally, we run back over the IR one last time and, using the
217 // computed liveness, check various safety conditions. For example,
218 // there must be no live nodes at the definition site for a variable
219 // unless it has an initializer. Similarly, each non-mutable local
220 // variable must not be assigned if there is some successor
221 // assignment. And so forth.
224 fn is_valid(&self) -> bool {
229 fn invalid_node() -> LiveNode { LiveNode(u32::MAX) }
236 #[derive(Copy, Clone, Debug)]
243 #[derive(Copy, Clone, Debug)]
245 Param(HirId, ast::Name),
250 struct IrMaps<'tcx> {
253 num_live_nodes: usize,
255 live_node_map: HirIdMap<LiveNode>,
256 variable_map: HirIdMap<Variable>,
257 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
258 var_kinds: Vec<VarKind>,
259 lnks: Vec<LiveNodeKind>,
263 fn new(tcx: TyCtxt<'tcx>, body_owner: DefId) -> IrMaps<'tcx> {
269 live_node_map: HirIdMap::default(),
270 variable_map: HirIdMap::default(),
271 capture_info_map: Default::default(),
272 var_kinds: Vec::new(),
277 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
278 let ln = LiveNode(self.num_live_nodes as u32);
280 self.num_live_nodes += 1;
282 debug!("{:?} is of kind {}", ln,
283 live_node_kind_to_string(lnk, self.tcx));
288 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
289 let ln = self.add_live_node(lnk);
290 self.live_node_map.insert(hir_id, ln);
292 debug!("{:?} is node {:?}", ln, hir_id);
295 fn add_variable(&mut self, vk: VarKind) -> Variable {
296 let v = Variable(self.num_vars as u32);
297 self.var_kinds.push(vk);
301 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) => {
302 self.variable_map.insert(node_id, v);
307 debug!("{:?} is {:?}", v, vk);
312 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
313 match self.variable_map.get(&hir_id) {
316 span_bug!(span, "no variable registered for id {:?}", hir_id);
321 fn variable_name(&self, var: Variable) -> String {
322 match self.var_kinds[var.get()] {
323 Local(LocalInfo { name, .. }) | Param(_, name) => {
326 CleanExit => "<clean-exit>".to_owned()
330 fn variable_is_shorthand(&self, var: Variable) -> bool {
331 match self.var_kinds[var.get()] {
332 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
333 Param(..) | CleanExit => false
337 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
338 self.capture_info_map.insert(hir_id, Rc::new(cs));
341 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
347 ir: &mut IrMaps<'tcx>,
349 decl: &'tcx hir::FnDecl,
350 body_id: hir::BodyId,
356 // swap in a new set of IR maps for this function body:
357 let def_id = ir.tcx.hir().local_def_id(id);
358 let mut fn_maps = IrMaps::new(ir.tcx, def_id);
360 // Don't run unused pass for #[derive()]
361 if let FnKind::Method(..) = fk {
362 let parent = ir.tcx.hir().get_parent_item(id);
363 if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
364 if i.attrs.iter().any(|a| a.check_name(sym::automatically_derived)) {
370 debug!("creating fn_maps: {:p}", &fn_maps);
372 let body = ir.tcx.hir().body(body_id);
374 for param in &body.params {
375 let is_shorthand = match param.pat.kind {
376 rustc::hir::PatKind::Struct(..) => true,
379 param.pat.each_binding(|_bm, hir_id, _x, ident| {
380 debug!("adding parameters {:?}", hir_id);
381 let var = if is_shorthand {
388 Param(hir_id, ident.name)
390 fn_maps.add_variable(var);
394 // gather up the various local variables, significant expressions,
396 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
399 let mut lsets = Liveness::new(&mut fn_maps, body_id);
400 let entry_ln = lsets.compute(&body.value);
402 // check for various error conditions
403 lsets.visit_body(body);
404 lsets.warn_about_unused_args(body, entry_ln);
407 fn add_from_pat(ir: &mut IrMaps<'_>, pat: &P<hir::Pat>) {
408 // For struct patterns, take note of which fields used shorthand
409 // (`x` rather than `x: x`).
410 let mut shorthand_field_ids = HirIdSet::default();
411 let mut pats = VecDeque::new();
413 while let Some(pat) = pats.pop_front() {
414 use rustc::hir::PatKind::*;
416 Binding(.., inner_pat) => {
417 pats.extend(inner_pat.iter());
419 Struct(_, fields, _) => {
420 let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
421 shorthand_field_ids.extend(ids);
423 Ref(inner_pat, _) | Box(inner_pat) => {
424 pats.push_back(inner_pat);
426 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
427 pats.extend(inner_pats.iter());
429 Slice(pre_pats, inner_pat, post_pats) => {
430 pats.extend(pre_pats.iter());
431 pats.extend(inner_pat.iter());
432 pats.extend(post_pats.iter());
438 pat.each_binding(|_, hir_id, _, 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<'tcx>(ir: &mut IrMaps<'tcx>, local: &'tcx hir::Local) {
449 add_from_pat(ir, &local.pat);
450 intravisit::walk_local(ir, local);
453 fn visit_arm<'tcx>(ir: &mut IrMaps<'tcx>, arm: &'tcx hir::Arm) {
454 add_from_pat(ir, &arm.pat);
455 intravisit::walk_arm(ir, arm);
458 fn visit_expr<'tcx>(ir: &mut IrMaps<'tcx>, expr: &'tcx Expr) {
460 // live nodes required for uses or definitions of variables:
461 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
462 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
463 if let Res::Local(var_hir_id) = path.res {
464 let upvars = ir.tcx.upvars(ir.body_owner);
465 if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hir_id)) {
466 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
469 intravisit::walk_expr(ir, expr);
471 hir::ExprKind::Closure(..) => {
472 // Interesting control flow (for loops can contain labeled
473 // breaks or continues)
474 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
476 // Make a live_node for each captured variable, with the span
477 // being the location that the variable is used. This results
478 // in better error messages than just pointing at the closure
479 // construction site.
480 let mut call_caps = Vec::new();
481 let closure_def_id = ir.tcx.hir().local_def_id(expr.hir_id);
482 if let Some(upvars) = ir.tcx.upvars(closure_def_id) {
483 let parent_upvars = ir.tcx.upvars(ir.body_owner);
484 call_caps.extend(upvars.iter().filter_map(|(&var_id, upvar)| {
485 let has_parent = parent_upvars
486 .map_or(false, |upvars| upvars.contains_key(&var_id));
488 let upvar_ln = ir.add_live_node(UpvarNode(upvar.span));
489 Some(CaptureInfo { ln: upvar_ln, var_hid: var_id })
495 ir.set_captures(expr.hir_id, call_caps);
496 let old_body_owner = ir.body_owner;
497 ir.body_owner = closure_def_id;
498 intravisit::walk_expr(ir, expr);
499 ir.body_owner = old_body_owner;
502 // live nodes required for interesting control flow:
503 hir::ExprKind::Match(..) |
504 hir::ExprKind::Loop(..) => {
505 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
506 intravisit::walk_expr(ir, expr);
508 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
509 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
510 intravisit::walk_expr(ir, expr);
513 // otherwise, live nodes are not required:
514 hir::ExprKind::Index(..) |
515 hir::ExprKind::Field(..) |
516 hir::ExprKind::Array(..) |
517 hir::ExprKind::Call(..) |
518 hir::ExprKind::MethodCall(..) |
519 hir::ExprKind::Tup(..) |
520 hir::ExprKind::Binary(..) |
521 hir::ExprKind::AddrOf(..) |
522 hir::ExprKind::Cast(..) |
523 hir::ExprKind::DropTemps(..) |
524 hir::ExprKind::Unary(..) |
525 hir::ExprKind::Break(..) |
526 hir::ExprKind::Continue(_) |
527 hir::ExprKind::Lit(_) |
528 hir::ExprKind::Ret(..) |
529 hir::ExprKind::Block(..) |
530 hir::ExprKind::Assign(..) |
531 hir::ExprKind::AssignOp(..) |
532 hir::ExprKind::Struct(..) |
533 hir::ExprKind::Repeat(..) |
534 hir::ExprKind::InlineAsm(..) |
535 hir::ExprKind::Box(..) |
536 hir::ExprKind::Yield(..) |
537 hir::ExprKind::Type(..) |
539 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
540 intravisit::walk_expr(ir, expr);
545 // ______________________________________________________________________
546 // Computing liveness sets
548 // Actually we compute just a bit more than just liveness, but we use
549 // the same basic propagation framework in all cases.
551 #[derive(Clone, Copy)]
558 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
559 /// very large, so it uses a more compact representation that takes advantage
560 /// of the fact that when the number of `RWU`s is large, most of them have an
561 /// invalid reader and an invalid writer.
563 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
564 /// an index into `unpacked_rwus`. In the common cases, this compacts the
565 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
568 /// More compact representations are possible -- e.g., use only 2 bits per
569 /// packed `RWU` and make the secondary table a HashMap that maps from
570 /// indices to `RWU`s -- but this one strikes a good balance between size
572 packed_rwus: Vec<u32>,
573 unpacked_rwus: Vec<RWU>,
576 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
577 const INV_INV_FALSE: u32 = u32::MAX;
579 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
580 const INV_INV_TRUE: u32 = u32::MAX - 1;
583 fn new(num_rwus: usize) -> RWUTable {
585 packed_rwus: vec![INV_INV_FALSE; num_rwus],
586 unpacked_rwus: vec![],
590 fn get(&self, idx: usize) -> RWU {
591 let packed_rwu = self.packed_rwus[idx];
593 INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false },
594 INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true },
595 _ => self.unpacked_rwus[packed_rwu as usize],
599 fn get_reader(&self, idx: usize) -> LiveNode {
600 let packed_rwu = self.packed_rwus[idx];
602 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
603 _ => self.unpacked_rwus[packed_rwu as usize].reader,
607 fn get_writer(&self, idx: usize) -> LiveNode {
608 let packed_rwu = self.packed_rwus[idx];
610 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
611 _ => self.unpacked_rwus[packed_rwu as usize].writer,
615 fn get_used(&self, idx: usize) -> bool {
616 let packed_rwu = self.packed_rwus[idx];
618 INV_INV_FALSE => false,
619 INV_INV_TRUE => true,
620 _ => self.unpacked_rwus[packed_rwu as usize].used,
625 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
626 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
629 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
630 if rwu.reader == invalid_node() && rwu.writer == invalid_node() {
631 // When we overwrite an indexing entry in `self.packed_rwus` with
632 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
633 // from `self.unpacked_rwus`; it's not worth the effort, and we
634 // can't have entries shifting around anyway.
635 self.packed_rwus[idx] = if rwu.used {
641 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
643 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
644 self.unpacked_rwus.push(rwu);
648 fn assign_inv_inv(&mut self, idx: usize) {
649 self.packed_rwus[idx] = if self.get_used(idx) {
657 #[derive(Copy, Clone)]
660 fallthrough_ln: LiveNode,
661 clean_exit_var: Variable
664 const ACC_READ: u32 = 1;
665 const ACC_WRITE: u32 = 2;
666 const ACC_USE: u32 = 4;
668 struct Liveness<'a, 'tcx> {
669 ir: &'a mut IrMaps<'tcx>,
670 tables: &'a ty::TypeckTables<'tcx>,
672 successors: Vec<LiveNode>,
675 // mappings from loop node ID to LiveNode
676 // ("break" label should map to loop node ID,
677 // it probably doesn't now)
678 break_ln: HirIdMap<LiveNode>,
679 cont_ln: HirIdMap<LiveNode>,
682 impl<'a, 'tcx> Liveness<'a, 'tcx> {
683 fn new(ir: &'a mut IrMaps<'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
684 // Special nodes and variables:
685 // - exit_ln represents the end of the fn, either by return or panic
686 // - implicit_ret_var is a pseudo-variable that represents
687 // an implicit return
688 let specials = Specials {
689 exit_ln: ir.add_live_node(ExitNode),
690 fallthrough_ln: ir.add_live_node(ExitNode),
691 clean_exit_var: ir.add_variable(CleanExit)
694 let tables = ir.tcx.body_tables(body);
696 let num_live_nodes = ir.num_live_nodes;
697 let num_vars = ir.num_vars;
703 successors: vec![invalid_node(); num_live_nodes],
704 rwu_table: RWUTable::new(num_live_nodes * num_vars),
705 break_ln: Default::default(),
706 cont_ln: Default::default(),
710 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
711 match self.ir.live_node_map.get(&hir_id) {
714 // This must be a mismatch between the ir_map construction
715 // above and the propagation code below; the two sets of
716 // code have to agree about which AST nodes are worth
717 // creating liveness nodes for.
720 "no live node registered for node {:?}",
726 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
727 self.ir.variable(hir_id, span)
730 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, mut succ: LiveNode) -> LiveNode {
731 // In an or-pattern, only consider the first pattern; any later patterns
732 // must have the same bindings, and we also consider the first pattern
733 // to be the "authoritative" set of ids.
734 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
735 let ln = self.live_node(hir_id, pat_sp);
736 let var = self.variable(hir_id, ident.span);
737 self.init_from_succ(ln, succ);
738 self.define(ln, var);
744 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
745 ln.get() * self.ir.num_vars + var.get()
748 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
749 assert!(ln.is_valid());
750 let reader = self.rwu_table.get_reader(self.idx(ln, var));
751 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
754 // Is this variable live on entry to any of its successor nodes?
755 fn live_on_exit(&self, ln: LiveNode, var: Variable)
756 -> Option<LiveNodeKind> {
757 let successor = self.successors[ln.get()];
758 self.live_on_entry(successor, var)
761 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
762 assert!(ln.is_valid());
763 self.rwu_table.get_used(self.idx(ln, var))
766 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
767 -> Option<LiveNodeKind> {
768 assert!(ln.is_valid());
769 let writer = self.rwu_table.get_writer(self.idx(ln, var));
770 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
773 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
774 -> Option<LiveNodeKind> {
775 let successor = self.successors[ln.get()];
776 self.assigned_on_entry(successor, var)
779 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
780 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
782 let node_base_idx = self.idx(ln, Variable(0));
783 let succ_base_idx = self.idx(succ_ln, Variable(0));
784 for var_idx in 0..self.ir.num_vars {
785 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
789 fn write_vars<F>(&self,
793 -> io::Result<()> where
794 F: FnMut(usize) -> LiveNode,
796 let node_base_idx = self.idx(ln, Variable(0));
797 for var_idx in 0..self.ir.num_vars {
798 let idx = node_base_idx + var_idx;
799 if test(idx).is_valid() {
800 write!(wr, " {:?}", Variable(var_idx as u32))?;
807 #[allow(unused_must_use)]
808 fn ln_str(&self, ln: LiveNode) -> String {
809 let mut wr = Vec::new();
811 let wr = &mut wr as &mut dyn Write;
812 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
813 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx));
814 write!(wr, " writes");
815 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx));
816 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
818 String::from_utf8(wr).unwrap()
821 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
822 self.successors[ln.get()] = succ_ln;
824 // It is not necessary to initialize the RWUs here because they are all
825 // set to INV_INV_FALSE when they are created, and the sets only grow
826 // during iterations.
829 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
830 // more efficient version of init_empty() / merge_from_succ()
831 self.successors[ln.get()] = succ_ln;
833 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
834 this.rwu_table.copy_packed(idx, succ_idx);
836 debug!("init_from_succ(ln={}, succ={})",
837 self.ln_str(ln), self.ln_str(succ_ln));
840 fn merge_from_succ(&mut self,
845 if ln == succ_ln { return false; }
847 let mut changed = false;
848 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
849 let mut rwu = this.rwu_table.get(idx);
850 let succ_rwu = this.rwu_table.get(succ_idx);
851 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
852 rwu.reader = succ_rwu.reader;
856 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
857 rwu.writer = succ_rwu.writer;
861 if succ_rwu.used && !rwu.used {
867 this.rwu_table.assign_unpacked(idx, rwu);
871 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
872 ln, self.ln_str(succ_ln), first_merge, changed);
876 // Indicates that a local variable was *defined*; we know that no
877 // uses of the variable can precede the definition (resolve checks
878 // this) so we just clear out all the data.
879 fn define(&mut self, writer: LiveNode, var: Variable) {
880 let idx = self.idx(writer, var);
881 self.rwu_table.assign_inv_inv(idx);
883 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
884 idx, self.ln_str(writer));
887 // Either read, write, or both depending on the acc bitset
888 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
889 debug!("{:?} accesses[{:x}] {:?}: {}",
890 ln, acc, var, self.ln_str(ln));
892 let idx = self.idx(ln, var);
893 let mut rwu = self.rwu_table.get(idx);
895 if (acc & ACC_WRITE) != 0 {
896 rwu.reader = invalid_node();
900 // Important: if we both read/write, must do read second
901 // or else the write will override.
902 if (acc & ACC_READ) != 0 {
906 if (acc & ACC_USE) != 0 {
910 self.rwu_table.assign_unpacked(idx, rwu);
913 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
914 debug!("compute: using id for body, {}",
915 self.ir.tcx.hir().hir_to_pretty_string(body.hir_id));
917 // the fallthrough exit is only for those cases where we do not
918 // explicitly return:
920 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
921 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
923 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
925 // hack to skip the loop unless debug! is enabled:
926 debug!("^^ liveness computation results for body {} (entry={:?})", {
927 for ln_idx in 0..self.ir.num_live_nodes {
928 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
937 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
939 if blk.targeted_by_break {
940 self.break_ln.insert(blk.hir_id, succ);
942 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
943 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
944 self.propagate_through_stmt(stmt, succ)
948 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
951 hir::StmtKind::Local(ref local) => {
952 // Note: we mark the variable as defined regardless of whether
953 // there is an initializer. Initially I had thought to only mark
954 // the live variable as defined if it was initialized, and then we
955 // could check for uninit variables just by scanning what is live
956 // at the start of the function. But that doesn't work so well for
957 // immutable variables defined in a loop:
958 // loop { let x; x = 5; }
959 // because the "assignment" loops back around and generates an error.
961 // So now we just check that variables defined w/o an
962 // initializer are not live at the point of their
963 // initialization, which is mildly more complex than checking
964 // once at the func header but otherwise equivalent.
966 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
967 self.define_bindings_in_pat(&local.pat, succ)
969 hir::StmtKind::Item(..) => succ,
970 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
971 self.propagate_through_expr(&expr, succ)
976 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
978 exprs.iter().rev().fold(succ, |succ, expr| {
979 self.propagate_through_expr(&expr, succ)
983 fn propagate_through_opt_expr(&mut self,
984 opt_expr: Option<&Expr>,
987 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
990 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode) -> LiveNode {
991 debug!("propagate_through_expr: {}", self.ir.tcx.hir().hir_to_pretty_string(expr.hir_id));
994 // Interesting cases with control flow or which gen/kill
995 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
996 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
999 hir::ExprKind::Field(ref e, _) => {
1000 self.propagate_through_expr(&e, succ)
1003 hir::ExprKind::Closure(..) => {
1004 debug!("{} is an ExprKind::Closure",
1005 self.ir.tcx.hir().hir_to_pretty_string(expr.hir_id));
1007 // the construction of a closure itself is not important,
1008 // but we have to consider the closed over variables.
1009 let caps = self.ir.capture_info_map.get(&expr.hir_id).cloned().unwrap_or_else(||
1010 span_bug!(expr.span, "no registered caps"));
1012 caps.iter().rev().fold(succ, |succ, cap| {
1013 self.init_from_succ(cap.ln, succ);
1014 let var = self.variable(cap.var_hid, expr.span);
1015 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1020 // Note that labels have been resolved, so we don't need to look
1021 // at the label ident
1022 hir::ExprKind::Loop(ref blk, _, _) => {
1023 self.propagate_through_loop(expr, &blk, succ)
1026 hir::ExprKind::Match(ref e, ref arms, _) => {
1041 let ln = self.live_node(expr.hir_id, expr.span);
1042 self.init_empty(ln, succ);
1043 let mut first_merge = true;
1045 let body_succ = self.propagate_through_expr(&arm.body, succ);
1047 let guard_succ = self.propagate_through_opt_expr(
1048 arm.guard.as_ref().map(|hir::Guard::If(e)| &**e),
1051 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
1052 self.merge_from_succ(ln, arm_succ, first_merge);
1053 first_merge = false;
1055 self.propagate_through_expr(&e, ln)
1058 hir::ExprKind::Ret(ref o_e) => {
1059 // ignore succ and subst exit_ln:
1060 let exit_ln = self.s.exit_ln;
1061 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1064 hir::ExprKind::Break(label, ref opt_expr) => {
1065 // Find which label this break jumps to
1066 let target = match label.target_id {
1067 Ok(hir_id) => self.break_ln.get(&hir_id),
1068 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1071 // Now that we know the label we're going to,
1072 // look it up in the break loop nodes table
1075 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1077 // FIXME: This should have been checked earlier. Once this is fixed,
1078 // replace with `delay_span_bug`. (#62480)
1079 self.ir.tcx.sess.struct_span_err(
1081 "`break` to unknown label",
1083 errors::FatalError.raise()
1088 hir::ExprKind::Continue(label) => {
1089 // Find which label this expr continues to
1090 let sc = label.target_id.unwrap_or_else(|err|
1091 span_bug!(expr.span, "loop scope error: {}", err));
1093 // Now that we know the label we're going to,
1094 // look it up in the continue loop nodes table
1095 self.cont_ln.get(&sc).cloned().unwrap_or_else(||
1096 span_bug!(expr.span, "continue to unknown label"))
1099 hir::ExprKind::Assign(ref l, ref r) => {
1100 // see comment on places in
1101 // propagate_through_place_components()
1102 let succ = self.write_place(&l, succ, ACC_WRITE);
1103 let succ = self.propagate_through_place_components(&l, succ);
1104 self.propagate_through_expr(&r, succ)
1107 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1108 // an overloaded assign op is like a method call
1109 if self.tables.is_method_call(expr) {
1110 let succ = self.propagate_through_expr(&l, succ);
1111 self.propagate_through_expr(&r, succ)
1113 // see comment on places in
1114 // propagate_through_place_components()
1115 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1116 let succ = self.propagate_through_expr(&r, succ);
1117 self.propagate_through_place_components(&l, succ)
1121 // Uninteresting cases: just propagate in rev exec order
1123 hir::ExprKind::Array(ref exprs) => {
1124 self.propagate_through_exprs(exprs, succ)
1127 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1128 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1129 fields.iter().rev().fold(succ, |succ, field| {
1130 self.propagate_through_expr(&field.expr, succ)
1134 hir::ExprKind::Call(ref f, ref args) => {
1135 let m = self.ir.tcx.hir().get_module_parent(expr.hir_id);
1136 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1141 let succ = self.propagate_through_exprs(args, succ);
1142 self.propagate_through_expr(&f, succ)
1145 hir::ExprKind::MethodCall(.., ref args) => {
1146 let m = self.ir.tcx.hir().get_module_parent(expr.hir_id);
1147 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1153 self.propagate_through_exprs(args, succ)
1156 hir::ExprKind::Tup(ref exprs) => {
1157 self.propagate_through_exprs(exprs, succ)
1160 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1161 let r_succ = self.propagate_through_expr(&r, succ);
1163 let ln = self.live_node(expr.hir_id, expr.span);
1164 self.init_from_succ(ln, succ);
1165 self.merge_from_succ(ln, r_succ, false);
1167 self.propagate_through_expr(&l, ln)
1170 hir::ExprKind::Index(ref l, ref r) |
1171 hir::ExprKind::Binary(_, ref l, ref r) => {
1172 let r_succ = self.propagate_through_expr(&r, succ);
1173 self.propagate_through_expr(&l, r_succ)
1176 hir::ExprKind::Box(ref e) |
1177 hir::ExprKind::AddrOf(_, _, ref e) |
1178 hir::ExprKind::Cast(ref e, _) |
1179 hir::ExprKind::Type(ref e, _) |
1180 hir::ExprKind::DropTemps(ref e) |
1181 hir::ExprKind::Unary(_, ref e) |
1182 hir::ExprKind::Yield(ref e, _) |
1183 hir::ExprKind::Repeat(ref e, _) => {
1184 self.propagate_through_expr(&e, succ)
1187 hir::ExprKind::InlineAsm(ref asm) => {
1188 let ia = &asm.inner;
1189 let outputs = &asm.outputs_exprs;
1190 let inputs = &asm.inputs_exprs;
1191 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1192 // see comment on places
1193 // in propagate_through_place_components()
1195 self.propagate_through_expr(output, succ)
1197 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1198 let succ = self.write_place(output, succ, acc);
1199 self.propagate_through_place_components(output, succ)
1203 // Inputs are executed first. Propagate last because of rev order
1204 self.propagate_through_exprs(inputs, succ)
1207 hir::ExprKind::Lit(..) | hir::ExprKind::Err |
1208 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
1212 // Note that labels have been resolved, so we don't need to look
1213 // at the label ident
1214 hir::ExprKind::Block(ref blk, _) => {
1215 self.propagate_through_block(&blk, succ)
1220 fn propagate_through_place_components(&mut self,
1226 // In general, the full flow graph structure for an
1227 // assignment/move/etc can be handled in one of two ways,
1228 // depending on whether what is being assigned is a "tracked
1229 // value" or not. A tracked value is basically a local
1230 // variable or argument.
1232 // The two kinds of graphs are:
1234 // Tracked place Untracked place
1235 // ----------------------++-----------------------
1239 // (rvalue) || (rvalue)
1242 // (write of place) || (place components)
1247 // ----------------------++-----------------------
1249 // I will cover the two cases in turn:
1253 // A tracked place is a local variable/argument `x`. In
1254 // these cases, the link_node where the write occurs is linked
1255 // to node id of `x`. The `write_place()` routine generates
1256 // the contents of this node. There are no subcomponents to
1259 // # Non-tracked places
1261 // These are places like `x[5]` or `x.f`. In that case, we
1262 // basically ignore the value which is written to but generate
1263 // reads for the components---`x` in these two examples. The
1264 // components reads are generated by
1265 // `propagate_through_place_components()` (this fn).
1269 // It is still possible to observe assignments to non-places;
1270 // these errors are detected in the later pass borrowck. We
1271 // just ignore such cases and treat them as reads.
1274 hir::ExprKind::Path(_) => succ,
1275 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1276 _ => self.propagate_through_expr(expr, succ)
1280 // see comment on propagate_through_place()
1281 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32) -> LiveNode {
1283 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1284 self.access_path(expr.hir_id, path, succ, acc)
1287 // We do not track other places, so just propagate through
1288 // to their subcomponents. Also, it may happen that
1289 // non-places occur here, because those are detected in the
1290 // later pass borrowck.
1295 fn access_var(&mut self, hir_id: HirId, var_hid: HirId, succ: LiveNode, acc: u32, span: Span)
1297 let ln = self.live_node(hir_id, span);
1299 self.init_from_succ(ln, succ);
1300 let var = self.variable(var_hid, span);
1301 self.acc(ln, var, acc);
1306 fn access_path(&mut self, hir_id: HirId, path: &hir::Path, succ: LiveNode, acc: u32)
1309 Res::Local(hid) => {
1310 let upvars = self.ir.tcx.upvars(self.ir.body_owner);
1311 if !upvars.map_or(false, |upvars| upvars.contains_key(&hid)) {
1312 self.access_var(hir_id, hid, succ, acc, path.span)
1321 fn propagate_through_loop(
1328 We model control flow like this:
1335 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1336 Meanwhile, a `break` expression will have a successor of `succ`.
1340 let mut first_merge = true;
1341 let ln = self.live_node(expr.hir_id, expr.span);
1342 self.init_empty(ln, succ);
1343 debug!("propagate_through_loop: using id for loop body {} {}",
1344 expr.hir_id, self.ir.tcx.hir().hir_to_pretty_string(body.hir_id));
1346 self.break_ln.insert(expr.hir_id, succ);
1348 self.cont_ln.insert(expr.hir_id, ln);
1350 let body_ln = self.propagate_through_block(body, ln);
1352 // repeat until fixed point is reached:
1353 while self.merge_from_succ(ln, body_ln, first_merge) {
1354 first_merge = false;
1355 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1362 // _______________________________________________________________________
1363 // Checking for error conditions
1365 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1366 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1367 NestedVisitorMap::None
1370 fn visit_local(&mut self, local: &'tcx hir::Local) {
1371 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1372 if local.init.is_some() {
1373 self.warn_about_dead_assign(spans, hir_id, ln, var);
1377 intravisit::walk_local(self, local);
1380 fn visit_expr(&mut self, ex: &'tcx Expr) {
1381 check_expr(self, ex);
1384 fn visit_arm(&mut self, arm: &'tcx hir::Arm) {
1385 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1386 intravisit::walk_arm(self, arm);
1390 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr) {
1392 hir::ExprKind::Assign(ref l, _) => {
1393 this.check_place(&l);
1396 hir::ExprKind::AssignOp(_, ref l, _) => {
1397 if !this.tables.is_method_call(expr) {
1398 this.check_place(&l);
1402 hir::ExprKind::InlineAsm(ref asm) => {
1403 for input in &asm.inputs_exprs {
1404 this.visit_expr(input);
1407 // Output operands must be places
1408 for (o, output) in asm.inner.outputs.iter().zip(&asm.outputs_exprs) {
1410 this.check_place(output);
1412 this.visit_expr(output);
1416 // no correctness conditions related to liveness
1417 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) |
1418 hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) |
1419 hir::ExprKind::Index(..) | hir::ExprKind::Field(..) |
1420 hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) |
1421 hir::ExprKind::Cast(..) | hir::ExprKind::DropTemps(..) | hir::ExprKind::Unary(..) |
1422 hir::ExprKind::Ret(..) | hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) |
1423 hir::ExprKind::Lit(_) | hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) |
1424 hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) |
1425 hir::ExprKind::Closure(..) | hir::ExprKind::Path(_) | hir::ExprKind::Yield(..) |
1426 hir::ExprKind::Box(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err => {}
1429 intravisit::walk_expr(this, expr);
1432 impl<'tcx> Liveness<'_, 'tcx> {
1433 fn check_place(&mut self, expr: &'tcx Expr) {
1435 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1436 if let Res::Local(var_hid) = path.res {
1437 let upvars = self.ir.tcx.upvars(self.ir.body_owner);
1438 if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hid)) {
1439 // Assignment to an immutable variable or argument: only legal
1440 // if there is no later assignment. If this local is actually
1441 // mutable, then check for a reassignment to flag the mutability
1443 let ln = self.live_node(expr.hir_id, expr.span);
1444 let var = self.variable(var_hid, expr.span);
1445 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1450 // For other kinds of places, no checks are required,
1451 // and any embedded expressions are actually rvalues
1452 intravisit::walk_expr(self, expr);
1457 fn should_warn(&self, var: Variable) -> Option<String> {
1458 let name = self.ir.variable_name(var);
1459 if name.is_empty() || name.as_bytes()[0] == b'_' {
1466 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1467 for p in &body.params {
1468 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1469 if self.live_on_entry(ln, var).is_none() {
1470 self.report_dead_assign(hir_id, spans, var, true);
1476 fn check_unused_vars_in_pat(
1479 entry_ln: Option<LiveNode>,
1480 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1482 // In an or-pattern, only consider the variable; any later patterns must have the same
1483 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1484 // However, we should take the spans of variables with the same name from the later
1485 // patterns so the suggestions to prefix with underscores will apply to those too.
1486 let mut vars: FxIndexMap<String, (LiveNode, Variable, HirId, Vec<Span>)> = <_>::default();
1488 pat.each_binding(|_, hir_id, pat_sp, ident| {
1489 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1490 let var = self.variable(hir_id, ident.span);
1491 vars.entry(self.ir.variable_name(var))
1492 .and_modify(|(.., spans)| spans.push(ident.span))
1493 .or_insert_with(|| (ln, var, hir_id, vec![ident.span]));
1496 for (_, (ln, var, id, spans)) in vars {
1497 if self.used_on_entry(ln, var) {
1498 on_used_on_entry(spans, id, ln, var);
1500 self.report_unused(spans, id, ln, var);
1505 fn report_unused(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1506 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1507 // annoying: for parameters in funcs like `fn(x: i32)
1508 // {ret}`, there is only one node, so asking about
1509 // assigned_on_exit() is not meaningful.
1510 let is_assigned = if ln == self.s.exit_ln {
1513 self.assigned_on_exit(ln, var).is_some()
1517 self.ir.tcx.lint_hir_note(
1518 lint::builtin::UNUSED_VARIABLES,
1521 &format!("variable `{}` is assigned to, but never used", name),
1522 &format!("consider using `_{}` instead", name),
1525 let mut err = self.ir.tcx.struct_span_lint_hir(
1526 lint::builtin::UNUSED_VARIABLES,
1529 &format!("unused variable: `{}`", name),
1532 if self.ir.variable_is_shorthand(var) {
1533 if let Node::Binding(pat) = self.ir.tcx.hir().get(hir_id) {
1534 // Handle `ref` and `ref mut`.
1535 let spans = spans.iter()
1536 .map(|_span| (pat.span, format!("{}: _", name)))
1539 err.multipart_suggestion(
1540 "try ignoring the field",
1542 Applicability::MachineApplicable,
1546 err.multipart_suggestion(
1547 "consider prefixing with an underscore",
1548 spans.iter().map(|span| (*span, format!("_{}", name))).collect(),
1549 Applicability::MachineApplicable,
1558 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1559 if self.live_on_exit(ln, var).is_none() {
1560 self.report_dead_assign(hir_id, spans, var, false);
1564 fn report_dead_assign(&self, hir_id: HirId, spans: Vec<Span>, var: Variable, is_param: bool) {
1565 if let Some(name) = self.should_warn(var) {
1567 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, spans,
1568 &format!("value passed to `{}` is never read", name))
1569 .help("maybe it is overwritten before being read?")
1572 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, spans,
1573 &format!("value assigned to `{}` is never read", name))
1574 .help("maybe it is overwritten before being read?")