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::*;
99 use errors::Applicability;
100 use rustc::hir::intravisit::{self, FnKind, NestedVisitorMap, Visitor};
102 use rustc::ty::query::Providers;
103 use rustc::ty::{self, TyCtxt};
104 use rustc_data_structures::fx::FxIndexMap;
105 use rustc_hir as hir;
106 use rustc_hir::def::*;
107 use rustc_hir::def_id::DefId;
108 use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet, Node};
109 use rustc_span::symbol::sym;
110 use rustc_span::Span;
113 use std::collections::VecDeque;
115 use std::io::prelude::*;
119 #[derive(Copy, Clone, PartialEq)]
120 struct Variable(u32);
122 #[derive(Copy, Clone, PartialEq)]
123 struct LiveNode(u32);
126 fn get(&self) -> usize {
132 fn get(&self) -> usize {
137 #[derive(Copy, Clone, PartialEq, Debug)]
145 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
146 let cm = tcx.sess.source_map();
148 UpvarNode(s) => format!("Upvar node [{}]", cm.span_to_string(s)),
149 ExprNode(s) => format!("Expr node [{}]", cm.span_to_string(s)),
150 VarDefNode(s) => format!("Var def node [{}]", cm.span_to_string(s)),
151 ExitNode => "Exit node".to_owned(),
155 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
156 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
157 NestedVisitorMap::OnlyBodies(&self.tcx.hir())
163 fd: &'tcx hir::FnDecl<'tcx>,
168 visit_fn(self, fk, fd, b, s, id);
171 fn visit_local(&mut self, l: &'tcx hir::Local<'tcx>) {
172 visit_local(self, l);
174 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
175 visit_expr(self, ex);
177 fn visit_arm(&mut self, a: &'tcx hir::Arm<'tcx>) {
182 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: DefId) {
183 tcx.hir().visit_item_likes_in_module(
185 &mut IrMaps::new(tcx, module_def_id).as_deep_visitor(),
189 pub fn provide(providers: &mut Providers<'_>) {
190 *providers = Providers { check_mod_liveness, ..*providers };
193 impl fmt::Debug for LiveNode {
194 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
195 write!(f, "ln({})", self.get())
199 impl fmt::Debug for Variable {
200 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
201 write!(f, "v({})", self.get())
205 // ______________________________________________________________________
208 // This is the first pass and the one that drives the main
209 // computation. It walks up and down the IR once. On the way down,
210 // we count for each function the number of variables as well as
211 // liveness nodes. A liveness node is basically an expression or
212 // capture clause that does something of interest: either it has
213 // interesting control flow or it uses/defines a local variable.
215 // On the way back up, at each function node we create liveness sets
216 // (we now know precisely how big to make our various vectors and so
217 // forth) and then do the data-flow propagation to compute the set
218 // of live variables at each program point.
220 // Finally, we run back over the IR one last time and, using the
221 // computed liveness, check various safety conditions. For example,
222 // there must be no live nodes at the definition site for a variable
223 // unless it has an initializer. Similarly, each non-mutable local
224 // variable must not be assigned if there is some successor
225 // assignment. And so forth.
228 fn is_valid(&self) -> bool {
233 fn invalid_node() -> LiveNode {
242 #[derive(Copy, Clone, Debug)]
249 #[derive(Copy, Clone, Debug)]
251 Param(HirId, ast::Name),
256 struct IrMaps<'tcx> {
259 num_live_nodes: usize,
261 live_node_map: HirIdMap<LiveNode>,
262 variable_map: HirIdMap<Variable>,
263 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
264 var_kinds: Vec<VarKind>,
265 lnks: Vec<LiveNodeKind>,
269 fn new(tcx: TyCtxt<'tcx>, body_owner: DefId) -> IrMaps<'tcx> {
275 live_node_map: HirIdMap::default(),
276 variable_map: HirIdMap::default(),
277 capture_info_map: Default::default(),
278 var_kinds: Vec::new(),
283 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
284 let ln = LiveNode(self.num_live_nodes as u32);
286 self.num_live_nodes += 1;
288 debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx));
293 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
294 let ln = self.add_live_node(lnk);
295 self.live_node_map.insert(hir_id, ln);
297 debug!("{:?} is node {:?}", ln, hir_id);
300 fn add_variable(&mut self, vk: VarKind) -> Variable {
301 let v = Variable(self.num_vars as u32);
302 self.var_kinds.push(vk);
306 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) => {
307 self.variable_map.insert(node_id, v);
312 debug!("{:?} is {:?}", v, vk);
317 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
318 match self.variable_map.get(&hir_id) {
321 span_bug!(span, "no variable registered for id {:?}", hir_id);
326 fn variable_name(&self, var: Variable) -> String {
327 match self.var_kinds[var.get()] {
328 Local(LocalInfo { name, .. }) | Param(_, name) => name.to_string(),
329 CleanExit => "<clean-exit>".to_owned(),
333 fn variable_is_shorthand(&self, var: Variable) -> bool {
334 match self.var_kinds[var.get()] {
335 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
336 Param(..) | CleanExit => false,
340 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
341 self.capture_info_map.insert(hir_id, Rc::new(cs));
344 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
350 ir: &mut IrMaps<'tcx>,
352 decl: &'tcx hir::FnDecl<'tcx>,
353 body_id: hir::BodyId,
359 // swap in a new set of IR maps for this function body:
360 let def_id = ir.tcx.hir().local_def_id(id);
361 let mut fn_maps = IrMaps::new(ir.tcx, def_id);
363 // Don't run unused pass for #[derive()]
364 if let FnKind::Method(..) = fk {
365 let parent = ir.tcx.hir().get_parent_item(id);
366 if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
367 if i.attrs.iter().any(|a| a.check_name(sym::automatically_derived)) {
373 debug!("creating fn_maps: {:p}", &fn_maps);
375 let body = ir.tcx.hir().body(body_id);
377 for param in body.params {
378 let is_shorthand = match param.pat.kind {
379 rustc_hir::PatKind::Struct(..) => true,
382 param.pat.each_binding(|_bm, hir_id, _x, ident| {
383 debug!("adding parameters {:?}", hir_id);
384 let var = if is_shorthand {
385 Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: true })
387 Param(hir_id, ident.name)
389 fn_maps.add_variable(var);
393 // gather up the various local variables, significant expressions,
395 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
398 let mut lsets = Liveness::new(&mut fn_maps, body_id);
399 let entry_ln = lsets.compute(&body.value);
401 // check for various error conditions
402 lsets.visit_body(body);
403 lsets.warn_about_unused_args(body, entry_ln);
406 fn add_from_pat(ir: &mut IrMaps<'_>, pat: &hir::Pat<'_>) {
407 // For struct patterns, take note of which fields used shorthand
408 // (`x` rather than `x: x`).
409 let mut shorthand_field_ids = HirIdSet::default();
410 let mut pats = VecDeque::new();
412 while let Some(pat) = pats.pop_front() {
413 use rustc_hir::PatKind::*;
415 Binding(.., inner_pat) => {
416 pats.extend(inner_pat.iter());
418 Struct(_, fields, _) => {
419 let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
420 shorthand_field_ids.extend(ids);
422 Ref(inner_pat, _) | Box(inner_pat) => {
423 pats.push_back(inner_pat);
425 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
426 pats.extend(inner_pats.iter());
428 Slice(pre_pats, inner_pat, post_pats) => {
429 pats.extend(pre_pats.iter());
430 pats.extend(inner_pat.iter());
431 pats.extend(post_pats.iter());
437 pat.each_binding(|_, hir_id, _, ident| {
438 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
439 ir.add_variable(Local(LocalInfo {
442 is_shorthand: shorthand_field_ids.contains(&hir_id),
447 fn visit_local<'tcx>(ir: &mut IrMaps<'tcx>, local: &'tcx hir::Local<'tcx>) {
448 add_from_pat(ir, &local.pat);
449 intravisit::walk_local(ir, local);
452 fn visit_arm<'tcx>(ir: &mut IrMaps<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
453 add_from_pat(ir, &arm.pat);
454 intravisit::walk_arm(ir, arm);
457 fn visit_expr<'tcx>(ir: &mut IrMaps<'tcx>, expr: &'tcx Expr<'tcx>) {
459 // live nodes required for uses or definitions of variables:
460 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
461 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
462 if let Res::Local(var_hir_id) = path.res {
463 let upvars = ir.tcx.upvars(ir.body_owner);
464 if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hir_id)) {
465 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 let closure_def_id = ir.tcx.hir().local_def_id(expr.hir_id);
481 if let Some(upvars) = ir.tcx.upvars(closure_def_id) {
482 let parent_upvars = ir.tcx.upvars(ir.body_owner);
483 call_caps.extend(upvars.iter().filter_map(|(&var_id, upvar)| {
485 parent_upvars.map_or(false, |upvars| upvars.contains_key(&var_id));
487 let upvar_ln = ir.add_live_node(UpvarNode(upvar.span));
488 Some(CaptureInfo { ln: upvar_ln, var_hid: var_id })
494 ir.set_captures(expr.hir_id, call_caps);
495 let old_body_owner = ir.body_owner;
496 ir.body_owner = closure_def_id;
497 intravisit::walk_expr(ir, expr);
498 ir.body_owner = old_body_owner;
501 // live nodes required for interesting control flow:
502 hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
503 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
504 intravisit::walk_expr(ir, expr);
506 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
507 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
508 intravisit::walk_expr(ir, expr);
511 // otherwise, live nodes are not required:
512 hir::ExprKind::Index(..)
513 | hir::ExprKind::Field(..)
514 | hir::ExprKind::Array(..)
515 | hir::ExprKind::Call(..)
516 | hir::ExprKind::MethodCall(..)
517 | hir::ExprKind::Tup(..)
518 | hir::ExprKind::Binary(..)
519 | hir::ExprKind::AddrOf(..)
520 | hir::ExprKind::Cast(..)
521 | hir::ExprKind::DropTemps(..)
522 | hir::ExprKind::Unary(..)
523 | hir::ExprKind::Break(..)
524 | hir::ExprKind::Continue(_)
525 | hir::ExprKind::Lit(_)
526 | hir::ExprKind::Ret(..)
527 | hir::ExprKind::Block(..)
528 | hir::ExprKind::Assign(..)
529 | hir::ExprKind::AssignOp(..)
530 | hir::ExprKind::Struct(..)
531 | hir::ExprKind::Repeat(..)
532 | hir::ExprKind::InlineAsm(..)
533 | hir::ExprKind::Box(..)
534 | hir::ExprKind::Yield(..)
535 | hir::ExprKind::Type(..)
537 | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
538 intravisit::walk_expr(ir, expr);
543 // ______________________________________________________________________
544 // Computing liveness sets
546 // Actually we compute just a bit more than just liveness, but we use
547 // the same basic propagation framework in all cases.
549 #[derive(Clone, Copy)]
556 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
557 /// very large, so it uses a more compact representation that takes advantage
558 /// of the fact that when the number of `RWU`s is large, most of them have an
559 /// invalid reader and an invalid writer.
561 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
562 /// an index into `unpacked_rwus`. In the common cases, this compacts the
563 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
566 /// More compact representations are possible -- e.g., use only 2 bits per
567 /// packed `RWU` and make the secondary table a HashMap that maps from
568 /// indices to `RWU`s -- but this one strikes a good balance between size
570 packed_rwus: Vec<u32>,
571 unpacked_rwus: Vec<RWU>,
574 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
575 const INV_INV_FALSE: u32 = u32::MAX;
577 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
578 const INV_INV_TRUE: u32 = u32::MAX - 1;
581 fn new(num_rwus: usize) -> RWUTable {
582 Self { packed_rwus: vec![INV_INV_FALSE; num_rwus], 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 { INV_INV_TRUE } else { INV_INV_FALSE }
632 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
634 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
635 self.unpacked_rwus.push(rwu);
639 fn assign_inv_inv(&mut self, idx: usize) {
640 self.packed_rwus[idx] = if self.get_used(idx) { INV_INV_TRUE } else { INV_INV_FALSE };
644 #[derive(Copy, Clone)]
647 fallthrough_ln: LiveNode,
648 clean_exit_var: Variable,
651 const ACC_READ: u32 = 1;
652 const ACC_WRITE: u32 = 2;
653 const ACC_USE: u32 = 4;
655 struct Liveness<'a, 'tcx> {
656 ir: &'a mut IrMaps<'tcx>,
657 tables: &'a ty::TypeckTables<'tcx>,
659 successors: Vec<LiveNode>,
662 // mappings from loop node ID to LiveNode
663 // ("break" label should map to loop node ID,
664 // it probably doesn't now)
665 break_ln: HirIdMap<LiveNode>,
666 cont_ln: HirIdMap<LiveNode>,
669 impl<'a, 'tcx> Liveness<'a, 'tcx> {
670 fn new(ir: &'a mut IrMaps<'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
671 // Special nodes and variables:
672 // - exit_ln represents the end of the fn, either by return or panic
673 // - implicit_ret_var is a pseudo-variable that represents
674 // an implicit return
675 let specials = Specials {
676 exit_ln: ir.add_live_node(ExitNode),
677 fallthrough_ln: ir.add_live_node(ExitNode),
678 clean_exit_var: ir.add_variable(CleanExit),
681 let tables = ir.tcx.body_tables(body);
683 let num_live_nodes = ir.num_live_nodes;
684 let num_vars = ir.num_vars;
690 successors: vec![invalid_node(); num_live_nodes],
691 rwu_table: RWUTable::new(num_live_nodes * num_vars),
692 break_ln: Default::default(),
693 cont_ln: Default::default(),
697 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
698 match self.ir.live_node_map.get(&hir_id) {
701 // This must be a mismatch between the ir_map construction
702 // above and the propagation code below; the two sets of
703 // code have to agree about which AST nodes are worth
704 // creating liveness nodes for.
705 span_bug!(span, "no live node registered for node {:?}", hir_id);
710 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
711 self.ir.variable(hir_id, span)
714 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
715 // In an or-pattern, only consider the first pattern; any later patterns
716 // must have the same bindings, and we also consider the first pattern
717 // to be the "authoritative" set of ids.
718 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
719 let ln = self.live_node(hir_id, pat_sp);
720 let var = self.variable(hir_id, ident.span);
721 self.init_from_succ(ln, succ);
722 self.define(ln, var);
728 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
729 ln.get() * self.ir.num_vars + var.get()
732 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
733 assert!(ln.is_valid());
734 let reader = self.rwu_table.get_reader(self.idx(ln, var));
735 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
738 // Is this variable live on entry to any of its successor nodes?
739 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
740 let successor = self.successors[ln.get()];
741 self.live_on_entry(successor, var)
744 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
745 assert!(ln.is_valid());
746 self.rwu_table.get_used(self.idx(ln, var))
749 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
750 assert!(ln.is_valid());
751 let writer = self.rwu_table.get_writer(self.idx(ln, var));
752 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
755 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
756 let successor = self.successors[ln.get()];
757 self.assigned_on_entry(successor, var)
760 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F)
762 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
764 let node_base_idx = self.idx(ln, Variable(0));
765 let succ_base_idx = self.idx(succ_ln, Variable(0));
766 for var_idx in 0..self.ir.num_vars {
767 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
771 fn write_vars<F>(&self, wr: &mut dyn Write, ln: LiveNode, mut test: F) -> io::Result<()>
773 F: FnMut(usize) -> LiveNode,
775 let node_base_idx = self.idx(ln, Variable(0));
776 for var_idx in 0..self.ir.num_vars {
777 let idx = node_base_idx + var_idx;
778 if test(idx).is_valid() {
779 write!(wr, " {:?}", Variable(var_idx as u32))?;
785 #[allow(unused_must_use)]
786 fn ln_str(&self, ln: LiveNode) -> String {
787 let mut wr = Vec::new();
789 let wr = &mut wr as &mut dyn Write;
790 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
791 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx));
792 write!(wr, " writes");
793 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx));
794 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
796 String::from_utf8(wr).unwrap()
799 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
800 self.successors[ln.get()] = succ_ln;
802 // It is not necessary to initialize the RWUs here because they are all
803 // set to INV_INV_FALSE when they are created, and the sets only grow
804 // during iterations.
807 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
808 // more efficient version of init_empty() / merge_from_succ()
809 self.successors[ln.get()] = succ_ln;
811 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
812 this.rwu_table.copy_packed(idx, succ_idx);
814 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
817 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode, first_merge: bool) -> bool {
822 let mut changed = false;
823 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
824 let mut rwu = this.rwu_table.get(idx);
825 let succ_rwu = this.rwu_table.get(succ_idx);
826 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
827 rwu.reader = succ_rwu.reader;
831 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
832 rwu.writer = succ_rwu.writer;
836 if succ_rwu.used && !rwu.used {
842 this.rwu_table.assign_unpacked(idx, rwu);
847 "merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
849 self.ln_str(succ_ln),
856 // Indicates that a local variable was *defined*; we know that no
857 // uses of the variable can precede the definition (resolve checks
858 // this) so we just clear out all the data.
859 fn define(&mut self, writer: LiveNode, var: Variable) {
860 let idx = self.idx(writer, var);
861 self.rwu_table.assign_inv_inv(idx);
863 debug!("{:?} defines {:?} (idx={}): {}", writer, var, idx, self.ln_str(writer));
866 // Either read, write, or both depending on the acc bitset
867 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
868 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
870 let idx = self.idx(ln, var);
871 let mut rwu = self.rwu_table.get(idx);
873 if (acc & ACC_WRITE) != 0 {
874 rwu.reader = invalid_node();
878 // Important: if we both read/write, must do read second
879 // or else the write will override.
880 if (acc & ACC_READ) != 0 {
884 if (acc & ACC_USE) != 0 {
888 self.rwu_table.assign_unpacked(idx, rwu);
891 fn compute(&mut self, body: &hir::Expr<'_>) -> LiveNode {
893 "compute: using id for body, {}",
894 self.ir.tcx.hir().hir_to_pretty_string(body.hir_id)
897 // the fallthrough exit is only for those cases where we do not
898 // explicitly return:
900 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
901 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
903 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
905 // hack to skip the loop unless debug! is enabled:
907 "^^ liveness computation results for body {} (entry={:?})",
909 for ln_idx in 0..self.ir.num_live_nodes {
910 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
920 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
921 if blk.targeted_by_break {
922 self.break_ln.insert(blk.hir_id, succ);
924 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
925 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
928 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
930 hir::StmtKind::Local(ref local) => {
931 // Note: we mark the variable as defined regardless of whether
932 // there is an initializer. Initially I had thought to only mark
933 // the live variable as defined if it was initialized, and then we
934 // could check for uninit variables just by scanning what is live
935 // at the start of the function. But that doesn't work so well for
936 // immutable variables defined in a loop:
937 // loop { let x; x = 5; }
938 // because the "assignment" loops back around and generates an error.
940 // So now we just check that variables defined w/o an
941 // initializer are not live at the point of their
942 // initialization, which is mildly more complex than checking
943 // once at the func header but otherwise equivalent.
945 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
946 self.define_bindings_in_pat(&local.pat, succ)
948 hir::StmtKind::Item(..) => succ,
949 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
950 self.propagate_through_expr(&expr, succ)
955 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
956 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
959 fn propagate_through_opt_expr(
961 opt_expr: Option<&Expr<'_>>,
964 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
967 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
968 debug!("propagate_through_expr: {}", self.ir.tcx.hir().hir_to_pretty_string(expr.hir_id));
971 // Interesting cases with control flow or which gen/kill
972 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
973 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
976 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
978 hir::ExprKind::Closure(..) => {
980 "{} is an ExprKind::Closure",
981 self.ir.tcx.hir().hir_to_pretty_string(expr.hir_id)
984 // the construction of a closure itself is not important,
985 // but we have to consider the closed over variables.
991 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
993 caps.iter().rev().fold(succ, |succ, cap| {
994 self.init_from_succ(cap.ln, succ);
995 let var = self.variable(cap.var_hid, expr.span);
996 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1001 // Note that labels have been resolved, so we don't need to look
1002 // at the label ident
1003 hir::ExprKind::Loop(ref blk, _, _) => self.propagate_through_loop(expr, &blk, succ),
1005 hir::ExprKind::Match(ref e, arms, _) => {
1020 let ln = self.live_node(expr.hir_id, expr.span);
1021 self.init_empty(ln, succ);
1022 let mut first_merge = true;
1024 let body_succ = self.propagate_through_expr(&arm.body, succ);
1026 let guard_succ = self.propagate_through_opt_expr(
1027 arm.guard.as_ref().map(|hir::Guard::If(e)| *e),
1030 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
1031 self.merge_from_succ(ln, arm_succ, first_merge);
1032 first_merge = false;
1034 self.propagate_through_expr(&e, ln)
1037 hir::ExprKind::Ret(ref o_e) => {
1038 // ignore succ and subst exit_ln:
1039 let exit_ln = self.s.exit_ln;
1040 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1043 hir::ExprKind::Break(label, ref opt_expr) => {
1044 // Find which label this break jumps to
1045 let target = match label.target_id {
1046 Ok(hir_id) => self.break_ln.get(&hir_id),
1047 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1051 // Now that we know the label we're going to,
1052 // look it up in the break loop nodes table
1055 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1057 // FIXME: This should have been checked earlier. Once this is fixed,
1058 // replace with `delay_span_bug`. (#62480)
1062 .struct_span_err(expr.span, "`break` to unknown label")
1064 errors::FatalError.raise()
1069 hir::ExprKind::Continue(label) => {
1070 // Find which label this expr continues to
1073 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
1075 // Now that we know the label we're going to,
1076 // look it up in the continue loop nodes table
1080 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
1083 hir::ExprKind::Assign(ref l, ref r, _) => {
1084 // see comment on places in
1085 // propagate_through_place_components()
1086 let succ = self.write_place(&l, succ, ACC_WRITE);
1087 let succ = self.propagate_through_place_components(&l, succ);
1088 self.propagate_through_expr(&r, succ)
1091 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1092 // an overloaded assign op is like a method call
1093 if self.tables.is_method_call(expr) {
1094 let succ = self.propagate_through_expr(&l, succ);
1095 self.propagate_through_expr(&r, succ)
1097 // see comment on places in
1098 // propagate_through_place_components()
1099 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
1100 let succ = self.propagate_through_expr(&r, succ);
1101 self.propagate_through_place_components(&l, succ)
1105 // Uninteresting cases: just propagate in rev exec order
1106 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
1108 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1109 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1113 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
1116 hir::ExprKind::Call(ref f, ref args) => {
1117 let m = self.ir.tcx.hir().get_module_parent(expr.hir_id);
1118 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1123 let succ = self.propagate_through_exprs(args, succ);
1124 self.propagate_through_expr(&f, succ)
1127 hir::ExprKind::MethodCall(.., ref args) => {
1128 let m = self.ir.tcx.hir().get_module_parent(expr.hir_id);
1129 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1135 self.propagate_through_exprs(args, succ)
1138 hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ),
1140 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1141 let r_succ = self.propagate_through_expr(&r, succ);
1143 let ln = self.live_node(expr.hir_id, expr.span);
1144 self.init_from_succ(ln, succ);
1145 self.merge_from_succ(ln, r_succ, false);
1147 self.propagate_through_expr(&l, ln)
1150 hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => {
1151 let r_succ = self.propagate_through_expr(&r, succ);
1152 self.propagate_through_expr(&l, r_succ)
1155 hir::ExprKind::Box(ref e)
1156 | hir::ExprKind::AddrOf(_, _, ref e)
1157 | hir::ExprKind::Cast(ref e, _)
1158 | hir::ExprKind::Type(ref e, _)
1159 | hir::ExprKind::DropTemps(ref e)
1160 | hir::ExprKind::Unary(_, ref e)
1161 | hir::ExprKind::Yield(ref e, _)
1162 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1164 hir::ExprKind::InlineAsm(ref asm) => {
1165 let ia = &asm.inner;
1166 let outputs = asm.outputs_exprs;
1167 let inputs = asm.inputs_exprs;
1168 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1169 // see comment on places
1170 // in propagate_through_place_components()
1172 self.propagate_through_expr(output, succ)
1174 let acc = if o.is_rw { ACC_WRITE | ACC_READ } else { ACC_WRITE };
1175 let succ = self.write_place(output, succ, acc);
1176 self.propagate_through_place_components(output, succ)
1180 // Inputs are executed first. Propagate last because of rev order
1181 self.propagate_through_exprs(inputs, succ)
1184 hir::ExprKind::Lit(..)
1185 | hir::ExprKind::Err
1186 | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => succ,
1188 // Note that labels have been resolved, so we don't need to look
1189 // at the label ident
1190 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1194 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1197 // In general, the full flow graph structure for an
1198 // assignment/move/etc can be handled in one of two ways,
1199 // depending on whether what is being assigned is a "tracked
1200 // value" or not. A tracked value is basically a local
1201 // variable or argument.
1203 // The two kinds of graphs are:
1205 // Tracked place Untracked place
1206 // ----------------------++-----------------------
1210 // (rvalue) || (rvalue)
1213 // (write of place) || (place components)
1218 // ----------------------++-----------------------
1220 // I will cover the two cases in turn:
1224 // A tracked place is a local variable/argument `x`. In
1225 // these cases, the link_node where the write occurs is linked
1226 // to node id of `x`. The `write_place()` routine generates
1227 // the contents of this node. There are no subcomponents to
1230 // # Non-tracked places
1232 // These are places like `x[5]` or `x.f`. In that case, we
1233 // basically ignore the value which is written to but generate
1234 // reads for the components---`x` in these two examples. The
1235 // components reads are generated by
1236 // `propagate_through_place_components()` (this fn).
1240 // It is still possible to observe assignments to non-places;
1241 // these errors are detected in the later pass borrowck. We
1242 // just ignore such cases and treat them as reads.
1245 hir::ExprKind::Path(_) => succ,
1246 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1247 _ => self.propagate_through_expr(expr, succ),
1251 // see comment on propagate_through_place()
1252 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1254 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1255 self.access_path(expr.hir_id, path, succ, acc)
1258 // We do not track other places, so just propagate through
1259 // to their subcomponents. Also, it may happen that
1260 // non-places occur here, because those are detected in the
1261 // later pass borrowck.
1274 let ln = self.live_node(hir_id, span);
1276 self.init_from_succ(ln, succ);
1277 let var = self.variable(var_hid, span);
1278 self.acc(ln, var, acc);
1286 path: &hir::Path<'_>,
1291 Res::Local(hid) => {
1292 let upvars = self.ir.tcx.upvars(self.ir.body_owner);
1293 if !upvars.map_or(false, |upvars| upvars.contains_key(&hid)) {
1294 self.access_var(hir_id, hid, succ, acc, path.span)
1303 fn propagate_through_loop(
1306 body: &hir::Block<'_>,
1310 We model control flow like this:
1317 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1318 Meanwhile, a `break` expression will have a successor of `succ`.
1322 let mut first_merge = true;
1323 let ln = self.live_node(expr.hir_id, expr.span);
1324 self.init_empty(ln, succ);
1326 "propagate_through_loop: using id for loop body {} {}",
1328 self.ir.tcx.hir().hir_to_pretty_string(body.hir_id)
1331 self.break_ln.insert(expr.hir_id, succ);
1333 self.cont_ln.insert(expr.hir_id, ln);
1335 let body_ln = self.propagate_through_block(body, ln);
1337 // repeat until fixed point is reached:
1338 while self.merge_from_succ(ln, body_ln, first_merge) {
1339 first_merge = false;
1340 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1347 // _______________________________________________________________________
1348 // Checking for error conditions
1350 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1351 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1352 NestedVisitorMap::None
1355 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1356 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1357 if local.init.is_some() {
1358 self.warn_about_dead_assign(spans, hir_id, ln, var);
1362 intravisit::walk_local(self, local);
1365 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1366 check_expr(self, ex);
1369 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1370 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1371 intravisit::walk_arm(self, arm);
1375 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1377 hir::ExprKind::Assign(ref l, ..) => {
1378 this.check_place(&l);
1381 hir::ExprKind::AssignOp(_, ref l, _) => {
1382 if !this.tables.is_method_call(expr) {
1383 this.check_place(&l);
1387 hir::ExprKind::InlineAsm(ref asm) => {
1388 for input in asm.inputs_exprs {
1389 this.visit_expr(input);
1392 // Output operands must be places
1393 for (o, output) in asm.inner.outputs.iter().zip(asm.outputs_exprs) {
1395 this.check_place(output);
1397 this.visit_expr(output);
1401 // no correctness conditions related to liveness
1402 hir::ExprKind::Call(..)
1403 | hir::ExprKind::MethodCall(..)
1404 | hir::ExprKind::Match(..)
1405 | hir::ExprKind::Loop(..)
1406 | hir::ExprKind::Index(..)
1407 | hir::ExprKind::Field(..)
1408 | hir::ExprKind::Array(..)
1409 | hir::ExprKind::Tup(..)
1410 | hir::ExprKind::Binary(..)
1411 | hir::ExprKind::Cast(..)
1412 | hir::ExprKind::DropTemps(..)
1413 | hir::ExprKind::Unary(..)
1414 | hir::ExprKind::Ret(..)
1415 | hir::ExprKind::Break(..)
1416 | hir::ExprKind::Continue(..)
1417 | hir::ExprKind::Lit(_)
1418 | hir::ExprKind::Block(..)
1419 | hir::ExprKind::AddrOf(..)
1420 | hir::ExprKind::Struct(..)
1421 | hir::ExprKind::Repeat(..)
1422 | hir::ExprKind::Closure(..)
1423 | hir::ExprKind::Path(_)
1424 | hir::ExprKind::Yield(..)
1425 | hir::ExprKind::Box(..)
1426 | hir::ExprKind::Type(..)
1427 | hir::ExprKind::Err => {}
1430 intravisit::walk_expr(this, expr);
1433 impl<'tcx> Liveness<'_, 'tcx> {
1434 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1436 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1437 if let Res::Local(var_hid) = path.res {
1438 let upvars = self.ir.tcx.upvars(self.ir.body_owner);
1439 if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hid)) {
1440 // Assignment to an immutable variable or argument: only legal
1441 // if there is no later assignment. If this local is actually
1442 // mutable, then check for a reassignment to flag the mutability
1444 let ln = self.live_node(expr.hir_id, expr.span);
1445 let var = self.variable(var_hid, expr.span);
1446 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1451 // For other kinds of places, no checks are required,
1452 // and any embedded expressions are actually rvalues
1453 intravisit::walk_expr(self, expr);
1458 fn should_warn(&self, var: Variable) -> Option<String> {
1459 let name = self.ir.variable_name(var);
1460 if name.is_empty() || name.as_bytes()[0] == b'_' { None } else { Some(name) }
1463 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1464 for p in body.params {
1465 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1466 if self.live_on_entry(ln, var).is_none() {
1467 self.report_dead_assign(hir_id, spans, var, true);
1473 fn check_unused_vars_in_pat(
1476 entry_ln: Option<LiveNode>,
1477 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1479 // In an or-pattern, only consider the variable; any later patterns must have the same
1480 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1481 // However, we should take the spans of variables with the same name from the later
1482 // patterns so the suggestions to prefix with underscores will apply to those too.
1483 let mut vars: FxIndexMap<String, (LiveNode, Variable, HirId, Vec<Span>)> = <_>::default();
1485 pat.each_binding(|_, hir_id, pat_sp, ident| {
1486 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1487 let var = self.variable(hir_id, ident.span);
1488 vars.entry(self.ir.variable_name(var))
1489 .and_modify(|(.., spans)| spans.push(ident.span))
1490 .or_insert_with(|| (ln, var, hir_id, vec![ident.span]));
1493 for (_, (ln, var, id, spans)) in vars {
1494 if self.used_on_entry(ln, var) {
1495 on_used_on_entry(spans, id, ln, var);
1497 self.report_unused(spans, id, ln, var);
1502 fn report_unused(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1503 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1504 // annoying: for parameters in funcs like `fn(x: i32)
1505 // {ret}`, there is only one node, so asking about
1506 // assigned_on_exit() is not meaningful.
1508 if ln == self.s.exit_ln { false } else { self.assigned_on_exit(ln, var).is_some() };
1511 self.ir.tcx.lint_hir_note(
1512 lint::builtin::UNUSED_VARIABLES,
1515 &format!("variable `{}` is assigned to, but never used", name),
1516 &format!("consider using `_{}` instead", name),
1519 let mut err = self.ir.tcx.struct_span_lint_hir(
1520 lint::builtin::UNUSED_VARIABLES,
1523 &format!("unused variable: `{}`", name),
1526 if self.ir.variable_is_shorthand(var) {
1527 if let Node::Binding(pat) = self.ir.tcx.hir().get(hir_id) {
1528 // Handle `ref` and `ref mut`.
1530 spans.iter().map(|_span| (pat.span, format!("{}: _", name))).collect();
1532 err.multipart_suggestion(
1533 "try ignoring the field",
1535 Applicability::MachineApplicable,
1539 err.multipart_suggestion(
1540 "consider prefixing with an underscore",
1541 spans.iter().map(|span| (*span, format!("_{}", name))).collect(),
1542 Applicability::MachineApplicable,
1551 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1552 if self.live_on_exit(ln, var).is_none() {
1553 self.report_dead_assign(hir_id, spans, var, false);
1557 fn report_dead_assign(&self, hir_id: HirId, spans: Vec<Span>, var: Variable, is_param: bool) {
1558 if let Some(name) = self.should_warn(var) {
1562 .struct_span_lint_hir(
1563 lint::builtin::UNUSED_ASSIGNMENTS,
1566 &format!("value passed to `{}` is never read", name),
1568 .help("maybe it is overwritten before being read?")
1573 .struct_span_lint_hir(
1574 lint::builtin::UNUSED_ASSIGNMENTS,
1577 &format!("value assigned to `{}` is never read", name),
1579 .help("maybe it is overwritten before being read?")