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::*;
100 use crate::hir::def::*;
101 use crate::hir::Node;
102 use crate::ty::{self, TyCtxt};
103 use crate::ty::query::Providers;
105 use crate::util::nodemap::{HirIdMap, HirIdSet};
107 use errors::Applicability;
108 use std::collections::{BTreeMap, VecDeque};
110 use std::io::prelude::*;
113 use syntax::ast::{self, NodeId};
115 use syntax::symbol::{kw, sym};
116 use syntax_pos::Span;
119 use crate::hir::{Expr, HirId};
120 use crate::hir::def_id::DefId;
121 use crate::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!("Upvar 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: HirId) {
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 provide(providers: &mut Providers<'_>) {
189 *providers = Providers {
195 impl fmt::Debug for LiveNode {
196 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
197 write!(f, "ln({})", self.get())
201 impl fmt::Debug for Variable {
202 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
203 write!(f, "v({})", self.get())
207 // ______________________________________________________________________
210 // This is the first pass and the one that drives the main
211 // computation. It walks up and down the IR once. On the way down,
212 // we count for each function the number of variables as well as
213 // liveness nodes. A liveness node is basically an expression or
214 // capture clause that does something of interest: either it has
215 // interesting control flow or it uses/defines a local variable.
217 // On the way back up, at each function node we create liveness sets
218 // (we now know precisely how big to make our various vectors and so
219 // forth) and then do the data-flow propagation to compute the set
220 // of live variables at each program point.
222 // Finally, we run back over the IR one last time and, using the
223 // computed liveness, check various safety conditions. For example,
224 // there must be no live nodes at the definition site for a variable
225 // unless it has an initializer. Similarly, each non-mutable local
226 // variable must not be assigned if there is some successor
227 // assignment. And so forth.
230 fn is_valid(&self) -> bool {
235 fn invalid_node() -> LiveNode { LiveNode(u32::MAX) }
242 #[derive(Copy, Clone, Debug)]
249 #[derive(Copy, Clone, Debug)]
251 Arg(HirId, ast::Name),
256 struct IrMaps<'a, 'tcx: 'a> {
257 tcx: TyCtxt<'a, 'tcx, 'tcx>,
258 num_live_nodes: usize,
260 live_node_map: HirIdMap<LiveNode>,
261 variable_map: HirIdMap<Variable>,
262 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
263 var_kinds: Vec<VarKind>,
264 lnks: Vec<LiveNodeKind>,
267 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
268 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
273 live_node_map: HirIdMap::default(),
274 variable_map: HirIdMap::default(),
275 capture_info_map: Default::default(),
276 var_kinds: Vec::new(),
281 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
282 let ln = LiveNode(self.num_live_nodes as u32);
284 self.num_live_nodes += 1;
286 debug!("{:?} is of kind {}", ln,
287 live_node_kind_to_string(lnk, self.tcx));
292 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
293 let ln = self.add_live_node(lnk);
294 self.live_node_map.insert(hir_id, ln);
296 debug!("{:?} is node {:?}", ln, hir_id);
299 fn add_variable(&mut self, vk: VarKind) -> Variable {
300 let v = Variable(self.num_vars as u32);
301 self.var_kinds.push(vk);
305 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
306 self.variable_map.insert(node_id, v);
311 debug!("{:?} is {:?}", v, vk);
316 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
317 match self.variable_map.get(&hir_id) {
320 span_bug!(span, "no variable registered for id {:?}", hir_id);
325 fn variable_name(&self, var: Variable) -> String {
326 match self.var_kinds[var.get()] {
327 Local(LocalInfo { name, .. }) | Arg(_, name) => {
330 CleanExit => "<clean-exit>".to_owned()
334 fn variable_is_shorthand(&self, var: Variable) -> bool {
335 match self.var_kinds[var.get()] {
336 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
337 Arg(..) | CleanExit => false
341 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
342 self.capture_info_map.insert(hir_id, Rc::new(cs));
345 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
350 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
352 decl: &'tcx hir::FnDecl,
353 body_id: hir::BodyId,
358 // swap in a new set of IR maps for this function body:
359 let mut fn_maps = IrMaps::new(ir.tcx);
361 // Don't run unused pass for #[derive()]
362 if let FnKind::Method(..) = fk {
363 let parent = ir.tcx.hir().get_parent_item(id);
364 if let Some(Node::Item(i)) = ir.tcx.hir().find_by_hir_id(parent) {
365 if i.attrs.iter().any(|a| a.check_name(sym::automatically_derived)) {
371 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps<'_, '_>);
373 let body = ir.tcx.hir().body(body_id);
375 for arg in &body.arguments {
376 let is_shorthand = match arg.pat.node {
377 crate::hir::PatKind::Struct(..) => true,
380 arg.pat.each_binding(|_bm, hir_id, _x, ident| {
381 debug!("adding argument {:?}", hir_id);
382 let var = if is_shorthand {
389 Arg(hir_id, ident.name)
391 fn_maps.add_variable(var);
395 // gather up the various local variables, significant expressions,
397 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
400 let mut lsets = Liveness::new(&mut fn_maps, body_id);
401 let entry_ln = lsets.compute(&body.value);
403 // check for various error conditions
404 lsets.visit_body(body);
405 lsets.warn_about_unused_args(body, entry_ln);
408 fn add_from_pat<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, pat: &P<hir::Pat>) {
409 // For struct patterns, take note of which fields used shorthand
410 // (`x` rather than `x: x`).
411 let mut shorthand_field_ids = HirIdSet::default();
412 let mut pats = VecDeque::new();
414 while let Some(pat) = pats.pop_front() {
415 use crate::hir::PatKind::*;
417 Binding(_, _, _, ref inner_pat) => {
418 pats.extend(inner_pat.iter());
420 Struct(_, ref fields, _) => {
421 for field in fields {
422 if field.node.is_shorthand {
423 shorthand_field_ids.insert(field.node.pat.hir_id);
427 Ref(ref inner_pat, _) |
428 Box(ref inner_pat) => {
429 pats.push_back(inner_pat);
431 TupleStruct(_, ref inner_pats, _) |
432 Tuple(ref inner_pats, _) => {
433 pats.extend(inner_pats.iter());
435 Slice(ref pre_pats, ref inner_pat, ref post_pats) => {
436 pats.extend(pre_pats.iter());
437 pats.extend(inner_pat.iter());
438 pats.extend(post_pats.iter());
444 pat.each_binding(|_bm, hir_id, _sp, ident| {
445 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
446 ir.add_variable(Local(LocalInfo {
449 is_shorthand: shorthand_field_ids.contains(&hir_id)
454 fn visit_local<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
455 add_from_pat(ir, &local.pat);
456 intravisit::walk_local(ir, local);
459 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
460 for pat in &arm.pats {
461 add_from_pat(ir, pat);
463 intravisit::walk_arm(ir, arm);
466 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
468 // live nodes required for uses or definitions of variables:
469 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
470 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
471 if let Res::Local(..) = path.res {
472 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
474 intravisit::walk_expr(ir, expr);
476 hir::ExprKind::Closure(..) => {
477 // Interesting control flow (for loops can contain labeled
478 // breaks or continues)
479 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
481 // Make a live_node for each captured variable, with the span
482 // being the location that the variable is used. This results
483 // in better error messages than just pointing at the closure
484 // construction site.
485 let mut call_caps = Vec::new();
486 let closure_def_id = ir.tcx.hir().local_def_id_from_hir_id(expr.hir_id);
487 if let Some(upvars) = ir.tcx.upvars(closure_def_id) {
488 call_caps.extend(upvars.iter().filter_map(|upvar| {
489 if let Res::Local(rv) = upvar.res {
490 let upvar_ln = ir.add_live_node(UpvarNode(upvar.span));
491 Some(CaptureInfo { ln: upvar_ln, var_hid: rv })
497 ir.set_captures(expr.hir_id, call_caps);
499 intravisit::walk_expr(ir, expr);
502 // live nodes required for interesting control flow:
503 hir::ExprKind::Match(..) |
504 hir::ExprKind::While(..) |
505 hir::ExprKind::Loop(..) => {
506 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
507 intravisit::walk_expr(ir, expr);
509 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
510 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
511 intravisit::walk_expr(ir, expr);
514 // otherwise, live nodes are not required:
515 hir::ExprKind::Index(..) |
516 hir::ExprKind::Field(..) |
517 hir::ExprKind::Array(..) |
518 hir::ExprKind::Call(..) |
519 hir::ExprKind::MethodCall(..) |
520 hir::ExprKind::Tup(..) |
521 hir::ExprKind::Binary(..) |
522 hir::ExprKind::AddrOf(..) |
523 hir::ExprKind::Cast(..) |
524 hir::ExprKind::DropTemps(..) |
525 hir::ExprKind::Unary(..) |
526 hir::ExprKind::Break(..) |
527 hir::ExprKind::Continue(_) |
528 hir::ExprKind::Lit(_) |
529 hir::ExprKind::Ret(..) |
530 hir::ExprKind::Block(..) |
531 hir::ExprKind::Assign(..) |
532 hir::ExprKind::AssignOp(..) |
533 hir::ExprKind::Struct(..) |
534 hir::ExprKind::Repeat(..) |
535 hir::ExprKind::InlineAsm(..) |
536 hir::ExprKind::Box(..) |
537 hir::ExprKind::Yield(..) |
538 hir::ExprKind::Type(..) |
540 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
541 intravisit::walk_expr(ir, expr);
546 // ______________________________________________________________________
547 // Computing liveness sets
549 // Actually we compute just a bit more than just liveness, but we use
550 // the same basic propagation framework in all cases.
552 #[derive(Clone, Copy)]
559 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
560 /// very large, so it uses a more compact representation that takes advantage
561 /// of the fact that when the number of `RWU`s is large, most of them have an
562 /// invalid reader and an invalid writer.
564 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
565 /// an index into `unpacked_rwus`. In the common cases, this compacts the
566 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
569 /// More compact representations are possible -- e.g., use only 2 bits per
570 /// packed `RWU` and make the secondary table a HashMap that maps from
571 /// indices to `RWU`s -- but this one strikes a good balance between size
573 packed_rwus: Vec<u32>,
574 unpacked_rwus: Vec<RWU>,
577 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
578 const INV_INV_FALSE: u32 = u32::MAX;
580 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
581 const INV_INV_TRUE: u32 = u32::MAX - 1;
584 fn new(num_rwus: usize) -> RWUTable {
586 packed_rwus: vec![INV_INV_FALSE; num_rwus],
587 unpacked_rwus: vec![],
591 fn get(&self, idx: usize) -> RWU {
592 let packed_rwu = self.packed_rwus[idx];
594 INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false },
595 INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true },
596 _ => self.unpacked_rwus[packed_rwu as usize],
600 fn get_reader(&self, idx: usize) -> LiveNode {
601 let packed_rwu = self.packed_rwus[idx];
603 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
604 _ => self.unpacked_rwus[packed_rwu as usize].reader,
608 fn get_writer(&self, idx: usize) -> LiveNode {
609 let packed_rwu = self.packed_rwus[idx];
611 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
612 _ => self.unpacked_rwus[packed_rwu as usize].writer,
616 fn get_used(&self, idx: usize) -> bool {
617 let packed_rwu = self.packed_rwus[idx];
619 INV_INV_FALSE => false,
620 INV_INV_TRUE => true,
621 _ => self.unpacked_rwus[packed_rwu as usize].used,
626 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
627 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
630 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
631 if rwu.reader == invalid_node() && rwu.writer == invalid_node() {
632 // When we overwrite an indexing entry in `self.packed_rwus` with
633 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
634 // from `self.unpacked_rwus`; it's not worth the effort, and we
635 // can't have entries shifting around anyway.
636 self.packed_rwus[idx] = if rwu.used {
642 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
644 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
645 self.unpacked_rwus.push(rwu);
649 fn assign_inv_inv(&mut self, idx: usize) {
650 self.packed_rwus[idx] = if self.get_used(idx) {
658 #[derive(Copy, Clone)]
661 fallthrough_ln: LiveNode,
662 clean_exit_var: Variable
665 const ACC_READ: u32 = 1;
666 const ACC_WRITE: u32 = 2;
667 const ACC_USE: u32 = 4;
669 struct Liveness<'a, 'tcx: 'a> {
670 ir: &'a mut IrMaps<'a, 'tcx>,
671 tables: &'a ty::TypeckTables<'tcx>,
673 successors: Vec<LiveNode>,
676 // mappings from loop node ID to LiveNode
677 // ("break" label should map to loop node ID,
678 // it probably doesn't now)
679 break_ln: HirIdMap<LiveNode>,
680 cont_ln: HirIdMap<LiveNode>,
683 impl<'a, 'tcx> Liveness<'a, 'tcx> {
684 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
685 // Special nodes and variables:
686 // - exit_ln represents the end of the fn, either by return or panic
687 // - implicit_ret_var is a pseudo-variable that represents
688 // an implicit return
689 let specials = Specials {
690 exit_ln: ir.add_live_node(ExitNode),
691 fallthrough_ln: ir.add_live_node(ExitNode),
692 clean_exit_var: ir.add_variable(CleanExit)
695 let tables = ir.tcx.body_tables(body);
697 let num_live_nodes = ir.num_live_nodes;
698 let num_vars = ir.num_vars;
704 successors: vec![invalid_node(); num_live_nodes],
705 rwu_table: RWUTable::new(num_live_nodes * num_vars),
706 break_ln: Default::default(),
707 cont_ln: Default::default(),
711 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
712 match self.ir.live_node_map.get(&hir_id) {
715 // This must be a mismatch between the ir_map construction
716 // above and the propagation code below; the two sets of
717 // code have to agree about which AST nodes are worth
718 // creating liveness nodes for.
721 "no live node registered for node {:?}",
727 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
728 self.ir.variable(hir_id, span)
731 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
732 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
734 pat.each_binding(|_bm, hir_id, sp, n| {
735 let ln = self.live_node(hir_id, sp);
736 let var = self.variable(hir_id, n.span);
737 f(self, ln, var, n.span, hir_id);
741 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
742 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
744 if let Some(pat) = pat {
745 self.pat_bindings(pat, f);
749 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
751 self.define_bindings_in_arm_pats(Some(pat), succ)
754 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
757 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
758 this.init_from_succ(ln, succ);
759 this.define(ln, var);
765 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
766 ln.get() * self.ir.num_vars + var.get()
769 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
770 assert!(ln.is_valid());
771 let reader = self.rwu_table.get_reader(self.idx(ln, var));
772 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
775 // Is this variable live on entry to any of its successor nodes?
776 fn live_on_exit(&self, ln: LiveNode, var: Variable)
777 -> Option<LiveNodeKind> {
778 let successor = self.successors[ln.get()];
779 self.live_on_entry(successor, var)
782 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
783 assert!(ln.is_valid());
784 self.rwu_table.get_used(self.idx(ln, var))
787 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
788 -> Option<LiveNodeKind> {
789 assert!(ln.is_valid());
790 let writer = self.rwu_table.get_writer(self.idx(ln, var));
791 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
794 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
795 -> Option<LiveNodeKind> {
796 let successor = self.successors[ln.get()];
797 self.assigned_on_entry(successor, var)
800 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
801 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
803 let node_base_idx = self.idx(ln, Variable(0));
804 let succ_base_idx = self.idx(succ_ln, Variable(0));
805 for var_idx in 0..self.ir.num_vars {
806 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
810 fn write_vars<F>(&self,
814 -> io::Result<()> where
815 F: FnMut(usize) -> LiveNode,
817 let node_base_idx = self.idx(ln, Variable(0));
818 for var_idx in 0..self.ir.num_vars {
819 let idx = node_base_idx + var_idx;
820 if test(idx).is_valid() {
821 write!(wr, " {:?}", Variable(var_idx as u32))?;
828 #[allow(unused_must_use)]
829 fn ln_str(&self, ln: LiveNode) -> String {
830 let mut wr = Vec::new();
832 let wr = &mut wr as &mut dyn Write;
833 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
834 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx));
835 write!(wr, " writes");
836 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx));
837 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
839 String::from_utf8(wr).unwrap()
842 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
843 self.successors[ln.get()] = succ_ln;
845 // It is not necessary to initialize the RWUs here because they are all
846 // set to INV_INV_FALSE when they are created, and the sets only grow
847 // during iterations.
850 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
851 // more efficient version of init_empty() / merge_from_succ()
852 self.successors[ln.get()] = succ_ln;
854 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
855 this.rwu_table.copy_packed(idx, succ_idx);
857 debug!("init_from_succ(ln={}, succ={})",
858 self.ln_str(ln), self.ln_str(succ_ln));
861 fn merge_from_succ(&mut self,
866 if ln == succ_ln { return false; }
868 let mut changed = false;
869 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
870 let mut rwu = this.rwu_table.get(idx);
871 let succ_rwu = this.rwu_table.get(succ_idx);
872 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
873 rwu.reader = succ_rwu.reader;
877 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
878 rwu.writer = succ_rwu.writer;
882 if succ_rwu.used && !rwu.used {
888 this.rwu_table.assign_unpacked(idx, rwu);
892 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
893 ln, self.ln_str(succ_ln), first_merge, changed);
897 // Indicates that a local variable was *defined*; we know that no
898 // uses of the variable can precede the definition (resolve checks
899 // this) so we just clear out all the data.
900 fn define(&mut self, writer: LiveNode, var: Variable) {
901 let idx = self.idx(writer, var);
902 self.rwu_table.assign_inv_inv(idx);
904 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
905 idx, self.ln_str(writer));
908 // Either read, write, or both depending on the acc bitset
909 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
910 debug!("{:?} accesses[{:x}] {:?}: {}",
911 ln, acc, var, self.ln_str(ln));
913 let idx = self.idx(ln, var);
914 let mut rwu = self.rwu_table.get(idx);
916 if (acc & ACC_WRITE) != 0 {
917 rwu.reader = invalid_node();
921 // Important: if we both read/write, must do read second
922 // or else the write will override.
923 if (acc & ACC_READ) != 0 {
927 if (acc & ACC_USE) != 0 {
931 self.rwu_table.assign_unpacked(idx, rwu);
934 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
935 debug!("compute: using id for body, {}",
936 self.ir.tcx.hir().hir_to_pretty_string(body.hir_id));
938 // the fallthrough exit is only for those cases where we do not
939 // explicitly return:
941 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
942 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
944 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
946 // hack to skip the loop unless debug! is enabled:
947 debug!("^^ liveness computation results for body {} (entry={:?})", {
948 for ln_idx in 0..self.ir.num_live_nodes {
949 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
958 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
960 if blk.targeted_by_break {
961 self.break_ln.insert(blk.hir_id, succ);
963 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
964 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
965 self.propagate_through_stmt(stmt, succ)
969 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
972 hir::StmtKind::Local(ref local) => {
973 // Note: we mark the variable as defined regardless of whether
974 // there is an initializer. Initially I had thought to only mark
975 // the live variable as defined if it was initialized, and then we
976 // could check for uninit variables just by scanning what is live
977 // at the start of the function. But that doesn't work so well for
978 // immutable variables defined in a loop:
979 // loop { let x; x = 5; }
980 // because the "assignment" loops back around and generates an error.
982 // So now we just check that variables defined w/o an
983 // initializer are not live at the point of their
984 // initialization, which is mildly more complex than checking
985 // once at the func header but otherwise equivalent.
987 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
988 self.define_bindings_in_pat(&local.pat, succ)
990 hir::StmtKind::Item(..) => succ,
991 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
992 self.propagate_through_expr(&expr, succ)
997 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
999 exprs.iter().rev().fold(succ, |succ, expr| {
1000 self.propagate_through_expr(&expr, succ)
1004 fn propagate_through_opt_expr(&mut self,
1005 opt_expr: Option<&Expr>,
1008 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
1011 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
1013 debug!("propagate_through_expr: {}", self.ir.tcx.hir().hir_to_pretty_string(expr.hir_id));
1016 // Interesting cases with control flow or which gen/kill
1017 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1018 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
1021 hir::ExprKind::Field(ref e, _) => {
1022 self.propagate_through_expr(&e, succ)
1025 hir::ExprKind::Closure(..) => {
1026 debug!("{} is an ExprKind::Closure",
1027 self.ir.tcx.hir().hir_to_pretty_string(expr.hir_id));
1029 // the construction of a closure itself is not important,
1030 // but we have to consider the closed over variables.
1031 let caps = self.ir.capture_info_map.get(&expr.hir_id).cloned().unwrap_or_else(||
1032 span_bug!(expr.span, "no registered caps"));
1034 caps.iter().rev().fold(succ, |succ, cap| {
1035 self.init_from_succ(cap.ln, succ);
1036 let var = self.variable(cap.var_hid, expr.span);
1037 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1042 hir::ExprKind::While(ref cond, ref blk, _) => {
1043 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1046 // Note that labels have been resolved, so we don't need to look
1047 // at the label ident
1048 hir::ExprKind::Loop(ref blk, _, _) => {
1049 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1052 hir::ExprKind::Match(ref e, ref arms, _) => {
1067 let ln = self.live_node(expr.hir_id, expr.span);
1068 self.init_empty(ln, succ);
1069 let mut first_merge = true;
1071 let body_succ = self.propagate_through_expr(&arm.body, succ);
1073 let guard_succ = self.propagate_through_opt_expr(
1074 arm.guard.as_ref().map(|hir::Guard::If(e)| &**e),
1077 // only consider the first pattern; any later patterns must have
1078 // the same bindings, and we also consider the first pattern to be
1079 // the "authoritative" set of ids
1081 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1083 self.merge_from_succ(ln, arm_succ, first_merge);
1084 first_merge = false;
1086 self.propagate_through_expr(&e, ln)
1089 hir::ExprKind::Ret(ref o_e) => {
1090 // ignore succ and subst exit_ln:
1091 let exit_ln = self.s.exit_ln;
1092 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1095 hir::ExprKind::Break(label, ref opt_expr) => {
1096 // Find which label this break jumps to
1097 let target = match label.target_id {
1098 Ok(hir_id) => self.break_ln.get(&hir_id),
1099 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1102 // Now that we know the label we're going to,
1103 // look it up in the break loop nodes table
1106 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1107 None => span_bug!(expr.span, "break to unknown label")
1111 hir::ExprKind::Continue(label) => {
1112 // Find which label this expr continues to
1113 let sc = label.target_id.unwrap_or_else(|err|
1114 span_bug!(expr.span, "loop scope error: {}", err));
1116 // Now that we know the label we're going to,
1117 // look it up in the continue loop nodes table
1118 self.cont_ln.get(&sc).cloned().unwrap_or_else(||
1119 span_bug!(expr.span, "continue to unknown label"))
1122 hir::ExprKind::Assign(ref l, ref r) => {
1123 // see comment on places in
1124 // propagate_through_place_components()
1125 let succ = self.write_place(&l, succ, ACC_WRITE);
1126 let succ = self.propagate_through_place_components(&l, succ);
1127 self.propagate_through_expr(&r, succ)
1130 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1131 // an overloaded assign op is like a method call
1132 if self.tables.is_method_call(expr) {
1133 let succ = self.propagate_through_expr(&l, succ);
1134 self.propagate_through_expr(&r, succ)
1136 // see comment on places in
1137 // propagate_through_place_components()
1138 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1139 let succ = self.propagate_through_expr(&r, succ);
1140 self.propagate_through_place_components(&l, succ)
1144 // Uninteresting cases: just propagate in rev exec order
1146 hir::ExprKind::Array(ref exprs) => {
1147 self.propagate_through_exprs(exprs, succ)
1150 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1151 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1152 fields.iter().rev().fold(succ, |succ, field| {
1153 self.propagate_through_expr(&field.expr, succ)
1157 hir::ExprKind::Call(ref f, ref args) => {
1158 let m = self.ir.tcx.hir().get_module_parent_by_hir_id(expr.hir_id);
1159 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1164 let succ = self.propagate_through_exprs(args, succ);
1165 self.propagate_through_expr(&f, succ)
1168 hir::ExprKind::MethodCall(.., ref args) => {
1169 let m = self.ir.tcx.hir().get_module_parent_by_hir_id(expr.hir_id);
1170 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1176 self.propagate_through_exprs(args, succ)
1179 hir::ExprKind::Tup(ref exprs) => {
1180 self.propagate_through_exprs(exprs, succ)
1183 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1184 let r_succ = self.propagate_through_expr(&r, succ);
1186 let ln = self.live_node(expr.hir_id, expr.span);
1187 self.init_from_succ(ln, succ);
1188 self.merge_from_succ(ln, r_succ, false);
1190 self.propagate_through_expr(&l, ln)
1193 hir::ExprKind::Index(ref l, ref r) |
1194 hir::ExprKind::Binary(_, ref l, ref r) => {
1195 let r_succ = self.propagate_through_expr(&r, succ);
1196 self.propagate_through_expr(&l, r_succ)
1199 hir::ExprKind::Box(ref e) |
1200 hir::ExprKind::AddrOf(_, ref e) |
1201 hir::ExprKind::Cast(ref e, _) |
1202 hir::ExprKind::Type(ref e, _) |
1203 hir::ExprKind::DropTemps(ref e) |
1204 hir::ExprKind::Unary(_, ref e) |
1205 hir::ExprKind::Yield(ref e) |
1206 hir::ExprKind::Repeat(ref e, _) => {
1207 self.propagate_through_expr(&e, succ)
1210 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1211 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1212 // see comment on places
1213 // in propagate_through_place_components()
1215 self.propagate_through_expr(output, succ)
1217 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1218 let succ = self.write_place(output, succ, acc);
1219 self.propagate_through_place_components(output, succ)
1222 // Inputs are executed first. Propagate last because of rev order
1223 self.propagate_through_exprs(inputs, succ)
1226 hir::ExprKind::Lit(..) | hir::ExprKind::Err |
1227 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
1231 // Note that labels have been resolved, so we don't need to look
1232 // at the label ident
1233 hir::ExprKind::Block(ref blk, _) => {
1234 self.propagate_through_block(&blk, succ)
1239 fn propagate_through_place_components(&mut self,
1245 // In general, the full flow graph structure for an
1246 // assignment/move/etc can be handled in one of two ways,
1247 // depending on whether what is being assigned is a "tracked
1248 // value" or not. A tracked value is basically a local
1249 // variable or argument.
1251 // The two kinds of graphs are:
1253 // Tracked place Untracked place
1254 // ----------------------++-----------------------
1258 // (rvalue) || (rvalue)
1261 // (write of place) || (place components)
1266 // ----------------------++-----------------------
1268 // I will cover the two cases in turn:
1272 // A tracked place is a local variable/argument `x`. In
1273 // these cases, the link_node where the write occurs is linked
1274 // to node id of `x`. The `write_place()` routine generates
1275 // the contents of this node. There are no subcomponents to
1278 // # Non-tracked places
1280 // These are places like `x[5]` or `x.f`. In that case, we
1281 // basically ignore the value which is written to but generate
1282 // reads for the components---`x` in these two examples. The
1283 // components reads are generated by
1284 // `propagate_through_place_components()` (this fn).
1288 // It is still possible to observe assignments to non-places;
1289 // these errors are detected in the later pass borrowck. We
1290 // just ignore such cases and treat them as reads.
1293 hir::ExprKind::Path(_) => succ,
1294 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1295 _ => self.propagate_through_expr(expr, succ)
1299 // see comment on propagate_through_place()
1300 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32) -> LiveNode {
1302 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1303 self.access_path(expr.hir_id, path, succ, acc)
1306 // We do not track other places, so just propagate through
1307 // to their subcomponents. Also, it may happen that
1308 // non-places occur here, because those are detected in the
1309 // later pass borrowck.
1314 fn access_var(&mut self, hir_id: HirId, nid: NodeId, succ: LiveNode, acc: u32, span: Span)
1316 let ln = self.live_node(hir_id, span);
1318 self.init_from_succ(ln, succ);
1319 let var_hid = self.ir.tcx.hir().node_to_hir_id(nid);
1320 let var = self.variable(var_hid, span);
1321 self.acc(ln, var, acc);
1326 fn access_path(&mut self, hir_id: HirId, path: &hir::Path, succ: LiveNode, acc: u32)
1329 Res::Local(hid) => {
1330 let nid = self.ir.tcx.hir().hir_to_node_id(hid);
1331 self.access_var(hir_id, nid, succ, acc, path.span)
1337 fn propagate_through_loop(&mut self,
1345 We model control flow like this:
1363 let mut first_merge = true;
1364 let ln = self.live_node(expr.hir_id, expr.span);
1365 self.init_empty(ln, succ);
1369 // If this is not a `loop` loop, then it's possible we bypass
1370 // the body altogether. Otherwise, the only way is via a `break`
1371 // in the loop body.
1372 self.merge_from_succ(ln, succ, first_merge);
1373 first_merge = false;
1376 debug!("propagate_through_loop: using id for loop body {} {}",
1377 expr.hir_id, self.ir.tcx.hir().hir_to_pretty_string(body.hir_id));
1379 self.break_ln.insert(expr.hir_id, succ);
1381 let cond_ln = match kind {
1383 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1386 self.cont_ln.insert(expr.hir_id, cond_ln);
1388 let body_ln = self.propagate_through_block(body, cond_ln);
1390 // repeat until fixed point is reached:
1391 while self.merge_from_succ(ln, body_ln, first_merge) {
1392 first_merge = false;
1394 let new_cond_ln = match kind {
1396 WhileLoop(ref cond) => {
1397 self.propagate_through_expr(&cond, ln)
1400 assert_eq!(cond_ln, new_cond_ln);
1401 assert_eq!(body_ln, self.propagate_through_block(body, cond_ln));
1408 // _______________________________________________________________________
1409 // Checking for error conditions
1411 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1412 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1413 NestedVisitorMap::None
1416 fn visit_local(&mut self, l: &'tcx hir::Local) {
1417 check_local(self, l);
1419 fn visit_expr(&mut self, ex: &'tcx Expr) {
1420 check_expr(self, ex);
1422 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1427 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1430 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1433 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1434 let span = local.pat.simple_ident().map_or(sp, |ident| ident.span);
1435 this.warn_about_unused(vec![span], id, ln, var);
1440 intravisit::walk_local(this, local);
1443 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1444 // Only consider the variable from the first pattern; any later patterns must have
1445 // the same bindings, and we also consider the first pattern to be the "authoritative" set of
1446 // ids. However, we should take the spans of variables with the same name from the later
1447 // patterns so the suggestions to prefix with underscores will apply to those too.
1448 let mut vars: BTreeMap<String, (LiveNode, Variable, HirId, Vec<Span>)> = Default::default();
1450 for pat in &arm.pats {
1451 this.arm_pats_bindings(Some(&*pat), |this, ln, var, sp, id| {
1452 let name = this.ir.variable_name(var);
1454 .and_modify(|(.., spans)| {
1457 .or_insert_with(|| {
1458 (ln, var, id, vec![sp])
1463 for (_, (ln, var, id, spans)) in vars {
1464 this.warn_about_unused(spans, id, ln, var);
1467 intravisit::walk_arm(this, arm);
1470 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1472 hir::ExprKind::Assign(ref l, _) => {
1473 this.check_place(&l);
1475 intravisit::walk_expr(this, expr);
1478 hir::ExprKind::AssignOp(_, ref l, _) => {
1479 if !this.tables.is_method_call(expr) {
1480 this.check_place(&l);
1483 intravisit::walk_expr(this, expr);
1486 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1487 for input in inputs {
1488 this.visit_expr(input);
1491 // Output operands must be places
1492 for (o, output) in ia.outputs.iter().zip(outputs) {
1494 this.check_place(output);
1496 this.visit_expr(output);
1499 intravisit::walk_expr(this, expr);
1502 // no correctness conditions related to liveness
1503 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) |
1504 hir::ExprKind::Match(..) | hir::ExprKind::While(..) | hir::ExprKind::Loop(..) |
1505 hir::ExprKind::Index(..) | hir::ExprKind::Field(..) |
1506 hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) |
1507 hir::ExprKind::Cast(..) | hir::ExprKind::DropTemps(..) | hir::ExprKind::Unary(..) |
1508 hir::ExprKind::Ret(..) | hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) |
1509 hir::ExprKind::Lit(_) | hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) |
1510 hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) |
1511 hir::ExprKind::Closure(..) | hir::ExprKind::Path(_) | hir::ExprKind::Yield(..) |
1512 hir::ExprKind::Box(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err => {
1513 intravisit::walk_expr(this, expr);
1518 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1519 fn check_place(&mut self, expr: &'tcx Expr) {
1521 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1522 if let Res::Local(var_hid) = path.res {
1523 // Assignment to an immutable variable or argument: only legal
1524 // if there is no later assignment. If this local is actually
1525 // mutable, then check for a reassignment to flag the mutability
1527 let ln = self.live_node(expr.hir_id, expr.span);
1528 let var = self.variable(var_hid, expr.span);
1529 self.warn_about_dead_assign(expr.span, expr.hir_id, ln, var);
1533 // For other kinds of places, no checks are required,
1534 // and any embedded expressions are actually rvalues
1535 intravisit::walk_expr(self, expr);
1540 fn should_warn(&self, var: Variable) -> Option<String> {
1541 let name = self.ir.variable_name(var);
1542 if name.is_empty() || name.as_bytes()[0] == b'_' {
1549 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1550 for arg in &body.arguments {
1551 arg.pat.each_binding(|_bm, hir_id, _, ident| {
1552 let sp = ident.span;
1553 let var = self.variable(hir_id, sp);
1554 // Ignore unused self.
1555 if ident.name != kw::SelfLower {
1556 if !self.warn_about_unused(vec![sp], hir_id, entry_ln, var) {
1557 if self.live_on_entry(entry_ln, var).is_none() {
1558 self.report_dead_assign(hir_id, sp, var, true);
1566 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1567 self.pat_bindings(pat, |this, ln, var, sp, id| {
1568 if !this.warn_about_unused(vec![sp], id, ln, var) {
1569 this.warn_about_dead_assign(sp, id, ln, var);
1574 fn warn_about_unused(&self,
1580 if !self.used_on_entry(ln, var) {
1581 let r = self.should_warn(var);
1582 if let Some(name) = r {
1583 // annoying: for parameters in funcs like `fn(x: i32)
1584 // {ret}`, there is only one node, so asking about
1585 // assigned_on_exit() is not meaningful.
1586 let is_assigned = if ln == self.s.exit_ln {
1589 self.assigned_on_exit(ln, var).is_some()
1593 self.ir.tcx.lint_hir_note(
1594 lint::builtin::UNUSED_VARIABLES,
1597 &format!("variable `{}` is assigned to, but never used", name),
1598 &format!("consider using `_{}` instead", name),
1600 } else if name != "self" {
1601 let mut err = self.ir.tcx.struct_span_lint_hir(
1602 lint::builtin::UNUSED_VARIABLES,
1605 &format!("unused variable: `{}`", name),
1608 if self.ir.variable_is_shorthand(var) {
1609 if let Node::Binding(pat) = self.ir.tcx.hir().get_by_hir_id(hir_id) {
1610 // Handle `ref` and `ref mut`.
1611 let spans = spans.iter()
1612 .map(|_span| (pat.span, format!("{}: _", name)))
1615 err.multipart_suggestion(
1616 "try ignoring the field",
1618 Applicability::MachineApplicable,
1622 err.multipart_suggestion(
1623 "consider prefixing with an underscore",
1624 spans.iter().map(|span| (*span, format!("_{}", name))).collect(),
1625 Applicability::MachineApplicable,
1638 fn warn_about_dead_assign(&self, sp: Span, hir_id: HirId, ln: LiveNode, var: Variable) {
1639 if self.live_on_exit(ln, var).is_none() {
1640 self.report_dead_assign(hir_id, sp, var, false);
1644 fn report_dead_assign(&self, hir_id: HirId, sp: Span, var: Variable, is_argument: bool) {
1645 if let Some(name) = self.should_warn(var) {
1647 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1648 &format!("value passed to `{}` is never read", name))
1649 .help("maybe it is overwritten before being read?")
1652 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1653 &format!("value assigned to `{}` is never read", name))
1654 .help("maybe it is overwritten before being read?")