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::{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::keywords;
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!("Free var node [{}]", cm.span_to_string(s))
160 format!("Expr node [{}]", cm.span_to_string(s))
163 format!("Var def node [{}]", cm.span_to_string(s))
165 ExitNode => "Exit node".to_owned(),
169 impl<'a, 'tcx> Visitor<'tcx> for IrMaps<'a, 'tcx> {
170 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
171 NestedVisitorMap::OnlyBodies(&self.tcx.hir())
174 fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
175 b: hir::BodyId, s: Span, id: NodeId) {
176 visit_fn(self, fk, fd, b, s, id);
179 fn visit_local(&mut self, l: &'tcx hir::Local) { visit_local(self, l); }
180 fn visit_expr(&mut self, ex: &'tcx Expr) { visit_expr(self, ex); }
181 fn visit_arm(&mut self, a: &'tcx hir::Arm) { visit_arm(self, a); }
184 fn check_mod_liveness<'tcx>(tcx: TyCtxt<'_, 'tcx, 'tcx>, module_def_id: DefId) {
185 tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor());
188 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
189 for &module in tcx.hir().krate().modules.keys() {
190 tcx.ensure().check_mod_liveness(tcx.hir().local_def_id(module));
194 pub fn provide(providers: &mut Providers<'_>) {
195 *providers = Providers {
201 impl fmt::Debug for LiveNode {
202 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
203 write!(f, "ln({})", self.get())
207 impl fmt::Debug for Variable {
208 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
209 write!(f, "v({})", self.get())
213 // ______________________________________________________________________
216 // This is the first pass and the one that drives the main
217 // computation. It walks up and down the IR once. On the way down,
218 // we count for each function the number of variables as well as
219 // liveness nodes. A liveness node is basically an expression or
220 // capture clause that does something of interest: either it has
221 // interesting control flow or it uses/defines a local variable.
223 // On the way back up, at each function node we create liveness sets
224 // (we now know precisely how big to make our various vectors and so
225 // forth) and then do the data-flow propagation to compute the set
226 // of live variables at each program point.
228 // Finally, we run back over the IR one last time and, using the
229 // computed liveness, check various safety conditions. For example,
230 // there must be no live nodes at the definition site for a variable
231 // unless it has an initializer. Similarly, each non-mutable local
232 // variable must not be assigned if there is some successor
233 // assignment. And so forth.
236 fn is_valid(&self) -> bool {
241 fn invalid_node() -> LiveNode { LiveNode(u32::MAX) }
248 #[derive(Copy, Clone, Debug)]
255 #[derive(Copy, Clone, Debug)]
257 Arg(HirId, ast::Name),
262 struct IrMaps<'a, 'tcx: 'a> {
263 tcx: TyCtxt<'a, 'tcx, 'tcx>,
264 num_live_nodes: usize,
266 live_node_map: HirIdMap<LiveNode>,
267 variable_map: HirIdMap<Variable>,
268 capture_info_map: NodeMap<Rc<Vec<CaptureInfo>>>,
269 var_kinds: Vec<VarKind>,
270 lnks: Vec<LiveNodeKind>,
273 impl<'a, 'tcx> IrMaps<'a, 'tcx> {
274 fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>) -> IrMaps<'a, 'tcx> {
279 live_node_map: HirIdMap::default(),
280 variable_map: HirIdMap::default(),
281 capture_info_map: Default::default(),
282 var_kinds: Vec::new(),
287 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
288 let ln = LiveNode(self.num_live_nodes as u32);
290 self.num_live_nodes += 1;
292 debug!("{:?} is of kind {}", ln,
293 live_node_kind_to_string(lnk, self.tcx));
298 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
299 let ln = self.add_live_node(lnk);
300 self.live_node_map.insert(hir_id, ln);
302 debug!("{:?} is node {:?}", ln, hir_id);
305 fn add_variable(&mut self, vk: VarKind) -> Variable {
306 let v = Variable(self.num_vars as u32);
307 self.var_kinds.push(vk);
311 Local(LocalInfo { id: node_id, .. }) | Arg(node_id, _) => {
312 self.variable_map.insert(node_id, v);
317 debug!("{:?} is {:?}", v, vk);
322 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
323 match self.variable_map.get(&hir_id) {
326 span_bug!(span, "no variable registered for id {:?}", hir_id);
331 fn variable_name(&self, var: Variable) -> String {
332 match self.var_kinds[var.get()] {
333 Local(LocalInfo { name, .. }) | Arg(_, name) => {
336 CleanExit => "<clean-exit>".to_owned()
340 fn variable_is_shorthand(&self, var: Variable) -> bool {
341 match self.var_kinds[var.get()] {
342 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
343 Arg(..) | CleanExit => false
347 fn set_captures(&mut self, node_id: NodeId, cs: Vec<CaptureInfo>) {
348 self.capture_info_map.insert(node_id, Rc::new(cs));
351 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
356 fn visit_fn<'a, 'tcx: 'a>(ir: &mut IrMaps<'a, 'tcx>,
358 decl: &'tcx hir::FnDecl,
359 body_id: hir::BodyId,
364 // swap in a new set of IR maps for this function body:
365 let mut fn_maps = IrMaps::new(ir.tcx);
367 // Don't run unused pass for #[derive()]
368 if let FnKind::Method(..) = fk {
369 let parent = ir.tcx.hir().get_parent(id);
370 if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
371 if i.attrs.iter().any(|a| a.check_name("automatically_derived")) {
377 debug!("creating fn_maps: {:?}", &fn_maps as *const IrMaps<'_, '_>);
379 let body = ir.tcx.hir().body(body_id);
381 for arg in &body.arguments {
382 arg.pat.each_binding(|_bm, hir_id, _x, ident| {
383 debug!("adding argument {:?}", hir_id);
384 fn_maps.add_variable(Arg(hir_id, ident.name));
388 // gather up the various local variables, significant expressions,
390 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
393 let mut lsets = Liveness::new(&mut fn_maps, body_id);
394 let entry_ln = lsets.compute(&body.value);
396 // check for various error conditions
397 lsets.visit_body(body);
398 lsets.warn_about_unused_args(body, entry_ln);
401 fn add_from_pat<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, pat: &P<hir::Pat>) {
402 // For struct patterns, take note of which fields used shorthand
403 // (`x` rather than `x: x`).
404 let mut shorthand_field_ids = HirIdSet::default();
405 let mut pats = VecDeque::new();
407 while let Some(pat) = pats.pop_front() {
408 use crate::hir::PatKind::*;
410 Binding(_, _, _, _, ref inner_pat) => {
411 pats.extend(inner_pat.iter());
413 Struct(_, ref fields, _) => {
414 for field in fields {
415 if field.node.is_shorthand {
416 shorthand_field_ids.insert(field.node.pat.hir_id);
420 Ref(ref inner_pat, _) |
421 Box(ref inner_pat) => {
422 pats.push_back(inner_pat);
424 TupleStruct(_, ref inner_pats, _) |
425 Tuple(ref inner_pats, _) => {
426 pats.extend(inner_pats.iter());
428 Slice(ref pre_pats, ref inner_pat, ref post_pats) => {
429 pats.extend(pre_pats.iter());
430 pats.extend(inner_pat.iter());
431 pats.extend(post_pats.iter());
437 pat.each_binding(|_bm, hir_id, _sp, 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<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, local: &'tcx hir::Local) {
448 add_from_pat(ir, &local.pat);
449 intravisit::walk_local(ir, local);
452 fn visit_arm<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, arm: &'tcx hir::Arm) {
453 for pat in &arm.pats {
454 add_from_pat(ir, pat);
456 intravisit::walk_arm(ir, arm);
459 fn visit_expr<'a, 'tcx>(ir: &mut IrMaps<'a, 'tcx>, expr: &'tcx Expr) {
461 // live nodes required for uses or definitions of variables:
462 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
463 debug!("expr {}: path that leads to {:?}", expr.id, path.def);
464 if let Def::Local(..) = path.def {
465 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
467 intravisit::walk_expr(ir, expr);
469 hir::ExprKind::Closure(..) => {
470 // Interesting control flow (for loops can contain labeled
471 // breaks or continues)
472 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
474 // Make a live_node for each captured variable, with the span
475 // being the location that the variable is used. This results
476 // in better error messages than just pointing at the closure
477 // construction site.
478 let mut call_caps = Vec::new();
479 ir.tcx.with_freevars(expr.id, |freevars| {
480 call_caps.extend(freevars.iter().filter_map(|fv| {
481 if let Def::Local(rv) = fv.def {
482 let fv_ln = ir.add_live_node(FreeVarNode(fv.span));
483 let var_hid = ir.tcx.hir().node_to_hir_id(rv);
484 Some(CaptureInfo { ln: fv_ln, var_hid })
490 ir.set_captures(expr.id, call_caps);
492 intravisit::walk_expr(ir, expr);
495 // live nodes required for interesting control flow:
496 hir::ExprKind::If(..) |
497 hir::ExprKind::Match(..) |
498 hir::ExprKind::While(..) |
499 hir::ExprKind::Loop(..) => {
500 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
501 intravisit::walk_expr(ir, expr);
503 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
504 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
505 intravisit::walk_expr(ir, expr);
508 // otherwise, live nodes are not required:
509 hir::ExprKind::Index(..) |
510 hir::ExprKind::Field(..) |
511 hir::ExprKind::Array(..) |
512 hir::ExprKind::Call(..) |
513 hir::ExprKind::MethodCall(..) |
514 hir::ExprKind::Tup(..) |
515 hir::ExprKind::Binary(..) |
516 hir::ExprKind::AddrOf(..) |
517 hir::ExprKind::Cast(..) |
518 hir::ExprKind::Unary(..) |
519 hir::ExprKind::Break(..) |
520 hir::ExprKind::Continue(_) |
521 hir::ExprKind::Lit(_) |
522 hir::ExprKind::Ret(..) |
523 hir::ExprKind::Block(..) |
524 hir::ExprKind::Assign(..) |
525 hir::ExprKind::AssignOp(..) |
526 hir::ExprKind::Struct(..) |
527 hir::ExprKind::Repeat(..) |
528 hir::ExprKind::InlineAsm(..) |
529 hir::ExprKind::Box(..) |
530 hir::ExprKind::Yield(..) |
531 hir::ExprKind::Type(..) |
533 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
534 intravisit::walk_expr(ir, expr);
539 // ______________________________________________________________________
540 // Computing liveness sets
542 // Actually we compute just a bit more than just liveness, but we use
543 // the same basic propagation framework in all cases.
545 #[derive(Clone, Copy)]
552 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
553 /// very large, so it uses a more compact representation that takes advantage
554 /// of the fact that when the number of `RWU`s is large, most of them have an
555 /// invalid reader and an invalid writer.
557 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
558 /// an index into `unpacked_rwus`. In the common cases, this compacts the
559 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
562 /// More compact representations are possible -- e.g., use only 2 bits per
563 /// packed `RWU` and make the secondary table a HashMap that maps from
564 /// indices to `RWU`s -- but this one strikes a good balance between size
566 packed_rwus: Vec<u32>,
567 unpacked_rwus: Vec<RWU>,
570 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
571 const INV_INV_FALSE: u32 = u32::MAX;
573 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
574 const INV_INV_TRUE: u32 = u32::MAX - 1;
577 fn new(num_rwus: usize) -> RWUTable {
579 packed_rwus: vec![INV_INV_FALSE; num_rwus],
580 unpacked_rwus: vec![],
584 fn get(&self, idx: usize) -> RWU {
585 let packed_rwu = self.packed_rwus[idx];
587 INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false },
588 INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true },
589 _ => self.unpacked_rwus[packed_rwu as usize],
593 fn get_reader(&self, idx: usize) -> LiveNode {
594 let packed_rwu = self.packed_rwus[idx];
596 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
597 _ => self.unpacked_rwus[packed_rwu as usize].reader,
601 fn get_writer(&self, idx: usize) -> LiveNode {
602 let packed_rwu = self.packed_rwus[idx];
604 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
605 _ => self.unpacked_rwus[packed_rwu as usize].writer,
609 fn get_used(&self, idx: usize) -> bool {
610 let packed_rwu = self.packed_rwus[idx];
612 INV_INV_FALSE => false,
613 INV_INV_TRUE => true,
614 _ => self.unpacked_rwus[packed_rwu as usize].used,
619 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
620 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
623 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
624 if rwu.reader == invalid_node() && rwu.writer == invalid_node() {
625 // When we overwrite an indexing entry in `self.packed_rwus` with
626 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
627 // from `self.unpacked_rwus`; it's not worth the effort, and we
628 // can't have entries shifting around anyway.
629 self.packed_rwus[idx] = if rwu.used {
635 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
637 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
638 self.unpacked_rwus.push(rwu);
642 fn assign_inv_inv(&mut self, idx: usize) {
643 self.packed_rwus[idx] = if self.get_used(idx) {
651 #[derive(Copy, Clone)]
654 fallthrough_ln: LiveNode,
655 clean_exit_var: Variable
658 const ACC_READ: u32 = 1;
659 const ACC_WRITE: u32 = 2;
660 const ACC_USE: u32 = 4;
662 struct Liveness<'a, 'tcx: 'a> {
663 ir: &'a mut IrMaps<'a, 'tcx>,
664 tables: &'a ty::TypeckTables<'tcx>,
666 successors: Vec<LiveNode>,
669 // mappings from loop node ID to LiveNode
670 // ("break" label should map to loop node ID,
671 // it probably doesn't now)
672 break_ln: NodeMap<LiveNode>,
673 cont_ln: NodeMap<LiveNode>,
676 impl<'a, 'tcx> Liveness<'a, 'tcx> {
677 fn new(ir: &'a mut IrMaps<'a, 'tcx>, body: hir::BodyId) -> Liveness<'a, 'tcx> {
678 // Special nodes and variables:
679 // - exit_ln represents the end of the fn, either by return or panic
680 // - implicit_ret_var is a pseudo-variable that represents
681 // an implicit return
682 let specials = Specials {
683 exit_ln: ir.add_live_node(ExitNode),
684 fallthrough_ln: ir.add_live_node(ExitNode),
685 clean_exit_var: ir.add_variable(CleanExit)
688 let tables = ir.tcx.body_tables(body);
690 let num_live_nodes = ir.num_live_nodes;
691 let num_vars = ir.num_vars;
697 successors: vec![invalid_node(); num_live_nodes],
698 rwu_table: RWUTable::new(num_live_nodes * num_vars),
699 break_ln: Default::default(),
700 cont_ln: Default::default(),
704 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
705 match self.ir.live_node_map.get(&hir_id) {
708 // This must be a mismatch between the ir_map construction
709 // above and the propagation code below; the two sets of
710 // code have to agree about which AST nodes are worth
711 // creating liveness nodes for.
714 "no live node registered for node {:?}",
720 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
721 self.ir.variable(hir_id, span)
724 fn pat_bindings<F>(&mut self, pat: &hir::Pat, mut f: F) where
725 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
727 pat.each_binding(|_bm, hir_id, sp, n| {
728 let ln = self.live_node(hir_id, sp);
729 let var = self.variable(hir_id, n.span);
730 f(self, ln, var, n.span, hir_id);
734 fn arm_pats_bindings<F>(&mut self, pat: Option<&hir::Pat>, f: F) where
735 F: FnMut(&mut Liveness<'a, 'tcx>, LiveNode, Variable, Span, HirId),
737 if let Some(pat) = pat {
738 self.pat_bindings(pat, f);
742 fn define_bindings_in_pat(&mut self, pat: &hir::Pat, succ: LiveNode)
744 self.define_bindings_in_arm_pats(Some(pat), succ)
747 fn define_bindings_in_arm_pats(&mut self, pat: Option<&hir::Pat>, succ: LiveNode)
750 self.arm_pats_bindings(pat, |this, ln, var, _sp, _id| {
751 this.init_from_succ(ln, succ);
752 this.define(ln, var);
758 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
759 ln.get() * self.ir.num_vars + var.get()
762 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
763 assert!(ln.is_valid());
764 let reader = self.rwu_table.get_reader(self.idx(ln, var));
765 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
768 // Is this variable live on entry to any of its successor nodes?
769 fn live_on_exit(&self, ln: LiveNode, var: Variable)
770 -> Option<LiveNodeKind> {
771 let successor = self.successors[ln.get()];
772 self.live_on_entry(successor, var)
775 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
776 assert!(ln.is_valid());
777 self.rwu_table.get_used(self.idx(ln, var))
780 fn assigned_on_entry(&self, ln: LiveNode, var: Variable)
781 -> Option<LiveNodeKind> {
782 assert!(ln.is_valid());
783 let writer = self.rwu_table.get_writer(self.idx(ln, var));
784 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
787 fn assigned_on_exit(&self, ln: LiveNode, var: Variable)
788 -> Option<LiveNodeKind> {
789 let successor = self.successors[ln.get()];
790 self.assigned_on_entry(successor, var)
793 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F) where
794 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
796 let node_base_idx = self.idx(ln, Variable(0));
797 let succ_base_idx = self.idx(succ_ln, Variable(0));
798 for var_idx in 0..self.ir.num_vars {
799 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
803 fn write_vars<F>(&self,
807 -> io::Result<()> where
808 F: FnMut(usize) -> LiveNode,
810 let node_base_idx = self.idx(ln, Variable(0));
811 for var_idx in 0..self.ir.num_vars {
812 let idx = node_base_idx + var_idx;
813 if test(idx).is_valid() {
814 write!(wr, " {:?}", Variable(var_idx as u32))?;
821 #[allow(unused_must_use)]
822 fn ln_str(&self, ln: LiveNode) -> String {
823 let mut wr = Vec::new();
825 let wr = &mut wr as &mut dyn Write;
826 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
827 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx));
828 write!(wr, " writes");
829 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx));
830 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
832 String::from_utf8(wr).unwrap()
835 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
836 self.successors[ln.get()] = succ_ln;
838 // It is not necessary to initialize the RWUs here because they are all
839 // set to INV_INV_FALSE when they are created, and the sets only grow
840 // during iterations.
843 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
844 // more efficient version of init_empty() / merge_from_succ()
845 self.successors[ln.get()] = succ_ln;
847 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
848 this.rwu_table.copy_packed(idx, succ_idx);
850 debug!("init_from_succ(ln={}, succ={})",
851 self.ln_str(ln), self.ln_str(succ_ln));
854 fn merge_from_succ(&mut self,
859 if ln == succ_ln { return false; }
861 let mut changed = false;
862 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
863 let mut rwu = this.rwu_table.get(idx);
864 let succ_rwu = this.rwu_table.get(succ_idx);
865 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
866 rwu.reader = succ_rwu.reader;
870 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
871 rwu.writer = succ_rwu.writer;
875 if succ_rwu.used && !rwu.used {
881 this.rwu_table.assign_unpacked(idx, rwu);
885 debug!("merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
886 ln, self.ln_str(succ_ln), first_merge, changed);
890 // Indicates that a local variable was *defined*; we know that no
891 // uses of the variable can precede the definition (resolve checks
892 // this) so we just clear out all the data.
893 fn define(&mut self, writer: LiveNode, var: Variable) {
894 let idx = self.idx(writer, var);
895 self.rwu_table.assign_inv_inv(idx);
897 debug!("{:?} defines {:?} (idx={}): {}", writer, var,
898 idx, self.ln_str(writer));
901 // Either read, write, or both depending on the acc bitset
902 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
903 debug!("{:?} accesses[{:x}] {:?}: {}",
904 ln, acc, var, self.ln_str(ln));
906 let idx = self.idx(ln, var);
907 let mut rwu = self.rwu_table.get(idx);
909 if (acc & ACC_WRITE) != 0 {
910 rwu.reader = invalid_node();
914 // Important: if we both read/write, must do read second
915 // or else the write will override.
916 if (acc & ACC_READ) != 0 {
920 if (acc & ACC_USE) != 0 {
924 self.rwu_table.assign_unpacked(idx, rwu);
927 fn compute(&mut self, body: &hir::Expr) -> LiveNode {
928 debug!("compute: using id for body, {}", self.ir.tcx.hir().node_to_pretty_string(body.id));
930 // the fallthrough exit is only for those cases where we do not
931 // explicitly return:
933 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
934 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
936 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
938 // hack to skip the loop unless debug! is enabled:
939 debug!("^^ liveness computation results for body {} (entry={:?})", {
940 for ln_idx in 0..self.ir.num_live_nodes {
941 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
950 fn propagate_through_block(&mut self, blk: &hir::Block, succ: LiveNode)
952 if blk.targeted_by_break {
953 self.break_ln.insert(blk.id, succ);
955 let succ = self.propagate_through_opt_expr(blk.expr.as_ref().map(|e| &**e), succ);
956 blk.stmts.iter().rev().fold(succ, |succ, stmt| {
957 self.propagate_through_stmt(stmt, succ)
961 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt, succ: LiveNode)
964 hir::StmtKind::Local(ref local) => {
965 // Note: we mark the variable as defined regardless of whether
966 // there is an initializer. Initially I had thought to only mark
967 // the live variable as defined if it was initialized, and then we
968 // could check for uninit variables just by scanning what is live
969 // at the start of the function. But that doesn't work so well for
970 // immutable variables defined in a loop:
971 // loop { let x; x = 5; }
972 // because the "assignment" loops back around and generates an error.
974 // So now we just check that variables defined w/o an
975 // initializer are not live at the point of their
976 // initialization, which is mildly more complex than checking
977 // once at the func header but otherwise equivalent.
979 let succ = self.propagate_through_opt_expr(local.init.as_ref().map(|e| &**e), succ);
980 self.define_bindings_in_pat(&local.pat, succ)
982 hir::StmtKind::Item(..) => succ,
983 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
984 self.propagate_through_expr(&expr, succ)
989 fn propagate_through_exprs(&mut self, exprs: &[Expr], succ: LiveNode)
991 exprs.iter().rev().fold(succ, |succ, expr| {
992 self.propagate_through_expr(&expr, succ)
996 fn propagate_through_opt_expr(&mut self,
997 opt_expr: Option<&Expr>,
1000 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
1003 fn propagate_through_expr(&mut self, expr: &Expr, succ: LiveNode)
1005 debug!("propagate_through_expr: {}", self.ir.tcx.hir().node_to_pretty_string(expr.id));
1008 // Interesting cases with control flow or which gen/kill
1009 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1010 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
1013 hir::ExprKind::Field(ref e, _) => {
1014 self.propagate_through_expr(&e, succ)
1017 hir::ExprKind::Closure(..) => {
1018 debug!("{} is an ExprKind::Closure",
1019 self.ir.tcx.hir().node_to_pretty_string(expr.id));
1021 // the construction of a closure itself is not important,
1022 // but we have to consider the closed over variables.
1023 let caps = self.ir.capture_info_map.get(&expr.id).cloned().unwrap_or_else(||
1024 span_bug!(expr.span, "no registered caps"));
1026 caps.iter().rev().fold(succ, |succ, cap| {
1027 self.init_from_succ(cap.ln, succ);
1028 let var = self.variable(cap.var_hid, expr.span);
1029 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1034 hir::ExprKind::If(ref cond, ref then, ref els) => {
1048 let else_ln = self.propagate_through_opt_expr(els.as_ref().map(|e| &**e), succ);
1049 let then_ln = self.propagate_through_expr(&then, succ);
1050 let ln = self.live_node(expr.hir_id, expr.span);
1051 self.init_from_succ(ln, else_ln);
1052 self.merge_from_succ(ln, then_ln, false);
1053 self.propagate_through_expr(&cond, ln)
1056 hir::ExprKind::While(ref cond, ref blk, _) => {
1057 self.propagate_through_loop(expr, WhileLoop(&cond), &blk, succ)
1060 // Note that labels have been resolved, so we don't need to look
1061 // at the label ident
1062 hir::ExprKind::Loop(ref blk, _, _) => {
1063 self.propagate_through_loop(expr, LoopLoop, &blk, succ)
1066 hir::ExprKind::Match(ref e, ref arms, _) => {
1081 let ln = self.live_node(expr.hir_id, expr.span);
1082 self.init_empty(ln, succ);
1083 let mut first_merge = true;
1085 let body_succ = self.propagate_through_expr(&arm.body, succ);
1087 let guard_succ = self.propagate_through_opt_expr(
1088 arm.guard.as_ref().map(|hir::Guard::If(e)| &**e),
1091 // only consider the first pattern; any later patterns must have
1092 // the same bindings, and we also consider the first pattern to be
1093 // the "authoritative" set of ids
1095 self.define_bindings_in_arm_pats(arm.pats.first().map(|p| &**p),
1097 self.merge_from_succ(ln, arm_succ, first_merge);
1098 first_merge = false;
1100 self.propagate_through_expr(&e, ln)
1103 hir::ExprKind::Ret(ref o_e) => {
1104 // ignore succ and subst exit_ln:
1105 let exit_ln = self.s.exit_ln;
1106 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1109 hir::ExprKind::Break(label, ref opt_expr) => {
1110 // Find which label this break jumps to
1111 let target = match label.target_id {
1112 Ok(node_id) => self.break_ln.get(&node_id),
1113 Err(err) => 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 break loop nodes table
1120 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1121 None => span_bug!(expr.span, "break to unknown label")
1125 hir::ExprKind::Continue(label) => {
1126 // Find which label this expr continues to
1127 let sc = label.target_id.unwrap_or_else(|err|
1128 span_bug!(expr.span, "loop scope error: {}", err));
1130 // Now that we know the label we're going to,
1131 // look it up in the continue loop nodes table
1132 self.cont_ln.get(&sc).cloned().unwrap_or_else(||
1133 span_bug!(expr.span, "continue to unknown label"))
1136 hir::ExprKind::Assign(ref l, ref r) => {
1137 // see comment on places in
1138 // propagate_through_place_components()
1139 let succ = self.write_place(&l, succ, ACC_WRITE);
1140 let succ = self.propagate_through_place_components(&l, succ);
1141 self.propagate_through_expr(&r, succ)
1144 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1145 // an overloaded assign op is like a method call
1146 if self.tables.is_method_call(expr) {
1147 let succ = self.propagate_through_expr(&l, succ);
1148 self.propagate_through_expr(&r, succ)
1150 // see comment on places in
1151 // propagate_through_place_components()
1152 let succ = self.write_place(&l, succ, ACC_WRITE|ACC_READ);
1153 let succ = self.propagate_through_expr(&r, succ);
1154 self.propagate_through_place_components(&l, succ)
1158 // Uninteresting cases: just propagate in rev exec order
1160 hir::ExprKind::Array(ref exprs) => {
1161 self.propagate_through_exprs(exprs, succ)
1164 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1165 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1166 fields.iter().rev().fold(succ, |succ, field| {
1167 self.propagate_through_expr(&field.expr, succ)
1171 hir::ExprKind::Call(ref f, ref args) => {
1172 let m = self.ir.tcx.hir().get_module_parent(expr.id);
1173 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1178 let succ = self.propagate_through_exprs(args, succ);
1179 self.propagate_through_expr(&f, succ)
1182 hir::ExprKind::MethodCall(.., ref args) => {
1183 let m = self.ir.tcx.hir().get_module_parent(expr.id);
1184 let succ = if self.ir.tcx.is_ty_uninhabited_from(m, self.tables.expr_ty(expr)) {
1190 self.propagate_through_exprs(args, succ)
1193 hir::ExprKind::Tup(ref exprs) => {
1194 self.propagate_through_exprs(exprs, succ)
1197 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1198 let r_succ = self.propagate_through_expr(&r, succ);
1200 let ln = self.live_node(expr.hir_id, expr.span);
1201 self.init_from_succ(ln, succ);
1202 self.merge_from_succ(ln, r_succ, false);
1204 self.propagate_through_expr(&l, ln)
1207 hir::ExprKind::Index(ref l, ref r) |
1208 hir::ExprKind::Binary(_, ref l, ref r) => {
1209 let r_succ = self.propagate_through_expr(&r, succ);
1210 self.propagate_through_expr(&l, r_succ)
1213 hir::ExprKind::Box(ref e) |
1214 hir::ExprKind::AddrOf(_, ref e) |
1215 hir::ExprKind::Cast(ref e, _) |
1216 hir::ExprKind::Type(ref e, _) |
1217 hir::ExprKind::Unary(_, ref e) |
1218 hir::ExprKind::Yield(ref e) |
1219 hir::ExprKind::Repeat(ref e, _) => {
1220 self.propagate_through_expr(&e, succ)
1223 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1224 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1225 // see comment on places
1226 // in propagate_through_place_components()
1228 self.propagate_through_expr(output, succ)
1230 let acc = if o.is_rw { ACC_WRITE|ACC_READ } else { ACC_WRITE };
1231 let succ = self.write_place(output, succ, acc);
1232 self.propagate_through_place_components(output, succ)
1235 // Inputs are executed first. Propagate last because of rev order
1236 self.propagate_through_exprs(inputs, succ)
1239 hir::ExprKind::Lit(..) | hir::ExprKind::Err |
1240 hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
1244 // Note that labels have been resolved, so we don't need to look
1245 // at the label ident
1246 hir::ExprKind::Block(ref blk, _) => {
1247 self.propagate_through_block(&blk, succ)
1252 fn propagate_through_place_components(&mut self,
1258 // In general, the full flow graph structure for an
1259 // assignment/move/etc can be handled in one of two ways,
1260 // depending on whether what is being assigned is a "tracked
1261 // value" or not. A tracked value is basically a local
1262 // variable or argument.
1264 // The two kinds of graphs are:
1266 // Tracked place Untracked place
1267 // ----------------------++-----------------------
1271 // (rvalue) || (rvalue)
1274 // (write of place) || (place components)
1279 // ----------------------++-----------------------
1281 // I will cover the two cases in turn:
1285 // A tracked place is a local variable/argument `x`. In
1286 // these cases, the link_node where the write occurs is linked
1287 // to node id of `x`. The `write_place()` routine generates
1288 // the contents of this node. There are no subcomponents to
1291 // # Non-tracked places
1293 // These are places like `x[5]` or `x.f`. In that case, we
1294 // basically ignore the value which is written to but generate
1295 // reads for the components---`x` in these two examples. The
1296 // components reads are generated by
1297 // `propagate_through_place_components()` (this fn).
1301 // It is still possible to observe assignments to non-places;
1302 // these errors are detected in the later pass borrowck. We
1303 // just ignore such cases and treat them as reads.
1306 hir::ExprKind::Path(_) => succ,
1307 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1308 _ => self.propagate_through_expr(expr, succ)
1312 // see comment on propagate_through_place()
1313 fn write_place(&mut self, expr: &Expr, succ: LiveNode, acc: u32) -> LiveNode {
1315 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1316 self.access_path(expr.hir_id, path, succ, acc)
1319 // We do not track other places, so just propagate through
1320 // to their subcomponents. Also, it may happen that
1321 // non-places occur here, because those are detected in the
1322 // later pass borrowck.
1327 fn access_var(&mut self, hir_id: HirId, nid: NodeId, succ: LiveNode, acc: u32, span: Span)
1329 let ln = self.live_node(hir_id, span);
1331 self.init_from_succ(ln, succ);
1332 let var_hid = self.ir.tcx.hir().node_to_hir_id(nid);
1333 let var = self.variable(var_hid, span);
1334 self.acc(ln, var, acc);
1339 fn access_path(&mut self, hir_id: HirId, path: &hir::Path, succ: LiveNode, acc: u32)
1342 Def::Local(nid) => {
1343 self.access_var(hir_id, nid, succ, acc, path.span)
1349 fn propagate_through_loop(&mut self,
1357 We model control flow like this:
1375 let mut first_merge = true;
1376 let ln = self.live_node(expr.hir_id, expr.span);
1377 self.init_empty(ln, succ);
1381 // If this is not a `loop` loop, then it's possible we bypass
1382 // the body altogether. Otherwise, the only way is via a `break`
1383 // in the loop body.
1384 self.merge_from_succ(ln, succ, first_merge);
1385 first_merge = false;
1388 debug!("propagate_through_loop: using id for loop body {} {}",
1389 expr.id, self.ir.tcx.hir().node_to_pretty_string(body.id));
1392 self.break_ln.insert(expr.id, succ);
1394 let cond_ln = match kind {
1396 WhileLoop(ref cond) => self.propagate_through_expr(&cond, ln),
1399 self.cont_ln.insert(expr.id, cond_ln);
1401 let body_ln = self.propagate_through_block(body, cond_ln);
1403 // repeat until fixed point is reached:
1404 while self.merge_from_succ(ln, body_ln, first_merge) {
1405 first_merge = false;
1407 let new_cond_ln = match kind {
1409 WhileLoop(ref cond) => {
1410 self.propagate_through_expr(&cond, ln)
1413 assert_eq!(cond_ln, new_cond_ln);
1414 assert_eq!(body_ln, self.propagate_through_block(body, cond_ln));
1421 // _______________________________________________________________________
1422 // Checking for error conditions
1424 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1425 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
1426 NestedVisitorMap::None
1429 fn visit_local(&mut self, l: &'tcx hir::Local) {
1430 check_local(self, l);
1432 fn visit_expr(&mut self, ex: &'tcx Expr) {
1433 check_expr(self, ex);
1435 fn visit_arm(&mut self, a: &'tcx hir::Arm) {
1440 fn check_local<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, local: &'tcx hir::Local) {
1443 this.warn_about_unused_or_dead_vars_in_pat(&local.pat);
1446 this.pat_bindings(&local.pat, |this, ln, var, sp, id| {
1447 let span = local.pat.simple_ident().map_or(sp, |ident| ident.span);
1448 this.warn_about_unused(vec![span], id, ln, var);
1453 intravisit::walk_local(this, local);
1456 fn check_arm<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, arm: &'tcx hir::Arm) {
1457 // Only consider the variable from the first pattern; any later patterns must have
1458 // the same bindings, and we also consider the first pattern to be the "authoritative" set of
1459 // ids. However, we should take the spans of variables with the same name from the later
1460 // patterns so the suggestions to prefix with underscores will apply to those too.
1461 let mut vars: BTreeMap<String, (LiveNode, Variable, HirId, Vec<Span>)> = Default::default();
1463 for pat in &arm.pats {
1464 this.arm_pats_bindings(Some(&*pat), |this, ln, var, sp, id| {
1465 let name = this.ir.variable_name(var);
1467 .and_modify(|(.., spans)| {
1470 .or_insert_with(|| {
1471 (ln, var, id, vec![sp])
1476 for (_, (ln, var, id, spans)) in vars {
1477 this.warn_about_unused(spans, id, ln, var);
1480 intravisit::walk_arm(this, arm);
1483 fn check_expr<'a, 'tcx>(this: &mut Liveness<'a, 'tcx>, expr: &'tcx Expr) {
1485 hir::ExprKind::Assign(ref l, _) => {
1486 this.check_place(&l);
1488 intravisit::walk_expr(this, expr);
1491 hir::ExprKind::AssignOp(_, ref l, _) => {
1492 if !this.tables.is_method_call(expr) {
1493 this.check_place(&l);
1496 intravisit::walk_expr(this, expr);
1499 hir::ExprKind::InlineAsm(ref ia, ref outputs, ref inputs) => {
1500 for input in inputs {
1501 this.visit_expr(input);
1504 // Output operands must be places
1505 for (o, output) in ia.outputs.iter().zip(outputs) {
1507 this.check_place(output);
1509 this.visit_expr(output);
1512 intravisit::walk_expr(this, expr);
1515 // no correctness conditions related to liveness
1516 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) | hir::ExprKind::If(..) |
1517 hir::ExprKind::Match(..) | hir::ExprKind::While(..) | hir::ExprKind::Loop(..) |
1518 hir::ExprKind::Index(..) | hir::ExprKind::Field(..) |
1519 hir::ExprKind::Array(..) | hir::ExprKind::Tup(..) | hir::ExprKind::Binary(..) |
1520 hir::ExprKind::Cast(..) | hir::ExprKind::Unary(..) | hir::ExprKind::Ret(..) |
1521 hir::ExprKind::Break(..) | hir::ExprKind::Continue(..) | hir::ExprKind::Lit(_) |
1522 hir::ExprKind::Block(..) | hir::ExprKind::AddrOf(..) |
1523 hir::ExprKind::Struct(..) | hir::ExprKind::Repeat(..) |
1524 hir::ExprKind::Closure(..) | hir::ExprKind::Path(_) | hir::ExprKind::Yield(..) |
1525 hir::ExprKind::Box(..) | hir::ExprKind::Type(..) | hir::ExprKind::Err => {
1526 intravisit::walk_expr(this, expr);
1531 impl<'a, 'tcx> Liveness<'a, 'tcx> {
1532 fn check_place(&mut self, expr: &'tcx Expr) {
1534 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1535 if let Def::Local(nid) = path.def {
1536 // Assignment to an immutable variable or argument: only legal
1537 // if there is no later assignment. If this local is actually
1538 // mutable, then check for a reassignment to flag the mutability
1540 let ln = self.live_node(expr.hir_id, expr.span);
1541 let var_hid = self.ir.tcx.hir().node_to_hir_id(nid);
1542 let var = self.variable(var_hid, expr.span);
1543 self.warn_about_dead_assign(expr.span, expr.hir_id, ln, var);
1547 // For other kinds of places, no checks are required,
1548 // and any embedded expressions are actually rvalues
1549 intravisit::walk_expr(self, expr);
1554 fn should_warn(&self, var: Variable) -> Option<String> {
1555 let name = self.ir.variable_name(var);
1556 if name.is_empty() || name.as_bytes()[0] == b'_' {
1563 fn warn_about_unused_args(&self, body: &hir::Body, entry_ln: LiveNode) {
1564 for arg in &body.arguments {
1565 arg.pat.each_binding(|_bm, hir_id, _, ident| {
1566 let sp = ident.span;
1567 let var = self.variable(hir_id, sp);
1568 // Ignore unused self.
1569 if ident.name != keywords::SelfLower.name() {
1570 if !self.warn_about_unused(vec![sp], hir_id, entry_ln, var) {
1571 if self.live_on_entry(entry_ln, var).is_none() {
1572 self.report_dead_assign(hir_id, sp, var, true);
1580 fn warn_about_unused_or_dead_vars_in_pat(&mut self, pat: &hir::Pat) {
1581 self.pat_bindings(pat, |this, ln, var, sp, id| {
1582 if !this.warn_about_unused(vec![sp], id, ln, var) {
1583 this.warn_about_dead_assign(sp, id, ln, var);
1588 fn warn_about_unused(&self,
1594 if !self.used_on_entry(ln, var) {
1595 let r = self.should_warn(var);
1596 if let Some(name) = r {
1597 // annoying: for parameters in funcs like `fn(x: i32)
1598 // {ret}`, there is only one node, so asking about
1599 // assigned_on_exit() is not meaningful.
1600 let is_assigned = if ln == self.s.exit_ln {
1603 self.assigned_on_exit(ln, var).is_some()
1607 self.ir.tcx.lint_hir_note(
1608 lint::builtin::UNUSED_VARIABLES,
1611 &format!("variable `{}` is assigned to, but never used", name),
1612 &format!("consider using `_{}` instead", name),
1614 } else if name != "self" {
1615 let mut err = self.ir.tcx.struct_span_lint_hir(
1616 lint::builtin::UNUSED_VARIABLES,
1619 &format!("unused variable: `{}`", name),
1622 if self.ir.variable_is_shorthand(var) {
1623 err.multipart_suggestion(
1624 "try ignoring the field",
1625 spans.iter().map(|span| (*span, format!("{}: _", name))).collect(),
1626 Applicability::MachineApplicable
1629 err.multipart_suggestion(
1630 "consider prefixing with an underscore",
1631 spans.iter().map(|span| (*span, format!("_{}", name))).collect(),
1632 Applicability::MachineApplicable,
1645 fn warn_about_dead_assign(&self, sp: Span, hir_id: HirId, ln: LiveNode, var: Variable) {
1646 if self.live_on_exit(ln, var).is_none() {
1647 self.report_dead_assign(hir_id, sp, var, false);
1651 fn report_dead_assign(&self, hir_id: HirId, sp: Span, var: Variable, is_argument: bool) {
1652 if let Some(name) = self.should_warn(var) {
1654 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1655 &format!("value passed to `{}` is never read", name))
1656 .help("maybe it is overwritten before being read?")
1659 self.ir.tcx.struct_span_lint_hir(lint::builtin::UNUSED_ASSIGNMENTS, hir_id, sp,
1660 &format!("value assigned to `{}` is never read", name))
1661 .help("maybe it is overwritten before being read?")