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 nodes and variables
81 //! We generate various special nodes for various, well, special purposes.
82 //! These are described in the `Specials` struct.
84 use self::LiveNodeKind::*;
87 use rustc_ast::InlineAsmOptions;
88 use rustc_data_structures::fx::FxIndexMap;
89 use rustc_errors::Applicability;
91 use rustc_hir::def::*;
92 use rustc_hir::def_id::LocalDefId;
93 use rustc_hir::intravisit::{self, FnKind, NestedVisitorMap, Visitor};
94 use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet, Node};
95 use rustc_middle::hir::map::Map;
96 use rustc_middle::ty::query::Providers;
97 use rustc_middle::ty::{self, TyCtxt};
98 use rustc_session::lint;
99 use rustc_span::symbol::{sym, Symbol};
100 use rustc_span::Span;
102 use std::collections::VecDeque;
105 use std::io::prelude::*;
108 #[derive(Copy, Clone, PartialEq)]
109 struct Variable(u32);
111 #[derive(Copy, Clone, PartialEq)]
112 struct LiveNode(u32);
115 fn get(&self) -> usize {
121 fn get(&self) -> usize {
126 #[derive(Copy, Clone, PartialEq, Debug)]
135 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
136 let sm = tcx.sess.source_map();
138 UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_string(s)),
139 ExprNode(s) => format!("Expr node [{}]", sm.span_to_string(s)),
140 VarDefNode(s) => format!("Var def node [{}]", sm.span_to_string(s)),
141 ClosureNode => "Closure node".to_owned(),
142 ExitNode => "Exit node".to_owned(),
146 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
147 type Map = Map<'tcx>;
149 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
150 NestedVisitorMap::OnlyBodies(self.tcx.hir())
156 fd: &'tcx hir::FnDecl<'tcx>,
161 visit_fn(self, fk, fd, b, s, id);
164 fn visit_local(&mut self, l: &'tcx hir::Local<'tcx>) {
165 visit_local(self, l);
167 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
168 visit_expr(self, ex);
170 fn visit_arm(&mut self, a: &'tcx hir::Arm<'tcx>) {
175 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
176 tcx.hir().visit_item_likes_in_module(
178 &mut IrMaps::new(tcx, module_def_id).as_deep_visitor(),
182 pub fn provide(providers: &mut Providers) {
183 *providers = Providers { check_mod_liveness, ..*providers };
186 impl fmt::Debug for LiveNode {
187 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
188 write!(f, "ln({})", self.get())
192 impl fmt::Debug for Variable {
193 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
194 write!(f, "v({})", self.get())
198 // ______________________________________________________________________
201 // This is the first pass and the one that drives the main
202 // computation. It walks up and down the IR once. On the way down,
203 // we count for each function the number of variables as well as
204 // liveness nodes. A liveness node is basically an expression or
205 // capture clause that does something of interest: either it has
206 // interesting control flow or it uses/defines a local variable.
208 // On the way back up, at each function node we create liveness sets
209 // (we now know precisely how big to make our various vectors and so
210 // forth) and then do the data-flow propagation to compute the set
211 // of live variables at each program point.
213 // Finally, we run back over the IR one last time and, using the
214 // computed liveness, check various safety conditions. For example,
215 // there must be no live nodes at the definition site for a variable
216 // unless it has an initializer. Similarly, each non-mutable local
217 // variable must not be assigned if there is some successor
218 // assignment. And so forth.
221 fn is_valid(&self) -> bool {
226 fn invalid_node() -> LiveNode {
235 #[derive(Copy, Clone, Debug)]
242 #[derive(Copy, Clone, Debug)]
244 Param(HirId, Symbol),
246 Upvar(HirId, Symbol),
249 struct IrMaps<'tcx> {
251 body_owner: LocalDefId,
252 num_live_nodes: usize,
254 live_node_map: HirIdMap<LiveNode>,
255 variable_map: HirIdMap<Variable>,
256 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
257 var_kinds: Vec<VarKind>,
258 lnks: Vec<LiveNodeKind>,
262 fn new(tcx: TyCtxt<'tcx>, body_owner: LocalDefId) -> IrMaps<'tcx> {
268 live_node_map: HirIdMap::default(),
269 variable_map: HirIdMap::default(),
270 capture_info_map: Default::default(),
271 var_kinds: Vec::new(),
276 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
277 let ln = LiveNode(self.num_live_nodes as u32);
279 self.num_live_nodes += 1;
281 debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx));
286 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
287 let ln = self.add_live_node(lnk);
288 self.live_node_map.insert(hir_id, ln);
290 debug!("{:?} is node {:?}", ln, hir_id);
293 fn add_variable(&mut self, vk: VarKind) -> Variable {
294 let v = Variable(self.num_vars as u32);
295 self.var_kinds.push(vk);
299 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) | Upvar(node_id, _) => {
300 self.variable_map.insert(node_id, v);
304 debug!("{:?} is {:?}", v, vk);
309 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
310 match self.variable_map.get(&hir_id) {
313 span_bug!(span, "no variable registered for id {:?}", hir_id);
318 fn variable_name(&self, var: Variable) -> String {
319 match self.var_kinds[var.get()] {
320 Local(LocalInfo { name, .. }) | Param(_, name) | Upvar(_, name) => name.to_string(),
324 fn variable_is_shorthand(&self, var: Variable) -> bool {
325 match self.var_kinds[var.get()] {
326 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
327 Param(..) | Upvar(..) => false,
331 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
332 self.capture_info_map.insert(hir_id, Rc::new(cs));
335 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
341 ir: &mut IrMaps<'tcx>,
343 decl: &'tcx hir::FnDecl<'tcx>,
344 body_id: hir::BodyId,
348 debug!("visit_fn {:?}", id);
350 // swap in a new set of IR maps for this function body:
351 let def_id = ir.tcx.hir().local_def_id(id);
352 let mut fn_maps = IrMaps::new(ir.tcx, def_id);
354 // Don't run unused pass for #[derive()]
355 if let FnKind::Method(..) = fk {
356 let parent = ir.tcx.hir().get_parent_item(id);
357 if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
358 if i.attrs.iter().any(|a| ir.tcx.sess.check_name(a, sym::automatically_derived)) {
364 debug!("creating fn_maps: {:p}", &fn_maps);
366 let body = ir.tcx.hir().body(body_id);
368 if let Some(upvars) = ir.tcx.upvars_mentioned(def_id) {
369 for (&var_hir_id, _upvar) in upvars {
370 debug!("adding upvar {:?}", var_hir_id);
371 let var_name = ir.tcx.hir().name(var_hir_id);
372 fn_maps.add_variable(Upvar(var_hir_id, var_name));
376 for param in body.params {
377 let is_shorthand = match param.pat.kind {
378 rustc_hir::PatKind::Struct(..) => true,
381 param.pat.each_binding(|_bm, hir_id, _x, ident| {
382 debug!("adding parameters {:?}", hir_id);
383 let var = if is_shorthand {
384 Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: true })
386 Param(hir_id, ident.name)
388 fn_maps.add_variable(var);
392 // gather up the various local variables, significant expressions,
394 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
397 let mut lsets = Liveness::new(&mut fn_maps, def_id);
398 let entry_ln = lsets.compute(fk, &body, sp, id);
399 lsets.log_liveness(entry_ln, id);
401 // check for various error conditions
402 lsets.visit_body(body);
403 lsets.warn_about_unused_upvars(entry_ln);
404 lsets.warn_about_unused_args(body, entry_ln);
407 fn add_from_pat(ir: &mut IrMaps<'_>, pat: &hir::Pat<'_>) {
408 // For struct patterns, take note of which fields used shorthand
409 // (`x` rather than `x: x`).
410 let mut shorthand_field_ids = HirIdSet::default();
411 let mut pats = VecDeque::new();
413 while let Some(pat) = pats.pop_front() {
414 use rustc_hir::PatKind::*;
416 Binding(.., inner_pat) => {
417 pats.extend(inner_pat.iter());
419 Struct(_, fields, _) => {
420 let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
421 shorthand_field_ids.extend(ids);
423 Ref(inner_pat, _) | Box(inner_pat) => {
424 pats.push_back(inner_pat);
426 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
427 pats.extend(inner_pats.iter());
429 Slice(pre_pats, inner_pat, post_pats) => {
430 pats.extend(pre_pats.iter());
431 pats.extend(inner_pat.iter());
432 pats.extend(post_pats.iter());
438 pat.each_binding(|_, hir_id, _, ident| {
439 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
440 ir.add_variable(Local(LocalInfo {
443 is_shorthand: shorthand_field_ids.contains(&hir_id),
448 fn visit_local<'tcx>(ir: &mut IrMaps<'tcx>, local: &'tcx hir::Local<'tcx>) {
449 add_from_pat(ir, &local.pat);
450 intravisit::walk_local(ir, local);
453 fn visit_arm<'tcx>(ir: &mut IrMaps<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
454 add_from_pat(ir, &arm.pat);
455 intravisit::walk_arm(ir, arm);
458 fn visit_expr<'tcx>(ir: &mut IrMaps<'tcx>, expr: &'tcx Expr<'tcx>) {
460 // live nodes required for uses or definitions of variables:
461 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
462 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
463 if let Res::Local(_var_hir_id) = path.res {
464 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
466 intravisit::walk_expr(ir, expr);
468 hir::ExprKind::Closure(..) => {
469 // Interesting control flow (for loops can contain labeled
470 // breaks or continues)
471 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
473 // Make a live_node for each captured variable, with the span
474 // being the location that the variable is used. This results
475 // in better error messages than just pointing at the closure
476 // construction site.
477 let mut call_caps = Vec::new();
478 let closure_def_id = ir.tcx.hir().local_def_id(expr.hir_id);
479 if let Some(upvars) = ir.tcx.upvars_mentioned(closure_def_id) {
480 call_caps.extend(upvars.iter().map(|(&var_id, upvar)| {
481 let upvar_ln = ir.add_live_node(UpvarNode(upvar.span));
482 CaptureInfo { ln: upvar_ln, var_hid: var_id }
485 ir.set_captures(expr.hir_id, call_caps);
486 let old_body_owner = ir.body_owner;
487 ir.body_owner = closure_def_id;
488 intravisit::walk_expr(ir, expr);
489 ir.body_owner = old_body_owner;
492 // live nodes required for interesting control flow:
493 hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
494 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
495 intravisit::walk_expr(ir, expr);
497 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
498 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
499 intravisit::walk_expr(ir, expr);
502 // otherwise, live nodes are not required:
503 hir::ExprKind::Index(..)
504 | hir::ExprKind::Field(..)
505 | hir::ExprKind::Array(..)
506 | hir::ExprKind::Call(..)
507 | hir::ExprKind::MethodCall(..)
508 | hir::ExprKind::Tup(..)
509 | hir::ExprKind::Binary(..)
510 | hir::ExprKind::AddrOf(..)
511 | hir::ExprKind::Cast(..)
512 | hir::ExprKind::DropTemps(..)
513 | hir::ExprKind::Unary(..)
514 | hir::ExprKind::Break(..)
515 | hir::ExprKind::Continue(_)
516 | hir::ExprKind::Lit(_)
517 | hir::ExprKind::Ret(..)
518 | hir::ExprKind::Block(..)
519 | hir::ExprKind::Assign(..)
520 | hir::ExprKind::AssignOp(..)
521 | hir::ExprKind::Struct(..)
522 | hir::ExprKind::Repeat(..)
523 | hir::ExprKind::InlineAsm(..)
524 | hir::ExprKind::LlvmInlineAsm(..)
525 | hir::ExprKind::Box(..)
526 | hir::ExprKind::Yield(..)
527 | hir::ExprKind::Type(..)
529 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
530 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
531 intravisit::walk_expr(ir, expr);
536 // ______________________________________________________________________
537 // Computing liveness sets
539 // Actually we compute just a bit more than just liveness, but we use
540 // the same basic propagation framework in all cases.
542 #[derive(Clone, Copy)]
549 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
550 /// very large, so it uses a more compact representation that takes advantage
551 /// of the fact that when the number of `RWU`s is large, most of them have an
552 /// invalid reader and an invalid writer.
554 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
555 /// an index into `unpacked_rwus`. In the common cases, this compacts the
556 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
559 /// More compact representations are possible -- e.g., use only 2 bits per
560 /// packed `RWU` and make the secondary table a HashMap that maps from
561 /// indices to `RWU`s -- but this one strikes a good balance between size
563 packed_rwus: Vec<u32>,
564 unpacked_rwus: Vec<RWU>,
567 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
568 const INV_INV_FALSE: u32 = u32::MAX;
570 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
571 const INV_INV_TRUE: u32 = u32::MAX - 1;
574 fn new(num_rwus: usize) -> RWUTable {
575 Self { packed_rwus: vec![INV_INV_FALSE; num_rwus], unpacked_rwus: vec![] }
578 fn get(&self, idx: usize) -> RWU {
579 let packed_rwu = self.packed_rwus[idx];
581 INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false },
582 INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true },
583 _ => self.unpacked_rwus[packed_rwu as usize],
587 fn get_reader(&self, idx: usize) -> LiveNode {
588 let packed_rwu = self.packed_rwus[idx];
590 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
591 _ => self.unpacked_rwus[packed_rwu as usize].reader,
595 fn get_writer(&self, idx: usize) -> LiveNode {
596 let packed_rwu = self.packed_rwus[idx];
598 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
599 _ => self.unpacked_rwus[packed_rwu as usize].writer,
603 fn get_used(&self, idx: usize) -> bool {
604 let packed_rwu = self.packed_rwus[idx];
606 INV_INV_FALSE => false,
607 INV_INV_TRUE => true,
608 _ => self.unpacked_rwus[packed_rwu as usize].used,
613 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
614 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
617 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
618 if rwu.reader == invalid_node() && rwu.writer == invalid_node() {
619 // When we overwrite an indexing entry in `self.packed_rwus` with
620 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
621 // from `self.unpacked_rwus`; it's not worth the effort, and we
622 // can't have entries shifting around anyway.
623 self.packed_rwus[idx] = if rwu.used { INV_INV_TRUE } else { INV_INV_FALSE }
625 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
627 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
628 self.unpacked_rwus.push(rwu);
632 fn assign_inv_inv(&mut self, idx: usize) {
633 self.packed_rwus[idx] = if self.get_used(idx) { INV_INV_TRUE } else { INV_INV_FALSE };
637 #[derive(Copy, Clone)]
639 /// A live node representing a point of execution before closure entry &
640 /// after closure exit. Used to calculate liveness of captured variables
641 /// through calls to the same closure. Used for Fn & FnMut closures only.
642 closure_ln: LiveNode,
643 /// A live node representing every 'exit' from the function, whether it be
644 /// by explicit return, panic, or other means.
648 const ACC_READ: u32 = 1;
649 const ACC_WRITE: u32 = 2;
650 const ACC_USE: u32 = 4;
652 struct Liveness<'a, 'tcx> {
653 ir: &'a mut IrMaps<'tcx>,
654 typeck_results: &'a ty::TypeckResults<'tcx>,
655 param_env: ty::ParamEnv<'tcx>,
657 successors: Vec<LiveNode>,
660 // mappings from loop node ID to LiveNode
661 // ("break" label should map to loop node ID,
662 // it probably doesn't now)
663 break_ln: HirIdMap<LiveNode>,
664 cont_ln: HirIdMap<LiveNode>,
667 impl<'a, 'tcx> Liveness<'a, 'tcx> {
668 fn new(ir: &'a mut IrMaps<'tcx>, def_id: LocalDefId) -> Liveness<'a, 'tcx> {
669 let specials = Specials {
670 closure_ln: ir.add_live_node(ClosureNode),
671 exit_ln: ir.add_live_node(ExitNode),
674 let typeck_results = ir.tcx.typeck(def_id);
675 let param_env = ir.tcx.param_env(def_id);
677 let num_live_nodes = ir.num_live_nodes;
678 let num_vars = ir.num_vars;
685 successors: vec![invalid_node(); num_live_nodes],
686 rwu_table: RWUTable::new(num_live_nodes * num_vars),
687 break_ln: Default::default(),
688 cont_ln: Default::default(),
692 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
693 match self.ir.live_node_map.get(&hir_id) {
696 // This must be a mismatch between the ir_map construction
697 // above and the propagation code below; the two sets of
698 // code have to agree about which AST nodes are worth
699 // creating liveness nodes for.
700 span_bug!(span, "no live node registered for node {:?}", hir_id);
705 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
706 self.ir.variable(hir_id, span)
709 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
710 // In an or-pattern, only consider the first pattern; any later patterns
711 // must have the same bindings, and we also consider the first pattern
712 // to be the "authoritative" set of ids.
713 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
714 let ln = self.live_node(hir_id, pat_sp);
715 let var = self.variable(hir_id, ident.span);
716 self.init_from_succ(ln, succ);
717 self.define(ln, var);
723 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
724 ln.get() * self.ir.num_vars + var.get()
727 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
728 assert!(ln.is_valid());
729 let reader = self.rwu_table.get_reader(self.idx(ln, var));
730 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
733 // Is this variable live on entry to any of its successor nodes?
734 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
735 let successor = self.successors[ln.get()];
736 self.live_on_entry(successor, var)
739 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
740 assert!(ln.is_valid());
741 self.rwu_table.get_used(self.idx(ln, var))
744 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
745 assert!(ln.is_valid());
746 let writer = self.rwu_table.get_writer(self.idx(ln, var));
747 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
750 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
751 let successor = self.successors[ln.get()];
752 self.assigned_on_entry(successor, var)
755 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F)
757 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
759 let node_base_idx = self.idx(ln, Variable(0));
760 let succ_base_idx = self.idx(succ_ln, Variable(0));
761 for var_idx in 0..self.ir.num_vars {
762 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
766 fn write_vars<F>(&self, wr: &mut dyn Write, ln: LiveNode, mut test: F) -> io::Result<()>
768 F: FnMut(usize) -> bool,
770 let node_base_idx = self.idx(ln, Variable(0));
771 for var_idx in 0..self.ir.num_vars {
772 let idx = node_base_idx + var_idx;
774 write!(wr, " {:?}", Variable(var_idx as u32))?;
780 #[allow(unused_must_use)]
781 fn ln_str(&self, ln: LiveNode) -> String {
782 let mut wr = Vec::new();
784 let wr = &mut wr as &mut dyn Write;
785 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
786 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx).is_valid());
787 write!(wr, " writes");
788 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx).is_valid());
790 self.write_vars(wr, ln, |idx| self.rwu_table.get_used(idx));
792 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
794 String::from_utf8(wr).unwrap()
797 fn log_liveness(&self, entry_ln: LiveNode, hir_id: hir::HirId) {
798 // hack to skip the loop unless debug! is enabled:
800 "^^ liveness computation results for body {} (entry={:?})",
802 for ln_idx in 0..self.ir.num_live_nodes {
803 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
811 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
812 self.successors[ln.get()] = succ_ln;
814 // It is not necessary to initialize the RWUs here because they are all
815 // set to INV_INV_FALSE when they are created, and the sets only grow
816 // during iterations.
819 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
820 // more efficient version of init_empty() / merge_from_succ()
821 self.successors[ln.get()] = succ_ln;
823 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
824 this.rwu_table.copy_packed(idx, succ_idx);
826 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
829 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode, first_merge: bool) -> bool {
834 let mut any_changed = false;
835 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
836 // This is a special case, pulled out from the code below, where we
837 // don't have to do anything. It occurs about 60-70% of the time.
838 if this.rwu_table.packed_rwus[succ_idx] == INV_INV_FALSE {
842 let mut changed = false;
843 let mut rwu = this.rwu_table.get(idx);
844 let succ_rwu = this.rwu_table.get(succ_idx);
845 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
846 rwu.reader = succ_rwu.reader;
850 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
851 rwu.writer = succ_rwu.writer;
855 if succ_rwu.used && !rwu.used {
861 this.rwu_table.assign_unpacked(idx, rwu);
867 "merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
869 self.ln_str(succ_ln),
876 // Indicates that a local variable was *defined*; we know that no
877 // uses of the variable can precede the definition (resolve checks
878 // this) so we just clear out all the data.
879 fn define(&mut self, writer: LiveNode, var: Variable) {
880 let idx = self.idx(writer, var);
881 self.rwu_table.assign_inv_inv(idx);
883 debug!("{:?} defines {:?} (idx={}): {}", writer, var, idx, self.ln_str(writer));
886 // Either read, write, or both depending on the acc bitset
887 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
888 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
890 let idx = self.idx(ln, var);
891 let mut rwu = self.rwu_table.get(idx);
893 if (acc & ACC_WRITE) != 0 {
894 rwu.reader = invalid_node();
898 // Important: if we both read/write, must do read second
899 // or else the write will override.
900 if (acc & ACC_READ) != 0 {
904 if (acc & ACC_USE) != 0 {
908 self.rwu_table.assign_unpacked(idx, rwu);
914 body: &hir::Body<'_>,
918 debug!("compute: using id for body, {:?}", body.value);
920 // # Liveness of captured variables
922 // When computing the liveness for captured variables we take into
923 // account how variable is captured (ByRef vs ByValue) and what is the
924 // closure kind (Generator / FnOnce vs Fn / FnMut).
926 // Variables captured by reference are assumed to be used on the exit
929 // In FnOnce closures, variables captured by value are known to be dead
930 // on exit since it is impossible to call the closure again.
932 // In Fn / FnMut closures, variables captured by value are live on exit
933 // if they are live on the entry to the closure, since only the closure
934 // itself can access them on subsequent calls.
936 if let Some(upvars) = self.ir.tcx.upvars_mentioned(self.ir.body_owner) {
937 // Mark upvars captured by reference as used after closure exits.
938 for (&var_hir_id, upvar) in upvars.iter().rev() {
939 let upvar_id = ty::UpvarId {
940 var_path: ty::UpvarPath { hir_id: var_hir_id },
941 closure_expr_id: self.ir.body_owner,
943 match self.typeck_results.upvar_capture(upvar_id) {
944 ty::UpvarCapture::ByRef(_) => {
945 let var = self.variable(var_hir_id, upvar.span);
946 self.acc(self.s.exit_ln, var, ACC_READ | ACC_USE);
948 ty::UpvarCapture::ByValue(_) => {}
953 let succ = self.propagate_through_expr(&body.value, self.s.exit_ln);
956 FnKind::Method(..) | FnKind::ItemFn(..) => return succ,
957 FnKind::Closure(..) => {}
960 let ty = self.typeck_results.node_type(id);
962 ty::Closure(_def_id, substs) => match substs.as_closure().kind() {
963 ty::ClosureKind::Fn => {}
964 ty::ClosureKind::FnMut => {}
965 ty::ClosureKind::FnOnce => return succ,
967 ty::Generator(..) => return succ,
969 span_bug!(span, "type of closure expr {:?} is not a closure {:?}", id, ty,);
973 // Propagate through calls to the closure.
974 let mut first_merge = true;
976 self.init_from_succ(self.s.closure_ln, succ);
977 for param in body.params {
978 param.pat.each_binding(|_bm, hir_id, _x, ident| {
979 let var = self.variable(hir_id, ident.span);
980 self.define(self.s.closure_ln, var);
984 if !self.merge_from_succ(self.s.exit_ln, self.s.closure_ln, first_merge) {
988 assert_eq!(succ, self.propagate_through_expr(&body.value, self.s.exit_ln));
994 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
995 if blk.targeted_by_break {
996 self.break_ln.insert(blk.hir_id, succ);
998 let succ = self.propagate_through_opt_expr(blk.expr.as_deref(), succ);
999 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
1002 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
1004 hir::StmtKind::Local(ref local) => {
1005 // Note: we mark the variable as defined regardless of whether
1006 // there is an initializer. Initially I had thought to only mark
1007 // the live variable as defined if it was initialized, and then we
1008 // could check for uninit variables just by scanning what is live
1009 // at the start of the function. But that doesn't work so well for
1010 // immutable variables defined in a loop:
1011 // loop { let x; x = 5; }
1012 // because the "assignment" loops back around and generates an error.
1014 // So now we just check that variables defined w/o an
1015 // initializer are not live at the point of their
1016 // initialization, which is mildly more complex than checking
1017 // once at the func header but otherwise equivalent.
1019 let succ = self.propagate_through_opt_expr(local.init.as_deref(), succ);
1020 self.define_bindings_in_pat(&local.pat, succ)
1022 hir::StmtKind::Item(..) => succ,
1023 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
1024 self.propagate_through_expr(&expr, succ)
1029 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
1030 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
1033 fn propagate_through_opt_expr(
1035 opt_expr: Option<&Expr<'_>>,
1038 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
1041 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1042 debug!("propagate_through_expr: {:?}", expr);
1045 // Interesting cases with control flow or which gen/kill
1046 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1047 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
1050 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1052 hir::ExprKind::Closure(..) => {
1053 debug!("{:?} is an ExprKind::Closure", expr);
1055 // the construction of a closure itself is not important,
1056 // but we have to consider the closed over variables.
1062 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
1064 caps.iter().rev().fold(succ, |succ, cap| {
1065 self.init_from_succ(cap.ln, succ);
1066 let var = self.variable(cap.var_hid, expr.span);
1067 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1072 // Note that labels have been resolved, so we don't need to look
1073 // at the label ident
1074 hir::ExprKind::Loop(ref blk, _, _) => self.propagate_through_loop(expr, &blk, succ),
1076 hir::ExprKind::Match(ref e, arms, _) => {
1091 let ln = self.live_node(expr.hir_id, expr.span);
1092 self.init_empty(ln, succ);
1093 let mut first_merge = true;
1095 let body_succ = self.propagate_through_expr(&arm.body, succ);
1097 let guard_succ = self.propagate_through_opt_expr(
1098 arm.guard.as_ref().map(|hir::Guard::If(e)| *e),
1101 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
1102 self.merge_from_succ(ln, arm_succ, first_merge);
1103 first_merge = false;
1105 self.propagate_through_expr(&e, ln)
1108 hir::ExprKind::Ret(ref o_e) => {
1109 // ignore succ and subst exit_ln:
1110 let exit_ln = self.s.exit_ln;
1111 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1114 hir::ExprKind::Break(label, ref opt_expr) => {
1115 // Find which label this break jumps to
1116 let target = match label.target_id {
1117 Ok(hir_id) => self.break_ln.get(&hir_id),
1118 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1122 // Now that we know the label we're going to,
1123 // look it up in the break loop nodes table
1126 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1127 None => span_bug!(expr.span, "`break` to unknown label"),
1131 hir::ExprKind::Continue(label) => {
1132 // Find which label this expr continues to
1135 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
1137 // Now that we know the label we're going to,
1138 // look it up in the continue loop nodes table
1142 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
1145 hir::ExprKind::Assign(ref l, ref r, _) => {
1146 // see comment on places in
1147 // propagate_through_place_components()
1148 let succ = self.write_place(&l, succ, ACC_WRITE);
1149 let succ = self.propagate_through_place_components(&l, succ);
1150 self.propagate_through_expr(&r, succ)
1153 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1154 // an overloaded assign op is like a method call
1155 if self.typeck_results.is_method_call(expr) {
1156 let succ = self.propagate_through_expr(&l, succ);
1157 self.propagate_through_expr(&r, succ)
1159 // see comment on places in
1160 // propagate_through_place_components()
1161 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
1162 let succ = self.propagate_through_expr(&r, succ);
1163 self.propagate_through_place_components(&l, succ)
1167 // Uninteresting cases: just propagate in rev exec order
1168 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
1170 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1171 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1175 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
1178 hir::ExprKind::Call(ref f, ref args) => {
1179 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1180 let succ = if self.ir.tcx.is_ty_uninhabited_from(
1182 self.typeck_results.expr_ty(expr),
1189 let succ = self.propagate_through_exprs(args, succ);
1190 self.propagate_through_expr(&f, succ)
1193 hir::ExprKind::MethodCall(.., ref args, _) => {
1194 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1195 let succ = if self.ir.tcx.is_ty_uninhabited_from(
1197 self.typeck_results.expr_ty(expr),
1205 self.propagate_through_exprs(args, succ)
1208 hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ),
1210 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1211 let r_succ = self.propagate_through_expr(&r, succ);
1213 let ln = self.live_node(expr.hir_id, expr.span);
1214 self.init_from_succ(ln, succ);
1215 self.merge_from_succ(ln, r_succ, false);
1217 self.propagate_through_expr(&l, ln)
1220 hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => {
1221 let r_succ = self.propagate_through_expr(&r, succ);
1222 self.propagate_through_expr(&l, r_succ)
1225 hir::ExprKind::Box(ref e)
1226 | hir::ExprKind::AddrOf(_, _, ref e)
1227 | hir::ExprKind::Cast(ref e, _)
1228 | hir::ExprKind::Type(ref e, _)
1229 | hir::ExprKind::DropTemps(ref e)
1230 | hir::ExprKind::Unary(_, ref e)
1231 | hir::ExprKind::Yield(ref e, _)
1232 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1234 hir::ExprKind::InlineAsm(ref asm) => {
1235 // Handle non-returning asm
1236 let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) {
1242 // Do a first pass for writing outputs only
1243 for op in asm.operands.iter().rev() {
1245 hir::InlineAsmOperand::In { .. }
1246 | hir::InlineAsmOperand::Const { .. }
1247 | hir::InlineAsmOperand::Sym { .. } => {}
1248 hir::InlineAsmOperand::Out { expr, .. } => {
1249 if let Some(expr) = expr {
1250 succ = self.write_place(expr, succ, ACC_WRITE);
1253 hir::InlineAsmOperand::InOut { expr, .. } => {
1254 succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE);
1256 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1257 if let Some(expr) = out_expr {
1258 succ = self.write_place(expr, succ, ACC_WRITE);
1264 // Then do a second pass for inputs
1265 let mut succ = succ;
1266 for op in asm.operands.iter().rev() {
1268 hir::InlineAsmOperand::In { expr, .. }
1269 | hir::InlineAsmOperand::Const { expr, .. }
1270 | hir::InlineAsmOperand::Sym { expr, .. } => {
1271 succ = self.propagate_through_expr(expr, succ)
1273 hir::InlineAsmOperand::Out { expr, .. } => {
1274 if let Some(expr) = expr {
1275 succ = self.propagate_through_place_components(expr, succ);
1278 hir::InlineAsmOperand::InOut { expr, .. } => {
1279 succ = self.propagate_through_place_components(expr, succ);
1281 hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
1282 if let Some(expr) = out_expr {
1283 succ = self.propagate_through_place_components(expr, succ);
1285 succ = self.propagate_through_expr(in_expr, succ);
1292 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1293 let ia = &asm.inner;
1294 let outputs = asm.outputs_exprs;
1295 let inputs = asm.inputs_exprs;
1296 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1297 // see comment on places
1298 // in propagate_through_place_components()
1300 self.propagate_through_expr(output, succ)
1302 let acc = if o.is_rw { ACC_WRITE | ACC_READ } else { ACC_WRITE };
1303 let succ = self.write_place(output, succ, acc);
1304 self.propagate_through_place_components(output, succ)
1308 // Inputs are executed first. Propagate last because of rev order
1309 self.propagate_through_exprs(inputs, succ)
1312 hir::ExprKind::Lit(..)
1313 | hir::ExprKind::Err
1314 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
1315 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => succ,
1317 // Note that labels have been resolved, so we don't need to look
1318 // at the label ident
1319 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1323 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1326 // In general, the full flow graph structure for an
1327 // assignment/move/etc can be handled in one of two ways,
1328 // depending on whether what is being assigned is a "tracked
1329 // value" or not. A tracked value is basically a local
1330 // variable or argument.
1332 // The two kinds of graphs are:
1334 // Tracked place Untracked place
1335 // ----------------------++-----------------------
1339 // (rvalue) || (rvalue)
1342 // (write of place) || (place components)
1347 // ----------------------++-----------------------
1349 // I will cover the two cases in turn:
1353 // A tracked place is a local variable/argument `x`. In
1354 // these cases, the link_node where the write occurs is linked
1355 // to node id of `x`. The `write_place()` routine generates
1356 // the contents of this node. There are no subcomponents to
1359 // # Non-tracked places
1361 // These are places like `x[5]` or `x.f`. In that case, we
1362 // basically ignore the value which is written to but generate
1363 // reads for the components---`x` in these two examples. The
1364 // components reads are generated by
1365 // `propagate_through_place_components()` (this fn).
1369 // It is still possible to observe assignments to non-places;
1370 // these errors are detected in the later pass borrowck. We
1371 // just ignore such cases and treat them as reads.
1374 hir::ExprKind::Path(_) => succ,
1375 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1376 _ => self.propagate_through_expr(expr, succ),
1380 // see comment on propagate_through_place()
1381 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1383 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1384 self.access_path(expr.hir_id, path, succ, acc)
1387 // We do not track other places, so just propagate through
1388 // to their subcomponents. Also, it may happen that
1389 // non-places occur here, because those are detected in the
1390 // later pass borrowck.
1403 let ln = self.live_node(hir_id, span);
1405 self.init_from_succ(ln, succ);
1406 let var = self.variable(var_hid, span);
1407 self.acc(ln, var, acc);
1415 path: &hir::Path<'_>,
1420 Res::Local(hid) => self.access_var(hir_id, hid, succ, acc, path.span),
1425 fn propagate_through_loop(
1428 body: &hir::Block<'_>,
1432 We model control flow like this:
1439 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1440 Meanwhile, a `break` expression will have a successor of `succ`.
1444 let mut first_merge = true;
1445 let ln = self.live_node(expr.hir_id, expr.span);
1446 self.init_empty(ln, succ);
1447 debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body);
1449 self.break_ln.insert(expr.hir_id, succ);
1451 self.cont_ln.insert(expr.hir_id, ln);
1453 let body_ln = self.propagate_through_block(body, ln);
1455 // repeat until fixed point is reached:
1456 while self.merge_from_succ(ln, body_ln, first_merge) {
1457 first_merge = false;
1458 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1465 // _______________________________________________________________________
1466 // Checking for error conditions
1468 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1469 type Map = intravisit::ErasedMap<'tcx>;
1471 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
1472 NestedVisitorMap::None
1475 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1476 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1477 if local.init.is_some() {
1478 self.warn_about_dead_assign(spans, hir_id, ln, var);
1482 intravisit::walk_local(self, local);
1485 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1486 check_expr(self, ex);
1489 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1490 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1491 intravisit::walk_arm(self, arm);
1495 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1497 hir::ExprKind::Assign(ref l, ..) => {
1498 this.check_place(&l);
1501 hir::ExprKind::AssignOp(_, ref l, _) => {
1502 if !this.typeck_results.is_method_call(expr) {
1503 this.check_place(&l);
1507 hir::ExprKind::InlineAsm(ref asm) => {
1508 for op in asm.operands {
1510 hir::InlineAsmOperand::Out { expr, .. } => {
1511 if let Some(expr) = expr {
1512 this.check_place(expr);
1515 hir::InlineAsmOperand::InOut { expr, .. } => {
1516 this.check_place(expr);
1518 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1519 if let Some(out_expr) = out_expr {
1520 this.check_place(out_expr);
1528 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1529 for input in asm.inputs_exprs {
1530 this.visit_expr(input);
1533 // Output operands must be places
1534 for (o, output) in asm.inner.outputs.iter().zip(asm.outputs_exprs) {
1536 this.check_place(output);
1538 this.visit_expr(output);
1542 // no correctness conditions related to liveness
1543 hir::ExprKind::Call(..)
1544 | hir::ExprKind::MethodCall(..)
1545 | hir::ExprKind::Match(..)
1546 | hir::ExprKind::Loop(..)
1547 | hir::ExprKind::Index(..)
1548 | hir::ExprKind::Field(..)
1549 | hir::ExprKind::Array(..)
1550 | hir::ExprKind::Tup(..)
1551 | hir::ExprKind::Binary(..)
1552 | hir::ExprKind::Cast(..)
1553 | hir::ExprKind::DropTemps(..)
1554 | hir::ExprKind::Unary(..)
1555 | hir::ExprKind::Ret(..)
1556 | hir::ExprKind::Break(..)
1557 | hir::ExprKind::Continue(..)
1558 | hir::ExprKind::Lit(_)
1559 | hir::ExprKind::Block(..)
1560 | hir::ExprKind::AddrOf(..)
1561 | hir::ExprKind::Struct(..)
1562 | hir::ExprKind::Repeat(..)
1563 | hir::ExprKind::Closure(..)
1564 | hir::ExprKind::Path(_)
1565 | hir::ExprKind::Yield(..)
1566 | hir::ExprKind::Box(..)
1567 | hir::ExprKind::Type(..)
1568 | hir::ExprKind::Err => {}
1571 intravisit::walk_expr(this, expr);
1574 impl<'tcx> Liveness<'_, 'tcx> {
1575 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1577 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1578 if let Res::Local(var_hid) = path.res {
1579 // Assignment to an immutable variable or argument: only legal
1580 // if there is no later assignment. If this local is actually
1581 // mutable, then check for a reassignment to flag the mutability
1583 let ln = self.live_node(expr.hir_id, expr.span);
1584 let var = self.variable(var_hid, expr.span);
1585 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1589 // For other kinds of places, no checks are required,
1590 // and any embedded expressions are actually rvalues
1591 intravisit::walk_expr(self, expr);
1596 fn should_warn(&self, var: Variable) -> Option<String> {
1597 let name = self.ir.variable_name(var);
1598 if name.is_empty() || name.as_bytes()[0] == b'_' { None } else { Some(name) }
1601 fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
1602 let upvars = match self.ir.tcx.upvars_mentioned(self.ir.body_owner) {
1604 Some(upvars) => upvars,
1606 for (&var_hir_id, upvar) in upvars.iter() {
1607 let var = self.variable(var_hir_id, upvar.span);
1608 let upvar_id = ty::UpvarId {
1609 var_path: ty::UpvarPath { hir_id: var_hir_id },
1610 closure_expr_id: self.ir.body_owner,
1612 match self.typeck_results.upvar_capture(upvar_id) {
1613 ty::UpvarCapture::ByValue(_) => {}
1614 ty::UpvarCapture::ByRef(..) => continue,
1616 if self.used_on_entry(entry_ln, var) {
1617 if self.live_on_entry(entry_ln, var).is_none() {
1618 if let Some(name) = self.should_warn(var) {
1619 self.ir.tcx.struct_span_lint_hir(
1620 lint::builtin::UNUSED_ASSIGNMENTS,
1624 lint.build(&format!("value captured by `{}` is never read", name))
1625 .help("did you mean to capture by reference instead?")
1632 if let Some(name) = self.should_warn(var) {
1633 self.ir.tcx.struct_span_lint_hir(
1634 lint::builtin::UNUSED_VARIABLES,
1638 lint.build(&format!("unused variable: `{}`", name))
1639 .help("did you mean to capture by reference instead?")
1648 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1649 for p in body.params {
1650 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1651 if self.live_on_entry(ln, var).is_none() {
1652 self.report_unsed_assign(hir_id, spans, var, |name| {
1653 format!("value passed to `{}` is never read", name)
1660 fn check_unused_vars_in_pat(
1663 entry_ln: Option<LiveNode>,
1664 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1666 // In an or-pattern, only consider the variable; any later patterns must have the same
1667 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1668 // However, we should take the ids and spans of variables with the same name from the later
1669 // patterns so the suggestions to prefix with underscores will apply to those too.
1670 let mut vars: FxIndexMap<String, (LiveNode, Variable, Vec<(HirId, Span)>)> = <_>::default();
1672 pat.each_binding(|_, hir_id, pat_sp, ident| {
1673 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1674 let var = self.variable(hir_id, ident.span);
1675 let id_and_sp = (hir_id, pat_sp);
1676 vars.entry(self.ir.variable_name(var))
1677 .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp))
1678 .or_insert_with(|| (ln, var, vec![id_and_sp]));
1681 for (_, (ln, var, hir_ids_and_spans)) in vars {
1682 if self.used_on_entry(ln, var) {
1683 let id = hir_ids_and_spans[0].0;
1684 let spans = hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect();
1685 on_used_on_entry(spans, id, ln, var);
1687 self.report_unused(hir_ids_and_spans, ln, var);
1692 fn report_unused(&self, hir_ids_and_spans: Vec<(HirId, Span)>, ln: LiveNode, var: Variable) {
1693 let first_hir_id = hir_ids_and_spans[0].0;
1695 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1696 // annoying: for parameters in funcs like `fn(x: i32)
1697 // {ret}`, there is only one node, so asking about
1698 // assigned_on_exit() is not meaningful.
1700 if ln == self.s.exit_ln { false } else { self.assigned_on_exit(ln, var).is_some() };
1703 self.ir.tcx.struct_span_lint_hir(
1704 lint::builtin::UNUSED_VARIABLES,
1706 hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect::<Vec<_>>(),
1708 lint.build(&format!("variable `{}` is assigned to, but never used", name))
1709 .note(&format!("consider using `_{}` instead", name))
1714 self.ir.tcx.struct_span_lint_hir(
1715 lint::builtin::UNUSED_VARIABLES,
1717 hir_ids_and_spans.iter().map(|(_, sp)| *sp).collect::<Vec<_>>(),
1719 let mut err = lint.build(&format!("unused variable: `{}`", name));
1721 let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
1722 hir_ids_and_spans.into_iter().partition(|(hir_id, span)| {
1723 let var = self.variable(*hir_id, *span);
1724 self.ir.variable_is_shorthand(var)
1727 let mut shorthands = shorthands
1729 .map(|(_, span)| (span, format!("{}: _", name)))
1730 .collect::<Vec<_>>();
1732 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1733 // the field" message, and suggest `_` for the non-shorthands. If we only
1734 // have non-shorthand, then prefix with an underscore instead.
1735 if !shorthands.is_empty() {
1739 .map(|(_, span)| (span, "_".to_string()))
1740 .collect::<Vec<_>>(),
1743 err.multipart_suggestion(
1744 "try ignoring the field",
1746 Applicability::MachineApplicable,
1749 err.multipart_suggestion(
1750 "if this is intentional, prefix it with an underscore",
1753 .map(|(_, span)| (span, format!("_{}", name)))
1754 .collect::<Vec<_>>(),
1755 Applicability::MachineApplicable,
1766 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1767 if self.live_on_exit(ln, var).is_none() {
1768 self.report_unsed_assign(hir_id, spans, var, |name| {
1769 format!("value assigned to `{}` is never read", name)
1774 fn report_unsed_assign(
1779 message: impl Fn(&str) -> String,
1781 if let Some(name) = self.should_warn(var) {
1782 self.ir.tcx.struct_span_lint_hir(
1783 lint::builtin::UNUSED_ASSIGNMENTS,
1787 lint.build(&message(&name))
1788 .help("maybe it is overwritten before being read?")