1 //! A classic liveness analysis based on dataflow over the AST. Computes,
2 //! for each local variable in a function, whether that variable is live
3 //! at a given point. Program execution points are identified by their
8 //! The basic model is that each local variable is assigned an index. We
9 //! represent sets of local variables using a vector indexed by this
10 //! index. The value in the vector is either 0, indicating the variable
11 //! is dead, or the ID of an expression that uses the variable.
13 //! We conceptually walk over the AST in reverse execution order. If we
14 //! find a use of a variable, we add it to the set of live variables. If
15 //! we find an assignment to a variable, we remove it from the set of live
16 //! variables. When we have to merge two flows, we take the union of
17 //! those two flows -- if the variable is live on both paths, we simply
18 //! pick one ID. In the event of loops, we continue doing this until a
19 //! fixed point is reached.
21 //! ## Checking initialization
23 //! At the function entry point, all variables must be dead. If this is
24 //! not the case, we can report an error using the ID found in the set of
25 //! live variables, which identifies a use of the variable which is not
26 //! dominated by an assignment.
30 //! After each explicit move, the variable must be dead.
32 //! ## Computing last uses
34 //! Any use of the variable where the variable is dead afterwards is a
37 //! # Implementation details
39 //! The actual implementation contains two (nested) walks over the AST.
40 //! The outer walk has the job of building up the ir_maps instance for the
41 //! enclosing function. On the way down the tree, it identifies those AST
42 //! nodes and variable IDs that will be needed for the liveness analysis
43 //! and assigns them contiguous IDs. The liveness ID for an AST node is
44 //! called a `live_node` (it's a newtype'd `u32`) and the ID for a variable
45 //! is called a `variable` (another newtype'd `u32`).
47 //! On the way back up the tree, as we are about to exit from a function
48 //! declaration we allocate a `liveness` instance. Now that we know
49 //! precisely how many nodes and variables we need, we can allocate all
50 //! the various arrays that we will need to precisely the right size. We then
51 //! perform the actual propagation on the `liveness` instance.
53 //! This propagation is encoded in the various `propagate_through_*()`
54 //! methods. It effectively does a reverse walk of the AST; whenever we
55 //! reach a loop node, we iterate until a fixed point is reached.
57 //! ## The `RWU` struct
59 //! At each live node `N`, we track three pieces of information for each
60 //! variable `V` (these are encapsulated in the `RWU` struct):
62 //! - `reader`: the `LiveNode` ID of some node which will read the value
63 //! that `V` holds on entry to `N`. Formally: a node `M` such
64 //! that there exists a path `P` from `N` to `M` where `P` does not
65 //! write `V`. If the `reader` is `invalid_node()`, then the current
66 //! value will never be read (the variable is dead, essentially).
68 //! - `writer`: the `LiveNode` ID of some node which will write the
69 //! variable `V` and which is reachable from `N`. Formally: a node `M`
70 //! such that there exists a path `P` from `N` to `M` and `M` writes
71 //! `V`. If the `writer` is `invalid_node()`, then there is no writer
72 //! of `V` that follows `N`.
74 //! - `used`: a boolean value indicating whether `V` is *used*. We
75 //! distinguish a *read* from a *use* in that a *use* is some read that
76 //! is not just used to generate a new value. For example, `x += 1` is
77 //! a read but not a use. This is used to generate better warnings.
79 //! ## Special Variables
81 //! We generate various special variables for various, well, special purposes.
82 //! These are described in the `specials` struct:
84 //! - `exit_ln`: a live node that is generated to represent every 'exit' from
85 //! the function, whether it be by explicit return, panic, or other means.
87 //! - `fallthrough_ln`: a live node that represents a fallthrough
89 //! - `clean_exit_var`: a synthetic variable that is only 'read' from the
90 //! fallthrough node. It is only live if the function could converge
91 //! via means other than an explicit `return` expression. That is, it is
92 //! only dead if the end of the function's block can never be reached.
93 //! It is the responsibility of typeck to ensure that there are no
94 //! `return` expressions in a function declared as diverging.
96 use self::LiveNodeKind::*;
99 use rustc_ast::ast::InlineAsmOptions;
100 use rustc_data_structures::fx::FxIndexMap;
101 use rustc_errors::Applicability;
102 use rustc_hir as hir;
103 use rustc_hir::def::*;
104 use rustc_hir::def_id::{DefId, LocalDefId};
105 use rustc_hir::intravisit::{self, FnKind, NestedVisitorMap, Visitor};
106 use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet, Node};
107 use rustc_middle::hir::map::Map;
108 use rustc_middle::ty::query::Providers;
109 use rustc_middle::ty::{self, TyCtxt};
110 use rustc_session::lint;
111 use rustc_span::symbol::{sym, Symbol};
112 use rustc_span::Span;
114 use std::collections::VecDeque;
117 use std::io::prelude::*;
120 #[derive(Copy, Clone, PartialEq)]
121 struct Variable(u32);
123 #[derive(Copy, Clone, PartialEq)]
124 struct LiveNode(u32);
127 fn get(&self) -> usize {
133 fn get(&self) -> usize {
138 #[derive(Copy, Clone, PartialEq, Debug)]
146 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
147 let sm = tcx.sess.source_map();
149 UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_string(s)),
150 ExprNode(s) => format!("Expr node [{}]", sm.span_to_string(s)),
151 VarDefNode(s) => format!("Var def node [{}]", sm.span_to_string(s)),
152 ExitNode => "Exit node".to_owned(),
156 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
157 type Map = Map<'tcx>;
159 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
160 NestedVisitorMap::OnlyBodies(self.tcx.hir())
166 fd: &'tcx hir::FnDecl<'tcx>,
171 visit_fn(self, fk, fd, b, s, id);
174 fn visit_local(&mut self, l: &'tcx hir::Local<'tcx>) {
175 visit_local(self, l);
177 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
178 visit_expr(self, ex);
180 fn visit_arm(&mut self, a: &'tcx hir::Arm<'tcx>) {
185 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: DefId) {
186 tcx.hir().visit_item_likes_in_module(
188 &mut IrMaps::new(tcx, module_def_id).as_deep_visitor(),
192 pub fn provide(providers: &mut Providers<'_>) {
193 *providers = Providers { check_mod_liveness, ..*providers };
196 impl fmt::Debug for LiveNode {
197 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
198 write!(f, "ln({})", self.get())
202 impl fmt::Debug for Variable {
203 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
204 write!(f, "v({})", self.get())
208 // ______________________________________________________________________
211 // This is the first pass and the one that drives the main
212 // computation. It walks up and down the IR once. On the way down,
213 // we count for each function the number of variables as well as
214 // liveness nodes. A liveness node is basically an expression or
215 // capture clause that does something of interest: either it has
216 // interesting control flow or it uses/defines a local variable.
218 // On the way back up, at each function node we create liveness sets
219 // (we now know precisely how big to make our various vectors and so
220 // forth) and then do the data-flow propagation to compute the set
221 // of live variables at each program point.
223 // Finally, we run back over the IR one last time and, using the
224 // computed liveness, check various safety conditions. For example,
225 // there must be no live nodes at the definition site for a variable
226 // unless it has an initializer. Similarly, each non-mutable local
227 // variable must not be assigned if there is some successor
228 // assignment. And so forth.
231 fn is_valid(&self) -> bool {
236 fn invalid_node() -> LiveNode {
245 #[derive(Copy, Clone, Debug)]
252 #[derive(Copy, Clone, Debug)]
254 Param(HirId, Symbol),
259 struct IrMaps<'tcx> {
262 num_live_nodes: usize,
264 live_node_map: HirIdMap<LiveNode>,
265 variable_map: HirIdMap<Variable>,
266 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
267 var_kinds: Vec<VarKind>,
268 lnks: Vec<LiveNodeKind>,
272 fn new(tcx: TyCtxt<'tcx>, body_owner: DefId) -> IrMaps<'tcx> {
278 live_node_map: HirIdMap::default(),
279 variable_map: HirIdMap::default(),
280 capture_info_map: Default::default(),
281 var_kinds: Vec::new(),
286 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
287 let ln = LiveNode(self.num_live_nodes as u32);
289 self.num_live_nodes += 1;
291 debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx));
296 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
297 let ln = self.add_live_node(lnk);
298 self.live_node_map.insert(hir_id, ln);
300 debug!("{:?} is node {:?}", ln, hir_id);
303 fn add_variable(&mut self, vk: VarKind) -> Variable {
304 let v = Variable(self.num_vars as u32);
305 self.var_kinds.push(vk);
309 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) => {
310 self.variable_map.insert(node_id, v);
315 debug!("{:?} is {:?}", v, vk);
320 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
321 match self.variable_map.get(&hir_id) {
324 span_bug!(span, "no variable registered for id {:?}", hir_id);
329 fn variable_name(&self, var: Variable) -> String {
330 match self.var_kinds[var.get()] {
331 Local(LocalInfo { name, .. }) | Param(_, name) => name.to_string(),
332 CleanExit => "<clean-exit>".to_owned(),
336 fn variable_is_shorthand(&self, var: Variable) -> bool {
337 match self.var_kinds[var.get()] {
338 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
339 Param(..) | CleanExit => false,
343 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
344 self.capture_info_map.insert(hir_id, Rc::new(cs));
347 fn lnk(&self, ln: LiveNode) -> LiveNodeKind {
353 ir: &mut IrMaps<'tcx>,
355 decl: &'tcx hir::FnDecl<'tcx>,
356 body_id: hir::BodyId,
362 // swap in a new set of IR maps for this function body:
363 let def_id = ir.tcx.hir().local_def_id(id);
364 let mut fn_maps = IrMaps::new(ir.tcx, def_id.to_def_id());
366 // Don't run unused pass for #[derive()]
367 if let FnKind::Method(..) = fk {
368 let parent = ir.tcx.hir().get_parent_item(id);
369 if let Some(Node::Item(i)) = ir.tcx.hir().find(parent) {
370 if i.attrs.iter().any(|a| a.check_name(sym::automatically_derived)) {
376 debug!("creating fn_maps: {:p}", &fn_maps);
378 let body = ir.tcx.hir().body(body_id);
380 for param in body.params {
381 let is_shorthand = match param.pat.kind {
382 rustc_hir::PatKind::Struct(..) => true,
385 param.pat.each_binding(|_bm, hir_id, _x, ident| {
386 debug!("adding parameters {:?}", hir_id);
387 let var = if is_shorthand {
388 Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: true })
390 Param(hir_id, ident.name)
392 fn_maps.add_variable(var);
396 // gather up the various local variables, significant expressions,
398 intravisit::walk_fn(&mut fn_maps, fk, decl, body_id, sp, id);
401 let mut lsets = Liveness::new(&mut fn_maps, def_id);
402 let entry_ln = lsets.compute(&body.value);
404 // check for various error conditions
405 lsets.visit_body(body);
406 lsets.warn_about_unused_args(body, entry_ln);
409 fn add_from_pat(ir: &mut IrMaps<'_>, pat: &hir::Pat<'_>) {
410 // For struct patterns, take note of which fields used shorthand
411 // (`x` rather than `x: x`).
412 let mut shorthand_field_ids = HirIdSet::default();
413 let mut pats = VecDeque::new();
415 while let Some(pat) = pats.pop_front() {
416 use rustc_hir::PatKind::*;
418 Binding(.., inner_pat) => {
419 pats.extend(inner_pat.iter());
421 Struct(_, fields, _) => {
422 let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
423 shorthand_field_ids.extend(ids);
425 Ref(inner_pat, _) | Box(inner_pat) => {
426 pats.push_back(inner_pat);
428 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
429 pats.extend(inner_pats.iter());
431 Slice(pre_pats, inner_pat, post_pats) => {
432 pats.extend(pre_pats.iter());
433 pats.extend(inner_pat.iter());
434 pats.extend(post_pats.iter());
440 pat.each_binding(|_, hir_id, _, ident| {
441 ir.add_live_node_for_node(hir_id, VarDefNode(ident.span));
442 ir.add_variable(Local(LocalInfo {
445 is_shorthand: shorthand_field_ids.contains(&hir_id),
450 fn visit_local<'tcx>(ir: &mut IrMaps<'tcx>, local: &'tcx hir::Local<'tcx>) {
451 add_from_pat(ir, &local.pat);
452 intravisit::walk_local(ir, local);
455 fn visit_arm<'tcx>(ir: &mut IrMaps<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
456 add_from_pat(ir, &arm.pat);
457 intravisit::walk_arm(ir, arm);
460 fn visit_expr<'tcx>(ir: &mut IrMaps<'tcx>, expr: &'tcx Expr<'tcx>) {
462 // live nodes required for uses or definitions of variables:
463 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
464 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
465 if let Res::Local(var_hir_id) = path.res {
466 let upvars = ir.tcx.upvars_mentioned(ir.body_owner);
467 if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hir_id)) {
468 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
471 intravisit::walk_expr(ir, expr);
473 hir::ExprKind::Closure(..) => {
474 // Interesting control flow (for loops can contain labeled
475 // breaks or continues)
476 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
478 // Make a live_node for each captured variable, with the span
479 // being the location that the variable is used. This results
480 // in better error messages than just pointing at the closure
481 // construction site.
482 let mut call_caps = Vec::new();
483 let closure_def_id = ir.tcx.hir().local_def_id(expr.hir_id);
484 if let Some(upvars) = ir.tcx.upvars_mentioned(closure_def_id) {
485 let parent_upvars = ir.tcx.upvars_mentioned(ir.body_owner);
486 call_caps.extend(upvars.iter().filter_map(|(&var_id, upvar)| {
488 parent_upvars.map_or(false, |upvars| upvars.contains_key(&var_id));
490 let upvar_ln = ir.add_live_node(UpvarNode(upvar.span));
491 Some(CaptureInfo { ln: upvar_ln, var_hid: var_id })
497 ir.set_captures(expr.hir_id, call_caps);
498 let old_body_owner = ir.body_owner;
499 ir.body_owner = closure_def_id.to_def_id();
500 intravisit::walk_expr(ir, expr);
501 ir.body_owner = old_body_owner;
504 // live nodes required for interesting control flow:
505 hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
506 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
507 intravisit::walk_expr(ir, expr);
509 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
510 ir.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
511 intravisit::walk_expr(ir, expr);
514 // otherwise, live nodes are not required:
515 hir::ExprKind::Index(..)
516 | hir::ExprKind::Field(..)
517 | hir::ExprKind::Array(..)
518 | hir::ExprKind::Call(..)
519 | hir::ExprKind::MethodCall(..)
520 | hir::ExprKind::Tup(..)
521 | hir::ExprKind::Binary(..)
522 | hir::ExprKind::AddrOf(..)
523 | hir::ExprKind::Cast(..)
524 | hir::ExprKind::DropTemps(..)
525 | hir::ExprKind::Unary(..)
526 | hir::ExprKind::Break(..)
527 | hir::ExprKind::Continue(_)
528 | hir::ExprKind::Lit(_)
529 | hir::ExprKind::Ret(..)
530 | hir::ExprKind::Block(..)
531 | hir::ExprKind::Assign(..)
532 | hir::ExprKind::AssignOp(..)
533 | hir::ExprKind::Struct(..)
534 | hir::ExprKind::Repeat(..)
535 | hir::ExprKind::InlineAsm(..)
536 | hir::ExprKind::LlvmInlineAsm(..)
537 | hir::ExprKind::Box(..)
538 | hir::ExprKind::Yield(..)
539 | hir::ExprKind::Type(..)
541 | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => {
542 intravisit::walk_expr(ir, expr);
547 // ______________________________________________________________________
548 // Computing liveness sets
550 // Actually we compute just a bit more than just liveness, but we use
551 // the same basic propagation framework in all cases.
553 #[derive(Clone, Copy)]
560 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
561 /// very large, so it uses a more compact representation that takes advantage
562 /// of the fact that when the number of `RWU`s is large, most of them have an
563 /// invalid reader and an invalid writer.
565 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
566 /// an index into `unpacked_rwus`. In the common cases, this compacts the
567 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
570 /// More compact representations are possible -- e.g., use only 2 bits per
571 /// packed `RWU` and make the secondary table a HashMap that maps from
572 /// indices to `RWU`s -- but this one strikes a good balance between size
574 packed_rwus: Vec<u32>,
575 unpacked_rwus: Vec<RWU>,
578 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: false }`.
579 const INV_INV_FALSE: u32 = u32::MAX;
581 // A constant representing `RWU { reader: invalid_node(); writer: invalid_node(); used: true }`.
582 const INV_INV_TRUE: u32 = u32::MAX - 1;
585 fn new(num_rwus: usize) -> RWUTable {
586 Self { packed_rwus: vec![INV_INV_FALSE; num_rwus], unpacked_rwus: vec![] }
589 fn get(&self, idx: usize) -> RWU {
590 let packed_rwu = self.packed_rwus[idx];
592 INV_INV_FALSE => RWU { reader: invalid_node(), writer: invalid_node(), used: false },
593 INV_INV_TRUE => RWU { reader: invalid_node(), writer: invalid_node(), used: true },
594 _ => self.unpacked_rwus[packed_rwu as usize],
598 fn get_reader(&self, idx: usize) -> LiveNode {
599 let packed_rwu = self.packed_rwus[idx];
601 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
602 _ => self.unpacked_rwus[packed_rwu as usize].reader,
606 fn get_writer(&self, idx: usize) -> LiveNode {
607 let packed_rwu = self.packed_rwus[idx];
609 INV_INV_FALSE | INV_INV_TRUE => invalid_node(),
610 _ => self.unpacked_rwus[packed_rwu as usize].writer,
614 fn get_used(&self, idx: usize) -> bool {
615 let packed_rwu = self.packed_rwus[idx];
617 INV_INV_FALSE => false,
618 INV_INV_TRUE => true,
619 _ => self.unpacked_rwus[packed_rwu as usize].used,
624 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
625 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
628 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
629 if rwu.reader == invalid_node() && rwu.writer == invalid_node() {
630 // When we overwrite an indexing entry in `self.packed_rwus` with
631 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
632 // from `self.unpacked_rwus`; it's not worth the effort, and we
633 // can't have entries shifting around anyway.
634 self.packed_rwus[idx] = if rwu.used { INV_INV_TRUE } else { INV_INV_FALSE }
636 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
638 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
639 self.unpacked_rwus.push(rwu);
643 fn assign_inv_inv(&mut self, idx: usize) {
644 self.packed_rwus[idx] = if self.get_used(idx) { INV_INV_TRUE } else { INV_INV_FALSE };
648 #[derive(Copy, Clone)]
651 fallthrough_ln: LiveNode,
652 clean_exit_var: Variable,
655 const ACC_READ: u32 = 1;
656 const ACC_WRITE: u32 = 2;
657 const ACC_USE: u32 = 4;
659 struct Liveness<'a, 'tcx> {
660 ir: &'a mut IrMaps<'tcx>,
661 tables: &'a ty::TypeckTables<'tcx>,
662 param_env: ty::ParamEnv<'tcx>,
664 successors: Vec<LiveNode>,
667 // mappings from loop node ID to LiveNode
668 // ("break" label should map to loop node ID,
669 // it probably doesn't now)
670 break_ln: HirIdMap<LiveNode>,
671 cont_ln: HirIdMap<LiveNode>,
674 impl<'a, 'tcx> Liveness<'a, 'tcx> {
675 fn new(ir: &'a mut IrMaps<'tcx>, def_id: LocalDefId) -> Liveness<'a, 'tcx> {
676 // Special nodes and variables:
677 // - exit_ln represents the end of the fn, either by return or panic
678 // - implicit_ret_var is a pseudo-variable that represents
679 // an implicit return
680 let specials = Specials {
681 exit_ln: ir.add_live_node(ExitNode),
682 fallthrough_ln: ir.add_live_node(ExitNode),
683 clean_exit_var: ir.add_variable(CleanExit),
686 let tables = ir.tcx.typeck_tables_of(def_id);
687 let param_env = ir.tcx.param_env(def_id);
689 let num_live_nodes = ir.num_live_nodes;
690 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.
712 span_bug!(span, "no live node registered for node {:?}", hir_id);
717 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
718 self.ir.variable(hir_id, span)
721 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
722 // In an or-pattern, only consider the first pattern; any later patterns
723 // must have the same bindings, and we also consider the first pattern
724 // to be the "authoritative" set of ids.
725 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
726 let ln = self.live_node(hir_id, pat_sp);
727 let var = self.variable(hir_id, ident.span);
728 self.init_from_succ(ln, succ);
729 self.define(ln, var);
735 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
736 ln.get() * self.ir.num_vars + var.get()
739 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
740 assert!(ln.is_valid());
741 let reader = self.rwu_table.get_reader(self.idx(ln, var));
742 if reader.is_valid() { Some(self.ir.lnk(reader)) } else { None }
745 // Is this variable live on entry to any of its successor nodes?
746 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
747 let successor = self.successors[ln.get()];
748 self.live_on_entry(successor, var)
751 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
752 assert!(ln.is_valid());
753 self.rwu_table.get_used(self.idx(ln, var))
756 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
757 assert!(ln.is_valid());
758 let writer = self.rwu_table.get_writer(self.idx(ln, var));
759 if writer.is_valid() { Some(self.ir.lnk(writer)) } else { None }
762 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
763 let successor = self.successors[ln.get()];
764 self.assigned_on_entry(successor, var)
767 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F)
769 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
771 let node_base_idx = self.idx(ln, Variable(0));
772 let succ_base_idx = self.idx(succ_ln, Variable(0));
773 for var_idx in 0..self.ir.num_vars {
774 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
778 fn write_vars<F>(&self, wr: &mut dyn Write, ln: LiveNode, mut test: F) -> io::Result<()>
780 F: FnMut(usize) -> LiveNode,
782 let node_base_idx = self.idx(ln, Variable(0));
783 for var_idx in 0..self.ir.num_vars {
784 let idx = node_base_idx + var_idx;
785 if test(idx).is_valid() {
786 write!(wr, " {:?}", Variable(var_idx as u32))?;
792 #[allow(unused_must_use)]
793 fn ln_str(&self, ln: LiveNode) -> String {
794 let mut wr = Vec::new();
796 let wr = &mut wr as &mut dyn Write;
797 write!(wr, "[ln({:?}) of kind {:?} reads", ln.get(), self.ir.lnk(ln));
798 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx));
799 write!(wr, " writes");
800 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx));
801 write!(wr, " precedes {:?}]", self.successors[ln.get()]);
803 String::from_utf8(wr).unwrap()
806 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
807 self.successors[ln.get()] = succ_ln;
809 // It is not necessary to initialize the RWUs here because they are all
810 // set to INV_INV_FALSE when they are created, and the sets only grow
811 // during iterations.
814 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
815 // more efficient version of init_empty() / merge_from_succ()
816 self.successors[ln.get()] = succ_ln;
818 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
819 this.rwu_table.copy_packed(idx, succ_idx);
821 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
824 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode, first_merge: bool) -> bool {
829 let mut any_changed = false;
830 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
831 // This is a special case, pulled out from the code below, where we
832 // don't have to do anything. It occurs about 60-70% of the time.
833 if this.rwu_table.packed_rwus[succ_idx] == INV_INV_FALSE {
837 let mut changed = false;
838 let mut rwu = this.rwu_table.get(idx);
839 let succ_rwu = this.rwu_table.get(succ_idx);
840 if succ_rwu.reader.is_valid() && !rwu.reader.is_valid() {
841 rwu.reader = succ_rwu.reader;
845 if succ_rwu.writer.is_valid() && !rwu.writer.is_valid() {
846 rwu.writer = succ_rwu.writer;
850 if succ_rwu.used && !rwu.used {
856 this.rwu_table.assign_unpacked(idx, rwu);
862 "merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
864 self.ln_str(succ_ln),
871 // Indicates that a local variable was *defined*; we know that no
872 // uses of the variable can precede the definition (resolve checks
873 // this) so we just clear out all the data.
874 fn define(&mut self, writer: LiveNode, var: Variable) {
875 let idx = self.idx(writer, var);
876 self.rwu_table.assign_inv_inv(idx);
878 debug!("{:?} defines {:?} (idx={}): {}", writer, var, idx, self.ln_str(writer));
881 // Either read, write, or both depending on the acc bitset
882 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
883 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
885 let idx = self.idx(ln, var);
886 let mut rwu = self.rwu_table.get(idx);
888 if (acc & ACC_WRITE) != 0 {
889 rwu.reader = invalid_node();
893 // Important: if we both read/write, must do read second
894 // or else the write will override.
895 if (acc & ACC_READ) != 0 {
899 if (acc & ACC_USE) != 0 {
903 self.rwu_table.assign_unpacked(idx, rwu);
906 fn compute(&mut self, body: &hir::Expr<'_>) -> LiveNode {
907 debug!("compute: using id for body, {:?}", body);
909 // the fallthrough exit is only for those cases where we do not
910 // explicitly return:
912 self.init_from_succ(s.fallthrough_ln, s.exit_ln);
913 self.acc(s.fallthrough_ln, s.clean_exit_var, ACC_READ);
915 let entry_ln = self.propagate_through_expr(body, s.fallthrough_ln);
917 // hack to skip the loop unless debug! is enabled:
919 "^^ liveness computation results for body {} (entry={:?})",
921 for ln_idx in 0..self.ir.num_live_nodes {
922 debug!("{:?}", self.ln_str(LiveNode(ln_idx as u32)));
932 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
933 if blk.targeted_by_break {
934 self.break_ln.insert(blk.hir_id, succ);
936 let succ = self.propagate_through_opt_expr(blk.expr.as_deref(), succ);
937 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
940 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
942 hir::StmtKind::Local(ref local) => {
943 // Note: we mark the variable as defined regardless of whether
944 // there is an initializer. Initially I had thought to only mark
945 // the live variable as defined if it was initialized, and then we
946 // could check for uninit variables just by scanning what is live
947 // at the start of the function. But that doesn't work so well for
948 // immutable variables defined in a loop:
949 // loop { let x; x = 5; }
950 // because the "assignment" loops back around and generates an error.
952 // So now we just check that variables defined w/o an
953 // initializer are not live at the point of their
954 // initialization, which is mildly more complex than checking
955 // once at the func header but otherwise equivalent.
957 let succ = self.propagate_through_opt_expr(local.init.as_deref(), succ);
958 self.define_bindings_in_pat(&local.pat, succ)
960 hir::StmtKind::Item(..) => succ,
961 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
962 self.propagate_through_expr(&expr, succ)
967 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
968 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
971 fn propagate_through_opt_expr(
973 opt_expr: Option<&Expr<'_>>,
976 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
979 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
980 debug!("propagate_through_expr: {:?}", expr);
983 // Interesting cases with control flow or which gen/kill
984 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
985 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
988 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
990 hir::ExprKind::Closure(..) => {
991 debug!("{:?} is an ExprKind::Closure", expr);
993 // the construction of a closure itself is not important,
994 // but we have to consider the closed over variables.
1000 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
1002 caps.iter().rev().fold(succ, |succ, cap| {
1003 self.init_from_succ(cap.ln, succ);
1004 let var = self.variable(cap.var_hid, expr.span);
1005 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1010 // Note that labels have been resolved, so we don't need to look
1011 // at the label ident
1012 hir::ExprKind::Loop(ref blk, _, _) => self.propagate_through_loop(expr, &blk, succ),
1014 hir::ExprKind::Match(ref e, arms, _) => {
1029 let ln = self.live_node(expr.hir_id, expr.span);
1030 self.init_empty(ln, succ);
1031 let mut first_merge = true;
1033 let body_succ = self.propagate_through_expr(&arm.body, succ);
1035 let guard_succ = self.propagate_through_opt_expr(
1036 arm.guard.as_ref().map(|hir::Guard::If(e)| *e),
1039 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
1040 self.merge_from_succ(ln, arm_succ, first_merge);
1041 first_merge = false;
1043 self.propagate_through_expr(&e, ln)
1046 hir::ExprKind::Ret(ref o_e) => {
1047 // ignore succ and subst exit_ln:
1048 let exit_ln = self.s.exit_ln;
1049 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), exit_ln)
1052 hir::ExprKind::Break(label, ref opt_expr) => {
1053 // Find which label this break jumps to
1054 let target = match label.target_id {
1055 Ok(hir_id) => self.break_ln.get(&hir_id),
1056 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1060 // Now that we know the label we're going to,
1061 // look it up in the break loop nodes table
1064 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1066 // FIXME: This should have been checked earlier. Once this is fixed,
1067 // replace with `delay_span_bug`. (#62480)
1071 .struct_span_err(expr.span, "`break` to unknown label")
1073 rustc_errors::FatalError.raise()
1078 hir::ExprKind::Continue(label) => {
1079 // Find which label this expr continues to
1082 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
1084 // Now that we know the label we're going to,
1085 // look it up in the continue loop nodes table
1089 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
1092 hir::ExprKind::Assign(ref l, ref r, _) => {
1093 // see comment on places in
1094 // propagate_through_place_components()
1095 let succ = self.write_place(&l, succ, ACC_WRITE);
1096 let succ = self.propagate_through_place_components(&l, succ);
1097 self.propagate_through_expr(&r, succ)
1100 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1101 // an overloaded assign op is like a method call
1102 if self.tables.is_method_call(expr) {
1103 let succ = self.propagate_through_expr(&l, succ);
1104 self.propagate_through_expr(&r, succ)
1106 // see comment on places in
1107 // propagate_through_place_components()
1108 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
1109 let succ = self.propagate_through_expr(&r, succ);
1110 self.propagate_through_place_components(&l, succ)
1114 // Uninteresting cases: just propagate in rev exec order
1115 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
1117 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1118 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1122 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
1125 hir::ExprKind::Call(ref f, ref args) => {
1126 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1127 let succ = if self.ir.tcx.is_ty_uninhabited_from(
1129 self.tables.expr_ty(expr),
1136 let succ = self.propagate_through_exprs(args, succ);
1137 self.propagate_through_expr(&f, succ)
1140 hir::ExprKind::MethodCall(.., ref args) => {
1141 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1142 let succ = if self.ir.tcx.is_ty_uninhabited_from(
1144 self.tables.expr_ty(expr),
1152 self.propagate_through_exprs(args, succ)
1155 hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ),
1157 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1158 let r_succ = self.propagate_through_expr(&r, succ);
1160 let ln = self.live_node(expr.hir_id, expr.span);
1161 self.init_from_succ(ln, succ);
1162 self.merge_from_succ(ln, r_succ, false);
1164 self.propagate_through_expr(&l, ln)
1167 hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => {
1168 let r_succ = self.propagate_through_expr(&r, succ);
1169 self.propagate_through_expr(&l, r_succ)
1172 hir::ExprKind::Box(ref e)
1173 | hir::ExprKind::AddrOf(_, _, ref e)
1174 | hir::ExprKind::Cast(ref e, _)
1175 | hir::ExprKind::Type(ref e, _)
1176 | hir::ExprKind::DropTemps(ref e)
1177 | hir::ExprKind::Unary(_, ref e)
1178 | hir::ExprKind::Yield(ref e, _)
1179 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1181 hir::ExprKind::InlineAsm(ref asm) => {
1182 // Handle non-returning asm
1183 let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) {
1189 // Do a first pass for writing outputs only
1190 for op in asm.operands.iter().rev() {
1192 hir::InlineAsmOperand::In { .. }
1193 | hir::InlineAsmOperand::Const { .. }
1194 | hir::InlineAsmOperand::Sym { .. } => {}
1195 hir::InlineAsmOperand::Out { expr, .. } => {
1196 if let Some(expr) = expr {
1197 succ = self.write_place(expr, succ, ACC_WRITE);
1200 hir::InlineAsmOperand::InOut { expr, .. } => {
1201 succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE);
1203 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1204 if let Some(expr) = out_expr {
1205 succ = self.write_place(expr, succ, ACC_WRITE);
1211 // Then do a second pass for inputs
1212 let mut succ = succ;
1213 for op in asm.operands.iter().rev() {
1215 hir::InlineAsmOperand::In { expr, .. }
1216 | hir::InlineAsmOperand::Const { expr, .. }
1217 | hir::InlineAsmOperand::Sym { expr, .. } => {
1218 succ = self.propagate_through_expr(expr, succ)
1220 hir::InlineAsmOperand::Out { expr, .. } => {
1221 if let Some(expr) = expr {
1222 succ = self.propagate_through_place_components(expr, succ);
1225 hir::InlineAsmOperand::InOut { expr, .. } => {
1226 succ = self.propagate_through_place_components(expr, succ);
1228 hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
1229 if let Some(expr) = out_expr {
1230 succ = self.propagate_through_place_components(expr, succ);
1232 succ = self.propagate_through_expr(in_expr, succ);
1239 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1240 let ia = &asm.inner;
1241 let outputs = asm.outputs_exprs;
1242 let inputs = asm.inputs_exprs;
1243 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1244 // see comment on places
1245 // in propagate_through_place_components()
1247 self.propagate_through_expr(output, succ)
1249 let acc = if o.is_rw { ACC_WRITE | ACC_READ } else { ACC_WRITE };
1250 let succ = self.write_place(output, succ, acc);
1251 self.propagate_through_place_components(output, succ)
1255 // Inputs are executed first. Propagate last because of rev order
1256 self.propagate_through_exprs(inputs, succ)
1259 hir::ExprKind::Lit(..)
1260 | hir::ExprKind::Err
1261 | hir::ExprKind::Path(hir::QPath::TypeRelative(..)) => succ,
1263 // Note that labels have been resolved, so we don't need to look
1264 // at the label ident
1265 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1269 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1272 // In general, the full flow graph structure for an
1273 // assignment/move/etc can be handled in one of two ways,
1274 // depending on whether what is being assigned is a "tracked
1275 // value" or not. A tracked value is basically a local
1276 // variable or argument.
1278 // The two kinds of graphs are:
1280 // Tracked place Untracked place
1281 // ----------------------++-----------------------
1285 // (rvalue) || (rvalue)
1288 // (write of place) || (place components)
1293 // ----------------------++-----------------------
1295 // I will cover the two cases in turn:
1299 // A tracked place is a local variable/argument `x`. In
1300 // these cases, the link_node where the write occurs is linked
1301 // to node id of `x`. The `write_place()` routine generates
1302 // the contents of this node. There are no subcomponents to
1305 // # Non-tracked places
1307 // These are places like `x[5]` or `x.f`. In that case, we
1308 // basically ignore the value which is written to but generate
1309 // reads for the components---`x` in these two examples. The
1310 // components reads are generated by
1311 // `propagate_through_place_components()` (this fn).
1315 // It is still possible to observe assignments to non-places;
1316 // these errors are detected in the later pass borrowck. We
1317 // just ignore such cases and treat them as reads.
1320 hir::ExprKind::Path(_) => succ,
1321 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1322 _ => self.propagate_through_expr(expr, succ),
1326 // see comment on propagate_through_place()
1327 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1329 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1330 self.access_path(expr.hir_id, path, succ, acc)
1333 // We do not track other places, so just propagate through
1334 // to their subcomponents. Also, it may happen that
1335 // non-places occur here, because those are detected in the
1336 // later pass borrowck.
1349 let ln = self.live_node(hir_id, span);
1351 self.init_from_succ(ln, succ);
1352 let var = self.variable(var_hid, span);
1353 self.acc(ln, var, acc);
1361 path: &hir::Path<'_>,
1366 Res::Local(hid) => {
1367 let upvars = self.ir.tcx.upvars_mentioned(self.ir.body_owner);
1368 if !upvars.map_or(false, |upvars| upvars.contains_key(&hid)) {
1369 self.access_var(hir_id, hid, succ, acc, path.span)
1378 fn propagate_through_loop(
1381 body: &hir::Block<'_>,
1385 We model control flow like this:
1392 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1393 Meanwhile, a `break` expression will have a successor of `succ`.
1397 let mut first_merge = true;
1398 let ln = self.live_node(expr.hir_id, expr.span);
1399 self.init_empty(ln, succ);
1400 debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body);
1402 self.break_ln.insert(expr.hir_id, succ);
1404 self.cont_ln.insert(expr.hir_id, ln);
1406 let body_ln = self.propagate_through_block(body, ln);
1408 // repeat until fixed point is reached:
1409 while self.merge_from_succ(ln, body_ln, first_merge) {
1410 first_merge = false;
1411 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1418 // _______________________________________________________________________
1419 // Checking for error conditions
1421 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1422 type Map = intravisit::ErasedMap<'tcx>;
1424 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
1425 NestedVisitorMap::None
1428 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1429 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1430 if local.init.is_some() {
1431 self.warn_about_dead_assign(spans, hir_id, ln, var);
1435 intravisit::walk_local(self, local);
1438 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1439 check_expr(self, ex);
1442 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1443 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1444 intravisit::walk_arm(self, arm);
1448 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1450 hir::ExprKind::Assign(ref l, ..) => {
1451 this.check_place(&l);
1454 hir::ExprKind::AssignOp(_, ref l, _) => {
1455 if !this.tables.is_method_call(expr) {
1456 this.check_place(&l);
1460 hir::ExprKind::InlineAsm(ref asm) => {
1461 for op in asm.operands {
1463 hir::InlineAsmOperand::Out { expr, .. } => {
1464 if let Some(expr) = expr {
1465 this.check_place(expr);
1468 hir::InlineAsmOperand::InOut { expr, .. } => {
1469 this.check_place(expr);
1471 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1472 if let Some(out_expr) = out_expr {
1473 this.check_place(out_expr);
1481 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1482 for input in asm.inputs_exprs {
1483 this.visit_expr(input);
1486 // Output operands must be places
1487 for (o, output) in asm.inner.outputs.iter().zip(asm.outputs_exprs) {
1489 this.check_place(output);
1491 this.visit_expr(output);
1495 // no correctness conditions related to liveness
1496 hir::ExprKind::Call(..)
1497 | hir::ExprKind::MethodCall(..)
1498 | hir::ExprKind::Match(..)
1499 | hir::ExprKind::Loop(..)
1500 | hir::ExprKind::Index(..)
1501 | hir::ExprKind::Field(..)
1502 | hir::ExprKind::Array(..)
1503 | hir::ExprKind::Tup(..)
1504 | hir::ExprKind::Binary(..)
1505 | hir::ExprKind::Cast(..)
1506 | hir::ExprKind::DropTemps(..)
1507 | hir::ExprKind::Unary(..)
1508 | hir::ExprKind::Ret(..)
1509 | hir::ExprKind::Break(..)
1510 | hir::ExprKind::Continue(..)
1511 | hir::ExprKind::Lit(_)
1512 | hir::ExprKind::Block(..)
1513 | hir::ExprKind::AddrOf(..)
1514 | hir::ExprKind::Struct(..)
1515 | hir::ExprKind::Repeat(..)
1516 | hir::ExprKind::Closure(..)
1517 | hir::ExprKind::Path(_)
1518 | hir::ExprKind::Yield(..)
1519 | hir::ExprKind::Box(..)
1520 | hir::ExprKind::Type(..)
1521 | hir::ExprKind::Err => {}
1524 intravisit::walk_expr(this, expr);
1527 impl<'tcx> Liveness<'_, 'tcx> {
1528 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1530 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1531 if let Res::Local(var_hid) = path.res {
1532 let upvars = self.ir.tcx.upvars_mentioned(self.ir.body_owner);
1533 if !upvars.map_or(false, |upvars| upvars.contains_key(&var_hid)) {
1534 // Assignment to an immutable variable or argument: only legal
1535 // if there is no later assignment. If this local is actually
1536 // mutable, then check for a reassignment to flag the mutability
1538 let ln = self.live_node(expr.hir_id, expr.span);
1539 let var = self.variable(var_hid, expr.span);
1540 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1545 // For other kinds of places, no checks are required,
1546 // and any embedded expressions are actually rvalues
1547 intravisit::walk_expr(self, expr);
1552 fn should_warn(&self, var: Variable) -> Option<String> {
1553 let name = self.ir.variable_name(var);
1554 if name.is_empty() || name.as_bytes()[0] == b'_' { None } else { Some(name) }
1557 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1558 for p in body.params {
1559 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1560 if self.live_on_entry(ln, var).is_none() {
1561 self.report_dead_assign(hir_id, spans, var, true);
1567 fn check_unused_vars_in_pat(
1570 entry_ln: Option<LiveNode>,
1571 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1573 // In an or-pattern, only consider the variable; any later patterns must have the same
1574 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1575 // However, we should take the ids and spans of variables with the same name from the later
1576 // patterns so the suggestions to prefix with underscores will apply to those too.
1577 let mut vars: FxIndexMap<String, (LiveNode, Variable, Vec<(HirId, Span)>)> = <_>::default();
1579 pat.each_binding(|_, hir_id, pat_sp, ident| {
1580 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1581 let var = self.variable(hir_id, ident.span);
1582 let id_and_sp = (hir_id, pat_sp);
1583 vars.entry(self.ir.variable_name(var))
1584 .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp))
1585 .or_insert_with(|| (ln, var, vec![id_and_sp]));
1588 for (_, (ln, var, hir_ids_and_spans)) in vars {
1589 if self.used_on_entry(ln, var) {
1590 let id = hir_ids_and_spans[0].0;
1591 let spans = hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect();
1592 on_used_on_entry(spans, id, ln, var);
1594 self.report_unused(hir_ids_and_spans, ln, var);
1599 fn report_unused(&self, hir_ids_and_spans: Vec<(HirId, Span)>, ln: LiveNode, var: Variable) {
1600 let first_hir_id = hir_ids_and_spans[0].0;
1602 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1603 // annoying: for parameters in funcs like `fn(x: i32)
1604 // {ret}`, there is only one node, so asking about
1605 // assigned_on_exit() is not meaningful.
1607 if ln == self.s.exit_ln { false } else { self.assigned_on_exit(ln, var).is_some() };
1610 self.ir.tcx.struct_span_lint_hir(
1611 lint::builtin::UNUSED_VARIABLES,
1613 hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect::<Vec<_>>(),
1615 lint.build(&format!("variable `{}` is assigned to, but never used", name))
1616 .note(&format!("consider using `_{}` instead", name))
1621 self.ir.tcx.struct_span_lint_hir(
1622 lint::builtin::UNUSED_VARIABLES,
1624 hir_ids_and_spans.iter().map(|(_, sp)| *sp).collect::<Vec<_>>(),
1626 let mut err = lint.build(&format!("unused variable: `{}`", name));
1628 let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
1629 hir_ids_and_spans.into_iter().partition(|(hir_id, span)| {
1630 let var = self.variable(*hir_id, *span);
1631 self.ir.variable_is_shorthand(var)
1634 let mut shorthands = shorthands
1636 .map(|(_, span)| (span, format!("{}: _", name)))
1637 .collect::<Vec<_>>();
1639 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1640 // the field" message, and suggest `_` for the non-shorthands. If we only
1641 // have non-shorthand, then prefix with an underscore instead.
1642 if !shorthands.is_empty() {
1646 .map(|(_, span)| (span, "_".to_string()))
1647 .collect::<Vec<_>>(),
1650 err.multipart_suggestion(
1651 "try ignoring the field",
1653 Applicability::MachineApplicable,
1656 err.multipart_suggestion(
1657 "if this is intentional, prefix it with an underscore",
1660 .map(|(_, span)| (span, format!("_{}", name)))
1661 .collect::<Vec<_>>(),
1662 Applicability::MachineApplicable,
1673 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1674 if self.live_on_exit(ln, var).is_none() {
1675 self.report_dead_assign(hir_id, spans, var, false);
1679 fn report_dead_assign(&self, hir_id: HirId, spans: Vec<Span>, var: Variable, is_param: bool) {
1680 if let Some(name) = self.should_warn(var) {
1682 self.ir.tcx.struct_span_lint_hir(
1683 lint::builtin::UNUSED_ASSIGNMENTS,
1687 lint.build(&format!("value passed to `{}` is never read", name))
1688 .help("maybe it is overwritten before being read?")
1693 self.ir.tcx.struct_span_lint_hir(
1694 lint::builtin::UNUSED_ASSIGNMENTS,
1698 lint.build(&format!("value assigned to `{}` is never read", name))
1699 .help("maybe it is overwritten before being read?")