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 `None`, 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 `None`, 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 `Liveness` 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, NestedVisitorMap, Visitor};
94 use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet};
95 use rustc_index::vec::IndexVec;
96 use rustc_middle::hir::map::Map;
97 use rustc_middle::ty::query::Providers;
98 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
99 use rustc_session::lint;
100 use rustc_span::symbol::{kw, sym, Symbol};
101 use rustc_span::Span;
103 use std::collections::VecDeque;
105 use std::io::prelude::*;
108 rustc_index::newtype_index! {
109 pub struct Variable {
110 DEBUG_FORMAT = "v({})",
114 rustc_index::newtype_index! {
115 pub struct LiveNode {
116 DEBUG_FORMAT = "ln({})",
120 #[derive(Copy, Clone, PartialEq, Debug)]
129 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
130 let sm = tcx.sess.source_map();
132 UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_string(s)),
133 ExprNode(s) => format!("Expr node [{}]", sm.span_to_string(s)),
134 VarDefNode(s) => format!("Var def node [{}]", sm.span_to_string(s)),
135 ClosureNode => "Closure node".to_owned(),
136 ExitNode => "Exit node".to_owned(),
140 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
141 tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor());
144 pub fn provide(providers: &mut Providers) {
145 *providers = Providers { check_mod_liveness, ..*providers };
148 // ______________________________________________________________________
151 // This is the first pass and the one that drives the main
152 // computation. It walks up and down the IR once. On the way down,
153 // we count for each function the number of variables as well as
154 // liveness nodes. A liveness node is basically an expression or
155 // capture clause that does something of interest: either it has
156 // interesting control flow or it uses/defines a local variable.
158 // On the way back up, at each function node we create liveness sets
159 // (we now know precisely how big to make our various vectors and so
160 // forth) and then do the data-flow propagation to compute the set
161 // of live variables at each program point.
163 // Finally, we run back over the IR one last time and, using the
164 // computed liveness, check various safety conditions. For example,
165 // there must be no live nodes at the definition site for a variable
166 // unless it has an initializer. Similarly, each non-mutable local
167 // variable must not be assigned if there is some successor
168 // assignment. And so forth.
175 #[derive(Copy, Clone, Debug)]
182 #[derive(Copy, Clone, Debug)]
184 Param(HirId, Symbol),
186 Upvar(HirId, Symbol),
189 struct IrMaps<'tcx> {
191 live_node_map: HirIdMap<LiveNode>,
192 variable_map: HirIdMap<Variable>,
193 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
194 var_kinds: IndexVec<Variable, VarKind>,
195 lnks: IndexVec<LiveNode, LiveNodeKind>,
199 fn new(tcx: TyCtxt<'tcx>) -> IrMaps<'tcx> {
202 live_node_map: HirIdMap::default(),
203 variable_map: HirIdMap::default(),
204 capture_info_map: Default::default(),
205 var_kinds: IndexVec::new(),
206 lnks: IndexVec::new(),
210 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
211 let ln = self.lnks.push(lnk);
213 debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx));
218 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
219 let ln = self.add_live_node(lnk);
220 self.live_node_map.insert(hir_id, ln);
222 debug!("{:?} is node {:?}", ln, hir_id);
225 fn add_variable(&mut self, vk: VarKind) -> Variable {
226 let v = self.var_kinds.push(vk);
229 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) | Upvar(node_id, _) => {
230 self.variable_map.insert(node_id, v);
234 debug!("{:?} is {:?}", v, vk);
239 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
240 match self.variable_map.get(&hir_id) {
243 span_bug!(span, "no variable registered for id {:?}", hir_id);
248 fn variable_name(&self, var: Variable) -> Symbol {
249 match self.var_kinds[var] {
250 Local(LocalInfo { name, .. }) | Param(_, name) | Upvar(_, name) => name,
254 fn variable_is_shorthand(&self, var: Variable) -> bool {
255 match self.var_kinds[var] {
256 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
257 Param(..) | Upvar(..) => false,
261 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
262 self.capture_info_map.insert(hir_id, Rc::new(cs));
265 fn add_from_pat(&mut self, pat: &hir::Pat<'tcx>) {
266 // For struct patterns, take note of which fields used shorthand
267 // (`x` rather than `x: x`).
268 let mut shorthand_field_ids = HirIdSet::default();
269 let mut pats = VecDeque::new();
271 while let Some(pat) = pats.pop_front() {
272 use rustc_hir::PatKind::*;
274 Binding(.., inner_pat) => {
275 pats.extend(inner_pat.iter());
277 Struct(_, fields, _) => {
278 let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
279 shorthand_field_ids.extend(ids);
281 Ref(inner_pat, _) | Box(inner_pat) => {
282 pats.push_back(inner_pat);
284 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
285 pats.extend(inner_pats.iter());
287 Slice(pre_pats, inner_pat, post_pats) => {
288 pats.extend(pre_pats.iter());
289 pats.extend(inner_pat.iter());
290 pats.extend(post_pats.iter());
296 pat.each_binding(|_, hir_id, _, ident| {
297 self.add_live_node_for_node(hir_id, VarDefNode(ident.span));
298 self.add_variable(Local(LocalInfo {
301 is_shorthand: shorthand_field_ids.contains(&hir_id),
307 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
308 type Map = Map<'tcx>;
310 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
311 NestedVisitorMap::OnlyBodies(self.tcx.hir())
314 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
315 debug!("visit_body {:?}", body.id());
317 // swap in a new set of IR maps for this body
318 let mut maps = IrMaps::new(self.tcx);
319 let hir_id = maps.tcx.hir().body_owner(body.id());
320 let local_def_id = maps.tcx.hir().local_def_id(hir_id);
321 let def_id = local_def_id.to_def_id();
323 // Don't run unused pass for #[derive()]
324 if let Some(parent) = self.tcx.parent(def_id) {
325 if let DefKind::Impl = self.tcx.def_kind(parent.expect_local()) {
326 if self.tcx.has_attr(parent, sym::automatically_derived) {
332 if let Some(captures) = maps.tcx.typeck(local_def_id).closure_captures.get(&def_id) {
333 for &var_hir_id in captures.keys() {
334 let var_name = maps.tcx.hir().name(var_hir_id);
335 maps.add_variable(Upvar(var_hir_id, var_name));
339 // gather up the various local variables, significant expressions,
341 intravisit::walk_body(&mut maps, body);
344 let mut lsets = Liveness::new(&mut maps, local_def_id);
345 let entry_ln = lsets.compute(&body, hir_id);
346 lsets.log_liveness(entry_ln, body.id().hir_id);
348 // check for various error conditions
349 lsets.visit_body(body);
350 lsets.warn_about_unused_upvars(entry_ln);
351 lsets.warn_about_unused_args(body, entry_ln);
354 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
355 self.add_from_pat(&local.pat);
356 intravisit::walk_local(self, local);
359 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
360 self.add_from_pat(&arm.pat);
361 intravisit::walk_arm(self, arm);
364 fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
365 let is_shorthand = match param.pat.kind {
366 rustc_hir::PatKind::Struct(..) => true,
369 param.pat.each_binding(|_bm, hir_id, _x, ident| {
370 let var = if is_shorthand {
371 Local(LocalInfo { id: hir_id, name: ident.name, is_shorthand: true })
373 Param(hir_id, ident.name)
375 self.add_variable(var);
377 intravisit::walk_param(self, param);
380 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
382 // live nodes required for uses or definitions of variables:
383 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
384 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
385 if let Res::Local(_var_hir_id) = path.res {
386 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
388 intravisit::walk_expr(self, expr);
390 hir::ExprKind::Closure(..) => {
391 // Interesting control flow (for loops can contain labeled
392 // breaks or continues)
393 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
395 // Make a live_node for each captured variable, with the span
396 // being the location that the variable is used. This results
397 // in better error messages than just pointing at the closure
398 // construction site.
399 let mut call_caps = Vec::new();
400 let closure_def_id = self.tcx.hir().local_def_id(expr.hir_id);
401 if let Some(captures) = self
403 .typeck(closure_def_id)
405 .get(&closure_def_id.to_def_id())
407 // If closure captures is Some, upvars_mentioned must also be Some
408 let upvars = self.tcx.upvars_mentioned(closure_def_id).unwrap();
409 call_caps.extend(captures.keys().map(|var_id| {
410 let upvar = upvars[var_id];
411 let upvar_ln = self.add_live_node(UpvarNode(upvar.span));
412 CaptureInfo { ln: upvar_ln, var_hid: *var_id }
415 self.set_captures(expr.hir_id, call_caps);
416 intravisit::walk_expr(self, expr);
419 // live nodes required for interesting control flow:
420 hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
421 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
422 intravisit::walk_expr(self, expr);
424 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
425 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
426 intravisit::walk_expr(self, expr);
429 // otherwise, live nodes are not required:
430 hir::ExprKind::Index(..)
431 | hir::ExprKind::Field(..)
432 | hir::ExprKind::Array(..)
433 | hir::ExprKind::Call(..)
434 | hir::ExprKind::MethodCall(..)
435 | hir::ExprKind::Tup(..)
436 | hir::ExprKind::Binary(..)
437 | hir::ExprKind::AddrOf(..)
438 | hir::ExprKind::Cast(..)
439 | hir::ExprKind::DropTemps(..)
440 | hir::ExprKind::Unary(..)
441 | hir::ExprKind::Break(..)
442 | hir::ExprKind::Continue(_)
443 | hir::ExprKind::Lit(_)
444 | hir::ExprKind::ConstBlock(..)
445 | hir::ExprKind::Ret(..)
446 | hir::ExprKind::Block(..)
447 | hir::ExprKind::Assign(..)
448 | hir::ExprKind::AssignOp(..)
449 | hir::ExprKind::Struct(..)
450 | hir::ExprKind::Repeat(..)
451 | hir::ExprKind::InlineAsm(..)
452 | hir::ExprKind::LlvmInlineAsm(..)
453 | hir::ExprKind::Box(..)
454 | hir::ExprKind::Yield(..)
455 | hir::ExprKind::Type(..)
457 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
458 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
459 intravisit::walk_expr(self, expr);
465 // ______________________________________________________________________
466 // Computing liveness sets
468 // Actually we compute just a bit more than just liveness, but we use
469 // the same basic propagation framework in all cases.
471 #[derive(Clone, Copy)]
473 reader: Option<LiveNode>,
474 writer: Option<LiveNode>,
478 /// Conceptually, this is like a `Vec<RWU>`. But the number of `RWU`s can get
479 /// very large, so it uses a more compact representation that takes advantage
480 /// of the fact that when the number of `RWU`s is large, most of them have an
481 /// invalid reader and an invalid writer.
483 /// Each entry in `packed_rwus` is either INV_INV_FALSE, INV_INV_TRUE, or
484 /// an index into `unpacked_rwus`. In the common cases, this compacts the
485 /// 65 bits of data into 32; in the uncommon cases, it expands the 65 bits
488 /// More compact representations are possible -- e.g., use only 2 bits per
489 /// packed `RWU` and make the secondary table a HashMap that maps from
490 /// indices to `RWU`s -- but this one strikes a good balance between size
492 packed_rwus: Vec<u32>,
493 unpacked_rwus: Vec<RWU>,
496 // A constant representing `RWU { reader: None; writer: None; used: false }`.
497 const INV_INV_FALSE: u32 = u32::MAX;
499 // A constant representing `RWU { reader: None; writer: None; used: true }`.
500 const INV_INV_TRUE: u32 = u32::MAX - 1;
503 fn new(num_rwus: usize) -> RWUTable {
504 Self { packed_rwus: vec![INV_INV_FALSE; num_rwus], unpacked_rwus: vec![] }
507 fn get(&self, idx: usize) -> RWU {
508 let packed_rwu = self.packed_rwus[idx];
510 INV_INV_FALSE => RWU { reader: None, writer: None, used: false },
511 INV_INV_TRUE => RWU { reader: None, writer: None, used: true },
512 _ => self.unpacked_rwus[packed_rwu as usize],
516 fn get_reader(&self, idx: usize) -> Option<LiveNode> {
517 let packed_rwu = self.packed_rwus[idx];
519 INV_INV_FALSE | INV_INV_TRUE => None,
520 _ => self.unpacked_rwus[packed_rwu as usize].reader,
524 fn get_writer(&self, idx: usize) -> Option<LiveNode> {
525 let packed_rwu = self.packed_rwus[idx];
527 INV_INV_FALSE | INV_INV_TRUE => None,
528 _ => self.unpacked_rwus[packed_rwu as usize].writer,
532 fn get_used(&self, idx: usize) -> bool {
533 let packed_rwu = self.packed_rwus[idx];
535 INV_INV_FALSE => false,
536 INV_INV_TRUE => true,
537 _ => self.unpacked_rwus[packed_rwu as usize].used,
542 fn copy_packed(&mut self, dst_idx: usize, src_idx: usize) {
543 self.packed_rwus[dst_idx] = self.packed_rwus[src_idx];
546 fn assign_unpacked(&mut self, idx: usize, rwu: RWU) {
547 if rwu.reader == None && rwu.writer == None {
548 // When we overwrite an indexing entry in `self.packed_rwus` with
549 // `INV_INV_{TRUE,FALSE}` we don't remove the corresponding entry
550 // from `self.unpacked_rwus`; it's not worth the effort, and we
551 // can't have entries shifting around anyway.
552 self.packed_rwus[idx] = if rwu.used { INV_INV_TRUE } else { INV_INV_FALSE }
554 // Add a new RWU to `unpacked_rwus` and make `packed_rwus[idx]`
556 self.packed_rwus[idx] = self.unpacked_rwus.len() as u32;
557 self.unpacked_rwus.push(rwu);
561 fn assign_inv_inv(&mut self, idx: usize) {
562 self.packed_rwus[idx] = if self.get_used(idx) { INV_INV_TRUE } else { INV_INV_FALSE };
566 const ACC_READ: u32 = 1;
567 const ACC_WRITE: u32 = 2;
568 const ACC_USE: u32 = 4;
570 struct Liveness<'a, 'tcx> {
571 ir: &'a mut IrMaps<'tcx>,
572 body_owner: LocalDefId,
573 typeck_results: &'a ty::TypeckResults<'tcx>,
574 param_env: ty::ParamEnv<'tcx>,
575 upvars: Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>>,
576 closure_captures: Option<&'tcx FxIndexMap<hir::HirId, ty::UpvarId>>,
577 successors: IndexVec<LiveNode, Option<LiveNode>>,
580 /// A live node representing a point of execution before closure entry &
581 /// after closure exit. Used to calculate liveness of captured variables
582 /// through calls to the same closure. Used for Fn & FnMut closures only.
583 closure_ln: LiveNode,
584 /// A live node representing every 'exit' from the function, whether it be
585 /// by explicit return, panic, or other means.
588 // mappings from loop node ID to LiveNode
589 // ("break" label should map to loop node ID,
590 // it probably doesn't now)
591 break_ln: HirIdMap<LiveNode>,
592 cont_ln: HirIdMap<LiveNode>,
595 impl<'a, 'tcx> Liveness<'a, 'tcx> {
596 fn new(ir: &'a mut IrMaps<'tcx>, body_owner: LocalDefId) -> Liveness<'a, 'tcx> {
597 let typeck_results = ir.tcx.typeck(body_owner);
598 let param_env = ir.tcx.param_env(body_owner);
599 let upvars = ir.tcx.upvars_mentioned(body_owner);
600 let closure_captures = typeck_results.closure_captures.get(&body_owner.to_def_id());
602 let closure_ln = ir.add_live_node(ClosureNode);
603 let exit_ln = ir.add_live_node(ExitNode);
605 let num_live_nodes = ir.lnks.len();
606 let num_vars = ir.var_kinds.len();
615 successors: IndexVec::from_elem_n(None, num_live_nodes),
616 rwu_table: RWUTable::new(num_live_nodes * num_vars),
619 break_ln: Default::default(),
620 cont_ln: Default::default(),
624 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
625 match self.ir.live_node_map.get(&hir_id) {
628 // This must be a mismatch between the ir_map construction
629 // above and the propagation code below; the two sets of
630 // code have to agree about which AST nodes are worth
631 // creating liveness nodes for.
632 span_bug!(span, "no live node registered for node {:?}", hir_id);
637 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
638 self.ir.variable(hir_id, span)
641 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
642 // In an or-pattern, only consider the first pattern; any later patterns
643 // must have the same bindings, and we also consider the first pattern
644 // to be the "authoritative" set of ids.
645 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
646 let ln = self.live_node(hir_id, pat_sp);
647 let var = self.variable(hir_id, ident.span);
648 self.init_from_succ(ln, succ);
649 self.define(ln, var);
655 fn idx(&self, ln: LiveNode, var: Variable) -> usize {
656 ln.index() * self.ir.var_kinds.len() + var.index()
659 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
660 if let Some(reader) = self.rwu_table.get_reader(self.idx(ln, var)) {
661 Some(self.ir.lnks[reader])
667 // Is this variable live on entry to any of its successor nodes?
668 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
669 let successor = self.successors[ln].unwrap();
670 self.live_on_entry(successor, var)
673 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
674 self.rwu_table.get_used(self.idx(ln, var))
677 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
678 if let Some(writer) = self.rwu_table.get_writer(self.idx(ln, var)) {
679 Some(self.ir.lnks[writer])
685 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> Option<LiveNodeKind> {
686 let successor = self.successors[ln].unwrap();
687 self.assigned_on_entry(successor, var)
690 fn indices2<F>(&mut self, ln: LiveNode, succ_ln: LiveNode, mut op: F)
692 F: FnMut(&mut Liveness<'a, 'tcx>, usize, usize),
694 let node_base_idx = self.idx(ln, Variable::from(0u32));
695 let succ_base_idx = self.idx(succ_ln, Variable::from(0u32));
696 for var_idx in 0..self.ir.var_kinds.len() {
697 op(self, node_base_idx + var_idx, succ_base_idx + var_idx);
701 fn write_vars<F>(&self, wr: &mut dyn Write, ln: LiveNode, mut test: F) -> io::Result<()>
703 F: FnMut(usize) -> bool,
705 let node_base_idx = self.idx(ln, Variable::from(0u32));
706 for var_idx in 0..self.ir.var_kinds.len() {
707 let idx = node_base_idx + var_idx;
709 write!(wr, " {:?}", Variable::from(var_idx))?;
715 #[allow(unused_must_use)]
716 fn ln_str(&self, ln: LiveNode) -> String {
717 let mut wr = Vec::new();
719 let wr = &mut wr as &mut dyn Write;
720 write!(wr, "[{:?} of kind {:?} reads", ln, self.ir.lnks[ln]);
721 self.write_vars(wr, ln, |idx| self.rwu_table.get_reader(idx).is_some());
722 write!(wr, " writes");
723 self.write_vars(wr, ln, |idx| self.rwu_table.get_writer(idx).is_some());
725 self.write_vars(wr, ln, |idx| self.rwu_table.get_used(idx));
727 write!(wr, " precedes {:?}]", self.successors[ln]);
729 String::from_utf8(wr).unwrap()
732 fn log_liveness(&self, entry_ln: LiveNode, hir_id: hir::HirId) {
733 // hack to skip the loop unless debug! is enabled:
735 "^^ liveness computation results for body {} (entry={:?})",
737 for ln_idx in 0..self.ir.lnks.len() {
738 debug!("{:?}", self.ln_str(LiveNode::from(ln_idx)));
746 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
747 self.successors[ln] = Some(succ_ln);
749 // It is not necessary to initialize the RWUs here because they are all
750 // set to INV_INV_FALSE when they are created, and the sets only grow
751 // during iterations.
754 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
755 // more efficient version of init_empty() / merge_from_succ()
756 self.successors[ln] = Some(succ_ln);
758 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
759 this.rwu_table.copy_packed(idx, succ_idx);
761 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
764 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode, first_merge: bool) -> bool {
769 let mut any_changed = false;
770 self.indices2(ln, succ_ln, |this, idx, succ_idx| {
771 // This is a special case, pulled out from the code below, where we
772 // don't have to do anything. It occurs about 60-70% of the time.
773 if this.rwu_table.packed_rwus[succ_idx] == INV_INV_FALSE {
777 let mut changed = false;
778 let mut rwu = this.rwu_table.get(idx);
779 let succ_rwu = this.rwu_table.get(succ_idx);
780 if succ_rwu.reader.is_some() && rwu.reader.is_none() {
781 rwu.reader = succ_rwu.reader;
785 if succ_rwu.writer.is_some() && rwu.writer.is_none() {
786 rwu.writer = succ_rwu.writer;
790 if succ_rwu.used && !rwu.used {
796 this.rwu_table.assign_unpacked(idx, rwu);
802 "merge_from_succ(ln={:?}, succ={}, first_merge={}, changed={})",
804 self.ln_str(succ_ln),
811 // Indicates that a local variable was *defined*; we know that no
812 // uses of the variable can precede the definition (resolve checks
813 // this) so we just clear out all the data.
814 fn define(&mut self, writer: LiveNode, var: Variable) {
815 let idx = self.idx(writer, var);
816 self.rwu_table.assign_inv_inv(idx);
818 debug!("{:?} defines {:?} (idx={}): {}", writer, var, idx, self.ln_str(writer));
821 // Either read, write, or both depending on the acc bitset
822 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
823 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
825 let idx = self.idx(ln, var);
826 let mut rwu = self.rwu_table.get(idx);
828 if (acc & ACC_WRITE) != 0 {
830 rwu.writer = Some(ln);
833 // Important: if we both read/write, must do read second
834 // or else the write will override.
835 if (acc & ACC_READ) != 0 {
836 rwu.reader = Some(ln);
839 if (acc & ACC_USE) != 0 {
843 self.rwu_table.assign_unpacked(idx, rwu);
846 fn compute(&mut self, body: &hir::Body<'_>, hir_id: HirId) -> LiveNode {
847 debug!("compute: for body {:?}", body.id().hir_id);
849 // # Liveness of captured variables
851 // When computing the liveness for captured variables we take into
852 // account how variable is captured (ByRef vs ByValue) and what is the
853 // closure kind (Generator / FnOnce vs Fn / FnMut).
855 // Variables captured by reference are assumed to be used on the exit
858 // In FnOnce closures, variables captured by value are known to be dead
859 // on exit since it is impossible to call the closure again.
861 // In Fn / FnMut closures, variables captured by value are live on exit
862 // if they are live on the entry to the closure, since only the closure
863 // itself can access them on subsequent calls.
865 if let Some(closure_captures) = self.closure_captures {
866 // Mark upvars captured by reference as used after closure exits.
867 // Since closure_captures is Some, upvars must exists too.
868 let upvars = self.upvars.unwrap();
869 for (&var_hir_id, upvar_id) in closure_captures {
870 let upvar = upvars[&var_hir_id];
871 match self.typeck_results.upvar_capture(*upvar_id) {
872 ty::UpvarCapture::ByRef(_) => {
873 let var = self.variable(var_hir_id, upvar.span);
874 self.acc(self.exit_ln, var, ACC_READ | ACC_USE);
876 ty::UpvarCapture::ByValue(_) => {}
881 let succ = self.propagate_through_expr(&body.value, self.exit_ln);
883 if self.closure_captures.is_none() {
884 // Either not a closure, or closure without any captured variables.
885 // No need to determine liveness of captured variables, since there
890 let ty = self.typeck_results.node_type(hir_id);
892 ty::Closure(_def_id, substs) => match substs.as_closure().kind() {
893 ty::ClosureKind::Fn => {}
894 ty::ClosureKind::FnMut => {}
895 ty::ClosureKind::FnOnce => return succ,
897 ty::Generator(..) => return succ,
901 "{} has upvars so it should have a closure type: {:?}",
908 // Propagate through calls to the closure.
909 let mut first_merge = true;
911 self.init_from_succ(self.closure_ln, succ);
912 for param in body.params {
913 param.pat.each_binding(|_bm, hir_id, _x, ident| {
914 let var = self.variable(hir_id, ident.span);
915 self.define(self.closure_ln, var);
919 if !self.merge_from_succ(self.exit_ln, self.closure_ln, first_merge) {
923 assert_eq!(succ, self.propagate_through_expr(&body.value, self.exit_ln));
929 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
930 if blk.targeted_by_break {
931 self.break_ln.insert(blk.hir_id, succ);
933 let succ = self.propagate_through_opt_expr(blk.expr.as_deref(), succ);
934 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
937 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
939 hir::StmtKind::Local(ref local) => {
940 // Note: we mark the variable as defined regardless of whether
941 // there is an initializer. Initially I had thought to only mark
942 // the live variable as defined if it was initialized, and then we
943 // could check for uninit variables just by scanning what is live
944 // at the start of the function. But that doesn't work so well for
945 // immutable variables defined in a loop:
946 // loop { let x; x = 5; }
947 // because the "assignment" loops back around and generates an error.
949 // So now we just check that variables defined w/o an
950 // initializer are not live at the point of their
951 // initialization, which is mildly more complex than checking
952 // once at the func header but otherwise equivalent.
954 let succ = self.propagate_through_opt_expr(local.init.as_deref(), succ);
955 self.define_bindings_in_pat(&local.pat, succ)
957 hir::StmtKind::Item(..) => succ,
958 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
959 self.propagate_through_expr(&expr, succ)
964 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
965 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
968 fn propagate_through_opt_expr(
970 opt_expr: Option<&Expr<'_>>,
973 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
976 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
977 debug!("propagate_through_expr: {:?}", expr);
980 // Interesting cases with control flow or which gen/kill
981 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
982 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
985 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
987 hir::ExprKind::Closure(..) => {
988 debug!("{:?} is an ExprKind::Closure", expr);
990 // the construction of a closure itself is not important,
991 // but we have to consider the closed over variables.
997 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
999 caps.iter().rev().fold(succ, |succ, cap| {
1000 self.init_from_succ(cap.ln, succ);
1001 let var = self.variable(cap.var_hid, expr.span);
1002 self.acc(cap.ln, var, ACC_READ | ACC_USE);
1007 // Note that labels have been resolved, so we don't need to look
1008 // at the label ident
1009 hir::ExprKind::Loop(ref blk, _, _) => self.propagate_through_loop(expr, &blk, succ),
1011 hir::ExprKind::Match(ref e, arms, _) => {
1026 let ln = self.live_node(expr.hir_id, expr.span);
1027 self.init_empty(ln, succ);
1028 let mut first_merge = true;
1030 let body_succ = self.propagate_through_expr(&arm.body, succ);
1032 let guard_succ = self.propagate_through_opt_expr(
1033 arm.guard.as_ref().map(|hir::Guard::If(e)| *e),
1036 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
1037 self.merge_from_succ(ln, arm_succ, first_merge);
1038 first_merge = false;
1040 self.propagate_through_expr(&e, ln)
1043 hir::ExprKind::Ret(ref o_e) => {
1044 // Ignore succ and subst exit_ln.
1045 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), self.exit_ln)
1048 hir::ExprKind::Break(label, ref opt_expr) => {
1049 // Find which label this break jumps to
1050 let target = match label.target_id {
1051 Ok(hir_id) => self.break_ln.get(&hir_id),
1052 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
1056 // Now that we know the label we're going to,
1057 // look it up in the break loop nodes table
1060 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
1061 None => span_bug!(expr.span, "`break` to unknown label"),
1065 hir::ExprKind::Continue(label) => {
1066 // Find which label this expr continues to
1069 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
1071 // Now that we know the label we're going to,
1072 // look it up in the continue loop nodes table
1076 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
1079 hir::ExprKind::Assign(ref l, ref r, _) => {
1080 // see comment on places in
1081 // propagate_through_place_components()
1082 let succ = self.write_place(&l, succ, ACC_WRITE);
1083 let succ = self.propagate_through_place_components(&l, succ);
1084 self.propagate_through_expr(&r, succ)
1087 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1088 // an overloaded assign op is like a method call
1089 if self.typeck_results.is_method_call(expr) {
1090 let succ = self.propagate_through_expr(&l, succ);
1091 self.propagate_through_expr(&r, succ)
1093 // see comment on places in
1094 // propagate_through_place_components()
1095 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
1096 let succ = self.propagate_through_expr(&r, succ);
1097 self.propagate_through_place_components(&l, succ)
1101 // Uninteresting cases: just propagate in rev exec order
1102 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
1104 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1105 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1109 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
1112 hir::ExprKind::Call(ref f, ref args) => {
1113 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1114 let succ = if self.ir.tcx.is_ty_uninhabited_from(
1116 self.typeck_results.expr_ty(expr),
1123 let succ = self.propagate_through_exprs(args, succ);
1124 self.propagate_through_expr(&f, succ)
1127 hir::ExprKind::MethodCall(.., ref args, _) => {
1128 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1129 let succ = if self.ir.tcx.is_ty_uninhabited_from(
1131 self.typeck_results.expr_ty(expr),
1139 self.propagate_through_exprs(args, succ)
1142 hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ),
1144 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1145 let r_succ = self.propagate_through_expr(&r, succ);
1147 let ln = self.live_node(expr.hir_id, expr.span);
1148 self.init_from_succ(ln, succ);
1149 self.merge_from_succ(ln, r_succ, false);
1151 self.propagate_through_expr(&l, ln)
1154 hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => {
1155 let r_succ = self.propagate_through_expr(&r, succ);
1156 self.propagate_through_expr(&l, r_succ)
1159 hir::ExprKind::Box(ref e)
1160 | hir::ExprKind::AddrOf(_, _, ref e)
1161 | hir::ExprKind::Cast(ref e, _)
1162 | hir::ExprKind::Type(ref e, _)
1163 | hir::ExprKind::DropTemps(ref e)
1164 | hir::ExprKind::Unary(_, ref e)
1165 | hir::ExprKind::Yield(ref e, _)
1166 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1168 hir::ExprKind::InlineAsm(ref asm) => {
1169 // Handle non-returning asm
1170 let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) {
1176 // Do a first pass for writing outputs only
1177 for (op, _op_sp) in asm.operands.iter().rev() {
1179 hir::InlineAsmOperand::In { .. }
1180 | hir::InlineAsmOperand::Const { .. }
1181 | hir::InlineAsmOperand::Sym { .. } => {}
1182 hir::InlineAsmOperand::Out { expr, .. } => {
1183 if let Some(expr) = expr {
1184 succ = self.write_place(expr, succ, ACC_WRITE);
1187 hir::InlineAsmOperand::InOut { expr, .. } => {
1188 succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE | ACC_USE);
1190 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1191 if let Some(expr) = out_expr {
1192 succ = self.write_place(expr, succ, ACC_WRITE);
1198 // Then do a second pass for inputs
1199 let mut succ = succ;
1200 for (op, _op_sp) in asm.operands.iter().rev() {
1202 hir::InlineAsmOperand::In { expr, .. }
1203 | hir::InlineAsmOperand::Const { expr, .. }
1204 | hir::InlineAsmOperand::Sym { expr, .. } => {
1205 succ = self.propagate_through_expr(expr, succ)
1207 hir::InlineAsmOperand::Out { expr, .. } => {
1208 if let Some(expr) = expr {
1209 succ = self.propagate_through_place_components(expr, succ);
1212 hir::InlineAsmOperand::InOut { expr, .. } => {
1213 succ = self.propagate_through_place_components(expr, succ);
1215 hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
1216 if let Some(expr) = out_expr {
1217 succ = self.propagate_through_place_components(expr, succ);
1219 succ = self.propagate_through_expr(in_expr, succ);
1226 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1227 let ia = &asm.inner;
1228 let outputs = asm.outputs_exprs;
1229 let inputs = asm.inputs_exprs;
1230 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1231 // see comment on places
1232 // in propagate_through_place_components()
1234 self.propagate_through_expr(output, succ)
1236 let acc = if o.is_rw { ACC_WRITE | ACC_READ } else { ACC_WRITE };
1237 let succ = self.write_place(output, succ, acc);
1238 self.propagate_through_place_components(output, succ)
1242 // Inputs are executed first. Propagate last because of rev order
1243 self.propagate_through_exprs(inputs, succ)
1246 hir::ExprKind::Lit(..)
1247 | hir::ExprKind::ConstBlock(..)
1248 | hir::ExprKind::Err
1249 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
1250 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => succ,
1252 // Note that labels have been resolved, so we don't need to look
1253 // at the label ident
1254 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1258 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1261 // In general, the full flow graph structure for an
1262 // assignment/move/etc can be handled in one of two ways,
1263 // depending on whether what is being assigned is a "tracked
1264 // value" or not. A tracked value is basically a local
1265 // variable or argument.
1267 // The two kinds of graphs are:
1269 // Tracked place Untracked place
1270 // ----------------------++-----------------------
1274 // (rvalue) || (rvalue)
1277 // (write of place) || (place components)
1282 // ----------------------++-----------------------
1284 // I will cover the two cases in turn:
1288 // A tracked place is a local variable/argument `x`. In
1289 // these cases, the link_node where the write occurs is linked
1290 // to node id of `x`. The `write_place()` routine generates
1291 // the contents of this node. There are no subcomponents to
1294 // # Non-tracked places
1296 // These are places like `x[5]` or `x.f`. In that case, we
1297 // basically ignore the value which is written to but generate
1298 // reads for the components---`x` in these two examples. The
1299 // components reads are generated by
1300 // `propagate_through_place_components()` (this fn).
1304 // It is still possible to observe assignments to non-places;
1305 // these errors are detected in the later pass borrowck. We
1306 // just ignore such cases and treat them as reads.
1309 hir::ExprKind::Path(_) => succ,
1310 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1311 _ => self.propagate_through_expr(expr, succ),
1315 // see comment on propagate_through_place()
1316 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1318 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1319 self.access_path(expr.hir_id, path, succ, acc)
1322 // We do not track other places, so just propagate through
1323 // to their subcomponents. Also, it may happen that
1324 // non-places occur here, because those are detected in the
1325 // later pass borrowck.
1338 let ln = self.live_node(hir_id, span);
1340 self.init_from_succ(ln, succ);
1341 let var = self.variable(var_hid, span);
1342 self.acc(ln, var, acc);
1350 path: &hir::Path<'_>,
1355 Res::Local(hid) => {
1356 let in_upvars = self.upvars.map_or(false, |u| u.contains_key(&hid));
1357 let in_captures = self.closure_captures.map_or(false, |c| c.contains_key(&hid));
1359 match (in_upvars, in_captures) {
1360 (false, _) | (true, true) => self.access_var(hir_id, hid, succ, acc, path.span),
1362 // This case is possible when with RFC-2229, a wild pattern
1363 // is used within a closure.
1364 // eg: `let _ = x`. The closure doesn't capture x here,
1365 // even though it's mentioned in the closure.
1374 fn propagate_through_loop(
1377 body: &hir::Block<'_>,
1381 We model control flow like this:
1388 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1389 Meanwhile, a `break` expression will have a successor of `succ`.
1393 let mut first_merge = true;
1394 let ln = self.live_node(expr.hir_id, expr.span);
1395 self.init_empty(ln, succ);
1396 debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body);
1398 self.break_ln.insert(expr.hir_id, succ);
1400 self.cont_ln.insert(expr.hir_id, ln);
1402 let body_ln = self.propagate_through_block(body, ln);
1404 // repeat until fixed point is reached:
1405 while self.merge_from_succ(ln, body_ln, first_merge) {
1406 first_merge = false;
1407 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1414 // _______________________________________________________________________
1415 // Checking for error conditions
1417 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1418 type Map = intravisit::ErasedMap<'tcx>;
1420 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
1421 NestedVisitorMap::None
1424 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1425 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1426 if local.init.is_some() {
1427 self.warn_about_dead_assign(spans, hir_id, ln, var);
1431 intravisit::walk_local(self, local);
1434 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1435 check_expr(self, ex);
1438 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1439 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1440 intravisit::walk_arm(self, arm);
1444 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1446 hir::ExprKind::Assign(ref l, ..) => {
1447 this.check_place(&l);
1450 hir::ExprKind::AssignOp(_, ref l, _) => {
1451 if !this.typeck_results.is_method_call(expr) {
1452 this.check_place(&l);
1456 hir::ExprKind::InlineAsm(ref asm) => {
1457 for (op, _op_sp) in asm.operands {
1459 hir::InlineAsmOperand::Out { expr, .. } => {
1460 if let Some(expr) = expr {
1461 this.check_place(expr);
1464 hir::InlineAsmOperand::InOut { expr, .. } => {
1465 this.check_place(expr);
1467 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1468 if let Some(out_expr) = out_expr {
1469 this.check_place(out_expr);
1477 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1478 for input in asm.inputs_exprs {
1479 this.visit_expr(input);
1482 // Output operands must be places
1483 for (o, output) in asm.inner.outputs.iter().zip(asm.outputs_exprs) {
1485 this.check_place(output);
1487 this.visit_expr(output);
1491 // no correctness conditions related to liveness
1492 hir::ExprKind::Call(..)
1493 | hir::ExprKind::MethodCall(..)
1494 | hir::ExprKind::Match(..)
1495 | hir::ExprKind::Loop(..)
1496 | hir::ExprKind::Index(..)
1497 | hir::ExprKind::Field(..)
1498 | hir::ExprKind::Array(..)
1499 | hir::ExprKind::Tup(..)
1500 | hir::ExprKind::Binary(..)
1501 | hir::ExprKind::Cast(..)
1502 | hir::ExprKind::DropTemps(..)
1503 | hir::ExprKind::Unary(..)
1504 | hir::ExprKind::Ret(..)
1505 | hir::ExprKind::Break(..)
1506 | hir::ExprKind::Continue(..)
1507 | hir::ExprKind::Lit(_)
1508 | hir::ExprKind::ConstBlock(..)
1509 | hir::ExprKind::Block(..)
1510 | hir::ExprKind::AddrOf(..)
1511 | hir::ExprKind::Struct(..)
1512 | hir::ExprKind::Repeat(..)
1513 | hir::ExprKind::Closure(..)
1514 | hir::ExprKind::Path(_)
1515 | hir::ExprKind::Yield(..)
1516 | hir::ExprKind::Box(..)
1517 | hir::ExprKind::Type(..)
1518 | hir::ExprKind::Err => {}
1521 intravisit::walk_expr(this, expr);
1524 impl<'tcx> Liveness<'_, 'tcx> {
1525 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1527 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1528 if let Res::Local(var_hid) = path.res {
1529 // Assignment to an immutable variable or argument: only legal
1530 // if there is no later assignment. If this local is actually
1531 // mutable, then check for a reassignment to flag the mutability
1533 let ln = self.live_node(expr.hir_id, expr.span);
1534 let var = self.variable(var_hid, expr.span);
1535 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1539 // For other kinds of places, no checks are required,
1540 // and any embedded expressions are actually rvalues
1541 intravisit::walk_expr(self, expr);
1546 fn should_warn(&self, var: Variable) -> Option<String> {
1547 let name = self.ir.variable_name(var);
1548 if name == kw::Invalid {
1551 let name: &str = &name.as_str();
1552 if name.as_bytes()[0] == b'_' {
1555 Some(name.to_owned())
1558 fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
1559 let closure_captures = match self.closure_captures {
1561 Some(closure_captures) => closure_captures,
1564 // If closure_captures is Some(), upvars must be Some() too.
1565 let upvars = self.upvars.unwrap();
1566 for &var_hir_id in closure_captures.keys() {
1567 let upvar = upvars[&var_hir_id];
1568 let var = self.variable(var_hir_id, upvar.span);
1569 let upvar_id = ty::UpvarId {
1570 var_path: ty::UpvarPath { hir_id: var_hir_id },
1571 closure_expr_id: self.body_owner,
1573 match self.typeck_results.upvar_capture(upvar_id) {
1574 ty::UpvarCapture::ByValue(_) => {}
1575 ty::UpvarCapture::ByRef(..) => continue,
1577 if self.used_on_entry(entry_ln, var) {
1578 if self.live_on_entry(entry_ln, var).is_none() {
1579 if let Some(name) = self.should_warn(var) {
1580 self.ir.tcx.struct_span_lint_hir(
1581 lint::builtin::UNUSED_ASSIGNMENTS,
1585 lint.build(&format!("value captured by `{}` is never read", name))
1586 .help("did you mean to capture by reference instead?")
1593 if let Some(name) = self.should_warn(var) {
1594 self.ir.tcx.struct_span_lint_hir(
1595 lint::builtin::UNUSED_VARIABLES,
1599 lint.build(&format!("unused variable: `{}`", name))
1600 .help("did you mean to capture by reference instead?")
1609 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1610 for p in body.params {
1611 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1612 if self.live_on_entry(ln, var).is_none() {
1613 self.report_unsed_assign(hir_id, spans, var, |name| {
1614 format!("value passed to `{}` is never read", name)
1621 fn check_unused_vars_in_pat(
1624 entry_ln: Option<LiveNode>,
1625 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1627 // In an or-pattern, only consider the variable; any later patterns must have the same
1628 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1629 // However, we should take the ids and spans of variables with the same name from the later
1630 // patterns so the suggestions to prefix with underscores will apply to those too.
1631 let mut vars: FxIndexMap<Symbol, (LiveNode, Variable, Vec<(HirId, Span)>)> = <_>::default();
1633 pat.each_binding(|_, hir_id, pat_sp, ident| {
1634 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1635 let var = self.variable(hir_id, ident.span);
1636 let id_and_sp = (hir_id, pat_sp);
1637 vars.entry(self.ir.variable_name(var))
1638 .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp))
1639 .or_insert_with(|| (ln, var, vec![id_and_sp]));
1642 for (_, (ln, var, hir_ids_and_spans)) in vars {
1643 if self.used_on_entry(ln, var) {
1644 let id = hir_ids_and_spans[0].0;
1645 let spans = hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect();
1646 on_used_on_entry(spans, id, ln, var);
1648 self.report_unused(hir_ids_and_spans, ln, var);
1653 fn report_unused(&self, hir_ids_and_spans: Vec<(HirId, Span)>, ln: LiveNode, var: Variable) {
1654 let first_hir_id = hir_ids_and_spans[0].0;
1656 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1657 // annoying: for parameters in funcs like `fn(x: i32)
1658 // {ret}`, there is only one node, so asking about
1659 // assigned_on_exit() is not meaningful.
1661 if ln == self.exit_ln { false } else { self.assigned_on_exit(ln, var).is_some() };
1664 self.ir.tcx.struct_span_lint_hir(
1665 lint::builtin::UNUSED_VARIABLES,
1667 hir_ids_and_spans.into_iter().map(|(_, sp)| sp).collect::<Vec<_>>(),
1669 lint.build(&format!("variable `{}` is assigned to, but never used", name))
1670 .note(&format!("consider using `_{}` instead", name))
1675 self.ir.tcx.struct_span_lint_hir(
1676 lint::builtin::UNUSED_VARIABLES,
1678 hir_ids_and_spans.iter().map(|(_, sp)| *sp).collect::<Vec<_>>(),
1680 let mut err = lint.build(&format!("unused variable: `{}`", name));
1682 let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
1683 hir_ids_and_spans.into_iter().partition(|(hir_id, span)| {
1684 let var = self.variable(*hir_id, *span);
1685 self.ir.variable_is_shorthand(var)
1688 let mut shorthands = shorthands
1690 .map(|(_, span)| (span, format!("{}: _", name)))
1691 .collect::<Vec<_>>();
1693 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1694 // the field" message, and suggest `_` for the non-shorthands. If we only
1695 // have non-shorthand, then prefix with an underscore instead.
1696 if !shorthands.is_empty() {
1700 .map(|(_, span)| (span, "_".to_string()))
1701 .collect::<Vec<_>>(),
1704 err.multipart_suggestion(
1705 "try ignoring the field",
1707 Applicability::MachineApplicable,
1710 err.multipart_suggestion(
1711 "if this is intentional, prefix it with an underscore",
1714 .map(|(_, span)| (span, format!("_{}", name)))
1715 .collect::<Vec<_>>(),
1716 Applicability::MachineApplicable,
1727 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1728 if self.live_on_exit(ln, var).is_none() {
1729 self.report_unsed_assign(hir_id, spans, var, |name| {
1730 format!("value assigned to `{}` is never read", name)
1735 fn report_unsed_assign(
1740 message: impl Fn(&str) -> String,
1742 if let Some(name) = self.should_warn(var) {
1743 self.ir.tcx.struct_span_lint_hir(
1744 lint::builtin::UNUSED_ASSIGNMENTS,
1748 lint.build(&message(&name))
1749 .help("maybe it is overwritten before being read?")