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, Visitor};
94 use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet};
95 use rustc_index::vec::IndexVec;
96 use rustc_middle::hir::nested_filter;
97 use rustc_middle::ty::query::Providers;
98 use rustc_middle::ty::{self, DefIdTree, RootVariableMinCaptureList, Ty, 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::*;
110 rustc_index::newtype_index! {
111 pub struct Variable {
112 DEBUG_FORMAT = "v({})",
116 rustc_index::newtype_index! {
117 pub struct LiveNode {
118 DEBUG_FORMAT = "ln({})",
122 #[derive(Copy, Clone, PartialEq, Debug)]
125 ExprNode(Span, HirId),
126 VarDefNode(Span, HirId),
131 fn live_node_kind_to_string(lnk: LiveNodeKind, tcx: TyCtxt<'_>) -> String {
132 let sm = tcx.sess.source_map();
134 UpvarNode(s) => format!("Upvar node [{}]", sm.span_to_diagnostic_string(s)),
135 ExprNode(s, _) => format!("Expr node [{}]", sm.span_to_diagnostic_string(s)),
136 VarDefNode(s, _) => format!("Var def node [{}]", sm.span_to_diagnostic_string(s)),
137 ClosureNode => "Closure node".to_owned(),
138 ExitNode => "Exit node".to_owned(),
142 fn check_mod_liveness(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
143 tcx.hir().visit_item_likes_in_module(module_def_id, &mut IrMaps::new(tcx).as_deep_visitor());
146 pub fn provide(providers: &mut Providers) {
147 *providers = Providers { check_mod_liveness, ..*providers };
150 // ______________________________________________________________________
153 // This is the first pass and the one that drives the main
154 // computation. It walks up and down the IR once. On the way down,
155 // we count for each function the number of variables as well as
156 // liveness nodes. A liveness node is basically an expression or
157 // capture clause that does something of interest: either it has
158 // interesting control flow or it uses/defines a local variable.
160 // On the way back up, at each function node we create liveness sets
161 // (we now know precisely how big to make our various vectors and so
162 // forth) and then do the data-flow propagation to compute the set
163 // of live variables at each program point.
165 // Finally, we run back over the IR one last time and, using the
166 // computed liveness, check various safety conditions. For example,
167 // there must be no live nodes at the definition site for a variable
168 // unless it has an initializer. Similarly, each non-mutable local
169 // variable must not be assigned if there is some successor
170 // assignment. And so forth.
177 #[derive(Copy, Clone, Debug)]
184 #[derive(Copy, Clone, Debug)]
186 Param(HirId, Symbol),
188 Upvar(HirId, Symbol),
191 struct IrMaps<'tcx> {
193 live_node_map: HirIdMap<LiveNode>,
194 variable_map: HirIdMap<Variable>,
195 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
196 var_kinds: IndexVec<Variable, VarKind>,
197 lnks: IndexVec<LiveNode, LiveNodeKind>,
200 impl<'tcx> IrMaps<'tcx> {
201 fn new(tcx: TyCtxt<'tcx>) -> IrMaps<'tcx> {
204 live_node_map: HirIdMap::default(),
205 variable_map: HirIdMap::default(),
206 capture_info_map: Default::default(),
207 var_kinds: IndexVec::new(),
208 lnks: IndexVec::new(),
212 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
213 let ln = self.lnks.push(lnk);
215 debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx));
220 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
221 let ln = self.add_live_node(lnk);
222 self.live_node_map.insert(hir_id, ln);
224 debug!("{:?} is node {:?}", ln, hir_id);
227 fn add_variable(&mut self, vk: VarKind) -> Variable {
228 let v = self.var_kinds.push(vk);
231 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) | Upvar(node_id, _) => {
232 self.variable_map.insert(node_id, v);
236 debug!("{:?} is {:?}", v, vk);
241 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
242 match self.variable_map.get(&hir_id) {
245 span_bug!(span, "no variable registered for id {:?}", hir_id);
250 fn variable_name(&self, var: Variable) -> Symbol {
251 match self.var_kinds[var] {
252 Local(LocalInfo { name, .. }) | Param(_, name) | Upvar(_, name) => name,
256 fn variable_is_shorthand(&self, var: Variable) -> bool {
257 match self.var_kinds[var] {
258 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
259 Param(..) | Upvar(..) => false,
263 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
264 self.capture_info_map.insert(hir_id, Rc::new(cs));
267 fn collect_shorthand_field_ids(&self, pat: &hir::Pat<'tcx>) -> HirIdSet {
268 // For struct patterns, take note of which fields used shorthand
269 // (`x` rather than `x: x`).
270 let mut shorthand_field_ids = HirIdSet::default();
271 let mut pats = VecDeque::new();
274 while let Some(pat) = pats.pop_front() {
275 use rustc_hir::PatKind::*;
277 Binding(.., inner_pat) => {
278 pats.extend(inner_pat.iter());
280 Struct(_, fields, _) => {
281 let (short, not_short): (Vec<&_>, Vec<&_>) =
282 fields.iter().partition(|f| f.is_shorthand);
283 shorthand_field_ids.extend(short.iter().map(|f| f.pat.hir_id));
284 pats.extend(not_short.iter().map(|f| f.pat));
286 Ref(inner_pat, _) | Box(inner_pat) => {
287 pats.push_back(inner_pat);
289 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
290 pats.extend(inner_pats.iter());
292 Slice(pre_pats, inner_pat, post_pats) => {
293 pats.extend(pre_pats.iter());
294 pats.extend(inner_pat.iter());
295 pats.extend(post_pats.iter());
301 return shorthand_field_ids;
304 fn add_from_pat(&mut self, pat: &hir::Pat<'tcx>) {
305 let shorthand_field_ids = self.collect_shorthand_field_ids(pat);
307 pat.each_binding(|_, hir_id, _, ident| {
308 self.add_live_node_for_node(hir_id, VarDefNode(ident.span, hir_id));
309 self.add_variable(Local(LocalInfo {
312 is_shorthand: shorthand_field_ids.contains(&hir_id),
318 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
319 type NestedFilter = nested_filter::OnlyBodies;
321 fn nested_visit_map(&mut self) -> Self::Map {
325 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
326 debug!("visit_body {:?}", body.id());
328 // swap in a new set of IR maps for this body
329 let mut maps = IrMaps::new(self.tcx);
330 let hir_id = maps.tcx.hir().body_owner(body.id());
331 let local_def_id = maps.tcx.hir().local_def_id(hir_id);
332 let def_id = local_def_id.to_def_id();
334 // Don't run unused pass for #[derive()]
335 if let Some(parent) = self.tcx.parent(def_id)
336 && let DefKind::Impl = self.tcx.def_kind(parent.expect_local())
337 && self.tcx.has_attr(parent, sym::automatically_derived)
342 // Don't run unused pass for #[naked]
343 if self.tcx.has_attr(def_id, sym::naked) {
347 if let Some(upvars) = maps.tcx.upvars_mentioned(def_id) {
348 for &var_hir_id in upvars.keys() {
349 let var_name = maps.tcx.hir().name(var_hir_id);
350 maps.add_variable(Upvar(var_hir_id, var_name));
354 // gather up the various local variables, significant expressions,
356 intravisit::walk_body(&mut maps, body);
359 let mut lsets = Liveness::new(&mut maps, local_def_id);
360 let entry_ln = lsets.compute(&body, hir_id);
361 lsets.log_liveness(entry_ln, body.id().hir_id);
363 // check for various error conditions
364 lsets.visit_body(body);
365 lsets.warn_about_unused_upvars(entry_ln);
366 lsets.warn_about_unused_args(body, entry_ln);
369 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
370 self.add_from_pat(&local.pat);
371 intravisit::walk_local(self, local);
374 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
375 self.add_from_pat(&arm.pat);
376 if let Some(hir::Guard::IfLet(ref pat, _)) = arm.guard {
377 self.add_from_pat(pat);
379 intravisit::walk_arm(self, arm);
382 fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
383 let shorthand_field_ids = self.collect_shorthand_field_ids(param.pat);
384 param.pat.each_binding(|_bm, hir_id, _x, ident| {
385 let var = match param.pat.kind {
386 rustc_hir::PatKind::Struct(..) => Local(LocalInfo {
389 is_shorthand: shorthand_field_ids.contains(&hir_id),
391 _ => Param(hir_id, ident.name),
393 self.add_variable(var);
395 intravisit::walk_param(self, param);
398 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
400 // live nodes required for uses or definitions of variables:
401 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
402 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
403 if let Res::Local(_var_hir_id) = path.res {
404 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
406 intravisit::walk_expr(self, expr);
408 hir::ExprKind::Closure(..) => {
409 // Interesting control flow (for loops can contain labeled
410 // breaks or continues)
411 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
413 // Make a live_node for each mentioned variable, with the span
414 // being the location that the variable is used. This results
415 // in better error messages than just pointing at the closure
416 // construction site.
417 let mut call_caps = Vec::new();
418 let closure_def_id = self.tcx.hir().local_def_id(expr.hir_id);
419 if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
420 call_caps.extend(upvars.keys().map(|var_id| {
421 let upvar = upvars[var_id];
422 let upvar_ln = self.add_live_node(UpvarNode(upvar.span));
423 CaptureInfo { ln: upvar_ln, var_hid: *var_id }
426 self.set_captures(expr.hir_id, call_caps);
427 intravisit::walk_expr(self, expr);
430 hir::ExprKind::Let(let_expr) => {
431 self.add_from_pat(let_expr.pat);
432 intravisit::walk_expr(self, expr);
435 // live nodes required for interesting control flow:
436 hir::ExprKind::If(..)
437 | hir::ExprKind::Match(..)
438 | hir::ExprKind::Loop(..)
439 | hir::ExprKind::Yield(..) => {
440 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
441 intravisit::walk_expr(self, expr);
443 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
444 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
445 intravisit::walk_expr(self, expr);
448 // otherwise, live nodes are not required:
449 hir::ExprKind::Index(..)
450 | hir::ExprKind::Field(..)
451 | hir::ExprKind::Array(..)
452 | hir::ExprKind::Call(..)
453 | hir::ExprKind::MethodCall(..)
454 | hir::ExprKind::Tup(..)
455 | hir::ExprKind::Binary(..)
456 | hir::ExprKind::AddrOf(..)
457 | hir::ExprKind::Cast(..)
458 | hir::ExprKind::DropTemps(..)
459 | hir::ExprKind::Unary(..)
460 | hir::ExprKind::Break(..)
461 | hir::ExprKind::Continue(_)
462 | hir::ExprKind::Lit(_)
463 | hir::ExprKind::ConstBlock(..)
464 | hir::ExprKind::Ret(..)
465 | hir::ExprKind::Block(..)
466 | hir::ExprKind::Assign(..)
467 | hir::ExprKind::AssignOp(..)
468 | hir::ExprKind::Struct(..)
469 | hir::ExprKind::Repeat(..)
470 | hir::ExprKind::InlineAsm(..)
471 | hir::ExprKind::Box(..)
472 | hir::ExprKind::Type(..)
474 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
475 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
476 intravisit::walk_expr(self, expr);
482 // ______________________________________________________________________
483 // Computing liveness sets
485 // Actually we compute just a bit more than just liveness, but we use
486 // the same basic propagation framework in all cases.
488 const ACC_READ: u32 = 1;
489 const ACC_WRITE: u32 = 2;
490 const ACC_USE: u32 = 4;
492 struct Liveness<'a, 'tcx> {
493 ir: &'a mut IrMaps<'tcx>,
494 typeck_results: &'a ty::TypeckResults<'tcx>,
495 param_env: ty::ParamEnv<'tcx>,
496 closure_min_captures: Option<&'tcx RootVariableMinCaptureList<'tcx>>,
497 successors: IndexVec<LiveNode, Option<LiveNode>>,
498 rwu_table: rwu_table::RWUTable,
500 /// A live node representing a point of execution before closure entry &
501 /// after closure exit. Used to calculate liveness of captured variables
502 /// through calls to the same closure. Used for Fn & FnMut closures only.
503 closure_ln: LiveNode,
504 /// A live node representing every 'exit' from the function, whether it be
505 /// by explicit return, panic, or other means.
508 // mappings from loop node ID to LiveNode
509 // ("break" label should map to loop node ID,
510 // it probably doesn't now)
511 break_ln: HirIdMap<LiveNode>,
512 cont_ln: HirIdMap<LiveNode>,
515 impl<'a, 'tcx> Liveness<'a, 'tcx> {
516 fn new(ir: &'a mut IrMaps<'tcx>, body_owner: LocalDefId) -> Liveness<'a, 'tcx> {
517 let typeck_results = ir.tcx.typeck(body_owner);
518 let param_env = ir.tcx.param_env(body_owner);
519 let closure_min_captures = typeck_results.closure_min_captures.get(&body_owner.to_def_id());
520 let closure_ln = ir.add_live_node(ClosureNode);
521 let exit_ln = ir.add_live_node(ExitNode);
523 let num_live_nodes = ir.lnks.len();
524 let num_vars = ir.var_kinds.len();
530 closure_min_captures,
531 successors: IndexVec::from_elem_n(None, num_live_nodes),
532 rwu_table: rwu_table::RWUTable::new(num_live_nodes, num_vars),
535 break_ln: Default::default(),
536 cont_ln: Default::default(),
540 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
541 match self.ir.live_node_map.get(&hir_id) {
544 // This must be a mismatch between the ir_map construction
545 // above and the propagation code below; the two sets of
546 // code have to agree about which AST nodes are worth
547 // creating liveness nodes for.
548 span_bug!(span, "no live node registered for node {:?}", hir_id);
553 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
554 self.ir.variable(hir_id, span)
557 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
558 // In an or-pattern, only consider the first pattern; any later patterns
559 // must have the same bindings, and we also consider the first pattern
560 // to be the "authoritative" set of ids.
561 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
562 let ln = self.live_node(hir_id, pat_sp);
563 let var = self.variable(hir_id, ident.span);
564 self.init_from_succ(ln, succ);
565 self.define(ln, var);
571 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
572 self.rwu_table.get_reader(ln, var)
575 // Is this variable live on entry to any of its successor nodes?
576 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> bool {
577 let successor = self.successors[ln].unwrap();
578 self.live_on_entry(successor, var)
581 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
582 self.rwu_table.get_used(ln, var)
585 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
586 self.rwu_table.get_writer(ln, var)
589 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> bool {
590 let successor = self.successors[ln].unwrap();
591 self.assigned_on_entry(successor, var)
594 fn write_vars<F>(&self, wr: &mut dyn Write, mut test: F) -> io::Result<()>
596 F: FnMut(Variable) -> bool,
598 for var_idx in 0..self.ir.var_kinds.len() {
599 let var = Variable::from(var_idx);
601 write!(wr, " {:?}", var)?;
607 #[allow(unused_must_use)]
608 fn ln_str(&self, ln: LiveNode) -> String {
609 let mut wr = Vec::new();
611 let wr = &mut wr as &mut dyn Write;
612 write!(wr, "[{:?} of kind {:?} reads", ln, self.ir.lnks[ln]);
613 self.write_vars(wr, |var| self.rwu_table.get_reader(ln, var));
614 write!(wr, " writes");
615 self.write_vars(wr, |var| self.rwu_table.get_writer(ln, var));
617 self.write_vars(wr, |var| self.rwu_table.get_used(ln, var));
619 write!(wr, " precedes {:?}]", self.successors[ln]);
621 String::from_utf8(wr).unwrap()
624 fn log_liveness(&self, entry_ln: LiveNode, hir_id: hir::HirId) {
625 // hack to skip the loop unless debug! is enabled:
627 "^^ liveness computation results for body {} (entry={:?})",
629 for ln_idx in 0..self.ir.lnks.len() {
630 debug!("{:?}", self.ln_str(LiveNode::from(ln_idx)));
638 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
639 self.successors[ln] = Some(succ_ln);
641 // It is not necessary to initialize the RWUs here because they are all
642 // empty when created, and the sets only grow during iterations.
645 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
646 // more efficient version of init_empty() / merge_from_succ()
647 self.successors[ln] = Some(succ_ln);
648 self.rwu_table.copy(ln, succ_ln);
649 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
652 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) -> bool {
657 let changed = self.rwu_table.union(ln, succ_ln);
658 debug!("merge_from_succ(ln={:?}, succ={}, changed={})", ln, self.ln_str(succ_ln), changed);
662 // Indicates that a local variable was *defined*; we know that no
663 // uses of the variable can precede the definition (resolve checks
664 // this) so we just clear out all the data.
665 fn define(&mut self, writer: LiveNode, var: Variable) {
666 let used = self.rwu_table.get_used(writer, var);
667 self.rwu_table.set(writer, var, rwu_table::RWU { reader: false, writer: false, used });
668 debug!("{:?} defines {:?}: {}", writer, var, self.ln_str(writer));
671 // Either read, write, or both depending on the acc bitset
672 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
673 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
675 let mut rwu = self.rwu_table.get(ln, var);
677 if (acc & ACC_WRITE) != 0 {
682 // Important: if we both read/write, must do read second
683 // or else the write will override.
684 if (acc & ACC_READ) != 0 {
688 if (acc & ACC_USE) != 0 {
692 self.rwu_table.set(ln, var, rwu);
695 fn compute(&mut self, body: &hir::Body<'_>, hir_id: HirId) -> LiveNode {
696 debug!("compute: for body {:?}", body.id().hir_id);
698 // # Liveness of captured variables
700 // When computing the liveness for captured variables we take into
701 // account how variable is captured (ByRef vs ByValue) and what is the
702 // closure kind (Generator / FnOnce vs Fn / FnMut).
704 // Variables captured by reference are assumed to be used on the exit
707 // In FnOnce closures, variables captured by value are known to be dead
708 // on exit since it is impossible to call the closure again.
710 // In Fn / FnMut closures, variables captured by value are live on exit
711 // if they are live on the entry to the closure, since only the closure
712 // itself can access them on subsequent calls.
714 if let Some(closure_min_captures) = self.closure_min_captures {
715 // Mark upvars captured by reference as used after closure exits.
716 for (&var_hir_id, min_capture_list) in closure_min_captures {
717 for captured_place in min_capture_list {
718 match captured_place.info.capture_kind {
719 ty::UpvarCapture::ByRef(_) => {
720 let var = self.variable(
722 captured_place.get_capture_kind_span(self.ir.tcx),
724 self.acc(self.exit_ln, var, ACC_READ | ACC_USE);
726 ty::UpvarCapture::ByValue => {}
732 let succ = self.propagate_through_expr(&body.value, self.exit_ln);
734 if self.closure_min_captures.is_none() {
735 // Either not a closure, or closure without any captured variables.
736 // No need to determine liveness of captured variables, since there
741 let ty = self.typeck_results.node_type(hir_id);
743 ty::Closure(_def_id, substs) => match substs.as_closure().kind() {
744 ty::ClosureKind::Fn => {}
745 ty::ClosureKind::FnMut => {}
746 ty::ClosureKind::FnOnce => return succ,
748 ty::Generator(..) => return succ,
752 "{} has upvars so it should have a closure type: {:?}",
759 // Propagate through calls to the closure.
761 self.init_from_succ(self.closure_ln, succ);
762 for param in body.params {
763 param.pat.each_binding(|_bm, hir_id, _x, ident| {
764 let var = self.variable(hir_id, ident.span);
765 self.define(self.closure_ln, var);
769 if !self.merge_from_succ(self.exit_ln, self.closure_ln) {
772 assert_eq!(succ, self.propagate_through_expr(&body.value, self.exit_ln));
778 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
779 if blk.targeted_by_break {
780 self.break_ln.insert(blk.hir_id, succ);
782 let succ = self.propagate_through_opt_expr(blk.expr, succ);
783 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
786 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
788 hir::StmtKind::Local(ref local) => {
789 // Note: we mark the variable as defined regardless of whether
790 // there is an initializer. Initially I had thought to only mark
791 // the live variable as defined if it was initialized, and then we
792 // could check for uninit variables just by scanning what is live
793 // at the start of the function. But that doesn't work so well for
794 // immutable variables defined in a loop:
795 // loop { let x; x = 5; }
796 // because the "assignment" loops back around and generates an error.
798 // So now we just check that variables defined w/o an
799 // initializer are not live at the point of their
800 // initialization, which is mildly more complex than checking
801 // once at the func header but otherwise equivalent.
803 let succ = self.propagate_through_opt_expr(local.init, succ);
804 self.define_bindings_in_pat(&local.pat, succ)
806 hir::StmtKind::Item(..) => succ,
807 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
808 self.propagate_through_expr(&expr, succ)
813 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
814 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
817 fn propagate_through_opt_expr(
819 opt_expr: Option<&Expr<'_>>,
822 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
825 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
826 debug!("propagate_through_expr: {:?}", expr);
829 // Interesting cases with control flow or which gen/kill
830 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
831 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
834 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
836 hir::ExprKind::Closure(..) => {
837 debug!("{:?} is an ExprKind::Closure", expr);
839 // the construction of a closure itself is not important,
840 // but we have to consider the closed over variables.
846 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
848 caps.iter().rev().fold(succ, |succ, cap| {
849 self.init_from_succ(cap.ln, succ);
850 let var = self.variable(cap.var_hid, expr.span);
851 self.acc(cap.ln, var, ACC_READ | ACC_USE);
856 hir::ExprKind::Let(let_expr) => {
857 let succ = self.propagate_through_expr(let_expr.init, succ);
858 self.define_bindings_in_pat(let_expr.pat, succ)
861 // Note that labels have been resolved, so we don't need to look
862 // at the label ident
863 hir::ExprKind::Loop(ref blk, ..) => self.propagate_through_loop(expr, &blk, succ),
865 hir::ExprKind::Yield(ref e, ..) => {
866 let yield_ln = self.live_node(expr.hir_id, expr.span);
867 self.init_from_succ(yield_ln, succ);
868 self.merge_from_succ(yield_ln, self.exit_ln);
869 self.propagate_through_expr(e, yield_ln)
872 hir::ExprKind::If(ref cond, ref then, ref else_opt) => {
887 self.propagate_through_opt_expr(else_opt.as_ref().map(|e| &**e), succ);
888 let then_ln = self.propagate_through_expr(&then, succ);
889 let ln = self.live_node(expr.hir_id, expr.span);
890 self.init_from_succ(ln, else_ln);
891 self.merge_from_succ(ln, then_ln);
892 self.propagate_through_expr(&cond, ln)
895 hir::ExprKind::Match(ref e, arms, _) => {
910 let ln = self.live_node(expr.hir_id, expr.span);
911 self.init_empty(ln, succ);
913 let body_succ = self.propagate_through_expr(&arm.body, succ);
915 let guard_succ = arm.guard.as_ref().map_or(body_succ, |g| match g {
916 hir::Guard::If(e) => self.propagate_through_expr(e, body_succ),
917 hir::Guard::IfLet(pat, e) => {
918 let let_bind = self.define_bindings_in_pat(pat, body_succ);
919 self.propagate_through_expr(e, let_bind)
922 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
923 self.merge_from_succ(ln, arm_succ);
925 self.propagate_through_expr(&e, ln)
928 hir::ExprKind::Ret(ref o_e) => {
929 // Ignore succ and subst exit_ln.
930 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), self.exit_ln)
933 hir::ExprKind::Break(label, ref opt_expr) => {
934 // Find which label this break jumps to
935 let target = match label.target_id {
936 Ok(hir_id) => self.break_ln.get(&hir_id),
937 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
941 // Now that we know the label we're going to,
942 // look it up in the break loop nodes table
945 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
946 None => span_bug!(expr.span, "`break` to unknown label"),
950 hir::ExprKind::Continue(label) => {
951 // Find which label this expr continues to
954 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
956 // Now that we know the label we're going to,
957 // look it up in the continue loop nodes table
961 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
964 hir::ExprKind::Assign(ref l, ref r, _) => {
965 // see comment on places in
966 // propagate_through_place_components()
967 let succ = self.write_place(&l, succ, ACC_WRITE);
968 let succ = self.propagate_through_place_components(&l, succ);
969 self.propagate_through_expr(&r, succ)
972 hir::ExprKind::AssignOp(_, ref l, ref r) => {
973 // an overloaded assign op is like a method call
974 if self.typeck_results.is_method_call(expr) {
975 let succ = self.propagate_through_expr(&l, succ);
976 self.propagate_through_expr(&r, succ)
978 // see comment on places in
979 // propagate_through_place_components()
980 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
981 let succ = self.propagate_through_expr(&r, succ);
982 self.propagate_through_place_components(&l, succ)
986 // Uninteresting cases: just propagate in rev exec order
987 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
989 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
990 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
994 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
997 hir::ExprKind::Call(ref f, ref args) => {
998 let succ = self.check_is_ty_uninhabited(expr, succ);
999 let succ = self.propagate_through_exprs(args, succ);
1000 self.propagate_through_expr(&f, succ)
1003 hir::ExprKind::MethodCall(.., ref args, _) => {
1004 let succ = self.check_is_ty_uninhabited(expr, succ);
1005 self.propagate_through_exprs(args, succ)
1008 hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ),
1010 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1011 let r_succ = self.propagate_through_expr(&r, succ);
1013 let ln = self.live_node(expr.hir_id, expr.span);
1014 self.init_from_succ(ln, succ);
1015 self.merge_from_succ(ln, r_succ);
1017 self.propagate_through_expr(&l, ln)
1020 hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => {
1021 let r_succ = self.propagate_through_expr(&r, succ);
1022 self.propagate_through_expr(&l, r_succ)
1025 hir::ExprKind::Box(ref e)
1026 | hir::ExprKind::AddrOf(_, _, ref e)
1027 | hir::ExprKind::Cast(ref e, _)
1028 | hir::ExprKind::Type(ref e, _)
1029 | hir::ExprKind::DropTemps(ref e)
1030 | hir::ExprKind::Unary(_, ref e)
1031 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1033 hir::ExprKind::InlineAsm(ref asm) => {
1034 // Handle non-returning asm
1035 let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) {
1041 // Do a first pass for writing outputs only
1042 for (op, _op_sp) in asm.operands.iter().rev() {
1044 hir::InlineAsmOperand::In { .. }
1045 | hir::InlineAsmOperand::Const { .. }
1046 | hir::InlineAsmOperand::SymFn { .. }
1047 | hir::InlineAsmOperand::SymStatic { .. } => {}
1048 hir::InlineAsmOperand::Out { expr, .. } => {
1049 if let Some(expr) = expr {
1050 succ = self.write_place(expr, succ, ACC_WRITE);
1053 hir::InlineAsmOperand::InOut { expr, .. } => {
1054 succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE | ACC_USE);
1056 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1057 if let Some(expr) = out_expr {
1058 succ = self.write_place(expr, succ, ACC_WRITE);
1064 // Then do a second pass for inputs
1065 let mut succ = succ;
1066 for (op, _op_sp) in asm.operands.iter().rev() {
1068 hir::InlineAsmOperand::In { expr, .. } => {
1069 succ = self.propagate_through_expr(expr, succ)
1071 hir::InlineAsmOperand::Out { expr, .. } => {
1072 if let Some(expr) = expr {
1073 succ = self.propagate_through_place_components(expr, succ);
1076 hir::InlineAsmOperand::InOut { expr, .. } => {
1077 succ = self.propagate_through_place_components(expr, succ);
1079 hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
1080 if let Some(expr) = out_expr {
1081 succ = self.propagate_through_place_components(expr, succ);
1083 succ = self.propagate_through_expr(in_expr, succ);
1085 hir::InlineAsmOperand::Const { .. }
1086 | hir::InlineAsmOperand::SymFn { .. }
1087 | hir::InlineAsmOperand::SymStatic { .. } => {}
1093 hir::ExprKind::Lit(..)
1094 | hir::ExprKind::ConstBlock(..)
1095 | hir::ExprKind::Err
1096 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
1097 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => succ,
1099 // Note that labels have been resolved, so we don't need to look
1100 // at the label ident
1101 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1105 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1108 // In general, the full flow graph structure for an
1109 // assignment/move/etc can be handled in one of two ways,
1110 // depending on whether what is being assigned is a "tracked
1111 // value" or not. A tracked value is basically a local
1112 // variable or argument.
1114 // The two kinds of graphs are:
1116 // Tracked place Untracked place
1117 // ----------------------++-----------------------
1121 // (rvalue) || (rvalue)
1124 // (write of place) || (place components)
1129 // ----------------------++-----------------------
1131 // I will cover the two cases in turn:
1135 // A tracked place is a local variable/argument `x`. In
1136 // these cases, the link_node where the write occurs is linked
1137 // to node id of `x`. The `write_place()` routine generates
1138 // the contents of this node. There are no subcomponents to
1141 // # Non-tracked places
1143 // These are places like `x[5]` or `x.f`. In that case, we
1144 // basically ignore the value which is written to but generate
1145 // reads for the components---`x` in these two examples. The
1146 // components reads are generated by
1147 // `propagate_through_place_components()` (this fn).
1151 // It is still possible to observe assignments to non-places;
1152 // these errors are detected in the later pass borrowck. We
1153 // just ignore such cases and treat them as reads.
1156 hir::ExprKind::Path(_) => succ,
1157 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1158 _ => self.propagate_through_expr(expr, succ),
1162 // see comment on propagate_through_place()
1163 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1165 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1166 self.access_path(expr.hir_id, path, succ, acc)
1169 // We do not track other places, so just propagate through
1170 // to their subcomponents. Also, it may happen that
1171 // non-places occur here, because those are detected in the
1172 // later pass borrowck.
1185 let ln = self.live_node(hir_id, span);
1187 self.init_from_succ(ln, succ);
1188 let var = self.variable(var_hid, span);
1189 self.acc(ln, var, acc);
1197 path: &hir::Path<'_>,
1202 Res::Local(hid) => self.access_var(hir_id, hid, succ, acc, path.span),
1207 fn propagate_through_loop(
1210 body: &hir::Block<'_>,
1214 We model control flow like this:
1221 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1222 Meanwhile, a `break` expression will have a successor of `succ`.
1226 let ln = self.live_node(expr.hir_id, expr.span);
1227 self.init_empty(ln, succ);
1228 debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body);
1230 self.break_ln.insert(expr.hir_id, succ);
1232 self.cont_ln.insert(expr.hir_id, ln);
1234 let body_ln = self.propagate_through_block(body, ln);
1236 // repeat until fixed point is reached:
1237 while self.merge_from_succ(ln, body_ln) {
1238 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1244 fn check_is_ty_uninhabited(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1245 let ty = self.typeck_results.expr_ty(expr);
1246 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1247 if self.ir.tcx.is_ty_uninhabited_from(m, ty, self.param_env) {
1248 match self.ir.lnks[succ] {
1249 LiveNodeKind::ExprNode(succ_span, succ_id) => {
1250 self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "expression");
1252 LiveNodeKind::VarDefNode(succ_span, succ_id) => {
1253 self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "definition");
1263 fn warn_about_unreachable(
1271 if !orig_ty.is_never() {
1272 // Unreachable code warnings are already emitted during type checking.
1273 // However, during type checking, full type information is being
1274 // calculated but not yet available, so the check for diverging
1275 // expressions due to uninhabited result types is pretty crude and
1276 // only checks whether ty.is_never(). Here, we have full type
1277 // information available and can issue warnings for less obviously
1278 // uninhabited types (e.g. empty enums). The check above is used so
1279 // that we do not emit the same warning twice if the uninhabited type
1282 self.ir.tcx.struct_span_lint_hir(
1283 lint::builtin::UNREACHABLE_CODE,
1287 let msg = format!("unreachable {}", descr);
1289 .span_label(expr_span, &msg)
1290 .span_label(orig_span, "any code following this expression is unreachable")
1294 "this expression has type `{}`, which is uninhabited",
1305 // _______________________________________________________________________
1306 // Checking for error conditions
1308 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1309 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1310 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1311 if local.init.is_some() {
1312 self.warn_about_dead_assign(spans, hir_id, ln, var);
1316 intravisit::walk_local(self, local);
1319 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1320 check_expr(self, ex);
1321 intravisit::walk_expr(self, ex);
1324 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1325 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1326 intravisit::walk_arm(self, arm);
1330 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1332 hir::ExprKind::Assign(ref l, ..) => {
1333 this.check_place(&l);
1336 hir::ExprKind::AssignOp(_, ref l, _) => {
1337 if !this.typeck_results.is_method_call(expr) {
1338 this.check_place(&l);
1342 hir::ExprKind::InlineAsm(ref asm) => {
1343 for (op, _op_sp) in asm.operands {
1345 hir::InlineAsmOperand::Out { expr, .. } => {
1346 if let Some(expr) = expr {
1347 this.check_place(expr);
1350 hir::InlineAsmOperand::InOut { expr, .. } => {
1351 this.check_place(expr);
1353 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1354 if let Some(out_expr) = out_expr {
1355 this.check_place(out_expr);
1363 hir::ExprKind::Let(let_expr) => {
1364 this.check_unused_vars_in_pat(let_expr.pat, None, |_, _, _, _| {});
1367 // no correctness conditions related to liveness
1368 hir::ExprKind::Call(..)
1369 | hir::ExprKind::MethodCall(..)
1370 | hir::ExprKind::Match(..)
1371 | hir::ExprKind::Loop(..)
1372 | hir::ExprKind::Index(..)
1373 | hir::ExprKind::Field(..)
1374 | hir::ExprKind::Array(..)
1375 | hir::ExprKind::Tup(..)
1376 | hir::ExprKind::Binary(..)
1377 | hir::ExprKind::Cast(..)
1378 | hir::ExprKind::If(..)
1379 | hir::ExprKind::DropTemps(..)
1380 | hir::ExprKind::Unary(..)
1381 | hir::ExprKind::Ret(..)
1382 | hir::ExprKind::Break(..)
1383 | hir::ExprKind::Continue(..)
1384 | hir::ExprKind::Lit(_)
1385 | hir::ExprKind::ConstBlock(..)
1386 | hir::ExprKind::Block(..)
1387 | hir::ExprKind::AddrOf(..)
1388 | hir::ExprKind::Struct(..)
1389 | hir::ExprKind::Repeat(..)
1390 | hir::ExprKind::Closure(..)
1391 | hir::ExprKind::Path(_)
1392 | hir::ExprKind::Yield(..)
1393 | hir::ExprKind::Box(..)
1394 | hir::ExprKind::Type(..)
1395 | hir::ExprKind::Err => {}
1399 impl<'tcx> Liveness<'_, 'tcx> {
1400 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1402 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1403 if let Res::Local(var_hid) = path.res {
1404 // Assignment to an immutable variable or argument: only legal
1405 // if there is no later assignment. If this local is actually
1406 // mutable, then check for a reassignment to flag the mutability
1408 let ln = self.live_node(expr.hir_id, expr.span);
1409 let var = self.variable(var_hid, expr.span);
1410 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1414 // For other kinds of places, no checks are required,
1415 // and any embedded expressions are actually rvalues
1416 intravisit::walk_expr(self, expr);
1421 fn should_warn(&self, var: Variable) -> Option<String> {
1422 let name = self.ir.variable_name(var);
1423 if name == kw::Empty {
1426 let name = name.as_str();
1427 if name.as_bytes()[0] == b'_' {
1430 Some(name.to_owned())
1433 fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
1434 let Some(closure_min_captures) = self.closure_min_captures else {
1438 // If closure_min_captures is Some(), upvars must be Some() too.
1439 for (&var_hir_id, min_capture_list) in closure_min_captures {
1440 for captured_place in min_capture_list {
1441 match captured_place.info.capture_kind {
1442 ty::UpvarCapture::ByValue => {}
1443 ty::UpvarCapture::ByRef(..) => continue,
1445 let span = captured_place.get_capture_kind_span(self.ir.tcx);
1446 let var = self.variable(var_hir_id, span);
1447 if self.used_on_entry(entry_ln, var) {
1448 if !self.live_on_entry(entry_ln, var) {
1449 if let Some(name) = self.should_warn(var) {
1450 self.ir.tcx.struct_span_lint_hir(
1451 lint::builtin::UNUSED_ASSIGNMENTS,
1455 lint.build(&format!(
1456 "value captured by `{}` is never read",
1459 .help("did you mean to capture by reference instead?")
1466 if let Some(name) = self.should_warn(var) {
1467 self.ir.tcx.struct_span_lint_hir(
1468 lint::builtin::UNUSED_VARIABLES,
1472 lint.build(&format!("unused variable: `{}`", name))
1473 .help("did you mean to capture by reference instead?")
1483 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1484 for p in body.params {
1485 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1486 if !self.live_on_entry(ln, var) {
1487 self.report_unused_assign(hir_id, spans, var, |name| {
1488 format!("value passed to `{}` is never read", name)
1495 fn check_unused_vars_in_pat(
1498 entry_ln: Option<LiveNode>,
1499 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1501 // In an or-pattern, only consider the variable; any later patterns must have the same
1502 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1503 // However, we should take the ids and spans of variables with the same name from the later
1504 // patterns so the suggestions to prefix with underscores will apply to those too.
1505 let mut vars: FxIndexMap<Symbol, (LiveNode, Variable, Vec<(HirId, Span, Span)>)> =
1508 pat.each_binding(|_, hir_id, pat_sp, ident| {
1509 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1510 let var = self.variable(hir_id, ident.span);
1511 let id_and_sp = (hir_id, pat_sp, ident.span);
1512 vars.entry(self.ir.variable_name(var))
1513 .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp))
1514 .or_insert_with(|| (ln, var, vec![id_and_sp]));
1517 for (_, (ln, var, hir_ids_and_spans)) in vars {
1518 if self.used_on_entry(ln, var) {
1519 let id = hir_ids_and_spans[0].0;
1521 hir_ids_and_spans.into_iter().map(|(_, _, ident_span)| ident_span).collect();
1522 on_used_on_entry(spans, id, ln, var);
1524 self.report_unused(hir_ids_and_spans, ln, var);
1531 hir_ids_and_spans: Vec<(HirId, Span, Span)>,
1535 let first_hir_id = hir_ids_and_spans[0].0;
1537 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1538 // annoying: for parameters in funcs like `fn(x: i32)
1539 // {ret}`, there is only one node, so asking about
1540 // assigned_on_exit() is not meaningful.
1542 if ln == self.exit_ln { false } else { self.assigned_on_exit(ln, var) };
1545 self.ir.tcx.struct_span_lint_hir(
1546 lint::builtin::UNUSED_VARIABLES,
1550 .map(|(_, _, ident_span)| ident_span)
1551 .collect::<Vec<_>>(),
1553 lint.build(&format!("variable `{}` is assigned to, but never used", name))
1554 .note(&format!("consider using `_{}` instead", name))
1559 let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
1560 hir_ids_and_spans.iter().copied().partition(|(hir_id, _, ident_span)| {
1561 let var = self.variable(*hir_id, *ident_span);
1562 self.ir.variable_is_shorthand(var)
1565 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1566 // the field" message, and suggest `_` for the non-shorthands. If we only
1567 // have non-shorthand, then prefix with an underscore instead.
1568 if !shorthands.is_empty() {
1569 let shorthands = shorthands
1571 .map(|(_, pat_span, _)| (pat_span, format!("{}: _", name)))
1575 .map(|(_, pat_span, _)| (pat_span, "_".to_string())),
1577 .collect::<Vec<_>>();
1579 self.ir.tcx.struct_span_lint_hir(
1580 lint::builtin::UNUSED_VARIABLES,
1584 .map(|(_, pat_span, _)| *pat_span)
1585 .collect::<Vec<_>>(),
1587 let mut err = lint.build(&format!("unused variable: `{}`", name));
1588 err.multipart_suggestion(
1589 "try ignoring the field",
1591 Applicability::MachineApplicable,
1597 let non_shorthands = non_shorthands
1599 .map(|(_, _, ident_span)| (ident_span, format!("_{}", name)))
1600 .collect::<Vec<_>>();
1602 self.ir.tcx.struct_span_lint_hir(
1603 lint::builtin::UNUSED_VARIABLES,
1607 .map(|(_, _, ident_span)| *ident_span)
1608 .collect::<Vec<_>>(),
1610 let mut err = lint.build(&format!("unused variable: `{}`", name));
1611 err.multipart_suggestion(
1612 "if this is intentional, prefix it with an underscore",
1614 Applicability::MachineApplicable,
1624 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1625 if !self.live_on_exit(ln, var) {
1626 self.report_unused_assign(hir_id, spans, var, |name| {
1627 format!("value assigned to `{}` is never read", name)
1632 fn report_unused_assign(
1637 message: impl Fn(&str) -> String,
1639 if let Some(name) = self.should_warn(var) {
1640 self.ir.tcx.struct_span_lint_hir(
1641 lint::builtin::UNUSED_ASSIGNMENTS,
1645 lint.build(&message(&name))
1646 .help("maybe it is overwritten before being read?")