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, RootVariableMinCaptureList, 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)]
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_string(s)),
135 ExprNode(s) => format!("Expr node [{}]", sm.span_to_string(s)),
136 VarDefNode(s) => format!("Var def node [{}]", sm.span_to_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>,
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 add_from_pat(&mut self, pat: &hir::Pat<'tcx>) {
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();
273 while let Some(pat) = pats.pop_front() {
274 use rustc_hir::PatKind::*;
276 Binding(.., inner_pat) => {
277 pats.extend(inner_pat.iter());
279 Struct(_, fields, _) => {
280 let ids = fields.iter().filter(|f| f.is_shorthand).map(|f| f.pat.hir_id);
281 shorthand_field_ids.extend(ids);
283 Ref(inner_pat, _) | Box(inner_pat) => {
284 pats.push_back(inner_pat);
286 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
287 pats.extend(inner_pats.iter());
289 Slice(pre_pats, inner_pat, post_pats) => {
290 pats.extend(pre_pats.iter());
291 pats.extend(inner_pat.iter());
292 pats.extend(post_pats.iter());
298 pat.each_binding(|_, hir_id, _, ident| {
299 self.add_live_node_for_node(hir_id, VarDefNode(ident.span));
300 self.add_variable(Local(LocalInfo {
303 is_shorthand: shorthand_field_ids.contains(&hir_id),
309 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
310 type Map = Map<'tcx>;
312 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
313 NestedVisitorMap::OnlyBodies(self.tcx.hir())
316 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
317 debug!("visit_body {:?}", body.id());
319 // swap in a new set of IR maps for this body
320 let mut maps = IrMaps::new(self.tcx);
321 let hir_id = maps.tcx.hir().body_owner(body.id());
322 let local_def_id = maps.tcx.hir().local_def_id(hir_id);
323 let def_id = local_def_id.to_def_id();
325 // Don't run unused pass for #[derive()]
326 if let Some(parent) = self.tcx.parent(def_id) {
327 if let DefKind::Impl = self.tcx.def_kind(parent.expect_local()) {
328 if self.tcx.has_attr(parent, sym::automatically_derived) {
334 if let Some(captures) = maps.tcx.typeck(local_def_id).closure_min_captures.get(&def_id) {
335 for &var_hir_id in captures.keys() {
336 let var_name = maps.tcx.hir().name(var_hir_id);
337 maps.add_variable(Upvar(var_hir_id, var_name));
341 // gather up the various local variables, significant expressions,
343 intravisit::walk_body(&mut maps, body);
346 let mut lsets = Liveness::new(&mut maps, local_def_id);
347 let entry_ln = lsets.compute(&body, hir_id);
348 lsets.log_liveness(entry_ln, body.id().hir_id);
350 // check for various error conditions
351 lsets.visit_body(body);
352 lsets.warn_about_unused_upvars(entry_ln);
353 lsets.warn_about_unused_args(body, entry_ln);
356 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
357 self.add_from_pat(&local.pat);
358 intravisit::walk_local(self, local);
361 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
362 self.add_from_pat(&arm.pat);
363 if let Some(hir::Guard::IfLet(ref pat, _)) = arm.guard {
364 self.add_from_pat(pat);
366 intravisit::walk_arm(self, arm);
369 fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
370 param.pat.each_binding(|_bm, hir_id, _x, ident| {
371 let var = match param.pat.kind {
372 rustc_hir::PatKind::Struct(_, fields, _) => Local(LocalInfo {
377 .find(|f| f.ident == ident)
378 .map_or(false, |f| f.is_shorthand),
380 _ => Param(hir_id, ident.name),
382 self.add_variable(var);
384 intravisit::walk_param(self, param);
387 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
389 // live nodes required for uses or definitions of variables:
390 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
391 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
392 if let Res::Local(_var_hir_id) = path.res {
393 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
395 intravisit::walk_expr(self, expr);
397 hir::ExprKind::Closure(..) => {
398 // Interesting control flow (for loops can contain labeled
399 // breaks or continues)
400 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
402 // Make a live_node for each captured variable, with the span
403 // being the location that the variable is used. This results
404 // in better error messages than just pointing at the closure
405 // construction site.
406 let mut call_caps = Vec::new();
407 let closure_def_id = self.tcx.hir().local_def_id(expr.hir_id);
408 if let Some(captures) = self
410 .typeck(closure_def_id)
411 .closure_min_captures
412 .get(&closure_def_id.to_def_id())
414 // If closure_min_captures is Some, upvars_mentioned must also be Some
415 let upvars = self.tcx.upvars_mentioned(closure_def_id).unwrap();
416 call_caps.extend(captures.keys().map(|var_id| {
417 let upvar = upvars[var_id];
418 let upvar_ln = self.add_live_node(UpvarNode(upvar.span));
419 CaptureInfo { ln: upvar_ln, var_hid: *var_id }
422 self.set_captures(expr.hir_id, call_caps);
423 intravisit::walk_expr(self, expr);
426 // live nodes required for interesting control flow:
427 hir::ExprKind::If(..) | hir::ExprKind::Match(..) | hir::ExprKind::Loop(..) => {
428 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
429 intravisit::walk_expr(self, expr);
431 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
432 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span));
433 intravisit::walk_expr(self, expr);
436 // otherwise, live nodes are not required:
437 hir::ExprKind::Index(..)
438 | hir::ExprKind::Field(..)
439 | hir::ExprKind::Array(..)
440 | hir::ExprKind::Call(..)
441 | hir::ExprKind::MethodCall(..)
442 | hir::ExprKind::Tup(..)
443 | hir::ExprKind::Binary(..)
444 | hir::ExprKind::AddrOf(..)
445 | hir::ExprKind::Cast(..)
446 | hir::ExprKind::DropTemps(..)
447 | hir::ExprKind::Unary(..)
448 | hir::ExprKind::Break(..)
449 | hir::ExprKind::Continue(_)
450 | hir::ExprKind::Lit(_)
451 | hir::ExprKind::ConstBlock(..)
452 | hir::ExprKind::Ret(..)
453 | hir::ExprKind::Block(..)
454 | hir::ExprKind::Assign(..)
455 | hir::ExprKind::AssignOp(..)
456 | hir::ExprKind::Struct(..)
457 | hir::ExprKind::Repeat(..)
458 | hir::ExprKind::InlineAsm(..)
459 | hir::ExprKind::LlvmInlineAsm(..)
460 | hir::ExprKind::Box(..)
461 | hir::ExprKind::Yield(..)
462 | hir::ExprKind::Type(..)
464 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
465 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
466 intravisit::walk_expr(self, expr);
472 // ______________________________________________________________________
473 // Computing liveness sets
475 // Actually we compute just a bit more than just liveness, but we use
476 // the same basic propagation framework in all cases.
478 const ACC_READ: u32 = 1;
479 const ACC_WRITE: u32 = 2;
480 const ACC_USE: u32 = 4;
482 struct Liveness<'a, 'tcx> {
483 ir: &'a mut IrMaps<'tcx>,
484 typeck_results: &'a ty::TypeckResults<'tcx>,
485 param_env: ty::ParamEnv<'tcx>,
486 upvars: Option<&'tcx FxIndexMap<hir::HirId, hir::Upvar>>,
487 closure_min_captures: Option<&'tcx RootVariableMinCaptureList<'tcx>>,
488 successors: IndexVec<LiveNode, Option<LiveNode>>,
489 rwu_table: rwu_table::RWUTable,
491 /// A live node representing a point of execution before closure entry &
492 /// after closure exit. Used to calculate liveness of captured variables
493 /// through calls to the same closure. Used for Fn & FnMut closures only.
494 closure_ln: LiveNode,
495 /// A live node representing every 'exit' from the function, whether it be
496 /// by explicit return, panic, or other means.
499 // mappings from loop node ID to LiveNode
500 // ("break" label should map to loop node ID,
501 // it probably doesn't now)
502 break_ln: HirIdMap<LiveNode>,
503 cont_ln: HirIdMap<LiveNode>,
506 impl<'a, 'tcx> Liveness<'a, 'tcx> {
507 fn new(ir: &'a mut IrMaps<'tcx>, body_owner: LocalDefId) -> Liveness<'a, 'tcx> {
508 let typeck_results = ir.tcx.typeck(body_owner);
509 let param_env = ir.tcx.param_env(body_owner);
510 let upvars = ir.tcx.upvars_mentioned(body_owner);
511 let closure_min_captures = typeck_results.closure_min_captures.get(&body_owner.to_def_id());
512 let closure_ln = ir.add_live_node(ClosureNode);
513 let exit_ln = ir.add_live_node(ExitNode);
515 let num_live_nodes = ir.lnks.len();
516 let num_vars = ir.var_kinds.len();
523 closure_min_captures,
524 successors: IndexVec::from_elem_n(None, num_live_nodes),
525 rwu_table: rwu_table::RWUTable::new(num_live_nodes, num_vars),
528 break_ln: Default::default(),
529 cont_ln: Default::default(),
533 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
534 match self.ir.live_node_map.get(&hir_id) {
537 // This must be a mismatch between the ir_map construction
538 // above and the propagation code below; the two sets of
539 // code have to agree about which AST nodes are worth
540 // creating liveness nodes for.
541 span_bug!(span, "no live node registered for node {:?}", hir_id);
546 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
547 self.ir.variable(hir_id, span)
550 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
551 // In an or-pattern, only consider the first pattern; any later patterns
552 // must have the same bindings, and we also consider the first pattern
553 // to be the "authoritative" set of ids.
554 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
555 let ln = self.live_node(hir_id, pat_sp);
556 let var = self.variable(hir_id, ident.span);
557 self.init_from_succ(ln, succ);
558 self.define(ln, var);
564 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
565 self.rwu_table.get_reader(ln, var)
568 // Is this variable live on entry to any of its successor nodes?
569 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> bool {
570 let successor = self.successors[ln].unwrap();
571 self.live_on_entry(successor, var)
574 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
575 self.rwu_table.get_used(ln, var)
578 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
579 self.rwu_table.get_writer(ln, var)
582 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> bool {
583 let successor = self.successors[ln].unwrap();
584 self.assigned_on_entry(successor, var)
587 fn write_vars<F>(&self, wr: &mut dyn Write, mut test: F) -> io::Result<()>
589 F: FnMut(Variable) -> bool,
591 for var_idx in 0..self.ir.var_kinds.len() {
592 let var = Variable::from(var_idx);
594 write!(wr, " {:?}", var)?;
600 #[allow(unused_must_use)]
601 fn ln_str(&self, ln: LiveNode) -> String {
602 let mut wr = Vec::new();
604 let wr = &mut wr as &mut dyn Write;
605 write!(wr, "[{:?} of kind {:?} reads", ln, self.ir.lnks[ln]);
606 self.write_vars(wr, |var| self.rwu_table.get_reader(ln, var));
607 write!(wr, " writes");
608 self.write_vars(wr, |var| self.rwu_table.get_writer(ln, var));
610 self.write_vars(wr, |var| self.rwu_table.get_used(ln, var));
612 write!(wr, " precedes {:?}]", self.successors[ln]);
614 String::from_utf8(wr).unwrap()
617 fn log_liveness(&self, entry_ln: LiveNode, hir_id: hir::HirId) {
618 // hack to skip the loop unless debug! is enabled:
620 "^^ liveness computation results for body {} (entry={:?})",
622 for ln_idx in 0..self.ir.lnks.len() {
623 debug!("{:?}", self.ln_str(LiveNode::from(ln_idx)));
631 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
632 self.successors[ln] = Some(succ_ln);
634 // It is not necessary to initialize the RWUs here because they are all
635 // empty when created, and the sets only grow during iterations.
638 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
639 // more efficient version of init_empty() / merge_from_succ()
640 self.successors[ln] = Some(succ_ln);
641 self.rwu_table.copy(ln, succ_ln);
642 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
645 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) -> bool {
650 let changed = self.rwu_table.union(ln, succ_ln);
651 debug!("merge_from_succ(ln={:?}, succ={}, changed={})", ln, self.ln_str(succ_ln), changed);
655 // Indicates that a local variable was *defined*; we know that no
656 // uses of the variable can precede the definition (resolve checks
657 // this) so we just clear out all the data.
658 fn define(&mut self, writer: LiveNode, var: Variable) {
659 let used = self.rwu_table.get_used(writer, var);
660 self.rwu_table.set(writer, var, rwu_table::RWU { reader: false, writer: false, used });
661 debug!("{:?} defines {:?}: {}", writer, var, self.ln_str(writer));
664 // Either read, write, or both depending on the acc bitset
665 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
666 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
668 let mut rwu = self.rwu_table.get(ln, var);
670 if (acc & ACC_WRITE) != 0 {
675 // Important: if we both read/write, must do read second
676 // or else the write will override.
677 if (acc & ACC_READ) != 0 {
681 if (acc & ACC_USE) != 0 {
685 self.rwu_table.set(ln, var, rwu);
688 fn compute(&mut self, body: &hir::Body<'_>, hir_id: HirId) -> LiveNode {
689 debug!("compute: for body {:?}", body.id().hir_id);
691 // # Liveness of captured variables
693 // When computing the liveness for captured variables we take into
694 // account how variable is captured (ByRef vs ByValue) and what is the
695 // closure kind (Generator / FnOnce vs Fn / FnMut).
697 // Variables captured by reference are assumed to be used on the exit
700 // In FnOnce closures, variables captured by value are known to be dead
701 // on exit since it is impossible to call the closure again.
703 // In Fn / FnMut closures, variables captured by value are live on exit
704 // if they are live on the entry to the closure, since only the closure
705 // itself can access them on subsequent calls.
707 if let Some(closure_min_captures) = self.closure_min_captures {
708 // Mark upvars captured by reference as used after closure exits.
709 for (&var_hir_id, min_capture_list) in closure_min_captures {
710 for captured_place in min_capture_list {
711 match captured_place.info.capture_kind {
712 ty::UpvarCapture::ByRef(_) => {
713 let var = self.variable(
715 captured_place.get_capture_kind_span(self.ir.tcx),
717 self.acc(self.exit_ln, var, ACC_READ | ACC_USE);
719 ty::UpvarCapture::ByValue(_) => {}
725 let succ = self.propagate_through_expr(&body.value, self.exit_ln);
727 if self.closure_min_captures.is_none() {
728 // Either not a closure, or closure without any captured variables.
729 // No need to determine liveness of captured variables, since there
734 let ty = self.typeck_results.node_type(hir_id);
736 ty::Closure(_def_id, substs) => match substs.as_closure().kind() {
737 ty::ClosureKind::Fn => {}
738 ty::ClosureKind::FnMut => {}
739 ty::ClosureKind::FnOnce => return succ,
741 ty::Generator(..) => return succ,
745 "{} has upvars so it should have a closure type: {:?}",
752 // Propagate through calls to the closure.
754 self.init_from_succ(self.closure_ln, succ);
755 for param in body.params {
756 param.pat.each_binding(|_bm, hir_id, _x, ident| {
757 let var = self.variable(hir_id, ident.span);
758 self.define(self.closure_ln, var);
762 if !self.merge_from_succ(self.exit_ln, self.closure_ln) {
765 assert_eq!(succ, self.propagate_through_expr(&body.value, self.exit_ln));
771 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
772 if blk.targeted_by_break {
773 self.break_ln.insert(blk.hir_id, succ);
775 let succ = self.propagate_through_opt_expr(blk.expr.as_deref(), succ);
776 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
779 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
781 hir::StmtKind::Local(ref local) => {
782 // Note: we mark the variable as defined regardless of whether
783 // there is an initializer. Initially I had thought to only mark
784 // the live variable as defined if it was initialized, and then we
785 // could check for uninit variables just by scanning what is live
786 // at the start of the function. But that doesn't work so well for
787 // immutable variables defined in a loop:
788 // loop { let x; x = 5; }
789 // because the "assignment" loops back around and generates an error.
791 // So now we just check that variables defined w/o an
792 // initializer are not live at the point of their
793 // initialization, which is mildly more complex than checking
794 // once at the func header but otherwise equivalent.
796 let succ = self.propagate_through_opt_expr(local.init.as_deref(), succ);
797 self.define_bindings_in_pat(&local.pat, succ)
799 hir::StmtKind::Item(..) => succ,
800 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
801 self.propagate_through_expr(&expr, succ)
806 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
807 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
810 fn propagate_through_opt_expr(
812 opt_expr: Option<&Expr<'_>>,
815 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
818 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
819 debug!("propagate_through_expr: {:?}", expr);
822 // Interesting cases with control flow or which gen/kill
823 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
824 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
827 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
829 hir::ExprKind::Closure(..) => {
830 debug!("{:?} is an ExprKind::Closure", expr);
832 // the construction of a closure itself is not important,
833 // but we have to consider the closed over variables.
839 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
841 caps.iter().rev().fold(succ, |succ, cap| {
842 self.init_from_succ(cap.ln, succ);
843 let var = self.variable(cap.var_hid, expr.span);
844 self.acc(cap.ln, var, ACC_READ | ACC_USE);
849 // Note that labels have been resolved, so we don't need to look
850 // at the label ident
851 hir::ExprKind::Loop(ref blk, ..) => self.propagate_through_loop(expr, &blk, succ),
853 hir::ExprKind::If(ref cond, ref then, ref else_opt) => {
868 self.propagate_through_opt_expr(else_opt.as_ref().map(|e| &**e), succ);
869 let then_ln = self.propagate_through_expr(&then, succ);
870 let ln = self.live_node(expr.hir_id, expr.span);
871 self.init_from_succ(ln, else_ln);
872 self.merge_from_succ(ln, then_ln);
873 self.propagate_through_expr(&cond, ln)
876 hir::ExprKind::Match(ref e, arms, _) => {
891 let ln = self.live_node(expr.hir_id, expr.span);
892 self.init_empty(ln, succ);
894 let body_succ = self.propagate_through_expr(&arm.body, succ);
896 let guard_succ = arm.guard.as_ref().map_or(body_succ, |g| match g {
897 hir::Guard::If(e) => self.propagate_through_expr(e, body_succ),
898 hir::Guard::IfLet(pat, e) => {
899 let let_bind = self.define_bindings_in_pat(pat, body_succ);
900 self.propagate_through_expr(e, let_bind)
903 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
904 self.merge_from_succ(ln, arm_succ);
906 self.propagate_through_expr(&e, ln)
909 hir::ExprKind::Ret(ref o_e) => {
910 // Ignore succ and subst exit_ln.
911 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), self.exit_ln)
914 hir::ExprKind::Break(label, ref opt_expr) => {
915 // Find which label this break jumps to
916 let target = match label.target_id {
917 Ok(hir_id) => self.break_ln.get(&hir_id),
918 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
922 // Now that we know the label we're going to,
923 // look it up in the break loop nodes table
926 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
927 None => span_bug!(expr.span, "`break` to unknown label"),
931 hir::ExprKind::Continue(label) => {
932 // Find which label this expr continues to
935 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
937 // Now that we know the label we're going to,
938 // look it up in the continue loop nodes table
942 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
945 hir::ExprKind::Assign(ref l, ref r, _) => {
946 // see comment on places in
947 // propagate_through_place_components()
948 let succ = self.write_place(&l, succ, ACC_WRITE);
949 let succ = self.propagate_through_place_components(&l, succ);
950 self.propagate_through_expr(&r, succ)
953 hir::ExprKind::AssignOp(_, ref l, ref r) => {
954 // an overloaded assign op is like a method call
955 if self.typeck_results.is_method_call(expr) {
956 let succ = self.propagate_through_expr(&l, succ);
957 self.propagate_through_expr(&r, succ)
959 // see comment on places in
960 // propagate_through_place_components()
961 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
962 let succ = self.propagate_through_expr(&r, succ);
963 self.propagate_through_place_components(&l, succ)
967 // Uninteresting cases: just propagate in rev exec order
968 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
970 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
971 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
975 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
978 hir::ExprKind::Call(ref f, ref args) => {
979 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
980 let succ = if self.ir.tcx.is_ty_uninhabited_from(
982 self.typeck_results.expr_ty(expr),
989 let succ = self.propagate_through_exprs(args, succ);
990 self.propagate_through_expr(&f, succ)
993 hir::ExprKind::MethodCall(.., ref args, _) => {
994 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
995 let succ = if self.ir.tcx.is_ty_uninhabited_from(
997 self.typeck_results.expr_ty(expr),
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::Yield(ref e, _)
1032 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1034 hir::ExprKind::InlineAsm(ref asm) => {
1035 // Handle non-returning asm
1036 let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) {
1042 // Do a first pass for writing outputs only
1043 for (op, _op_sp) in asm.operands.iter().rev() {
1045 hir::InlineAsmOperand::In { .. }
1046 | hir::InlineAsmOperand::Const { .. }
1047 | hir::InlineAsmOperand::Sym { .. } => {}
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 | hir::InlineAsmOperand::Const { expr, .. }
1070 | hir::InlineAsmOperand::Sym { expr, .. } => {
1071 succ = self.propagate_through_expr(expr, succ)
1073 hir::InlineAsmOperand::Out { expr, .. } => {
1074 if let Some(expr) = expr {
1075 succ = self.propagate_through_place_components(expr, succ);
1078 hir::InlineAsmOperand::InOut { expr, .. } => {
1079 succ = self.propagate_through_place_components(expr, succ);
1081 hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
1082 if let Some(expr) = out_expr {
1083 succ = self.propagate_through_place_components(expr, succ);
1085 succ = self.propagate_through_expr(in_expr, succ);
1092 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1093 let ia = &asm.inner;
1094 let outputs = asm.outputs_exprs;
1095 let inputs = asm.inputs_exprs;
1096 let succ = ia.outputs.iter().zip(outputs).rev().fold(succ, |succ, (o, output)| {
1097 // see comment on places
1098 // in propagate_through_place_components()
1100 self.propagate_through_expr(output, succ)
1102 let acc = if o.is_rw { ACC_WRITE | ACC_READ } else { ACC_WRITE };
1103 let succ = self.write_place(output, succ, acc);
1104 self.propagate_through_place_components(output, succ)
1108 // Inputs are executed first. Propagate last because of rev order
1109 self.propagate_through_exprs(inputs, succ)
1112 hir::ExprKind::Lit(..)
1113 | hir::ExprKind::ConstBlock(..)
1114 | hir::ExprKind::Err
1115 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
1116 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => succ,
1118 // Note that labels have been resolved, so we don't need to look
1119 // at the label ident
1120 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1124 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1127 // In general, the full flow graph structure for an
1128 // assignment/move/etc can be handled in one of two ways,
1129 // depending on whether what is being assigned is a "tracked
1130 // value" or not. A tracked value is basically a local
1131 // variable or argument.
1133 // The two kinds of graphs are:
1135 // Tracked place Untracked place
1136 // ----------------------++-----------------------
1140 // (rvalue) || (rvalue)
1143 // (write of place) || (place components)
1148 // ----------------------++-----------------------
1150 // I will cover the two cases in turn:
1154 // A tracked place is a local variable/argument `x`. In
1155 // these cases, the link_node where the write occurs is linked
1156 // to node id of `x`. The `write_place()` routine generates
1157 // the contents of this node. There are no subcomponents to
1160 // # Non-tracked places
1162 // These are places like `x[5]` or `x.f`. In that case, we
1163 // basically ignore the value which is written to but generate
1164 // reads for the components---`x` in these two examples. The
1165 // components reads are generated by
1166 // `propagate_through_place_components()` (this fn).
1170 // It is still possible to observe assignments to non-places;
1171 // these errors are detected in the later pass borrowck. We
1172 // just ignore such cases and treat them as reads.
1175 hir::ExprKind::Path(_) => succ,
1176 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1177 _ => self.propagate_through_expr(expr, succ),
1181 // see comment on propagate_through_place()
1182 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1184 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1185 self.access_path(expr.hir_id, path, succ, acc)
1188 // We do not track other places, so just propagate through
1189 // to their subcomponents. Also, it may happen that
1190 // non-places occur here, because those are detected in the
1191 // later pass borrowck.
1204 let ln = self.live_node(hir_id, span);
1206 self.init_from_succ(ln, succ);
1207 let var = self.variable(var_hid, span);
1208 self.acc(ln, var, acc);
1216 path: &hir::Path<'_>,
1221 Res::Local(hid) => {
1222 let in_upvars = self.upvars.map_or(false, |u| u.contains_key(&hid));
1223 let in_captures = self.closure_min_captures.map_or(false, |c| c.contains_key(&hid));
1225 match (in_upvars, in_captures) {
1226 (false, _) | (true, true) => self.access_var(hir_id, hid, succ, acc, path.span),
1228 // This case is possible when with RFC-2229, a wild pattern
1229 // is used within a closure.
1230 // eg: `let _ = x`. The closure doesn't capture x here,
1231 // even though it's mentioned in the closure.
1240 fn propagate_through_loop(
1243 body: &hir::Block<'_>,
1247 We model control flow like this:
1254 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1255 Meanwhile, a `break` expression will have a successor of `succ`.
1259 let ln = self.live_node(expr.hir_id, expr.span);
1260 self.init_empty(ln, succ);
1261 debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body);
1263 self.break_ln.insert(expr.hir_id, succ);
1265 self.cont_ln.insert(expr.hir_id, ln);
1267 let body_ln = self.propagate_through_block(body, ln);
1269 // repeat until fixed point is reached:
1270 while self.merge_from_succ(ln, body_ln) {
1271 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1278 // _______________________________________________________________________
1279 // Checking for error conditions
1281 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1282 type Map = intravisit::ErasedMap<'tcx>;
1284 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
1285 NestedVisitorMap::None
1288 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1289 self.check_unused_vars_in_pat(&local.pat, None, |spans, hir_id, ln, var| {
1290 if local.init.is_some() {
1291 self.warn_about_dead_assign(spans, hir_id, ln, var);
1295 intravisit::walk_local(self, local);
1298 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1299 check_expr(self, ex);
1302 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1303 self.check_unused_vars_in_pat(&arm.pat, None, |_, _, _, _| {});
1304 intravisit::walk_arm(self, arm);
1308 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1310 hir::ExprKind::Assign(ref l, ..) => {
1311 this.check_place(&l);
1314 hir::ExprKind::AssignOp(_, ref l, _) => {
1315 if !this.typeck_results.is_method_call(expr) {
1316 this.check_place(&l);
1320 hir::ExprKind::InlineAsm(ref asm) => {
1321 for (op, _op_sp) in asm.operands {
1323 hir::InlineAsmOperand::Out { expr, .. } => {
1324 if let Some(expr) = expr {
1325 this.check_place(expr);
1328 hir::InlineAsmOperand::InOut { expr, .. } => {
1329 this.check_place(expr);
1331 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1332 if let Some(out_expr) = out_expr {
1333 this.check_place(out_expr);
1341 hir::ExprKind::LlvmInlineAsm(ref asm) => {
1342 for input in asm.inputs_exprs {
1343 this.visit_expr(input);
1346 // Output operands must be places
1347 for (o, output) in asm.inner.outputs.iter().zip(asm.outputs_exprs) {
1349 this.check_place(output);
1351 this.visit_expr(output);
1355 // no correctness conditions related to liveness
1356 hir::ExprKind::Call(..)
1357 | hir::ExprKind::MethodCall(..)
1358 | hir::ExprKind::Match(..)
1359 | hir::ExprKind::Loop(..)
1360 | hir::ExprKind::Index(..)
1361 | hir::ExprKind::Field(..)
1362 | hir::ExprKind::Array(..)
1363 | hir::ExprKind::Tup(..)
1364 | hir::ExprKind::Binary(..)
1365 | hir::ExprKind::Cast(..)
1366 | hir::ExprKind::If(..)
1367 | hir::ExprKind::DropTemps(..)
1368 | hir::ExprKind::Unary(..)
1369 | hir::ExprKind::Ret(..)
1370 | hir::ExprKind::Break(..)
1371 | hir::ExprKind::Continue(..)
1372 | hir::ExprKind::Lit(_)
1373 | hir::ExprKind::ConstBlock(..)
1374 | hir::ExprKind::Block(..)
1375 | hir::ExprKind::AddrOf(..)
1376 | hir::ExprKind::Struct(..)
1377 | hir::ExprKind::Repeat(..)
1378 | hir::ExprKind::Closure(..)
1379 | hir::ExprKind::Path(_)
1380 | hir::ExprKind::Yield(..)
1381 | hir::ExprKind::Box(..)
1382 | hir::ExprKind::Type(..)
1383 | hir::ExprKind::Err => {}
1386 intravisit::walk_expr(this, expr);
1389 impl<'tcx> Liveness<'_, 'tcx> {
1390 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1392 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1393 if let Res::Local(var_hid) = path.res {
1394 // Assignment to an immutable variable or argument: only legal
1395 // if there is no later assignment. If this local is actually
1396 // mutable, then check for a reassignment to flag the mutability
1398 let ln = self.live_node(expr.hir_id, expr.span);
1399 let var = self.variable(var_hid, expr.span);
1400 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1404 // For other kinds of places, no checks are required,
1405 // and any embedded expressions are actually rvalues
1406 intravisit::walk_expr(self, expr);
1411 fn should_warn(&self, var: Variable) -> Option<String> {
1412 let name = self.ir.variable_name(var);
1413 if name == kw::Empty {
1416 let name: &str = &name.as_str();
1417 if name.as_bytes()[0] == b'_' {
1420 Some(name.to_owned())
1423 fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
1424 let closure_min_captures = match self.closure_min_captures {
1426 Some(closure_min_captures) => closure_min_captures,
1429 // If closure_min_captures is Some(), upvars must be Some() too.
1430 for (&var_hir_id, min_capture_list) in closure_min_captures {
1431 for captured_place in min_capture_list {
1432 match captured_place.info.capture_kind {
1433 ty::UpvarCapture::ByValue(_) => {}
1434 ty::UpvarCapture::ByRef(..) => continue,
1436 let span = captured_place.get_capture_kind_span(self.ir.tcx);
1437 let var = self.variable(var_hir_id, span);
1438 if self.used_on_entry(entry_ln, var) {
1439 if !self.live_on_entry(entry_ln, var) {
1440 if let Some(name) = self.should_warn(var) {
1441 self.ir.tcx.struct_span_lint_hir(
1442 lint::builtin::UNUSED_ASSIGNMENTS,
1446 lint.build(&format!(
1447 "value captured by `{}` is never read",
1450 .help("did you mean to capture by reference instead?")
1457 if let Some(name) = self.should_warn(var) {
1458 self.ir.tcx.struct_span_lint_hir(
1459 lint::builtin::UNUSED_VARIABLES,
1463 lint.build(&format!("unused variable: `{}`", name))
1464 .help("did you mean to capture by reference instead?")
1474 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1475 for p in body.params {
1476 self.check_unused_vars_in_pat(&p.pat, Some(entry_ln), |spans, hir_id, ln, var| {
1477 if !self.live_on_entry(ln, var) {
1478 self.report_unsed_assign(hir_id, spans, var, |name| {
1479 format!("value passed to `{}` is never read", name)
1486 fn check_unused_vars_in_pat(
1489 entry_ln: Option<LiveNode>,
1490 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1492 // In an or-pattern, only consider the variable; any later patterns must have the same
1493 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1494 // However, we should take the ids and spans of variables with the same name from the later
1495 // patterns so the suggestions to prefix with underscores will apply to those too.
1496 let mut vars: FxIndexMap<Symbol, (LiveNode, Variable, Vec<(HirId, Span, Span)>)> =
1499 pat.each_binding(|_, hir_id, pat_sp, ident| {
1500 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1501 let var = self.variable(hir_id, ident.span);
1502 let id_and_sp = (hir_id, pat_sp, ident.span);
1503 vars.entry(self.ir.variable_name(var))
1504 .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp))
1505 .or_insert_with(|| (ln, var, vec![id_and_sp]));
1508 for (_, (ln, var, hir_ids_and_spans)) in vars {
1509 if self.used_on_entry(ln, var) {
1510 let id = hir_ids_and_spans[0].0;
1512 hir_ids_and_spans.into_iter().map(|(_, _, ident_span)| ident_span).collect();
1513 on_used_on_entry(spans, id, ln, var);
1515 self.report_unused(hir_ids_and_spans, ln, var);
1522 hir_ids_and_spans: Vec<(HirId, Span, Span)>,
1526 let first_hir_id = hir_ids_and_spans[0].0;
1528 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1529 // annoying: for parameters in funcs like `fn(x: i32)
1530 // {ret}`, there is only one node, so asking about
1531 // assigned_on_exit() is not meaningful.
1533 if ln == self.exit_ln { false } else { self.assigned_on_exit(ln, var) };
1536 self.ir.tcx.struct_span_lint_hir(
1537 lint::builtin::UNUSED_VARIABLES,
1541 .map(|(_, _, ident_span)| ident_span)
1542 .collect::<Vec<_>>(),
1544 lint.build(&format!("variable `{}` is assigned to, but never used", name))
1545 .note(&format!("consider using `_{}` instead", name))
1550 let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
1551 hir_ids_and_spans.iter().copied().partition(|(hir_id, _, ident_span)| {
1552 let var = self.variable(*hir_id, *ident_span);
1553 self.ir.variable_is_shorthand(var)
1556 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1557 // the field" message, and suggest `_` for the non-shorthands. If we only
1558 // have non-shorthand, then prefix with an underscore instead.
1559 if !shorthands.is_empty() {
1560 let shorthands = shorthands
1562 .map(|(_, pat_span, _)| (pat_span, format!("{}: _", name)))
1566 .map(|(_, pat_span, _)| (pat_span, "_".to_string())),
1568 .collect::<Vec<_>>();
1570 self.ir.tcx.struct_span_lint_hir(
1571 lint::builtin::UNUSED_VARIABLES,
1575 .map(|(_, pat_span, _)| *pat_span)
1576 .collect::<Vec<_>>(),
1578 let mut err = lint.build(&format!("unused variable: `{}`", name));
1579 err.multipart_suggestion(
1580 "try ignoring the field",
1582 Applicability::MachineApplicable,
1588 let non_shorthands = non_shorthands
1590 .map(|(_, _, ident_span)| (ident_span, format!("_{}", name)))
1591 .collect::<Vec<_>>();
1593 self.ir.tcx.struct_span_lint_hir(
1594 lint::builtin::UNUSED_VARIABLES,
1598 .map(|(_, _, ident_span)| *ident_span)
1599 .collect::<Vec<_>>(),
1601 let mut err = lint.build(&format!("unused variable: `{}`", name));
1602 err.multipart_suggestion(
1603 "if this is intentional, prefix it with an underscore",
1605 Applicability::MachineApplicable,
1615 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1616 if !self.live_on_exit(ln, var) {
1617 self.report_unsed_assign(hir_id, spans, var, |name| {
1618 format!("value assigned to `{}` is never read", name)
1623 fn report_unsed_assign(
1628 message: impl Fn(&str) -> String,
1630 if let Some(name) = self.should_warn(var) {
1631 self.ir.tcx.struct_span_lint_hir(
1632 lint::builtin::UNUSED_ASSIGNMENTS,
1636 lint.build(&message(&name))
1637 .help("maybe it is overwritten before being read?")