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;
90 use rustc_errors::Diagnostic;
92 use rustc_hir::def::*;
93 use rustc_hir::def_id::{DefId, LocalDefId};
94 use rustc_hir::intravisit::{self, Visitor};
95 use rustc_hir::{Expr, HirId, HirIdMap, HirIdSet};
96 use rustc_index::vec::IndexVec;
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::{BytePos, 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_liveness(tcx: TyCtxt<'_>, def_id: DefId) {
143 let local_def_id = match def_id.as_local() {
145 Some(def_id) => def_id,
148 // Don't run unused pass for #[derive()]
149 let parent = tcx.local_parent(local_def_id);
150 if let DefKind::Impl = tcx.def_kind(parent)
151 && tcx.has_attr(parent.to_def_id(), sym::automatically_derived)
156 // Don't run unused pass for #[naked]
157 if tcx.has_attr(def_id, sym::naked) {
161 let mut maps = IrMaps::new(tcx);
162 let body_id = tcx.hir().body_owned_by(local_def_id);
163 let hir_id = tcx.hir().body_owner(body_id);
164 let body = tcx.hir().body(body_id);
166 if let Some(upvars) = tcx.upvars_mentioned(def_id) {
167 for &var_hir_id in upvars.keys() {
168 let var_name = tcx.hir().name(var_hir_id);
169 maps.add_variable(Upvar(var_hir_id, var_name));
173 // gather up the various local variables, significant expressions,
175 maps.visit_body(body);
178 let mut lsets = Liveness::new(&mut maps, local_def_id);
179 let entry_ln = lsets.compute(&body, hir_id);
180 lsets.log_liveness(entry_ln, body_id.hir_id);
182 // check for various error conditions
183 lsets.visit_body(body);
184 lsets.warn_about_unused_upvars(entry_ln);
185 lsets.warn_about_unused_args(body, entry_ln);
188 pub fn provide(providers: &mut Providers) {
189 *providers = Providers { check_liveness, ..*providers };
192 // ______________________________________________________________________
195 // This is the first pass and the one that drives the main
196 // computation. It walks up and down the IR once. On the way down,
197 // we count for each function the number of variables as well as
198 // liveness nodes. A liveness node is basically an expression or
199 // capture clause that does something of interest: either it has
200 // interesting control flow or it uses/defines a local variable.
202 // On the way back up, at each function node we create liveness sets
203 // (we now know precisely how big to make our various vectors and so
204 // forth) and then do the data-flow propagation to compute the set
205 // of live variables at each program point.
207 // Finally, we run back over the IR one last time and, using the
208 // computed liveness, check various safety conditions. For example,
209 // there must be no live nodes at the definition site for a variable
210 // unless it has an initializer. Similarly, each non-mutable local
211 // variable must not be assigned if there is some successor
212 // assignment. And so forth.
219 #[derive(Copy, Clone, Debug)]
226 #[derive(Copy, Clone, Debug)]
228 Param(HirId, Symbol),
230 Upvar(HirId, Symbol),
233 struct CollectLitsVisitor<'tcx> {
234 lit_exprs: Vec<&'tcx hir::Expr<'tcx>>,
237 impl<'tcx> Visitor<'tcx> for CollectLitsVisitor<'tcx> {
238 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
239 if let hir::ExprKind::Lit(_) = expr.kind {
240 self.lit_exprs.push(expr);
242 intravisit::walk_expr(self, expr);
246 struct IrMaps<'tcx> {
248 live_node_map: HirIdMap<LiveNode>,
249 variable_map: HirIdMap<Variable>,
250 capture_info_map: HirIdMap<Rc<Vec<CaptureInfo>>>,
251 var_kinds: IndexVec<Variable, VarKind>,
252 lnks: IndexVec<LiveNode, LiveNodeKind>,
255 impl<'tcx> IrMaps<'tcx> {
256 fn new(tcx: TyCtxt<'tcx>) -> IrMaps<'tcx> {
259 live_node_map: HirIdMap::default(),
260 variable_map: HirIdMap::default(),
261 capture_info_map: Default::default(),
262 var_kinds: IndexVec::new(),
263 lnks: IndexVec::new(),
267 fn add_live_node(&mut self, lnk: LiveNodeKind) -> LiveNode {
268 let ln = self.lnks.push(lnk);
270 debug!("{:?} is of kind {}", ln, live_node_kind_to_string(lnk, self.tcx));
275 fn add_live_node_for_node(&mut self, hir_id: HirId, lnk: LiveNodeKind) {
276 let ln = self.add_live_node(lnk);
277 self.live_node_map.insert(hir_id, ln);
279 debug!("{:?} is node {:?}", ln, hir_id);
282 fn add_variable(&mut self, vk: VarKind) -> Variable {
283 let v = self.var_kinds.push(vk);
286 Local(LocalInfo { id: node_id, .. }) | Param(node_id, _) | Upvar(node_id, _) => {
287 self.variable_map.insert(node_id, v);
291 debug!("{:?} is {:?}", v, vk);
296 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
297 match self.variable_map.get(&hir_id) {
300 span_bug!(span, "no variable registered for id {:?}", hir_id);
305 fn variable_name(&self, var: Variable) -> Symbol {
306 match self.var_kinds[var] {
307 Local(LocalInfo { name, .. }) | Param(_, name) | Upvar(_, name) => name,
311 fn variable_is_shorthand(&self, var: Variable) -> bool {
312 match self.var_kinds[var] {
313 Local(LocalInfo { is_shorthand, .. }) => is_shorthand,
314 Param(..) | Upvar(..) => false,
318 fn set_captures(&mut self, hir_id: HirId, cs: Vec<CaptureInfo>) {
319 self.capture_info_map.insert(hir_id, Rc::new(cs));
322 fn collect_shorthand_field_ids(&self, pat: &hir::Pat<'tcx>) -> HirIdSet {
323 // For struct patterns, take note of which fields used shorthand
324 // (`x` rather than `x: x`).
325 let mut shorthand_field_ids = HirIdSet::default();
326 let mut pats = VecDeque::new();
329 while let Some(pat) = pats.pop_front() {
330 use rustc_hir::PatKind::*;
332 Binding(.., inner_pat) => {
333 pats.extend(inner_pat.iter());
335 Struct(_, fields, _) => {
336 let (short, not_short): (Vec<_>, _) =
337 fields.iter().partition(|f| f.is_shorthand);
338 shorthand_field_ids.extend(short.iter().map(|f| f.pat.hir_id));
339 pats.extend(not_short.iter().map(|f| f.pat));
341 Ref(inner_pat, _) | Box(inner_pat) => {
342 pats.push_back(inner_pat);
344 TupleStruct(_, inner_pats, _) | Tuple(inner_pats, _) | Or(inner_pats) => {
345 pats.extend(inner_pats.iter());
347 Slice(pre_pats, inner_pat, post_pats) => {
348 pats.extend(pre_pats.iter());
349 pats.extend(inner_pat.iter());
350 pats.extend(post_pats.iter());
359 fn add_from_pat(&mut self, pat: &hir::Pat<'tcx>) {
360 let shorthand_field_ids = self.collect_shorthand_field_ids(pat);
362 pat.each_binding(|_, hir_id, _, ident| {
363 self.add_live_node_for_node(hir_id, VarDefNode(ident.span, hir_id));
364 self.add_variable(Local(LocalInfo {
367 is_shorthand: shorthand_field_ids.contains(&hir_id),
373 impl<'tcx> Visitor<'tcx> for IrMaps<'tcx> {
374 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
375 self.add_from_pat(&local.pat);
376 if local.els.is_some() {
377 self.add_live_node_for_node(local.hir_id, ExprNode(local.span, local.hir_id));
379 intravisit::walk_local(self, local);
382 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
383 self.add_from_pat(&arm.pat);
384 if let Some(hir::Guard::IfLet(ref let_expr)) = arm.guard {
385 self.add_from_pat(let_expr.pat);
387 intravisit::walk_arm(self, arm);
390 fn visit_param(&mut self, param: &'tcx hir::Param<'tcx>) {
391 let shorthand_field_ids = self.collect_shorthand_field_ids(param.pat);
392 param.pat.each_binding(|_bm, hir_id, _x, ident| {
393 let var = match param.pat.kind {
394 rustc_hir::PatKind::Struct(..) => Local(LocalInfo {
397 is_shorthand: shorthand_field_ids.contains(&hir_id),
399 _ => Param(hir_id, ident.name),
401 self.add_variable(var);
403 intravisit::walk_param(self, param);
406 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
408 // live nodes required for uses or definitions of variables:
409 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
410 debug!("expr {}: path that leads to {:?}", expr.hir_id, path.res);
411 if let Res::Local(_var_hir_id) = path.res {
412 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
414 intravisit::walk_expr(self, expr);
416 hir::ExprKind::Closure { .. } => {
417 // Interesting control flow (for loops can contain labeled
418 // breaks or continues)
419 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
421 // Make a live_node for each mentioned variable, with the span
422 // being the location that the variable is used. This results
423 // in better error messages than just pointing at the closure
424 // construction site.
425 let mut call_caps = Vec::new();
426 let closure_def_id = self.tcx.hir().local_def_id(expr.hir_id);
427 if let Some(upvars) = self.tcx.upvars_mentioned(closure_def_id) {
428 call_caps.extend(upvars.keys().map(|var_id| {
429 let upvar = upvars[var_id];
430 let upvar_ln = self.add_live_node(UpvarNode(upvar.span));
431 CaptureInfo { ln: upvar_ln, var_hid: *var_id }
434 self.set_captures(expr.hir_id, call_caps);
435 intravisit::walk_expr(self, expr);
438 hir::ExprKind::Let(let_expr) => {
439 self.add_from_pat(let_expr.pat);
440 intravisit::walk_expr(self, expr);
443 // live nodes required for interesting control flow:
444 hir::ExprKind::If(..)
445 | hir::ExprKind::Match(..)
446 | hir::ExprKind::Loop(..)
447 | hir::ExprKind::Yield(..) => {
448 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
449 intravisit::walk_expr(self, expr);
451 hir::ExprKind::Binary(op, ..) if op.node.is_lazy() => {
452 self.add_live_node_for_node(expr.hir_id, ExprNode(expr.span, expr.hir_id));
453 intravisit::walk_expr(self, expr);
456 // otherwise, live nodes are not required:
457 hir::ExprKind::Index(..)
458 | hir::ExprKind::Field(..)
459 | hir::ExprKind::Array(..)
460 | hir::ExprKind::Call(..)
461 | hir::ExprKind::MethodCall(..)
462 | hir::ExprKind::Tup(..)
463 | hir::ExprKind::Binary(..)
464 | hir::ExprKind::AddrOf(..)
465 | hir::ExprKind::Cast(..)
466 | hir::ExprKind::DropTemps(..)
467 | hir::ExprKind::Unary(..)
468 | hir::ExprKind::Break(..)
469 | hir::ExprKind::Continue(_)
470 | hir::ExprKind::Lit(_)
471 | hir::ExprKind::ConstBlock(..)
472 | hir::ExprKind::Ret(..)
473 | hir::ExprKind::Block(..)
474 | hir::ExprKind::Assign(..)
475 | hir::ExprKind::AssignOp(..)
476 | hir::ExprKind::Struct(..)
477 | hir::ExprKind::Repeat(..)
478 | hir::ExprKind::InlineAsm(..)
479 | hir::ExprKind::Box(..)
480 | hir::ExprKind::Type(..)
482 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
483 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => {
484 intravisit::walk_expr(self, expr);
490 // ______________________________________________________________________
491 // Computing liveness sets
493 // Actually we compute just a bit more than just liveness, but we use
494 // the same basic propagation framework in all cases.
496 const ACC_READ: u32 = 1;
497 const ACC_WRITE: u32 = 2;
498 const ACC_USE: u32 = 4;
500 struct Liveness<'a, 'tcx> {
501 ir: &'a mut IrMaps<'tcx>,
502 typeck_results: &'a ty::TypeckResults<'tcx>,
503 param_env: ty::ParamEnv<'tcx>,
504 closure_min_captures: Option<&'tcx RootVariableMinCaptureList<'tcx>>,
505 successors: IndexVec<LiveNode, Option<LiveNode>>,
506 rwu_table: rwu_table::RWUTable,
508 /// A live node representing a point of execution before closure entry &
509 /// after closure exit. Used to calculate liveness of captured variables
510 /// through calls to the same closure. Used for Fn & FnMut closures only.
511 closure_ln: LiveNode,
512 /// A live node representing every 'exit' from the function, whether it be
513 /// by explicit return, panic, or other means.
516 // mappings from loop node ID to LiveNode
517 // ("break" label should map to loop node ID,
518 // it probably doesn't now)
519 break_ln: HirIdMap<LiveNode>,
520 cont_ln: HirIdMap<LiveNode>,
523 impl<'a, 'tcx> Liveness<'a, 'tcx> {
524 fn new(ir: &'a mut IrMaps<'tcx>, body_owner: LocalDefId) -> Liveness<'a, 'tcx> {
525 let typeck_results = ir.tcx.typeck(body_owner);
526 let param_env = ir.tcx.param_env(body_owner);
527 let closure_min_captures = typeck_results.closure_min_captures.get(&body_owner);
528 let closure_ln = ir.add_live_node(ClosureNode);
529 let exit_ln = ir.add_live_node(ExitNode);
531 let num_live_nodes = ir.lnks.len();
532 let num_vars = ir.var_kinds.len();
538 closure_min_captures,
539 successors: IndexVec::from_elem_n(None, num_live_nodes),
540 rwu_table: rwu_table::RWUTable::new(num_live_nodes, num_vars),
543 break_ln: Default::default(),
544 cont_ln: Default::default(),
548 fn live_node(&self, hir_id: HirId, span: Span) -> LiveNode {
549 match self.ir.live_node_map.get(&hir_id) {
552 // This must be a mismatch between the ir_map construction
553 // above and the propagation code below; the two sets of
554 // code have to agree about which AST nodes are worth
555 // creating liveness nodes for.
556 span_bug!(span, "no live node registered for node {:?}", hir_id);
561 fn variable(&self, hir_id: HirId, span: Span) -> Variable {
562 self.ir.variable(hir_id, span)
565 fn define_bindings_in_pat(&mut self, pat: &hir::Pat<'_>, mut succ: LiveNode) -> LiveNode {
566 // In an or-pattern, only consider the first pattern; any later patterns
567 // must have the same bindings, and we also consider the first pattern
568 // to be the "authoritative" set of ids.
569 pat.each_binding_or_first(&mut |_, hir_id, pat_sp, ident| {
570 let ln = self.live_node(hir_id, pat_sp);
571 let var = self.variable(hir_id, ident.span);
572 self.init_from_succ(ln, succ);
573 self.define(ln, var);
579 fn live_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
580 self.rwu_table.get_reader(ln, var)
583 // Is this variable live on entry to any of its successor nodes?
584 fn live_on_exit(&self, ln: LiveNode, var: Variable) -> bool {
585 let successor = self.successors[ln].unwrap();
586 self.live_on_entry(successor, var)
589 fn used_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
590 self.rwu_table.get_used(ln, var)
593 fn assigned_on_entry(&self, ln: LiveNode, var: Variable) -> bool {
594 self.rwu_table.get_writer(ln, var)
597 fn assigned_on_exit(&self, ln: LiveNode, var: Variable) -> bool {
598 let successor = self.successors[ln].unwrap();
599 self.assigned_on_entry(successor, var)
602 fn write_vars<F>(&self, wr: &mut dyn Write, mut test: F) -> io::Result<()>
604 F: FnMut(Variable) -> bool,
606 for var_idx in 0..self.ir.var_kinds.len() {
607 let var = Variable::from(var_idx);
609 write!(wr, " {:?}", var)?;
615 #[allow(unused_must_use)]
616 fn ln_str(&self, ln: LiveNode) -> String {
617 let mut wr = Vec::new();
619 let wr = &mut wr as &mut dyn Write;
620 write!(wr, "[{:?} of kind {:?} reads", ln, self.ir.lnks[ln]);
621 self.write_vars(wr, |var| self.rwu_table.get_reader(ln, var));
622 write!(wr, " writes");
623 self.write_vars(wr, |var| self.rwu_table.get_writer(ln, var));
625 self.write_vars(wr, |var| self.rwu_table.get_used(ln, var));
627 write!(wr, " precedes {:?}]", self.successors[ln]);
629 String::from_utf8(wr).unwrap()
632 fn log_liveness(&self, entry_ln: LiveNode, hir_id: hir::HirId) {
633 // hack to skip the loop unless debug! is enabled:
635 "^^ liveness computation results for body {} (entry={:?})",
637 for ln_idx in 0..self.ir.lnks.len() {
638 debug!("{:?}", self.ln_str(LiveNode::from(ln_idx)));
646 fn init_empty(&mut self, ln: LiveNode, succ_ln: LiveNode) {
647 self.successors[ln] = Some(succ_ln);
649 // It is not necessary to initialize the RWUs here because they are all
650 // empty when created, and the sets only grow during iterations.
653 fn init_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) {
654 // more efficient version of init_empty() / merge_from_succ()
655 self.successors[ln] = Some(succ_ln);
656 self.rwu_table.copy(ln, succ_ln);
657 debug!("init_from_succ(ln={}, succ={})", self.ln_str(ln), self.ln_str(succ_ln));
660 fn merge_from_succ(&mut self, ln: LiveNode, succ_ln: LiveNode) -> bool {
665 let changed = self.rwu_table.union(ln, succ_ln);
666 debug!("merge_from_succ(ln={:?}, succ={}, changed={})", ln, self.ln_str(succ_ln), changed);
670 // Indicates that a local variable was *defined*; we know that no
671 // uses of the variable can precede the definition (resolve checks
672 // this) so we just clear out all the data.
673 fn define(&mut self, writer: LiveNode, var: Variable) {
674 let used = self.rwu_table.get_used(writer, var);
675 self.rwu_table.set(writer, var, rwu_table::RWU { reader: false, writer: false, used });
676 debug!("{:?} defines {:?}: {}", writer, var, self.ln_str(writer));
679 // Either read, write, or both depending on the acc bitset
680 fn acc(&mut self, ln: LiveNode, var: Variable, acc: u32) {
681 debug!("{:?} accesses[{:x}] {:?}: {}", ln, acc, var, self.ln_str(ln));
683 let mut rwu = self.rwu_table.get(ln, var);
685 if (acc & ACC_WRITE) != 0 {
690 // Important: if we both read/write, must do read second
691 // or else the write will override.
692 if (acc & ACC_READ) != 0 {
696 if (acc & ACC_USE) != 0 {
700 self.rwu_table.set(ln, var, rwu);
703 fn compute(&mut self, body: &hir::Body<'_>, hir_id: HirId) -> LiveNode {
704 debug!("compute: for body {:?}", body.id().hir_id);
706 // # Liveness of captured variables
708 // When computing the liveness for captured variables we take into
709 // account how variable is captured (ByRef vs ByValue) and what is the
710 // closure kind (Generator / FnOnce vs Fn / FnMut).
712 // Variables captured by reference are assumed to be used on the exit
715 // In FnOnce closures, variables captured by value are known to be dead
716 // on exit since it is impossible to call the closure again.
718 // In Fn / FnMut closures, variables captured by value are live on exit
719 // if they are live on the entry to the closure, since only the closure
720 // itself can access them on subsequent calls.
722 if let Some(closure_min_captures) = self.closure_min_captures {
723 // Mark upvars captured by reference as used after closure exits.
724 for (&var_hir_id, min_capture_list) in closure_min_captures {
725 for captured_place in min_capture_list {
726 match captured_place.info.capture_kind {
727 ty::UpvarCapture::ByRef(_) => {
728 let var = self.variable(
730 captured_place.get_capture_kind_span(self.ir.tcx),
732 self.acc(self.exit_ln, var, ACC_READ | ACC_USE);
734 ty::UpvarCapture::ByValue => {}
740 let succ = self.propagate_through_expr(&body.value, self.exit_ln);
742 if self.closure_min_captures.is_none() {
743 // Either not a closure, or closure without any captured variables.
744 // No need to determine liveness of captured variables, since there
749 let ty = self.typeck_results.node_type(hir_id);
751 ty::Closure(_def_id, substs) => match substs.as_closure().kind() {
752 ty::ClosureKind::Fn => {}
753 ty::ClosureKind::FnMut => {}
754 ty::ClosureKind::FnOnce => return succ,
756 ty::Generator(..) => return succ,
760 "{} has upvars so it should have a closure type: {:?}",
767 // Propagate through calls to the closure.
769 self.init_from_succ(self.closure_ln, succ);
770 for param in body.params {
771 param.pat.each_binding(|_bm, hir_id, _x, ident| {
772 let var = self.variable(hir_id, ident.span);
773 self.define(self.closure_ln, var);
777 if !self.merge_from_succ(self.exit_ln, self.closure_ln) {
780 assert_eq!(succ, self.propagate_through_expr(&body.value, self.exit_ln));
786 fn propagate_through_block(&mut self, blk: &hir::Block<'_>, succ: LiveNode) -> LiveNode {
787 if blk.targeted_by_break {
788 self.break_ln.insert(blk.hir_id, succ);
790 let succ = self.propagate_through_opt_expr(blk.expr, succ);
791 blk.stmts.iter().rev().fold(succ, |succ, stmt| self.propagate_through_stmt(stmt, succ))
794 fn propagate_through_stmt(&mut self, stmt: &hir::Stmt<'_>, succ: LiveNode) -> LiveNode {
796 hir::StmtKind::Local(ref local) => {
797 // Note: we mark the variable as defined regardless of whether
798 // there is an initializer. Initially I had thought to only mark
799 // the live variable as defined if it was initialized, and then we
800 // could check for uninit variables just by scanning what is live
801 // at the start of the function. But that doesn't work so well for
802 // immutable variables defined in a loop:
803 // loop { let x; x = 5; }
804 // because the "assignment" loops back around and generates an error.
806 // So now we just check that variables defined w/o an
807 // initializer are not live at the point of their
808 // initialization, which is mildly more complex than checking
809 // once at the func header but otherwise equivalent.
811 if let Some(els) = local.els {
812 // Eventually, `let pat: ty = init else { els };` is mostly equivalent to
813 // `let (bindings, ...) = match init { pat => (bindings, ...), _ => els };`
814 // except that extended lifetime applies at the `init` location.
828 if let Some(init) = local.init {
829 let else_ln = self.propagate_through_block(els, succ);
830 let ln = self.live_node(local.hir_id, local.span);
831 self.init_from_succ(ln, succ);
832 self.merge_from_succ(ln, else_ln);
833 let succ = self.propagate_through_expr(init, ln);
834 self.define_bindings_in_pat(&local.pat, succ)
838 "variable is uninitialized but an unexpected else branch is found"
842 let succ = self.propagate_through_opt_expr(local.init, succ);
843 self.define_bindings_in_pat(&local.pat, succ)
846 hir::StmtKind::Item(..) => succ,
847 hir::StmtKind::Expr(ref expr) | hir::StmtKind::Semi(ref expr) => {
848 self.propagate_through_expr(&expr, succ)
853 fn propagate_through_exprs(&mut self, exprs: &[Expr<'_>], succ: LiveNode) -> LiveNode {
854 exprs.iter().rev().fold(succ, |succ, expr| self.propagate_through_expr(&expr, succ))
857 fn propagate_through_opt_expr(
859 opt_expr: Option<&Expr<'_>>,
862 opt_expr.map_or(succ, |expr| self.propagate_through_expr(expr, succ))
865 fn propagate_through_expr(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
866 debug!("propagate_through_expr: {:?}", expr);
869 // Interesting cases with control flow or which gen/kill
870 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
871 self.access_path(expr.hir_id, path, succ, ACC_READ | ACC_USE)
874 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
876 hir::ExprKind::Closure { .. } => {
877 debug!("{:?} is an ExprKind::Closure", expr);
879 // the construction of a closure itself is not important,
880 // but we have to consider the closed over variables.
886 .unwrap_or_else(|| span_bug!(expr.span, "no registered caps"));
888 caps.iter().rev().fold(succ, |succ, cap| {
889 self.init_from_succ(cap.ln, succ);
890 let var = self.variable(cap.var_hid, expr.span);
891 self.acc(cap.ln, var, ACC_READ | ACC_USE);
896 hir::ExprKind::Let(let_expr) => {
897 let succ = self.propagate_through_expr(let_expr.init, succ);
898 self.define_bindings_in_pat(let_expr.pat, succ)
901 // Note that labels have been resolved, so we don't need to look
902 // at the label ident
903 hir::ExprKind::Loop(ref blk, ..) => self.propagate_through_loop(expr, &blk, succ),
905 hir::ExprKind::Yield(ref e, ..) => {
906 let yield_ln = self.live_node(expr.hir_id, expr.span);
907 self.init_from_succ(yield_ln, succ);
908 self.merge_from_succ(yield_ln, self.exit_ln);
909 self.propagate_through_expr(e, yield_ln)
912 hir::ExprKind::If(ref cond, ref then, ref else_opt) => {
927 self.propagate_through_opt_expr(else_opt.as_ref().map(|e| &**e), succ);
928 let then_ln = self.propagate_through_expr(&then, succ);
929 let ln = self.live_node(expr.hir_id, expr.span);
930 self.init_from_succ(ln, else_ln);
931 self.merge_from_succ(ln, then_ln);
932 self.propagate_through_expr(&cond, ln)
935 hir::ExprKind::Match(ref e, arms, _) => {
950 let ln = self.live_node(expr.hir_id, expr.span);
951 self.init_empty(ln, succ);
953 let body_succ = self.propagate_through_expr(&arm.body, succ);
955 let guard_succ = arm.guard.as_ref().map_or(body_succ, |g| match g {
956 hir::Guard::If(e) => self.propagate_through_expr(e, body_succ),
957 hir::Guard::IfLet(let_expr) => {
958 let let_bind = self.define_bindings_in_pat(let_expr.pat, body_succ);
959 self.propagate_through_expr(let_expr.init, let_bind)
962 let arm_succ = self.define_bindings_in_pat(&arm.pat, guard_succ);
963 self.merge_from_succ(ln, arm_succ);
965 self.propagate_through_expr(&e, ln)
968 hir::ExprKind::Ret(ref o_e) => {
969 // Ignore succ and subst exit_ln.
970 self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), self.exit_ln)
973 hir::ExprKind::Break(label, ref opt_expr) => {
974 // Find which label this break jumps to
975 let target = match label.target_id {
976 Ok(hir_id) => self.break_ln.get(&hir_id),
977 Err(err) => span_bug!(expr.span, "loop scope error: {}", err),
981 // Now that we know the label we're going to,
982 // look it up in the break loop nodes table
985 Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
986 None => span_bug!(expr.span, "`break` to unknown label"),
990 hir::ExprKind::Continue(label) => {
991 // Find which label this expr continues to
994 .unwrap_or_else(|err| span_bug!(expr.span, "loop scope error: {}", err));
996 // Now that we know the label we're going to,
997 // look it up in the continue loop nodes table
1001 .unwrap_or_else(|| span_bug!(expr.span, "continue to unknown label"))
1004 hir::ExprKind::Assign(ref l, ref r, _) => {
1005 // see comment on places in
1006 // propagate_through_place_components()
1007 let succ = self.write_place(&l, succ, ACC_WRITE);
1008 let succ = self.propagate_through_place_components(&l, succ);
1009 self.propagate_through_expr(&r, succ)
1012 hir::ExprKind::AssignOp(_, ref l, ref r) => {
1013 // an overloaded assign op is like a method call
1014 if self.typeck_results.is_method_call(expr) {
1015 let succ = self.propagate_through_expr(&l, succ);
1016 self.propagate_through_expr(&r, succ)
1018 // see comment on places in
1019 // propagate_through_place_components()
1020 let succ = self.write_place(&l, succ, ACC_WRITE | ACC_READ);
1021 let succ = self.propagate_through_expr(&r, succ);
1022 self.propagate_through_place_components(&l, succ)
1026 // Uninteresting cases: just propagate in rev exec order
1027 hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
1029 hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
1030 let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
1034 .fold(succ, |succ, field| self.propagate_through_expr(&field.expr, succ))
1037 hir::ExprKind::Call(ref f, ref args) => {
1038 let succ = self.check_is_ty_uninhabited(expr, succ);
1039 let succ = self.propagate_through_exprs(args, succ);
1040 self.propagate_through_expr(&f, succ)
1043 hir::ExprKind::MethodCall(.., receiver, ref args, _) => {
1044 let succ = self.check_is_ty_uninhabited(expr, succ);
1045 let succ = self.propagate_through_exprs(args, succ);
1046 self.propagate_through_expr(receiver, succ)
1049 hir::ExprKind::Tup(ref exprs) => self.propagate_through_exprs(exprs, succ),
1051 hir::ExprKind::Binary(op, ref l, ref r) if op.node.is_lazy() => {
1052 let r_succ = self.propagate_through_expr(&r, succ);
1054 let ln = self.live_node(expr.hir_id, expr.span);
1055 self.init_from_succ(ln, succ);
1056 self.merge_from_succ(ln, r_succ);
1058 self.propagate_through_expr(&l, ln)
1061 hir::ExprKind::Index(ref l, ref r) | hir::ExprKind::Binary(_, ref l, ref r) => {
1062 let r_succ = self.propagate_through_expr(&r, succ);
1063 self.propagate_through_expr(&l, r_succ)
1066 hir::ExprKind::Box(ref e)
1067 | hir::ExprKind::AddrOf(_, _, ref e)
1068 | hir::ExprKind::Cast(ref e, _)
1069 | hir::ExprKind::Type(ref e, _)
1070 | hir::ExprKind::DropTemps(ref e)
1071 | hir::ExprKind::Unary(_, ref e)
1072 | hir::ExprKind::Repeat(ref e, _) => self.propagate_through_expr(&e, succ),
1074 hir::ExprKind::InlineAsm(ref asm) => {
1075 // Handle non-returning asm
1076 let mut succ = if asm.options.contains(InlineAsmOptions::NORETURN) {
1082 // Do a first pass for writing outputs only
1083 for (op, _op_sp) in asm.operands.iter().rev() {
1085 hir::InlineAsmOperand::In { .. }
1086 | hir::InlineAsmOperand::Const { .. }
1087 | hir::InlineAsmOperand::SymFn { .. }
1088 | hir::InlineAsmOperand::SymStatic { .. } => {}
1089 hir::InlineAsmOperand::Out { expr, .. } => {
1090 if let Some(expr) = expr {
1091 succ = self.write_place(expr, succ, ACC_WRITE);
1094 hir::InlineAsmOperand::InOut { expr, .. } => {
1095 succ = self.write_place(expr, succ, ACC_READ | ACC_WRITE | ACC_USE);
1097 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1098 if let Some(expr) = out_expr {
1099 succ = self.write_place(expr, succ, ACC_WRITE);
1105 // Then do a second pass for inputs
1106 let mut succ = succ;
1107 for (op, _op_sp) in asm.operands.iter().rev() {
1109 hir::InlineAsmOperand::In { expr, .. } => {
1110 succ = self.propagate_through_expr(expr, succ)
1112 hir::InlineAsmOperand::Out { expr, .. } => {
1113 if let Some(expr) = expr {
1114 succ = self.propagate_through_place_components(expr, succ);
1117 hir::InlineAsmOperand::InOut { expr, .. } => {
1118 succ = self.propagate_through_place_components(expr, succ);
1120 hir::InlineAsmOperand::SplitInOut { in_expr, out_expr, .. } => {
1121 if let Some(expr) = out_expr {
1122 succ = self.propagate_through_place_components(expr, succ);
1124 succ = self.propagate_through_expr(in_expr, succ);
1126 hir::InlineAsmOperand::Const { .. }
1127 | hir::InlineAsmOperand::SymFn { .. }
1128 | hir::InlineAsmOperand::SymStatic { .. } => {}
1134 hir::ExprKind::Lit(..)
1135 | hir::ExprKind::ConstBlock(..)
1136 | hir::ExprKind::Err
1137 | hir::ExprKind::Path(hir::QPath::TypeRelative(..))
1138 | hir::ExprKind::Path(hir::QPath::LangItem(..)) => succ,
1140 // Note that labels have been resolved, so we don't need to look
1141 // at the label ident
1142 hir::ExprKind::Block(ref blk, _) => self.propagate_through_block(&blk, succ),
1146 fn propagate_through_place_components(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1149 // In general, the full flow graph structure for an
1150 // assignment/move/etc can be handled in one of two ways,
1151 // depending on whether what is being assigned is a "tracked
1152 // value" or not. A tracked value is basically a local
1153 // variable or argument.
1155 // The two kinds of graphs are:
1157 // Tracked place Untracked place
1158 // ----------------------++-----------------------
1162 // (rvalue) || (rvalue)
1165 // (write of place) || (place components)
1170 // ----------------------++-----------------------
1172 // I will cover the two cases in turn:
1176 // A tracked place is a local variable/argument `x`. In
1177 // these cases, the link_node where the write occurs is linked
1178 // to node id of `x`. The `write_place()` routine generates
1179 // the contents of this node. There are no subcomponents to
1182 // # Non-tracked places
1184 // These are places like `x[5]` or `x.f`. In that case, we
1185 // basically ignore the value which is written to but generate
1186 // reads for the components---`x` in these two examples. The
1187 // components reads are generated by
1188 // `propagate_through_place_components()` (this fn).
1192 // It is still possible to observe assignments to non-places;
1193 // these errors are detected in the later pass borrowck. We
1194 // just ignore such cases and treat them as reads.
1197 hir::ExprKind::Path(_) => succ,
1198 hir::ExprKind::Field(ref e, _) => self.propagate_through_expr(&e, succ),
1199 _ => self.propagate_through_expr(expr, succ),
1203 // see comment on propagate_through_place()
1204 fn write_place(&mut self, expr: &Expr<'_>, succ: LiveNode, acc: u32) -> LiveNode {
1206 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1207 self.access_path(expr.hir_id, path, succ, acc)
1210 // We do not track other places, so just propagate through
1211 // to their subcomponents. Also, it may happen that
1212 // non-places occur here, because those are detected in the
1213 // later pass borrowck.
1226 let ln = self.live_node(hir_id, span);
1228 self.init_from_succ(ln, succ);
1229 let var = self.variable(var_hid, span);
1230 self.acc(ln, var, acc);
1238 path: &hir::Path<'_>,
1243 Res::Local(hid) => self.access_var(hir_id, hid, succ, acc, path.span),
1248 fn propagate_through_loop(
1251 body: &hir::Block<'_>,
1255 We model control flow like this:
1262 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1263 Meanwhile, a `break` expression will have a successor of `succ`.
1267 let ln = self.live_node(expr.hir_id, expr.span);
1268 self.init_empty(ln, succ);
1269 debug!("propagate_through_loop: using id for loop body {} {:?}", expr.hir_id, body);
1271 self.break_ln.insert(expr.hir_id, succ);
1273 self.cont_ln.insert(expr.hir_id, ln);
1275 let body_ln = self.propagate_through_block(body, ln);
1277 // repeat until fixed point is reached:
1278 while self.merge_from_succ(ln, body_ln) {
1279 assert_eq!(body_ln, self.propagate_through_block(body, ln));
1285 fn check_is_ty_uninhabited(&mut self, expr: &Expr<'_>, succ: LiveNode) -> LiveNode {
1286 let ty = self.typeck_results.expr_ty(expr);
1287 let m = self.ir.tcx.parent_module(expr.hir_id).to_def_id();
1288 if self.ir.tcx.is_ty_uninhabited_from(m, ty, self.param_env) {
1289 match self.ir.lnks[succ] {
1290 LiveNodeKind::ExprNode(succ_span, succ_id) => {
1291 self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "expression");
1293 LiveNodeKind::VarDefNode(succ_span, succ_id) => {
1294 self.warn_about_unreachable(expr.span, ty, succ_span, succ_id, "definition");
1304 fn warn_about_unreachable(
1312 if !orig_ty.is_never() {
1313 // Unreachable code warnings are already emitted during type checking.
1314 // However, during type checking, full type information is being
1315 // calculated but not yet available, so the check for diverging
1316 // expressions due to uninhabited result types is pretty crude and
1317 // only checks whether ty.is_never(). Here, we have full type
1318 // information available and can issue warnings for less obviously
1319 // uninhabited types (e.g. empty enums). The check above is used so
1320 // that we do not emit the same warning twice if the uninhabited type
1323 let msg = format!("unreachable {}", descr);
1324 self.ir.tcx.struct_span_lint_hir(
1325 lint::builtin::UNREACHABLE_CODE,
1330 diag.span_label(expr_span, &msg)
1331 .span_label(orig_span, "any code following this expression is unreachable")
1335 "this expression has type `{}`, which is uninhabited",
1345 // _______________________________________________________________________
1346 // Checking for error conditions
1348 impl<'a, 'tcx> Visitor<'tcx> for Liveness<'a, 'tcx> {
1349 fn visit_local(&mut self, local: &'tcx hir::Local<'tcx>) {
1350 self.check_unused_vars_in_pat(&local.pat, None, None, |spans, hir_id, ln, var| {
1351 if local.init.is_some() {
1352 self.warn_about_dead_assign(spans, hir_id, ln, var);
1356 intravisit::walk_local(self, local);
1359 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
1360 check_expr(self, ex);
1361 intravisit::walk_expr(self, ex);
1364 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
1365 self.check_unused_vars_in_pat(&arm.pat, None, None, |_, _, _, _| {});
1366 intravisit::walk_arm(self, arm);
1370 fn check_expr<'tcx>(this: &mut Liveness<'_, 'tcx>, expr: &'tcx Expr<'tcx>) {
1372 hir::ExprKind::Assign(ref l, ..) => {
1373 this.check_place(&l);
1376 hir::ExprKind::AssignOp(_, ref l, _) => {
1377 if !this.typeck_results.is_method_call(expr) {
1378 this.check_place(&l);
1382 hir::ExprKind::InlineAsm(ref asm) => {
1383 for (op, _op_sp) in asm.operands {
1385 hir::InlineAsmOperand::Out { expr, .. } => {
1386 if let Some(expr) = expr {
1387 this.check_place(expr);
1390 hir::InlineAsmOperand::InOut { expr, .. } => {
1391 this.check_place(expr);
1393 hir::InlineAsmOperand::SplitInOut { out_expr, .. } => {
1394 if let Some(out_expr) = out_expr {
1395 this.check_place(out_expr);
1403 hir::ExprKind::Let(let_expr) => {
1404 this.check_unused_vars_in_pat(let_expr.pat, None, None, |_, _, _, _| {});
1407 // no correctness conditions related to liveness
1408 hir::ExprKind::Call(..)
1409 | hir::ExprKind::MethodCall(..)
1410 | hir::ExprKind::Match(..)
1411 | hir::ExprKind::Loop(..)
1412 | hir::ExprKind::Index(..)
1413 | hir::ExprKind::Field(..)
1414 | hir::ExprKind::Array(..)
1415 | hir::ExprKind::Tup(..)
1416 | hir::ExprKind::Binary(..)
1417 | hir::ExprKind::Cast(..)
1418 | hir::ExprKind::If(..)
1419 | hir::ExprKind::DropTemps(..)
1420 | hir::ExprKind::Unary(..)
1421 | hir::ExprKind::Ret(..)
1422 | hir::ExprKind::Break(..)
1423 | hir::ExprKind::Continue(..)
1424 | hir::ExprKind::Lit(_)
1425 | hir::ExprKind::ConstBlock(..)
1426 | hir::ExprKind::Block(..)
1427 | hir::ExprKind::AddrOf(..)
1428 | hir::ExprKind::Struct(..)
1429 | hir::ExprKind::Repeat(..)
1430 | hir::ExprKind::Closure { .. }
1431 | hir::ExprKind::Path(_)
1432 | hir::ExprKind::Yield(..)
1433 | hir::ExprKind::Box(..)
1434 | hir::ExprKind::Type(..)
1435 | hir::ExprKind::Err => {}
1439 impl<'tcx> Liveness<'_, 'tcx> {
1440 fn check_place(&mut self, expr: &'tcx Expr<'tcx>) {
1442 hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) => {
1443 if let Res::Local(var_hid) = path.res {
1444 // Assignment to an immutable variable or argument: only legal
1445 // if there is no later assignment. If this local is actually
1446 // mutable, then check for a reassignment to flag the mutability
1448 let ln = self.live_node(expr.hir_id, expr.span);
1449 let var = self.variable(var_hid, expr.span);
1450 self.warn_about_dead_assign(vec![expr.span], expr.hir_id, ln, var);
1454 // For other kinds of places, no checks are required,
1455 // and any embedded expressions are actually rvalues
1456 intravisit::walk_expr(self, expr);
1461 fn should_warn(&self, var: Variable) -> Option<String> {
1462 let name = self.ir.variable_name(var);
1463 if name == kw::Empty {
1466 let name = name.as_str();
1467 if name.as_bytes()[0] == b'_' {
1470 Some(name.to_owned())
1473 fn warn_about_unused_upvars(&self, entry_ln: LiveNode) {
1474 let Some(closure_min_captures) = self.closure_min_captures else {
1478 // If closure_min_captures is Some(), upvars must be Some() too.
1479 for (&var_hir_id, min_capture_list) in closure_min_captures {
1480 for captured_place in min_capture_list {
1481 match captured_place.info.capture_kind {
1482 ty::UpvarCapture::ByValue => {}
1483 ty::UpvarCapture::ByRef(..) => continue,
1485 let span = captured_place.get_capture_kind_span(self.ir.tcx);
1486 let var = self.variable(var_hir_id, span);
1487 if self.used_on_entry(entry_ln, var) {
1488 if !self.live_on_entry(entry_ln, var) {
1489 if let Some(name) = self.should_warn(var) {
1490 self.ir.tcx.struct_span_lint_hir(
1491 lint::builtin::UNUSED_ASSIGNMENTS,
1494 format!("value captured by `{}` is never read", name),
1495 |lint| lint.help("did you mean to capture by reference instead?"),
1500 if let Some(name) = self.should_warn(var) {
1501 self.ir.tcx.struct_span_lint_hir(
1502 lint::builtin::UNUSED_VARIABLES,
1505 format!("unused variable: `{}`", name),
1506 |lint| lint.help("did you mean to capture by reference instead?"),
1514 fn warn_about_unused_args(&self, body: &hir::Body<'_>, entry_ln: LiveNode) {
1515 for p in body.params {
1516 self.check_unused_vars_in_pat(
1520 |spans, hir_id, ln, var| {
1521 if !self.live_on_entry(ln, var) {
1522 self.report_unused_assign(hir_id, spans, var, |name| {
1523 format!("value passed to `{}` is never read", name)
1531 fn check_unused_vars_in_pat(
1534 entry_ln: Option<LiveNode>,
1535 opt_body: Option<&hir::Body<'_>>,
1536 on_used_on_entry: impl Fn(Vec<Span>, HirId, LiveNode, Variable),
1538 // In an or-pattern, only consider the variable; any later patterns must have the same
1539 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1540 // However, we should take the ids and spans of variables with the same name from the later
1541 // patterns so the suggestions to prefix with underscores will apply to those too.
1542 let mut vars: FxIndexMap<Symbol, (LiveNode, Variable, Vec<(HirId, Span, Span)>)> =
1545 pat.each_binding(|_, hir_id, pat_sp, ident| {
1546 let ln = entry_ln.unwrap_or_else(|| self.live_node(hir_id, pat_sp));
1547 let var = self.variable(hir_id, ident.span);
1548 let id_and_sp = (hir_id, pat_sp, ident.span);
1549 vars.entry(self.ir.variable_name(var))
1550 .and_modify(|(.., hir_ids_and_spans)| hir_ids_and_spans.push(id_and_sp))
1551 .or_insert_with(|| (ln, var, vec![id_and_sp]));
1554 let can_remove = matches!(&pat.kind, hir::PatKind::Struct(_, _, true));
1556 for (_, (ln, var, hir_ids_and_spans)) in vars {
1557 if self.used_on_entry(ln, var) {
1558 let id = hir_ids_and_spans[0].0;
1560 hir_ids_and_spans.into_iter().map(|(_, _, ident_span)| ident_span).collect();
1561 on_used_on_entry(spans, id, ln, var);
1563 self.report_unused(hir_ids_and_spans, ln, var, can_remove, pat, opt_body);
1568 #[instrument(skip(self), level = "INFO")]
1571 hir_ids_and_spans: Vec<(HirId, Span, Span)>,
1576 opt_body: Option<&hir::Body<'_>>,
1578 let first_hir_id = hir_ids_and_spans[0].0;
1580 if let Some(name) = self.should_warn(var).filter(|name| name != "self") {
1581 // annoying: for parameters in funcs like `fn(x: i32)
1582 // {ret}`, there is only one node, so asking about
1583 // assigned_on_exit() is not meaningful.
1585 if ln == self.exit_ln { false } else { self.assigned_on_exit(ln, var) };
1588 self.ir.tcx.struct_span_lint_hir(
1589 lint::builtin::UNUSED_VARIABLES,
1593 .map(|(_, _, ident_span)| ident_span)
1594 .collect::<Vec<_>>(),
1595 format!("variable `{}` is assigned to, but never used", name),
1596 |lint| lint.note(&format!("consider using `_{}` instead", name)),
1598 } else if can_remove {
1599 self.ir.tcx.struct_span_lint_hir(
1600 lint::builtin::UNUSED_VARIABLES,
1602 hir_ids_and_spans.iter().map(|(_, pat_span, _)| *pat_span).collect::<Vec<_>>(),
1603 format!("unused variable: `{}`", name),
1605 lint.multipart_suggestion(
1606 "try removing the field",
1609 .map(|(_, pat_span, _)| {
1615 .span_extend_to_next_char(*pat_span, ',', true);
1616 (span.with_hi(BytePos(span.hi().0 + 1)), String::new())
1619 Applicability::MachineApplicable,
1624 let (shorthands, non_shorthands): (Vec<_>, Vec<_>) =
1625 hir_ids_and_spans.iter().copied().partition(|(hir_id, _, ident_span)| {
1626 let var = self.variable(*hir_id, *ident_span);
1627 self.ir.variable_is_shorthand(var)
1630 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1631 // the field" message, and suggest `_` for the non-shorthands. If we only
1632 // have non-shorthand, then prefix with an underscore instead.
1633 if !shorthands.is_empty() {
1634 let shorthands = shorthands
1636 .map(|(_, pat_span, _)| (pat_span, format!("{}: _", name)))
1640 .map(|(_, pat_span, _)| (pat_span, "_".to_string())),
1642 .collect::<Vec<_>>();
1644 self.ir.tcx.struct_span_lint_hir(
1645 lint::builtin::UNUSED_VARIABLES,
1649 .map(|(_, pat_span, _)| *pat_span)
1650 .collect::<Vec<_>>(),
1651 format!("unused variable: `{}`", name),
1653 lint.multipart_suggestion(
1654 "try ignoring the field",
1656 Applicability::MachineApplicable,
1661 let non_shorthands = non_shorthands
1663 .map(|(_, _, ident_span)| (ident_span, format!("_{}", name)))
1664 .collect::<Vec<_>>();
1666 self.ir.tcx.struct_span_lint_hir(
1667 lint::builtin::UNUSED_VARIABLES,
1671 .map(|(_, _, ident_span)| *ident_span)
1672 .collect::<Vec<_>>(),
1673 format!("unused variable: `{}`", name),
1675 if self.has_added_lit_match_name_span(&name, opt_body, lint) {
1676 lint.span_label(pat.span, "unused variable");
1678 lint.multipart_suggestion(
1679 "if this is intentional, prefix it with an underscore",
1681 Applicability::MachineApplicable,
1690 fn has_added_lit_match_name_span(
1693 opt_body: Option<&hir::Body<'_>>,
1694 err: &mut Diagnostic,
1696 let mut has_litstring = false;
1697 let Some(opt_body) = opt_body else {return false;};
1698 let mut visitor = CollectLitsVisitor { lit_exprs: vec![] };
1699 intravisit::walk_body(&mut visitor, opt_body);
1700 for lit_expr in visitor.lit_exprs {
1701 let hir::ExprKind::Lit(litx) = &lit_expr.kind else { continue };
1702 let rustc_ast::LitKind::Str(syb, _) = litx.node else{ continue; };
1703 let name_str: &str = syb.as_str();
1704 let mut name_pa = String::from("{");
1705 name_pa.push_str(&name);
1707 if name_str.contains(&name_pa) {
1710 "you might have meant to use string interpolation in this string literal",
1712 err.multipart_suggestion(
1713 "string interpolation only works in `format!` invocations",
1715 (lit_expr.span.shrink_to_lo(), "format!(".to_string()),
1716 (lit_expr.span.shrink_to_hi(), ")".to_string()),
1718 Applicability::MachineApplicable,
1720 has_litstring = true;
1726 fn warn_about_dead_assign(&self, spans: Vec<Span>, hir_id: HirId, ln: LiveNode, var: Variable) {
1727 if !self.live_on_exit(ln, var) {
1728 self.report_unused_assign(hir_id, spans, var, |name| {
1729 format!("value assigned to `{}` is never read", name)
1734 fn report_unused_assign(
1739 message: impl Fn(&str) -> String,
1741 if let Some(name) = self.should_warn(var) {
1742 self.ir.tcx.struct_span_lint_hir(
1743 lint::builtin::UNUSED_ASSIGNMENTS,
1747 |lint| lint.help("maybe it is overwritten before being read?"),