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 #[debug_format = "v({})"]
112 pub struct Variable {}
115 rustc_index
::newtype_index
! {
116 #[debug_format = "ln({})"]
117 pub struct LiveNode {}
120 #[derive(Copy, Clone, PartialEq, Debug)]
123 ExprNode(Span
, HirId
),
124 VarDefNode(Span
, HirId
),
129 fn live_node_kind_to_string(lnk
: LiveNodeKind
, tcx
: TyCtxt
<'_
>) -> String
{
130 let sm
= tcx
.sess
.source_map();
132 UpvarNode(s
) => format
!("Upvar node [{}]", sm
.span_to_diagnostic_string(s
)),
133 ExprNode(s
, _
) => format
!("Expr node [{}]", sm
.span_to_diagnostic_string(s
)),
134 VarDefNode(s
, _
) => format
!("Var def node [{}]", sm
.span_to_diagnostic_string(s
)),
135 ClosureNode
=> "Closure node".to_owned(),
136 ExitNode
=> "Exit node".to_owned(),
140 fn check_liveness(tcx
: TyCtxt
<'_
>, def_id
: DefId
) {
141 let local_def_id
= match def_id
.as_local() {
143 Some(def_id
) => def_id
,
146 // Don't run unused pass for #[derive()]
147 let parent
= tcx
.local_parent(local_def_id
);
148 if let DefKind
::Impl
= tcx
.def_kind(parent
)
149 && tcx
.has_attr(parent
.to_def_id(), sym
::automatically_derived
)
154 // Don't run unused pass for #[naked]
155 if tcx
.has_attr(def_id
, sym
::naked
) {
159 let mut maps
= IrMaps
::new(tcx
);
160 let body_id
= tcx
.hir().body_owned_by(local_def_id
);
161 let hir_id
= tcx
.hir().body_owner(body_id
);
162 let body
= tcx
.hir().body(body_id
);
164 if let Some(upvars
) = tcx
.upvars_mentioned(def_id
) {
165 for &var_hir_id
in upvars
.keys() {
166 let var_name
= tcx
.hir().name(var_hir_id
);
167 maps
.add_variable(Upvar(var_hir_id
, var_name
));
171 // gather up the various local variables, significant expressions,
173 maps
.visit_body(body
);
176 let mut lsets
= Liveness
::new(&mut maps
, local_def_id
);
177 let entry_ln
= lsets
.compute(&body
, hir_id
);
178 lsets
.log_liveness(entry_ln
, body_id
.hir_id
);
180 // check for various error conditions
181 lsets
.visit_body(body
);
182 lsets
.warn_about_unused_upvars(entry_ln
);
183 lsets
.warn_about_unused_args(body
, entry_ln
);
186 pub fn provide(providers
: &mut Providers
) {
187 *providers
= Providers { check_liveness, ..*providers }
;
190 // ______________________________________________________________________
193 // This is the first pass and the one that drives the main
194 // computation. It walks up and down the IR once. On the way down,
195 // we count for each function the number of variables as well as
196 // liveness nodes. A liveness node is basically an expression or
197 // capture clause that does something of interest: either it has
198 // interesting control flow or it uses/defines a local variable.
200 // On the way back up, at each function node we create liveness sets
201 // (we now know precisely how big to make our various vectors and so
202 // forth) and then do the data-flow propagation to compute the set
203 // of live variables at each program point.
205 // Finally, we run back over the IR one last time and, using the
206 // computed liveness, check various safety conditions. For example,
207 // there must be no live nodes at the definition site for a variable
208 // unless it has an initializer. Similarly, each non-mutable local
209 // variable must not be assigned if there is some successor
210 // assignment. And so forth.
217 #[derive(Copy, Clone, Debug)]
224 #[derive(Copy, Clone, Debug)]
226 Param(HirId
, Symbol
),
228 Upvar(HirId
, Symbol
),
231 struct CollectLitsVisitor
<'tcx
> {
232 lit_exprs
: Vec
<&'tcx hir
::Expr
<'tcx
>>,
235 impl<'tcx
> Visitor
<'tcx
> for CollectLitsVisitor
<'tcx
> {
236 fn visit_expr(&mut self, expr
: &'tcx Expr
<'tcx
>) {
237 if let hir
::ExprKind
::Lit(_
) = expr
.kind
{
238 self.lit_exprs
.push(expr
);
240 intravisit
::walk_expr(self, expr
);
244 struct IrMaps
<'tcx
> {
246 live_node_map
: HirIdMap
<LiveNode
>,
247 variable_map
: HirIdMap
<Variable
>,
248 capture_info_map
: HirIdMap
<Rc
<Vec
<CaptureInfo
>>>,
249 var_kinds
: IndexVec
<Variable
, VarKind
>,
250 lnks
: IndexVec
<LiveNode
, LiveNodeKind
>,
253 impl<'tcx
> IrMaps
<'tcx
> {
254 fn new(tcx
: TyCtxt
<'tcx
>) -> IrMaps
<'tcx
> {
257 live_node_map
: HirIdMap
::default(),
258 variable_map
: HirIdMap
::default(),
259 capture_info_map
: Default
::default(),
260 var_kinds
: IndexVec
::new(),
261 lnks
: IndexVec
::new(),
265 fn add_live_node(&mut self, lnk
: LiveNodeKind
) -> LiveNode
{
266 let ln
= self.lnks
.push(lnk
);
268 debug
!("{:?} is of kind {}", ln
, live_node_kind_to_string(lnk
, self.tcx
));
273 fn add_live_node_for_node(&mut self, hir_id
: HirId
, lnk
: LiveNodeKind
) {
274 let ln
= self.add_live_node(lnk
);
275 self.live_node_map
.insert(hir_id
, ln
);
277 debug
!("{:?} is node {:?}", ln
, hir_id
);
280 fn add_variable(&mut self, vk
: VarKind
) -> Variable
{
281 let v
= self.var_kinds
.push(vk
);
284 Local(LocalInfo { id: node_id, .. }
) | Param(node_id
, _
) | Upvar(node_id
, _
) => {
285 self.variable_map
.insert(node_id
, v
);
289 debug
!("{:?} is {:?}", v
, vk
);
294 fn variable(&self, hir_id
: HirId
, span
: Span
) -> Variable
{
295 match self.variable_map
.get(&hir_id
) {
298 span_bug
!(span
, "no variable registered for id {:?}", hir_id
);
303 fn variable_name(&self, var
: Variable
) -> Symbol
{
304 match self.var_kinds
[var
] {
305 Local(LocalInfo { name, .. }
) | Param(_
, name
) | Upvar(_
, name
) => name
,
309 fn variable_is_shorthand(&self, var
: Variable
) -> bool
{
310 match self.var_kinds
[var
] {
311 Local(LocalInfo { is_shorthand, .. }
) => is_shorthand
,
312 Param(..) | Upvar(..) => false,
316 fn set_captures(&mut self, hir_id
: HirId
, cs
: Vec
<CaptureInfo
>) {
317 self.capture_info_map
.insert(hir_id
, Rc
::new(cs
));
320 fn collect_shorthand_field_ids(&self, pat
: &hir
::Pat
<'tcx
>) -> HirIdSet
{
321 // For struct patterns, take note of which fields used shorthand
322 // (`x` rather than `x: x`).
323 let mut shorthand_field_ids
= HirIdSet
::default();
324 let mut pats
= VecDeque
::new();
327 while let Some(pat
) = pats
.pop_front() {
328 use rustc_hir
::PatKind
::*;
330 Binding(.., inner_pat
) => {
331 pats
.extend(inner_pat
.iter());
333 Struct(_
, fields
, _
) => {
334 let (short
, not_short
): (Vec
<_
>, _
) =
335 fields
.iter().partition(|f
| f
.is_shorthand
);
336 shorthand_field_ids
.extend(short
.iter().map(|f
| f
.pat
.hir_id
));
337 pats
.extend(not_short
.iter().map(|f
| f
.pat
));
339 Ref(inner_pat
, _
) | Box(inner_pat
) => {
340 pats
.push_back(inner_pat
);
342 TupleStruct(_
, inner_pats
, _
) | Tuple(inner_pats
, _
) | Or(inner_pats
) => {
343 pats
.extend(inner_pats
.iter());
345 Slice(pre_pats
, inner_pat
, post_pats
) => {
346 pats
.extend(pre_pats
.iter());
347 pats
.extend(inner_pat
.iter());
348 pats
.extend(post_pats
.iter());
357 fn add_from_pat(&mut self, pat
: &hir
::Pat
<'tcx
>) {
358 let shorthand_field_ids
= self.collect_shorthand_field_ids(pat
);
360 pat
.each_binding(|_
, hir_id
, _
, ident
| {
361 self.add_live_node_for_node(hir_id
, VarDefNode(ident
.span
, hir_id
));
362 self.add_variable(Local(LocalInfo
{
365 is_shorthand
: shorthand_field_ids
.contains(&hir_id
),
371 impl<'tcx
> Visitor
<'tcx
> for IrMaps
<'tcx
> {
372 fn visit_local(&mut self, local
: &'tcx hir
::Local
<'tcx
>) {
373 self.add_from_pat(&local
.pat
);
374 if local
.els
.is_some() {
375 self.add_live_node_for_node(local
.hir_id
, ExprNode(local
.span
, local
.hir_id
));
377 intravisit
::walk_local(self, local
);
380 fn visit_arm(&mut self, arm
: &'tcx hir
::Arm
<'tcx
>) {
381 self.add_from_pat(&arm
.pat
);
382 if let Some(hir
::Guard
::IfLet(ref let_expr
)) = arm
.guard
{
383 self.add_from_pat(let_expr
.pat
);
385 intravisit
::walk_arm(self, arm
);
388 fn visit_param(&mut self, param
: &'tcx hir
::Param
<'tcx
>) {
389 let shorthand_field_ids
= self.collect_shorthand_field_ids(param
.pat
);
390 param
.pat
.each_binding(|_bm
, hir_id
, _x
, ident
| {
391 let var
= match param
.pat
.kind
{
392 rustc_hir
::PatKind
::Struct(..) => Local(LocalInfo
{
395 is_shorthand
: shorthand_field_ids
.contains(&hir_id
),
397 _
=> Param(hir_id
, ident
.name
),
399 self.add_variable(var
);
401 intravisit
::walk_param(self, param
);
404 fn visit_expr(&mut self, expr
: &'tcx Expr
<'tcx
>) {
406 // live nodes required for uses or definitions of variables:
407 hir
::ExprKind
::Path(hir
::QPath
::Resolved(_
, ref path
)) => {
408 debug
!("expr {}: path that leads to {:?}", expr
.hir_id
, path
.res
);
409 if let Res
::Local(_var_hir_id
) = path
.res
{
410 self.add_live_node_for_node(expr
.hir_id
, ExprNode(expr
.span
, expr
.hir_id
));
412 intravisit
::walk_expr(self, expr
);
414 hir
::ExprKind
::Closure(closure
) => {
415 // Interesting control flow (for loops can contain labeled
416 // breaks or continues)
417 self.add_live_node_for_node(expr
.hir_id
, ExprNode(expr
.span
, expr
.hir_id
));
419 // Make a live_node for each mentioned variable, with the span
420 // being the location that the variable is used. This results
421 // in better error messages than just pointing at the closure
422 // construction site.
423 let mut call_caps
= Vec
::new();
424 if let Some(upvars
) = self.tcx
.upvars_mentioned(closure
.def_id
) {
425 call_caps
.extend(upvars
.keys().map(|var_id
| {
426 let upvar
= upvars
[var_id
];
427 let upvar_ln
= self.add_live_node(UpvarNode(upvar
.span
));
428 CaptureInfo { ln: upvar_ln, var_hid: *var_id }
431 self.set_captures(expr
.hir_id
, call_caps
);
432 intravisit
::walk_expr(self, expr
);
435 hir
::ExprKind
::Let(let_expr
) => {
436 self.add_from_pat(let_expr
.pat
);
437 intravisit
::walk_expr(self, expr
);
440 // live nodes required for interesting control flow:
441 hir
::ExprKind
::If(..)
442 | hir
::ExprKind
::Match(..)
443 | hir
::ExprKind
::Loop(..)
444 | hir
::ExprKind
::Yield(..) => {
445 self.add_live_node_for_node(expr
.hir_id
, ExprNode(expr
.span
, expr
.hir_id
));
446 intravisit
::walk_expr(self, expr
);
448 hir
::ExprKind
::Binary(op
, ..) if op
.node
.is_lazy() => {
449 self.add_live_node_for_node(expr
.hir_id
, ExprNode(expr
.span
, expr
.hir_id
));
450 intravisit
::walk_expr(self, expr
);
453 // otherwise, live nodes are not required:
454 hir
::ExprKind
::Index(..)
455 | hir
::ExprKind
::Field(..)
456 | hir
::ExprKind
::Array(..)
457 | hir
::ExprKind
::Call(..)
458 | hir
::ExprKind
::MethodCall(..)
459 | hir
::ExprKind
::Tup(..)
460 | hir
::ExprKind
::Binary(..)
461 | hir
::ExprKind
::AddrOf(..)
462 | hir
::ExprKind
::Cast(..)
463 | hir
::ExprKind
::DropTemps(..)
464 | hir
::ExprKind
::Unary(..)
465 | hir
::ExprKind
::Break(..)
466 | hir
::ExprKind
::Continue(_
)
467 | hir
::ExprKind
::Lit(_
)
468 | hir
::ExprKind
::ConstBlock(..)
469 | hir
::ExprKind
::Ret(..)
470 | hir
::ExprKind
::Block(..)
471 | hir
::ExprKind
::Assign(..)
472 | hir
::ExprKind
::AssignOp(..)
473 | hir
::ExprKind
::Struct(..)
474 | hir
::ExprKind
::Repeat(..)
475 | hir
::ExprKind
::InlineAsm(..)
476 | hir
::ExprKind
::Box(..)
477 | hir
::ExprKind
::Type(..)
479 | hir
::ExprKind
::Path(hir
::QPath
::TypeRelative(..))
480 | hir
::ExprKind
::Path(hir
::QPath
::LangItem(..)) => {
481 intravisit
::walk_expr(self, expr
);
487 // ______________________________________________________________________
488 // Computing liveness sets
490 // Actually we compute just a bit more than just liveness, but we use
491 // the same basic propagation framework in all cases.
493 const ACC_READ
: u32 = 1;
494 const ACC_WRITE
: u32 = 2;
495 const ACC_USE
: u32 = 4;
497 struct Liveness
<'a
, 'tcx
> {
498 ir
: &'a
mut IrMaps
<'tcx
>,
499 typeck_results
: &'a ty
::TypeckResults
<'tcx
>,
500 param_env
: ty
::ParamEnv
<'tcx
>,
501 closure_min_captures
: Option
<&'tcx RootVariableMinCaptureList
<'tcx
>>,
502 successors
: IndexVec
<LiveNode
, Option
<LiveNode
>>,
503 rwu_table
: rwu_table
::RWUTable
,
505 /// A live node representing a point of execution before closure entry &
506 /// after closure exit. Used to calculate liveness of captured variables
507 /// through calls to the same closure. Used for Fn & FnMut closures only.
508 closure_ln
: LiveNode
,
509 /// A live node representing every 'exit' from the function, whether it be
510 /// by explicit return, panic, or other means.
513 // mappings from loop node ID to LiveNode
514 // ("break" label should map to loop node ID,
515 // it probably doesn't now)
516 break_ln
: HirIdMap
<LiveNode
>,
517 cont_ln
: HirIdMap
<LiveNode
>,
520 impl<'a
, 'tcx
> Liveness
<'a
, 'tcx
> {
521 fn new(ir
: &'a
mut IrMaps
<'tcx
>, body_owner
: LocalDefId
) -> Liveness
<'a
, 'tcx
> {
522 let typeck_results
= ir
.tcx
.typeck(body_owner
);
523 let param_env
= ir
.tcx
.param_env(body_owner
);
524 let closure_min_captures
= typeck_results
.closure_min_captures
.get(&body_owner
);
525 let closure_ln
= ir
.add_live_node(ClosureNode
);
526 let exit_ln
= ir
.add_live_node(ExitNode
);
528 let num_live_nodes
= ir
.lnks
.len();
529 let num_vars
= ir
.var_kinds
.len();
535 closure_min_captures
,
536 successors
: IndexVec
::from_elem_n(None
, num_live_nodes
),
537 rwu_table
: rwu_table
::RWUTable
::new(num_live_nodes
, num_vars
),
540 break_ln
: Default
::default(),
541 cont_ln
: Default
::default(),
545 fn live_node(&self, hir_id
: HirId
, span
: Span
) -> LiveNode
{
546 match self.ir
.live_node_map
.get(&hir_id
) {
549 // This must be a mismatch between the ir_map construction
550 // above and the propagation code below; the two sets of
551 // code have to agree about which AST nodes are worth
552 // creating liveness nodes for.
553 span_bug
!(span
, "no live node registered for node {:?}", hir_id
);
558 fn variable(&self, hir_id
: HirId
, span
: Span
) -> Variable
{
559 self.ir
.variable(hir_id
, span
)
562 fn define_bindings_in_pat(&mut self, pat
: &hir
::Pat
<'_
>, mut succ
: LiveNode
) -> LiveNode
{
563 // In an or-pattern, only consider the first pattern; any later patterns
564 // must have the same bindings, and we also consider the first pattern
565 // to be the "authoritative" set of ids.
566 pat
.each_binding_or_first(&mut |_
, hir_id
, pat_sp
, ident
| {
567 let ln
= self.live_node(hir_id
, pat_sp
);
568 let var
= self.variable(hir_id
, ident
.span
);
569 self.init_from_succ(ln
, succ
);
570 self.define(ln
, var
);
576 fn live_on_entry(&self, ln
: LiveNode
, var
: Variable
) -> bool
{
577 self.rwu_table
.get_reader(ln
, var
)
580 // Is this variable live on entry to any of its successor nodes?
581 fn live_on_exit(&self, ln
: LiveNode
, var
: Variable
) -> bool
{
582 let successor
= self.successors
[ln
].unwrap();
583 self.live_on_entry(successor
, var
)
586 fn used_on_entry(&self, ln
: LiveNode
, var
: Variable
) -> bool
{
587 self.rwu_table
.get_used(ln
, var
)
590 fn assigned_on_entry(&self, ln
: LiveNode
, var
: Variable
) -> bool
{
591 self.rwu_table
.get_writer(ln
, var
)
594 fn assigned_on_exit(&self, ln
: LiveNode
, var
: Variable
) -> bool
{
595 let successor
= self.successors
[ln
].unwrap();
596 self.assigned_on_entry(successor
, var
)
599 fn write_vars
<F
>(&self, wr
: &mut dyn Write
, mut test
: F
) -> io
::Result
<()>
601 F
: FnMut(Variable
) -> bool
,
603 for var_idx
in 0..self.ir
.var_kinds
.len() {
604 let var
= Variable
::from(var_idx
);
606 write
!(wr
, " {:?}", var
)?
;
612 #[allow(unused_must_use)]
613 fn ln_str(&self, ln
: LiveNode
) -> String
{
614 let mut wr
= Vec
::new();
616 let wr
= &mut wr
as &mut dyn Write
;
617 write
!(wr
, "[{:?} of kind {:?} reads", ln
, self.ir
.lnks
[ln
]);
618 self.write_vars(wr
, |var
| self.rwu_table
.get_reader(ln
, var
));
619 write
!(wr
, " writes");
620 self.write_vars(wr
, |var
| self.rwu_table
.get_writer(ln
, var
));
622 self.write_vars(wr
, |var
| self.rwu_table
.get_used(ln
, var
));
624 write
!(wr
, " precedes {:?}]", self.successors
[ln
]);
626 String
::from_utf8(wr
).unwrap()
629 fn log_liveness(&self, entry_ln
: LiveNode
, hir_id
: hir
::HirId
) {
630 // hack to skip the loop unless debug! is enabled:
632 "^^ liveness computation results for body {} (entry={:?})",
634 for ln_idx
in 0..self.ir
.lnks
.len() {
635 debug
!("{:?}", self.ln_str(LiveNode
::from(ln_idx
)));
643 fn init_empty(&mut self, ln
: LiveNode
, succ_ln
: LiveNode
) {
644 self.successors
[ln
] = Some(succ_ln
);
646 // It is not necessary to initialize the RWUs here because they are all
647 // empty when created, and the sets only grow during iterations.
650 fn init_from_succ(&mut self, ln
: LiveNode
, succ_ln
: LiveNode
) {
651 // more efficient version of init_empty() / merge_from_succ()
652 self.successors
[ln
] = Some(succ_ln
);
653 self.rwu_table
.copy(ln
, succ_ln
);
654 debug
!("init_from_succ(ln={}, succ={})", self.ln_str(ln
), self.ln_str(succ_ln
));
657 fn merge_from_succ(&mut self, ln
: LiveNode
, succ_ln
: LiveNode
) -> bool
{
662 let changed
= self.rwu_table
.union(ln
, succ_ln
);
663 debug
!("merge_from_succ(ln={:?}, succ={}, changed={})", ln
, self.ln_str(succ_ln
), changed
);
667 // Indicates that a local variable was *defined*; we know that no
668 // uses of the variable can precede the definition (resolve checks
669 // this) so we just clear out all the data.
670 fn define(&mut self, writer
: LiveNode
, var
: Variable
) {
671 let used
= self.rwu_table
.get_used(writer
, var
);
672 self.rwu_table
.set(writer
, var
, rwu_table
::RWU { reader: false, writer: false, used }
);
673 debug
!("{:?} defines {:?}: {}", writer
, var
, self.ln_str(writer
));
676 // Either read, write, or both depending on the acc bitset
677 fn acc(&mut self, ln
: LiveNode
, var
: Variable
, acc
: u32) {
678 debug
!("{:?} accesses[{:x}] {:?}: {}", ln
, acc
, var
, self.ln_str(ln
));
680 let mut rwu
= self.rwu_table
.get(ln
, var
);
682 if (acc
& ACC_WRITE
) != 0 {
687 // Important: if we both read/write, must do read second
688 // or else the write will override.
689 if (acc
& ACC_READ
) != 0 {
693 if (acc
& ACC_USE
) != 0 {
697 self.rwu_table
.set(ln
, var
, rwu
);
700 fn compute(&mut self, body
: &hir
::Body
<'_
>, hir_id
: HirId
) -> LiveNode
{
701 debug
!("compute: for body {:?}", body
.id().hir_id
);
703 // # Liveness of captured variables
705 // When computing the liveness for captured variables we take into
706 // account how variable is captured (ByRef vs ByValue) and what is the
707 // closure kind (Generator / FnOnce vs Fn / FnMut).
709 // Variables captured by reference are assumed to be used on the exit
712 // In FnOnce closures, variables captured by value are known to be dead
713 // on exit since it is impossible to call the closure again.
715 // In Fn / FnMut closures, variables captured by value are live on exit
716 // if they are live on the entry to the closure, since only the closure
717 // itself can access them on subsequent calls.
719 if let Some(closure_min_captures
) = self.closure_min_captures
{
720 // Mark upvars captured by reference as used after closure exits.
721 for (&var_hir_id
, min_capture_list
) in closure_min_captures
{
722 for captured_place
in min_capture_list
{
723 match captured_place
.info
.capture_kind
{
724 ty
::UpvarCapture
::ByRef(_
) => {
725 let var
= self.variable(
727 captured_place
.get_capture_kind_span(self.ir
.tcx
),
729 self.acc(self.exit_ln
, var
, ACC_READ
| ACC_USE
);
731 ty
::UpvarCapture
::ByValue
=> {}
737 let succ
= self.propagate_through_expr(&body
.value
, self.exit_ln
);
739 if self.closure_min_captures
.is_none() {
740 // Either not a closure, or closure without any captured variables.
741 // No need to determine liveness of captured variables, since there
746 let ty
= self.typeck_results
.node_type(hir_id
);
748 ty
::Closure(_def_id
, substs
) => match substs
.as_closure().kind() {
749 ty
::ClosureKind
::Fn
=> {}
750 ty
::ClosureKind
::FnMut
=> {}
751 ty
::ClosureKind
::FnOnce
=> return succ
,
753 ty
::Generator(..) => return succ
,
757 "{} has upvars so it should have a closure type: {:?}",
764 // Propagate through calls to the closure.
766 self.init_from_succ(self.closure_ln
, succ
);
767 for param
in body
.params
{
768 param
.pat
.each_binding(|_bm
, hir_id
, _x
, ident
| {
769 let var
= self.variable(hir_id
, ident
.span
);
770 self.define(self.closure_ln
, var
);
774 if !self.merge_from_succ(self.exit_ln
, self.closure_ln
) {
777 assert_eq
!(succ
, self.propagate_through_expr(&body
.value
, self.exit_ln
));
783 fn propagate_through_block(&mut self, blk
: &hir
::Block
<'_
>, succ
: LiveNode
) -> LiveNode
{
784 if blk
.targeted_by_break
{
785 self.break_ln
.insert(blk
.hir_id
, succ
);
787 let succ
= self.propagate_through_opt_expr(blk
.expr
, succ
);
788 blk
.stmts
.iter().rev().fold(succ
, |succ
, stmt
| self.propagate_through_stmt(stmt
, succ
))
791 fn propagate_through_stmt(&mut self, stmt
: &hir
::Stmt
<'_
>, succ
: LiveNode
) -> LiveNode
{
793 hir
::StmtKind
::Local(ref local
) => {
794 // Note: we mark the variable as defined regardless of whether
795 // there is an initializer. Initially I had thought to only mark
796 // the live variable as defined if it was initialized, and then we
797 // could check for uninit variables just by scanning what is live
798 // at the start of the function. But that doesn't work so well for
799 // immutable variables defined in a loop:
800 // loop { let x; x = 5; }
801 // because the "assignment" loops back around and generates an error.
803 // So now we just check that variables defined w/o an
804 // initializer are not live at the point of their
805 // initialization, which is mildly more complex than checking
806 // once at the func header but otherwise equivalent.
808 if let Some(els
) = local
.els
{
809 // Eventually, `let pat: ty = init else { els };` is mostly equivalent to
810 // `let (bindings, ...) = match init { pat => (bindings, ...), _ => els };`
811 // except that extended lifetime applies at the `init` location.
825 if let Some(init
) = local
.init
{
826 let else_ln
= self.propagate_through_block(els
, succ
);
827 let ln
= self.live_node(local
.hir_id
, local
.span
);
828 self.init_from_succ(ln
, succ
);
829 self.merge_from_succ(ln
, else_ln
);
830 let succ
= self.propagate_through_expr(init
, ln
);
831 self.define_bindings_in_pat(&local
.pat
, succ
)
835 "variable is uninitialized but an unexpected else branch is found"
839 let succ
= self.propagate_through_opt_expr(local
.init
, succ
);
840 self.define_bindings_in_pat(&local
.pat
, succ
)
843 hir
::StmtKind
::Item(..) => succ
,
844 hir
::StmtKind
::Expr(ref expr
) | hir
::StmtKind
::Semi(ref expr
) => {
845 self.propagate_through_expr(&expr
, succ
)
850 fn propagate_through_exprs(&mut self, exprs
: &[Expr
<'_
>], succ
: LiveNode
) -> LiveNode
{
851 exprs
.iter().rev().fold(succ
, |succ
, expr
| self.propagate_through_expr(&expr
, succ
))
854 fn propagate_through_opt_expr(
856 opt_expr
: Option
<&Expr
<'_
>>,
859 opt_expr
.map_or(succ
, |expr
| self.propagate_through_expr(expr
, succ
))
862 fn propagate_through_expr(&mut self, expr
: &Expr
<'_
>, succ
: LiveNode
) -> LiveNode
{
863 debug
!("propagate_through_expr: {:?}", expr
);
866 // Interesting cases with control flow or which gen/kill
867 hir
::ExprKind
::Path(hir
::QPath
::Resolved(_
, ref path
)) => {
868 self.access_path(expr
.hir_id
, path
, succ
, ACC_READ
| ACC_USE
)
871 hir
::ExprKind
::Field(ref e
, _
) => self.propagate_through_expr(&e
, succ
),
873 hir
::ExprKind
::Closure { .. }
=> {
874 debug
!("{:?} is an ExprKind::Closure", expr
);
876 // the construction of a closure itself is not important,
877 // but we have to consider the closed over variables.
883 .unwrap_or_else(|| span_bug
!(expr
.span
, "no registered caps"));
885 caps
.iter().rev().fold(succ
, |succ
, cap
| {
886 self.init_from_succ(cap
.ln
, succ
);
887 let var
= self.variable(cap
.var_hid
, expr
.span
);
888 self.acc(cap
.ln
, var
, ACC_READ
| ACC_USE
);
893 hir
::ExprKind
::Let(let_expr
) => {
894 let succ
= self.propagate_through_expr(let_expr
.init
, succ
);
895 self.define_bindings_in_pat(let_expr
.pat
, succ
)
898 // Note that labels have been resolved, so we don't need to look
899 // at the label ident
900 hir
::ExprKind
::Loop(ref blk
, ..) => self.propagate_through_loop(expr
, &blk
, succ
),
902 hir
::ExprKind
::Yield(ref e
, ..) => {
903 let yield_ln
= self.live_node(expr
.hir_id
, expr
.span
);
904 self.init_from_succ(yield_ln
, succ
);
905 self.merge_from_succ(yield_ln
, self.exit_ln
);
906 self.propagate_through_expr(e
, yield_ln
)
909 hir
::ExprKind
::If(ref cond
, ref then
, ref else_opt
) => {
923 let else_ln
= self.propagate_through_opt_expr(else_opt
.as_deref(), succ
);
924 let then_ln
= self.propagate_through_expr(&then
, succ
);
925 let ln
= self.live_node(expr
.hir_id
, expr
.span
);
926 self.init_from_succ(ln
, else_ln
);
927 self.merge_from_succ(ln
, then_ln
);
928 self.propagate_through_expr(&cond
, ln
)
931 hir
::ExprKind
::Match(ref e
, arms
, _
) => {
946 let ln
= self.live_node(expr
.hir_id
, expr
.span
);
947 self.init_empty(ln
, succ
);
949 let body_succ
= self.propagate_through_expr(&arm
.body
, succ
);
951 let guard_succ
= arm
.guard
.as_ref().map_or(body_succ
, |g
| match g
{
952 hir
::Guard
::If(e
) => self.propagate_through_expr(e
, body_succ
),
953 hir
::Guard
::IfLet(let_expr
) => {
954 let let_bind
= self.define_bindings_in_pat(let_expr
.pat
, body_succ
);
955 self.propagate_through_expr(let_expr
.init
, let_bind
)
958 let arm_succ
= self.define_bindings_in_pat(&arm
.pat
, guard_succ
);
959 self.merge_from_succ(ln
, arm_succ
);
961 self.propagate_through_expr(&e
, ln
)
964 hir
::ExprKind
::Ret(ref o_e
) => {
965 // Ignore succ and subst exit_ln.
966 self.propagate_through_opt_expr(o_e
.as_deref(), self.exit_ln
)
969 hir
::ExprKind
::Break(label
, ref opt_expr
) => {
970 // Find which label this break jumps to
971 let target
= match label
.target_id
{
972 Ok(hir_id
) => self.break_ln
.get(&hir_id
),
973 Err(err
) => span_bug
!(expr
.span
, "loop scope error: {}", err
),
977 // Now that we know the label we're going to,
978 // look it up in the break loop nodes table
981 Some(b
) => self.propagate_through_opt_expr(opt_expr
.as_deref(), b
),
982 None
=> span_bug
!(expr
.span
, "`break` to unknown label"),
986 hir
::ExprKind
::Continue(label
) => {
987 // Find which label this expr continues to
990 .unwrap_or_else(|err
| span_bug
!(expr
.span
, "loop scope error: {}", err
));
992 // Now that we know the label we're going to,
993 // look it up in the continue loop nodes table
997 .unwrap_or_else(|| span_bug
!(expr
.span
, "continue to unknown label"))
1000 hir
::ExprKind
::Assign(ref l
, ref r
, _
) => {
1001 // see comment on places in
1002 // propagate_through_place_components()
1003 let succ
= self.write_place(&l
, succ
, ACC_WRITE
);
1004 let succ
= self.propagate_through_place_components(&l
, succ
);
1005 self.propagate_through_expr(&r
, succ
)
1008 hir
::ExprKind
::AssignOp(_
, ref l
, ref r
) => {
1009 // an overloaded assign op is like a method call
1010 if self.typeck_results
.is_method_call(expr
) {
1011 let succ
= self.propagate_through_expr(&l
, succ
);
1012 self.propagate_through_expr(&r
, succ
)
1014 // see comment on places in
1015 // propagate_through_place_components()
1016 let succ
= self.write_place(&l
, succ
, ACC_WRITE
| ACC_READ
);
1017 let succ
= self.propagate_through_expr(&r
, succ
);
1018 self.propagate_through_place_components(&l
, succ
)
1022 // Uninteresting cases: just propagate in rev exec order
1023 hir
::ExprKind
::Array(ref exprs
) => self.propagate_through_exprs(exprs
, succ
),
1025 hir
::ExprKind
::Struct(_
, ref fields
, ref with_expr
) => {
1026 let succ
= self.propagate_through_opt_expr(with_expr
.as_deref(), succ
);
1030 .fold(succ
, |succ
, field
| self.propagate_through_expr(&field
.expr
, succ
))
1033 hir
::ExprKind
::Call(ref f
, ref args
) => {
1034 let succ
= self.check_is_ty_uninhabited(expr
, succ
);
1035 let succ
= self.propagate_through_exprs(args
, succ
);
1036 self.propagate_through_expr(&f
, succ
)
1039 hir
::ExprKind
::MethodCall(.., receiver
, ref args
, _
) => {
1040 let succ
= self.check_is_ty_uninhabited(expr
, succ
);
1041 let succ
= self.propagate_through_exprs(args
, succ
);
1042 self.propagate_through_expr(receiver
, succ
)
1045 hir
::ExprKind
::Tup(ref exprs
) => self.propagate_through_exprs(exprs
, succ
),
1047 hir
::ExprKind
::Binary(op
, ref l
, ref r
) if op
.node
.is_lazy() => {
1048 let r_succ
= self.propagate_through_expr(&r
, succ
);
1050 let ln
= self.live_node(expr
.hir_id
, expr
.span
);
1051 self.init_from_succ(ln
, succ
);
1052 self.merge_from_succ(ln
, r_succ
);
1054 self.propagate_through_expr(&l
, ln
)
1057 hir
::ExprKind
::Index(ref l
, ref r
) | hir
::ExprKind
::Binary(_
, ref l
, ref r
) => {
1058 let r_succ
= self.propagate_through_expr(&r
, succ
);
1059 self.propagate_through_expr(&l
, r_succ
)
1062 hir
::ExprKind
::Box(ref e
)
1063 | hir
::ExprKind
::AddrOf(_
, _
, ref e
)
1064 | hir
::ExprKind
::Cast(ref e
, _
)
1065 | hir
::ExprKind
::Type(ref e
, _
)
1066 | hir
::ExprKind
::DropTemps(ref e
)
1067 | hir
::ExprKind
::Unary(_
, ref e
)
1068 | hir
::ExprKind
::Repeat(ref e
, _
) => self.propagate_through_expr(&e
, succ
),
1070 hir
::ExprKind
::InlineAsm(ref asm
) => {
1071 // Handle non-returning asm
1072 let mut succ
= if asm
.options
.contains(InlineAsmOptions
::NORETURN
) {
1078 // Do a first pass for writing outputs only
1079 for (op
, _op_sp
) in asm
.operands
.iter().rev() {
1081 hir
::InlineAsmOperand
::In { .. }
1082 | hir
::InlineAsmOperand
::Const { .. }
1083 | hir
::InlineAsmOperand
::SymFn { .. }
1084 | hir
::InlineAsmOperand
::SymStatic { .. }
=> {}
1085 hir
::InlineAsmOperand
::Out { expr, .. }
=> {
1086 if let Some(expr
) = expr
{
1087 succ
= self.write_place(expr
, succ
, ACC_WRITE
);
1090 hir
::InlineAsmOperand
::InOut { expr, .. }
=> {
1091 succ
= self.write_place(expr
, succ
, ACC_READ
| ACC_WRITE
| ACC_USE
);
1093 hir
::InlineAsmOperand
::SplitInOut { out_expr, .. }
=> {
1094 if let Some(expr
) = out_expr
{
1095 succ
= self.write_place(expr
, succ
, ACC_WRITE
);
1101 // Then do a second pass for inputs
1102 let mut succ
= succ
;
1103 for (op
, _op_sp
) in asm
.operands
.iter().rev() {
1105 hir
::InlineAsmOperand
::In { expr, .. }
=> {
1106 succ
= self.propagate_through_expr(expr
, succ
)
1108 hir
::InlineAsmOperand
::Out { expr, .. }
=> {
1109 if let Some(expr
) = expr
{
1110 succ
= self.propagate_through_place_components(expr
, succ
);
1113 hir
::InlineAsmOperand
::InOut { expr, .. }
=> {
1114 succ
= self.propagate_through_place_components(expr
, succ
);
1116 hir
::InlineAsmOperand
::SplitInOut { in_expr, out_expr, .. }
=> {
1117 if let Some(expr
) = out_expr
{
1118 succ
= self.propagate_through_place_components(expr
, succ
);
1120 succ
= self.propagate_through_expr(in_expr
, succ
);
1122 hir
::InlineAsmOperand
::Const { .. }
1123 | hir
::InlineAsmOperand
::SymFn { .. }
1124 | hir
::InlineAsmOperand
::SymStatic { .. }
=> {}
1130 hir
::ExprKind
::Lit(..)
1131 | hir
::ExprKind
::ConstBlock(..)
1132 | hir
::ExprKind
::Err
1133 | hir
::ExprKind
::Path(hir
::QPath
::TypeRelative(..))
1134 | hir
::ExprKind
::Path(hir
::QPath
::LangItem(..)) => succ
,
1136 // Note that labels have been resolved, so we don't need to look
1137 // at the label ident
1138 hir
::ExprKind
::Block(ref blk
, _
) => self.propagate_through_block(&blk
, succ
),
1142 fn propagate_through_place_components(&mut self, expr
: &Expr
<'_
>, succ
: LiveNode
) -> LiveNode
{
1145 // In general, the full flow graph structure for an
1146 // assignment/move/etc can be handled in one of two ways,
1147 // depending on whether what is being assigned is a "tracked
1148 // value" or not. A tracked value is basically a local
1149 // variable or argument.
1151 // The two kinds of graphs are:
1153 // Tracked place Untracked place
1154 // ----------------------++-----------------------
1158 // (rvalue) || (rvalue)
1161 // (write of place) || (place components)
1166 // ----------------------++-----------------------
1168 // I will cover the two cases in turn:
1172 // A tracked place is a local variable/argument `x`. In
1173 // these cases, the link_node where the write occurs is linked
1174 // to node id of `x`. The `write_place()` routine generates
1175 // the contents of this node. There are no subcomponents to
1178 // # Non-tracked places
1180 // These are places like `x[5]` or `x.f`. In that case, we
1181 // basically ignore the value which is written to but generate
1182 // reads for the components---`x` in these two examples. The
1183 // components reads are generated by
1184 // `propagate_through_place_components()` (this fn).
1188 // It is still possible to observe assignments to non-places;
1189 // these errors are detected in the later pass borrowck. We
1190 // just ignore such cases and treat them as reads.
1193 hir
::ExprKind
::Path(_
) => succ
,
1194 hir
::ExprKind
::Field(ref e
, _
) => self.propagate_through_expr(&e
, succ
),
1195 _
=> self.propagate_through_expr(expr
, succ
),
1199 // see comment on propagate_through_place()
1200 fn write_place(&mut self, expr
: &Expr
<'_
>, succ
: LiveNode
, acc
: u32) -> LiveNode
{
1202 hir
::ExprKind
::Path(hir
::QPath
::Resolved(_
, ref path
)) => {
1203 self.access_path(expr
.hir_id
, path
, succ
, acc
)
1206 // We do not track other places, so just propagate through
1207 // to their subcomponents. Also, it may happen that
1208 // non-places occur here, because those are detected in the
1209 // later pass borrowck.
1222 let ln
= self.live_node(hir_id
, span
);
1224 self.init_from_succ(ln
, succ
);
1225 let var
= self.variable(var_hid
, span
);
1226 self.acc(ln
, var
, acc
);
1234 path
: &hir
::Path
<'_
>,
1239 Res
::Local(hid
) => self.access_var(hir_id
, hid
, succ
, acc
, path
.span
),
1244 fn propagate_through_loop(
1247 body
: &hir
::Block
<'_
>,
1251 We model control flow like this:
1258 Note that a `continue` expression targeting the `loop` will have a successor of `expr`.
1259 Meanwhile, a `break` expression will have a successor of `succ`.
1263 let ln
= self.live_node(expr
.hir_id
, expr
.span
);
1264 self.init_empty(ln
, succ
);
1265 debug
!("propagate_through_loop: using id for loop body {} {:?}", expr
.hir_id
, body
);
1267 self.break_ln
.insert(expr
.hir_id
, succ
);
1269 self.cont_ln
.insert(expr
.hir_id
, ln
);
1271 let body_ln
= self.propagate_through_block(body
, ln
);
1273 // repeat until fixed point is reached:
1274 while self.merge_from_succ(ln
, body_ln
) {
1275 assert_eq
!(body_ln
, self.propagate_through_block(body
, ln
));
1281 fn check_is_ty_uninhabited(&mut self, expr
: &Expr
<'_
>, succ
: LiveNode
) -> LiveNode
{
1282 let ty
= self.typeck_results
.expr_ty(expr
);
1283 let m
= self.ir
.tcx
.parent_module(expr
.hir_id
).to_def_id();
1284 if ty
.is_inhabited_from(self.ir
.tcx
, m
, self.param_env
) {
1287 match self.ir
.lnks
[succ
] {
1288 LiveNodeKind
::ExprNode(succ_span
, succ_id
) => {
1289 self.warn_about_unreachable(expr
.span
, ty
, succ_span
, succ_id
, "expression");
1291 LiveNodeKind
::VarDefNode(succ_span
, succ_id
) => {
1292 self.warn_about_unreachable(expr
.span
, ty
, succ_span
, succ_id
, "definition");
1299 fn warn_about_unreachable(
1307 if !orig_ty
.is_never() {
1308 // Unreachable code warnings are already emitted during type checking.
1309 // However, during type checking, full type information is being
1310 // calculated but not yet available, so the check for diverging
1311 // expressions due to uninhabited result types is pretty crude and
1312 // only checks whether ty.is_never(). Here, we have full type
1313 // information available and can issue warnings for less obviously
1314 // uninhabited types (e.g. empty enums). The check above is used so
1315 // that we do not emit the same warning twice if the uninhabited type
1318 let msg
= format
!("unreachable {}", descr
);
1319 self.ir
.tcx
.struct_span_lint_hir(
1320 lint
::builtin
::UNREACHABLE_CODE
,
1325 diag
.span_label(expr_span
, &msg
)
1326 .span_label(orig_span
, "any code following this expression is unreachable")
1330 "this expression has type `{}`, which is uninhabited",
1340 // _______________________________________________________________________
1341 // Checking for error conditions
1343 impl<'a
, 'tcx
> Visitor
<'tcx
> for Liveness
<'a
, 'tcx
> {
1344 fn visit_local(&mut self, local
: &'tcx hir
::Local
<'tcx
>) {
1345 self.check_unused_vars_in_pat(&local
.pat
, None
, None
, |spans
, hir_id
, ln
, var
| {
1346 if local
.init
.is_some() {
1347 self.warn_about_dead_assign(spans
, hir_id
, ln
, var
);
1351 intravisit
::walk_local(self, local
);
1354 fn visit_expr(&mut self, ex
: &'tcx Expr
<'tcx
>) {
1355 check_expr(self, ex
);
1356 intravisit
::walk_expr(self, ex
);
1359 fn visit_arm(&mut self, arm
: &'tcx hir
::Arm
<'tcx
>) {
1360 self.check_unused_vars_in_pat(&arm
.pat
, None
, None
, |_
, _
, _
, _
| {}
);
1361 intravisit
::walk_arm(self, arm
);
1365 fn check_expr
<'tcx
>(this
: &mut Liveness
<'_
, 'tcx
>, expr
: &'tcx Expr
<'tcx
>) {
1367 hir
::ExprKind
::Assign(ref l
, ..) => {
1368 this
.check_place(&l
);
1371 hir
::ExprKind
::AssignOp(_
, ref l
, _
) => {
1372 if !this
.typeck_results
.is_method_call(expr
) {
1373 this
.check_place(&l
);
1377 hir
::ExprKind
::InlineAsm(ref asm
) => {
1378 for (op
, _op_sp
) in asm
.operands
{
1380 hir
::InlineAsmOperand
::Out { expr, .. }
=> {
1381 if let Some(expr
) = expr
{
1382 this
.check_place(expr
);
1385 hir
::InlineAsmOperand
::InOut { expr, .. }
=> {
1386 this
.check_place(expr
);
1388 hir
::InlineAsmOperand
::SplitInOut { out_expr, .. }
=> {
1389 if let Some(out_expr
) = out_expr
{
1390 this
.check_place(out_expr
);
1398 hir
::ExprKind
::Let(let_expr
) => {
1399 this
.check_unused_vars_in_pat(let_expr
.pat
, None
, None
, |_
, _
, _
, _
| {}
);
1402 // no correctness conditions related to liveness
1403 hir
::ExprKind
::Call(..)
1404 | hir
::ExprKind
::MethodCall(..)
1405 | hir
::ExprKind
::Match(..)
1406 | hir
::ExprKind
::Loop(..)
1407 | hir
::ExprKind
::Index(..)
1408 | hir
::ExprKind
::Field(..)
1409 | hir
::ExprKind
::Array(..)
1410 | hir
::ExprKind
::Tup(..)
1411 | hir
::ExprKind
::Binary(..)
1412 | hir
::ExprKind
::Cast(..)
1413 | hir
::ExprKind
::If(..)
1414 | hir
::ExprKind
::DropTemps(..)
1415 | hir
::ExprKind
::Unary(..)
1416 | hir
::ExprKind
::Ret(..)
1417 | hir
::ExprKind
::Break(..)
1418 | hir
::ExprKind
::Continue(..)
1419 | hir
::ExprKind
::Lit(_
)
1420 | hir
::ExprKind
::ConstBlock(..)
1421 | hir
::ExprKind
::Block(..)
1422 | hir
::ExprKind
::AddrOf(..)
1423 | hir
::ExprKind
::Struct(..)
1424 | hir
::ExprKind
::Repeat(..)
1425 | hir
::ExprKind
::Closure { .. }
1426 | hir
::ExprKind
::Path(_
)
1427 | hir
::ExprKind
::Yield(..)
1428 | hir
::ExprKind
::Box(..)
1429 | hir
::ExprKind
::Type(..)
1430 | hir
::ExprKind
::Err
=> {}
1434 impl<'tcx
> Liveness
<'_
, 'tcx
> {
1435 fn check_place(&mut self, expr
: &'tcx Expr
<'tcx
>) {
1437 hir
::ExprKind
::Path(hir
::QPath
::Resolved(_
, ref path
)) => {
1438 if let Res
::Local(var_hid
) = path
.res
{
1439 // Assignment to an immutable variable or argument: only legal
1440 // if there is no later assignment. If this local is actually
1441 // mutable, then check for a reassignment to flag the mutability
1443 let ln
= self.live_node(expr
.hir_id
, expr
.span
);
1444 let var
= self.variable(var_hid
, expr
.span
);
1445 self.warn_about_dead_assign(vec
![expr
.span
], expr
.hir_id
, ln
, var
);
1449 // For other kinds of places, no checks are required,
1450 // and any embedded expressions are actually rvalues
1451 intravisit
::walk_expr(self, expr
);
1456 fn should_warn(&self, var
: Variable
) -> Option
<String
> {
1457 let name
= self.ir
.variable_name(var
);
1458 if name
== kw
::Empty
{
1461 let name
= name
.as_str();
1462 if name
.as_bytes()[0] == b'_'
{
1465 Some(name
.to_owned())
1468 fn warn_about_unused_upvars(&self, entry_ln
: LiveNode
) {
1469 let Some(closure_min_captures
) = self.closure_min_captures
else {
1473 // If closure_min_captures is Some(), upvars must be Some() too.
1474 for (&var_hir_id
, min_capture_list
) in closure_min_captures
{
1475 for captured_place
in min_capture_list
{
1476 match captured_place
.info
.capture_kind
{
1477 ty
::UpvarCapture
::ByValue
=> {}
1478 ty
::UpvarCapture
::ByRef(..) => continue,
1480 let span
= captured_place
.get_capture_kind_span(self.ir
.tcx
);
1481 let var
= self.variable(var_hir_id
, span
);
1482 if self.used_on_entry(entry_ln
, var
) {
1483 if !self.live_on_entry(entry_ln
, var
) {
1484 if let Some(name
) = self.should_warn(var
) {
1485 self.ir
.tcx
.struct_span_lint_hir(
1486 lint
::builtin
::UNUSED_ASSIGNMENTS
,
1489 format
!("value captured by `{}` is never read", name
),
1490 |lint
| lint
.help("did you mean to capture by reference instead?"),
1495 if let Some(name
) = self.should_warn(var
) {
1496 self.ir
.tcx
.struct_span_lint_hir(
1497 lint
::builtin
::UNUSED_VARIABLES
,
1500 format
!("unused variable: `{}`", name
),
1501 |lint
| lint
.help("did you mean to capture by reference instead?"),
1509 fn warn_about_unused_args(&self, body
: &hir
::Body
<'_
>, entry_ln
: LiveNode
) {
1510 for p
in body
.params
{
1511 self.check_unused_vars_in_pat(
1515 |spans
, hir_id
, ln
, var
| {
1516 if !self.live_on_entry(ln
, var
) {
1517 self.report_unused_assign(hir_id
, spans
, var
, |name
| {
1518 format
!("value passed to `{}` is never read", name
)
1526 fn check_unused_vars_in_pat(
1529 entry_ln
: Option
<LiveNode
>,
1530 opt_body
: Option
<&hir
::Body
<'_
>>,
1531 on_used_on_entry
: impl Fn(Vec
<Span
>, HirId
, LiveNode
, Variable
),
1533 // In an or-pattern, only consider the variable; any later patterns must have the same
1534 // bindings, and we also consider the first pattern to be the "authoritative" set of ids.
1535 // However, we should take the ids and spans of variables with the same name from the later
1536 // patterns so the suggestions to prefix with underscores will apply to those too.
1537 let mut vars
: FxIndexMap
<Symbol
, (LiveNode
, Variable
, Vec
<(HirId
, Span
, Span
)>)> =
1540 pat
.each_binding(|_
, hir_id
, pat_sp
, ident
| {
1541 let ln
= entry_ln
.unwrap_or_else(|| self.live_node(hir_id
, pat_sp
));
1542 let var
= self.variable(hir_id
, ident
.span
);
1543 let id_and_sp
= (hir_id
, pat_sp
, ident
.span
);
1544 vars
.entry(self.ir
.variable_name(var
))
1545 .and_modify(|(.., hir_ids_and_spans
)| hir_ids_and_spans
.push(id_and_sp
))
1546 .or_insert_with(|| (ln
, var
, vec
![id_and_sp
]));
1549 let can_remove
= match pat
.kind
{
1550 hir
::PatKind
::Struct(_
, fields
, true) => {
1551 // if all fields are shorthand, remove the struct field, otherwise, mark with _ as prefix
1552 fields
.iter().all(|f
| f
.is_shorthand
)
1557 for (_
, (ln
, var
, hir_ids_and_spans
)) in vars
{
1558 if self.used_on_entry(ln
, var
) {
1559 let id
= hir_ids_and_spans
[0].0;
1561 hir_ids_and_spans
.into_iter().map(|(_
, _
, ident_span
)| ident_span
).collect();
1562 on_used_on_entry(spans
, id
, ln
, var
);
1564 self.report_unused(hir_ids_and_spans
, ln
, var
, can_remove
, pat
, opt_body
);
1569 #[instrument(skip(self), level = "INFO")]
1572 hir_ids_and_spans
: Vec
<(HirId
, Span
, Span
)>,
1577 opt_body
: Option
<&hir
::Body
<'_
>>,
1579 let first_hir_id
= hir_ids_and_spans
[0].0;
1581 if let Some(name
) = self.should_warn(var
).filter(|name
| name
!= "self") {
1582 // annoying: for parameters in funcs like `fn(x: i32)
1583 // {ret}`, there is only one node, so asking about
1584 // assigned_on_exit() is not meaningful.
1586 if ln
== self.exit_ln { false }
else { self.assigned_on_exit(ln, var) }
;
1589 self.ir
.tcx
.struct_span_lint_hir(
1590 lint
::builtin
::UNUSED_VARIABLES
,
1594 .map(|(_
, _
, ident_span
)| ident_span
)
1595 .collect
::<Vec
<_
>>(),
1596 format
!("variable `{}` is assigned to, but never used", name
),
1597 |lint
| lint
.note(&format
!("consider using `_{}` instead", name
)),
1599 } else if can_remove
{
1600 self.ir
.tcx
.struct_span_lint_hir(
1601 lint
::builtin
::UNUSED_VARIABLES
,
1603 hir_ids_and_spans
.iter().map(|(_
, pat_span
, _
)| *pat_span
).collect
::<Vec
<_
>>(),
1604 format
!("unused variable: `{}`", name
),
1606 lint
.multipart_suggestion(
1607 "try removing the field",
1610 .map(|(_
, pat_span
, _
)| {
1616 .span_extend_to_next_char(*pat_span
, '
,'
, true);
1617 (span
.with_hi(BytePos(span
.hi().0 + 1)), String
::new())
1620 Applicability
::MachineApplicable
,
1625 let (shorthands
, non_shorthands
): (Vec
<_
>, Vec
<_
>) =
1626 hir_ids_and_spans
.iter().copied().partition(|(hir_id
, _
, ident_span
)| {
1627 let var
= self.variable(*hir_id
, *ident_span
);
1628 self.ir
.variable_is_shorthand(var
)
1631 // If we have both shorthand and non-shorthand, prefer the "try ignoring
1632 // the field" message, and suggest `_` for the non-shorthands. If we only
1633 // have non-shorthand, then prefix with an underscore instead.
1634 if !shorthands
.is_empty() {
1635 let shorthands
= shorthands
1637 .map(|(_
, pat_span
, _
)| (pat_span
, format
!("{}: _", name
)))
1641 .map(|(_
, pat_span
, _
)| (pat_span
, "_".to_string())),
1643 .collect
::<Vec
<_
>>();
1645 self.ir
.tcx
.struct_span_lint_hir(
1646 lint
::builtin
::UNUSED_VARIABLES
,
1650 .map(|(_
, pat_span
, _
)| *pat_span
)
1651 .collect
::<Vec
<_
>>(),
1652 format
!("unused variable: `{}`", name
),
1654 lint
.multipart_suggestion(
1655 "try ignoring the field",
1657 Applicability
::MachineApplicable
,
1662 let non_shorthands
= non_shorthands
1664 .map(|(_
, _
, ident_span
)| (ident_span
, format
!("_{}", name
)))
1665 .collect
::<Vec
<_
>>();
1667 self.ir
.tcx
.struct_span_lint_hir(
1668 lint
::builtin
::UNUSED_VARIABLES
,
1672 .map(|(_
, _
, ident_span
)| *ident_span
)
1673 .collect
::<Vec
<_
>>(),
1674 format
!("unused variable: `{}`", name
),
1676 if self.has_added_lit_match_name_span(&name
, opt_body
, lint
) {
1677 lint
.span_label(pat
.span
, "unused variable");
1679 lint
.multipart_suggestion(
1680 "if this is intentional, prefix it with an underscore",
1682 Applicability
::MachineApplicable
,
1691 fn has_added_lit_match_name_span(
1694 opt_body
: Option
<&hir
::Body
<'_
>>,
1695 err
: &mut Diagnostic
,
1697 let mut has_litstring
= false;
1698 let Some(opt_body
) = opt_body
else {return false;}
;
1699 let mut visitor
= CollectLitsVisitor { lit_exprs: vec![] }
;
1700 intravisit
::walk_body(&mut visitor
, opt_body
);
1701 for lit_expr
in visitor
.lit_exprs
{
1702 let hir
::ExprKind
::Lit(litx
) = &lit_expr
.kind
else { continue }
;
1703 let rustc_ast
::LitKind
::Str(syb
, _
) = litx
.node
else{ continue; }
;
1704 let name_str
: &str = syb
.as_str();
1705 let mut name_pa
= String
::from("{");
1706 name_pa
.push_str(&name
);
1708 if name_str
.contains(&name_pa
) {
1711 "you might have meant to use string interpolation in this string literal",
1713 err
.multipart_suggestion(
1714 "string interpolation only works in `format!` invocations",
1716 (lit_expr
.span
.shrink_to_lo(), "format!(".to_string()),
1717 (lit_expr
.span
.shrink_to_hi(), ")".to_string()),
1719 Applicability
::MachineApplicable
,
1721 has_litstring
= true;
1727 fn warn_about_dead_assign(&self, spans
: Vec
<Span
>, hir_id
: HirId
, ln
: LiveNode
, var
: Variable
) {
1728 if !self.live_on_exit(ln
, var
) {
1729 self.report_unused_assign(hir_id
, spans
, var
, |name
| {
1730 format
!("value assigned to `{}` is never read", name
)
1735 fn report_unused_assign(
1740 message
: impl Fn(&str) -> String
,
1742 if let Some(name
) = self.should_warn(var
) {
1743 self.ir
.tcx
.struct_span_lint_hir(
1744 lint
::builtin
::UNUSED_ASSIGNMENTS
,
1748 |lint
| lint
.help("maybe it is overwritten before being read?"),