1 //! This file builds up the `ScopeTree`, which describes
2 //! the parent links in the region hierarchy.
4 //! For more information about how MIR-based region-checking works,
5 //! see the [rustc dev guide].
7 //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
9 use rustc_ast
::walk_list
;
10 use rustc_data_structures
::fx
::FxHashSet
;
12 use rustc_hir
::def_id
::DefId
;
13 use rustc_hir
::intravisit
::{self, Visitor}
;
14 use rustc_hir
::{Arm, Block, Expr, Local, Pat, PatKind, Stmt}
;
15 use rustc_index
::vec
::Idx
;
16 use rustc_middle
::middle
::region
::*;
17 use rustc_middle
::ty
::TyCtxt
;
18 use rustc_span
::source_map
;
23 #[derive(Debug, Copy, Clone)]
25 /// The scope that contains any new variables declared, plus its depth in
27 var_parent
: Option
<(Scope
, ScopeDepth
)>,
29 /// Region parent of expressions, etc., plus its depth in the scope tree.
30 parent
: Option
<(Scope
, ScopeDepth
)>,
33 struct RegionResolutionVisitor
<'tcx
> {
36 // The number of expressions and patterns visited in the current body.
37 expr_and_pat_count
: usize,
38 // When this is `true`, we record the `Scopes` we encounter
39 // when processing a Yield expression. This allows us to fix
41 pessimistic_yield
: bool
,
42 // Stores scopes when `pessimistic_yield` is `true`.
43 fixup_scopes
: Vec
<Scope
>,
44 // The generated scope tree.
45 scope_tree
: ScopeTree
,
49 /// `terminating_scopes` is a set containing the ids of each
50 /// statement, or conditional/repeating expression. These scopes
51 /// are calling "terminating scopes" because, when attempting to
52 /// find the scope of a temporary, by default we search up the
53 /// enclosing scopes until we encounter the terminating scope. A
54 /// conditional/repeating expression is one which is not
55 /// guaranteed to execute exactly once upon entering the parent
56 /// scope. This could be because the expression only executes
57 /// conditionally, such as the expression `b` in `a && b`, or
58 /// because the expression may execute many times, such as a loop
59 /// body. The reason that we distinguish such expressions is that,
60 /// upon exiting the parent scope, we cannot statically know how
61 /// many times the expression executed, and thus if the expression
62 /// creates temporaries we cannot know statically how many such
63 /// temporaries we would have to cleanup. Therefore, we ensure that
64 /// the temporaries never outlast the conditional/repeating
65 /// expression, preventing the need for dynamic checks and/or
66 /// arbitrary amounts of stack space. Terminating scopes end
67 /// up being contained in a DestructionScope that contains the
68 /// destructor's execution.
69 terminating_scopes
: FxHashSet
<hir
::ItemLocalId
>,
72 /// Records the lifetime of a local variable as `cx.var_parent`
73 fn record_var_lifetime(
74 visitor
: &mut RegionResolutionVisitor
<'_
>,
75 var_id
: hir
::ItemLocalId
,
78 match visitor
.cx
.var_parent
{
80 // this can happen in extern fn declarations like
82 // extern fn isalnum(c: c_int) -> c_int
84 Some((parent_scope
, _
)) => visitor
.scope_tree
.record_var_scope(var_id
, parent_scope
),
88 fn resolve_block
<'tcx
>(visitor
: &mut RegionResolutionVisitor
<'tcx
>, blk
: &'tcx hir
::Block
<'tcx
>) {
89 debug
!("resolve_block(blk.hir_id={:?})", blk
.hir_id
);
91 let prev_cx
= visitor
.cx
;
93 // We treat the tail expression in the block (if any) somewhat
94 // differently from the statements. The issue has to do with
95 // temporary lifetimes. Consider the following:
98 // let inner = ... (&bar()) ...;
100 // (... (&foo()) ...) // (the tail expression)
101 // }, other_argument());
103 // Each of the statements within the block is a terminating
104 // scope, and thus a temporary (e.g., the result of calling
105 // `bar()` in the initializer expression for `let inner = ...;`)
106 // will be cleaned up immediately after its corresponding
107 // statement (i.e., `let inner = ...;`) executes.
109 // On the other hand, temporaries associated with evaluating the
110 // tail expression for the block are assigned lifetimes so that
111 // they will be cleaned up as part of the terminating scope
112 // *surrounding* the block expression. Here, the terminating
113 // scope for the block expression is the `quux(..)` call; so
114 // those temporaries will only be cleaned up *after* both
115 // `other_argument()` has run and also the call to `quux(..)`
116 // itself has returned.
118 visitor
.enter_node_scope_with_dtor(blk
.hir_id
.local_id
);
119 visitor
.cx
.var_parent
= visitor
.cx
.parent
;
122 // This block should be kept approximately in sync with
123 // `intravisit::walk_block`. (We manually walk the block, rather
124 // than call `walk_block`, in order to maintain precise
125 // index information.)
127 for (i
, statement
) in blk
.stmts
.iter().enumerate() {
128 match statement
.kind
{
129 hir
::StmtKind
::Local(..) | hir
::StmtKind
::Item(..) => {
130 // Each declaration introduces a subscope for bindings
131 // introduced by the declaration; this subscope covers a
132 // suffix of the block. Each subscope in a block has the
133 // previous subscope in the block as a parent, except for
134 // the first such subscope, which has the block itself as a
136 visitor
.enter_scope(Scope
{
137 id
: blk
.hir_id
.local_id
,
138 data
: ScopeData
::Remainder(FirstStatementIndex
::new(i
)),
140 visitor
.cx
.var_parent
= visitor
.cx
.parent
;
142 hir
::StmtKind
::Expr(..) | hir
::StmtKind
::Semi(..) => {}
144 visitor
.visit_stmt(statement
)
146 walk_list
!(visitor
, visit_expr
, &blk
.expr
);
149 visitor
.cx
= prev_cx
;
152 fn resolve_arm
<'tcx
>(visitor
: &mut RegionResolutionVisitor
<'tcx
>, arm
: &'tcx hir
::Arm
<'tcx
>) {
153 let prev_cx
= visitor
.cx
;
155 visitor
.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node }
);
156 visitor
.cx
.var_parent
= visitor
.cx
.parent
;
158 visitor
.terminating_scopes
.insert(arm
.body
.hir_id
.local_id
);
160 if let Some(hir
::Guard
::If(ref expr
)) = arm
.guard
{
161 visitor
.terminating_scopes
.insert(expr
.hir_id
.local_id
);
164 intravisit
::walk_arm(visitor
, arm
);
166 visitor
.cx
= prev_cx
;
169 fn resolve_pat
<'tcx
>(visitor
: &mut RegionResolutionVisitor
<'tcx
>, pat
: &'tcx hir
::Pat
<'tcx
>) {
170 visitor
.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node }
);
172 // If this is a binding then record the lifetime of that binding.
173 if let PatKind
::Binding(..) = pat
.kind
{
174 record_var_lifetime(visitor
, pat
.hir_id
.local_id
, pat
.span
);
177 debug
!("resolve_pat - pre-increment {} pat = {:?}", visitor
.expr_and_pat_count
, pat
);
179 intravisit
::walk_pat(visitor
, pat
);
181 visitor
.expr_and_pat_count
+= 1;
183 debug
!("resolve_pat - post-increment {} pat = {:?}", visitor
.expr_and_pat_count
, pat
);
186 fn resolve_stmt
<'tcx
>(visitor
: &mut RegionResolutionVisitor
<'tcx
>, stmt
: &'tcx hir
::Stmt
<'tcx
>) {
187 let stmt_id
= stmt
.hir_id
.local_id
;
188 debug
!("resolve_stmt(stmt.id={:?})", stmt_id
);
190 // Every statement will clean up the temporaries created during
191 // execution of that statement. Therefore each statement has an
192 // associated destruction scope that represents the scope of the
193 // statement plus its destructors, and thus the scope for which
194 // regions referenced by the destructors need to survive.
195 visitor
.terminating_scopes
.insert(stmt_id
);
197 let prev_parent
= visitor
.cx
.parent
;
198 visitor
.enter_node_scope_with_dtor(stmt_id
);
200 intravisit
::walk_stmt(visitor
, stmt
);
202 visitor
.cx
.parent
= prev_parent
;
205 fn resolve_expr
<'tcx
>(visitor
: &mut RegionResolutionVisitor
<'tcx
>, expr
: &'tcx hir
::Expr
<'tcx
>) {
206 debug
!("resolve_expr - pre-increment {} expr = {:?}", visitor
.expr_and_pat_count
, expr
);
208 let prev_cx
= visitor
.cx
;
209 visitor
.enter_node_scope_with_dtor(expr
.hir_id
.local_id
);
212 let terminating_scopes
= &mut visitor
.terminating_scopes
;
213 let mut terminating
= |id
: hir
::ItemLocalId
| {
214 terminating_scopes
.insert(id
);
217 // Conditional or repeating scopes are always terminating
218 // scopes, meaning that temporaries cannot outlive them.
219 // This ensures fixed size stacks.
220 hir
::ExprKind
::Binary(
221 source_map
::Spanned { node: hir::BinOpKind::And, .. }
,
225 | hir
::ExprKind
::Binary(
226 source_map
::Spanned { node: hir::BinOpKind::Or, .. }
,
230 // For shortcircuiting operators, mark the RHS as a terminating
231 // scope since it only executes conditionally.
232 terminating(r
.hir_id
.local_id
);
235 hir
::ExprKind
::If(_
, ref then
, Some(ref otherwise
)) => {
236 terminating(then
.hir_id
.local_id
);
237 terminating(otherwise
.hir_id
.local_id
);
240 hir
::ExprKind
::If(_
, ref then
, None
) => {
241 terminating(then
.hir_id
.local_id
);
244 hir
::ExprKind
::Loop(ref body
, _
, _
, _
) => {
245 terminating(body
.hir_id
.local_id
);
248 hir
::ExprKind
::DropTemps(ref expr
) => {
249 // `DropTemps(expr)` does not denote a conditional scope.
250 // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
251 terminating(expr
.hir_id
.local_id
);
254 hir
::ExprKind
::AssignOp(..)
255 | hir
::ExprKind
::Index(..)
256 | hir
::ExprKind
::Unary(..)
257 | hir
::ExprKind
::Call(..)
258 | hir
::ExprKind
::MethodCall(..) => {
259 // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
261 // The lifetimes for a call or method call look as follows:
269 // The idea is that call.callee_id represents *the time when
270 // the invoked function is actually running* and call.id
271 // represents *the time to prepare the arguments and make the
272 // call*. See the section "Borrows in Calls" borrowck/README.md
273 // for an extended explanation of why this distinction is
276 // record_superlifetime(new_cx, expr.callee_id);
283 let prev_pessimistic
= visitor
.pessimistic_yield
;
285 // Ordinarily, we can rely on the visit order of HIR intravisit
286 // to correspond to the actual execution order of statements.
287 // However, there's a weird corner case with compound assignment
288 // operators (e.g. `a += b`). The evaluation order depends on whether
289 // or not the operator is overloaded (e.g. whether or not a trait
290 // like AddAssign is implemented).
292 // For primitive types (which, despite having a trait impl, don't actually
293 // end up calling it), the evaluation order is right-to-left. For example,
294 // the following code snippet:
297 // *{println!("LHS!"); y} += {println!("RHS!"); 1};
304 // However, if the operator is used on a non-primitive type,
305 // the evaluation order will be left-to-right, since the operator
306 // actually get desugared to a method call. For example, this
307 // nearly identical code snippet:
309 // let y = &mut String::new();
310 // *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
316 // To determine the actual execution order, we need to perform
317 // trait resolution. Unfortunately, we need to be able to compute
318 // yield_in_scope before type checking is even done, as it gets
319 // used by AST borrowcheck.
321 // Fortunately, we don't need to know the actual execution order.
322 // It suffices to know the 'worst case' order with respect to yields.
323 // Specifically, we need to know the highest 'expr_and_pat_count'
324 // that we could assign to the yield expression. To do this,
325 // we pick the greater of the two values from the left-hand
326 // and right-hand expressions. This makes us overly conservative
327 // about what types could possibly live across yield points,
328 // but we will never fail to detect that a type does actually
329 // live across a yield point. The latter part is critical -
330 // we're already overly conservative about what types will live
331 // across yield points, as the generated MIR will determine
332 // when things are actually live. However, for typecheck to work
333 // properly, we can't miss any types.
336 // Manually recurse over closures and inline consts, because they are the only
337 // case of nested bodies that share the parent environment.
338 hir
::ExprKind
::Closure { body, .. }
339 | hir
::ExprKind
::ConstBlock(hir
::AnonConst { body, .. }
) => {
340 let body
= visitor
.tcx
.hir().body(body
);
341 visitor
.visit_body(body
);
343 hir
::ExprKind
::AssignOp(_
, ref left_expr
, ref right_expr
) => {
345 "resolve_expr - enabling pessimistic_yield, was previously {}",
349 let start_point
= visitor
.fixup_scopes
.len();
350 visitor
.pessimistic_yield
= true;
352 // If the actual execution order turns out to be right-to-left,
353 // then we're fine. However, if the actual execution order is left-to-right,
354 // then we'll assign too low a count to any `yield` expressions
355 // we encounter in 'right_expression' - they should really occur after all of the
356 // expressions in 'left_expression'.
357 visitor
.visit_expr(&right_expr
);
358 visitor
.pessimistic_yield
= prev_pessimistic
;
360 debug
!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic
);
361 visitor
.visit_expr(&left_expr
);
362 debug
!("resolve_expr - fixing up counts to {}", visitor
.expr_and_pat_count
);
364 // Remove and process any scopes pushed by the visitor
365 let target_scopes
= visitor
.fixup_scopes
.drain(start_point
..);
367 for scope
in target_scopes
{
369 visitor
.scope_tree
.yield_in_scope
.get_mut(&scope
).unwrap().last_mut().unwrap();
370 let count
= yield_data
.expr_and_pat_count
;
371 let span
= yield_data
.span
;
373 // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
374 // before walking the left-hand side, it should be impossible for the recorded
375 // count to be greater than the left-hand side count.
376 if count
> visitor
.expr_and_pat_count
{
378 "Encountered greater count {} at span {:?} - expected no greater than {}",
381 visitor
.expr_and_pat_count
384 let new_count
= visitor
.expr_and_pat_count
;
386 "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
387 scope
, count
, new_count
, span
390 yield_data
.expr_and_pat_count
= new_count
;
394 hir
::ExprKind
::If(ref cond
, ref then
, Some(ref otherwise
)) => {
395 let expr_cx
= visitor
.cx
;
396 visitor
.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen }
);
397 visitor
.cx
.var_parent
= visitor
.cx
.parent
;
398 visitor
.visit_expr(cond
);
399 visitor
.visit_expr(then
);
400 visitor
.cx
= expr_cx
;
401 visitor
.visit_expr(otherwise
);
404 hir
::ExprKind
::If(ref cond
, ref then
, None
) => {
405 let expr_cx
= visitor
.cx
;
406 visitor
.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen }
);
407 visitor
.cx
.var_parent
= visitor
.cx
.parent
;
408 visitor
.visit_expr(cond
);
409 visitor
.visit_expr(then
);
410 visitor
.cx
= expr_cx
;
413 _
=> intravisit
::walk_expr(visitor
, expr
),
416 visitor
.expr_and_pat_count
+= 1;
418 debug
!("resolve_expr post-increment {}, expr = {:?}", visitor
.expr_and_pat_count
, expr
);
420 if let hir
::ExprKind
::Yield(_
, source
) = &expr
.kind
{
421 // Mark this expr's scope and all parent scopes as containing `yield`.
422 let mut scope
= Scope { id: expr.hir_id.local_id, data: ScopeData::Node }
;
424 let span
= match expr
.kind
{
425 hir
::ExprKind
::Yield(expr
, hir
::YieldSource
::Await { .. }
) => {
426 expr
.span
.shrink_to_hi().to(expr
.span
)
431 YieldData { span, expr_and_pat_count: visitor.expr_and_pat_count, source: *source }
;
432 match visitor
.scope_tree
.yield_in_scope
.get_mut(&scope
) {
433 Some(yields
) => yields
.push(data
),
435 visitor
.scope_tree
.yield_in_scope
.insert(scope
, vec
![data
]);
439 if visitor
.pessimistic_yield
{
440 debug
!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope
);
441 visitor
.fixup_scopes
.push(scope
);
444 // Keep traversing up while we can.
445 match visitor
.scope_tree
.parent_map
.get(&scope
) {
446 // Don't cross from closure bodies to their parent.
447 Some(&(superscope
, _
)) => match superscope
.data
{
448 ScopeData
::CallSite
=> break,
449 _
=> scope
= superscope
,
456 visitor
.cx
= prev_cx
;
459 fn resolve_local
<'tcx
>(
460 visitor
: &mut RegionResolutionVisitor
<'tcx
>,
461 pat
: Option
<&'tcx hir
::Pat
<'tcx
>>,
462 init
: Option
<&'tcx hir
::Expr
<'tcx
>>,
464 debug
!("resolve_local(pat={:?}, init={:?})", pat
, init
);
466 let blk_scope
= visitor
.cx
.var_parent
.map(|(p
, _
)| p
);
468 // As an exception to the normal rules governing temporary
469 // lifetimes, initializers in a let have a temporary lifetime
470 // of the enclosing block. This means that e.g., a program
471 // like the following is legal:
473 // let ref x = HashMap::new();
475 // Because the hash map will be freed in the enclosing block.
477 // We express the rules more formally based on 3 grammars (defined
478 // fully in the helpers below that implement them):
480 // 1. `E&`, which matches expressions like `&<rvalue>` that
481 // own a pointer into the stack.
483 // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
484 // y)` that produce ref bindings into the value they are
485 // matched against or something (at least partially) owned by
486 // the value they are matched against. (By partially owned,
487 // I mean that creating a binding into a ref-counted or managed value
488 // would still count.)
490 // 3. `ET`, which matches both rvalues like `foo()` as well as places
491 // based on rvalues like `foo().x[2].y`.
493 // A subexpression `<rvalue>` that appears in a let initializer
494 // `let pat [: ty] = expr` has an extended temporary lifetime if
495 // any of the following conditions are met:
497 // A. `pat` matches `P&` and `expr` matches `ET`
498 // (covers cases where `pat` creates ref bindings into an rvalue
499 // produced by `expr`)
500 // B. `ty` is a borrowed pointer and `expr` matches `ET`
501 // (covers cases where coercion creates a borrow)
502 // C. `expr` matches `E&`
503 // (covers cases `expr` borrows an rvalue that is then assigned
504 // to memory (at least partially) owned by the binding)
506 // Here are some examples hopefully giving an intuition where each
507 // rule comes into play and why:
509 // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
510 // would have an extended lifetime, but not `foo()`.
512 // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
515 // In some cases, multiple rules may apply (though not to the same
516 // rvalue). For example:
518 // let ref x = [&a(), &b()];
520 // Here, the expression `[...]` has an extended lifetime due to rule
521 // A, but the inner rvalues `a()` and `b()` have an extended lifetime
524 if let Some(expr
) = init
{
525 record_rvalue_scope_if_borrow_expr(visitor
, &expr
, blk_scope
);
527 if let Some(pat
) = pat
{
528 if is_binding_pat(pat
) {
529 visitor
.scope_tree
.record_rvalue_candidate(
531 RvalueCandidateType
::Pattern
{
532 target
: expr
.hir_id
.local_id
,
540 // Make sure we visit the initializer first, so expr_and_pat_count remains correct
541 if let Some(expr
) = init
{
542 visitor
.visit_expr(expr
);
544 if let Some(pat
) = pat
{
545 visitor
.visit_pat(pat
);
548 /// Returns `true` if `pat` match the `P&` non-terminal.
552 /// | StructName { ..., P&, ... }
553 /// | VariantName(..., P&, ...)
554 /// | [ ..., P&, ... ]
555 /// | ( ..., P&, ... )
556 /// | ... "|" P& "|" ...
559 fn is_binding_pat(pat
: &hir
::Pat
<'_
>) -> bool
{
560 // Note that the code below looks for *explicit* refs only, that is, it won't
561 // know about *implicit* refs as introduced in #42640.
563 // This is not a problem. For example, consider
565 // let (ref x, ref y) = (Foo { .. }, Bar { .. });
567 // Due to the explicit refs on the left hand side, the below code would signal
568 // that the temporary value on the right hand side should live until the end of
569 // the enclosing block (as opposed to being dropped after the let is complete).
571 // To create an implicit ref, however, you must have a borrowed value on the RHS
572 // already, as in this example (which won't compile before #42640):
574 // let Foo { x, .. } = &Foo { x: ..., ... };
578 // let Foo { ref x, .. } = Foo { ... };
580 // In the former case (the implicit ref version), the temporary is created by the
581 // & expression, and its lifetime would be extended to the end of the block (due
582 // to a different rule, not the below code).
584 PatKind
::Binding(hir
::BindingAnnotation
::Ref
, ..)
585 | PatKind
::Binding(hir
::BindingAnnotation
::RefMut
, ..) => true,
587 PatKind
::Struct(_
, ref field_pats
, _
) => {
588 field_pats
.iter().any(|fp
| is_binding_pat(&fp
.pat
))
591 PatKind
::Slice(ref pats1
, ref pats2
, ref pats3
) => {
592 pats1
.iter().any(|p
| is_binding_pat(&p
))
593 || pats2
.iter().any(|p
| is_binding_pat(&p
))
594 || pats3
.iter().any(|p
| is_binding_pat(&p
))
597 PatKind
::Or(ref subpats
)
598 | PatKind
::TupleStruct(_
, ref subpats
, _
)
599 | PatKind
::Tuple(ref subpats
, _
) => subpats
.iter().any(|p
| is_binding_pat(&p
)),
601 PatKind
::Box(ref subpat
) => is_binding_pat(&subpat
),
605 hir
::BindingAnnotation
::Unannotated
| hir
::BindingAnnotation
::Mutable
,
611 | PatKind
::Range(_
, _
, _
) => false,
615 /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
619 /// | StructName { ..., f: E&, ... }
620 /// | [ ..., E&, ... ]
621 /// | ( ..., E&, ... )
627 fn record_rvalue_scope_if_borrow_expr
<'tcx
>(
628 visitor
: &mut RegionResolutionVisitor
<'tcx
>,
629 expr
: &hir
::Expr
<'_
>,
630 blk_id
: Option
<Scope
>,
633 hir
::ExprKind
::AddrOf(_
, _
, subexpr
) => {
634 record_rvalue_scope_if_borrow_expr(visitor
, subexpr
, blk_id
);
635 visitor
.scope_tree
.record_rvalue_candidate(
637 RvalueCandidateType
::Borrow
{
638 target
: subexpr
.hir_id
.local_id
,
643 hir
::ExprKind
::Struct(_
, fields
, _
) => {
644 for field
in fields
{
645 record_rvalue_scope_if_borrow_expr(visitor
, &field
.expr
, blk_id
);
648 hir
::ExprKind
::Array(subexprs
) | hir
::ExprKind
::Tup(subexprs
) => {
649 for subexpr
in subexprs
{
650 record_rvalue_scope_if_borrow_expr(visitor
, &subexpr
, blk_id
);
653 hir
::ExprKind
::Cast(ref subexpr
, _
) => {
654 record_rvalue_scope_if_borrow_expr(visitor
, &subexpr
, blk_id
)
656 hir
::ExprKind
::Block(ref block
, _
) => {
657 if let Some(ref subexpr
) = block
.expr
{
658 record_rvalue_scope_if_borrow_expr(visitor
, &subexpr
, blk_id
);
661 hir
::ExprKind
::Call(..) | hir
::ExprKind
::MethodCall(..) => {
662 // FIXME(@dingxiangfei2009): choose call arguments here
663 // for candidacy for extended parameter rule application
665 hir
::ExprKind
::Index(..) => {
666 // FIXME(@dingxiangfei2009): select the indices
667 // as candidate for rvalue scope rules
674 impl<'tcx
> RegionResolutionVisitor
<'tcx
> {
675 /// Records the current parent (if any) as the parent of `child_scope`.
676 /// Returns the depth of `child_scope`.
677 fn record_child_scope(&mut self, child_scope
: Scope
) -> ScopeDepth
{
678 let parent
= self.cx
.parent
;
679 self.scope_tree
.record_scope_parent(child_scope
, parent
);
680 // If `child_scope` has no parent, it must be the root node, and so has
681 // a depth of 1. Otherwise, its depth is one more than its parent's.
682 parent
.map_or(1, |(_p
, d
)| d
+ 1)
685 /// Records the current parent (if any) as the parent of `child_scope`,
686 /// and sets `child_scope` as the new current parent.
687 fn enter_scope(&mut self, child_scope
: Scope
) {
688 let child_depth
= self.record_child_scope(child_scope
);
689 self.cx
.parent
= Some((child_scope
, child_depth
));
692 fn enter_node_scope_with_dtor(&mut self, id
: hir
::ItemLocalId
) {
693 // If node was previously marked as a terminating scope during the
694 // recursive visit of its parent node in the AST, then we need to
695 // account for the destruction scope representing the scope of
696 // the destructors that run immediately after it completes.
697 if self.terminating_scopes
.contains(&id
) {
698 self.enter_scope(Scope { id, data: ScopeData::Destruction }
);
700 self.enter_scope(Scope { id, data: ScopeData::Node }
);
704 impl<'tcx
> Visitor
<'tcx
> for RegionResolutionVisitor
<'tcx
> {
705 fn visit_block(&mut self, b
: &'tcx Block
<'tcx
>) {
706 resolve_block(self, b
);
709 fn visit_body(&mut self, body
: &'tcx hir
::Body
<'tcx
>) {
710 let body_id
= body
.id();
711 let owner_id
= self.tcx
.hir().body_owner_def_id(body_id
);
714 "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
716 self.tcx
.sess
.source_map().span_to_diagnostic_string(body
.value
.span
),
721 // Save all state that is specific to the outer function
722 // body. These will be restored once down below, once we've
724 let outer_ec
= mem
::replace(&mut self.expr_and_pat_count
, 0);
725 let outer_cx
= self.cx
;
726 let outer_ts
= mem
::take(&mut self.terminating_scopes
);
727 // The 'pessimistic yield' flag is set to true when we are
728 // processing a `+=` statement and have to make pessimistic
729 // control flow assumptions. This doesn't apply to nested
730 // bodies within the `+=` statements. See #69307.
731 let outer_pessimistic_yield
= mem
::replace(&mut self.pessimistic_yield
, false);
732 self.terminating_scopes
.insert(body
.value
.hir_id
.local_id
);
734 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite }
);
735 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments }
);
737 // The arguments and `self` are parented to the fn.
738 self.cx
.var_parent
= self.cx
.parent
.take();
739 for param
in body
.params
{
740 self.visit_pat(¶m
.pat
);
743 // The body of the every fn is a root scope.
744 self.cx
.parent
= self.cx
.var_parent
;
745 if self.tcx
.hir().body_owner_kind(owner_id
).is_fn_or_closure() {
746 self.visit_expr(&body
.value
)
748 // Only functions have an outer terminating (drop) scope, while
749 // temporaries in constant initializers may be 'static, but only
750 // according to rvalue lifetime semantics, using the same
751 // syntactical rules used for let initializers.
753 // e.g., in `let x = &f();`, the temporary holding the result from
754 // the `f()` call lives for the entirety of the surrounding block.
756 // Similarly, `const X: ... = &f();` would have the result of `f()`
757 // live for `'static`, implying (if Drop restrictions on constants
758 // ever get lifted) that the value *could* have a destructor, but
759 // it'd get leaked instead of the destructor running during the
760 // evaluation of `X` (if at all allowed by CTFE).
762 // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
763 // would *not* let the `f()` temporary escape into an outer scope
764 // (i.e., `'static`), which means that after `g` returns, it drops,
765 // and all the associated destruction scope rules apply.
766 self.cx
.var_parent
= None
;
767 resolve_local(self, None
, Some(&body
.value
));
770 if body
.generator_kind
.is_some() {
771 self.scope_tree
.body_expr_count
.insert(body_id
, self.expr_and_pat_count
);
774 // Restore context we had at the start.
775 self.expr_and_pat_count
= outer_ec
;
777 self.terminating_scopes
= outer_ts
;
778 self.pessimistic_yield
= outer_pessimistic_yield
;
781 fn visit_arm(&mut self, a
: &'tcx Arm
<'tcx
>) {
782 resolve_arm(self, a
);
784 fn visit_pat(&mut self, p
: &'tcx Pat
<'tcx
>) {
785 resolve_pat(self, p
);
787 fn visit_stmt(&mut self, s
: &'tcx Stmt
<'tcx
>) {
788 resolve_stmt(self, s
);
790 fn visit_expr(&mut self, ex
: &'tcx Expr
<'tcx
>) {
791 resolve_expr(self, ex
);
793 fn visit_local(&mut self, l
: &'tcx Local
<'tcx
>) {
794 resolve_local(self, Some(&l
.pat
), l
.init
);
798 /// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
799 /// in the case of closures, this will be redirected to the enclosing function.
801 /// Performance: This is a query rather than a simple function to enable
802 /// re-use in incremental scenarios. We may sometimes need to rerun the
803 /// type checker even when the HIR hasn't changed, and in those cases
804 /// we can avoid reconstructing the region scope tree.
805 pub fn region_scope_tree(tcx
: TyCtxt
<'_
>, def_id
: DefId
) -> &ScopeTree
{
806 let typeck_root_def_id
= tcx
.typeck_root_def_id(def_id
);
807 if typeck_root_def_id
!= def_id
{
808 return tcx
.region_scope_tree(typeck_root_def_id
);
811 let id
= tcx
.hir().local_def_id_to_hir_id(def_id
.expect_local());
812 let scope_tree
= if let Some(body_id
) = tcx
.hir().maybe_body_owned_by(id
) {
813 let mut visitor
= RegionResolutionVisitor
{
815 scope_tree
: ScopeTree
::default(),
816 expr_and_pat_count
: 0,
817 cx
: Context { parent: None, var_parent: None }
,
818 terminating_scopes
: Default
::default(),
819 pessimistic_yield
: false,
820 fixup_scopes
: vec
![],
823 let body
= tcx
.hir().body(body_id
);
824 visitor
.scope_tree
.root_body
= Some(body
.value
.hir_id
);
825 visitor
.visit_body(body
);
831 tcx
.arena
.alloc(scope_tree
)