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::Local { els: Some(els), .. }) => {
130 // Let-else has a special lexical structure for variables.
131 // First we take a checkpoint of the current scope context here.
132 let mut prev_cx = visitor.cx;
134 visitor.enter_scope(Scope {
135 id: blk.hir_id.local_id,
136 data: ScopeData::Remainder(FirstStatementIndex::new(i)),
138 visitor.cx.var_parent = visitor.cx.parent;
139 visitor.visit_stmt(statement);
140 // We need to back out temporarily to the last enclosing scope
141 // for the `else` block, so that even the temporaries receiving
142 // extended lifetime will be dropped inside this block.
143 // We are visiting the `else` block in this order so that
144 // the sequence of visits agree with the order in the default
145 // `hir::intravisit` visitor.
146 mem::swap(&mut prev_cx, &mut visitor.cx);
147 visitor.terminating_scopes.insert(els.hir_id.local_id);
148 visitor.visit_block(els);
149 // From now on, we continue normally.
150 visitor.cx = prev_cx;
152 hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
153 // Each declaration introduces a subscope for bindings
154 // introduced by the declaration; this subscope covers a
155 // suffix of the block. Each subscope in a block has the
156 // previous subscope in the block as a parent, except for
157 // the first such subscope, which has the block itself as a
159 visitor.enter_scope(Scope {
160 id: blk.hir_id.local_id,
161 data: ScopeData::Remainder(FirstStatementIndex::new(i)),
163 visitor.cx.var_parent = visitor.cx.parent;
164 visitor.visit_stmt(statement)
166 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
169 walk_list!(visitor, visit_expr, &blk.expr);
172 visitor.cx = prev_cx;
175 fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
176 let prev_cx = visitor.cx;
178 visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node });
179 visitor.cx.var_parent = visitor.cx.parent;
181 visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
183 if let Some(hir::Guard::If(ref expr)) = arm.guard {
184 visitor.terminating_scopes.insert(expr.hir_id.local_id);
187 intravisit::walk_arm(visitor, arm);
189 visitor.cx = prev_cx;
192 fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
193 visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node });
195 // If this is a binding then record the lifetime of that binding.
196 if let PatKind::Binding(..) = pat.kind {
197 record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
200 debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
202 intravisit::walk_pat(visitor, pat);
204 visitor.expr_and_pat_count += 1;
206 debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
209 fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
210 let stmt_id = stmt.hir_id.local_id;
211 debug!("resolve_stmt(stmt.id={:?})", stmt_id);
213 // Every statement will clean up the temporaries created during
214 // execution of that statement. Therefore each statement has an
215 // associated destruction scope that represents the scope of the
216 // statement plus its destructors, and thus the scope for which
217 // regions referenced by the destructors need to survive.
218 visitor.terminating_scopes.insert(stmt_id);
220 let prev_parent = visitor.cx.parent;
221 visitor.enter_node_scope_with_dtor(stmt_id);
223 intravisit::walk_stmt(visitor, stmt);
225 visitor.cx.parent = prev_parent;
228 fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
229 debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
231 let prev_cx = visitor.cx;
232 visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
235 let terminating_scopes = &mut visitor.terminating_scopes;
236 let mut terminating = |id: hir::ItemLocalId| {
237 terminating_scopes.insert(id);
240 // Conditional or repeating scopes are always terminating
241 // scopes, meaning that temporaries cannot outlive them.
242 // This ensures fixed size stacks.
243 hir::ExprKind::Binary(
244 source_map::Spanned { node: hir::BinOpKind::And, .. },
248 | hir::ExprKind::Binary(
249 source_map::Spanned { node: hir::BinOpKind::Or, .. },
253 // For shortcircuiting operators, mark the RHS as a terminating
254 // scope since it only executes conditionally.
256 // `Let` expressions (in a let-chain) shouldn't be terminating, as their temporaries
257 // should live beyond the immediate expression
258 if !matches!(r.kind, hir::ExprKind::Let(_)) {
259 terminating(r.hir_id.local_id);
262 hir::ExprKind::If(_, ref then, Some(ref otherwise)) => {
263 terminating(then.hir_id.local_id);
264 terminating(otherwise.hir_id.local_id);
267 hir::ExprKind::If(_, ref then, None) => {
268 terminating(then.hir_id.local_id);
271 hir::ExprKind::Loop(ref body, _, _, _) => {
272 terminating(body.hir_id.local_id);
275 hir::ExprKind::DropTemps(ref expr) => {
276 // `DropTemps(expr)` does not denote a conditional scope.
277 // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
278 terminating(expr.hir_id.local_id);
281 hir::ExprKind::AssignOp(..)
282 | hir::ExprKind::Index(..)
283 | hir::ExprKind::Unary(..)
284 | hir::ExprKind::Call(..)
285 | hir::ExprKind::MethodCall(..) => {
286 // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
288 // The lifetimes for a call or method call look as follows:
296 // The idea is that call.callee_id represents *the time when
297 // the invoked function is actually running* and call.id
298 // represents *the time to prepare the arguments and make the
299 // call*. See the section "Borrows in Calls" borrowck/README.md
300 // for an extended explanation of why this distinction is
303 // record_superlifetime(new_cx, expr.callee_id);
310 let prev_pessimistic = visitor.pessimistic_yield;
312 // Ordinarily, we can rely on the visit order of HIR intravisit
313 // to correspond to the actual execution order of statements.
314 // However, there's a weird corner case with compound assignment
315 // operators (e.g. `a += b`). The evaluation order depends on whether
316 // or not the operator is overloaded (e.g. whether or not a trait
317 // like AddAssign is implemented).
319 // For primitive types (which, despite having a trait impl, don't actually
320 // end up calling it), the evaluation order is right-to-left. For example,
321 // the following code snippet:
324 // *{println!("LHS!"); y} += {println!("RHS!"); 1};
331 // However, if the operator is used on a non-primitive type,
332 // the evaluation order will be left-to-right, since the operator
333 // actually get desugared to a method call. For example, this
334 // nearly identical code snippet:
336 // let y = &mut String::new();
337 // *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
343 // To determine the actual execution order, we need to perform
344 // trait resolution. Unfortunately, we need to be able to compute
345 // yield_in_scope before type checking is even done, as it gets
346 // used by AST borrowcheck.
348 // Fortunately, we don't need to know the actual execution order.
349 // It suffices to know the 'worst case' order with respect to yields.
350 // Specifically, we need to know the highest 'expr_and_pat_count'
351 // that we could assign to the yield expression. To do this,
352 // we pick the greater of the two values from the left-hand
353 // and right-hand expressions. This makes us overly conservative
354 // about what types could possibly live across yield points,
355 // but we will never fail to detect that a type does actually
356 // live across a yield point. The latter part is critical -
357 // we're already overly conservative about what types will live
358 // across yield points, as the generated MIR will determine
359 // when things are actually live. However, for typecheck to work
360 // properly, we can't miss any types.
363 // Manually recurse over closures and inline consts, because they are the only
364 // case of nested bodies that share the parent environment.
365 hir::ExprKind::Closure(&hir::Closure { body, .. })
366 | hir::ExprKind::ConstBlock(hir::AnonConst { body, .. }) => {
367 let body = visitor.tcx.hir().body(body);
368 visitor.visit_body(body);
370 hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
372 "resolve_expr - enabling pessimistic_yield, was previously {}",
376 let start_point = visitor.fixup_scopes.len();
377 visitor.pessimistic_yield = true;
379 // If the actual execution order turns out to be right-to-left,
380 // then we're fine. However, if the actual execution order is left-to-right,
381 // then we'll assign too low a count to any `yield` expressions
382 // we encounter in 'right_expression' - they should really occur after all of the
383 // expressions in 'left_expression'.
384 visitor.visit_expr(&right_expr);
385 visitor.pessimistic_yield = prev_pessimistic;
387 debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
388 visitor.visit_expr(&left_expr);
389 debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
391 // Remove and process any scopes pushed by the visitor
392 let target_scopes = visitor.fixup_scopes.drain(start_point..);
394 for scope in target_scopes {
396 visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap().last_mut().unwrap();
397 let count = yield_data.expr_and_pat_count;
398 let span = yield_data.span;
400 // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
401 // before walking the left-hand side, it should be impossible for the recorded
402 // count to be greater than the left-hand side count.
403 if count > visitor.expr_and_pat_count {
405 "Encountered greater count {} at span {:?} - expected no greater than {}",
408 visitor.expr_and_pat_count
411 let new_count = visitor.expr_and_pat_count;
413 "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
414 scope, count, new_count, span
417 yield_data.expr_and_pat_count = new_count;
421 hir::ExprKind::If(ref cond, ref then, Some(ref otherwise)) => {
422 let expr_cx = visitor.cx;
423 visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
424 visitor.cx.var_parent = visitor.cx.parent;
425 visitor.visit_expr(cond);
426 visitor.visit_expr(then);
427 visitor.cx = expr_cx;
428 visitor.visit_expr(otherwise);
431 hir::ExprKind::If(ref cond, ref then, None) => {
432 let expr_cx = visitor.cx;
433 visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
434 visitor.cx.var_parent = visitor.cx.parent;
435 visitor.visit_expr(cond);
436 visitor.visit_expr(then);
437 visitor.cx = expr_cx;
440 _ => intravisit::walk_expr(visitor, expr),
443 visitor.expr_and_pat_count += 1;
445 debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
447 if let hir::ExprKind::Yield(_, source) = &expr.kind {
448 // Mark this expr's scope and all parent scopes as containing `yield`.
449 let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
451 let span = match expr.kind {
452 hir::ExprKind::Yield(expr, hir::YieldSource::Await { .. }) => {
453 expr.span.shrink_to_hi().to(expr.span)
458 YieldData { span, expr_and_pat_count: visitor.expr_and_pat_count, source: *source };
459 match visitor.scope_tree.yield_in_scope.get_mut(&scope) {
460 Some(yields) => yields.push(data),
462 visitor.scope_tree.yield_in_scope.insert(scope, vec![data]);
466 if visitor.pessimistic_yield {
467 debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
468 visitor.fixup_scopes.push(scope);
471 // Keep traversing up while we can.
472 match visitor.scope_tree.parent_map.get(&scope) {
473 // Don't cross from closure bodies to their parent.
474 Some(&(superscope, _)) => match superscope.data {
475 ScopeData::CallSite => break,
476 _ => scope = superscope,
483 visitor.cx = prev_cx;
486 fn resolve_local<'tcx>(
487 visitor: &mut RegionResolutionVisitor<'tcx>,
488 pat: Option<&'tcx hir::Pat<'tcx>>,
489 init: Option<&'tcx hir::Expr<'tcx>>,
491 debug!("resolve_local(pat={:?}, init={:?})", pat, init);
493 let blk_scope = visitor.cx.var_parent.map(|(p, _)| p);
495 // As an exception to the normal rules governing temporary
496 // lifetimes, initializers in a let have a temporary lifetime
497 // of the enclosing block. This means that e.g., a program
498 // like the following is legal:
500 // let ref x = HashMap::new();
502 // Because the hash map will be freed in the enclosing block.
504 // We express the rules more formally based on 3 grammars (defined
505 // fully in the helpers below that implement them):
507 // 1. `E&`, which matches expressions like `&<rvalue>` that
508 // own a pointer into the stack.
510 // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
511 // y)` that produce ref bindings into the value they are
512 // matched against or something (at least partially) owned by
513 // the value they are matched against. (By partially owned,
514 // I mean that creating a binding into a ref-counted or managed value
515 // would still count.)
517 // 3. `ET`, which matches both rvalues like `foo()` as well as places
518 // based on rvalues like `foo().x[2].y`.
520 // A subexpression `<rvalue>` that appears in a let initializer
521 // `let pat [: ty] = expr` has an extended temporary lifetime if
522 // any of the following conditions are met:
524 // A. `pat` matches `P&` and `expr` matches `ET`
525 // (covers cases where `pat` creates ref bindings into an rvalue
526 // produced by `expr`)
527 // B. `ty` is a borrowed pointer and `expr` matches `ET`
528 // (covers cases where coercion creates a borrow)
529 // C. `expr` matches `E&`
530 // (covers cases `expr` borrows an rvalue that is then assigned
531 // to memory (at least partially) owned by the binding)
533 // Here are some examples hopefully giving an intuition where each
534 // rule comes into play and why:
536 // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
537 // would have an extended lifetime, but not `foo()`.
539 // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
542 // In some cases, multiple rules may apply (though not to the same
543 // rvalue). For example:
545 // let ref x = [&a(), &b()];
547 // Here, the expression `[...]` has an extended lifetime due to rule
548 // A, but the inner rvalues `a()` and `b()` have an extended lifetime
551 if let Some(expr) = init {
552 record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope);
554 if let Some(pat) = pat {
555 if is_binding_pat(pat) {
556 visitor.scope_tree.record_rvalue_candidate(
558 RvalueCandidateType::Pattern {
559 target: expr.hir_id.local_id,
567 // Make sure we visit the initializer first, so expr_and_pat_count remains correct.
568 // The correct order, as shared between generator_interior, drop_ranges and intravisitor,
569 // is to walk initializer, followed by pattern bindings, finally followed by the `else` block.
570 if let Some(expr) = init {
571 visitor.visit_expr(expr);
573 if let Some(pat) = pat {
574 visitor.visit_pat(pat);
577 /// Returns `true` if `pat` match the `P&` non-terminal.
581 /// | StructName { ..., P&, ... }
582 /// | VariantName(..., P&, ...)
583 /// | [ ..., P&, ... ]
584 /// | ( ..., P&, ... )
585 /// | ... "|" P& "|" ...
588 fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
589 // Note that the code below looks for *explicit* refs only, that is, it won't
590 // know about *implicit* refs as introduced in #42640.
592 // This is not a problem. For example, consider
594 // let (ref x, ref y) = (Foo { .. }, Bar { .. });
596 // Due to the explicit refs on the left hand side, the below code would signal
597 // that the temporary value on the right hand side should live until the end of
598 // the enclosing block (as opposed to being dropped after the let is complete).
600 // To create an implicit ref, however, you must have a borrowed value on the RHS
601 // already, as in this example (which won't compile before #42640):
603 // let Foo { x, .. } = &Foo { x: ..., ... };
607 // let Foo { ref x, .. } = Foo { ... };
609 // In the former case (the implicit ref version), the temporary is created by the
610 // & expression, and its lifetime would be extended to the end of the block (due
611 // to a different rule, not the below code).
613 PatKind::Binding(hir::BindingAnnotation(hir::ByRef::Yes, _), ..) => true,
615 PatKind::Struct(_, ref field_pats, _) => {
616 field_pats.iter().any(|fp| is_binding_pat(&fp.pat))
619 PatKind::Slice(ref pats1, ref pats2, ref pats3) => {
620 pats1.iter().any(|p| is_binding_pat(&p))
621 || pats2.iter().any(|p| is_binding_pat(&p))
622 || pats3.iter().any(|p| is_binding_pat(&p))
625 PatKind::Or(ref subpats)
626 | PatKind::TupleStruct(_, ref subpats, _)
627 | PatKind::Tuple(ref subpats, _) => subpats.iter().any(|p| is_binding_pat(&p)),
629 PatKind::Box(ref subpat) => is_binding_pat(&subpat),
632 | PatKind::Binding(hir::BindingAnnotation(hir::ByRef::No, _), ..)
636 | PatKind::Range(_, _, _) => false,
640 /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
644 /// | StructName { ..., f: E&, ... }
645 /// | [ ..., E&, ... ]
646 /// | ( ..., E&, ... )
652 fn record_rvalue_scope_if_borrow_expr<'tcx>(
653 visitor: &mut RegionResolutionVisitor<'tcx>,
654 expr: &hir::Expr<'_>,
655 blk_id: Option<Scope>,
658 hir::ExprKind::AddrOf(_, _, subexpr) => {
659 record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
660 visitor.scope_tree.record_rvalue_candidate(
662 RvalueCandidateType::Borrow {
663 target: subexpr.hir_id.local_id,
668 hir::ExprKind::Struct(_, fields, _) => {
669 for field in fields {
670 record_rvalue_scope_if_borrow_expr(visitor, &field.expr, blk_id);
673 hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
674 for subexpr in subexprs {
675 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
678 hir::ExprKind::Cast(ref subexpr, _) => {
679 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id)
681 hir::ExprKind::Block(ref block, _) => {
682 if let Some(ref subexpr) = block.expr {
683 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
686 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) => {
687 // FIXME(@dingxiangfei2009): choose call arguments here
688 // for candidacy for extended parameter rule application
690 hir::ExprKind::Index(..) => {
691 // FIXME(@dingxiangfei2009): select the indices
692 // as candidate for rvalue scope rules
699 impl<'tcx> RegionResolutionVisitor<'tcx> {
700 /// Records the current parent (if any) as the parent of `child_scope`.
701 /// Returns the depth of `child_scope`.
702 fn record_child_scope(&mut self, child_scope: Scope) -> ScopeDepth {
703 let parent = self.cx.parent;
704 self.scope_tree.record_scope_parent(child_scope, parent);
705 // If `child_scope` has no parent, it must be the root node, and so has
706 // a depth of 1. Otherwise, its depth is one more than its parent's.
707 parent.map_or(1, |(_p, d)| d + 1)
710 /// Records the current parent (if any) as the parent of `child_scope`,
711 /// and sets `child_scope` as the new current parent.
712 fn enter_scope(&mut self, child_scope: Scope) {
713 let child_depth = self.record_child_scope(child_scope);
714 self.cx.parent = Some((child_scope, child_depth));
717 fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) {
718 // If node was previously marked as a terminating scope during the
719 // recursive visit of its parent node in the AST, then we need to
720 // account for the destruction scope representing the scope of
721 // the destructors that run immediately after it completes.
722 if self.terminating_scopes.contains(&id) {
723 self.enter_scope(Scope { id, data: ScopeData::Destruction });
725 self.enter_scope(Scope { id, data: ScopeData::Node });
729 impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
730 fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
731 resolve_block(self, b);
734 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
735 let body_id = body.id();
736 let owner_id = self.tcx.hir().body_owner_def_id(body_id);
739 "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
741 self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
746 // Save all state that is specific to the outer function
747 // body. These will be restored once down below, once we've
749 let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
750 let outer_cx = self.cx;
751 let outer_ts = mem::take(&mut self.terminating_scopes);
752 // The 'pessimistic yield' flag is set to true when we are
753 // processing a `+=` statement and have to make pessimistic
754 // control flow assumptions. This doesn't apply to nested
755 // bodies within the `+=` statements. See #69307.
756 let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false);
757 self.terminating_scopes.insert(body.value.hir_id.local_id);
759 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite });
760 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments });
762 // The arguments and `self` are parented to the fn.
763 self.cx.var_parent = self.cx.parent.take();
764 for param in body.params {
765 self.visit_pat(¶m.pat);
768 // The body of the every fn is a root scope.
769 self.cx.parent = self.cx.var_parent;
770 if self.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() {
771 self.visit_expr(&body.value)
773 // Only functions have an outer terminating (drop) scope, while
774 // temporaries in constant initializers may be 'static, but only
775 // according to rvalue lifetime semantics, using the same
776 // syntactical rules used for let initializers.
778 // e.g., in `let x = &f();`, the temporary holding the result from
779 // the `f()` call lives for the entirety of the surrounding block.
781 // Similarly, `const X: ... = &f();` would have the result of `f()`
782 // live for `'static`, implying (if Drop restrictions on constants
783 // ever get lifted) that the value *could* have a destructor, but
784 // it'd get leaked instead of the destructor running during the
785 // evaluation of `X` (if at all allowed by CTFE).
787 // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
788 // would *not* let the `f()` temporary escape into an outer scope
789 // (i.e., `'static`), which means that after `g` returns, it drops,
790 // and all the associated destruction scope rules apply.
791 self.cx.var_parent = None;
792 resolve_local(self, None, Some(&body.value));
795 if body.generator_kind.is_some() {
796 self.scope_tree.body_expr_count.insert(body_id, self.expr_and_pat_count);
799 // Restore context we had at the start.
800 self.expr_and_pat_count = outer_ec;
802 self.terminating_scopes = outer_ts;
803 self.pessimistic_yield = outer_pessimistic_yield;
806 fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
807 resolve_arm(self, a);
809 fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
810 resolve_pat(self, p);
812 fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
813 resolve_stmt(self, s);
815 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
816 resolve_expr(self, ex);
818 fn visit_local(&mut self, l: &'tcx Local<'tcx>) {
819 resolve_local(self, Some(&l.pat), l.init)
823 /// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
824 /// in the case of closures, this will be redirected to the enclosing function.
826 /// Performance: This is a query rather than a simple function to enable
827 /// re-use in incremental scenarios. We may sometimes need to rerun the
828 /// type checker even when the HIR hasn't changed, and in those cases
829 /// we can avoid reconstructing the region scope tree.
830 pub fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
831 let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
832 if typeck_root_def_id != def_id {
833 return tcx.region_scope_tree(typeck_root_def_id);
836 let scope_tree = if let Some(body_id) = tcx.hir().maybe_body_owned_by(def_id.expect_local()) {
837 let mut visitor = RegionResolutionVisitor {
839 scope_tree: ScopeTree::default(),
840 expr_and_pat_count: 0,
841 cx: Context { parent: None, var_parent: None },
842 terminating_scopes: Default::default(),
843 pessimistic_yield: false,
844 fixup_scopes: vec![],
847 let body = tcx.hir().body(body_id);
848 visitor.scope_tree.root_body = Some(body.value.hir_id);
849 visitor.visit_body(body);
855 tcx.arena.alloc(scope_tree)