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.visit_block(els);
148 // From now on, we continue normally.
149 visitor.cx = prev_cx;
151 hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
152 // Each declaration introduces a subscope for bindings
153 // introduced by the declaration; this subscope covers a
154 // suffix of the block. Each subscope in a block has the
155 // previous subscope in the block as a parent, except for
156 // the first such subscope, which has the block itself as a
158 visitor.enter_scope(Scope {
159 id: blk.hir_id.local_id,
160 data: ScopeData::Remainder(FirstStatementIndex::new(i)),
162 visitor.cx.var_parent = visitor.cx.parent;
163 visitor.visit_stmt(statement)
165 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
168 walk_list!(visitor, visit_expr, &blk.expr);
171 visitor.cx = prev_cx;
174 fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
175 let prev_cx = visitor.cx;
177 visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node });
178 visitor.cx.var_parent = visitor.cx.parent;
180 visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
182 if let Some(hir::Guard::If(ref expr)) = arm.guard {
183 visitor.terminating_scopes.insert(expr.hir_id.local_id);
186 intravisit::walk_arm(visitor, arm);
188 visitor.cx = prev_cx;
191 fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
192 visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node });
194 // If this is a binding then record the lifetime of that binding.
195 if let PatKind::Binding(..) = pat.kind {
196 record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
199 debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
201 intravisit::walk_pat(visitor, pat);
203 visitor.expr_and_pat_count += 1;
205 debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
208 fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
209 let stmt_id = stmt.hir_id.local_id;
210 debug!("resolve_stmt(stmt.id={:?})", stmt_id);
212 // Every statement will clean up the temporaries created during
213 // execution of that statement. Therefore each statement has an
214 // associated destruction scope that represents the scope of the
215 // statement plus its destructors, and thus the scope for which
216 // regions referenced by the destructors need to survive.
217 visitor.terminating_scopes.insert(stmt_id);
219 let prev_parent = visitor.cx.parent;
220 visitor.enter_node_scope_with_dtor(stmt_id);
222 intravisit::walk_stmt(visitor, stmt);
224 visitor.cx.parent = prev_parent;
227 fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
228 debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
230 let prev_cx = visitor.cx;
231 visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
234 let terminating_scopes = &mut visitor.terminating_scopes;
235 let mut terminating = |id: hir::ItemLocalId| {
236 terminating_scopes.insert(id);
239 // Conditional or repeating scopes are always terminating
240 // scopes, meaning that temporaries cannot outlive them.
241 // This ensures fixed size stacks.
242 hir::ExprKind::Binary(
243 source_map::Spanned { node: hir::BinOpKind::And, .. },
247 | hir::ExprKind::Binary(
248 source_map::Spanned { node: hir::BinOpKind::Or, .. },
252 // For shortcircuiting operators, mark the RHS as a terminating
253 // scope since it only executes conditionally.
254 terminating(r.hir_id.local_id);
257 hir::ExprKind::If(_, ref then, Some(ref otherwise)) => {
258 terminating(then.hir_id.local_id);
259 terminating(otherwise.hir_id.local_id);
262 hir::ExprKind::If(_, ref then, None) => {
263 terminating(then.hir_id.local_id);
266 hir::ExprKind::Loop(ref body, _, _, _) => {
267 terminating(body.hir_id.local_id);
270 hir::ExprKind::DropTemps(ref expr) => {
271 // `DropTemps(expr)` does not denote a conditional scope.
272 // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
273 terminating(expr.hir_id.local_id);
276 hir::ExprKind::AssignOp(..)
277 | hir::ExprKind::Index(..)
278 | hir::ExprKind::Unary(..)
279 | hir::ExprKind::Call(..)
280 | hir::ExprKind::MethodCall(..) => {
281 // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
283 // The lifetimes for a call or method call look as follows:
291 // The idea is that call.callee_id represents *the time when
292 // the invoked function is actually running* and call.id
293 // represents *the time to prepare the arguments and make the
294 // call*. See the section "Borrows in Calls" borrowck/README.md
295 // for an extended explanation of why this distinction is
298 // record_superlifetime(new_cx, expr.callee_id);
305 let prev_pessimistic = visitor.pessimistic_yield;
307 // Ordinarily, we can rely on the visit order of HIR intravisit
308 // to correspond to the actual execution order of statements.
309 // However, there's a weird corner case with compound assignment
310 // operators (e.g. `a += b`). The evaluation order depends on whether
311 // or not the operator is overloaded (e.g. whether or not a trait
312 // like AddAssign is implemented).
314 // For primitive types (which, despite having a trait impl, don't actually
315 // end up calling it), the evaluation order is right-to-left. For example,
316 // the following code snippet:
319 // *{println!("LHS!"); y} += {println!("RHS!"); 1};
326 // However, if the operator is used on a non-primitive type,
327 // the evaluation order will be left-to-right, since the operator
328 // actually get desugared to a method call. For example, this
329 // nearly identical code snippet:
331 // let y = &mut String::new();
332 // *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
338 // To determine the actual execution order, we need to perform
339 // trait resolution. Unfortunately, we need to be able to compute
340 // yield_in_scope before type checking is even done, as it gets
341 // used by AST borrowcheck.
343 // Fortunately, we don't need to know the actual execution order.
344 // It suffices to know the 'worst case' order with respect to yields.
345 // Specifically, we need to know the highest 'expr_and_pat_count'
346 // that we could assign to the yield expression. To do this,
347 // we pick the greater of the two values from the left-hand
348 // and right-hand expressions. This makes us overly conservative
349 // about what types could possibly live across yield points,
350 // but we will never fail to detect that a type does actually
351 // live across a yield point. The latter part is critical -
352 // we're already overly conservative about what types will live
353 // across yield points, as the generated MIR will determine
354 // when things are actually live. However, for typecheck to work
355 // properly, we can't miss any types.
358 // Manually recurse over closures and inline consts, because they are the only
359 // case of nested bodies that share the parent environment.
360 hir::ExprKind::Closure(&hir::Closure { body, .. })
361 | hir::ExprKind::ConstBlock(hir::AnonConst { body, .. }) => {
362 let body = visitor.tcx.hir().body(body);
363 visitor.visit_body(body);
365 hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
367 "resolve_expr - enabling pessimistic_yield, was previously {}",
371 let start_point = visitor.fixup_scopes.len();
372 visitor.pessimistic_yield = true;
374 // If the actual execution order turns out to be right-to-left,
375 // then we're fine. However, if the actual execution order is left-to-right,
376 // then we'll assign too low a count to any `yield` expressions
377 // we encounter in 'right_expression' - they should really occur after all of the
378 // expressions in 'left_expression'.
379 visitor.visit_expr(&right_expr);
380 visitor.pessimistic_yield = prev_pessimistic;
382 debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
383 visitor.visit_expr(&left_expr);
384 debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
386 // Remove and process any scopes pushed by the visitor
387 let target_scopes = visitor.fixup_scopes.drain(start_point..);
389 for scope in target_scopes {
391 visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap().last_mut().unwrap();
392 let count = yield_data.expr_and_pat_count;
393 let span = yield_data.span;
395 // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
396 // before walking the left-hand side, it should be impossible for the recorded
397 // count to be greater than the left-hand side count.
398 if count > visitor.expr_and_pat_count {
400 "Encountered greater count {} at span {:?} - expected no greater than {}",
403 visitor.expr_and_pat_count
406 let new_count = visitor.expr_and_pat_count;
408 "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
409 scope, count, new_count, span
412 yield_data.expr_and_pat_count = new_count;
416 hir::ExprKind::If(ref cond, ref then, Some(ref otherwise)) => {
417 let expr_cx = visitor.cx;
418 visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
419 visitor.cx.var_parent = visitor.cx.parent;
420 visitor.visit_expr(cond);
421 visitor.visit_expr(then);
422 visitor.cx = expr_cx;
423 visitor.visit_expr(otherwise);
426 hir::ExprKind::If(ref cond, ref then, None) => {
427 let expr_cx = visitor.cx;
428 visitor.enter_scope(Scope { id: then.hir_id.local_id, data: ScopeData::IfThen });
429 visitor.cx.var_parent = visitor.cx.parent;
430 visitor.visit_expr(cond);
431 visitor.visit_expr(then);
432 visitor.cx = expr_cx;
435 _ => intravisit::walk_expr(visitor, expr),
438 visitor.expr_and_pat_count += 1;
440 debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
442 if let hir::ExprKind::Yield(_, source) = &expr.kind {
443 // Mark this expr's scope and all parent scopes as containing `yield`.
444 let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
446 let span = match expr.kind {
447 hir::ExprKind::Yield(expr, hir::YieldSource::Await { .. }) => {
448 expr.span.shrink_to_hi().to(expr.span)
453 YieldData { span, expr_and_pat_count: visitor.expr_and_pat_count, source: *source };
454 match visitor.scope_tree.yield_in_scope.get_mut(&scope) {
455 Some(yields) => yields.push(data),
457 visitor.scope_tree.yield_in_scope.insert(scope, vec![data]);
461 if visitor.pessimistic_yield {
462 debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
463 visitor.fixup_scopes.push(scope);
466 // Keep traversing up while we can.
467 match visitor.scope_tree.parent_map.get(&scope) {
468 // Don't cross from closure bodies to their parent.
469 Some(&(superscope, _)) => match superscope.data {
470 ScopeData::CallSite => break,
471 _ => scope = superscope,
478 visitor.cx = prev_cx;
481 fn resolve_local<'tcx>(
482 visitor: &mut RegionResolutionVisitor<'tcx>,
483 pat: Option<&'tcx hir::Pat<'tcx>>,
484 init: Option<&'tcx hir::Expr<'tcx>>,
486 debug!("resolve_local(pat={:?}, init={:?})", pat, init);
488 let blk_scope = visitor.cx.var_parent.map(|(p, _)| p);
490 // As an exception to the normal rules governing temporary
491 // lifetimes, initializers in a let have a temporary lifetime
492 // of the enclosing block. This means that e.g., a program
493 // like the following is legal:
495 // let ref x = HashMap::new();
497 // Because the hash map will be freed in the enclosing block.
499 // We express the rules more formally based on 3 grammars (defined
500 // fully in the helpers below that implement them):
502 // 1. `E&`, which matches expressions like `&<rvalue>` that
503 // own a pointer into the stack.
505 // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
506 // y)` that produce ref bindings into the value they are
507 // matched against or something (at least partially) owned by
508 // the value they are matched against. (By partially owned,
509 // I mean that creating a binding into a ref-counted or managed value
510 // would still count.)
512 // 3. `ET`, which matches both rvalues like `foo()` as well as places
513 // based on rvalues like `foo().x[2].y`.
515 // A subexpression `<rvalue>` that appears in a let initializer
516 // `let pat [: ty] = expr` has an extended temporary lifetime if
517 // any of the following conditions are met:
519 // A. `pat` matches `P&` and `expr` matches `ET`
520 // (covers cases where `pat` creates ref bindings into an rvalue
521 // produced by `expr`)
522 // B. `ty` is a borrowed pointer and `expr` matches `ET`
523 // (covers cases where coercion creates a borrow)
524 // C. `expr` matches `E&`
525 // (covers cases `expr` borrows an rvalue that is then assigned
526 // to memory (at least partially) owned by the binding)
528 // Here are some examples hopefully giving an intuition where each
529 // rule comes into play and why:
531 // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
532 // would have an extended lifetime, but not `foo()`.
534 // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
537 // In some cases, multiple rules may apply (though not to the same
538 // rvalue). For example:
540 // let ref x = [&a(), &b()];
542 // Here, the expression `[...]` has an extended lifetime due to rule
543 // A, but the inner rvalues `a()` and `b()` have an extended lifetime
546 if let Some(expr) = init {
547 record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope);
549 if let Some(pat) = pat {
550 if is_binding_pat(pat) {
551 visitor.scope_tree.record_rvalue_candidate(
553 RvalueCandidateType::Pattern {
554 target: expr.hir_id.local_id,
562 // Make sure we visit the initializer first, so expr_and_pat_count remains correct.
563 // The correct order, as shared between generator_interior, drop_ranges and intravisitor,
564 // is to walk initializer, followed by pattern bindings, finally followed by the `else` block.
565 if let Some(expr) = init {
566 visitor.visit_expr(expr);
568 if let Some(pat) = pat {
569 visitor.visit_pat(pat);
572 /// Returns `true` if `pat` match the `P&` non-terminal.
576 /// | StructName { ..., P&, ... }
577 /// | VariantName(..., P&, ...)
578 /// | [ ..., P&, ... ]
579 /// | ( ..., P&, ... )
580 /// | ... "|" P& "|" ...
583 fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
584 // Note that the code below looks for *explicit* refs only, that is, it won't
585 // know about *implicit* refs as introduced in #42640.
587 // This is not a problem. For example, consider
589 // let (ref x, ref y) = (Foo { .. }, Bar { .. });
591 // Due to the explicit refs on the left hand side, the below code would signal
592 // that the temporary value on the right hand side should live until the end of
593 // the enclosing block (as opposed to being dropped after the let is complete).
595 // To create an implicit ref, however, you must have a borrowed value on the RHS
596 // already, as in this example (which won't compile before #42640):
598 // let Foo { x, .. } = &Foo { x: ..., ... };
602 // let Foo { ref x, .. } = Foo { ... };
604 // In the former case (the implicit ref version), the temporary is created by the
605 // & expression, and its lifetime would be extended to the end of the block (due
606 // to a different rule, not the below code).
608 PatKind::Binding(hir::BindingAnnotation(hir::ByRef::Yes, _), ..) => true,
610 PatKind::Struct(_, ref field_pats, _) => {
611 field_pats.iter().any(|fp| is_binding_pat(&fp.pat))
614 PatKind::Slice(ref pats1, ref pats2, ref pats3) => {
615 pats1.iter().any(|p| is_binding_pat(&p))
616 || pats2.iter().any(|p| is_binding_pat(&p))
617 || pats3.iter().any(|p| is_binding_pat(&p))
620 PatKind::Or(ref subpats)
621 | PatKind::TupleStruct(_, ref subpats, _)
622 | PatKind::Tuple(ref subpats, _) => subpats.iter().any(|p| is_binding_pat(&p)),
624 PatKind::Box(ref subpat) => is_binding_pat(&subpat),
627 | PatKind::Binding(hir::BindingAnnotation(hir::ByRef::No, _), ..)
631 | PatKind::Range(_, _, _) => false,
635 /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
639 /// | StructName { ..., f: E&, ... }
640 /// | [ ..., E&, ... ]
641 /// | ( ..., E&, ... )
647 fn record_rvalue_scope_if_borrow_expr<'tcx>(
648 visitor: &mut RegionResolutionVisitor<'tcx>,
649 expr: &hir::Expr<'_>,
650 blk_id: Option<Scope>,
653 hir::ExprKind::AddrOf(_, _, subexpr) => {
654 record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
655 visitor.scope_tree.record_rvalue_candidate(
657 RvalueCandidateType::Borrow {
658 target: subexpr.hir_id.local_id,
663 hir::ExprKind::Struct(_, fields, _) => {
664 for field in fields {
665 record_rvalue_scope_if_borrow_expr(visitor, &field.expr, blk_id);
668 hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
669 for subexpr in subexprs {
670 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
673 hir::ExprKind::Cast(ref subexpr, _) => {
674 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id)
676 hir::ExprKind::Block(ref block, _) => {
677 if let Some(ref subexpr) = block.expr {
678 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
681 hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) => {
682 // FIXME(@dingxiangfei2009): choose call arguments here
683 // for candidacy for extended parameter rule application
685 hir::ExprKind::Index(..) => {
686 // FIXME(@dingxiangfei2009): select the indices
687 // as candidate for rvalue scope rules
694 impl<'tcx> RegionResolutionVisitor<'tcx> {
695 /// Records the current parent (if any) as the parent of `child_scope`.
696 /// Returns the depth of `child_scope`.
697 fn record_child_scope(&mut self, child_scope: Scope) -> ScopeDepth {
698 let parent = self.cx.parent;
699 self.scope_tree.record_scope_parent(child_scope, parent);
700 // If `child_scope` has no parent, it must be the root node, and so has
701 // a depth of 1. Otherwise, its depth is one more than its parent's.
702 parent.map_or(1, |(_p, d)| d + 1)
705 /// Records the current parent (if any) as the parent of `child_scope`,
706 /// and sets `child_scope` as the new current parent.
707 fn enter_scope(&mut self, child_scope: Scope) {
708 let child_depth = self.record_child_scope(child_scope);
709 self.cx.parent = Some((child_scope, child_depth));
712 fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) {
713 // If node was previously marked as a terminating scope during the
714 // recursive visit of its parent node in the AST, then we need to
715 // account for the destruction scope representing the scope of
716 // the destructors that run immediately after it completes.
717 if self.terminating_scopes.contains(&id) {
718 self.enter_scope(Scope { id, data: ScopeData::Destruction });
720 self.enter_scope(Scope { id, data: ScopeData::Node });
724 impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
725 fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
726 resolve_block(self, b);
729 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
730 let body_id = body.id();
731 let owner_id = self.tcx.hir().body_owner_def_id(body_id);
734 "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
736 self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
741 // Save all state that is specific to the outer function
742 // body. These will be restored once down below, once we've
744 let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
745 let outer_cx = self.cx;
746 let outer_ts = mem::take(&mut self.terminating_scopes);
747 // The 'pessimistic yield' flag is set to true when we are
748 // processing a `+=` statement and have to make pessimistic
749 // control flow assumptions. This doesn't apply to nested
750 // bodies within the `+=` statements. See #69307.
751 let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false);
752 self.terminating_scopes.insert(body.value.hir_id.local_id);
754 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite });
755 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments });
757 // The arguments and `self` are parented to the fn.
758 self.cx.var_parent = self.cx.parent.take();
759 for param in body.params {
760 self.visit_pat(¶m.pat);
763 // The body of the every fn is a root scope.
764 self.cx.parent = self.cx.var_parent;
765 if self.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() {
766 self.visit_expr(&body.value)
768 // Only functions have an outer terminating (drop) scope, while
769 // temporaries in constant initializers may be 'static, but only
770 // according to rvalue lifetime semantics, using the same
771 // syntactical rules used for let initializers.
773 // e.g., in `let x = &f();`, the temporary holding the result from
774 // the `f()` call lives for the entirety of the surrounding block.
776 // Similarly, `const X: ... = &f();` would have the result of `f()`
777 // live for `'static`, implying (if Drop restrictions on constants
778 // ever get lifted) that the value *could* have a destructor, but
779 // it'd get leaked instead of the destructor running during the
780 // evaluation of `X` (if at all allowed by CTFE).
782 // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
783 // would *not* let the `f()` temporary escape into an outer scope
784 // (i.e., `'static`), which means that after `g` returns, it drops,
785 // and all the associated destruction scope rules apply.
786 self.cx.var_parent = None;
787 resolve_local(self, None, Some(&body.value));
790 if body.generator_kind.is_some() {
791 self.scope_tree.body_expr_count.insert(body_id, self.expr_and_pat_count);
794 // Restore context we had at the start.
795 self.expr_and_pat_count = outer_ec;
797 self.terminating_scopes = outer_ts;
798 self.pessimistic_yield = outer_pessimistic_yield;
801 fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
802 resolve_arm(self, a);
804 fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
805 resolve_pat(self, p);
807 fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
808 resolve_stmt(self, s);
810 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
811 resolve_expr(self, ex);
813 fn visit_local(&mut self, l: &'tcx Local<'tcx>) {
814 resolve_local(self, Some(&l.pat), l.init)
818 /// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
819 /// in the case of closures, this will be redirected to the enclosing function.
821 /// Performance: This is a query rather than a simple function to enable
822 /// re-use in incremental scenarios. We may sometimes need to rerun the
823 /// type checker even when the HIR hasn't changed, and in those cases
824 /// we can avoid reconstructing the region scope tree.
825 pub fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
826 let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
827 if typeck_root_def_id != def_id {
828 return tcx.region_scope_tree(typeck_root_def_id);
831 let scope_tree = if let Some(body_id) = tcx.hir().maybe_body_owned_by(def_id.expect_local()) {
832 let mut visitor = RegionResolutionVisitor {
834 scope_tree: ScopeTree::default(),
835 expr_and_pat_count: 0,
836 cx: Context { parent: None, var_parent: None },
837 terminating_scopes: Default::default(),
838 pessimistic_yield: false,
839 fixup_scopes: vec![],
842 let body = tcx.hir().body(body_id);
843 visitor.scope_tree.root_body = Some(body.value.hir_id);
844 visitor.visit_body(body);
850 tcx.arena.alloc(scope_tree)