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, NestedVisitorMap, Visitor};
14 use rustc_hir::{Arm, Block, Expr, Local, Node, Pat, PatKind, Stmt};
15 use rustc_index::vec::Idx;
16 use rustc_middle::middle::region::*;
17 use rustc_middle::ty::query::Providers;
18 use rustc_middle::ty::TyCtxt;
19 use rustc_span::source_map;
24 #[derive(Debug, Copy, Clone)]
26 /// The root of the current region tree. This is typically the id
27 /// of the innermost fn body. Each fn forms its own disjoint tree
28 /// in the region hierarchy. These fn bodies are themselves
29 /// arranged into a tree. See the "Modeling closures" section of
30 /// the README in `rustc_trait_selection::infer::region_constraints`
32 root_id: Option<hir::ItemLocalId>,
34 /// The scope that contains any new variables declared, plus its depth in
36 var_parent: Option<(Scope, ScopeDepth)>,
38 /// Region parent of expressions, etc., plus its depth in the scope tree.
39 parent: Option<(Scope, ScopeDepth)>,
42 struct RegionResolutionVisitor<'tcx> {
45 // The number of expressions and patterns visited in the current body.
46 expr_and_pat_count: usize,
47 // When this is `true`, we record the `Scopes` we encounter
48 // when processing a Yield expression. This allows us to fix
50 pessimistic_yield: bool,
51 // Stores scopes when `pessimistic_yield` is `true`.
52 fixup_scopes: Vec<Scope>,
53 // The generated scope tree.
54 scope_tree: ScopeTree,
58 /// `terminating_scopes` is a set containing the ids of each
59 /// statement, or conditional/repeating expression. These scopes
60 /// are calling "terminating scopes" because, when attempting to
61 /// find the scope of a temporary, by default we search up the
62 /// enclosing scopes until we encounter the terminating scope. A
63 /// conditional/repeating expression is one which is not
64 /// guaranteed to execute exactly once upon entering the parent
65 /// scope. This could be because the expression only executes
66 /// conditionally, such as the expression `b` in `a && b`, or
67 /// because the expression may execute many times, such as a loop
68 /// body. The reason that we distinguish such expressions is that,
69 /// upon exiting the parent scope, we cannot statically know how
70 /// many times the expression executed, and thus if the expression
71 /// creates temporaries we cannot know statically how many such
72 /// temporaries we would have to cleanup. Therefore, we ensure that
73 /// the temporaries never outlast the conditional/repeating
74 /// expression, preventing the need for dynamic checks and/or
75 /// arbitrary amounts of stack space. Terminating scopes end
76 /// up being contained in a DestructionScope that contains the
77 /// destructor's execution.
78 terminating_scopes: FxHashSet<hir::ItemLocalId>,
81 /// Records the lifetime of a local variable as `cx.var_parent`
82 fn record_var_lifetime(
83 visitor: &mut RegionResolutionVisitor<'_>,
84 var_id: hir::ItemLocalId,
87 match visitor.cx.var_parent {
89 // this can happen in extern fn declarations like
91 // extern fn isalnum(c: c_int) -> c_int
93 Some((parent_scope, _)) => visitor.scope_tree.record_var_scope(var_id, parent_scope),
97 fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx hir::Block<'tcx>) {
98 debug!("resolve_block(blk.hir_id={:?})", blk.hir_id);
100 let prev_cx = visitor.cx;
102 // We treat the tail expression in the block (if any) somewhat
103 // differently from the statements. The issue has to do with
104 // temporary lifetimes. Consider the following:
107 // let inner = ... (&bar()) ...;
109 // (... (&foo()) ...) // (the tail expression)
110 // }, other_argument());
112 // Each of the statements within the block is a terminating
113 // scope, and thus a temporary (e.g., the result of calling
114 // `bar()` in the initializer expression for `let inner = ...;`)
115 // will be cleaned up immediately after its corresponding
116 // statement (i.e., `let inner = ...;`) executes.
118 // On the other hand, temporaries associated with evaluating the
119 // tail expression for the block are assigned lifetimes so that
120 // they will be cleaned up as part of the terminating scope
121 // *surrounding* the block expression. Here, the terminating
122 // scope for the block expression is the `quux(..)` call; so
123 // those temporaries will only be cleaned up *after* both
124 // `other_argument()` has run and also the call to `quux(..)`
125 // itself has returned.
127 visitor.enter_node_scope_with_dtor(blk.hir_id.local_id);
128 visitor.cx.var_parent = visitor.cx.parent;
131 // This block should be kept approximately in sync with
132 // `intravisit::walk_block`. (We manually walk the block, rather
133 // than call `walk_block`, in order to maintain precise
134 // index information.)
136 for (i, statement) in blk.stmts.iter().enumerate() {
137 match statement.kind {
138 hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {
139 // Each declaration introduces a subscope for bindings
140 // introduced by the declaration; this subscope covers a
141 // suffix of the block. Each subscope in a block has the
142 // previous subscope in the block as a parent, except for
143 // the first such subscope, which has the block itself as a
145 visitor.enter_scope(Scope {
146 id: blk.hir_id.local_id,
147 data: ScopeData::Remainder(FirstStatementIndex::new(i)),
149 visitor.cx.var_parent = visitor.cx.parent;
151 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
153 visitor.visit_stmt(statement)
155 walk_list!(visitor, visit_expr, &blk.expr);
158 visitor.cx = prev_cx;
161 fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
162 let prev_cx = visitor.cx;
164 visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node });
165 visitor.cx.var_parent = visitor.cx.parent;
167 visitor.terminating_scopes.insert(arm.body.hir_id.local_id);
169 if let Some(hir::Guard::If(ref expr)) = arm.guard {
170 visitor.terminating_scopes.insert(expr.hir_id.local_id);
173 intravisit::walk_arm(visitor, arm);
175 visitor.cx = prev_cx;
178 fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
179 visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node });
181 // If this is a binding then record the lifetime of that binding.
182 if let PatKind::Binding(..) = pat.kind {
183 record_var_lifetime(visitor, pat.hir_id.local_id, pat.span);
186 debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
188 intravisit::walk_pat(visitor, pat);
190 visitor.expr_and_pat_count += 1;
192 debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
195 fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
196 let stmt_id = stmt.hir_id.local_id;
197 debug!("resolve_stmt(stmt.id={:?})", stmt_id);
199 // Every statement will clean up the temporaries created during
200 // execution of that statement. Therefore each statement has an
201 // associated destruction scope that represents the scope of the
202 // statement plus its destructors, and thus the scope for which
203 // regions referenced by the destructors need to survive.
204 visitor.terminating_scopes.insert(stmt_id);
206 let prev_parent = visitor.cx.parent;
207 visitor.enter_node_scope_with_dtor(stmt_id);
209 intravisit::walk_stmt(visitor, stmt);
211 visitor.cx.parent = prev_parent;
214 fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
215 debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
217 let prev_cx = visitor.cx;
218 visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
221 let terminating_scopes = &mut visitor.terminating_scopes;
222 let mut terminating = |id: hir::ItemLocalId| {
223 terminating_scopes.insert(id);
226 // Conditional or repeating scopes are always terminating
227 // scopes, meaning that temporaries cannot outlive them.
228 // This ensures fixed size stacks.
229 hir::ExprKind::Binary(
230 source_map::Spanned { node: hir::BinOpKind::And, .. },
234 | hir::ExprKind::Binary(
235 source_map::Spanned { node: hir::BinOpKind::Or, .. },
239 // For shortcircuiting operators, mark the RHS as a terminating
240 // scope since it only executes conditionally.
241 terminating(r.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 evluation 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, because they are the only
337 // case of nested bodies that share the parent environment.
338 hir::ExprKind::Closure(.., body, _, _) => {
339 let body = visitor.tcx.hir().body(body);
340 visitor.visit_body(body);
342 hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
344 "resolve_expr - enabling pessimistic_yield, was previously {}",
348 let start_point = visitor.fixup_scopes.len();
349 visitor.pessimistic_yield = true;
351 // If the actual execution order turns out to be right-to-left,
352 // then we're fine. However, if the actual execution order is left-to-right,
353 // then we'll assign too low a count to any `yield` expressions
354 // we encounter in 'right_expression' - they should really occur after all of the
355 // expressions in 'left_expression'.
356 visitor.visit_expr(&right_expr);
357 visitor.pessimistic_yield = prev_pessimistic;
359 debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
360 visitor.visit_expr(&left_expr);
361 debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
363 // Remove and process any scopes pushed by the visitor
364 let target_scopes = visitor.fixup_scopes.drain(start_point..);
366 for scope in target_scopes {
367 let mut yield_data = visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap();
368 let count = yield_data.expr_and_pat_count;
369 let span = yield_data.span;
371 // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
372 // before walking the left-hand side, it should be impossible for the recorded
373 // count to be greater than the left-hand side count.
374 if count > visitor.expr_and_pat_count {
376 "Encountered greater count {} at span {:?} - expected no greater than {}",
379 visitor.expr_and_pat_count
382 let new_count = visitor.expr_and_pat_count;
384 "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
385 scope, count, new_count, span
388 yield_data.expr_and_pat_count = new_count;
392 _ => intravisit::walk_expr(visitor, expr),
395 visitor.expr_and_pat_count += 1;
397 debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
399 if let hir::ExprKind::Yield(_, source) = &expr.kind {
400 // Mark this expr's scope and all parent scopes as containing `yield`.
401 let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
403 let data = YieldData {
405 expr_and_pat_count: visitor.expr_and_pat_count,
408 visitor.scope_tree.yield_in_scope.insert(scope, data);
409 if visitor.pessimistic_yield {
410 debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
411 visitor.fixup_scopes.push(scope);
414 // Keep traversing up while we can.
415 match visitor.scope_tree.parent_map.get(&scope) {
416 // Don't cross from closure bodies to their parent.
417 Some(&(superscope, _)) => match superscope.data {
418 ScopeData::CallSite => break,
419 _ => scope = superscope,
426 visitor.cx = prev_cx;
429 fn resolve_local<'tcx>(
430 visitor: &mut RegionResolutionVisitor<'tcx>,
431 pat: Option<&'tcx hir::Pat<'tcx>>,
432 init: Option<&'tcx hir::Expr<'tcx>>,
434 debug!("resolve_local(pat={:?}, init={:?})", pat, init);
436 let blk_scope = visitor.cx.var_parent.map(|(p, _)| p);
438 // As an exception to the normal rules governing temporary
439 // lifetimes, initializers in a let have a temporary lifetime
440 // of the enclosing block. This means that e.g., a program
441 // like the following is legal:
443 // let ref x = HashMap::new();
445 // Because the hash map will be freed in the enclosing block.
447 // We express the rules more formally based on 3 grammars (defined
448 // fully in the helpers below that implement them):
450 // 1. `E&`, which matches expressions like `&<rvalue>` that
451 // own a pointer into the stack.
453 // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
454 // y)` that produce ref bindings into the value they are
455 // matched against or something (at least partially) owned by
456 // the value they are matched against. (By partially owned,
457 // I mean that creating a binding into a ref-counted or managed value
458 // would still count.)
460 // 3. `ET`, which matches both rvalues like `foo()` as well as places
461 // based on rvalues like `foo().x[2].y`.
463 // A subexpression `<rvalue>` that appears in a let initializer
464 // `let pat [: ty] = expr` has an extended temporary lifetime if
465 // any of the following conditions are met:
467 // A. `pat` matches `P&` and `expr` matches `ET`
468 // (covers cases where `pat` creates ref bindings into an rvalue
469 // produced by `expr`)
470 // B. `ty` is a borrowed pointer and `expr` matches `ET`
471 // (covers cases where coercion creates a borrow)
472 // C. `expr` matches `E&`
473 // (covers cases `expr` borrows an rvalue that is then assigned
474 // to memory (at least partially) owned by the binding)
476 // Here are some examples hopefully giving an intuition where each
477 // rule comes into play and why:
479 // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
480 // would have an extended lifetime, but not `foo()`.
482 // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
485 // In some cases, multiple rules may apply (though not to the same
486 // rvalue). For example:
488 // let ref x = [&a(), &b()];
490 // Here, the expression `[...]` has an extended lifetime due to rule
491 // A, but the inner rvalues `a()` and `b()` have an extended lifetime
494 if let Some(expr) = init {
495 record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope);
497 if let Some(pat) = pat {
498 if is_binding_pat(pat) {
499 record_rvalue_scope(visitor, &expr, blk_scope);
504 // Make sure we visit the initializer first, so expr_and_pat_count remains correct
505 if let Some(expr) = init {
506 visitor.visit_expr(expr);
508 if let Some(pat) = pat {
509 visitor.visit_pat(pat);
512 /// Returns `true` if `pat` match the `P&` non-terminal.
516 /// | StructName { ..., P&, ... }
517 /// | VariantName(..., P&, ...)
518 /// | [ ..., P&, ... ]
519 /// | ( ..., P&, ... )
520 /// | ... "|" P& "|" ...
523 fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
524 // Note that the code below looks for *explicit* refs only, that is, it won't
525 // know about *implicit* refs as introduced in #42640.
527 // This is not a problem. For example, consider
529 // let (ref x, ref y) = (Foo { .. }, Bar { .. });
531 // Due to the explicit refs on the left hand side, the below code would signal
532 // that the temporary value on the right hand side should live until the end of
533 // the enclosing block (as opposed to being dropped after the let is complete).
535 // To create an implicit ref, however, you must have a borrowed value on the RHS
536 // already, as in this example (which won't compile before #42640):
538 // let Foo { x, .. } = &Foo { x: ..., ... };
542 // let Foo { ref x, .. } = Foo { ... };
544 // In the former case (the implicit ref version), the temporary is created by the
545 // & expression, and its lifetime would be extended to the end of the block (due
546 // to a different rule, not the below code).
548 PatKind::Binding(hir::BindingAnnotation::Ref, ..)
549 | PatKind::Binding(hir::BindingAnnotation::RefMut, ..) => true,
551 PatKind::Struct(_, ref field_pats, _) => {
552 field_pats.iter().any(|fp| is_binding_pat(&fp.pat))
555 PatKind::Slice(ref pats1, ref pats2, ref pats3) => {
556 pats1.iter().any(|p| is_binding_pat(&p))
557 || pats2.iter().any(|p| is_binding_pat(&p))
558 || pats3.iter().any(|p| is_binding_pat(&p))
561 PatKind::Or(ref subpats)
562 | PatKind::TupleStruct(_, ref subpats, _)
563 | PatKind::Tuple(ref subpats, _) => subpats.iter().any(|p| is_binding_pat(&p)),
565 PatKind::Box(ref subpat) => is_binding_pat(&subpat),
569 hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
575 | PatKind::Range(_, _, _) => false,
579 /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
583 /// | StructName { ..., f: E&, ... }
584 /// | [ ..., E&, ... ]
585 /// | ( ..., E&, ... )
591 fn record_rvalue_scope_if_borrow_expr<'tcx>(
592 visitor: &mut RegionResolutionVisitor<'tcx>,
593 expr: &hir::Expr<'_>,
594 blk_id: Option<Scope>,
597 hir::ExprKind::AddrOf(_, _, ref subexpr) => {
598 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
599 record_rvalue_scope(visitor, &subexpr, blk_id);
601 hir::ExprKind::Struct(_, fields, _) => {
602 for field in fields {
603 record_rvalue_scope_if_borrow_expr(visitor, &field.expr, blk_id);
606 hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
607 for subexpr in subexprs {
608 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
611 hir::ExprKind::Cast(ref subexpr, _) => {
612 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id)
614 hir::ExprKind::Block(ref block, _) => {
615 if let Some(ref subexpr) = block.expr {
616 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
623 /// Applied to an expression `expr` if `expr` -- or something owned or partially owned by
624 /// `expr` -- is going to be indirectly referenced by a variable in a let statement. In that
625 /// case, the "temporary lifetime" or `expr` is extended to be the block enclosing the `let`
628 /// More formally, if `expr` matches the grammar `ET`, record the rvalue scope of the matching
629 /// `<rvalue>` as `blk_id`:
639 /// Note: ET is intended to match "rvalues or places based on rvalues".
640 fn record_rvalue_scope<'tcx>(
641 visitor: &mut RegionResolutionVisitor<'tcx>,
642 expr: &hir::Expr<'_>,
643 blk_scope: Option<Scope>,
647 // Note: give all the expressions matching `ET` with the
648 // extended temporary lifetime, not just the innermost rvalue,
649 // because in codegen if we must compile e.g., `*rvalue()`
650 // into a temporary, we request the temporary scope of the
652 visitor.scope_tree.record_rvalue_scope(expr.hir_id.local_id, blk_scope);
655 hir::ExprKind::AddrOf(_, _, ref subexpr)
656 | hir::ExprKind::Unary(hir::UnOp::UnDeref, ref subexpr)
657 | hir::ExprKind::Field(ref subexpr, _)
658 | hir::ExprKind::Index(ref subexpr, _) => {
669 impl<'tcx> RegionResolutionVisitor<'tcx> {
670 /// Records the current parent (if any) as the parent of `child_scope`.
671 /// Returns the depth of `child_scope`.
672 fn record_child_scope(&mut self, child_scope: Scope) -> ScopeDepth {
673 let parent = self.cx.parent;
674 self.scope_tree.record_scope_parent(child_scope, parent);
675 // If `child_scope` has no parent, it must be the root node, and so has
676 // a depth of 1. Otherwise, its depth is one more than its parent's.
677 parent.map_or(1, |(_p, d)| d + 1)
680 /// Records the current parent (if any) as the parent of `child_scope`,
681 /// and sets `child_scope` as the new current parent.
682 fn enter_scope(&mut self, child_scope: Scope) {
683 let child_depth = self.record_child_scope(child_scope);
684 self.cx.parent = Some((child_scope, child_depth));
687 fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) {
688 // If node was previously marked as a terminating scope during the
689 // recursive visit of its parent node in the AST, then we need to
690 // account for the destruction scope representing the scope of
691 // the destructors that run immediately after it completes.
692 if self.terminating_scopes.contains(&id) {
693 self.enter_scope(Scope { id, data: ScopeData::Destruction });
695 self.enter_scope(Scope { id, data: ScopeData::Node });
699 impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
700 type Map = intravisit::ErasedMap<'tcx>;
702 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
703 NestedVisitorMap::None
706 fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
707 resolve_block(self, b);
710 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
711 let body_id = body.id();
712 let owner_id = self.tcx.hir().body_owner(body_id);
715 "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
717 self.tcx.sess.source_map().span_to_string(body.value.span),
722 // Save all state that is specific to the outer function
723 // body. These will be restored once down below, once we've
725 let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
726 let outer_cx = self.cx;
727 let outer_ts = mem::take(&mut self.terminating_scopes);
728 // The 'pessimistic yield' flag is set to true when we are
729 // processing a `+=` statement and have to make pessimistic
730 // control flow assumptions. This doesn't apply to nested
731 // bodies within the `+=` statements. See #69307.
732 let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false);
733 self.terminating_scopes.insert(body.value.hir_id.local_id);
735 if let Some(root_id) = self.cx.root_id {
736 self.scope_tree.record_closure_parent(body.value.hir_id.local_id, root_id);
738 self.cx.root_id = Some(body.value.hir_id.local_id);
740 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite });
741 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments });
743 // The arguments and `self` are parented to the fn.
744 self.cx.var_parent = self.cx.parent.take();
745 for param in body.params {
746 self.visit_pat(¶m.pat);
749 // The body of the every fn is a root scope.
750 self.cx.parent = self.cx.var_parent;
751 if self.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() {
752 self.visit_expr(&body.value)
754 // Only functions have an outer terminating (drop) scope, while
755 // temporaries in constant initializers may be 'static, but only
756 // according to rvalue lifetime semantics, using the same
757 // syntactical rules used for let initializers.
759 // e.g., in `let x = &f();`, the temporary holding the result from
760 // the `f()` call lives for the entirety of the surrounding block.
762 // Similarly, `const X: ... = &f();` would have the result of `f()`
763 // live for `'static`, implying (if Drop restrictions on constants
764 // ever get lifted) that the value *could* have a destructor, but
765 // it'd get leaked instead of the destructor running during the
766 // evaluation of `X` (if at all allowed by CTFE).
768 // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
769 // would *not* let the `f()` temporary escape into an outer scope
770 // (i.e., `'static`), which means that after `g` returns, it drops,
771 // and all the associated destruction scope rules apply.
772 self.cx.var_parent = None;
773 resolve_local(self, None, Some(&body.value));
776 if body.generator_kind.is_some() {
777 self.scope_tree.body_expr_count.insert(body_id, self.expr_and_pat_count);
780 // Restore context we had at the start.
781 self.expr_and_pat_count = outer_ec;
783 self.terminating_scopes = outer_ts;
784 self.pessimistic_yield = outer_pessimistic_yield;
787 fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
788 resolve_arm(self, a);
790 fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
791 resolve_pat(self, p);
793 fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
794 resolve_stmt(self, s);
796 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
797 resolve_expr(self, ex);
799 fn visit_local(&mut self, l: &'tcx Local<'tcx>) {
800 resolve_local(self, Some(&l.pat), l.init.as_deref());
804 fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
805 let closure_base_def_id = tcx.closure_base_def_id(def_id);
806 if closure_base_def_id != def_id {
807 return tcx.region_scope_tree(closure_base_def_id);
810 let id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
811 let scope_tree = if let Some(body_id) = tcx.hir().maybe_body_owned_by(id) {
812 let mut visitor = RegionResolutionVisitor {
814 scope_tree: ScopeTree::default(),
815 expr_and_pat_count: 0,
816 cx: Context { root_id: None, parent: None, var_parent: None },
817 terminating_scopes: Default::default(),
818 pessimistic_yield: false,
819 fixup_scopes: vec![],
822 let body = tcx.hir().body(body_id);
823 visitor.scope_tree.root_body = Some(body.value.hir_id);
825 // If the item is an associated const or a method,
826 // record its impl/trait parent, as it can also have
827 // lifetime parameters free in this body.
828 match tcx.hir().get(id) {
829 Node::ImplItem(_) | Node::TraitItem(_) => {
830 visitor.scope_tree.root_parent = Some(tcx.hir().get_parent_item(id));
835 visitor.visit_body(body);
842 tcx.arena.alloc(scope_tree)
845 pub fn provide(providers: &mut Providers) {
846 *providers = Providers { region_scope_tree, ..*providers };