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::If(ref expr, ref then, Some(ref otherwise)) => {
245 terminating(expr.hir_id.local_id);
246 terminating(then.hir_id.local_id);
247 terminating(otherwise.hir_id.local_id);
250 hir::ExprKind::If(ref expr, ref then, None) => {
251 terminating(expr.hir_id.local_id);
252 terminating(then.hir_id.local_id);
255 hir::ExprKind::Loop(ref body, _, _, _) => {
256 terminating(body.hir_id.local_id);
259 hir::ExprKind::DropTemps(ref expr) => {
260 // `DropTemps(expr)` does not denote a conditional scope.
261 // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
262 terminating(expr.hir_id.local_id);
265 hir::ExprKind::AssignOp(..)
266 | hir::ExprKind::Index(..)
267 | hir::ExprKind::Unary(..)
268 | hir::ExprKind::Call(..)
269 | hir::ExprKind::MethodCall(..) => {
270 // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
272 // The lifetimes for a call or method call look as follows:
280 // The idea is that call.callee_id represents *the time when
281 // the invoked function is actually running* and call.id
282 // represents *the time to prepare the arguments and make the
283 // call*. See the section "Borrows in Calls" borrowck/README.md
284 // for an extended explanation of why this distinction is
287 // record_superlifetime(new_cx, expr.callee_id);
294 let prev_pessimistic = visitor.pessimistic_yield;
296 // Ordinarily, we can rely on the visit order of HIR intravisit
297 // to correspond to the actual execution order of statements.
298 // However, there's a weird corner case with compound assignment
299 // operators (e.g. `a += b`). The evaluation order depends on whether
300 // or not the operator is overloaded (e.g. whether or not a trait
301 // like AddAssign is implemented).
303 // For primitive types (which, despite having a trait impl, don't actually
304 // end up calling it), the evluation order is right-to-left. For example,
305 // the following code snippet:
308 // *{println!("LHS!"); y} += {println!("RHS!"); 1};
315 // However, if the operator is used on a non-primitive type,
316 // the evaluation order will be left-to-right, since the operator
317 // actually get desugared to a method call. For example, this
318 // nearly identical code snippet:
320 // let y = &mut String::new();
321 // *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
327 // To determine the actual execution order, we need to perform
328 // trait resolution. Unfortunately, we need to be able to compute
329 // yield_in_scope before type checking is even done, as it gets
330 // used by AST borrowcheck.
332 // Fortunately, we don't need to know the actual execution order.
333 // It suffices to know the 'worst case' order with respect to yields.
334 // Specifically, we need to know the highest 'expr_and_pat_count'
335 // that we could assign to the yield expression. To do this,
336 // we pick the greater of the two values from the left-hand
337 // and right-hand expressions. This makes us overly conservative
338 // about what types could possibly live across yield points,
339 // but we will never fail to detect that a type does actually
340 // live across a yield point. The latter part is critical -
341 // we're already overly conservative about what types will live
342 // across yield points, as the generated MIR will determine
343 // when things are actually live. However, for typecheck to work
344 // properly, we can't miss any types.
347 // Manually recurse over closures, because they are the only
348 // case of nested bodies that share the parent environment.
349 hir::ExprKind::Closure(.., body, _, _) => {
350 let body = visitor.tcx.hir().body(body);
351 visitor.visit_body(body);
353 hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => {
355 "resolve_expr - enabling pessimistic_yield, was previously {}",
359 let start_point = visitor.fixup_scopes.len();
360 visitor.pessimistic_yield = true;
362 // If the actual execution order turns out to be right-to-left,
363 // then we're fine. However, if the actual execution order is left-to-right,
364 // then we'll assign too low a count to any `yield` expressions
365 // we encounter in 'right_expression' - they should really occur after all of the
366 // expressions in 'left_expression'.
367 visitor.visit_expr(&right_expr);
368 visitor.pessimistic_yield = prev_pessimistic;
370 debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
371 visitor.visit_expr(&left_expr);
372 debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
374 // Remove and process any scopes pushed by the visitor
375 let target_scopes = visitor.fixup_scopes.drain(start_point..);
377 for scope in target_scopes {
378 let mut yield_data = visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap();
379 let count = yield_data.expr_and_pat_count;
380 let span = yield_data.span;
382 // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
383 // before walking the left-hand side, it should be impossible for the recorded
384 // count to be greater than the left-hand side count.
385 if count > visitor.expr_and_pat_count {
387 "Encountered greater count {} at span {:?} - expected no greater than {}",
390 visitor.expr_and_pat_count
393 let new_count = visitor.expr_and_pat_count;
395 "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
396 scope, count, new_count, span
399 yield_data.expr_and_pat_count = new_count;
403 _ => intravisit::walk_expr(visitor, expr),
406 visitor.expr_and_pat_count += 1;
408 debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
410 if let hir::ExprKind::Yield(_, source) = &expr.kind {
411 // Mark this expr's scope and all parent scopes as containing `yield`.
412 let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node };
414 let data = YieldData {
416 expr_and_pat_count: visitor.expr_and_pat_count,
419 visitor.scope_tree.yield_in_scope.insert(scope, data);
420 if visitor.pessimistic_yield {
421 debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
422 visitor.fixup_scopes.push(scope);
425 // Keep traversing up while we can.
426 match visitor.scope_tree.parent_map.get(&scope) {
427 // Don't cross from closure bodies to their parent.
428 Some(&(superscope, _)) => match superscope.data {
429 ScopeData::CallSite => break,
430 _ => scope = superscope,
437 visitor.cx = prev_cx;
440 fn resolve_local<'tcx>(
441 visitor: &mut RegionResolutionVisitor<'tcx>,
442 pat: Option<&'tcx hir::Pat<'tcx>>,
443 init: Option<&'tcx hir::Expr<'tcx>>,
445 debug!("resolve_local(pat={:?}, init={:?})", pat, init);
447 let blk_scope = visitor.cx.var_parent.map(|(p, _)| p);
449 // As an exception to the normal rules governing temporary
450 // lifetimes, initializers in a let have a temporary lifetime
451 // of the enclosing block. This means that e.g., a program
452 // like the following is legal:
454 // let ref x = HashMap::new();
456 // Because the hash map will be freed in the enclosing block.
458 // We express the rules more formally based on 3 grammars (defined
459 // fully in the helpers below that implement them):
461 // 1. `E&`, which matches expressions like `&<rvalue>` that
462 // own a pointer into the stack.
464 // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
465 // y)` that produce ref bindings into the value they are
466 // matched against or something (at least partially) owned by
467 // the value they are matched against. (By partially owned,
468 // I mean that creating a binding into a ref-counted or managed value
469 // would still count.)
471 // 3. `ET`, which matches both rvalues like `foo()` as well as places
472 // based on rvalues like `foo().x[2].y`.
474 // A subexpression `<rvalue>` that appears in a let initializer
475 // `let pat [: ty] = expr` has an extended temporary lifetime if
476 // any of the following conditions are met:
478 // A. `pat` matches `P&` and `expr` matches `ET`
479 // (covers cases where `pat` creates ref bindings into an rvalue
480 // produced by `expr`)
481 // B. `ty` is a borrowed pointer and `expr` matches `ET`
482 // (covers cases where coercion creates a borrow)
483 // C. `expr` matches `E&`
484 // (covers cases `expr` borrows an rvalue that is then assigned
485 // to memory (at least partially) owned by the binding)
487 // Here are some examples hopefully giving an intuition where each
488 // rule comes into play and why:
490 // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
491 // would have an extended lifetime, but not `foo()`.
493 // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
496 // In some cases, multiple rules may apply (though not to the same
497 // rvalue). For example:
499 // let ref x = [&a(), &b()];
501 // Here, the expression `[...]` has an extended lifetime due to rule
502 // A, but the inner rvalues `a()` and `b()` have an extended lifetime
505 if let Some(expr) = init {
506 record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope);
508 if let Some(pat) = pat {
509 if is_binding_pat(pat) {
510 record_rvalue_scope(visitor, &expr, blk_scope);
515 // Make sure we visit the initializer first, so expr_and_pat_count remains correct
516 if let Some(expr) = init {
517 visitor.visit_expr(expr);
519 if let Some(pat) = pat {
520 visitor.visit_pat(pat);
523 /// Returns `true` if `pat` match the `P&` non-terminal.
527 /// | StructName { ..., P&, ... }
528 /// | VariantName(..., P&, ...)
529 /// | [ ..., P&, ... ]
530 /// | ( ..., P&, ... )
531 /// | ... "|" P& "|" ...
534 fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
535 // Note that the code below looks for *explicit* refs only, that is, it won't
536 // know about *implicit* refs as introduced in #42640.
538 // This is not a problem. For example, consider
540 // let (ref x, ref y) = (Foo { .. }, Bar { .. });
542 // Due to the explicit refs on the left hand side, the below code would signal
543 // that the temporary value on the right hand side should live until the end of
544 // the enclosing block (as opposed to being dropped after the let is complete).
546 // To create an implicit ref, however, you must have a borrowed value on the RHS
547 // already, as in this example (which won't compile before #42640):
549 // let Foo { x, .. } = &Foo { x: ..., ... };
553 // let Foo { ref x, .. } = Foo { ... };
555 // In the former case (the implicit ref version), the temporary is created by the
556 // & expression, and its lifetime would be extended to the end of the block (due
557 // to a different rule, not the below code).
559 PatKind::Binding(hir::BindingAnnotation::Ref, ..)
560 | PatKind::Binding(hir::BindingAnnotation::RefMut, ..) => true,
562 PatKind::Struct(_, ref field_pats, _) => {
563 field_pats.iter().any(|fp| is_binding_pat(&fp.pat))
566 PatKind::Slice(ref pats1, ref pats2, ref pats3) => {
567 pats1.iter().any(|p| is_binding_pat(&p))
568 || pats2.iter().any(|p| is_binding_pat(&p))
569 || pats3.iter().any(|p| is_binding_pat(&p))
572 PatKind::Or(ref subpats)
573 | PatKind::TupleStruct(_, ref subpats, _)
574 | PatKind::Tuple(ref subpats, _) => subpats.iter().any(|p| is_binding_pat(&p)),
576 PatKind::Box(ref subpat) => is_binding_pat(&subpat),
580 hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable,
586 | PatKind::Range(_, _, _) => false,
590 /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
594 /// | StructName { ..., f: E&, ... }
595 /// | [ ..., E&, ... ]
596 /// | ( ..., E&, ... )
602 fn record_rvalue_scope_if_borrow_expr<'tcx>(
603 visitor: &mut RegionResolutionVisitor<'tcx>,
604 expr: &hir::Expr<'_>,
605 blk_id: Option<Scope>,
608 hir::ExprKind::AddrOf(_, _, ref subexpr) => {
609 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
610 record_rvalue_scope(visitor, &subexpr, blk_id);
612 hir::ExprKind::Struct(_, fields, _) => {
613 for field in fields {
614 record_rvalue_scope_if_borrow_expr(visitor, &field.expr, blk_id);
617 hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
618 for subexpr in subexprs {
619 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
622 hir::ExprKind::Cast(ref subexpr, _) => {
623 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id)
625 hir::ExprKind::Block(ref block, _) => {
626 if let Some(ref subexpr) = block.expr {
627 record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id);
634 /// Applied to an expression `expr` if `expr` -- or something owned or partially owned by
635 /// `expr` -- is going to be indirectly referenced by a variable in a let statement. In that
636 /// case, the "temporary lifetime" or `expr` is extended to be the block enclosing the `let`
639 /// More formally, if `expr` matches the grammar `ET`, record the rvalue scope of the matching
640 /// `<rvalue>` as `blk_id`:
650 /// Note: ET is intended to match "rvalues or places based on rvalues".
651 fn record_rvalue_scope<'tcx>(
652 visitor: &mut RegionResolutionVisitor<'tcx>,
653 expr: &hir::Expr<'_>,
654 blk_scope: Option<Scope>,
658 // Note: give all the expressions matching `ET` with the
659 // extended temporary lifetime, not just the innermost rvalue,
660 // because in codegen if we must compile e.g., `*rvalue()`
661 // into a temporary, we request the temporary scope of the
663 visitor.scope_tree.record_rvalue_scope(expr.hir_id.local_id, blk_scope);
666 hir::ExprKind::AddrOf(_, _, ref subexpr)
667 | hir::ExprKind::Unary(hir::UnOp::UnDeref, ref subexpr)
668 | hir::ExprKind::Field(ref subexpr, _)
669 | hir::ExprKind::Index(ref subexpr, _) => {
680 impl<'tcx> RegionResolutionVisitor<'tcx> {
681 /// Records the current parent (if any) as the parent of `child_scope`.
682 /// Returns the depth of `child_scope`.
683 fn record_child_scope(&mut self, child_scope: Scope) -> ScopeDepth {
684 let parent = self.cx.parent;
685 self.scope_tree.record_scope_parent(child_scope, parent);
686 // If `child_scope` has no parent, it must be the root node, and so has
687 // a depth of 1. Otherwise, its depth is one more than its parent's.
688 parent.map_or(1, |(_p, d)| d + 1)
691 /// Records the current parent (if any) as the parent of `child_scope`,
692 /// and sets `child_scope` as the new current parent.
693 fn enter_scope(&mut self, child_scope: Scope) {
694 let child_depth = self.record_child_scope(child_scope);
695 self.cx.parent = Some((child_scope, child_depth));
698 fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) {
699 // If node was previously marked as a terminating scope during the
700 // recursive visit of its parent node in the AST, then we need to
701 // account for the destruction scope representing the scope of
702 // the destructors that run immediately after it completes.
703 if self.terminating_scopes.contains(&id) {
704 self.enter_scope(Scope { id, data: ScopeData::Destruction });
706 self.enter_scope(Scope { id, data: ScopeData::Node });
710 impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
711 type Map = intravisit::ErasedMap<'tcx>;
713 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
714 NestedVisitorMap::None
717 fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
718 resolve_block(self, b);
721 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
722 let body_id = body.id();
723 let owner_id = self.tcx.hir().body_owner(body_id);
726 "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
728 self.tcx.sess.source_map().span_to_string(body.value.span),
733 // Save all state that is specific to the outer function
734 // body. These will be restored once down below, once we've
736 let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
737 let outer_cx = self.cx;
738 let outer_ts = mem::take(&mut self.terminating_scopes);
739 // The 'pessimistic yield' flag is set to true when we are
740 // processing a `+=` statement and have to make pessimistic
741 // control flow assumptions. This doesn't apply to nested
742 // bodies within the `+=` statements. See #69307.
743 let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false);
744 self.terminating_scopes.insert(body.value.hir_id.local_id);
746 if let Some(root_id) = self.cx.root_id {
747 self.scope_tree.record_closure_parent(body.value.hir_id.local_id, root_id);
749 self.cx.root_id = Some(body.value.hir_id.local_id);
751 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite });
752 self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments });
754 // The arguments and `self` are parented to the fn.
755 self.cx.var_parent = self.cx.parent.take();
756 for param in body.params {
757 self.visit_pat(¶m.pat);
760 // The body of the every fn is a root scope.
761 self.cx.parent = self.cx.var_parent;
762 if self.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() {
763 self.visit_expr(&body.value)
765 // Only functions have an outer terminating (drop) scope, while
766 // temporaries in constant initializers may be 'static, but only
767 // according to rvalue lifetime semantics, using the same
768 // syntactical rules used for let initializers.
770 // e.g., in `let x = &f();`, the temporary holding the result from
771 // the `f()` call lives for the entirety of the surrounding block.
773 // Similarly, `const X: ... = &f();` would have the result of `f()`
774 // live for `'static`, implying (if Drop restrictions on constants
775 // ever get lifted) that the value *could* have a destructor, but
776 // it'd get leaked instead of the destructor running during the
777 // evaluation of `X` (if at all allowed by CTFE).
779 // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
780 // would *not* let the `f()` temporary escape into an outer scope
781 // (i.e., `'static`), which means that after `g` returns, it drops,
782 // and all the associated destruction scope rules apply.
783 self.cx.var_parent = None;
784 resolve_local(self, None, Some(&body.value));
787 if body.generator_kind.is_some() {
788 self.scope_tree.body_expr_count.insert(body_id, self.expr_and_pat_count);
791 // Restore context we had at the start.
792 self.expr_and_pat_count = outer_ec;
794 self.terminating_scopes = outer_ts;
795 self.pessimistic_yield = outer_pessimistic_yield;
798 fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
799 resolve_arm(self, a);
801 fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
802 resolve_pat(self, p);
804 fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
805 resolve_stmt(self, s);
807 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
808 resolve_expr(self, ex);
810 fn visit_local(&mut self, l: &'tcx Local<'tcx>) {
811 resolve_local(self, Some(&l.pat), l.init.as_deref());
815 fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
816 let closure_base_def_id = tcx.closure_base_def_id(def_id);
817 if closure_base_def_id != def_id {
818 return tcx.region_scope_tree(closure_base_def_id);
821 let id = tcx.hir().local_def_id_to_hir_id(def_id.expect_local());
822 let scope_tree = if let Some(body_id) = tcx.hir().maybe_body_owned_by(id) {
823 let mut visitor = RegionResolutionVisitor {
825 scope_tree: ScopeTree::default(),
826 expr_and_pat_count: 0,
827 cx: Context { root_id: None, parent: None, var_parent: None },
828 terminating_scopes: Default::default(),
829 pessimistic_yield: false,
830 fixup_scopes: vec![],
833 let body = tcx.hir().body(body_id);
834 visitor.scope_tree.root_body = Some(body.value.hir_id);
836 // If the item is an associated const or a method,
837 // record its impl/trait parent, as it can also have
838 // lifetime parameters free in this body.
839 match tcx.hir().get(id) {
840 Node::ImplItem(_) | Node::TraitItem(_) => {
841 visitor.scope_tree.root_parent = Some(tcx.hir().get_parent_item(id));
846 visitor.visit_body(body);
853 tcx.arena.alloc(scope_tree)
856 pub fn provide(providers: &mut Providers) {
857 *providers = Providers { region_scope_tree, ..*providers };