rustc_hir_analysis/check/region.rs
1//! This file builds up the `ScopeTree`, which describes
2//! the parent links in the region hierarchy.
3//!
4//! For more information about how MIR-based region-checking works,
5//! see the [rustc dev guide].
6//!
7//! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html
8
9use std::mem;
10
11use rustc_data_structures::fx::FxHashMap;
12use rustc_hir as hir;
13use rustc_hir::def::{CtorKind, DefKind, Res};
14use rustc_hir::def_id::DefId;
15use rustc_hir::intravisit::{self, Visitor};
16use rustc_hir::{Arm, Block, Expr, LetStmt, Pat, PatKind, Stmt};
17use rustc_index::Idx;
18use rustc_middle::middle::region::*;
19use rustc_middle::ty::TyCtxt;
20use rustc_session::lint;
21use rustc_span::source_map;
22use tracing::debug;
23
24#[derive(Debug, Copy, Clone)]
25struct Context {
26 /// The scope that contains any new variables declared.
27 var_parent: Option<Scope>,
28
29 /// Region parent of expressions, etc.
30 parent: Option<Scope>,
31}
32
33struct ScopeResolutionVisitor<'tcx> {
34 tcx: TyCtxt<'tcx>,
35
36 // The generated scope tree.
37 scope_tree: ScopeTree,
38
39 cx: Context,
40
41 extended_super_lets: FxHashMap<hir::ItemLocalId, Option<Scope>>,
42}
43
44/// Records the lifetime of a local variable as `cx.var_parent`
45fn record_var_lifetime(visitor: &mut ScopeResolutionVisitor<'_>, var_id: hir::ItemLocalId) {
46 match visitor.cx.var_parent {
47 None => {
48 // this can happen in extern fn declarations like
49 //
50 // extern fn isalnum(c: c_int) -> c_int
51 }
52 Some(parent_scope) => visitor.scope_tree.record_var_scope(var_id, parent_scope),
53 }
54}
55
56fn resolve_block<'tcx>(
57 visitor: &mut ScopeResolutionVisitor<'tcx>,
58 blk: &'tcx hir::Block<'tcx>,
59 terminating: bool,
60) {
61 debug!("resolve_block(blk.hir_id={:?})", blk.hir_id);
62
63 let prev_cx = visitor.cx;
64
65 // We treat the tail expression in the block (if any) somewhat
66 // differently from the statements. The issue has to do with
67 // temporary lifetimes. Consider the following:
68 //
69 // quux({
70 // let inner = ... (&bar()) ...;
71 //
72 // (... (&foo()) ...) // (the tail expression)
73 // }, other_argument());
74 //
75 // Each of the statements within the block is a terminating
76 // scope, and thus a temporary (e.g., the result of calling
77 // `bar()` in the initializer expression for `let inner = ...;`)
78 // will be cleaned up immediately after its corresponding
79 // statement (i.e., `let inner = ...;`) executes.
80 //
81 // On the other hand, temporaries associated with evaluating the
82 // tail expression for the block are assigned lifetimes so that
83 // they will be cleaned up as part of the terminating scope
84 // *surrounding* the block expression. Here, the terminating
85 // scope for the block expression is the `quux(..)` call; so
86 // those temporaries will only be cleaned up *after* both
87 // `other_argument()` has run and also the call to `quux(..)`
88 // itself has returned.
89
90 visitor.enter_node_scope_with_dtor(blk.hir_id.local_id, terminating);
91 visitor.cx.var_parent = visitor.cx.parent;
92
93 {
94 // This block should be kept approximately in sync with
95 // `intravisit::walk_block`. (We manually walk the block, rather
96 // than call `walk_block`, in order to maintain precise
97 // index information.)
98
99 for (i, statement) in blk.stmts.iter().enumerate() {
100 match statement.kind {
101 hir::StmtKind::Let(LetStmt { els: Some(els), .. }) => {
102 // Let-else has a special lexical structure for variables.
103 // First we take a checkpoint of the current scope context here.
104 let mut prev_cx = visitor.cx;
105
106 visitor.enter_scope(Scope {
107 local_id: blk.hir_id.local_id,
108 data: ScopeData::Remainder(FirstStatementIndex::new(i)),
109 });
110 visitor.cx.var_parent = visitor.cx.parent;
111 visitor.visit_stmt(statement);
112 // We need to back out temporarily to the last enclosing scope
113 // for the `else` block, so that even the temporaries receiving
114 // extended lifetime will be dropped inside this block.
115 // We are visiting the `else` block in this order so that
116 // the sequence of visits agree with the order in the default
117 // `hir::intravisit` visitor.
118 mem::swap(&mut prev_cx, &mut visitor.cx);
119 resolve_block(visitor, els, true);
120 // From now on, we continue normally.
121 visitor.cx = prev_cx;
122 }
123 hir::StmtKind::Let(..) => {
124 // Each declaration introduces a subscope for bindings
125 // introduced by the declaration; this subscope covers a
126 // suffix of the block. Each subscope in a block has the
127 // previous subscope in the block as a parent, except for
128 // the first such subscope, which has the block itself as a
129 // parent.
130 visitor.enter_scope(Scope {
131 local_id: blk.hir_id.local_id,
132 data: ScopeData::Remainder(FirstStatementIndex::new(i)),
133 });
134 visitor.cx.var_parent = visitor.cx.parent;
135 visitor.visit_stmt(statement)
136 }
137 hir::StmtKind::Item(..) => {
138 // Don't create scopes for items, since they won't be
139 // lowered to THIR and MIR.
140 }
141 hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
142 }
143 }
144 if let Some(tail_expr) = blk.expr {
145 let local_id = tail_expr.hir_id.local_id;
146 let edition = blk.span.edition();
147 let terminating = edition.at_least_rust_2024();
148 if !terminating
149 && !visitor
150 .tcx
151 .lints_that_dont_need_to_run(())
152 .contains(&lint::LintId::of(lint::builtin::TAIL_EXPR_DROP_ORDER))
153 {
154 // If this temporary scope will be changing once the codebase adopts Rust 2024,
155 // and we are linting about possible semantic changes that would result,
156 // then record this node-id in the field `backwards_incompatible_scope`
157 // for future reference.
158 visitor
159 .scope_tree
160 .backwards_incompatible_scope
161 .insert(local_id, Scope { local_id, data: ScopeData::Node });
162 }
163 resolve_expr(visitor, tail_expr, terminating);
164 }
165 }
166
167 visitor.cx = prev_cx;
168}
169
170fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
171 fn has_let_expr(expr: &Expr<'_>) -> bool {
172 match &expr.kind {
173 hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
174 hir::ExprKind::Let(..) => true,
175 _ => false,
176 }
177 }
178
179 let prev_cx = visitor.cx;
180
181 visitor.enter_node_scope_with_dtor(arm.hir_id.local_id, true);
182 visitor.cx.var_parent = visitor.cx.parent;
183
184 resolve_pat(visitor, arm.pat);
185 if let Some(guard) = arm.guard {
186 resolve_expr(visitor, guard, !has_let_expr(guard));
187 }
188 resolve_expr(visitor, arm.body, false);
189
190 visitor.cx = prev_cx;
191}
192
193#[tracing::instrument(level = "debug", skip(visitor))]
194fn resolve_pat<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
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);
198 }
199
200 intravisit::walk_pat(visitor, pat);
201}
202
203fn resolve_stmt<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
204 let stmt_id = stmt.hir_id.local_id;
205 debug!("resolve_stmt(stmt.id={:?})", stmt_id);
206
207 if let hir::StmtKind::Let(LetStmt { super_: Some(_), .. }) = stmt.kind {
208 // `super let` statement does not start a new scope, such that
209 //
210 // { super let x = identity(&temp()); &x }.method();
211 //
212 // behaves exactly as
213 //
214 // (&identity(&temp()).method();
215 intravisit::walk_stmt(visitor, stmt);
216 } else {
217 // Every statement will clean up the temporaries created during
218 // execution of that statement. Therefore each statement has an
219 // associated destruction scope that represents the scope of the
220 // statement plus its destructors, and thus the scope for which
221 // regions referenced by the destructors need to survive.
222
223 let prev_parent = visitor.cx.parent;
224 visitor.enter_node_scope_with_dtor(stmt_id, true);
225
226 intravisit::walk_stmt(visitor, stmt);
227
228 visitor.cx.parent = prev_parent;
229 }
230}
231
232#[tracing::instrument(level = "debug", skip(visitor))]
233fn resolve_expr<'tcx>(
234 visitor: &mut ScopeResolutionVisitor<'tcx>,
235 expr: &'tcx hir::Expr<'tcx>,
236 terminating: bool,
237) {
238 let prev_cx = visitor.cx;
239 visitor.enter_node_scope_with_dtor(expr.hir_id.local_id, terminating);
240
241 match expr.kind {
242 // Conditional or repeating scopes are always terminating
243 // scopes, meaning that temporaries cannot outlive them.
244 // This ensures fixed size stacks.
245 hir::ExprKind::Binary(
246 source_map::Spanned { node: hir::BinOpKind::And | hir::BinOpKind::Or, .. },
247 left,
248 right,
249 ) => {
250 // expr is a short circuiting operator (|| or &&). As its
251 // functionality can't be overridden by traits, it always
252 // processes bool sub-expressions. bools are Copy and thus we
253 // can drop any temporaries in evaluation (read) order
254 // (with the exception of potentially failing let expressions).
255 // We achieve this by enclosing the operands in a terminating
256 // scope, both the LHS and the RHS.
257
258 // We optimize this a little in the presence of chains.
259 // Chains like a && b && c get lowered to AND(AND(a, b), c).
260 // In here, b and c are RHS, while a is the only LHS operand in
261 // that chain. This holds true for longer chains as well: the
262 // leading operand is always the only LHS operand that is not a
263 // binop itself. Putting a binop like AND(a, b) into a
264 // terminating scope is not useful, thus we only put the LHS
265 // into a terminating scope if it is not a binop.
266
267 let terminate_lhs = match left.kind {
268 // let expressions can create temporaries that live on
269 hir::ExprKind::Let(_) => false,
270 // binops already drop their temporaries, so there is no
271 // need to put them into a terminating scope.
272 // This is purely an optimization to reduce the number of
273 // terminating scopes.
274 hir::ExprKind::Binary(
275 source_map::Spanned { node: hir::BinOpKind::And | hir::BinOpKind::Or, .. },
276 ..,
277 ) => false,
278 // otherwise: mark it as terminating
279 _ => true,
280 };
281
282 // `Let` expressions (in a let-chain) shouldn't be terminating, as their temporaries
283 // should live beyond the immediate expression
284 let terminate_rhs = !matches!(right.kind, hir::ExprKind::Let(_));
285
286 resolve_expr(visitor, left, terminate_lhs);
287 resolve_expr(visitor, right, terminate_rhs);
288 }
289 // Manually recurse over closures, because they are nested bodies
290 // that share the parent environment. We handle const blocks in
291 // `visit_inline_const`.
292 hir::ExprKind::Closure(&hir::Closure { body, .. }) => {
293 let body = visitor.tcx.hir_body(body);
294 visitor.visit_body(body);
295 }
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).
302 //
303 // For primitive types (which, despite having a trait impl, don't actually
304 // end up calling it), the evaluation order is right-to-left. For example,
305 // the following code snippet:
306 //
307 // let y = &mut 0;
308 // *{println!("LHS!"); y} += {println!("RHS!"); 1};
309 //
310 // will print:
311 //
312 // RHS!
313 // LHS!
314 //
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:
319 //
320 // let y = &mut String::new();
321 // *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
322 //
323 // will print:
324 // LHS String
325 // RHS String
326 //
327 // To determine the actual execution order, we need to perform
328 // trait resolution. Fortunately, we don't need to know the actual execution order.
329 hir::ExprKind::AssignOp(_, left_expr, right_expr) => {
330 visitor.visit_expr(right_expr);
331 visitor.visit_expr(left_expr);
332 }
333
334 hir::ExprKind::If(cond, then, Some(otherwise)) => {
335 let expr_cx = visitor.cx;
336 let data = if expr.span.at_least_rust_2024() {
337 ScopeData::IfThenRescope
338 } else {
339 ScopeData::IfThen
340 };
341 visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
342 visitor.cx.var_parent = visitor.cx.parent;
343 visitor.visit_expr(cond);
344 resolve_expr(visitor, then, true);
345 visitor.cx = expr_cx;
346 resolve_expr(visitor, otherwise, true);
347 }
348
349 hir::ExprKind::If(cond, then, None) => {
350 let expr_cx = visitor.cx;
351 let data = if expr.span.at_least_rust_2024() {
352 ScopeData::IfThenRescope
353 } else {
354 ScopeData::IfThen
355 };
356 visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
357 visitor.cx.var_parent = visitor.cx.parent;
358 visitor.visit_expr(cond);
359 resolve_expr(visitor, then, true);
360 visitor.cx = expr_cx;
361 }
362
363 hir::ExprKind::Loop(body, _, _, _) => {
364 resolve_block(visitor, body, true);
365 }
366
367 hir::ExprKind::DropTemps(expr) => {
368 // `DropTemps(expr)` does not denote a conditional scope.
369 // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
370 resolve_expr(visitor, expr, true);
371 }
372
373 _ => intravisit::walk_expr(visitor, expr),
374 }
375
376 visitor.cx = prev_cx;
377}
378
379#[derive(Copy, Clone, PartialEq, Eq, Debug)]
380enum LetKind {
381 Regular,
382 Super,
383}
384
385fn resolve_local<'tcx>(
386 visitor: &mut ScopeResolutionVisitor<'tcx>,
387 pat: Option<&'tcx hir::Pat<'tcx>>,
388 init: Option<&'tcx hir::Expr<'tcx>>,
389 let_kind: LetKind,
390) {
391 debug!("resolve_local(pat={:?}, init={:?}, let_kind={:?})", pat, init, let_kind);
392
393 // As an exception to the normal rules governing temporary
394 // lifetimes, initializers in a let have a temporary lifetime
395 // of the enclosing block. This means that e.g., a program
396 // like the following is legal:
397 //
398 // let ref x = HashMap::new();
399 //
400 // Because the hash map will be freed in the enclosing block.
401 //
402 // We express the rules more formally based on 3 grammars (defined
403 // fully in the helpers below that implement them):
404 //
405 // 1. `E&`, which matches expressions like `&<rvalue>` that
406 // own a pointer into the stack.
407 //
408 // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
409 // y)` that produce ref bindings into the value they are
410 // matched against or something (at least partially) owned by
411 // the value they are matched against. (By partially owned,
412 // I mean that creating a binding into a ref-counted or managed value
413 // would still count.)
414 //
415 // 3. `ET`, which matches both rvalues like `foo()` as well as places
416 // based on rvalues like `foo().x[2].y`.
417 //
418 // A subexpression `<rvalue>` that appears in a let initializer
419 // `let pat [: ty] = expr` has an extended temporary lifetime if
420 // any of the following conditions are met:
421 //
422 // A. `pat` matches `P&` and `expr` matches `ET`
423 // (covers cases where `pat` creates ref bindings into an rvalue
424 // produced by `expr`)
425 // B. `ty` is a borrowed pointer and `expr` matches `ET`
426 // (covers cases where coercion creates a borrow)
427 // C. `expr` matches `E&`
428 // (covers cases `expr` borrows an rvalue that is then assigned
429 // to memory (at least partially) owned by the binding)
430 //
431 // Here are some examples hopefully giving an intuition where each
432 // rule comes into play and why:
433 //
434 // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
435 // would have an extended lifetime, but not `foo()`.
436 //
437 // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
438 // lifetime.
439 //
440 // In some cases, multiple rules may apply (though not to the same
441 // rvalue). For example:
442 //
443 // let ref x = [&a(), &b()];
444 //
445 // Here, the expression `[...]` has an extended lifetime due to rule
446 // A, but the inner rvalues `a()` and `b()` have an extended lifetime
447 // due to rule C.
448
449 if let_kind == LetKind::Super {
450 if let Some(scope) = visitor.extended_super_lets.remove(&pat.unwrap().hir_id.local_id) {
451 // This expression was lifetime-extended by a parent let binding. E.g.
452 //
453 // let a = {
454 // super let b = temp();
455 // &b
456 // };
457 //
458 // (Which needs to behave exactly as: let a = &temp();)
459 //
460 // Processing of `let a` will have already decided to extend the lifetime of this
461 // `super let` to its own var_scope. We use that scope.
462 visitor.cx.var_parent = scope;
463 } else {
464 // This `super let` is not subject to lifetime extension from a parent let binding. E.g.
465 //
466 // identity({ super let x = temp(); &x }).method();
467 //
468 // (Which needs to behave exactly as: identity(&temp()).method();)
469 //
470 // Iterate up to the enclosing destruction scope to find the same scope that will also
471 // be used for the result of the block itself.
472 while let Some(s) = visitor.cx.var_parent {
473 let parent = visitor.scope_tree.parent_map.get(&s).cloned();
474 if let Some(Scope { data: ScopeData::Destruction, .. }) = parent {
475 break;
476 }
477 visitor.cx.var_parent = parent;
478 }
479 }
480 }
481
482 if let Some(expr) = init {
483 record_rvalue_scope_if_borrow_expr(visitor, expr, visitor.cx.var_parent);
484
485 if let Some(pat) = pat {
486 if is_binding_pat(pat) {
487 visitor.scope_tree.record_rvalue_candidate(
488 expr.hir_id,
489 RvalueCandidate {
490 target: expr.hir_id.local_id,
491 lifetime: visitor.cx.var_parent,
492 },
493 );
494 }
495 }
496 }
497
498 // Make sure we visit the initializer first.
499 // The correct order, as shared between drop_ranges and intravisitor,
500 // is to walk initializer, followed by pattern bindings, finally followed by the `else` block.
501 if let Some(expr) = init {
502 visitor.visit_expr(expr);
503 }
504
505 if let Some(pat) = pat {
506 visitor.visit_pat(pat);
507 }
508
509 /// Returns `true` if `pat` match the `P&` non-terminal.
510 ///
511 /// ```text
512 /// P& = ref X
513 /// | StructName { ..., P&, ... }
514 /// | VariantName(..., P&, ...)
515 /// | [ ..., P&, ... ]
516 /// | ( ..., P&, ... )
517 /// | ... "|" P& "|" ...
518 /// | box P&
519 /// | P& if ...
520 /// ```
521 fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
522 // Note that the code below looks for *explicit* refs only, that is, it won't
523 // know about *implicit* refs as introduced in #42640.
524 //
525 // This is not a problem. For example, consider
526 //
527 // let (ref x, ref y) = (Foo { .. }, Bar { .. });
528 //
529 // Due to the explicit refs on the left hand side, the below code would signal
530 // that the temporary value on the right hand side should live until the end of
531 // the enclosing block (as opposed to being dropped after the let is complete).
532 //
533 // To create an implicit ref, however, you must have a borrowed value on the RHS
534 // already, as in this example (which won't compile before #42640):
535 //
536 // let Foo { x, .. } = &Foo { x: ..., ... };
537 //
538 // in place of
539 //
540 // let Foo { ref x, .. } = Foo { ... };
541 //
542 // In the former case (the implicit ref version), the temporary is created by the
543 // & expression, and its lifetime would be extended to the end of the block (due
544 // to a different rule, not the below code).
545 match pat.kind {
546 PatKind::Binding(hir::BindingMode(hir::ByRef::Yes(_), _), ..) => true,
547
548 PatKind::Struct(_, field_pats, _) => field_pats.iter().any(|fp| is_binding_pat(fp.pat)),
549
550 PatKind::Slice(pats1, pats2, pats3) => {
551 pats1.iter().any(|p| is_binding_pat(p))
552 || pats2.iter().any(|p| is_binding_pat(p))
553 || pats3.iter().any(|p| is_binding_pat(p))
554 }
555
556 PatKind::Or(subpats)
557 | PatKind::TupleStruct(_, subpats, _)
558 | PatKind::Tuple(subpats, _) => subpats.iter().any(|p| is_binding_pat(p)),
559
560 PatKind::Box(subpat) | PatKind::Deref(subpat) | PatKind::Guard(subpat, _) => {
561 is_binding_pat(subpat)
562 }
563
564 PatKind::Ref(_, _)
565 | PatKind::Binding(hir::BindingMode(hir::ByRef::No, _), ..)
566 | PatKind::Missing
567 | PatKind::Wild
568 | PatKind::Never
569 | PatKind::Expr(_)
570 | PatKind::Range(_, _, _)
571 | PatKind::Err(_) => false,
572 }
573 }
574
575 /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
576 ///
577 /// ```text
578 /// E& = & ET
579 /// | StructName { ..., f: E&, ... }
580 /// | [ ..., E&, ... ]
581 /// | ( ..., E&, ... )
582 /// | {...; E&}
583 /// | { super let ... = E&; ... }
584 /// | if _ { ...; E& } else { ...; E& }
585 /// | match _ { ..., _ => E&, ... }
586 /// | box E&
587 /// | E& as ...
588 /// | ( E& )
589 /// ```
590 fn record_rvalue_scope_if_borrow_expr<'tcx>(
591 visitor: &mut ScopeResolutionVisitor<'tcx>,
592 expr: &hir::Expr<'_>,
593 blk_id: Option<Scope>,
594 ) {
595 match expr.kind {
596 hir::ExprKind::AddrOf(_, _, subexpr) => {
597 record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
598 visitor.scope_tree.record_rvalue_candidate(
599 subexpr.hir_id,
600 RvalueCandidate { target: subexpr.hir_id.local_id, lifetime: blk_id },
601 );
602 }
603 hir::ExprKind::Struct(_, fields, _) => {
604 for field in fields {
605 record_rvalue_scope_if_borrow_expr(visitor, field.expr, blk_id);
606 }
607 }
608 hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
609 for subexpr in subexprs {
610 record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
611 }
612 }
613 hir::ExprKind::Cast(subexpr, _) => {
614 record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id)
615 }
616 hir::ExprKind::Block(block, _) => {
617 if let Some(subexpr) = block.expr {
618 record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
619 }
620 for stmt in block.stmts {
621 if let hir::StmtKind::Let(local) = stmt.kind
622 && let Some(_) = local.super_
623 {
624 visitor.extended_super_lets.insert(local.pat.hir_id.local_id, blk_id);
625 }
626 }
627 }
628 hir::ExprKind::If(_, then_block, else_block) => {
629 record_rvalue_scope_if_borrow_expr(visitor, then_block, blk_id);
630 if let Some(else_block) = else_block {
631 record_rvalue_scope_if_borrow_expr(visitor, else_block, blk_id);
632 }
633 }
634 hir::ExprKind::Match(_, arms, _) => {
635 for arm in arms {
636 record_rvalue_scope_if_borrow_expr(visitor, arm.body, blk_id);
637 }
638 }
639 hir::ExprKind::Call(func, args) => {
640 // Recurse into tuple constructors, such as `Some(&temp())`.
641 //
642 // That way, there is no difference between `Some(..)` and `Some { 0: .. }`,
643 // even though the former is syntactically a function call.
644 if let hir::ExprKind::Path(path) = &func.kind
645 && let hir::QPath::Resolved(None, path) = path
646 && let Res::SelfCtor(_) | Res::Def(DefKind::Ctor(_, CtorKind::Fn), _) = path.res
647 {
648 for arg in args {
649 record_rvalue_scope_if_borrow_expr(visitor, arg, blk_id);
650 }
651 }
652 }
653 _ => {}
654 }
655 }
656}
657
658impl<'tcx> ScopeResolutionVisitor<'tcx> {
659 /// Records the current parent (if any) as the parent of `child_scope`.
660 fn record_child_scope(&mut self, child_scope: Scope) {
661 let parent = self.cx.parent;
662 self.scope_tree.record_scope_parent(child_scope, parent);
663 }
664
665 /// Records the current parent (if any) as the parent of `child_scope`,
666 /// and sets `child_scope` as the new current parent.
667 fn enter_scope(&mut self, child_scope: Scope) {
668 self.record_child_scope(child_scope);
669 self.cx.parent = Some(child_scope);
670 }
671
672 fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId, terminating: bool) {
673 // If node was previously marked as a terminating scope during the
674 // recursive visit of its parent node in the HIR, then we need to
675 // account for the destruction scope representing the scope of
676 // the destructors that run immediately after it completes.
677 if terminating {
678 self.enter_scope(Scope { local_id: id, data: ScopeData::Destruction });
679 }
680 self.enter_scope(Scope { local_id: id, data: ScopeData::Node });
681 }
682
683 fn enter_body(&mut self, hir_id: hir::HirId, f: impl FnOnce(&mut Self)) {
684 let outer_cx = self.cx;
685
686 self.enter_scope(Scope { local_id: hir_id.local_id, data: ScopeData::CallSite });
687 self.enter_scope(Scope { local_id: hir_id.local_id, data: ScopeData::Arguments });
688
689 f(self);
690
691 // Restore context we had at the start.
692 self.cx = outer_cx;
693 }
694}
695
696impl<'tcx> Visitor<'tcx> for ScopeResolutionVisitor<'tcx> {
697 fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
698 resolve_block(self, b, false);
699 }
700
701 fn visit_body(&mut self, body: &hir::Body<'tcx>) {
702 let body_id = body.id();
703 let owner_id = self.tcx.hir_body_owner_def_id(body_id);
704
705 debug!(
706 "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
707 owner_id,
708 self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
709 body_id,
710 self.cx.parent
711 );
712
713 self.enter_body(body.value.hir_id, |this| {
714 if this.tcx.hir_body_owner_kind(owner_id).is_fn_or_closure() {
715 // The arguments and `self` are parented to the fn.
716 this.cx.var_parent = this.cx.parent;
717 for param in body.params {
718 this.visit_pat(param.pat);
719 }
720
721 // The body of the every fn is a root scope.
722 resolve_expr(this, body.value, true);
723 } else {
724 // Only functions have an outer terminating (drop) scope, while
725 // temporaries in constant initializers may be 'static, but only
726 // according to rvalue lifetime semantics, using the same
727 // syntactical rules used for let initializers.
728 //
729 // e.g., in `let x = &f();`, the temporary holding the result from
730 // the `f()` call lives for the entirety of the surrounding block.
731 //
732 // Similarly, `const X: ... = &f();` would have the result of `f()`
733 // live for `'static`, implying (if Drop restrictions on constants
734 // ever get lifted) that the value *could* have a destructor, but
735 // it'd get leaked instead of the destructor running during the
736 // evaluation of `X` (if at all allowed by CTFE).
737 //
738 // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
739 // would *not* let the `f()` temporary escape into an outer scope
740 // (i.e., `'static`), which means that after `g` returns, it drops,
741 // and all the associated destruction scope rules apply.
742 this.cx.var_parent = None;
743 this.enter_scope(Scope {
744 local_id: body.value.hir_id.local_id,
745 data: ScopeData::Destruction,
746 });
747 resolve_local(this, None, Some(body.value), LetKind::Regular);
748 }
749 })
750 }
751
752 fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
753 resolve_arm(self, a);
754 }
755 fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
756 resolve_pat(self, p);
757 }
758 fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
759 resolve_stmt(self, s);
760 }
761 fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
762 resolve_expr(self, ex, false);
763 }
764 fn visit_local(&mut self, l: &'tcx LetStmt<'tcx>) {
765 let let_kind = match l.super_ {
766 Some(_) => LetKind::Super,
767 None => LetKind::Regular,
768 };
769 resolve_local(self, Some(l.pat), l.init, let_kind);
770 }
771 fn visit_inline_const(&mut self, c: &'tcx hir::ConstBlock) {
772 let body = self.tcx.hir_body(c.body);
773 self.visit_body(body);
774 }
775}
776
777/// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
778/// in the case of closures, this will be redirected to the enclosing function.
779///
780/// Performance: This is a query rather than a simple function to enable
781/// re-use in incremental scenarios. We may sometimes need to rerun the
782/// type checker even when the HIR hasn't changed, and in those cases
783/// we can avoid reconstructing the region scope tree.
784pub(crate) fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
785 let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
786 if typeck_root_def_id != def_id {
787 return tcx.region_scope_tree(typeck_root_def_id);
788 }
789
790 let scope_tree = if let Some(body) = tcx.hir_maybe_body_owned_by(def_id.expect_local()) {
791 let mut visitor = ScopeResolutionVisitor {
792 tcx,
793 scope_tree: ScopeTree::default(),
794 cx: Context { parent: None, var_parent: None },
795 extended_super_lets: Default::default(),
796 };
797
798 visitor.scope_tree.root_body = Some(body.value.hir_id);
799 visitor.visit_body(&body);
800 visitor.scope_tree
801 } else {
802 ScopeTree::default()
803 };
804
805 tcx.arena.alloc(scope_tree)
806}