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::FxHashSet;
12use rustc_hir as hir;
13use rustc_hir::def_id::DefId;
14use rustc_hir::intravisit::{self, Visitor};
15use rustc_hir::{Arm, Block, Expr, LetStmt, Pat, PatKind, Stmt};
16use rustc_index::Idx;
17use rustc_middle::bug;
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 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
40    // up their indices.
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,
46
47    cx: Context,
48
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>,
70}
71
72/// Records the lifetime of a local variable as `cx.var_parent`
73fn record_var_lifetime(visitor: &mut ScopeResolutionVisitor<'_>, var_id: hir::ItemLocalId) {
74    match visitor.cx.var_parent {
75        None => {
76            // this can happen in extern fn declarations like
77            //
78            // extern fn isalnum(c: c_int) -> c_int
79        }
80        Some(parent_scope) => visitor.scope_tree.record_var_scope(var_id, parent_scope),
81    }
82}
83
84fn resolve_block<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, blk: &'tcx hir::Block<'tcx>) {
85    debug!("resolve_block(blk.hir_id={:?})", blk.hir_id);
86
87    let prev_cx = visitor.cx;
88
89    // We treat the tail expression in the block (if any) somewhat
90    // differently from the statements. The issue has to do with
91    // temporary lifetimes. Consider the following:
92    //
93    //    quux({
94    //        let inner = ... (&bar()) ...;
95    //
96    //        (... (&foo()) ...) // (the tail expression)
97    //    }, other_argument());
98    //
99    // Each of the statements within the block is a terminating
100    // scope, and thus a temporary (e.g., the result of calling
101    // `bar()` in the initializer expression for `let inner = ...;`)
102    // will be cleaned up immediately after its corresponding
103    // statement (i.e., `let inner = ...;`) executes.
104    //
105    // On the other hand, temporaries associated with evaluating the
106    // tail expression for the block are assigned lifetimes so that
107    // they will be cleaned up as part of the terminating scope
108    // *surrounding* the block expression. Here, the terminating
109    // scope for the block expression is the `quux(..)` call; so
110    // those temporaries will only be cleaned up *after* both
111    // `other_argument()` has run and also the call to `quux(..)`
112    // itself has returned.
113
114    visitor.enter_node_scope_with_dtor(blk.hir_id.local_id);
115    visitor.cx.var_parent = visitor.cx.parent;
116
117    {
118        // This block should be kept approximately in sync with
119        // `intravisit::walk_block`. (We manually walk the block, rather
120        // than call `walk_block`, in order to maintain precise
121        // index information.)
122
123        for (i, statement) in blk.stmts.iter().enumerate() {
124            match statement.kind {
125                hir::StmtKind::Let(LetStmt { els: Some(els), .. }) => {
126                    // Let-else has a special lexical structure for variables.
127                    // First we take a checkpoint of the current scope context here.
128                    let mut prev_cx = visitor.cx;
129
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                    // We need to back out temporarily to the last enclosing scope
137                    // for the `else` block, so that even the temporaries receiving
138                    // extended lifetime will be dropped inside this block.
139                    // We are visiting the `else` block in this order so that
140                    // the sequence of visits agree with the order in the default
141                    // `hir::intravisit` visitor.
142                    mem::swap(&mut prev_cx, &mut visitor.cx);
143                    visitor.terminating_scopes.insert(els.hir_id.local_id);
144                    visitor.visit_block(els);
145                    // From now on, we continue normally.
146                    visitor.cx = prev_cx;
147                }
148                hir::StmtKind::Let(..) => {
149                    // Each declaration introduces a subscope for bindings
150                    // introduced by the declaration; this subscope covers a
151                    // suffix of the block. Each subscope in a block has the
152                    // previous subscope in the block as a parent, except for
153                    // the first such subscope, which has the block itself as a
154                    // parent.
155                    visitor.enter_scope(Scope {
156                        local_id: blk.hir_id.local_id,
157                        data: ScopeData::Remainder(FirstStatementIndex::new(i)),
158                    });
159                    visitor.cx.var_parent = visitor.cx.parent;
160                    visitor.visit_stmt(statement)
161                }
162                hir::StmtKind::Item(..) => {
163                    // Don't create scopes for items, since they won't be
164                    // lowered to THIR and MIR.
165                }
166                hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
167            }
168        }
169        if let Some(tail_expr) = blk.expr {
170            let local_id = tail_expr.hir_id.local_id;
171            let edition = blk.span.edition();
172            if edition.at_least_rust_2024() {
173                visitor.terminating_scopes.insert(local_id);
174            } else if !visitor
175                .tcx
176                .lints_that_dont_need_to_run(())
177                .contains(&lint::LintId::of(lint::builtin::TAIL_EXPR_DROP_ORDER))
178            {
179                // If this temporary scope will be changing once the codebase adopts Rust 2024,
180                // and we are linting about possible semantic changes that would result,
181                // then record this node-id in the field `backwards_incompatible_scope`
182                // for future reference.
183                visitor
184                    .scope_tree
185                    .backwards_incompatible_scope
186                    .insert(local_id, Scope { local_id, data: ScopeData::Node });
187            }
188            visitor.visit_expr(tail_expr);
189        }
190    }
191
192    visitor.cx = prev_cx;
193}
194
195fn resolve_arm<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
196    fn has_let_expr(expr: &Expr<'_>) -> bool {
197        match &expr.kind {
198            hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
199            hir::ExprKind::Let(..) => true,
200            _ => false,
201        }
202    }
203
204    let prev_cx = visitor.cx;
205
206    visitor.terminating_scopes.insert(arm.hir_id.local_id);
207
208    visitor.enter_node_scope_with_dtor(arm.hir_id.local_id);
209    visitor.cx.var_parent = visitor.cx.parent;
210
211    if let Some(expr) = arm.guard
212        && !has_let_expr(expr)
213    {
214        visitor.terminating_scopes.insert(expr.hir_id.local_id);
215    }
216
217    intravisit::walk_arm(visitor, arm);
218
219    visitor.cx = prev_cx;
220}
221
222fn resolve_pat<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) {
223    // If this is a binding then record the lifetime of that binding.
224    if let PatKind::Binding(..) = pat.kind {
225        record_var_lifetime(visitor, pat.hir_id.local_id);
226    }
227
228    debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
229
230    intravisit::walk_pat(visitor, pat);
231
232    visitor.expr_and_pat_count += 1;
233
234    debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat);
235}
236
237fn resolve_stmt<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) {
238    let stmt_id = stmt.hir_id.local_id;
239    debug!("resolve_stmt(stmt.id={:?})", stmt_id);
240
241    // Every statement will clean up the temporaries created during
242    // execution of that statement. Therefore each statement has an
243    // associated destruction scope that represents the scope of the
244    // statement plus its destructors, and thus the scope for which
245    // regions referenced by the destructors need to survive.
246    visitor.terminating_scopes.insert(stmt_id);
247
248    let prev_parent = visitor.cx.parent;
249    visitor.enter_node_scope_with_dtor(stmt_id);
250
251    intravisit::walk_stmt(visitor, stmt);
252
253    visitor.cx.parent = prev_parent;
254}
255
256fn resolve_expr<'tcx>(visitor: &mut ScopeResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
257    debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
258
259    let prev_cx = visitor.cx;
260    visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
261
262    {
263        let terminating_scopes = &mut visitor.terminating_scopes;
264        let mut terminating = |id: hir::ItemLocalId| {
265            terminating_scopes.insert(id);
266        };
267        match expr.kind {
268            // Conditional or repeating scopes are always terminating
269            // scopes, meaning that temporaries cannot outlive them.
270            // This ensures fixed size stacks.
271            hir::ExprKind::Binary(
272                source_map::Spanned { node: hir::BinOpKind::And | hir::BinOpKind::Or, .. },
273                l,
274                r,
275            ) => {
276                // expr is a short circuiting operator (|| or &&). As its
277                // functionality can't be overridden by traits, it always
278                // processes bool sub-expressions. bools are Copy and thus we
279                // can drop any temporaries in evaluation (read) order
280                // (with the exception of potentially failing let expressions).
281                // We achieve this by enclosing the operands in a terminating
282                // scope, both the LHS and the RHS.
283
284                // We optimize this a little in the presence of chains.
285                // Chains like a && b && c get lowered to AND(AND(a, b), c).
286                // In here, b and c are RHS, while a is the only LHS operand in
287                // that chain. This holds true for longer chains as well: the
288                // leading operand is always the only LHS operand that is not a
289                // binop itself. Putting a binop like AND(a, b) into a
290                // terminating scope is not useful, thus we only put the LHS
291                // into a terminating scope if it is not a binop.
292
293                let terminate_lhs = match l.kind {
294                    // let expressions can create temporaries that live on
295                    hir::ExprKind::Let(_) => false,
296                    // binops already drop their temporaries, so there is no
297                    // need to put them into a terminating scope.
298                    // This is purely an optimization to reduce the number of
299                    // terminating scopes.
300                    hir::ExprKind::Binary(
301                        source_map::Spanned {
302                            node: hir::BinOpKind::And | hir::BinOpKind::Or, ..
303                        },
304                        ..,
305                    ) => false,
306                    // otherwise: mark it as terminating
307                    _ => true,
308                };
309                if terminate_lhs {
310                    terminating(l.hir_id.local_id);
311                }
312
313                // `Let` expressions (in a let-chain) shouldn't be terminating, as their temporaries
314                // should live beyond the immediate expression
315                if !matches!(r.kind, hir::ExprKind::Let(_)) {
316                    terminating(r.hir_id.local_id);
317                }
318            }
319            hir::ExprKind::If(_, then, Some(otherwise)) => {
320                terminating(then.hir_id.local_id);
321                terminating(otherwise.hir_id.local_id);
322            }
323
324            hir::ExprKind::If(_, then, None) => {
325                terminating(then.hir_id.local_id);
326            }
327
328            hir::ExprKind::Loop(body, _, _, _) => {
329                terminating(body.hir_id.local_id);
330            }
331
332            hir::ExprKind::DropTemps(expr) => {
333                // `DropTemps(expr)` does not denote a conditional scope.
334                // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`.
335                terminating(expr.hir_id.local_id);
336            }
337
338            hir::ExprKind::AssignOp(..)
339            | hir::ExprKind::Index(..)
340            | hir::ExprKind::Unary(..)
341            | hir::ExprKind::Call(..)
342            | hir::ExprKind::MethodCall(..) => {
343                // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls
344                //
345                // The lifetimes for a call or method call look as follows:
346                //
347                // call.id
348                // - arg0.id
349                // - ...
350                // - argN.id
351                // - call.callee_id
352                //
353                // The idea is that call.callee_id represents *the time when
354                // the invoked function is actually running* and call.id
355                // represents *the time to prepare the arguments and make the
356                // call*. See the section "Borrows in Calls" borrowck/README.md
357                // for an extended explanation of why this distinction is
358                // important.
359                //
360                // record_superlifetime(new_cx, expr.callee_id);
361            }
362
363            _ => {}
364        }
365    }
366
367    let prev_pessimistic = visitor.pessimistic_yield;
368
369    // Ordinarily, we can rely on the visit order of HIR intravisit
370    // to correspond to the actual execution order of statements.
371    // However, there's a weird corner case with compound assignment
372    // operators (e.g. `a += b`). The evaluation order depends on whether
373    // or not the operator is overloaded (e.g. whether or not a trait
374    // like AddAssign is implemented).
375
376    // For primitive types (which, despite having a trait impl, don't actually
377    // end up calling it), the evaluation order is right-to-left. For example,
378    // the following code snippet:
379    //
380    //    let y = &mut 0;
381    //    *{println!("LHS!"); y} += {println!("RHS!"); 1};
382    //
383    // will print:
384    //
385    // RHS!
386    // LHS!
387    //
388    // However, if the operator is used on a non-primitive type,
389    // the evaluation order will be left-to-right, since the operator
390    // actually get desugared to a method call. For example, this
391    // nearly identical code snippet:
392    //
393    //     let y = &mut String::new();
394    //    *{println!("LHS String"); y} += {println!("RHS String"); "hi"};
395    //
396    // will print:
397    // LHS String
398    // RHS String
399    //
400    // To determine the actual execution order, we need to perform
401    // trait resolution. Unfortunately, we need to be able to compute
402    // yield_in_scope before type checking is even done, as it gets
403    // used by AST borrowcheck.
404    //
405    // Fortunately, we don't need to know the actual execution order.
406    // It suffices to know the 'worst case' order with respect to yields.
407    // Specifically, we need to know the highest 'expr_and_pat_count'
408    // that we could assign to the yield expression. To do this,
409    // we pick the greater of the two values from the left-hand
410    // and right-hand expressions. This makes us overly conservative
411    // about what types could possibly live across yield points,
412    // but we will never fail to detect that a type does actually
413    // live across a yield point. The latter part is critical -
414    // we're already overly conservative about what types will live
415    // across yield points, as the generated MIR will determine
416    // when things are actually live. However, for typecheck to work
417    // properly, we can't miss any types.
418
419    match expr.kind {
420        // Manually recurse over closures, because they are nested bodies
421        // that share the parent environment. We handle const blocks in
422        // `visit_inline_const`.
423        hir::ExprKind::Closure(&hir::Closure { body, .. }) => {
424            let body = visitor.tcx.hir_body(body);
425            visitor.visit_body(body);
426        }
427        hir::ExprKind::AssignOp(_, left_expr, right_expr) => {
428            debug!(
429                "resolve_expr - enabling pessimistic_yield, was previously {}",
430                prev_pessimistic
431            );
432
433            let start_point = visitor.fixup_scopes.len();
434            visitor.pessimistic_yield = true;
435
436            // If the actual execution order turns out to be right-to-left,
437            // then we're fine. However, if the actual execution order is left-to-right,
438            // then we'll assign too low a count to any `yield` expressions
439            // we encounter in 'right_expression' - they should really occur after all of the
440            // expressions in 'left_expression'.
441            visitor.visit_expr(right_expr);
442            visitor.pessimistic_yield = prev_pessimistic;
443
444            debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic);
445            visitor.visit_expr(left_expr);
446            debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count);
447
448            // Remove and process any scopes pushed by the visitor
449            let target_scopes = visitor.fixup_scopes.drain(start_point..);
450
451            for scope in target_scopes {
452                let yield_data =
453                    visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap().last_mut().unwrap();
454                let count = yield_data.expr_and_pat_count;
455                let span = yield_data.span;
456
457                // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope
458                // before walking the left-hand side, it should be impossible for the recorded
459                // count to be greater than the left-hand side count.
460                if count > visitor.expr_and_pat_count {
461                    bug!(
462                        "Encountered greater count {} at span {:?} - expected no greater than {}",
463                        count,
464                        span,
465                        visitor.expr_and_pat_count
466                    );
467                }
468                let new_count = visitor.expr_and_pat_count;
469                debug!(
470                    "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}",
471                    scope, count, new_count, span
472                );
473
474                yield_data.expr_and_pat_count = new_count;
475            }
476        }
477
478        hir::ExprKind::If(cond, then, Some(otherwise)) => {
479            let expr_cx = visitor.cx;
480            let data = if expr.span.at_least_rust_2024() {
481                ScopeData::IfThenRescope
482            } else {
483                ScopeData::IfThen
484            };
485            visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
486            visitor.cx.var_parent = visitor.cx.parent;
487            visitor.visit_expr(cond);
488            visitor.visit_expr(then);
489            visitor.cx = expr_cx;
490            visitor.visit_expr(otherwise);
491        }
492
493        hir::ExprKind::If(cond, then, None) => {
494            let expr_cx = visitor.cx;
495            let data = if expr.span.at_least_rust_2024() {
496                ScopeData::IfThenRescope
497            } else {
498                ScopeData::IfThen
499            };
500            visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
501            visitor.cx.var_parent = visitor.cx.parent;
502            visitor.visit_expr(cond);
503            visitor.visit_expr(then);
504            visitor.cx = expr_cx;
505        }
506
507        _ => intravisit::walk_expr(visitor, expr),
508    }
509
510    visitor.expr_and_pat_count += 1;
511
512    debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr);
513
514    if let hir::ExprKind::Yield(_, source) = &expr.kind {
515        // Mark this expr's scope and all parent scopes as containing `yield`.
516        let mut scope = Scope { local_id: expr.hir_id.local_id, data: ScopeData::Node };
517        loop {
518            let span = match expr.kind {
519                hir::ExprKind::Yield(expr, hir::YieldSource::Await { .. }) => {
520                    expr.span.shrink_to_hi().to(expr.span)
521                }
522                _ => expr.span,
523            };
524            let data =
525                YieldData { span, expr_and_pat_count: visitor.expr_and_pat_count, source: *source };
526            match visitor.scope_tree.yield_in_scope.get_mut(&scope) {
527                Some(yields) => yields.push(data),
528                None => {
529                    visitor.scope_tree.yield_in_scope.insert(scope, vec![data]);
530                }
531            }
532
533            if visitor.pessimistic_yield {
534                debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope);
535                visitor.fixup_scopes.push(scope);
536            }
537
538            // Keep traversing up while we can.
539            match visitor.scope_tree.parent_map.get(&scope) {
540                // Don't cross from closure bodies to their parent.
541                Some(&superscope) => match superscope.data {
542                    ScopeData::CallSite => break,
543                    _ => scope = superscope,
544                },
545                None => break,
546            }
547        }
548    }
549
550    visitor.cx = prev_cx;
551}
552
553fn resolve_local<'tcx>(
554    visitor: &mut ScopeResolutionVisitor<'tcx>,
555    pat: Option<&'tcx hir::Pat<'tcx>>,
556    init: Option<&'tcx hir::Expr<'tcx>>,
557) {
558    debug!("resolve_local(pat={:?}, init={:?})", pat, init);
559
560    let blk_scope = visitor.cx.var_parent;
561
562    // As an exception to the normal rules governing temporary
563    // lifetimes, initializers in a let have a temporary lifetime
564    // of the enclosing block. This means that e.g., a program
565    // like the following is legal:
566    //
567    //     let ref x = HashMap::new();
568    //
569    // Because the hash map will be freed in the enclosing block.
570    //
571    // We express the rules more formally based on 3 grammars (defined
572    // fully in the helpers below that implement them):
573    //
574    // 1. `E&`, which matches expressions like `&<rvalue>` that
575    //    own a pointer into the stack.
576    //
577    // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref
578    //    y)` that produce ref bindings into the value they are
579    //    matched against or something (at least partially) owned by
580    //    the value they are matched against. (By partially owned,
581    //    I mean that creating a binding into a ref-counted or managed value
582    //    would still count.)
583    //
584    // 3. `ET`, which matches both rvalues like `foo()` as well as places
585    //    based on rvalues like `foo().x[2].y`.
586    //
587    // A subexpression `<rvalue>` that appears in a let initializer
588    // `let pat [: ty] = expr` has an extended temporary lifetime if
589    // any of the following conditions are met:
590    //
591    // A. `pat` matches `P&` and `expr` matches `ET`
592    //    (covers cases where `pat` creates ref bindings into an rvalue
593    //     produced by `expr`)
594    // B. `ty` is a borrowed pointer and `expr` matches `ET`
595    //    (covers cases where coercion creates a borrow)
596    // C. `expr` matches `E&`
597    //    (covers cases `expr` borrows an rvalue that is then assigned
598    //     to memory (at least partially) owned by the binding)
599    //
600    // Here are some examples hopefully giving an intuition where each
601    // rule comes into play and why:
602    //
603    // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)`
604    // would have an extended lifetime, but not `foo()`.
605    //
606    // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended
607    // lifetime.
608    //
609    // In some cases, multiple rules may apply (though not to the same
610    // rvalue). For example:
611    //
612    //     let ref x = [&a(), &b()];
613    //
614    // Here, the expression `[...]` has an extended lifetime due to rule
615    // A, but the inner rvalues `a()` and `b()` have an extended lifetime
616    // due to rule C.
617
618    if let Some(expr) = init {
619        record_rvalue_scope_if_borrow_expr(visitor, expr, blk_scope);
620
621        if let Some(pat) = pat {
622            if is_binding_pat(pat) {
623                visitor.scope_tree.record_rvalue_candidate(
624                    expr.hir_id,
625                    RvalueCandidate { target: expr.hir_id.local_id, lifetime: blk_scope },
626                );
627            }
628        }
629    }
630
631    // Make sure we visit the initializer first, so expr_and_pat_count remains correct.
632    // The correct order, as shared between coroutine_interior, drop_ranges and intravisitor,
633    // is to walk initializer, followed by pattern bindings, finally followed by the `else` block.
634    if let Some(expr) = init {
635        visitor.visit_expr(expr);
636    }
637    if let Some(pat) = pat {
638        visitor.visit_pat(pat);
639    }
640
641    /// Returns `true` if `pat` match the `P&` non-terminal.
642    ///
643    /// ```text
644    ///     P& = ref X
645    ///        | StructName { ..., P&, ... }
646    ///        | VariantName(..., P&, ...)
647    ///        | [ ..., P&, ... ]
648    ///        | ( ..., P&, ... )
649    ///        | ... "|" P& "|" ...
650    ///        | box P&
651    ///        | P& if ...
652    /// ```
653    fn is_binding_pat(pat: &hir::Pat<'_>) -> bool {
654        // Note that the code below looks for *explicit* refs only, that is, it won't
655        // know about *implicit* refs as introduced in #42640.
656        //
657        // This is not a problem. For example, consider
658        //
659        //      let (ref x, ref y) = (Foo { .. }, Bar { .. });
660        //
661        // Due to the explicit refs on the left hand side, the below code would signal
662        // that the temporary value on the right hand side should live until the end of
663        // the enclosing block (as opposed to being dropped after the let is complete).
664        //
665        // To create an implicit ref, however, you must have a borrowed value on the RHS
666        // already, as in this example (which won't compile before #42640):
667        //
668        //      let Foo { x, .. } = &Foo { x: ..., ... };
669        //
670        // in place of
671        //
672        //      let Foo { ref x, .. } = Foo { ... };
673        //
674        // In the former case (the implicit ref version), the temporary is created by the
675        // & expression, and its lifetime would be extended to the end of the block (due
676        // to a different rule, not the below code).
677        match pat.kind {
678            PatKind::Binding(hir::BindingMode(hir::ByRef::Yes(_), _), ..) => true,
679
680            PatKind::Struct(_, field_pats, _) => field_pats.iter().any(|fp| is_binding_pat(fp.pat)),
681
682            PatKind::Slice(pats1, pats2, pats3) => {
683                pats1.iter().any(|p| is_binding_pat(p))
684                    || pats2.iter().any(|p| is_binding_pat(p))
685                    || pats3.iter().any(|p| is_binding_pat(p))
686            }
687
688            PatKind::Or(subpats)
689            | PatKind::TupleStruct(_, subpats, _)
690            | PatKind::Tuple(subpats, _) => subpats.iter().any(|p| is_binding_pat(p)),
691
692            PatKind::Box(subpat) | PatKind::Deref(subpat) | PatKind::Guard(subpat, _) => {
693                is_binding_pat(subpat)
694            }
695
696            PatKind::Ref(_, _)
697            | PatKind::Binding(hir::BindingMode(hir::ByRef::No, _), ..)
698            | PatKind::Wild
699            | PatKind::Never
700            | PatKind::Expr(_)
701            | PatKind::Range(_, _, _)
702            | PatKind::Err(_) => false,
703        }
704    }
705
706    /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
707    ///
708    /// ```text
709    ///     E& = & ET
710    ///        | StructName { ..., f: E&, ... }
711    ///        | [ ..., E&, ... ]
712    ///        | ( ..., E&, ... )
713    ///        | {...; E&}
714    ///        | if _ { ...; E& } else { ...; E& }
715    ///        | match _ { ..., _ => E&, ... }
716    ///        | box E&
717    ///        | E& as ...
718    ///        | ( E& )
719    /// ```
720    fn record_rvalue_scope_if_borrow_expr<'tcx>(
721        visitor: &mut ScopeResolutionVisitor<'tcx>,
722        expr: &hir::Expr<'_>,
723        blk_id: Option<Scope>,
724    ) {
725        match expr.kind {
726            hir::ExprKind::AddrOf(_, _, subexpr) => {
727                record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
728                visitor.scope_tree.record_rvalue_candidate(
729                    subexpr.hir_id,
730                    RvalueCandidate { target: subexpr.hir_id.local_id, lifetime: blk_id },
731                );
732            }
733            hir::ExprKind::Struct(_, fields, _) => {
734                for field in fields {
735                    record_rvalue_scope_if_borrow_expr(visitor, field.expr, blk_id);
736                }
737            }
738            hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => {
739                for subexpr in subexprs {
740                    record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
741                }
742            }
743            hir::ExprKind::Cast(subexpr, _) => {
744                record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id)
745            }
746            hir::ExprKind::Block(block, _) => {
747                if let Some(subexpr) = block.expr {
748                    record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
749                }
750            }
751            hir::ExprKind::If(_, then_block, else_block) => {
752                record_rvalue_scope_if_borrow_expr(visitor, then_block, blk_id);
753                if let Some(else_block) = else_block {
754                    record_rvalue_scope_if_borrow_expr(visitor, else_block, blk_id);
755                }
756            }
757            hir::ExprKind::Match(_, arms, _) => {
758                for arm in arms {
759                    record_rvalue_scope_if_borrow_expr(visitor, arm.body, blk_id);
760                }
761            }
762            hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) => {
763                // FIXME(@dingxiangfei2009): choose call arguments here
764                // for candidacy for extended parameter rule application
765            }
766            hir::ExprKind::Index(..) => {
767                // FIXME(@dingxiangfei2009): select the indices
768                // as candidate for rvalue scope rules
769            }
770            _ => {}
771        }
772    }
773}
774
775impl<'tcx> ScopeResolutionVisitor<'tcx> {
776    /// Records the current parent (if any) as the parent of `child_scope`.
777    fn record_child_scope(&mut self, child_scope: Scope) {
778        let parent = self.cx.parent;
779        self.scope_tree.record_scope_parent(child_scope, parent);
780    }
781
782    /// Records the current parent (if any) as the parent of `child_scope`,
783    /// and sets `child_scope` as the new current parent.
784    fn enter_scope(&mut self, child_scope: Scope) {
785        self.record_child_scope(child_scope);
786        self.cx.parent = Some(child_scope);
787    }
788
789    fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) {
790        // If node was previously marked as a terminating scope during the
791        // recursive visit of its parent node in the HIR, then we need to
792        // account for the destruction scope representing the scope of
793        // the destructors that run immediately after it completes.
794        if self.terminating_scopes.contains(&id) {
795            self.enter_scope(Scope { local_id: id, data: ScopeData::Destruction });
796        }
797        self.enter_scope(Scope { local_id: id, data: ScopeData::Node });
798    }
799
800    fn enter_body(&mut self, hir_id: hir::HirId, f: impl FnOnce(&mut Self)) {
801        // Save all state that is specific to the outer function
802        // body. These will be restored once down below, once we've
803        // visited the body.
804        let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0);
805        let outer_cx = self.cx;
806        let outer_ts = mem::take(&mut self.terminating_scopes);
807        // The 'pessimistic yield' flag is set to true when we are
808        // processing a `+=` statement and have to make pessimistic
809        // control flow assumptions. This doesn't apply to nested
810        // bodies within the `+=` statements. See #69307.
811        let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false);
812        self.terminating_scopes.insert(hir_id.local_id);
813
814        self.enter_scope(Scope { local_id: hir_id.local_id, data: ScopeData::CallSite });
815        self.enter_scope(Scope { local_id: hir_id.local_id, data: ScopeData::Arguments });
816
817        f(self);
818
819        // Restore context we had at the start.
820        self.expr_and_pat_count = outer_ec;
821        self.cx = outer_cx;
822        self.terminating_scopes = outer_ts;
823        self.pessimistic_yield = outer_pessimistic_yield;
824    }
825}
826
827impl<'tcx> Visitor<'tcx> for ScopeResolutionVisitor<'tcx> {
828    fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
829        resolve_block(self, b);
830    }
831
832    fn visit_body(&mut self, body: &hir::Body<'tcx>) {
833        let body_id = body.id();
834        let owner_id = self.tcx.hir_body_owner_def_id(body_id);
835
836        debug!(
837            "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
838            owner_id,
839            self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
840            body_id,
841            self.cx.parent
842        );
843
844        self.enter_body(body.value.hir_id, |this| {
845            if this.tcx.hir_body_owner_kind(owner_id).is_fn_or_closure() {
846                // The arguments and `self` are parented to the fn.
847                this.cx.var_parent = this.cx.parent;
848                for param in body.params {
849                    this.visit_pat(param.pat);
850                }
851
852                // The body of the every fn is a root scope.
853                this.visit_expr(body.value)
854            } else {
855                // Only functions have an outer terminating (drop) scope, while
856                // temporaries in constant initializers may be 'static, but only
857                // according to rvalue lifetime semantics, using the same
858                // syntactical rules used for let initializers.
859                //
860                // e.g., in `let x = &f();`, the temporary holding the result from
861                // the `f()` call lives for the entirety of the surrounding block.
862                //
863                // Similarly, `const X: ... = &f();` would have the result of `f()`
864                // live for `'static`, implying (if Drop restrictions on constants
865                // ever get lifted) that the value *could* have a destructor, but
866                // it'd get leaked instead of the destructor running during the
867                // evaluation of `X` (if at all allowed by CTFE).
868                //
869                // However, `const Y: ... = g(&f());`, like `let y = g(&f());`,
870                // would *not* let the `f()` temporary escape into an outer scope
871                // (i.e., `'static`), which means that after `g` returns, it drops,
872                // and all the associated destruction scope rules apply.
873                this.cx.var_parent = None;
874                resolve_local(this, None, Some(body.value));
875            }
876        })
877    }
878
879    fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) {
880        resolve_arm(self, a);
881    }
882    fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) {
883        resolve_pat(self, p);
884    }
885    fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) {
886        resolve_stmt(self, s);
887    }
888    fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) {
889        resolve_expr(self, ex);
890    }
891    fn visit_local(&mut self, l: &'tcx LetStmt<'tcx>) {
892        resolve_local(self, Some(l.pat), l.init)
893    }
894    fn visit_inline_const(&mut self, c: &'tcx hir::ConstBlock) {
895        let body = self.tcx.hir_body(c.body);
896        self.visit_body(body);
897    }
898}
899
900/// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
901/// in the case of closures, this will be redirected to the enclosing function.
902///
903/// Performance: This is a query rather than a simple function to enable
904/// re-use in incremental scenarios. We may sometimes need to rerun the
905/// type checker even when the HIR hasn't changed, and in those cases
906/// we can avoid reconstructing the region scope tree.
907pub(crate) fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
908    let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
909    if typeck_root_def_id != def_id {
910        return tcx.region_scope_tree(typeck_root_def_id);
911    }
912
913    let scope_tree = if let Some(body) = tcx.hir_maybe_body_owned_by(def_id.expect_local()) {
914        let mut visitor = ScopeResolutionVisitor {
915            tcx,
916            scope_tree: ScopeTree::default(),
917            expr_and_pat_count: 0,
918            cx: Context { parent: None, var_parent: None },
919            terminating_scopes: Default::default(),
920            pessimistic_yield: false,
921            fixup_scopes: vec![],
922        };
923
924        visitor.scope_tree.root_body = Some(body.value.hir_id);
925        visitor.visit_body(&body);
926        visitor.scope_tree
927    } else {
928        ScopeTree::default()
929    };
930
931    tcx.arena.alloc(scope_tree)
932}