| //! This file builds up the `ScopeTree`, which describes |
| //! the parent links in the region hierarchy. |
| //! |
| //! For more information about how MIR-based region-checking works, |
| //! see the [rustc dev guide]. |
| //! |
| //! [rustc dev guide]: https://rustc-dev-guide.rust-lang.org/borrow_check.html |
| |
| use rustc_ast::walk_list; |
| use rustc_data_structures::fx::FxHashSet; |
| use rustc_hir as hir; |
| use rustc_hir::def_id::DefId; |
| use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor}; |
| use rustc_hir::{Arm, Block, Expr, Local, Node, Pat, PatKind, Stmt}; |
| use rustc_index::vec::Idx; |
| use rustc_middle::middle::region::*; |
| use rustc_middle::ty::query::Providers; |
| use rustc_middle::ty::TyCtxt; |
| use rustc_span::source_map; |
| use rustc_span::Span; |
| |
| use std::mem; |
| |
| #[derive(Debug, Copy, Clone)] |
| pub struct Context { |
| /// The root of the current region tree. This is typically the id |
| /// of the innermost fn body. Each fn forms its own disjoint tree |
| /// in the region hierarchy. These fn bodies are themselves |
| /// arranged into a tree. See the "Modeling closures" section of |
| /// the README in `rustc_trait_selection::infer::region_constraints` |
| /// for more details. |
| root_id: Option<hir::ItemLocalId>, |
| |
| /// The scope that contains any new variables declared, plus its depth in |
| /// the scope tree. |
| var_parent: Option<(Scope, ScopeDepth)>, |
| |
| /// Region parent of expressions, etc., plus its depth in the scope tree. |
| parent: Option<(Scope, ScopeDepth)>, |
| } |
| |
| struct RegionResolutionVisitor<'tcx> { |
| tcx: TyCtxt<'tcx>, |
| |
| // The number of expressions and patterns visited in the current body. |
| expr_and_pat_count: usize, |
| // When this is `true`, we record the `Scopes` we encounter |
| // when processing a Yield expression. This allows us to fix |
| // up their indices. |
| pessimistic_yield: bool, |
| // Stores scopes when `pessimistic_yield` is `true`. |
| fixup_scopes: Vec<Scope>, |
| // The generated scope tree. |
| scope_tree: ScopeTree, |
| |
| cx: Context, |
| |
| /// `terminating_scopes` is a set containing the ids of each |
| /// statement, or conditional/repeating expression. These scopes |
| /// are calling "terminating scopes" because, when attempting to |
| /// find the scope of a temporary, by default we search up the |
| /// enclosing scopes until we encounter the terminating scope. A |
| /// conditional/repeating expression is one which is not |
| /// guaranteed to execute exactly once upon entering the parent |
| /// scope. This could be because the expression only executes |
| /// conditionally, such as the expression `b` in `a && b`, or |
| /// because the expression may execute many times, such as a loop |
| /// body. The reason that we distinguish such expressions is that, |
| /// upon exiting the parent scope, we cannot statically know how |
| /// many times the expression executed, and thus if the expression |
| /// creates temporaries we cannot know statically how many such |
| /// temporaries we would have to cleanup. Therefore, we ensure that |
| /// the temporaries never outlast the conditional/repeating |
| /// expression, preventing the need for dynamic checks and/or |
| /// arbitrary amounts of stack space. Terminating scopes end |
| /// up being contained in a DestructionScope that contains the |
| /// destructor's execution. |
| terminating_scopes: FxHashSet<hir::ItemLocalId>, |
| } |
| |
| /// Records the lifetime of a local variable as `cx.var_parent` |
| fn record_var_lifetime( |
| visitor: &mut RegionResolutionVisitor<'_>, |
| var_id: hir::ItemLocalId, |
| _sp: Span, |
| ) { |
| match visitor.cx.var_parent { |
| None => { |
| // this can happen in extern fn declarations like |
| // |
| // extern fn isalnum(c: c_int) -> c_int |
| } |
| Some((parent_scope, _)) => visitor.scope_tree.record_var_scope(var_id, parent_scope), |
| } |
| } |
| |
| fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx hir::Block<'tcx>) { |
| debug!("resolve_block(blk.hir_id={:?})", blk.hir_id); |
| |
| let prev_cx = visitor.cx; |
| |
| // We treat the tail expression in the block (if any) somewhat |
| // differently from the statements. The issue has to do with |
| // temporary lifetimes. Consider the following: |
| // |
| // quux({ |
| // let inner = ... (&bar()) ...; |
| // |
| // (... (&foo()) ...) // (the tail expression) |
| // }, other_argument()); |
| // |
| // Each of the statements within the block is a terminating |
| // scope, and thus a temporary (e.g., the result of calling |
| // `bar()` in the initializer expression for `let inner = ...;`) |
| // will be cleaned up immediately after its corresponding |
| // statement (i.e., `let inner = ...;`) executes. |
| // |
| // On the other hand, temporaries associated with evaluating the |
| // tail expression for the block are assigned lifetimes so that |
| // they will be cleaned up as part of the terminating scope |
| // *surrounding* the block expression. Here, the terminating |
| // scope for the block expression is the `quux(..)` call; so |
| // those temporaries will only be cleaned up *after* both |
| // `other_argument()` has run and also the call to `quux(..)` |
| // itself has returned. |
| |
| visitor.enter_node_scope_with_dtor(blk.hir_id.local_id); |
| visitor.cx.var_parent = visitor.cx.parent; |
| |
| { |
| // This block should be kept approximately in sync with |
| // `intravisit::walk_block`. (We manually walk the block, rather |
| // than call `walk_block`, in order to maintain precise |
| // index information.) |
| |
| for (i, statement) in blk.stmts.iter().enumerate() { |
| match statement.kind { |
| hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => { |
| // Each declaration introduces a subscope for bindings |
| // introduced by the declaration; this subscope covers a |
| // suffix of the block. Each subscope in a block has the |
| // previous subscope in the block as a parent, except for |
| // the first such subscope, which has the block itself as a |
| // parent. |
| visitor.enter_scope(Scope { |
| id: blk.hir_id.local_id, |
| data: ScopeData::Remainder(FirstStatementIndex::new(i)), |
| }); |
| visitor.cx.var_parent = visitor.cx.parent; |
| } |
| hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {} |
| } |
| visitor.visit_stmt(statement) |
| } |
| walk_list!(visitor, visit_expr, &blk.expr); |
| } |
| |
| visitor.cx = prev_cx; |
| } |
| |
| fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) { |
| let prev_cx = visitor.cx; |
| |
| visitor.enter_scope(Scope { id: arm.hir_id.local_id, data: ScopeData::Node }); |
| visitor.cx.var_parent = visitor.cx.parent; |
| |
| visitor.terminating_scopes.insert(arm.body.hir_id.local_id); |
| |
| if let Some(hir::Guard::If(ref expr)) = arm.guard { |
| visitor.terminating_scopes.insert(expr.hir_id.local_id); |
| } |
| |
| intravisit::walk_arm(visitor, arm); |
| |
| visitor.cx = prev_cx; |
| } |
| |
| fn resolve_pat<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, pat: &'tcx hir::Pat<'tcx>) { |
| visitor.record_child_scope(Scope { id: pat.hir_id.local_id, data: ScopeData::Node }); |
| |
| // If this is a binding then record the lifetime of that binding. |
| if let PatKind::Binding(..) = pat.kind { |
| record_var_lifetime(visitor, pat.hir_id.local_id, pat.span); |
| } |
| |
| debug!("resolve_pat - pre-increment {} pat = {:?}", visitor.expr_and_pat_count, pat); |
| |
| intravisit::walk_pat(visitor, pat); |
| |
| visitor.expr_and_pat_count += 1; |
| |
| debug!("resolve_pat - post-increment {} pat = {:?}", visitor.expr_and_pat_count, pat); |
| } |
| |
| fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx hir::Stmt<'tcx>) { |
| let stmt_id = stmt.hir_id.local_id; |
| debug!("resolve_stmt(stmt.id={:?})", stmt_id); |
| |
| // Every statement will clean up the temporaries created during |
| // execution of that statement. Therefore each statement has an |
| // associated destruction scope that represents the scope of the |
| // statement plus its destructors, and thus the scope for which |
| // regions referenced by the destructors need to survive. |
| visitor.terminating_scopes.insert(stmt_id); |
| |
| let prev_parent = visitor.cx.parent; |
| visitor.enter_node_scope_with_dtor(stmt_id); |
| |
| intravisit::walk_stmt(visitor, stmt); |
| |
| visitor.cx.parent = prev_parent; |
| } |
| |
| fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) { |
| debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr); |
| |
| let prev_cx = visitor.cx; |
| visitor.enter_node_scope_with_dtor(expr.hir_id.local_id); |
| |
| { |
| let terminating_scopes = &mut visitor.terminating_scopes; |
| let mut terminating = |id: hir::ItemLocalId| { |
| terminating_scopes.insert(id); |
| }; |
| match expr.kind { |
| // Conditional or repeating scopes are always terminating |
| // scopes, meaning that temporaries cannot outlive them. |
| // This ensures fixed size stacks. |
| hir::ExprKind::Binary( |
| source_map::Spanned { node: hir::BinOpKind::And, .. }, |
| _, |
| ref r, |
| ) |
| | hir::ExprKind::Binary( |
| source_map::Spanned { node: hir::BinOpKind::Or, .. }, |
| _, |
| ref r, |
| ) => { |
| // For shortcircuiting operators, mark the RHS as a terminating |
| // scope since it only executes conditionally. |
| terminating(r.hir_id.local_id); |
| } |
| |
| hir::ExprKind::Loop(ref body, _, _) => { |
| terminating(body.hir_id.local_id); |
| } |
| |
| hir::ExprKind::DropTemps(ref expr) => { |
| // `DropTemps(expr)` does not denote a conditional scope. |
| // Rather, we want to achieve the same behavior as `{ let _t = expr; _t }`. |
| terminating(expr.hir_id.local_id); |
| } |
| |
| hir::ExprKind::AssignOp(..) |
| | hir::ExprKind::Index(..) |
| | hir::ExprKind::Unary(..) |
| | hir::ExprKind::Call(..) |
| | hir::ExprKind::MethodCall(..) => { |
| // FIXME(https://github.com/rust-lang/rfcs/issues/811) Nested method calls |
| // |
| // The lifetimes for a call or method call look as follows: |
| // |
| // call.id |
| // - arg0.id |
| // - ... |
| // - argN.id |
| // - call.callee_id |
| // |
| // The idea is that call.callee_id represents *the time when |
| // the invoked function is actually running* and call.id |
| // represents *the time to prepare the arguments and make the |
| // call*. See the section "Borrows in Calls" borrowck/README.md |
| // for an extended explanation of why this distinction is |
| // important. |
| // |
| // record_superlifetime(new_cx, expr.callee_id); |
| } |
| |
| _ => {} |
| } |
| } |
| |
| let prev_pessimistic = visitor.pessimistic_yield; |
| |
| // Ordinarily, we can rely on the visit order of HIR intravisit |
| // to correspond to the actual execution order of statements. |
| // However, there's a weird corner case with compound assignment |
| // operators (e.g. `a += b`). The evaluation order depends on whether |
| // or not the operator is overloaded (e.g. whether or not a trait |
| // like AddAssign is implemented). |
| |
| // For primitive types (which, despite having a trait impl, don't actually |
| // end up calling it), the evluation order is right-to-left. For example, |
| // the following code snippet: |
| // |
| // let y = &mut 0; |
| // *{println!("LHS!"); y} += {println!("RHS!"); 1}; |
| // |
| // will print: |
| // |
| // RHS! |
| // LHS! |
| // |
| // However, if the operator is used on a non-primitive type, |
| // the evaluation order will be left-to-right, since the operator |
| // actually get desugared to a method call. For example, this |
| // nearly identical code snippet: |
| // |
| // let y = &mut String::new(); |
| // *{println!("LHS String"); y} += {println!("RHS String"); "hi"}; |
| // |
| // will print: |
| // LHS String |
| // RHS String |
| // |
| // To determine the actual execution order, we need to perform |
| // trait resolution. Unfortunately, we need to be able to compute |
| // yield_in_scope before type checking is even done, as it gets |
| // used by AST borrowcheck. |
| // |
| // Fortunately, we don't need to know the actual execution order. |
| // It suffices to know the 'worst case' order with respect to yields. |
| // Specifically, we need to know the highest 'expr_and_pat_count' |
| // that we could assign to the yield expression. To do this, |
| // we pick the greater of the two values from the left-hand |
| // and right-hand expressions. This makes us overly conservative |
| // about what types could possibly live across yield points, |
| // but we will never fail to detect that a type does actually |
| // live across a yield point. The latter part is critical - |
| // we're already overly conservative about what types will live |
| // across yield points, as the generated MIR will determine |
| // when things are actually live. However, for typecheck to work |
| // properly, we can't miss any types. |
| |
| match expr.kind { |
| // Manually recurse over closures, because they are the only |
| // case of nested bodies that share the parent environment. |
| hir::ExprKind::Closure(.., body, _, _) => { |
| let body = visitor.tcx.hir().body(body); |
| visitor.visit_body(body); |
| } |
| hir::ExprKind::AssignOp(_, ref left_expr, ref right_expr) => { |
| debug!( |
| "resolve_expr - enabling pessimistic_yield, was previously {}", |
| prev_pessimistic |
| ); |
| |
| let start_point = visitor.fixup_scopes.len(); |
| visitor.pessimistic_yield = true; |
| |
| // If the actual execution order turns out to be right-to-left, |
| // then we're fine. However, if the actual execution order is left-to-right, |
| // then we'll assign too low a count to any `yield` expressions |
| // we encounter in 'right_expression' - they should really occur after all of the |
| // expressions in 'left_expression'. |
| visitor.visit_expr(&right_expr); |
| visitor.pessimistic_yield = prev_pessimistic; |
| |
| debug!("resolve_expr - restoring pessimistic_yield to {}", prev_pessimistic); |
| visitor.visit_expr(&left_expr); |
| debug!("resolve_expr - fixing up counts to {}", visitor.expr_and_pat_count); |
| |
| // Remove and process any scopes pushed by the visitor |
| let target_scopes = visitor.fixup_scopes.drain(start_point..); |
| |
| for scope in target_scopes { |
| let mut yield_data = visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap(); |
| let count = yield_data.expr_and_pat_count; |
| let span = yield_data.span; |
| |
| // expr_and_pat_count never decreases. Since we recorded counts in yield_in_scope |
| // before walking the left-hand side, it should be impossible for the recorded |
| // count to be greater than the left-hand side count. |
| if count > visitor.expr_and_pat_count { |
| bug!( |
| "Encountered greater count {} at span {:?} - expected no greater than {}", |
| count, |
| span, |
| visitor.expr_and_pat_count |
| ); |
| } |
| let new_count = visitor.expr_and_pat_count; |
| debug!( |
| "resolve_expr - increasing count for scope {:?} from {} to {} at span {:?}", |
| scope, count, new_count, span |
| ); |
| |
| yield_data.expr_and_pat_count = new_count; |
| } |
| } |
| |
| _ => intravisit::walk_expr(visitor, expr), |
| } |
| |
| visitor.expr_and_pat_count += 1; |
| |
| debug!("resolve_expr post-increment {}, expr = {:?}", visitor.expr_and_pat_count, expr); |
| |
| if let hir::ExprKind::Yield(_, source) = &expr.kind { |
| // Mark this expr's scope and all parent scopes as containing `yield`. |
| let mut scope = Scope { id: expr.hir_id.local_id, data: ScopeData::Node }; |
| loop { |
| let data = YieldData { |
| span: expr.span, |
| expr_and_pat_count: visitor.expr_and_pat_count, |
| source: *source, |
| }; |
| visitor.scope_tree.yield_in_scope.insert(scope, data); |
| if visitor.pessimistic_yield { |
| debug!("resolve_expr in pessimistic_yield - marking scope {:?} for fixup", scope); |
| visitor.fixup_scopes.push(scope); |
| } |
| |
| // Keep traversing up while we can. |
| match visitor.scope_tree.parent_map.get(&scope) { |
| // Don't cross from closure bodies to their parent. |
| Some(&(superscope, _)) => match superscope.data { |
| ScopeData::CallSite => break, |
| _ => scope = superscope, |
| }, |
| None => break, |
| } |
| } |
| } |
| |
| visitor.cx = prev_cx; |
| } |
| |
| fn resolve_local<'tcx>( |
| visitor: &mut RegionResolutionVisitor<'tcx>, |
| pat: Option<&'tcx hir::Pat<'tcx>>, |
| init: Option<&'tcx hir::Expr<'tcx>>, |
| ) { |
| debug!("resolve_local(pat={:?}, init={:?})", pat, init); |
| |
| let blk_scope = visitor.cx.var_parent.map(|(p, _)| p); |
| |
| // As an exception to the normal rules governing temporary |
| // lifetimes, initializers in a let have a temporary lifetime |
| // of the enclosing block. This means that e.g., a program |
| // like the following is legal: |
| // |
| // let ref x = HashMap::new(); |
| // |
| // Because the hash map will be freed in the enclosing block. |
| // |
| // We express the rules more formally based on 3 grammars (defined |
| // fully in the helpers below that implement them): |
| // |
| // 1. `E&`, which matches expressions like `&<rvalue>` that |
| // own a pointer into the stack. |
| // |
| // 2. `P&`, which matches patterns like `ref x` or `(ref x, ref |
| // y)` that produce ref bindings into the value they are |
| // matched against or something (at least partially) owned by |
| // the value they are matched against. (By partially owned, |
| // I mean that creating a binding into a ref-counted or managed value |
| // would still count.) |
| // |
| // 3. `ET`, which matches both rvalues like `foo()` as well as places |
| // based on rvalues like `foo().x[2].y`. |
| // |
| // A subexpression `<rvalue>` that appears in a let initializer |
| // `let pat [: ty] = expr` has an extended temporary lifetime if |
| // any of the following conditions are met: |
| // |
| // A. `pat` matches `P&` and `expr` matches `ET` |
| // (covers cases where `pat` creates ref bindings into an rvalue |
| // produced by `expr`) |
| // B. `ty` is a borrowed pointer and `expr` matches `ET` |
| // (covers cases where coercion creates a borrow) |
| // C. `expr` matches `E&` |
| // (covers cases `expr` borrows an rvalue that is then assigned |
| // to memory (at least partially) owned by the binding) |
| // |
| // Here are some examples hopefully giving an intuition where each |
| // rule comes into play and why: |
| // |
| // Rule A. `let (ref x, ref y) = (foo().x, 44)`. The rvalue `(22, 44)` |
| // would have an extended lifetime, but not `foo()`. |
| // |
| // Rule B. `let x = &foo().x`. The rvalue `foo()` would have extended |
| // lifetime. |
| // |
| // In some cases, multiple rules may apply (though not to the same |
| // rvalue). For example: |
| // |
| // let ref x = [&a(), &b()]; |
| // |
| // Here, the expression `[...]` has an extended lifetime due to rule |
| // A, but the inner rvalues `a()` and `b()` have an extended lifetime |
| // due to rule C. |
| |
| if let Some(expr) = init { |
| record_rvalue_scope_if_borrow_expr(visitor, &expr, blk_scope); |
| |
| if let Some(pat) = pat { |
| if is_binding_pat(pat) { |
| record_rvalue_scope(visitor, &expr, blk_scope); |
| } |
| } |
| } |
| |
| // Make sure we visit the initializer first, so expr_and_pat_count remains correct |
| if let Some(expr) = init { |
| visitor.visit_expr(expr); |
| } |
| if let Some(pat) = pat { |
| visitor.visit_pat(pat); |
| } |
| |
| /// Returns `true` if `pat` match the `P&` non-terminal. |
| /// |
| /// ```text |
| /// P& = ref X |
| /// | StructName { ..., P&, ... } |
| /// | VariantName(..., P&, ...) |
| /// | [ ..., P&, ... ] |
| /// | ( ..., P&, ... ) |
| /// | ... "|" P& "|" ... |
| /// | box P& |
| /// ``` |
| fn is_binding_pat(pat: &hir::Pat<'_>) -> bool { |
| // Note that the code below looks for *explicit* refs only, that is, it won't |
| // know about *implicit* refs as introduced in #42640. |
| // |
| // This is not a problem. For example, consider |
| // |
| // let (ref x, ref y) = (Foo { .. }, Bar { .. }); |
| // |
| // Due to the explicit refs on the left hand side, the below code would signal |
| // that the temporary value on the right hand side should live until the end of |
| // the enclosing block (as opposed to being dropped after the let is complete). |
| // |
| // To create an implicit ref, however, you must have a borrowed value on the RHS |
| // already, as in this example (which won't compile before #42640): |
| // |
| // let Foo { x, .. } = &Foo { x: ..., ... }; |
| // |
| // in place of |
| // |
| // let Foo { ref x, .. } = Foo { ... }; |
| // |
| // In the former case (the implicit ref version), the temporary is created by the |
| // & expression, and its lifetime would be extended to the end of the block (due |
| // to a different rule, not the below code). |
| match pat.kind { |
| PatKind::Binding(hir::BindingAnnotation::Ref, ..) |
| | PatKind::Binding(hir::BindingAnnotation::RefMut, ..) => true, |
| |
| PatKind::Struct(_, ref field_pats, _) => { |
| field_pats.iter().any(|fp| is_binding_pat(&fp.pat)) |
| } |
| |
| PatKind::Slice(ref pats1, ref pats2, ref pats3) => { |
| pats1.iter().any(|p| is_binding_pat(&p)) |
| || pats2.iter().any(|p| is_binding_pat(&p)) |
| || pats3.iter().any(|p| is_binding_pat(&p)) |
| } |
| |
| PatKind::Or(ref subpats) |
| | PatKind::TupleStruct(_, ref subpats, _) |
| | PatKind::Tuple(ref subpats, _) => subpats.iter().any(|p| is_binding_pat(&p)), |
| |
| PatKind::Box(ref subpat) => is_binding_pat(&subpat), |
| |
| PatKind::Ref(_, _) |
| | PatKind::Binding( |
| hir::BindingAnnotation::Unannotated | hir::BindingAnnotation::Mutable, |
| .., |
| ) |
| | PatKind::Wild |
| | PatKind::Path(_) |
| | PatKind::Lit(_) |
| | PatKind::Range(_, _, _) => false, |
| } |
| } |
| |
| /// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate: |
| /// |
| /// ```text |
| /// E& = & ET |
| /// | StructName { ..., f: E&, ... } |
| /// | [ ..., E&, ... ] |
| /// | ( ..., E&, ... ) |
| /// | {...; E&} |
| /// | box E& |
| /// | E& as ... |
| /// | ( E& ) |
| /// ``` |
| fn record_rvalue_scope_if_borrow_expr<'tcx>( |
| visitor: &mut RegionResolutionVisitor<'tcx>, |
| expr: &hir::Expr<'_>, |
| blk_id: Option<Scope>, |
| ) { |
| match expr.kind { |
| hir::ExprKind::AddrOf(_, _, ref subexpr) => { |
| record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id); |
| record_rvalue_scope(visitor, &subexpr, blk_id); |
| } |
| hir::ExprKind::Struct(_, fields, _) => { |
| for field in fields { |
| record_rvalue_scope_if_borrow_expr(visitor, &field.expr, blk_id); |
| } |
| } |
| hir::ExprKind::Array(subexprs) | hir::ExprKind::Tup(subexprs) => { |
| for subexpr in subexprs { |
| record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id); |
| } |
| } |
| hir::ExprKind::Cast(ref subexpr, _) => { |
| record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id) |
| } |
| hir::ExprKind::Block(ref block, _) => { |
| if let Some(ref subexpr) = block.expr { |
| record_rvalue_scope_if_borrow_expr(visitor, &subexpr, blk_id); |
| } |
| } |
| _ => {} |
| } |
| } |
| |
| /// Applied to an expression `expr` if `expr` -- or something owned or partially owned by |
| /// `expr` -- is going to be indirectly referenced by a variable in a let statement. In that |
| /// case, the "temporary lifetime" or `expr` is extended to be the block enclosing the `let` |
| /// statement. |
| /// |
| /// More formally, if `expr` matches the grammar `ET`, record the rvalue scope of the matching |
| /// `<rvalue>` as `blk_id`: |
| /// |
| /// ```text |
| /// ET = *ET |
| /// | ET[...] |
| /// | ET.f |
| /// | (ET) |
| /// | <rvalue> |
| /// ``` |
| /// |
| /// Note: ET is intended to match "rvalues or places based on rvalues". |
| fn record_rvalue_scope<'tcx>( |
| visitor: &mut RegionResolutionVisitor<'tcx>, |
| expr: &hir::Expr<'_>, |
| blk_scope: Option<Scope>, |
| ) { |
| let mut expr = expr; |
| loop { |
| // Note: give all the expressions matching `ET` with the |
| // extended temporary lifetime, not just the innermost rvalue, |
| // because in codegen if we must compile e.g., `*rvalue()` |
| // into a temporary, we request the temporary scope of the |
| // outer expression. |
| visitor.scope_tree.record_rvalue_scope(expr.hir_id.local_id, blk_scope); |
| |
| match expr.kind { |
| hir::ExprKind::AddrOf(_, _, ref subexpr) |
| | hir::ExprKind::Unary(hir::UnOp::UnDeref, ref subexpr) |
| | hir::ExprKind::Field(ref subexpr, _) |
| | hir::ExprKind::Index(ref subexpr, _) => { |
| expr = &subexpr; |
| } |
| _ => { |
| return; |
| } |
| } |
| } |
| } |
| } |
| |
| impl<'tcx> RegionResolutionVisitor<'tcx> { |
| /// Records the current parent (if any) as the parent of `child_scope`. |
| /// Returns the depth of `child_scope`. |
| fn record_child_scope(&mut self, child_scope: Scope) -> ScopeDepth { |
| let parent = self.cx.parent; |
| self.scope_tree.record_scope_parent(child_scope, parent); |
| // If `child_scope` has no parent, it must be the root node, and so has |
| // a depth of 1. Otherwise, its depth is one more than its parent's. |
| parent.map_or(1, |(_p, d)| d + 1) |
| } |
| |
| /// Records the current parent (if any) as the parent of `child_scope`, |
| /// and sets `child_scope` as the new current parent. |
| fn enter_scope(&mut self, child_scope: Scope) { |
| let child_depth = self.record_child_scope(child_scope); |
| self.cx.parent = Some((child_scope, child_depth)); |
| } |
| |
| fn enter_node_scope_with_dtor(&mut self, id: hir::ItemLocalId) { |
| // If node was previously marked as a terminating scope during the |
| // recursive visit of its parent node in the AST, then we need to |
| // account for the destruction scope representing the scope of |
| // the destructors that run immediately after it completes. |
| if self.terminating_scopes.contains(&id) { |
| self.enter_scope(Scope { id, data: ScopeData::Destruction }); |
| } |
| self.enter_scope(Scope { id, data: ScopeData::Node }); |
| } |
| } |
| |
| impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> { |
| type Map = intravisit::ErasedMap<'tcx>; |
| |
| fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> { |
| NestedVisitorMap::None |
| } |
| |
| fn visit_block(&mut self, b: &'tcx Block<'tcx>) { |
| resolve_block(self, b); |
| } |
| |
| fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) { |
| let body_id = body.id(); |
| let owner_id = self.tcx.hir().body_owner(body_id); |
| |
| debug!( |
| "visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})", |
| owner_id, |
| self.tcx.sess.source_map().span_to_string(body.value.span), |
| body_id, |
| self.cx.parent |
| ); |
| |
| // Save all state that is specific to the outer function |
| // body. These will be restored once down below, once we've |
| // visited the body. |
| let outer_ec = mem::replace(&mut self.expr_and_pat_count, 0); |
| let outer_cx = self.cx; |
| let outer_ts = mem::take(&mut self.terminating_scopes); |
| // The 'pessimistic yield' flag is set to true when we are |
| // processing a `+=` statement and have to make pessimistic |
| // control flow assumptions. This doesn't apply to nested |
| // bodies within the `+=` statements. See #69307. |
| let outer_pessimistic_yield = mem::replace(&mut self.pessimistic_yield, false); |
| self.terminating_scopes.insert(body.value.hir_id.local_id); |
| |
| if let Some(root_id) = self.cx.root_id { |
| self.scope_tree.record_closure_parent(body.value.hir_id.local_id, root_id); |
| } |
| self.cx.root_id = Some(body.value.hir_id.local_id); |
| |
| self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::CallSite }); |
| self.enter_scope(Scope { id: body.value.hir_id.local_id, data: ScopeData::Arguments }); |
| |
| // The arguments and `self` are parented to the fn. |
| self.cx.var_parent = self.cx.parent.take(); |
| for param in body.params { |
| self.visit_pat(¶m.pat); |
| } |
| |
| // The body of the every fn is a root scope. |
| self.cx.parent = self.cx.var_parent; |
| if self.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() { |
| self.visit_expr(&body.value) |
| } else { |
| // Only functions have an outer terminating (drop) scope, while |
| // temporaries in constant initializers may be 'static, but only |
| // according to rvalue lifetime semantics, using the same |
| // syntactical rules used for let initializers. |
| // |
| // e.g., in `let x = &f();`, the temporary holding the result from |
| // the `f()` call lives for the entirety of the surrounding block. |
| // |
| // Similarly, `const X: ... = &f();` would have the result of `f()` |
| // live for `'static`, implying (if Drop restrictions on constants |
| // ever get lifted) that the value *could* have a destructor, but |
| // it'd get leaked instead of the destructor running during the |
| // evaluation of `X` (if at all allowed by CTFE). |
| // |
| // However, `const Y: ... = g(&f());`, like `let y = g(&f());`, |
| // would *not* let the `f()` temporary escape into an outer scope |
| // (i.e., `'static`), which means that after `g` returns, it drops, |
| // and all the associated destruction scope rules apply. |
| self.cx.var_parent = None; |
| resolve_local(self, None, Some(&body.value)); |
| } |
| |
| if body.generator_kind.is_some() { |
| self.scope_tree.body_expr_count.insert(body_id, self.expr_and_pat_count); |
| } |
| |
| // Restore context we had at the start. |
| self.expr_and_pat_count = outer_ec; |
| self.cx = outer_cx; |
| self.terminating_scopes = outer_ts; |
| self.pessimistic_yield = outer_pessimistic_yield; |
| } |
| |
| fn visit_arm(&mut self, a: &'tcx Arm<'tcx>) { |
| resolve_arm(self, a); |
| } |
| fn visit_pat(&mut self, p: &'tcx Pat<'tcx>) { |
| resolve_pat(self, p); |
| } |
| fn visit_stmt(&mut self, s: &'tcx Stmt<'tcx>) { |
| resolve_stmt(self, s); |
| } |
| fn visit_expr(&mut self, ex: &'tcx Expr<'tcx>) { |
| resolve_expr(self, ex); |
| } |
| fn visit_local(&mut self, l: &'tcx Local<'tcx>) { |
| resolve_local(self, Some(&l.pat), l.init.as_deref()); |
| } |
| } |
| |
| fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree { |
| let closure_base_def_id = tcx.closure_base_def_id(def_id); |
| if closure_base_def_id != def_id { |
| return tcx.region_scope_tree(closure_base_def_id); |
| } |
| |
| let id = tcx.hir().as_local_hir_id(def_id.expect_local()); |
| let scope_tree = if let Some(body_id) = tcx.hir().maybe_body_owned_by(id) { |
| let mut visitor = RegionResolutionVisitor { |
| tcx, |
| scope_tree: ScopeTree::default(), |
| expr_and_pat_count: 0, |
| cx: Context { root_id: None, parent: None, var_parent: None }, |
| terminating_scopes: Default::default(), |
| pessimistic_yield: false, |
| fixup_scopes: vec![], |
| }; |
| |
| let body = tcx.hir().body(body_id); |
| visitor.scope_tree.root_body = Some(body.value.hir_id); |
| |
| // If the item is an associated const or a method, |
| // record its impl/trait parent, as it can also have |
| // lifetime parameters free in this body. |
| match tcx.hir().get(id) { |
| Node::ImplItem(_) | Node::TraitItem(_) => { |
| visitor.scope_tree.root_parent = Some(tcx.hir().get_parent_item(id)); |
| } |
| _ => {} |
| } |
| |
| visitor.visit_body(body); |
| |
| visitor.scope_tree |
| } else { |
| ScopeTree::default() |
| }; |
| |
| tcx.arena.alloc(scope_tree) |
| } |
| |
| pub fn provide(providers: &mut Providers) { |
| *providers = Providers { region_scope_tree, ..*providers }; |
| } |
