rustc_hir_analysis/check/region.rs
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//! 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 std::mem;
use rustc_data_structures::fx::FxHashSet;
use rustc_hir as hir;
use rustc_hir::def_id::DefId;
use rustc_hir::intravisit::{self, Visitor};
use rustc_hir::{Arm, Block, Expr, LetStmt, Pat, PatKind, Stmt};
use rustc_index::Idx;
use rustc_middle::bug;
use rustc_middle::middle::region::*;
use rustc_middle::ty::TyCtxt;
use rustc_session::lint;
use rustc_span::source_map;
use tracing::debug;
#[derive(Debug, Copy, Clone)]
struct Context {
/// 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) {
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::Let(LetStmt { els: Some(els), .. }) => {
// Let-else has a special lexical structure for variables.
// First we take a checkpoint of the current scope context here.
let mut prev_cx = visitor.cx;
visitor.enter_scope(Scope {
local_id: blk.hir_id.local_id,
data: ScopeData::Remainder(FirstStatementIndex::new(i)),
});
visitor.cx.var_parent = visitor.cx.parent;
visitor.visit_stmt(statement);
// We need to back out temporarily to the last enclosing scope
// for the `else` block, so that even the temporaries receiving
// extended lifetime will be dropped inside this block.
// We are visiting the `else` block in this order so that
// the sequence of visits agree with the order in the default
// `hir::intravisit` visitor.
mem::swap(&mut prev_cx, &mut visitor.cx);
visitor.terminating_scopes.insert(els.hir_id.local_id);
visitor.visit_block(els);
// From now on, we continue normally.
visitor.cx = prev_cx;
}
hir::StmtKind::Let(..) => {
// 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 {
local_id: blk.hir_id.local_id,
data: ScopeData::Remainder(FirstStatementIndex::new(i)),
});
visitor.cx.var_parent = visitor.cx.parent;
visitor.visit_stmt(statement)
}
hir::StmtKind::Item(..) => {
// Don't create scopes for items, since they won't be
// lowered to THIR and MIR.
}
hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => visitor.visit_stmt(statement),
}
}
if let Some(tail_expr) = blk.expr {
let local_id = tail_expr.hir_id.local_id;
let edition = blk.span.edition();
if edition.at_least_rust_2024() {
visitor.terminating_scopes.insert(local_id);
} else if !visitor
.tcx
.lints_that_dont_need_to_run(())
.contains(&lint::LintId::of(lint::builtin::TAIL_EXPR_DROP_ORDER))
{
// If this temporary scope will be changing once the codebase adopts Rust 2024,
// and we are linting about possible semantic changes that would result,
// then record this node-id in the field `backwards_incompatible_scope`
// for future reference.
visitor
.scope_tree
.backwards_incompatible_scope
.insert(local_id, Scope { local_id, data: ScopeData::Node });
}
visitor.visit_expr(tail_expr);
}
}
visitor.cx = prev_cx;
}
fn resolve_arm<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, arm: &'tcx hir::Arm<'tcx>) {
fn has_let_expr(expr: &Expr<'_>) -> bool {
match &expr.kind {
hir::ExprKind::Binary(_, lhs, rhs) => has_let_expr(lhs) || has_let_expr(rhs),
hir::ExprKind::Let(..) => true,
_ => false,
}
}
let prev_cx = visitor.cx;
visitor.terminating_scopes.insert(arm.hir_id.local_id);
visitor.enter_node_scope_with_dtor(arm.hir_id.local_id);
visitor.cx.var_parent = visitor.cx.parent;
if let Some(expr) = arm.guard
&& !has_let_expr(expr)
{
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 { local_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);
}
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 | hir::BinOpKind::Or, .. },
l,
r,
) => {
// expr is a short circuiting operator (|| or &&). As its
// functionality can't be overridden by traits, it always
// processes bool sub-expressions. bools are Copy and thus we
// can drop any temporaries in evaluation (read) order
// (with the exception of potentially failing let expressions).
// We achieve this by enclosing the operands in a terminating
// scope, both the LHS and the RHS.
// We optimize this a little in the presence of chains.
// Chains like a && b && c get lowered to AND(AND(a, b), c).
// In here, b and c are RHS, while a is the only LHS operand in
// that chain. This holds true for longer chains as well: the
// leading operand is always the only LHS operand that is not a
// binop itself. Putting a binop like AND(a, b) into a
// terminating scope is not useful, thus we only put the LHS
// into a terminating scope if it is not a binop.
let terminate_lhs = match l.kind {
// let expressions can create temporaries that live on
hir::ExprKind::Let(_) => false,
// binops already drop their temporaries, so there is no
// need to put them into a terminating scope.
// This is purely an optimization to reduce the number of
// terminating scopes.
hir::ExprKind::Binary(
source_map::Spanned {
node: hir::BinOpKind::And | hir::BinOpKind::Or, ..
},
..,
) => false,
// otherwise: mark it as terminating
_ => true,
};
if terminate_lhs {
terminating(l.hir_id.local_id);
}
// `Let` expressions (in a let-chain) shouldn't be terminating, as their temporaries
// should live beyond the immediate expression
if !matches!(r.kind, hir::ExprKind::Let(_)) {
terminating(r.hir_id.local_id);
}
}
hir::ExprKind::If(_, then, Some(otherwise)) => {
terminating(then.hir_id.local_id);
terminating(otherwise.hir_id.local_id);
}
hir::ExprKind::If(_, then, None) => {
terminating(then.hir_id.local_id);
}
hir::ExprKind::Loop(body, _, _, _) => {
terminating(body.hir_id.local_id);
}
hir::ExprKind::DropTemps(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 evaluation 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 and inline consts, because they are the only
// case of nested bodies that share the parent environment.
hir::ExprKind::Closure(&hir::Closure { body, .. })
| hir::ExprKind::ConstBlock(hir::ConstBlock { body, .. }) => {
let body = visitor.tcx.hir().body(body);
visitor.visit_body(body);
}
hir::ExprKind::AssignOp(_, left_expr, 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 yield_data =
visitor.scope_tree.yield_in_scope.get_mut(&scope).unwrap().last_mut().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;
}
}
hir::ExprKind::If(cond, then, Some(otherwise)) => {
let expr_cx = visitor.cx;
let data = if expr.span.at_least_rust_2024() {
ScopeData::IfThenRescope
} else {
ScopeData::IfThen
};
visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
visitor.cx.var_parent = visitor.cx.parent;
visitor.visit_expr(cond);
visitor.visit_expr(then);
visitor.cx = expr_cx;
visitor.visit_expr(otherwise);
}
hir::ExprKind::If(cond, then, None) => {
let expr_cx = visitor.cx;
let data = if expr.span.at_least_rust_2024() {
ScopeData::IfThenRescope
} else {
ScopeData::IfThen
};
visitor.enter_scope(Scope { local_id: then.hir_id.local_id, data });
visitor.cx.var_parent = visitor.cx.parent;
visitor.visit_expr(cond);
visitor.visit_expr(then);
visitor.cx = expr_cx;
}
_ => 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 { local_id: expr.hir_id.local_id, data: ScopeData::Node };
loop {
let span = match expr.kind {
hir::ExprKind::Yield(expr, hir::YieldSource::Await { .. }) => {
expr.span.shrink_to_hi().to(expr.span)
}
_ => expr.span,
};
let data =
YieldData { span, expr_and_pat_count: visitor.expr_and_pat_count, source: *source };
match visitor.scope_tree.yield_in_scope.get_mut(&scope) {
Some(yields) => yields.push(data),
None => {
visitor.scope_tree.yield_in_scope.insert(scope, vec![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) {
visitor.scope_tree.record_rvalue_candidate(
expr.hir_id,
RvalueCandidateType::Pattern {
target: expr.hir_id.local_id,
lifetime: blk_scope,
},
);
}
}
}
// Make sure we visit the initializer first, so expr_and_pat_count remains correct.
// The correct order, as shared between coroutine_interior, drop_ranges and intravisitor,
// is to walk initializer, followed by pattern bindings, finally followed by the `else` block.
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::BindingMode(hir::ByRef::Yes(_), _), ..) => true,
PatKind::Struct(_, field_pats, _) => field_pats.iter().any(|fp| is_binding_pat(fp.pat)),
PatKind::Slice(pats1, pats2, 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(subpats)
| PatKind::TupleStruct(_, subpats, _)
| PatKind::Tuple(subpats, _) => subpats.iter().any(|p| is_binding_pat(p)),
PatKind::Box(subpat) | PatKind::Deref(subpat) => is_binding_pat(subpat),
PatKind::Ref(_, _)
| PatKind::Binding(hir::BindingMode(hir::ByRef::No, _), ..)
| PatKind::Wild
| PatKind::Never
| PatKind::Path(_)
| PatKind::Lit(_)
| PatKind::Range(_, _, _)
| PatKind::Err(_) => false,
}
}
/// If `expr` matches the `E&` grammar, then records an extended rvalue scope as appropriate:
///
/// ```text
/// E& = & ET
/// | StructName { ..., f: E&, ... }
/// | [ ..., E&, ... ]
/// | ( ..., E&, ... )
/// | {...; E&}
/// | if _ { ...; E& } else { ...; E& }
/// | match _ { ..., _ => 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(_, _, subexpr) => {
record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
visitor.scope_tree.record_rvalue_candidate(
subexpr.hir_id,
RvalueCandidateType::Borrow {
target: subexpr.hir_id.local_id,
lifetime: 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(subexpr, _) => {
record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id)
}
hir::ExprKind::Block(block, _) => {
if let Some(subexpr) = block.expr {
record_rvalue_scope_if_borrow_expr(visitor, subexpr, blk_id);
}
}
hir::ExprKind::If(_, then_block, else_block) => {
record_rvalue_scope_if_borrow_expr(visitor, then_block, blk_id);
if let Some(else_block) = else_block {
record_rvalue_scope_if_borrow_expr(visitor, else_block, blk_id);
}
}
hir::ExprKind::Match(_, arms, _) => {
for arm in arms {
record_rvalue_scope_if_borrow_expr(visitor, arm.body, blk_id);
}
}
hir::ExprKind::Call(..) | hir::ExprKind::MethodCall(..) => {
// FIXME(@dingxiangfei2009): choose call arguments here
// for candidacy for extended parameter rule application
}
hir::ExprKind::Index(..) => {
// FIXME(@dingxiangfei2009): select the indices
// as candidate for rvalue scope rules
}
_ => {}
}
}
}
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 HIR, 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 { local_id: id, data: ScopeData::Destruction });
}
self.enter_scope(Scope { local_id: id, data: ScopeData::Node });
}
fn enter_body(&mut self, hir_id: hir::HirId, f: impl FnOnce(&mut Self)) {
// 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(hir_id.local_id);
self.enter_scope(Scope { local_id: hir_id.local_id, data: ScopeData::CallSite });
self.enter_scope(Scope { local_id: hir_id.local_id, data: ScopeData::Arguments });
f(self);
// 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;
}
}
impl<'tcx> Visitor<'tcx> for RegionResolutionVisitor<'tcx> {
fn visit_block(&mut self, b: &'tcx Block<'tcx>) {
resolve_block(self, b);
}
fn visit_body(&mut self, body: &hir::Body<'tcx>) {
let body_id = body.id();
let owner_id = self.tcx.hir().body_owner_def_id(body_id);
debug!(
"visit_body(id={:?}, span={:?}, body.id={:?}, cx.parent={:?})",
owner_id,
self.tcx.sess.source_map().span_to_diagnostic_string(body.value.span),
body_id,
self.cx.parent
);
self.enter_body(body.value.hir_id, |this| {
if this.tcx.hir().body_owner_kind(owner_id).is_fn_or_closure() {
// The arguments and `self` are parented to the fn.
this.cx.var_parent = this.cx.parent.take();
for param in body.params {
this.visit_pat(param.pat);
}
// The body of the every fn is a root scope.
this.cx.parent = this.cx.var_parent;
this.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.
this.cx.var_parent = None;
resolve_local(this, None, Some(body.value));
}
})
}
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 LetStmt<'tcx>) {
resolve_local(self, Some(l.pat), l.init)
}
}
/// Per-body `region::ScopeTree`. The `DefId` should be the owner `DefId` for the body;
/// in the case of closures, this will be redirected to the enclosing function.
///
/// Performance: This is a query rather than a simple function to enable
/// re-use in incremental scenarios. We may sometimes need to rerun the
/// type checker even when the HIR hasn't changed, and in those cases
/// we can avoid reconstructing the region scope tree.
pub(crate) fn region_scope_tree(tcx: TyCtxt<'_>, def_id: DefId) -> &ScopeTree {
let typeck_root_def_id = tcx.typeck_root_def_id(def_id);
if typeck_root_def_id != def_id {
return tcx.region_scope_tree(typeck_root_def_id);
}
let scope_tree = if let Some(body) = tcx.hir().maybe_body_owned_by(def_id.expect_local()) {
let mut visitor = RegionResolutionVisitor {
tcx,
scope_tree: ScopeTree::default(),
expr_and_pat_count: 0,
cx: Context { parent: None, var_parent: None },
terminating_scopes: Default::default(),
pessimistic_yield: false,
fixup_scopes: vec![],
};
visitor.scope_tree.root_body = Some(body.value.hir_id);
visitor.visit_body(&body);
visitor.scope_tree
} else {
ScopeTree::default()
};
tcx.arena.alloc(scope_tree)
}