Destructors

When an initialized variable or temporary goes out of scope, its destructor is run, or it is dropped. Assignment also runs the destructor of its left-hand operand, if it’s initialized. If a variable has been partially initialized, only its initialized fields are dropped.

The destructor of a type T consists of:

  1. If T: Drop, calling <T as std::ops::Drop>::drop
  2. Recursively running the destructor of all of its fields.
    • The fields of a struct are dropped in declaration order.
    • The fields of the active enum variant are dropped in declaration order.
    • The fields of a tuple are dropped in order.
    • The elements of an array or owned slice are dropped from the first element to the last.
    • The variables that a closure captures by move are dropped in an unspecified order.
    • Trait objects run the destructor of the underlying type.
    • Other types don’t result in any further drops.

If a destructor must be run manually, such as when implementing your own smart pointer, std::ptr::drop_in_place can be used.

Some examples:

#![allow(unused)]
fn main() {
struct PrintOnDrop(&'static str);

impl Drop for PrintOnDrop {
    fn drop(&mut self) {
        println!("{}", self.0);
    }
}

let mut overwritten = PrintOnDrop("drops when overwritten");
overwritten = PrintOnDrop("drops when scope ends");

let tuple = (PrintOnDrop("Tuple first"), PrintOnDrop("Tuple second"));

let moved;
// No destructor run on assignment.
moved = PrintOnDrop("Drops when moved");
// Drops now, but is then uninitialized.
moved;

// Uninitialized does not drop.
let uninitialized: PrintOnDrop;

// After a partial move, only the remaining fields are dropped.
let mut partial_move = (PrintOnDrop("first"), PrintOnDrop("forgotten"));
// Perform a partial move, leaving only `partial_move.0` initialized.
core::mem::forget(partial_move.1);
// When partial_move's scope ends, only the first field is dropped.
}

Drop scopes

Each variable or temporary is associated to a drop scope. When control flow leaves a drop scope all variables associated to that scope are dropped in reverse order of declaration (for variables) or creation (for temporaries).

Drop scopes are determined after replacing for, if let, and while let expressions with the equivalent expressions using match.

Overloaded operators are not distinguished from built-in operators and binding modes are not considered.

Given a function, or closure, there are drop scopes for:

  • The entire function
  • Each block, including the function body
    • In the case of a block expression, the scope for the block and the expression are the same scope.
  • Each arm of a match expression

Drop scopes are nested within one another as follows. When multiple scopes are left at once, such as when returning from a function, variables are dropped from the inside outwards.

  • The entire function scope is the outer most scope.
[destructors.scope.nesting.function-body]
  • The function body block is contained within the scope of the entire function.
  • The parent of the expression in an expression statement is the scope of the statement.
  • The parent of the initializer of a let statement is the let statement’s scope.
  • The parent of a statement scope is the scope of the block that contains the statement.
  • The parent of the expression for a match guard is the scope of the arm that the guard is for.
  • The parent of the expression after the => in a match expression is the scope of the arm that it’s in.
  • The parent of the arm scope is the scope of the match expression that it belongs to.
  • The parent of all other scopes is the scope of the immediately enclosing expression.

Scopes of function parameters

All function parameters are in the scope of the entire function body, so are dropped last when evaluating the function. Each actual function parameter is dropped after any bindings introduced in that parameter’s pattern.

#![allow(unused)]
fn main() {
struct PrintOnDrop(&'static str);
impl Drop for PrintOnDrop {
    fn drop(&mut self) {
        println!("drop({})", self.0);
    }
}
// Drops `y`, then the second parameter, then `x`, then the first parameter
fn patterns_in_parameters(
    (x, _): (PrintOnDrop, PrintOnDrop),
    (_, y): (PrintOnDrop, PrintOnDrop),
) {}

// drop order is 3 2 0 1
patterns_in_parameters(
    (PrintOnDrop("0"), PrintOnDrop("1")),
    (PrintOnDrop("2"), PrintOnDrop("3")),
);
}

Scopes of local variables

Local variables declared in a let statement are associated to the scope of the block that contains the let statement. Local variables declared in a match expression are associated to the arm scope of the match arm that they are declared in.

#![allow(unused)]
fn main() {
struct PrintOnDrop(&'static str);
impl Drop for PrintOnDrop {
    fn drop(&mut self) {
        println!("drop({})", self.0);
    }
}
let declared_first = PrintOnDrop("Dropped last in outer scope");
{
    let declared_in_block = PrintOnDrop("Dropped in inner scope");
}
let declared_last = PrintOnDrop("Dropped first in outer scope");
}

If multiple patterns are used in the same arm for a match expression, then an unspecified pattern will be used to determine the drop order.

Temporary scopes

The temporary scope of an expression is the scope that is used for the temporary variable that holds the result of that expression when used in a place context, unless it is promoted.

Apart from lifetime extension, the temporary scope of an expression is the smallest scope that contains the expression and is one of the following:

  • The entire function.
  • A statement.
  • The body of an if, while or loop expression.
  • The else block of an if expression.
  • The condition expression of an if or while expression, or a match guard.
  • The body expression for a match arm.
  • The second operand of a lazy boolean expression.

Notes:

Temporaries that are created in the final expression of a function body are dropped after any named variables bound in the function body. Their drop scope is the entire function, as there is no smaller enclosing temporary scope.

The scrutinee of a match expression is not a temporary scope, so temporaries in the scrutinee can be dropped after the match expression. For example, the temporary for 1 in match 1 { ref mut z => z }; lives until the end of the statement.

Some examples:

#![allow(unused)]
fn main() {
struct PrintOnDrop(&'static str);
impl Drop for PrintOnDrop {
    fn drop(&mut self) {
        println!("drop({})", self.0);
    }
}
let local_var = PrintOnDrop("local var");

// Dropped once the condition has been evaluated
if PrintOnDrop("If condition").0 == "If condition" {
    // Dropped at the end of the block
    PrintOnDrop("If body").0
} else {
    unreachable!()
};

// Dropped at the end of the statement
(PrintOnDrop("first operand").0 == ""
// Dropped at the )
|| PrintOnDrop("second operand").0 == "")
// Dropped at the end of the expression
|| PrintOnDrop("third operand").0 == "";

// Dropped at the end of the function, after local variables.
// Changing this to a statement containing a return expression would make the
// temporary be dropped before the local variables. Binding to a variable
// which is then returned would also make the temporary be dropped first.
match PrintOnDrop("Matched value in final expression") {
    // Dropped once the condition has been evaluated
    _ if PrintOnDrop("guard condition").0 == "" => (),
    _ => (),
}
}

Operands

Temporaries are also created to hold the result of operands to an expression while the other operands are evaluated. The temporaries are associated to the scope of the expression with that operand. Since the temporaries are moved from once the expression is evaluated, dropping them has no effect unless one of the operands to an expression breaks out of the expression, returns, or panics.

#![allow(unused)]
fn main() {
struct PrintOnDrop(&'static str);
impl Drop for PrintOnDrop {
    fn drop(&mut self) {
        println!("drop({})", self.0);
    }
}
loop {
    // Tuple expression doesn't finish evaluating so operands drop in reverse order
    (
        PrintOnDrop("Outer tuple first"),
        PrintOnDrop("Outer tuple second"),
        (
            PrintOnDrop("Inner tuple first"),
            PrintOnDrop("Inner tuple second"),
            break,
        ),
        PrintOnDrop("Never created"),
    );
}
}

Constant promotion

Promotion of a value expression to a 'static slot occurs when the expression could be written in a constant and borrowed, and that borrow could be dereferenced where the expression was originally written, without changing the runtime behavior. That is, the promoted expression can be evaluated at compile-time and the resulting value does not contain interior mutability or destructors (these properties are determined based on the value where possible, e.g. &None always has the type &'static Option<_>, as it contains nothing disallowed).

Temporary lifetime extension

Note: The exact rules for temporary lifetime extension are subject to change. This is describing the current behavior only.

The temporary scopes for expressions in let statements are sometimes extended to the scope of the block containing the let statement. This is done when the usual temporary scope would be too small, based on certain syntactic rules. For example:

#![allow(unused)]
fn main() {
let x = &mut 0;
// Usually a temporary would be dropped by now, but the temporary for `0` lives
// to the end of the block.
println!("{}", x);
}

Lifetime extension also applies to static and const items, where it makes temporaries live until the end of the program. For example:

#![allow(unused)]
fn main() {
const C: &Vec<i32> = &Vec::new();
// Usually this would be a dangling reference as the `Vec` would only
// exist inside the initializer expression of `C`, but instead the
// borrow gets lifetime-extended so it effectively has `'static` lifetime.
println!("{:?}", C);
}

If a borrow, dereference, field, or tuple indexing expression has an extended temporary scope then so does its operand. If an indexing expression has an extended temporary scope then the indexed expression also has an extended temporary scope.

Extending based on patterns

An extending pattern is either

So ref x, V(ref x) and [ref x, y] are all extending patterns, but x, &ref x and &(ref x,) are not.

If the pattern in a let statement is an extending pattern then the temporary scope of the initializer expression is extended.

Extending based on expressions

For a let statement with an initializer, an extending expression is an expression which is one of the following:

So the borrow expressions in &mut 0, (&1, &mut 2), and Some { 0: &mut 3 } are all extending expressions. The borrows in &0 + &1 and Some(&mut 0) are not: the latter is syntactically a function call expression.

The operand of any extending borrow expression has its temporary scope extended.

Examples

Here are some examples where expressions have extended temporary scopes:

#![allow(unused)]
fn main() {
fn temp() {}
trait Use { fn use_temp(&self) -> &Self { self } }
impl Use for () {}
// The temporary that stores the result of `temp()` lives in the same scope
// as x in these cases.
let x = &temp();
let x = &temp() as &dyn Send;
let x = (&*&temp(),);
let x = { [Some { 0: &temp(), }] };
let ref x = temp();
let ref x = *&temp();
x;
}

Here are some examples where expressions don’t have extended temporary scopes:

#![allow(unused)]
fn main() {
fn temp() {}
trait Use { fn use_temp(&self) -> &Self { self } }
impl Use for () {}
// The temporary that stores the result of `temp()` only lives until the
// end of the let statement in these cases.

let x = Some(&temp());         // ERROR
let x = (&temp()).use_temp();  // ERROR
x;
}

Not running destructors

std::mem::forget can be used to prevent the destructor of a variable from being run, and std::mem::ManuallyDrop provides a wrapper to prevent a variable or field from being dropped automatically.

Note: Preventing a destructor from being run via std::mem::forget or other means is safe even if it has a type that isn’t 'static. Besides the places where destructors are guaranteed to run as defined by this document, types may not safely rely on a destructor being run for soundness.