Static TAIL_EXPR_DROP_ORDER

pub static TAIL_EXPR_DROP_ORDER: &'static Lint

The tail_expr_drop_order lint looks for those values generated at the tail expression location, that runs a custom Drop destructor. Some of them may be dropped earlier in Edition 2024 that they used to in Edition 2021 and prior. This lint detects those cases and provides you information on those values and their custom destructor implementations. Your discretion on this information is required.

Example

#![warn(tail_expr_drop_order)]
struct Droppy(i32);
impl Droppy {
    fn get(&self) -> i32 {
        self.0
    }
}
impl Drop for Droppy {
    fn drop(&mut self) {
        // This is a custom destructor and it induces side-effects that is observable
        // especially when the drop order at a tail expression changes.
        println!("loud drop {}", self.0);
    }
}
fn edition_2021() -> i32 {
    let another_droppy = Droppy(0);
    Droppy(1).get()
}
fn main() {
    edition_2021();
}

{{produces}}

Explanation

In tail expression of blocks or function bodies, values of type with significant Drop implementation has an ill-specified drop order before Edition 2024 so that they are dropped only after dropping local variables. Edition 2024 introduces a new rule with drop orders for them, so that they are dropped first before dropping local variables.

A significant Drop::drop destructor here refers to an explicit, arbitrary implementation of the Drop trait on the type, with exceptions including Vec, Box, Rc, BTreeMap and HashMap that are marked by the compiler otherwise so long that the generic types have no significant destructor recursively. In other words, a type has a significant drop destructor when it has a Drop implementation or its destructor invokes a significant destructor on a type. Since we cannot completely reason about the change by just inspecting the existence of a significant destructor, this lint remains only a suggestion and is set to allow by default.

This lint only points out the issue with Droppy, which will be dropped before another_droppy does in Edition 2024. No fix will be proposed by this lint. However, the most probable fix is to hoist Droppy into its own local variable binding.

struct Droppy(i32);
impl Droppy {
    fn get(&self) -> i32 {
        self.0
    }
}
fn edition_2024() -> i32 {
    let value = Droppy(0);
    let another_droppy = Droppy(1);
    value.get()
}