Static DYN_DROP

pub static DYN_DROP: &Lint

The dyn_drop lint checks for trait objects with std::ops::Drop.

Example

fn foo(_x: Box<dyn Drop>) {}

{{produces}}

Explanation

A trait object bound of the form dyn Drop is most likely misleading and not what the programmer intended.

Drop bounds do not actually indicate whether a type can be trivially dropped or not, because a composite type containing Drop types does not necessarily implement Drop itself. Naïvely, one might be tempted to write a deferred drop system, to pull cleaning up memory out of a latency-sensitive code path, using dyn Drop trait objects. However, this breaks down e.g. when T is String, which does not implement Drop, but should probably be accepted.

To write a trait object bound that accepts anything, use a placeholder trait with a blanket implementation.

trait Placeholder {}
impl<T> Placeholder for T {}
fn foo(_x: Box<dyn Placeholder>) {}