Based on the conversation in rust-lang#86747.

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.

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