The Copy trait was implemented on a type with a Drop implementation.

Erroneous code example:

#[derive(Copy)]
struct Foo; // error!

impl Drop for Foo {
    fn drop(&mut self) {
    }
}

Explicitly implementing both Drop and Copy trait on a type is currently disallowed. This feature can make some sense in theory, but the current implementation is incorrect and can lead to memory unsafety (see issue #20126), so it has been disabled for now.