An attempt was made to implement `Drop` on a concrete specialization of a
generic type. An example is shown below:
```compile_fail,E0366
struct Foo<T> {
t: T
}
impl Drop for Foo<u32> {
fn drop(&mut self) {}
}
```
This code is not legal: it is not possible to specialize `Drop` to a subset of
implementations of a generic type. One workaround for this is to wrap the
generic type, as shown below:
```
struct Foo<T> {
t: T
}
struct Bar {
t: Foo<u32>
}
impl Drop for Bar {
fn drop(&mut self) {}
}
```