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An attempt was made to implement `Drop` on a specialization of a generic type.
Erroneous code example:
```compile_fail,E0367
trait Foo {}
struct MyStruct<T> {
t: T
}
impl<T: Foo> Drop for MyStruct<T> {
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. In order for this code to work, `MyStruct`
must also require that `T` implements `Foo`. Alternatively, another option is
to wrap the generic type in another that specializes appropriately:
```
trait Foo{}
struct MyStruct<T> {
t: T
}
struct MyStructWrapper<T: Foo> {
t: MyStruct<T>
}
impl <T: Foo> Drop for MyStructWrapper<T> {
fn drop(&mut self) {}
}
```
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