A negative implementation is one that excludes a type from implementing a
particular trait. Not being able to use a trait is always a safe operation,
so negative implementations are always safe and never need to be marked as
unsafe.
```compile_fail
#![feature(optin_builtin_traits)]
struct Foo;
// unsafe is unnecessary
unsafe impl !Clone for Foo { }
```
This will compile:
```ignore (ignore auto_trait future compatibility warning)
#![feature(optin_builtin_traits)]
struct Foo;
auto trait Enterprise {}
impl !Enterprise for Foo { }
```
Please note that negative impls are only allowed for auto traits.