-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.
+A negative implementation was marked as unsafe.
-```compile_fail
-#![feature(optin_builtin_traits)]
+Erroneous code example:
+```compile_fail
struct Foo;
-// unsafe is unnecessary
-unsafe impl !Clone for Foo { }
+unsafe impl !Clone for Foo { } // error!
```
+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.
+
This will compile:
```ignore (ignore auto_trait future compatibility warning)