A negative implementation was marked as unsafe.
Erroneous code example:
struct 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:
#![feature(optin_builtin_traits)]
struct Foo;
auto trait Enterprise {}
impl !Enterprise for Foo { }Please note that negative impls are only allowed for auto traits.