Understanding Rust Error E0198 (Negative Implementation Unsafety)

Rust Error E0198: Negative Implementation Unsafety

This article aims to help you understand and resolve Rust Error E0198. This error occurs when a negative implementation is incorrectly marked as unsafe.

The official Rust documentation provides a brief definition of Error E0198 and an erroneous code example:

struct Foo;

unsafe impl !Clone for Foo { } // error!

Negative Implementations in Rust

A negative implementation in Rust is used to exclude a type from implementing a particular trait. Since not being able to use a trait is always a safe operation, negative implementations are inherently safe and do not require an unsafe marker.

Auto Traits and Negative Implementations

Negative implementations are allowed only for auto traits in Rust. An auto trait is a trait automatically implemented for all types, unless explicitly opted out. Here is a correct example using negative implementation with an auto trait:

#![feature(auto_traits)]

struct Foo;

auto trait Enterprise {}

impl !Enterprise for Foo { }

Resolving Rust Error E0198

To resolve Rust Error E0198, you should remove the 'unsafe' marker from the negative implementation. The following example demonstrates this, where the previous erroneous code is corrected:

struct Foo;

impl !Clone for Foo { } // no error

In conclusion, remember that negative implementations are inherently safe and do not require an unsafe marker. Be mindful of using auto traits when working with negative implementations, as they are only permitted with auto traits. By adhering to these guidelines, you can prevent and resolve Rust Error E0198 in your code.