Understanding Rust Error E0749: Items in Negative Impls

Introduction to Rust Error E0749

Rust Error E0749 occurs when an item is added on a negative impl. Negative impls are used to declare that a trait is not implemented and never will be for a specific type. Therefore, there is no need to specify the values for trait methods or other items in negative impls.

Erroneous Code Example

#![allow(unused)]
#![feature(negative_impls)]
trait MyTrait {
    type Foo;
}

impl !MyTrait for u32 {
    type Foo = i32; // error!
}

In the code above, the negative impl is trying to define an associated type (type Foo = i32;) within the negative impl declaration, which is the cause of Rust Error E0749.

Fixing Rust Error E0749

To fix Rust Error E0749, simply remove the items in the negative impls. Following is a corrected version of the erroneous code:

#![allow(unused)]
#![feature(negative_impls)]
trait MyTrait {
    type Foo;
}

impl !MyTrait for u32 {}

In the fixed example, the negative impl no longer has any items. This corrected code explicitly states that the MyTrait trait is not, and never will be, implemented for the u32 type without providing any associated items.

When to Use Negative Impls

Negative impls are useful in situations where you want to explicitly state that a specific type will never implement a certain trait. This can help prevent incorrect trait implementations, clarify your design intentions, and provide better error messages in case someone tries to use the trait with the restricted type.