Rust Error E0750: Negative Impl as Default Impl

Understanding Rust Error E0750

In Rust, error E0750 occurs when a negative implementation (impl) is used as a default implementation for a trait. A default impl provides default values for the items within to be used by other impls, while a negative impl declares that there are no other impls. Combining these two concepts is not logical and will result in an E0750 error.

Erroneous Code Example

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

default impl !MyTrait for u32 {} // error!
fn main() {}

In this code, the negative impl is declared as a default impl for the trait MyTrait. This will cause Rust Error E0750.

Fixing Rust Error E0750

To resolve Rust Error E0750, you need to separate the negative impl from the default impl. There are several ways to handle this depending on your requirements.

Method 1: Removing the default keyword

If you do not require a default impl for MyTrait, simply remove the default keyword from the negative impl declaration:

impl !MyTrait for u32 {}
fn main() {}

Method 2: Separating the impls

If you need a separate default impl for MyTrait, declare the default impl and the negative impl separately:

default impl MyTrait for u32 {
    type Foo = ();
}

impl !MyTrait for i32 {}
fn main() {}

In this example, we have declared a default impl for u32 with a specific default type for Foo and a separate negative impl for i32 indicating that MyTrait will not be implemented for i32.

Conclusion

Rust Error E0750 occurs when a negative impl is used as a default impl, which is an illogical combination. By separating the negative impl from the default impl, you can successfully avoid this error. Remember to consider your requirements when implementing the solutions mentioned above.