Understanding Rust Error E0199: Safe Traits and Unsafe Implementations

Introduction to Rust Error E0199

Rust Error E0199 occurs when a trait implementation for a safe trait is marked unsafe. Safe traits should not have unsafe implementations; marking such an implementation as unsafe will cause a compiler error.

Example of Erroneous Code

Consider the following code example that triggers Rust Error E0199:

#![allow(unused)]
fn main() {
    struct Foo;

    trait Bar { }

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

Explanation of the Error

In the above example, a safe trait, Bar, is defined, and an unsafe implementation, impl Bar for Foo, is created. According to Rust's safety guidelines, safe traits should only have safe implementations. Since the implementation is marked as unsafe, the compiler produces Error E0199.

Fixing the Error

To fix Rust Error E0199, remove the unsafe marker from the trait implementation. Modifying the previous example:

#![allow(unused)]
fn main() {
    struct Foo;

    trait Bar { }

    impl Bar for Foo { } // ok!
}

With the unsafe marker removed, the code is now compliant with Rust's safety guidelines and compiles without error.

Summary

Rust Error E0199 is triggered when an unsafe implementation is created for a safe trait. To resolve this error, simply remove the unsafe marker from the implementation. By ensuring that safe traits only have safe implementations, you adhere to Rust's safety guidelines and create more reliable and secure code.