Understanding Rust Error E0208 in Detail

Rust Error E0208

Error E0208 is a Rust compiler error code that used to be associated with the variance of a type's generic parameters. However, this error code has been deprecated and is no longer emitted by the Rust compiler. The information regarding the error code can still be useful for understanding variance and how it affects Rust code.

Understanding Variance

In Rust, variance describes how the subtyping relationship between types changes based on the relationship of their generic parameters. In other words, it's a way of categorizing how the compiler should treat the relationship between types when considering lifetimes and type parameters.

Rust Compiler and the Deprecated Error E0208

As mentioned earlier, the E0208 error code is no longer emitted by the Rust compiler. However, the attribute #[rustc_variance] still exists and is used internally by the compiler. By applying the #[rustc_variance] attribute, you can observe the variance of a type's generic parameters for testing purposes. Remember, you must enable the feature with #![feature(rustc_attrs)] to use this attribute.

An Example of the Deprecated Error E0208

The code block below illustrates how the deprecated E0208 error used to be produced:

#![allow(unused)]
#![feature(rustc_attrs)]

#[rustc_variance]
struct Foo<'a, T> { // error: deliberate error to display type's variance
    t: &'a mut T,
}

This code would previously trigger a deliberate E0208 error to display the type's variance: 'error: [-, o]' (where '-' represents invariance and 'o' represents covariance). However, as the error is deprecated, it's no longer necessary to deal with it directly.

Resolving the Error for Legacy Rust Code