Error code E0208

This error code is internal to the compiler and will not be emitted with normal Rust code.

Note: this error code is no longer emitted by the compiler.

This error code shows the variance of a type's generic parameters.

Erroneous code example:

#![allow(unused)]
fn main() {
// NOTE: this feature is perma-unstable and should *only* be used for
//       testing purposes.
#![allow(internal_features)]
#![feature(rustc_attrs)]

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

which produces the following error:

error: [-, o]
 --> <anon>:4:1
  |
4 | struct Foo<'a, T> {
  | ^^^^^^^^^^^^^^^^^

Note that while #[rustc_variance] still exists and is used within the compiler, it no longer is marked as E0208 and instead has no error code.

This error is deliberately triggered with the #[rustc_variance] attribute (#![feature(rustc_attrs)] must be enabled) and helps to show you the variance of the type's generic parameters. You can read more about variance and subtyping in this section of the Rustonomicon. For a more in depth look at variance (including a more complete list of common variances) see this section of the Reference. For information on how variance is implemented in the compiler, see this section of rustc-dev-guide.

This error can be easily fixed by removing the #[rustc_variance] attribute, the compiler's suggestion to comment it out can be applied automatically with rustfix.