compiler/rustc_error_codes/src/error_codes/E0208.md

#### 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:

```compile_fail
// 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:

```text
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`.

[this section of the Rustonomicon]: https://doc.rust-lang.org/nomicon/subtyping.html
[this section of the Reference]: https://doc.rust-lang.org/reference/subtyping.html#variance
[this section of `rustc-dev-guide`]: https://rustc-dev-guide.rust-lang.org/variance.html