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-rw-r--r-- | compiler/rustc_error_codes/src/error_codes/E0208.md | 46 |
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diff --git a/compiler/rustc_error_codes/src/error_codes/E0208.md b/compiler/rustc_error_codes/src/error_codes/E0208.md new file mode 100644 index 000000000..1ae01106f --- /dev/null +++ b/compiler/rustc_error_codes/src/error_codes/E0208.md @@ -0,0 +1,46 @@ +#### 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. +#![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 Rustnomicon]. 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 Rustnomicon]: 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 |