diff --git a/src/doc/nomicon/subtyping.md b/src/doc/nomicon/subtyping.md index 0253a1fb03539..5def5c3903353 100644 --- a/src/doc/nomicon/subtyping.md +++ b/src/doc/nomicon/subtyping.md @@ -44,10 +44,11 @@ subtyping of its outputs. There are two kinds of variance in Rust: * F is *invariant* over `T` otherwise (no subtyping relation can be derived) (For those of you who are familiar with variance from other languages, what we -refer to as "just" variance is in fact *covariance*. Rust does not have -contravariance. Historically Rust did have some contravariance but it was -scrapped due to poor interactions with other features. If you experience -contravariance in Rust call your local compiler developer for medical advice.) +refer to as "just" variance is in fact *covariance*. Rust has *contravariance* +for functions. The future of contravariance is uncertain and it may be +scrapped. For now, `fn(T)` is contravariant in `T`, which is used in matching +methods in trait implementations to the trait definition. Traits don't have +inferred variance, so `Fn(T)` is invariant in `T`). Some important variances: