From ef1cd4dd45675cd77366ab642303c44f381d02b3 Mon Sep 17 00:00:00 2001 From: Ulrik Sverdrup Date: Fri, 11 Dec 2015 18:23:31 +0100 Subject: [PATCH] nomicon: Mention contravariance --- subtyping.md | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/subtyping.md b/subtyping.md index 0253a1f..5def5c3 100644 --- a/subtyping.md +++ b/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: