Nobody writes variance at hand in Rust because it's impossible (since before 1.0). You sometimes need to enforce stricter variance than the default (can cause unsoundness and is very subtle but actual unsoundness here is rare) or give more permissive variance (harder and also cannot cause unsoundness, just annoyance).

When implementing the type system, however, variance does have to be treated with care. Historically it wasn't much, now it is.