Description
In https://doc.rust-lang.org/nightly/nomicon/subtyping.html:
The beginning of the chapter, up to "Variance", is easy to follow. Cat extends Animal, lifetimes are bigger or smaller = easy.
But then the language suddenly changes to purely abstract academic terminology like "type constructor in Rust is any generic type with unbound arguments", and you've lost me. When you say "constructors in Rust", I think of pub fn new() -> Self and that Vec::new() has an generic argument… but that doesn't explain variance to me at all.
The variance section switches to using F<Sub> and F<Super> instead of continuing the analogy of Cat and Animal. This makes it harder to understand, because you've just explained to me relationship based on cats and dogs, so now I wonder "is F<Sub> the Cat?"
Note about contravariance mentions "higher rank lifetimes" without any prior introduction to what "higher rank" is, how lifetimes have a "rank", etc.
The section later comes back to cats and dogs example, but at this point I'm completely confused and exhausted.
I think it'd help to fully explain variance based on cats and dogs before explaining variance in academic terms.