Rust contravariance, Regarding the contravariance of `fn(&'a i32) -> ()`

[Cerber-Ursi]

@phil-skillwon

when we define a function signature that requires a type as a parameter, only supertype can be converted to subtype.

Nope. It's always "subtype -> supertype", no matter what. It's just that for function types, "type of function A can be converted to type of function B" means "argument for function B can be converted to argument for function A".