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

[nerditation]

@phil-skillwon

... because the Rust compiler will automatically downcast 'static to 'a. Thus, &'static i32 can naturally become &'a i32.

only for the "return" value, NOT for the arguments.

the information of the arguments are not "contained" within the value of fn(&'static i32). you can "downcast" when you "give", but you must "upcast" when you "demand".