The reason Rust requires types here is because global inference is computationally expensive and causes spooky action at a distance. The only real difference between your F# and this is the braces, which I do personally think makes code easier to read.
Of course there’s more overhead in smaller functions. I’d never write this code in Rust. I’d either inline it, or use a closure, which does allow for inference, and dropping the braces given that this is a one liner:
It's a closure, not just an anonymous function, though it of course has an empty environment so it's not a significant difference in this case. Rust doesn't have syntax for anonymous functions, only closures.
I thought that the Rust playground was requiring me to use return, but I was mistaken. However, it actually lends to my original point though because I would wager that dropping the return was part of the influence of ML on the beginnings of Rust.
Of course there’s more overhead in smaller functions. I’d never write this code in Rust. I’d either inline it, or use a closure, which does allow for inference, and dropping the braces given that this is a one liner:
Different needs for different things.