How to understand the sentence "However, both input and return types can be inferred and input variable names must be specified"?

[liby]

I read this passage in rust-by-example/fn/closures.

The syntax and capabilities of closures make them very convenient for on the fly usage. Calling a closure is exactly like calling a function. However, both input and return types can be inferred and input variable names must be specified.

What do "input variable names must be specified" refer to? I can't figure it out.

[H2CO3]

I'm guessing they contrast this with some other languages where "magic" placeholders create implicit closures, something that you deliberately can't do in Rust. For example, one could imagine the following syntax:

let numbers = vec![1, 2, 3];
let bigger_numbers = numbers.into_iter().map(_ + 1);
                                             ^^^^^

This would cause the _ nameless placeholder to stand in for the single parameter for an automatically-created closure, of which the body is the expression noname_parameter + 1. I think this and similar features have been proposed (and rejected) before, and here the RBE guide might be trying to preemptively address such expectations.