_ expressions

^Syntax
UnderscoreExpression :
_

Underscore expressions, denoted with the symbol _, are used to signify a placeholder in a destructuring assignment. They may only appear in the left-hand side of an assignment.

An example of an _ expression:

#![allow(unused)]
fn main() {
let p = (1, 2);
let mut a = 0;
(_, a) = p;
}