r[expr.placeholder]

_ expressions

r[expr.placeholder.syntax]

UnderscoreExpression -> `_`

r[expr.placeholder.intro] Underscore expressions, denoted with the symbol _, are used to signify a placeholder in a destructuring assignment.

r[expr.placeholder.lhs-assignment-only] They may only appear in the left-hand side of an assignment.

r[expr.placeholder.pattern] Note that this is distinct from the wildcard pattern.

Examples of _ expressions:

let p = (1, 2);
let mut a = 0;
(_, a) = p;

struct Position {
    x: u32,
    y: u32,
}

Position { x: a, y: _ } = Position{ x: 2, y: 3 };

// unused result, assignment to `_` used to declare intent and remove a warning
_ = 2 + 2;
// triggers unused_must_use warning
// 2 + 2;

// equivalent technique using a wildcard pattern in a let-binding
let _ = 2 + 2;