Irrefutable Patterns
In day 1 we briefly saw how patterns can be used to destructure compound values. Let’s review that and talk about a few other things patterns can express:
// Copyright 2025 Google LLC
// SPDX-License-Identifier: Apache-2.0
fn takes_tuple(tuple: (char, i32, bool)) {
let a = tuple.0;
let b = tuple.1;
let c = tuple.2;
// This does the same thing as above.
let (a, b, c) = tuple;
// Ignore the first element, only bind the second and third.
let (_, b, c) = tuple;
// Ignore everything but the last element.
let (.., c) = tuple;
}
fn main() {
takes_tuple(('a', 777, true));
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All of the demonstrated patterns are irrefutable, meaning that they will always match the value on the right hand side.
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Patterns are type-specific, including irrefutable patterns. Try adding or removing an element to the tuple and look at the resulting compiler errors.
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Variable names are patterns that always match and bind the matched value into a new variable with that name.
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_is a pattern that always matches any value, discarding the matched value. -
..allows you to ignore multiple values at once.
More to Explore
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You can also demonstrate more advanced usages of
.., such as ignoring the middle elements of a tuple.#![allow(unused)] fn main() { // Copyright 2025 Google LLC // SPDX-License-Identifier: Apache-2.0 fn takes_tuple(tuple: (char, i32, bool, u8)) { let (first, .., last) = tuple; } } -
All of these patterns work with arrays as well:
#![allow(unused)] fn main() { // Copyright 2025 Google LLC // SPDX-License-Identifier: Apache-2.0 fn takes_array(array: [u8; 5]) { let [first, .., last] = array; } }