Trait core::ops::IndexMut

1.0.0 · source ·
pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
    // Required method
    fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
Expand description

Used for indexing operations (container[index]) in mutable contexts.

container[index] is actually syntactic sugar for *container.index_mut(index), but only when used as a mutable value. If an immutable value is requested, the Index trait is used instead. This allows nice things such as v[index] = value.


A very simple implementation of a Balance struct that has two sides, where each can be indexed mutably and immutably.

use std::ops::{Index, IndexMut};

enum Side {

#[derive(Debug, PartialEq)]
enum Weight {

struct Balance {
    pub left: Weight,
    pub right: Weight,

impl Index<Side> for Balance {
    type Output = Weight;

    fn index(&self, index: Side) -> &Self::Output {
        println!("Accessing {index:?}-side of balance immutably");
        match index {
            Side::Left => &self.left,
            Side::Right => &self.right,

impl IndexMut<Side> for Balance {
    fn index_mut(&mut self, index: Side) -> &mut Self::Output {
        println!("Accessing {index:?}-side of balance mutably");
        match index {
            Side::Left => &mut self.left,
            Side::Right => &mut self.right,

let mut balance = Balance {
    right: Weight::Kilogram(2.5),
    left: Weight::Pound(1.5),

// In this case, `balance[Side::Right]` is sugar for
// `*balance.index(Side::Right)`, since we are only *reading*
// `balance[Side::Right]`, not writing it.
assert_eq!(balance[Side::Right], Weight::Kilogram(2.5));

// However, in this case `balance[Side::Left]` is sugar for
// `*balance.index_mut(Side::Left)`, since we are writing
// `balance[Side::Left]`.
balance[Side::Left] = Weight::Kilogram(3.0);

Required Methods§

1.0.0 · source

fn index_mut(&mut self, index: Idx) -> &mut Self::Output

Performs the mutable indexing (container[index]) operation.


May panic if the index is out of bounds.


1.0.0 · source§

impl<I> IndexMut<I> for str
where I: SliceIndex<str>,


impl<I, T, const N: usize> IndexMut<I> for Simd<T, N>

1.0.0 · source§

impl<T, I> IndexMut<I> for [T]
where I: SliceIndex<[T]>,

1.50.0 · source§

impl<T, I, const N: usize> IndexMut<I> for [T; N]
where [T]: IndexMut<I>,