Derive Macro derive_more::IndexMut

#[derive(IndexMut)]
{
    // Attributes available to this derive:
    #[index_mut]
}

Available on crate feature index_mut only.

What #[derive(IndexMut)] generates

Deriving IndexMut only works for a single field of a struct. Furthermore it requires that the type also implements Index, so usually Index should also be derived. The result is that you will mutably index it’s member directly.

With #[index_mut] or #[index_mut(ignore)] it’s possible to indicate the field that you want to derive IndexMut for.

Example usage

#[derive(Index, IndexMut)]
struct MyVec(Vec<i32>);

#[derive(Index, IndexMut)]
struct Numbers {
    #[index]
    #[index_mut]
    numbers: Vec<i32>,
    useless: bool,
}

let mut myvec = MyVec(vec![5, 8]);
myvec[0] = 50;
assert_eq!(50, myvec[0]);

let mut numbers = Numbers{numbers: vec![100, 200], useless: false};
numbers[1] = 400;
assert_eq!(400, numbers[1]);

Regular structs

When deriving IndexMut for a struct:

#[derive(Index, IndexMut)]
struct Numbers {
    #[index]
    #[index_mut]
    numbers: Vec<i32>,
    useless: bool,
}

Code like this will be generated to implement IndexMut:

impl<__IdxT> ::core::ops::IndexMut<__IdxT> for Numbers
where
    Vec<i32>: ::core::ops::IndexMut<__IdxT>,
{
    #[inline]
    fn index_mut(&mut self, idx: __IdxT) -> &mut Self::Output {
        <Vec<i32> as ::core::ops::IndexMut<__IdxT>>::index_mut(&mut self.numbers, idx)
    }
}

Enums

Deriving IndexMut is not supported for enums.