another-boids-in-rust/vendor/derive_more-impl/doc/index_mut.md

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

# use derive_more::{Index, IndexMut};
#
#[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:

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

Code like this will be generated to implement IndexMut:

# use ::core::ops::Index;
# struct Numbers {
#     numbers: Vec<i32>,
#     useless: bool,
# }
# impl<__IdxT> Index<__IdxT> for Numbers
# where
#     Vec<i32>: Index<__IdxT>,
# {
#     type Output = <Vec<i32> as Index<__IdxT>>::Output;
#     #[inline]
#     fn index(&self, idx: __IdxT) -> &Self::Output {
#         <Vec<i32> as Index<__IdxT>>::index(&self.numbers, idx)
#     }
# }
impl<__IdxT> derive_more::IndexMut<__IdxT> for Numbers
where
    Vec<i32>: derive_more::IndexMut<__IdxT>,
{
    #[inline]
    fn index_mut(&mut self, idx: __IdxT) -> &mut Self::Output {
        <Vec<i32> as derive_more::IndexMut<__IdxT>>::index_mut(&mut self.numbers, idx)
    }
}

Enums

Deriving IndexMut is not supported for enums.