What #[derive(Index)] generates

• 1 Example usage
• 2 Structs
• 3 Enums

Deriving Index only works for a single field of a struct. The result is that you will index it’s member directly.

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

1 Example usage

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

// You can specify the field you want to derive Index for
#[derive(Index)]
struct Numbers {
    #[index]
    numbers: Vec<i32>,
    useless: bool,
}

fn main() {
    assert_eq!(5, MyVec(vec![5, 8])[0]);
    assert_eq!(200, Numbers{numbers: vec![100, 200], useless: false}[1]);
}

2 Structs

When deriving Index for a struct:

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

Code like this will be generated:

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

3 Enums

Deriving Index is not supported for enums.