typed_index_derive

Custom derive to easily create newtype index types.

A frequent pattern in Rust is to store objects in a vector and use integer indexes as handlers to them. While using usize works, it could become confusing if there are several flavors of indexes. To make the meaning of each index clear the newtype wrappers like FooIdx(usize) are useful, but require a fair amount of boilerplate. This crate derives the boilerplate for you:

#[macro_use]
extern crate typed_index_derive;

struct Spam(String);

#[derive(
    // Usual derives for plain old data
    Debug, Copy, Clone, Ord, PartialOrd, Eq, PartialEq, Hash,
    // this crate
    TypedIndex
)]
#[typed_index(Spam)] // index into `&[Spam]`
struct SpamIdx(usize); // could be `u32` instead of `usize`

fn main() {
    let spams = vec![Spam("foo".into()), Spam("bar".into()), Spam("baz".into())];

    // Conversions between `usize` and `SpamIdx`
    let idx: SpamIdx = 1.into();
    assert_eq!(usize::from(idx), 1);

    // We can index `Vec<Spam>` with SpamIdx
    assert_eq!(&spams[idx].0, "bar");

    // However, we can't index `Vec<usize>`
    // vec![1, 2, 3][idx]
    // error: slice indices are of type `usize` or ranges of `usize`

    // You can add/subtract `usize` from an index
    assert_eq!(&spams[idx - 1].0, "foo");

    // The difference between two indices is `usize`
    assert_eq!(idx - idx, 0usize);
}