The iota! macro constructs a set of related constants.
[dependencies] iota = "0.2"
use iota::iota; iota! { const A: u8 = 1 << iota; , B , C , D } fn main() { assert_eq!(A, 1); assert_eq!(B, 2); assert_eq!(C, 4); assert_eq!(D, 8); }
Within an iota! block, the iota variable is an untyped integer constant whose value begins at 0 and increments by 1 for every constant declared in the block.
use iota::iota; iota! { const A: u8 = 1 << iota; , B const C: i32 = -1; // iota is not used but still incremented pub const D: u8 = iota * 2; , E , F } // `iota` begins again from 0 in this block iota! { const G: usize = 1 << (iota + 10); , H } fn main() { assert_eq!(A, 1 << 0); assert_eq!(B, 1 << 1); assert_eq!(C, -1); assert_eq!(D, 3 * 2); assert_eq!(E, 4 * 2); assert_eq!(F, 5 * 2); assert_eq!(G, 1 << (0 + 10)); assert_eq!(H, 1 << (1 + 10)); }