Macro seq 

macro_rules! seq {
    [$first:expr $(, $value:expr)* $(,)?] => { ... };
    [] => { ... };
    [$value:expr; $amount:expr] => { ... };
}

General-purpose macro for creating a sequence of items

let seq: Vec<_> = seq![1, 2, 3];
assert_eq!(seq, vec![1, 2, 3]);

seq![] relies on type inference to determine which sequence to become.

See the crate-level documentation for more info.