generic_array::arr_impl!

macro_rules! arr_impl {
    ($T:ty; $N:ty, [$($x:expr),*], []) => { ... };
    ($T:ty; $N:ty, [], [$x1:expr]) => { ... };
    ($T:ty; $N:ty, [], [$x1:expr, $($x:expr),+]) => { ... };
    ($T:ty; $N:ty, [$($y:expr),+], [$x1:expr]) => { ... };
    ($T:ty; $N:ty, [$($y:expr),+], [$x1:expr, $($x:expr),+]) => { ... };
}

Proper implementation of arr!