Is there something like C99 designated range initializer for arrays?

The similar question is asked in Is there a way to get C99 array designators or alternative in Rust?, but the answers there cannot initialize a value for a give range.

For example,

int a[10] = { [4 ... 8] = 10 };

How do const arrays in Rust use an initializer on the range? If we don't use const, it can be done by fill() method:

let mut a: [i32; 10] = [0; 10];
a[4..9].fill(10);

But fill() cannot be used in const block.

Solution

There is no built-in way to do this, but it's possible to create it yourself with a macro:

macro_rules! array_ranged_init {
    (
        $default_value:expr
        $(,
            [
                $($min:literal)?
                $( .. $($max:literal)? )?
            ] = $value:expr
        )*;
        $len:expr
    ) => {
        {
            let mut array = [$default_value; $len];
            $(
                #[allow(unused_variables)]
                let min = 0;
                $(let min = $min;)?
                
                #[allow(unused_variables)]
                let max = min + 1;
                $(
                    #[allow(unused_variables)]
                    let max = $len;
                    $(let max = $max;)?
                )?
                
                let mut i = min;
                while i < max {
                    array[i] = $value;
                    i += 1;
                }
            )*
            array
        }
    }
}

You can then use the macro like this:

static DATA: [i32; 10] = array_ranged_init![0, [4..9] = 10; 10];

#[test]
fn test() {
    assert_eq!(DATA, [0, 0, 0, 0, 10, 10, 10, 10, 10, 0])
}

This macro allows you to include multiple ranges:

static DATA: [i32; 12] = array_ranged_init![
    0,
    [1..6] = 4,
    [10..] = 6,
    [..1] = 1,
    [3] = 9;
    12
];

#[test]
fn test() {
    assert_eq!(DATA, [1, 4, 4, 9, 4, 4, 0, 0, 0, 0, 6, 6])
}