Can array lengths be inferred in Rust?

Question

I can do this:

let a: [f32; 3] = [0.0, 1.0, 2.0];

But why doesn\'t this work?

let a: [f32; _] = [0.0, 1.0, 2.0];
         


        

Answer 1

Since 1.39 it's possible using a simple macro

macro_rules! arr {
    ($id: ident $name: ident: [$ty: ty; _] = $value: expr) => {
        $id $name: [$ty; $value.len()] = $value;
    }
}

Usage

arr!(static BYTES: [u8; _] = *b"foo");
arr!(let floats: [f32; _] = [0., 1.]);