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

_ can only be used in two contexts: in patterns, to match a value to ignore, and as a placeholder for a type. In array types, the length is not a type, but an expression, and _ cannot be used in expressions.

What you can do, though, is append f32 to only one of the literals and omit the type completely. Since all the items of an array must have the same type, the compiler will infer the correct element type for the array.

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

Answer 2

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.]);