Invalid parse on array size expr inference

[CohenArthur]

In this code

#[allow(unused)]
macro_rules! simd_shuffle {
    ($x:expr, $y:expr, $idx:expr $(,)?) => {{
        simd_shuffle(
            $x,
            $y,
            const {
                let v: [u32; _] = $idx;
                v
            },
        )
    }};
}

and more specifically on the following statement:

let v: [u32; _] = $idx;

we get the following error:

test.rs:9:18: error: use of ‘_’ is not allowed on the right-side of an assignment
    9 |     let a: [u32; _] = [15u32];
      |                  ^

an easy testcase is this one:

fn main() {
    let a: [u32; _] = [15u32];
}

Note that this is technically hidden behind #![feature(generic_arg_infer)]

[philberty]

#3886