Changing base unit in Length

[chungwong]

use uom::si::f32::Length;
use uom::si::length::millimeter;
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize, Debug)]
struct Unit {
    // length is in millimeter but the base unit of Length is meter
    length: Length,
}

fn main() {
    let data = r#"
    {
        "length": 0.4
    }"#;

    let v: Unit = serde_json::from_str(data).unwrap();

    dbg!(v);
}

// Output
[src/main.rs:26] v = Unit {
    length: 0.4 m^1,
}

Upon deserialization, how do I specify Length to be in millimeter? That is, 0.4mm rather than 0.4m