Force Rust serde to deserialize JSON Number to u16

Question

How can I force serde to parse a JSON Number as a Rust u16 type?

Below I parse a JSON file. json_data is of type Value. This only has a as_u64() method, no as_u16(). As a result I first have to parse the JSON Number as a u64, then try and parse that as u16. This seems fairly convoluted. Is there some way I can force serde_json to parse this value as a u16 to avoid this?

(As a side note, the JSON data has already been validated using a JSON schema, which checks the value of x is within the bounds of u16).

$ cat test.json
{
    "x": 100
}

use serde_json::Value;
use std::fs::File;
use std::io::BufReader;

pub struct Test {
    pub x: u16,
}

fn main() {
    let filename = "test.json";
    let file =
        File::open(filename).unwrap_or_else(|e| panic!("Unable to open file {}: {}", filename, e));
    let reader = BufReader::new(file);
    let json_data: Value = serde_json::from_reader(reader)
        .unwrap_or_else(|e| panic!("Unable to parse JSON file {}: {}", filename, e));

    assert!(json_data["x"].is_u64());

    let mut t = Test { x: 5 };

    if json_data["x"].is_u64() {
        let _new_x: u64 = json_data["x"].as_u64().expect("Unable to parse x");
        t.x = u16::try_from(_new_x).expect("Unable to convert x to u16");
    }
}

Finomnis · Accepted Answer · 2025-03-14 17:11:38Z

5

Don't use the raw serde_json::Value type for this usecase.

While it is possible, it is not what serde is intended for; instead, serde is meant to deserialize directly into your struct.

The magic here is adding a #[derive(Deserialize)] to your struct. Be aware that you need to activate the derive feature of serde for this.

Like so:

use serde::Deserialize;
use std::fs::File;
use std::io::BufReader;

#[derive(Debug, Deserialize)]
pub struct Test {
    pub x: u16,
}

fn main() {
    let filename = "test.json";
    let file =
        File::open(filename).unwrap_or_else(|e| panic!("Unable to open file {}: {}", filename, e));
    let reader = BufReader::new(file);
    let json_data: Test = serde_json::from_reader(reader)
        .unwrap_or_else(|e| panic!("Unable to parse JSON file {}: {}", filename, e));

    println!("{:?}", json_data)
}

Test { x: 100 }

If you now change the test.json to:

{
    "x": 123456
}

You get:

thread 'main' panicked at src\main.rs:16:29:
Unable to parse JSON file test.json: invalid value: integer `123456`, expected u16 at line 2 column 16

answered Mar 14 at 17:11

Finomnis

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2 Comments

Baldrick Mar 15 at 10:04

Thanks, this has fixed my issues exactly as desired. Out of curiosity, I did read the serde docs, in order to get as far as I did, but I clearly misunderstood something. I had already seen that I could add the #[derive(Deserialize)] attribute, but the solution also requires that I specify the return type of serde_json::from_reader() as my Struct (Test). Both of these are needed for this to work as desired. I don't see this clearly explained in the serde docs anywhere, did I miss it, or is it not clearly documented anywhere?

Finomnis Mar 15 at 18:57

The second part (specifying the return type) is not really serde specific. It's a Rust mechanism. If the return type is a generic, you need to specify it somewhere. Instead of specifying it in the target object, you could also annotate the function itself, like so: serde_json::from_reader::<_, Test>(reader). But I find this one less intuitive to read.

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