how to use U256 in near smart contract in rust language

Question

I am trying to use U256 in rust smart contract for near protocol with a library called ethnum. But it always give errors like:

the trait `JsonSchema` is not implemented for `U256`
 │     |
 │     = help: the following other types implement trait `JsonSchema`:
 │               &'a T
 │               &'a mut T
 │               ()
 │               (T0, T1)
 │               (T0, T1, T2)
 │               (T0, T1, T2, T3)
 │               (T0, T1, T2, T3, T4)
 │               (T0, T1, T2, T3, T4, T5)

I expect a smart contract which use U256 library to sum, change, multiply etc. so that it can be used for other smart contracts too in NEAR protocol's rust smart contract.

Please provide enough code so others can better understand or reproduce the problem. — Community, May 07 '23 at 23:10

score 0 · Answer 1 · answered May 06 '23 at 14:19

The error is raised, because ethnum::U256 does not implement the schemars::JsonSchema trait. Unfortunately you can't implement JsonSchema on U256 yourself (trying to implement a foreign trait on foreign type would raise another error, namely E0117). But you can create a wrapper type around U256 (this is called the newtype pattern) and implement JsonSchema for it:

/*
[dependencies]
ethnum = { version = "1", features = ["serde"] }
schemars = "0.8"
serde = { version = "1", features = ["derive"] }
*/
use ethnum::U256 as _U256;
use schemars::{
    gen::SchemaGenerator,
    schema::{InstanceType, Schema, SchemaObject},
    schema_for, JsonSchema,
};
use serde::{Serialize, Deserialize};

#[derive(Serialize, Deserialize)]
struct U256(_U256);

impl JsonSchema for U256 {
    fn schema_name() -> String {
        "u256".to_owned()
    }

    fn json_schema(_: &mut SchemaGenerator) -> Schema {
        let mut schema = SchemaObject {
            instance_type: Some(InstanceType::Integer.into()),
            format: Some(Self::schema_name()),
            ..Default::default()
        };
        schema.number().minimum = Some(0.0);
        schema.into()
    }
}

fn main() {
    let u256_schema = schema_for!(U256);
    println!("{u256_schema:?}");
}

Rustexplorer.

when i run the same code at https://remix.ethereum.org/, it also give similer error: the trait bound `ethnum::U256: Serialize` is not satisfied — Aakash Singh Rajput, May 06 '23 at 15:30
Have you imported `ethnum` like I did? With the `serde` feature? I.e. is this how you import it in your `Cargo.toml` file: `ethnum = { version = "1", features = ["serde"] }` — Jonas Fassbender, May 06 '23 at 17:59

how to use U256 in near smart contract in rust language

1 Answers1