Implement PSP22 contract

First, we will cover the implementation of PSP22 token used by our smart contract, which will represent the stable coin that we will be lending and another PSP22 token which we will be using as collateral. These are used just to test our example, you will not be creating an actual PSP22 implementation of stable coin or collateral token in your lending protocol, but this will also showcase how to implement a basic implementation of a fungible token with OpenBrush.

Definition of the StableCoin trait

In the traits/stable_coin.rs, we will define a StableCoin trait. That trait contains only two super traits: PSP22 and PSP22Metadata, without any other method. That shows that StableCoin is simple PSP22. In the implementation of the contract we will implement that trait to be sure that all super traits are also implemented. StableCoinRef can be used by other developers to do a cross contract call to StableCoinContract.

use openbrush::contracts::traits::psp22::{
    extensions::metadata::*,
    extensions::mintable::*,
    *,
};

#[openbrush::wrapper]
pub type StableCoinRef = dyn PSP22 + PSP22Metadata + PSP22Mintable;

#[openbrush::trait_definition]
pub trait StableCoin: PSP22 + PSP22Metadata + PSP22Mintable {}

Add dependencies

First we will add the dependencies used in our PSP22 contract to the Cargo.toml file. You will import the same dependencies as in the PSP22 documentation, so we will not show it here to keep it simple.

Implement the contract

We want a basic PSP22 token with metadata, so we will add the PSP22 Metadata extension to our contract. We will add a openbrush::contract macro to our contract and add some imports:

#![cfg_attr(not(feature = "std"), no_std, no_main)]

/// This is a simple `PSP22` which will be used as a stable coin and a collateral token in our lending contract
#[openbrush::implementation(PSP22, PSP22Metadata, PSP22Mintable)]
#[openbrush::contract]
pub mod token {
    use openbrush::traits::String;
    use lending_project::traits::stable_coin::*;
    use openbrush::traits::Storage;

Define the storage

We will derive the storage traits related to PSP22 and PSP22Metadata and declare the fields related to these traits.

/// Define the storage for PSP22 data and Metadata data
#[ink(storage)]
#[derive(Default, Storage)]
pub struct StableCoinContract {
    #[storage_field]
    psp22: psp22::Data,
    #[storage_field]
    metadata: metadata::Data,
}

Implement the PSP22 and PSP22Metadata traits and define the constructor

We will implement the PSP22Metadata trait and define the constructor where we will set the name and the symbol for our token. Also, we will mint the initial supply of tokens to the caller of the constructor.

PSP22Metadata and similar traits imported from OpenBrush should be implemented with #[openbrush::implementation] macro.

#[openbrush::implementation(PSP22, PSP22Metadata, PSP22Mintable)]
#[openbrush::contract]
mod contract {
    ...
}

Below we are implementing StableCoin trait itself and defining the constructor.

// It forces the compiler to check that you implemented all super traits
impl StableCoin for StableCoinContract {}

impl StableCoinContract {
    /// Constructor with name and symbol
    #[ink(constructor)]
    pub fn new(name: Option<String>, symbol: Option<String>) -> Self {
        let mut instance = Self::default();

        instance.metadata.name = name;
        instance.metadata.symbol = symbol;
        instance.metadata.decimals = 18;
        let total_supply = 1_000_000 * 10_u128.pow(18);
        assert!(instance._mint_to(Self::env().caller(), total_supply).is_ok());
        
        instance
    }
}