`near-bindgen`

Rust library for writing NEAR smart contracts.

Features | Pre-requisites | Writing Rust Contract | Building Rust Contract | Running Rust Contract | Limitations and Future Work

Example

Wrap a struct in #[near_bindgen] and it generates a smart contract compatible with the NEAR blockchain:

#[near_bindgen]
#[derive(Default, BorshDeserialize, BorshSerialize)]
pub struct StatusMessage {
    records: HashMap<String, String>,
}

#[near_bindgen]
impl StatusMessage {
    pub fn set_status(&mut self, message: String) {
        let account_id = env::signer_account_id();
        self.records.insert(account_id, message);
    }

    pub fn get_status(&self, account_id: String) -> Option<String> {
        self.records.get(&account_id).cloned()
    }
}

Features

Unit-testable. Writing unit tests is easy with near-bindgen:

#[test]
fn set_get_message() {
    // Use VMContext to setup gas, balance, storage usage, account id, etc.
    let context = VMContext { ... };
    let config = Config::default();
    testing_env!(context, config);
    let mut contract = StatusMessage::default();
    contract.set_status(&mut env, "hello".to_string());
    assert_eq!("hello".to_string(), contract.get_status("bob.near".to_string()).unwrap());
}

To run unit tests include env_test feature:

cargo test --package status-message --features env_test

Asynchronous cross-contract calls. Asynchronous cross-contract calls allow parallel execution of multiple contracts in parallel with subsequent aggregation on another contract. env exposes the following methods:
- promise_create -- schedules an execution of a function on some contract;
- promise_then -- attaches the callback back to the current contract once the function is executed;
- promise_and -- combinator, allows waiting on several promises simultaneously, before executing the callback;
- promise_return -- treats the result of execution of the promise as the result of the current function.
Follow examples/cross-contract to see various usages of cross contract calls, including system-level actions done from inside the contract like balance transfer (examples of other system-level actions are: account creation, access key creation/deletion, contract deployment, etc).

Initialization methods. We can define an initialization method that can be used to initialize the state of the contract.

#[near_bindgen(init => new)]
impl StatusMessage {
  pub fn new(user: String, status: String) -> Self {
      let mut res = Self::default();
      res.records.insert(user, status);
      res
  }
}

Pre-requisites

To develop Rust contracts you would need to:

Install Rustup:

curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh

Install the nightly Rust compiler:

rustup install nightly

Use the nightly compiler for your repo:

cd ./myproject
rustup override set nightly

Writing Rust Contract

You can follow the examples/status-message crate that shows a simple Rust contract.

The general workflow is the following:

Create a crate and configure the Cargo.toml similarly to how it is configured in examples/status-message/Cargo.toml;
Crate needs to have one pub struct that will represent the smart contract itself:
- The struct needs to implement Default trait which NEAR will use to create the initial state of the contract upon its first usage;
- The struct also needs to implement BorshSerialize and BorshDeserialize traits which NEAR will use to save/load contract's internal state;
Here is an example of a smart contract struct:
```
#[near_bindgen]
#[derive(Default, BorshSerialize, BorshDeserialize)]
pub struct MyContract {
    data: HashMap<u64, u64>
}
```
Define methods that NEAR will expose as smart contract methods:
- You are free to define any methods for the struct but only public methods will be exposed as smart contract methods;
- Methods need to use either &self, &mut self, or self;
- Decorate the impl section with #[near_bindgen] macro. That is where all the M.A.G.I.C. (Macros-Auto-Generated Injected Code) is happening
- If you need to use blockchain interface, e.g. to get the current account id then you can access it with env::*;
Here is an example of smart contract methods:
```
#[near_bindgen]
impl MyContract {
   pub fn insert_data(&mut self, key: u64, value: u64) -> Option<u64> {
       self.data.insert(key)
   }
   pub fn get_data(&self, key: u64) -> Option<u64> {
       self.data.get(&key).cloned()
   }
}
```

Building Rust Contract

We can build the contract using rustc:

RUSTFLAGS='-C link-arg=-s' cargo +nightly build --target wasm32-unknown-unknown --release

Running Rust Contract

If you skipped the previous steps you can use the already built contract from examples/status-message/res/status-message.wasm.

Start the local testnet

Let's start the local Near testnet to run the contract on it.

Make sure you have Docker installed;
Clone the nearprotocol/nearcore;
Make sure you are in master branch, then run
```
rm -rf testdir; ./scripts/start_unittest.py
```
It might take a minute to start if you machine have not downloaded the docker image yet.

Note, the locally running node will create testdir directory where it will keep the node state and the configs, including the secret key of the validator's account which we can use to create new accounts later.

Create the project and deploy the contract

Make sure you have the newest version of near-shell installed by running:
```
npm install -g near-shell
```
Create the near-shell project. This will allow having configuration like URL of the node in the config file instead of passing it with each near-shell command.
```
near new_project ./myproject; cd ./myproject
```
Modify the config to point to the local node: open ./src/config.js in ./myproject and change nodeUrl under development to be http://localhost:3030. This is how it should look like:
```
case 'development':
return {
   networkId: 'default',
   nodeUrl: 'http://localhost:3030',
   ...
}
```
Create account for your smart contract, e.g. we can use status_message as the account identifier:
```
near create_account status_message --masterAccount=test.near --homeDir=../nearcore/testdir
```
Note, homeDir should point to the home directory of the node which contains the secret key which we will use to sign transactions.

Deploy the contract code to the newly created account:

near deploy --accountId=status_message --homeDir=../nearcore/testdir --wasmFile=../examples/status-message/res/status_message.wasm

Call contract functions

Let's call the set_status function on the smart contract:
```
near call status_message set_status "{\"message\": \"Hello\"}" --accountId=test.near --homeDir=../nearcore/testdir
```
Notice that we use account id test.near to call a smart contract deployed to status_message account id. The smart contract will remember that account test.near left the message "Hello", see the implementation in examples/status-message/src/lib.rs.

Do another call to get_status function to check that the message was correctly recorded:

    near call status_message get_status "{\"account_id\": \"test.near\"}" --accountId=test.near --homeDir=../nearcore/testdir

Observe the output:

Result: Hello

Do another call to get_status but this time inquire about the account that have not left any messages:

    near call status_message get_status "{\"account_id\": \"some_other_account\"}" --accountId=test.near --homeDir=../nearcore/testdir

Observe the output:

Result: null

Cleaning up

Stop the node using docker commands:

docker stop nearcore watchtower
docker rm nearcore watchtower

Remove the node project directory:
```
rm -rf myproject
```
Remove the node data:
```
rm -rf testdir
```

Limitations and Future Work

The current implementation of wasm_bindgen has the following limitations:

The smart contract struct should be serializable with borsh which is true for most of the structs;
The method arguments and the return type should be json-serializable, which is true for most of the types, with some exceptions. For instance, a HashMap<MyEnum, SomeValue> where MyEnum is a non-trivial tagged-union with field-structs in variants will not serialize into json, you would need to convert it to Vec<(MyEnum, SomeValue)> first. Require arguments and the return type to be json-serializable for compatiblity with contracts written in other languages, like TypeScript;
Smart contract can use std but cannot use wasm-incompatible OS-level features, like threads, file system, network, etc. In the future we will support the file system too;
Smart contracts should be deterministic and time-independent, e.g. we cannot use Instant::now. In the future we will expose Instant::now;

We also have the following temporary inefficiencies:

Current smart contracts do not utilize the trie and do not use state storage efficiently. It is okay for small collections, but in the future we will provide an alternative near::collections::{HashMap, HashSet, Vec} that will be using storage in an efficient way;
The current smart contract size is around typically ~80-180Kb, which happens because we compile-in the bincode and serde-json libraries. In the future, we will cherry-pick only the necessary components from these libraries. For now you can use wasm-opt to slightly shrink the size:
```
wasm-opt -Oz --output ./pkg/optimized_contract.wasm ./pkg/contract.wasm
```
See Binaryen for the installation instructions.

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