Example

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

[near_bindgen]

[derive(Default, BorshDeserialize, BorshSerialize)]

pub struct StatusMessage { records: HashMap, }

[near_bindgen]

impl StatusMessage { pub fn setstatus(&mut self, message: String) { let accountid = env::signeraccountid(); 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:

  ```rust

  [test]

  fn setgetmessage() { // Use VMContext to setup gas, balance, storage usage, account id, etc. let context = VMContext { ... }; let config = Config::default(); testingenv!(context, config); let mut contract = StatusMessage::default(); contract.setstatus(&mut env, "hello".tostring()); asserteq!("hello".tostring(), contract.getstatus("bob.near".to_string()).unwrap()); } ```

  To run unit tests include env_test feature: bash 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.

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

  ```rust

  [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 have: * Rustup installed and switched to nightly Rust compiler: bash curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh rustup default nightly * WASM pack installed: bash curl https://rustwasm.github.io/wasm-pack/installer/init.sh -sSf | sh

Writing Rust Contract

You can follow the test-contract crate that shows a simple Rust contract.

The general workflow is the following: 1. Create a crate and configure the Cargo.toml similarly to how it is configured in examples/status-message/Cargo.toml; 2. 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: rust #[near_bindgen] #[derive(Default, BorshSerialize, BorshDeserialize)] pub struct MyContract { data: HashMap<u64, u64> }

1. 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: ```rust

   [near_bindgen]

   impl MyContract { pub fn insertdata(&mut self, key: u64, value: u64) -> Option { self.data.insert(key) } pub fn getdata(&self, key: u64) -> Option { self.data.get(&key).cloned() } } ```

Building Rust Contract

We can build the contract using rustc: bash cargo +nightly build --target wasm32-unknown-unknown --release But then we would want to compress it using wasm-opt and wasm-gc tools that we installed with wasm-pack, see examples/status-message/build.sh.

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: bash wasm-opt -Oz --output ./pkg/optimized_contract.wasm ./pkg/contract.wasm See Binaryen for the installation instructions.