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Rust library for writing NEAR smart contracts
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near-bindgen Add readme symlink Jul 22, 2019


Rust library for writing NEAR smart contracts. version Download Join the community on Spectrum Join the community on Discord

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


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

#[derive(Default, Serialize, Deserialize)]
pub struct StatusMessage {
    records: HashMap<Vec<u8>, String>,

impl StatusMessage {
    pub fn set_status(&mut self, message: String) {
        let account_id = ENV.originator_id();
        self.records.insert(account_id, message);

    pub fn get_status(&self, account_id: Vec<u8>) -> Option<String> {


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

    fn set_get_message() {
        let account_id = b"alice";
        let mut contract = StatusMessage::default();
        assert_eq!(Some("Hello".to_owned()), contract.get_status(account_id.to_vec()));

    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;
    • return_promise -- treats the result of execution of the promise as the result of the current function.


To develop Rust contracts you would need have:

  • Rustup installed and switched to nightly Rust compiler:
curl --proto '=https' --tlsv1.2 -sSf | sh
rustup default nightly
curl -sSf | sh

To test Rust contracts you would need a locally running NEAR testnet, which we launch using Docker.

To communicate with the NEAR network we recommend using the NEAR shell which you can install with:

npm install -g near-shell

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 test-contract/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 Serialize and Deserialize traits which NEAR will use to save/load contract's internal state;

    Here is an example of a smart contract struct:

    #[derive(Default, Serialize, Deserialize)]
    pub struct MyContract {
        data: HashMap<u64, u64>
  3. Define methods that NEAR will expose as smart contract methods:

    • Are you free to define any methods for the struct but only non-static public methods will be exposed as smart contract methods;
    • Methods need to use either &self or &mut 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

    Here is an example of smart contract methods:

    impl MyContract {
       pub fn insert_data(&mut self, key: u64, value: u64) -> Option<u64> {
       pub fn get_data(&self, key: u64) -> Option<u64> {

Building Rust Contract

We can build the contract using the wasm-pack like this:

wasm-pack build --no-typescript --release

This will build the contract code in the pkg subfolder.

The error messages are currently WIP, so please reach directly to the maintainers until this is fixed.

Run the Contract

If you skipped the previous steps you can use the already built contract from test-contract/res/mission_control.wasm.

Let's start the local NEAR testnet and run the smart contract on it.

  • Start the local testnet:
  • Create an account into which we will deploy the contract using NEAR shell:
    near create_account --useDevAccount "missioncontrol"
  • Deploy the smart contract code:
    near deploy --accountId missioncontrol --wasmFile test-contract/res/mission_control.wasm
  • Let's create some other account that will be calling our contract:
    near create_account --useDevAccount "purplebot"
  • Using purplebot account call the method of the smart contract:
    near call missioncontrol add_agent "{}" --accountId purplebot
  • Using missioncontrol account call its own method:
    near call missioncontrol simulate "{\"account_id\":\"purplebot\"}" --accountId missioncontrol
    Observe that the returned result is true which corresponds to the bot being "alive".
  • Using purplebot account call the view method of the smart contract:
    near view missioncontrol assets_quantity "{\"account_id\":\"purplebot\",\"asset\":\"MissionTime\"}" --accountId missioncontrol
    Observe that the returned result is 2 (which is the correct expected value).

Note, smart contract methods that use &mut self modify the state of the smart contract and therefore the only way for them to be called is using near call which results in a transaction being created and propagated into a block. On the other hand, smart contract methods &self do not modify the state of the smart contract and can be also called with near view which does not create transactions.

Note, currently NEAR shell creates a neardev folder with public and secret keys. You need to cleanup this folder after you restart the local NEAR testnet.

Limitations and Future Work

The current implementation of wasm_bindgen has the following limitations:

  • The smart contract struct should be serializable with bincode 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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