Cosmwasm Starter Pack

This is a template to build smart contracts in Rust to run inside a Cosmos SDK module on all chains that enable it. To understand the framework better, please read the overview in the cosmwasm repo. This assumes you understand the theory and just want to get coding.

Creating a new repo from template

Before starting, make sure you have rustup along with a recent rustc and cargo version installed. Currently, we are testing on 1.37+.

And you need to have the wasm32-unknown-unknown target installed as well.

You can check that via:

rustc --version
cargo --version
rustup target list --installed
# if wasm32 is not listed above, run this
rustup target add wasm32-unknown-unknown

You will also need to have cargo generate installed. Unless you did that before, run this line now:

cargo install cargo-generate --features vendored-openssl

Now that the prerequisites are over, you can create your new contract. Go to the folder in which you want to place it and run:

cargo generate --git https://github.com/confio/cosmwasm-template.git --name YOUR_NAME_HERE

You will now have a new folder called YOUR_NAME_HERE (I hope you changed that to something else) containing a simple working contract and build system that you can customize.

Compiling and running tests

Now that you created your custom contract, make sure you can compile and run it before making any changes. Go into the

# this will produce a wasm build in ./target/wasm32-unknown-unknown/release/YOUR_NAME_HERE.wasm
cargo wasm

# this runs unit tests with helpful backtraces
RUST_BACKTRACE=1 cargo unit-test
RUST_BACKTRACE=1 cargo test --lib --features backtraces

# this runs integration tests with cranelift backend (uses rust stable)
cargo test

# this runs integration tests with singlepass backend (needs rust nightly)
cargo test --no-default-features --features singlepass

The wasmer engine, embedded in cosmwasm-vm supports multiple backends: singlepass and cranelift. Singlepass has fast compile times and slower run times, and supportes gas metering. It also requires rust nightly. This is used as default when embedding cosmwasm-vm in go-cosmwasm and is needed to use if you want to check the gas usage.

However, when just building contacts, if you don't want to worry about installing two rust toolchains, you can run all tests with cranelift. The integration tests may take a small bit longer, but the results will be the same. The only difference is that you can not check gas usage here, so if you wish to optimize gas, you must switch to nightly and run with cranelift.

Understanding the tests

The main code is in src/contract.rs and the unit tests there run in pure rust, which makes them very quick to execute and give nice output on failures, especially if you do RUST_BACKTRACE=1 cargo test.

However, we don't just want to test the logic rust, but also the compiled wasm artifact inside a vm. You can look in tests/integration.rs to see some examples there. They load the wasm binary into the vm and call the contract externally. Effort has been made that the syntax is very similar to the calls in the native rust contract and quite easy to code. In fact, usually you can just copy a few unit tests and modify a few lines to make an integration test (this should get even easier in a future release).

We consider testing critical for anything on a blockchain, and recommend to always keep the tests up to date. While doing active development, it is often simplest to disable the integration tests completely and iterate rapidly on the code in contract.rs, both the logic and the tests. Once the code is finalized, you can copy over some unit tests into the integration.rs and make the needed changes. This ensures the compiled wasm also behaves as desired in the real system.

Preparing the wasm for production

Before we upload it to a chain, we need to ensure the smallest output size possible, as this will be included in the body of a transaction. We also want to have a reproducible build process, so third parties can verify that the uploaded wasm code did indeed come from the claimed rust code.

To solve both these issues, we have produced cosmwasm-opt, a docker image to produce an extremely small build output in a consistent manner. To use it,

Linux: docker run --rm -u $(id -u):$(id -g) -v $(pwd):/code confio/cosmwasm-opt:0.4.1

This produces a contract.wasm file in the current directory (which must be the root directory of your rust project, the one with Cargo.toml inside). The current sample contract compiles down to around 48kB wasm file.

Note this will take a while, as it doesn't share the cargo registry nor the incremental compilation cache with your host system, in order to provide the most consistent setup.

We also track the versions of cosmwasm that we aim for compatibility. The most important aspect is the same version of wasm-pack and wasm-bindgen. For 0.4.1 we are tied to wasm-pack 0.8.1, wasm-bindgen 0.2.53, and rust 1.38.