Skip to main content
Version: 0.16

Downloading and Compiling Contract

In this section, we will download a sample contract, and compile to it to a wasm binary executable.

Please first review the client setup instructions, and configure a client before proceeding. Either the Node.js REPL or Go CLI will work.

Compiling and Testing Contract

Let's download the repo in which we collect cw-examples and try out an existing simple name service contract where mimics a name service marketplace. Also this tutorials is the defacto cosmos-sdk entrance tutorial. First, clone the repo and try to build the wasm bundle:

# get the code
git clone
cd cw-contracts
git fetch --tags
git checkout nameservice-0.11.0
cd contracts/nameservice

# compile the wasm contract with stable toolchain
rustup default stable
cargo wasm

After this compiles, it should produce a file in target/wasm32-unknown-unknown/release/cw_nameservice.wasm. A quick ls -lh should show around 1.7MB. This is a release build, but not stripped of all unneeded code. To produce a much smaller version, you can run this which tells the compiler to strip all unused code out:

RUSTFLAGS='-C link-arg=-s' cargo wasm

This produces a file about 162kB. We use this and another optimizer in the next last section to produce the final product uploaded to the blockchain. You don't need to worry about running this yourself (unless you are curious), but you should have an idea of the final size of your contract this way.

Unit Tests

Let's try running the unit tests:

RUST_BACKTRACE=1 cargo unit-test

After some compilation steps, you should see:

running 5 tests
test contract::tests::cannot_initialize_expired ... ok
test contract::tests::proper_initialization ... ok
test contract::tests::init_and_query ... ok
test contract::tests::handle_refund ... ok
test contract::tests::handle_approve ... ok

test result: ok. 5 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

RUST_BACKTRACE=1 will provide you with full stack traces on any error, which is super useful. This only works for unit tests (which test native rust code, not the compiled wasm). Also, if you want to know where cargo wasm and cargo unit-test come from, they are just aliases defined in .cargo/config. Take a look there to understand the cargo flags better.

Optimized Compilation

To reduce gas costs, the binary size should be as small as possible. This will result in a less costly deployment, and lower fees on every interaction. Luckily, there is tooling to help with this. You can optimize production code using cosmwasm/rust-optimizer. rust-optimizer produces reproducible builds of cosmwasm smart contracts. This means third parties can verify the contract is actually the claimed code.

docker run --rm -v "$(pwd)":/code \
--mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
--mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \

Binary will be at artifacts and its size will be 137k.