Compile a Rust smart contract module#

This guide will show you how to compile smart contract module written in Rust to a Wasm module.


Make sure to have Rust and Cargo installed and the wasm32-unknown-unknown target, together with cargo-concordium and the Rust source code for a smart contract module, you wish to compile.

See also

For instructions on how to install the developer tools see Install tools for development.

Compiling to Wasm#

To help building smart contract modules and to take advantage of features such as contract schemas, Concordium recommends using the cargo-concordium tool for building Rust smart contracts.

In order to build a smart contract for deployment, run:

$cargo concordium build --verifiable --out contract.wasm.v1

This uses Cargo for building, but runs further optimizations on the result.

The --verifiable option is available on cargo-concordium version 3.1 or newer. It will cause cargo-concordium to build the sources in a fixed container to make the build reproducible. It will additionally produce a tar file with packaged sources that were used for the build. The list of available docker images can be found on DockerHub.

The tar archive should be uploaded and made publicly available, and its link should be embedded into the deployed module using the cargo concordium edit-build-info command. For example

$cargo concordium edit-build-info --module module.wasm.v1 --source-link --verify

The --out option is required for verifiable builds, and instructs cargo-concordium to output the build artifact at the specified location.

Verifiable builds require a container runtime such as Docker to be installed.

Additional information about verifiable builds can be found on Github.


If you receive an error similiar to the one below, you need to update your cargo-concordium to version 3.0 or later.

Error: Could not build smart contract.

Caused by:
   0: Could not build module schema.
   1: Could not generate module schema from Wasm module.
   2: Unsupported instruction 0xc0

If you want a smaller code footprint, enable the wee_alloc feature by using:

$cargo concordium build --verifiable --out contract.wasm.v1 -- --features wee_alloc

See also

For building the schema for a smart contract module, some further preparation is required. Building and embedding the schema can and should be combined with a verifiable build using the --verifiable option.


It is also possible to compile using Cargo directly by running:

$cargo build --target=wasm32-unknown-unknown [--release]

Note that even with --release set, the produced Wasm module includes debug information.

Non-verifiable builds#

Omitting the --verifiable option from the build command will make a build on the host machine. This is typically not verifiable since the Rust toolchain is affected by the build environment. However, for local development, non-verifiable builds can be useful.

By default, running the cargo concordium build command will produce a smart contract module which can be found relative to your project root folder in ./target/concordium/wasm32-unknown-unknown/release/my_module.wasm.v1. The --out option can be supplied also for non-verifiable builds. For example, running the following command will output your smart contract module into the root folder of your project in a file called my_module.wasm.v1.

$cargo concordium build --out ./my_module.wasm.v1


cargo-concordium produces several smart contract modules with different suffixes. The suffix corresponds to the smart contract version, i.e. my_module.wasm/my_module.wasm.v0 for V0 contracts and my_module.wasm.v1 for V1 contracts. Concordium recommends using the wasm module with the .v1 extension (the most-up-to date smart contract version). The file my_module.wasm.v1 will be used when deploying a smart contract on-chain.

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