Compile a Rust smart contract module#

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

Preparation#

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.

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, run:

$cargo concordium build

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

Removing host information from build#

The compiled Wasm module can contain information from the host machine building the binary; information such as the absolute path of the .cargo directory.

For most people this is not sensitive information, but it is important to be aware of it.

On Linux the paths can be inspected by running:

strings contract.wasm | grep /home/

The solution

The ideal solution would be to remove this path entirely, but that is unfortunately not a trivial task in general.

It is possible to work around the issue by using the --remap-path-prefix flag when compiling the contract. On Unix-like systems the flag can be passed directly to the cargo concordium invocation using the RUSTFLAGS environment variable:

$RUSTFLAGS="--remap-path-prefix=$HOME=" cargo concordium build

Which will replace the users home path with the empty string. Other paths could be mapped in a similar way. In general using --remap-path-prefix=from=to will map from to to at the beginning of any embedded path.

The flag can also be set permanently in the .cargo/config file in your crate, under the build section:

[build]
rustflags = ["--remap-path-prefix=/home/<user>="]

where <user> should be replaced with the user building the wasm module.

Caveats#

The above will likely not fix the issue if the rust-src component is installed for the Rust toolchain. This component is needed by some Rust tools such as the rust-analyzer.