Generating Candid files for Rust canisters

Beginner

Rust

Candid interface files (.did) are not generated automatically for Rust canisters like they are for Motoko canisters. For Rust canisters, you must write the .did file yourself or you can use a tool like the candid-extractor to extract the Candid from the canister's Wasm.

Install the candid-extractor with cargo:

cargo install candid-extractor

Then, in your Rust canister code, call the export_candid macro at the end of your lib.rs file:

lib.rs

use ic_cdk::query;
use ic_cdk::update;

#[query]
fn hello(name: String) -> String {
    format!("Hello, {}!", name)
}

#[update]
fn world(name: String) -> String {
    format!("World, {}!", name)
}

// Enable Candid export
ic_cdk::export_candid!();

Before extracting the Candid interface, you must first compile the canister's Wasm module:

cargo build --release --target wasm32-unknown-unknown --package <CANISTER>

The Wasm module will be stored at:

target/wasm32-unknown-unknown/release/<CANISTER>.wasm

Then you can extract the Candid interface from the Wasm module and save it to a file:

candid-extractor target/wasm32-unknown-unknown/release/<CANISTER>.wasm > <CANISTER>.did