software-mansion/scarb v2.21.0 on GitHub

Welcome to the release notes for Scarb v2.21.0!

This release updates Scarb to Cairo v2.21.0. It brings static contract forwarding for Starknet contracts, more correct dependency resolution when a lockfile is present, and more accurate proc macro diagnostics in the language server.

Static Contract Forwarding

Want a contract that exposes another contract's functions without copying their code? With static forwarding, your contract can pass calls straight through to another contract's class. Unlike a typical proxy, which reads the target class hash from storage, the hash is baked in at compile time: no storage reads, no setup.

Here's how to use it. Say you have a counter_contract that implements ICounterContract, and you want static_proxy to expose the same interface:

#[starknet::contract]
#[feature("forward-impl")]
pub mod static_proxy {
    #[storage]
    struct Storage {}

    // Which class to forward to: the compiled `counter_contract`.
    impl CounterClassHash =
        super::counter_contract__class_hash__::ForwardingClassHashImpl<ContractState>;

    // Expose every `ICounterContract` function by forwarding it to that class.
    #[abi(embed_v0)]
    impl ForwardedImpl = super::ICounterContractForwardImpl<ContractState>;
}

You don't write either of those impls yourself. Cairo generates ICounterContractForwardImpl from the interface, and Scarb generates counter_contract__class_hash__ from the contract.

Then turn forwarding on in Scarb.toml:

[[target.starknet-contract]]
forwarding = true

That's it. Scarb compiles counter_contract first, computes its class hash, and embeds it into static_proxy automatically. Test targets pick up the same setting, so your tests run against the same hashes.

A few things to keep in mind:

The contract you forward to must be built together with yours. It should be defined in your package or listed in build-external-contracts.
Contracts can't forward to each other in a loop.

See the Starknet contract target docs for the full details.

Cairo Version

This version of Scarb comes with Cairo v2.21.0.

What's Changed

Bump Cairo by @wawel37 in #3257
Impl forwarding by @wawel37 in #3251
Locked dependency sources by @wawel37 in #3255
Pass a call site per derive to the proc macro server by @hakiers in #3275

Full Changelog: v2.20.1...v2.21.0