Foundry: Forge, Cast, Anvil

Foundry is a Solidity development toolkit written in Rust, originally created by Georgios Konstantopoulos and open-sourced through Paradigm in 2021. It is now the default toolchain across most serious Ethereum protocols (Compound, Lido, Aave, Optimism, ENS, and so on). The reasons are concrete and not religious. The previous dominant stack — Hardhat plus various JavaScript testing libraries — required you to write tests in JavaScript or TypeScript while your contracts were in Solidity. That meant a constant context-switch between languages, plus the need to serialize and deserialize complex Solidity types (structs, fixed-size arrays) across the JS↔Solidity boundary. It worked, but it was slow (Node.js startup + an ethers.js call for every test action) and the test code looked nothing like the production code it was testing. Foundry tests are themselves Solidity contracts. The same patterns you use to write production code — structs, mappings, custom errors, modifiers — are available in your tests. The test runner is Rust, so the entire suite runs in seconds even for large codebases. Aave's full Foundry suite, with thousands of tests across hundreds of files, runs in roughly half a minute on a laptop. Foundry is three tools that share infrastructure: - **forge** — the compiler, test runner, and project manager. Most of what you do daily. - **cast** — a swiss-army knife CLI for interacting with any EVM chain. Replaces the maze of `web3 console`, `ethers.js` scripts, and curl-to-RPC commands. - **anvil** — a local devnet with full mainnet-fork capability. Replaces `ganache` and `hardhat node`. Installing the whole toolkit is one command: `curl -L https://foundry.paradigm.xyz | bash; foundryup`. The next sections assume you've done that.