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Rust Development

Guidance for AI agents working with Rust code in the Optimism monorepo. See dev-workflow.md for tool versions, PR workflow, and other cross-language guidance.

Workspace Layout

All Rust code lives under rust/. This is a unified Cargo workspace — always run Rust commands from this directory. The workspace contains three main component groups:

  • Kona — Proof system and rollup node (rust/kona/)
  • Op-Reth — OP Stack execution client built on reth (rust/op-reth/)
  • Op-Alloy / Alloy extensions — OP Stack types and providers

Check rust/Cargo.toml for the full workspace member list, dependency versions, and lint configuration. The Rust toolchain version is pinned in rust/rust-toolchain.toml.

Build System

Run just --list in rust/ to see all available targets. The key ones:

cd rust

# Build the workspace
just build

# Build in release mode
just build-release

# Build specific binaries
just build-node      # kona-node
just build-op-reth   # op-reth

Running Tests

Tests use cargo-nextest (not cargo test) for unit tests:

cd rust

# Run all tests (unit + doc tests)
just test

# Unit tests only (excludes online tests)
just test-unit

# Doc tests only
just test-docs

Running op-reth E2E Tests

The op-reth E2E tests (rust/op-reth/tests/proofs/) run a full devnet with op-geth (sequencer) and op-reth (validator). They require two build prerequisites:

  1. Forge artifacts — the devnet deploys contracts from compiled artifacts:

    cd packages/contracts-bedrock
mise exec -- just build-no-tests

  2. op-reth release binary — the test harness (op-devstack/sysgo/rust_binary.go) only searches target/release/, not target/debug/. Options:

    # Option A: let the test build it (slow first run, cached after)
RUST_JIT_BUILD=1 go test -v -run TestName ./rust/op-reth/tests/proofs/core/

# Option B: pre-build the binary
cd rust && just build-op-reth

Run from the monorepo root:

mise exec -- go test -v -run TestExecutePayloadSuccess -count=1 ./rust/op-reth/tests/proofs/core/

Generating Prestates

Kona prestates are built via Docker:

cd rust
just build-kona-prestates

Linting

cd rust

# Run all lints (format check + clippy + doc lints)
just lint

# Individual lint steps
just fmt-check      # formatting (requires nightly)
just lint-clippy    # clippy with all features, -D warnings
just lint-docs      # rustdoc warnings

Lint configuration lives in rust/Cargo.toml (workspace lints section), rust/clippy.toml, and rust/rustfmt.toml.

Formatting Requires Nightly

Formatting uses a pinned nightly toolchain (defined as NIGHTLY in rust/justfile). If the nightly isn't installed:

cd rust
just install-nightly

Then use just fmt-fix to auto-format, or just fmt-check to verify.

no_std Compatibility

Many kona and alloy crates must compile without the standard library (for the fault proof VM). If you modify these crates, verify no_std builds:

cd rust
just check-no-std

This builds affected crates for the riscv32imac-unknown-none-elf target.

Dependency Auditing

The workspace uses cargo-deny for license, advisory, and dependency checks. Configuration is in rust/deny.toml.

cd rust
just deny

Before Every Commit

Run these checks from rust/. Fix all issues — CI enforces zero warnings.

  1. Lint — this checks formatting, clippy, and doc lints:

    just lint

  2. Test — run tests for changed packages:

    just test-unit

  3. no_std — if you changed any proof, protocol, or alloy crate:

    just check-no-std

CI

Op-reth requires clang / libclang-dev for reth-mdbx-sys bindgen. CI installs this automatically — if you see bindgen errors locally, install clang.

Skills

  • Fix Rust Formatting (.claude/skills/fix-rust-fmt/SKILL.md): Fixes rust-fmt CI failures by installing the pinned nightly toolchain and running just fmt-fix. Invoke with /fix-rust-fmt.