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Smart Contract Style Guide

This document provides guidance on how we organize and write our smart contracts. For cases where this document does not provide guidance, please refer to existing contracts for guidance, with priority on the L2OutputOracle and OptimismPortal.

Standards and Conventions

Style

Comments

Optimism smart contracts follow the triple-slash solidity natspec comment style with additional rules. These are:

  • Always use @notice since it has the same general effect as @dev but avoids confusion about when to use one over the other.
  • Include a newline between @notice and the first @param.
  • Include a newline between @param and the first @return.
  • Use a line-length of 100 characters.

We also have the following custom tags:

  • @custom:proxied: Add to a contract whenever it's meant to live behind a proxy.
  • @custom:upgradeable: Add to a contract whenever it's meant to be inherited by an upgradeable contract.
  • @custom:semver: Add to version variable which indicate the contracts semver.
  • @custom:legacy: Add to an event or function when it only exists for legacy support.
  • @custom:network-specific: Add to state variables which vary between OP Chains.

Errors

  • Use require statements when making simple assertions.
  • Use revert(string) if throwing an error where an assertion is not being made (no custom errors). See here for an example of this in practice.
  • Error strings MUST have the format "{ContractName}: {message}" where message is a lower case string.

Function Parameters

  • Function parameters should be prefixed with an underscore.

Function Return Arguments

  • Arguments returned by functions should be suffixed with an underscore.

Event Parameters

  • Event parameters should NOT be prefixed with an underscore.

Immutable variables

Immutable variables:

  • should be in SCREAMING_SNAKE_CASE
  • should be internal
  • should have a hand written getter function

This approach clearly indicates to the developer that the value is immutable, without exposing the non-standard casing to the interface. It also ensures that we don’t need to break the ABIs if we switch between values being in storage and immutable.

Spacers

We use spacer variables to account for old storage slots that are no longer being used. The name of a spacer variable MUST be in the format spacer_<slot>_<offset>_<length> where <slot> is the original storage slot number, <offset> is the original offset position within the storage slot, and <length> is the original size of the variable. Spacers MUST be private.

Proxy by Default

All contracts should be assumed to live behind proxies (except in certain special circumstances). This means that new contracts MUST be built under the assumption of upgradeability. We use a minimal Proxy contract designed to be owned by a corresponding ProxyAdmin which follow the interfaces of OpenZeppelin's Proxy and ProxyAdmin contracts, respectively.

Unless explicitly discussed otherwise, you MUST include the following basic upgradeability pattern for each new implementation contract:

  1. Extend OpenZeppelin's Initializable base contract.
  2. Include a function initialize with the modifier initializer().
  3. In the constructor:
    1. Call _disableInitializers() to ensure the implementation contract cannot be initialized.
    2. Set any immutables. However, we generally prefer to not use immutables to ensure the same implementation contracts can be used for all chains, and to allow chain operators to dynamically configure parameters

Because reinitializer(uint64 version) is not used, the process for upgrading the implementation is to atomically:

  1. Upgrade the implementation to the StorageSetter contract.
  2. Use that to set the initialized slot (typically slot 0) to zero.
  3. Upgrade the implementation to the desired new implementation and initialize it.

Versioning

All (non-library and non-abstract) contracts MUST inherit the ISemver interface which exposes a version() function that returns a semver-compliant version string.

Contracts must have a version of 1.0.0 or greater to be production ready.

Additionally, contracts MUST use the following versioning scheme when incrementing their version:

  • patch releases are to be used only for changes that do NOT modify contract bytecode (such as updating comments).
  • minor releases are to be used for changes that modify bytecode OR changes that expand the contract ABI provided that these changes do NOT break the existing interface.
  • major releases are to be used for changes that break the existing contract interface OR changes that modify the security model of a contract.

The remainder of the contract versioning and release process can be found in `VERSIONING.md.

Exceptions

We have made an exception to the Semver rule for the WETH contract to avoid making changes to a well-known, simple, and recognizable contract.

Additionally, bumping the patch version does change the bytecode, so another exception is carved out for this. In other words, changing comments increments the patch version, which changes bytecode. This bytecode change implies a minor version increment is needed, but because it's just a version change, only a patch increment should be used.

Dependencies

Where basic functionality is already supported by an existing contract in the OpenZeppelin library, we should default to using the Upgradeable version of that contract.

Source Code

The following guidelines should be followed for all contracts in the src/ directory:

  • All state changing functions should emit a corresponding event. This ensures that all actions are transparent, can be easily monitored, and can be reconstructed from the event logs.

Tests

Tests are written using Foundry.

All test contracts and functions should be organized and named according to the following guidelines.

These guidelines are also encoded in a script which can be run with:

tsx scripts/checks/check-test-names.ts

Expect Revert with Low Level Calls

There is a non-intuitive behavior in foundry tests, which is documented here. When testing for a revert on a low-level call, please use the revertsAsExpected pattern suggested there.

Note: This is a work in progress, not all test files are compliant with these guidelines.

Organizing Principles

  • Solidity contracts are used to organize the test suite similar to how mocha uses describe.
  • Every non-trivial state changing function should have a separate contract for happy and sad path tests. This helps to make it very obvious where there are not yet sad path tests.
  • Simpler functions like getters and setters are grouped together into test contracts.

Test function naming convention

Test function names are split by underscores, into 3 or 4 parts. An example function name is test_onlyOwner_callerIsNotOwner_reverts().

The parts are: [method]_[FunctionName]_[reason]_[status], where:

  • [method] is either test, testFuzz, or testDiff
  • [FunctionName] is the name of the function or higher level behavior being tested.
  • [reason] is an optional description for the behavior being tested.
  • [status] must be one of:
    • succeeds: used for most happy path cases
    • reverts: used for most sad path cases
    • works: used for tests which include a mix of happy and sad assertions (these should be broken up if possible)
    • fails: used for tests which 'fail' in some way other than reverting
    • benchmark: used for tests intended to establish gas costs

Contract Naming Conventions

Test contracts should be named one of the following according to their use:

  • TargetContract_Init for contracts that perform basic setup to be reused in other test contracts.
  • TargetContract_Function_Test for contracts containing happy path tests for a given function.
  • TargetContract_Function_TestFail for contracts containing sad path tests for a given function.

To minimize clutter, getter functions can be grouped together into a single test contract, ie. TargetContract_Getters_Test.

Withdrawing From Fee Vaults

See the file scripts/FeeVaultWithdrawal.s.sol to withdraw from the L2 fee vaults. It includes instructions on how to run it. foundry is required.