[Actionable List] Replace revert strings with custom errors

🧐 Motivation

Custom Errors, which were introduced in 0.8.4, contribute to reduce the contract size and improve clarity of revert reasons.

A quick study shows that replacing a (short) revert string with a custom error can save 8.5k gas, and slightly decrease the run cost.

error SomeError();
error SomeErrorWithArgs(address);

contract A { function test() external { revert("some short message"); } }
// deploy: 90599
// run: 21288

contract B { function test() external { revert("some long message that will cause deployment to be more expensive, but hopefully not execution"); } }
// deploy: 109401
// run: 21318

contract C { function test() external { revert SomeError(); } }
// deploy: 81953
// run: 21228

contract D { function test() external { revert SomeErrorWithArgs(msg.sender); } }
// deploy: 83225
// run: 21245

📝 Next steps

There are some question to resolve before moving forward with that change:

• Check/validate custom error support by developer tools (hardhat + truffle / hardhat + waffle, etherscan, …)
  • Adapt our tests if needed
• List all revert string, and decide what custom error to use for each one
  • some custom errors might be reusable between contracts (ex: InsufficientBalance for ERCs 20, 777, 1155)
  • decide which parameter (if any) to include in the custom error
• Decide where to define the errors
  • In each file: makes re-use difficult
  • In a global file: messy
  • Other?

See

• #3183 (uses custom errors)
• #2725 (rewrite of access control around custom errors)

[wiasliaw]

There are two testing development tools had supported the custom error.

• waffle
• hardhat-chai-matchers

I recommend hardhat-chai-matchers. it provide better readability.

// hardhat-chai-matchers
await expect(contract.call()).to.be.revertedWithCustomError(
  contract,
  "SomeCustomError"
);

but waffle provider a matcher combined reason string and custom error

// waffle
// reason string
await expect(token.checkRole('ADMIN'))
  .to.be.revertedWith(/AccessControl: account .* is missing role .*/);

// custom error
await expect(token.transfer(receiver, 100))
    .to.be.revertedWith('InsufficientBalance')
    .withArgs(0, 100);

[ItsShadowl]

hardhat-chai-matchers should be preferred over Waffle nowadays.

I believe both supports Custom Errors, and the implementation can be tested with either.

We use Custom Error all over our codebase where possible. It is lighter.