Truffle's default optimizer settings actually have an effect

[haltman-at]

Issue

When Truffle compiles Solidity, it merges the settings from the Truffle Config into some default settings, those default settings being:

settings: {
  optimizer: {
    enabled: false
    runs: 200
  }
}

Seems harmless, right? Well... turns out that on Solidity versions prior to 0.8.6, having enabled set to false, and having it absent entirely, had different results. Having it set to false, as opposed to absent entirely, actually turned on some optimizations (though evidently not much).

This poses a problem when it comes to contract verification; see discussion on this Blockscout issue. Unfortunately I don't think we can just get rid of this default without potentially breaking things. But we can perhaps discard it in fetch-and-compile specifically. However if we do so, we'll have to update the Etherscan fetcher (and possibly the Sourcify fetcher?) per the linked discussion (like, the Etherscan fetcher will have to convert enabled being absent to it being false; the runs: 200 default may also be necessary).

All of this is basically OK for now, but it will need to be solved if we want to do something about #4779.

Environment

• Truffle version (truffle version): 5.5.26