Accept empty optimizer sequence even with Yul optimizer disabled

[cameel]

Original comment: https://github.com/ethereum/solidity/pull/14149#discussion_r1174026509.

Description

One weird consequence of #14149 is that now, counter-intuitively, this in Standard JSON optimizer settings:

yul: false,

executes more optimizer steps than this:

yul: true,
yulDetails: {optimizerSteps: ":"}

This is not obvious to users as can be seen in #14619 or #14470. From UI perspective it would be less confusing to allow setting optimizerSteps: ":" regardless of whether the optimizer is enabled or not.

Specification

• On the CLI --yul-optimizations:":" should not trigger an error when used with --no-optimize-yul (or without --optimize, which implies --no-optimize-yul).
• In Standard JSON yulDetails: {optimizerSteps: ":"} should not trigger an error when used with yul: false.
  • Changing other options inside yulDetails should not be allowed when Yul optimizer is disabled.
• In both cases, a completely empty sequence (i.e. :) should be the only accepted value with Yul optimizer disabled. Allowing adding steps would make things even more confusing.
  • We might also allow the default sequence, i.e. (u:).
• Behavior should be identical to the case where the Yul optimizer is enabled, but metadata should reflect the options that were actually used (e.g. it should not say that --optimize-yul was used if it was not).

[ekpyron]

In principle, the nicer way to do this in the longer term would be something like: Deprecate the current yul-related settings altogether (we won't allow disabling stack optimizations in the long-run making the optimizer steps the only remaining setting specific to the yul optimizer) and just have a single setting yul: "...sequenceOfSteps...".... maybe with a string alias for the default sequence... Then "enabling" the yul optimizer would be having a non-empty yul optimizer sequence and "disabling" the yul optimizer would be setting the empty sequence...

But that'd be a breaking change and we won't yet remove the stack optimization setting, so for the time being the plan suggested above should be the best solution.

[cameel]

Then "enabling" the yul optimizer would be having a non-empty yul optimizer sequence and "disabling" the yul optimizer would be setting the empty sequence...

But that would make it impossible to run without UnusedPruner, which is what #14619 was about, wouldn't it? I.e. if we do it that way and yul: "" still runs UnusedPruner then users can no longer have a completely empty sequence. And if it does not, then you can run into "Stack too deep".

I think we still need to distinguish these two cases. The problem is really that currently it's possible to have even less optimization than with the optimizer nominally disabled, which is weird.

[ekpyron]

I don't follow, why? ":" as a sequence would fully disable the yul optimizer (including the unused pruner). The sequence would just have two different default values (neither of them ":") depending on the general enabled setting of the optimizer.

[cameel]

It would not fully disable the optimizer but it would not run UnusedPruner because there's no u in it. That was enough in #14619.