fix(build): emptyOutDir should happen for watch rebuilds

[sapphi-red]

Rollup seems to run options hook for rebuilds, but Rolldown doesn't seem to.

fixes #22205
refs rolldown/rolldown#9053

[bluwy]

Why choose not to run it for optimization? It lends Rollup's plugin interface so it should prioritize compatibility IMO, and optimizations come additively.

[sapphi-red]

I assumed there's a significant performance difference. @IWANABETHATGUY Would you elaborate the rationale behind the decision?

[sapphi-red]

I forgot to mention but we still need to use watchChange instead of options because options is called for each output in Rolldown (while Rollup only calls it once). This is due to its architecture.

[hyfdev]

Some notes:

• Calling options hook require rust to call js. It's less performant and I don't see any necessity here(will explain below)
• If every rebuild in watch is a fresh rebuild, then maybe it makes some sense to call options hook to allow users to do some customization. But this isn't true, I think even for Rollup, this isn't not a good idea. It causes buggy behavior if users try to changes options during watch.
• Considering the incremental build feature of Rolldown,
  • changing options will lead to a fresh rebuild to recreate everything. This could be supported, but not worth it in paratice.
  • Options is already sealed for watch mode, it shouldn't be allowed to change.

[bluwy]

I forgot to mention but we still need to use watchChange instead of options because options is called for each output in Rolldown (while Rollup only calls it once). This is due to its architecture.

Ok yeah that makes sense to me, not blocking this PR then.

• If every rebuild in watch is a fresh rebuild, then maybe it makes some sense to call options hook to allow users to do some customization. But this isn't true, I think even for Rollup, this isn't not a good idea. It causes buggy behavior if users try to changes options during watch.

• Considering the incremental build feature of Rolldown,

  • changing options will lead to a fresh rebuild to recreate everything. This could be supported, but not worth it in paratice.
  • Options is already sealed for watch mode, it shouldn't be allowed to change.

Thanks, I think all that makes sense to me. It is indeed rare for the options hook to change between watch reruns, and should be fine if it's "cached" between runs.