Feat: Adding a warning in the upgrade CLI when a config file contains non-migratable values.

[baptisthecht]

I encountered an issue in several of my projects during migration. I was unable to migrate correctly because my config file contained the now-deprecated "safelist" key. I had to reverse-engineer the problem to identify what was blocking me. To help others facing the same issue, I added this warning to inform them that they need to edit their config file for a smooth automatic migration.

[philipp-spiess]

@baptisthecht Thanks! It would be good if we can add some warnings to the other cases where we opt-out of migrating to a CSS file as well and add a quick test to ensure this works. Would you be interested to take a stab at this?

[baptisthecht]

@baptisthecht Thanks! It would be good if we can add some warnings to the other cases where we opt-out of migrating to a CSS file as well and add a quick test to ensure this works. Would you be interested to take a stab at this?

Sure !

[baptisthecht]

Should we keep the non-migratable Tailwind properties by linking the old tailwind.config.xxs file and warning that they are deprecated, or should we ignore the deprecated properties and proceed with the migration anyway?

I think we could migrate by skipping extra properties like the corePlugins property:

if ('corePlugins' in unresolvedConfig) {
  info(
    `The \`corePlugins\` option is no longer supported as of Tailwind CSS v4.0, so it has been removed from your configuration.`
  );
}

So, we may exclude the knownProperties part of the canMigrateConfig function and just skip non-migratable properties.

Then, we can add warnings for other non-migratable cases, such as static plugins or presets.

[philipp-spiess]

I thin it's fine to skip corePlugins like you said but I'm unsure of skipping all unknown properties without an allow list. Could lead to some confusion since plugins in v3 you can read the whole config object so there might be some plugin specific settings: https://play.tailwindcss.com/1ZICA06UpK?file=config