Migration: Wrap `:has/not/where/is(...)` selectors in `:global(...)` if necessary

[dummdidumm]

Describe the problem

These are scoped in Svelte 5, but weren't in Svelte 4

Describe the proposed solution

Adjust migration script to add global if necessary

Importance

nice to have

[paoloricciuti]

What do we do if we have this AND postcss? Bail out and add a migration task?

[dummdidumm]

What does postcss change about this? Can you give an example?

[paoloricciuti]

What does postcss change about this? Can you give an example?

That we are blanking out css in the migration script because it was erroring out (i'm assuming because svelte is not able to parse it without a preprocessor)

[Conduitry]

This is something that people need to change even in non-runes mode, right? Should this be part of a separate class of (simpler) migrations that people need to perform upon upgrading to v5, rather than the more complex and fiddly and error-prone ones involving switching to runes?

See also #11313, which feels like something that a migration would be nice for, but which should happen along with this :has() etc. change, and not along with switching to runes.

[dominikg]

What does postcss change about this? Can you give an example?

i'm assuming because svelte is not able to parse it without a preprocessor

uhh we do have the preprocessors of the project available and can use them, right? (just import svelte.config.js and use preprocess). that might not give us the input css but the output css should be parsable by svelte and then we can inspect it to learn if it uses a feature that needs user attention and add a migration marker comment to that style block without nuking its source.

[dummdidumm]

I'm confident a conservative regex will get us far enough

[Conduitry]

Does anyone have any thoughts about breaking up migrations into two phases: 'needs to be done in non-runes mode due to breaking changes' and 'can be done later as you gradually switch to runes, and more likely to need manual attention'? It feels weird, but I don't have another great solution here. Combining obligatory non-runes changes together with optional runes rewrites into a single migration script rubs me the wrong way.

[dummdidumm]

I think it's a good point, and my proposal is to make this an option in our migration function, and we can then prompt in the migration script "convert components to new syntax?" with the default being Yes