fix: component filling a named slot does not honor let: directives

[dimfeld]

Fixes #9797, a regression in Svelte 5 in which let: directives on a component filling a named slot do not work.

<Nested {things}>
  <!-- Throws error: thing is undefined -->
  <SlotInner slot="foo" let:thing {thing} />
</Nested>

This fix mostly works but is not complete yet. I'm hoping someone more knowledgable can help fill in the exact details on how to do things.

Some problems:

• On the client side, the value passed to the inner component is an object instead of the actual value. It looks like this is because attribute.metadata.dynamic === false when it should probably be true. I haven't yet figured out the right way to accomplish this. Simon fixed this one :)
• When multiple elements fill the same named slot, it renders const thing = $$slotProps.thing twice, although I think that's an issue with the existing implementation as well. Might just need some kind of deduplication when rendering the elements in context.state.init, but maybe there's a better way? update: as described below, the code that rendered to this wasn't supposed to be allowed in the first place.
• It's a bit ambiguous as to whether a let: directive in this case applies to the parent's named slot being filled, or the current component's default slot. This change is only additive, pulling in values from the parent's named slot, and never cancelling the rendering of the current component's default slot. I think that ends up ok, worst case it's a bit of wasted code. EDIT: I tested on Svelte 4 and it removes the bindings to the default slot in this case, so I've done the same here.

Svelte 5 rewrite

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[dummdidumm]

Could you clarify with examples what you mean by the second-last bullet points (multiple fill the same)? It sounds like you found a related bug while trying to fix this?

[dimfeld]

Thanks for the assistance here!

Could you clarify with examples what you mean by the second-last bullet points (multiple fill the same)? It sounds like you found a related bug while trying to fix this?

Actually this turns out to be simpler than I thought, since the code that causes the error shouldn't be allowed in the first place.

REPL that shows the behavior

If I use 2 divs instead, I get a "duplicate slot" error on both Svelte 4 and 5, so I think the issue here is actually just that slot attributes on components aren't participating in that part of the analysis.

[dummdidumm]

Yeah this throws a validation error in Svelte 4 but currently doesn't in Svelte 5. You can add it to the PR if you want or create a separate one.

[dimfeld]

Sounds good, I'll just add it into this current PR. Should have more time to work on this tonight I think. Thanks!

[dimfeld]

Turned out to be a small change to get that working. Thanks again for your help here!

[dummdidumm]

Thank you!