fix: prevent memory leaking signals in legacy mode

[7nik]

Closes #16040

Alternative to #16141

Looks like there is no need for tracking these signals, they just need to use safe_equals equality check.

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Playground

pnpm add https://pkg.pr.new/svelte@16145

[Rich-Harris]

nice fix

[paoloricciuti]

Wait I was sure I left a comment here but apparently I did not: if your item is an object and you mutate it before update needs to be invoked...this currently doesn't work so dunno if we want to make it work but technically it works in svelte 4

[Rich-Harris]

do you have a repro?

[paoloricciuti]

Yup: this works and invoke before update in svelte 4 but doesn't work in svelte 5 legacy mode (and it will definitely not invoke before after this change.

Same for an each block where the item is an object

[7nik]

I've looked in that case, and think there, like in #each in case of mutation, should be invoked invalidate_inner_signals. But using it requests a bunch of metadata that the #await block currently lacks.

[paoloricciuti]

Tbf I don't think anyone is relying on this behavior so we could just decide this is fine

[Rich-Harris]

Yeah, there'd be no harm in fixing it but since this PR doesn't introduce a new bug and since it's likely that no-one relies on the Svelte 4 behaviour I'm inclined to ignore it