'invalid SlotMap key used' when removing an item with nested reactivity from a list that has cleanup

[lukechu10]

Reproducible example:

use sycamore::prelude::*;

fn main() {
    let _ = create_root(move || {
        let items = create_signal(vec![create_signal(()), create_signal(())]);

        map_indexed(items, |x| on_cleanup(move || x.dispose()));
        items.set(items.get_clone()[1..].to_vec());
    });
}

This is a pretty critical error since this is a common pattern for managing nested reactivity.

The error that appears is:

 panicked at /home/_/src/sycamore/packages/sycamore-reactive/src/root.rs:298:36:
invalid SlotMap key used

[lukechu10]

This turns out to actually be a super subtle bug where we accidentally track the nested signal inside of map_indexed/map_keyed because we perform a equality check between items which counts as a reactive access (i.e. the PartialEq implementation of Signal is reactive). This then tracks the nested signal as a dependency of the mapped memo. However, if the item is removed, it is disposed because of the on_cleanup and so we run into #741.

The solution would be to make sure that the update function of the memo only depends on the input list and nothing else.