Investigate using SlotMap

[kanashimia]

Currently slint uses Rc<RefCell<_>> in lots of places, it causes unnecessary memory fragmentation, and is very error prone, as it essentially moves borrow checking errors to runtime.
In Rust it is typical to use arenas instead of smart pointers for self recursive cases.
SlotMap is basically a generational arena with free list, so it doesn't suffer from memory leaks.
I hope is that it will make code simpler and will help with having less runtime borrow errors.

Example: https://github.com/orlp/slotmap/blob/master/examples/doubly_linked_list.rs

Leptos uses slotmaps for their reactive code, although there are still many Rc<RefCell<_>> there.

The reactive system is implemented as a single data structure that exists at runtime. In exchange for giving ownership over a value to the reactive system (by creating a signal), you receive a Copy + 'static identifier for its location in the reactive system. This enables most of the ergonomics of storing and sharing state, the use of callback closures without lifetime issues, etc. This is implemented by storing signals in a slotmap arena. The signal, memo, and scope types that are exposed to users simply carry around an index into that slotmap.

Their code: https://github.com/leptos-rs/leptos/tree/main/leptos_reactive

[ogoffart]

Which particular use of Rc<RefCell>> would you consider problematic?
I'm not seeing how this would help in our case.

[kanashimia]

Sorry my mention of Rc<RefCell<_>> was dubious, will remove it.
In particular I think it may be helpful for internal/core/model.rs and core/internal/item_tree.rs, to simplify those abstractions.
It also may fix issues like #3215 more apparent and easier to fix, but I'm not sure about it.
You can close this issue if you don't think it can help, I mostly just wanted to mention the possibility.

[ogoffart]

I don't think this would help for #3215
The model really need to be a Rc because it is shared with user's code.
But this issue isn't really actionable as is so i'm going to close it.
Thanks anyway for the suggestion. And feel free to open other issue if you have more concrete suggestions.