Expose Rust Global Allocator to LVGL C code

[rafaelcaricio]

We could enable the LVGL side to use Rust memory allocator, if it's available and the feature "alloc" enabled in lvgl-rs. That way we wouldn't need to use our custom Box implementation for that case. We would still need this implementation for cases where all features in lvgl-rs are disabled and we do not have a Box abstraction to handover memory to LVGL C.

Some code example of how this could work is available at: https://github.com/ezrosent/allocators-rs/tree/master/malloc-bind We could get inspiration from the malloc-bind project and implement a similar solution to be used in embedded devices.

The problem we want to solve is that LVGL C always support "dynamically" allocated memory backed by a static array-based backend, just like wee_alloc. LVGL-rs is a library and the choice of allocator must be made by the firmware/application developer. Besides that, the developer might decide that they don't want to have an allocator at all on their Rust firmware. LVGL C will still use their own memory management implementation.

One option is to always require users of LVGL-rs to define a global allocator in their project. This would be a limitation for some use cases?! In that scenario, we wouldn't need the code in mem module at all, since the memory space would be shared by LVGL C and Rust.

[rafaelcaricio]

On this topic, this is a very interesting option to consider using in LVGL-rs and recommending to users:

• https://github.com/vcombey/fallible_collections

This allows to handle allocation errors when using collections.

[rafaelcaricio]

There is an ongoing support in Rust for explicit allocator usage, this could help support the use of the LVGL internal allocator through lvgl-rs.

Example API: https://doc.rust-lang.org/alloc/boxed/struct.Box.html#method.new_in