pub struct DrawBuffer<const N: usize>

[fulup-bzh]

Forcing this to a constant prevent from having simulator screen size store in a config object. Is there any special reason to enforce const here ?

pub struct DrawBuffer<const N: usize>

[nia-e]

Unfortunately I'm unsure if that can practically be removed; it would require having an allocator at runtime which we can only guarantee with the alloc feature (and duplicating code would be a massive pain). I've considered implementing a basic allocator based on LVGL's as a nonglobal fallback, but it would be a huge source of bugs I worry. I'll keep this open for the future

[fulup-bzh]

I'm not sure to understand, this is an integer and not a pointer it is copies once also the CONTS does not provide any functionnality

[nia-e]

The const N is used to determine the size of the allocation for the buffer; we store a Pin<Box<[MaybeUninit<lvgl_sys::lv_color_t>; N]>> inside of the actual DrawBuffer struct (note that this is not the same Box as in the rust stdlib; we use our own implementation, see mem.rs). Because the innermost value is a sized slice [T; N] we can't actually allocate it at runtime; we'd need a Vec<T> impl or similar, which is considerably more complex than just our boutique Box<T>. Arguably this is a Rust limitation, but short of doing some extremely unsafe things I don't see how to get around it short of said custom Vec implementation

[fulup-bzh]

I understand your point, it is a before everything a Rust limitation. Where you cannot force a specific element of a structure to be constance. If is was a string, we could use &'static str for integer it does not exist.

The point I do not get is that for me they is not difference in between the two following options

• DrawBuffer (255*128)
• sz= config.h*config.v; DrawBuffer(sz)

[nia-e]

In the latter case, you're getting the values at runtime (since the compiler doesn't know the value of config.h or of config.v) so the actual size of the final DrawBuffer isn't known by the compiler. You could work around this with a build script that sets some constants, or by using const fns to calculate the size (if the config-reading crate you use supports it).

[fulup-bzh]

I understand your point. I will try to move around by checking if a const fn thank you for your very reactive support.