GuiAction with a well-defined initial state

[m4c0]

I was watching your video and I noticed you want to avoid uninitialised state in GuiAction. You had to use MaybeUninit in Rust for Rust reasons, but that introduces an UB in your code.

I think the trick you wanted is an empty struct as the default enum state in GuiAction. It is as simple as struct {} none;, but I added as an explicit struct because it is easier to remember and explicitly informs your intent (it is also nicer if you use an automatic code formatting).

The empty struct should be fine in both sides because empty structs are actually modelled as a single byte in memory. It is an old C standard trick to make that memory byte's address as an identity for an empty struct.

Since it is on top of the enum, it will be the default state - therefore making any value in memory as a valid and well-defined union state.

Disclaimer: I don't know Zig or Rust but I did my best to follow the how the GuiAction is used in both sides.

Zig seems fine because it doesn't use the value of the union when the action is "none". So I left it as-is.

Rust might not need the MaybeUninit trick because the empty struct should make it a valid enum. So I think it is enough to initialise the tag in make_action.

Unfortunately, my Windows machine has VIM and Clang, but no Rust - so I could not test this change. But I checked if gui.h compiles via clang -x c. :)