Introduce DiplomatChar

[robertbastian]

Fixes #318

[sffc]

Why did you do type DiplomatChar = u32 instead of struct DiplomatChar(pub u32) ?

[robertbastian]

This type goes on the ABI, and struct DiplomatChar(pub u32) is not generally ABI compatible with u32

[sffc]

Ok. It just struck me as slightly weird because type means that the things are 100% equivalent and can be swapped out at will, but in this case DiplomatChar actually has different semantics than char because we read it in a proc macro in a layer above Rust code compilation.

[robertbastian]

The typedef is basically a signal to the macro to treat this as a char (i.e. generate uchar32_t instead of uint32_t in C), and a signal to the Rust implementation that this is meant to be a char.

I could have actually done pub struct DiplomatChar(pub u32) and have the macro wrap and unwrap, but:

• that's extra complexity in the macro
• it's nice that Rust code can just pass this type to ...32 methods without having .0 everywhere
• it's nice that Rust code can use primitive casts with it, i.e. a suffixed as DiplomatChar instead of DiplomatChar( at the beginning of the expression

[sffc]

Okay on point 1. Neutral on point 2. On point 3, I don't generally like as casts in Rust because they hide away footguns like converting signed to unsigned and converting larger ints to smaller ints. I find DiplomatChar( less error-prone (and it is even a few characters shorter).