Pay for what you use: varadic functions

[0xekez]

Varadic functions require that we heap allocate function arguments. This is slow. Most existing Lust programs do not use varadic functions - in that situation we don't need to heap allocate arguments.

Trouble with doing this is that it adds a lot of code complexity (two implementations of function calls) and once there is a standard library which will use varadic functions most Lust programs are liable to use varadic functions.