[ltungv]

Oh right. I read about this once or twice and have never looked at it again. I've made the (wrong) assumption that this function needs some compile-time help. Is it all done at runtime? Or does that depend on the actual C implementation?

[bombela]

Stack allocation is always done at runtime. Baring special optimizations, the CPU and the code work together to (stack grows downward) decrement the CPU stack pointer register. This register keeps track of where the bottom of the stack is.

What is done at compile time by the compiler, is calculating the amount to decrement the CPU stack pointer register at runtime for every stack variables.

The compiler and alloca must work together, because optimization can easily break something here. For example, when a function return, the stack pointer register better be restored properly. Furthermore, in Rust you must not forget to call Drop::drop() for all types that have such a destructor.