So one important question about Rust that is relevant here is how it handles dynamic stack allocation. It is in the language? C++ has alloca of course but needs a couple of macros to work across compilers. Memory allocation ends up being a very big performance concern in renderers. PBRT uses both memory arenas and an ALLOCA macro.
User-defined placement new and custom allocators for existing data structures are still being worked on. `box` can do a form of placement new already, and while you can use your own allocator, you can't use it with already-existing types.
That is a valid and useful technique, but being able to dynamically allocate stack memory in the first place is separate from using placement new on stack memory.
Of course, I was just pointing out it's not always necessary to use alloca - I've written a production-level path-tracer without needing to allocate stack memory dynamically.
