How then does Rust avoid the if('thingy') issue? Or, for that matter, a language like C, where presumably a and b would be allocated on the stack. In C would they both hang around until the function returns? Or would I have to copy them into the array anyway, which would be heap-allocated?
They're stack allocated in Rust too, so they're both going to stick around. Arrays in Rust are stack allocated, but this code would probably use a vector, which is heap allocated, and so yeah, you'd end up copying them from the stack to the heap.
