> C has no choice but to have MyFunction allocate a couple kilobytes on the stack and have main copy the structure from the stack to where it should eventually go. The equivalent Rust code, however, can pass the address of the object x to MyFunction, as if it were actually void MyFunction(bigstruct &output).

What sort of C implementation (ABI, compiler, etc.) are you thinking of here? gcc (x86, x86-64, ARM) is perfectly capable of doing the exact optimization that you describe Rust being able to do.