cpp.cpp

struct X {
    int x[100];
};

X hi() {
    return {0};
}

cpp.s

hi():
  mov rdx, rdi
  mov QWORD PTR [rdi], 0
  lea rdi, [rdi+8]
  xor eax, eax
  mov QWORD PTR [rdi+384], 0
  mov rcx, rdx
  and rdi, -8
  sub rcx, rdi
  add ecx, 400
  shr ecx, 3
  rep stosq
  mov rax, rdx
  ret

rust.rs

pub fn hi() -> [i32; 100] {
    [0; 100]
}

rust.s

example::hi:
        push    rbx
        mov     rbx, rdi
        xor     esi, esi
        mov     edx, 400
        call    memset@PLT
        mov     rax, rbx
        pop     rbx
        ret

C++ strictly follows the calling convention when returning huge objects
not the case for Rust
where the function is transformed into something likevoid hi(int *p);