FFI guaranties between pointers and references

fn test1(arg: *const u8) {}
fn test2(arg: Option<&u8>) {}
fn test3(arg: &u8) (}

Assuming arg is a valid pointer, are test1 and test2 equivalent? Also, assuming arg can’t be null, are they equivalent to test3? mem::size_of tells me they have the same size, but is it a guaranty of the language?
Same thing with structs:

struct S1 {
    a: *const u8,
struct S2 {
    a: Option<&u8>,
struct S3 {
    a: &u8,

Nullable Pointers in FFI

More background: I’m doing an implementation of libvulkan.so. The error checking in Vulkan is very minimal, the driver don’t have to validate every piece of input. During development, you can active middlewares to verify that the application effectively uses correctly the API. For example, in vkCreateBuffer, the Valid Usage section dictates that pBuffer must be a pointer to a valid instance of VkBuffer. For convenience, I would like pBuffer to be of type &mut VkBuffer instead of *const VkBuffer. As I can have C code calling my code, I can do that if they are equivalent.