I'm working with FFI, some native structures may have new fields added with updated version. I don't want to define a new struct for each version but want to use derive/macro to handle this automatically.

For example, I can create a new "Verisioned" derive with "version" attribute for a struct to describe the fields and their version requirments:

#[derive(Versioned)]
#[repr(C)]
pub struct InnerStruct {
    pub value1: u32,
    #[version(added = 2)] 
    pub value2: u32,
    #[version(added = 3, removed = 6)]
    pub value3: String,
}

I want to return an Option::None if the version is not matched with "version" attribute, otherwise, return the value type. The problem for this is, the struct itself doesn't have the required "version" field, because the original struct doesn't have version at all. This "version" field is specified by a "outer" struct.

pub struct OuterStruct {
    pub version: u32,
    pub inner: InnerStruct,
    pub other: u32,
}

I want to reexport fields of InnerStruct basically for OuterStruct. For example,

impl OuterStruct {

    pub fn value2(&self) -> Option<u32> {
        if self.version >= 2 {
            Some(self.inner.value2)
        } else {
            None
    }
}

As you can see, the implementation for the value2 function is quite trivial, though it needs access the version attribute to know what version is required for the field. I want to see if derive/macro can be used to generate these automatically, so I can use these functions directly?

AFAIK, proc-macro can only access the struct it applies to, so the inner struct with "Versioned" derive can't access the outer struct (it doesn't even know there is one exist or not though), the outer struct can't access inner struct either with macro expansion. Am I understanding this right? Is there any way to solve this problem?

Edit: Added more details.

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