Handling Enums with Data Variants

Rust enums are often more complex than Java enums since they can carry data. Java needs to map Rust enums to a compatible structure, identifying active variants to avoid misinterpreting memory.

Example: Enum with Multiple Variants

Rust Side:

#![allow(unused)]
fn main() {
#[repr(C)]
pub enum Status {
    Ok(i32),
    Error(String),
}

#[no_mangle]
pub extern "C" fn get_status() -> Status {
    Status::Ok(200)
}
}

Java Side Solution: To handle this enum, Java needs to use a layout that supports both an enum tag (discriminator) and the associated data.

StructLayout statusLayout = MemoryLayout.structLayout(
    ValueLayout.JAVA_INT.withName("tag"),     // Enum discriminator
    ValueLayout.JAVA_INT.withName("value")    // Holds Ok value or error pointer
);

VarHandle tagHandle = statusLayout.varHandle(PathElement.groupElement("tag"));
VarHandle valueHandle = statusLayout.varHandle(PathElement.groupElement("value"));

MemorySegment statusSegment = arena.allocate(statusLayout);
int tag = (int) tagHandle.get(statusSegment);

if (tag == 0) {  // Ok variant
    int okValue = (int) valueHandle.get(statusSegment);
    System.out.println("Status OK: " + okValue);
} else {  // Error variant
    // Process error value appropriately
    System.out.println("Status Error");
}

Explanation and Solution:

Discriminator and Value Fields: tag differentiates between Ok and Error, while value holds associated data. By reading tag, Java can branch to handle each case correctly.

Memory Layout Compatibility: Using a StructLayout with specific VarHandles ensures memory alignment and prevents misinterpretation of data.

Why It’s Tricky:

Enums in Rust can carry various data types for each variant, which Java enums don’t support. The solution requires careful layout management and handling each variant’s data accordingly.