enum case containing Box<T> is not lowered to a pointer

[morrisonlevi]

The example below generates two functions. The Box<u32> in make_option's return type is correctly lowered to uint32_t * but for the EntryResult in make_result the codegen generates an opaque type that it uses incorrectly:

#[repr(C)]
pub enum EntryResult {
    Ok(Box<u32>),
    Err(u32),
}

#[no_mangle]
pub extern "C" fn make_option() -> Option<Box<u32>> {
    None
}

#[no_mangle]
pub extern "C" fn make_result() -> EntryResult {
    EntryResult::Err(0)
}

Generated C code (though I've added comments):

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

typedef struct Box_u32 Box_u32; // uh-oh

typedef enum EntryResult_Tag {
  Ok,
  Err,
} EntryResult_Tag;

typedef struct EntryResult {
  EntryResult_Tag tag;
  union {
    struct {
      // this is an incomplete type; it cannot be used here. Expected uint32 *.
      struct Box_u32 ok;
    };
    struct {
      uint32_t err;
    };
  };
} EntryResult;

uint32_t *make_option(void);

struct EntryResult make_result(void);

My actual use case is pretty close to what's shown here: I want to return a Result type and since Result isn't repr(C) I roll my own. I've done this with other types that are repr(C) before, but this is the first time I've wanted to use a Box<T> and was surprised at the result (ha).