fix: vx_struct_fields_field_dtype takes usize instead of u64

Signed-off-by: Robert Kruszewski <[email protected]>

Author: robert3005

vortex-ffi/cinclude/vortex.h

@@ -805,7 +805,7 @@ const vx_string *vx_struct_fields_field_name(const vx_struct_fields *dtype, size
  *
  * Returns null if the index is out of bounds or if the field dtype cannot be parsed.
  */
-const vx_dtype *vx_struct_fields_field_dtype(const vx_struct_fields *dtype, uint64_t idx);
+const vx_dtype *vx_struct_fields_field_dtype(const vx_struct_fields *dtype, size_t idx);
 
 /**
  * Free an owned [`vx_struct_fields_builder`] object.

vortex-ffi/src/struct_fields.rs

@@ -56,7 +56,7 @@ pub unsafe extern "C-unwind" fn vx_struct_fields_field_name(
 #[unsafe(no_mangle)]
 pub unsafe extern "C-unwind" fn vx_struct_fields_field_dtype(
     dtype: *const vx_struct_fields,
-    idx: u64,
+    idx: usize,
 ) -> *const vx_dtype {
     let ptr = unsafe { dtype.as_ref() }.vortex_expect("null ptr");
     let struct_dtype = &ptr.0;