Function lean_object_data_byte_size 

pub unsafe extern "C" fn lean_object_data_byte_size(
    o: *mut lean_object,
) -> usize

Returns the size of the salient part of an object’s storage, i.e. the parts that contribute to the value representation; padding or unused capacity is excluded. Operations that read from an object’s storage must only access these parts, since the non-salient parts may not be initialized.