Declaring External Types

Note: The facility described in this document is not yet available for all foreign language bindings.

It is possible to use types defined by UniFFI in an external crate. For example, let's assume that you have an existing crate named demo_crate with the following UDL:

dictionary DemoDict {
  string string_val,
  boolean bool_val,

And further, assume that you have another crate called consuming-crate which would like to use this dictionary. Inside consuming-crate's UDL file you can reference DemoDict by using a typedef with an External attribute, as shown below.

typedef extern DemoDict;

// Now define our own dictionary which references the imported type.
dictionary ConsumingDict {
  DemoDict demo_dict,
  boolean another_bool,

(Note the special type extern used on the typedef. It is not currently enforced that the literal extern is used, but we hope to enforce this later, so please use it!)

Inside consuming-crate's Rust code you must use that struct as normal - for example, consuming-crate's might look like:

fn main() {
use demo_crate::DemoDict;

pub struct ConsumingDict {
    demo_dict: DemoDict,
    another_bool: bool,

include!(concat!(env!("OUT_DIR"), "/"));

Your Cargo.toml must reference the external crate as normal.

The External attribute can be specified on dictionaries, enums and errors.