External types

External types are types implemented by UniFFI but outside of this UDL file.

They are similar to, but different from custom types which wrap UniFFI primitive types.

But like custom types, external types are all declared using a typedef with attributes giving more detail.

Types in another crate

There's a whole page about that!

Types from procmacros in this crate.

If your crate has types defined via #[uniffi::export] etc you can make them available to the UDL file in your own crate via a typedef with a [Rust=] attribute. Eg, your Rust might have:

pub struct One {
    inner: i32,

you can use it in your UDL:

typedef extern One;

namespace app {
    // use the procmacro type.
    One get_one(One? one);

Supported values:

  • "enum", "trait", "callback", "trait_with_foreign"
  • For records, either "record" or "dictionary"
  • For objects, either "object" or "interface"