Byte buffers (&[u8])

A function argument typed &[u8] (or [ByRef] bytes in UDL) lets Rust borrow a foreign-owned byte buffer for the duration of the FFI call — no copy is made. This is useful for large payloads (images, network frames, audio buffers) that would otherwise be duplicated on every call.

For owned bytes, or for any case the constraints below disallow, use Vec<u8> / bytes instead.

Usage

In Rust, declare the parameter as a borrow in the usual way:

#[uniffi::export]
pub fn parse_frame(data: &[u8]) -> Frame {
    // `data` borrows the foreign buffer; valid for this call only.
    ...
}

In UDL, mark the parameter [ByRef] bytes:

namespace example {
    Frame parse_frame([ByRef] bytes data);
};

Constraints

• Direction. &[u8] only flows foreign → Rust. A function may take it as an argument; it cannot be returned, used in a callback result, or used as a trait-method return value.
• Argument position only. &[u8] cannot be nested inside a record, sequence, map, Option, or Result. The borrow lives only as long as the FFI call, so structures that could outlive the call are rejected.
• Lifetime. The slice is valid only inside the body of the Rust function. Don't store it, don't hand it to a spawned task, and don't capture it in async work that outlives the call. If you need the data afterwards, copy it (data.to_vec()).

Foreign-language types

Each binding maps [ByRef] bytes to a natural zero-copy type:

Binding	Foreign type	Note
Kotlin	java.nio.ByteBuffer	Must be a direct buffer (ByteBuffer.allocateDirect(...)).
Swift	Data	The call runs inside Data.withUnsafeBytes.
Python	bytes	The bytes object's stable internal pointer is used.

Passing a non-direct ByteBuffer from Kotlin raises an IllegalArgumentException at the call site, matching JNA's getDirectBufferPointer requirement.