Struct Desc 

#[repr(C)]
pub struct Desc {
    pub ptr: usize,
    pub len: usize,
}

How a guest hands a (pointer, length) pair back to the host.

The obvious alternative is packing both into the single i64 a wasm export can return. That works only while pointers are 32 bits. Returning a pointer to this struct instead costs one extra memory read per call and keeps every export signature unchanged under a 64-bit guest, where usize simply widens.

Do not replace this with bit-packing; that is precisely what it exists to avoid. The host reads exactly 2 * size_of::<usize>() bytes at the returned address and splits them down the middle, so this layout is load-bearing — hence #[repr(C)], and hence the tests pinning its size and field order.

Fields

ptr: usize

Address of the payload.

len: usize

Payload length, in bytes.