Avoid copy when the plugin returns

[arnaudgolfouse]

This should avoid copying the data until another function is called.

[arnaudgolfouse]

Some notes:

Once in a lifetime !

Functions now return ReturnedData<'a>, preventing any other function being called while this structure is alive.

Technically the data returned by a function should no longer be accessible from inside the plugin after the return (if the plugin respect the spirit of the protocol), so maybe we could get away with the lifetime if absolutely needed (using a Rc<RefCell>/Arc<Mutex> for example).

Why not return ptr/len directly ?

The call to wasm_minimal_protocol_send_result_to_host seems redundant, does it not ? Well, I tried having plugin functions be something like

pub extern "C" fn hello() -> (*const u8, usize, i32) {
    let message = String::from("Hello world!");
    let (ptr, len, code) = (message.as_ptr(), message.len(), 0);
    std::mem::forget(message);
    (ptr, len, code)
}

But it does not seems quite ready yet... It works on wasm32-unknown-unknown by setting the env variable RUSTFLAGS="-C target-feature=+multivalue", which is already not ideal ; but then this breaks when using wasm32-wasi. See this (opened) issue: rust-lang/rust#73755

[astrale-sharp]

This looks great, thanks a ton, it doesn't seem to be missing anything either, are you okay with me merging it?

<3

[arnaudgolfouse]

Oh wait, rereading this I see I slightly messed up the documentation

[arnaudgolfouse]

Ok, looks good to me !