panic on trying to convert ArrayBuffer -> Uint8Array -> Vec<u8>

[Ivshti]

I'm using the copy_to method introduced in #1147 to try to convert a result from a fetch into an owned Vec<u8>

tihs is the full code:

let window = web_sys::window().unwrap();
let pr = window.fetch_with_str("https://v3-cinemeta.strem.io/catalog/movie/top.json");
let fut = JsFuture::from(pr)
    .and_then(|resp_value| {
	// @TODO: reduce number of copies
	assert!(resp_value.is_instance_of::<Response>());
	let resp: Response = resp_value.dyn_into().unwrap();
	let buf_promise = resp.array_buffer().unwrap();
	JsFuture::from(buf_promise).map(|buf_val| {
	    assert!(buf_val.is_instance_of::<ArrayBuffer>());
	    let typebuf: js_sys::Uint8Array = js_sys::Uint8Array::new(&buf_val);
	    let mut body: Vec<u8> = Vec::with_capacity(typebuf.length() as usize);
	    typebuf.copy_to(&mut body);
	    body
	})
    })

And this is the result:

wasm-0000006e-3320:2 Uncaught (in promise) RuntimeError: unreachable
    at __rust_start_panic (wasm-function[3320]:1)
    at rust_panic (wasm-function[3319]:31)
    at std::panicking::rust_panic_with_hook::h6143ca5bdd7761ea (wasm-function[3314]:305)
    at std::panicking::continue_panic_fmt::h7ca9995d457595c0 (wasm-function[3313]:116)
    at std::panicking::begin_panic_fmt::h0539ff1194a858ad (wasm-function[3298]:95)
    at js_sys::Uint8Array::copy_to::hf6b245de5260e8be (wasm-function[1831]:563)

What am I missing?

There's a bit of code, given as an example, that seems to do the same here: #1160

[Ivshti]

However, this works:

let mut body = vec![0; typebuf.length() as usize];
typebuf.copy_to(&mut body[..]);

I guess the memory needs to be allocated, which makes sense.

However, the copy_to behavior does seem unsafe

[alexcrichton]

Ah yes indeed! As the documentation mentions, the two arrays must have the same length and in the first example the target array has length 0 (with_capacity doesn't allocate any space).

I'd also recommend console_error_panic_hook as that should make the error much more debuggable too!