Error when using Shuffle in a wasm project

[will-sloan]

When I try to shuffle a vec, it causes an error when passing it to JS. Without using rand, the problem goes away.
The error:
Error importing index.js: RuntimeError: unreachable

To reproduce the error:
Cargo.toml

[dependencies]
rand = "0.7.3"
serde = "1.0.114"
serde_derive = "1.0.114"

[dependencies.wasm-bindgen]
wasm-bindgen = "0.2.63"
features = ["serde-serialize"]

lib.rs

mod utils;

use rand::seq::SliceRandom;
use rand::thread_rng;
use serde::de::{self, Deserialize, Deserializer, MapAccess,, Visitor};
use serde::ser::{Serialize, SerializeStruct, Serializer};
use std::fmt;
use wasm_bindgen::prelude::*;
use wasm_bindgen::JsValue;

#[macro_use]
extern crate serde_derive;

struct Person {
    name: String,
    age: u8,
    phones: Vec<String>,
}

// Impl serialize for struct
impl Serialize for Person {
    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
    where
        S: Serializer,
    {
        let mut s = serializer.serialize_struct("Person", 3)?;
        s.serialize_field("name", &self.name)?;
        s.serialize_field("age", &self.age)?;
        s.serialize_field("phones", &self.phones)?;
        s.end()
    }
}
#[wasm_bindgen]
pub fn pass_value_to_js() -> JsValue {
    let mut p = Person {
        name: String::from("Hello"),
        age: 13,
        phones: vec![
            String::from("phone"),
            String::from("AAAAA"),
            String::from("Guacamole"),
        ],
    };
    p.phones.shuffle(&mut thread_rng()); // this causes the error
    JsValue::from_serde(&p).unwrap()
}

Without the line p.phones.shuffle(&mut thread_rng());, the program works.
Is this the right place for this kind of error?

[bjorn3]

This is probably getrandom returning an error, which rand then turns into a panic. Because you don't have a panic handler registered the panic turns into a silent abort. Depending on getrandom and enabling the wasm-bindgen feature for it should fix this. See https://docs.rs/getrandom/0.1.14/getrandom/#support-for-webassembly-and-asmjs for more info.

[will-sloan]

That solved it. Thanks.

[vks]

While this is more or less "intended behavior", I think our documentation for WASM could be improved though:

• In the Rand book, we claim that getrandom does not use feature flags, which is confusing (and arguably wrong).
• We don't mention WASM in the rand readme or documentation. I think we should at least refer to consulting getrandom's documentation.