`*arg0` does not live long enough - wasm_bindgen

Question

When running the following code:

Cargo.toml

[lib]
crate-type = ["cdylib"]

[dependencies]
serde = { version = "1.0", features = ["derive"] }
wasm-bindgen = {version = "0.2.67", features = ["serde-serialize"] }
wasm-bindgen-futures = "0.4.17"

lib.rs

use serde::{Deserialize, Serialize};
use wasm_bindgen::prelude::*;

#[derive(Serialize, Deserialize)]
struct Output {
    strings: Vec<String>,
}

#[wasm_bindgen] //error occuring here
pub async fn return_strings(_input: &str) -> JsValue {
    
    //example function that returns a js value
    let strings: Vec<String> = Default::default();
    let output = Output { strings };
    
    JsValue::from_serde(&output).unwrap()
}

I get the following error:

*arg0 does not live long enough
borrowed value does not live long enoughargument requires that *arg0 is borrowed for 'static

If anyone could let me know why, I would be a huge help.

matthew-mcallister matthew-mcallister · Accepted Answer · 2020-08-30T08:08:14

The error message is caused when Rust assumes that the future returned by return_strings has the same lifetime as _input even though it doesn't actually borrow from it. Basically, the de-sugared function signature looks like:

pub fn return_strings<'a>(_input: &'a str) -> impl Future<Output = JsValue> + 'a;

The generated code wants a future with 'static lifetime, but the returned future actually winds up with the lifetime of a temporary string variable.

The developers are aware of this limitation. Probably the easiest solution is to take an owned String or Box<str> argument instead of a borrowed &str.

`*arg0` does not live long enough - wasm_bindgen

1 Answers