Why does #[wasm_bindgen] allow &T in synchronous functions but not in async functions?

[cspinetta]

Summary

I'm encountering an issue when using #[wasm_bindgen] with functions that accept references (&T). Here's a simplified example:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub fn sync_function(x: &ExportedNamedStruct) {
    println!("sync_function: {:?}", x);
}

The above code works fine, and I can call sync_function from JavaScript without any issues. However, when I make the function async:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
pub async fn async_function(x: &ExportedNamedStruct) {
    println!("async_function: {:?}", x);
}

I get the following error:

error[E0433]: failed to resolve: could not find `borrow` in `core`
  --> src/lib.rs:20:1
   |
20 | #[wasm_bindgen]
   | ^^^^^^^^^^^^^^^ could not find `borrow` in `core`
   |
   = note: this error originates in the attribute macro `wasm_bindgen` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider importing this module
   |
6  + use std::borrow;
   |

From my understanding, wasm-bindgen supports borrowing (&T) in synchronous functions (as specified here), but this does not seem to extend to async functions. I assume this limitation is due to how Rust manages lifetimes and how WebAssembly ABI works, especially with async functions potentially suspending execution.

Questions:

• Could you provide insights into why wasm-bindgen does not support &T in async functions?
• Are there any best practices or idiomatic patterns for handling this situation?
• Are there plans to support this in the future?

Additional Details

• Rust version: 1.82.0
• wasm-bindgen version: 0.2.95

Example Struct:

use wasm_bindgen::prelude::*;

#[wasm_bindgen]
#[derive(Debug)]
pub struct ExportedNamedStruct {
    pub inner: u32,
}

Thank you for your time and insights!

[daxpedda]

It seems to me that the issue is rather that the proc-macro generates some invalid path to core::borrow. So it probably got fixed by #4005. In any case, this should work and I was unable to produce the same error with the newest version!

Try updating wasm-bindgen, but please let me know if this didn't fix the issue for you!