#[wasm-bindgen] macro does not work for extern functions not in the root of the crate

[rbalicki2]

Hello. Thank you so much for your help these past two weeks.

So, as of today (perhaps due to upgrading to the latest nightly, I don't know), wasm-bindgen does not seem to work for extern functions not in the root of the crate. I have the following in the root of my crate:

#![feature(proc_macro, wasm_custom_section, wasm_import_module)]

extern crate wasm_bindgen;

use wasm_bindgen::prelude::wasm_bindgen;

#[macro_use]
mod js;

// if the following is uncommented, everything works
// #[wasm_bindgen]
// extern {
//   #[wasm_bindgen]
//   pub fn set_timeout();
// }

and the following in js.rs

use wasm_bindgen::prelude::wasm_bindgen;
#[wasm_bindgen]
extern {
  // #[wasm_bindgen]
  // pub fn set_timeout();
}

If I uncomment that block in the root of the crate, everything compiles, whether or not the pub fn set_timeout is uncommented in js.rs. However, if the block in the root of the crate is commented, and the function in js.rs is uncommented (and only then), do I get the following error:

thread 'main' panicked at 'failed to run export: Function("Module doesn\'t have export __wbindgen_describe___wbg_f_set_timeout_set_timeout_n")', libcore/result.rs:945:5
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
   1: std::sys_common::backtrace::print
   2: std::panicking::default_hook::{{closure}}
   3: std::panicking::default_hook
   4: std::panicking::rust_panic_with_hook
   5: std::panicking::begin_panic_fmt
   6: rust_begin_unwind
   7: core::panicking::panic_fmt
   8: core::result::unwrap_failed
   9: wasm_bindgen_cli_support::Bindgen::_generate::{{closure}}
  10: wasm_bindgen_cli_support::js::Context::describe
  11: wasm_bindgen_cli_support::js::SubContext::generate
  12: wasm_bindgen_cli_support::Bindgen::_generate
  13: wasm_bindgen::main
  14: std::rt::lang_start::{{closure}}
  15: std::panicking::try::do_call
  16: __rust_maybe_catch_panic
  17: std::rt::lang_start_internal
  18: main

I'm running these commands:

cargo +nightly build --target wasm32-unknown-unknown
RUST_BACKTRACE=1 wasm-bindgen ./target/wasm32-unknown-unknown/debug/wasm_website_frontend.wasm --out-dir ./dist

versions, etc:

rustc 1.27.0-nightly (ac3c2288f 2018-04-18)
wasm-bindgen = "0.2.5"
wasm-bindgen --version
wasm-bindgen 0.2.5

Thank you so much!

[rbalicki2]

Additional finding: it doesn't seem to matter whether the function is in another file, per se, only that it's in another module. Anyway, I got my main project by moving the (relatively few) js imports into the root.

[jsheard]

I ran into the same problem, the catch seems to be that #[wasm_bindgen] extern functions need to be public all the way up to the crate root. If you replace mod js with pub mod js then it should work.

[rbalicki2]

Thank you! That fixed it! 🙌

[alexcrichton]

Ah yeah this is an unfortunate limitation of wasm-bindgen right now. This has to do with the weird details of symbol visibility in rustc, but @jsheard's solution is indeed the best option for now. Now long-term I think that's definitely not a great solution and we'll probably want toexplore different options, but I'm not sure what to do differently at this time :(