Error in compiling Rust into webassembly using emscripten on Windows

Question

I just tried to compile Rust example project into webassembly using emscripten on Windows, but it causes error. How can I fix it?

What I did:

1. Install emscripten

>git clone https://github.com/emscripten-core/emsdk.git
>cd emsdk
>git pull
>emsdk install latest
>emsdk activate latest
>emsdk_env.bat

2. Install rustup Download rustup-init.exe from https://www.rust-lang.org/ja/tools/install and run it
3. Install wasm32 target by rustup

>rustup target add wasm32-unknown-emscripten

4. Create example project

>cargo new --bin web_assembly
>cd web_assembly

5. Try to compile, but error occurs

>cargo build --target wasm32-unknown-emscripten
   Compiling web_assembly v0.1.0 (F:\github\rusttest\web_assembly)
error: linking with `emcc.bat` failed: exit code: 1
  |
  = note: "cmd" "/c" "emcc.bat" "-s" "DISABLE_EXCEPTION_CATCHING=0" "-L" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib" "-L" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\self-contained" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.0.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.1.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.2.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.3.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.4.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.5.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.6.rcgu.o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.web_assembly.egwz9uoo-cgu.7.rcgu.o" "-o" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.js" "-s" "EXPORTED_FUNCTIONS=[\"_main\",\"_rust_eh_personality\"]" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps\\web_assembly.3a2jxscq1lvbzwaf.rcgu.o" "-O0" "--memory-init-file" "0" "-g4" "-s" "DEFAULT_LIBRARY_FUNCS_TO_INCLUDE=[]" "-L" "F:\\github\\rusttest\\web_assembly\\target\\wasm32-unknown-emscripten\\debug\\deps" "-L" "F:\\github\\rusttest\\web_assembly\\target\\debug\\deps" "-L" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libstd-28368703ab79076a.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libpanic_unwind-c6cbccdff18b55f7.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\librustc_demangle-a5ffc5310c14c91c.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libhashbrown-08c349e57dac68c0.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\librustc_std_workspace_alloc-ce3363c7b27912b6.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libunwind-686deac84d1c117a.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libcfg_if-1aefc0615f4a87c4.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\liblibc-35a51890c8428321.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\liballoc-d41456f5d5a426f6.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\librustc_std_workspace_core-2a7cc4e4deb2c4e8.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libcore-997b7450818c8186.rlib" "C:\\Users\\Username\\.rustup\\toolchains\\stable-x86_64-pc-windows-msvc\\lib\\rustlib\\wasm32-unknown-emscripten\\lib\\libcompiler_builtins-68f9d21fef2c27f2.rlib" "-l" "c" "-s" "ERROR_ON_UNDEFINED_SYMBOLS=1" "-s" "ASSERTIONS=1" "-s" "ABORTING_MALLOC=0" "-Wl,--fatal-warnings"
  = note: shared:INFO: (Emscripten: Running sanity checks)
          emcc: warning: please replace -g4 with -gsource-map [-Wdeprecated]
          error: undefined symbol: __gxx_personality_v0 (referenced by top-level compiled C/C++ code)
          warning: Link with `-s LLD_REPORT_UNDEFINED` to get more information on undefined symbols
          warning: To disable errors for undefined symbols use `-s ERROR_ON_UNDEFINED_SYMBOLS=0`
          warning: ___gxx_personality_v0 may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library
          Error: Aborting compilation due to previous errors
          emcc: error: 'F:/github/emsdk/node/14.15.5_64bit/bin/node.exe F:\github\emsdk\upstream\emscripten\src\compiler.js C:\Users\Username\AppData\Local\Temp\tmpang8muuo.txt' failed (1)


error: aborting due to previous error

error: could not compile `web_assembly`

To learn more, run the command again with --verbose.

Versions of tools:

>emcc.bat --version
emcc (Emscripten gcc/clang-like replacement + linker emulating GNU ld) 2.0.20-git (8ed0f4fee2abd04923c2fd2bf44835169ce2ac2e)
Copyright (C) 2014 the Emscripten authors (see AUTHORS.txt)
This is free and open source software under the MIT license.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

>rustup --version
rustup 1.24.1 (a01bd6b0d 2021-04-27)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.52.1 (9bc8c42bb 2021-05-09)`

Answer 1

If you want/need emscripten (to use c or c++ based crates or enjoy working dwarf debug) here is how to fix the build error.

The error cause is described here: https://github.com/rust-lang/rust/issues/85821

Workaround is to:

Create stub for the missing function (save as file gxx_personality_v0_stub.cpp):

#include "unwind.h"
#include "stdint.h"

extern "C" _Unwind_Reason_Code __gxx_personality_v0   (int version, _Unwind_Action actions, uint64_t exceptionClass, _Unwind_Exception* unwind_exception, _Unwind_Context* context) {
    return  _URC_NO_REASON;
}

Build using emcc:

emcc -c gxx_personality_v0_stub.cpp

Create/modify .cargo/config file:

[build]
rustflags = ["-C", "link-args=gxx_personality_v0_stub.o"]

Enjoy working latest rust (tested with v1.55.0) and emscripten (v2.0.29)

Note: Stack unwind on panic won't work with this stub. But since the function was stubbed anyway in emscripten you are not missing any functionality.