"unknown file type lib.rmeta" when compiling Emscripten side module

[hoodmane]

When compiling an Emscripten side module as follows:

export RUSTFLAGS="-C link-arg=-s -C link-arg=SIDE_MODULE=1 -C relocation-model=pic -C target-feature=+mutable-globals"
cargo +nightly -Z build-std build --target=wasm32-unknown-emscripten

I get the error

wasm-ld: error: unknown file type: lib.rmeta

This error occurs so long as emcc is invoked to link at least one .rlib file. I am using

rustc 1.59.0-nightly (f1ce0e6a0 2022-01-05)

I have tested that the error occurs with both emscripten v3.1.0 and emscripten v2.0.16. I have had success with patching the emscripten linker to run ar -d some_library.rlib lib.rmeta to fix this.

I am compiling lib.rs which looks as follows:

lib.rs

#![no_std]

use core::panic::PanicInfo;

#[no_mangle]
pub extern "C" fn main() -> i32 {
    0	
}

#[panic_handler]
fn panic(_info: &PanicInfo) -> ! {
    loop {}
}

and my Cargo.toml is:

Cargo.toml

[package]
name = "rust_test"
version = "0.1.0"
edition = "2021"

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

[profile.dev]
panic = 'abort'

[bjorn3]

The linker should skip lib.rmeta as it isn't an object file. Normally this happens automatically. I don't know why this doesn't happen here.

[MolotovCherry]

Also happens when compiling with MAIN_MODULE=1

[hoodmane]

As a workaround, I patch Emscripten as follows:

https://github.com/pyodide/pyodide/blob/cfafcda3f6dde8797266ac02c65e51c4614090a9/emsdk/patches/0001-fix-up-rust-linker-errors.patch

[MolotovCherry]

Thanks for posting the patch! I'll check it out asap

[hoodmane]

As an update, the solution to this is to use -sSIDE_MODULE=2. If you also want everything to be exported, you could try -sSIDE_MODULE=2 -sEXPORT_ALL.