Unreal support : Error LNK2019 : unresolved external symbol ****$cxxbridge1$func

[tarnishablec]

I was testing integrating cxx to Unreal Engine custom plugin to call a simple rust function from c++.

this is my file structure

this is src/lib.rs

#[cxx::bridge(namespace = "dwebble")]
pub mod ffi {
    extern "Rust" {
        pub fn test() -> u8;
    }
}

pub fn test() -> u8 {
    return 22;
}

build.rs

extern crate cxx_build;

use std::fs;


fn main() {
    let out_lib_dir = "Bindings";
    fs::remove_dir_all(out_lib_dir).unwrap();

    cxx_build::bridge("src/lib.rs")
        .out_dir(out_lib_dir)
        .compile("dwebble");
}

unreal build.cs

public class Dwebble : ModuleRules
{
	public Dwebble(ReadOnlyTargetRules Target) : base(Target)
	{
		PCHUsage = PCHUsageMode.UseExplicitOrSharedPCHs;
		Type = ModuleType.CPlusPlus;

		PublicAdditionalLibraries.AddRange(
		[
			Path.Combine(PluginDirectory, @"Bindings\dwebble.lib"),
		]);

		// PublicPreBuildLibraries.Add(Path.Combine(PluginDirectory, @"Bindings\dwebble.lib"));


		PublicIncludePaths.AddRange(
			new string[]
			{
				// ... add public include paths required here ...
				Path.Combine(PluginDirectory, @"target\cxxbridge")
			}
		);
...

c++ code in unreal

#include "DwebbleSubsystem.h"
#include "dwebble/src/lib.rs.h"

void UDwebbleSubsystem::Initialize(FSubsystemCollectionBase& Collection)
{
    Super::Initialize(Collection);

    UE_LOG(LogTemp, Warning, TEXT("UDwebbleSubsystem::Initialize with %d"), dwebble::test());
}

when building in unreal a compiler error prompted

11>dwebble.lib(08496bdb74dbd239-lib.rs.o): Error LNK2019 : unresolved external symbol dwebble$cxxbridge1$test referenced in function "unsigned char __cdecl dwebble::test(void)" (?test@dwebble@@YAEXZ)
11>UnrealEditor-Dwebble-Win64-DebugGame.dll: Error LNK1120 : 1 unresolved externals

What am i missing to do?
using msvc 14.44.

[msundvick]

We've been building a DLL for use with Unreal, and this is certainly a bit of a PITA, looks like the same issue.

This issue appears related to #1153. There are some workarounds in there, but for us they didn't work very well, with a mess of conflicting symbols coming from our dependencies (although for your basic case, they might be fine, our dependencies aren't trivial). There's probably an elegant solution, but I couldn't find one. Some other issues that seem potentially relevant:

• Re-exporting C symbols for cdylib rust-lang/rfcs#2771
• Linking a static C library does not include all symbols rust-lang/rust#78827

Instead of an elegant solution, what I've currently got is a somewhat horrific hack:

• Output a staticlib from rust. This will generally include all the symbols without issues.
• Build a dll wrapper around this staticlib using a super barebones c++ file. We then manually configure the re-exported symbols using https://learn.microsoft.com/en-us/cpp/build/reference/def-specify-module-definition-file?view=msvc-170 (for example, in you're example you would have a single line file with ?test@dwebble@@YAEXZ I think

Not pretty, but is does work. Would love to hear if anyone's got other workarounds.