C SwiftPM Target Fails, 'missing target.o' File

Using Swift
,
1

I'm trying to wrap a C library in a Swift package, and have come across an error I've never seen before and can't seem to resolve. I'm trying to wrap the xxHash library, and I have a compiling library with a Swift target wrapping the C API in some nice syntax. It compiles regularly, but when I go to try and run tests on it or include it in another package, I get the following error:

Build input file cannot be found: '/Users/.../Build/Products/Debug/CxxHash.o'. Did you forget to declare this file as an output of a script phase or custom build rule which produces it?

My project structure is set up like so:

import PackageDescription

let package = Package(
    name: "xxHash",
    products: [
        .library(
            name: "xxHash",
            targets: ["xxHash", "CxxHash"]
        ),
    ],
    targets: [
        .target(
            name: "xxHash",
            dependencies: ["CxxHash"]
        ),
        .target(
            name: "CxxHash"
        ),
        .testTarget(
            name: "xxHashTests",
            dependencies: ["xxHash"]
        ),
    ]
)

Package.swift
Sources/
  CxxHash/include/
    xxhash.h
    module.modulemap
    xxhash_swift.h
  xxHash/
    xxHash.swift
Tests/xxHashTests
  xxHashTests.swift

My modulemap:

module CxxHash {
    umbrella header "xxhash_swift.h"
    export *
}

And the xxhash_swift.h is just setting some compile flags and including the xxhash.h header:

#ifndef __xxhash_swift_h__
#define __xxhash_swift_h__

#define XXH_INLINE_ALL
#define XXH_PRIVATE_API
#define XXH_STATIC_LINKING_ONLY

#include "xxhash.h"

#endif /* __xxhash_swift_h__ */

Really not sure where to go from here, I've uploaded the project to GitHub if anyone wants to take a look and maybe spot something obvious I'm missing!

2

Ah I might have posted a little early, swapping .target with .systemLibrary made it compile. Not entirely sure what the difference is there.