Using ZMQ on iOS

Question

I am trying to build an iOS app that is exposed to a fairly extensive Rust project, but I am having issues with one of the dependencies in the project. The Rust project has a ZMQ dependency, and while I am able to build the project on my Mac, when I try to incorporate the Rust project I get the following error:

error: failed to run custom build command for `zmq-sys v0.8.3`

Caused by:
  process didn't exit successfully: `/Users/lisa/company/project/project_detail/target/debug/build/zmq-sys-16dfc88dff84d855/build-script-build` (exit status: 101)
  --- stdout
  cargo:rerun-if-env-changed=LIBZMQ_NO_PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS_aarch64-apple-ios
  cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS_aarch64_apple_ios
  cargo:rerun-if-env-changed=TARGET_PKG_CONFIG_ALLOW_CROSS
  cargo:rerun-if-env-changed=PKG_CONFIG_ALLOW_CROSS
  cargo:rerun-if-env-changed=PKG_CONFIG_aarch64-apple-ios
  cargo:rerun-if-env-changed=PKG_CONFIG_aarch64_apple_ios
  cargo:rerun-if-env-changed=TARGET_PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_aarch64-apple-ios
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR_aarch64_apple_ios
  cargo:rerun-if-env-changed=TARGET_PKG_CONFIG_SYSROOT_DIR
  cargo:rerun-if-env-changed=PKG_CONFIG_SYSROOT_DIR

  --- stderr
  thread 'main' panicked at 'Unable to locate libzmq:
  pkg-config has not been configured to support cross-compilation.

  Install a sysroot for the target platform and configure it via
  PKG_CONFIG_SYSROOT_DIR and PKG_CONFIG_PATH, or install a
  cross-compiling wrapper for pkg-config and set it via
  PKG_CONFIG environment variable.', /Users/lisa/.cargo/registry/src/github.com-1ecc6299db9ec823/zmq-sys-0.8.3/build.rs:31:17
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
warning: build failed, waiting for other jobs to finish...

I am trying to build the appropriate library for the iOS and iOS simulator architectures without relying on pkg-config, but if someone can provide useful instructions for configuring pkg-config for cross-compilation, I am not opposed to it, I just don't know how and can't find any up to date resources.

I have cloned the libzmq project from this github: https://github.com/zeromq/libzmq.git, then navigate into libzmq.

Inside libzmq, I run ./autogen.sh followed by ./configure --prefix [directory location] --target "aarch64-apple-ios", then make and make install.

The above procedure without the --target flag is how I got the project to compile on my Mac in the first place, and with the target flag in place make and make install both complete successfully, but when I try to build the project using PROJECT_DIR="${PWD}/ProjectName" ./ProjectName/build-rust.sh, I get the previously posted error.

I am following the Swift-Bridge Book (https://chinedufn.github.io/swift-bridge/building/xcode-and-cargo/index.html) to try to load the Rust libraries into Xcode.

One thing to note: In the libzmq git hub, there is a folder called builds/ios/ and inside this folder is what looks to be a build file for all of the iOS archs, but I haven't been able to get the repo to build with that file. I think figuring out how to use the build_ios.sh build file as opposed to the standard build.sh provided might be the answer, but I've tried dropping build_ios.sh where build.sh is, and ./autogen.sh doesn't work, because it complains that it can't find configure.ac, even though this file does exist.