Builds fail on my Apple Silicon MacBook

[colinbdclark]

This may be an issue with Big Sur or with running on M1 hardware, I'm not sure. However, when I try to build flocking-cpal, I get the following error

The following warnings were emitted during compilation:

warning: couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
warning: set the LLVM_CONFIG_PATH environment variable to the full path to a valid `llvm-config` executable (including the executable itself)

error: failed to run custom build command for `coreaudio-sys v0.2.6`

Caused by:
  process didn't exit successfully: `~/code/flocking-core/target/debug/build/coreaudio-sys-ba8cad63afa3ce94/build-script-build` (exit code: 101)
  --- stdout
  cargo:rerun-if-env-changed=COREAUDIO_SDK_PATH
  cargo:rustc-link-lib=framework=AudioToolbox
  cargo:rustc-link-lib=framework=AudioToolbox
  cargo:rustc-link-lib=framework=CoreAudio
  cargo:rerun-if-env-changed=BINDGEN_EXTRA_CLANG_ARGS
  cargo:warning=couldn't execute `llvm-config --prefix` (error: No such file or directory (os error 2))
  cargo:warning=set the LLVM_CONFIG_PATH environment variable to the full path to a valid `llvm-config` executable (including the executable itself)

  --- stderr
  thread 'main' panicked at 'libclang error; possible causes include:
  - Invalid flag syntax
  - Unrecognized flags
  - Invalid flag arguments
  - File I/O errors
  - Host vs. target architecture mismatch
  If you encounter an error missing from this list, please file an issue or a PR!', /Users/colin/.cargo/registry/src/github.com-1ecc6299db9ec823/bindgen-0.53.3/src/ir/context.rs:573:15
  note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This seems to have to with the ASIO feature being turned on by default, and the Windows-specific specification of an older version of clang in flocking-cpal's Cargo.toml. Why this is the case only on M1, I'm not sure.

[colinbdclark]

In practice, I think it makes sense to follow how CPAL's build system handles it, and build ASIO support only in CI using the --feature asio flag.

[colinbdclark]

But the question is still why the older version of clang is included even on macOS? The only way I can get the system to build on Apple Silicon is to both remove the Windows-specific dependency block and to not specify the ASIO feature to CPAL.

[colinbdclark]

I can't find any way to make platform-conditional dependencies work correctly, at least like this:

[dependencies]
flocking = { path = "../flocking" }

[target.'cfg(target_os = "macos")'.dependencies]
cpal = "0.13.3"

[target.'cfg(target_os = "windows")'.dependencies]
cpal = {version = "0.13.3", features = ["asio"]}
clang-sys = "0.29.3"

[target.'cfg(target_os = "linux")'.dependencies]
cpal = {version = "0.13.3", features = ["jack"]}

In this case, as best as I can tell I always end build building with the "asio" feature enabled and including the 0.29.3 version of clang-sys that is specified for Windows, even on macOS. Either of the two being present will cause this error on Apple Silicon.

Realistically, I think this means we're going to have to drop support for ASIO on Windows until this can be resolved (and ideally CPAL sorts out its crashing issue with ASIO, too).