Improve asio sys build script

[alisomay]

Improvements

• vcvarsall.bat is executed exhaustively for any supported host and target architecture combinations now.
• Downloading the ASIO SDK is working on all platforms now which enables cross compilation.
• A section on the README.md is added which explains cross compilation scenarios for cpal with the asio feature.
• vcvarsall.bat search is improved greatly to execute smarter and faster to cover for CI scenarios where the disk I/O might be a bottle neck.
• A type mismatch bug which was apparent on using the mingw-w64 tool chain is fixed.

With these changes I want to enable:

• Better building experience in CI
• Improve existing changes to be cross compilation ready.

What I have tried:

• macOS aarch64 (host) -> windows x86, x86_64 (target) ✅
• linux (ubuntu) aarch64 (host) -> windows x86, x86_64 (target) ✅
• windows aarch64 (host)-> windows x86, x86_64, aarch64 (target) ✅
• windows x86_64 (host)-> windows x86, x86_64, aarch64 (target) ✅

If I missed something, I'll correct it along the way.
Thank you for the last merge, and I look forward to your review 🙏

[alisomay]

I've also checked a bit about the reason why asmjs-wasm32-test fails the CI.
This snippet

#[cfg(target_os = "emscripten")]
impl wasm_bindgen::convert::IntoWasmAbi for BufferSize {
    type Abi = wasm_bindgen::convert::WasmOption<u32>;
    fn into_abi(self) -> Self::Abi {
        match self {
            Self::Default => None,
            Self::Fixed(fc) => Some(fc),
        }
        .into_abi()
    }
}

Has this type wasm_bindgen::convert::WasmOption<u32>.

But that type does not exist anymore in the module.

The changes made to that snippet was 12 months ago and the last time I have checked the CI was green a few weeks ago.

I wonder if my changes made a side effect..

I don't know how it is used in the wild but this may provide a solution:

#[cfg(target_os = "emscripten")]
impl wasm_bindgen::convert::IntoWasmAbi for BufferSize {
    type Abi = u32;
    fn into_abi(self) -> Self::Abi {
        match self {
            Self::Default => 0x00FF_FFFFu32,
            Self::Fixed(fc) => fc,
        }
        .into_abi()
    }
}

I'm trying to mimic https://github.com/rustwasm/wasm-bindgen/blob/962f580d6f5def2df4223588de96feeb30ce7572/src/convert/impls.rs#L114-L140

Where 0x00FF_FFFFu32 is used to bridge None.
It compiles but I don't know if it is the expected behavior.

[alisomay]

No significant rush but when approximately you may look at this?
Thanks 🙏

[est31]

why these blank lines?

[alisomay]

Probably forgotten, I'll just remove them.

[alisomay]

2d6b063

[est31]

this can just be a &[...] right?

[alisomay]

Probably it can. 👍

[alisomay]

2d6b063

[est31]

target_arch in a build script also refers to the arch of the host, as the build scripts are always compiled for the host.

[alisomay]

This is indeed confusing. I didn't catch that thanks!

Running this build script on macos aarch64
with the command cargo build --target=x86_64-pc-windows-gnu :

fn main() {
    let target_arch = std::env::var("CARGO_CFG_TARGET_ARCH").unwrap();
    let host_architecture = std::env::consts::ARCH;

    if cfg!(target_arch = "x86_64") {
        println!(r#"cfg!(target_arch = "x86_64")"#);
    }

    if cfg!(target_arch = "aarch64") {
        println!(r#"cfg!(target_arch = "aarch64")"#);
    }

    println!(
        "CARGO_CFG_TARGET_ARCH: {}\nstd::env::consts::ARCH: {}",
        target_arch, host_architecture
    );

    panic!();
}

yields

  --- stdout
  cfg!(target_arch = "aarch64")
  CARGO_CFG_TARGET_ARCH: x86_64
  std::env::consts::ARCH: aarch64

which indeed proves the target_arch in the cfg! macro is actually the host arch from my point of view.

I'll apply the changes accordingly replacing those with the target arch parsed from the triple.
I'll also double check if it usesCARGO_CFG_* variables.
rust-lang/cargo#4302

[alisomay]

2d6b063
Fixes it.

[est31]

I wonder if my changes made a side effect..

I don't think so.

I don't know how it is used in the wild but this may provide a solution:

feel free to make a PR!