rust: fix host build on aarch64 darwin

[jonasjelonek]

rust/host failed to compile on macOS running on Apple Silicon M1 Pro because the host target triple is autogenerated to be 'arm64-unknown-linux-'. Rust doesn't have such a target triple, thus the build failes because there are no pre-built artifacts for bootstrapping.

Fix this by setting RUSTC_HOST_ARCH to 'aarch64-apple-darwin' in case our host is HOST_ARCH=arm64 and HOST_OS=darwin.

Maintainer: @lu-zero
Tested: macOS 14 on MacBook M1 Pro Apple Silicon, OpenWrt trunk

Description:

I just tried to compile OpenWrt with a Rust package that I included in my testing package feed. This package depends Rust/host, and it tries to bootstrap Rust environment for this. This bootstrapping failed due to an error in x.py with message:

src/stage0.json doesn't contain a checksum for dist/2023-06-01/rust-std-1.70.0-arm64-unknown-linux-.tar.xz

I did a bit debugging and saw, that the target triple 'arm64-unknown-linux-' isn't properly generated and doesn't come close to a matching Rust target triple. The proper target triple would be 'aarch64-apple-darwin'. Hard-specifying this in rust-values.mk is then working properly, the rust/host build and building the Rust package is working then.

I'm aware of that this fix only fixes build with Apple Silicon macOS. I guess, similar problems could occur on ARM64 based Linux because Rust target triples always use aarch64, but the auto-generated triples use 'uname -m' and end up with 'arm64'. Any thoughts how this could be extended to all aarch64 hosts?

cc @PolynomialDivision

[lu-zero]

I'm aware of that this fix only fixes build with Apple Silicon macOS. I guess, similar problems could occur on ARM64 based Linux because Rust target triples always use aarch64, but the auto-generated triples use 'uname -m' and end up with 'arm64'. Any thoughts how this could be extended to all aarch64 hosts?

Linux correctly reports aarch64, I'm surprised the rest works fine on non-linux :)

[lu-zero]

The patch is fine for me.

[jonasjelonek]

I'm aware of that this fix only fixes build with Apple Silicon macOS. I guess, similar problems could occur on ARM64 based Linux because Rust target triples always use aarch64, but the auto-generated triples use 'uname -m' and end up with 'arm64'. Any thoughts how this could be extended to all aarch64 hosts?

Linux correctly reports aarch64, I'm surprised the rest works fine on non-linux :)

Yes, you're right. Could have checked that too ^^. So Linux is safe and it only affects macOS.

[PolynomialDivision]

Nice. Thanks a lot. Please also backport to 23.05. :)

[BKPepe]

Returning to this pull request after a few years, as it definitely deserves a follow-up and a proper cross-reference.

Thanks to this PR, I have no issues compiling rust/host on macOS since I'm on aarch64. However, it turns out that a similar tweak is also needed for x86_64 on MacOS, as pointed out in this comment: #29673 (comment)

Since it's all connected, it makes sense to leave these references linked here.