aarch64-apple-ios cannot be cross-compiled from Linux and causes cargo doc failures #76584

All up there seems to two issues:

• Docs cannot be generated for iOS from non-macOS hosts because of a dependency on a binary for generating linker flags that only exists on macOS
• Errors are not being propagated from this part of the build process to rustbuild, leaving for a headscratcher

I added an aarch64-apple-ios requirement to the docs.rs metadata in one of my crates, which caused a build failure on docs.rs. rust-lang/docs.rs#1040

Upon trying to reproduce this issue and find a fix, I hit the issue while trying to build rustc with the target aarch64-apple-ios from an x86_64 Linux host.

$ ./x.py build --target aarch64-apple-ios
Updating only changed submodules
Submodules updated in 0.02 seconds
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s


error occurred: No such file or directory (os error 2)


failed to run: /.../rust/build/bootstrap/debug/bootstrap build --target aarch64-apple-ios
Build completed unsuccessfully in 0:00:00

Not ideal output. 😛

The reason this happens is quite straightforward: ios and tvos targets use the apple_sdk_base module to resolve the pre_link_args field, and it is fallable.

The logic in the get_sdk_root function seems a little faulty, as it does a simple check for existence of a directory from the environment variable SDKROOT, then if that fails, it tries to run the xcrun binary which only exists on macOS hosts. That xcrun failure is a hard failure.

As stated in rust-lang/docs.rs#1040, a workaround for at least generating documentation is to provide SDKROOT with a path to an empty directory to iPhoneOS.platform.

No such issue occurs on *-apple-darwin because an entirely different codepath is taken.

I am not sure of the right course of action to fix this:

1. Change ios and tvos targets to not require pre_link_flags to be set, like macOS, and let build failures occur after printing a warning so the user can at least fix the issue
2. Add a hack for doc so that at least docs.rs can generate documentation for ios
3. Don't change this behaviour at all, but fix the error message so that cross-compiling devs won't just stare blindly at their screen asking "why me"

My preference is for the first option, and I'm quite happy to contribute a fix for it, though I would need assistance in successfully cross-compiling rustc for macOS from Linux. 😄

It is probable that this is the same issue: #42084.