Apple Rust link actions pass conflicting deployment targets to clang

[csmulhern]

rules_rust forwards Apple C++ toolchain linker args to rustc, including clang targets like:

-C link-arg=-target
-C link-arg=arm64-apple-macosx26.0

When developing on macOS, these linker arguments commonly come from the Xcode toolchain created by apple_support. With apple_support, the minimum deployment version defaults to whatever the installed SDK version is.

That target argument encodes the Apple deployment target for clang. However, rustc does its own minimum deployment target detection. This is mediated through the MACOSX_DEPLOYMENT_TARGET env variable. If MACOSX_DEPLOYMENT_TARGET is unset, rustc uses its built-in default and injects its own -mmacosx-version-min (which defaults to 11.0.0 on arm64)

Source: https://doc.rust-lang.org/nightly/rustc/platform-support/apple-darwin.html#deployment-target

This can make clang receive conflicting deployment targets, for example:

-mmacosx-version-min=11.0.0
-target arm64-apple-macosx26.0

With Rust 1.97 surfacing linker stderr, this issue is now more visible, with the following warning printed for every rust target:

warning: linker stderr: clang: overriding '-mmacosx-version-min=11.0.0' option with '-target arm64-apple-macosx26.5' [-Woverriding-option]
  |
  = note: `#[warn(linker_messages)]` on by default

I believe rules_rust should continue to forward linker arguments from the C++ toolchain, but should also pass MACOSX_DEPLOYMENT_TARGET to rustc when compiling for macOS targets. Similar behavior should be done for other Apple platforms as well (e.g. IPHONEOS_DEPLOYMENT_TARGET).

[scentini]

Thanks! If someone can figure out how the MACOSX_DEPLOYMENT_TARGET gets set/propagated for the C++ rules, we'd be happy to accept a PR that does the same for Rust.

[UebelAndre]

Thanks! If someone can figure out how the MACOSX_DEPLOYMENT_TARGET gets set/propagated for the C++ rules, we'd be happy to accept a PR that does the same for Rust.

@keith do you know?

[csmulhern]

I don’t think MACOSX_DEPLOYMENT_TARGET is set for C++ targets. The toolchain only specifies the linker args.

The problem arises because when the linker is invoked through rustc, rustc is detecting that MACOSX_DEPLOYMENT_TARGET is unset and setting it itself (to 11.0). rustc is responsible for this env variable being sent.

We either need to stop rustc from doing this, which I believe requires a change to rustc. Or we need to stop rustc from doing the wrong thing by setting MACOSX_DEPLOYMENT_TARGET when invoking rustc so that it matches the linker target argument being supplied by the toolchain.

[keith]

++ yea we don't set it in the toolchain because everything else accepts the triple. I kinda think this has to be done in rules_rust because if we did it in the apple_support toolchain that wouldn't apply to other toolchains that could also target macOS.

I poked around the rust source for this and since the -link-arg flags are opaque it's not clear to me the best way to solve this there. I think we should file an upstream issue since it seems to me like the triple containing all the info should be respected somehow.

In the short term it seems like maybe the best bet is to extract the triple from the command line args and set the env vars ourselves in rules_rust