Add aarch64-apple-visionos and aarch64-apple-visionos-sim tier 3 targets

[agg23]

Introduces aarch64-apple-visionos and aarch64-apple-visionos-sim as tier 3 targets. This allows native development for the Apple Vision Pro's visionOS platform.

This work has been tracked in rust-lang/compiler-team#642. There is a corresponding libc change rust-lang/libc#3568 that is not required for merge.

Ideally we would be able to incorporate this change to the object crate, but the author has stated that a release will not be cut for quite a while. Therefore, the two locations that would reference the xrOS constant from object are hardcoded to their MachO values of 11 and 12, accompanied by TODOs to mark the code as needing change. I am open to suggestions on what to do here to get this checked in.

Tier 3 Target Policy

At this tier, the Rust project provides no official support for a target, so we place minimal requirements on the introduction of targets.

A tier 3 target must have a designated developer or developers (the "target maintainers") on record to be CCed when issues arise regarding the target. (The mechanism to track and CC such developers may evolve over time.)

See src/doc/rustc/src/platform-support/apple-visionos.md

Targets must use naming consistent with any existing targets; for instance, a target for the same CPU or OS as an existing Rust target should use the same name for that CPU or OS. Targets should normally use the same names and naming conventions as used elsewhere in the broader ecosystem beyond Rust (such as in other toolchains), unless they have a very good reason to diverge. Changing the name of a target can be highly disruptive, especially once the target reaches a higher tier, so getting the name right is important even for a tier 3 target.

• Target names should not introduce undue confusion or ambiguity unless absolutely necessary to maintain ecosystem compatibility. For example, if the name of the target makes people extremely likely to form incorrect beliefs about what it targets, the name should be changed or augmented to disambiguate it.
• If possible, use only letters, numbers, dashes and underscores for the name. Periods (.) are known to cause issues in Cargo.

This naming scheme matches $ARCH-$VENDOR-$OS-$ABI which is matches the iOS Apple Silicon simulator (aarch64-apple-ios-sim) and other Apple targets.

Tier 3 targets may have unusual requirements to build or use, but must not
create legal issues or impose onerous legal terms for the Rust project or for
Rust developers or users.

• The target must not introduce license incompatibilities.
• Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
• The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to besubject to any new license requirements.
• Compiling, linking, and emitting functional binaries, libraries, or other code for the target (whether hosted on the target itself or cross-compiling from another target) must not depend on proprietary (non-FOSS) libraries. Host tools built for the target itself may depend on the ordinary runtime libraries supplied by the platform and commonly used by other applications built for the target, but those libraries must not be required for code generation for the target; cross-compilation to the target must not require such libraries at all. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.
• "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

This contribution is fully available under the standard Rust license with no additional legal restrictions whatsoever. This PR does not introduce any new dependency less permissive than the Rust license policy.

The new targets do not depend on proprietary libraries.

Tier 3 targets should attempt to implement as much of the standard libraries as possible and appropriate (core for most targets, alloc for targets that can support dynamic memory allocation, std for targets with an operating system or equivalent layer of system-provided functionality), but may leave some code unimplemented (either unavailable or stubbed out as appropriate), whether because the target makes it impossible to implement or challenging to implement. The authors of pull requests are not obligated to avoid calling any portions of the standard library on the basis of a tier 3 target not implementing those portions.

This new target mirrors the standard library for watchOS and iOS, with minor divergences.

The target must provide documentation for the Rust community explaining how to build for the target, using cross-compilation if possible. If the target supports running binaries, or running tests (even if they do not pass), the documentation must explain how to run such binaries or tests for the target, using emulation if possible or dedicated hardware if necessary.

Documentation is provided in src/doc/rustc/src/platform-support/apple-visionos.md

Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.

• This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.

Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.

• Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.

Patches adding or updating tier 3 targets must not break any existing tier 2 or tier 1 target, and must not knowingly break another tier 3 target without approval of either the compiler team or the maintainers of the other tier 3 target.

• In particular, this may come up when working on closely related targets, such as variations of the same architecture with different features. Avoid introducing unconditional uses of features that another variation of the target may not have; use conditional compilation or runtime detection, as appropriate, to let each target run code supported by that target.

I acknowledge these requirements and intend to ensure that they are met.

This target does not touch any existing tier 2 or tier 1 targets and should not break any other targets.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @Mark-Simulacrum (or someone else) some time within the next two weeks.

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

This PR changes how LLVM is built. Consider updating src/bootstrap/download-ci-llvm-stamp.

[rustbot]

There are merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

You can start a rebase with the following commands:

$ # rebase
$ git rebase -i master
$ # delete any merge commits in the editor that appears
$ git push --force-with-lease

The following commits are merge commits:

• 67e7d4a

[eugenehp]

Great work @agg23
Looking forward to seeing this PR merged!

[eugenehp]

Tried to compile this PR inside a nightly rust and build a simple binary with it, here's the error I got:

Looks like rustup toolchain has to be connected to this to be able to pull this target and get it installed first.

[agg23]

@eugenehp It sounds like you might have built something incorrectly. Since you're using cargo directly, are you sure you've selected the newly built toolchain with rustup?

If you run (note, this needs to be the custom built toolchain's rustc) rustc --print target-list, you should see:

aarch64-apple-xros
aarch64-apple-xros-sim

amongst the other options.

Beware that you cannot build libc, and thus std, so you can't actually build anything of use really. This requires my libc PR.

If you want to work on visionOS now, I have published instructions on how to set it up. I have tested this both in the simulator and on an actual headset.

[madsmtm]

I suspect that this can be much higher, possibly even apple-a15 or apple-m2?

Same for the simulator target.

[agg23]

Very good point. Do you know how I would find out what is acceptable? Just poking around in LLVM?

[nikic]

See https://github.com/llvm/llvm-project/blob/fde344aef20bc4280f01294ac6e14a5c2db2d572/clang/lib/Driver/ToolChains/Arch/AArch64.cpp#L50-L60

[agg23]

I am unsure why, but in use rustc reports that apple-m1 is not a valid feature for this target. I see nothing obvious in the codebase where we whitelist LLVM architectures.

Reverting to the latest available in rustc, apple-a16

[agg23]

Is there a recommended way for testing a no libc target? I previously was testing static libraries generated with a rustc built with my libc PR, so I know the compiler works, but I want to make sure that it's building something usable still.

[rustbot]

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

triagebot.toml has been modified, there may have been changes to the review queue.

cc @davidtwco, @wesleywiser

[madsmtm]

The libc that core/std is built with when using -Zbuild-std cannot be overridden (as far as I know) using Cargo, you will have to build a custom sysroot with a patched libc instead.

1. Edit both rust/Cargo.toml and your own Cargo.toml to contain:

   [patch.crates-io]
   libc = { git = "https://github.com/agg23/libc.git", branch = "main" }
   # Optional: Use `cc` with customizations for visionOS
   cc = { git = "https://github.com/madsmtm/cc-rs.git", branch = "visionos" }

2. Build a stage1 sysroot using that, see https://rustc-dev-guide.rust-lang.org/building/how-to-build-and-run.html. It'll be roughly:

   cargo +nightly update -plibc -pcc
   rustup toolchain link stage1 build/host/stage1
   ./x build library --target=aarch64-apple-darwin,aarch64-apple-visionos,aarch64-apple-visionos-sim

3. Run cargo +stage1 build --target=aarch64-apple-visionos in the project that you want to build.

Note that to do all of this, you'll need to also grab the changes from #123721 and #123723, as those are needed to build the compiler without warnings.

---

Or you can wait until #123721 and rust-lang/libc#3568 are merged and released, and rust/Cargo.lock has been updated to use a newer version of libc, then -Zbuild-std should work (most real-world applications will likely also need rust-lang/cc-rs#1029).

[agg23]

You definitely can do this with the shipping tier 3 compiler; no need to build Rust yourself. As far as I know, all I'm doing is running with xargo, patching libc to my custom version, and forcing panic-unwind

[madsmtm]

no need to build Rust yourself.

I have never touched xargo, but if you say so then it's probably true, especially after #123721 is fixed, and by using a compatible version of libc (like the ones in my fork).

[coolbluewater]

Is a build with no patches or xargo needed likely within days, or is it expected to take a lot longer?

[agg23]

Is a build with no patches or xargo needed likely within days, or is it expected to take a lot longer?

You will not need patches or xargo once libc merges and is updated in the compiler. However, given that the PR is not passing CI and the maintainer hasn't been around for a while, who knows how long that will take.

[coolbluewater]

The maintainer of libc is just one guy? That's a tad concerning,

[coolbluewater]

@agg23 jokes apart, might be a good thing to escalate this up the rust org.