Add Apple WatchOS compile targets

[vladimir-ea]

Hello,

I would like to add the following target triples for Apple WatchOS as Tier 3 platforms:

armv7k-apple-watchos
arm64_32-apple-watchos
x86_64-apple-watchos-sim
There are some pre-requisites Pull Requests:
rust-lang/compiler-builtins#456 (merged)
rust-lang/cc-rs#662 (pending)
rust-lang/libc#2717 (merged)

There will be a subsequent PR with standard library changes for WatchOS. Previous compiler and library changes were in a single PR (#94736) which is now closed in favour of separate PRs.

Many thanks!
Vlad.

Tier 3 Target Requirements

Adds support for Apple WatchOS compile targets.

Below are details on how this target meets the requirements for tier 3:

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.)

@deg4uss3r has volunteered to be the target maintainer. I am also happy to help if a second maintainer is required.

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.

Uses the same naming as the LLVM target, and the same convention as other Apple targets.

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.

I don't believe there is any ambiguity here.

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.

I don't see any legal issues here.

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 be subject to any new license requirements.
If the target supports building host tools (such as rustc or cargo), those host tools must not depend on proprietary (non-FOSS) libraries, other than ordinary runtime libraries supplied by the platform and commonly used by other binaries built for the target. 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.
Targets should not require proprietary (non-FOSS) components to link a functional binary or library.
"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.

I see no issues with any of the above.

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.

Only relevant to those making approval decisions.

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.

core and alloc can be used. std support will be added in a subsequent PR.

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 tests (even if they do not pass), the documentation must explain how to run tests for the target, using emulation if possible or dedicated hardware if necessary.

Use --target= option to cross compile, just like any target. Tests can be run using the WatchOS simulator (see https://developer.apple.com/documentation/xcode/running-your-app-in-the-simulator-or-on-a-device).

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.

I don't foresee this being a problem.

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.

No other targets should be affected by the pull request.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @estebank (or someone else) soon.

Please see the contribution instructions for more information.

[rust-highfive]

⚠️ Warning ⚠️

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

[estebank]

Re-shuffling

r? rust-lang/compiler

[cjgillot]

Thanks for the PR @vladimir-ea. Unfortunately, I am not an appropriate reviewer for this.
@nagisa, could you take this review? Thanks.

[bors]

☔ The latest upstream changes (presumably #95624) made this pull request unmergeable. Please resolve the merge conflicts.

[rudyrichter]

@vladimir-ea would it be possible to add an additional simulator target to this list? x86_64-apple-watchos-sim will work great on intel macs but Apple silicon probably needs something like aarch64-apple-watchos-sim

[pnkfelix]

@vladimir-ea according to the target tier policy, we need some conditions to be satisfied; you can see them here: https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html#tier-3-target-policy

In particular, are you willing to be the target maintainer on record? Do you know of anyone else who is also willing to serve in that capacity?

You can see #92670 , specifically the discussion thread on that PR, as an example for how other PR's have handled documenting this.

[deg4uss3r]

I am happy to be a maintainer for this target 😀

[deg4uss3r]

@pnkfelix I would love to get this MR off the ground I know there's a lot of pieces:

• adding the target (this MR)
• the std part of the MR advised to be broken up by @m-ou-se
• the cc-rs parts that Alex cannot maintain anymore and suggested that might be rolled into the libs team/searching for maintainers (maybe @yaahc can help me figure out if this should be rolled into the libs, I think it should for sure @m-ou-se is definitely correct!)

I am a current target co-maintainer plus I have a small team I can help get these MRs in and worked on as I am very interested in getting some code running on the Apple Watch.

I want to keep @vladimir-ea's commits and they are welcome to jump back in if they reappear for sure but I think myself and team will start working on this PR and the others to help get this target sorted unless anyone disagrees 😄

[JohnCSimon]

@vladimir-ea can you please address the merge conflict?

FYI: when a PR is ready for review, send a message containing
@rustbot ready to switch to S-waiting-on-review so the PR is in the reviewer's backlog.

[vladimir-ea]

@vladimir-ea according to the target tier policy, we need some conditions to be satisfied; you can see them here: https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html#tier-3-target-policy

In particular, are you willing to be the target maintainer on record? Do you know of anyone else who is also willing to serve in that capacity?

You can see #92670 , specifically the discussion thread on that PR, as an example for how other PR's have handled documenting this.

I have added this to the main comment. above.

[vladimir-ea]

@pnkfelix I would love to get this MR off the ground I know there's a lot of pieces:

• adding the target (this MR)
• the std part of the MR advised to be broken up by @m-ou-se
• the cc-rs parts that Alex cannot maintain anymore and suggested that might be rolled into the libs team/searching for maintainers (maybe @yaahc can help me figure out if this should be rolled into the libs, I think it should for sure @m-ou-se is definitely correct!)

I am a current target co-maintainer plus I have a small team I can help get these MRs in and worked on as I am very interested in getting some code running on the Apple Watch.

I want to keep @vladimir-ea's commits and they are welcome to jump back in if they reappear for sure but I think myself and team will start working on this PR and the others to help get this target sorted unless anyone disagrees 😄

I would be very happy for you to take over this target and be its maintainer - I put you down as target maintainer in the tier 3 comment above - hope that is ok.

I have the changes ready for the std support but I did not see a way to stack PRs, so I can raise that if /when this one is merged.

Note that cc-rs changes are not required to build the targets but they are required to build anything with those targets..

[vladimir-ea]

NB: seems like submodule changes have snuck in. You'll want to remove them from this PR. Changes to the Cargo.lock also seem accidental.

@nagisa sorry about that, messed up the rebase - should be fixed now

[nagisa]

@bors r+

[bors]

📌 Commit 83dedd15abee72b390f9b75d0c008c1977ebb5d3 has been approved by nagisa

[klensy]

NB: seems like submodule changes have snuck in. You'll want to remove them from this PR. Changes to the Cargo.lock also seem accidental.

@nagisa sorry about that, messed up the rebase - should be fixed now

Commits touching unneeded files and submodules still will be in history of commits, why not squash?

[nagisa]

Oh, I didn't notice that. Yeah, please squash.

@bors r-

[vladimir-ea]

Oh, I didn't notice that. Yeah, please squash.

@bors r-

@nagisa squashed

[nagisa]

@bors r+

[bors]

📌 Commit dc5c610 has been approved by nagisa