Promote arm64ec-pc-windows-msvc to tier 2

[dpaoliello]

Proposal

Promote the arm64ec-pc-windows-msvc tier 3 target to tier 2

A tier 2 target must have value to people other than its maintainers. (It may still be a niche target, but it must not be exclusively useful for an inherently closed group.)

Arm64EC is used both within Microsoft and by external developers as well - either to enable gradual migration of an x64 application to Arm64, or to allow an Arm64 application to load 3rd party x64 plug-ins/addons (e.g., Microsoft Office).

A tier 2 target must have a designated team of developers (the "target maintainers") available to consult on target-specific build-breaking issues, or if necessary to develop target-specific language or library implementation details. This team must have at least 2 developers.

I will be the primary maintainer for this target, and Microsoft will be providing support as per the other *-msvc targets

The target maintainers should not only fix target-specific issues, but should use any such issue as an opportunity to educate the Rust community about portability to their target, and enhance documentation of the target.

Agreed.

The Arm64EC platform documentation for Rust already has guidance on how to think about the target and re-use other code: https://doc.rust-lang.org/nightly/rustc/platform-support/arm64ec-pc-windows-msvc.html#reusing-code-from-other-architectures---x86_64-or-aarch64

Microsoft has also provided general guidance for Arm64EC: https://learn.microsoft.com/en-us/windows/arm/arm64ec

The target must not place undue burden on Rust developers not specifically concerned with that target. Rust developers are expected to not gratuitously break a tier 2 target, but are not expected to become experts in every tier 2 target, and are not expected to provide target-specific implementations for every tier 2 target.

All Arm64EC code is under #[cfg(target_arch = "arm64ec")]. As mentioned above, I have provided guidance on how to re-use existing architecture-specific code for Arm64EC (where possible).

The target must provide documentation for the Rust community explaining how to build for the target using cross-compilation, and explaining how to run tests for the target. If at all possible, this documentation should show how to run Rust programs and tests for the target using emulation, to allow anyone to do so. If the target cannot be feasibly emulated, the documentation should explain how to obtain and work with physical hardware, cloud systems, or equivalent.

Documentation for using Arm64EC with Rust has already been provided, and Microsoft has documentation for setting up an Arm64EC build environment as well.

The target must document its baseline expectations for the features or versions of CPUs, operating systems, libraries, runtime environments, and similar.

This is included in the Rust platform documentation linked above: Arm64EC requires Windows 11 on an Arm64 device.

If introducing a new tier 2 or higher target that is identical to an existing Rust target except for the baseline expectations for the features or versions of CPUs, operating systems, libraries, runtime environments, and similar, then the proposed target must document to the satisfaction of the approving teams why the specific difference in baseline expectations provides sufficient value to justify a separate target.

Not applicable. We've already had a discussion on why Arm64EC being its own target_arch makes sense: rust-lang/stdarch#1550 (comment)

Tier 2 targets must not leave any significant portions of core or the standard library unimplemented or stubbed out, unless they cannot possibly be supported on the target.

This target supports the full standard library.

The code generation backend for the target should not have deficiencies that invalidate Rust safety properties, as evaluated by the Rust compiler team. (This requirement does not apply to arbitrary security enhancements or mitigations provided by code generation backends, only to those properties needed to ensure safe Rust code cannot cause undefined behavior or other unsoundness.) If this requirement does not hold, the target must clearly and prominently document any such limitations as part of the target's entry in the target tier list, and ideally also via a failing test in the testsuite. The Rust compiler team must be satisfied with the balance between these limitations and the difficulty of implementing the necessary features.

Arm64EC uses LLVM as its code generation backend.

If the target supports C code, and the target has an interoperable calling convention for C code, the Rust target must support that C calling convention for the platform via extern "C". The C calling convention does not need to be the default Rust calling convention for the target, however.

Yes.

The target must build reliably in CI, for all components that Rust's CI considers mandatory.

Yes.

The approving teams may additionally require that a subset of tests pass in CI, such as enough to build a functional "hello world" program, ./x.py test --no-run, or equivalent "smoke tests". In particular, this requirement may apply if the target builds host tools, or if the tests in question provide substantial value via early detection of critical problems.
Building the target in CI must not take substantially longer than the current slowest target in CI, and should not substantially raise the maintenance burden of the CI infrastructure. This requirement is subjective, to be evaluated by the infrastructure team, and will take the community importance of the target into account.

Yes.

Tier 2 targets should, if at all possible, support cross-compiling. Tier 2 targets should not require using the target as the host for builds, even if the target supports host tools.

Cross compiling is supported on x64 Windows using the Windows 11 SDK 22000 (or later) with VS build tools 17.3+ - these requirements are met with Rust's existing Windows builders.

In addition to the legal requirements for all targets (specified in the tier 3 requirements), because a tier 2 target typically involves the Rust project building and supplying various compiled binaries, incorporating the target and redistributing any resulting compiled binaries (e.g. built libraries, host tools if any) must not impose any onerous license requirements on any members of the Rust project, including infrastructure team members and those operating CI systems. This is a subjective requirement, to be evaluated by the approving teams.

Licensing requirements for Arm64EC are the same as any other *-msvc target.

Tier 2 targets must not impose burden on the authors of pull requests, or other developers in the community, to ensure that tests pass for 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 tests failing for the target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding the PR breaking tests on a tier 2 target, unless they have opted into such messages.

Yes.

The target maintainers should regularly run the testsuite for the target, and should fix any test failures in a reasonably timely fashion.

Yes.

All requirements for tier 3 apply.

arm64ec-pc-windows-msvc is already a tier 3 target.

Mentors or Reviewers

@dpaoliello

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

Concerns or objections to the proposal should be discussed on Zulip and formally registered here by adding a comment with the following syntax:

@rustbot concern reason-for-concern 
<description of the concern> 

Concerns can be lifted with:

@rustbot resolve reason-for-concern 

See documentation at https://forge.rust-lang.org

cc @rust-lang/compiler @rust-lang/compiler-contributors

[davidtwco]

@rustbot second

[dpaoliello]

@apiraino ping...

[apiraino]

@rustbot label -final-comment-period +major-change-accepted