RFC: Promote aarch64-pc-windows-msvc to Tier 1

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+- Feature Name: promote-aarch64-pc-windows-msvc-to-tier-1
+- Start Date: 2025-05-22
+- RFC PR: [rust-lang/rfcs#3817](https://github.com/rust-lang/rfcs/pull/3817)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Promote aarch64-pc-windows-msvc to Tier 1 with Host Tools.
+
+# Motivation
+[motivation]: #motivation
+
+About [30% of Rust users use Windows][survey-2024], while the majority of these developers and their
+customers are using x64 hardware, the usage of Arm64 Windows has been growing since it was first
+made available in Windows 10, and has been accelerating, especially with the availability of the
+SnapDragon X processors.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+No changes required: Rust tooling for Arm64 Windows has been available for a while now so this
+doesn't affect the end user experience.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Tier 1 targets must adhere to the [Tier 1 Target Policy][tier-1-policy]. Going through these
+requirements point-by-point:
+
+> Tier 1 targets must have substantial, widespread interest within the developer community, and must
+> serve the ongoing needs of multiple production users of Rust across multiple organizations or
+> projects. These requirements are subjective, and determined by consensus of the approving teams.
+
+As mentioned above, Windows users comprise a substantial proportion of Rust developers, and Arm64
+hardware is increasingly being used by them and their customers.
+
+For the past two years, Arm64 PCs have accounted for 10-14% of Windows sales:
+- <https://www.prnewswire.com/news-releases/2025-will-see-ai-pcs-become-the-new-normal-but-arm-based-pcs-will-not-grow-out-of-its-minority-segment-302340341.html>
+- <https://www.counterpointresearch.com/insights/arm-based-pcs-to-nearly-double-market-share-by-2027/>
+- <https://www.tomshardware.com/pc-components/cpus/arm-pc-market-share-shrinks-mercury-research>
+
+Overall, they are estimated to account for 1 to 1.5% of the Windows population:
+- <https://www.canalys.com/insights/arming-your-pc-for-the-upcoming-ai-era>
+- <https://www.techpowerup.com/329255/snapdragon-x-failed-qualcomm-sold-720-000-pcs-in-q3-around-0-8-market-share>
+
+While that's a small relative number, in absolute terms it works out to 140 to 210 million devices.
+
+For Rust itself, per the [Rust download dashboard][download-dashboard] `aarch64-pc-windows-msvc` is
+the third most downloaded rustc non-tier 1 flavor (after x64 and Arm64 Linux musl flavors) and sees
+~3% the number of downloads of `x86_64-pc-windows-msvc`.
+
+> The target maintainer team must include at least 3 developers.
+
+`aarch64-pc-windows-msvc` is supported by [the 5 `*-pc-windows-msvc` maintainers][msvc-support].
+
+> The target must build and pass tests reliably in CI, for all components that Rust's CI considers
+> mandatory.
+> The target must not disable an excessive number of tests or pieces of tests in the testsuite in
+> order to do so. This is a subjective requirement.
+
+[The `dist-aarch64-msvc` CI job has been running reliably for over 4 years now][promote-tier-2],
+and I have [new CI jobs working where Rust is built and tested on Arm64 Windows runners][ci-draft-pr].
+
+The following tests had to be disabled for `aarch64-pc-windows-msvc`:
+- [Tests in `std::fs` that require symlinks][disable-fs]: this is a limitation of the runner image
+  and I've [filed an issue to have it fixed][fix-symlinks].
+- [Various debug info tests][disable-debuginfo]
+  - `tests/debuginfo/step-into-match.rs`: Stepping out of functions behaves differently.
+  - `tests/debuginfo/type-names.rs`: Arm64 Windows cdb doesn't support JavaScript extensions. I've
+    filed a bug internally with the debugger team to have this fixed.
+  - `tests/ui/runtime/backtrace-debuginfo.rs`: Backtraces are truncated. I've filed
+    [an issue to investigate this][backtrace-issue].
+
+> The target must provide as much of the Rust standard library as is feasible and appropriate to
+> provide.
+
+The full Standard Library is available.
+
+> Building the target and running the testsuite for the target must not take substantially longer
+> than other targets, and should not substantially raise the maintenance burden of the CI
+> infrastructure.
+
+[A `try` run of the new CI jobs completed in under 2 hours.][try-job]
+
+> If running the testsuite requires additional infrastructure (such as physical systems running the
+> target), the target maintainers must arrange to provide such resources to the Rust project, to the
+> satisfaction and approval of the Rust infrastructure team.
+> Such resources may be provided via cloud systems, via emulation, or via physical hardware.
+
+The new CI jobs use the free [`windows-11-arm` runners provided by GitHub][runner-announcement].
+
+> Tier 1 targets must not have a hard requirement for signed, verified, or otherwise "approved"
+> binaries. Developers must be able to build, run, and test binaries for the target on systems they
+> control, or provide such binaries for others to run. (Doing so may require enabling some
+> appropriate "developer mode" on such systems, but must not require the payment of any additional
+> fee or other consideration, or agreement to any onerous legal agreements.)
+
+There are no differences between x64 and Arm64 Windows in this regard.
+
+> All requirements for tier 2 apply.
+
+Going through the Tier 2 policies:
+
+> 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.
+
+Understood.
+
+> 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 al
+> 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.
+> The target must document its baseline expectations for the features or versions of CPUs, operating
+> systems, libraries, runtime environments, and similar.
+
+Understood, as part of the promotion PR I will add a page to Platform Support.
+
+> The code generation backend for the target should not have deficiencies that invalidate Rust
+> safety properties, as evaluated by the Rust compiler team.
+
+There are no known deficiencies in LLVM's support for Arm64 Windows.
+
+> 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.
+
+`extern "C"` correctly works for calling C code.
+
+> 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.
+
+`aarch64-pc-windows-msvc` can be cross-compiled from x86 and x64 Windows, or other platforms that
+can run those tools.
+
+> 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.
+
+There are no such license requirements for Arm64 Windows code.
+
+> 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.
+
+Understood.
+
+> The target maintainers should regularly run the testsuite for the target, and should fix any test
+> failures in a reasonably timely fashion.
+
+Understood, and this will be automated once promoted to Tier 1.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The `windows-11-arm` runners provided by GitHub are relatively new, and so we do not know what the
+availability or reliability of these runners will be.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+`aarch64-pc-windows-msvc` could be left as a Tier 2 with Host Tools target, but given the importance
+of this target to Microsoft and the increasing usage of Arm64 by Windows users, it will become more
+and more likely that issues with this target will need to be treated as critical. Catching issues
+early in development will prevent the need to Beta and Stable backports.
+
+# Prior art
+[prior-art]: #prior-art
+
+- [RFC 2959][rfc-2959] promoted `aarch64-unknown-linux-gnu` to Tier 1.
+- [RFC 3671][rfc-3671] promoted `aarch64-apple-darwin` to Tier 1.
+- [`stdarch` has been using using `windows-11-arm` runners][stdarch-pr] since early May.
+- LLVM has dedicated [Arm64 Windows builders][llvm-builders].
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+* Adding Arm64 Windows jobs to more Rust repos, such as `cargo`.
+* Promoting `arm64ec-pc-windows-msvc` to Tier 1.
+* Add a `aarch64-pc-windows-gnu` target.
+* Promote `aarch64-pc-windows-gnullvm` to Tier 1.
+
+[backtrace-issue]: https://github.com/rust-lang/rust/issues/140489
+[ci-draft-pr]: https://github.com/rust-lang/rust/pull/140136
+[disable-debuginfo]: https://github.com/rust-lang/rust/pull/140755
+[disable-fs]: https://github.com/rust-lang/rust/pull/140759
+[download-dashboard]: https://p.datadoghq.com/sb/3a172e20-e9e1-11ed-80e3-da7ad0900002-60425c7cb1b7beb2e8959a305a301c0c?fromUser=false&refresh_mode=sliding&from_ts=1747503249629&to_ts=1750095249629&live=true
+[fix-symlinks]: https://github.com/actions/partner-runner-images/issues/94
+[llvm-builders]: https://lab.llvm.org/buildbot/#/builders/161
+[msvc-support]: https://doc.rust-lang.org/nightly/rustc/platform-support/windows-msvc.html
+[platform-support]: https://github.com/rust-lang/rust/blob/e3892a40a9d06034fdf2432a9d3d29fa97726299/src/doc/rustc/src/platform-support.md?plain=1#:~:text=aarch64%2Dpc%2Dwindows%2Dmsvc
+[promote-tier-2]: https://github.com/rust-lang/rust/pull/75914
+[rfc-2959]: https://rust-lang.github.io/rfcs/2959-promote-aarch64-unknown-linux-gnu-to-tier1.html
+[rfc-3671]: https://rust-lang.github.io/rfcs/3671-promote-aarch64-apple-darwin-to-tier-1.html
+[runner-announcement]: https://github.com/orgs/community/discussions/155713
+[stdarch-pr]: https://github.com/rust-lang/stdarch/pull/1785
+[survey-2024]: https://blog.rust-lang.org/2025/02/13/2024-State-Of-Rust-Survey-results
+[tier-1-policy]: https://doc.rust-lang.org/rustc/target-tier-policy.html#tier-1-target-policy
+[try-job]: https://github.com/rust-lang-ci/rust/actions/runs/14871501014