Promote aarch64-pc-windows-msvc to Tier 1

Author: dpaoliello

text/3817-promote-aarch64-pc-windows-msvc-to-tier-1.md

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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 substatial propotion of Rust developers, and Arm64
+hardware is increasingly being used by them and their customers.
+
+> The target maintainer team must include at least 3 developers.
+
+There is an existing [Windows team for Rust][windows-team] and, within Microsoft, Arm64 Windows is
+considered to be a Tier 1 target for internal Rust usage.
+
+> 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 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.
+
+[`aarch64-pc-windows-msvc` is already a Tier 2 target.][platform-support]
+
+# 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.
+- 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 `stdarch`.
+* Promoting Arm64EC to Tier 1.
+
+[ci-draft-pr][https://github.com/rust-lang/rust/pull/140136]
+[llvm-builders][https://lab.llvm.org/buildbot/#/builders/161]
+[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
+[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]
+[windows-team]: https://github.com/rust-lang/team/blob/055aa9ddb4e9a10c760042baa54da71b1585a821/teams/windows.toml