Promote aarch64-apple-darwin to Tier 1

Author: shepmaster

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+- Feature Name: promote-aarch64-apple-darwin-to-tier-1
+- Start Date: 2024-07-09
+- RFC PR: [rust-lang/rfcs#0000](https://github.com/rust-lang/rfcs/pull/0000)
+- Rust Issue: [rust-lang/rust#0000](https://github.com/rust-lang/rust/issues/0000)
+
+# Summary
+[summary]: #summary
+
+Promote aarch64-apple-darwin to Tier 1.
+
+# Motivation
+[motivation]: #motivation
+
+Approximately [33% of Rust users][survey-2023] use macOS for
+development. Hardware using Apple Silicon CPUs is noticeably more
+performant than previous x86\_64 Apple hardware and many developers
+have already transitioned to using aarch64-apple-darwin. This number
+is expected to increase as Apple no longer produces x86\_64 hardware.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This change will not require additional explanation to Rust
+programmers as many people believe that aarch64-apple-darwin is
+_already_ Tier 1. As such, I expect this change will reduce potential
+confusion.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Tier 1 targets must adhere to the [Tier 1 Target
+Policy][tier-1-policy]. This RFC intends that aarch64-apple-darwin
+will be Tier 1 [with host tools][host-tools]. 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.
+
+As [stated above][motivation], macOS users comprise a non-trivial
+percentage of overall Rust users.
+
+> The target maintainer team must include at least 3 developers.
+
+There is an existing [team for Apple and macOS specific
+concerns][apple-team]. The aarch64-apple-darwin target is actively
+used and maintained. Rust has been [tracking and fixing Apple Silicon
+specific issues][silicon-issues] and the LLVM team has been doing the
+same.
+
+> The target must build and pass tests reliably in CI, for all
+> components that Rust's CI considers mandatory.
+
+Since [2024-02-06][enabled-m1], Rust continuous integration has been
+building and testing the aarch64-apple-darwin compiler and host tools
+with roughly the same settings as x86\_64.
+
+> The target must provide as much of the Rust standard library as is
+> feasible and appropriate to provide.
+
+No material difference exists between the x86\_64-apple-darwin and
+aarch64-apple-darwin targets in this regard.
+
+> 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.
+
+Due to improved hardware performance, aarch64-apple-darwin is usually
+faster than x86\_64-apple-darwin. As a recent example,
+[aarch64-apple-darwin took 61 minutes][dist-build-aarch64] while
+[x86\_64-apple-darwin took 118 minutes][dist-build-x86\_64].
+
+> Tier 1 targets must not have a hard requirement for signed, verified, or
+> otherwise "approved" binaries.
+
+No material difference exists between the x86\_64-apple-darwin and
+aarch64-apple-darwin targets in this regard.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Tier 1 status requires that we are able to build and run binaries for
+the platform. While x86\_64 machines have been available in continuous
+integration workflows for many years, aarch64 machines are relatively
+new. The first Apple Silicon runners for GitHub Actions were [released
+on 2023-10-02][runner-m1] with free runners for open source projects
+[released on 2024-01-03][runner-m1-oss]. Availability or robustness of
+these runners may be lower compared to x86\_64.
+
+Tier 1 status requires increased continuous integration resource usage
+which means increased cost to the project. However, the
+aarch64-apple-darwin target has been treated as Tier 1 since
+[2024-02-06][enabled-m1] without causing financial concern.
+
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+Apple Silicon is the _de facto_ path forward for macOS.
+
+# Prior art
+[prior-art]: #prior-art
+
+- [RFC 2959][rfc-2959] promoted `aarch64-unknown-linux-gnu` to Tier 1.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+It is expected that **x86\_64**-apple-darwin will be demoted to Tier 2
+at some future point as hardware for this platform is [no longer being
+produced][transition]. This may reduce our continuous integration
+costs, offsetting any increases from adding
+aarch64-apple-darwin. There are **no concrete plans** to demote
+x86\_64-apple-darwin at this time and any such demotion would need its
+own well-publicized RFC.
+
+[apple-team]: https://github.com/rust-lang/team/blob/16fc8a96bf2733bc0e7ca553a645f3840ed0a7a4/teams/apple.toml
+[dist-build-aarch64]: https://github.com/rust-lang-ci/rust/actions/runs/9856130302/job/27212491241
+[dist-build-x86\_64]: https://github.com/rust-lang-ci/rust/actions/runs/9856130302/job/27212490161
+[enabled-m1]: https://github.com/rust-lang/rust/pull/120509
+[host-tools]: https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html#tier-1-with-host-tools
+[rfc-2959]: https://rust-lang.github.io/rfcs/2959-promote-aarch64-unknown-linux-gnu-to-tier1.html
+[runner-m1-oss]: https://github.blog/changelog/2024-01-30-github-actions-introducing-the-new-m1-macos-runner-available-to-open-source/
+[runner-m1]: https://github.blog/2023-10-02-introducing-the-new-apple-silicon-powered-m1-macos-larger-runner-for-github-actions/
+[silicon-issues]: https://github.com/rust-lang/rust/issues?q=is%3Aissue+sort%3Aupdated-desc+label%3AO-macos+label%3AO-AArch64
+[survey-2023]: https://blog.rust-lang.org/2024/02/19/2023-Rust-Annual-Survey-2023-results.html
+[tier-1-policy]: https://doc.rust-lang.org/nightly/rustc/target-tier-policy.html#tier-1-target-policy
+[transition]: https://en.wikipedia.org/wiki/Mac_transition_to_Apple_silicon