Demote x86_64-apple-darwin from Tier 1 to Tier 2 with host tools

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+- Feature Name: none
+- Start Date: 2025-07-23
+- RFC PR: [rust-lang/rfcs#3841](https://github.com/rust-lang/rfcs/pull/3841)
+- Rust Issue: [rust-lang/rust#145252](https://github.com/rust-lang/rust/pull/145252)
+
+# Summary
+[summary]: #summary
+
+Demote target `x86_64-apple-darwin` from Tier 1 to Tier 2 with host tools as this platform's lifetime is limited.
+
+Tier 2 with host tools means that the `x86_64-apple-darwin` target,
+including tools like `rustc` and `cargo`,
+is guaranteed to build but is not guaranteed to pass tests.
+
+This RFC does **not** propose removing the target completely from the codebase.
+
+# Motivation
+[motivation]: #motivation
+
+The `x86_64-apple-darwin` target has no long-term future.
+Upcoming changes will affect Rust's ability to ensure that the target meets the Tier 1 requirements,
+so we should demote it to Tier 2 with host tools in a controlled fashion.
+
+The most immediate critical change is that the free GitHub Actions macOS x86\_64 runners that the Rust project relies on will be [discontinued soon][macos-13-sunset].
+There is no known long-term replacement for these runners.
+
+## A brief timeline
+
+- 2020-06-22: Apple [announced plans][trans] to shift away from the x86\_64 architecture.
+- 2020-12-31: Rust [promoted `aarch64-apple-darwin`][aarch-tier-2] to Tier 2 with host tools.
+- 2023-06-05: Apple [announced the replacement][trans] of the last x86\_64 hardware.
+- 2023-10-02: GitHub [announces public GitHub Actions runners][m1-runners] for Apple silicon.
+- 2024-10-17: Rust [promoted `aarch64-apple-darwin`][aarch-tier-1] to Tier 1.
+- **2025-07-23**: This RFC opened.
+- 2025-09-01: GitHub [will discontinue][macos-13-sunset] providing free macOS x86\_64 runners for public repositories.
+- 2025 (Fall): [macOS 26][tahoe] will be the last macOS to support the x86\_64 architecture.
+- 2027: The [Rosetta 2][trans] compatibility layer will be mostly removed.
+
+[trans]: https://en.wikipedia.org/wiki/Mac_transition_to_Apple_silicon
+[aarch-tier-2]: https://blog.rust-lang.org/2020/12/31/Rust-1.49.0/#64-bit-arm-macos-and-windows-reach-tier-2
+[aarch-tier-1]: https://blog.rust-lang.org/2024/10/17/Rust-1.82.0/#macos-on-64-bit-arm-is-now-tier-1
+[m1-runners]: https://github.blog/changelog/2023-10-02-github-actions-apple-silicon-m1-macos-runners-are-now-available-in-public-beta/
+[macos-13-sunset]: https://github.blog/changelog/2025-07-11-upcoming-changes-to-macos-hosted-runners-macos-latest-migration-and-xcode-support-policy-updates/#macos-13-is-closing-down
+[tahoe]: https://en.wikipedia.org/wiki/MacOS_Tahoe
+
+## `x86_64-apple-darwin` popularity
+
+Looking at the [public download statistics][dl-stats] for the previous month (retrieved on 2025-07-21),
+we can see that `x86_64-apple-darwin` has substantially fewer downloads than `aarch64-apple-darwin`:
+
+[dl-stats]: https://p.datadoghq.com/sb/3a172e20-e9e1-11ed-80e3-da7ad0900002-60425c7cb1b7beb2e8959a305a301c0c?fromUser=false&refresh_mode=sliding&from_ts=1750525313022&to_ts=1753117313022&live=true
+
+### `rustc`
+
+| platform                   | downloads | percentage |
+|----------------------------|----------:|-----------:|
+| `x86_64-unknown-linux-gnu` |   194.38M |     81.58% |
+| `aarch64-apple-darwin`     |     7.15M |      3.00% |
+| `x86_64-apple-darwin`      |     2.74M |      1.15% |
+
+### `std`
+
+| platform                   | downloads | percentage |
+|----------------------------|----------:|-----------:|
+| `x86_64-unknown-linux-gnu` |    95.12M |     66.20% |
+| `aarch64-apple-darwin`     |     4.82M |      3.35% |
+| `x86_64-apple-darwin`      |     2.76M |      1.92% |
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The first release after this RFC is merged will be the last one with Tier 1 support for the `x86_64-apple-darwin` target.
+The release after that will demote the target to Tier 2 with host tools,
+which means we no longer guarantee that it will be tested by CI.
+
+Once this RFC is merged,
+a blog post will be published on the main Rust Blog announcing the change to alert users of the demotion.
+
+The demotion will also be mentioned in the release announcement for the last
+release with Tier 1 support, as well as the first release with Tier
+2 support.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The CI setup for [`rust-lang/rust`][r-l/r] will be modified to change the `dist-x86_64-apple` builder to no longer build the tests or run them.
+
+[r-l/r]: https://github.com/rust-lang/rust
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Without automated testing,
+this target will likely deteriorate more quickly.
+
+Users may be relying on Rust's Tier 1 support to provide confidence for their own artifacts.
+These users will be stuck on an old compiler version.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+- Rust CI could use emulation,
+  such as that provided by [Rosetta 2][rosetta-2].
+  Lightweight experiments show that this may increase CI times by a factor of 3
+  (e.g. a step taking 200 seconds would now take 600 seconds).
+  This would be a temporary solution,
+  as eventually Apple will [sunset Rosetta 2][trans].
+
+  We may choose to run tests in emulation even after the target is demoted to Tier 2 with host tools.
+  That change would be evaluated independently from this RFC and in a similar fashion to other non-Tier-1 targets with extra testing.
+  This evaluation would include aspects like CI complexity, test flakiness, test execution time, ability of contributors to have access to the hardware to fix issues, etc.
+  Any extra testing would be at the whim of various Rust teams to reduce or remove at any point with no prior notice.
+
+- The Rust Foundation could pay for GitHub Actions runners that will continue to use the x86\_64 architecture,
+  such as `macos-13-large`, `macos-14-large`, or `macos-15-large`.
+  This would be a temporary solution,
+  as eventually GitHub will [sunset all x86\_64-compatible runners][n-1-policy].
+
+- A third party could indefinitely provide all appropriate CI resources for the x86\_64 architecture.
+  No such third party has made themselves known,
+  nor has the Rust infrastructure team determined how to best integrate such resources.
+
+[rosetta-2]: https://en.wikipedia.org/wiki/Rosetta_(software)
+[n-1-policy]: https://github.com/actions/runner-images?tab=readme-ov-file#software-and-image-support
+
+# Prior art
+[prior-art]: #prior-art
+
+- The `i686-pc-windows-gnu` target was demoted in [RFC 3771][rfc-3771].
+  Similar to this RFC,
+  the ability to reliably test the target was questionable.
+
+- The `i686-apple-darwin` target was demoted in [RFC 2837][rfc-2837].
+  Similar to this RFC,
+  relevant hardware was no longer produced and it had been announced that upcoming operating systems would no longer support the architecture.
+
+[rfc-2837]: https://rust-lang.github.io/rfcs/2837-demote-apple-32bit.html
+[rfc-3771]: https://rust-lang.github.io/rfcs/3771-demote-i686-pc-windows-gnu.html
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+- None
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+`x86_64-apple-darwin` could be demoted to Tier 3 or support completely removed.
+There's no strong technical or financial reason to do this at this point in time.
+Should further demotions be proposed,
+those will be evaluated separately and on thier own merits,
+using the [target tier policy][tier-policy] as guidance.
+
+[tier-policy]: https://doc.rust-lang.org/stable/rustc/target-tier-policy.html