demote-apple-32bit: initial draft

Author: pietroalbini

text/0000-demote-apple-32bit.md

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+- Feature Name: `demote_apple_32bit`
+- Start Date: 2019-12-10
+- 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
+
+This RFC proposes to demote the `i686-apple-darwin` rustc target from Tier 1 to
+Tier 3, and to demote the `armv7-apple-ios`, `armv7s-apple-ios` and
+`i386-apple-ios` rustc targets from Tier 2 to Tier 3.
+
+# Motivation
+[motivation]: #motivation
+
+Apple [publicly announced][macos-announcement] that macOS 10.14 Mojave is the
+last OS supporting the execution of 32bit binaries, and macOS 10.15 (and later)
+prevents running them at all. It's been years since the last 32bit Apple
+hardware was sold, so providing 64bit binaries should cover most of the macOS
+userbase.
+
+Apple [also announced][ios-announcement] that iOS 10 is the last one supporting
+the execution of 32bit apps, and they won't work at all on iOS 11 and later.
+All iPhones after the iPhone 5 and the iPhone 5C support 64bit apps, which
+means all the supported ones can run them.
+
+Along with the deprecation, Apple removed support for building 32bit binaries
+since Xcode 10, and that makes building rustc itself on the project's CI harder
+(as we're limited to providers still offering Xcode 9).
+
+It makes little sense for the Rust team to continue providing support for a
+platform the upstream vendor abandoned, especially when it requires extra
+effort from us infrastructure-wise.
+
+[macos-announcement]: https://support.apple.com/en-us/HT208436
+[ios-announcement]: https://developer.apple.com/documentation/uikit/app_and_environment/updating_your_app_from_32-bit_to_64-bit_architecture
+
+# 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 `i686-apple-darwin` target and Tier 2 support for the
+`armv7-apple-ios`, `armv7s-apple-ios` and `i386-apple-ios` targets. The release
+after that will demote the targets to Tier 3, which means no official build
+will be available for them, and they will not 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 the users of the target of the demotion. The
+demotion will also be mentioned in the release announcement for the last
+release with Tier 1 and Tier 2 support, as well as the first release with Tier
+3 support.
+
+This RFC does **not** propose removing the targets completely from the
+codebase: that will be decided either by another RFC just for those targets, or
+by an RFC defining a general policy for Tier 3 target removal.
+
+Once the targets are demoted to Tier 3, users on other platforms with one of
+those targets' `rust-std` installed won't be able to update the toolchain until
+they remove that target. Users using an Apple 32bit compiler as their host
+platforms will instead be prevented from updating at all, as no new binary
+artifact will be available.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `i686-apple-darwin`, `armv7-apple-ios`, `armv7s-apple-ios` and
+`i386-apple-ios` targets will be considered Tier 3 from the second release
+after this RFC is merged. The code supporting the target will not be removed
+from the compiler, even though we won't guarantee it will continue to work.
+
+The following CI builders will be removed:
+
+- `dist-i686-apple`
+- `i686-apple`
+
+In addition, the `armv7-apple-ios`, `armv7s-apple-ios` and `i386-apple-ios`
+targets will be removed from the `dist-x86_64-apple` builder.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Users might depend on the target, and approving this RFC means they'll be stuck
+on an old compiler version forever, unless they build their own compiler and
+fix the regressions introduced in newer releases themselves.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+Support for building 32bit binaries for Apple targets is shrinking as time goes
+by: the latest SDKs from Apple don't support building them at all, and CI
+providers are slowly starting to upgrade their minimum supported SDK versions:
+
+* Azure Pipelines (the provider rustc currently use) doesn't have any public
+  information on when Xcode 9 will be deprecated.
+* GitHub Actions doesn't support Xcode 9 at all.
+* Travis CI [deprecated Xcode older than 9.2][travis-ci-xcode-deprecation] in
+  July 2018.
+
+If this RFC is not accepted, we'll eventually reach a point when we'll have to
+make considerable investments both in terms of money and time to keep building
+on Apple 32bit.
+
+[travis-ci-xcode-deprecation]: https://blog.travis-ci.com/2018-07-19-xcode9-4-default-announce
+
+# Prior art
+[prior-art]: #prior-art
+
+There is no precedent inside the project for the deprecation of Tier 1 targets
+to Tier 3.
+
+Go is taking a [similar approach to us][go-34749], documenting that the last
+release supporting Apple 32bit is going to be Go 1.14 (the next one), with
+support for the target being dropped in Go 1.15.
+
+[go-34749]: https://github.com/golang/go/issues/34749
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+*Nothing here.*
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+*Nothing here.*