Document minimum supported versions for apple operating systems to platform documentation

[simlay]

In #124089 (review), it was pointed out that there's not an easy way to know the minimum version of an apple OS that is supported. One must kind of grep for *_DEPLOYMENT_TARGET and then parse the output to retrieve an answer and get an answer.

If I read the source correctly, the minimum supported versions currently are:

• watchOS 5.0
• tvOS 10.0
• visionOS 1.0
• macOS 10.12 for non arm and 11.0 for arm macOS
• iOS 14 for arm64e, 13.1 for mac catalyst and 10.0 otherwise

The versions are occasionally bumped up as in rust-lang/compiler-team#556 and so it would be good write them down in a more human readable form.

This has lead to various code comments about version support for various platforms being a little inconsistent. It is possible this comment is to mean "minimum version of this syscall" but that is unclear.

I will admit that I'm not certain as to the best way to do this. Adding these to the platform support docs seems like the right choice. Perhaps also in the specific platform support page

Some of the tier 3 pages list a minimum and some do not. arm64e-apple-ios for example lists 12.0+ and arm64e-apple-darwin lists 11+ however watchOS and tvOS do not.

Perhaps related: I am unfamiliar with target triples arm64e-apple-ios and arm64e-apple-darwin. Are these actually used?

CC: @madsmtm

[madsmtm]

We give the arm64e-apple-* triples a higher default deployment target since the hardware is only supported on those newer OS versions. It's technically the same story with x86_64h-apple-darwin, although there the version is macOS 10.8, which is lower than the minimum Rust already requires.

I think it's a good idea to add to the platform support docs, I was considering splitting it into the following pages / "categories":

• *-apple-darwin
• *-apple-ios
• *-apple-tvos
• *-apple-watchos
• *-apple-visionos
• *-apple-ios-macabi (Mac Catalyst is sufficiently different from iOS that it makes sense to document it separately)
• x86_64h-apple-darwin (Documents only the things relating to x86_64h, links to *-apple-darwin for the rest)
• arm64e-apple-* (Same idea as above)

Do you think that's a good idea? In any case, I'd like to do this work:
@rustbot claim

CC @Nilstrieb who is working on target docs
CC @thomcc

@rustbot label O-macos O-ios O-tvos O-watchos O-visionos

[madsmtm]

Related: cc might soon require setting the deployment target when cross-compiling these platforms from non-macOS, so maybe it makes sense to have a top-level *-apple-* page explaining these targets in general, including cross compilation stuff?

[Noratrieb]

The platform support docs are the right place for this, yes. I think adding it to the platform support docs and then also leaving a comment next to them linking to DEPLOYMENT_TARGET and the other way around should ensure that they stay in sync.