Apple: Re-implement SDK discovery instead of using xcrun

[madsmtm]

Shelling out to a global command line tool like this makes it difficult for rustc to properly implement change-tracking of the involved variables and paths (e.g. we could tell Cargo to relink after the user runs xcode-select --switch or installs/updates Xcode) as required for #118204, and makes error handling more difficult. So instead, this PR re-implements the SDK discovery logic that xcrun does, to avoid depending on that.

The logic is now roughly:

let sdkroot = if let Some(sdkroot) = env::var("SDKROOT") && sdk_matches(sdkroot, target_os) {
    sdkroot
} else if let Some(dir) = env::var("DEVELOPER_DIR") {
    search_for_platform_and_sdk(dir, sdk_name)
} else if let Some(path) = fs::read_link("/var/db/xcode_select_link")? { // Special path that `xcode-select --switch` configures, undocumented
    search_for_platform_and_sdk(path, sdk_name)
} else if fs::exists("/Applications/Xcode.app")? { // Hard-coded fall back
    search_for_platform_and_sdk("/Applications/Xcode.app/Contents/Developer", sdk_name)
} else if fs::exists("/Library/Developer/CommandLineTools")? { // Hard-coded fall back
    search_for_sdk("/Library/Developer/CommandLineTools/SDKs", sdk_name)
} else {
    todo!("tell the user that they need to install Xcode")
};

I have tested this in a newly created virtual machine, and verified that:

• When neither Xcode nor the command line tools are installed, an error message is shown which directs the user towards xcode-select --install, which fixes Detect linkage error when xcode-select --install hasn't been run #84534.
• When using a newly installed Xcode that hasn't yet been launched, linking fails and the error message shows a readable note about agreeing to the license with sudo xcodebuild -license, which fixes For *-apple-ios targets, if license agreement is not agreed to (yet), better behaviour is necessary #56829.

Part of #129432.

I'm not sure, but I don't think release notes are desirable here? The behaviour should be mostly identical apart from better error messages, and besides, this was very under-documented before.

CC @BlackHoleFox @thomcc.

[rustbot]

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[workingjubilee]

not saying we shouldn't add this, but how long do you think is left until we get fed up with shoving this kind of code into cg_ssa and split out rustc_linker? ...or would it be rustc_archiver?

[madsmtm]

not saying we shouldn't add this, but how long do you think is left until we get fed up with shoving this kind of code into cg_ssa and split out rustc_linker? ...or would it be rustc_archiver?

No idea honestly, I'm not really familiar enough with the overall crate structure to say.

[BlackHoleFox]

Short initial review since I was tagged, but not an actual compiler reviewer etc etc.

[rustbot]

Some changes occurred in src/doc/rustc/src/platform-support

cc @Noratrieb

[workingjubilee]

r? compiler

[BlackHoleFox]

macOS changes here look fine to me.

[bors]

☔ The latest upstream changes (presumably #132497) made this pull request unmergeable. Please resolve the merge conflicts.