Pass Apple SDK root to compiler driver via SDKROOT env var

This is more in-line with what Apple's tooling expects, and allows us to
better support custom compiler drivers (such as certain Homebrew and
Nixpkgs compilers) that prefer their own `-isysroot` flag.

Effectively, we now invoke the compiler driver as-if it was invoked as
`xcrun -sdk $sdk_name $tool`.

Author: madsmtm

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -3208,15 +3208,37 @@ fn add_apple_sdk(cmd: &mut dyn Linker, sess: &Session, flavor: LinkerFlavor) ->
     let sdkroot = sess.time("get_apple_sdk_root", || get_apple_sdk_root(sess))?;
 
     if cc == Cc::Yes {
-        // Use `-isysroot` instead of `--sysroot`, as only the former
-        // makes Clang treat it as a platform SDK.
+        // There are a few options to pass the SDK root when linking with a C/C++ compiler:
+        // - The `--sysroot` flag.
+        // - The `-isysroot` flag.
+        // - The `SDKROOT` environment variable.
         //
-        // This is admittedly a bit strange, as on most targets
-        // `-isysroot` only applies to include header files, but on Apple
-        // targets this also applies to libraries and frameworks.
-        cmd.cc_arg("-isysroot");
-        cmd.cc_arg(&sdk_root);
+        // `--sysroot` isn't actually enough to get Clang to treat it as a platform SDK, you need
+        // to specify `-isysroot`. This is admittedly a bit strange, as on most targets `-isysroot`
+        // only applies to include header files, but on Apple targets it also applies to libraries
+        // and frameworks.
+        //
+        // This leaves the choice between `-isysroot` and `SDKROOT`. Both are supported by Clang and
+        // GCC, though they may not be supported by all compiler drivers. We choose `SDKROOT`,
+        // primarily because that is the same interface that is used when invoking the tool under
+        // `xcrun -sdk macosx $tool`.
+        //
+        // In that sense, if a given compiler driver does not support `SDKROOT`, the blame is fairly
+        // clearly in the tool in question, since they also don't support being run under `xcrun`.
+        //
+        // Additionally, `SDKROOT` is an environment variable and thus optional. It also has lower
+        // precedence than `-isysroot`, so a custom compiler driver that does not support it and
+        // instead figures out the SDK on their own can easily do so by using `-isysroot`.
+        //
+        // (This in particular affects Clang built with the `DEFAULT_SYSROOT` CMake flag, such as
+        // the one provided by some versions of Homebrew's `llvm` package. Those will end up
+        // ignoring the value we set here, and instead use their built-in sysroot).
+        cmd.cmd().env("SDKROOT", &sdkroot);
     } else {
+        // When invoking the linker directly, we use the `-syslibroot` parameter. `SDKROOT` is not
+        // read by the linker, so it's really the only option.
+        //
+        // This is also what Clang does.
         cmd.link_arg("-syslibroot");
         cmd.link_arg(&sdkroot);
     }
@@ -3250,7 +3272,13 @@ fn get_apple_sdk_root(sess: &Session) -> Option<PathBuf> {
             }
             "macosx"
                 if sdkroot.contains("iPhoneOS.platform")
-                    || sdkroot.contains("iPhoneSimulator.platform") => {}
+                    || sdkroot.contains("iPhoneSimulator.platform")
+                    || sdkroot.contains("AppleTVOS.platform")
+                    || sdkroot.contains("AppleTVSimulator.platform")
+                    || sdkroot.contains("WatchOS.platform")
+                    || sdkroot.contains("WatchSimulator.platform")
+                    || sdkroot.contains("XROS.platform")
+                    || sdkroot.contains("XRSimulator.platform") => {}
             "watchos"
                 if sdkroot.contains("WatchSimulator.platform")
                     || sdkroot.contains("MacOSX.platform") => {}

compiler/rustc_target/src/spec/base/apple/mod.rs

@@ -1,5 +1,4 @@
 use std::borrow::Cow;
-use std::env;
 use std::fmt::{Display, from_fn};
 use std::num::ParseIntError;
 use std::str::FromStr;
@@ -209,29 +208,10 @@ fn link_env_remove(os: &'static str) -> StaticCow<[StaticCow<str>]> {
     // that's only applicable to cross-OS compilation. Always leave anything for the
     // host OS alone though.
     if os == "macos" {
-        let mut env_remove = Vec::with_capacity(2);
-        // Remove the `SDKROOT` environment variable if it's clearly set for the wrong platform, which
-        // may occur when we're linking a custom build script while targeting iOS for example.
-        if let Ok(sdkroot) = env::var("SDKROOT") {
-            if sdkroot.contains("iPhoneOS.platform")
-                || sdkroot.contains("iPhoneSimulator.platform")
-                || sdkroot.contains("AppleTVOS.platform")
-                || sdkroot.contains("AppleTVSimulator.platform")
-                || sdkroot.contains("WatchOS.platform")
-                || sdkroot.contains("WatchSimulator.platform")
-                || sdkroot.contains("XROS.platform")
-                || sdkroot.contains("XRSimulator.platform")
-            {
-                env_remove.push("SDKROOT".into())
-            }
-        }
-        // Additionally, `IPHONEOS_DEPLOYMENT_TARGET` must not be set when using the Xcode linker at
+        // `IPHONEOS_DEPLOYMENT_TARGET` must not be set when using the Xcode linker at
         // "/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ld",
         // although this is apparently ignored when using the linker at "/usr/bin/ld".
-        env_remove.push("IPHONEOS_DEPLOYMENT_TARGET".into());
-        env_remove.push("TVOS_DEPLOYMENT_TARGET".into());
-        env_remove.push("XROS_DEPLOYMENT_TARGET".into());
-        env_remove.into()
+        cvs!["IPHONEOS_DEPLOYMENT_TARGET", "TVOS_DEPLOYMENT_TARGET", "XROS_DEPLOYMENT_TARGET"]
     } else {
         // Otherwise if cross-compiling for a different OS/SDK (including Mac Catalyst), remove any part
         // of the linking environment that's wrong and reversed.