Don't use the value of SDKROOT if it doesn't match the target. (#1047)

* Don't use the value of SDKROOT if it doesn't match the target.

The SDKROOT environment variable can be problematic on Mac.
In cases where you are compiling for multiple targets, SDKROOT
cannot be right for all of them. Furthermore, the system Python
interpreter sets SDKROOT to the MacOSX SDK if not already set.

So, if you are using a Python script to run a build, and you need
to build for multiple targets, the logic in the cc crate doesn't work.
This is precisely what is happening with rustc itself, and so we can't
upgrade the version of the cc crate used by the bootstrap code.

(Unsetting SDKROOT doesn't work either because the custom clang
build that rustc uses for CI depends on it being set)

* use env::var_os instead of env::var

* Fix compilation

* Fix compilation error

* fix compilation

---------

Co-authored-by: Jiahao XU <Jiahao_XU@outlook.com>

src/lib.rs

@@ -3651,8 +3651,43 @@ impl Build {
     }
 
     fn apple_sdk_root(&self, sdk: &str) -> Result<OsString, Error> {
+        // Code copied from rustc's compiler/rustc_codegen_ssa/src/back/link.rs.
         if let Some(sdkroot) = env::var_os("SDKROOT") {
-            return Ok(sdkroot);
+            let p = PathBuf::from(sdkroot);
+            let sdkroot = p.to_string_lossy();
+            match sdk {
+                // Ignore `SDKROOT` if it's clearly set for the wrong platform.
+                "appletvos"
+                    if sdkroot.contains("TVSimulator.platform")
+                        || sdkroot.contains("MacOSX.platform") => {}
+                "appletvsimulator"
+                    if sdkroot.contains("TVOS.platform") || sdkroot.contains("MacOSX.platform") => {
+                }
+                "iphoneos"
+                    if sdkroot.contains("iPhoneSimulator.platform")
+                        || sdkroot.contains("MacOSX.platform") => {}
+                "iphonesimulator"
+                    if sdkroot.contains("iPhoneOS.platform")
+                        || sdkroot.contains("MacOSX.platform") => {}
+                "macosx10.15"
+                    if sdkroot.contains("iPhoneOS.platform")
+                        || sdkroot.contains("iPhoneSimulator.platform") => {}
+                "watchos"
+                    if sdkroot.contains("WatchSimulator.platform")
+                        || sdkroot.contains("MacOSX.platform") => {}
+                "watchsimulator"
+                    if sdkroot.contains("WatchOS.platform")
+                        || sdkroot.contains("MacOSX.platform") => {}
+                "xros"
+                    if sdkroot.contains("XRSimulator.platform")
+                        || sdkroot.contains("MacOSX.platform") => {}
+                "xrsimulator"
+                    if sdkroot.contains("XROS.platform") || sdkroot.contains("MacOSX.platform") => {
+                }
+                // Ignore `SDKROOT` if it's not a valid path.
+                _ if !p.is_absolute() || p == Path::new("/") || !p.exists() => {}
+                _ => return Ok(p.into()),
+            }
         }
 
         let mut cache = self

tests/test.rs

@@ -626,6 +626,32 @@ fn clang_apple_visionos() {
     test.cmd(0).must_not_have("-mxrsimulator-version-min=1.0");
 }
 
+#[cfg(target_os = "macos")]
+#[test]
+fn apple_sdkroot_wrong() {
+    let output = std::process::Command::new("xcrun")
+        .args(["--show-sdk-path", "--sdk", "iphoneos"])
+        .output()
+        .unwrap();
+    if !output.status.success() {
+        return;
+    }
+
+    let wrong_sdkroot = "/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk";
+    let test = Test::clang();
+    test.gcc()
+        .__set_env("SDKROOT", wrong_sdkroot)
+        .target("aarch64-apple-ios")
+        .file("foo.c")
+        .compile("foo");
+
+    dbg!(test.cmd(0).args);
+
+    test.cmd(0)
+        .must_have(std::str::from_utf8(&output.stdout).unwrap().trim());
+    test.cmd(0).must_not_have(wrong_sdkroot);
+}
+
 #[test]
 fn compile_intermediates() {
     let test = Test::gnu();