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

src/lib.rs

@@ -3653,8 +3653,43 @@ 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();