handle wrong SDKROOT for macos->macos build

fixes rust-lang#530

src/lib.rs

@@ -1172,6 +1172,7 @@ impl Build {
             cmd.arg("-c");
         }
         cmd.arg(&obj.src);
+        self.fix_env_for_apple_os(&mut cmd)?;
 
         run(&mut cmd, &name)?;
         Ok(())
@@ -1887,27 +1888,9 @@ impl Build {
         };
 
         self.print(&format!("Detecting iOS SDK path for {}", sdk));
-        let sdk_path = self
-            .cmd("xcrun")
-            .arg("--show-sdk-path")
-            .arg("--sdk")
-            .arg(sdk)
-            .stderr(Stdio::inherit())
-            .output()?
-            .stdout;
-
-        let sdk_path = match String::from_utf8(sdk_path) {
-            Ok(p) => p,
-            Err(_) => {
-                return Err(Error::new(
-                    ErrorKind::IOError,
-                    "Unable to determine iOS SDK path.",
-                ));
-            }
-        };
-
+        let sdk_path = self.apple_sdk_root(sdk)?;
         cmd.args.push("-isysroot".into());
-        cmd.args.push(sdk_path.trim().into());
+        cmd.args.push(sdk_path);
         cmd.args.push("-fembed-bitcode".into());
         /*
          * TODO we probably ultimately want the -fembed-bitcode-marker flag
@@ -2467,6 +2450,54 @@ impl Build {
             println!("{}", s);
         }
     }
+
+    #[cfg(target_os = "macos")]
+    fn fix_env_for_apple_os(&self, cmd: &mut Command) -> Result<(), Error> {
+        let target = self.get_target()?;
+        let host = self.get_host()?;
+        if host.contains("apple-darwin") && target.contains("apple-darwin") {
+            // Replace 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.
+            // Removing is not enough, because of cc from XCode can not find include files in such case
+            if let Ok(sdkroot) = env::var("SDKROOT") {
+                if sdkroot.contains("iPhone") {
+                    let macos_sdk = self.apple_sdk_root("macosx")?;
+                    cmd.env("SDKROOT", macos_sdk);
+                }
+            }
+            // Additionally, `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".
+            cmd.env_remove("IPHONEOS_DEPLOYMENT_TARGET");
+        }
+        Ok(())
+    }
+    #[cfg(not(target_os = "macos"))]
+    fn fix_env_for_apple_os(&self, _cmd: &mut Command) -> Result<(), Error> {
+        Ok(())
+    }
+
+    fn apple_sdk_root(&self, sdk: &str) -> Result<OsString, Error> {
+        let sdk_path = self
+            .cmd("xcrun")
+            .arg("--show-sdk-path")
+            .arg("--sdk")
+            .arg(sdk)
+            .stderr(Stdio::inherit())
+            .output()?
+            .stdout;
+
+        let sdk_path = match String::from_utf8(sdk_path) {
+            Ok(p) => p,
+            Err(_) => {
+                return Err(Error::new(
+                    ErrorKind::IOError,
+                    "Unable to determine iOS SDK path.",
+                ));
+            }
+        };
+        Ok(sdk_path.trim().into())
+    }
 }
 
 impl Default for Build {