Fix sysroot on macOS when cross-compiling and SDKROOT is set

src/librustc_target/spec/apple_base.rs

@@ -1,4 +1,4 @@
-use std::env;
+use std::{env, path::Path};
 
 use crate::spec::{LinkArgs, TargetOptions};
 
@@ -51,3 +51,63 @@ pub fn macos_llvm_target(arch: &str) -> String {
     let (major, minor) = macos_deployment_target();
     format!("{}-apple-macosx{}.{}.0", arch, major, minor)
 }
+
+#[cfg(target_os = "macos")]
+pub fn sysroot(sdk: &str) -> Result<Option<String>, String> {
+    // Like Clang, allow the `SDKROOT` environment variable used by Xcode to define the sysroot.
+    if let Some(sdk_root) = env::var("SDKROOT").ok() {
+        let actual_sdk_path = sdk_path(sdk)?;
+        let sdk_root_p = Path::new(&sdk_root);
+        // Ignore `SDKROOT` if it's not a valid path.
+        if !sdk_root_p.is_absolute() || sdk_root_p == Path::new("/") || !sdk_root_p.exists() {
+            return Ok(Some(actual_sdk_path));
+        }
+        // Ignore `SDKROOT` if it's clearly set for the wrong platform, which may occur when we're
+        // compiling a custom build script while targeting iOS for example.
+        return Ok(Some(match sdk {
+            "iphoneos" if sdk_root.contains("iPhoneSimulator.platform")
+                || sdk_root.contains("MacOSX.platform") => actual_sdk_path,
+            "iphonesimulator" if sdk_root.contains("iPhoneOS.platform")
+                || sdk_root.contains("MacOSX.platform") => actual_sdk_path,
+            "macosx" | "macosx10.15" if sdk_root.contains("iPhoneOS.platform")
+                || sdk_root.contains("iPhoneSimulator.platform") => actual_sdk_path,
+            _ => sdk_root,
+        }))
+    }
+    Ok(None)
+}
+
+// `xcrun` is only available on macOS.
+#[cfg(not(target_os = "macos"))]
+pub fn sysroot(_sdk: &str) -> Result<Option<String>, String> {
+    if let Some(sdk_root) = env::var("SDKROOT").ok() {
+        let sdk_root_p = Path::new(&sdk_root);
+        // Use `SDKROOT` only if it's a valid path.
+        if sdk_root_p.is_absolute() && sdk_root_p != Path::new("/") && sdk_root_p.exists() {
+            return Ok(Some(sdk_root));
+        }
+    }
+    Ok(None)
+}
+
+#[cfg(target_os = "macos")]
+fn sdk_path(sdk_name: &str) -> Result<String, String> {
+    let res = std::process::Command::new("xcrun")
+        .arg("--show-sdk-path")
+        .arg("-sdk")
+        .arg(sdk_name)
+        .output()
+        .and_then(|output| {
+            if output.status.success() {
+                Ok(String::from_utf8(output.stdout).unwrap())
+            } else {
+                let error = String::from_utf8(output.stderr);
+                let error = format!("process exit with error: {}", error.unwrap());
+                Err(std::io::Error::new(std::io::ErrorKind::Other, &error[..]))
+            }
+        });
+    match res {
+        Ok(output) => Ok(output.trim().to_string()),
+        Err(e) => Err(format!("failed to get {} SDK path: {}", sdk_name, e)),
+    }
+}

src/librustc_target/spec/apple_ios_base.rs

@@ -1,7 +1,3 @@
-use std::env;
-use std::io;
-use std::path::Path;
-use std::process::Command;
 use crate::spec::{LinkArgs, LinkerFlavor, TargetOptions};
 
 use Arch::*;
@@ -30,42 +26,6 @@ impl Arch {
     }
 }
 
-pub fn get_sdk_root(sdk_name: &str) -> Result<String, String> {
-    // Following what clang does
-    // (https://github.com/llvm/llvm-project/blob/
-    // 296a80102a9b72c3eda80558fb78a3ed8849b341/clang/lib/Driver/ToolChains/Darwin.cpp#L1661-L1678)
-    // to allow the SDK path to be set. (For clang, xcrun sets
-    // SDKROOT; for rustc, the user or build system can set it, or we
-    // can fall back to checking for xcrun on PATH.)
-    if let Some(sdkroot) = env::var("SDKROOT").ok() {
-        let sdkroot_path = Path::new(&sdkroot);
-        if sdkroot_path.is_absolute() && sdkroot_path != Path::new("/") && sdkroot_path.exists() {
-            return Ok(sdkroot);
-        }
-    }
-    let res = Command::new("xcrun")
-                      .arg("--show-sdk-path")
-                      .arg("-sdk")
-                      .arg(sdk_name)
-                      .output()
-                      .and_then(|output| {
-                          if output.status.success() {
-                              Ok(String::from_utf8(output.stdout).unwrap())
-                          } else {
-                              let error = String::from_utf8(output.stderr);
-                              let error = format!("process exit with error: {}",
-                                                  error.unwrap());
-                              Err(io::Error::new(io::ErrorKind::Other,
-                                                 &error[..]))
-                          }
-                      });
-
-    match res {
-        Ok(output) => Ok(output.trim().to_string()),
-        Err(e) => Err(format!("failed to get {} SDK path: {}", sdk_name, e))
-    }
-}
-
 fn build_pre_link_args(arch: Arch) -> Result<LinkArgs, String> {
     let sdk_name = match arch {
         Armv7 | Armv7s | Arm64 => "iphoneos",
@@ -75,16 +35,19 @@ fn build_pre_link_args(arch: Arch) -> Result<LinkArgs, String> {
 
     let arch_name = arch.to_string();
 
-    let sdk_root = get_sdk_root(sdk_name)?;
-
     let mut args = LinkArgs::new();
-    args.insert(LinkerFlavor::Gcc,
-                vec!["-arch".to_string(),
-                     arch_name.to_string(),
-                     "-isysroot".to_string(),
-                     sdk_root.clone(),
-                     "-Wl,-syslibroot".to_string(),
-                     sdk_root]);
+    args.insert(LinkerFlavor::Gcc, vec!["-arch".to_string(), arch_name.to_string()]);
+    if let Some(sdk_root) = super::apple_base::sysroot(sdk_name)? {
+        args.insert(
+            LinkerFlavor::Gcc,
+            vec![
+                "-isysroot".to_string(),
+                sdk_root.clone(),
+                "-Wl,-syslibroot".to_string(),
+                sdk_root
+            ],
+        );
+    }
 
     Ok(args)
 }

src/librustc_target/spec/i686_apple_darwin.rs

@@ -5,6 +5,12 @@ pub fn target() -> TargetResult {
     base.cpu = "yonah".to_string();
     base.max_atomic_width = Some(64);
     base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-m32".to_string()]);
+    if let Some(sysroot) = super::apple_base::sysroot("macosx")? {
+        base.pre_link_args.insert(
+            LinkerFlavor::Gcc,
+            vec!["-isysroot".to_string(), sysroot.clone(), "-Wl,-syslibroot".to_string(), sysroot],
+        );
+    }
     base.stack_probes = true;
     base.eliminate_frame_pointer = false;
 

src/librustc_target/spec/x86_64_apple_darwin.rs

@@ -6,6 +6,12 @@ pub fn target() -> TargetResult {
     base.max_atomic_width = Some(128); // core2 support cmpxchg16b
     base.eliminate_frame_pointer = false;
     base.pre_link_args.insert(LinkerFlavor::Gcc, vec!["-m64".to_string()]);
+    if let Some(sysroot) = super::apple_base::sysroot("macosx")? {
+        base.pre_link_args.insert(
+            LinkerFlavor::Gcc,
+            vec!["-isysroot".to_string(), sysroot.clone(), "-Wl,-syslibroot".to_string(), sysroot],
+        );
+    }
     base.stack_probes = true;
 
     // Clang automatically chooses a more specific target based on