Provide cross-compiling guidance when Apple SDK is missing

The SDK, along with a cross-linker, are the two main components required
to link pure-Rust applications from non-macOS systems, but this can be
hard to figure out.

Author: madsmtm

compiler/rustc_codegen_ssa/messages.ftl

@@ -404,6 +404,23 @@ codegen_ssa_visual_studio_not_installed = you may need to install Visual Studio
 codegen_ssa_xcrun_command_line_tools_insufficient =
     when compiling for iOS, tvOS, visionOS or watchOS, you need a full installation of Xcode
 
+codegen_ssa_xcrun_cross_download_sdk =
+    the SDK can be downloaded and extracted from https://developer.apple.com/download/all/?q=xcode (requires an Apple ID).
+
+    The full Xcode bundle should contain the SDK in Xcode.app/Contents/Developer/Platforms/{$sdk_name}.platform/Developer/SDKs/{$sdk_name}.sdk{ $sdk_name ->
+        [MacOSX] , but downloading just the "Command Line Tools for Xcode" should also be sufficient to obtain the macOS SDK.
+        *[other] .
+    }
+
+codegen_ssa_xcrun_cross_env_var =
+    pass the path to the SDK using the SDKROOT environment variable
+
+codegen_ssa_xcrun_cross_ill_supported_target =
+    cross-compiling to iOS, tvOS, visionOS or watchOS (in particular the linking step) is ill supported on non-macOS hosts
+
+codegen_ssa_xcrun_cross_linker_not_explicitly_set =
+    you will also need to use a linker capable of linking Mach-O files, consider using the bundled `lld` with `-Clinker=rust-lld`
+
 codegen_ssa_xcrun_failed_invoking = invoking `{$command_formatted}` to find {$sdk_name}.sdk failed: {$error}
 
 codegen_ssa_xcrun_found_developer_dir = found active developer directory at "{$developer_dir}"

compiler/rustc_codegen_ssa/src/back/apple.rs

@@ -172,16 +172,29 @@ pub(super) fn get_sdk_root(sess: &Session) -> Option<PathBuf> {
             let mut diag = sess.dcx().create_err(err);
 
             // Recognize common error cases, and give more Rust-specific error messages for those.
-            if let Some(developer_dir) = xcode_select_developer_dir() {
-                diag.arg("developer_dir", &developer_dir);
-                diag.note(fluent::codegen_ssa_xcrun_found_developer_dir);
-                if developer_dir.as_os_str().to_string_lossy().contains("CommandLineTools") {
-                    if sdk_name != "MacOSX" {
-                        diag.help(fluent::codegen_ssa_xcrun_command_line_tools_insufficient);
+            if sess.host.os == "macos" {
+                if let Some(developer_dir) = xcode_select_developer_dir() {
+                    diag.arg("developer_dir", &developer_dir);
+                    diag.note(fluent::codegen_ssa_xcrun_found_developer_dir);
+                    if developer_dir.as_os_str().to_string_lossy().contains("CommandLineTools") {
+                        if sdk_name != "MacOSX" {
+                            diag.help(fluent::codegen_ssa_xcrun_command_line_tools_insufficient);
+                        }
                     }
+                } else {
+                    diag.help(fluent::codegen_ssa_xcrun_no_developer_dir);
                 }
             } else {
-                diag.help(fluent::codegen_ssa_xcrun_no_developer_dir);
+                diag.help(fluent::codegen_ssa_xcrun_cross_env_var);
+                diag.help(fluent::codegen_ssa_xcrun_cross_download_sdk);
+
+                if sess.opts.cg.linker.is_none() {
+                    diag.warn(fluent::codegen_ssa_xcrun_cross_linker_not_explicitly_set);
+                }
+
+                if sess.target.os != "macos" {
+                    diag.warn(fluent::codegen_ssa_xcrun_cross_ill_supported_target);
+                }
             }
 
             diag.emit();