disable LTO on proc-macro crates when cross compiling

When cross-compiling with LTO=thin/fat, Cargo invokes rustc
with LTO enabled even for proc-macro crates, which causes
compilation to fail.

See #110296 for more information.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

Author: onur-ozkan

src/bootstrap/bin/rustc.rs

@@ -79,6 +79,15 @@ fn main() {
         cmd.args(lint_flags.split_whitespace());
     }
 
+    let crate_type = arg("--crate-type");
+
+    // When cross-compiling with LTO=thin/fat, Cargo invokes rustc
+    // with LTO enabled even for proc-macro crates, which causes compilation to fail.
+    // See https://github.com/rust-lang/rust/issues/110296 for more information.
+    if crate_type == Some("proc-macro") {
+        cmd.arg("-C").arg("lto=off");
+    }
+
     if target.is_some() {
         // The stage0 compiler has a special sysroot distinct from what we
         // actually downloaded, so we just always pass the `--sysroot` option,
@@ -102,7 +111,7 @@ fn main() {
         // `-Ztls-model=initial-exec` must not be applied to proc-macros, see
         // issue https://github.com/rust-lang/rust/issues/100530
         if env::var("RUSTC_TLS_MODEL_INITIAL_EXEC").is_ok()
-            && arg("--crate-type") != Some("proc-macro")
+            && crate_type != Some("proc-macro")
             && !matches!(crate_name, Some("proc_macro2" | "quote" | "syn" | "synstructure"))
         {
             cmd.arg("-Ztls-model=initial-exec");