disable download-rustc if LLVM submodule has changes in CI

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

src/bootstrap/src/core/config/config.rs

@@ -2399,6 +2399,19 @@ impl Config {
                 Some(commit) => {
                     self.download_ci_rustc(commit);
 
+                    // We never enable `download-rustc` without `download-ci-llvm`. If `self.llvm_from_ci` is false,
+                    // it means the "if-unchanged" logic has detected some changes in the LLVM submodule.
+                    if CiEnv::is_ci() && !self.llvm_from_ci {
+                        // This happens when LLVM submodule is updated in CI, we should disable ci-rustc without an error
+                        // to not break CI. For non-CI environments, we should return an error.
+                        if CiEnv::is_ci() {
+                            println!("WARNING: LLVM submodule has changes, `download-rustc` will be disabled.");
+                            return None;
+                        } else {
+                            panic!("ERROR: LLVM submodule has changes, `download-rustc` can't be used.");
+                        }
+                    }
+
                     if let Some(config_path) = &self.config {
                         let ci_config_toml = match self.get_builder_toml("ci-rustc") {
                             Ok(ci_config_toml) => ci_config_toml,