make llvm::is_ci_llvm_modified logic more precise

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

Author: onur-ozkan

src/bootstrap/src/core/build_steps/llvm.rs

@@ -242,15 +242,30 @@ pub(crate) fn is_ci_llvm_available(config: &Config, asserts: bool) -> bool {
 
 /// Returns true if we're running in CI with modified LLVM (and thus can't download it)
 pub(crate) fn is_ci_llvm_modified(config: &Config) -> bool {
-    CiEnv::is_rust_lang_managed_ci_job() && config.rust_info.is_managed_git_subrepository() && {
-        // We assume we have access to git, so it's okay to unconditionally pass
-        // `true` here.
-        let llvm_sha = detect_llvm_sha(config, true);
-        let head_sha =
-            output(helpers::git(Some(&config.src)).arg("rev-parse").arg("HEAD").as_command_mut());
-        let head_sha = head_sha.trim();
-        llvm_sha == head_sha
+    // If not running in a CI environment, return false.
+    if !CiEnv::is_ci() {
+        return false;
+    }
+
+    // In rust-lang/rust managed CI, assert the existence of the LLVM submodule.
+    if CiEnv::is_rust_lang_managed_ci_job() {
+        assert!(
+            config.in_tree_llvm_info.is_managed_git_subrepository(),
+            "LLVM submodule must be fetched in rust-lang/rust managed CI builders."
+        );
     }
+    // If LLVM submodule isn't present, skip the change check as it won't work.
+    else if !config.in_tree_llvm_info.is_managed_git_subrepository() {
+        return false;
+    }
+
+    // We assume we have access to git (as this runs in CI), so it's okay
+    // to unconditionally pass `true` here.
+    let llvm_sha = detect_llvm_sha(config, true);
+    let head_sha =
+        output(helpers::git(Some(&config.src)).arg("rev-parse").arg("HEAD").as_command_mut());
+    let head_sha = head_sha.trim();
+    llvm_sha == head_sha
 }
 
 #[derive(Debug, Clone, Hash, PartialEq, Eq)]