check if the LLVM submodule is fetched in is_ci_llvm_modified

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

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

@@ -241,7 +241,17 @@ 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_ci() && config.rust_info.is_managed_git_subrepository() && {
+    // If the LLVM submodule is unavailable (which is the case when `llvm.download-ci-llvm` is set to true),
+    // LLVM cannot be modified which means it is unnecessary to run the git logic below.
+    //
+    // This is very unlikely to happen on our (rust-lang/rust) CI runners, as we intentionally fetch all
+    // submodules in CI and most of the time (probably always) prefer `llvm.download-ci-llvm` to be set
+    // to "if-unchanged" or true.
+    if config.in_tree_llvm_info.is_managed_git_subrepository() {
+        return false;
+    }
+
+    CiEnv::is_ci() && {
         // We assume we have access to git, so it's okay to unconditionally pass
         // `true` here.
         let llvm_sha = detect_llvm_sha(config, true);

src/bootstrap/src/lib.rs

@@ -312,8 +312,6 @@ impl Build {
         let miri_info = config.miri_info.clone();
         let rustfmt_info = config.rustfmt_info.clone();
         let enzyme_info = config.enzyme_info.clone();
-
-        // we always try to use git for LLVM builds
         let in_tree_llvm_info = config.in_tree_llvm_info.clone();
         let in_tree_gcc_info = config.in_tree_gcc_info.clone();