check if the LLVM submodule is fetched in is_ci_llvm_modified

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

Author: onur-ozkan

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)fnis_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(){
+returnfalse;
+}
+
+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();