improve ci-rustc finding logic

Signed-off-by: onur-ozkan <[email protected]>

Author: onur-ozkan

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

@@ -13,6 +13,7 @@ use std::str::FromStr;
 use std::sync::OnceLock;
 use std::{cmp, env, fs};
 
+use build_helper::ci::CiEnv;
 use build_helper::exit;
 use build_helper::git::{GitConfig, get_closest_merge_commit, output_result};
 use serde::{Deserialize, Deserializer};
@@ -2718,21 +2719,28 @@ impl Config {
         download_rustc: Option<StringOrBool>,
         llvm_assertions: bool,
     ) -> Option<String> {
+        if !is_download_ci_available(&self.build.triple, llvm_assertions) {
+            return None;
+        }
+
         // If `download-rustc` is not set, default to rebuilding.
         let if_unchanged = match download_rustc {
             None | Some(StringOrBool::Bool(false)) => return None,
             Some(StringOrBool::Bool(true)) => false,
-            Some(StringOrBool::String(s)) if s == "if-unchanged" => {
-                is_download_ci_available(&self.build.triple, llvm_assertions)
-            }
+            Some(StringOrBool::String(s)) if s == "if-unchanged" => true,
             Some(StringOrBool::String(other)) => {
                 panic!("unrecognized option for download-rustc: {other}")
             }
         };
 
         // Look for a version to compare to based on the current commit.
         // Only commits merged by bors will have CI artifacts.
-        let commit = get_closest_merge_commit(Some(&self.src), &self.git_config(), &[]).unwrap();
+        let commit = get_closest_merge_commit(
+            Some(&self.src),
+            &self.git_config(),
+            &[self.src.join("compiler"), self.src.join("library")],
+        )
+        .unwrap();
         if commit.is_empty() {
             println!("ERROR: could not find commit hash for downloading rustc");
             println!("HELP: maybe your repository history is too shallow?");
@@ -2741,6 +2749,19 @@ impl Config {
             crate::exit!(1);
         }
 
+        if CiEnv::is_ci() && {
+            let head_sha =
+                output(helpers::git(Some(&self.src)).arg("rev-parse").arg("HEAD").as_command_mut());
+            let head_sha = head_sha.trim();
+            commit == head_sha
+        } {
+            eprintln!("CI rustc commit matches with HEAD and we are in CI.");
+            eprintln!(
+                "`rustc.download-ci` functionality will be skipped as artifacts are not available."
+            );
+            return None;
+        }
+
         // Warn if there were changes to the compiler or standard library since the ancestor commit.
         let has_changes = !t!(helpers::git(Some(&self.src))
             .args(["diff-index", "--quiet", &commit])