diff --git a/src/bootstrap/src/core/builder/tests.rs b/src/bootstrap/src/core/builder/tests.rs
index 363a9c82699..3d2e3daf915 100644
--- a/src/bootstrap/src/core/builder/tests.rs
+++ b/src/bootstrap/src/core/builder/tests.rs
@@ -1,5 +1,7 @@
 use std::thread;
 
+use build_helper::git::get_closest_merge_commit;
+
 use super::*;
 use crate::Flags;
 use crate::core::build_steps::doc::DocumentationFormat;
@@ -223,31 +225,30 @@ fn ci_rustc_if_unchanged_logic() {
         |&_| Ok(Default::default()),
     );
 
-    let build = Build::new(config.clone());
-    let builder = Builder::new(&build);
-
     if config.rust_info.is_from_tarball() {
         return;
     }
 
+    let build = Build::new(config.clone());
+    let builder = Builder::new(&build);
+
     if config.out.exists() {
         fs::remove_dir_all(&config.out).unwrap();
     }
 
     builder.run_step_descriptions(&Builder::get_step_descriptions(config.cmd.kind()), &[]);
 
-    let commit = helpers::get_closest_merge_base_commit(
+    let compiler_path = build.src.join("compiler");
+    let library_path = build.src.join("compiler");
+
+    let commit = get_closest_merge_commit(
         Some(&builder.config.src),
         &builder.config.git_config(),
-        &builder.config.stage0_metadata.config.git_merge_commit_email,
-        &[],
+        &[compiler_path.clone(), library_path.clone()],
     )
     .unwrap();
 
-    let compiler_path = build.src.join("compiler");
-    let library_path = build.src.join("library");
-
-    let has_changes = helpers::git(Some(&builder.src))
+    let has_changes = !helpers::git(Some(&builder.src))
         .args(["diff-index", "--quiet", &commit])
         .arg("--")
         .args([compiler_path, library_path])
@@ -256,9 +257,7 @@ fn ci_rustc_if_unchanged_logic() {
         .unwrap()
         .success();
 
-    assert!(
-        has_changes != config.out.join(config.build.to_string()).join("ci-rustc-sysroot").exists()
-    );
+    assert!(has_changes == config.download_rustc_commit.is_none());
 }
 
 mod defaults {
diff --git a/src/bootstrap/src/core/config/config.rs b/src/bootstrap/src/core/config/config.rs
index e3641254b19..76e3343e86f 100644
--- a/src/bootstrap/src/core/config/config.rs
+++ b/src/bootstrap/src/core/config/config.rs
@@ -2422,8 +2422,9 @@ impl Config {
                             ci_config_toml,
                         );
 
-                        let disable_ci_rustc_if_incompatible =
-                            env::var_os("DISABLE_CI_RUSTC_IF_INCOMPATIBLE")
+                        // Primarily used by CI runners to avoid handling download-rustc incompatible
+                        // options one by one on shell scripts.
+                        let disable_ci_rustc_if_incompatible = env::var_os("DISABLE_CI_RUSTC_IF_INCOMPATIBLE")
                             .is_some_and(|s| s == "1" || s == "true");
 
                         if disable_ci_rustc_if_incompatible && res.is_err() {
diff --git a/src/ci/docker/host-x86_64/mingw-check/Dockerfile b/src/ci/docker/host-x86_64/mingw-check/Dockerfile
index 97fe56fbfcb..467ca1dac67 100644
--- a/src/ci/docker/host-x86_64/mingw-check/Dockerfile
+++ b/src/ci/docker/host-x86_64/mingw-check/Dockerfile
@@ -55,7 +55,7 @@ ENV SCRIPT \
            git config --global user.name \"dummy\" && \
            git add ../compiler/rustc/src/main.rs && \
            git commit -m \"test commit for rust.download-rustc=if-unchanged logic\" && \
-           python3 ../x.py test bootstrap -- core::builder::tests::ci_rustc_if_unchanged_logic && \
+           DISABLE_CI_RUSTC_IF_INCOMPATIBLE=0 python3 ../x.py test bootstrap -- core::builder::tests::ci_rustc_if_unchanged_logic && \
            # Revert the dummy commit
            git reset --hard HEAD~1 && \
 
diff --git a/src/ci/run.sh b/src/ci/run.sh
index 8a8d26f8302..457f36bdd20 100755
--- a/src/ci/run.sh
+++ b/src/ci/run.sh
@@ -52,7 +52,8 @@ if [ "$CI" != "" ]; then
     RUST_CONFIGURE_ARGS="$RUST_CONFIGURE_ARGS --set change-id=99999999"
 fi
 
-# If runner uses an incompatible option and `FORCE_CI_RUSTC` is not defined, switch to in-tree rustc.
+# If runner uses an incompatible option and `FORCE_CI_RUSTC` is not defined,
+# switch to in-tree rustc.
 if [ "$FORCE_CI_RUSTC" == "" ]; then
     DISABLE_CI_RUSTC_IF_INCOMPATIBLE=1
 fi