do not allow using local llvm while using rustc from ci

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

Author: onur-ozkan

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

@@ -1787,6 +1787,7 @@ impl Config {
                 download_ci_llvm,
                 build_config,
             } = llvm;
+
             match ccache {
                 Some(StringOrBool::String(ref s)) => config.ccache = Some(s.to_string()),
                 Some(StringOrBool::Bool(true)) => {
@@ -2483,9 +2484,18 @@ impl Config {
                 llvm::is_ci_llvm_available(self, asserts)
             }
         };
+
         match download_ci_llvm {
             None => self.channel == "dev" && if_unchanged(),
-            Some(StringOrBool::Bool(b)) => b,
+            Some(StringOrBool::Bool(b)) => {
+                if !b && self.download_rustc_commit.is_some() {
+                    panic!(
+                        "`llvm.download-ci-llvm` cannot be set to `false` if `rust.download-rustc` is set to `true` or `if-unchanged`."
+                    );
+                }
+
+                b
+            }
             // FIXME: "if-available" is deprecated. Remove this block later (around mid 2024)
             // to not break builds between the recent-to-old checkouts.
             Some(StringOrBool::String(s)) if s == "if-available" => {