Assume non-git LLVM is fresh if the stamp file exists

src/bootstrap/native.rs

@@ -81,26 +81,29 @@ impl Step for Llvm {
             (info, "src/llvm-project/llvm", builder.llvm_out(target), dir.join("bin"))
         };
 
-        if !llvm_info.is_git() {
-            println!(
-                "git could not determine the LLVM submodule commit hash. \
-                Assuming that an LLVM build is necessary.",
-            );
-        }
-
         let build_llvm_config = llvm_config_ret_dir
             .join(exe("llvm-config", &*builder.config.build));
         let done_stamp = out_dir.join("llvm-finished-building");
 
-        if let Some(llvm_commit) = llvm_info.sha() {
-            if done_stamp.exists() {
+        if done_stamp.exists() {
+            if let Some(llvm_commit) = llvm_info.sha() {
                 let done_contents = t!(fs::read(&done_stamp));
 
                 // If LLVM was already built previously and the submodule's commit didn't change
                 // from the previous build, then no action is required.
                 if done_contents == llvm_commit.as_bytes() {
-                    return build_llvm_config
+                    return build_llvm_config;
                 }
+            } else {
+                builder.info(
+                    "Could not determine the LLVM submodule commit hash. \
+                     Assuming that an LLVM rebuild is not necessary.",
+                );
+                builder.info(&format!(
+                    "To force LLVM to rebuild, remove the file `{}`",
+                    done_stamp.display()
+                ));
+                return build_llvm_config;
             }
         }
 
@@ -303,9 +306,7 @@ impl Step for Llvm {
 
         cfg.build();
 
-        if let Some(llvm_commit) = llvm_info.sha() {
-            t!(fs::write(&done_stamp, llvm_commit));
-        }
+        t!(fs::write(&done_stamp, llvm_info.sha().unwrap_or("")));
 
         build_llvm_config
     }