Rollup merge of rust-lang#64156 - cuviper:gitless-llvm, r=alexcrichton

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

Rustbuild usually writes the LLVM submodule commit in a stamp file, so
we can avoid rebuilding it unnecessarily. However, for builds from a
source tarball (non-git), we were assuming a rebuild is always needed.
This can cause a lot of extra work if any environment like `CFLAGS`
changed between steps like build and install, which are often separate
in distro builds.

Now we also write an empty stamp file if the git commit is unknown, and
its presence is trusted to indicate that no rebuild is needed. An info
message reports that this is happening, along with the stamp file path
that can be deleted to force a rebuild anyway.

Fixes rust-lang#61206.

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
     }