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Add more descriotion message and optimize code

Author: LuuuXXX

src/bootstrap/src/core/build_steps/dist.rs

@@ -2222,6 +2222,8 @@ impl Step for RustDev {
         // If updating this, you likely want to change
         // src/bootstrap/download-ci-llvm-stamp as well, otherwise local users
         // will not pick up the extra file until LLVM gets bumped.
+        // We should include all the build artifacts that get from a source build,
+        // so that you can use the downloadable LLVM as if you’ve just run a full source build.
         if src_bindir.exists() {
             for entry in walkdir::WalkDir::new(&src_bindir) {
                 let entry = t!(entry);

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

@@ -2108,16 +2108,16 @@ impl Config {
                 llvm::is_ci_llvm_available(&self, asserts)
             }
             Some(StringOrBool::String(s)) if s == "if-unchanged" => {
-                if !self.rust_info.is_from_tarball() {
-                    if self
+                // Git is needed to track modifications here, but tarball source is not available.
+                // If not modified here or built through tarball source, we maintain consistency
+                // with '"if available"'.
+                if !self.rust_info.is_from_tarball()
+                    && self
                         .last_modified_commit(&["src/llvm-project"], "download-ci-llvm", true)
                         .is_none()
-                    {
-                        // there are some untracked changes in the the given paths.
-                        false
-                    } else {
-                        llvm::is_ci_llvm_available(&self, asserts)
-                    }
+                {
+                    // there are some untracked changes in the the given paths.
+                    false
                 } else {
                     llvm::is_ci_llvm_available(&self, asserts)
                 }