rustbuild: Fix copy helper with existing files

This erroneously truncated files when the destination already existed and was an
existing hard link to the source. This in turn caused weird bugs!

Closes #37745

src/bootstrap/util.rs

@@ -41,6 +41,12 @@ pub fn mtime(path: &Path) -> FileTime {
 /// Copies a file from `src` to `dst`, attempting to use hard links and then
 /// falling back to an actually filesystem copy if necessary.
 pub fn copy(src: &Path, dst: &Path) {
+    // A call to `hard_link` will fail if `dst` exists, so remove it if it
+    // already exists so we can try to help `hard_link` succeed.
+    let _ = fs::remove_file(&dst);
+
+    // Attempt to "easy copy" by creating a hard link (symlinks don't work on
+    // windows), but if that fails just fall back to a slow `copy` operation.
     let res = fs::hard_link(src, dst);
     let res = res.or_else(|_| fs::copy(src, dst).map(|_| ()));
     if let Err(e) = res {