Implement suggestions from the PR

  - Move loading of atomic bool outside the loop
  - Add comment about TryFrom for future improvement

Author: nicokoch

src/libstd/sys/unix/fs.rs

@@ -818,14 +818,16 @@ pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
         (metadata.permissions(), metadata.size())
     };
 
+    let has_copy_file_range = HAS_COPY_FILE_RANGE.load(Ordering::Relaxed);
     let mut written = 0u64;
     while written < len {
+        // TODO should ideally use TryFrom
         let bytes_to_copy = if len - written > usize::max_value() as u64 {
             usize::max_value()
         } else {
             (len - written) as usize
         };
-        let copy_result = if HAS_COPY_FILE_RANGE.load(Ordering::Relaxed) {
+        let copy_result = if has_copy_file_range {
             let copy_result = unsafe {
                 // We actually don't have to adjust the offsets,
                 // because copy_file_range adjusts the file offset automatically