Fix large file copies on 32 bit platforms

Author: nicokoch

src/libstd/sys/unix/fs.rs

@@ -815,15 +815,19 @@ pub fn copy(from: &Path, to: &Path) -> io::Result<u64> {
 
     let mut written = 0u64;
     while written < len {
-        let bytes_to_copy = len - written;
+        let bytes_to_copy = if len - written > usize::max_value() as u64 {
+            usize::max_value()
+        } else {
+            (len - written) as usize
+        };
         let copy_result = unsafe {
             // We actually don't have to adjust the offsets,
             // because copy_file_range adjusts the file offset automatically
             cvt(copy_file_range(reader.as_raw_fd(),
                                 ptr::null_mut(),
                                 writer.as_raw_fd(),
                                 ptr::null_mut(),
-                                bytes_to_copy as usize,
+                                bytes_to_copy,
                                 0)
                 )
         };