add and use rename_or_copy_remove fn that fallback to copy & remove

src/librustc/util/fs.rs

@@ -82,6 +82,31 @@ pub fn link_or_copy<P: AsRef<Path>, Q: AsRef<Path>>(p: P, q: Q) -> io::Result<Li
     }
 }
 
+#[derive(Debug)]
+pub enum RenameOrCopyRemove {
+    Rename,
+    CopyRemove
+}
+
+/// Rename `p` into `q`, preferring to use `rename` if possible.
+/// If `rename` fails (rename may fail for reasons such as crossing filesystem), fallback to copy & remove
+pub fn rename_or_copy_remove<P: AsRef<Path>, Q: AsRef<Path>>(p: P, q: Q) -> io::Result<RenameOrCopyRemove> {
+    let p = p.as_ref();
+    let q = q.as_ref();
+    match fs::rename(p, q) {
+        Ok(()) => Ok(RenameOrCopyRemove::Rename),
+        Err(_) => {
+            match fs::copy(p, q) {
+                Ok(_) => {
+                    fs::remove_file(p)?;
+                    Ok(RenameOrCopyRemove::CopyRemove)
+                },
+                Err(e) => Err(e)
+            }
+        }
+    }
+}
+
 // Like std::fs::create_dir_all, except handles concurrent calls among multiple
 // threads or processes.
 pub fn create_dir_racy(path: &Path) -> io::Result<()> {

src/librustc_driver/driver.rs

@@ -22,6 +22,7 @@ use rustc::middle::privacy::AccessLevels;
 use rustc::ty::{self, TyCtxt, Resolutions, GlobalArenas};
 use rustc::util::common::time;
 use rustc::util::nodemap::{NodeSet, NodeMap};
+use rustc::util::fs::rename_or_copy_remove;
 use rustc_borrowck as borrowck;
 use rustc_incremental::{self, IncrementalHashesMap};
 use rustc_incremental::ich::Fingerprint;
@@ -1084,7 +1085,7 @@ pub fn phase_5_run_llvm_passes(sess: &Session,
         // are going to build an executable
         if sess.opts.output_types.contains_key(&OutputType::Exe) {
             let f = outputs.path(OutputType::Object);
-            fs::rename(&f,
+            rename_or_copy_remove(&f,
                      f.with_file_name(format!("{}.0.o",
                                               f.file_stem().unwrap().to_string_lossy()))).unwrap();
         }