Issue rust-lang#13933: Remove transmute_mut from IO

The IO libraries casted self to mut so they can pass it to seek(SEEK_CUR, 0).
Fix this by introducing a private seek function that takes &self
  - of course one should be careful with it if he lacks an
    exclusive reference to self.

Author: Ariel Ben-Yehuda

src/libnative/io/file_win32.rs

@@ -90,6 +90,25 @@ impl FileDesc {
     pub fn handle(&self) -> libc::HANDLE {
         unsafe { libc::get_osfhandle(self.fd()) as libc::HANDLE }
     }
+
+    // A version of seek that takes &self so that tell can call it
+    //   - the private seek should of course take &mut self.
+    fn seek_common(&self, pos: i64, style: io::SeekStyle) -> Result<u64, IoError> {
+        let whence = match style {
+            io::SeekSet => libc::FILE_BEGIN,
+            io::SeekEnd => libc::FILE_END,
+            io::SeekCur => libc::FILE_CURRENT,
+        };
+        unsafe {
+            let mut newpos = 0;
+            match libc::SetFilePointerEx(self.handle(), pos, &mut newpos,
+                                         whence) {
+                0 => Err(super::last_error()),
+                _ => Ok(newpos as u64),
+            }
+        }
+    }
+
 }
 
 impl io::Reader for FileDesc {
@@ -151,26 +170,13 @@ impl rtio::RtioFileStream for FileDesc {
         }
         Ok(())
     }
+
     fn seek(&mut self, pos: i64, style: io::SeekStyle) -> Result<u64, IoError> {
-        let whence = match style {
-            io::SeekSet => libc::FILE_BEGIN,
-            io::SeekEnd => libc::FILE_END,
-            io::SeekCur => libc::FILE_CURRENT,
-        };
-        unsafe {
-            let mut newpos = 0;
-            match libc::SetFilePointerEx(self.handle(), pos, &mut newpos,
-                                         whence) {
-                0 => Err(super::last_error()),
-                _ => Ok(newpos as u64),
-            }
-        }
+        self.seek_common(pos, style)
     }
+
     fn tell(&self) -> Result<u64, IoError> {
-        // This transmute is fine because our seek implementation doesn't
-        // actually use the mutable self at all.
-        // FIXME #13933: Remove/justify all `&T` to `&mut T` transmutes
-        unsafe { mem::transmute::<&_, &mut FileDesc>(self).seek(0, io::SeekCur) }
+        self.seek_common(0, io::SeekCur)
     }
 
     fn fsync(&mut self) -> Result<(), IoError> {

src/librustuv/file.rs

@@ -377,7 +377,7 @@ impl FileWatcher {
         let r = FsRequest::write(&self.loop_, self.fd, buf, offset);
         r.map_err(uv_error_to_io_error)
     }
-    fn seek_common(&mut self, pos: i64, whence: c_int) ->
+    fn seek_common(&self, pos: i64, whence: c_int) ->
         Result<u64, IoError>{
         unsafe {
             match libc::lseek(self.fd, pos as libc::off_t, whence) {
@@ -446,10 +446,8 @@ impl rtio::RtioFileStream for FileWatcher {
     }
     fn tell(&self) -> Result<u64, IoError> {
         use libc::SEEK_CUR;
-        // this is temporary
-        // FIXME #13933: Remove/justify all `&T` to `&mut T` transmutes
-        let self_ = unsafe { mem::transmute::<&_, &mut FileWatcher>(self) };
-        self_.seek_common(0, SEEK_CUR)
+
+        self.seek_common(0, SEEK_CUR)
     }
     fn fsync(&mut self) -> Result<(), IoError> {
         let _m = self.fire_homing_missile();