revert change incorrectly identified as "hack"

put back negative impls to ensure `Span` is not `Send` nor `Sync`

src/libsyntax_pos/lib.rs

@@ -227,6 +227,14 @@ impl SpanData {
     }
 }
 
+// The interner is pointed to by a thread local value which is only set on the main thread
+// with parallelization is disabled. So we don't allow `Span` to transfer between threads
+// to avoid panics and other errors, even though it would be memory safe to do so.
+#[cfg(not(parallel_compiler))]
+impl !Send for Span {}
+#[cfg(not(parallel_compiler))]
+impl !Sync for Span {}
+
 impl PartialOrd for Span {
     fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
         PartialOrd::partial_cmp(&self.data(), &rhs.data())