Work around invalid spans in imported FileMaps

Author: michaelwoerister

src/librustc/middle/astencode.rs

@@ -235,12 +235,27 @@ impl<'a, 'b, 'tcx> DecodeContext<'a, 'b, 'tcx> {
     pub fn tr_span(&self, span: Span) -> Span {
         let imported_filemaps = &self.cdata.codemap_import_info[..];
 
+        let span = if span.lo > span.hi {
+            // Currently macro expansion sometimes produces invalid Span values
+            // where lo > hi. In order not to crash the compiler when trying to
+            // translate these values, let's transform them into something we
+            // can handle (and which will produce useful debug locations at
+            // least some of the time).
+            // This workaround is only necessary as long as macro expansion is
+            // not fixed. FIXME(#23480)
+            codemap::mk_sp(span.lo, span.lo)
+        } else {
+            span
+        };
+
         let filemap_index = {
             // Optimize for the case that most spans within a translated item
             // originate from the same filemap.
             let last_filemap_index = self.last_filemap_index.get();
 
             if span.lo >= imported_filemaps[last_filemap_index].original_start_pos &&
+               span.lo <= imported_filemaps[last_filemap_index].original_end_pos &&
+               span.hi >= imported_filemaps[last_filemap_index].original_start_pos &&
                span.hi <= imported_filemaps[last_filemap_index].original_end_pos {
                 last_filemap_index
             } else {