Test that names of variables in external macros are not shown on a borrow error

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -317,6 +317,14 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
         opt: DescribePlaceOpt,
     ) -> Option<String> {
         let local = place.local;
+        if self.body.local_decls[local]
+            .source_info
+            .span
+            .in_external_macro(self.infcx.tcx.sess.source_map())
+        {
+            return None;
+        }
+
         let mut autoderef_index = None;
         let mut buf = String::new();
         let mut ok = self.append_local_to_string(local, &mut buf);