rustc_metadata: Do not encode empty reexport lists

This is more a cleanup than optimization

Author: petrochenkov

compiler/rustc_metadata/src/rmeta/encoder.rs

@@ -1267,13 +1267,6 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
         // the crate root for consistency with other crates (some of the resolver
         // code uses it). However, we skip encoding anything relating to child
         // items - we encode information about proc-macros later on.
-        let reexports = if !self.is_proc_macro {
-            tcx.module_reexports(local_def_id).unwrap_or(&[])
-        } else {
-            &[]
-        };
-
-        record_array!(self.tables.module_reexports[def_id] <- reexports);
         if self.is_proc_macro {
             // Encode this here because we don't do it in encode_def_ids.
             record!(self.tables.expn_that_defined[def_id] <- tcx.expn_that_defined(local_def_id));
@@ -1305,6 +1298,11 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                     }
                 }
             }));
+
+            if let Some(reexports) = tcx.module_reexports(local_def_id) {
+                assert!(!reexports.is_empty());
+                record_array!(self.tables.module_reexports[def_id] <- reexports);
+            }
         }
     }