Update E0389 to the new format. #35630

src/librustc_borrowck/borrowck/mod.rs

@@ -919,9 +919,11 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                     "{} in a static location", prefix)
             }
             mc::AliasableBorrowed => {
-                struct_span_err!(
+                let mut e = struct_span_err!(
                     self.tcx.sess, span, E0389,
-                    "{} in a `&` reference", prefix)
+                    "{} in a `&` reference", prefix);
+                e.span_label(span, &"assignment into an immutable reference");
+                e
             }
         };
 

src/test/compile-fail/E0389.rs

@@ -16,5 +16,6 @@ fn main() {
     let mut fancy = FancyNum{ num: 5 };
     let fancy_ref = &(&mut fancy);
     fancy_ref.num = 6; //~ ERROR E0389
+                       //~^ NOTE assignment into an immutable reference
     println!("{}", fancy_ref.num);
 }