Change label to "consider changing this to mut f"

Change the wording of mutable borrow on immutable binding from "this
should be `mut`" to "consider changing this to `mut f`".

src/librustc_borrowck/borrowck/mod.rs

@@ -748,7 +748,9 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
             db.span_label(span, &msg);
         }
         if let Some(span) = local_def {
-            db.span_label(span, &"this should be `mut`");
+            if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(span) {
+                db.span_label(span, &format!("consider changing this to `mut {}`", snippet));
+            }
         }
         db
     }

src/test/ui/did_you_mean/issue-39544.stderr

@@ -2,7 +2,7 @@ error: cannot borrow immutable field `z.x` as mutable
   --> $DIR/issue-39544.rs:21:18
    |
 20 |     let z = Z { x: X::Y };
-   |         - this should be `mut`
+   |         - consider changing this to `mut z`
 21 |     let _ = &mut z.x;
    |                  ^^^