Fix #33819 and update ui test

Author: Jonathan Turner

src/librustc_borrowck/borrowck/mod.rs

@@ -977,7 +977,11 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                         if let Categorization::Local(local_id) = err.cmt.cat {
                             let span = self.tcx.map.span(local_id);
                             if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(span) {
-                                if snippet != "self" {
+                                if snippet.starts_with("ref ") {
+                                    db.span_label(span,
+                                        &format!("use `{}` here to make mutable",
+                                            snippet.replace("ref ", "ref mut ")));
+                                } else if snippet != "self" {
                                     db.span_label(span,
                                         &format!("use `mut {}` here to make mutable", snippet));
                                 }

src/test/compile-fail/issue-33819.rs

@@ -0,0 +1,9 @@
+fn main() {
+    let mut op = Some(2);
+    match op {
+        Some(ref v) => { let a = &mut v; },
+        //~^ ERROR:cannot borrow immutable
+        //~| use `ref mut v` here to make mutable
+        None => {},
+    }
+}

src/test/ui/mismatched_types/main.stderr

@@ -1,8 +1,10 @@
 error: mismatched types [--explain E0308]
   --> $DIR/main.rs:14:18
+   |>
 14 |>     let x: u32 = (
    |>                  ^ expected u32, found ()
 note: expected type `u32`
 note:    found type `()`
 
+
 error: aborting due to previous error