Fix wrong test replacemen in ref mut suggestion

src/librustc_borrowck/borrowck/mod.rs

@@ -1196,14 +1196,18 @@ impl<'a, 'tcx> BorrowckCtxt<'a, 'tcx> {
                 let let_span = self.tcx.hir.span(node_id);
                 match self.local_binding_mode(node_id) {
                     ty::BindByReference(..) => {
-                        let snippet = self.tcx.sess.codemap().span_to_snippet(let_span);
-                        if let Ok(snippet) = snippet {
-                            db.span_label(
+                        if let Ok(snippet) = self.tcx.sess.codemap().span_to_snippet(let_span) {
+                            let replace_str = if snippet.starts_with("ref ") {
+                                snippet.replacen("ref ", "ref mut ", 1)
+                            } else {
+                                snippet
+                            };
+                            db.span_suggestion(
                                 let_span,
-                                format!("consider changing this to `{}`",
-                                         snippet.replace("ref ", "ref mut "))
+                                "use a mutable reference instead",
+                                replace_str,
                             );
-                        }
+                        };
                     }
                     ty::BindByValue(..) => {
                         if let (Some(local_ty), is_implicit_self) = self.local_ty(node_id) {

src/test/ui/nll/issue-51244.rs

@@ -0,0 +1,16 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(nll)]
+
+fn main() {
+    let ref my_ref @ _ = 0;
+    *my_ref = 0; //~ ERROR cannot assign to data in a `&` reference [E0594]
+}

src/test/ui/nll/issue-51244.stderr

@@ -0,0 +1,11 @@
+error[E0594]: cannot assign to data in a `&` reference
+  --> $DIR/issue-51244.rs:15:5
+   |
+LL |     let ref my_ref @ _ = 0;
+   |         -------------- help: consider changing this to be a mutable reference: `&mut ef my_ref @ _`
+LL |     *my_ref = 0; //~ ERROR cannot assign to data in a `&` reference [E0594]
+   |     ^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.

src/test/ui/rfc-2005-default-binding-mode/enum.stderr

@@ -2,23 +2,23 @@ error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:19:5
    |
 LL |     let Wrap(x) = &Wrap(3);
-   |              - consider changing this to `x`
+   |              - help: use a mutable reference instead: `x`
 LL |     *x += 1; //~ ERROR cannot assign to immutable
    |     ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:23:9
    |
 LL |     if let Some(x) = &Some(3) {
-   |                 - consider changing this to `x`
+   |                 - help: use a mutable reference instead: `x`
 LL |         *x += 1; //~ ERROR cannot assign to immutable
    |         ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*x`
   --> $DIR/enum.rs:29:9
    |
 LL |     while let Some(x) = &Some(3) {
-   |                    - consider changing this to `x`
+   |                    - help: use a mutable reference instead: `x`
 LL |         *x += 1; //~ ERROR cannot assign to immutable
    |         ^^^^^^^ cannot borrow as mutable
 

src/test/ui/rfc-2005-default-binding-mode/explicit-mut.stderr

@@ -2,23 +2,23 @@ error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:17:13
    |
 LL |         Some(n) => {
-   |              - consider changing this to `n`
+   |              - help: use a mutable reference instead: `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:25:13
    |
 LL |         Some(n) => {
-   |              - consider changing this to `n`
+   |              - help: use a mutable reference instead: `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 
 error[E0594]: cannot assign to immutable borrowed content `*n`
   --> $DIR/explicit-mut.rs:33:13
    |
 LL |         Some(n) => {
-   |              - consider changing this to `n`
+   |              - help: use a mutable reference instead: `n`
 LL |             *n += 1; //~ ERROR cannot assign to immutable
    |             ^^^^^^^ cannot borrow as mutable
 

src/test/ui/suggestions/issue-51244.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let ref my_ref @ _ = 0;
+    *my_ref = 0; //~ ERROR cannot assign to immutable borrowed content `*my_ref` [E0594]
+}

src/test/ui/suggestions/issue-51244.stderr

@@ -0,0 +1,11 @@
+error[E0594]: cannot assign to immutable borrowed content `*my_ref`
+  --> $DIR/issue-51244.rs:13:5
+   |
+LL |     let ref my_ref @ _ = 0;
+   |         -------------- help: use a mutable reference instead: `ref mut my_ref @ _`
+LL |     *my_ref = 0; //~ ERROR cannot assign to immutable borrowed content `*my_ref` [E0594]
+   |     ^^^^^^^^^^^ cannot borrow as mutable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.