Add regression test for negative case

Author: estebank

src/test/ui/suggestions/mut-ref-reassignment.rs

@@ -1,5 +1,17 @@
-fn change_opt(opt: &mut Option<String>){
+fn suggestion(opt: &mut Option<String>) {
+    opt = None; //~ ERROR mismatched types
+}
+
+fn no_suggestion(opt: &mut Result<String, ()>) {
     opt = None //~ ERROR mismatched types
 }
 
+fn suggestion2(opt: &mut Option<String>) {
+    opt = Some(String::new())//~ ERROR mismatched types
+}
+
+fn no_suggestion2(opt: &mut Option<String>) {
+    opt = Some(42)//~ ERROR mismatched types
+}
+
 fn main() {}

src/test/ui/suggestions/mut-ref-reassignment.stderr

@@ -1,16 +1,47 @@
 error[E0308]: mismatched types
   --> $DIR/mut-ref-reassignment.rs:2:11
    |
-LL |     opt = None
+LL |     opt = None;
    |           ^^^^ expected mutable reference, found enum `std::option::Option`
    |
    = note: expected type `&mut std::option::Option<std::string::String>`
               found type `std::option::Option<_>`
 help: consider dereferencing here to assign to the mutable borrowed piece of memory
    |
-LL |     *opt = None
+LL |     *opt = None;
    |     ^^^^
 
-error: aborting due to previous error
+error[E0308]: mismatched types
+  --> $DIR/mut-ref-reassignment.rs:6:11
+   |
+LL |     opt = None
+   |           ^^^^ expected mutable reference, found enum `std::option::Option`
+   |
+   = note: expected type `&mut std::result::Result<std::string::String, ()>`
+              found type `std::option::Option<_>`
+
+error[E0308]: mismatched types
+  --> $DIR/mut-ref-reassignment.rs:10:11
+   |
+LL |     opt = Some(String::new())
+   |           ^^^^^^^^^^^^^^^^^^^ expected mutable reference, found enum `std::option::Option`
+   |
+   = note: expected type `&mut std::option::Option<std::string::String>`
+              found type `std::option::Option<std::string::String>`
+help: consider dereferencing here to assign to the mutable borrowed piece of memory
+   |
+LL |     *opt = Some(String::new())
+   |     ^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/mut-ref-reassignment.rs:14:11
+   |
+LL |     opt = Some(42)
+   |           ^^^^^^^^ expected mutable reference, found enum `std::option::Option`
+   |
+   = note: expected type `&mut std::option::Option<std::string::String>`
+              found type `std::option::Option<{integer}>`
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0308`.