Add test

src/test/ui/suggestions/suggest-mut-method-for-loop.rs

@@ -0,0 +1,17 @@
+use std::collections::HashMap;
+struct X(usize);
+struct Y {
+    v: u32
+}
+
+fn main() {
+    let mut buzz = HashMap::new();
+    buzz.insert("a", Y { v: 0 });
+
+    for mut t in buzz.values() {
+                  //~^ HELP
+                  //~| SUGGESTION values_mut()
+        t.v += 1;
+        //~^ ERROR cannot assign
+    }
+}

src/test/ui/suggestions/suggest-mut-method-for-loop.stderr

@@ -0,0 +1,15 @@
+error[E0594]: cannot assign to `t.v` which is behind a `&` reference
+  --> $DIR/suggest-mut-method-for-loop.rs:14:9
+   |
+LL |     for mut t in buzz.values() {
+   |                  -------------
+   |                  |    |
+   |                  |    help: use mutable method: `values_mut()`
+   |                  this iterator yields `&` references
+...
+LL |         t.v += 1;
+   |         ^^^^^^^^ `t` is a `&` reference, so the data it refers to cannot be written
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0594`.