Fix while_let_on_iterator

Reborrow mutable references rather then take a reference to them.

tests/ui/while_let_on_iterator.fixed

@@ -334,6 +334,29 @@ fn issue7249() {
     x();
 }
 
+fn issue7510() {
+    let mut it = 0..10;
+    let it = &mut it;
+    // Needs to reborrow `it` as the binding isn't mutable
+    for x in &mut *it {
+        if x % 2 == 0 {
+            break;
+        }
+    }
+    println!("{}", it.next().unwrap());
+
+    struct S<T>(T);
+    let mut it = 0..10;
+    let it = S(&mut it);
+    // Needs to reborrow `it.0` as the binding isn't mutable
+    for x in &mut *it.0 {
+        if x % 2 == 0 {
+            break;
+        }
+    }
+    println!("{}", it.0.next().unwrap());
+}
+
 fn main() {
     let mut it = 0..20;
     for _ in it {

tests/ui/while_let_on_iterator.rs

@@ -334,6 +334,29 @@ fn issue7249() {
     x();
 }
 
+fn issue7510() {
+    let mut it = 0..10;
+    let it = &mut it;
+    // Needs to reborrow `it` as the binding isn't mutable
+    while let Some(x) = it.next() {
+        if x % 2 == 0 {
+            break;
+        }
+    }
+    println!("{}", it.next().unwrap());
+
+    struct S<T>(T);
+    let mut it = 0..10;
+    let it = S(&mut it);
+    // Needs to reborrow `it.0` as the binding isn't mutable
+    while let Some(x) = it.0.next() {
+        if x % 2 == 0 {
+            break;
+        }
+    }
+    println!("{}", it.0.next().unwrap());
+}
+
 fn main() {
     let mut it = 0..20;
     while let Some(..) = it.next() {

tests/ui/while_let_on_iterator.stderr

@@ -111,10 +111,22 @@ LL |         while let Some(x) = it.next() {
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for x in &mut it`
 
 error: this loop could be written as a `for` loop
-  --> $DIR/while_let_on_iterator.rs:339:5
+  --> $DIR/while_let_on_iterator.rs:341:5
+   |
+LL |     while let Some(x) = it.next() {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for x in &mut *it`
+
+error: this loop could be written as a `for` loop
+  --> $DIR/while_let_on_iterator.rs:352:5
+   |
+LL |     while let Some(x) = it.0.next() {
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for x in &mut *it.0`
+
+error: this loop could be written as a `for` loop
+  --> $DIR/while_let_on_iterator.rs:362:5
    |
 LL |     while let Some(..) = it.next() {
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `for _ in it`
 
-error: aborting due to 19 previous errors
+error: aborting due to 21 previous errors