Move issue-42106 test from compile-fail to ui

src/test/ui/issue-42106.rs

@@ -10,7 +10,7 @@
 
 fn do_something<T>(collection: &mut Vec<T>) {
     let _a = &collection;
-    collection.swap(1, 2); //~ ERROR E0502
+    collection.swap(1, 2);
 }
 
 fn main() {}

src/test/ui/issue-42106.stderr

@@ -0,0 +1,12 @@
+error[E0502]: cannot borrow `*collection` as mutable because `collection` is also borrowed as immutable
+  --> $DIR/issue-42106.rs:13:5
+   |
+12 |     let _a = &collection;
+   |               ---------- immutable borrow occurs here
+13 |     collection.swap(1, 2);
+   |     ^^^^^^^^^^ mutable borrow occurs here
+14 | }
+   | - immutable borrow ends here
+
+error: aborting due to 2 previous errors
+