Add test for issue-44405

Author: JohnTitor

src/test/ui/issues/issue-44405.rs

@@ -0,0 +1,22 @@
+use std::ops::Index;
+
+struct Test;
+struct Container(Test);
+
+impl Test {
+    fn test(&mut self) {}
+}
+
+impl<'a> Index<&'a bool> for Container {
+    type Output = Test;
+
+    fn index(&self, _index: &'a bool) -> &Test {
+        &self.0
+    }
+}
+
+fn main() {
+    let container = Container(Test);
+    let mut val = true;
+    container[&mut val].test(); //~ ERROR: cannot borrow data
+}

src/test/ui/issues/issue-44405.stderr

@@ -0,0 +1,11 @@
+error[E0596]: cannot borrow data in an index of `Container` as mutable
+  --> $DIR/issue-44405.rs:21:5
+   |
+LL |     container[&mut val].test();
+   |     ^^^^^^^^^^^^^^^^^^^ cannot borrow as mutable
+   |
+   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `Container`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0596`.