Add regression test for rust-lang#28935

Author: JohnTitor

src/test/ui/inference/issue-28935.rs

@@ -0,0 +1,9 @@
+// check-pass
+
+use std::cell::RefCell;
+
+pub fn f(v: Vec<RefCell<u8>>) {
+    let _t = &mut *v[0].borrow_mut();
+}
+
+fn main() {}