Add regression test for #25954 (cyclic closure type), which is now

impossible.

src/test/compile-fail/closure-referencing-itself-issue-25954.rs

@@ -0,0 +1,28 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #25954: detect and reject a closure type that
+// references itself.
+
+use std::cell::{Cell, RefCell};
+
+struct A<T: Fn()> {
+    x: RefCell<Option<T>>,
+    b: Cell<i32>,
+}
+
+fn main() {
+    let mut p = A{x: RefCell::new(None), b: Cell::new(4i32)};
+
+    // This is an error about types of infinite size:
+    let q = || p.b.set(5i32); //~ ERROR mismatched types
+
+    *(p.x.borrow_mut()) = Some(q);
+}