Add test for impl Fn associativity

Author: WaffleLapkin

src/test/ui/impl-trait/impl_fn_associativity.rs

@@ -0,0 +1,16 @@
+// run-pass
+use std::fmt::Debug;
+
+fn f_debug() -> impl Fn() -> impl Debug {
+    || ()
+}
+
+fn ff_debug() -> impl Fn() -> impl Fn() -> impl Debug {
+    || f_debug()
+}
+
+fn main() {
+    // Check that `ff_debug` is `() -> (() -> Debug)` and not `(() -> ()) -> Debug`
+    let debug = ff_debug()()();
+    assert_eq!(format!("{:?}", debug), "()");
+}