Insert missing maybe_print_comment call for class destructors

While it wasn't the original issue, the first program from #2487
failed the pretty-printing test because of this, so it's still a good
test case :-)

It also turns out that the second program from #2487 now triggers a
kind error, so I figured I might as well add it as a test case.

Author: catamorphism

src/libsyntax/print/pprust.rs

@@ -539,6 +539,7 @@ fn print_item(s: ps, &&item: @ast::item) {
           print_block(s, ctor.node.body);
           option::iter(m_dtor) {|dtor|
             hardbreak_if_not_bol(s);
+            maybe_print_comment(s, dtor.span.lo);
             head(s, "drop");
             print_block(s, dtor.node.body);
           }

src/test/compile-fail/issue-2487-b.rs

@@ -0,0 +1,19 @@
+class socket {
+    let sock: int;
+
+    new() { self.sock = 1; }
+
+    drop { }
+
+    fn set_identity()  {
+      closure { ||
+        setsockopt_bytes(self.sock) //! ERROR copying a noncopyable value
+      } 
+    }
+}
+
+fn closure(f: fn@()) { f() }
+
+fn setsockopt_bytes(+_sock: int) { }
+
+fn main() {}

src/test/run-pass/issue-2487-a.rs

@@ -0,0 +1,19 @@
+class socket {
+    let sock: int;
+
+    new() { self.sock = 1; }
+
+    drop { }
+
+    fn set_identity()  {
+        closure { ||
+            setsockopt_bytes(copy self.sock)
+        }
+    }
+}
+
+fn closure(f: fn()) { f() }
+
+fn setsockopt_bytes(_sock: int) { }
+
+fn main() {}