Add a regression test for #313

Author: oli-obk

tests/run-pass/btreemap.rs

@@ -0,0 +1,14 @@
+#[derive(PartialEq, Eq, PartialOrd, Ord)]
+pub enum Foo {
+    A(&'static str),
+    _B,
+    _C,
+}
+
+pub fn main() {
+    let mut b = std::collections::BTreeSet::new();
+    b.insert(Foo::A("\'"));
+    b.insert(Foo::A("/="));
+    b.insert(Foo::A("#"));
+    b.insert(Foo::A("0o"));
+}