Adding xfailed test for #3874

Author: catamorphism

src/test/run-pass/issue-3874.rs

@@ -0,0 +1,9 @@
+// xfail-test
+enum PureCounter { PureCounter(uint) }
+
+pure fn each(self: PureCounter, blk: fn(v: &uint)) {
+    let PureCounter(ref x) = self;
+    blk(x);
+}
+
+fn main() {}