Add niche-in-generator test

Author: tmandry

src/test/run-pass/generator/niche-in-generator.rs

@@ -0,0 +1,17 @@
+// Test that niche finding works with captured generator upvars.
+
+#![feature(generators)]
+
+use std::mem::size_of_val;
+
+fn take<T>(_: T) {}
+
+fn main() {
+    let x = false;
+    let gen1 = || {
+        yield;
+        take(x);
+    };
+
+    assert_eq!(size_of_val(&gen1), size_of_val(&Some(gen1)));
+}