Add test for rust-lang#53678.

Author: Centril

src/test/ui/existential_types/issue-53678-generator-and-const-fn.rs

@@ -0,0 +1,19 @@
+// check-pass
+
+#![feature(const_fn, generators, generator_trait, existential_type)]
+
+use std::ops::Generator;
+
+existential type GenOnce<Y, R>: Generator<Yield = Y, Return = R>;
+
+const fn const_generator<Y, R>(yielding: Y, returning: R) -> GenOnce<Y, R> {
+    move || {
+        yield yielding;
+
+        return returning;
+    }
+}
+
+const FOO: GenOnce<usize, usize> = const_generator(10, 100);
+
+fn main() {}