Add test case for rust-lang#57478

Author: eholk

src/test/ui/generator/issue-57478.rs

@@ -0,0 +1,14 @@
+#![feature(negative_impls, generators)]
+
+struct Foo;
+impl !Send for Foo {}
+
+fn main() {
+    assert_send(|| {
+        let guard = Foo;
+        drop(guard);
+        yield;
+    })
+}
+
+fn assert_send<T: Send>(_: T) {}