add regression test for rust-lang#60629

Author: hellow554

src/test/incremental/issue-60629.rs

@@ -0,0 +1,10 @@
+// revisions:rpass1 rpass2
+
+struct A;
+
+#[cfg(rpass2)]
+impl From<A> for () {
+    fn from(_: A) {}
+}
+
+fn main() {}