add a regression test

Author: WaffleLapkin

tests/ui/explicit-tail-calls/two-phase.rs

@@ -0,0 +1,11 @@
+// regression test for <https://github.com/rust-lang/rust/issues/112788>
+//@ check-pass
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+fn f(x: &mut ()) {
+    let _y = String::new();
+    become f(x);
+}
+
+fn main() {}