Add tests to demonstrate explicit tail call bug

tests/ui/explicit-tail-calls/become-cast-return.rs

@@ -0,0 +1,22 @@
+//@ check-pass
+//@ ignore-backends: gcc
+//@ known-bug: #148239
+//@ compile-flags: -Zno-codegen
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+#[inline(never)]
+fn leaf(_: &Box<u8>) -> [u8; 1] {
+    [1]
+}
+
+#[inline(never)]
+fn dispatch(param: &Box<u8>) -> [u8; 1] {
+    become leaf(param)
+}
+
+fn main() {
+    let data = Box::new(0);
+    let out = dispatch(&data);
+    assert_eq!(out, [1]);
+}

tests/ui/explicit-tail-calls/become-indirect-return.rs

@@ -0,0 +1,21 @@
+//@ run-pass
+//@ ignore-backends: gcc
+//@ known-bug: #148239
+#![expect(incomplete_features)]
+#![feature(explicit_tail_calls)]
+
+#[inline(never)]
+fn op_dummy(_param: &Box<u8>) -> [u8; 24] {
+    [1; 24]
+}
+
+#[inline(never)]
+fn dispatch(param: &Box<u8>) -> [u8; 24] {
+    become op_dummy(param)
+}
+
+fn main() {
+    let param = Box::new(0);
+    let result = dispatch(&param);
+    assert_ne!(result, [1; 24]); // the data is not right!
+}