add a test for combining RPIT with explicit tail calls

tests/ui/explicit-tail-calls/rpit.rs

@@ -0,0 +1,20 @@
+#![feature(explicit_tail_calls)]
+#![expect(incomplete_features)]
+
+// Regression test for https://github.com/rust-lang/rust/issues/139305.
+//
+// Combining return position impl trait (RPIT) with guaranteed tail calls does not
+// currently work, but at least it does not ICE.
+
+fn foo(x: u32, y: u32) -> u32 {
+    x + y
+}
+
+fn bar(x: u32, y: u32) -> impl ToString {
+    become foo(x, y);
+    //~^ ERROR mismatched signatures
+}
+
+fn main() {
+    assert_eq!(bar(1, 2).to_string(), "3");
+}

tests/ui/explicit-tail-calls/rpit.stderr

@@ -0,0 +1,12 @@
+error: mismatched signatures
+  --> $DIR/rpit.rs:14:5
+   |
+LL |     become foo(x, y);
+   |     ^^^^^^^^^^^^^^^^
+   |
+   = note: `become` requires caller and callee to have matching signatures
+   = note: caller signature: `fn(u32, u32) -> impl ToString`
+   = note: callee signature: `fn(u32, u32) -> u32`
+
+error: aborting due to 1 previous error
+