Add test that uninhabited repr(transparent) type has same function return ABI as wrapped type.

Author: zachs18

tests/ui/uninhabited/uninhabited-transparent-return-abi.rs

@@ -0,0 +1,33 @@
+//@ run-pass
+//@ needs-unwind
+// See https://github.com/rust-lang/rust/issues/135802
+
+enum Void {}
+
+// Should be ABI-compatible with T, but wasn't prior to the PR adding this test.
+#[repr(transparent)]
+struct NoReturn<T>(T, Void);
+
+// Returned by invisible reference (in most ABIs)
+#[allow(dead_code)]
+struct Large(u64, u64, u64);
+
+// Prior to the PR adding this test, this function had a different ABI than
+// `fn() -> Large` (on `x86_64-unknown-linux-gnu` at least), so calling it as `fn() -> Large`
+// would pass the return place pointer in rdi and `correct` in rsi, but the function
+// would expect `correct` in rdi.
+fn never(correct: &mut bool) -> NoReturn<Large> {
+    *correct = true;
+    panic!("catch this")
+}
+
+fn main() {
+    let mut correct = false;
+    let never: fn(&mut bool) -> NoReturn<Large> = never;
+    // Safety: `NoReturn<Large>` is a `repr(transparent)` wrapper around `Large`,
+    // so they should be ABI-compatible.
+    let never: fn(&mut bool) -> Large = unsafe { std::mem::transmute(never) };
+    let result = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| never(&mut correct)));
+    assert!(result.is_err(), "function should have panicked");
+    assert!(correct, "function should have stored `true` into `correct`");
+}