Add tests for Result<u32, (u32, !)> layout. The comments will be accurate after the commits which add the relevant layout optimizations.

Author: zachs18

tests/ui/consts/const-eval/ub-enum.rs

@@ -100,6 +100,11 @@ const BAD_UNINHABITED_WITH_DATA1: Result<(i32, Never), (i32, !)> = unsafe { mem:
 const BAD_UNINHABITED_WITH_DATA2: Result<(i32, !), (i32, Never)> = unsafe { mem::transmute(0u64) };
 //~^ ERROR uninhabited enum variant
 
+// All variants have same-size data but only one inhabited.
+// Use `0` as constant to make behavior endianness-independent.
+const GOOD_SEMIINHABITED_WITH_DATA1: Result<i32, (i32, !)> = unsafe { mem::transmute(0u64) };
+const GOOD_SEMIINHABITED_WITH_DATA2: Result<(i32, !), i32> = unsafe { mem::transmute(1u64) };
+
 const TEST_ICE_89765: () = {
     // This is a regression test for https://github.com/rust-lang/rust/issues/89765.
     unsafe {

tests/ui/consts/const-eval/ub-enum.stderr

@@ -121,7 +121,7 @@ LL | const BAD_UNINHABITED_WITH_DATA2: Result<(i32, !), (i32, Never)> = unsafe {
    |                                                                             ^^^^^^^^^^^^^^^^^^^^ evaluation of `BAD_UNINHABITED_WITH_DATA2` failed here
 
 error[E0080]: read discriminant of an uninhabited enum variant
-  --> $DIR/ub-enum.rs:106:9
+  --> $DIR/ub-enum.rs:111:9
    |
 LL |         std::mem::discriminant(&*(&() as *const () as *const Never));
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `TEST_ICE_89765` failed inside this call