Add UI test for #49296

Author: memoryruins

src/test/ui/consts/const-eval/issue-49296.rs

@@ -0,0 +1,23 @@
+// issue-49296: Unsafe shenigans in constants can result in missing errors
+
+#![feature(const_fn)]
+#![feature(const_fn_union)]
+
+const unsafe fn transmute<T: Copy, U: Copy>(t: T) -> U {
+    union Transmute<T: Copy, U: Copy> {
+        from: T,
+        to: U,
+    }
+
+    Transmute { from: t }.to
+}
+
+const fn wat(x: u64) -> &'static u64 {
+    unsafe { transmute(&x) }
+}
+const X: u64 = *wat(42);
+//~^ ERROR any use of this value will cause an error
+
+fn main() {
+    println!("{}", X);
+}

src/test/ui/consts/const-eval/issue-49296.stderr

@@ -0,0 +1,12 @@
+error: any use of this value will cause an error
+  --> $DIR/issue-49296.rs:18:1
+   |
+LL | const X: u64 = *wat(42);
+   | ^^^^^^^^^^^^^^^--------^
+   |                |
+   |                dangling pointer was dereferenced
+   |
+   = note: #[deny(const_err)] on by default
+
+error: aborting due to previous error
+