add regression test for something we fixed

tests/ui/consts/issue-105536-const-val-roundtrip-ptr-eq.rs

@@ -0,0 +1,31 @@
+// run-pass
+
+// This does not reflect a stable guarantee (we guarantee very little for equality of pointers
+// around `const`), but it would be good to understand what is happening if these assertions ever
+// fail.
+use std::ptr::NonNull;
+use std::slice::from_raw_parts;
+
+const PTR_U8: *const u8 = NonNull::dangling().as_ptr();
+const CONST_U8_REF: &[u8] = unsafe { from_raw_parts(PTR_U8, 0) };
+const CONST_U8_PTR: *const u8 = unsafe { from_raw_parts(PTR_U8, 0).as_ptr() };
+static STATIC_U8_REF: &[u8] = unsafe { from_raw_parts(PTR_U8, 0) };
+
+const PTR_U16: *const u16 = NonNull::dangling().as_ptr();
+const CONST_U16_REF: &[u16] = unsafe { from_raw_parts(PTR_U16, 0) };
+
+const fn const_u8_fn() -> &'static [u8] {
+    unsafe { from_raw_parts(PTR_U8, 0) }
+}
+
+fn main() {
+    let ptr_u8 = unsafe { from_raw_parts(PTR_U8, 0) }.as_ptr();
+    let ptr_u16 = unsafe { from_raw_parts(PTR_U16, 0) }.as_ptr();
+
+    assert_eq!(ptr_u8, PTR_U8);
+    assert_eq!(ptr_u8, CONST_U8_PTR);
+    assert_eq!(ptr_u8, const_u8_fn().as_ptr());
+    assert_eq!(ptr_u8, STATIC_U8_REF.as_ptr());
+    assert_eq!(ptr_u16, CONST_U16_REF.as_ptr());
+    assert_eq!(ptr_u8, CONST_U8_REF.as_ptr());
+}