add test for nasty example

tests/compile-fail/strict_provenance_transmute.rs

@@ -0,0 +1,27 @@
+// compile-flags: -Zmiri-strict-provenance
+#![feature(strict_provenance)]
+
+use std::mem;
+
+// This is the example from
+// <https://github.com/rust-lang/unsafe-code-guidelines/issues/286#issuecomment-1085144431>.
+
+unsafe fn deref(left: *const u8, right: *const u8) {
+    let left_int: usize = mem::transmute(left); //~ERROR expected initialized plain (non-pointer) bytes
+    let right_int: usize = mem::transmute(right);
+    if left_int == right_int {
+        // The compiler is allowed to replace `left_int` by `right_int` here...
+        let left_ptr: *const u8 = mem::transmute(left_int);
+        // ...which however means here it could be dereferencing the wrong pointer.
+        let _val = *left_ptr;
+    }
+}
+
+fn main() {
+    let ptr1 = &0u8 as *const u8;
+    let ptr2 = &1u8 as *const u8;
+    unsafe {
+        // Two pointers with the same address but different provenance.
+        deref(ptr1, ptr2.with_addr(ptr1.addr()));
+    }
+}