Fix null pointer t est

Author: carbotaniuman

src/intptrcast.rs

@@ -121,8 +121,7 @@ impl<'mir, 'tcx> GlobalStateInner {
 
         let global_state = ecx.machine.intptrcast.borrow();
 
-        // Special-case NULL, it is always an invalid pointer.
-        if addr == 0 || global_state.provenance_mode == ProvenanceMode::Strict {
+        if global_state.provenance_mode == ProvenanceMode::Strict {
             Pointer::new(None, Size::from_bytes(addr))
         } else if global_state.provenance_mode == ProvenanceMode::Legacy {
             let alloc_id = Self::alloc_id_from_addr(ecx, addr);

tests/compile-fail/permissive_provenance_null.rs

@@ -41,8 +41,22 @@ fn ptr_roundtrip_imperfect() {
     assert_eq!(unsafe { *ptr }, 3);
 }
 
+/// Ensure that we can roundtrip through a pointer with an address of 0
+fn ptr_roundtrip_null() {
+    let x = &42;
+    let x_ptr = x as *const i32;
+    let x_null_ptr = x_ptr.with_addr(0); // addr 0, but still the provenance of x
+    let null = x_null_ptr.expose_addr();
+    assert_eq!(null, 0);
+
+    let x_null_ptr_copy = ptr::from_exposed_addr::<i32>(null); // just a roundtrip, so has provenance of x (angelically)
+    let x_ptr_copy = x_null_ptr_copy.with_addr(x_ptr.addr()); // addr of x and provenance of x
+    assert_eq!(unsafe { *x_ptr_copy }, 42);
+}
+
 fn main() {
     ptr_roundtrip_out_of_bounds();
     ptr_roundtrip_confusion();
     ptr_roundtrip_imperfect();
+    ptr_roundtrip_null();
 }