Rollup merge of rust-lang#130107 - RalfJung:const-ptr-is-null, r=oli-obk

const: make ptr.is_null() stop execution on ambiguity

This seems better than saying `false` -- saying `false` is in fact actively unsound if `NonNull` then uses this to permit putting this pointer inside of it, but at runtime it turns out to be null.

Part of rust-lang#74939
Cc ```@rust-lang/wg-const-eval```

core/src/ptr/const_ptr.rs

@@ -40,15 +40,17 @@ impl<T: ?Sized> *const T {
 
         #[inline]
         const fn const_impl(ptr: *const u8) -> bool {
-            // Compare via a cast to a thin pointer, so fat pointers are only
-            // considering their "data" part for null-ness.
             match (ptr).guaranteed_eq(null_mut()) {
-                None => false,
                 Some(res) => res,
+                // To remain maximally convervative, we stop execution when we don't
+                // know whether the pointer is null or not.
+                // We can *not* return `false` here, that would be unsound in `NonNull::new`!
+                None => panic!("null-ness of this pointer cannot be determined in const context"),
             }
         }
 
-        #[allow(unused_unsafe)]
+        // Compare via a cast to a thin pointer, so fat pointers are only
+        // considering their "data" part for null-ness.
         const_eval_select((self as *const u8,), const_impl, runtime_impl)
     }
 

core/src/ptr/mut_ptr.rs

@@ -33,22 +33,7 @@ impl<T: ?Sized> *mut T {
     #[rustc_diagnostic_item = "ptr_is_null"]
     #[inline]
     pub const fn is_null(self) -> bool {
-        #[inline]
-        fn runtime_impl(ptr: *mut u8) -> bool {
-            ptr.addr() == 0
-        }
-
-        #[inline]
-        const fn const_impl(ptr: *mut u8) -> bool {
-            // Compare via a cast to a thin pointer, so fat pointers are only
-            // considering their "data" part for null-ness.
-            match (ptr).guaranteed_eq(null_mut()) {
-                None => false,
-                Some(res) => res,
-            }
-        }
-
-        const_eval_select((self as *mut u8,), const_impl, runtime_impl)
+        self.cast_const().is_null()
     }
 
     /// Casts to a pointer of another type.