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

RalfJung · RalfJung · commit cc4242b209fd · 2024-09-08T19:07:46.000+02:00
diff --git a/core/src/ptr/const_ptr.rs b/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)
     }
 
diff --git a/core/src/ptr/mut_ptr.rs b/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.

Original file line number	Diff line number	Diff line change
`@@ -40,15 +40,17 @@ impl<T: ?Sized> *const T {`
`40`	`40`
`41`	`41`	`#[inline]`
`42`	`42`	`const fn const_impl(ptr: *const u8) -> bool {`
`43`		`- // Compare via a cast to a thin pointer, so fat pointers are only`
`44`		`- // considering their "data" part for null-ness.`
`45`	`43`	`match (ptr).guaranteed_eq(null_mut()) {`
`46`		`- None => false,`
`47`	`44`	`Some(res) => res,`
	`45`	`+ // To remain maximally convervative, we stop execution when we don't`
	`46`	`+ // know whether the pointer is null or not.`
	`47`	+ // We can not return `false` here, that would be unsound in `NonNull::new`!
	`48`	`+ None => panic!("null-ness of this pointer cannot be determined in const context"),`
`48`	`49`	`}`
`49`	`50`	`}`
`50`	`51`
`51`		`- #[allow(unused_unsafe)]`
	`52`	`+ // Compare via a cast to a thin pointer, so fat pointers are only`
	`53`	`+ // considering their "data" part for null-ness.`
`52`	`54`	`const_eval_select((self as *const u8,), const_impl, runtime_impl)`
`53`	`55`	`}`
`54`	`56`