offset_from: "the difference must fit in an isize" is a corollary

also, isize::MIN is an impossible distance

Author: RalfJung

core/src/ptr/const_ptr.rs

@@ -611,8 +611,7 @@ impl<T: ?Sized> *const T {
     ///
     /// # Safety
     ///
-    /// If any of the following conditions are violated, the result is Undefined
-    /// Behavior:
+    /// If any of the following conditions are violated, the result is Undefined Behavior:
     ///
     /// * `self` and `origin` must either
     ///
@@ -623,26 +622,10 @@ impl<T: ?Sized> *const T {
     /// * The distance between the pointers, in bytes, must be an exact multiple
     ///   of the size of `T`.
     ///
-    /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
-    ///
-    /// * The distance being in bounds cannot rely on "wrapping around" the address space.
-    ///
-    /// Rust types are never larger than `isize::MAX` and Rust allocations never wrap around the
-    /// address space, so two pointers within some value of any Rust type `T` will always satisfy
-    /// the last two conditions. The standard library also generally ensures that allocations
-    /// never reach a size where an offset is a concern. For instance, `Vec` and `Box` ensure they
-    /// never allocate more than `isize::MAX` bytes, so `ptr_into_vec.offset_from(vec.as_ptr())`
-    /// always satisfies the last two conditions.
-    ///
-    /// Most platforms fundamentally can't even construct such a large allocation.
-    /// For instance, no known 64-bit platform can ever serve a request
-    /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
-    /// However, some 32-bit and 16-bit platforms may successfully serve a request for
-    /// more than `isize::MAX` bytes with things like Physical Address
-    /// Extension. As such, memory acquired directly from allocators or memory
-    /// mapped files *may* be too large to handle with this function.
-    /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on
-    /// such large allocations either.)
+    /// As a consequence, the absolute distance between the pointers, **in bytes**, computed on
+    /// mathematical integers (without "wrapping around"), cannot overflow an `isize`. This is
+    /// implied by the in-bounds requirement, and the fact that no allocated object can be larger
+    /// than `isize::MAX` bytes.
     ///
     /// The requirement for pointers to be derived from the same allocated object is primarily
     /// needed for `const`-compatibility: the distance between pointers into *different* allocated

core/src/ptr/mut_ptr.rs

@@ -836,8 +836,7 @@ impl<T: ?Sized> *mut T {
     ///
     /// # Safety
     ///
-    /// If any of the following conditions are violated, the result is Undefined
-    /// Behavior:
+    /// If any of the following conditions are violated, the result is Undefined Behavior:
     ///
     /// * `self` and `origin` must either
     ///
@@ -848,26 +847,10 @@ impl<T: ?Sized> *mut T {
     /// * The distance between the pointers, in bytes, must be an exact multiple
     ///   of the size of `T`.
     ///
-    /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
-    ///
-    /// * The distance being in bounds cannot rely on "wrapping around" the address space.
-    ///
-    /// Rust types are never larger than `isize::MAX` and Rust allocations never wrap around the
-    /// address space, so two pointers within some value of any Rust type `T` will always satisfy
-    /// the last two conditions. The standard library also generally ensures that allocations
-    /// never reach a size where an offset is a concern. For instance, `Vec` and `Box` ensure they
-    /// never allocate more than `isize::MAX` bytes, so `ptr_into_vec.offset_from(vec.as_ptr())`
-    /// always satisfies the last two conditions.
-    ///
-    /// Most platforms fundamentally can't even construct such a large allocation.
-    /// For instance, no known 64-bit platform can ever serve a request
-    /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
-    /// However, some 32-bit and 16-bit platforms may successfully serve a request for
-    /// more than `isize::MAX` bytes with things like Physical Address
-    /// Extension. As such, memory acquired directly from allocators or memory
-    /// mapped files *may* be too large to handle with this function.
-    /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on
-    /// such large allocations either.)
+    /// As a consequence, the absolute distance between the pointers, in bytes, computed on
+    /// mathematical integers (without "wrapping around"), cannot overflow an `isize`. This is
+    /// implied by the in-bounds requirement, and the fact that no allocated object can be larger
+    /// than `isize::MAX` bytes.
     ///
     /// The requirement for pointers to be derived from the same allocated object is primarily
     /// needed for `const`-compatibility: the distance between pointers into *different* allocated

core/src/ptr/non_null.rs

@@ -761,38 +761,21 @@ impl<T: ?Sized> NonNull<T> {
     ///
     /// # Safety
     ///
-    /// If any of the following conditions are violated, the result is Undefined
-    /// Behavior:
+    /// If any of the following conditions are violated, the result is Undefined Behavior:
     ///
-    /// * Both `self` and `origin` must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * `self` and `origin` must either
     ///
-    /// * Both pointers must be *derived from* a pointer to the same object.
-    ///   (See below for an example.)
+    ///   * both be *derived from* a pointer to the same [allocated object], and the memory range between
+    ///     the two pointers must be either empty or in bounds of that object. (See below for an example.)
+    ///   * or both be derived from an integer literal/constant, and point to the same address.
     ///
     /// * The distance between the pointers, in bytes, must be an exact multiple
     ///   of the size of `T`.
     ///
-    /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
-    ///
-    /// * The distance being in bounds cannot rely on "wrapping around" the address space.
-    ///
-    /// Rust types are never larger than `isize::MAX` and Rust allocations never wrap around the
-    /// address space, so two pointers within some value of any Rust type `T` will always satisfy
-    /// the last two conditions. The standard library also generally ensures that allocations
-    /// never reach a size where an offset is a concern. For instance, `Vec` and `Box` ensure they
-    /// never allocate more than `isize::MAX` bytes, so `ptr_into_vec.offset_from(vec.as_ptr())`
-    /// always satisfies the last two conditions.
-    ///
-    /// Most platforms fundamentally can't even construct such a large allocation.
-    /// For instance, no known 64-bit platform can ever serve a request
-    /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
-    /// However, some 32-bit and 16-bit platforms may successfully serve a request for
-    /// more than `isize::MAX` bytes with things like Physical Address
-    /// Extension. As such, memory acquired directly from allocators or memory
-    /// mapped files *may* be too large to handle with this function.
-    /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on
-    /// such large allocations either.)
+    /// As a consequence, the absolute distance between the pointers, in bytes, computed on
+    /// mathematical integers (without "wrapping around"), cannot overflow an `isize`. This is
+    /// implied by the in-bounds requirement, and the fact that no allocated object can be larger
+    /// than `isize::MAX` bytes.
     ///
     /// The requirement for pointers to be derived from the same allocated object is primarily
     /// needed for `const`-compatibility: the distance between pointers into *different* allocated