offset_from / sub_ptr docs: emphasize that pointers must be in the same allocation

Author: RalfJung

Diff for: core/src/ptr/const_ptr.rs

@@ -582,7 +582,7 @@ impl<T: ?Sized> *const T {
         intrinsics::ptr_mask(self.cast::<()>(), mask).with_metadata_of(self)
     }
 
-    /// Calculates the distance between two pointers. The returned value is in
+    /// Calculates the distance between two pointers within the same allocation. The returned value is in
     /// units of T: the distance in bytes divided by `mem::size_of::<T>()`.
     ///
     /// This is equivalent to `(self as isize - origin as isize) / (mem::size_of::<T>() as isize)`,
@@ -677,7 +677,7 @@ impl<T: ?Sized> *const T {
         unsafe { intrinsics::ptr_offset_from(self, origin) }
     }
 
-    /// Calculates the distance between two pointers. The returned value is in
+    /// Calculates the distance between two pointers within the same allocation. The returned value is in
     /// units of **bytes**.
     ///
     /// This is purely a convenience for casting to a `u8` pointer and
@@ -695,7 +695,7 @@ impl<T: ?Sized> *const T {
         unsafe { self.cast::<u8>().offset_from(origin.cast::<u8>()) }
     }
 
-    /// Calculates the distance between two pointers, *where it's known that
+    /// Calculates the distance between two pointers within the same allocation, *where it's known that
     /// `self` is equal to or greater than `origin`*. The returned value is in
     /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
     ///
@@ -790,7 +790,7 @@ impl<T: ?Sized> *const T {
         unsafe { intrinsics::ptr_offset_from_unsigned(self, origin) }
     }
 
-    /// Calculates the distance between two pointers, *where it's known that
+    /// Calculates the distance between two pointers within the same allocation, *where it's known that
     /// `self` is equal to or greater than `origin`*. The returned value is in
     /// units of **bytes**.
     ///

Diff for: core/src/ptr/mut_ptr.rs

@@ -746,7 +746,7 @@ impl<T: ?Sized> *mut T {
         (self as *const T).guaranteed_ne(other as _)
     }
 
-    /// Calculates the distance between two pointers. The returned value is in
+    /// Calculates the distance between two pointers within the same allocation. The returned value is in
     /// units of T: the distance in bytes divided by `mem::size_of::<T>()`.
     ///
     /// This is equivalent to `(self as isize - origin as isize) / (mem::size_of::<T>() as isize)`,
@@ -839,7 +839,7 @@ impl<T: ?Sized> *mut T {
         unsafe { (self as *const T).offset_from(origin) }
     }
 
-    /// Calculates the distance between two pointers. The returned value is in
+    /// Calculates the distance between two pointers within the same allocation. The returned value is in
     /// units of **bytes**.
     ///
     /// This is purely a convenience for casting to a `u8` pointer and
@@ -857,7 +857,7 @@ impl<T: ?Sized> *mut T {
         unsafe { self.cast::<u8>().offset_from(origin.cast::<u8>()) }
     }
 
-    /// Calculates the distance between two pointers, *where it's known that
+    /// Calculates the distance between two pointers within the same allocation, *where it's known that
     /// `self` is equal to or greater than `origin`*. The returned value is in
     /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
     ///
@@ -930,7 +930,7 @@ impl<T: ?Sized> *mut T {
         unsafe { (self as *const T).sub_ptr(origin) }
     }
 
-    /// Calculates the distance between two pointers, *where it's known that
+    /// Calculates the distance between two pointers within the same allocation, *where it's known that
     /// `self` is equal to or greater than `origin`*. The returned value is in
     /// units of **bytes**.
     ///

Diff for: core/src/ptr/non_null.rs

@@ -676,7 +676,7 @@ impl<T: ?Sized> NonNull<T> {
         unsafe { NonNull { pointer: self.pointer.byte_sub(count) } }
     }
 
-    /// Calculates the distance between two pointers. The returned value is in
+    /// Calculates the distance between two pointers within the same allocation. The returned value is in
     /// units of T: the distance in bytes divided by `mem::size_of::<T>()`.
     ///
     /// This is equivalent to `(self as isize - origin as isize) / (mem::size_of::<T>() as isize)`,
@@ -773,7 +773,7 @@ impl<T: ?Sized> NonNull<T> {
         unsafe { self.pointer.offset_from(origin.pointer) }
     }
 
-    /// Calculates the distance between two pointers. The returned value is in
+    /// Calculates the distance between two pointers within the same allocation. The returned value is in
     /// units of **bytes**.
     ///
     /// This is purely a convenience for casting to a `u8` pointer and
@@ -793,7 +793,7 @@ impl<T: ?Sized> NonNull<T> {
 
     // N.B. `wrapping_offset``, `wrapping_add`, etc are not implemented because they can wrap to null
 
-    /// Calculates the distance between two pointers, *where it's known that
+    /// Calculates the distance between two pointers within the same allocation, *where it's known that
     /// `self` is equal to or greater than `origin`*. The returned value is in
     /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
     ///
@@ -866,7 +866,7 @@ impl<T: ?Sized> NonNull<T> {
         unsafe { self.pointer.sub_ptr(subtracted.pointer) }
     }
 
-    /// Calculates the distance between two pointers, *where it's known that
+    /// Calculates the distance between two pointers within the same allocation, *where it's known that
     /// `self` is equal to or greater than `origin`*. The returned value is in
     /// units of **bytes**.
     ///