offset: allow zero-byte offset on arbitrary pointers

Author: RalfJung

library/core/src/intrinsics.rs

@@ -1421,10 +1421,10 @@ extern "rust-intrinsic" {
     ///
     /// # Safety
     ///
-    /// Both the starting and resulting pointer must be either in bounds or one
-    /// byte past the end of an allocated object. If either pointer is out of
-    /// bounds or arithmetic overflow occurs then any further use of the
-    /// returned value will result in undefined behavior.
+    /// If the computed offset is non-zero, then both the starting and resulting pointer must be
+    /// either in bounds or one byte past the end of an allocated object. If either pointer is out
+    /// of bounds or arithmetic overflow occurs then any further use of the returned value will
+    /// result in undefined behavior.
     ///
     /// The stabilized version of this intrinsic is [`pointer::offset`].
     #[must_use = "returns a new pointer rather than modifying its argument"]

library/core/src/ptr/const_ptr.rs

@@ -416,8 +416,8 @@ impl<T: ?Sized> *const T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * If the computed offset is non-zero, then both the starting and resulting pointer must be
+    ///   either in bounds or one byte past the end of the same [allocated object].
     ///
     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
     ///
@@ -891,8 +891,8 @@ impl<T: ?Sized> *const T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * If the computed offset is non-zero, then both the starting and resulting pointer must be
+    ///   either in bounds or one byte past the end of the same [allocated object].
     ///
     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
     ///
@@ -975,8 +975,8 @@ impl<T: ?Sized> *const T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * If the computed offset is non-zero, then both the starting and resulting pointer must be
+    ///   either in bounds or one byte past the end of the same [allocated object].
     ///
     /// * The computed offset cannot exceed `isize::MAX` **bytes**.
     ///

library/core/src/ptr/mut_ptr.rs

@@ -429,8 +429,8 @@ impl<T: ?Sized> *mut T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * If the computed offset is non-zero, then both the starting and resulting pointer must be
+    ///   either in bounds or one byte past the end of the same [allocated object].
     ///
     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
     ///
@@ -992,8 +992,8 @@ impl<T: ?Sized> *mut T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * If the computed offset is non-zero, then both the starting and resulting pointer must be
+    ///   either in bounds or one byte past the end of the same [allocated object].
     ///
     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
     ///
@@ -1076,8 +1076,8 @@ impl<T: ?Sized> *mut T {
     /// If any of the following conditions are violated, the result is Undefined
     /// Behavior:
     ///
-    /// * Both the starting and resulting pointer must be either in bounds or one
-    ///   byte past the end of the same [allocated object].
+    /// * If the computed offset is non-zero, then both the starting and resulting pointer must be
+    ///   either in bounds or one byte past the end of the same [allocated object].
     ///
     /// * The computed offset cannot exceed `isize::MAX` **bytes**.
     ///