Rollup merge of rust-lang#82751 - RalfJung:offset_from, r=dtolnay

improve offset_from docs

`@thomcc` pointed out that the current docs leave it kind of unclear how one can satisfy the "no wrapping around `isize` or the address space" requirement of `offset_from`, so make the docs clearer about that.

FWIW, I don't think I entirely agree with that second paragraph about large objects (that I left mostly unchanged here). LLVM, to my knowledge, fundamentally assumes that all allocations fit into an `isize::MAX`. So in that sense creating a larger allocation is simply UB. I would expect a guarantee that Rust heap allocation methods will never return allocations larger than `isize::MAX` (or rather, Rust heap allocation methods should require that the `Layout` is no larger than `isize::MAX`). However, I cannot find any such requirement documented currently. Large allocations are not mentioned at all in the allocator docs, which is quite surprising -- even if we say that such allocations are not insta-UB (which I think is incompatible with LLVM), they are still extremely footgunny since `ptr::offset`/`ptr::add` do not support offsetting by more than `isize::MAX` bytes.

Furthermore, the allocator docs don't even say anything about allocations wrapping around the address space. But that is certainly something allocators must ensure never happens; we cannot expect clients to defend against this.

Cc `@rust-lang/wg-allocators`

library/core/src/ptr/const_ptr.rs

@@ -320,25 +320,31 @@ impl<T: ?Sized> *const T {
     /// * Both pointers must be *derived from* a pointer to the same object.
     ///   (See below for an example.)
     ///
-    /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
-    ///
     /// * 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.
     ///
-    /// The compiler and standard library generally try to ensure 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())` is always safe.
+    /// 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 an allocation.
+    /// 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.)
+    ///
+    /// [`add`]: #method.add
     ///
     /// # Panics
     ///

library/core/src/ptr/mut_ptr.rs

@@ -491,25 +491,31 @@ impl<T: ?Sized> *mut T {
     /// * Both pointers must be *derived from* a pointer to the same object.
     ///   (See below for an example.)
     ///
-    /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
-    ///
     /// * 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.
     ///
-    /// The compiler and standard library generally try to ensure 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())` is always safe.
+    /// 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 an allocation.
+    /// 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.)
+    ///
+    /// [`add`]: #method.add
     ///
     /// # Panics
     ///