Rollup merge of rust-lang#75454 - ltratt:option_optimisation_guarante…

…es, r=dtolnay

Explicitly document the size guarantees that Option makes.

Triggered by a discussion on wg-unsafe-code-guidelines about which layouts of `Option<T>` one can guarantee are optimised to a single pointer.

CC @RalfJung

library/core/src/option.rs

@@ -70,10 +70,23 @@
 //! }
 //! ```
 //!
-//! This usage of [`Option`] to create safe nullable pointers is so
-//! common that Rust does special optimizations to make the
-//! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
-//! in Rust are stored as efficiently as any other pointer type.
+//! # Representation
+//!
+//! Rust guarantees to optimize the following types `T` such that
+//! [`Option<T>`] has the same size as `T`:
+//!
+//! * [`Box<U>`]
+//! * `&U`
+//! * `&mut U`
+//! * `fn`, `extern "C" fn`
+//! * [`num::NonZero*`]
+//! * [`ptr::NonNull<U>`]
+//! * `#[repr(transparent)]` struct around one of the types in this list.
+//!
+//! It is further guaranteed that, for the cases above, one can
+//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
+//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
+//! is undefined behaviour).
 //!
 //! # Examples
 //!