Auto merge of #115333 - joshlf:patch-5, r=RalfJung

Guarantee representation of None in NPO

This allows users to soundly transmute zeroes into `Option` types subject to the null pointer optimization (NPO). It unblocks google/zerocopy#293.

Author: bors

library/core/src/option.rs

@@ -119,15 +119,21 @@
 //! # Representation
 //!
 //! Rust guarantees to optimize the following types `T` such that
-//! [`Option<T>`] has the same size and alignment as `T`:
-//!
-//! * [`Box<U>`]
-//! * `&U`
-//! * `&mut U`
-//! * `fn`, `extern "C" fn`[^extern_fn]
-//! * [`num::NonZero*`]
-//! * [`ptr::NonNull<U>`]
-//! * `#[repr(transparent)]` struct around one of the types in this list.
+//! [`Option<T>`] has the same size and alignment as `T`. In some
+//! of these cases, Rust further guarantees that
+//! `transmute::<_, Option<T>>([0u8; size_of::<T>()])` is sound and
+//! produces `Option::<T>::None`. These cases are identified by the
+//! second column:
+//!
+//! | `T`                                                                 | `transmute::<_, Option<T>>([0u8; size_of::<T>()])` sound? |
+//! |---------------------------------------------------------------------|----------------------------------------------------------------------|
+//! | [`Box<U>`]                                                          | when `U: Sized`                                                      |
+//! | `&U`                                                                | when `U: Sized`                                                      |
+//! | `&mut U`                                                            | when `U: Sized`                                                      |
+//! | `fn`, `extern "C" fn`[^extern_fn]                                   | always                                                               |
+//! | [`num::NonZero*`]                                                   | always                                                               |
+//! | [`ptr::NonNull<U>`]                                                 | when `U: Sized`                                                      |
+//! | `#[repr(transparent)]` struct around one of the types in this list. | when it holds for the inner type                                     |
 //!
 //! [^extern_fn]: this remains true for any other ABI: `extern "abi" fn` (_e.g._, `extern "system" fn`)
 //!