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Rollup merge of #75454 - ltratt:option_optimisation_guarantees, r=dtolnay

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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
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Ralf Jung 2020-09-26 12:58:12 +02:00 committed by GitHub
commit 1e62382a4f
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21
library/core/src/option.rs
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@ -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
//!