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Explicitly document the size guarantees that Option makes.

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Triggered by a discussion on wg-unsafe-code-guidelines about which layouts of
`Option<T>` one can guarantee are optimised to a single pointer.
This commit is contained in:
Laurence Tratt 2020-08-12 15:07:38 +01:00
parent 5989bf4872
commit 73ada2d404
1 changed files with 12 additions and 4 deletions
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16
library/core/src/option.rs
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@ -70,10 +70,18 @@
//! }
//! ```
//!
//! 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 optimise the following inner types such that an [`Option`] which contains
//! them has the same size as a pointer:
//!
//! * `&T`
//! * `&mut T`
//! * `extern "C" fn`
//! * [`num::NonZero*`]
//! * [`ptr::NonNull<T>`]
//! * `#[repr(transparent)]` struct around one of the types in this list.
//! * [`Box<T>`]
//!
//! # Examples
//!