Rollup merge of #75454 - ltratt:option_optimisation_guarantees, 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
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//! }
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//! }
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//! ```
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//! ```
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//!
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//!
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//! This usage of [`Option`] to create safe nullable pointers is so
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//! # Representation
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//! common that Rust does special optimizations to make the
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//!
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//! representation of [`Option`]`<`[`Box<T>`]`>` a single pointer. Optional pointers
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//! Rust guarantees to optimize the following types `T` such that
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//! in Rust are stored as efficiently as any other pointer type.
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//! [`Option<T>`] has the same size as `T`:
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//!
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//! * [`Box<U>`]
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//! * `&U`
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//! * `&mut U`
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//! * `fn`, `extern "C" fn`
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//! * [`num::NonZero*`]
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//! * [`ptr::NonNull<U>`]
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//! * `#[repr(transparent)]` struct around one of the types in this list.
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//!
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//! It is further guaranteed that, for the cases above, one can
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//! [`mem::transmute`] from all valid values of `T` to `Option<T>` and
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//! from `Some::<T>(_)` to `T` (but transmuting `None::<T>` to `T`
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//! is undefined behaviour).
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//!
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//!
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//! # Examples
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//! # Examples
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//!
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//!
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