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Explicitly document the size guarantees that Option makes. #75454

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Merged
merged 8 commits into from
Sep 26, 2020
Merged

Explicitly document the size guarantees that Option makes. #75454

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73ada2d
Explicitly document the size guarantees that Option makes.
ltratt Aug 12, 2020
f5118a5
Clarify and add guarantee about `transmute`.
ltratt Aug 12, 2020
83f47aa
Be clear about the reverse `transmute` guarantees.
ltratt Aug 12, 2020
f3d7196
Be clearer about Some/None transmute.
ltratt Aug 12, 2020
8cb8955
Change notation.
ltratt Aug 12, 2020
55802e3
Add Rust function pointers.
ltratt Aug 12, 2020
68209c3
Rename the types for clarity.
ltratt Aug 17, 2020
9bac577
Grammar tweak.
ltratt Aug 17, 2020
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21 changes: 17 additions & 4 deletions library/core/src/option.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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
//!
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