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Explicitly document the size guarantees that Option makes. #75454

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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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19 changes: 15 additions & 4 deletions library/core/src/option.rs
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Expand Up @@ -70,10 +70,21 @@
//! }
//! ```
//!
//! 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 an
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Suggested change
//! Rust guarantees to optimize the following types `<T>` such that an
//! Rust guarantees to optimize the following types `T` such that an

I never saw <T> as notation for a type, not sure what that is supposed to indicate.

//! [`Option<T>`] has the same size as `T`:
//!
//! * [`Box<T>`]
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This wording is pretty confusing to me, I think because it uses the same name T to mean two different things. For example this line is talking about what happens when T = Box<T> ... which is never true.

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I struggled to find a good phrasing here; I agree the current phrasing isn't great.

Might something like:

//! Rust guarantees that the following types are optimised such that their size
//! is identical whether or not they are surrounded by an `Option`:

be clearer?

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Maybe just use Box<_> or Box<U>. Naming things is generally a good idea, so I'd not remove the T in the introduction.

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Fixed in 68209c3.

//! * `&T`
//! * `&mut T`
//! * `extern "C" fn`
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//! * [`num::NonZero*`]
//! * [`ptr::NonNull<T>`]
//! * `#[repr(transparent)]` struct around one of the types in this list.
//!
//! For the above cases, it is guaranteed that one can use [`mem::transmute`]
//! between `T` and `Option<T>` and vice versa.
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No, not vice versa! If you transmute a None from Option<T> to T, you get UB.

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Good point. Should there be a guarantee for non-None in such a case, or just rule out the reverse direction entirely?

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Guarantee for non-None seems reasonable.

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Fixed in 83f47aa.

//!
//! # Examples
//!
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