Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Explicitly document the size guarantees that Option makes. #75454

Merged
merged 8 commits into from
Sep 26, 2020
Merged
Changes from 4 commits
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
21 changes: 17 additions & 4 deletions library/core/src/option.rs
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 an
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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>`]
Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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.

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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?

Copy link
Member

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

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

Copy link
Contributor Author

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Fixed in 68209c3.

//! * `&T`
//! * `&mut T`
//! * `extern "C" fn`
ltratt marked this conversation as resolved.
Show resolved Hide resolved
//! * [`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 [`mem::transmute`]
//! from all valid values of `T` to `Option<T>` but only from
//! `Option::Some(T)` to `T` (i.e. transmuting `None` to `<T>` is undefined
ltratt marked this conversation as resolved.
Show resolved Hide resolved
//! behaviour).
//!
//! # Examples
//!
Expand Down