Unstable Rust Features


What is Rust's #![feature(...)]?

When using the nightly channel of Rust or when compiling Rust manually with the appropriate flags, it‘s possible to use unstable features. These features include language additions, library additions, compiler features, and other capabilities not subject to Rust’s usual stability guarantees. Most of these features are temporarily-unstable additions that will become stable after a period of time has passed during which testing, discussion, and further design has completed. Some features, however, are intentionally permanently-unstable features intended for internal compiler use. Other features may be removed completely when a better solution has been found or when it was determined that the downsides of the feature outweighed the advantages. Each feature has an associated tracking issue on the rust-lang/rust Github repository.

Our Rust Versioning Process

Fuchsia currently builds using a pinned revision of upstream Rust's master branch. We mirror Rust into this repository. The version used to compile Fuchsia is set in buildtools/fuchsia.ensure. The latest revision of Rust which can be set in buildtools is the most recently completed build here. We currently update the Rust version fairly often to pick up new changes we upstream, such as changes to syscalls used by the standard library.

The Goal

We want to be able to roll forward or backward to other versions of Rust to pick up bugfixes or roll back problem-inducing changes. Depending on too many unstable nightly features could make this process extremely painful.

We also want to have code that is clear and easy to use, and use of unstable or rapidly changing features can make code harder to understand or modify. Unstable features are often poorly documented, and what documentation exists is often out of date.

That said, there are also a number of features that are designed explicitly for Fuchsia's use cases. These features provide great readability or performance benefits, and our use of them helps to prove them out and move them further along the path to stabilization.

The Process

Unstable feature requests should be sent to the rust-fuchsia@fuchsia.com mailing list. They should be hosted on docs.google.com to allow for comments and suggestion on the document itself. Proposals should include the following information:

  • A quick summary of the feature
  • What the feature is used for in Fuchsia
  • A summary of what is left before the feature can be stabilized
  • A person in charge of owning a particular feature who will follow the tracking issue, participate in discussion on how to modify or stabilize the feature, and manage any necessary updates to Fuchsia code that result from breaking changes to the feature or removal of the feature.

Following this email is a week-long comment period during which any arguments for or against a feature should be laid out on the doc. Once this period is over, a group of reviewers will meet and come to a consensus decision on whether or not to allow use of the feature. This decision will be based on arguments previously discussed on the doc, and will not include new arguments brought by the review board members. If new arguments surface, they will be added to the doc and more time will be given for others to respond.

If the feature is approved, the feature summary, usage, stabilization report, and owner listed in the doc are added to the “Currently Used Features” section listed below. This documentation must be checked in before the feature can be used.

The current list of reviewers is as follows:

Currently Used Features

This list includes all of the unstable features currently used in Fuchsia.

  • Rust 2018 Edition
    • Summary: The Rust 2018 edition is the upcoming release of Rust that makes several breaking changes while still allowing interop with non-Rust 2018 crates. The main source of breakages are (1) changes to the module system and (2) the addition of new keywords. For more details on how editions work, see the edition guide.
    • Use in Fuchsia: An “edition” key has been added to the rustc_xxx set of GN templates. All Rust targets will gradually move towards the use of edition = “2018” to prepare for the upcoming release and allow for early use of the async/await feature. async blocks and closures are only allowed in Rust 2018 because async was not a keyword in Rust 2015.
    • Remaining before stabilization: There’s some ongoing discussion around final non-breaking tweaks to the new module system and perhaps some in-band-lifetimes lints against single-letter lifetimes in impl body headers, but overall the edition is shaping up fast and is set to ship later this year. For more details, see Rust 2018: an early preview and Rust 2018: the home stretch. There are no major breaking changes expected before stabilization, and shipping the 2018 edition is a primary goal for this year, so the risk of never stabilizing is extremely low.
    • Owner: cramertj@
  • async_await and await_macro
    • Summary: These two features enable the syntactic transformation from code that looks like move async { await!(x); await!(y), await!(z); } into an anonymous enum State { First(X, Y, Z), Second(Y, Z), Third(Z), Done } that implements the Future trait. This also makes it possible to await non-’static futures by internally creating a self-referential generator, allowing for significantly more performant, idiomatic, readable, and writable code than the current futures combinators.
    • Use in Fuchsia: These features will be used in fuchsia-async and in applications which make use of the async/await feature in order to achieve more performant, ergonomic, and idiomatic async code.
    • Remaining before stabilization: Apart from being dependent on all the other features, there are no major additional stability risks (read: no major unresolved design decisions) introduced by these features. There are changes planned to the implementation, but they’re mostly polish, performance optimizations, and a few scattered bugfixes.
    • Owner: cramertj@
  • pin
    • Summary: This feature includes the PinMut type and the Unpin trait. PinMut is a mutable reference to an object that does not allow moving the underlying object unless it implements the Unpin trait. This is used to create sound self-referential types.
    • Use in Fuchsia: Needed for async_await
    • Remaining before stabilization: This feature has gone through a number of design iterations and is the most likely to receive backwards-incompatible changes -- there are a number of design ideas floating around about the potential to make it safe to project from PinMut<MyType> to PinMut<FieldOfMyType>, but these have so far been fruitless (mostly depending on features that aren’t even RFC’d, let alone implemented). It’s likely that PinMut will stabilize without these extensions, but cramertj@ will be in charge of managing any transition necessary. This feature has also been proposed for stabilization.
    • Owner: cramertj@
  • arbitrary_self_types
    • Summary: This simple feature enables the use of self: PinMut<Self> which is necessary to write implementations of the Future trait.
    • Use in Fuchsia: Needed for async_await
    • Remaining before stabilization: This is a particularly simple feature and likely won’t see any breaking changes (especially to our minimal usage).
    • Owner: cramertj@
  • futures_api
    • Summary: This feature enables the std::task and std::future modules which provide the Future interface for the standard library.
    • Use in Fuchsia: Needed for async_await
    • Remaining before stabilization: As in async/await, cramertj@ wrote the implementation of this feature, and while its surface area is large, there aren’t significant planned changes to it. cramertj@ will manage any necessary transition.
    • Owner: cramertj@