This is a project group RFC version of lang-team#29.

This RFC establishes a new project group, under the libs team, to produce a portable SIMD API in a new rust-lang/stdsimd repository, exposed through a new std::simd (and core::simd) module in the standard library in the same manner as stdarch. The output of this project group will be the finalization of RFC 2948 and stabilization of std::simd.


The current stable core::arch module is described by RFC 2325, which considers a portable API desirable but out-of-scope. The current RFC 2948 provides a good motivation for this API. Various ecosystem implementations of portable SIMD have appeared over the years, including packed_simd, and wide, each taking a different set of trade-offs in implementation while retaining some similarities in their public API. The group will pull together a “blessed” implementation in the standard library with the explicit goal of stabilization for the 2021 edition.



  • Determine the shape of the portable SIMD API.
  • Get an unstable std::simd and core::simd API in the standard library. This may mean renaming packed_simd to stdsimd and working directly on it, or creating a new repository and pulling in chunks of code as needed.
  • Produce a stabilization plan to allow portions of std::simd to be stabilized when they’re ready, and coordinate with other unstable features.
  • Respond to user feedback and review contributions to the API.
  • Update RFC 2948 based on the final API and stabilization plan.
  • Stabilize std::simd!

Non Goals

  • This group isn’t directly attempting to build out more core::arch APIs.

Membership Requirements

  • Group membership is open, any interested party can participate in discussions, repeat contributors will be added to appropriate teams.

Additional Questions

What support do you need, and separately want, from the Rust organization?

Support scaffolding a space to work and integrating stdsimd into libcore and input from engineers who are familiar with this space.

Why should this be a project group over a community effort?

Community efforts have already produced libraries that are in use, but pulling those together in the standard library needs a group with permissions to get things merged.

What do you expect the relationship to the team be?

The project group will regularly update libs on how things are going, whether there are any blockers

Who are the initial shepherds/leaders? (This is preferably 2–3 individuals, but not required.)

  • @BurntSushi
  • @calebzulawski
  • @hsivonen
  • @KodrAus
  • @Lokathor

Is your group long-running or temporary?


If it is temporary, how long do you see it running for?

Until the 2021 edition, which is probably mid 2021.

If applicable, which other groups or teams do you expect to have close contact with?

The project group will interact with:

  • libs
  • compiler

Where do you see your group needing help?

There will be lots of feedback to gather from users and input from compiler developers on how to approach implementation.