Sized Hierarchy and Scalable Vectors

[nikomatsakis]

Metadata
Contact	@davidtwco
Team(s)	compiler (@davidtwco), lang (@nikomatsakis), libs (@Amanieu), types (@lcnr)
Goal document	2026/scalable-vectors
Funding contact	Funding team

Summary

Over the next year, we will build on the foundational work from 2025 to stabilize
the Sized trait hierarchy and continue nightly support for scalable vectors:

• Stabilize the refined Sized trait hierarchy (without constness), unblocking extern types
• Propose and implement const Sized to support scalable vectors
• Achieve RFC acceptance for [RFC #3838] (Scalable Vectors)
• Land SVE types and intrinsics in stdarch for nightly experimentation
• Continue addressing stabilization blockers for SVE itself
• Begin design work for supporting the Scalable Matrix Extension (SME)

The const Sized work (Part II of [RFC #3729]) is deferred to a future goal,
allowing us to deliver value sooner through the trait hierarchy stabilization.
This future work interacts with ongoing const generics
efforts, as const Sized depends on progress in const traits.

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This issue is intended for status updates only.

For general questions or comments, please contact a goal owner or post in the Zulip topic.

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[davidtwco]

An initial update on what we've been up to and some background:

• This goal is submitted on behalf of the Rust team at Arm, but primarily worked on by myself (@davidtwco) and @JamieCunliffe. Anyone interested can always contact me for updates and I'll keep this issue up-to-date.
• @JamieCunliffe been working on supporting Arm's scalable vector extension (SVE) for a couple years - primarily in RFC: Add a scalable representation to allow support for scalable vectors rfcs#3268 and its implementation Support for a scalable simd representation rust#118917.
  • Through this work, we've discovered other changes to the language necessary to be able to support these types without special cases in the type system, which we're also working on (see below).
  • Jamie is still resolving feedback on this RFC and its implementation, and keeping it rebased. We hope that it can be landed experimentally now that there's a feasible path to remove the special cases in the type system (see below).
  • The next steps for this RFC and implementation are..
    • ..to continue to respond to feedback on the RFC and implementation.
• I've (@davidtwco) been working on Hierarchy of Sized traits rfcs#3729 which improves Rust's support for exotically sized types, and would allow scalable vectors to be represented in the type system without special cases.
  • We've had two design meetings with the language team about the RFC and had a broadly positive reception.
    • There is a non-strict dependency on const traits (Make trait methods callable in const contexts rfcs#3762) which has created uncertainty as to whether this RFC could be accepted without the specifics of const traits being nailed down.
  • I've been working on implementing the RFC: an initial implementation of the non-const traits has been completed and adding the const traits is in-progress.
    • The language team have indicated interest in seeing this land experimentally, but this will depend on whether the implementors of const traits are okay with this, as it would add to the work they need to do to make any syntactic changes requested by the language team in Make trait methods callable in const contexts rfcs#3762.
  • I'm continuing to respond to feedback on the RFC, but as this has largely trailed off, the next steps for this RFC are..
    • ..for the language team to decide to accept, reject, or request further changes to the RFC.
    • ..for progress on the implementation to continue.

[davidtwco]

A small update, we've opened a draft PR for the initial implementation of this - rust-lang/rust#137944. Otherwise, just continued to address feedback on the RFCs.