2025-06-16 — Last Week in Carbon

[josh11b]

News

Chandler Carruth will be giving a Marquee Talk at RustConf 2025 on Carbon, full title and abstract coming soon!

Updates on leads issues

• Decisions
  • (none)
• New issues
  • (none)
• Active discussions
  • #5522: Immutable value semantic returns questions from #5434
    • potential resolution proposed
  • #5523: Orthogonality in the binding design questions prompted by #5434

Updates on proposals

• Decisions
  • #5670: Guidance on AI coding tools
    • New proposal, already approved
• New RFC proposals
  • #5661: Progressive disclosure principle
• Active discussions
  • #5434: ref parameters, arguments, returns and let returns
  • #5448: Carbon/C++ Interop: Primitive Types proposal

Updates on moderation across our community spaces (Discord and GitHub)

• 10 trained moderation & conduct team members + community lead, in theory available from UTC-7 to UTC+5.5.
  • WIP: Talking community journey with Zig folks (3 interviews so far, more TBC).
  • BLOCKED: resuming conversation about how we can be more intentional about providing good first issues for newcomers
  • No conduct related moderation intervention for a whole month! 👏
• Last week’s stats across our spaces:
  • In the last 7 days in our repo: 8 authors pushed 37 commits to all branches, 36 of which to trunk, we changed 395 files with 10,072 additions and 12,864 deletions, with 36 merged PRs, 8 open PRs, have closed 2 issues and opened 1 new one.

Activity on toolchain

• 06-10 Iteration on value stores; LLVM symbolizer;
• 06-10 #5401 PR to alphabetize type_inst.h scope has grown substantially
• #generics-and-templates Currently deduce doesn't look at rewrite rules
• #generics-and-templates Interaction between rewrite resolution and eval
• #generics-and-templates Storing more data in rewrite constraints, and the interaction with import
• Discord #toolchain (begin, end)
  • June 9: Which semir nodes to mark as errors to get good error handling, @danakj and @jonmeow from #5620
  • June 10: Roundtrippability of Inst to/from AnyFoo
  • June 11: Continuation of previous, centered around importing FacetType containing an error
  • June 12: Canonicalization of constants involving errors
  • June 12: Carbon Copy toolchain update summary
• PRs:
  • test infrastructure #5629 #5635 #5648 #5657
  • rewrites in facet types in #5620 #5639 #5660
  • other facet work #5647 #5644 #5664
  • value stores in #5633 #5638 #5636
  • C++ import in #5614 #5468
  • dependencies and tools in #5637 #5665 #5669 #5652 #5671
  • dev workflow #5658 #5646
  • deferred definition worklist in #5608 #5609
  • lowering generics & thunks in #5631 #5642
  • test updates in #5594 #5615
  • others in #5641 #5656 #5659 #5666 #5643
  • 🎉 heterogeneous operators in #5653

Activity on C++ interop

• 06-10 Discussed reconciling a C++ type included in different files and exposed on API boundaries.
• 06-10 Discussed how to select an overload and call it. Decided to keep thunk generation independent and do it after we choose an overload.

Other active discussions

These include Discord Chat, GitHub Discussions, or open discussion sessions.

• #infra: GitHub actions are failing due to an issue with the GitHub CLI. It's fixed upstream, but not clear when the fix will show up in action runners.
  • Local workaround implemented in #5669 and #5671.
• #infra: mold reportedly gives better linking performance than lld.
  • You can try it by adding build --linkopt=-fuse-ld=mold to your user.bazelrc.
• #infra: "Fix missing deps" pre-commit can make breaking changes to the build graph.
  • #5658 Adds a late pre-commit check that the build graph is valid.
• #llvm: Link to discussion about why Zig is moving away from LLVM, including explanation from Zig lead.
• #style: Discussion of style rule about reference vs. pointer members, and whether the option of a private reference accessor changes the tradeoffs.
  • Consensus for no change to the style rule. Mixed opinions on private accessors.
• #general-design: Discussion of whether/how the rule that forms don't affect name lookup illustrates "progressive disclosure"
• #design-reading-group: Link to A Case for Feminism in Programming Language Design.
• #infra: It's hard to get the toolchain version in Compiler Explorer. Adding it to crash logging.
• #infra: libc++ hardening may have accidentally been disabled in our builds.
• #infra: Has there been a change that would affect GitHub notification emails?
• #generics-and-templates Cycle detection in rewrite rules
• 06-09 Definition of auto needs some way to be generic across template.
• 06-09 Mutable compile-time state within an otherwise runtime function?
• 06-10 Updating the IndexWith operator interface (for [...]) to use forms means we need to support the opposite variance for associated constants
• 06-10 06-11 Constrain types independently of form
  • Use case: controlling the category of return of as, when the type is specified by the argument to the As interface.
  • Decided on: let out ResultForm:! Form where .type = Dest;
  • #generics-and-templates
• 06-10 Named constraints provide progressive disclosure
• 06-13 How to specify returns for ref proposal
  • Perhaps compound returns should be the default?
  • Syntax for returning values
• 06-13 How do we deal with IndexWith that is naturally written as a constraint requiring IndexWithPrimitive twice, once with ref and once with value parameter and return forms.
• 06-13 Issues with using anchor with named constraints

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