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Tracking issue for MCP 945: Parse unstable keywords for experimental syntax #153839

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Tracking issue for MCP 945: Parse unstable keywords for experimental syntax#153839
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fmeasedianne
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B-MCP-approvedBlocker: Approved by an accepted MCP but not yet fully implemented.B-unstableBlocker: Implemented in the nightly compiler and unstable.C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCS-tracking-perma-unstableStatus: The feature will stay unstable indefinitely.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

Description

@rustbot
rustbot
opened on Mar 13, 2026

This is a tracking issue for MCP "Parse unstable keywords for experimental syntax" (rust-lang/compiler-team#945).

Note

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.

A tracking issue is however not meant for large scale discussion, questions, or bug reports.
Instead, open a dedicated issue for the specific matter, link to this tracking issue and ping the MCP author.
Alternatively, post in the Zulip stream linked from the MCP.

Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

Steps

  • Implement the MCP
  • Replace builtin # …(…) (gated behind builtin_syntax, Tracking issue for internal syntax #110680) with k#…(…) (gated behind internal_syntax & forced_keywords): Replace builtin # with k# #162232
  • Parse k#gen { … } as gen block in Rust 2021 (don't forget to make rustfmt aware of this, too!)
  • Migrate various built-in macros to internal syntax where appropriate (strictly speaking, this step belongs to Tracking issue for internal syntax #110680 where it's already listed)
  • Adjust documentation
    • In the Rust Compiler Development Guide if applicable (e.g., for refactorings)
      • Add a tutorial section somewhere teaching the reader how to introduce "experimental keywords" (e.g. is, only, own) without breaking the world using forced_keywords
    • In the Rust Forge if applicable (e.g., for policy changes)
    • In the rustc book if applicable (e.g., for target tier or CLI changes)

Unresolved questions

None so far.

Implementation history

Activity

  1. rustbot
    added
    T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.
    C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFC
    S-tracking-unimplementedStatus: The feature has not been implemented.
    B-MCP-approvedBlocker: Approved by an accepted MCP but not yet fully implemented.
    on Mar 13, 2026
  2. rust-lang
    deleted a comment from khalidsaidi on Mar 14, 2026
  3. dianne
    self-assigned this
    on Mar 15, 2026
  4. rust-bors
    added a commit that references this issue on Sep 28, 2026

    Auto merge of #161775 - fmease:kay-hash, r=petrochenkov

    2269776
  5. pull
    added a commit that references this issue on Sep 28, 2026

    Auto merge of #161775 - fmease:kay-hash, r=petrochenkov

    3e31383
  6. fmease
    self-assigned this
    on Sep 28, 2026
  7. fmease
    added
    B-unstableBlocker: Implemented in the nightly compiler and unstable.
    S-tracking-perma-unstableStatus: The feature will stay unstable indefinitely.
    and removed
    S-tracking-unimplementedStatus: The feature has not been implemented.
    on Sep 28, 2026
  8. swimming001
    added a commit that references this issue on Oct 12, 2026

    Auto merge of #161775 - fmease:kay-hash, r=petrochenkov

    b0bdf6f
  9. JonathanBrouwer
    added 5 commits that reference this issue on Nov 3, 2026

    Rollup merge of rust-lang#162232 - fmease:yeet-builtin-hash, r=JohnTitor

    754474b

    Rollup merge of rust-lang#162232 - fmease:yeet-builtin-hash, r=JohnTitor

    bb2b761

    Rollup merge of rust-lang#162232 - fmease:yeet-builtin-hash, r=JohnTitor

    c2da812

    Rollup merge of rust-lang#162232 - fmease:yeet-builtin-hash, r=JohnTitor

    0a0d6ce

    Rollup merge of rust-lang#162232 - fmease:yeet-builtin-hash, r=JohnTitor

    fa0cc7f
  10. rust-bors
    added a commit that references this issue on Nov 3, 2026

    test(rustfmt): specify edition 2021 for test involving `forced_keywords`

    5676e60
  11. jieyouxu
    added a commit that references this issue on Nov 3, 2026

    test(rustfmt): specify edition 2021 for test involving `forced_keywords`

    6da7b55
  12. fmease
    added a commit that references this issue on Nov 3, 2026

    test(rustfmt): specify edition 2021 for test involving `forced_keywords`

    fbd4136
  13. JonathanBrouwer
    added a commit that references this issue on Nov 3, 2026

    Rollup merge of rust-lang#162232 - fmease:yeet-builtin-hash, r=JohnTitor

    75c8895
  14. rust-bors
    added a commit that references this issue on Nov 4, 2026

    Unrolled build for #162232 in rollup 165436

    fe189c5
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B-MCP-approvedBlocker: Approved by an accepted MCP but not yet fully implemented.B-unstableBlocker: Implemented in the nightly compiler and unstable.C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCS-tracking-perma-unstableStatus: The feature will stay unstable indefinitely.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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